JP2008134663A - Apparatus and method for controlling optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a control system compensating displacement of an operating point of the optical modulator to be driven by 2 Vπ voltage amplitude. <P>SOLUTION: An apparatus for controlling the optical modulator, in which the signal light of a CS-RZ modulation system or the like is generated by using two LN modulators 10, 20 connected in series and which has a bias supply part 27 for supplying a DC bias to the LN modulator 20 to adjust the operating point, is provided with: an output monitor part 60 for detecting a change of the signal light to be outputted from the poststage LN modulator 20; and a control part 70 for determining displacement of the operating point of the optical modulator on the basis of the result detected by the output monitor part 30 and controlling the bias supply part 27 to reduce the displacement of the operating point. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a control technique for an optical modulator used in optical communication, and more particularly to a control technique for compensating for a phase shift between a plurality of drive signals for driving the optical modulator and an operating point shift of the optical modulator.

  Currently, optical transmission systems with optical signal transmission speeds of 10 Gb / s, etc. have been put into practical use. However, due to the rapid increase in network usage in recent years, further increase in network capacity is required, and ultra-long distance The demand is also increasing.

  In an optical transmission system with a transmission rate of 10 Gb / s or higher, the influence of the wavelength dispersion on the waveform is increased and the optical spectrum is widened, so that WDM transmission in which channel light is arranged at high density becomes difficult. In particular, in a 40 Gb / s optical transmission system or the like, chromatic dispersion is one of the factors that limit the transmission distance.

  As one means for solving the above problems, a dispersion compensation technique for accurately measuring and compensating for a dispersion value of an optical transmission line has been studied (see, for example, Patent Document 1 and Patent Document 2). In addition, in order to realize the above-described optical transmission system, it is indispensable to develop a modulation method having a large dispersion tolerance. Specifically, a good optical signal-to-noise ratio can be ensured for increasing the distance of the optical transmission system, that is, it is strong in self-phase modulation (SPM) effect, and the upper limit of the optical input power to the optical transmission line A modulation scheme that can increase the frequency is required. Furthermore, in order to increase the capacity, a modulation method having a narrow optical spectrum capable of high-density WDM optical transmission is required.

  In recent years, a carrier-suppressed return-to-zero (hereinafter referred to as CS-RZ) modulation method has been studied as a new modulation method (for example, see Non-Patent Document 1). As will be described later, this CS-RZ modulation system has a wide chromatic dispersion tolerance because the optical spectrum width is 2/3 times that of an RZ (Return-to-Zero) modulation system, and a high-density channel arrangement in WDM. Has the advantage of being possible. In addition, since the waveform deterioration due to the self-phase modulation effect is small, it is possible to secure an optical SN ratio for increasing the distance.

FIG. 25 is a diagram illustrating a basic configuration for generating a 40 Gb / s CS-RZ modulated signal.
In FIG. 25, the light source 100 generates continuous light. The continuous light output from the light source 100 is sequentially input and modulated into two LiNbO 3 modulators (hereinafter referred to as LN modulators) 110 and 120 connected in series.

  For example, the LN modulator 110 in the previous stage is configured such that a data signal corresponding to the NRZ modulation method with a bit rate generated by the data signal generation unit 111 of 40 Gb / s is applied as a drive signal to a signal electrode (not shown). The continuous light from the light source 100 is modulated in accordance with the data signal, and a 40 Gb / s NRZ signal light having a waveform illustrated in FIG. 26A is output to the LN modulator 120 at the subsequent stage.

  The LN modulator 120 at the subsequent stage uses, for example, a Mach-Zehnder (MZ) type modulator having two signal electrodes and the like, and is generated based on a clock signal having a frequency that is ½ times the bit rate of the data signal. By applying the first drive signal and the second drive signal to each signal electrode, the NRZ signal light from the LN modulator 110 in the previous stage is further modulated, and a waveform of 40 Gb / s having the waveform illustrated in FIG. s CS-RZ signal light is output. Here, a clock signal having a frequency of 20 GHz and a waveform such as a sine wave is generated by the clock signal generation unit 121, and the clock signal is bifurcated by the branching device 124, and then by the phase shifters 125 A and 125 B. The phase difference is adjusted to be approximately 180 °, and the respective amplitudes are adjusted by the amplifiers 126A and 126B to become the first and second drive signals applied to the signal electrodes of the LN modulator 120.

  In addition, the phases of the data signal and the clock signal are synchronized with each other by branching a part of the clock signal generated by the clock signal generation unit 121 by the branching unit 122 and transmitting it to the data signal generation unit 111. The phase difference between the signals is controlled by adjusting the phase of the clock signal by the phase shifter 123.

  Here, the principle of generating 40 Gb / s CS-RZ signal light will be briefly described using the light intensity characteristics with respect to the drive voltage of the LN modulator shown in FIG.

  In general, when signal light corresponding to the NRZ modulation method or the RZ modulation method is generated using an optical modulator whose light intensity characteristic with respect to the drive voltage periodically changes, Modulation is performed by applying a drive voltage corresponding to “valley, mountain” (hereinafter, this drive voltage is referred to as Vπ) to the optical modulator. Here, the “mountain” of the light intensity characteristic is the peak of light emission, and the “valley” is the peak of extinction.

  On the other hand, in the case of generating signal light corresponding to the CS-RZ modulation method, 40 Gb / s NRZ signal light modulated according to the data signal by the LN modulator 110 in the preceding stage shown in FIG. In this case, the LN modulator 120 in the subsequent stage is driven as shown in the left side of FIG. 27. A driving voltage corresponding to “mountain, valley, mountain” of the light intensity characteristic with respect to the voltage (hereinafter, this driving voltage is set to 2Vπ) is given. This light modulation is performed in accordance with the -1, 0, 1 levels of the clock signal corresponding to the on, off, and on states of the light, respectively, and the generated CS-RZ signal light is shown in the upper right of FIG. A binary optical waveform as shown in FIG. Since this CS-RZ modulation signal light has an optical phase value of 0 or π, for example, as shown in the calculation result of the optical spectrum at the lower right in FIG. The carrier component is suppressed.

  The signal light of the CS-RZ modulation method generated as described above is an optical waveform having substantially the same shape as the optical waveform of the RZ modulation method, as shown in each experimental result of the optical spectrum and optical waveform shown in FIG. And the optical spectrum width is narrower than in the RZ modulation method. For example, as shown in the experimental results regarding the chromatic dispersion tolerance shown in FIG. 29, the total chromatic dispersion range where the power penalty value is 1 dB or less is about 40 ps / nm in the case of the RZ modulation method. In the case of the CS-RZ modulation method, it is about 50 ps / nm, and it can be seen that the dispersion tolerance of the signal light of the CS-RZ modulation method is increased compared to the signal light of the RZ modulation method.

JP-A-11-72761 JP 2002-077053 A Y.Miyamoto et.al., “320Gbit / s (8x40Gbit / s) WDM transmission over 367-km zero-dispersion-flattened line with 120-km repeater spacing using carrier-suppressed return-to-zero pulse format”, OAA ' 99 PD, PdP4

  By the way, the signal light corresponding to the CS-RZ modulation system has the above-described advantages, but the phase between the first and second drive signals given to the subsequent optical modulator driven based on the clock signal. Must be adjusted precisely, and the phase between the data signal used to drive the optical modulator in the preceding stage and the clock signal must also be adjusted precisely. Furthermore, since a phase shift may occur due to environmental changes such as temperature, it is essential to perform feedback control by detecting the phase change of each signal during system operation.

  Therefore, for example, as shown in FIG. 30, the applicant monitors the optical spectrum of the signal light output from the optical modulator by the monitor unit 130, and based on the intensity change of a specific frequency component in the optical spectrum. Thus, a method of feedback control of the phase shift between the drive signals as described above is proposed by the control circuit 140 (see Japanese Patent Application No. 2002-087017). According to this prior application, by paying attention to the intensity change of the specific frequency component of the output light spectrum, it is possible to reliably detect the phase shift between the signals of the drive system, and the optimum drive condition can be stably obtained. Thus, the phase difference between the drive signals can be controlled.

  However, the control method of the optical modulator according to the invention of the prior application has the following problems. That is, in the above control method, the specific frequency component of the output light spectrum is extracted using the narrow-band optical filter 132, and the intensity change is monitored. At this time, the optical filter having a sufficiently narrow bandwidth in the transmission band is used. If the specific frequency component is not extracted by using, there is a problem that the accuracy of monitoring the intensity change is lowered. In general, since it is not easy to realize an optical filter having a sufficiently narrow bandwidth, it is difficult to stably feedback control the phase difference between drive signals due to the above-described decrease in monitoring accuracy. there is a possibility.

