JP5208539B2 - Polarization control apparatus and polarization control method - Google Patents

Description

  The present invention relates to an apparatus and a control method for controlling the polarization state of an optical signal.

  A polarization multiplexing method is known in which two optical signals in orthogonal polarization states are multiplexed and transmitted. In the polarization multiplexing method, it is necessary to separate two orthogonal optical signals in the receiving apparatus. In a polarization separation device that separates an optical signal, polarization control is realized so that the characteristics are optimum (maximum or minimum) while observing some characteristics.

  For example, the polarization separation device controls two wave plates, a λ / 2 plate and a λ / 4 plate, and a rotation angle of each wave plate to make an output optical signal an appropriate polarization state. A control apparatus and a polarization beam splitter (PBS) which is a polarization separation device are provided. At the time of polarization separation, it is necessary to control the rotation angles of the two wave plates in the polarization control device so that the polarization is linearly polarized with respect to the axis of the polarization beam splitter.

  In general, in order to control the wave plate, the polarization controller obtains an index value indicating the polarization state based on the output light of the polarization separation device, and the index value is optimal based on this index value. Adjust the optimal rotation angle to show the value. As the index value, for example, the signal strength of a beat signal (Non-Patent Document 1) is known.

In addition, as the communication speed increases, the pulse width per bit becomes smaller and it is more susceptible to PMD (polarization mode dispersion) of the transmission line optical fiber. Therefore, it is necessary to perform PMD compensation. is there. The polarization control device described above can also be applied when performing PMD compensation. In this case, the polarization controller is used to appropriately separate an optical signal affected by PMD (polarization mode dispersion) into two optical signals. In addition, as the index value, the degree of polarization (DOP: see Non-Patent Document 2), the signal strength of the clock signal (see Non-Patent Document 3), the Q value of the eye waveform (see Non-Patent Document 4), and the like can be used. .
“Interference detectionenabling 2 × 20 Gbit / s RZ polarization division multiplex transmission”, S.C. Hinz et al., Electronics Letters, vol. 37, no.8, pp. 511-512, August 2001 “Simple dynamic polarization mode dispersion compensator”, C.I. Francia et al., Electronics Letters, vol.35, no.5, Mar 1999, May 1999. “Automaticpolarization-mode dispersion compensation in 40-Gbit / s transmission”, H.C. Ooi et al., OFC 1999, WE5-1, February 1999 “Adaptive PMDcompensation in a 160 Gb / s RZ transmission system using eye monitor feedback”, F.R. Buchali et al., EcoC2002, W7.1.3, September 2002 "Study of feedback control of polarization mode dispersion compensator using random step width climbing method for polarization mode dispersion adaptive compensation", Takeshi Tanizawa et al., Shingaku Technique, OCS 2007-30, July 2007

  In a conventional polarization control device, control using a simple and well-known hill-climbing method is generally performed so that the index value is an optimum value. However, when the hill-climbing method is used, if the observed index value becomes a local value (local maximum or minimum), the local value is set to the optimum value (maximum value, minimum value). ), The characteristic cannot be sufficiently obtained for the optimum value to be reached, and the optimization cannot proceed (local solution).

  A technique that can avoid this local solution and search for the optimum values (maximum value, minimum value) to be reached is required. For example, Non-Patent Document 5 randomly changes the search step width as a method for avoiding a local solution that is a problem in PMD compensation control. In the technique proposed in Non-Patent Document 5, as control, the λ / 2 plate and λ / 4 plate of the polarization control device are rotated by random step widths, respectively, and an optimum value is searched. If this technique is used, the step width changes randomly even if it falls into the local solution, so that it is possible to escape from the local solution when a large step width is adopted.

  However, in the above technique, since a random step width is used, it may take time to reach the target value when the target value is close to the target value (optimum value) that should be originally reached. That is, the step width is selected by a random function (uniform or Gaussian distribution). Therefore, when a large step width is selected even when the state is close to the target value, more time is required to reach the target value in order to perform a search larger than necessary. Furthermore, it is possible to get out of the local solution only when a large step width is selected. Since this selection is random, it takes time until a large value is selected. However, it may take time to reach the target value.

  The present invention provides a polarization control device and a polarization control method capable of easily deviating from a local solution even when an index value falls into a local solution and capable of reaching a target value in a short time. The purpose is to provide.

An object of the present invention is to arrange a λ / 2 plate disposed on the optical axis of an optical signal and rotatable about the optical axis;
A λ / 4 plate disposed on the same optical axis as the λ / 2 plate and rotatable about the optical axis;
Λ / 2 plate control means for rotating the λ / 2 plate with a first step width;
Λ / 4 plate control means for rotating the λ / 4 plate with a second step width;
Wavelength plate simultaneous control means for simultaneously rotating the λ / 2 plate and the λ / 4 plate with a third step width and a fourth step width, respectively;
Based on the index value based on the signal state of the optical signal polarization-controlled by the polarization control device and the target value acquired in advance and stored in the storage device, the λ / 2 plate control means, λ / 4 plate Control means, and processing means for operating the wavelength plate simultaneous control means,
The processing means operates at least one of the λ / 2 plate control means and the λ / 4 plate control means a predetermined number of times, and the acquired index value is determined based on the acquired index value and the target value. This is achieved by a polarization control device that operates the wavelength plate simultaneous control means when it is determined that the wave plate has fallen.

  In a preferred embodiment, the processing means sequentially operates the λ / 2 plate control means and the λ / 4 plate control means, or sequentially operates the λ / 4 plate control means and the λ / 2 plate control means. .

  In the above invention, it is desirable that the third step width and the fourth step width are equal to or larger than the first step width and the second step width.

Another object of the present invention is to arrange a λ / 2 plate disposed on the optical axis of an optical signal and rotatable about the optical axis;
A λ / 4 plate disposed on the same optical axis as the λ / 2 plate and rotatable about the optical axis;
Λ / 2 plate control means for rotating the λ / 2 plate with a first step width;
Λ / 4 plate control means for rotating the λ / 4 plate with a second step width;
Wavelength plate simultaneous control means for simultaneously rotating the λ / 2 plate and the λ / 4 plate with a third step width and a fourth step width, respectively;
Based on the index value based on the signal state of the optical signal polarization-controlled by the polarization control device and the target value acquired in advance and stored in the storage device, the λ / 2 plate control means, λ / 4 plate Control means, and processing means for operating the wavelength plate simultaneous control means,
When the processing unit operates the wavelength plate simultaneous control unit a predetermined number of times and determines that the acquired index value falls into a local solution based on the acquired index value and the target value, the λ / 2 plate control means and at least one of the λ / 4 plate control means are operated.

