JP5811531B2 - Optical transmitter, optical communication system, and optical transmission method - Google Patents

Description

  The present invention relates to an optical communication technique, and more particularly to an optical transmitter, an optical communication system, and an optical transmission method for transmitting an optical signal based on an analog signal.

  2. Description of the Related Art Conventionally, in an optical communication system, OOK (On Off Keying), which is an intensity modulation method in which 1-bit information is put on the light intensity, and DPSK (Differential), which is a differential phase shift keying method in which 1-bit information is put on two phase differences. Phase shift keying) DQPSK (Differential Quadrature Phase Shift Keying), which is a differential four-phase phase shift keying method in which 2-bit information is loaded on four phase differences, has been adopted and studied. Any of the modulation signals is digital signal modulation that handles a binary code of 1 or 0.

  In OOK and DPSK, an MZ (Mach-Zehnder) type optical modulator (hereinafter referred to as an MZ modulator) using a change in refractive index by applying an electric field, that is, an electro-optic effect is used. In DQPSK, a two parallel MZ modulator (hereinafter referred to as an IQ modulator) composed of two MZ modulators is used. In this IQ modulator, an MZ modulator as an MZ interferometer is connected in parallel, and an In-phase channel (hereinafter referred to as I channel) which is a real part and a Quadrature-phase channel (hereinafter referred to as Q channel) which is an imaginary part. ) Are combined with a carrier phase difference of 90 degrees to generate a complex optical electric field.

  On the other hand, in the MZ modulator, since the optimum bias voltage changes under the influence of environmental temperature and time fluctuation, high-precision optical modulator bias control is desired to stabilize the signal quality. There have been reported examples of feedback control related to bias voltages of an MZ modulator used for OOK and an IQ modulator used for DQPSK (see, for example, Patent Document 1 and Patent Document 2).

JP 2000-171766 A JP 2008-92172 A

In the future, as communication traffic increases, high-speed optical transceivers such as 40 Gbps and 100 Gbps are desired. The modulation method applied to such a high-speed optical transceiver is preferably one that can increase the amount of information that can be transmitted with one symbol, with the background that it is difficult to increase the speed of the component parts. As such a modulation method, OFDM (Orthogonal Frequency Division Multiplexing), which is orthogonal frequency division multiplexing that reduces the symbol rate by dividing a carrier into a plurality of subcarriers, and the amplitudes of two carriers that are 90 degrees out of phase independently. QAM (Quadrature Amplitude Modulation), which is quadrature amplitude modulation in which each carrier wave is modulated with a plurality of amplitude values, is promising. In QAM taking 2 ^N values, N bits can be transmitted in one symbol, and the frequency utilization efficiency can be improved N times compared to OOK.

  In the pre-equalization technique, waveform distortion in an optical transceiver or optical transmission line is electrically equalized and transmitted in advance by digital signal processing on the transmission side. This pre-equalization technique has advantages in that the system configuration can be simplified compared to optical dispersion compensation and that the amount of equalization can be increased compared to dispersion compensation on the receiving side, which is 40 Gbps and 100 Gbps. As described above, even in a high-speed optical transmission technique in which the influence of waveform distortion in the optical transmission path becomes large, it can be an effective compensation means.

  In any modulation scheme of OFDM, QAM, and pre-equalization, an optical signal having an arbitrary amplitude level is generated on the transmission side. Unlike conventional digital signal modulation, linear amplification operation and electro-optical conversion are performed. An operation is performed, and an optical signal generation capability as analog signal modulation is desired. In digital signal modulation, the degree of modulation is almost constant for each optical waveform with respect to a binary code, whereas in analog signal modulation, an optical signal has an arbitrary amplitude level, and the degree of modulation differs for each optical waveform. become.

  However, in the conventional optical transmitters disclosed in Patent Documents 1 and 2, the feedback control circuit for the MZ modulator bias has been designed on the premise of digital signal modulation with a constant degree of modulation. If an OFDM, QAM, or pre-equalization modulation scheme is applied to such a conventional optical transmitter, the degree of modulation differs for each optical waveform in analog signal modulation, and control suitable for bias control according to the degree of modulation. Since the parameter may change, there is a problem that signal quality is deteriorated.

  The present invention has been made to solve the above-described problems, and realizes stable optical modulator bias control even when applied to analog signal modulation in which the degree of modulation differs for each optical waveform. It is an object.