  Further, the control system of the prior invention has a problem that control corresponding to the operating point variation of the optical modulator is not realized.

  The present invention has been made paying attention to the above problems, and an object of the present invention is to provide a control method capable of compensating for an operating point shift of an optical modulator driven with a voltage amplitude of 2Vπ.

  An optical modulator to be controlled by the control device of the present invention for achieving the above object includes a portion for branching an optical waveguide into a first branch optical waveguide and a second branch optical waveguide, and the first and second branches. And a refractive index of each of the first and second branch optical waveguides is controlled by using a first electrode and a second electrode provided in the first and second branch optical waveguides, respectively. A modulator configured to obtain a periodic light intensity characteristic corresponding to the difference in refractive index, and the first and second electrodes so that the modulator performs a modulation operation corresponding to one period of the light intensity characteristic A drive unit that supplies a drive signal to at least one of the above, and a bias supply unit that adjusts an operating point by supplying a DC bias to the modulation unit.

  For such an optical modulator, the present control device includes an output monitor unit that detects a change in signal light output from the optical modulator unit, and an operation of the modulator unit based on a detection result of the output monitor unit. And a controller that controls the bias supply unit so as to determine a point shift and reduce the operating point shift. Alternatively, in the control method for such an optical modulator, a change in the signal light output from the optical modulation unit is detected, an operating point shift of the modulation unit is determined based on the detection result, and the determined operation The bias supply unit is controlled so as to reduce the point deviation.

  In such a configuration, the operating point deviation is determined based on the change in the output light from the optical modulator, and the bias supply unit is controlled based on the result to adjust the DC bias applied to the modulation unit. This makes it possible to compensate for the operating point shift of the optical modulator driven with a voltage amplitude of 2Vπ.

  As described above, according to the control method of the optical modulator according to the present invention, the operating point deviation of the modulation unit is determined based on the change of the output light from the optical modulator, and the operating point deviation is reduced. By controlling the bias supply unit and adjusting the DC bias applied to the modulation unit, it is possible to compensate for the operating point deviation of the optical modulator driven with a voltage amplitude of 2Vπ. As a result, the optical modulator can be stably driven under optimum conditions.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the same reference numerals denote the same or corresponding parts throughout the drawings.

FIG. 1 is a block diagram illustrating a configuration example of a control device for an optical modulator.
In FIG. 1, an optical modulator to which the control device of this example is applied, for example, continuous light generated by a light source 1 is applied to LN modulators 10 and 20 as first and second modulators connected in series. The signals are sequentially input and modulated, and signal light of the CS-RZ modulation method is output. With respect to this optical modulator, the present control device is a phase shift detector that detects a phase shift between the drive signal applied to the preceding LN modulator 10 and the drive signal applied to the subsequent LN modulator 20. A comparator 30 and a control circuit 31 as a control unit that controls the phase shifter 23 so as to reduce the phase shift detected by the phase comparator 30 are optimized to optimize the relative phase of each drive signal. . Hereinafter, each component will be specifically described.

  The front-stage LN modulator 10 is a general optical modulator configured using a lithium niobate (LiNbO3: LN) substrate. The front-stage LN modulator 10 is a data signal (for example, a 40 Gb / s data signal) generated by the data signal generation unit 11 and having a bit rate of B (b / s) corresponding to the NRZ modulation method, for example. By being driven according to DATA, the continuous light from the light source 1 is modulated and the B (b / s) NRZ signal light is output to the LN modulator 20 at the subsequent stage.

  The data signal generation unit 11 uses the NRZ modulation method based on a data signal having a bit rate of B / n (b / s) corresponding to a plurality of channels (here, n) given from the outside. A corresponding B (b / s) data signal DATA is generated, and the data signal DATA is supplied to the LN modulator 10. At the same time, a clock component is extracted from the B (b / s) data signal DATA. The generated clock signal CLKd having a frequency of B / 2 (Hz) is output to the phase comparator 30.

  The latter-stage LN modulator 20 is a known Mach-Zehnder type optical modulator configured using a lithium niobate substrate. Specifically, the LN modulator 20 includes a portion that branches the optical waveguide into a first branch optical waveguide and a second branch optical waveguide, and a portion that combines the first and second branch optical waveguides. The refractive index of the first and second branch optical waveguides is controlled by using the first electrode 20A and the second electrode 20B provided in the first and second branch optical waveguides, respectively, and periodically according to the refractive index difference The light intensity characteristics can be obtained. Here, an example in which LN substrates are used as the front-stage and rear-stage modulators is shown, but the substrate material is not limited to the above, and the front-stage and rear-stage modulators are made of a known material having an electro-optic effect. A modulator may be configured.

  Each electrode 20A, 20B of the LN modulator 20 has a frequency corresponding to ½ times the bit rate of the data signal generated by the data signal generation unit 11, that is, a frequency of B / 2 (Hz). Clock signals (for example, 20 GHz clock signals) CLK1 and CLK2 are given as drive signals. Further, the B / 2 (Hz) clock signals CLK1 and CLK2 are supplied to the LN modulator 20, and the potential difference between the electrodes 20A and 20B is caused by the periodic light intensity characteristics of the LN modulator 20 ( The phase difference between the respective signals is adjusted by the phase shifter 25 and the amplitudes of the respective signals are adjusted by the amplifiers 26A and 26B so as to correspond to one period of FIG. The latter-stage LN modulator 20 driven according to each of the clock signals CLK1 and CLK2 further modulates the NRZ signal light from the former-stage LN modulator 10, and B (b / s) CS-RZ signal light. Is output.

  For example, the clock signal generation unit 21 generates a B / 2 (Hz) clock signal CLK0 having a waveform such as a sine wave. The B / 2 (Hz) clock signal CLK0 is branched into two by the branching device 22 and sent to the data signal generating unit 11 and the phase shifter 23, respectively. The signal sent from the branching device 22 to the data signal generation unit 11 is used as a synchronization signal of the B (b / s) data signal DATA generated by the data signal generation unit 11.

  The phase shifter 23 adjusts the phase of the clock signal CLK0 sent from the branching device 22 in accordance with a control signal SFB output from the control circuit 31 as will be described later. As the phase shifter 23, for example, a variable length coaxial tube or a voltage control type device can be used. The B / 2 (Hz) clock signal CLK0 whose phase is adjusted by the phase shifter 23 is branched into three clock signals CLK1, CLK2 and CLK3 by the branching device 24, and the phase shifter 25, the amplifier 26B and the phase comparator. 30 respectively.

  The phase shifter 25 adjusts the phase of the clock signal CLK1 sent from the branching device 24 here in order to control the phase difference between the drive signals respectively applied to the electrodes 20A and 20B of the LN modulator 20 at the subsequent stage. It is. As the phase shifter 25, for example, a variable length coaxial tube or a voltage control type device can be used. The clock signal CLK1 whose phase is adjusted by the phase shifter 25 is supplied to the first electrode 20A of the LN modulator 20 after the amplitude thereof is adjusted to a required level by the amplifier 26A. On the other hand, the clock signal CLK2 branched by the branching device 24 is sent to the amplifier 26B without passing through the phase shifter here, and after its amplitude is adjusted to a required level, it is applied to the second electrode 20B of the LN modulator 20. Given.

  Although the phase shifter 25 is provided only on the clock signal CLK1 side here, a phase shifter is provided between the branching device 24 and the amplifier 26B on the clock signal CLK2 side, and both the clock signals CLK1 and CLK2 are provided. Phase adjustment may be performed.

  The phase comparator 30 performs a phase comparison between the B / 2 (Hz) clock signal CLKd output from the data signal generator 11 and the B / 2 (Hz) clock signal CLK3 branched by the branching device 24. The phase shift between the clock signals CLKd and CLK3 is detected, and a signal indicating the detection result is output to the control circuit 31. In this phase comparator 30, the physical length LCLK3 of the signal line propagating the clock signal CLK3 to and from the branching device 24 is changed between the clock signal CLK1 and the electrodes 20A and 20B of the branching device 24 and the LN modulator 20. The signal lines propagating CLK2 are arranged so as to be equal to the physical lengths LCLK1 and LCLK2 (LCLK1 = LCLK2 = LCLK3). Since the physical lengths LCLK1 to LCLK3 of each signal line are set to be equal to each other in this way, even if the physical lengths LCLK1 to LCLK3 of each signal line change due to temperature fluctuation or the like, there is almost no difference in the amount of change. Even when the clock signal CLK3 for monitoring is used, the phase comparison between the data signal and the clock signal can be performed with high accuracy.