  In the above invention, it is desirable that the first step width and the second step width are equal to or larger than the third step width and the fourth step width.

  In another preferred embodiment, the processing means determines that a local solution has fallen when the difference between the predetermined index value and the target value is outside a predetermined range even after the predetermined number of operations. To do.

In a more preferred embodiment, the storage device has the index value and the first value so that the first step width and the second step width become smaller as the index value approaches a target value. A table in which the step width and the second step width are associated with each other,
The processing means refers to the table and obtains the first step width and the second step width associated with the index value.

In another preferred embodiment, the storage device has the first step width and the second step as the absolute value of the difference value between the index value acquired last time and the index value acquired this time decreases. A table in which the index value is associated with the first step width and the second step width so as to reduce the width;
The processing means refers to the table and obtains the first step width and the second step width associated with the difference value.

Another object of the present invention is to arrange a λ / 2 plate disposed on the optical axis of an optical signal and rotatable about the optical axis;
A λ / 4 plate disposed on the same optical axis as the λ / 2 plate and rotatable about the optical axis;
Λ / 2 plate control means for rotating the λ / 2 plate with a first step width;
Λ / 4 plate control means for rotating the λ / 4 plate with a second step width;
Wavelength plate simultaneous control means for simultaneously rotating the λ / 2 plate and the λ / 4 plate with a third step width and a fourth step width, respectively;
Based on the index value based on the signal state of the optical signal polarization-controlled by the polarization control device and the target value acquired in advance and stored in the storage device, the λ / 2 plate control means, λ / 4 plate In the polarization control device comprising the control means and the processing means for operating the wavelength plate simultaneous control means,
In the processing means,
Operating at least one of the λ / 2 plate control means and the λ / 4 plate control means a predetermined number of times;
Determining whether the acquired index value falls into a local solution based on the acquired index value and the target value;
This is achieved by a polarization control method comprising the step of operating the wave plate simultaneous control means when it is determined that the local solution has fallen.

In a preferred embodiment, in the step of operating the predetermined number of times,
The step of sequentially operating the λ / 2 plate control means and the λ / 4 plate control means or the step of sequentially operating the λ / 4 plate control means and the λ / 2 plate control means is performed the predetermined number of times.

Another object of the present invention is to arrange a λ / 2 plate disposed on the optical axis of an optical signal and rotatable about the optical axis;
A λ / 4 plate disposed on the same optical axis as the λ / 2 plate and rotatable about the optical axis;
Λ / 2 plate control means for rotating the λ / 2 plate with a first step width;
Λ / 4 plate control means for rotating the λ / 4 plate with a second step width;
Wavelength plate simultaneous control means for simultaneously rotating the λ / 2 plate and the λ / 4 plate with a third step width and a fourth step width, respectively;
Based on the index value based on the signal state of the optical signal polarization-controlled by the polarization control device and the target value acquired in advance and stored in the storage device, the λ / 2 plate control means, λ / 4 plate In the polarization control device comprising the control means and the processing means for operating the wavelength plate simultaneous control means,
In the processing means,
Operating the wavelength plate simultaneous control means a predetermined number of times;
Determining whether the acquired index value falls into a local solution based on the acquired index value and the target value;
A polarization control method comprising: operating at least one of the λ / 2 plate control means and the λ / 4 plate control means when it is determined that the local solution falls into the local solution. Achieved.

In a preferred embodiment, the step of determining whether or not it falls into the local solution comprises:
The step of determining that the difference between the predetermined index value and the target value is out of a predetermined range even after the predetermined number of operations is in a local solution.

  According to the present invention, even when an index value falls into a local solution, the polarization control device and the polarization control that can easily escape from the local solution and can reach the target value in a short time It becomes possible to provide a method.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a block diagram schematically showing the configuration of the optical communication system according to the first embodiment of the present invention. In FIG. 1, in an optical communication system employing a polarization multiplexing transmission system, the polarization control device according to the present invention is applied to a polarization separation device on the receiving side. In the first embodiment, the polarization control device performs control so that the polarization states of two optical signals multiplexed by the polarization multiplexing method are appropriate.

  As shown in FIG. 1, the optical communication system includes transmitters 10-1 and 10-2, and the polarization multiplexing device 12 orthogonalizes signals output from the transmitters 10-1 and 10-2. Multiplexed as two optical signals and sent to the transmission line 14. .., 16-n are arranged on the transmission line 14 and are transmitted through the transmission line 14 via the repeater (hereinafter simply denoted by reference numeral 16) 16. The wave-multiplexed optical signal reaches the receiving side.

  A polarization separation device 20, receivers 26-1 and 26-2 and a polarization state monitor 28 are provided on the reception side of the optical communication system. The polarization separation device 20 appropriately controls the polarization state of the polarization multiplexed optical signal transmitted through the transmission line 14 and the output optical signal from the polarization control device 22. And a polarization beam splitter (PBS) 24 that extracts each of the orthogonal optical signals. The two optical signals from the polarization beam splitter 24 are received by the receivers 26-1 and 26-2, respectively.

  For example, the polarization state monitor 28 receives an optical signal output from the polarization beam splitter 24, calculates an index value based on the optical signal, and outputs the index value to the polarization controller 22. For example, in this embodiment, the intensity of the beat signal of the X polarization and the Y polarization is used as the index value, and the polarization state monitor 28 calculates the intensity of the beat signal and sends it to the polarization controller 22. Output. Therefore, in the polarization control device 22 according to the first embodiment, processing for optimizing the strength of the beat signal, that is, minimizing the strength of the beat signal is executed.

  In the present embodiment, the polarization state monitor 28 monitors the state of the signal after polarization control, obtains an index value based on the signal state, calculates other index values, It may be output to the polarization controller 22.

  FIG. 2 is a block diagram showing the configuration of the polarization control device according to this embodiment. As shown in FIG. 2, the polarization controller 22 according to this embodiment includes a λ / 2 plate (HWP) 32, a λ / 4 plate (QWP) 33, an HWP control unit 34 that controls the rotation of the HWP 32, and a QWP 33. QWP control unit 35 that controls the rotation of the HWP 32, a simultaneous control unit 36 that controls the simultaneous rotation of both the HWP 32 and the QWP 33, and a sequence control that controls the operation sequence of the HWP control unit 34, the QWP control unit 35 and the simultaneous control unit 36 A portion 37 is provided.

