JP2002016548A - System for compensating polarized wave mode dispersion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a system for compensating a polarized wave mode that can surely and with high accuracy prevent deterioration in the signal quality due to polarized mode dispersion. SOLUTION: The system is provided with a polarized wave controller 1 that applies polarized plane control to a received optical signal, a polarized wave separator 2 that splits the optical signal into two orthogonally polarized wave components, a variable optical delay device 5 that compensates a delay time difference between the two orthogonally polarized wave components, a polarized wave multiplexer 6 that applies polarized wave multiplexing to optical signals of a 1st optical path 3 and a 2nd optical path 4, an optical signal monitor 7a, that detects the optical signal with one orthogonally polarized wave component, an optical signal monitor 7b, that detects the optical signal resulting from the polarized wave synthesis, a polarized wave controller control circuit 8 that controls the polarized wave controller 1, on the basis of the optical signal detected by the optical signal monitor 7a, and a variable optical delay unit control circuit 9 that controls the variable optical delay device 5, on the basis of the optical signal detected by the optical signal monitor 7b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system, and more particularly to a polarization mode dispersion compensator capable of improving high-speed optical signal transmission characteristics on an optical transmission line having polarization-dependent transmission characteristics.

[0002]

2. Description of the Related Art A single mode optical fiber has a pair of orthogonal polarization states. This orthogonal polarization state is referred to as PSP (Prin
Called ciple States of Polarization). Generally, an arbitrary polarization state (SOP: States Of Porarization) can be described by a combination of these two orthogonal polarization states. Transmission lines and optical components such as optical fibers are dependent on the polarization dependence of propagation delay time due to the asymmetry of the core and optical components (Polarization Mode Disp.
ersion). Therefore, the optical signal that has undergone PMD is the sum of orthogonally polarized signals that have undergone different time delays. Since the PMD causes waveform distortion, it is known that it becomes a main signal deterioration factor particularly in an ultra-high-speed optical transmission system.

If the interaction between the higher-order PMD and other deteriorating factors can be neglected, each orthogonal polarization signal is not subjected to waveform distortion, so that an appropriate delay is applied to the two orthogonal polarization signals. By multiplexing after giving, it is possible to recover waveform deterioration caused by PMD.

FIG. 9 is a diagram for explaining the concept of waveform deterioration due to PMD and PMD compensation. In FIG. 9, in the optical signal output from the optical transmitter, the p-polarized signal and the s-polarized signal are temporally overlapped as shown in FIG. 9A. Here, when an optical signal propagates through a transmission line having PMD, FIG.
As shown in (b), the p-polarized signal and the s-polarized signal are:
It becomes a waveform that is added with a delay of time τ. In order to compensate the added waveform, that is, the waveform having deteriorated waveform, to the original waveform, as shown in FIG. What is necessary is just to give a delay time difference of time “−τ”. However, since the polarization state (SOP) of light propagating through the optical fiber transmission line continues to change due to environmental temperature and stress, it is necessary to dynamically compensate for PMD.

Here, a description will be given of a conventional PMD compensator for providing the above-described delay time difference. FIG. 10 is a block diagram showing a schematic configuration of a conventional polarization mode dispersion (PMD) compensator. The polarization mode dispersion compensator shown in FIG. 10 is described in, for example, the document “Limitation of Optical Fi
rst-Order PMD Compensation ", H. Bulow, WE1-
2, OFC99. In FIG. 10, the polarization mode dispersion compensator first includes a polarization controller 101.
Controls the polarization state of the input optical signal, and is separated by the polarization splitter 102 into two orthogonal polarization components.
After one of the two polarization components is given a time delay of the above-described time delay difference by the variable optical delay unit 105, the two orthogonal polarization components are converted by the polarization combiner 106. Are multiplexed. This means that the PMD compensation by providing the delay time difference shown in FIG. 9 has been performed.

On the other hand, the variable optical delay device 105 shown in FIG.
, And by giving a fixed delay time difference,
Some perform D compensation. FIG. 11 is a block diagram showing a configuration of another conventional polarization mode dispersion compensator. The polarization mode dispersion compensator shown in FIG.
Document `` Automatic polarization-mode Dispersion compe
nsation in 40Gbit / s transmission ", Hiroki Ooi et al.
WE5-2, OFC99. In FIG. 11, the polarization mode dispersion compensator includes a polarization controller 201.
Controls the polarization state of the input optical signal. The polarization maintaining fiber 224 acts as a birefringent medium having a large PMD. Therefore, it has essentially the same function as that in which the fixed delay time difference provided by the variable optical delay device 105 shown in FIG. 10 is fixed.

Assuming that the PMD amount of the polarization maintaining fiber 224 is T [ps], a delay amount of either T [ps] or -T [ps] can be generated by setting the polarization controller 201. Therefore, the PM of -2T to 2T [ps]
D can be compensated from -T to T [ps]. In FIG. 11, the polarization maintaining fiber 224
A part of the optical signal output from is extracted and detected by the optical signal monitor 207, and the polarization controller control circuit 208
The PMD is controlled by controlling the polarization controller 201 so that the clock signal component of the detected optical signal is maximized.
I am trying to compensate.