  The control circuit 31 generates a control signal SFB for feedback control of the phase adjustment amount in the phase shifter 23 so that the phase shift between the clock signals CLKd and CLK3 is reduced in accordance with the output signal from the phase comparator 30. .

  In the optical modulator to which the control device configured as described above is applied, continuous light from the light source 1 is input to the preceding LN modulator 10. The LN modulator 10 is supplied with the B (b / s) data signal DATA generated by the data signal generator 11 as a drive signal. The continuous light input to the LN modulator 10 is the data signal DATA. Are modulated to become B (b / s) NRZ signal light, output from the front-stage LN modulator 10 and sent to the rear-stage LN modulator 20.

  The LN modulator 20 at the subsequent stage is adjusted by the phase shifter 23 after the B / 2 (Hz) clock signal CLK0 generated by the clock signal generation unit 21 is branched by the branching unit 24, and further, the phase shifter 25 and Clock signals CLK1 and CLK2 whose phases and amplitudes are adjusted by the amplifiers 26A and 26B are respectively supplied as drive signals to the first and second electrodes 20A and 20B. At this time, the phase adjustment with respect to the clock signal CLK0 performed by the phase shifter 23 is feedback-controlled according to the control signal SFB output from the control circuit 31.

  Specifically, this feedback control is performed by the phase shift detected by the phase comparator 30, that is, B / 2 (extracted from the B (b / s) data signal DATA that drives the LN modulator 10 in the previous stage. Hz) and the clock signal CLK3 equivalent to the B / 2 (Hz) clock signals CLK1 and CLK2 that drive the LN modulator 20 in the subsequent stage are reduced, and finally substantially zero. The phase adjustment amount in the phase shifter 23 is optimized so that the phase shift between the data signal DATA and the clock signals CLK1 and CLK2 is automatically compensated.

  The B / 2 (Hz) clock signals CLK1 and CLK2 compensated for the phase shift from the B (b / s) data signal DATA by the feedback control as described above are further applied to the LN modulator 20 at the subsequent stage. Then, the phase difference is adjusted by the phase shifter 25 so that the potential difference between the electrodes 20A and 20B corresponds to one period of the periodic light intensity characteristic of the LN modulator 20, and the amplifiers 26A and 26B. The amplitude of each is adjusted. In the LN modulator 20 in which the clock signals CLK1 and CLK2 adjusted in this way are applied to the respective electrodes as drive signals, the NRZ signal light from the LN modulator 10 in the previous stage is modulated according to the clock signal of B / 2 (Hz). Then, B (b / s) CS-RZ signal light in which waveform deterioration due to a phase shift between the data signal DATA and the clock signals CLK1 and CLK2 is suppressed is output to the outside.

  As a specific operation mode of such an optical modulator, for example, when the system is introduced, the phase adjustment amount of each of the phase shifters 23 and 25 is set to a value at which the output waveform is best by manual operation or the like. In this state, the feedback control by the phase comparator 30 and the control circuit 31 is started, so that the phase shift between the data signal DATA and the clock signals CLK1 and CLK2 generated due to temperature fluctuation or the like during system operation is reliably detected. It becomes possible to automatically compensate for the phase shift.

  As described above, according to the control device for the optical modulator, the B / 2 (Hz) clock signal CLKd extracted from the B (b / s) data signal DATA that drives the LN modulator 10 in the preceding stage, and the subsequent stage The phase shift between the B / 2 (Hz) clock signals CLK1 and CLK2 for driving the LN modulator 20 and the equivalent clock signal CLK3 is detected, and the phase shifter 23 is feedback-controlled based on the detection result. As a result, the data signal DATA and the clock signals CLK1, CLK1 and CLK1 can be obtained only by a simple electric circuit without monitoring the optical spectrum of the signal light output from the latter-stage LN modulator 20 as in the above-described prior invention. It becomes possible to automatically compensate for the phase shift between CLK2.

  In the above embodiment, the front-stage LN modulator 10 is driven by a B (b / s) data signal, and the rear-stage LN modulator 20 is driven by a B / 2 (Hz) clock signal, so that B ( Although the optical modulator for generating the CS-RZ signal light of b / s) has been described, the present invention is not limited to this. For example, as shown in FIG. 2, a clock signal CLK1 ′ having a frequency of B (Hz) corresponding to the bit rate of the data signal DATA for driving the front-stage LN modulator 10 is used as one electrode 20A of the rear-stage LN modulator 20. The other electrode 20B is grounded or the like, so that the control device can be applied to the modulator or the like that generates the B (b / s) RZ signal light in the same manner as in the above embodiment. It is. In this case, the data signal generation unit 11 outputs the B (Hz) clock signal CLKd ′ extracted from the B (b / s) data signal DATA to the phase comparator 30, and the phase comparator 30 outputs the clock signal CLKd ′. It is assumed that the phase comparison between CLKd ′ and the clock signal CLK2 ′ branched by the branching device 24 is performed. Here again, the physical length LCLK1 ′ of the signal line that propagates the clock signal CLK1 ′ between the branching device 24 and the electrode 20A of the LN modulator 20 at the subsequent stage, and the clock signal between the branching device 24 and the phase comparator 30. The physical length LCLK2 ′ of the signal line propagating CLK2 ′ is set to be equal (LCLK1 ′ = LCLK2 ′). As a specific configuration of the phase comparator 30, for example, a D flip-flop can be used, and the clock signal CLK 2 ′ from the branching device 24 and the clock signal CLKd ′ from the data signal generation unit 11 are converted to the D flip-flop. A configuration may be adopted in which the control signal SFB is generated by the control circuit 31 using the output signals from the D flip-flops respectively supplied to the data input terminal and the clock input terminal.

  In the above embodiment, the configuration in which the front-stage LN modulator is driven by the data signal and the rear-stage LN modulator is driven by the clock signal has been described. However, for example, as shown in FIG. It is also possible to replace the LN modulator in the previous stage with a clock signal and drive the LN modulator in the subsequent stage with a data signal. The same applies to other embodiments described below.

  Next, another configuration example of the optical modulator control device will be described.

FIG. 4 is a block diagram illustrating another configuration example of the control device for the optical modulator.
In FIG. 4, the control device of this example drives the LN modulator 10 at the front stage by the data signal DATA and drives the LN modulator 20 at the rear stage by the clock signals CLK1 and CLK2 in the same manner as in the above embodiment. The present invention is applied to an optical modulator that outputs signal light of the CS-RZ modulation method. The configuration of the present control device is different from that in the above embodiment in that a part of the signal light output from the LN modulator 20 in the subsequent stage is branched as monitor light instead of the phase comparator 30 and the control circuit 31 in the above embodiment. Then, photoelectric conversion is performed to obtain an electrical spectrum, and an output monitor unit 40 that monitors the intensity of a specific frequency component of the electrical spectrum, and data based on the intensity change of the specific frequency component monitored by the output monitor unit 40 A control circuit 50 is provided that determines a phase shift between the signal DATA and the clock signals CLK1 and CLK2 and feedback-controls the phase shifter 23. Other configurations are the same as those in the case of the above embodiment, and the description thereof is omitted here.

  The output monitor unit 40 includes, for example, an optical coupler 41, a light receiving circuit 42, an electric filter 43, and an electric power sensor 44. The optical coupler 41 branches a part of the CS-RZ signal light output from the LN modulator 20 at the subsequent stage as monitor light and sends it to the light receiving circuit 42. The light receiving circuit 42 is a circuit that photoelectrically converts the monitor light branched by the optical coupler 41 to obtain an electrical spectrum. The electrical filter 43 is an electrical bandpass filter capable of extracting a specific frequency component whose intensity changes most greatly according to the phase shift between the data signal and the clock signal from the electrical spectrum acquired by the light receiving circuit 42. is there. The specific frequency component will be described later. The electric power sensor 44 measures the intensity of the electric signal extracted by the electric filter 43 and outputs the result to the control circuit 50.

  The control circuit 50 generates a control signal SFB for feedback control of the phase adjustment amount of the phase shifter 23 so that the intensity of the specific frequency component measured by the electric power sensor 44 is maximized. The feedback control by the control circuit 50 is performed based on the change characteristic of the electric spectrum of the CS-RZ signal light with respect to the phase change between the data signal DATA and the clock signals CLK1 and CLK2 as described below.