  In the present embodiment, the HWP control unit 34 can rotate the HWP 32 with a predetermined step width (Δθ) around the optical axis (arrow in FIG. 2). The simultaneous control unit 36 can rotate the HWP 32 with a predetermined step width (Δθ ′) around the optical axis. Further, the QWP control unit 35 can rotate the QWP 33 around the optical axis (arrow in FIG. 2) with a predetermined step width (Δθ). The simultaneous control unit 36 can rotate the QWP 33 with a predetermined step width (Δθ ′) around the optical axis.

The polarization controller 22 according to the present embodiment basically employs the following logic to search for the optimum value (minimum value) of the beat signal intensity.
(1) Normally, the HWP 32 is rotated to search for the optimum value, and then the QWP 33 is rotated to search for the optimum value. Of course, the order may be reversed (firstly, the QWP 33 is rotated to search for the optimum value).
(2) If it is determined that the DOP has fallen into a local solution, an attempt is made to avoid the local solution by rotating the HWP 32 and the QWP 33 simultaneously to expand the search space.

  FIG. 3 is a flowchart illustrating an example of the optimum value search process executed in the polarization control device according to the present embodiment. As shown in FIG. 3, the sequence control unit 37 of the polarization control device 22 refers to the intensity of the beat signal from the polarization state monitor 28 (step 301), and the intensity of the beat signal that is the index value is the target value. It is determined whether the value is within a certain range (step 302).

Here, the target value will be briefly described. In the present embodiment, since there are two parameters, the rotation angle θ _{HWP of HWP} 32 and the rotation angle θ _QWP of _QWP 33, there is a two-dimensional search space specified by (θ _HWP , θ _QWP ) (FIG. 7 (a) and (b)). Therefore, for all (θ _HWP , θ _QWP ), the corresponding index value F (θ _HWP , θ _QWP ) is observed, and (θ _{HWP_MIN} , θ _{QWP_MIN} ) that indicates the optimum value (minimum value) is always obtained. It is desirable that However, it takes a very long time to calculate the corresponding index value F (θ _HWP , θ _QWP ) for all (θ _HWP , θ _QWP ) in the search space. At this time, if time is required, the polarization state of the optical signal propagated through the transmission path changes, and the characteristics of the search space change. As a result, it is impossible to obtain the correct optimum value condition angle (θ _{HWP_MIN} , θ _{QWP_MIN} ). Therefore, in the present embodiment, prior to the processing executed by the polarization control device 22, the index values F (θ _HWP , θ _QWP ) corresponding to all the above (θ _HWP , θ _QWP ) are set in advance. Observe and _store only the minimum value F (θ _{HWP_MIN} , θ _{QWP_MIN} ) as a target value in a memory (not shown).

  Therefore, in step 302, the sequence control unit 37 compares the target value stored in the memory with the index value acquired from the polarization state monitor 28. If it is determined Yes in step 302, the process waits until the next monitor timing (step 302 to be executed next). If it is determined No in step 302, the sequence control unit 37 activates the HWP control unit 34 to perform HWP control (step 303).

FIG. 4 is a flowchart showing an example of the HWP control process according to the present embodiment. As shown in FIG. 4, the HWP control unit 34 rotates the HWP 32 to the reference position (θ _HWPi , θ _QWPi ) when necessary (step 401). Note that when the HWP control unit 34 is activated (initially), the HWP 32 is in the reference position, so this step can be omitted. The HWP control unit 34 acquires the index value F _i = (θ _HWPi , θ _QWPi ) from the polarization state monitor 28 in the state of the current reference position, and determines the step width Δθ based on the index value. (Step 402). The determination of the step width Δθ will be described in detail later.

Next, the HWP control unit 34 rotates the HWP 32 in the positive direction from the reference position by Δθ to obtain θ _{HWPi + 1} = θ _HWPi + Δθ (step 403). Since the position of the QWP 33 is not changed, θ _{QWPi + 1} = θ _QWPi . In this state, the HWP control unit 34 _{acquires the} index value F _{i + 1} = (θ _{HWPi + 1} , θ _{QWPi + 1} ) from the polarization state monitor 28 (step 404). The HWP control unit 34 rotates the HWP 32 in the negative direction from the reference position by Δθ to obtain θ _{HWPi + 2} = θ _HWPi −Δθ (step 405). Since the position of the QWP 33 is not changed, θ _{QWPi + 2} = θ _QWPi . In this state, the HWP control unit 34 _{acquires the} index value F _{i + 2} = (θ _{HWPi + 2} , θ _{QWPi + 2} ) from the polarization state monitor 28 (step 406).

The HWP control unit 34 compares the obtained index values F _i , F _{i + 1} and F _{i + 2} (step 407), and selects the best value (minimum value) among them (step 408). The best value is selected, if an improvement over F _i corresponding to the current value at the reference position (Yes in step 409), HWP control unit 34, the angle a new reference position of the conditions of the best values (theta _HWPi , θ _QWPi ) (step 410), and the process returns to step 401. On the other hand, if No is determined in step 409, the process is terminated.

In the HWP control process, as long as F _i is improved, the HWP32 both by the positive and negative directions step width Δθ is rotated continuously found the best value.

Following the HWP control (step 303 in FIG. 3), QWP control is executed (step 304). Although not shown, the QWP control is the same as the HWP control, and the angle θ _QWP of the _QWP 33 is rotated in the positive direction and the negative direction by the step width Δθ in the search space. As will be described in detail later, in the present embodiment, the step width Δθ in the HWP control and the step width Δθ in the QWP control are determined to be the same for the same index value.

The QWP control unit 35 rotates the QWP 33 to the reference position (θ _HWPi , θ _QWPi ) when necessary. Similar to step 401 in FIG. 4, when the QWP control unit 35 is activated (initially), this step can be omitted because the QWP 33 is in the reference position. The QWP control unit 35 acquires the index value F _i = (θ _HWPi , θ _QWPi ) from the polarization state monitor 28 while the QWP 33 is at the reference position, and determines the step width Δθ based on the index value. To do.

Next, the QWP control unit 35 rotates the QWP 33 in the positive direction from the reference position by Δθ, so that θ _{QWPi + 1} = θ _QWPi + Δθ. Since the position of the HWP 32 is not changed, θ _{HWPi + 1} = θ _HWPi . In this state, the QWP control unit 35 _{acquires the} index value F _{i + 1} = (θ _{HWPi + 1} , θ _{QWPi + 1} ) from the polarization state monitor 28. The QWP control unit 35 rotates the QWP 33 in the negative direction from the reference position by Δθ, so that θ _{QWPi + 2} = θ _QWPi −Δθ. Since the position of the HWP 32 is not changed, θ _{HWPi + 2} = θ _HWPi . In this state, the QWP control unit 35 _{acquires the} index value F _{i + 2} = (θ _{HWPi + 2} , θ _{QWPi + 2} ) from the polarization state monitor 28.