An optical transmitter according to the present invention includes an analog signal generation unit that generates an analog signal based on an input transmission data sequence, and an optical modulator that modulates light based on the analog signal and outputs the modulated optical signal A bias control circuit for performing feedback control of a bias voltage applied to the optical modulator, and an adjustment for adjusting a control parameter used for controlling the bias voltage based on modulation degree information indicating a modulation degree corresponding to the optical signal And the adjustment circuit adjusts the control parameter based on the modulation degree information that is changed according to setting information for generating the analog signal.

  According to the present invention, for example, in an optical transmitter having an optical modulator that performs analog signal modulation such that the modulation degree differs for each optical waveform by OFDM, QAM, and pre-equalization, the control parameter can be adjusted according to the modulation degree By doing so, there is an effect that stable control of the optical modulator bias becomes possible.

1 is a block diagram showing an optical transmitter according to Embodiment 1 of the present invention. Explanatory drawing for demonstrating the optical transmitter by Embodiment 1 of this invention Explanatory drawing for demonstrating the optical transmitter by Embodiment 1 of this invention Configuration diagram showing an optical transmitter according to a second embodiment of the present invention.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing an optical transmitter according to Embodiment 1 of the present invention. In each figure, the same numerals indicate the same or corresponding parts. In FIG. 1, the optical transmitter according to the first embodiment converts an analog signal generation unit 1 that generates an analog signal as an analog modulated electric signal based on a transmission data sequence, a light source 2, and the modulation signal into an optical signal. MZ (Mach-Zehnder) type optical modulator (hereinafter referred to as MZ modulator) 3, a driver 4 that linearly amplifies an analog signal and outputs it as a modulation signal, and a light detection unit 5 that detects an optical signal output intensity, The control signal source 6 provided for constant control of the bias voltage of the MZ modulator 3 and the synchronization provided for extracting a control error signal as an error component from the control signal included in the monitor signal detected after optical modulation. A detection circuit 7; a bias control circuit 8 that controls the bias voltage of the MZ modulator 3 so that an error component detected after synchronous detection is eliminated; and a control signal that is superimposed on the bias voltage of the MZ modulator 3. Consisting of regulating circuit 9 to adjust the width.

  In FIG. 1, the light source 2, the light input / output unit of the MZ modulator 3, and the light input unit of the light detection unit 5 are optically connected via, for example, an optical fiber cable. The analog signal generator 1, the modulation and bias electrodes of the MZ modulator 3, the driver 4, the electrical output unit of the light detection unit 5, the control signal source 6, the synchronous detection circuit 7, the bias control circuit 8, and the adjustment circuit 9 For example, they are electrically connected via electric wiring.

  The analog signal generation unit 1 converts, for example, an OFDM (Orthogonal Frequency Division Multiplexing), QAM (Quadrature Amplitude Modulation), pre-etc. And a digital circuit for performing digital signal processing by the digital technology and a digital / analog conversion circuit for converting a digital electric signal from the digital circuit into an analog signal and outputting the analog signal. The digital circuit performs digital signal processing using setting information for generating an analog signal read from the memory. The configuration of the analog signal generation unit 1 is not limited to this. In short, the analog signal generation unit 1 is for generating an arbitrary analog signal corresponding to an optical signal whose degree of modulation changes according to the optical waveform. It is applicable as Embodiment 1 of the invention.

Note that, as described above, OFDM is an orthogonal frequency division multiplexing system that reduces a symbol rate by dividing a carrier into a plurality of subcarriers. QAM is a quadrature amplitude modulation method in which the amplitudes of two carriers that are 90 degrees out of phase are independently modulated, and each carrier is modulated with a plurality of amplitude values. In QAM taking 2 ^N values, N bits can be transmitted in one symbol, and the frequency utilization efficiency can be improved N times compared to OOK. In the analog signal generation unit 1, these modulation methods can realize high-speed optical communication of 40 Gbps or 100 Gbps or more by increasing the amount of information that can be transmitted in one symbol without increasing the speed of the components. Has an advantage in terms.

  In addition, as described above, the pre-equalization technique is to electrically equalize and transmit waveform distortion in an optical transmitter / receiver or an optical transmission line in advance by digital signal processing on the transmission side. In the analog signal generation unit 1, this pre-equalization technique has an advantage in that the system configuration can be simplified as compared with optical dispersion compensation and the amount of equalization can be increased as compared with dispersion compensation on the reception side. As a high-speed optical transmission technique in which the influence of waveform distortion in the optical transmission line is large, such as 40 Gbps or 100 Gbps or more, it is an effective compensation means.

