JP4935540B2 - Polarization mode dispersion compensation method and polarization mode dispersion compensation apparatus - Google Patents

Description

  The present invention relates to a polarization mode dispersion compensation method and a polarization mode dispersion compensation apparatus that remove distortion of a time waveform of an optical pulse caused by polarization mode dispersion in an optical transmission system.

  The higher the transmission speed in an optical transmission system, the greater the influence of optical pulse time waveform distortion caused by Polarization Mode Dispersion (PMD) on transmission quality, thus compensating for PMD. Increased need. PMD has birefringence in the optical fiber due to bending stress applied to the optical fiber that constitutes the optical transmission line, temperature change, or the cross-sectional shape of the core becomes slightly elliptical in the optical fiber manufacturing process. Due to expression. With reference to FIGS. 1 (A) to (C), a phenomenon in which the time waveform of an optical pulse propagating through the optical fiber is distorted by the PMD will be described.

  FIGS. 1 (A) to 1 (C) are diagrams showing changes in the time waveform before and after an optical pulse propagates through an optical fiber having birefringence. FIG. 1 (A) shows the time waveform of the polarization component in the x and y axis directions of the optical pulse before propagating through the optical transmission line 10 and the deviation in the x and y axis directions after propagating through the optical transmission line 10. The time waveform about a wave component is shown. FIG. 1B is a diagram illustrating a time waveform obtained by combining the polarization components in the x and y axis directions of an optical pulse before propagating through the optical transmission line 10. FIG. 1 (C) is a diagram showing a time waveform obtained by combining the polarization components in the x and y axis directions of the optical pulse after propagating through the optical transmission line 10. In FIGS. 1 (A) to 1 (C), the time waveform takes the time axis in the horizontal axis direction, and its light intensity is schematically shown.

The polarization components of the optical pulse 20 before propagating through the optical transmission line 10 in the x and y axis directions (hereinafter sometimes referred to as intrinsic polarization modes) have the same peak position on the time axis. Therefore, since the time waveform of the optical pulse 20 constituting the signal before transmission is a combination of the polarization components in the x and y axis directions, it is a unimodal pulse waveform as shown in FIG. is there. When the optical pulse 20 propagates through the optical transmission line 10, due to the birefringence of the optical transmission line 10, the propagation time difference, that is, the group delay difference between the polarization components of the optical pulse 20 in the x and y axis directions ( DGD: Differential Group Delay) occurs. This phenomenon is PMD. As shown in Fig. 1 (C), the time waveform of the optical pulse 22 composing the post-transmission signal output from the optical transmission line 10 by PMD is distorted with multiple bimodal peaks instead of unimodal. It becomes a light pulse.

  Conventionally, there are two types of polarization mode dispersion compensators (hereinafter sometimes referred to as PMD compensators) for reducing the distortion of the optical pulse time waveform caused by transmission: optical stage compensation method and electrical stage compensation method. The method is known. As an optical stage compensation method, a method of compensating for DGD by monitoring degree of polarization (DOP: Degree of Polarization) (see, for example, Patent Documents 1 and 2 and Non-Patent Document 1), or spectral hole burning (SHB: There is known a method (see Non-Patent Document 1) for compensating for DGD by monitoring (Spectrum Hole Burning).

  The method of monitoring DOP and compensating for DGD can be performed without depending on the transmission bit rate of the optical signal that is the object of waveform distortion compensation, and for any RZ (Return to Zero) format optical signal Is also applicable. A PMD compensator for carrying out this method includes a polarization plane controller, a group delay difference compensator (hereinafter also referred to as a DGD compensator), a polarimeter, and a controller for controlling them. Is done. The PMD compensation method using this PMD compensation device uses an algorithm that adaptively controls the polarization controller and the DGD compensator so as to maximize the DOP, and compensates for the temporal waveform distortion of the optical pulse due to PMD. Yes.

  With reference to FIG. 2, the configuration and operation of a conventional PMD compensation device will be described. FIG. 2 is a schematic block diagram of a conventional PMD compensation device. The optical signal path is indicated by a thick line, and the electrical signal path is indicated by a thin line.

  The PMD compensation apparatus 100 includes a polarization plane controller 102, a DGD compensator 104, a control signal generation unit 106, an optical coupler 108, and a polarimeter 110. The polarization plane controller 102 can arbitrarily convert the polarization state of the input optical signal 101 in accordance with the control signal 107a supplied from the control signal generation unit 106. In response to the control signal 107b supplied from the control signal generation unit 106, the DGD compensator 104 sets the group delay amount for one polarization mode component of the intrinsic polarization mode of the optical signal 103 output from the polarization plane controller 102. It can be given arbitrarily.

  In the PMD compensation apparatus 100, the PMD compensation method including the following first to third steps is performed on the input optical signal 101.

  The first step is for the input optical signal 101, the polarization direction of the intrinsic polarization mode component that has reached the PMD compensation device 100 first of the two intrinsic polarization modes of the input optical signal 101, and the delay of the DGD compensator. In this step, the polarization plane controller 102 is controlled so that the direction of the phase axis (Slow axis) matches.

  The optical signal 105 output from the DGD compensator is partially branched by the optical coupler 108 and input to the polarimeter 110 as an optical signal 109b. The second step is a step of calculating the DOP in the polarimeter 110 by extracting the Stokes parameter of the optical signal 109b.

  In the third step, in the control signal generation unit 106, based on the DOP signal 111 output from the polarimeter 110, the polarization plane controller 102 and the DGD compensation are performed so that the DOP calculated by the polarimeter 110 is maximized. In this step, the control signals 107a and 107b to be supplied to the device 104 are calculated. It is known that the time waveform distortion of an optical pulse caused by PMD becomes minimum when DOP is maximum (see, for example, Non-Patent Document 1).

  By performing the feedback control by repeating the first to third steps described above, the PMD compensation apparatus 100 performs compensation for the time waveform distortion of the optical pulse.

  In addition, as a method to reduce the number of optical parts and compensate PMD in a short time, while controlling the polarization plane controller, the two eigenpolarization modes are separated by the polarization beam splitter, and the intensity of both polarization components A method is also known in which analog / digital conversion is performed on a value, a correlation value between the two is calculated based on an electrical method, and the polarization plane controller is adaptively controlled to increase the correlation value (for example, (See Patent Document 2).

