JP2006271009A - Optical transmission system, optical multiplexing transmission system, and its peripheral technique - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique for optimizing transmission conditions to achieve large-capacity transmission. <P>SOLUTION: Wavelength converters 118 in addition to optical amplifiers 104 are installed on the midway of the optical transmission line 102 to optimize the optical wavelength λs of the signal in each amplification relay zone. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical transmission system and related technology, in particular, an optical transmission system having a transmission path in which transmission conditions are optimized for large-capacity transmission, and optical time division multiplexing for enabling large-capacity transmission ( The present invention relates to an optical transmission system employing an optical multiplexing technology such as Optical Time Division Multiplexing (OTDM) and related technologies for its realization.

  As the transmission speed increases, the transmission distance is severely limited due to waveform degradation due to group-velocity dispersion (GVD) of the optical fiber. Furthermore, when the transmission rate increases, it is necessary to increase the transmission optical power in order to ensure a transmission / reception level difference. Then, the influence of the self-phase modulation (SPM) effect which is a fiber nonlinear effect becomes large, and the waveform deterioration becomes more complicated due to the interaction with the group velocity dispersion (SPM-GVD effect).

When the waveform deterioration due to the SPM-GVD effect is dominant, a scaling law as shown in the following equation is substantially established.
DB ² P _av L ² = const. (1)
D: Dispersion value (ps / nm / km)
B: Transmission speed (Gb / s)
P _av : Average optical power in transmission line (mW)
L: Transmission distance (km)
const. : Determined by the required penalty.
For example, when the transmission rate B is quadrupled from 10 Gb / s to 40 Gb / s, the average optical power P _av in the transmission path needs to be quadrupled. Therefore, in order to ensure the same transmission distance L, the dispersion value D at the signal light wavelength must be set to 1/64.

Therefore, in order to minimize the dispersion value of the signal light, a dispersion shifted fiber (DSF) in which the zero dispersion wavelength λ ₀ of the fiber is shifted to the 1.55 μ band where the transmission loss of the fiber is minimized is used. Transmission in the 1.55 μ band is currently underway. Note that the zero-dispersion wavelength λ ₀ means that the chromatic dispersion D (ps / nm / km), which is the amount of change in transmission delay time with respect to a slight change in wavelength, is changed from a negative value (normal dispersion) to a positive value (abnormal dispersion). This is the wavelength at the point of rotation, and the absolute value of chromatic dispersion is minimized in the vicinity of this wavelength λ ₀ , so waveform distortion due to chromatic dispersion is minimized.

However, the zero dispersion wavelength λ ₀ of the DSF transmission line is inevitably changed in the length direction due to a minute change in the fiber core diameter in the drawing process. Furthermore, multi-core cables of several km segments are connected to the transmission line cable, and λ ₀ between adjacent segments has no continuity and is randomly distributed. In addition, λ ₀ also changes due to changes over time due to changes in environmental temperature and the like.

Therefore, conventionally, the worst design has been adopted in which the distribution of λ ₀ and the change with time are taken into consideration so that even if the worst value continues from the start point to the end point, the predetermined transmission quality can be satisfied. For this reason, an increase in the cost of the transmission path is unavoidable, which has been an obstacle to increasing the capacity.
On the other hand, in signal processing such as modulation and demodulation of optical signals, these signal processing are usually performed at the stage of electrical signals, and optical transmission is performed by speeding up electrical signals for modulating optical signals. The mainstream was to speed up the system. However, recently, it has become a problem that it is difficult to increase the speed at an electric signal level by an electronic device. Research and development of optical communication devices at 10 to 40 Gb / s using Si, GaAs, HBT, HEMT, etc. are underway, but it is said that the current level of practical use is up to 10 to 20 Gb / s. It has been broken.

  For this reason, in order to increase the transmission speed of the optical transmission system beyond the operating speed of the electronic device, multiplexing technology in the optical domain is effective. One is a method using wavelength division multiplexing (Wavelength Division Multiplexing: WDM), and the other is a method using time axis multiplexing (Optical Time Division Multiplexing: OTDM), but either method is practical. It is necessary to develop the peripheral technology for the development.

Accordingly, a first object of the present invention is to provide a technique for optimizing transmission conditions in order to enable large-capacity transmission.
A second object of the present invention is to provide a peripheral technique for practical use of optical multiplexing to enable large-capacity transmission.

  According to the present invention, an optical transmitter that generates an optical signal, an optical transmission path that transmits the optical signal generated by the optical transmitter, and an optical receiver that recognizes the optical signal transmitted by the optical transmission path, , An optical amplification repeater that optically amplifies an optical signal transmitted in the optical transmission path, and a wavelength converter that converts the wavelength of the optical signal optically amplified by the optical amplification repeater An optical transmission system is provided.

  According to the present invention, there is provided a drift compensation circuit for an optical modulator of an optical multiplexing system that multiplexes a plurality of optical signals that are respectively modulated by baseband signals in the plurality of optical modulators. A plurality of drive circuits for amplitude-modulating each baseband signal supplied to the modulator with a low-frequency signal; an optical branching device for branching a part of an optical multiplexed signal obtained by multiplexing the optical signals; A photodetector that converts a part of the optical multiplexed signal branched by the splitter into an electrical signal, and a low-frequency signal component included in the output of the photodetector is used in each of the plurality of drive circuits. There is also provided a drift compensation circuit comprising control means for generating a bias signal for drift compensation of each optical modulator by phase detection with the frequency signal.

According to the present invention, the optical time division multiplexing means for time-division multiplexing a plurality of optical signals, and the receiving side can directly extract the clock of the optical signal before multiplexing from the optical multiplexed signal. There is also provided an optical transmitter comprising amplitude difference providing means for applying a difference to the amplitude of each optical signal in the optical multiplexed signal.
According to the present invention, optical time division multiplexing means for time division multiplexing a plurality of optical signal channels, and identification information for identifying each optical signal channel is added to the optical multiplexed signal generated by the optical time division multiplexing means. An identification information extraction circuit for extracting identification information included in the optical signal channel, and an output so that each optical signal channel is output to a predetermined output destination according to the identification information extracted by the identification information extraction circuit. An optical transmission system comprising a control circuit for changing the destination is also provided.

According to the present invention, an identification information extraction circuit that extracts identification information included in an optical signal channel, and each optical signal channel is output to a predetermined output destination according to the identification information extracted by the identification information extraction circuit. An optical receiver comprising a control circuit for changing the output destination is also provided.
According to the present invention, the clock recovery circuit that recovers the clock of each optical signal channel from the optical multiplexed signal, and the light that separates each optical signal channel from the optical multiplexed signal according to the clock recovered by the clock recovery circuit. A switch, an identification information extraction circuit that extracts identification information included in the optical signal channel separated by the optical switch, and each optical signal channel is output to a predetermined output destination according to the identification information extracted by the identification information extraction circuit An optical separator comprising a control circuit for changing the output destination is also provided.

According to the present invention, optical time division multiplexing means for time division multiplexing a plurality of optical signal channels, and identification information for identifying each optical signal channel in the optical multiplexed signal generated by the optical time division multiplexing means are provided. An optical transmitter comprising means for providing is also provided.
According to the present invention, an optical receiver for receiving an optical time division multiplexed signal in which a plurality of optical signals are time division multiplexed and a low frequency signal is superimposed in a time slot of a specific optical signal, An optical switch that separates the optical time division multiplexed signal into optical signals, clock generation means for generating a clock for controlling the optical switch, and a low-frequency signal superimposed on the optical time division multiplexed signal are used. There is also provided an optical receiver comprising clock phase control means for controlling the phase of the clock generated by the clock generation means so as to be synchronized with the optical time division multiplexed signal.

  FIG. 1 is a block diagram of an example of an optical signal transmission system according to the present invention. In FIG. 1, 11 is an optical transmitter, 12 is an optical receiver, 13 is an optical fiber, 14 is a variable wavelength light source, 15 is a variable wavelength filter, 16 and 17 are optical amplifiers or optical repeaters having optical amplifiers, A light receiving unit 19 is a drive circuit.

As the wavelength tunable light source 14 of the optical transmitter 11, for example, a wavelength tunable semiconductor laser having various configurations such as a three-electrode type tunable semiconductor laser already proposed and an external diffraction grating control type tunable semiconductor laser is used. Can do.
FIG. 2 is an explanatory diagram of a wavelength tunable semiconductor laser and shows an outline of a three-electrode type wavelength tunable semiconductor laser. The wavelength tunable semiconductor laser shown in FIG. 2 has an InGaAsP / InP laser configuration. A laser oscillation region 27 including the active layer 25 is formed between the common electrode 21 and the electrode 22, a wavelength fine tuning region 28 is formed between the common electrode 21 and the electrode 23, and the space between the common electrode 21 and the electrode 24 is formed. A rough wavelength tuning region 29 including the diffraction grating 26 is formed. The light output can be controlled by adjusting the current Ip applied to the electrode 23 and the current Id applied to the electrode 24 to change the emission wavelength, and the current Ia applied to the electrode 22. Therefore, by controlling the currents Ia, Ip, and Id from the drive circuit 19, it is possible to control the emission wavelength and output an optical signal modulated according to the transmission information.

The wavelength variable light source 14 is not limited to the direct modulation type that directly drives the light source as described above, but can also be an external modulation type that modulates light from the light source with an external modulator. It is also possible to amplify an optical signal from the wavelength tunable light source 14 with an optical amplifier and send it to the optical fiber 13.
As the optical amplifiers 16 and 17, for example, a rare earth doped optical fiber amplifier doped with Er, Nd, or the like can be used. For example, the Er doped optical fiber amplifier can transmit an optical signal of 1.5 μm band to 1.48 μm or 0 It can be directly amplified by 98 μm excitation light.

  The optical receiver 12 includes, for example, an optical amplifier 17, a wavelength tunable filter 15, and a light receiver 18. The optical amplifier 17 and the wavelength tunable filter 15 can be omitted. The light receiving unit 18 can be configured by various photodiodes, phototransistors, and the like that convert optical signals into electrical signals. In addition, various known configurations can be adopted for the information processing unit that converts it into an electrical signal by the light receiving unit 18, performs equalization processing, performs level identification, reproduces transmission information, and performs reception processing, The illustration is omitted.

  Further, as the wavelength tunable filter 15, filters having various known configurations can be used. For example, B-1055 “Fabry-Perot type wavelength using Si, B-1055 Spring Proceedings of the IEICE Spring Conference” It is also possible to use a tunable filter with temperature control as described under the heading “Review of Selected Optical Filter”. It is also possible to use a fixed wavelength characteristic filter having a pass wavelength characteristic capable of covering the entire wavelength variable range of the wavelength variable light source 14.

  If the distance between the optical transmitter 11 and the optical receiver 12 is not a long distance, the optical amplifier 16 can be omitted. In this case, there is no light between the optical transmitter 11 and the optical receiver 12. They are connected only by the fiber 13. The wavelength tunable light source 14 is an optical fiber for each splice section (segment section) when there is fluctuation of a zero dispersion wavelength along the length direction of the optical fiber 13 or when an optical fiber having a manufacturing unit length is connected. When there are 13 variations in the zero dispersion wavelength, for example, an average value of fluctuations or variations in the zero dispersion wavelength is obtained over the entire length between the optical transmitter 11 and the optical receiver 12, and this is used as the emission wavelength. Adjusted to As will be described later, the signal light wavelength that provides the best transmission characteristics is not always the zero dispersion wavelength.

  When the optical signal is amplified by a rare earth doped optical fiber amplifier and input to the optical fiber for long-distance transmission, if the wavelength of the signal light is an anomalous dispersion region near the zero dispersion wavelength of the optical fiber, Four-wave mixing (FWM) occurs with spontaneously emitted light, and spontaneously emitted light is amplified and S / N deteriorates due to modulation instability. In order to prevent this, the emission wavelength is adjusted to the normal dispersion region while avoiding the vicinity of the zero dispersion wavelength of the optical fiber 13.

  In the case of a long-distance transmission system, optical amplifiers 16 and 17 are generally provided. Since these optical amplifiers 16 and 17 amplify the optical signal and spontaneously emitted light is generated, it is desirable to provide a filter in front of the light receiving unit 18, and this is the wavelength tunable filter 15 capable of adjusting the pass wavelength characteristic. More desirable. In that case, when adjusting the emission wavelength of the wavelength tunable light source 14 of the optical transmitter 11 as described above, the transmission characteristic is further improved by adjusting the pass wavelength characteristic of the wavelength tunable filter 15 according to the emission wavelength. Can be improved.

  FIG. 3 is a block diagram of another example of the optical signal transmission system according to the present invention. In FIG. 3, 30 is a repeater, 31 is an optical transmitter, 32 is an optical receiver, 33 is an optical fiber, 34 is a wavelength variable light source, 35 is a wavelength variable filter, 36 and 37 are optical direct amplifiers, and 38 is a light receiver. , 39 is a drive circuit, and 40 is a wavelength tunable filter. Parts having the same names as those of the embodiment shown in FIG. 1 have the same functions.

  In this example, the repeater 30 is configured by an optical amplifier 36 such as a rare earth-doped optical fiber amplifier and a wavelength tunable filter 40. The wavelength tunable filter 40, together with the wavelength tunable filter 35 of the optical receiver 32, adjusts the wavelength pass characteristics, thereby removing optical components other than the optical signal such as spontaneous emission light generated from the optical amplifier 36 for each repeater 30. The transmission characteristics can be improved. Accordingly, long-distance transmission is further facilitated. As the wavelength tunable filter 40, a fixed wavelength filter having a pass wavelength characteristic that covers the entire variable range of the emission wavelength of the wavelength tunable light source 34 can be used. Further, although the case where the wavelength tunable filter 40 is provided in the subsequent stage of the optical amplifier 36 is illustrated, the present invention is not limited to such a configuration.

  FIG. 4 is a block diagram of still another example of the optical signal transmission system according to the present invention. In FIG. 4, 41 is an optical transmitter, 42 is an optical receiver, 43 is an optical fiber, 44 is a wavelength variable light source, 45 is a wavelength variable filter, 46 and 47 are optical amplifiers, 48 is a light receiver, 49 is a drive circuit, 50 is a wavelength tunable filter, 51 is a repeater, 52 is a sweep control unit, and 53 is a transmission characteristic measurement unit.

  This example corresponds to the case where the transmission characteristic measurement unit 53 and the sweep control unit 52 are added to the embodiment shown in FIG. 3. The drive circuit 49 is controlled by the sweep control unit 52, and the wavelength tunable light source 44. Sweeps the emission wavelength. For example, when the wavelength tunable light source 44 is the wavelength tunable semiconductor laser shown in FIG. 2, the currents Ip and Id are changed. In the case of a semiconductor laser having another configuration, the temperature is sequentially changed to change the emission wavelength. Can be swept. Then, the optical signal whose emission wavelength has been swept is transmitted through the optical fiber 43 and the repeater 51, and the result received by the light receiving unit 48 of the optical receiving unit 42 is added to the transmission characteristic measuring unit 53. The transmission characteristics between the optical transmitter 11 and the optical receiver 12 are measured. Based on the measurement result of the transmission characteristics, the emission wavelength of the wavelength variable light source 44 and the wavelength pass characteristics of the wavelength variable filters 45 and 50 are set so that the transmission characteristics are the best.

  When the wavelength tunable light source 44 and the wavelength tunable filters 45 and 50 are automatically adjusted from the transmission characteristic measuring unit 53 side, the emission wavelength of the wavelength tunable light source 44 is swept from the sweep control unit 52 via the drive circuit 49. The sweep control information to be transmitted is transmitted to the transmission characteristic measuring unit 53 as indicated by a dotted line. The transmission characteristic measurement unit 53 controls the pass wavelength characteristics of the wavelength tunable filters 45 and 50 as indicated by the dotted line in accordance with the sweep control information. The transmission characteristic measuring unit 53 determines the light emission wavelength with the best transmission characteristic from the transmission characteristic in the light receiving unit 48 during the sweep control, transmits the control information to the drive circuit 49 so as to obtain the value, and the light emission wavelength. Control information is added to the wavelength tunable filters 45 and 50 so as to have a pass wavelength characteristic corresponding to.

  Therefore, when the system is started up, by starting the sweep control unit 52, the emission wavelength of the wavelength tunable light source 44 can be automatically set so that the transmission characteristics are the best. During the system operation, the sweep control unit 52 is stopped. However, the transmission characteristic is measured by the transmission characteristic measurement unit 53 periodically or continuously, and the wavelength variable light source 44 is set so that the transmission characteristic is the best. It is also possible to adjust the wavelength transmission characteristics of the wavelength tunable filters 45 and 50.

  Transmission of control information for adjusting the wavelength pass characteristic from the transmission characteristic control unit 53 to each repeater 51 and control information of the best point of the transmission characteristic to be applied to the drive circuit 49, or transmission from the sweep control unit 52 to the transmission characteristic control unit 53 Since the sweep control information can be transmitted at a relatively low speed, it can be transmitted by a control signal line or the like laid between the optical transmitter 11 and the optical receiver 12, and can be transmitted in both directions. In the case of a system for transmitting a signal, it is also possible to superimpose and transmit an optical signal as a sub signal.

When the transmission characteristic is measured by measuring the code error rate in the transmission characteristic control unit 53, the result of measuring the code error rate at each wavelength by sweeping the emission wavelength of the wavelength variable light source 44 is shown in FIG. When the allowable error rate is 10 ⁻¹¹ , for example, the emission wavelength of the wavelength tunable light source 44 is set approximately at the center of the wavelength range that provides the allowable error rate. That is, as the best point of the transmission characteristics, the emission wavelength of the wavelength variable light source 44 is set via the drive circuit 49, and the wavelength pass characteristics of the wavelength variable filters 45 and 50 are set. As a result, the code error rate can be kept below the allowable value even if there is a characteristic variation of the optical fiber 43 due to a temperature change or a secular change.

For the measurement of the code error rate, means for measuring the error rate in a normal transmission system can be applied. If the transmission rate of the optical signal is high, for example, 10 Gb / s or higher, the error rate at each wavelength can be measured in a short time even when the allowable error rate is smaller than 10 ⁻¹¹ . It is also possible to transmit with the parity check bit added and measure the error rate using the parity check bit.

  The transmission characteristic can be measured by using the eye pattern in the transmission characteristic measuring unit 53. For example, FIG. 6 is an explanatory diagram of an eye mask pattern. If an eye mask pattern indicated by a thick line is used as a threshold value, an eye pattern of a received signal is formed outside the thick line eye mask pattern. The light emission wavelength of the wavelength tunable light source 44 is adjusted so that exceeds a predetermined threshold value. If the transmission characteristics are good, the eye pattern opens widely. Therefore, the emission wavelength of the wavelength tunable light source 44 may be adjusted so that the opening of the eye pattern is maximized. As the adjusting means in this case, it is possible to adopt means by observing the eye pattern and performing manual control or by automatic control using computer processing or the like.