  In FIG. 1, a light source 2 is a semiconductor laser having, for example, an InP compound semiconductor mixed crystal as a constituent material, and emits and outputs a laser beam as a continuous wave in a 1.55 μm wavelength band. The configuration of the light source 2 is not limited to this. For example, a 1.3 μm wavelength band, a solid-state laser, or the like may be used, and in short, emits light for realizing desired optical communication. Is applicable as Embodiment 1 of the present invention.

In FIG. 1, an MZ modulator 3 is an optical modulator that uses, for example, a LiNbO ₃ crystal as a constituent material and uses a change in refractive index by applying an electric field, that is, a so-called electro-optic effect. The MZ modulator 3 is configured as a so-called MZ interferometer by connecting in parallel two optical waveguides having electrodes (not shown) between two Y branch optical waveguides. The MZ modulator 3 uses a modulation signal input to the modulation electrode and a refractive index change caused by a bias voltage input to the bias electrode for light passing through the MZ interferometer. A light intensity change corresponding to the phase difference is given and output. The MZ modulator 3 is an optical modulator that can achieve both high signal quality such as low chirp and high speed. The configuration of the MZ modulator 3 is not limited to this. For example, an InP-based compound semiconductor mixed crystal or the like may be used as a constituent material, and in short, bias control is performed according to a change in the degree of modulation. An optical modulator in which a suitable control parameter changes can be applied as the first embodiment of the present invention.

  The light detection unit 5 includes a monitor PD (Photo-Diode) having, for example, an InP compound semiconductor mixed crystal as a constituent material. The light detection unit 5 photoelectrically converts a part of the optical output branched from the optical output of the MZ modulator 3 into an electrical signal by the monitor PD and outputs the electrical signal. The configuration of the light detection unit 5 is not limited to this. For example, a Ge semiconductor crystal or the like may be used as a constituent material for the monitor PD.

  In FIG. 1, the driver 4, the control signal source 6, the synchronous detection circuit 7, the bias control circuit 8, and the adjustment circuit 9 are configured as an electric circuit. The adjustment circuit 9 may be configured to be realized by software processing using a computer program executed by a microcomputer or the like.

  Next, the operation will be described. In FIG. 1, a transmission data series is input to an analog signal generation unit 1 from the outside of an optical transmitter. The analog signal generation unit 1 generates an analog signal based on the above-described OFDM or pre-equalization based on the input transmission data sequence and outputs the analog signal to the driver 4. The driver 4 linearly amplifies the input analog signal and outputs it to the modulation electrode of the MZ modulator 3 as a modulation signal which is a main signal. On the other hand, the light source 2 emits laser light and outputs it to the MZ modulator 3. The MZ modulator 3 to which the bias voltage controlled by the bias control circuit 8 is applied to the bias electrode converts the modulation signal input to the modulation electrode into an optical signal by the operation of modulating the input laser light. The signal is converted and output to the outside of the optical transmitter as an optical signal by OFDM or pre-equalization.

  Next, operations related to bias control will be described in detail. In FIG. 1, the bias control circuit 8 applies a bias voltage on which a control signal as a low frequency component outside the main signal band called dither generated from the control signal source 6 is superimposed to the bias electrode of the MZ modulator 3, Feedback control is performed so that the control signal included in the monitor signal detected by the light detection unit 5 is minimized, and the variation from the optimum bias voltage due to the influence of the environmental temperature and time variation is compensated.

  FIG. 2 shows a case where the MZ modulator 3 having the maximum drive amplitude of 2Vπ is driven around the NULL point with the drive amplitude Vsignal [Vpp] of the main signal and the bias control is performed with the control signal amplitude Vdither [Vpp]. It is a figure which shows an example of the relationship between a drive voltage and optical output intensity. The curve in FIG. 2 shows the relationship between the drive voltage of the MZ modulator 3 and the light output intensity. The electric drive waveform of the MZ modulator 3 is shown below the curve in FIG. 2, and the main signal component of the optical output waveform output from the MZ modulator 3 is shown on the right side of the curve in FIG. In the case of driving with a driving amplitude of 2Vπ, the maximum value of the light output intensity can be used, so that the modulation degree is 100%.