[0008]

However, FIG.
In the conventional polarization mode dispersion compensator shown in (1), since both the polarization controller 101 and the variable optical delay device 105 are controlled, if either one deviates from the optimum value, PMD cannot be compensated and However, the control directions of both the polarization controller 101 and the variable optical delay unit 105 cannot be estimated based on the waveform of the output optical signal, and good PMD compensation cannot be performed. Was.

On the other hand, the conventional polarization mode dispersion compensator shown in FIG. 11 has a function of fixing the delay amount by the variable optical delay unit 105, so that the waveform deterioration due to PMD cannot be completely suppressed. There was a problem.

[0010] The present invention has been made in view of the above,
An object of the present invention is to provide a polarization mode dispersion compensator capable of accurately and reliably deteriorating signal quality due to polarization mode dispersion.

[0011]

To achieve the above object, a polarization mode dispersion compensator according to the present invention comprises: a polarization control means for controlling a polarization plane of an input optical signal; and the polarization control means. The optical signal based on the polarization control by
A polarization separation unit that separates into two orthogonal polarization components, and two orthogonal polarization components separated by the polarization separation unit are input, one of the orthogonal polarization components is delayed, and A delay compensating means for compensating the delay time difference and outputting the compensated two orthogonal polarization components; and a quadrature polarization component of one of the two orthogonal polarization components input to the delay compensating means. First optical signal detecting means for detecting an optical signal, second optical signal detecting means for detecting an optical signal output from the delay compensating means, and an optical signal detected by the first optical signal detecting means. First control means for controlling the polarization control means based on the first optical signal, and second control means for controlling the delay compensation means based on the optical signal detected by the second optical signal detection means. It is characterized by having.

According to the present invention, the first and second control means are respectively controlled based on the optical signals detected by the two independent first optical signal detection means and the second optical signal detection means. They are controlled independently.

[0013] A polarization mode dispersion compensator according to the next invention comprises a polarization control means for controlling the polarization plane of an input optical signal, and the optical signal based on the polarization control by the polarization control means. A polarization separation unit that separates into two orthogonal polarization components, and two orthogonal polarization components separated by the polarization separation unit are input, and one of the orthogonal polarization components is delayed so that each of the orthogonal polarization components is delayed. Is compensated for, and the compensated 2
Delay compensating means for outputting two orthogonal polarization components, and a first orthogonal polarization component of one of the two orthogonal polarization components.
A light extraction means for extracting a part of the optical signal of the optical signal and the second optical signal comprising the two orthogonal polarization components output from the delay compensating means, and the light extraction means extracted by the light extraction means. An optical switch that switches and outputs a first optical signal and the second optical signal in a time-division manner, a first controller that controls the polarization controller based on the first optical signal, A second control unit for controlling the delay compensation unit based on a second optical signal, and detecting the first optical signal and the second optical signal output from the optical switch;
Optical signal detecting means for outputting detection results to the corresponding first and second control means, and switching control means for performing time-division switching control of the optical switch and the optical signal detecting means. It is characterized by having.

According to the present invention, the plurality of first and second control means can be independently controlled by only one optical signal detecting means for detecting the optical signal time-divisionally switched and output by the optical switch. .

[0015] The polarization mode dispersion compensator according to the next invention is the above-mentioned invention, wherein the optical attenuator for attenuating an optical signal of one of the two orthogonal polarization components is provided; Third optical signal detecting means for detecting the optical signal output by the compensating means, and third control for controlling the amount of attenuation of the optical attenuating means based on the optical signal detected by the third optical signal detecting means. Means are further provided.

According to this invention, the third optical signal detecting means detects the optical signal, and the third control means controls the attenuation of the optical attenuating means based on the detection result of the third optical signal detecting means. To control.

According to another aspect of the present invention, there is provided a polarization mode dispersion compensator for controlling a polarization plane of an input optical signal, and converting the optical signal based on the polarization plane control by the polarization control section. A polarization separation unit that separates into two orthogonal polarization components, and two orthogonal polarization components separated by the polarization separation unit are input, and one of the orthogonal polarization components is delayed so that each of the orthogonal polarization components is delayed. Is compensated for, and the compensated 2
Delay compensating means for outputting two orthogonal polarization components, optical attenuating means for attenuating an optical signal of one orthogonal polarization component of the two orthogonal polarization components, and Light extraction means for extracting a part of each of an optical signal of one orthogonal polarization component and a second optical signal of the two orthogonal polarization components output from the delay compensation means; And outputting the first optical signal and the second optical signal extracted by the light extracting means as one first optical signal and two second optical signals in a time-division manner. An optical switch; first control means for controlling the polarization control means based on the first optical signal; and second control means for controlling the delay compensation means based on one of the second optical signals. And the light attenuating means based on the other second optical signal. Third control means for controlling an amount, detecting one of the first optical signals and two of the second optical signals output from the optical switch, and comparing the detection results with the corresponding first to fourth signals. 3. An optical signal detecting means for outputting to the control means of No. 3, and a switching control means for performing time-division switching control of the optical switch and the optical signal detecting means.