Compensation work for optical pulse time waveform distortion is desirable as the time to converge to the optimal control point that minimizes the amount of time waveform distortion of the optical pulse is shorter with respect to PMD time fluctuations due to changes in the external environment of the optical transmission line . This is because the transmission quality deteriorates during the compensation work of the temporal waveform distortion of the optical pulse. Further, it is desirable that the compensation operation for the time waveform distortion of the optical pulse is executed by a method that does not affect the propagation state of the input optical signal even during the compensation work for converging to the optimum control point.
Special Table 2006-527386 Japanese Unexamined Patent Publication No. 2006-211507 Special Table 2005-512455 Akihiko Tsujimura, Shoji Ishikawa “Current Status and Issues of Automatic Polarization Mode Dispersion Compensation Technology” OPTRONICS, 2003, No. 10, pp. 126-129.

  However, the conventional PMD compensator based on the method of compensating DGD by monitoring DOP has the following problems (1) to (3).

  (1) This method searches for the optimal control point by applying a change to the polarization state or phase of the externally input optical signal that is the compensated optical signal, so it converges to the optimal control point. Until then, waveform distortion occurs in the input optical signal, and transmission quality deteriorates. In order to obtain the distortion parameters (Stokes parameters, etc.) necessary for calculating DOP, etc., as described above, it is necessary to operate the polarization plane controller and the DGD compensator. This is because the time waveform of the compensated optical signal is distorted.

  (2) Since the operation of acquiring the distortion parameter necessary for calculating DOP or the like is performed while affecting the time waveform of the compensated optical signal, it is desirable to minimize this influence as much as possible. For this reason, a technique has been adopted in which the time required for the operation of acquiring the distortion parameter necessary for calculating DOP or the like is lengthened.

  For example, a technique such as dithering search, which is a technique for slightly changing a parameter for determining the polarization state of the compensated optical signal, may be used (for example, see Non-Patent Document 1). According to this dithering search, when the dithering amplitude is small, temporary waveform distortion is suppressed in the process until convergence to the optimal control point, but the time until convergence to the optimal control point is increased. . On the other hand, when the dithering amplitude is large, the time until convergence to the optimal control point is shortened, but distortion appears significantly in the time waveform of the compensated optical signal. That is, the time until convergence to the optimal control point and the amount of distortion appearing in the time waveform of the compensated optical signal are in a trade-off relationship.

  (3) In a situation where the polarization state of the compensated optical signal fluctuates greatly in a short time, it may take a long time to converge to the optimal control point. Also, in the PMD compensation method by controlling the polarization plane controller and the DGD compensator, the polarization state of the compensated optical signal fluctuates in a shorter time than the time until convergence to the optimal control point. Can be. In such a case, PMD compensation of the compensated optical signal cannot be performed.

  In other words, with the conventional PMD compensation method, it is difficult to quickly converge to the optimal control point where the amount of distortion of the compensated optical signal is minimized without distorting the compensated optical signal. there were.

  Accordingly, an object of the present invention is to detect a distortion correction parameter that maximizes DOP without distorting the time waveform of a compensated optical signal, and to compensate for PMD in a short time, and to this method An object of the present invention is to provide a PMD compensation device for realizing the above.

  In order to achieve the above object, the PMD compensation apparatus of the present invention comprises an optical splitter, a main optical signal compensator, and a compensation signal generator. The main optical signal compensator includes a first polarization plane controller and a first DGD compensator.

  The optical branching device branches an input optical signal input from the outside into a main optical signal and a monitor optical signal and outputs them. The compensation signal generation unit generates a first compensation parameter signal and a second compensation parameter signal from the monitor light signal, and supplies them to the first polarization plane controller and the first DGD compensator, respectively.

  The first compensation parameter signal is a signal that instructs the first polarization plane controller about the amount of rotation of the polarization plane of the main optical signal, and the second compensation parameter signal is the polarization plane control for the first DGD compensator. This signal indicates the DGD of the main light signal.

  The first polarization plane controller rotationally controls the polarization plane of the main optical signal based on the first compensation parameter signal to generate and output a polarization plane control main optical signal. The first DGD compensator generates a polarization mode dispersion compensation main optical signal (hereinafter also referred to as PMD compensation main optical signal) by reducing the DGD of the polarization control main optical signal based on the second compensation parameter signal. And output.

  The compensation signal generation unit can be configured as follows, for example. That is, the compensation signal generation unit includes a second polarization plane controller, a second DGD compensator, a polarization analyzer, and a control signal generation unit.

  The second polarization plane controller rotates and controls the polarization plane of the monitor optical signal, and generates and outputs a polarization plane control monitor optical signal. The second DGD compensator reduces the DGD of the polarization plane control monitor optical signal to generate and output a polarization mode dispersion compensation monitor optical signal (hereinafter also referred to as a PMD compensation monitor optical signal). The polarization analyzer calculates a Stokes parameter of the PMD compensated monitor optical signal, converts the Stokes parameter into a Stokes parameter signal that is an electrical signal, and outputs the signal. The control signal generation unit generates, from the Stokes parameter signal, a first compensation parameter signal that indicates the amount of rotation of the polarization plane of the main optical signal and a second compensation parameter signal that indicates the DGD of the polarization control main optical signal. To the first polarization plane controller and the first DGD compensator.

  To achieve the above object, the PMD compensation method of the present invention comprises an optical branching step, a compensation signal generation step, a first polarization plane control step, and a first group delay difference compensation step. The

  The optical branching step is a step of branching an input optical signal input from the outside into a main optical signal and a monitor optical signal and outputting them. The compensation signal generation step generates a first compensation parameter signal that indicates the amount of rotation of the polarization plane of the main optical signal and a second compensation parameter signal that indicates the DGD of the polarization control main optical signal from the monitor optical signal. This is the output step. The first polarization plane control step is a step in which the polarization plane of the main optical signal is rotationally controlled based on the first compensation parameter signal to generate and output a polarization plane control main optical signal. The first DGD compensation step is a step of generating and outputting a PMD compensation main optical signal by reducing the DGD of the polarization plane control main optical signal based on the second compensation parameter signal.

  The PMD compensation method of the present invention described above can be implemented using the PMD compensation apparatus of the present invention. In this case, it is preferable to include the following steps as the compensation signal generation step. That is, the PMD compensation method using the PMD compensation apparatus of the present invention is the above-described PMD compensation method, wherein the compensation signal generation step includes a polarization plane rotation angle calculation step, a DGD calculation step, a DOP comparison step, and a compensation parameter signal. It comprises a supply step and a PMD compensation value setting step.

  The polarization plane rotation angle calculating step measures the DOP of the polarization plane control monitor optical signal while rotating the direction of the optical axis of the second polarization plane controller while monitoring the polarization plane of the monitor optical signal, and the monitor that maximizes the DOP This is a step of calculating the rotation angle of the polarization plane of the optical signal.