In order to suppress the control error due to the bias control circuit 8 to the minimum, the control signal superposed on the bias electrode of the MZ modulator 3 when the deviation from the optimum bias voltage is applied to the photodetector 5 and the synchronous detection circuit 7. It is preferable that detection is possible with a good S / N (Signal / Noise) ratio. From the viewpoint of the S / N ratio of the control signal component, a larger control signal amplitude is better. However, if there is an unnecessary control signal amplitude at the time of reception recognition of the main signal, the S / N ratio of the main signal component is deteriorated and reception sensitivity is deteriorated. Therefore, in order to prevent deterioration of the signal quality of the main signal while superimposing the control signal, it is desirable to suppress the control signal amplitude ratio A with respect to the drive amplitude of the main signal to a certain value or less. The relationship between A, the drive amplitude Vsignal of the main signal, and the control signal amplitude Vdither is as shown in Expression (1).
A = Vdither / Vsignal (1)

On the other hand, when an arbitrary analog signal is handled, the degree of modulation m differs for each design value of the optical waveform. The relationship between the modulation degree m, the driving amplitude Vsignal of the main signal, and the maximum driving amplitude 2Vπ of the modulator is as shown in Expression (2). From the equations (1) and (2), the control signal amplitude Vdither can be obtained as shown in the equation (3).
m = Vsignal / 2Vπ (2)
Vdither = 2Vπ × m × A (3)

  Therefore, it is desirable to adjust the amplitude Vdither of the control signal for each set value of the modulation factor m based on the equation (3) so that A in the equation (1) is suppressed to a predetermined value. In the first embodiment, the adjustment circuit 9 has a function of adjusting the amplitude value of the control signal for each modulation degree, thereby realizing bias control in which signal degradation is suppressed for each arbitrary waveform amplitude. it can. Note that the modulation degree information used in the adjustment circuit 9 may be combined with setting information for generating an analog signal used in the analog signal generation unit 1. Further, the modulation degree information used in the adjustment circuit 9 may be input from the outside of the optical transmitter.

  In the first embodiment, the adjustment circuit 9 may be provided with a memory in which the relationship between a plurality of modulation degrees m and the control signal amplitude Vdither is previously recorded as table information. Further, since the voltage amplitude value of the maximum drive amplitude 2Vπ is different for each optical modulator used in the MZ modulator 3, by writing different information of 2Vπ in the memory, each MZ is based on the modulation degree information. It is possible to adjust the control signal amplitude taking into account variations due to individual differences in the modulator.

In the first embodiment, the control signal amplitude may be adjusted based on the modulation degree information detectable by the light detection unit 5. The relationship between the modulation degree m in the MZ modulator 3 and the light output P (m) based on the light output with the modulation degree of 100% is expressed by Expression (4) and is as shown in FIG.
P (m) = sin ² (m × π / 2) + sin ² ((−1) × m × π / 2) (4)

  In FIG. 3, the relationship between the modulation power m and the light output P (m) based on the light output of 100% modulation is determined by the extinction characteristic of the MZ modulator 3. The modulation degree m may be obtained from the light intensity P (m) detected by the light detection unit 5 based on the equation (4), and the control signal amplitude Vdither may be adjusted based on the equation (3). Thereby, the control signal amplitude can be adjusted autonomously inside the transmitter without inputting modulation degree information from the outside of the optical transmitter. In order to detect the optical output on the output side of the MZ modulator 3, the optical output of the light source 2 constituting the optical transmitter, the insertion loss of the MZ modulator 3, the monitor PD sensitivity of the light detection unit 5, the MZ modulator 3 Therefore, it is possible to detect in real time a change in the modulation factor m caused by a change in the operating environment of each component of the optical transmitter that incorporates up to 2 Vπ of the driving amplitude of the driver 4. As a result, it is possible to adjust the control signal amplitude Vdither including variations due to individual differences among components and temperature variations.

  In the first embodiment, the adjustment circuit 9 controls not only the modulation degree information but also the bit error information indicating the bit error rate determined by the optical receiver that has received the main signal component of the present optical transmitter. The signal amplitude Vdither may be adjusted. That is, after adjusting the control signal amplitude value based on the modulation degree information read from the optical output of the MZ modulator 3 first, the control signal amplitude may be finely adjusted based on the bit error information of the optical receiver. . Thereby, the modulation degree information read from the optical output based on the equation (4) is only the DC component, but the control signal amplitude can be adjusted with high accuracy including the AC component by reading the actual bit error information. It becomes possible.