There is also a method of measuring the Q value (= electrical SNR) instead of the code error rate. The definition of the Q value is shown in FIG. That is,
Q = 20 log ₁₀ [(μ ₁ −μ ₀ ) / (σ ₁ + σ ₀ )]
However, μ ₁ : Average level at “light emission”
μ ₀ : Average level when “no light emission”
σ ₁ : Standard deviation of level at “light emission”
σ ₀ : standard deviation of the level at “non-light emission”. The Q value is expressed by using the difference between the signal level of light emission and non-light emission (= signal amplitude) for the numerator and the sum of the standard deviations of the noise of light emission and non-light emission for the denominator. Assuming a Gaussian distribution as the noise distribution, the code error rate given by the Q value defined by the above equation matches the minimum value of the actually measured code error rate. The Q factor measurement system has almost the same configuration as the optical receiver, uses an identification circuit having a variable reference voltage function, changes the identification level of the equalized waveform up and down from the optimum level, measures the code error rate, By obtaining the intersection of two straight lines obtained from the measurement, the minimum point of the code error rate can be estimated and the Q value can be obtained. Furthermore, a method of measuring a transmission waveform and using a standard of equal code error rate curve can be applied.

  FIG. 8 is a block diagram of another example of the optical signal transmission system according to the present invention. In FIG. 8, 61 is an optical transmitter, 62 is an optical receiver, 63 is an optical fiber, 64 is a variable wavelength light source, 65 is an optical branching unit, 66a and 66b are external modulators, 67a and 67b are drive circuits, and 68 is An optical multiplexing unit, 69 and 70 are optical amplifiers, 71 is a wavelength tunable filter, 72 is an optical branching unit, 73a and 73b are light receiving units, and 74 is a transmission characteristic measuring unit.

The wavelength tunable light source 64 can be realized by, for example, a wavelength tunable semiconductor laser, as in the above examples. The optical branching unit 65 divides the output light of the wavelength tunable light source 64 into two and adds them to the external modulators 66a and 66b, respectively. .
Clock signals CLKa and CLKb and transmission information not shown are added to the drive circuits 67a and 67b, modulation signals synchronized with the clock signals CLKa and CLKb are added to the external modulators 66a and 66b, and the two-branched light is modulated. Each modulated optical signal is multiplexed by the optical multiplexing unit 68, amplified by the optical amplifier 69, and sent to the optical fiber 63. As the external modulators 66a and 66b, for example, a Mach-Zehnder optical modulator using a LiNbO ₃ substrate, a semiconductor absorption optical modulator, or the like can be used.

  Various multiplexing means such as bit multiplexing, byte multiplexing, and frame multiplexing can be applied to the multiplexing of the optical signal in the optical multiplexing unit 68, and the external modulators 66a and 66b corresponding to such multiplexing means. In the optical multiplexing unit 68, the optical signal is multiplexed by controlling the modulation timing in the optical modulator to control the phase of the output modulated optical signals of the external modulators 66a and 66b to be different.

  The optical branching unit 65 is, for example, a Mach-Zehnder type optical modulator having two output ports separated from the output port side, and the output light of the wavelength tunable light source 64 is incident on this optical modulator, for example, 10 Gb / s. When modulated with the clock signal, 10 Gb / s optical clock signals having opposite phases are output from the two output ports and applied to the external modulators 66a and 66b, respectively. Then, when modulated by the transmission information in the external modulators 66a and 66b and optically multiplexed in the optical multiplexer 68, a multiplexed optical signal having a transmission rate of 20 Gb / s is obtained.

  In the optical receiver 62, when two systems of transmission information are time-division multiplexed, they can be separated after being converted into electrical signals, but in this example, they are amplified by the optical amplifier 70. Then, noise light or the like is removed by the wavelength tunable filter 71, branched by the light branching unit 72, and input to the light receiving units 73a and 73b, respectively. A clock signal similar to the clock signals CLKa and CLKb in the optical transmission unit 61 can be obtained based on the clock signal from the clock recovery unit (not shown), and can be used to output the light receiving units 73a and 73b. Information of two transmission systems is reproduced from the signal.

  Although the transmission characteristic measuring unit 74 can be provided corresponding to each of the light receiving units 73a and 73b, it is provided for any one of the light receiving units to measure the transmission characteristic at the time of system startup or during operation. The emission wavelength of the wavelength tunable light source 64 is set and the pass wavelength characteristic of the wavelength tunable filter 71 is set so that the transmission characteristics are the best. Thereby, high-speed long-distance transmission becomes easy. The optical amplifier 69, the wavelength variable filter 71, etc. can be omitted.

  FIG. 9 is a block diagram of another example of the optical transmission system according to the present invention. In FIG. 9, 81 is an optical transmitter, 82 is an optical receiver, 83 is an optical fiber, 84 is a variable wavelength light source, 85 is an optical branching unit, 86a and 86b are external modulators, 87a and 87b are drive circuits, and 88 is An optical multiplexing unit 89 is an optical direct amplifier, 90 is an optical branching unit, 91a and 91b are optical direct amplifiers, 92a and 92b are wavelength variable filters, 93a and 93b are light receiving units, and 94 is a transmission characteristic measuring unit.

  The optical transmission unit 81 has the same configuration as the transmission unit 61 of the above-described embodiment and operates in the same manner. In the optical receiving unit 82, the optical signal received via the optical fiber 83 is branched by the optical branching unit 90, amplified by the optical amplifiers 91a and 91b, respectively, and the light receiving units 93a and 93b via the wavelength variable filters 92a and 92b. Add to. Accordingly, the two systems of transmission information are subjected to light reception processing by the respective light receiving sections 93a and 93b.

  Similarly to the above-described embodiment, the transmission characteristic measuring unit 94 measures the transmission characteristic between the optical transmission unit 81 and the optical reception unit 82 using the output signal of one of the light receiving units 93a and 93b. The wavelength tunable light source 84 is controlled so that the emission wavelength at the best point of the transmission characteristics is obtained, and the wavelength pass characteristics of the wavelength tunable filters 92a and 92b are controlled. Therefore, high-speed transmission is possible by time-division multiplexing of optical signals, and long-distance transmission is possible by controlling the emission wavelength to have the best transmission characteristics even if there is variation in zero dispersion wavelength. Become.

  The present invention is not limited to the above-described example, and various additions and modifications can be made. In the embodiments of FIGS. 8 and 9, the output light of the wavelength variable light source is branched into two. Although the case is shown, it can be further branched into a large number, and an external modulator is provided for each, and transmission information of multiple systems is time-division multiplexed as a modulated optical signal to enable high-speed transmission. Further, although an embodiment in the case of bit multiplexing is shown, multiplexing means such as byte multiplexing and frame multiplexing can also be adopted. Further, in the embodiment shown in FIGS. 8 and 9, an optical amplifier can be connected at every predetermined distance of the optical fibers 63 and 83 to enable transmission over a longer distance.

  In the examples described so far, the transmission conditions are optimized by using a wavelength tunable light source and adjusting the wavelength of the signal light to an optimum value for a given transmission path. Optimal transmission conditions for a given wavelength can be obtained using a variable dispersion compensator that can fix the wavelength of light and adjust the amount of chromatic dispersion. An example of such an optical transmission system will be described below.

In the examples of FIGS. 10 and 11, the variable dispersion compensator is arranged in the transmission unit, and in the examples of FIGS. 12 and 13, it is arranged in the reception unit. 10 and 12 show a configuration example in the case of a repeaterless transmission system, and FIGS. 11 and 13 show a configuration example in the case of an optical amplification multiple relay system. In the figure, 100 is an optical transmitter, 101 is a variable dispersion compensator capable of varying the amount of dispersion, 102 is a transmission path, 103 is an optical receiver, and 104 is a relay amplifier. In the following description, the same reference numbers indicate the same components.
The dispersion variable compensator 101 used here is a Mach-Zehnder interferometer type dispersion compensator using a PLC (Planar Lightwave Circuit) (for example, the 1994 IEICE Spring Conference C-337 “PLC type optical dispersion equalizer”). Dispersion Compensation Experiment using ”Higuchi et al.) Or optical resonator type dispersion compensator (for example, 1994 IEICE Autumn Meeting B-935“ Discussion of Dispersion Compensation Method Using Optical Resonator ”Fukayo et al.) Etc. can be used.

  FIG. 14 shows an embodiment in which a variable dispersion compensator is also arranged in the repeater in the optical amplification multiple repeater system. However, the present invention is not limited to the configuration in which the dispersion variable compensator is disposed in all the repeaters and the transceivers as in the example of FIG. 14. For example, the configuration in which the dispersion variable compensator is disposed only in the repeater, the transmission A configuration in which a variable dispersion compensator is arranged in the transmitter and the repeater, or a configuration in which a variable dispersion compensator is arranged in the repeater and the receiver is also possible. Moreover, even when arrange | positioning to a repeater, you may arrange | position only to the one part.

  Regarding the dispersion compensation techniques used in FIGS. 10 to 14, various dispersion compensators and dispersions using them have been already applied to any of the land system, submarine system, repeaterless system, and multi-relay system. Although compensation methods have been proposed and implemented, the point of the present invention is that the dispersion compensation amount that can vary the dispersion amount is used to optimize the dispersion compensation amount for each relay section to a value with good transmission characteristics. It is in.

As a method for this optimization, when the zero dispersion wavelength of the transmission line including the fluctuation in the length direction can be grasped in advance, the optimum dispersion compensation amount can be determined from a simulation or the like. To do.
FIG. 15 shows another example. In this example, the dispersion compensation amount is swept while measuring the transmission characteristic on the receiving side, and the variable dispersion compensator 101 is set to a value at which the transmission characteristic is good. Here, since the variable dispersion compensator 101 is disposed in the receiving unit, the transmission characteristic may be measured while sweeping the dispersion compensation amount in the receiver, and set to the optimum dispersion compensation amount. The transmission characteristic measurement method in the transmission characteristic measurement unit 105 may be the same as the transmission characteristic measurement units 53, 74, and 94 already described.

  Another example is shown in FIGS. In this example, in an optical transmission system, a control signal is fed back to a transmitter or a repeater based on transmission characteristics measured on the receiving side, and the dispersion compensation amount of the dispersion variable compensator 101 installed therein is optimized. is there. FIG. 16 shows a configuration in which a variable dispersion compensator is disposed only in the transmission unit. The optimum dispersion compensation amount is obtained by measuring transmission characteristics on the reception side while feeding back the dispersion compensation amount on the transmission side and feeding back the information. Can be set to FIG. 17 shows a case where variable dispersion compensators are arranged in all of the transmitter, receiver, and repeater. Note that in a configuration in which a plurality of dispersion compensators are arranged in the system, all of them need not be variable dispersion compensators, and some of them may use fixed dispersion compensators. In the case of a fixed dispersion compensator, it can also be realized by a dispersion compensating fiber (DCF).

The dispersion compensation amount is controlled by the variable dispersion compensator, not only at the time of system startup, but also during system operation, by controlling the dispersion compensation amount while monitoring the transmission characteristics, thereby changing the wavelength variation and transmission of the light source LD. It is also possible to cope with temperature change and temporal change of the zero dispersion wavelength of the path.
These processes may be performed manually or automatically by the CPU. In addition to providing a CPU for each regenerative relay section between the optical transmitter and the optical receiver and controlling them independently, a plurality of regenerative relay sections can be controlled intensively while adjusting the mutual relationship with one CPU. May be.

  18 and 19 show another example. In this example, the variable dispersion compensator and the variable wavelength light source 106 are used together. FIG. 18 shows a configuration example in the case of a repeaterless transmission system, and FIG. However, as shown in the figure, the present invention is not limited to the configuration in which the variable dispersion compensator is disposed on all of the transmission side, the repeater, and the reception side, and various combinations are possible as in the case of FIG.

  In this optical transmission system, together with the dispersion variable compensator, the wavelength tunable light source 106 is used for the transmission unit, and while measuring the transmission characteristics on the reception side, the transmission wavelength is also swept to set the value so that the transmission characteristics are good. Alternatively, the control signal is fed back to the transmitter based on the transmission characteristics measured on the receiving side, and the wavelength of the wavelength tunable light source 106 is set to an optimum value.

  As described above, particularly when performing transmission at a relatively high optical power level using an optical amplifier (both repeaterless transmission and multi-relay transmission), the signal light wavelength is close to the zero dispersion wavelength of the optical fiber transmission line. In addition, when an abnormal (positive) dispersion region is set, modulation instability occurs due to four-wave mixing (FWM) between signal light and spontaneous emission light (ASE). Thereby, ASE is amplified and S / N of signal light deteriorates. In order to avoid this, it is known that a method in which the signal light wavelength is set in a normal (negative) dispersion region and positive dispersion compensation is performed at the receiving side or a repeater is effective. That is, the wavelength of the wavelength tunable light source is set so that the dispersion value is negative with respect to the transmission line and the FWM can be suppressed, and at the same time, the dispersion amount of the dispersion compensator is set to a positive value. Compensate. By setting the wavelength of the wavelength tunable light source so that the dispersion value is positive and the FWM can be suppressed with respect to the transmission line, and at the same time, the dispersion amount of the dispersion compensator is set to a negative value. Dispersion compensation may be performed. These transmission wavelengths may be set automatically. The transmission wavelength may be set when the system is started up. Alternatively, the transmission wavelength may be set during system operation.

If there is a protection line that has almost the same dispersion conditions and installation environment as the operation line, first optimize the dispersion compensation amount and transmission wavelength in the protection line, and then refer to them to determine the operation line. It is also possible to apply this method. As a result, each optimization can be performed without bringing down the service.
The parameters for controlling the conditions of the transmission line include the pre-chirping amount in addition to the signal light wavelength (FIGS. 1, 3, 4, 8, 9, 18, and 19) and the dispersion compensation amount (FIGS. 10 to 19). And the power of light input to the fiber.

  The pre-chirping method is a method of controlling a change in transmission waveform due to chromatic dispersion and nonlinear effect by giving a wavelength (frequency) distribution in one pulse of a transmission wavelength in advance. Various methods have been proposed so far. Yes. When, for example, a Mach-Zehnder type optical modulator is used as the external modulator, the relationship between the applied voltage and the optical output becomes a sine curve as shown in FIG. When a positive pulse as shown in FIGS. 21 (1) and 21 (a) is applied using the vicinity of Vb1 as the applied voltage, a positive light pulse with the same phase as the applied voltage as shown in FIGS. 21 (2) and (a). Is output. At this time, as shown in FIGS. 21 (3) (a), the wavelength of light is shorter than the average value at the rising portion of the optical pulse and is longer at the falling portion. That is, in one optical pulse, the wavelength shifts with time from a short wavelength (blue side) to a long wavelength (red side). This phenomenon is called red shift. On the other hand, when a negative pulse as shown in FIGS. 21 (1) and 21 (b) is applied using the vicinity of Vb2, the phase is opposite to the applied voltage phase as shown in FIGS. 21 (2) and 21 (b). Are output. At this time, as shown in FIGS. 21 (3) and 21 (b), the wavelength of light shifts to the long wavelength side at the rising portion of the optical pulse, and shifts to the short wavelength side at the falling portion. That is, in one light pulse, the wavelength shifts from a long wavelength (red side) to a short wavelength (blue side) with time. This phenomenon is called blue shift. Assuming that the parameter representing the chirping amount is α, α> 0 for red shift and α <0 for blue shift. When the wavelength of the signal light is shorter than the zero dispersion wavelength and the transmission condition of the optical fiber is in the normal dispersion (D <0) region, the long wavelength light travels faster in the optical fiber than the short wavelength light. If pre-chirping of α> 0 (red shift) is given in advance, an effect of sharpening the pulse waveform is obtained, and the waveform deterioration is improved. On the contrary, short wavelength light is faster when it is in the region of anomalous dispersion (D> 0). Therefore, if pre-chirping of α <0 (blue shift) is applied in advance, the waveform deterioration is improved. Further, the transmission condition of the entire optical system can be optimized by adjusting the value of α according to the condition of the transmission path. In the Mach-Zehnder optical modulator, the sign of α can be switched depending on whether the operating point Vb1 or the operating point Vb2 is used as described above. Also, as shown in FIG. 22, if the applied voltage of the phase modulation unit 108 is varied using a Mach-Zehnder type optical modulator in which the intensity modulation unit 107 and the phase modulation unit 108 are connected in tandem, the pre-chirping amount α can be reduced. It can be continuously varied. In the illustrated example, the intensity modulation unit and the phase modulation unit are integrally integrated, but individual devices may be connected.

  As for the power of the fiber input light, the state of the waveform change due to the interaction between the self-phase modulation effect and the chromatic dispersion (SPM-GVD effect) in the transmission line can be achieved by changing the transmission light power and the repeater light output power. change. In the case of WDM transmission, the amount of crosstalk by FWM (described later) also changes. Note that these optical power changes can be easily realized by controlling the optical output power of the transmission light source and the optical amplifier (optical post-amplifier, optical in-line amplifier).

The control of the pre-chirping amount and / or the optical power is controlled in place of or in combination with the control of the signal light wavelength and / or the dispersion compensation amount in the examples of FIGS. Can be implemented.
In the examples described so far, in order to cope with the change with time of the zero dispersion wavelength λ ₀ of the transmission line, the transmission characteristics are measured periodically or continuously, and the control parameters such as the wavelength of the signal light are adjusted. ing. One of the causes of the change of λ ₀ with time is a change in the temperature of the transmission line. As for this, it is possible to optimize the transmission condition by estimating the shift amount of the zero dispersion wavelength by evaluating the temperature of the transmission line and correcting the control parameter based on the estimated shift amount.

In the optical amplification multi-relay WDM system using a band near the zero-dispersion wavelength of an optical fiber, there is a crosstalk due to four-wave mixing between signal lights as a deterioration factor of transmission characteristics. In order to avoid this crosstalk, it is necessary to separate the signal band from the zero dispersion wavelength of the transmission line, as opposed to the case of single wave transmission. An arrangement example of signal light wavelengths is shown in FIG. Also in this case, it is important to grasp the variation in the length direction of λ ₀ of the actual optical fiber transmission line.

FIG. 24 shows a measurement example of the temperature dependence of λ ₀ .
Source: H. Onaka et al., “Measuring the Longitudial Distribution of Four-Wave Mix
ing Efficiency in Dispersion-Shifted Fibers ", IEEE Photonics Technology
Letters, Vol. 6, No. 12, 1994.
Here, for a DSF having a length of 1.1 km, λ ₀ is obtained from the generation efficiency of four-wave mixing (FWM). Thus, a change of 2.4 nm (change rate: 0.03 nm / ° C.) is shown in the temperature range of −20 to + 60 ° C. Since the dispersion slope of the DSF used here is 0.07 ps / nm ² / km, the chromatic dispersion value changes by 0.168 ps / nm / km. This change may have to be fully taken into account in system design, along with length variations, at transmission rates of 10 Gb / s and higher.