According to the present invention, the plurality of first to third control means can be independently controlled by only one optical signal detecting means for detecting the optical signal time-divisionally switched and output by the optical switch. .

In the polarization mode dispersion compensator according to the next invention, in the above invention, the delay compensation means is a variable birefringence medium.

According to the present invention, it is possible to realize a device that performs delay control using a variable birefringence medium, is resistant to mechanical vibration, and promotes reduction in size and weight.

[0021] In the polarization mode dispersion compensating apparatus according to the next invention, in the above invention, the first to third optical signal detecting means or the optical signal detecting means converts an input optical signal into an electric signal. Converting means, a first signal level detecting means for detecting a signal level of the electric signal converted by the converting means, a clock signal extracting means for extracting a clock signal from the electric signal converted by the converting means, And a second signal level detecting means for detecting a signal level extracted by the clock signal extracting means.

According to the invention, in the first to third optical signal detecting means or the optical signal detecting means, the converting means comprises:
The input optical signal is converted into an electric signal, the first signal level converting means detects the signal level of the electric signal converted by the converting means, and the clock signal extracting means is converted by the converting means. A clock signal is extracted from the electric signal, and the second signal level detection means detects the signal level extracted by the clock signal extraction means, and monitors the clock extraction efficiency.

In the polarization mode dispersion compensating apparatus according to the next invention, in the above invention, the first to third optical signal detecting means or the optical signal detecting means measures the degree of polarization of the input optical signal. It is characterized by doing.

According to the present invention, the first to third optical signal detecting means or the optical signal detecting means measures the degree of polarization of the input optical signal.

In the polarization mode dispersion compensator according to the next invention, in the above invention, the first to third control means perform synchronous detection using a dither signal.

According to the present invention, the first to third control means perform synchronous detection using a dither signal.

[0027] In the polarization mode dispersion compensating apparatus according to the next invention, the dither signal is 500
It is a different signal of less than kHz.

According to the present invention, the dither signals in the first to third control means are different signals of 500 kHz or less.

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the polarization mode dispersion compensator according to the present invention will be described below in detail with reference to the accompanying drawings.

Embodiment 1 FIG. 1 is a block diagram showing a configuration of a polarization mode dispersion compensator according to Embodiment 1 of the present invention. In FIG. 1, an optical signal that has undergone PMD by a transmission line is input to a polarization controller 1. The polarization controller 1 controls a polarization state of an input optical signal. The optical signal whose polarization state is controlled is separated by the polarization splitter 2 into two polarization components having independent orthogonal polarization states. An optical signal having one of the two polarization states is output to the first optical path 3, and an optical signal having the other polarization state is output to the second optical path 4.

A part of the optical signal input to the first optical path 3 is monitored by an optical signal monitor 7a via a beam splitter. The polarization controller control circuit 8 controls the polarization controller 1 based on the signal output from the optical signal monitor 7a.

On the other hand, the optical signal input to the second optical path 4 is
The variable optical delay 5 is input to the variable optical delay 5, and the variable optical delay 5 gives a delay amount to the optical signal under the control of the variable optical delay control circuit 9, and outputs the signal to the polarization combiner 6.

The polarization combiner 6 combines the optical signals input from the first optical path 3 and the second optical path 4 and outputs them. A part of the optical signal that is polarization-combined by the polarization combiner 6 is monitored by an optical signal monitor 7b via a beam splitter. Then, as described above, the variable optical delay device control circuit 9 controls the amount of delay by the variable optical delay device 5 based on the signal output from the optical signal monitor 7b.

Here, the polarization controller 1 and the variable optical delay unit 5 are controlled based on the monitoring results by the independent optical signal monitors 7a and 7b. For this reason,
Even if the waveform is degraded by the PMD, the waveform of each independent orthogonal polarization state (PSP) is guaranteed by the fact that it is not distorted.

The polarization controller 1 may control the polarization state so that the polarization separator 2 separates the polarization state into orthogonal polarization states. Here, the optical signal monitor 7a operates such that an optical signal in a predetermined polarization state is input to the first optical path 3, or
A signal for controlling the polarization controller 1 so as to minimize distortion of the optical signal input to the optical path 3 is output to the polarization controller control circuit 8.

On the other hand, the variable optical delay unit 5 needs to generate a delay of a sign opposite to that of the PMD that the optical signal input to the polarization mode dispersion compensator suffers. This variable optical delay device 5
The optical signal monitor 7b detects the distortion of the optical signal output from the polarization combiner 6 and indicates the distortion because the optical signal that has been subjected to the optimal delay and the delay compensated has the minimum distortion. The signal is output to the variable optical delay device control circuit 9, and the variable optical delay device control circuit 9 controls the optical signal based on this signal so that distortion of the optical signal is minimized, whereby an optimal delay is given, and PMD is reduced. Decrease.