  Further, in the first embodiment, the offset adjustment of the bias control circuit 8 may be performed by the adjustment circuit 9. In the analog circuit for actual bias control, there are offset components of the monitor PD of the light detection unit 5, DUTY variations of the analog signal components, offset components in the control block, and so on. In some cases, the error component of the detected control signal does not become zero. In this case, it is desirable that an optimum drive state is obtained by intentionally providing an offset component for the convergence point that is the target value to be converged by the bias control circuit 8. Therefore, in the first embodiment, after the adjustment circuit 9 adjusts the amplitude value of the control signal based on the modulation degree information and the bit error information, the bias control circuit set in advance based on the modulation degree information and the bit error information. A mechanism for adjusting an offset component with respect to 8 convergence points may be added. As a result, it is possible to realize bias control that maintains a high signal quality such that the bit error rate is minimized with respect to an arbitrary analog signal.

  As described above, in the optical transmitter according to Embodiment 1 of the present invention, the adjustment circuit 9 uses the amplitude of the control signal as the control parameter used for feedback control of the bias voltage of the MZ modulator 3 based on the modulation degree information. The offset component of the value and target value is adjusted. Thus, for example, in an optical modulator that performs analog signal modulation in which the degree of modulation differs for each optical waveform by OFDM or pre-equalization, there is an effect that stable bias control is possible. As a result, for example, optical communication that maintains high signal quality by OFDM or pre-equalization can be realized.

Embodiment 2. FIG.
4 is a block diagram showing an optical transmitter according to the second embodiment of the present invention. In the configuration of the optical transmitter shown in FIG. 1, instead of the MZ modulator 3, the driver 4, and the bias control circuit 8, FIG. Two parallel MZ type optical modulators (hereinafter referred to as IQ modulators) 3a composed of MZ modulators, a driver 4a for an in-phase channel (hereinafter referred to as I channel) which is a real part, and an imaginary part It shows that a driver 4b for quadrature-phase channel (hereinafter referred to as Q channel) and a bias control circuit 8a for the IQ modulator 3a are provided. In each figure, the same numerals indicate the same or corresponding parts.

  In FIG. 4, the optical transmitter according to the second embodiment includes an IQ modulator 3a that can modulate an I channel and a Q channel independently and includes an I channel modulation electrode and a Q channel modulation electrode (not shown), and an I channel. Monitor signal detected from the optical output of the IQ modulator 3a, the I channel driver 4a that supplies the Ich modulation signal to the modulation electrode, the Q channel driver 4b that supplies the Qch modulation signal to the Q channel modulation electrode Is provided with a bias control circuit 8a for controlling the Ich bias, the Qch bias, and the phase bias, and the rest of the configuration is the same as that of the optical transmitter according to the first embodiment. Hereinafter, description of the same operation with the same configuration as that of the first embodiment may be omitted.

  In FIG. 4, an IQ modulator 3a connects two MZ modulators in parallel as an MZ interferometer, and an I channel Ich modulation signal that is a real part and a Qch Qch modulation signal that is an imaginary part, A complex optical electric field is generated by multiplexing by applying a carrier phase difference of 90 degrees. The IQ modulator 3a includes a modulation electrode and a bias electrode in each MZ modulator, and a phase control electrode on one of optical waveguides connecting the MZ modulators in parallel. The I-channel driver 4a, the Q-channel driver 4b, and the bias control circuit 8a are configured as analog circuits.

  Next, the operation will be described. In FIG. 4, the IQ modulator 3a modulates the light input from the light source 2 based on the Ich modulation signal or the Qch modulation signal input independently from the driver 4a or the driver 4b, respectively, by QAM or pre-equalization. For example, the optical signal is output to an optical transmission line as an optical fiber.

  When a two-parallel MZ type optical modulator is used as an IQ modulator, a waveform having an amplitude that is twice (2Vπ) the amplitude of the driving voltage corresponding to the phase π, ie, from the peak to the valley of the driving voltage versus the optical output intensity. When driven by voltage, the modulation factor is 100%, which is the maximum.