The QWP control unit 34 compares the obtained index values F _i , F _{i + 1,} and F _{i + 2} and selects the best value (minimum value) among them. The best value is selected, if an improvement over F _i corresponding to the current value at the reference position, QWP control unit 35 sets the angle of the best value condition new reference position (θ _HWPi, θ _QWPi) to Then, the process returns to the top. Meanwhile, the best value is selected, if not improved over F _i corresponding to the current value at the reference position, the process ends. Also in QWP control process, as long as F _i is improved, the QWP33 both by the positive and negative directions step width Δθ is rotated continuously found the best value.

  After QWP control (step 304), the sequence control unit 37 determines whether or not the index value from the polarization state monitor 28 is within a certain range from the target value (step 305). If it is determined Yes in step 305, the process returns to step 301. On the other hand, when it is determined No in step 305, the sequence control unit 37 determines whether or not the combination of the two processing steps of step 303 and step 305 has been executed K times or more in succession (step 306). If NO is determined in step 306, the process returns to step 303. On the other hand, when it is determined No in step 306, the sequence control unit 37 causes the simultaneous control unit 36 to perform simultaneous control of the HWP 32 and the QWP 33 (step 307).

5 and 6 are flowcharts showing an example of simultaneous control according to the present embodiment. As shown in FIG. 5, the simultaneous control unit 36 rotates the HWP 32 and the QWP 33 to the reference positions (θ _HWPi , θ _QWPi ) when necessary (step 501). The simultaneous control unit 36 acquires the index value F _i = (θ _HWPi , θ _QWPi ) from the polarization state monitor 28 with the HWP 32 at the reference position, and sets the step width Δθ ′ based on the index value. Determination is made (step 502).

Next, the simultaneous control unit 36 rotates both the HWP 32 and the QWP 33 by Δθ ′ in the positive direction from the reference position, so that θ _{HWPi + 1} = θ _HWPi + Δθ ′ and θ _{QWPi + 1} = θ _QWPi + Δθ ′ (step 503). In this state, the simultaneous control unit 36 _{acquires the} index value F _{i + 1} = (θ _{HWPi + 1} , θ _{QWPi + 1} ) from the polarization state monitor 28 (step 504). Simultaneous control unit 36, both HWP32 and QWP33 'is rotated _{by, θ HWPi + 2 = θ HWPi} -Δθ' Δθ in the negative direction from the reference position, and _{θ QWPi + 2 = θ QWPi -Δθ} '( step 505). In this state, the simultaneous control unit 36 _{acquires the} index value F _{i + 2} = (θ _{HWPi + 2} , θ _{QWPi + 2} ) from the polarization state monitor 28 (step 506).

The simultaneous control unit 36 compares the obtained index values F _i , F _{i + 1} and F _{i + 2} (step 507), and selects the best value (minimum value) among them (step 508). The best value is selected, if an improvement over F _i corresponding to the current value at the reference position (Yes in step 509), simultaneous control unit 36, the angle a new reference position of the best value condition (θ _HWPi , Θ _QWPi ) (step 510), and the process ends.

On the other hand, if it is determined No in step 509, the process proceeds to FIG. As shown in FIG. 6, the simultaneous control unit 36 rotates the HWP 32 and the QWP 33 to the reference positions (θ _HWPi , θ _QWPi ) when necessary (Step 601), and again returns the index value F _i from the polarization state monitor 28. = (Θ _HWPi , θ _QWPi ) is acquired, and the step width Δθ ′ is determined based on the index value (step 602).

The simultaneous control unit 36 rotates the HWP 32 by Δθ ′ in the positive direction from the reference position, and rotates the QWP 33 by Δθ ′ in the negative direction from the reference position so that _{θHWPi + 1} = _θHWPi + Δθ ′, _{θQWPi + 1} = _θQWPi −Δθ ′ (step 603). In this state, the simultaneous control unit 36 _{acquires the} index value F _{i + 1} = (θ _{HWPi + 1} , θ _{QWPi + 1} ) from the polarization state monitor 28 (step 604). The simultaneous control unit 36 rotates the HWP 32 by Δθ ′ in the negative direction from the reference position, and rotates the QWP 33 by Δθ ′ in the positive direction from the reference position, so that θ _{HWPi + 2} = θ _HWPi − _Δθ ′, θ _{QWPi + 2} = θ _QWPi + Δθ ′ is set (step 605). In this state, the simultaneous control unit 36 _{acquires the} index value F _{i + 2} = (θ _{HWPi + 2} , θ _{QWPi + 2} ) from the polarization state monitor 28 (step 606).

The simultaneous control unit 36 compares the obtained index values F _i , F _{i + 1} and F _{i + 2} (step 607), and selects the best value (minimum value) among them (step 608). Next, the simultaneous control unit 36 sets the angle of the best value condition to a new reference position (θ _HWPi , θ _QWPi ) (step 609), and ends the process.

FIG. 7 is a diagram illustrating the HWP control process, the QWP control process, and the simultaneous control process with reference to the search space. 7A and 7B, the horizontal axis represents θ _HWP and the vertical axis represents θ _QWP . In the HWP control process, in FIG. 7A, θ _HWP is changed while θ _QWP is fixed, and the index value acquisition position is moved laterally from the reference position 700 (see reference numeral 701). In the QWP control process, in FIG. 7B, θ _QWP is changed while θ _HWP is fixed, and the index value acquisition position is moved vertically from the reference position 700 (see reference numeral 702).

  On the other hand, in the simultaneous control process, the index value acquisition position is moved obliquely from the reference position 700 as shown in FIG. 7B, reference numerals 703 and 704 denote movements in the search space when both HWP 32 and QWP 33 are rotated by Δθ ′ in the positive direction from the reference position (step 503 in FIG. 5), respectively, in the simultaneous control process. In addition, both HWP 32 and QWP 33 show movement in the search space when they are rotated by Δθ ′ in the negative direction from the reference position (step 505 in FIG. 5). Reference numerals 705 and 706 respectively denote movement in the search space when the HWP 32 is rotated by Δθ ′ from the reference position in the positive direction and the QWP 33 from the reference position in the negative direction (step 603 in FIG. 6), and the HWP 32 The movement in the search space when the QWP 33 is rotated by Δθ ′ in the negative direction from the reference position and in the positive direction from the reference position (step 605 in FIG. 6) is shown.