  FIG. 5 is a diagram illustrating an example of the electrical spectrum of the CS-RZ signal light generated when the phase between the 40 Gb / s data signal and the 20 GHz clock signal is changed. FIG. 6 is a diagram showing an example of an optical waveform of CS-RZ signal light generated when the phase is changed in the same manner as in FIG. Here, based on the state when the phase between the data signal and the clock signal is optimized (the delay time due to the phase shift between the signals is 0 ps), the phase shift is increased to perform the original optimum The phase of the clock signal is changed until it returns to the phase state (delay time is 25 ps).

  First, as shown in FIG. 6, even if the phase shift corresponding to only 5 ps (1 mm in terms of coaxial cable length) is made as the delay time between the data signal and the clock signal, the optical waveform of the CS-RZ signal light is It turns out that it deteriorates greatly. At this time, as shown in FIG. 5, the electrical spectrum of the CS-RZ signal light is lower than the frequency of 40 GHz corresponding to the bit rate of the data signal when the phase between the data signal and the clock signal is shifted from the optimum point. It turns out that the intensity | strength about the area | region (Frequency area | region over several GHz centering on about 25 GHz in the example of FIG. 5) distant to the zone side reduces.

  Therefore, in this embodiment, attention is paid to a specific frequency component whose intensity changes greatly according to the phase shift between the data signal and the clock signal as described above, and a phase shift occurs based on the intensity change of the specific frequency component. The state is determined, and feedback control of the phase shifter 23 is performed so that the phase difference between the data signal and the clock signal is optimized.

  Specifically, when an electric spectrum corresponding to the 40 Gb / s CS-RZ signal light as described above is obtained by the light receiving circuit 42, the intensity increases according to the phase shift as shown by the broken line portion in FIG. 7. The center frequency of the transmission band of the electric filter 43 is set so as to match the maximum frequency of about 25 GHz. As apparent from FIG. 7, the intensity of the specific frequency component extracted by the electric filter 43 increases as the phase shift between the data signal and the clock signal becomes smaller. Therefore, the intensity measured by the electric power sensor 44 By controlling the phase shifter 23 so as to maximize the phase difference, the phase difference between the data signal and the clock signal can be optimized.

  The electrical filter 43 has a characteristic in which the bandwidth of the transmission band is narrower and the transmittance changes sharply at both ends of the transmission band, so that the gap between the data signal and the clock signal is increased. It becomes possible to detect the phase shift with higher accuracy. Realization of such an electric filter having a narrow band and sharp filter characteristics is easier than the optical filter used in the above-mentioned prior invention. For this reason, the phase difference between the data signal and the clock signal can be optimized more stably.

  Furthermore, in the control circuit 50, the traveling direction of the phase shift can be detected by detecting a maximum value by applying a known process such as dithering to the intensity measured by the electric power sensor 44. If the phase shift 23 is detected in this way and the feedback control of the phase shifter 23 is performed, the phase difference between the data signal and the clock signal can be optimized at a higher speed.

  As described above, according to this embodiment, the phase shift between the data signal and the clock signal is ensured by monitoring the intensity change of the specific frequency component in the electrical spectrum of the output light from the LN modulator 20 at the subsequent stage. Since detection and feedback control can be performed, CS-RZ signal light can be generated under stable driving conditions.

  In the present embodiment, the case where a bandpass filter is used as the electrical filter 43 has been described. However, the electrical filter that extracts a specific frequency component from the electrical spectrum of the output light is not limited to the above, and for example, a data signal It is also possible to use a low-pass filter having a cutoff frequency on the lower frequency side than the frequency corresponding to the bit rate and on the higher frequency side than the frequency whose intensity changes to the maximum according to the phase shift. However, it is desirable to use a bandpass filter in order to detect the phase shift with higher accuracy.

  Next, still another embodiment of the control device for the optical modulator will be described. In the form of FIG. 4 described above, the feedback control is performed by detecting the phase shift between the data signal and the clock signal based on the electrical spectrum of the output light. In the following embodiment, a control device that performs feedback control by detecting a phase shift between the clock signals CLK1 and CLK2 that drive the subsequent-stage LN modulator 20 based on the electrical spectrum of output light will be described.

FIG. 8 is a block diagram showing the configuration of the control device for the optical modulator according to the present embodiment.
In FIG. 8, the configuration of the present embodiment is different from the configuration of the configuration shown in FIG. 4 described above in that an output monitor 40 ′ in which the electric filter 43 is omitted from the output monitor 40 used in the configuration of FIG. It is a point provided. The monitoring result of the output monitor unit 40 ′ is sent to the control circuit 50, and the phase shifters 25A and 25B are feedback-controlled by the control circuit 50 to optimize the phase difference between the clock signals CLK1 and CLK2. Here, a phase shifter 25A is arranged between the branching device 24 and the amplifier 26A so as to correspond to the clock signals CLK1 and CLK2 applied to the respective electrodes 20A and 20B of the LN modulator 20 at the subsequent stage, and the branching device. Although the phase shifter 25B is arranged between the amplifier 24 and the amplifier 26B and the phase difference between the clock signals CLK1 and CLK2 is adjusted by the two phase shifters 25A and 25B, the above two forms are shown. Similarly, the phase difference between the two clock signals may be relatively controlled by adjusting the phase of one clock signal with a phase shifter.

  The output monitor unit 40 ′ branches a part of the CS-RZ signal light output from the LN modulator 20 at the subsequent stage as monitor light by the optical coupler 41, and photoelectrically converts the monitor light by the light receiving circuit 42 to obtain an electric spectrum. In this case, the electric spectrum is directly sent to the electric power sensor 44 without passing through an electric filter, and the electric power sensor 44 measures the intensity (hereinafter referred to as total power) over the entire frequency band of the electric spectrum. Then, a signal indicating the measurement result is output to the control circuit 50.

  The control circuit 50 generates a control signal SFB for feedback control of the phase adjustment amounts of the phase shifters 25A and 25B so that the total power measured by the electric power sensor 44 is maximized. The feedback control by the control circuit 50 is performed based on the change characteristic of the electrical spectrum of the CS-RZ signal light with respect to the phase change between the clock signal CLK1 and the clock signal CLK2 as described below.

  FIG. 9 is a diagram illustrating an example of the electrical spectrum of the CS-RZ signal light generated when the relative phase between the 20 GHz clock signals CLK1 and CLK2 is changed. FIG. 10 is a diagram showing an example of an optical waveform of CS-RZ signal light generated when the phase is changed in the same manner as in FIG. Here, based on the state when the phase between the clock signals CLK1 and CLK2 is optimized (the phase shift between the signals is 0 degree), the phase shift is gradually increased to obtain the original optimum phase state. The phase difference between the clock signals is changed until the phase shift returns to 360 degrees.

  As shown in FIG. 10, when the phase shift between the clock signals CLK1 and CLK2 increases, the output light waveform greatly deteriorates, and when the phase shift reaches 180 degrees, the output light disappears. At this time, as shown in FIG. 9, it is clear that the total power of the electrical spectrum of the output light decreases as the phase shift between the clock signals CLK1 and CLK2 increases.

  Therefore, in the present embodiment, the occurrence state of the phase shift is determined based on the change in the total power that changes according to the phase shift between the clock signals CLK1 and CLK2 as described above, and the phase difference between the clock signals CLK1 and CLK2 is determined. The phase shifters 25A and 25B are feedback-controlled so as to be optimized.

  Specifically, when an electrical spectrum corresponding to the 40 Gb / s CS-RZ signal light as described above is obtained by the light receiving circuit 42, as is clear from the change in the electrical spectrum extracted in FIG. 11. Since the total power measured by the electric power sensor 44 increases as the phase shift between the clock signals CLK1 and CLK2 decreases, the phase shifters 25A and 25B are feedback-controlled so that the total power is maximized. The phase difference between the clock signals CLK1 and CLK2 can be optimized.

  In addition, the control circuit 50 can detect the maximum value detection by applying a known process such as dithering to the total power measured by the electric power sensor 44 to detect the traveling direction of the phase shift. Thus, by detecting the traveling direction of the phase shift and performing the feedback control of the phase shifters 25A and 25B, the phase difference between the clock signals CLK1 and CLK2 can be optimized at a higher speed. Become.

  As described above, according to this embodiment, by monitoring the total power of the electrical spectrum of the output light from the latter-stage LN modulator 20, the two clock signals CLK1, CLK2 that drive the latter-stage LN modulator 20 are monitored. Since the phase shift can be reliably detected and feedback control can be performed, the CS-RZ signal light can be generated under stable driving conditions.