  The temperature is evaluated at an appropriate point 110 in the optical fiber transmission line 102 laid between the optical transmitter 100 and the optical receiver 103 as shown in FIG. 25 or at a plurality of points 110 as shown in FIG. If the temperature is evaluated at a plurality of points, the distribution of the shift amount of the zero dispersion wavelength can be known. In the case of the optical amplification repeater transmission system, as shown in FIG. 27, a part or all of the repeater transmission line 102 is performed at one point or a plurality of points.

  As a method of temperature evaluation, in addition to the method of directly measuring the temperature of the optical fiber cable in the transmission line using an appropriate temperature sensor, the pipe temperature and the surface of the optical fiber cable when it is buried in the ground If the temperature is laid in the sea, the temperature of the optical fiber cable can be estimated by measuring the water temperature. It can also be inferred from measurements of temperature and surface temperature at the terminal station or relay station. If an optical fiber for temperature measurement is laid along the optical fiber cable and the Raman scattered light is measured using an OTDR (Optical Time Domain Reflectometry) method, a continuous temperature distribution can be measured.

The amount of variation of λ ₀ is grasped from the above temperature evaluation values, and the control parameters such as the signal light wavelength are corrected accordingly. In addition, a method of creating an average seasonal variation and day / night variation calendar from the result of past temperature evaluation and changing the control parameters based on it (may be programmed in advance) is also conceivable.
FIG. 28 and FIG. 29 show an example in which the wavelength variable light source 106 is controlled based on the temperature evaluation value, thereby correcting the wavelength of the signal light for each regenerative repeat section to obtain the optimum transmission condition. FIG. 28 shows an example of a repeaterless transmission system, and FIG. 29 shows a case of an optical amplification repeater transmission system. FIG. 30 shows an example of correcting the pre-chirping amount α based on the temperature evaluation value.

  31 to 33 are examples of correcting the dispersion compensation amount. Although the case of an optical amplification repeater transmission system is shown, it is needless to say that it can be applied to a repeaterless transmission system. 31 shows an example in which the variable dispersion compensator 101 is installed in the transmission unit, FIG. 32 shows an example in which the dispersion variable compensator 101 is installed in the reception unit, and FIG. 33 shows the dispersion compensator 101 in the transmission unit, reception unit, and each repeater. An example of installing is shown.

FIG. 34 shows an example in which the waveform change due to the SPM-GVD effect is improved by correcting the transmission power and the repeater optical output power based on the temperature evaluation value. The light source may be controlled instead of controlling the amplifier. FIG. 35 shows an example of correcting the signal light wavelength, the pre-chirping amount, the dispersion compensation amount, and the optical power.
These correction processes may be performed manually or automatically by the CPU. In addition to providing a CPU for each regenerative repeater section between the optical transmitter and the optical receiver and controlling them independently, a plurality of regenerative repeater sections are controlled centrally while adjusting the mutual relationship with one CPU. May be.

The SPM effect described above is supposed to occur because the refractive index of the fiber changes abruptly when the light intensity changes abruptly. Therefore, if the rise time and fall time of the optical pulse are forcibly extended and the change in the intensity of the optical signal is transmitted smoothly, the waveform deterioration due to the SPM effect can be reduced. In this case, rather than simply changing only the light intensity and extending the rise / fall transition time, it is better to forcibly generate chromatic dispersion and extend the transition time by means such as dispersion compensation at a later stage. Can be compensated for. As means for forcible chromatic dispersion, there are a method in which the signal light wavelength is intentionally shifted from the zero dispersion wavelength λ ₀ and dispersed by GVD, and a method in which a dispersion compensator is inserted on the transmission side.

36 and 37 show an example in which the wavelength λ _{s of the} signal light is set to a value away from the zero dispersion wavelength λ ₀ of the DSF, and the dispersion compensator 112 having a fixed dispersion value is arranged on the receiving side. 36 shows the case of relayless transmission, and FIG. 37 shows the case of multi-relay transmission. The dispersion value D of the dispersion compensator 112 is a value that can compensate for GVD due to λ _s ≠ λ ₀ .
38 and 39 show the side where the dispersion compensator 112 is arranged on the transmission side, FIG. 38 shows the case of non-relay transmission, and FIG. 39 shows the case of multi-relay transmission. Also in this case, the dispersion value D of the dispersion compensator 112 is a value that can compensate for the GVD due to λ _s ≠ λ ₀ .

In the example of FIG. 36 or FIG. 37, as shown in FIG. 40, particularly when λ _s <λ ₀ and D> 0 are set, λ _s is in the negative dispersion region, so signal light and spontaneous emission light And four-wave mixing is prevented. Of course, as shown in FIG. 41, a combination of λ _s > λ ₀ and D <0 may be used. As shown in FIGS. 42A and 42B, dispersion compensators having opposite polarities of the dispersion value D may be arranged on both the reception side and the transmission side. Furthermore, as shown in FIG. 43, dispersion compensators may be arranged on all or part of the transmission side, the reception side, and each repeater.

As described above, in order to suppress the SPM effect, the transmission characteristic is measured after disposing a dispersion compensator assuming that the wavelength λ _{s of the} signal light is away from the zero dispersion wavelength λ ₀ , and according to the result, λ Further optimization can be performed by correcting _s to an optimum value. 44 to 46 schematically show some examples of the system configuration. All of the many variations already described can be applied to the measurement method and control mode of the transmission characteristics. Further, as shown in FIGS. 47 to 49, the signal light wavelength λ _s may be fixed and the pre-chirping amount may be controlled. Further, as shown in FIGS. 50 to 52, both the signal light wavelength λ _s and the pre-chirping amount may be controlled. As described above, the control of the pre-chirping amount can be realized by a Mach-Zehnder type optical modulator.

In the example described so far, in one regenerative repeating section sandwiched between an optical transmitter and an optical receiver, a single signal light wavelength is used even if an optical amplification repeater is provided. In the example described below, the wavelength converter is arranged in the optical amplifying repeater to optimize the signal light wavelength λ _s for each amplification repeating section.
FIG. 53 shows an example of an optical transmission system in which the wavelength converter 118 is arranged in the optical amplification repeater and the signal light wavelength is optimized for each amplification repeater section. In FIG. 53, the wavelength converter 118 is arranged in all the amplification relay sections, but this is not a limitation. As shown in FIG. 54, the wavelength variable light source 106 may be arranged in the transmission unit, and the signal light wavelength in the section from the transmission unit to the first optical amplification repeater may be further optimized.

  The wavelength converter 118 can be realized by using a wavelength conversion laser by an optical bistable laser shown in FIG. 55, for example. The left part of the figure is an optical bistable region, and the electrode on the active layer 120 is separated into two, and this part is used as a saturable absorption region. When the current in the gain regions 122 and 124 is adjusted to keep the element in a state immediately before oscillation, when the input light is injected, the saturable absorption region 126 becomes transparent and laser oscillation occurs, and output light of different wavelengths is generated. can get. On the other hand, the right part of the figure is an oscillation wavelength control region, which consists of a DBR region 130 having a phase shift region 128 and a diffraction grating 129. When a current is injected into the DBR region 130, the refractive index of the light guide layer 132 decreases due to the plasma effect by carriers, and the Bragg wavelength can be moved to the short wavelength side. Further, by changing the injection current to the phase shift region 128, the equivalent optical path length of this region can be changed, and the phase of light can be adjusted to the oscillation condition. Therefore, the wavelength of the output light can be controlled over a wide range by appropriately changing the currents in the two regions.

As a second implementation example of the wavelength converter 112, there is one that positively utilizes the phenomenon of four-wave mixing. When light having two wavelengths λ ₀ and λ _in near the zero dispersion wavelength is incident on the DSF, light of λ _out = λ ₀ + (λ ₀ −λ _in ) is generated by four-wave mixing. If λ ₀ is generated from a wavelength tunable light source and is made variable, and only λ _out light is extracted from the output light by a filter, λ _in can be converted into λ _out and the wavelength can be controlled.

By setting the signal light wavelength for each optical amplification repeater section, the transmission cost can be reduced by increasing the transmission speed by further suppressing the chromatic dispersion, and by expanding the allowable range of variation of the zero dispersion wavelength λ ₀ . Reduction is planned. In addition, it does not bother to regenerate and re-set the signal light wavelength, but uses the wavelength conversion while maintaining the high-speed optical signal in the optical amplifying repeater, so that the two-time photoelectric conversion and the processing by the high-speed electronic circuit are performed. Therefore, the system can be reduced in size and cost can be reduced.

  When the zero-dispersion wavelength of the transmission line including the fluctuation in the length direction can be grasped in advance, the optimum signal light wavelength is set for each optical amplification repeater section from simulation or the like. If the zero-dispersion wavelength of the transmission line is unknown, measure the transmission characteristics on the receiving side while sweeping the wavelength tunable light source and wavelength converter at system startup, and set the wavelength so that the transmission characteristics are optimal. . At this time, as shown in FIG. 56, a method of performing wavelength sweep while feeding back a control signal from the transmission measuring unit 105 is also conceivable. In this case, for example, first, each wavelength converter is set to zero wavelength shift amount, the wavelength variable light source is swept, and the wavelength with the best transmission characteristics is set. At this time, if the transmission characteristics do not satisfy the standard, a method of sweeping the wavelength converters in order from the side closer to the transmission side and setting the transmission characteristics to the best wavelengths for each may be considered. In this case, all of the many variations already described can be applied to the measurement method of transmission characteristics and the mode of control at the time of system startup and operation.

In the example shown in FIG. 57, as already described, a dispersion compensator 112 for intentionally creating a GVD to reduce the SPM effect is further arranged in the transmission unit. A dispersion compensator may be further arranged in each repeater.
Next, peripheral techniques for putting optical multiplexing to practical use will be described.
The Mach-Zehnder type optical modulator used to generate an optical signal by modulating a light beam from a light source with an electric signal has a sinusoidal characteristic as already described with reference to FIG. Since it drifts with changes and changes over time, it is necessary to compensate for the drift so that the change range (operating point) of the applied voltage is always appropriate. Zehnder by Japanese Patent Laid-Open No. 3-251815 that the applied voltage (high-frequency electrical signal) amplitude modulated by the low frequency of the frequency f _0, f ₀ component included in the output light controls the bias of the applied voltage to be zero A technique for compensating for modulator drift is disclosed. That does not include f ₀ component because changes in opposite phases to each other in the envelope frequency 2f ₀ of the upper and lower output optical signal as shown in FIG. 58 when the range V ₀ -V ₁ driving voltage is appropriate On the other hand, when the operating point fluctuates, the upper and lower envelopes of the output optical signal change in phase with each other at the frequency f ₀ as shown in FIGS. 59 and 60, so that the f ₀ component is included. Therefore, a part of the output optical signal is branched by a coupler to be converted into an electric signal, and the operating point is stabilized by controlling the bias of the optical modulator with the output phase-detected at f ₀ .

  When such a drift compensation technique is applied to an optical multiplexing system, since an optical modulator is provided for each optical channel, a drift compensation circuit is also required for each. Therefore, if the drift compensation technique is applied to an optical multiplexing system as it is, there is a problem that a large number of couplers for branching an optical signal, many photodetectors for converting the branched optical signal into an electrical signal, and the like are required. .

FIG. 61 shows an example of an optical multiplexing system having the drift compensation circuit of the present invention. In this example, parallel to the optical modulator 201 of _a plurality arranged MZ, 201 ₂ ... the same wavelength lambda ₀ of the laser beam type, respectively, the optical modulator 201 _1, 201 ₂ ... of the drive circuit 203 _1, In 203 ₂ ..., The drive signal (modulation signal) is amplitude-modulated with low frequency signals of different frequencies f ₁ , f ₂ ... Generated by the low frequency oscillators 204 ₁ , 204 ₂ .

The output light from each of the optical modulators 201 ₁ , 201 ₂ ... Is optically multiplexed and then sent to the optical transmission line, and part of the output light is split as monitor light by the optical splitter 205, and the split light is detected by the photodetector. Optical / electrical conversion is performed at 206 and further branched at an electric signal level, and the branched signals are supplied to corresponding phase detection / bias supply circuits 202 ₁ , 202 ₂ ... Through band-pass filters 208 ₁ , 208 ₂ . The band-pass filter 208 _k (where k = 1, 2,..., The same applies below) passes the frequency f _k of the low-frequency superimposed component of the corresponding optical modulator 201 _k .

In the phase detection / bias supply circuit 202 _k , the low frequency component in the output light photoelectrically converted and extracted by the bandpass filter 208 _k is phase-detected by the output of the oscillator 204 _k to control the operating point of the optical modulator 201 _k. Generate a signal. Each of the optical modulators 201 ₁ , 201 ₂ ... Performs this control simultaneously.
With this configuration, the phase detection and bias control of the supply circuit 202 ₁ of the optical modulator 201 ₁ is carried out at a low frequency f ₁ component which is branched by the band filter 208 _1, Similarly, the optical modulator 201 _{and second} phase detection Since the bias supply circuit 202 ₂ is controlled by the low-frequency f ₂ component branched by the band filter 208 ₂ , bias control of each of the optical modulators 201 ₁ , 201 ₂ . .

This configuration is effective when a plurality of optical signals are optical time division multiplexed (OTDM). Multiple optical modulators can be controlled simultaneously by output light branching and photoelectric conversion in one place. In this example, band filters 208 ₁ , 208 ₂ ... For extracting each frequency component are used after photoelectric conversion and branching, but they may be omitted if stable operation is possible.

In the example of FIG. 61, all the optical modulators 201 ₁ , 201 ₂ ... Operate in parallel and simultaneously control the operating point drift by low frequency amplitude modulation. The drive circuit that performs low-frequency amplitude modulation is switched over time so that only one drive circuit is performing low-frequency amplitude modulation at any given time, and in conjunction with this, the low-frequency amplitude modulation is performed. It is also possible to detect and control the operating point drift of only the optical modulator performing the operation, while fixing the operating points of the remaining optical modulators. If it does in this way, the signal of the same frequency can be used as a low frequency signal.

FIG. 62 shows an example of such an optical multiplexing system. In this example, control to the optical modulators arranged in parallel is switched at a constant time interval T ₀ . That is, a plurality of Mach-Zehnder type optical modulators 201 ₁ , 201 ₂ ... Are arranged in parallel, and after the optical signals of the same wavelength λ ₀ are modulated by the optical modulators 201 ₁ , 201 ₂ . Combine. Low frequency oscillator 204 is prepared only one generates a single low frequency f _0, which changeover switch 209 the drive circuits time for each interval T ₀ at 203 _1, 203 ₂ ... supplied by switching in time to the The drive circuits 203 ₁ , 203 ₂ ... Are sequentially switched in time to perform low frequency amplitude modulation with a single frequency f ₀ .

On the output side of the optical modulators 201 ₁ , 201 ₂ ..., The combined output light is branched by the optical splitter 205, photoelectrically converted by the photodetector 206, and supplied to the phase detection / bias supply circuit 202. The phase detection / bias supply circuit 202 generates and outputs a bias voltage by phase-detecting a low-frequency component in a signal obtained by branching and photoelectrically converting the output light with a low-frequency f ₀ signal from the low-frequency oscillator 204.

The output of the phase detection / bias supply circuit 202 is supplied to each of the optical modulators 201 ₁ , 201 ₂ . The changeover switch 210 works in conjunction with the changeover switch 209 to control the operating point drift by supplying a bias voltage only to the optical modulator performing low frequency amplitude modulation in the drive circuit, The operating points of the remaining optical modulators are fixed (for example, fixed by a latch or the like; the same applies hereinafter).

Similar to the example of FIG. 61, this example is effective in the case of optical time division multiplexing of a plurality of optical signals, and has an advantage that it can be controlled by one phase detection / bias supply circuit. The time T ₀ is set as short as possible within a sufficiently long range compared to the control time constant so that no drift occurs in the uncontrolled optical modulator.
FIG. 63 shows another example of the optical multiplexing system of the present invention. In this example, optical modulators 201 ₁ , 201 ₂ ... Are arranged in series. That is, a plurality of Mach-Zehnder type optical modulators 201 ₁ , 201 ₂ ... Are arranged in series, and the system is configured to apply modulation at least twice to the light of wavelength λ ₀ from the light source. Since this system does not multiplex optical signals, it should not be called an optical multiplexing system. However, for convenience, this system will be called an optical multiplexing system.

The drive circuits 203 ₁ , 203 ₂ ... Of the optical modulators 201 ₁ , 201 ₂ ... Perform low frequency amplitude modulation at different frequencies f ₁ , f ₂ . The output light from the optical modulator at the last stage is branched by the optical branching unit 205, photoelectrically converted by the photodetector 206, and this electric signal is phase-detected / biased via the band-pass filters 208 ₁ , 208 ₂ . These are supplied to the circuits 202 ₁ , 202 ₂ . The band-pass filter 208 _k passes the frequency f _k of the low-frequency superimposed component of the corresponding optical modulator 201 _k .

The phase detection / bias supply circuit 202 _k detects the operating point drift by detecting the phase of the low frequency component in the signal branched from the output light using the low frequency f _k signal from the oscillator 204 _k , and the corresponding optical modulator. Control the operating point of 201 _k . This operating point control is performed simultaneously in each of the optical modulators 201 ₁ , 201 ₂ . If the operation can be stabilized, the band filters 208 ₁ , 208 ₂ .

In the example of FIG. 63, all the optical modulators 201 ₁ , 201 ₂ ... Simultaneously control the operating point drift by performing the low-frequency superposition, but as another example, as an arbitrary time The drive circuit that performs low-frequency amplitude modulation is switched over time so that there is only one drive circuit that performs low-frequency amplitude modulation in FIG. The operating point drift of only the modulator may be detected and controlled, and the operating points of the remaining optical modulators may be fixed during that time.

FIG. 64 shows an example of such an optical multiplexing system. In this example, control to the optical modulators 201 ₁ , 201 ₂ ... Arranged in series is switched at a constant time interval T ₀ . That is, a plurality of Mach-Zehnder type optical modulators 201 ₁ , 201 ₂ ... Are arranged in series to constitute a system that modulates light from a light source twice or more. Low frequency oscillator 204 is prepared only one generates a single low frequency f _0, which changeover switch 209 the drive circuits time for each interval T ₀ at 203 _1, 203 ₂ ... supplied by switching in time to the The drive circuits 203 ₁ , 203 ₂ ... Are sequentially switched in time to perform low frequency amplitude modulation with a single frequency f ₀ .

The output light of the optical modulator at the final stage is branched by the optical branching unit 205, photoelectrically converted by the photodetector 206, and supplied to the phase detection / bias supply circuit 202. The phase detection / bias supply circuit 202 generates and outputs a bias voltage by phase-detecting a low frequency component in a signal obtained by branching and photoelectrically converting the output light with a single low frequency f ₀ signal from the low frequency oscillator 204. .

The output of the phase detection / bias supply circuit 202 is supplied to each of the optical modulators 201 ₁ , 201 ₂ . The changeover switch 210 works in conjunction with the changeover switch 209 to control the operating point drift by supplying a bias voltage only to the optical modulator performing low frequency amplitude modulation in the drive circuit, The operating points of the remaining optical modulators are fixed.