The polarization controller 1 has a half-wave plate and a λ / 4
This is realized by a polarization controller in combination with a wave plate. Further, the polarization controller 1 can be realized by a polarization controller using a dielectric or a semiconductor. Polarization separator 2
The polarization combiner 6 can be realized by using a device called a polarization beam splitter. The polarization beam splitter can be realized by a fiber fusion type or a type using a bulk crystal. The variable optical delay unit 5 may be a device that changes the physical optical path length by a movable unit, or may use a device that changes the optical path length by temperature, stress, or the like.

Here, a detailed configuration of the optical signal monitors 7a and 7b will be described with reference to FIGS. FIG.
FIG. 3 is a block diagram showing an example of a detailed configuration of the optical signal monitors 7a and 7b. In FIG. 2, optical signal monitors 7a, 7
b is realized by a circuit for measuring the clock extraction efficiency. First, the input optical signal is transmitted to the photodiode 1
3 is converted into an electric signal. Variable gain amplifier 14
Variably amplifies the input electric signal under the control of the level detector 15a. The level detector 15a controls the gain of the variable gain amplifier 14 so that the signal level from the variable gain amplifier 14 becomes constant. Clock extractor 16
Extracts the clock of the input signal and outputs the clock to the level detector 15b.
Output to The signal input to the level detector 15b is
Since the input signal has a predetermined level under the control of the level detector 15a, the clock extraction level for the predetermined level input signal is detected. The level detectors 15a and 15b can be realized by a diode type peak detector, a power meter, or the like. Further, the clock extractor 16 includes the optical signal monitors 7a,
When the optical signal input to 7b is an NRZ signal,
It can be realized by a non-linear clock extraction circuit using a doubler or a mixer. When the input optical signal is an RZ signal, it can be realized by a linear clock extraction circuit such as a narrow band filter.

FIG. 3 is a block diagram showing another example of the detailed configuration of the optical signal monitors 7a and 7b. 3, the optical signal monitors 7a and 7b are realized by a circuit for measuring clock extraction efficiency, similarly to the optical signal monitors 7a and 7b shown in FIG. First, the input optical signal is converted into an electric signal by the photodiode 13. The variable gain amplifier 14 variably amplifies the input electric signal. The level detector 15a includes the variable gain amplifier 14
, And outputs the signal level to the divider 17. The clock extractor 16 extracts the clock of the output signal of the variable gain amplifier 14 and outputs the clock to the level detector 15b. The level detector 15b detects a signal level output from the clock extractor 16. The divider 17 divides the output B from the level detector 15b by the output A from the level detector 15a, and outputs a division result. Since the output of the divider 17 is proportional to the clock extraction level per unit signal, the output of the optical signal monitor 7 shown in FIG.
Reference numerals a and 7b denote circuits for measuring clock extraction efficiency similarly to the optical signal monitors 7a and 7b shown in FIG.

The optical signal monitors 7a and 7b are not limited to the configuration shown in FIG. 2 or FIG. 3, but may be realized using a polarization degree monitor. When the polarization mode dispersion compensator is functioning ideally, the output degree of polarization is maximized. Therefore, desired control can be realized by feeding back such that the degree of polarization is maximized.

According to the first embodiment, the optical signal monitors 7a and 7b independently detect optical signals, and based on the detection results, the polarization controller control circuit 8 and the variable optical delay control circuit 9 Control the polarization controller 1 and the variable optical delay unit 5 independently of each other, so that highly accurate and reliable PMD compensation can be performed.

Embodiment 2 Next, a second embodiment of the present invention will be described. In the second embodiment, the configuration of the variable optical delay unit control circuit 9 in the first embodiment is realized by using a synchronous detection method. Other configurations are the same as those of the first embodiment, and the same components are denoted by the same reference numerals.

FIG. 4 is a block diagram showing a configuration of the polarization mode dispersion compensator according to the second embodiment of the present invention.
In FIG. 4, an oscillator 23 outputs a sine wave dither signal of, for example, 2 KHz. The mixer 18 mixes the sine wave dither signal output from the oscillator 23 with the signal input from the optical signal monitor 7b, and performs synchronous detection. The mixed signal is amplified by an amplifier 19, and then input to a low-pass filter 20. The error signal output from the low-pass filter 20 is converted into a sine wave dither signal input to a mixer 18 and an optical signal monitor. 7b has a different sign depending on whether the phase is opposite to that of the signal inputted from 7b. The adder 21 adds the error signal output from the low-pass filter 20 and the sine wave dither signal output from the oscillator 23, and the addition result is output to the variable optical delay device 5 by the drive amplifier 22. The optical signal provided with the delay amount by the variable optical delay unit 5 is monitored again by the optical signal monitor 7b, and the monitored signal is output to the variable optical delay unit control circuit 9.