  4 and 8 described above, the front-stage LN modulator 10 is driven by a data signal of B (b / s), and the rear-stage LN modulator 20 is driven by a clock signal of B / 2 (Hz). The optical modulator that drives and generates the B- (b / s) CS-RZ signal light has been described. However, the present invention is not limited to this, and in the same manner as illustrated in FIG. By applying a clock signal having a frequency of B (Hz) corresponding to the bit rate of the data signal for driving the LN modulator 10 to one electrode of the LN modulator 20 in the subsequent stage, the RZ signal light of B (b / s) The present invention can also be applied to a modulator that generates

  Next, a first embodiment of a control device for an optical modulator according to the present invention will be described. In the first embodiment, a control device capable of realizing control corresponding to the operating point variation of the optical modulator will be described.

  First, the operating point variation of the optical modulator to be controlled in the first embodiment will be briefly described. In general, a Mach-Zehnder optical modulator is used to generate signal light corresponding to the CS-RZ modulation method or the like. This optical modulator has the advantage that the wavelength variation of the transmitted light is small. However, there is a problem that the operating point of the electro-optical conversion characteristic varies with time due to a temperature change or a change with time of a material (for example, lithium niobate) used for the substrate.

  In order to suppress this operating point variation, conventionally, with respect to the generation of signal light corresponding to the NRZ modulation method, for example, a drive signal on which a low-frequency signal is superimposed is applied to a Mach-Zehnder optical modulator, and the above-mentioned included in output light A technique is known in which a low-frequency signal component is extracted to detect an operating point variation, and the DC bias of the optical modulator is feedback-controlled based on the detection result (for details, see Japanese Patent Application Laid-Open No. 3-251815). See

  FIG. 12 is a block diagram illustrating a schematic configuration of a control device to which the above-described known technique is applied. FIG. 13 is a diagram for explaining the principle of operating point compensation by the control device of FIG.

  In the configuration of FIG. 12, a low-frequency signal (with a frequency of f0 = 1 kHz or the like) generated by the oscillator 227 is given as a gain control voltage to the drive circuit 211 that drives the Mach-Zehnder type optical modulator 210. NRZ data signal amplitude-modulated according to the low-frequency signal is generated as shown in the lower left of FIG. 6, and the input light from the light source 201 is externally modulated by applying the NRZ data signal to the electrode of the modulator 210. A part of the NRZ signal light output from the modulator 210 is branched as monitor light by the optical coupler 221 and then converted into an electric signal by the light receiver (PD) 222, and the frequency f0 component included in the electric signal is The signal is selectively amplified by the amplifier 223 and sent to the phase comparator 224. The phase comparator 224 performs phase comparison between the output signal from the amplifier 223 and the low-frequency signal from the oscillator 227, and a signal indicating the result is sent to the bias supply circuit 226 via the low-pass filter 225 that removes unnecessary components. Given, the control of the DC bias for adjusting the operating point of the modulator 210 is performed.

  The optimum operating point of the Mach-Zehnder type optical modulator in NRZ modulation is that the high level and low level of the waveform of the drive signal whose amplitude is set to Vπ, as shown by the curve a in the upper left of FIG. And the point that gives the minimum power. When the modulator 210 is driven at this optimum operating point, the NRZ signal light output from the modulator 210 does not include the frequency f0 component as shown in the upper right of FIG. 13, but is twice the frequency f0. The components are generated.

  On the other hand, as shown by the curves b and c in the upper left of FIG. 13, when the operating point of the modulator 210 deviates from the optimum point, the drive waveform and output for the high level or low level envelope depending on the direction of the deviation. The phase is inverted between the optical waveforms. The output light at this time has a waveform in which high-level and low-level envelopes are modulated in the same phase, as shown in the middle and lower stages on the right side of FIG. 13, and includes a frequency f0 component. Since the phase of the frequency f0 component included in the output light is inverted when the operating direction changes, the phase of the operating point is detected by comparing the phase with the low-frequency signal superimposed on the drive signal. It becomes possible. Therefore, it is possible to drive the modulator 210 at the optimum operating point by performing feedback control of the DC bias applied to the modulator 210 according to the result of the phase comparison in the phase comparator 224.

  When the operating point compensation technique related to the generation of the signal light corresponding to the NRZ modulation method as described above is applied to the generation of the signal light of the CS-RZ modulation method using the two-stage modulator, the NRZ modulation is performed by the data signal. It is possible to effectively perform the compensation of the operating point for one of the modulators that performs the above. However, for the other modulator driven by the clock signal, a drive amplitude 2Vπ that is twice that of NRZ modulation is used (see FIG. 27). The envelopes of the output light waveform corresponding to the high level side and the low level side of the modulated drive signal are in opposite phases and cancel each other, and the frequency f0 component cannot be detected from the output light. For this reason, a modulation scheme that is driven between two light emission vertices or two extinction vertexes of the electro-optical conversion characteristics of a Mach-Zehnder optical modulator, such as the CS-RZ modulation method and the RZ modulation method. Cannot apply the conventional operating point compensation technique as described above.

  Therefore, in the first embodiment of the present invention, for example, an optical modulator that generates signal light corresponding to the CS (R / B) modulation scheme of B (b / s) is driven with a voltage amplitude of 2Vπ using a clock signal. A control device that realizes the operating point compensation of the modulator will be described.

FIG. 14 is a block diagram illustrating a configuration of the control device for the optical modulator according to the first embodiment.
In FIG. 14, the control device of the first embodiment drives the LN modulator 10 at the preceding stage with the data signal DATA of B (b / s) and the LN modulator 20 at the subsequent stage, as in the case of each of the above-described forms. Is applied to an optical modulator that outputs a signal light of the CS-RZ modulation system by driving the signal with clock signals CLK1 and CLK2 of B / 2 (Hz). The configuration of the present control device is different from that of the other embodiment in that a bias supply circuit 27 that provides a DC bias for adjusting the operating point to the LN modulator 20 in the subsequent stage is provided. The operation point 27 is feedback-controlled by the output monitor unit 60 and the control circuit 70 so that the operating point compensation of the LN modulator 20 at the subsequent stage is performed.

  Specifically, the output monitor unit 60 includes, for example, an optical coupler 61, a light receiving circuit 62, an electric filter 63, and an electric power sensor 64. The optical coupler 61 branches a part of the CS-RZ signal light output from the LN modulator 20 at the subsequent stage as monitor light and sends it to the light receiving circuit 62. The light receiving circuit 62 is a circuit that obtains an electric spectrum by photoelectrically converting the monitor light branched by the optical coupler 61. The electrical filter 63 is a narrow-band electrical bandpass filter that can extract a frequency component having B / 2 (Hz) as a center frequency from the electrical spectrum acquired by the light receiving circuit 62. The electric power sensor 64 measures the intensity of the electric signal extracted by the electric filter 63 and outputs the result to the control circuit 70.

  The control circuit 70 feedback-controls the operation setting of the bias supply circuit 27 so that the intensity of the frequency component having B / 2 (Hz) as the center frequency measured by the electric power sensor 64 is minimized. SFB is generated.

  Although not shown here, for the operating point shift of the LN modulator 10 in the previous stage, a conventional compensation method in which the above-described low frequency signal is superimposed on the driving signal to perform operating point compensation is applied. And

  Next, the principle of operating point compensation for the latter-stage LN modulator 20 in the present embodiment will be specifically described.

FIG. 15 is a diagram for explaining a change in output light when an operating point shift occurs in a modulator driven with a voltage amplitude of 2Vπ.
For example, when the 20 GHz clock signals CLK1 and CLK2 are applied to the electrodes 20A and 20B of the LN modulator 20 at the subsequent stage, the potential difference between the electrodes 20A and 20B is as shown on the left side of FIG. It changes with an amplitude of 2Vπ corresponding to one period of 20 periodic light intensity characteristics. At this time, if the operating point of the LN modulator 20 deviates from the optimum point, the waveform of the 40 Gb / s CS-RZ signal light output from the LN modulator 20 is as shown in the upper right side of FIG. It can be seen that there is a deviation in the level between each bit. As for the electrical spectrum of the output light, as shown in the middle part on the right side of FIG. 15, when the operating point is optimally set (see, for example, FIG. 5 or FIG. 9), a peak is observed at a frequency of 20 GHz. It can be seen that it occurs. Further, as shown in the lower right part of FIG. 15, the optical spectrum of the output light corresponds to the center optical frequency that was not found when the operating point was optimally set (see, for example, the lower right part of FIG. 27). It can be seen that a carrier component is generated.