  In the DGD calculation step, the rotation angle of the polarization plane of the monitor optical signal is fixed to the rotation angle of the polarization plane calculated in the polarization plane rotation angle calculation step, and the DGD of the polarization plane control monitor optical signal is obtained by the second DGD compensator. In this step, the DOP of the PMD compensated monitor optical signal is measured while changing the value, and the DGD of the PMD compensated monitor optical signal that maximizes the DOP is calculated.

  In the DOP comparison step, the calculated DGD is set in the second DGD compensator, the DOP of the PMD compensation monitor optical signal is measured, and the DOP is a reference DOP that is the minimum reference value of the preset DOP. This is a step of determining whether or not the value exceeds.

  The compensation parameter signal supply step is a step of generating and outputting a first compensation parameter signal and generating and outputting a second compensation parameter signal. The first compensation parameter signal indicates the rotation angle of the polarization plane of the monitor optical signal as the rotation angle amount of the polarization plane of the main optical signal when the DOP measured in the DOP comparison step exceeds the reference DOP. Signal. The second compensation parameter signal is a signal that indicates the DGD of the polarization control optical signal as the DGD of the polarization control main optical signal when the DOP measured in the DOP comparison step exceeds the reference DOP. .

  The PMD compensation value setting step is a step of setting the rotation angle of the polarization plane of the main optical signal by the first compensation parameter signal and setting the DGD of the polarization control main optical signal by the second compensation parameter signal.

  Further, the above-described PMD compensation method of the present invention comprises steps S10 to S34 shown below, and can be implemented using the PMD compensation apparatus of the present invention.

  Step S10 is a reference DOP setting step for determining a reference DOP that is a minimum reference value of DOP of the PMD compensated monitor optical signal output from the second DGD compensator.

  Step S12 is a first DOP measurement step for measuring the DOP of the polarization plane control monitor optical signal output from the second polarization plane controller with respect to a plurality of different rotation angles of the second polarization plane controller. It is.

  Step S14 is a rotation amount calculation step for calculating a rotation angle that gives the maximum DOP from the DOP values obtained in step S12 for a plurality of different rotation angles.

  Step S16 is a rotation step for rotating and fixing the second polarization plane controller by the rotation amount obtained in step S14.

  Step S22 is a second DOP measurement step of measuring the DOP of the PMD compensation monitor optical signal output from the second DGD compensator with respect to a plurality of different group delay amounts of the second DGD compensator, respectively. .

  Step S24 is a delay amount calculation step for calculating the group delay amount that gives the maximum DOP from the DOP values for the plurality of different group delay amounts acquired in step S22.

  Step S26 is a group delay amount setting step for setting and fixing the group delay amount obtained in step S24 to the second DGD compensator.

  Step S28 is a PMD compensation monitor that is output from the second DGD compensator while the second polarization plane controller remains fixed in step S16 and the second DGD compensator remains fixed in step S26. This is a DOP measurement step for measuring the DOP of the optical signal.

  Step S30 is a DOP comparison step for determining whether or not the DOP measured in step S28 exceeds a reference DOP.

  If NO is determined in step S30, the process returns to step S12. If the reference DOP is exceeded, the following steps S32 and S34 are executed.

  Step S32 generates and outputs the rotation angle that gives the rotation amount calculated in Step S14 as a first compensation parameter signal that indicates the rotation amount of the polarization plane of the main optical signal, and outputs the DGD calculated in Step S24. The step of generating and outputting as a second compensation parameter signal for instructing the DGD of the polarization control main optical signal.

  Step S34 is a PMD compensation step in which the rotation angle of the polarization plane of the main optical signal is set by the first compensation parameter signal, and the DGD of the polarization control main optical signal is set by the second compensation parameter signal.

  The following steps may be provided between step S10 and step S12. That is, following step S10, the normal DOP measurement step (step S100) for measuring the DOP of the PMD compensation monitor optical signal output from the second DGD compensator, and the DOP measured in step S100 exceeds the reference DOP. This is a DOP comparison step (step S102) for determining whether or not the In step S102, when it is determined that the DOP measured in step S100 exceeds the reference DOP, the process returns to step S10, and when it is determined that the DOP does not exceed the reference DOP, the process proceeds to step S12. Good to do.

  According to the PMD compensation device of the present invention, the input optical signal input from the outside is branched into the main optical signal and the monitor optical signal by the optical splitter. The DOP measurement of the input optical signal is performed using the monitor optical signal, and the main optical signal is not affected during the DOP measurement. Since the main optical signal is a compensated optical signal whose time waveform may be distorted, the DOP of the monitor optical signal is measured without distorting the time waveform of the compensated optical signal. Since the monitor optical signal is an optical signal generated by branching the input optical signal, the DOP of the monitor optical signal is similarly the DOP of the main optical signal generated by branching the input optical signal. equal.

  Therefore, the DOP of the main light signal is measured using the monitor light signal. When the DOP of the main optical signal is measured, as will be described later, it is possible to detect a distortion correction parameter that maximizes the DOP based on the DOP value.

  If the distortion correction parameter that maximizes the DOP is detected, the PMD of the main optical signal can be compensated in a short time based on the distortion parameter. That is, the main optical signal is not used at all during the above-mentioned DOP measurement, and the distortion correction parameter that maximizes the DOP of the main optical signal is detected by the monitor optical signal, so that the distortion that maximizes the DOP is performed. If a correction parameter is detected, the PMD of the main light signal can be compensated by only one control operation using this parameter. That is, the PMD compensation of the main light signal can be executed in a short time.

  The above-described distortion correction parameter that maximizes the DOP is detected by, for example, a compensation signal generation unit including a second polarization plane controller, a second DGD compensator, a polarization analyzer, and a control signal generation unit. It is possible.

  Since the Stokes parameter of the PMD compensated monitor optical signal is calculated by the polarization analyzer, DOP is calculated from the Stokes parameter. On the other hand, by controlling the rotation of the polarization plane of the monitor optical signal by the second polarization plane controller, the relationship of DOP with respect to the rotation amount of the polarization plane of the PMD compensated monitor optical signal can be detected. In addition, the second DGD compensator can detect the relationship of the DOP to the DGD of the PMD compensated monitor optical signal.

  That is, from the amount of rotation of the polarization plane of the PMD compensation monitor optical signal and the relationship of DOP with respect to DGD, the amount of rotation of the polarization plane of the PMD compensation monitor optical signal and DGD that maximizes DOP are calculated in the control signal generator. By outputting the conditions for maximizing the DOP to the first polarization plane controller and the first DGD compensator and controlling them, it becomes possible to generate an optical signal with PMD compensated. Setting the conditions for maximizing DOP in the first polarization controller and the first DGD compensator can be done in a short time compared to the time required to find the condition for maximizing DOP from the main optical signal in the conventional method. is there.