Similar to the example of FIG. 63, this example is effective in optical time division multiplexing, and has the advantage that it can be controlled by a single phase detection / bias supply circuit.
FIG. 65 shows another example of the optical multiplexing system of the present invention. In this example, light having different wavelengths λ ₁ , λ ₂ ... Is modulated and wavelength-multiplexed by optical modulators 201 ₁ , 201 ₂ ... Arranged in parallel, and the optical modulators 201 ₁ , 201 ₂ . The circuits 203 ₁ , 203 ₂ ... Perform low frequency amplitude modulation at different frequencies f ₁ , f ₂ . That is, arranged a plurality of Mach-Zehnder type optical modulator 201 _1, 201 ₂ ... in parallel, different wavelengths lambda _1, constitutes a system for performing waveform multiplexing of lambda ₂ ... of the optical signal, the optical modulators 201 _1, 201 ₂ ... drive circuit 203 _1, 203 ₂ ... different frequencies f ₁ in, f ₂ ... perform low-frequency amplitude modulation of the wavelength multiplexing the optical modulators 201 _1, 201 ₂ ... of the output optical multiplexing to Outputs output light.

This wavelength-multiplexed output light is branched by the optical splitter 205 and subjected to photoelectric conversion by the photodetector 206. The photoelectrically converted signals are supplied to phase detection / bias supply circuits 202 ₁ , 202 ₂ ... Via band-pass filters 208 ₁ , 208 ₂ . The band-pass filter 208 _k passes the frequency f _k of the low-frequency superimposed component of the corresponding optical modulator 201 _k . The phase detection / bias supply circuit 202 _k detects the operating point drift by detecting the phase of the low-frequency component in the signal branched from the output light using the low-frequency f _k signal of the oscillator 204 _k , and the optical modulator 201 _k . Control the operating point. This control is performed simultaneously by the optical modulators 201 ₁ , 201 ₂ .

This example is effective in the case of wavelength multiplexing, and a plurality of optical modulators can be controlled simultaneously on the same principle as the example of FIG. If stable operation is possible, the band filters 208 ₁ , 208 ₂ .
In the example of FIG. 65, all the optical modulators 201 ₁ , 202 ₂ ... Simultaneously control the operating point drift by performing the low-frequency amplitude modulation. The drive circuit that performs low-frequency amplitude modulation is switched over time so that only one drive circuit is performing low-frequency amplitude modulation at the time, and the low-frequency amplitude modulation is performed in conjunction therewith. The operating point drift of only the optical modulator may be detected and controlled, and the operating points of the remaining optical modulators may be fixed during that time.

FIG. 66 shows an example of such an optical multiplexing system. In this example, control to optical modulators 201 ₁ , 201 ₂ ... Arranged in parallel and using light of different wavelengths λ ₁ , λ ₂ ... Is switched at a constant time interval T ₀ . That is, arranged optical modulator 201 ₁ of a plurality of Mach-Zehnder type, 201 ₂ ... in parallel, performs modulation of the optical modulators 201 _1, 201 ₂ ... at different wavelengths lambda _1, respectively, lambda ₂ ... of the optical signal Then combine them. Low frequency oscillator 204 is prepared only one generates a single low frequency f _0, which changeover switch 209 the drive circuits time for each interval T ₀ at 203 _1, 203 ₂ ... supplied by switching in time to the The drive circuits 203 ₁ , 203 ₂ ... Are sequentially switched in time to perform low frequency amplitude modulation with a single frequency f ₀ .

On the output side of the optical modulators 201 ₁ , 201 ₂ ..., The combined output light is branched by the optical splitter 205, photoelectrically converted by the photodetector 206, and supplied to the phase detection / bias supply circuit 202. The phase detection / bias supply circuit 202 generates and outputs a bias voltage by phase-detecting a low frequency component in a signal obtained by branching and photoelectrically converting the output light with a single low frequency f ₀ signal from the low frequency oscillator 204. .

The output of the phase detection / bias supply circuit 202 is supplied to each of the optical modulators 201 ₁ , 201 ₂ . The changeover switch 210 works in conjunction with the changeover switch 209 to control the operating point drift by supplying a bias voltage only to the optical modulator performing low frequency amplitude modulation in the drive circuit, The operating points of the remaining optical modulators are fixed.

Similar to the example of FIG. 65, this example is effective in wavelength multiplexing and has the advantage that it can be controlled by a single phase detection / bias supply circuit.
FIG. 67 shows another example of the optical multiplexing system of the present invention. In this example, before the electrical output of the combined output light of the optical modulators 201 ₁ , 201 ₂ ... Arranged in parallel and using light of different wavelengths λ ₁ , λ ₂ . .. Are separated into signals from optical modulators 201 ₁ , 201 ₂ ... With wavelengths λ ₁ , λ ₂ . That is, a plurality of Mach-Zehnder optical modulators 201 ₁ , 201 _2, ... Are arranged in parallel to constitute a system that performs wavelength multiplexing of optical signals having different wavelengths λ ₁ , λ ₂ . Only one low frequency oscillator 204 is prepared to generate a single low frequency f ₀ . Each of the drive circuits 203 ₁ , 203 ₂ ... Performs low frequency amplitude modulation at a single frequency f ₀ .
The output light from each of the optical modulators 201 ₁ , 201 ₂ ... Is multiplexed to produce wavelength multiplexed output light, the wavelength multiplexed output light is branched by the optical splitter 205, and further passed through the wavelength separation element 212. The optical signals of λ ₁ , λ ₂ ... are separated. The separated optical signals are passed through photodetectors 206 ₁ , 206 ₂ ..., And photoelectrically converted signals are supplied to phase detection / bias supply circuits 202 ₁ , 202 ₂ . The phase detection / bias supply circuit 202 _k detects the operating point drift by detecting the phase of the low frequency component in the wavelength separated signal using the low frequency f ₀ signal, and controls the operating point of the corresponding optical modulator 201 _k. . This control is performed simultaneously by the optical modulators 201 ₁ , 201 ₂ . This embodiment is effective in wavelength multiplexing as in the example of FIG. 65, and can be applied particularly when the separability is better at the optical wavelength.

FIG. 68 shows another example of the optical multiplexing system of the present invention. In this example, are arranged in parallel, different wavelengths lambda _1, switching in lambda ₂ ... optical modulator 201 _1, 201 ₂ ... interval T ₀ fixed time control to using light of, the temporally transmission wavelength Only the wavelength component of the optical modulator to be controlled is extracted by the wavelength tunable filter 213 to be switched.

That is, a plurality of Mach-Zehnder optical modulators 201 ₁ , 201 ₂ ... Are arranged in parallel to constitute a system for performing wavelength multiplexing of optical signals having different wavelengths λ ₁ , λ ₂ . Only one low frequency oscillator 204 is prepared to generate a single low frequency f ₀ . Each drive circuit 203 ₁ , 203 ₂ ... Performs low frequency amplitude modulation with a single frequency f ₀ .
The output light from each of the optical modulators 201 ₁ , 201 ₂ . Perform photoelectric conversion. The optical wavelength variable filter 213 is a filter whose transmission wavelength is switched over time, and extracts and outputs only one wavelength component at an arbitrary time. The output signal of the photodetector 206 is supplied to each phase detection / bias supply circuit 202 ₁ , 202 ₂ ... The switching of the changeover switch 214 is interlocked with the optical wavelength variable filter 213, when the optical wavelength variable filter 213 is switched so as to transmit the wavelength lambda _k switches switch 214 is phase detection and bias the output signal It is switched to supply to the supply circuit 202 _k .

The phase detection / bias supply circuit 202 _k detects the operating point drift of the optical modulator 201 _k corresponding to the extracted wavelength λ _k by detecting the phase of the low frequency component in the photoelectrically converted signal with the low frequency f ₀ signal. And control. In the meantime, the operating points of the remaining optical modulators are fixed.
Similar to the example of FIG. 65, this example is effective in wavelength multiplexing, and can be applied particularly when wavelength selection by an optical wavelength variable filter is easier. In this example, it is not necessary to switch the drive circuit that performs low-frequency amplitude modulation in terms of time, but in order to more clearly separate low-frequency components that appear due to operating point drift for each optical modulator, switching is performed. It doesn't matter.

FIG. 69 shows another example of the optical multiplexing system of the present invention. This optical multiplexing system is _obtained by adding sign inversion circuits 215 _k and 216 _k for chirping control described in JP-A-4-140712 to the system shown in FIG. As described with reference to FIGS. 20 and 21, in the Mach-Zehnder type optical modulator, the pre-chirping direction can be switched from red shift to blue shift by changing the operating point from Vb1 to Vb2. The waveform can be improved by giving a red shift when the wavelength of the signal light is in the normal dispersion region of the DSF and giving a blue shift when it is in the anomalous dispersion region. The sign inverting circuit 215 _k changes the operating point from Vb1 to Vb2 by inverting the polarity of the low-frequency signal supplied from the oscillator 204 _k to the driving circuit 203 _{k in} accordance with the operating point switching signal. Since the logic of the logic and the optical signal of the modulation signal operating point is changed to Vb2 is reversed in synchronization with the switching of the sign inversion circuit 215 _k inverts the logic of the modulation signal in the sign inversion circuit 216 _k. It should be noted that even if the phase of the signal supplied from the oscillator 204 _k to the phase detection / bias supply circuit 202 _k is inverted instead of inverting the phase of the signal supplied from the oscillator 204 _k to the drive circuit 203 _k , the phase detection / The result of phase detection in the bias supply circuit 202 _k may be inverted. Further, the operating point may be switched simultaneously for all the optical modulators 201 _k .

  The embodiment in FIG. 69 is obtained by adding a sign inversion circuit to the optical multiplexing system in FIG. 61 described above, but the present invention is not limited to this, and each of the optical multiplexing systems described thus far. Similarly, a sign inversion circuit can be added to the control system (FIGS. 62 to 68). At this time, the operation switching signals of the respective sign inversion circuits can be performed independently or linked together. You may make it perform.

  The change of the operating point by the sign inverting circuit (operating point shift circuit) may be a method of providing a changeover switch from the outside, a method of the system performing automatic switching by checking transmission characteristics on the receiving side, for example. When transmission is performed at a wavelength close to the zero dispersion wavelength of the fiber, the sign of chromatic dispersion in the transmission can be positive or negative due to variations in the zero dispersion wavelength of the fiber or variations in the light source wavelength. In such a case, it may be convenient to switch the operating point of each optical modulator independently. In wavelength multiplexing, when the magnitude relationship between the zero dispersion wavelength of the fiber and each wavelength of the signal light is known, it is more convenient to switch the operating points of the optical modulators having the same magnitude relationship. There are cases. Even in the case of optical time division multiplexing, the wavelength variation of the output light can be reversed by switching the operating points of the optical modulators arranged in series at once.

Next, a clock extraction technique in optical time division multiplexing (OTDM) transmission will be described.
FIG. 70 shows an optical time division multiplex transmission system to which the clock signal extraction according to the present invention is applied. This embodiment shows a system configuration for realizing a transmission rate of 40 Gb / s by two-wave multiplexing. FIG. 71 is a time chart of each signal on the transmission side of this system, and shows the waveform of each signal indicated by alphabets a to i in FIG.

  First, 20 GHz optical signals c and d whose phases are opposite to each other are generated from an optical signal a of a light source LD (laser diode) 240 by a 1-input 2-output optical switch 241 operating with a single sine wave b of 20 GHz. Next, 20 Gb / s RZ optical signals g and h are generated by externally modulating the clock optical signals c and d with 20 Gb / s NRZ signals e and f by the external modulators 244 and 245, respectively. . These signals are bit-multiplexed (optical MUX) by the multiplexer 246 to generate a 40 Gb / s optical multiplexed signal i. With this optical time division multiplexing (OTDM) system, 40 Gb / s optical transmission can be realized without requiring an ultra-wideband electronic device equivalent to 40 Gb / s.

As other configurations, instead of the light source LD 240 and the optical switch 241 in FIG. 70, a configuration using a short pulse light source or an LD with a semiconductor optical modulator, or an optical switch 241 for optical branching on the transmission side, A configuration in which a simple passive optical power branching element or an external modulator is driven by a sine wave is also possible.
On the other hand, on the receiving side, it is necessary to separate the 40 Gb / s optical multiplexed signal i into two 20 Gb / s RZ optical signals (optical DEMUX). Recently, proposals and experiments of an optical DEMUX method using an ultrafast PLL using nonlinear effects such as four-wave mixing (FWM) and cross-phase modulation (XPM) phenomenon have been actively conducted. There is also a problem in terms of stability.

  Therefore, as shown in FIG. 70, it is considered that the simplest method is to perform bit separation alternately for each bit by a 1-input 2-output optical switch used on the transmission side. In FIG. 70, the optical multiplexed signal received from the transmission line 248 is input to the optical switch 252 for bit separation through the optical preamplifier 249, and a part thereof is branched by the optical branching device 250 and input to the clock extraction circuit 251. Is done. For example, as shown in FIG. 82, the clock extraction circuit 251 directly extracts a clock signal by a narrow band electric filter (dielectric resonance filter, SAW filter, etc.) 262 after photoelectrically converting the input signal by the photodetector 260. The extracted clock signal is supplied as a signal for giving bit separation timing to the optical switch 252. In response to this clock signal, the optical switch 252 separates the received 40 Gb / s optical multiplexed signal i into two 20 Gb / s RZ optical signals (optical DEMUX) and inputs them to the respective optical receivers 253 and 254.

However, in this receiver configuration, in order to perform not only code identification but also optical switch operation in the optical switch 252, a 20 GHz clock signal synchronized with the data main signal is required, and the received optical multiplexed signal itself has 20 GHz. Ingredients must be included.
Therefore, in the present invention, a 20 GHz component having a size sufficient for clock signal extraction is included in the optical multiplexed signal i itself transmitted by the following method. That is, as shown in FIG. 72, an amplitude difference is provided between two RZ signals g and h on the transmission side, and a clock is extracted from an optical multiplexed signal i of 40 GHz obtained by combining the two. As shown in the figure, the multiplexed optical multiplexed signal i sufficiently includes a 20 GHz clock signal component as indicated by a dotted line in the figure.

  Next, various methods for providing an amplitude difference in an optical signal multiplexed to include a clock signal component in the optical multiplexed signal will be described. Here, for convenience of explanation, as a method of optical multiplexing, another light source LD is used, and the configuration in which each output light is externally modulated and then combined is case A, and one light source as in the above-described example of FIG. Case B is a configuration in which the output light of the LD is branched and then multiplexed after external modulation.

FIG. 74 shows an example of case A. If light sources LD for inputting optical signals to the external modulators 244 and 245 are prepared and the output powers of the light sources LD 240a and 240b are set to be different, an amplitude difference is provided for the multiplexed optical signals g and h. Can do.
FIG. 75 shows another example of case A. As shown in the figure, an optical attenuator 256 is inserted in one of the optical paths from the light source LD to the multiplexer 246 to provide an amplitude difference between the two multiplexed optical signals g and h. be able to. In the illustrated example, the optical attenuator 256 is provided between the external modulator 244 and the multiplexer 246, but it may be provided between the light source LD 240 a and the external modulator 244. Of course, it may be provided in the optical path on the external modulator 245 side. Instead of the optical attenuator 256, a configuration using an optical amplifier is also possible. Furthermore, not only the external modulation system, but also these methods are effective when an LD direct modulation or a modulator-integrated LD is used.

FIG. 76 shows an example in which the method of FIG. 75 described above is applied to Case B. The case B is the same as that described above except that the number of the light sources LD is one, and a detailed description thereof will be omitted.
Furthermore, in both cases A and B, in the optical time division multiplex transmission system employing the external modulation system, the optical intensity amplitude differences of a plurality of optical signals are inserted as external modulators 244 and 245, respectively. This can be realized by using different loss factors.

  When a Mach-Zehnder type optical modulator is used as the external modulator, the amplitude of the output light can be changed by changing the voltage amplitude for driving the optical modulator or changing its bias point. 77 and 78 show this state. FIG. 77 shows how the light output intensity changes by changing the amplitude value of the drive voltage (applied voltage) from Ve to Vf. FIG. 78 shows how the light output intensity changes by changing the bias voltage of the drive voltage from VB-e to VB-f. As described above, only when the Mach-Zehnder type optical modulator is used as the external modulator, the output light intensity of the external modulators 244 and 245 can be changed by changing the drive voltage amplitude or the bias voltage.

In the case B, the output ratio of the output light from the light source LD 240 by the optical switch 241 (or an alternative passive optical power branching element or the like) is not set to 1: 1 so that the outputs of the external modulators 244 and 245 are output. An amplitude difference can be provided in the light.
FIG. 79 shows another example of case B. In this example, the polarization states of the output lights of the external modulators 244 and 245 are set so that the principal axes of linearly polarized waves are orthogonal. As described above, when two RZ signals having different polarization states, for example, the main axes of linearly polarized waves are orthogonal to each other, are optically multiplexed in the optical path after the optical multiplexing in the transmission unit (multiplexer 246), By inserting the polarization-dependent optical element 257, the optical intensity of the two multiplexed optical signals can have an amplitude difference after passing through the optical element 257, and the optical amplitude differs by alternating bits. An optical multiplexed signal can be realized.

Further, instead of inserting the above-described polarization-dependent optical element 257, a multiplexer having a polarization dependency in the multiplexing ratio is used as a multiplexer due to the structure or the incident polarization axis of the optical signal. Configuration is also possible.
Further, in a transmission system in which the relationship between the polarization states between alternating bits on the transmission side is maintained to some extent on the reception side, as shown in FIG. 80, before the optical separation in the optical switch 252 in the reception unit. In addition, an optical element 258 having polarization dependency may be inserted.

  In this embodiment, the case of two-wave multiplexing in which the alternating bits have a light intensity amplitude difference has been described, but an N-wave multiplexing configuration is also possible. For example, in the case of an optical multiplexing transmission system that performs four-wave multiplexing, a clock signal can be extracted from the optical multiplexing signal. FIG. 73 shows an example of such four-wave multiplexing. When optical signals to be multiplexed four-wave are g1, g2, g3, and g4, amplitude differences are provided in the relationship of g1> g2 = g4> g3. Are multiplexed to create an optical multiplexed signal i, the optical multiplexed signal i includes a clock signal component as indicated by a dotted line in the figure. Further, in this case, a plurality of types of clock signal components can be included by an amplitude difference setting method.

  Moreover, the following point is mentioned as one characteristic point of this invention. That is, in the conventional optical transmission system up to 10 Gb / s, after receiving the signal light (photoelectric conversion), the main signal is branched at the electric stage to perform clock extraction. On the other hand, in the present invention, as shown in FIG. 81, clock extraction is performed by the above-described methods from the optical multiplexed signal branched from the main signal at the optical stage, and optical separation is performed using the clock signal. The point is a feature.