  In consideration of the change characteristics of the output light when the operating point deviation occurs as described above, in this embodiment, the frequency component centered on 20 GHz of the electrical spectrum, that is, the clock signals CLK1 and CLK2 for driving the LN modulator 20 are used. The intensity of the frequency component centered on B / 2 (Hz) corresponding to the frequency of the frequency is monitored, and the operating point deviation is determined based on the monitoring result, and the operating point is optimized. Thus, feedback control of DC bias is performed.

  Specifically, when an electrical spectrum corresponding to the 40 Gb / s CS-RZ signal light illustrated in FIG. 15 is obtained by the light receiving circuit 62, as shown in the broken line portion of FIG. The center frequency of the transmission band of the electric filter 63 is set so as to coincide with 20 GHz which is the frequency of CLK2. As is clear from FIG. 16, the intensity of the frequency component extracted by the electric filter 63 decreases as the operating point approaches the optimum point, so that the intensity measured by the electric power sensor 64 is minimized. By performing feedback control of the operation setting of the bias supply circuit 27, the operating point of the LN modulator 20 can be optimized.

  Further, in the control circuit 70, the minimum value is detected by applying a known process such as dithering to the intensity measured by the electric power sensor 64, whereby the traveling direction of the operating point deviation can be detected. As described above, if the operating direction of the operating point deviation is detected and the DC bias feedback control is performed, the operating point compensation of the LN modulator 20 can be performed at higher speed.

  As described above, according to the first embodiment, the LN modulator 20 driven by the clock signals CLK1 and CLK2 of B / 2 (Hz) is centered on B / 2 (Hz) of the electrical spectrum of the output light. By monitoring the change in the intensity of the frequency component, it becomes possible to reliably detect the state of occurrence of the operating point deviation and perform feedback control of the DC bias. Therefore, the conventional low frequency signal is superimposed on the drive signal. It becomes possible to realize the operating point compensation for the modulator driven with the voltage amplitude of 2Vπ, which is difficult with the method of operating point compensation. This makes it possible to generate CS-RZ signal light under stable driving conditions.

  Next, a second embodiment of the control device for the optical modulator according to the present invention will be described. In the first embodiment, the case where the operating point compensation is performed paying attention to the change of the electric spectrum among the change characteristics of the output light due to the operating point shift shown in FIG. 15 has been described. In the second embodiment, a case will be described in which operating point compensation is performed by paying attention to the change in the optical spectrum among the change characteristics of the output light.

FIG. 17 is a block diagram illustrating a configuration of a control device for an optical modulator according to the second embodiment.
In FIG. 17, the configuration of the present embodiment is different from that of the first embodiment shown in FIG. 14 described above, except that the output monitor unit 60 and the control circuit 70 are replaced by the LN modulator 20 at the subsequent stage. An output monitor unit 80 that extracts a central optical frequency component after branching a part of the signal light as monitor light and monitors the optical power, and a change in optical power of the central optical frequency component monitored by the output monitor unit 80 And a control circuit 90 for determining the deviation of the operating point of the LN modulator 20 and controlling the bias supply circuit 27 in a feedback manner. Other configurations other than those described above are the same as those in the first embodiment.

  The output monitor unit 80 includes, for example, an optical coupler 81, a narrow band optical filter 82, and an optical power meter 83. The optical coupler 81 branches a part of the CS-RZ signal light output from the LN modulator 20 at the subsequent stage as monitor light and sends it to the narrowband optical filter 82. The narrowband optical filter 82 has a filter characteristic in which the bandwidth of the transmission band is sufficiently narrow, and extracts only the center optical frequency component from the monitor light branched by the optical coupler 81. The optical power meter 83 measures the power of the monitor light extracted by the narrow band optical filter 82 and outputs the result to the control circuit 90.

  The control circuit 90 feedback-controls the operation setting of the bias supply circuit 27 so that the monitor light power measured by the optical power meter 83 is minimized. This feedback control is performed based on a change in the carrier component corresponding to the center optical frequency generated in the output optical spectrum due to the operating point shift as shown in FIG. The carrier component of the center optical frequency is generated when the symmetry of the two clock signals CLK1 and CLK2 applied to the electrodes 20A and 20B of the LN modulator 20 is lost and carrier suppression is not performed.

  Specifically, when a part of the 40 Gb / s CS-RZ signal light having the optical spectrum illustrated in the lower right side of FIG. 15 is branched as the monitor light by the optical coupler 81, the broken line part of FIG. As shown in FIG. 4, the center frequency of the transmission band of the narrow-band optical filter 82 is set to match the center optical frequency fc of the optical spectrum of the CS-RZ signal light. As apparent from FIG. 18, the optical power of the optical frequency component extracted by the narrow-band optical filter 82 becomes smaller as the operating point approaches the optimum point, so that the optical power measured by the optical power meter 83 is reduced. The operation point of the LN modulator 20 can be optimized by performing feedback control of the operation setting of the bias supply circuit 27 so as to be minimized.

  Further, the control circuit 90 can detect the traveling direction of the operating point deviation by detecting a minimum value by applying a known process such as dithering to the optical power measured by the optical power meter 83. . As described above, if the operating direction of the operating point deviation is detected and the DC bias feedback control is performed, the operating point compensation of the LN modulator 20 can be performed at higher speed.

  As described above, according to the second embodiment, the LN modulator 20 driven by the B / 2 (Hz) clock signals CLK1 and CLK2 has the optical power of the carrier component generated at the center optical frequency of the output optical spectrum. By monitoring the change, it is possible to reliably detect the occurrence state of the operating point deviation and perform feedback control of the DC bias, so that the operating point compensation for the modulator driven with the voltage amplitude of 2Vπ is performed. And CS-RZ signal light can be generated under stable driving conditions.

  In the above second embodiment, the configuration example in which the optical power meter 83 measures the optical power of the central optical frequency component extracted by the narrowband optical filter 82 has been shown. However, for example, as shown in FIG. A light receiving element 84 and an electric power sensor 85 may be provided in place of the power meter 83 to measure the optical power of the central optical frequency component.

  In each of the above-described embodiments, an example in which individual components are connected in series as the front-stage and rear-stage LN modulators 10 and 20 has been shown. For example, the front-stage and rear-stage LN modulators 10 and 20 You may make it form 20 continuously.

  Further, with regard to the above-described form, the respective configurations are appropriately combined to compensate for the phase shift between the data signal that drives the front-stage LN modulator 10 and the clock signal that drives the rear-stage LN modulator 20, It is also possible to simultaneously perform two or more compensations among the phase shift compensation between the two systems of clock signals that drive the LN modulator 20 and the operating point compensation of the LN modulator 20 at the subsequent stage. Below, the specific Example regarding said combination is enumerated.

  FIG. 20 is a block diagram illustrating an example of an optical modulator control device that combines the configurations of FIG. 1, FIG. 8, and the first embodiment. In the configuration of this embodiment, compensation of the phase shift between the data signal and the clock signal realized by feedback control of the phase shifter 23 by the phase comparator 30 and the control circuit 31, and the output monitor unit 40 ′ and the control circuit 50 This is realized by the compensation of the phase shift between the clock signals CLK1 and CLK2 realized by the feedback control of the phase shifter 25 and the feedback control of the bias supply circuit 27 based on the electric spectrum of the output light by the output monitor unit 60 and the control circuit 70. The operating point compensation of the subsequent LN modulator 20 is executed simultaneously.

  Further, with respect to the configuration of the embodiment shown in FIG. 20, for example, as shown in FIG. 21, the optical coupler 41, the light receiving circuit 42 and the electric power sensor 44 of the output monitor unit 40 ′, and the light of the output monitor unit 60 It is possible to simplify the configuration by sharing the coupler 61, the light receiving circuit 62, and the electric power sensor 64, and providing the control CPU 91 having the functions of both the control circuits 50 and 70.

  FIG. 22 is a block diagram illustrating an example of an optical modulator control device in which the configurations of FIG. 1, FIG. 8, and the second embodiment are combined. In the configuration of this embodiment, compensation of the phase shift between the data signal and the clock signal realized by feedback control of the phase shifter 23 by the phase comparator 30 and the control circuit 31, and the output monitor unit 40 ′ and the control circuit 50 This is realized by the compensation of the phase shift between the clock signals CLK1 and CLK2 realized by the feedback control of the phase shifter 25, and the feedback control of the bias supply circuit 27 based on the optical spectrum of the output light by the output monitor unit 80 and the control circuit 90. The operating point compensation of the subsequent LN modulator 20 is executed simultaneously.