  Further, according to the PMD compensation method of the present invention, in the compensation signal generation step, the first compensation parameter signal for instructing the rotation amount of the polarization plane of the optical signal by the monitor optical signal without using the main optical signal, and the polarization plane control A second compensation parameter signal indicating the DGD of the main light signal is generated. While the first and second parameter signal generation steps are being performed, the main optical signal is not affected at all. The polarization plane of the main optical signal is rotationally controlled based on the first compensation parameter signal, and the DGD of the polarization control main optical signal can be reduced based on the second compensation parameter signal.

  By implementing the PMD compensation method of the present invention using the PMD compensation device of the present invention, a polarization plane rotation angle calculation step, a DGD calculation step, a DOP comparison step, a compensation parameter signal supply step, a PMD compensation value A compensation signal generation step comprising a setting step is realized.

  Through the polarization plane rotation angle calculation step and the DGD calculation step, the rotation angle of the polarization plane of the monitor optical signal that maximizes the DOP of the PMD compensation optical signal and the DGD of the polarization plane control monitor optical signal are calculated. The DOP comparison step determines whether or not the DOP at this time exceeds the reference DOP. When it is determined that the DOP exceeds the first DOP, first and second compensation parameter signals are generated. In the PMD compensation value setting step, the rotation angle of the polarization plane of the main optical signal and the DGD of the polarization control main optical signal are set based on the first and second compensation parameter signals, respectively.

  Further, according to the PMD compensation method comprising steps S10 to S34 described above, the second polarization plane controller and the second DGD compensation are performed by steps S12, S14, S16, S22, S24, S26, S28, and S30. The setting states of the second polarization plane controller and the second DGD compensator that maximize the DOP of the PMD compensation monitor optical signal are determined. In step S32, first and second compensation parameter signals are generated. In step S34, the rotation angle of the polarization plane of the main optical signal and the DGD of the polarization control main optical signal are set.

  As described above, according to the PMD compensation method of the present invention, the main optical signal itself is not used until the condition for maximizing the DOP of the PMD compensated monitor optical signal is determined. Therefore, it is possible to detect the first and second parameter signals, which are distortion correction parameters that maximize the DOP, without distorting the time waveform of the main optical signal that is the compensated optical signal.

  Also, setting the states of the first polarization plane controller and the first DGD compensator based on the first and second parameter signals, respectively, which gives the condition for maximizing the DOP, from the main optical signal in the conventional method Compared to the time to find the condition that maximizes DOP, this is possible in a short time.

  Further, by providing step S100 and step S102, the DOP of the main optical signal, that is, the input optical signal is always monitored using the monitor optical signal. In step S102, if an emergency is detected that the DOP of the PMD compensation monitor optical signal, that is, the input optical signal is lower than the reference DOP, step S12 is executed to immediately start the PMD compensation method of the present invention. The

  In the above-mentioned constant observation of the DOP of the monitor optical signal, the polarization state of the input optical signal is not affected at all. Then, the PMD compensation method of the present invention is started when an emergency situation is detected in which the polarization state of the input optical signal falls below the reference DOP. As a result, as described above, there is an effect that the polarization state of the input optical signal is compensated in a short time without distorting the time waveform of the input optical signal.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Each figure shows one configuration example according to the present invention, and only schematically shows the arrangement relationship of each component to the extent that the present invention can be understood. It is not limited to. In the following description, specific elements and operating conditions may be taken up. However, these elements and operating conditions are only one of preferred examples, and thus are not limited to these. Moreover, in each figure, the same component is shown with the same number, and the overlapping description may be omitted.

<Configuration of PMD compensation device>
With reference to FIG. 3, the configuration of the PMD compensation apparatus of the present invention will be described. FIG. 3 is a schematic block diagram of the PMD compensation device of the present invention. The optical signal path is indicated by a thick line, and the electrical signal path is indicated by a thin line.

  The PMD compensation apparatus 300 includes an optical branching device 302, a main optical signal compensation unit 304, and a compensation signal generation unit 306. The main optical signal compensation unit 304 includes a first polarization plane controller 310 and a first DGD compensator 312.

  The optical splitter 302 branches an input optical signal 301 input from the outside into a main optical signal 303a and a monitor optical signal 303b and outputs them. As the optical splitter 302, a commercially available optical fiber type optical coupler or the like can be used as appropriate.

  The compensation signal generation unit 306 generates a first compensation parameter signal 321a and a second compensation parameter signal 321b from the monitor light signal 303b, and supplies them to the first polarization plane controller 310 and the first DGD compensator 312 respectively.

  The first compensation parameter signal 321a is a signal that instructs the first polarization plane controller 310 the amount of rotation of the polarization plane of the main optical signal 303a, and the second compensation parameter signal 321b is to the first DGD compensator 312 Thus, it is a signal for instructing DGD of the polarization plane control main light signal 311.

  The first polarization plane controller 310 rotationally controls the polarization plane of the main optical signal 303a based on the first compensation parameter signal 321a, and generates and outputs a polarization plane control main optical signal 311. The first DGD compensator 312 reduces the DGD of the polarization control main optical signal 311 based on the second compensation parameter signal 321b, and generates and outputs the PMD compensation main optical signal 313.

  The compensation signal generation unit 306 includes a second polarization plane controller 314, a second DGD compensator 316, a polarization analyzer 318, and a control signal generation unit 320.

  The second polarization plane controller 314 rotationally controls the polarization plane of the monitor optical signal 303b to generate and output a polarization plane control monitor optical signal 315. The second DGD compensator 316 reduces the DGD of the polarization plane control monitor optical signal 315 to generate and output the PMD compensation monitor optical signal 317. The polarization analyzer 318 calculates a Stokes parameter of the PMD compensation monitor optical signal 317, converts the Stokes parameter into a Stokes parameter signal 319, which is an electric signal, and outputs it. The control signal generator 320, from the Stokes parameter signal 319, a first compensation parameter signal 321a that indicates the amount of rotation of the polarization plane of the main optical signal 303a, and a second compensation parameter that indicates the DGD of the polarization plane control main optical signal 311 A signal 321b is generated and output to the first polarization plane controller 310 and the first DGD compensator 312.

  In addition, the control signal generation unit 320 instructs the rotation amount of the polarization plane of the monitor light signal 303b in the process of generating the first compensation parameter signal 321a and the second compensation parameter signal 321b from the monitor light signal 303b. The first compensation subparameter signal 321c and the second compensation subparameter signal 321d for instructing the DGD of the polarization plane control monitor optical signal 315 are generated and output to the second polarization plane controller 314 and the second DGD compensator 316, respectively. .