  As a next problem in the multiplexed transmission system, it is generally required that the correspondence between each channel before multiplexing on the transmitting side and each channel after demultiplexing on the receiving side is fixedly determined. For example, in FIG. 70, the signal supplied to the drive circuit 242 is always received by the optical receiver 253, and the signal supplied to the drive circuit 243 is always received by the optical receiver 254. However, since the conventional OTDM transmission system does not distinguish between channels on the receiving side, the correspondence may change each time the system is started up, and therefore there is a problem that the line cannot be managed.

  FIG. 83 shows a configuration of an optical separator suitable for use on the receiving side of the OTDM communication system according to the present invention. An optical branch circuit 300 that branches the received optical signal into two, a clock signal recovery circuit 302 that recovers a clock signal from one of the branched optical signals, and separates the received optical signal into two channels at an optical level according to the recovered clock signal The configuration including the optical switch 304 and the two optical receiving units 306 and 308 for reproducing data from the optical signals of the separated channels is the same as that of the receiving side of the system of FIG.

  Data transmitted from the transmission side follows, for example, the format shown in FIG. In FIG. 84, 310 is frame synchronization data for establishing frame synchronization in the optical receivers 306 and 308, and 312 is identification data for identifying a channel. The line identification data extraction circuits 314 and 316 extract the identification data 312, and the control circuit 318 controls the signal switching circuit 320 according to the identification data extracted by the line identification data extraction circuits 314 and 316 and outputs it to the output line 1. The connection in the signal switching circuit 320 is controlled so that data to be output is output to the output line 1 and data to be output to the output line 2 is output to the output line 2. The input signals of the line identification data extraction circuits 314 and 316 may be extracted from the output of the signal switching circuit 320. The control circuit 318 can be easily realized using a microcomputer.

  In the optical separator shown in FIG. 85, instead of switching the connection of the output of the optical receiver, the phase shifter 322 is controlled to change the phase of the clock signal applied to the optical switch 304, thereby substantially switching the connection. The effect is gained. In the case of double multiplexing, the switching of the connection is substantially achieved by shifting the phase of the clock signal by 180 °.

  FIG. 86 shows an example in which the optical separator of FIG. 85 is expanded from 2 channels to 4 channels. The clock signal regenerated by the clock signal generation circuit 302 is supplied to the optical switch 304 through the phase variable device 322, and is also divided by a half by the 1/2 frequency divider 303, and the phase variable devices 323 and 323 'are supplied. The signals are supplied to the optical switches 305 and 305 ′ through the respective devices. If four-channel optical signals CH1 to CH4 are multiplexed in the order of CH1 → CH2 → CH3 → CH4 as shown in FIG. 115 (a), the optical switch 304 is 1 with the clock shown in FIG. 115 (b). Since switching is performed for each time slot, CH1 and CH3 are alternated from one output as shown in FIG. 115 (c), and CH2 and CH4 are alternated every two time slots as shown in FIG. 115 (d) from the other output. Is output. Since the optical switches 305 and 305 ′ are switched every two time slots by the clock shown in FIGS. 115 (e) and 115 (h), CH1 and CH3 are separated as shown in FIGS. 115 (f) and 115 (g), As shown in FIGS. 115 (i) and (j), CH2 and CH4 are separated. The control circuit 318 outputs, for example, CH1 from the output line 1, CH2 from the output line 2, CH3 from the output line 3, and CH4 from the output line 4 in accordance with the identification data extracted by the line identification data extraction circuits 314 to 317. In this way, the phase shifters 322, 323 and 323 'are controlled.

  In the optical separator shown in FIG. 87, instead of shifting the phase by 180 ° with the phase variable device 322 in FIG. Only delay or advance. The same effect can be obtained by delaying or advancing the optical signal by a corresponding time instead of changing the phase of the clock signal. The optical delay circuit 324 can be realized, for example, by changing the optical path length by mechanically moving the corner cube 326 as shown in FIG.

FIG. 89 shows another example of the optical separator of the present invention. In the example of FIG. 89, instead of the identification data 312 of FIG. 84, identification is enabled by superimposing low frequency signals f ₁ -f ₄ having different frequencies for each channel as shown in FIG. However, FIG. 89 shows the case of double multiplexing.
The optical signals separated by the optical switch 304 are branched by optical branching circuits 328 and 330, converted into electrical signals by optical / electrical conversion circuits 332 and 334, and superimposed on the low frequency detection units 336 and 338. Is detected. The control circuit 318 recognizes the channel from the frequency of the low-frequency signal detected by the low-frequency detection units 336 and 338, and the signal so that the signal of the predetermined channel is output from the output line 1 and the output line 2. The connection in the exchange circuit 320 is switched. As described above, instead of switching the connection, the phase of the clock signal may be changed as shown in FIG. 91, or the optical signal may be delayed or advanced as shown in FIG. . In the case of double multiplexing, it is sufficient to recognize one channel, so the optical branching circuit 330, the optical / electrical conversion circuit 334, and the low-frequency detection unit 338 are not necessarily required, and are shown in FIGS. 91 and 92. Thus, although only one channel may be used, it may be provided as a backup when a failure occurs on one side. Also, as shown in FIG. 93, the optical receiver 306 or 308 may be provided with a photocurrent monitor circuit 342 for monitoring the current flowing to the optical / electrical conversion element 340, and a low frequency signal may be extracted from this output. it can. In this case, the optical branch circuits 328 and 330 and the optical / electrical converters 332 and 334 can be eliminated.

FIG. 94 shows the configuration of an optical transmitter that superimposes and transmits a low frequency signal f _i having a different frequency for each channel. Light from the light source 400 is punched out with a clock signal in the external optical modulator 402, and is branched by a necessary number (2 in the example in the figure) in the branch circuit 404. The first branched light is modulated at the frequency f ₁ by the external light modulator 406 and further modulated by the first main signal at the external light modulator 408. The second branched light is also modulated at the frequency by the external light modulator 410. Modulating with f ₂ , modulating with the second main signal in the external optical modulator 412, creating a phase difference with the first optical signal with the delay unit 414, and adding with the adder 416. As a result, the frequency f ₁ is superimposed on the amplitude in the period (time slot) modulated with the first main signal, and the frequency f ₂ is superimposed on the amplitude in the time slot modulated with the second main signal. The signal punching in the external optical modulator 402 is preferably performed so that the multiplexed signal has the waveform shown in FIG. 107, that is, the pulse of each channel occupies one time slot. Advantages obtained by this will be described later.

In the optical multiplexing method shown in FIG. 94, the optical signal is split in phase and modulated, and then combined to produce a phase difference. In the optical multiplexing method shown in FIG. After branching in the reverse phase by the switch 241, the signals are multiplexed as they are. Also in the latter method, if an external optical modulator that modulates at the frequency f ₁ or f ₂ is provided in series with the external optical modulators 244 and 245, low-frequency signals having different frequencies can be superimposed on each channel. The external optical modulators 402, 406, 408, 410, and 412 can be realized by a LiNbO ₃ Mach-Zehnder optical modulator or an EA optical modulator (electroabsorption optical modulator).

FIG. 95 shows another example of the optical transmitter of the present invention. By preliminarily superimposing the frequencies f ₁ and f ₂ on the main signal in the drive circuits 418 and 420, the number of external optical modulators can be reduced. The drive circuits 418 and 420 can be realized by a dual gate FET as shown in FIG. A drive waveform is shown in FIG.
If a part of the configuration of the optical separator for the optical receiver described with reference to FIG. 83 to FIG. 93 is modified, a certain type of optical switch for exchanging optical signals according to the identification information included in the optical multiplexed signal. Can be used. For example, as shown in FIG. 116 or FIG. 117, the optical receiving units 306 and 308 in FIG. 85 or 87 are replaced with optical branch circuits 600 and 602, respectively, and the other outputs of the optical branch circuits 600 and 602 are replaced with optical output lines. If it connects to 1 and 2, it can be used as an optical switch. With respect to the circuit of FIG. 91 or 92, the outputs of the optical branch circuits 328 and 330 may be directly connected to the optical output lines 1 and 2 as shown in FIG. 118 or FIG.

When a low frequency signal is superimposed on the received signal as shown in FIG. 90, it is possible to control the stabilization of the phase of the clock signal for optical separation in addition to channel identification using this. FIG. 97 shows the configuration of an optical receiver having a phase control unit that stabilizes a clock signal using a superimposed low-frequency signal.
An optical branching unit 430 for branching the received Qbits optical signal into two, a timing recovery unit 432 for recovering a Q / 2 Hz clock signal from one of the branched signals, and an optical signal 2 at an optical level according to the recovered clock signal The configuration of the optical switch 434 that separates the optical signal into two Q / 2 bit / s optical signals and the optical receivers 436 and 438 that reproduce the data signal from the separated optical signal is the same as that in FIG.

The phase of the clock signal supplied from the timing recovery unit 432 to the optical switch 434 is varied by the phase variable circuit 439, and the phase variable circuit 439 is controlled by the phase control unit 440. The phase control unit 440 includes an optical branching unit 442 that branches one of the separated Q / 2 bit optical signals, a light receiving element 444 that converts the branched optical signal into an electric signal, and a specific output among outputs from the light receiving element 444. bandpass filter 446 for passing only signals of frequency f _1, the oscillation frequency g ₁ of the oscillator 448, a synchronous detection circuit for phase-synchronous detection output of the band pass filter 446 in signal frequency g ₁ 450, detection of the synchronous detection circuit 450 The output is compared with a predetermined reference value, the comparator 452 that generates a control voltage according to the comparison result, and the output of the oscillator 448 and the output of the comparator 452 are added to output the control signal of the phase variable circuit 439. An adder 454 is included. Note that it is not necessary that low-frequency signals having different frequencies are superimposed on all the optical signals of the respective channels, and it is sufficient if the low-frequency signals are superimposed only on specific channels. In the latter case, the bandpass filter 446 is not necessary. However, even in this case, the signal-to-noise ratio of the signal input to the synchronous detection circuit 450 can be improved by inserting the bandpass filter 446.

As shown in FIG. 98 (a), it is assumed that the frequency f ₁ is superimposed only on CH1. When the phase of the clock signal completely matches the phase of the optical signal in the optical switch 434 as shown in FIG. 98 (b), the frequency f ₁ output from the light receiving element 444 as shown in FIG. 98 (c). The signal strength of is maximized. As shown in FIG. 98 (d), when the phase of the clock signal is shifted, not all the f ₁ signals received from the optical switch 434 are cut out, so the strength of the f ₁ signal is lowered. That is, as the phase difference θ between the optical signal and the clock signal defined in FIG. 99 changes from 0 ° to ± 180 °, the intensity of the f ₁ component decreases linearly as shown in FIG. Since the phase variable circuit 409 is controlled by the output of the oscillator 448, the clock signal is finely phase-modulated at a frequency g _1. Now, when the center of the phase change is referred to as being point (b) of FIG. 100, the intensity of the f ₁ component varies with the frequency g _1, and, since the straight line is diagonally right up, the intensity variation of the f ₁ component phase Is in phase with or opposite to the phase of the frequency g ₁ output from the oscillator 448. Therefore, when the synchronous detection circuit 450 performs phase synchronous detection at the frequency g ₁ , the output takes a positive value or a negative value having a certain absolute value. When the center of the phase fluctuation is at point (c) in FIG. 100, the straight line is descending to the right, so that the phase detection output has the same value as that at point (b) and the sign is reversed. When it is at point (a), half of the fluctuation is folded back, so that the fluctuation component of the frequency g ₁ disappears and the phase detection output becomes zero. Therefore, the comparator 452 compares the output of the synchronous detection circuit 450 with a reference value, for example, 0 level, and adds the comparison result to the output of the oscillator 448 to control the phase of the clock signal. It is possible to control so that the point (a) becomes. The control signal input to the comparator 452 is for inverting the polarity of the output of the comparator 452, and the center of the control is reversed by inverting the polarity of the output of the comparator 452 in FIG. Since the point (maximum value) shifts to the minimum value, the phase of the clock signal is shifted by 180 °. This makes it possible to easily achieve channel switching.

The circuit shown in FIG. 101 is a modification of the optical receiver of FIG. In the circuit of FIG. 101, not only the optical signal going to the optical receiver 438 but also the optical signal going to the optical receiver 436 is branched by the optical branching device 460 and sent to the phase control unit 462 having the same configuration as the phase control unit 440. Entered. However, the center frequency of the bandpass filter included in the phase control unit 440 is f ₁ , whereas the phase control unit 462 locks to another channel, so the center frequency of the bandpass filter is f ₂ . . The outputs of the bandpass filters of the respective phase control units 440 and 462 are input to the comparator 464 and compared with the reference value. As a result, any unused channel can be recognized. When one of the channels is unused, the switch 466 is controlled to control the phase of the clock with the phase amount control signal obtained for the channel in use, and either one of the phase control units 440 and 462 is output. Select and output a signal. The phase amount control signal selected by the switch 466 is supplied to the phase variable circuit 439. When there are a plurality of channels recognized as being used, the switcher 466 is controlled in accordance with a predetermined priority order.

FIG. 102 is also a modification of FIG. Instead of branching the optical signal and converting it to an electrical signal in order to detect the low frequency signal in the phase controller 440, as described in FIG. 93, the optical / electrical conversion element 470 provided in the optical receiver 438 is used. The low-frequency signal is detected from the output of the photocurrent monitor unit 472.
FIG. 103 shows an example extended to four multiplexes. The two Q / 2 bit / s optical signals separated by the optical switch 434 are further separated into four Q / 4 bit / s optical signals by the optical switches 474 and 476. The Q / 4 Hz clock signal obtained by dividing the Q / 2 Hz clock signal output from the phase variable circuit 439 by the 1/2 frequency divider 478 is supplied to the optical switches 474 and 476 via the phase variable circuits 480 and 482, respectively. The The phase control unit 440 controls the phase of the Q / 2 Hz clock signal with the frequency f ₁ as a control target. For example, as shown in FIG. 104, the frequency f ₁ is superimposed on the CH1 and CH3 time slots, so that the phase of the Q / 2 Hz clock signal is synchronized with the CH1 or CH3 time slot by the action of the phase control unit 440. Therefore, CH1 + CH3 and CH2 + CH4 can be stably separated by the optical switch 434. Note that f ₁ does not necessarily have to be superimposed on CH3.

In the example of FIG. 103, it is assumed that CH1 + CH3 is input to the optical switch 476 and CH2 + CH4 is input to the optical switch 474. The phase control unit 484 controls the phase of the Q / 4 Hz clock signal input to the optical switch 476 using the frequency f ₂ as a control target. Since the frequency f ₂ is superimposed on the time slot of CH1 in the example of FIG. 104, the optical switch 476 can stably separate CH1 and CH3. The phase control unit 486 controls the phase of the Q / 4 Hz clock signal input to the optical switch 474 using the frequency f ₃ as a control target. Since the frequency f ₃ in the example of FIG. 104 is superimposed on the CH2 time slot, the optical switch 474 may be stably separated CH2 and CH4.

FIG. 103 shows an example of 4 multiplexing, but in the case of 8 multiplexing and 16 multiplexing, the optical switch, the frequency divider, and the phase control unit may be connected in cascade.
As a modification of the circuit of FIG. 103, when the phase of the Q / 2 Hz clock applied to the optical switch 434 is optimal, the optical path at the time of manufacture is optimized so that the phase of the Q / 4 Hz clock applied to the optical switches 474 and 476 is optimal. If the phase of the electrical path is adjusted, the phase variable circuits 480 and 482 and the phase control units 484 and 486 therefor can be omitted as shown in FIG. In this case, since only the low frequency signal to be multiplexed is f ₁ , the band pass filter 446 can be omitted.

In the configuration of FIG. 103, the frequencies f ₁ , f ₂ , and f ₃ necessary for the phase control units 440, 484, and 486 are output from the photocurrent monitor unit of the optical / electrical conversion element provided in the optical receiver in the same manner as in FIG. You may make it detect from. In this case, f ₁ and f ₂ are detected from the optical receiver 453 that is scheduled to receive CH1, and f ₃ is detected from the optical receiver 455 that is scheduled to receive CH3. Configure as follows.

  FIG. 106 shows an example of the detailed configuration of the timing recovery unit 432 and the phase variable circuit 439 in FIGS. 97, 101, 102, 103 and 105. The timing reproduction unit 432 is a light receiving element 490 that performs optical / electrical conversion, a non-linear extraction circuit 492 for generating a QHz component from the output of the light receiving element 490, a timing filter 494 that extracts only the QHz component from the output of the non-linear extraction circuit 492, It comprises a limiter amplifier 496 and a 1/2 frequency divider 498 for making the output amplitude of the timing filter 494 constant. The phase variable circuit 439 is preferably inserted between the timing filter 494 and the limiter amplifier 496.

  The nonlinear extraction circuit 492 further includes a differentiation circuit 500 and a full wave rectification circuit 502. When a signal in which the pulse of each channel is in one time slot is optically multiplexed, the waveform is as shown in FIG. 107, so that the differentiation circuit 500 and the full-wave rectification circuit 502 for performing nonlinear extraction processing are not required. The timing reproducing unit 432 can be simplified.

  Further, when such simplification is performed, the light receiving element 490 that performs optical / electrical conversion may have a characteristic having a resonance frequency at QHz as shown in FIG. A light receiving element having a flat and wide frequency characteristic is not necessary, and manufacturing of the light receiving element is facilitated. Further, since the light receiving element has a filter characteristic, the out-of-band attenuation characteristic of the timing filter 494 can be relaxed.

  FIG. 109 is a modification of the circuit of FIG. A part of the signal is branched by the branch circuit 504, the level of the received signal is detected by the level detector 506, and compared with the reference value by the comparator 508, an input interruption alarm is output if it is below the reference value. Since the input to the comparator 508 are composed of only passive components with few failures, it is possible to reliably detect optical input interruption. This makes it possible to distinguish whether the optical input signal is in a disconnected state or out of synchronization when the optical receiver is in a signal disconnected state.

An optical switch that separates an input optical signal into two optical signals with opposite phases according to a clock signal is realized by an active optical directional coupler using a Ti-diffused LiNbO ₃ crystal waveguide described in Japanese Patent Laid-Open No. 55-7315 it can. As shown in FIG. 110, two gate type optical switches 510 and 512, an optical branch circuit 514 that distributes the optical signal to them, and a phase shifter that shifts the phase of the clock signal to one gate type optical switch 510 by 180 °. 516 may be used. In this case, the polarization dependence can be made smaller than that of the active optical directional coupler. A gate-type optical switch refers to a device that is also used as an optical modulator and whose light transmittance changes depending on an applied voltage, and is, for example, an electroabsorption optical modulator (EA modulator).

In the optical receiver described with reference to FIGS. 97 to 105, the optimum clock is obtained by controlling the phase of the clock recovered from the received optical signal. The clock is generated by a VCO (voltage controlled oscillator), It can also be configured to optimize the frequency and phase. In FIG. 111, the configuration and operation of the phase control unit 440 are the same as those in FIG. A clock signal supplied to the optical switch 434 is generated by the VCO 520, and a control signal output from the phase control unit 440 is supplied to the VCO 520.
In the following, it will be described that the clock signal generated by the VCO 520 is controlled to an optimum value in the circuit of FIG.
It is defined as follows.