  In the same way as the bias control of the MZ modulator 3 shown in the first embodiment, the bias control circuit 8a receives control signals for I channel and Q channel, which are low frequency components outside the main signal band called dither. The Ich bias and Qch bias respectively superimposed on the bias voltage are applied to each bias electrode of the IQ modulator 3a, and feedback control is performed so that each control signal included in the monitor signal detected by the light detection unit 5 is minimized. A circuit to perform is provided. Note that the two MZ modulators can be controlled independently by dividing the dither frequency for the I channel and the Q channel.

  Further, the bias control circuit 8a applies a phase bias to the phase control electrode of the IQ modulator 3a so as to give a carrier phase difference of 90 degrees to each optical signal modulated based on the Ich modulation signal or the Qch modulation signal. It also has a circuit.

  Here, when the drive amplitudes of the main signal as the Ich modulation signal and the Qch modulation signal are the same value, and 2Vπ of each MZ modulator is the same value, the equations (1), (2), and (3) The same holds true for the first embodiment, and the control signal amplitude for the I channel and the Q channel may be set to the same value. In the second embodiment, the adjustment circuit 9 is provided with a function of adjusting the control signal amplitude for the I channel and the Q channel for each modulation degree so as to have the same value, so that the signal degradation is performed for each arbitrary waveform amplitude. It is possible to realize bias control in which is suppressed. The modulation degree information used in the adjustment circuit 9 may be combined with setting information for generating an analog signal used in the analog signal generation unit 1. Further, the modulation degree information used in the adjustment circuit 9 may be input from the outside of the optical transmitter.

  In the second embodiment, a peak detection circuit (not shown) is provided at each output section of the driver 4a and the driver 4b. Based on each driver output amplitude information read from the peak detection circuit, the I channel and Q channel The channel control signal amplitude may be individually adjusted to a different value. The modulation degree information that can be read from the light detection unit 5 is the sum of the modulation degrees of the I channel and the Q channel, and the modulation degree information of the I channel and the Q channel cannot be separated into each. By additionally providing the peak detection circuit at the output part of the drivers 4a and 4b, the modulation degree of the I channel and the Q channel can be grasped individually. Thereby, by individually adjusting the control signal amplitudes for the I channel and the Q channel, signal degradation of the main signal due to the control signal is suppressed in both the I channel and the Q channel, and high signal quality can be maintained.

  The IQ modulator 3a may be either a single-phase drive or a differential drive for the modulation electrode of one MZ modulator. When the MZ modulator is a differential drive type, each of the driver 4a and the driver 4b may use two single-phase drive types with one input / output or one differential drive type with two input / outputs.

  In the second embodiment, as in the first embodiment, the adjustment circuit 9 may be provided with a memory in which a relationship between a plurality of modulation degrees and control signal amplitudes is recorded in advance as table information. Based on the modulation degree information, the same effect is obtained that the control signal amplitude can be adjusted in consideration of variations due to individual differences among the MZ modulators.

  In the second embodiment, similarly to the first embodiment, the control signal amplitude may be adjusted based on the modulation degree information that can be detected by the light detection unit 5. The relationship between the modulation degree and the optical output in the IQ modulator 3a is the same as that shown in the equation (4) and FIG. 3, and the modulation degree is obtained from the light intensity detected by the light detection unit 5 and the control signal amplitude is adjusted. You may do it. Thus, the control signal amplitude can be adjusted autonomously inside the transmitter without inputting modulation degree information from the outside of the transmitter. Then, it is possible to adjust the control signal amplitude including variations due to individual differences between components and temperature variations.

  In the second embodiment, similarly to the first embodiment, not only the modulation degree information but also the bit error information indicating the bit error rate determined by the optical receiver that has received the main signal component of the present optical transmitter. Thus, the adjustment circuit 9 may adjust the control signal amplitude. That is, after adjusting the control signal amplitude value based on the modulation degree information, the control signal amplitude is finely adjusted based on the bit error information of the optical receiver, so that the control signal with high accuracy including the AC component of the signal is obtained. The amplitude can be adjusted.

  In the second embodiment, the adjustment of the offset of the bias control circuit 8a may be performed by the adjustment circuit 9 as in the first embodiment. High signal quality that minimizes the bit error rate by adjusting the control signal amplitude based on modulation degree information and bit error information and then intentionally providing an offset component for the target value to be converged by the bias control circuit Bias control that maintains the above can be realized.