As is well known, DGD is a value defined as the ratio of the intensity of the completely polarized light component in the total light intensity, so that it is represented by the Stokes parameters (S ₀ , S ₁ , S ₂ , S ₃ ) It is given by equation (1).
DGD = {S ₁ ² + S ₂ ² + S ₃ ² } ^1/2 / S ₀ (1)
Therefore, if the Stokes parameter is calculated, the DGD is easily calculated by the equation (1).

  In principle, the first polarization plane controller 310 and the second polarization plane controller 314 can be configured by combining a half-wave plate and a quarter-wave plate, and are commercially available under the names of polarization plane controllers and the like. It is possible to use the device as appropriate. Also, the first DGD compensator 312 and the second DGD compensator 316 are commercially available devices that control the DGD value by applying stress to the optical fiber core according to a control signal such as a voltage. Is possible.

  Here, the first polarization plane controller 310, the second polarization plane controller 314, the first DGD compensator 312 and the second DGD compensator 316 are provided with both a function as a polarization plane controller and a function as a DGD compensator. The PMD emulator PMDE-301 manufactured by General Photonics Corporation was used. According to this PMD emulator, it is possible to control the rotation of the polarization plane and to control and output the DGD in accordance with an external control signal. In other words, this PMD emulator PMDE-301 is formed with specifications for adjusting and outputting the polarization plane direction and DGD of input light in accordance with an external control signal.

  Specifically, by connecting a logic control circuit such as a personal computer to the I / O connection terminal provided in the PMD emulator PMDE-301 and supplying a TTL level signal to the PMD emulator PMDE-301, It is possible to adjust the direction of the polarization plane of the input light and DGD for output.

  As the polarization analyzer 318, the DOP measuring instrument POD-101A of General Photonics was used. The Stokes parameter signal 319 output from the DOP measuring instrument POD-101A is supplied to the control signal generator 320 via the USB interface (not shown). The control signal generation unit 320 uses a personal computer in which a program for calculating DOP from Stokes parameters is installed. That is, the DOP was calculated from the Stokes parameter signal 319 output from the DOP measuring instrument POD-101A by this personal computer.

<Operation of PMD compensator>
According to the PMD compensation apparatus of the present invention described above, the PMD compensation method of the present invention described below can be implemented. The PMD compensation method of the present invention comprises an optical branching step, a compensation signal generating step, a first polarization plane control step, and a first group delay difference compensation step.

  The optical branching step is performed by the optical branching unit 302, which branches the input optical signal 301 input from the outside into the main optical signal 303a and the monitoring optical signal 303b and outputs them. From the monitor optical signal 303b, a first compensation parameter signal 321a that indicates the amount of rotation of the polarization plane of the main optical signal 303a and a second compensation parameter signal 321b that indicates the DGD of the polarization control main optical signal 311 are generated and output. The compensation signal generation step to be performed is executed by the compensation signal generation unit 306.

  Based on the first compensation parameter signal 321a, the first polarization plane control step of rotating and controlling the polarization plane of the main optical signal 303a to generate and output the polarization plane control main optical signal 311 includes the first polarization plane controller 310. Is executed. Further, the first DGD compensation unit 312 generates and outputs the PMD compensation main optical signal 313 by reducing the DGD of the polarization control main optical signal 311 based on the second compensation parameter signal 321b. Executed.

  Since the PMD emulator PMDE-301 includes the first polarization plane controller 310 and the first DGD compensator 312, the first polarization plane control step and the first DGD compensation step described above are performed with respect to the PMD emulator PMDE-301. This is performed by supplying a first compensation parameter signal 321a and a second compensation parameter signal 321b.

  Specifically, the compensation signal generation step executed by the compensation signal generation unit 306 includes a polarization plane rotation angle calculation step, a DGD calculation step, a DOP comparison step, a compensation parameter signal supply step, and a PMD compensation value setting step. And is composed.

  In the polarization plane rotation angle calculation step, the DOP of the polarization plane control monitor optical signal 315 is measured while rotating the direction of the optical axis of the second polarization plane controller 314 while the polarization plane of the monitor optical signal 303b is rotated. The rotation angle of the polarization plane of the monitor optical signal is calculated.

  In the DGD calculation step, the rotation angle of the polarization plane of the monitor optical signal 303b is fixed to the rotation angle of the polarization plane calculated in the polarization plane rotation angle calculation step, and the polarization control control monitor optical signal is output by the second DGD compensator 316. This is a step of measuring the DOP of the PMD compensation monitor optical signal 317 while changing the DGD of 315, and calculating the DGD of the PMD compensation monitor optical signal 317 that maximizes this DOP.

  The DOP comparison step sets the DGD calculated in the DGD calculation step to the second DGD compensator 316, measures the DOP of the PMD compensation monitor optical signal 317, and this DOP exceeds the preset reference DOP. This is a step of determining whether or not the

  The compensation parameter signal supply step is a step of generating and outputting the first compensation parameter signal 321a and generating and outputting the second compensation parameter signal 321b. The first compensation parameter signal 321a indicates the rotation angle of the polarization plane of the monitor optical signal as the rotation angle amount of the polarization plane of the main optical signal when the DOP measured in the DOP comparison step exceeds the reference DOP. Signal. The second compensation parameter signal 321b indicates the DGD of the polarization plane control monitor optical signal 315 when the DOP measured in the DOP comparison step exceeds the reference DOP as the DGD of the polarization plane control main optical signal 311. Signal.

  The PMD compensation value setting step sets the rotation angle of the polarization plane of the main optical signal 303a by the first compensation parameter signal 321a, and sets the DGD of the polarization control main optical signal 311 by the second compensation parameter signal 321b. It is a step.

  Step S10 is a reference DOP setting step for determining a reference DOP which is a minimum reference value of DOP of the PMD compensation monitor optical signal 317 output from the second DGD compensator 316. In the optical communication system, since it is clear in design how much distortion of the time waveform of the optical pulse is allowed, first, the DOP is set according to the optical communication system in which the PMD compensation device of the present invention is used. . The higher the transmission rate of the optical communication system, the greater the influence of the distortion of the time waveform of the optical pulse constituting the optical signal. Therefore, the reference DOP is set according to the transmission rate of the optical communication system in which the PMD compensation apparatus of the present invention is used.

  While normal communication is being performed in the optical communication system, the DOP of the input optical signal input to the optical communication system does not frequently fall below the reference DOP. Therefore, the PMD compensation method of the present invention is executed when it is observed that the DOP is constantly observed and below the reference DOP.

  With reference to FIG. 4, the constant monitoring of the DOP of the input optical signal will be described. FIG. 4 is a flowchart for explaining the step of constantly monitoring the DOP of the input optical signal.