φ (t): phase of clock signal input to optical switch 434 (output signal of VCO 520
Phase)
α (t): Phase of optical signal input to optical switch (for simplicity, 1010 alternating signal is input
Thinks
Channel 1 is the reference.

θ (t): φ (t) −α (t) (see FIG. 99)
ω ₀ : VCO free-running angular frequency V (t): VCO control voltage × angular frequency modulation sensitivity of VCO Vo: output value of synchronous detection circuit 450 K: constant f passing through bandpass filter 446 (center frequency f ₁ ) The intensity of _one component changes as shown in FIG. 100 with respect to the phase difference θ.

Here, by adding the low frequency signal I ₀ cos2πg ₁ t from the oscillator 448 to the control voltage of the VCO 520, perturbation is given to θ.
dφ / dt = ω ₀ + V (t) = ω ₀ + I ₀ cos 2πg ₁ t + KVo (1) Now, assuming that Vo is changing slowly or in a steady state, it is assumed to be constant.

When the formula (1) is integrated, the formula (2) is obtained, and it can be seen that φ is phase-modulated at the frequency g ₁ .
φ = (ω ₀ + KVo) t + I ₁ sin 2πg ₁ t + C
(C: integration constant, I ₁ : constant) (2)
Therefore, assuming that the time derivative of α (t) is constant, θ (t) is phase-modulated at the frequency g ₁ as in the case of FIG.

When phase modulation is applied to θ, the f ₁ component passing through the bandpass filter 446 responds as shown in FIG. That is, at point (a), since the waveform is folded, the sin 2πg ₁ t component disappears in the fluctuation of the f ₁ component. The point (b) is in phase with the input signal, and the point (c) is out of phase.
Therefore, the output Vo (t) when synchronous detection is performed using the output I ₀ sin2πg ₁ t from the oscillator 448 is as shown in FIG.
(It is assumed that +1 is output in the case of in-phase and -1 is output in the case of opposite phase.)
Next, we will see where the movement converges. For simplification, excluding the perturbation component, the following is applied.

dφ / dt = ω ₀ + V (t) = ω ₀ + KVo (t) (3)
Further deformation,
dθ / dt = dφ / dt−dα / dt = ω ₀ −dα / dt + Vo (t)
= Δω ₀ + KVo (t)
Since dα / dt is a constant value, Δω ₀ is a constant value. If K is positive and K is large and Δω ₀ / K≈0,
When θ> 0, since Vo (t) <0 and dθ / dt <0, θ converges to 0.

When θ <0, θ converges to 0 because Vo (t)> 0 and dθ / dt> 0.
From the above, it is shown that θ converges to 0, and the optical path can be switched at an optimal timing with the optical switch.
In the configuration of FIG. 111, FIG. 113 shows a circuit configuration for detecting a light input interruption. The received optical signal is branched at the optical branching unit 522, and one is input to the optical switch 434 (FIG. 111). The other is converted into an electric signal by the light receiving element 524, the low frequency components f ₁ and f ₂ are detected by the band pass filters 526 and 528, respectively, and divided by the direct current value by the dividers 530 and 532, and normalized. In 534 and 536, it is compared with the reference value, and AND of the comparison result is used as an input disconnection alarm signal. In the circuit of FIG. 111, it is assumed that f ₁ is superimposed on one channel and f ₂ is superimposed on the other channel. Only _one of f ₁ and f ₂ may be monitored, but if both are monitored, only one line can be used. Dividers 530 and 532 are for eliminating the influence of fluctuations in input power, and are not necessary when the input power is stable.

FIG. 114 is an extension of the circuit of FIG. 111 to four multiplexes. The clocks of the optical switches 540 and 542 are generated by dividing the output of the VCO 520 by ½ by the ½ divider 544. The low frequency signal f ₁ is superimposed on CH1 and CH2.