  In the simultaneous control process, by moving the index value acquisition position in the search space in an oblique direction, the state that has fallen into the local solution when moving in the search space in the horizontal or vertical direction The possibility of breaking through can be increased.

  Next, the search space movement step width Δθ in the HWP control process, the QWP control process, and the simultaneous control process will be described. In the present embodiment, the interval between the minimum value and the maximum value of the index value is divided into a plurality of ranges, and a step width is assigned to each range. In the example of FIG. 8A, the maximum value (100 in this example) and the minimum value (0 in this example) are divided equally (linearly), and each index value is in a range. A step width as shown in FIG. 8A is assigned. That is, in the example of FIG. 8A, the step widths 2, 4, 6, 8 and 10 are respectively set for the index value ranges of 0 to 20, 20 to 40, 40 to 60, 60 to 80 and 80 to 100. Assigned.

  The object of the present embodiment is to find a position in the search space where the intensity of the beat signal is the minimum value. Therefore, as the index value becomes smaller, it can be predicted that the position in the search space showing the minimum value is approaching, so the step width is made smaller.

  In the example of FIG. 8B, the minimum value and the maximum value are non-linearly divided, and step widths as shown in FIG. 8B are assigned to the respective index value ranges. In the example of FIG. 8B, step widths 2, 4, 6, 8, and 10 are assigned to the index value ranges of 0 to 5, 5 to 15, 15 to 35, 35 to 65, and 65 to 100, respectively. ing. In the example shown in FIG. 8B, when the index value becomes smaller than in the example shown in FIG. 8A, the degree of decreasing the step width is increased.

  In actual processing, a table storing index value ranges and corresponding step widths is provided in a memory (not shown), and the HWP control unit 34, the QWP control unit 35, and the simultaneous control unit 36 are respectively provided with HWP, What is necessary is just to acquire the step width matched with the range containing an index value with reference to a table, when rotating QWP.

  In the present embodiment, the same table is used for the step width Δθ in HWP control and the step width Δθ in QWP control. Therefore, when the index values are the same, the step width Δθ matches. On the other hand, with respect to Δθ ′ in the simultaneous control process, it is desirable that values that satisfy Δθ ≦ Δθ ′ are stored in the table for the same index value. This is because the simultaneous control process aims to break out from a state that has fallen into a local solution, and thus it is desirable to increase the movement in the search space.

  In the present embodiment, since the randomness as in Non-Patent Document 4 is not used, it is possible to definitely avoid the local solution, and even when the state is close to the target value, a step with a large value is possible. The width is not selected, and the time to reach the target value can be shortened.

  In the present embodiment, the step width is determined based on the index value, and the index value is brought close to the target value. In general, the maximum gradient method is known as one of the methods applying the hill-climbing method. In the maximum gradient method, the step width is determined according to the current gradient. In order to use the maximum gradient method in the polarization control device, it is necessary to change the angle of the wave plate to obtain index values of the current position and its surrounding positions. However, since the polarization state fluctuates in a short time, it is difficult to observe the gradient by obtaining a plurality of index values. On the other hand, in the present invention, since the step width can be determined based on the index value, processing in a short time is possible.

  Next, a second embodiment of the present invention will be described. In the first embodiment, the polarization control device according to the present invention is applied to the polarization separation device on the receiving side in the optical communication system employing the polarization multiplexing transmission method. In the second embodiment, the polarization control device according to the present invention is applied to the PMD compensation device provided on the reception side for PMD (polarization mode dispersion) compensation of the transmission line. In the second embodiment, the polarization control device separates an optical signal affected by PMD (polarization mode dispersion) into two optical signals when performing PMD (polarization mode dispersion) compensation. In addition, the optimum polarization state is controlled.

  FIG. 9 is a block diagram showing an apparatus on the receiving side according to the second embodiment. The optical signal transmitted through the transmission line via the repeater 16 is incident on the PMD compensation device 120. PMD (Polarization Mode Dispersion) is a phenomenon in which a propagation time difference occurs between two orthogonal polarization mode components. Since transmission characteristics deteriorate as transmission speed increases, PMD compensation device 120 is required. The PMD compensation device 120 is provided to compensate for the characteristic deterioration due to the propagation time difference. The output of the PMD compensation device 120 is received by the receiver 26.

  As shown in FIG. 9, the PMD compensation device 120 according to the present embodiment includes a polarization control device 22, a polarization beam splitter (PBS) 122, a DGD (differential group delay) device 126, a polarization beam. It has a splitter (PBS) 124. Further, the output of the PMD compensation device 120 is received by the receiver 26 and also observed by the polarization state monitor 28.

  In the second embodiment, the optical signal received by the PMD compensator 120 is incident on the polarization controller 22, and the optimum polarization state is set so that the X polarization and the Y polarization can be appropriately separated in the PBS 122. It is controlled to become. The output of the polarization controller 22 is given to the PBS 122 and separated into an X polarization and a Y polarization. One of these polarizations is input to the DGD device 126 and given a certain delay, and adjusted so that the propagation speeds of the X polarization and the Y polarization are equal. One polarization from the polarization beam splitter 122 and the polarization delayed in the DGD device 126 are combined in the PBS 124, and the combined optical signal is supplied to the receiver 26.

  In the second embodiment, DOP (Degree of Polarization) is used as an index value, and the polarization state monitor 28 calculates DOP and outputs it to the polarization controller 22. Therefore, in the second embodiment, processing for optimizing DOP, that is, setting DOP to the maximum value is executed. Of course, the polarization state monitor 28 monitors the state of the signal after polarization control, obtains an index value based on the signal state, calculates another index value, and outputs it to the polarization controller 22. You may do it.

  For example, the signal strength of the clock signal may be used as the index value. In the case of a 40 Gbit / S optical signal, a signal of 40 GHz component can be extracted by passing it through a bandpass filter having the same frequency as the modulation speed after being converted into an electric signal in the optical receiver. This signal is called a clock signal. An index value indicating the signal strength can be obtained from the voltage or power of the extracted clock signal. Alternatively, the Q value or the opening degree of the eye waveform may be used as the index value.