  In the flowchart shown in FIG. 4, it is assumed that the reference DOP is set in step S10, and then the optical communication system starts operating. Therefore, when the optical communication system starts operation, the continuous observation of the DOP of the input optical signal can be started. The continuous observation of the DOP of the input optical signal is executed by step S100 which is a normal DOP measurement step and step S102 which is a DOP comparison step.

  Step S100 is a step in which the polarization analyzer 318 measures the DOP of the PMD compensation monitor optical signal 317 output from the second DGD compensator 316. Step S102 is a step of determining whether or not the DOP measured in step S100 is greater than or equal to the reference DOP. As long as the DOP measured in step S100 is greater than or equal to the reference DOP, step S100 is executed, and the DOP of the PMD compensation monitor optical signal 317 is continuously monitored.

  On the other hand, if it is determined in step S102 that the DOP measured in step S100 is lower than the reference DOP, the PMD compensation method of the present invention is executed. For this purpose, step S12 is executed.

  Next, the PMD compensation method of the present invention will be described more specifically with reference to FIG. 5, assuming that it is determined that the DOP measured in step S100 is lower than the reference DOP. FIG. 5 is a flowchart for explaining the PMD compensation method of the present invention. In FIG. 5, step S100 and step S102 constituting the constant monitoring step of DOP of the input optical signal between step S10 and step S12 are omitted.

  As described above, when it is observed that the DOP falls below the reference DOP in the constant observation of the DOP, step S12 of the PMD compensation method of the present invention is executed.

  Step S12 is a first DOP measurement in which the DOP of the PMD compensation monitor optical signal 317 output from the second DGD compensator 316 is measured with respect to a plurality of different rotation angles of the second polarization plane controller 314, respectively. It is a step. The different rotation angles of the second polarization plane controller 314 are, for example, three points of 2π / 5 radians, 4π / 5 radians, and π radians, and the PMD compensation monitor optical signal 317 for each rotation amount. Is measured by the polarization analyzer 318. At this time, the DGD set in the second DGD compensator 316 is fixed to an arbitrary value. The DGD set in the second DGD compensator 316 is changed in step S24 described later.

  Step S14 is a rotation amount calculation step of calculating a rotation angle that gives the maximum DOP from the DOP values obtained in step S12 for a plurality of different rotations of the second polarization plane controller 314. The rotation amount calculation step here uses, for example, a secondary interpolation method described later.

  Step S16 is a rotation step for fixing the second polarization plane controller 316 to the rotation angle obtained in step S14. When step S16 ends, the rotation angle of the second polarization plane controller is provisionally determined. With the second polarization plane controller fixed in this state, the steps after step S22 described below are executed.

  Step S22 is a second DOP measurement in which the DOP of the PMD compensation monitor optical signal 317 output from the second DGD compensator 316 is measured with respect to a plurality of different group delay amounts of the second DGD compensator 316, respectively. It is a step.

  Step S24 is a delay amount calculation step for calculating the group delay amount that gives the maximum DOP from the DOP values for the plurality of different group delay amounts acquired in step S22. Also in the delay amount calculation step, similar to the rotation amount calculation step described above, the delay amount calculation step is executed using a secondary interpolation method or the like.

  Step S26 is a group delay amount setting step for setting and fixing the group delay amount obtained in step S24 to the second DGD compensator 316. That is, the group delay amount obtained in step S24 is set in the second DGD compensator 316.

  Step S28 is output from the second DGD compensator 316, leaving the second polarization controller 314 fixed in step S16 and the second DGD compensator 316 fixed in step S26. This is a DOP measurement step for measuring the DOP of the PMD compensation monitor optical signal 317. This step is performed by the polarization analyzer 318. The polarization analyzer 318 calculates a Stokes parameter of the PMD compensation monitor optical signal 317, converts the Stokes parameter into a Stokes parameter signal 319 that is an electric signal, outputs the converted Stokes parameter, and supplies the Stokes parameter to the control signal generator 320.

  Step S30 is a DOP comparison step for determining whether or not the DOP measured in step S28 exceeds a reference DOP. That is, in the control signal generation unit 320, the first compensation parameter signal 321a and the second compensation parameter signal 321b are generated from the Stokes parameter signal 319 and supplied to the first polarization plane controller 310 and the first DGD compensator 312, respectively. .

  In step S30, which will be described later, the first compensation subparameter signal 321c, which is the same signal as the first compensation parameter signal 321a, is supplied to the second polarization plane controller 314 in preparation for the determination to return to step S12. The second compensation subparameter signal 321d, which is the same signal as the second compensation parameter signal 321b, is supplied to the second DGD compensator 316.

  If it is determined in step S30 that the DOP measured in step S28 does not exceed the reference DOP, the process returns to step S12. If the DOP exceeds the reference DOP, the following steps S32 and S34 are executed. To do.

  Step S32 generates and outputs the rotation angle that gives the amount of rotation calculated in Step S14 as the first compensation parameter signal 321a that indicates the amount of rotation of the polarization plane of the main light signal 303a, and is calculated in Step S24. This is a step of generating and outputting the DGD as the second compensation parameter signal 321b for instructing the DGD of the polarization control main light signal 311.

  Step S34 is a PMD compensation step for setting the rotation angle of the polarization plane of the main light signal 303a by the first compensation parameter signal 321a and setting the DGD of the polarization control main light signal 311 by the second compensation parameter signal 321b. It is.

  As described above, when step S34 is completed, the DOP of the PMD compensation main light signal 313 is in a state exceeding the reference DOP. That is, by executing steps S12 to S34 described above, the PMD compensated main optical signal 313 that has been below the reference DOP has been compensated to be above the reference DOP.

  With reference to FIGS. 6 (A) and 6 (B), the time variation of the DOP of the PMD compensation main optical signal 313 during the PMD compensation operation in the case of the PMD compensation device of the present invention and the case of the conventional PMD compensation device will be described. To do. FIGS. 6 (A) and 6 (B) are diagrams for explaining the time variation of the DOP of the PMD compensated main optical signal. FIG. 6 (A) is a diagram showing the state of time variation of the DOP of the optical signal during the PMD compensation operation by the PMD compensation device of the present invention, and FIG. 6 (B) is a PMD compensation operation by the conventional PMD compensation device. It is a figure which shows the mode of the time fluctuation | variation of DOP of the optical signal in the inside. The horizontal axis is a time axis scaled at an arbitrary scale, and the vertical axis shows DOP in%.