As described above, according to the present invention, transmission of large capacity is possible by optimizing transmission conditions, and several peripheral technologies for realizing optical multiplexing are established.
(Supplementary note 1) an optical transmitter that generates an optical signal;
An optical transmission line for transmitting an optical signal generated by the optical transmission unit;
An optical receiver for recognizing an optical signal transmitted through the optical transmission path;
Characteristic adjusting means suitable for adapting the characteristic of the optical signal to the characteristic of the optical transmission line by adjusting at least one of the characteristic value of the optical signal and the characteristic value of the optical transmission line; Optical transmission system.
(Supplementary note 2) The optical transmission system according to supplementary note 1, wherein the characteristic adjusting means includes a wavelength variable light source that adjusts a wavelength of the optical signal by adjusting a wavelength of the light source as a characteristic value of the optical signal.
(Additional remark 3) The said characteristic adjustment means is provided in the end point of the said optical transmission line, and includes the wavelength variable filter which can adjust a passage wavelength as a characteristic value of the said optical transmission line according to the wavelength of an optical signal. 2. The optical transmission system according to 2.
(Appendix 4) The optical transmission line includes a plurality of optical fibers and an optical amplifier that is provided between the optical fibers and optically amplifies an optical signal transmitted through the optical fiber,
The characteristic adjustment means includes a second tunable filter provided on the output side of the optical amplifier and capable of adjusting a passing wavelength as a characteristic value of the optical transmission line according to the wavelength of the optical signal. 3. The optical transmission system according to 3.
(Appendix 5) The optical transmission line includes an optical fiber,
The characteristic adjusting means includes a variable dispersion compensator that is provided in the optical transmission line and compensates the chromatic dispersion characteristic of the optical fiber by adjusting the chromatic dispersion characteristic value as the characteristic value of the optical transmission line. 5. The optical transmission system according to claim 4.
(Appendix 6) The optical transmission line includes an optical amplifier that is provided between the optical fibers and amplifies an optical signal transmitted through the optical fiber,
The light according to claim 5, wherein the wavelength of the optical signal is a value that is far from the zero-dispersion wavelength of the optical fiber to such an extent that the four-wave mixing between the spontaneous emission light generated by the optical amplifier and the optical signal can be suppressed. Transmission system.
(Supplementary note 7) Any one of Supplementary notes 1 to 5, wherein the characteristic adjusting unit includes a variable chirping unit that adjusts a pre-chirping amount given to the optical signal generated by the optical transmission unit as a characteristic value of the optical signal. The optical transmission system described.
(Appendix 8) The optical transmission line includes an optical fiber,
8. The optical transmission system according to appendix 1, 2, 3, 4, 5 or 7, wherein the characteristic adjusting means includes means for adjusting power of an optical signal incident on the optical fiber as a characteristic value of the optical signal.
(Supplementary note 9) Further includes transmission characteristic measuring means for measuring transmission characteristics by evaluating the quality of the optical signal recognized by the optical receiver,
In the characteristic adjusting means, at least one of the characteristic value of the optical signal and the characteristic value of the optical transmission line is adjusted so that the transmission characteristic measured by the transmission characteristic measuring means is optimized. The optical transmission system according to claim 1.
(Supplementary Note 10) Characteristic value sweeping means that makes it possible to automatically find the best characteristic value by sweeping at least one of the characteristic value of the optical signal and the characteristic value of the optical transmission line within a predetermined range The optical transmission system according to appendix 9, further comprising:
(Additional remark 11) The said transmission characteristic measurement means is an optical transmission system of Additional remark 9 which evaluates quality by evaluating the code error rate in an optical signal.
(Supplementary note 12) The optical transmission system according to supplementary note 9, wherein the transmission characteristic measuring means evaluates the quality by evaluating the opening of the eye pattern of the optical signal.
(Supplementary note 13) The optical transmission system according to supplementary note 9, wherein the transmission characteristic measuring means evaluates quality by measuring a Q value of an optical signal.
(Supplementary note 14) The optical transmission system according to supplementary note 9, wherein the transmission characteristic measuring means evaluates the quality by checking a parity bit in the optical signal.
(Supplementary Note 15) An optical multiplexing unit that optically multiplexes a plurality of optical signals and supplies the optical signals to the optical transmission line;
15. The optical transmission system according to any one of appendices 1 to 14, further comprising: an optical dispersion unit that optically disperses the multiplexed optical signal transmitted through the optical transmission line.
(Supplementary Note 16) The optical transmission line includes a working line and a protection line,
The optical transmission system according to any one of appendices 10 to 15, wherein the characteristic value sweeping means sweeps characteristic values for the protection line.
(Supplementary Note 17) The temperature of the optical transmission line is evaluated in order to cause the characteristic adjusting unit to adjust at least one of the characteristic value of the optical signal and the characteristic value of the optical transmission line in accordance with a temperature change of the optical transmission line. The optical transmission system according to appendix 1, further comprising temperature evaluation means.
(Supplementary note 18) The optical transmission system according to supplementary note 17, wherein the temperature evaluation means evaluates temperature at a plurality of points of the optical transmission line.
(Supplementary note 19) The optical transmission system according to supplementary note 17 or 18, wherein the temperature evaluation means includes a temperature sensor that directly measures the temperature of the optical transmission line.
(Supplementary note 20) The optical transmission system according to supplementary note 17 or 18, wherein the temperature evaluation means evaluates the temperature of the optical transmission line by measuring the temperature in the vicinity of the optical transmission line.
(Supplementary note 21) The optical transmission system according to supplementary note 17 or 18, wherein the temperature evaluation means evaluates the temperature of the optical transmission line by measuring the temperature at a terminal station or a relay station.
(Supplementary note 22) The light according to any one of supplementary notes 17 to 21, wherein the characteristic adjusting means includes a wavelength variable light source that adjusts a wavelength of the optical signal by adjusting a wavelength of the light source as a characteristic value of the optical signal. Transmission system.
(Supplementary note 23) The optical transmission line includes an optical fiber,
The characteristic adjustment means includes a variable dispersion compensator that is provided in the optical transmission line and compensates the chromatic dispersion characteristic of the optical fiber by adjusting the chromatic dispersion characteristic value as the characteristic value of the optical transmission line. The optical transmission system according to any one of 22.
(Supplementary Note 24) Any one of Supplementary Notes 17 to 23, wherein the characteristic adjustment unit includes a variable chirping unit that adjusts a pre-chirping amount given to the optical signal generated by the optical transmission unit as a characteristic value of the optical signal. The optical transmission system described.
(Supplementary Note 25) The optical transmission line includes an optical fiber,
The optical transmission system according to any one of appendices 17 to 24, wherein the characteristic adjusting means includes means for adjusting the power of an optical signal incident on the optical fiber as a characteristic value of the optical signal.
(Supplementary note 26) The optical transmission according to any one of supplementary notes 17 to 25, further comprising an automatic adjustment unit that automatically adjusts the characteristic adjustment unit based on an evaluation value of the temperature of the optical transmission line by the temperature evaluation unit. system.
(Supplementary note 27) an optical transmitter that generates an optical signal;
An optical transmission line for transmitting an optical signal generated by the optical transmission unit;
An optical receiver for recognizing an optical signal transmitted through the optical transmission path;
An optical transmission system comprising: means for reducing nonlinear effects by smoothing a change in intensity of an optical signal transmitted through the optical transmission line.
(Supplementary note 28) The nonlinear effect reducing means includes means for imparting chromatic dispersion sufficient to reduce the nonlinear effect to the optical signal,
28. The optical transmission system according to appendix 27, further comprising means for compensating for chromatic dispersion generated by the chromatic dispersion providing means.
(Supplementary note 29) The chromatic dispersion imparting means includes an optical fiber having a zero dispersion wavelength that is far enough from the signal light wavelength to impart sufficient chromatic dispersion to reduce the nonlinear effect on the optical signal,
29. The optical transmission system according to appendix 28, wherein the chromatic dispersion compensation means includes a dispersion compensator.
(Supplementary note 30) The optical transmission system according to supplementary note 29, wherein the optical fiber exhibits a negative dispersion value at a wavelength of the optical signal.
(Supplementary note 31) The optical transmission system according to supplementary note 29, wherein the optical fiber exhibits a positive dispersion value at a wavelength of the optical signal.
(Supplementary Note 32) The chromatic dispersion imparting unit includes a dispersion compensator that imparts sufficient chromatic dispersion to reduce the nonlinear effect on the optical signal,
29. The optical transmission system according to appendix 28, wherein the chromatic dispersion compensation means includes an optical fiber having a dispersion value that compensates for chromatic dispersion provided by the dispersion compensator.
(Supplementary Note 33) The chromatic dispersion imparting unit includes a first dispersion compensator that imparts sufficient chromatic dispersion to reduce the nonlinear effect on the optical signal,
29. The optical transmission system according to appendix 28, wherein the chromatic dispersion compensation means includes a second dispersion compensator having a dispersion value with a sign opposite to the sign of the dispersion value of the first dispersion compensator.
(Supplementary note 34) The optical transmission system according to supplementary note 33, wherein the first dispersion compensator has a positive dispersion value with respect to the optical signal.
(Supplementary note 35) The optical transmission system according to supplementary note 33, wherein the first dispersion compensator has a negative dispersion value with respect to the optical signal.
(Supplementary note 36) The optical transmission system according to supplementary note 28, further comprising optical signal characteristic adjusting means for optimizing the quality of the optical signal recognized by the optical receiver by adjusting the characteristic value of the optical signal.
(Supplementary note 37) The optical transmission system according to supplementary note 36, wherein the optical signal characteristic adjusting means includes a wavelength variable light source that adjusts a wavelength of the optical signal by adjusting a wavelength of the light source as a characteristic value of the optical signal.
(Supplementary note 38) The optical transmission system according to supplementary note 36, wherein the optical signal characteristic adjusting means includes variable chirping means for adjusting a pre-chirping amount given to the optical signal generated by the optical transmitter as the characteristic value of the optical signal. .
(Supplementary note 39) The optical transmission line includes an optical fiber,
37. The optical transmission system according to appendix 36, wherein the optical signal characteristic adjusting means includes means for adjusting the power of the optical signal incident on the optical fiber as the characteristic value of the optical signal.
(Supplementary note 40) Further comprising transmission characteristic measuring means for measuring transmission characteristics by evaluating the quality of an optical signal recognized by the optical receiver,
40. The optical transmission system according to any one of supplementary notes 36 to 39, wherein in the optical signal characteristic adjusting means, the characteristic value of the optical signal is adjusted so that the transmission characteristic measured by the transmission characteristic measuring means becomes the best.
(Supplementary note 41) The optical transmission system according to supplementary note 40, further comprising characteristic value sweeping means that makes it possible to automatically find the best characteristic value by sweeping the characteristic value of the optical signal within a predetermined range.
(Additional remark 42) The said transmission characteristic measurement means is an optical transmission system of Additional remark 40 which evaluates quality by evaluating the code error rate in an optical signal.
(Supplementary note 43) The optical transmission system according to supplementary note 40, wherein the transmission characteristic measuring means evaluates quality by evaluating an opening of an eye pattern of an optical signal.
(Supplementary note 44) The optical transmission system according to supplementary note 40, wherein the transmission characteristic measurement means evaluates quality by measuring a Q value of an optical signal.
(Supplementary note 45) The optical transmission system according to supplementary note 40, wherein the transmission characteristic measuring means evaluates the quality by checking a parity bit in the optical signal.
(Supplementary Note 46) The optical transmission line includes a working line and a protection line,
46. The optical transmission system according to any one of appendices 40 to 45, wherein the characteristic value sweeping means sweeps characteristic values for the protection line.
(Supplementary Note 47) An optical transmitter that generates an optical signal;
An optical transmission line for transmitting an optical signal generated by the optical transmission unit;
An optical receiver for recognizing an optical signal transmitted through the optical transmission path;
An optical amplification repeater that is provided in the middle of the optical transmission line and optically amplifies an optical signal transmitted through the optical transmission line;
An optical transmission system comprising: a wavelength converter for converting a wavelength of an optical signal optically amplified by the optical amplification repeater.
(Supplementary note 48) The optical transmission system according to supplementary note 47, wherein the optical transmission unit includes a variable wavelength light source that adjusts a wavelength of the optical signal by adjusting a wavelength of the light source.
(Supplementary note 49) Further comprising transmission characteristic measuring means for measuring transmission characteristics by evaluating the quality of the optical signal recognized by the optical receiver,
49. The optical transmission system according to appendix 48, wherein the wavelength of the output light is adjusted so that the transmission characteristic measured by the transmission characteristic measuring means is the best in the wavelength converter and the wavelength tunable light source.
(Supplementary note 50) The light according to supplementary note 49, further comprising wavelength sweeping means that enables the wavelength converter and the wavelength tunable light source to automatically find the best wavelength by sweeping the wavelength of the output light within a predetermined range. Transmission system.
(Supplementary note 51) The optical transmission system according to supplementary note 49, wherein the transmission characteristic measurement means evaluates quality by evaluating a code error rate in an optical signal.
(Supplementary note 52) The optical transmission system according to supplementary note 49, wherein the transmission characteristic measuring means evaluates the quality by evaluating the opening of the eye pattern of the optical signal.
(Supplementary note 53) The optical transmission system according to supplementary note 49, wherein the transmission characteristic measurement means evaluates quality by measuring a Q value of an optical signal.
(Supplementary note 54) The optical transmission system according to supplementary note 49, wherein the transmission characteristic measuring means evaluates the quality by checking a parity bit in the optical signal.
(Supplementary Note 55) The optical transmission line includes a working line and a protection line,
56. The optical transmission system according to any one of appendices 50 to 55, wherein the wavelength sweeping means sweeps the wavelength of the protection line.
(Supplementary note 56) A drift compensation circuit for an optical modulator of an optical multiplexing system that multiplexes a plurality of optical signals respectively modulated by baseband signals in the plurality of optical modulators,
A plurality of drive circuits for amplitude-modulating each baseband signal supplied to the plurality of optical modulators with a low-frequency signal;
An optical branching device for branching a part of the optical multiplexed signal obtained by multiplexing the optical signals;
A photodetector for converting a part of the multiplexed optical signal branched by the optical splitter into an electrical signal;
Generates a bias signal for drift compensation of each optical modulator by phase-detecting the low frequency signal component included in the output of the photodetector with the low frequency signal used in each of the plurality of driving circuits. And a drift compensation circuit.
(Supplementary note 57) The drift compensation circuit according to supplementary note 56, wherein the plurality of optical signals are time-division multiplexed at the same wavelength.
(Supplementary Note 58) Low frequency signals used in the plurality of drive circuits have different frequencies,
The control means includes
And a plurality of phase detection / bias supply circuits that generate a bias signal for each optical modulator by phase-detecting the output of the photodetector with the low-frequency signals used in the plurality of drive circuits. 58. The drift compensation circuit according to 57.
(Supplementary Note 59) The control means includes:
A first changeover switch for providing a low-frequency signal to the plurality of drive circuits in a time-sharing manner;
A phase detection / bias supply circuit for generating a bias signal for drift compensation of the optical modulator by detecting the phase of the output of the photodetector with the low frequency signal;
58. The drift compensation according to claim 57, further comprising: a second changeover switch that distributes a bias signal generated by the phase detection / bias supply circuit to each optical modulator by operating in conjunction with the first changeover switch. circuit.
(Supplementary note 60) The drift compensation circuit according to supplementary note 58 or 59, wherein the plurality of optical modulators are connected in parallel.
(Supplementary note 61) The drift compensation circuit according to supplementary note 58 or 59, wherein the plurality of optical modulators are connected in series.
(Supplementary note 62) The drift compensation circuit according to supplementary note 56, wherein the plurality of optical signals are wavelength-multiplexed at different wavelengths.
(Supplementary Note 63) Low frequency signals used in the plurality of drive circuits have different frequencies,
The control means includes
A plurality of phase detection / bias supply circuits for generating a bias signal for each of the optical modulators by phase-detecting the output of the photodetector with the low-frequency signals used in the plurality of drive circuits, respectively. The drift compensation circuit according to appendix 62.
(Supplementary Note 64) The control means includes:
A first changeover switch for providing a low-frequency signal to the plurality of drive circuits in a time-sharing manner;
A phase detection / bias supply circuit for generating a bias signal for drift compensation of the optical modulator by detecting the phase of the output of the photodetector with the low frequency signal;
62. A drift compensation according to claim 62, further comprising: a second changeover switch that distributes a bias signal generated by the phase detection / bias supply circuit to each optical modulator by operating in conjunction with the first changeover switch. circuit.
(Supplementary Note 65) The photodetector includes a plurality of photodetectors,
The control means includes
A wavelength dispersion element that disperses a part of the optical multiplexed signal branched by the optical branching unit for each wavelength and supplies each of the plurality of photodetectors;
A plurality of phase detection / bias generating a bias signal for each optical modulator by phase-detecting a low frequency component output from each of the photodetectors with a low frequency signal used in the drive circuit 63. A drift compensation circuit according to appendix 62, including a supply circuit.
(Supplementary Note 66) The control means includes:
A plurality of phase detection / bias for generating a bias optical signal for each optical modulator by detecting a phase of a low frequency component included in the output of each optical modulator with the low frequency signal used in the drive circuit A supply circuit;
An optical wavelength tunable filter provided between the optical branching device and the photodetector, wherein a passing wavelength is switched in a time division manner,
By operating in conjunction with switching of the pass wavelength in the optical wavelength tunable filter, the output of the photodetector is distributed to each of the phase detection / bias supply circuits as a low frequency component included in the output of each optical modulator. 63. A drift compensation circuit according to appendix 62, including a change-over switch for
(Supplementary note 67) The drift compensation circuit according to any one of supplementary notes 56 to 66, further comprising a sign inversion circuit for switching an operating point of the optical modulation circuit.
(Supplementary Note 68) Optical time division multiplexing means for time division multiplexing a plurality of optical signals;
An optical transmission line for transmitting the optical multiplexed signal generated by the optical time division multiplexing means, and clock extraction for directly extracting the clock of the optical signal before multiplexing from the optical multiplexed signal transmitted by the optical transmission path Means,
An optical transmission system comprising: an amplitude difference providing unit that enables clock extraction by the clock extracting unit by adding a difference to the amplitude of each optical signal in the optical multiplexed optical signal supplied to the clock extracting unit.
(Supplementary note 69) The optical transmission system according to supplementary note 68, wherein the amplitude difference providing unit includes a plurality of light sources each having a different light output power, which is a light source for each of the plurality of optical signals.
(Supplementary note 70) The optical transmission system according to supplementary note 68, wherein the amplitude difference giving means includes an optical attenuator for attenuating at least one of the optical signals before being multiplexed by the optical time division multiplexing means.
(Supplementary note 71) The optical transmission system according to supplementary note 68, wherein the amplitude difference providing means includes an optical amplifier that amplifies at least one of the optical signals before being multiplexed by the optical time division multiplexing means.
(Supplementary note 72) The optical transmission system according to supplementary note 68, wherein the amplitude difference providing means includes a plurality of optical modulators that generate the plurality of optical signals and that are provided with differences in the amplitudes of the drive signals. .
(Supplementary note 73) The optical transmission according to supplementary note 68, wherein the amplitude difference providing means includes a plurality of optical modulators that generate the plurality of optical signals and that are provided with differences in bias voltages of the drive signals. system.
(Supplementary note 74) The optical transmission system according to supplementary note 68, wherein the amplitude difference providing unit includes an optical switch that branches the light from the light source at different branching ratios.
(Supplementary Note 75) The amplitude difference providing unit is an optical modulator that generates the plurality of optical signals, and a plurality of optical modulators having different polarization main axes;
69. The optical transmission system according to appendix 68, further comprising a polarization dependent optical element through which the optical multiplexed signal passes.
(Supplementary note 76) The optical transmission system according to supplementary note 75, wherein the polarization-dependent optical element is provided on a transmission side of the optical transmission line.
(Supplementary note 77) The optical transmission system according to supplementary note 75, wherein the polarization-dependent optical element is provided on a reception side of the optical transmission line.
(Supplementary Note 78) Optical time division multiplexing means for time division multiplexing with a plurality of optical signals;
In order to enable the receiving side to directly extract the clock of the optical signal before multiplexing from the optical multiplexed signal, an amplitude difference providing unit for adding and adding a difference to the amplitude of each optical signal in the optical multiplexed signal is provided. Optical transmitter.
(Supplementary note 79) The optical transmitter according to supplementary note 78, wherein the amplitude difference providing unit includes a plurality of light sources each having a different light output power, which are light sources for the plurality of optical signals.
(Supplementary note 80) The optical transmitter according to supplementary note 78, wherein the amplitude difference providing unit includes an optical attenuator that attenuates at least one of the optical signals before being multiplexed by the optical time division multiplexing unit.
(Supplementary note 81) The optical transmitter according to supplementary note 78, wherein the amplitude difference providing unit includes an optical amplifier that amplifies at least one of the optical signals before being multiplexed by the optical time division multiplexing unit.
(Supplementary note 82) The optical transmitter according to supplementary note 78, wherein the amplitude difference providing unit includes a plurality of optical modulators that generate the plurality of optical signals and to which a difference is given to the amplitude of the drive signal. .
(Supplementary note 83) The optical transmission according to supplementary note 78, wherein the amplitude difference providing means includes a plurality of optical modulators that generate the plurality of optical signals and that are provided with differences in bias voltages of the drive signals. Machine.
(Supplementary note 84) The optical transmitter according to supplementary note 78, wherein the amplitude difference providing unit includes an optical switch that branches light from the light source at different dispersion ratios.
(Supplementary Note 85) The amplitude difference providing means includes:
An optical modulator that generates the plurality of optical signals, and a plurality of optical modulators having different polarization main axes;
79. The optical transmitter according to appendix 78, comprising a polarization dependent optical element through which the optical multiplexed signal passes.
(Supplementary Note 86) Optical time division multiplexing means for time division multiplexing a plurality of optical signal channels;
Means for giving identification information for identifying each optical signal channel to the optical multiplexed signal generated by the optical time division multiplexing means;
An identification information extraction circuit for extracting identification information contained in the optical signal channel;
An optical transmission system comprising: a control circuit that changes an output destination so that each optical signal channel is output to a predetermined output destination according to the identification information extracted by the identification information extraction circuit.
(Supplementary Note 87) A clock recovery circuit for recovering a clock of each optical signal channel from the optical multiplexed signal;
90. The optical transmission system according to supplementary note 86, further comprising: an optical switch that separates each optical signal channel from the optical multiplexed signal in accordance with a clock regenerated by the clock regenerating circuit.
(Supplementary Note 88) The identification information adding unit adds identification data to a data signal transmitted by the optical signal,
90. The optical transmission system according to appendix 87, wherein the identification information extraction circuit extracts identification data included in a data signal reproduced from each optical signal channel.
(Supplementary note 89) The optical transmission system according to supplementary note 88, wherein the control circuit changes an output destination by changing a connection between an output of each optical signal channel and the output destination.
(Supplementary note 90) The optical transmission system according to supplementary note 88, wherein the control circuit changes an output destination by changing a phase of a clock supplied to the optical switch.
(Supplementary note 91) The optical transmission system according to supplementary note 88, wherein the control circuit changes an output destination by delaying or advancing an optical multiplexed signal input to the optical switch.
(Supplementary Note 92) The identification information providing unit weights the low frequency signal to the optical multiplexed signal in a time slot of a specific optical signal channel,
90. The optical transmission system according to appendix 87, wherein the identification information extraction circuit extracts identification information by detecting a low-frequency signal weighted by the optical multiplexed signal.
(Supplementary note 93) The optical transmission system according to supplementary note 92, wherein the identification information extraction circuit detects the low-frequency signal from a signal obtained by converting the optical signal channel separated by the optical switch into an electrical signal.
(Supplementary note 94) The light according to supplementary note 92, wherein the identification information extraction circuit detects the low-frequency signal from a current flowing to an optical / electrical conversion element for converting the optical signal channel separated by the optical switch into an electrical signal. Transmission system.
(Supplementary Note 95) Further comprising a signal switching circuit provided between the output of the optical switch and the output destination,
95. The optical transmission system according to any one of appendices 92 to 94, wherein the control circuit changes an output destination by changing a connection relationship between an output of the optical switch and the output destination in the signal switching circuit.
(Supplementary note 96) The optical transmission system according to any one of supplementary notes 92 to 94, wherein the control circuit changes an output destination by changing a phase of a clock supplied to the optical switch.
(Supplementary note 97) The optical transmission system according to any one of supplementary notes 92 to 94, wherein the control circuit changes an output destination by delaying or advancing an optical multiplexed signal input to the optical switch.
(Supplementary Note 98) An identification information extraction circuit that extracts identification information included in the optical signal channel;
An optical receiver comprising: a control circuit that changes an output destination so that each optical signal channel is output to a predetermined output destination according to the identification information extracted by the identification information extraction circuit.
(Supplementary Note 99) A clock recovery circuit for recovering a clock of each optical signal channel from an optical multiplexed signal;
99. The optical receiver according to supplementary note 98, further comprising: an optical switch that separates each optical signal channel from the optical multiplexed signal in accordance with the clock recovered by the clock recovery circuit.
(Supplementary Note 100) The identification information addition is added as identification data to a data signal transmitted by the optical signal,
99. The optical receiver according to appendix 99, wherein the identification information extraction circuit extracts identification data included in a data signal reproduced from each optical signal channel.
(Supplementary Note 101) A signal exchange circuit provided between the output and the output destination of the optical switch is further provided,
100. The optical receiver according to appendix 100, wherein the control circuit changes an output destination by changing a connection relationship between an output of the optical switch and the output destination in the signal switching circuit.
(Supplementary note 102) The optical receiver according to supplementary note 100, wherein the control circuit changes an output destination by changing a phase of a clock supplied to the optical switch.
(Supplementary note 103) The optical receiver according to supplementary note 100, wherein the control circuit changes an output destination by delaying or advancing an optical multiplexed signal input to the optical switch.
(Supplementary Note 104) The identification information is obtained by weighting the optical multiplexed signal with a low frequency signal in a time slot of a specific optical signal channel,
The optical receiver according to supplementary note 99, wherein the identification information extraction circuit extracts identification information by detecting a low-frequency signal weighted by the optical multiplexed signal.
(Supplementary note 105) The optical receiver according to supplementary note 104, wherein the identification information extraction circuit detects the low-frequency signal from a signal obtained by converting the optical signal channel separated by the optical switch into an electrical signal.
(Additional remark 106) The said identification information extraction circuit detects the said low frequency signal from the electric current which flows into the optical / electrical conversion element for converting the optical signal channel isolate | separated by the said optical switch into an electrical signal, The light of Additional remark 104 Receiving machine.
(Supplementary Note 107) Further comprising a signal switching circuit provided between the output of the optical switch and the output destination,
107. The optical receiver according to any one of appendices 104 to 106, wherein the control circuit changes an output destination by changing a connection relationship between an output of the optical switch and the output destination in the signal switching circuit.
(Supplementary note 108) The optical receiver according to any one of supplementary notes 104 to 106, wherein the control circuit changes an output destination by changing a phase of a clock supplied to the optical switch.
(Supplementary note 109) The optical receiver according to any one of supplementary notes 104 to 106, wherein the control circuit changes an output destination by delaying or advancing an optical multiplexed signal input to the optical switch.
(Supplementary Note 110) A clock recovery circuit for recovering a clock of each optical signal channel from an optical multiplexed signal;
An optical switch that separates each optical signal channel from the optical multiplexed signal in accordance with a clock recovered by the clock recovery circuit;
An identification information extraction circuit for extracting identification information included in the optical signal channel separated by the optical switch;
An optical separator comprising: a control circuit that changes an output destination so that each optical signal channel is output to a predetermined output destination according to the identification information extracted by the identification information extraction circuit.
(Supplementary Note 111) The identification information addition is added as identification data to a data signal represented by the optical signal,
111. The optical separator according to appendix 110, wherein the identification information extraction circuit extracts identification data included in a data signal reproduced from each optical signal channel.
(Supplementary note 112) Further comprising a signal switching circuit provided between the output of the optical switch and the output destination,
112. The optical disperser according to appendix 111, wherein the control circuit changes an output destination by changing a connection relationship between an output of the optical switch and the output destination in the signal switching circuit.
(Supplementary note 113) The optical separator according to supplementary note 111, wherein the control circuit changes an output destination by changing a phase of a clock supplied to the optical switch.
(Supplementary note 114) The optical separator according to supplementary note 111, wherein the control circuit changes an output destination by delaying or advancing an optical multiplexed signal input to the optical switch.
(Supplementary Note 115) The identification information is obtained by weighting a low frequency signal to the optical multiplexed signal in a time slot of a specific optical signal channel,
111. The optical separator according to supplementary note 110, wherein the identification information extraction circuit extracts identification information by detecting a low-frequency signal weighted by the optical multiplexed signal.
(Supplementary note 116) The optical separator according to supplementary note 115, wherein the identification information extraction circuit detects the low-frequency signal from a signal obtained by converting the optical signal channel separated by the optical switch into an electrical signal.
(Supplementary note 117) The light according to supplementary note 115, wherein the identification information extraction circuit detects the low-frequency signal from a current flowing to an optical / electrical conversion element for converting the optical signal channel separated by the optical switch into an electrical signal. Separator.
(Supplementary note 118) Further comprising a signal switching circuit provided between the output of the optical switch and the output destination,
118. The optical separator according to any one of appendices 115 to 117, wherein the control circuit changes an output destination by changing a connection relationship between an output of the optical switch and the output destination in the signal switching circuit.
(Supplementary note 119) The optical separator according to any one of supplementary notes 115 to 117, wherein the control circuit changes an output destination by changing a phase of a clock supplied to the optical switch.
(Supplementary note 120) The optical separator according to any one of supplementary notes 115 to 117, wherein the control circuit changes an output destination by delaying or advancing an optical multiplexed signal input to the optical switch.
(Supplementary note 121) Optical time division multiplexing means for time division multiplexing a plurality of optical signal channels;
An optical transmitter comprising: means for giving identification information for identifying each optical signal channel to the optical multiplexed optical signal generated by the optical time division multiplexing means.
(Supplementary note 122) The optical transmitter according to supplementary note 121, wherein the identification information adding unit superimposes a low-frequency signal on the optical multiplexed signal in a time slot of a specific optical signal channel.
(Supplementary Note 123) The supplementary note includes a second optical modulator connected in series to an optical modulator for generating the specific optical signal channel and supplied with the low-frequency signal as a modulation signal. 122. An optical transmitter according to 122.
(Supplementary Note 124) The drive for supplying the identification information as the modulation signal to the optical modulator for generating the optical signal channel by superimposing the low frequency signal on the modulation signal for the specific optical signal channel 122. The optical transmitter according to appendix 122, including a circuit.
(Supplementary Note 125) An optical receiver for receiving an optical time division multiplexed signal in which a plurality of optical signals are time division multiplexed and a low frequency signal is superimposed in a time slot of a specific optical signal,
An optical switch for distributing the optical time division multiplexed signal to each optical signal;
Clock generating means for generating a clock for controlling the optical switch;
An optical circuit comprising: clock phase control means for controlling the phase of the clock generated by the clock generating means to be synchronized with the optical time division multiplexed signal using a low frequency signal superimposed on the optical time division multiplexed signal; Receiving machine.
(Supplementary Note 126) The clock generation means includes a timing recovery unit that generates a clock by extracting a clock component from the received optical time division multiplexed signal,
The clock phase control means includes
A phase variable circuit that changes the phase of the clock output by the timing recovery unit according to a phase control signal;
125. The optical receiver according to appendix 125, further including a phase control unit that generates a phase control signal for controlling the phase variable circuit so that the clock is synchronized with the optical time division multiplexed signal using the low-frequency signal.
(Supplementary Note 127) The phase control unit includes:
A light receiving element that converts one of the optical signals dispersed by the optical switch into an electrical signal;
An oscillator that outputs a signal of a predetermined frequency;
A synchronous detection circuit for phase detection of the output of the light receiving element by the output of the oscillator;
A comparator for comparing the output of the synchronous detection circuit with a predetermined threshold;
127. The optical receiver according to claim 126, further comprising: an adder that adds the comparison result of the comparator to the output of the oscillator and outputs the result as the phase control signal.
(Supplementary note 128) The optical receiver according to supplementary note 127, wherein the phase control unit further includes a band-pass filter that is provided between the light receiving element and the synchronous detection circuit and passes only the low-frequency signal component.
(Supplementary note 129) The optical receiver according to supplementary note 127, wherein the comparator inverts and outputs a comparison result in accordance with a control signal.
(Supplementary Note 130) The timing reproduction unit includes:
A level detector for detecting the level of the extracted clock component;
131. The optical receiver according to any one of appendices 126 to 129, including a comparator that compares the output of the level detector with a predetermined reference value and outputs an alarm signal when the level detector is below the reference value.
(Supplementary Note 131) The clock generation means includes a voltage controlled oscillator that generates a clock having a frequency according to a control voltage,
125. The supplementary note 125, wherein the clock phase control means includes a phase control unit that uses the low-frequency signal to generate a control voltage for controlling the voltage controlled oscillator so that the clock is synchronized with the optical time division multiplexed signal. Optical receiver.
(Supplementary Note 132) The phase control unit includes:
A light receiving element that converts one of the optical signals separated by the optical switch into an electrical signal;
An oscillator that outputs a signal of a predetermined frequency;
A synchronous detection circuit that compares the phase of the output of the light receiving element with the output of the oscillator;
A comparator for comparing the output of the synchronous detection circuit with a predetermined threshold;
134. The optical receiver according to appendix 131, further comprising: an adder that adds the comparison result of the comparator to the output of the oscillator and outputs the result as the control voltage.
(Additional remark 133) The said phase control part is an optical receiver of Additional remark 132 further provided with the band pass filter which is provided between the said light receiving element and the said synchronous detection circuit, and allows only the said low frequency signal component to pass through.
(Supplementary note 134) The optical receiver according to supplementary note 132, wherein the comparator inverts and outputs the comparison result in accordance with a control signal.
(Supplementary note 135) a low frequency signal detection circuit for detecting the low frequency component superimposed on the optical time division multiplexed signal;
135. The optical receiver according to any one of appendices 131 to 134, further comprising a comparator that compares a detection level of the low-frequency signal detection circuit with a predetermined reference value and outputs an alarm signal when the detection level is equal to or lower than the reference value. .