With such a control loop, the signal output from the optical signal monitor 7b becomes maximum, and the set delay amount of the variable optical delay unit 5 is stabilized. Here, since the variable optical delay unit control circuit 9 employs the synchronous detection method, it is possible to realize highly sensitive and highly accurate automatic control. The response time constant of this control loop is determined by the cutoff frequency of the low-pass filter 20. Typically, the cutoff frequency of the low-pass filter 20 is about 100 Hz.

The configuration of the variable optical delay unit control circuit 9 described above can be applied to the polarization controller control circuit 8. Here, since it is not preferable to make the response time constant of the variable optical delay device control circuit 9 close to the response time constant of the polarization controller control circuit 8, for example, the response time constant of the variable optical delay device control circuit 9 is set to 100 Hz. It is preferable to set the response time constant of the polarization controller control circuit 8 to 1 kHz. In this case, the frequency of the sine wave dither signal used in the variable optical delay device control circuit 9 is preferably 2 kHz, and the frequency of the sine wave dither signal used in the polarization controller control circuit 8 is preferably 20 kHz. Since the frequency of the dither signal needs to be lower than the response speed of the device to be controlled and sufficiently higher than the time constant of the control loop, a frequency of 500 kHz or less can be generally used.

In the second embodiment, since a control signal necessary for delay compensation of PMD is obtained by the synchronous detection method, PMD by delay compensation can be performed with high accuracy and high speed.
Compensation can be made.

Embodiment 3 Next, a third embodiment of the present invention will be described. In the first embodiment described above,
Although the optical signal monitor 7a monitors the optical signal on the first optical path 3 separated by the polarization splitter 2,
In the third embodiment, the optical signal monitor 7a monitors an optical signal on the second optical path 4.

FIG. 5 is a block diagram showing a configuration of the polarization mode dispersion compensator according to the third embodiment of the present invention.
In FIG. 5, an optical signal monitor 37a monitors an optical signal on the second optical path 4, and outputs the monitoring result to the polarization controller control circuit 8
Output to

This is because, depending on the characteristics of the polarization splitter 2, an optical signal in a desired polarization state is converted into either an optical signal transmitted on the first optical path 3 or an optical signal transmitted on the second optical path 4. This is because, when input, an optical signal in a desired polarization state is also input to the other first optical path 3 or second optical path 4. That is, the optical signal monitored by the optical signal monitor 37a is equivalent whether it is on the first optical path 3 or the second optical path 4.

In FIG. 5, the optical signal monitor 37a
Monitors the optical signal on the input side of the variable optical delay unit 5, but may monitor the optical signal on the output side of the variable optical delay unit 5.

Embodiment 4 FIG. Next, a fourth embodiment of the present invention will be described. In the above-described first to third embodiments, the polarization controller 1 and the variable optical delay unit 5 are controlled using the two optical signal monitors 7a and 7b. The two optical signal monitors 7 are used to control the polarization controller 1 and the variable optical delay unit 5 by time division multiplexing.

FIG. 6 is a block diagram showing the configuration of the polarization mode dispersion compensator according to the fourth embodiment of the present invention.
In FIG. 6, the polarization mode dispersion compensator is newly provided with a two-input one-output optical switch 42 and an optical signal monitor 7.
In addition to one optical signal monitor 7 corresponding to a and 7b, an optical switch 42 and a switching control unit 43 for performing switching control of the optical signal monitor 7 are provided. Other configurations are the same as those of the third embodiment, and the same components are denoted by the same reference numerals.

The second input of the optical switch 42 is connected to the second optical path 4.
The upper optical signal and the optical signal after the polarization combining by the polarization combiner 6 are input, and one optical signal selected by the switching instruction of the switching controller 43 is output to the optical signal monitor 7. The optical signal monitor 7 outputs the optical signal to the polarization controller control circuit 8 when the optical signal on the second optical path 4 is selected by the switching instruction of the switching control 43, and When the optical signal after the polarization combining is selected, the optical signal is output to the variable optical delay unit control circuit 9. The switching control unit 43
It instructs the optical switch 42 and the optical signal monitor 7 to perform time division multiplex switching.

In general, the optical signal monitors 7a, 7b, 7
When an ultra-high-speed optical signal is handled, the cost increases. Therefore, by using one optical signal monitor 7, a simple and inexpensive polarization mode dispersion compensator can be realized.

Embodiment 5 Next, a fifth embodiment of the present invention will be described. In the fifth embodiment, not only PMD compensation by delay compensation and polarization plane control but also compensation of a loss difference caused by independent orthogonal polarization components on a transmission line, that is, polarization dependent loss PDL (Polarization Depending Loss) can be compensated. Like that.