  As described above, in the optical transmitter according to the second embodiment of the present invention, the adjustment circuit 9 is for the I channel as the control parameter used for feedback control of the bias voltage of the IQ modulator 3a based on the modulation degree information. The offset value of the amplitude value and target value of the control signal for the Q channel is adjusted. As a result, for example, an optical modulator that performs analog modulation in which the degree of modulation differs for each optical waveform by QAM or pre-equalization has an effect of enabling stable bias control. As a result, for example, optical communication maintaining high signal quality by QAM or pre-equalization can be realized.

  In addition, two or more optical transmitters described in the first embodiment and / or the second embodiment are provided, and the optical signals transmitted from the two or more optical transmitters are wavelength-multiplexed to transmit the optical fiber. In addition, it may be applied to a WDM (Wavelength Division Multiplexing) optical communication system in which wavelength separation is performed on the receiving side and reception is performed for each wavelength by two or more optical receivers. This makes it possible to realize a high-speed and large-capacity WDM optical communication system that maintains high signal quality by, for example, OFDM, QAM, and pre-equalization.

DESCRIPTION OF SYMBOLS 1 Analog signal production | generation part 2 Light source 3, 3a Optical modulator 4, 4a, 4b Driver 5 Photodetection part 6 Control signal source 7 Synchronous detection circuit 8, 8a Bias control circuit 9 Adjustment circuit

Claims (7)

An analog signal generator for generating an analog signal based on the input transmission data sequence;
An optical modulator that modulates light based on the analog signal and outputs the modulated optical signal;
A bias control circuit for controlling a bias voltage applied to the optical modulator;
An adjustment circuit for adjusting a control parameter used for feedback control of the bias voltage based on modulation degree information indicating a modulation degree corresponding to the optical signal;
With
The optical transmitter is characterized in that the adjustment circuit adjusts the control parameter based on the modulation degree information changed according to setting information for generating the analog signal. A light source for inputting emitted light to the light modulator;
A driver that supplies the optical modulator as a modulation signal obtained by linearly amplifying the analog signal generated by the analog signal generation unit;
A light detection unit for detecting the light intensity of the optical signal output by the optical modulator;
A control signal source for generating a control signal superimposed on the bias voltage;
A synchronous detection circuit that detects a control error signal indicating an error from a target value by performing synchronous detection of the light intensity detected by the detection unit and the control signal generated by the control signal source;
The bias control circuit performs feedback control of the bias voltage based on the control error signal,
The optical transmitter according to claim 1, wherein the adjustment circuit adjusts the control parameter as an amplitude of the control signal based on the modulation degree information.   The adjustment circuit adjusts the amplitude of the control signal based on the modulation degree information as table information indicating a relationship between the modulation degree read from the memory and the amplitude of the control signal. Optical transmitter. The optical transmitter according to claim 2, wherein the adjustment circuit adjusts the control parameter as an offset component of the amplitude of the control signal and the target value based on the modulation degree information and bit error information. . The optical modulator divides the input light into two, and each of the two branched lights is used for an In-phase channel (hereinafter referred to as I channel) which is a real part and a Quadrature-phase channel (hereinafter referred to as Q hereinafter) which is an imaginary part. This is an IQ modulator that generates and outputs a complex optical electric field by applying a 90-degree carrier wave phase difference to each modulated optical signal and combining the modulated optical signals. ,
The driver is an I-channel driver and a Q-channel driver that supply the I-channel and Q-channel modulated signals obtained by linearly amplifying the analog signal to the IQ modulator, respectively.
The control signal source generates I channel and Q channel control signals having different dither frequencies,
5. The optical transmitter according to claim 2, wherein the bias control circuit controls the three bias voltages of an I channel bias, a Q channel bias, and a phase bias. 6. .   Two or more optical transmitters according to any one of claims 1 to 5 are provided, optical signals transmitted from these two or more optical transmitters are wavelength-multiplexed and transmitted through an optical fiber, An optical communication system, wherein wavelength division is performed on a receiving side and reception is performed for each wavelength by two or more optical receivers. An analog signal generation step for generating an analog signal based on the input transmission data sequence;
An optical modulation step of modulating light based on the analog signal by an optical modulator and outputting the modulated optical signal;
A bias control step for controlling a bias voltage applied to the optical modulator;
An adjustment step of adjusting a control parameter used for feedback control of the bias voltage based on modulation degree information indicating a modulation degree corresponding to the optical signal;
With
The optical transmission method characterized in that the adjustment step adjusts the control parameter based on the modulation degree information that is changed according to setting information for generating the analog signal.

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