  As shown in FIG. 6 (A), according to the PMD compensation device of the present invention, when the characteristic of the optical transmission line suddenly changes, that is, when the DOP of the input optical signal sharply decreases, the above-mentioned step S12 Until step S34 is completed, the DOP of the input optical signal is a constant value in a lowered state. When step S34 ends, the DOP of the input optical signal suddenly increases, and the PMD of the input optical signal is compensated.

  On the other hand, as shown in FIG. 6 (B), according to the conventional PMD compensation device, when the DOP of the input optical signal rapidly decreases, the input optical signal itself is used to perform the PMD compensation operation. The DOP of the input optical signal gradually decreases and gradually increases to reach the reference DOP. To shorten the recovery time from when the DOP of the input optical signal suddenly drops until it reaches the reference DOP, the minimum value of the DOP change curve of the input optical signal shown in FIG. There will be a time zone in which the time waveform of the optical pulse of the input optical signal is temporarily greatly distorted. In order not to greatly distort the time waveform of the optical pulse of the input optical signal, it is necessary to lengthen the recovery time from when the DOP of the input optical signal rapidly decreases until the reference DOP is reached.

  In the PMD compensation device of the present invention shown in FIG. 6 (A), in order to shorten the time zone in which the DOP of the input optical signal is reduced, it is necessary to shorten the time from step S12 to step S34. However, this can be solved by improving the polarization analyzer 318 or the control signal generator 320 to a device that operates at high speed. On the other hand, shortening the time zone in which the DOP of the input optical signal is decreasing in the conventional PMD compensation device shown in FIG. 6 (B) is, in principle, the temporal waveform of the optical pulse of the input optical signal temporarily It is necessary to accept that there is a very distorted time zone.

  With reference to FIG. 7, a method of determining the rotation angle of the first polarization plane controller that gives the maximum DOP of the main optical signal by the secondary interpolation method will be described. FIG. 7 is a diagram for explaining a method of determining the rotation angle of the polarization plane controller that gives the maximum DOP of the optical signal by the quadratic interpolation method. The horizontal axis shows the rotation angle of the polarization controller in radians (rad), and the vertical axis shows the DOP of the optical signal in%.

The curve shown in FIG. 7 indicates that the DOP of the PMD compensation monitor optical signal 317 output from the second DGD compensator 316 is rotated by 3 points of 2π / 5 radians, 4π / 5 radians, and π radians of the second polarization plane controller 314. The results of measuring DOP with respect to the angle are shown. The observed DOP is indicated by a downward arrow. From these three points, the relationship between the rotation angle φ of the second polarization plane controller 314 and the polarization degree function DOP (φ) as a function of the rotation angle φ is found. According to FIG. 7, DOP (2π / 5) = 90%, DOP (4π / 5) = 40%, and DOP (π) = 0%. Therefore, if the relationship between φ and DOP (φ) is given by DOP (φ) = Aφ ² + Bφ + C, the values of DOP (φ) at φ = 2π / 5, 4π / 5 and π described above are used. Coefficients A, B and C are obtained. A quadratic curve DOP (φ) given by the coefficients A, B, and C determined in this manner is shown by a broken line in FIG.

  On the other hand, the DOP value indicated by a black circle is a value obtained by actual measurement, and in FIG. 7, the dots indicated by the black circle are connected by a solid line. It turns out that both agreement is very good.

  Based on the quadratic curve DOP (φ) given by the above-described coefficients A, B and C shown in FIG. 7, the rotation angle giving the maximum DOP was obtained by the binary search method. In the example shown in FIG. 7, φ = 0.2π (= π / 5) was calculated as the rotation angle that gives the maximum DOP. That is, in the above-described step S14, it has been confirmed that it is effective to use the secondary interpolation method and the binary search method to calculate the rotation angle that gives the maximum DOP. Similarly, in step S24 described above, it has been confirmed that it is effective to use the quadratic interpolation method to calculate the group delay amount that gives the maximum DOP.

It is a diagram showing the state of the change of the time waveform before and after the optical pulse propagates through the optical fiber having birefringence, (A) is the x and y of the optical pulse before propagating through the optical transmission line 10 A time waveform for the polarization component in the axial direction and a time waveform for the polarization component in the x and y axis directions after propagating through the optical transmission line 10 are shown, and (B) propagates through the optical transmission line 10. The time waveform of the polarization components in the x and y axis directions of the previous optical pulse is shown. (C) shows the polarization components in the x and y axis directions of the optical pulse after propagation through the optical transmission line 10. It is a figure which shows the performed time waveform. It is a schematic block diagram of a conventional PMD compensation device. 1 is a schematic block diagram of a PMD compensation device of the present invention. It is a flowchart for demonstrating the continuous monitoring step of DOP of an input optical signal. 3 is a flowchart for explaining a PMD compensation method of the present invention. It is a diagram for explaining the time variation of the DOP of the PMD compensation main optical signal, (A) shows the state of the time variation of the DOP of the optical signal during the PMD compensation operation by the PMD compensation device of the present invention, (B) The state of time variation of the DOP of the optical signal during the PMD compensation operation by the conventional PMD compensation device is shown. FIG. 6 is a diagram for explaining a method of determining a rotation angle of a first polarization plane controller that gives the maximum DOP of the main optical signal by a secondary interpolation method.

Explanation of symbols

10: Optical transmission line
100, 300: PMD compensator
102: Polarization plane controller
104: DGD compensator
106: Control signal generator
108: Optical coupler
110: Polarimeter
302: Optical splitter
304: Main light signal compensator
306: Compensation signal generator
310: First polarization plane controller
312: 1st DGD compensator
314: Second polarization plane controller
316: Second DGD compensator
318: Polarization analyzer
320: Control signal generator

Claims (4)