FIG. 7 is a block diagram showing a configuration of the polarization mode dispersion compensator according to the fifth embodiment of the present invention.
In FIG. 7, the polarization mode dispersion compensator includes a variable optical attenuator 50 provided on the first optical path 3, an optical signal monitor 7c for monitoring an optical signal obtained by the polarization combining of the polarization combiner 6, and an optical signal. Variable optical attenuator 5 based on the monitor result of monitor 7c
A variable optical attenuator control circuit 51 for controlling 0 is newly provided. Other configurations are the same as those of the first embodiment, and the same components are denoted by the same reference numerals.

The optical signal monitor 7c monitors a part of the optical signal that has been polarization-synthesized by the polarization synthesizer 6 via a beam splitter, and outputs the monitored signal to the variable optical attenuator control circuit 51. . The variable optical attenuator control circuit 51 controls the attenuation of the variable optical attenuator 50 based on the monitoring result.
The configuration of the optical signal monitor 7c is the same as that of the optical signal monitors 7a and 7b.

Incidentally, the configuration of the variable optical attenuator control circuit 51 may be a circuit configuration for detecting the clock extraction efficiency and measuring the degree of polarization, similarly to the variable optical delay device control circuit 9 in the second embodiment. A circuit configuration may be used. Also,
The variable optical attenuator 50 may be provided on the second optical path 4. The response time constant of the variable optical attenuator control circuit 51 needs to be set to a value different from the response time constant of the variable optical delay device control circuit 9 and the response time constant of the polarization controller control circuit 8.

Further, the fourth embodiment may be applied to the fifth embodiment, and an optical switch may be used to output optical signals monitored by the optical monitors 7a to 7c. Thus, a polarization mode dispersion compensator having a simple configuration can be realized.

According to the fifth embodiment, not only the PMD compensation by the delay compensation and the polarization plane control, but also the compensation of the difference of the loss caused by the independent orthogonal polarization components on the transmission line, that is, the polarization dependent loss PDL. can do.

Embodiment 6 FIG. Next, a sixth embodiment of the present invention will be described. In the first to fifth embodiments described above, the delay time difference is compensated for by using a mirror system. However, in the sixth embodiment, the delay time difference is compensated for by using a variable birefringence medium. ing.

FIG. 8 is a block diagram showing the configuration of the polarization mode dispersion compensator according to the sixth embodiment of the present invention.
8, this polarization mode dispersion compensator is provided with a variable birefringence medium 60 instead of the polarization splitter 2, the variable optical delay unit 5, and the polarization combiner 6 shown in FIG. The birefringence medium 60 compensates for the delay time difference.

In FIG. 8, an optical signal that has undergone PMD via a transmission line is input to a polarization controller 1. The polarization controller 1 controls the polarization state of the input optical signal, that is, the polarization plane. An optical signal having a polarization plane controlled by the polarization controller 1 is supplied to a terminal 71 of a three-terminal optical circulator 71.
is input to a. The optical signal input to the terminal 71a is input from the terminal 71b to the input / output end 64 of the variable birefringence medium 60, and reciprocates in the variable birefringence medium 60, thereby causing a delay time difference between the optical signals of the respective polarization planes. And the optical signal whose delay time difference has been compensated is input from the input / output terminal 64 to the terminal 71b of the optical circulator 71 again, and the terminal 71c
Output from

The variable birefringence medium 60 is provided with a grating 62 in an optical waveguide 61, and gives a different group delay time to the polarization of each polarization plane of an optical signal input into the optical waveguide 61. Compensates for the delay time difference between each polarization. Here, the wavelength (black wavelength) λb reflected by the grating 62 is
Can be expressed by the following equation using the equivalent birefringence neff and the grating pitch Λ. That is, λb = 2nef
f · Λ. In this case, since the grating pitch Λ and the equivalent birefringence neff are constant, the heating unit 63
a, 63b are provided, and the amount of heating of the heating units 63a, 63b is controlled to give a temperature gradient to the optical waveguide 61, and the delay time difference between the polarized lights is adjusted. The adjustment of the delay time difference is controlled by the temperature control unit 63.

Here, the optical signal monitor 7b is connected to the terminal 71c of the optical circulator 71 via the beam splitter 72.
A part of the optical signal output from is output and output to the variable birefringence medium control circuit 75. The variable birefringence medium control circuit 75 controls the delay time difference of the variable birefringence medium 60 by controlling the temperature control unit 63.

On the other hand, the input / output terminal 6 of the variable birefringence medium 60
A part of the optical signal output from 4 is extracted by the beam splitter 73 and output to the polarization splitter 74.
The polarization separator 74 separates the polarized light having one polarization plane of the extracted part of the optical signal, and outputs it to the optical signal monitor 7a. The optical signal monitor 7a controls the signal to be controlled by the polarization controller 1 so that the polarization state of the separated polarization becomes a predetermined polarization or minimizes the polarization distortion. Output to the circuit 8.

Thus, Embodiments 1 to 5 described above are performed.
Similarly to the above, the control of the polarization controller 1 and the control of the delay time difference compensation by the variable birefringence medium 60 are controlled by the monitoring results by the independent optical signal monitors 7a and 7b.