The polarization control device 22 according to the second embodiment operates in substantially the same manner as that of the first embodiment. That is, the following logic is employed to search for the optimum value (maximum value) of DOP.
(1) Normally, the HWP 32 is rotated to search for the optimum value, and then the QWP 33 is rotated to search for the optimum value. Of course, the order may be reversed (firstly, the QWP 33 is rotated to search for the optimum value).
(2) If it is determined that the DOP has fallen into a local solution, an attempt is made to avoid the local solution by rotating the HWP 32 and the QWP 33 simultaneously to expand the search space.

  The processing executed by the polarization control device according to the second embodiment is also shown in FIGS. Therefore, further detailed description is omitted.

  As described above, by applying the polarization control device 22 according to the present invention to the PMD compensation device 120, it is possible to output an optical signal in a polarization state that can be appropriately separated to the beam splitter 122. .

  The present invention is not limited to the above embodiments, and various modifications can be made within the scope of the invention described in the claims, and these are also included in the scope of the present invention. Needless to say.

  For example, in the above embodiment, the wave plate is controlled in the order of HWP control (step 303 in FIG. 3) and QWP control (step 304 in FIG. 3). However, the present invention is not limited to this. The wave plate may be controlled in the order of control and HWP control.

  In the above embodiment, when the HWP control and the QWP control are performed and it is determined that the index value falls into the local solution, the simultaneous control of the HWP 32 and the QWP 33 is performed. However, the processing order is not limited to the above. FIG. 10 is a flowchart illustrating an example of the optimum value search process executed in the polarization control device according to another embodiment. As shown in FIG. 10, the sequence control unit 37 of the polarization controller 22 refers to the DOP from the polarization state monitor 28 (step 1001), and the DOP as the index value is within a certain range from the target value. Is determined (step 1002). When it is determined No in step 1002, the sequence control unit 37 activates the simultaneous control unit 36 to perform simultaneous control (step 1003). The simultaneous control is the same as that described with reference to FIGS.

  Thereafter, the sequence control unit 37 determines whether or not the index value from the polarization state monitor 28 is within a certain range from the target value (step 1004). If it is determined YES in step 1004, the process returns to step 1001. On the other hand, when it is determined No in step 1004, the sequence control unit 37 determines whether step 1003 is continued and executed K times or more (step 1005). If NO in step 1005, the process returns to step 1003.

  On the other hand, when it is determined Yes in step 1005, the sequence control unit 37 activates the HWP control unit 34 to execute the HWP control process (step 1006). The HWP control process is the same as that described with reference to FIG.

  Next, the sequence control unit 37 determines whether or not the index value is within a certain range from the target value by the HWP control process (step 1007). If it is determined YES in step 1007, the process returns to step 1001. On the other hand, if it is determined No in step 1007, the QWP control unit 35 is activated to execute the QWP control process (step 1008). The QWP control process is the same as that in the first embodiment. After the QWP control process, the process returns to step 1001.

  In the other embodiments, the step width Δθ in the HWP control process and the QWP control process and the step width Δθ ′ in the simultaneous control process are such that Δθ ≧ Δθ ′ with respect to the same index value. Is preferably stored in a table. In other words, in another embodiment, the HWP control process and the QWP control process are executed to overcome a state in which a local solution has fallen. Therefore, it is desirable that the step width Δθ be larger in the search space.

  Also in the other embodiments described above, the order of the HWP control process and the QWP control process may be reversed.

  In the first and second embodiments, the index value does not fall within a certain range from the target value even though the HWP control process and the QWP control process that are initially executed are executed K times. In some cases, simultaneous control is being performed. In another embodiment, the HWP control is executed when the index value does not fall within a certain range from the target value even though the simultaneous control to be executed initially is executed K times. . The number of executions of the K times can be set to a desired numerical value.

  In the embodiment, the step width Δθ in the HWP control and the step width Δθ in the QWP control are determined to be the same value if the index values are the same. However, the present invention is not limited to this, and a configuration in which different step widths can be obtained even if the index values are the same may be employed.

Furthermore, in the above embodiment, in the simultaneous control, the step width for changing the angle of HWP 32 and the step width for changing the angle of QWP 33 are determined to be the same value Δθ ′ if the index values are the same. The Thereby, in the search space, the position where the index value is acquired moves in the directions of 45 degrees, 135 degrees, 225 degrees, and 315 degrees from the search position. However, the step width is not limited to this, and the step width Δθ _{HWP of the HWP} 32 and the step width Δθ _QWP of the _QWP 33 may be set independently.

  Furthermore, in the first embodiment, in order to find the position in the search space where the index value becomes the minimum value, as the index value becomes smaller, the position approaches the position in the search space showing the minimum value. Therefore, the step width Δθ is reduced. Further, in the second embodiment, in order to find the position in the search space where the index value becomes the maximum value, as the index value increases, the position approaches the position in the search space indicating the maximum value. Therefore, the step width Δθ is reduced. FIG. 11A shows an example in which the minimum value and the maximum value are divided equally (linearly) in the second embodiment, and step widths are assigned to the respective ranges. FIG. Is an example in which the minimum value and the maximum value are non-linearly divided and step widths are assigned to the respective ranges. Also in these examples, a table storing index value ranges and corresponding step widths is provided in a memory (not shown), and the HWP control unit 34, the QWP control unit 35, and the simultaneous control unit 36 are each provided with an HWP 32. When the QWP 33 is rotated, the step width associated with the range including the index value may be acquired by referring to the table.

  However, the present invention is not limited to the configuration described above. For example, as the difference value between the target value and the search value becomes smaller, it can be predicted that the position in the search space showing the maximum value is approaching, so the step width Δθ may be made smaller. . In this case, a table in which the minimum value and the maximum value of the difference value are linearly divided and a step width is assigned to each difference value range, or between the minimum value and the maximum value of the difference value, A table may be provided in a memory (not shown) in which a space is divided nonlinearly and a step width is assigned to each difference value range, and the table is referred to. Note that the difference value table has substantially the same structure as that shown in FIGS. 8A and 8B.

  Furthermore, instead of obtaining the step width from the difference between the index value and the target value, the step width may be determined from the difference value between the index values. Here, as the absolute value of the difference value between the index value acquired last time and the index value acquired this time becomes smaller, the step width corresponding to the range of the absolute value of the difference value so that the step width becomes smaller Are stored in a memory (not shown), and when the HWP control unit 34, the QWP control unit 35, and the simultaneous control unit 36 rotate the HWP 32 and the QWP 33, respectively, The absolute value of the difference value from the index value acquired this time is calculated, and the step width corresponding to the absolute value may be acquired by referring to the table.