An optical splitter, a main optical signal compensator, and a compensation signal generator ;
Before SL optical splitter is branched and output an input optical signal inputted from the outside into the main light signal and the monitor light signal,
The main optical signal compensator reduces the group delay difference between the first polarization plane controller that controls the polarization plane of the main optical signal to generate the polarization control main optical signal, and the polarization control main optical signal. A first group delay difference compensator that generates a polarization mode dispersion compensation main optical signal,
The compensation signal generation unit rotationally controls the polarization plane of the monitor optical signal, generates a polarization plane control monitor optical signal, and outputs the second polarization plane controller;
A second group delay difference compensator that reduces the group delay difference of the polarization plane control monitor optical signal and generates and outputs a polarization mode dispersion compensation monitor optical signal;
A polarization analyzer that calculates a Stokes parameter of the polarization mode dispersion compensation monitor optical signal, converts the Stokes parameter into a Stokes parameter signal that is an electrical signal, and outputs the Stokes parameter signal;
From the Stokes parameter signal, generate a first compensation parameter signal that indicates the amount of rotation of the polarization plane of the main optical signal, and a second compensation parameter signal that indicates the group delay difference of the polarization control main optical signal, A control signal generator for outputting to each of the first polarization plane controller and the first group delay difference compensator;
With
From the monitoring optical signal, the first compensation parameter signal for indicating the amount of rotation of the polarization plane of the main optical signal, and generates the second compensation parameter signal indicating a differential group delay of the polarization control master optical signal And supplying the first compensation parameter signal to the first polarization plane controller, supplying the second compensation parameter signal to the first group delay difference compensator,
The first polarization plane controller rotationally controls the polarization plane of the main optical signal based on the first compensation parameter signal, generates and outputs the polarization plane control main optical signal,
The first group delay difference compensator, the reduces the group delay difference of the polarization control master optical signal based on the second compensation parameter signal, generates and outputs the polarization mode dispersion compensator main optical signal A polarization mode dispersion compensator. A polarization mode dispersion compensation method using the polarization mode dispersion compensation device according to claim 1 ,
An optical branching step of branching and outputting the input optical signal input from the outside into the main optical signal and the monitor optical signal;
From the monitor optical signal, the first compensation parameter signal indicating the amount of rotation of the polarization plane of the main optical signal and the second compensation parameter signal indicating the group delay difference of the polarization control main optical signal are generated. Generating a compensation signal to be output,
Based on the first compensation parameter signal, rotation control of the polarization plane of the main optical signal, to generate and output the polarization plane control main optical signal, and a first polarization plane control step,
A first group delay difference compensation step of generating and outputting the polarization mode dispersion compensation main optical signal by reducing the group delay difference of the polarization plane control main optical signal based on the second compensation parameter signal; Prepared,
The compensation signal generation step includes:
While rotating the direction of the optical axis of the second polarization plane controller, the polarization degree of the polarization plane control monitor optical signal is measured, and the rotation angle of the polarization plane of the monitor optical signal that maximizes the polarization degree is calculated. A step of calculating the polarization plane rotation angle;
The polarization plane control monitor optical signal is fixed by the second group delay difference compensator by fixing the rotation angle of the polarization plane of the monitor optical signal to the rotation angle of the polarization plane calculated in the polarization plane rotation angle calculation step. Measuring the polarization degree of the polarization mode dispersion-compensated monitor optical signal while changing the group delay difference, and calculating the group delay difference of the polarization plane control monitor optical signal that maximizes the polarization degree Steps,
The calculated group delay difference is set in the second group delay difference compensator, the degree of polarization of the polarization mode dispersion compensation monitor optical signal is measured, and the degree of polarization is set in advance. A polarization degree comparison step for determining whether or not a reference polarization degree value is exceeded, which is a minimum reference value of
When the polarization degree measured in the polarization degree comparison step exceeds the reference polarization degree, the rotation angle of the polarization plane of the monitor optical signal is designated as the rotation angle amount of the polarization plane of the main optical signal. Generating and outputting the first compensation parameter signal, and when the degree of polarization measured in the degree-of-polarization comparison step exceeds the reference degree of polarization, the group delay difference of the polarization plane control monitor optical signal A compensation parameter signal supply step for generating and outputting the second compensation parameter signal, instructing as a group delay difference of the polarization control main optical signal,
Polarization mode dispersion compensation that sets the rotation angle of the polarization plane of the main optical signal by the first compensation parameter signal and sets the group delay difference of the polarization control main optical signal by the second compensation parameter signal A polarization mode dispersion compensation method comprising: a value setting step. A polarization mode dispersion compensation method using the polarization mode dispersion compensation device according to claim 1 ,
A reference polarization degree setting step (step S10) for determining a reference polarization degree that is a minimum reference value of the polarization degree of the polarization mode dispersion compensation monitor optical signal output from the second group delay difference compensator;
A first polarization degree measurement step for measuring the polarization degree of the polarization plane control monitor optical signal output from the second polarization plane controller with respect to a plurality of different rotation angles of the second polarization plane controller; Step S12)
A rotation amount calculating step (step S14) for calculating a rotation angle that gives the maximum degree of polarization from the value of the degree of polarization with respect to a plurality of different rotation angles obtained in step S12,
Rotating and fixing the second polarization plane controller by the amount of rotation determined in step S14, a rotation step (step S16),
The degree of polarization of the polarization mode dispersion compensation monitor optical signal output from the second group delay difference compensator is different from that of the plurality of different group delay amounts of the second group delay difference compensator. A second polarization degree measuring step (step S22) for measuring
A delay amount calculating step (step S24) for calculating a group delay amount that gives the maximum degree of polarization, from the value of the polarization degree for different group delay amounts obtained in step S22.
A group delay amount setting step (step S26) for setting and fixing the group delay amount obtained in step S24 to the second group delay difference compensator;
While the second polarization plane controller remains fixed in the step S16 and the second group delay difference compensator remains fixed in the step S26, the second group delay difference compensator Measuring the degree of polarization of the polarization mode dispersion compensation monitor optical signal output from the polarization degree measurement step (step S28);
A degree of polarization comparison step (step S30) for determining whether the degree of polarization measured in step S28 exceeds the reference degree of polarization;
The rotation angle that gives the amount of rotation calculated in step S14 is generated and output as the first compensation parameter signal that indicates the amount of rotation of the polarization plane of the main optical signal, and the group calculated in step S24 A compensation parameter signal supply step (step S32) for generating and outputting a delay difference as the second compensation parameter signal indicating a group delay difference of the polarization plane control main optical signal;
The rotation angle of the polarization plane of the main optical signal is set by the first compensation parameter signal generated in step S32, and the polarization plane control main unit is set by the second compensation parameter signal generated in step S32. A polarization mode dispersion compensation step (step S34) for setting a group delay difference of the optical signal,
If it is determined in step S30 that the reference polarization degree is not exceeded, the process returns to step S12, and if it is determined that the reference polarization degree is exceeded, the steps S32 and S34 are performed. A polarization mode dispersion compensation method characterized by comprising: The polarization mode dispersion compensation method according to claim 3 , further comprising:
Measuring the degree of polarization of the polarization mode dispersion compensation monitor optical signal output from the second group delay difference compensator, a normal degree of polarization measurement step (step S100),
A polarization degree comparison step (step S102) for determining whether or not the degree of polarization measured in step S100 exceeds the reference polarization degree,
Subsequent to step S10, execute step S100 and step S102,
In Step S102, when it is determined that the degree of polarization measured in Step S100 exceeds the reference degree of polarization, the process returns to Step S10.
If it is determined in step S102 that the polarization degree measured in step S100 does not exceed the reference polarization degree, the polarization mode dispersion compensation method proceeds to step S12.

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