In the sixth embodiment, the optical waveguide 61 is provided with a temperature gradient by the temperature controller 63 to adjust the delay time difference. However, the present invention is not limited to this. The delay time difference may be adjusted by giving a gradient or the like.

In the sixth embodiment, a part of the optical signal output from the variable birefringence medium 60 is extracted by the beam splitter 73, and polarized light having a desired polarization plane is extracted by the polarization splitter 74. However, a part of the optical signal input to the variable birefringence medium 60 may be extracted, and the polarization of a desired polarization plane in the extracted optical signal may be extracted.

According to the sixth embodiment, the first embodiment
5, the compensation of the delay time difference by the mirror system formed by the polarization splitter 2, the variable optical delay unit 5, and the polarization combiner 6 can be performed by the variable birefringence medium 60. Further, by using the variable birefringence medium 60, higher mechanical strength can be obtained as compared with a mirror system, and furthermore, reduction in size and weight can be promoted.

[0071]

As described above, according to the present invention, each of the first and second optical signal detecting means independently detects the first and second optical signal based on the detected optical signal. Since the first control means and the second control means are independently controlled, there is an effect that highly accurate and reliable PMD compensation can be performed.

According to the next invention, the plurality of first and second control means can be independently controlled by only one optical signal detecting means for detecting the optical signal time-divisionally switched and output by the optical switch. Therefore, there is an effect that highly accurate and reliable PMD compensation can be performed with a simple configuration.

According to the next invention, the third optical signal detecting means detects the optical signal, and the third control means controls the attenuation of the optical attenuating means based on the detection result of the third optical signal detecting means. Since the amount is controlled, not only PMD compensation by delay compensation and polarization plane control, but also compensation for the loss difference caused by independent orthogonal polarization components on the transmission line, that is, polarization dependent loss PLD. This has the effect that it can be performed.

According to the next invention, the plurality of first to third control means can be independently controlled by only one optical signal detecting means for detecting the optical signal time-divisionally switched and output by the optical switch. Therefore, there is an effect that highly accurate and reliable PMD compensation can be performed with a simple configuration.

According to the next invention, since the delay control is performed using the variable birefringence medium, it is resistant to mechanical vibration,
In addition, there is an effect that a device that promotes reduction in size and weight can be realized.

According to the next invention, in the first to third optical signal detecting means or the optical signal detecting means, the converting means converts the input optical signal into an electric signal and performs the first signal level conversion. Means for detecting a signal level of the electric signal converted by the converting means, clock signal extracting means extracting a clock signal from the electric signal converted by the converting means, and second signal level detecting means, Since the signal level extracted by the clock signal extracting means is detected and the clock extraction efficiency is monitored, a control signal required for PMD compensation such as delay compensation can be reliably obtained. .

According to the next invention, the first to third optical signal detecting means or the optical signal detecting means measure the degree of polarization of the input optical signal, so that delay compensation and the like can be performed. There is an effect that a control signal necessary for PMD compensation can be reliably obtained.

According to the next invention, since the first to third control means perform synchronous detection using a dither signal, PMD compensation such as delay compensation can be performed with high accuracy and high speed by synchronous detection. Is achieved.

According to the next invention, since the dither signals in the first to third control means are different signals of 500 kHz or less, PMD compensation such as delay compensation can be performed with high accuracy and high speed. It has the effect of being able to.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a polarization mode dispersion compensator according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing an example of a detailed configuration of optical signal monitors 7a and 7b shown in FIG.

FIG. 3 is a block diagram showing another example of the detailed configuration of the optical signal monitors 7a and 7b shown in FIG.

FIG. 4 is a block diagram illustrating a configuration of a polarization mode dispersion compensator according to a second embodiment of the present invention.

FIG. 5 is a block diagram showing a configuration of a polarization mode dispersion compensator according to a third embodiment of the present invention.

FIG. 6 is a block diagram showing a configuration of a polarization mode dispersion compensator according to a fourth embodiment of the present invention.

FIG. 7 is a block diagram illustrating a configuration of a polarization mode dispersion compensator according to a fifth embodiment of the present invention.

FIG. 8 is a block diagram illustrating a configuration of a polarization mode dispersion compensator according to a sixth embodiment of the present invention.

FIG. 9 is a diagram illustrating the concept of polarization mode dispersion compensation.

FIG. 10 is a block diagram showing a configuration of a conventional polarization mode dispersion compensator.

FIG. 11 is a block diagram showing another configuration of the conventional polarization mode dispersion compensator.