FIG. 1 is a block diagram schematically showing the configuration of the optical communication system according to the first embodiment. FIG. 2 is a block diagram showing the configuration of the polarization control device according to this embodiment. FIG. 3 is a flowchart illustrating an example of the optimum value search process executed in the polarization control device according to the present embodiment. FIG. 4 is a flowchart showing an example of the HWP control process according to the present embodiment. FIG. 5 is a flowchart illustrating an example of simultaneous control according to the present embodiment. FIG. 6 is a flowchart illustrating an example of simultaneous control according to the present embodiment. FIG. 7 is a diagram illustrating the HWP control process, the QWP control process, and the simultaneous control process with reference to the search space. FIG. 8 is a diagram for explaining an example of the step width in the present embodiment. FIG. 9 is a block diagram showing an apparatus on the receiving side according to the second embodiment. FIG. 10 is a flowchart illustrating an example of the optimum value search process executed in the polarization control device according to another embodiment. FIG. 11 is a diagram illustrating another example of the step width in the present embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Transmitter 12 Polarization multiplexing apparatus 14 Transmission path 16 Repeater 20 Polarization separation apparatus 22 Polarization control apparatus 24 Polarization beam splitter 26 Receiver 28 Polarization state monitor 32 λ / 2 plate 33 λ / 4 plate 34 HWP control Unit 35 QWP control unit 36 simultaneous control unit 37 sequence control unit 120 PMD compensator 122, 124 polarization beam splitter 126 DGD device

Claims (14)