  According to the present invention, an optical transmitter that generates an optical signal, an optical transmission path that transmits the optical signal generated by the optical transmitter, and an optical receiver that recognizes the optical signal transmitted by the optical transmission path, Characteristic adjusting means suitable for adapting the characteristic of the optical signal to the characteristic of the optical transmission line by adjusting at least one of the characteristic value of the optical signal and the characteristic value of the optical transmission line. An optical transmission system is provided.

According to the present invention, an optical transmitter that generates an optical signal, an optical transmission path that transmits the optical signal generated by the optical transmitter, and an optical receiver that recognizes the optical signal transmitted by the optical transmission path, There is also provided an optical transmission system comprising means for reducing nonlinear effects by smoothing a change in the intensity of an optical signal transmitted through the optical transmission line.
According to the present invention, an optical transmitter that generates an optical signal, an optical transmission path that transmits the optical signal generated by the optical transmitter, and an optical receiver that recognizes the optical signal transmitted by the optical transmission path, An optical amplification repeater that is provided in the middle of the optical transmission line and optically amplifies an optical signal transmitted through the optical transmission line, and a wavelength converter that converts the wavelength of the optical signal optically amplified by the optical amplification repeater An optical transmission system comprising:

  According to the present invention, there is provided a drift compensation circuit for an optical modulator of an optical multiplexing system that multiplexes a plurality of optical signals that are respectively modulated by baseband signals in the plurality of optical modulators. A plurality of drive circuits for amplitude-modulating each baseband signal supplied to the modulator with a low-frequency signal; an optical branching device for branching a part of an optical multiplexed signal obtained by multiplexing the optical signals; A photodetector that converts a part of the optical multiplexed signal branched by the splitter into an electrical signal, and a low-frequency signal component included in the output of the photodetector is used in each of the plurality of drive circuits. There is also provided a drift compensation circuit comprising control means for generating a bias signal for drift compensation of each optical modulator by phase detection with the frequency signal.

  According to the present invention, an optical time division multiplexing means for time division multiplexing a plurality of optical signals, an optical transmission line for transmitting an optical multiplexed signal generated by the optical time division multiplexing means, and transmission by the optical transmission path A clock extraction unit that directly extracts a clock of the optical signal before multiplexing from the multiplexed optical signal and a difference in amplitude of each optical signal in the optical multiplexed signal supplied to the clock extraction unit There is also provided an optical transmission system comprising amplitude difference providing means for enabling clock extraction by the clock extracting means.

According to the present invention, the optical time division multiplexing means for time-division multiplexing a plurality of optical signals, and the receiving side can directly extract the clock of the optical signal before multiplexing from the optical multiplexed signal. There is also provided an optical transmitter comprising amplitude difference providing means for applying a difference to the amplitude of each optical signal in the optical multiplexed signal.
According to the present invention, optical time division multiplexing means for time division multiplexing a plurality of optical signal channels, and identification information for identifying each optical signal channel is added to the optical multiplexed signal generated by the optical time division multiplexing means. An identification information extraction circuit for extracting identification information included in the optical signal channel, and an output so that each optical signal channel is output to a predetermined output destination according to the identification information extracted by the identification information extraction circuit. An optical transmission system comprising a control circuit for changing the destination is also provided.

According to the present invention, an identification information extraction circuit that extracts identification information included in an optical signal channel, and each optical signal channel is output to a predetermined output destination according to the identification information extracted by the identification information extraction circuit. An optical receiver comprising a control circuit for changing the output destination is also provided.
According to the present invention, the clock recovery circuit that recovers the clock of each optical signal channel from the optical multiplexed signal, and the light that separates each optical signal channel from the optical multiplexed signal according to the clock recovered by the clock recovery circuit. A switch, an identification information extraction circuit that extracts identification information included in the optical signal channel separated by the optical switch, and each optical signal channel is output to a predetermined output destination according to the identification information extracted by the identification information extraction circuit An optical separator comprising a control circuit for changing the output destination is also provided.

According to the present invention, optical time division multiplexing means for time division multiplexing a plurality of optical signal channels, and identification information for identifying each optical signal channel in the optical multiplexed signal generated by the optical time division multiplexing means are provided. An optical transmitter comprising means for providing is also provided.
According to the present invention, an optical receiver for receiving an optical time division multiplexed signal in which a plurality of optical signals are time division multiplexed and a low frequency signal is superimposed in a time slot of a specific optical signal, An optical switch that separates the optical time division multiplexed signal into optical signals, clock generation means for generating a clock for controlling the optical switch, and a low-frequency signal superimposed on the optical time division multiplexed signal are used. There is also provided an optical receiver comprising clock phase control means for controlling the phase of the clock generated by the clock generation means so as to be synchronized with the optical time division multiplexed signal.

It is a block diagram which shows an example of the optical transmission system of this invention provided with the wavelength variable light source. It is a perspective view which shows the wavelength variable semiconductor laser as an example of a wavelength variable light source. It is a block diagram which shows the other example of the optical transmission system of this invention which further provided the wavelength variable filter for every repeater. It is a block diagram which shows the other example of the optical transmission system of this invention which further provided the transmission characteristic part. It is a figure explaining the method to determine a wavelength from the measured value of a code error rate. It is a figure explaining the measurement of the transmission characteristic by an eye pattern. It is a figure explaining Q value. It is a block diagram which shows the other example of the optical transmission system of this invention. It is a block diagram which shows the other example of the optical transmission system of this invention. It is a block diagram which shows an example of the optical transmission system of this invention with which the dispersion variable compensator was provided in the transmission side. It is a block diagram which shows the other example of the optical transmission system of this invention. It is a block diagram which shows an example of the optical transmission system of this invention in which the dispersion variable compensator was provided in the receiving side. It is a block diagram which shows the other example of the optical transmission system of this invention. It is a block diagram which shows an example of the optical transmission system of this invention in which the dispersion variable compensator was provided also in the repeater. It is a block diagram which shows an example of the optical transmission system of this invention in which the transmission characteristic measurement part was further provided. It is a block diagram which shows the other example of the optical transmission system of this invention. It is a block diagram which shows the other example of the optical transmission system of this invention. It is a block diagram which shows the other example of the optical transmission system of this invention. It is a block diagram which shows the other example of the optical transmission system of this invention. It is a figure which shows the characteristic of a Mach-Zehnder type optical modulator. It is a figure explaining the red shift and blue shift in a Mach-Zehnder type optical modulator. It is a figure which shows the Mach-Zehnder type | mold optical modulator which connected the intensity | strength modulation part and the phase modulation part in tandem for control of the amount of pre-chirping. It is a figure which shows an example of arrangement | positioning of the signal number wavelength in a wavelength multiplexing system. It is a figure which shows the temperature dependence of zero dispersion wavelength (lambda) ο. It is a figure which shows an example of the temperature evaluation of an optical fiber. It is a figure which shows the other example of the temperature evaluation of an optical fiber. It is a figure which shows the other example of the temperature evaluation of an optical fiber. It is a figure which shows an example of the optical transmission system which changes a signal light wavelength based on temperature evaluation. It is a figure which shows the other example of the optical transmission system of this invention. It is a figure which shows an example of the optical transmission system which changes the amount of pre-chirping based on temperature evaluation. It is a figure which shows an example of the optical transmission system which changes dispersion compensation amount based on temperature evaluation. It is a figure which shows an example of an optical transmission system arrange | positioned at a dispersion variable compensator and the receiving side. It is a figure which shows an example of the optical transmission system which has arrange | positioned the dispersion variable compensator in the transmission part, the receiving part, and the repeater. It is a figure which shows an example of the optical transmission system which changes the amplification degree of an optical amplifier based on temperature evaluation. It is a figure which shows an example of the optical transmission system which changes a signal light wavelength, the amount of pre-chirping, a dispersion compensation amount, and an optical amplification degree based on temperature evaluation. It is a figure which shows an example of the optical transmission system which reduced the nonlinear effect by arrange | positioning a dispersion compensator on the receiving side. It is a figure which shows the other example of an optical transmission system. It is a figure which shows an example of the optical transmission system which has arrange | positioned the dispersion compensator on the transmission side. It is a figure which shows the other example of an optical transmission system. It is a figure which shows an example of the optical transmission system which made positive the compensation amount D of the dispersion compensator arrange | positioned at the receiving side. It is a figure which shows an example of the optical transmission system which made the compensation amount D of the dispersion compensator arrange | positioned at the receiving side negative. It is a figure which shows an example of the optical transmission system which has arrange | positioned the dispersion compensator from which the polarity of a dispersion value is mutually opposite on the transmission side and the receiving side. It is a figure which shows an example of the optical transmission system which has arrange | positioned the dispersion compensator also to the repeater. It is a figure which shows an example of the optical transmission system which arrange | positions a dispersion compensator and also measures a transmission characteristic and optimizes a signal light wavelength. It is a figure which shows an example of the optical transmission system which has arrange | positioned the dispersion compensator on the transmission side. It is a figure which shows an example of the optical transmission system which has arrange | positioned the dispersion compensator in the transmission part, the receiving part, and the repeater. It is a figure which shows an example of the optical transmission system which arrange | positions a dispersion compensator and also measures a transmission characteristic and controls the amount of pre-chirping optimally. It is a figure which shows an example of the optical transmission system which has arrange | positioned the dispersion compensator on the transmission side. It is a figure which shows an example of the optical transmission system which has arrange | positioned the dispersion compensator in the transmission part, the receiving part, and the repeater. It is a figure which shows an example of the optical transmission system which arrange | positions a dispersion compensator and measures a transmission characteristic, and controls a signal light wavelength and the amount of pre-chirping optimally. It is a figure which shows an example of the optical transmission system which has arrange | positioned the dispersion compensator on the transmission side. It is a figure which shows an example of the optical transmission system which has arrange | positioned the dispersion compensator in the transmission part, the receiving part, and the repeater. It is a figure which shows an example of the optical transmission system which has arrange | positioned the wavelength converter to the optical amplification repeater. It is a figure which shows an example of the optical transmission system which also made the wavelength in a transmission part variable. It is sectional drawing of the wavelength conversion laser as an example of a wavelength converter. It is a figure which shows an example of the optical transmission system which measures a transmission characteristic and optimizes a signal light wavelength for every optical amplification repeater section. Furthermore, it is a figure which shows an example of the optical transmission system which has arrange | positioned the dispersion compensator. It is a figure explaining operation | movement of a drift compensation circuit when an operating point is appropriate. It is a figure explaining operation | movement of a drift compensation circuit when an operating point fluctuates. It is a figure explaining operation | movement of a drift compensation circuit when an operating point fluctuates. It is a block diagram which shows an example of the optical multiplexing system which has the drift compensation circuit of this invention. It is a block diagram which shows the other example of the optical multiplexing system which has the drift compensation circuit of this invention. It is a block diagram which shows the other example of the optical multiplexing system which has the drift compensation circuit of this invention. It is a block diagram which shows the other example of the optical multiplexing system which has the drift compensation circuit of this invention. It is a block diagram which shows the other example of the optical multiplexing system which has the drift compensation circuit of this invention. It is a block diagram which shows the other example of the optical multiplexing system which has the drift compensation circuit of this invention. It is a block diagram which shows the other example of the optical multiplexing system which has the drift compensation circuit of this invention. It is a block diagram which shows the other example of the optical multiplexing system which has the drift compensation circuit of this invention. It is a block diagram which shows the other example of the optical multiplexing system which has the drift compensation circuit of this invention. It is a block diagram which shows an example of the optical time division multiplex transmission system to which the clock extraction technique of this invention is applied. 71 is a timing chart for explaining the operation of the system of FIG. It is a wave form diagram for demonstrating the clock extraction technique of this invention. It is a wave form diagram for demonstrating the other example of the clock extraction technique of this invention. It is a block diagram which shows an example of the optical transmitter of this invention. It is a block diagram which shows the other example of the optical transmitter of this invention. It is a block diagram which shows the other example of the optical transmitter of this invention. It is a figure explaining the change of the intensity | strength of output light by changing the amplitude of the drive voltage of a Mach-Zehnder type optical modulator. It is a figure explaining the change of the intensity | strength of output light by changing the bias of the drive voltage of a Mach-Zehnder type optical modulator. It is a block diagram which shows the other example of the optical transmitter of this invention. It is a block diagram which shows an example of the optical receiver of this invention. It is a block diagram which shows the other example of the optical receiver of this invention. It is a block diagram which shows the detail of a clock extraction circuit. It is a block diagram which shows an example of the optical separator of this invention. It is a figure which shows an example of the format of the transmission data containing channel identification data. It is a block diagram which shows the other example of the optical separator of this invention. It is a block diagram which shows the other example of the optical separator of this invention. It is a block diagram which shows the other example of the optical separator of this invention. It is a figure which shows an example of an optical delay circuit. It is a block diagram which shows the other example of the optical separator of this invention. It is a figure which shows the low frequency signal multiplexed by the optical signal. It is a block diagram which shows the other example of the optical separator of this invention. It is a block diagram which shows the other example of the optical separator of this invention. It is a block diagram which shows the other example of the optical separator of this invention. It is a block diagram which shows an example of the optical transmitter of this invention. It is a block diagram which shows the other example of the optical transmitter of this invention. 4 is a circuit diagram showing details of drive circuits 418 and 420. FIG. It is a block diagram which shows the optical receiver which performs clock phase stabilization control of this invention. 98 is a timing chart for explaining the operation of the circuit of FIG. It is a figure explaining phase difference theta. Is a graph showing the relationship between the intensity of the phase difference θ and f ₁ component. It is a figure which shows the other example of the optical receiver of this invention. It is a figure which shows the other example of the optical receiver of this invention. It is a figure which shows the other example of the optical receiver of this invention. It is a figure which shows an example of the low frequency signal weighted by a received signal in the optical receiver of FIG. It is a block diagram which shows the other example of the optical receiver of this invention. It is a block diagram which shows an example of the detail of a timing reproduction part. It is a wave form diagram of an example of an optical multiplexing signal. It is a figure which shows an example of the characteristic of a light receiving element. It is a block diagram which shows the other example of the detail of a timing reproduction part. It is a block diagram which shows the detail of an optical switch. It is a block diagram which shows the other example of the optical receiver of this invention. It is a figure which shows the relationship of the period detection output value with respect to phase difference (theta). It is a figure which shows an example of the circuit for an input disconnection alarm detection. It is a block diagram which shows the other example of the optical receiver of this invention. FIG. 87 is a timing chart illustrating operation of the circuit of FIG. 86. It is a block diagram showing an example of the optical switch of this invention. It is a block diagram showing the other example of the optical switch of this invention. It is a block diagram showing the other example of the optical switch of this invention. It is a block diagram showing the other example of the optical switch of this invention. FIG. 99 is a waveform diagram showing an operation of the circuit of FIG. 96.

Explanation of symbols

13, 33, 43, 63, 83, 102 Optical fiber 16, 17, 36, 37, 46, 47, 69, 70, 89, 91a, 91b, 104
Optical amplifier

Claims (12)

A drift compensation circuit for an optical modulator of an optical multiplexing system that multiplexes a plurality of optical signals each modulated by a baseband signal in a plurality of optical modulators,
A plurality of drive circuits for amplitude-modulating each baseband signal supplied to the plurality of optical modulators with a low-frequency signal;
An optical branching device for branching a part of the optical multiplexed signal obtained by multiplexing the optical signals;
A photodetector for converting a part of the multiplexed optical signal branched by the optical splitter into an electrical signal;
Generates a bias signal for drift compensation of each optical modulator by phase-detecting the low frequency signal component included in the output of the photodetector with the low frequency signal used in each of the plurality of driving circuits. And a drift compensation circuit.   The drift compensation circuit according to claim 1, wherein the plurality of optical signals are time-division multiplexed at the same wavelength. The low frequency signals used in the plurality of drive circuits have different frequencies,
The control means includes
A plurality of phase detection / bias supply circuits for generating a bias signal for each optical modulator by phase-detecting the output of the photodetector with a low-frequency signal used in the plurality of drive circuits. Item 3. The drift compensation circuit according to Item 2. The control means includes
A first changeover switch for providing a low-frequency signal to the plurality of drive circuits in a time-sharing manner;
A phase detection / bias supply circuit for generating a bias signal for drift compensation of the optical modulator by detecting the phase of the output of the photodetector with the low frequency signal;
The drift according to claim 2, further comprising: a second changeover switch that operates in conjunction with the first changeover switch and distributes a bias signal generated by the phase detection / bias supply circuit to each optical modulator. Compensation circuit.   The drift compensation circuit according to claim 3 or 4, wherein the plurality of optical modulators are connected in parallel.   The drift compensation circuit according to claim 3 or 4, wherein the plurality of optical modulators are connected in series.   The drift compensation circuit according to claim 1, wherein the plurality of optical signals are wavelength-multiplexed at different wavelengths. The low frequency signals used in the plurality of drive circuits have different frequencies,
The control means includes
A plurality of phase detection / bias supply circuits for generating bias signals for the respective optical modulators by phase-detecting the outputs of the photodetectors with the low-frequency signals used in the plurality of drive circuits, respectively. The drift compensation circuit according to claim 7. The control means includes
A first changeover switch for providing a low-frequency signal to the plurality of drive circuits in a time-sharing manner;
A phase detection / bias supply circuit for generating a bias signal for drift compensation of the optical modulator by detecting the phase of the output of the photodetector with the low frequency signal;
The drift according to claim 7, further comprising: a second changeover switch that operates in conjunction with the first changeover switch to distribute the bias signal generated by the phase detection / bias supply circuit to each optical modulator. Compensation circuit. The photodetector includes a plurality of photodetectors;
The control means includes
A wavelength dispersion element that disperses a part of the optical multiplexed signal branched by the optical branching unit for each wavelength and supplies each of the plurality of photodetectors;
A plurality of phase detection / bias generating a bias signal for each optical modulator by phase-detecting a low frequency component output from each of the photodetectors with a low frequency signal used in the drive circuit The drift compensation circuit according to claim 7, further comprising a supply circuit. The control means includes
A plurality of phase detection / bias for generating a bias optical signal for each optical modulator by detecting a phase of a low frequency component included in the output of each optical modulator with the low frequency signal used in the drive circuit A supply circuit;
An optical wavelength tunable filter provided between the optical branching device and the photodetector, wherein a passing wavelength is switched in a time division manner,
By operating in conjunction with switching of the pass wavelength in the optical wavelength tunable filter, the output of the photodetector is distributed to each of the phase detection / bias supply circuits as a low frequency component included in the output of each optical modulator. The drift compensation circuit according to claim 7, further comprising: a changeover switch that performs switching.   The drift compensation circuit according to claim 1, further comprising a sign inversion circuit for switching an operating point of the optical modulation circuit.

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