  FIG. 23 is a block diagram illustrating an example of an optical modulator control device that combines the configurations of FIG. 4, FIG. 8, and the first embodiment. In this combination, each compensation is performed based on the electrical spectrum of the output light. Therefore, the optical coupler, the light receiving circuit, and the electrical power sensor of the output monitor unit corresponding to each compensation are shared, and the electrical power sensor. It is possible to simplify the configuration by controlling the phase shifters 23 and 25 and the bias supply circuit 27 by the control CPU 92 based on the measurement results in FIG.

  FIG. 24 is a block diagram illustrating an example of an optical modulator control device that combines the configurations of FIGS. 4, 8, and the second embodiment. In the configuration of this embodiment, the change of the electrical spectrum of the output light is monitored by the output monitor unit that shares the optical coupler, the light receiving circuit, and the electric power sensor, and the control CPU 93 controls the phase shifters 23 and 25 based on the monitoring result. Are respectively subjected to feedback control, so that the phase shift between the data signal and the clock signal and the phase shift between the clock signals CLK1 and CLK2 are compensated. At the same time, the operating point compensation of the LN modulator 20 in the subsequent stage is performed by feedback control of the bias supply circuit 27 based on the optical spectrum of the output light by the output monitor unit 80 and the control circuit 90.

  Each of the configurations shown in FIG. 20 to FIG. 24 described above is a preferable specific example of the combination of the above-described embodiments. Similarly to these, the optical modulator can be formed by other combinations other than those described above. Of course, it is also possible to constitute the control device.

  The main inventions disclosed in this specification are summarized as follows.

(Supplementary note 1) a first modulation unit and a second modulation unit connected in series;
A drive unit that provides drive signals whose phases are synchronized to the first and second modulation units, respectively.
The second modulation section has a portion for branching the optical waveguide into a first branch optical waveguide and a second branch optical waveguide, and a portion for synthesizing the first and second branch optical waveguides. Using the first electrode and the second electrode respectively provided in the branch optical waveguide, the refractive indexes of the first and second branch optical waveguides are controlled, and a periodic light intensity characteristic corresponding to the difference in refractive index is obtained. Consisting of composition
The drive unit is capable of providing a drive signal to at least one of the first and second electrodes so that the second modulation unit performs a modulation operation corresponding to one cycle of the light intensity characteristic. A control device for the modulator,
A phase shift detector for detecting a phase shift by comparing the phases of the drive signals applied to the first and second modulators;
An optical modulator control device comprising: a control unit that controls the drive unit so that the phase shift detected by the phase shift detection unit is reduced.

(Supplementary note 2) The optical modulator control device according to supplementary note 1, wherein
In the optical modulator, a data signal having a predetermined bit rate is supplied from the driving unit to the first modulating unit, and a clock signal having a frequency corresponding to ½ times the bit rate of the data signal is driven. When applied to the first and second electrodes of the second modulation unit from the unit,
The phase shift detection unit includes a phase of a clock signal having a frequency corresponding to ½ times the bit rate extracted from the data signal, and each of the phases supplied to the first and second electrodes of the second modulation unit. Compare the phase of the clock signal equivalent to the clock signal to detect the phase shift,
The control unit adjusts the phase of at least one of the data signal and the clock signal supplied to the first and second modulation units so that the phase shift detected by the phase shift detection unit is small. An optical modulator control device.

(Supplementary note 3) The optical modulator control device according to supplementary note 1, wherein
The optical modulator receives a data signal having a predetermined bit rate from the driving unit to the first modulating unit, and receives a clock signal having a frequency corresponding to the bit rate of the data signal from the driving unit. When given to one electrode of the modulator,
The phase shift detection unit includes a phase of a clock signal having a frequency corresponding to the bit rate extracted from the data signal and a phase of a clock signal equivalent to a clock signal applied to one electrode of the second modulation unit. To detect phase shift,
The control unit adjusts the phase of at least one of the data signal and the clock signal supplied to the first and second modulation units so that the phase shift detected by the phase shift detection unit is small. An optical modulator control device.

(Supplementary note 4) a first modulation unit and a second modulation unit connected in series;
A drive unit that provides drive signals whose phases are synchronized to the first and second modulation units, respectively.
The second modulation section has a portion for branching the optical waveguide into a first branch optical waveguide and a second branch optical waveguide, and a portion for synthesizing the first and second branch optical waveguides. Using the first electrode and the second electrode respectively provided in the branch optical waveguide, the refractive indexes of the first and second branch optical waveguides are controlled, and a periodic light intensity characteristic corresponding to the difference in refractive index is obtained. Consisting of composition
The drive unit is capable of providing a drive signal to at least one of the first and second electrodes so that the second modulation unit performs a modulation operation corresponding to one cycle of the light intensity characteristic. A control device for the modulator,
An output monitor for photoelectrically converting the signal light output from the optical modulator to obtain an electrical spectrum, and detecting information on the intensity of the electrical spectrum;
A control unit that determines a phase shift between drive signals of the optical modulator based on intensity information detected by the output monitor unit, and controls the drive unit so that the phase shift is reduced; An optical modulator control device characterized by the above.

(Supplementary note 5) The optical modulator control device according to supplementary note 4, wherein
The output monitor unit extracts a specific frequency component from the electrical spectrum and detects the intensity,
The control unit determines a phase shift between the drive signals given to the first and second modulation units according to the intensity of a specific frequency component detected by the output monitor unit, and the intensity becomes maximum. As described above, the optical modulator control device adjusts the phase of at least one of the drive signals applied to the first and second modulators.

(Supplementary note 6) The optical modulator control device according to supplementary note 4, wherein
When the driving unit gives a driving signal having a predetermined phase difference to the first and second electrodes of the second modulation unit, respectively.
The output monitor unit detects the intensity over the entire frequency band of the electrical spectrum,
The control unit determines a phase shift between the drive signals applied to the first and second electrodes of the second modulation unit according to the intensity detected by the output monitor unit so that the intensity is maximized. And adjusting the phase of at least one of the drive signals applied to the first and second electrodes of the second modulator.

(Supplementary note 7) The optical modulator control device according to supplementary note 4, wherein
In the optical modulator, a data signal having a predetermined bit rate is supplied from the driving unit to the first modulating unit, and a clock signal having a frequency corresponding to ½ times the bit rate of the data signal is driven. The optical modulator control device is characterized in that signal light of a carrier suppression RZ modulation system is generated by being applied to the first and second electrodes of the second modulation unit from the unit.

(Supplementary note 8) The optical modulator control device according to supplementary note 4, wherein
The optical modulator receives a data signal having a predetermined bit rate from the driving unit to the first modulating unit, and receives a clock signal having a frequency corresponding to the bit rate of the data signal from the driving unit. An optical modulator control device that generates RZ-modulated signal light when applied to one electrode of a modulation unit.

(Supplementary Note 9) First and second branch optical waveguides having a portion that branches the optical waveguide into a first branch optical waveguide and a second branch optical waveguide, and a portion that synthesizes the first and second branch optical waveguides Modulation of a configuration in which the first and second electrodes respectively provided on the first and second electrodes are used to control the refractive indexes of the first and second branch optical waveguides to obtain periodic light intensity characteristics according to the refractive index difference And
A drive unit that applies a drive signal to at least one of the first and second electrodes so that the modulation unit performs a modulation operation corresponding to one period of the light intensity characteristic;
A control device for an optical modulator, comprising: a bias supply unit that adjusts an operating point by supplying a DC bias to the modulation unit;
An output monitor for detecting a change in signal light output from the light modulator;
And a control unit that determines an operating point deviation of the modulation unit based on a detection result of the output monitor unit and controls the bias supply unit so that the operating point deviation is reduced. Control device for optical modulator.

(Supplementary note 10) The optical modulator control device according to supplementary note 9, wherein
The output monitor unit photoelectrically converts the signal light output from the light modulation unit to obtain an electric spectrum, and extracts a frequency component matching the frequency of the drive signal supplied to the modulation unit from the electric spectrum. Detect the intensity,
The control unit of the optical modulator, wherein the control unit determines an operating point shift of the modulation unit according to an intensity detected by the output monitor unit.

(Supplementary note 11) The optical modulator control device according to supplementary note 9, wherein
The output monitor unit acquires an optical spectrum of the signal light output from the optical modulation unit, extracts an optical frequency component that matches a center optical frequency of the signal light from the optical spectrum, and detects the optical power. ,
The control unit of the optical modulator, wherein the control unit determines an operating point shift of the modulation unit according to the optical power detected by the output monitor unit.