A λ / 2 plate disposed on the optical axis of the optical signal and rotatable about the optical axis;
A λ / 4 plate disposed on the same optical axis as the λ / 2 plate and rotatable about the optical axis;
Λ / 2 plate control means for rotating the λ / 2 plate with a first step width;
Λ / 4 plate control means for rotating the λ / 4 plate with a second step width;
Wavelength plate simultaneous control means for simultaneously rotating the λ / 2 plate and the λ / 4 plate with a third step width and a fourth step width, respectively;
Based on the index value based on the signal state of the optical signal polarization-controlled by the polarization control device and the target value acquired in advance and stored in the storage device, the λ / 2 plate control means, λ / 4 plate Control means, and processing means for operating the wavelength plate simultaneous control means,
The index value is associated with the first step width and the second step width so that the first step width and the second step width become smaller as the index value approaches the target value. Storage means for storing the table,
With
The processing means refers to the table stored in the storage means, obtains the first step width and the second step width associated with the index value, and the λ / 2 plate control means And at least one of the λ / 4 plate control means is operated a predetermined number of times, and based on the acquired index value and the target value, it is determined that the acquired index value falls into a local solution, the wavelength A polarization controller characterized by operating a plate simultaneous control means.   A λ / 2 plate disposed on the optical axis of the optical signal and rotatable about the optical axis;
  A λ / 4 plate disposed on the same optical axis as the λ / 2 plate and rotatable about the optical axis;
  Λ / 2 plate control means for rotating the λ / 2 plate with a first step width;
  Λ / 4 plate control means for rotating the λ / 4 plate with a second step width;
  Wavelength plate simultaneous control means for simultaneously rotating the λ / 2 plate and the λ / 4 plate with a third step width and a fourth step width, respectively;
  Based on the index value based on the signal state of the optical signal polarization-controlled by the polarization control device and the target value acquired in advance and stored in the storage device, the λ / 2 plate control means, λ / 4 plate Control means, and processing means for operating the wavelength plate simultaneous control means,
  As the absolute value of the difference value between the index value acquired last time and the index value acquired this time becomes smaller, the first step width and the second step width become smaller, Storage means for storing a table in which one step width and a second step width are associated with each other;
  With
  The processing means refers to the table stored in the storage means, obtains the first step width and the second step width associated with the difference value, and the λ / 2 plate control means And at least one of the λ / 4 plate control means is operated a predetermined number of times, and based on the acquired index value and the target value, it is determined that the acquired index value falls into a local solution, the wavelength A polarization controller characterized by operating a plate simultaneous control means.   The processing means sequentially operates the λ / 2 plate control means and the λ / 4 plate control means, or sequentially operates the λ / 4 plate control means and the λ / 2 plate control means. Item 3. The polarization control device according to Item 1 or 2.   4. The deviation according to claim 1, wherein the third step width and the fourth step width are equal to or larger than the first step width and the second step width. 5. Wave control device. A λ / 2 plate disposed on the optical axis of the optical signal and rotatable about the optical axis;
A λ / 4 plate disposed on the same optical axis as the λ / 2 plate and rotatable about the optical axis;
Λ / 2 plate control means for rotating the λ / 2 plate with a first step width;
Λ / 4 plate control means for rotating the λ / 4 plate with a second step width;
Wavelength plate simultaneous control means for simultaneously rotating the λ / 2 plate and the λ / 4 plate with a third step width and a fourth step width, respectively;
Based on the index value based on the signal state of the optical signal polarization-controlled by the polarization control device and the target value acquired in advance and stored in the storage device, the λ / 2 plate control means, λ / 4 plate Control means, and processing means for operating the wavelength plate simultaneous control means,
The index value is associated with the first step width and the second step width so that the first step width and the second step width become smaller as the index value approaches the target value. Storage means for storing the table,
With
The processing means refers to the table stored in the storage means to obtain the first step width and the second step width associated with the index value, and the wavelength plate simultaneous control means The λ / 2 plate control means and the λ / 4 plate control when it is determined that the acquired index value falls into a local solution based on the acquired index value and the target value. A polarization controller characterized by operating at least one of the means.   A λ / 2 plate disposed on the optical axis of the optical signal and rotatable about the optical axis;
  A λ / 4 plate disposed on the same optical axis as the λ / 2 plate and rotatable about the optical axis;
  Λ / 2 plate control means for rotating the λ / 2 plate with a first step width;
  Λ / 4 plate control means for rotating the λ / 4 plate with a second step width;
  Wavelength plate simultaneous control means for simultaneously rotating the λ / 2 plate and the λ / 4 plate with a third step width and a fourth step width, respectively;
  Based on the index value based on the signal state of the optical signal polarization-controlled by the polarization control device and the target value acquired in advance and stored in the storage device, the λ / 2 plate control means, λ / 4 plate Control means, and processing means for operating the wavelength plate simultaneous control means,
  As the absolute value of the difference value between the index value acquired last time and the index value acquired this time becomes smaller, the index value and the index value so that the first step width and the second step width become smaller Storage means for storing a table in which the first step width and the second step width are associated with each other;
  With
  The processing means refers to the table stored in the storage means, the wavelength plate simultaneous control means for obtaining the first step width and the second step width associated with the difference value, The λ / 2 plate control means and the λ / 4 plate control means when operated a predetermined number of times and based on the acquired index value and the target value, when it is determined that the acquired index value falls into a local solution A polarization control device characterized in that at least one of them is operated.   The polarization control apparatus according to claim 5 or 6, wherein the first step width and the second step width are equal to or larger than the third step width and the fourth step width.   The processing means determines that a local solution has fallen when the difference between the predetermined index value and the target value is outside a predetermined range even after the predetermined number of operations. The polarization control device according to any one of 1 to 7. A λ / 2 plate disposed on the optical axis of the optical signal and rotatable about the optical axis;
A λ / 4 plate disposed on the same optical axis as the λ / 2 plate and rotatable about the optical axis;
Λ / 2 plate control means for rotating the λ / 2 plate with a first step width;
Λ / 4 plate control means for rotating the λ / 4 plate with a second step width;
Wavelength plate simultaneous control means for simultaneously rotating the λ / 2 plate and the λ / 4 plate with a third step width and a fourth step width, respectively;
Based on the index value based on the signal state of the optical signal polarization-controlled by the polarization control device and the target value acquired in advance and stored in the storage device, the λ / 2 plate control means, λ / 4 plate In the polarization control device comprising the control means and the processing means for operating the wavelength plate simultaneous control means,
In the processing means,
The index value is associated with the first step width and the second step width so that the first step width and the second step width become smaller as the index value approaches the target value. Obtaining the first step width and the second step width associated with the index value by referring to the table from the storage means storing the table;
Operating at least one of the λ / 2 plate control means and the λ / 4 plate control means a predetermined number of times;
Determining whether the acquired index value falls into a local solution based on the acquired index value and the target value;
And a step of operating the wavelength plate simultaneous control means when it is determined that the local solution has fallen.   A λ / 2 plate disposed on the optical axis of the optical signal and rotatable about the optical axis;
  A λ / 4 plate disposed on the same optical axis as the λ / 2 plate and rotatable about the optical axis;
  Λ / 2 plate control means for rotating the λ / 2 plate with a first step width;
  Λ / 4 plate control means for rotating the λ / 4 plate with a second step width;
  Wavelength plate simultaneous control means for simultaneously rotating the λ / 2 plate and the λ / 4 plate with a third step width and a fourth step width, respectively;
  Based on the index value based on the signal state of the optical signal polarization-controlled by the polarization control device and the target value acquired in advance and stored in the storage device, the λ / 2 plate control means, λ / 4 plate In the polarization control device comprising the control means and the processing means for operating the wavelength plate simultaneous control means,
  In the processing means,
  As the absolute value of the difference value between the index value acquired last time and the index value acquired this time becomes smaller, the index value and the index value so that the first step width and the second step width become smaller The first step width and the second step width associated with the difference value with reference to the table from the storage means for storing a table in which the first step width and the second step width are associated with each other Step to get the
  Operating at least one of the λ / 2 plate control means and the λ / 4 plate control means a predetermined number of times;
  Determining whether the acquired index value falls into a local solution based on the acquired index value and the target value;
  And a step of operating the wavelength plate simultaneous control means when it is determined that the local solution has fallen. In the step of operating the predetermined number of times,
The step of sequentially operating the λ / 2 plate control means and the λ / 4 plate control means or the step of sequentially operating the λ / 4 plate control means and the λ / 2 plate control means is performed the predetermined number of times. The polarization control method according to claim 9 or 10 . A λ / 2 plate disposed on the optical axis of the optical signal and rotatable about the optical axis;
A λ / 4 plate disposed on the same optical axis as the λ / 2 plate and rotatable about the optical axis;
Λ / 2 plate control means for rotating the λ / 2 plate with a first step width;
Λ / 4 plate control means for rotating the λ / 4 plate with a second step width;
Wavelength plate simultaneous control means for simultaneously rotating the λ / 2 plate and the λ / 4 plate with a third step width and a fourth step width, respectively;
Based on the index value based on the signal state of the optical signal polarization-controlled by the polarization control device and the target value acquired in advance and stored in the storage device, the λ / 2 plate control means, λ / 4 plate In the polarization control device comprising the control means and the processing means for operating the wavelength plate simultaneous control means,
In the processing means,
The index value is associated with the first step width and the second step width so that the first step width and the second step width become smaller as the index value approaches the target value. Obtaining the first step width and the second step width associated with the index value by referring to the table from the storage means storing the table;
Operating the wavelength plate simultaneous control means a predetermined number of times;
Determining whether the acquired index value falls into a local solution based on the acquired index value and the target value;
And a step of operating at least one of the λ / 2 plate control means and the λ / 4 plate control means when it is determined that the local solution has fallen.   A λ / 2 plate disposed on the optical axis of the optical signal and rotatable about the optical axis;
  A λ / 4 plate disposed on the same optical axis as the λ / 2 plate and rotatable about the optical axis;
  Λ / 2 plate control means for rotating the λ / 2 plate with a first step width;
  Λ / 4 plate control means for rotating the λ / 4 plate with a second step width;
  Wavelength plate simultaneous control means for simultaneously rotating the λ / 2 plate and the λ / 4 plate with a third step width and a fourth step width, respectively;
  Based on the index value based on the signal state of the optical signal polarization-controlled by the polarization control device and the target value acquired in advance and stored in the storage device, the λ / 2 plate control means, λ / 4 plate In the polarization control device comprising the control means and the processing means for operating the wavelength plate simultaneous control means,
  In the processing means,
  As the absolute value of the difference value between the index value acquired last time and the index value acquired this time becomes smaller, the index value and the index value so that the first step width and the second step width become smaller The first step width and the second step width associated with the difference value with reference to the table from the storage means for storing a table in which the first step width and the second step width are associated with each other Step to get the
  Operating the wavelength plate simultaneous control means a predetermined number of times;
  Determining whether the acquired index value falls into a local solution based on the acquired index value and the target value;
  And a step of operating at least one of the λ / 2 plate control means and the λ / 4 plate control means when it is determined that the local solution has fallen. The step of determining whether or not it falls into the local solution,
10. The method according to claim 9, further comprising a step of determining that a local solution has fallen when the difference between the predetermined index value and the target value is outside a predetermined range even after the predetermined number of operations. The polarization control method according to claim 13 .

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