[Explanation of symbols]

1 polarization controller, 2,74 polarization separator, 3 first optical path, 4 second optical path, 5 variable optical delay device, 6 polarization combiner,
7, 7a, 7b, 7c, 37a Optical signal monitor, 8 Polarization controller control circuit, 9 Variable optical delay control circuit, 13
Photodiode, 14 Variable gain amplifier, 15a, 1
5b level detector, 16 clock extractor, 17 divider, 18 multiplier, 19 amplifier, 20 low-pass filter, 21 adder, 22 drive amplifier, 23 oscillator, 42 optical switch, 43 switching controller, 50 variable light Attenuator, 51 variable optical attenuator control circuit, 60 variable birefringence medium, 63 temperature control unit, 71 optical circulator, 75 variable birefringence medium control circuit.

Claims (9)

[Claims] A polarization controller configured to control a polarization plane of an input optical signal; and a polarization controller configured to separate the optical signal into two orthogonal polarization components based on the polarization plane control by the polarization controller. Separation means, two orthogonal polarization components separated by the polarization separation means are input, and one of the orthogonal polarization components is delayed to compensate for a delay time difference between the respective orthogonal polarization components. Delay compensating means for outputting two orthogonal polarization components, and a first optical signal for detecting an optical signal of any one of the two orthogonal polarization components input to the delay compensating means Detecting means for detecting an optical signal output from the delay compensating means;
Optical signal detecting means, first control means for controlling the polarization control means based on the optical signal detected by the first optical signal detecting means, and detecting by the second optical signal detecting means And a second control means for controlling the delay compensation means on the basis of the optical signal. 2. A polarization controller for controlling the polarization plane of an input optical signal, and a polarization unit for separating the optical signal into two orthogonal polarization components based on the polarization plane control by the polarization controller. Separation means, two orthogonal polarization components separated by the polarization separation means are input, and one of the orthogonal polarization components is delayed to compensate for a delay time difference between the respective orthogonal polarization components. Delay compensating means for outputting two orthogonal polarization components; a first optical signal which is one orthogonal polarization component of the two orthogonal polarization components; and the two orthogonal signals output from the delay compensating means. A light extraction means for extracting a part of an optical signal with a second optical signal comprising a polarization component; and time-divisionally dividing the first optical signal and the second optical signal extracted by the light extraction means. An optical switch for switching and outputting the first optical signal; First control means for controlling the polarization control means; second control means for controlling the delay compensation means based on the second optical signal; and the first control means output from the optical switch. Optical signal detecting means for detecting the optical signal and the second optical signal, and outputting the detection results to the corresponding first control means and the second control means, respectively, the optical switch and the optical signal detection A polarization mode dispersion compensator, comprising: switching control means for performing time-division switching control of the means. 3. An optical attenuator for attenuating an optical signal of one of the two orthogonal polarization components, and a third optical signal detector for detecting an optical signal output by the delay compensator. And means for controlling an amount of attenuation of the light attenuating means based on the optical signal detected by the third optical signal detecting means. 3. The polarization mode dispersion compensator according to 2. 4. A polarization controller for controlling the polarization plane of the input optical signal, and a polarization controller for separating the optical signal into two orthogonal polarization components based on the polarization plane control by the polarization controller. Separation means, two orthogonal polarization components separated by the polarization separation means are input, and one of the orthogonal polarization components is delayed to compensate for a delay time difference between the respective orthogonal polarization components. Delay compensation means for outputting two orthogonal polarization components; optical attenuation means for attenuating an optical signal of one orthogonal polarization component of the two orthogonal polarization components; A light extracting means for extracting a part of each of a first optical signal which is one of the orthogonal polarization components and a second optical signal comprising the two orthogonal polarization components output from the delay compensating means And the first optical signal extracted by the light extraction means and An optical switch which switches and outputs the second optical signal as one first optical signal and two second optical signals in a time-division manner; First control means for controlling the wave control means, second control means for controlling the delay compensation means based on one of the second optical signals, and based on the other second optical signal. Third control means for controlling the amount of attenuation of the light attenuating means, and detecting one of the first optical signal and two of the second optical signals outputted from the optical switch, and Polarization, comprising: optical signal detecting means for outputting to the corresponding first to third control means; and switching control means for performing time-division switching control of the optical switch and the optical signal detecting means. Modal dispersion compensator. 5. The polarization mode dispersion compensator according to claim 1, wherein the delay compensator is a variable birefringence medium. 6. The first to third optical signal detecting means or the optical signal detecting means, a converting means for converting an input optical signal into an electric signal, and a signal of the electric signal converted by the converting means. First signal level detecting means for detecting a level; clock signal extracting means for extracting a clock signal from the electric signal converted by the converting means; and second signal detecting the signal level extracted by the clock signal extracting means. The polarization mode dispersion compensator according to any one of claims 1 to 5, further comprising: signal level detection means. 7. The optical signal detecting device according to claim 1, wherein the first to third optical signal detecting means or the optical signal detecting means measures a degree of polarization of the input optical signal. 3. The polarization mode dispersion compensator according to item 1. 8. The polarization mode dispersion compensator according to claim 1, wherein the first to third control means perform synchronous detection using a dither signal. 9. The polarization mode dispersion compensator according to claim 8, wherein the dither signal is a different signal of 500 kHz or less.