(Supplementary note 12) a first modulation unit and a second modulation unit connected in series;
A drive unit that provides drive signals whose phases are synchronized to the first and second modulation units, respectively.
The second modulation section has a portion for branching the optical waveguide into a first branch optical waveguide and a second branch optical waveguide, and a portion for synthesizing the first and second branch optical waveguides. Using the first electrode and the second electrode respectively provided in the branch optical waveguide, the refractive indexes of the first and second branch optical waveguides are controlled, and a periodic light intensity characteristic corresponding to the difference in refractive index is obtained. Consisting of composition
The drive unit is capable of providing a drive signal to at least one of the first and second electrodes so that the second modulation unit performs a modulation operation corresponding to one cycle of the light intensity characteristic. A control method for a modulator,
Detecting the phase shift by comparing the phases of the drive signals applied to the first and second modulators;
A method of controlling an optical modulator, wherein the drive unit is controlled so that the detected phase shift is reduced.

(Supplementary note 13) a first modulation unit and a second modulation unit connected in series;
A drive unit that provides drive signals whose phases are synchronized to the first and second modulation units, respectively.
The second modulation section has a portion for branching the optical waveguide into a first branch optical waveguide and a second branch optical waveguide, and a portion for synthesizing the first and second branch optical waveguides. Using the first electrode and the second electrode respectively provided in the branch optical waveguide, the refractive indexes of the first and second branch optical waveguides are controlled, and a periodic light intensity characteristic corresponding to the difference in refractive index is obtained. Consisting of composition
The drive unit is capable of providing a drive signal to at least one of the first and second electrodes so that the second modulation unit performs a modulation operation corresponding to one cycle of the light intensity characteristic. A control method for a modulator,
Photoelectrically converting the signal light output from the optical modulator to obtain an electrical spectrum;
Detecting information on the intensity of the acquired electrical spectrum;
A control method of an optical modulator, wherein the drive unit is controlled based on the detected intensity information.

(Supplementary Note 14) First and second branch optical waveguides having a portion that branches the optical waveguide into a first branch optical waveguide and a second branch optical waveguide, and a portion that synthesizes the first and second branch optical waveguides A modulation unit that controls the refractive indexes of the first and second branch optical waveguides by using the first electrode and the second electrode, respectively, and obtains a periodic light intensity characteristic according to the difference in refractive index; ,
A drive unit that applies a drive signal to at least one of the first and second electrodes so that the modulation unit performs a modulation operation corresponding to one period of the light intensity characteristic;
A control method for an optical modulator comprising a bias supply unit that adjusts an operating point by supplying a DC bias to the modulation unit,
Detecting a change in signal light output from the light modulation unit,
Based on the detection result, determine the operating point deviation of the modulation unit,
The method of controlling an optical modulator, comprising: controlling the bias supply unit so that the determined operating point deviation is reduced.

It is a block diagram which shows one form of the control apparatus of an optical modulator. It is a block diagram which shows the other structural example relevant to said form. It is a block diagram which shows another structural example relevant to said form. It is a block diagram which shows the other form of the control apparatus of an optical modulator. It is a figure which shows an example of the electrical spectrum of the output light produced | generated when the phase between a data signal and a clock signal is changed. It is a figure which shows an example of the optical waveform of the output light produced | generated when the phase between a data signal and a clock signal is changed. It is a figure for demonstrating the detection operation | movement of the phase shift in the form of FIG. It is a block diagram which shows another form of the control apparatus of an optical modulator. It is a figure which shows an example of the electrical spectrum of the output light produced | generated when the phase between two systems of clock signals is changed. It is a figure which shows an example of the optical waveform of the output light produced | generated when the phase between two systems of clock signals is changed. It is a figure for demonstrating the detection operation | movement of the phase shift in said form. It is a block diagram which shows schematic structure of the control apparatus to which a well-known operating point compensation system is applied. It is a figure for demonstrating the principle of the operating point stabilization by the control apparatus of FIG. It is a block diagram which shows the structure of 1st Embodiment of this invention. It is a figure for demonstrating the change of output light when an operating point shift generate | occur | produces in the modulator driven by voltage amplitude 2V (pi). It is a figure for demonstrating the detection operation | movement of the operating point deviation in said 1st Embodiment. It is a block diagram which shows the structure of 2nd Embodiment of this invention. It is a figure for demonstrating the detection operation | movement of the operating point deviation in said 2nd Embodiment. It is a block diagram which shows the other structural example relevant to said 2nd Embodiment. It is a block diagram which shows the Example which combined each structure of said FIG. 1, FIG. 8, and 1st Embodiment. It is a block diagram which shows the application example relevant to the Example of FIG. It is a block diagram which shows the Example which combined each structure of said FIG. 1, FIG. 8, and 2nd Embodiment. It is a block diagram which shows the Example which combined each structure of said FIG. 4, FIG. 8, and 1st Embodiment. It is a block diagram which shows the Example which combined each structure of said FIG. 4, FIG. 8, and 2nd Embodiment. It is a figure which shows the basic composition of the conventional optical modulator for CS-RZ modulation. FIG. 26 is a diagram illustrating a waveform example of signal light generated in the basic configuration of FIG. 25. It is a figure for demonstrating the principle by which CS-RZ signal light is produced | generated. It is a figure which shows the experimental result for demonstrating the characteristic of the optical spectrum and optical waveform about CS-RZ signal light. It is a figure which shows the experimental result for demonstrating the characteristic regarding the wavelength dispersion tolerance about CS-RZ signal light. It is a block diagram which shows the control apparatus of the optical modulator by prior invention.

Explanation of symbols

10, 20 LN modulator 11 Data signal generator 20A, 20B Signal electrode 21 Clock signal generator 22, 24 Brancher 23, 25, 25A, 25B Phaser 26A, 26B Amplifier 27 Bias supply circuit 30 Phase comparator 31, 50 , 70, 90 Control circuit 40, 40 ', 60, 80 Output monitor unit 41, 61, 81 Optical coupler 42, 62 Light receiving circuit 43, 63 Electrical filter 44, 64 Electrical power sensor 82 Narrow band optical filter 83 Optical power meter 91 ~ 93 CPU for control

Claims (4)

The optical waveguide has a portion that branches into a first branch optical waveguide and a second branch optical waveguide, and a portion that synthesizes the first and second branch optical waveguides, and is provided in each of the first and second branch optical waveguides. A modulation unit configured to control the refractive indexes of the first and second branch optical waveguides using the first electrode and the second electrode, and to obtain a periodic light intensity characteristic according to the refractive index difference;
A drive unit that applies a drive signal to at least one of the first and second electrodes so that the modulation unit performs a modulation operation corresponding to one period of the light intensity characteristic;
A bias supply unit that adjusts an operating point by supplying a DC bias to the modulation unit;
A control device for an optical modulator comprising:
An output monitor for detecting a change in signal light output from the light modulator;
A control unit that determines an operating point shift of the modulation unit based on a detection result of the output monitor unit and controls the bias supply unit so that the operating point shift is reduced;
A control device for an optical modulator, comprising: The optical modulator control device according to claim 1,
The output monitor unit photoelectrically converts the signal light output from the light modulation unit to obtain an electric spectrum, and extracts a frequency component matching the frequency of the drive signal supplied to the modulation unit from the electric spectrum. Detect the intensity,
The control unit of the optical modulator, wherein the control unit determines an operating point shift of the modulation unit according to an intensity detected by the output monitor unit. The optical modulator control device according to claim 1,
The output monitor unit acquires an optical spectrum of the signal light output from the optical modulation unit, extracts an optical frequency component that matches a center optical frequency of the signal light from the optical spectrum, and detects the optical power. ,
The control unit of the optical modulator, wherein the control unit determines an operating point shift of the modulation unit according to the optical power detected by the output monitor unit. The optical waveguide has a portion that branches into a first branch optical waveguide and a second branch optical waveguide, and a portion that synthesizes the first and second branch optical waveguides, and is provided in each of the first and second branch optical waveguides. A modulator that controls the refractive indexes of the first and second branch optical waveguides using the first electrode and the second electrode, and obtains a periodic light intensity characteristic according to the difference in refractive index;
A drive unit that applies a drive signal to at least one of the first and second electrodes so that the modulation unit performs a modulation operation corresponding to one period of the light intensity characteristic;
A bias supply unit that adjusts an operating point by supplying a DC bias to the modulation unit;
A control method for an optical modulator comprising:
Detecting a change in signal light output from the light modulation unit,
Based on the detection result, determine the operating point deviation of the modulation unit,
The method of controlling an optical modulator, comprising: controlling the bias supply unit so that the determined operating point deviation is reduced.

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