JP4366000B2 - Polarization mode dispersion compensation method and optical receiver - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for compensating polarization mode dispersion received by an optical signal propagating in an optical fiber, and an optical receiver using the method.
[0002]
[Prior art]
An optical fiber is one in which light is confined and transmitted in a high refractive index core surrounded by a clad. When this core is a perfect circle, no residual birefringence occurs in the optical fiber. There is no group delay difference between the polarized waves. However, when distortion occurs in the core of the optical fiber, residual birefringence occurs in the optical fiber, and a difference in signal arrival time, that is, a group delay difference occurs between the orthogonally polarized waves.
[0003]
The following two are the main causes of the occurrence of core distortion in optical fibers. That is, one is distortion at the time of manufacture, and the other is temperature variation and seasonal variation after laying.
[0004]
Among these, the distortion at the time of manufacturing has a small group delay difference and has a constant value in time, so there is almost no influence on high-speed transmission. However, the group delay difference caused by temperature fluctuations and seasonal fluctuations after laying has a statistical value of Maxwell distribution, and may take a value much larger than the average value at any time.
[0005]
Polarization mode dispersion (PMD: Polarization Mode Dispersion) occurs due to such a randomly varying group delay difference and polarization direction, which greatly affects high-speed transmission. For example, in an optical transmission system using an optical fiber having an average group delay difference of 20 ps, a transmission path error of 35 hours may occur per year. In order to compensate for the influence of PMD, it is necessary to perform adaptive control according to the amount of change.
[0006]
As a method for compensating PMD, a so-called optical compensation method or electrical compensation method is conventionally known. For example, Japanese Patent Laid-Open No. 7-231297 discloses an optical compensation method. This method performs compensation using a single (N = 1) or plural (N> 1) polarization maintaining fibers and a polarization controller before converting an optical signal into an electrical signal. Also, “Polarization Mode Dispersion Compensation by Phase Diversity Detection” (IEEE PHOTONICS TECHNOLOGY LETTERS, VOL.9, NO.1, JANUARY 1997, pp.121-123) discloses an electrical compensation method. In this method, the optical signal after passing through the polarization controller is converted into an electrical signal, and then compensated using a phase comparator.
[0007]
[Problems to be solved by the invention]
However, when a single (N = 1) polarization controller or polarization maintaining fiber is used as in the optical compensation method described above, the group delay difference of the polarization maintaining fiber is a constant value. There is a problem in that a residual group delay difference corresponding to a difference from PMD generated on the road occurs, and waveform deterioration occurs on the receiving side. Further, the PMD to which this method can be applied is limited to a relatively small amount. On the other hand, when PMD compensation is performed using a plurality of (N> 1) polarization controllers or polarization maintaining fibers, it is necessary to control a plurality of polarization controllers at the same time, which complicates the control and algorithm. . Furthermore, since the time until the control converges increases as the number N of the polarization controllers constituting the control increases, the control in the compensator does not follow the PMD fluctuation occurring in the transmission path. Oscillation and transmission path errors may occur.
[0008]
In addition, when a polarization controller or phase comparator is used as in the electrical compensation method described above, the polarization direction control by the polarization controller and the group delay amount control by the delay difference detection circuit are determined from the phase difference information. Therefore, there is a problem that the control and the algorithm become complicated.
[0009]
Accordingly, an object of the present invention is to provide a polarization mode dispersion compensation method and an optical receiver capable of performing compensation control simply and stably.
[0010]
[Means for Solving the Problems]
The above object is achieved by a polarization mode dispersion compensation method comprising the steps of optically compensating the polarization direction of input light and electrically compensating the group delay difference of the input light. Here, it is preferable that a part of the group delay difference is optically compensated using a polarization maintaining fiber.
[0011]
An optical receiver according to the present invention includes an optical compensation unit that optically compensates for the polarization direction of input light and a part of the group delay difference, and an electrical compensation that electrically compensates for residual group delay difference of the input light. And configured. Here, the optical compensation unit is configured using, for example, a polarization controller and a polarization maintaining fiber, and the electrical compensation unit is configured using, for example, a delay circuit.
[0012]
Thus, the present invention adopts a compensation method (hybrid compensation method) in which an optical compensation method and an electrical compensation method are hybridized. As a result, the polarization direction control by the optical compensation method and the residual group delay difference control by the electrical compensation method can be performed independently, so that the control can be simplified and stabilized. Is possible.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram showing an example of an optical transmission system to which a polarization mode dispersion compensation method according to the present invention is applied. As shown in the figure, the signal light transmitted from the optical transmitter 1 propagates through a transmission path 3 formed of an optical fiber via a plurality of optical amplifiers 4 and is received by the optical receiver 2. In this optical transmission system, the optical receiver 2 compensates for the influence of polarization mode dispersion (PMD) caused by the transmission path 3 constituted by this optical fiber. The optical receiver 2 performs PMD compensation control using a hybrid compensation method that combines an optical compensation method and an electrical compensation method, as described below.
[0014]
FIG. 2 is a diagram showing an embodiment of an optical receiver having a polarization mode dispersion compensation function according to the present invention. In this embodiment, a hybrid compensation method is used in which PMD optical compensation is performed first, followed by electrical compensation. As shown in the figure, the optical receiver 2 includes an optical input unit 9, a polarization controller 5 for controlling an arbitrary polarization state, a polarization maintaining fiber 6 which is a fiber having a certain amount of group delay difference, a polarization A polarization maintaining optical coupler 8 that branches the optical power while maintaining the state, a polarization maintaining fiber 12 having a small group delay difference that does not affect the signal light, and a polarization beam splitter 11 that separates the incident polarization component , Optical detectors 7a, 7b and 7c for performing optical / electrical conversion, a frequency extractor 13 for extracting frequency components from the received electrical waveform, delay circuits 14a and 14b for providing an electrical delay amount, and a phase difference from the two received waveforms. It comprises a phase difference detector 17 for detecting, and a discriminator 16 for discriminating the “1” level and “0” level of the received waveform. Here, the orthogonal polarization axis at the output end of the compensating polarization maintaining fiber 6 needs to coincide with the horizontal axis and the vertical axis of the polarization beam splitter 11. The frequency extractor 13 is realized by a clock extraction circuit that extracts a clock component from the received electrical waveform, for example.
[0015]
In the present embodiment, first, signal light having an arbitrary polarization direction and group delay difference is input from the optical input unit 9 and transmitted through the polarization controller 5 and the polarization holding fiber 6, and then a part of the signal light is transmitted. Branches at the polarization maintaining optical coupler 8. The photodetector 7a optically / electrically converts the branched optical waveform. The frequency extractor 13 extracts a frequency component included in the signal from the converted electric waveform. Here, as the polarization controller 5, for example, a combination of a half-wave plate and a quarter-wave plate is used. FIG. 5 is a diagram showing an embodiment of the frequency extractor 13 according to the present invention. The frequency extractor 13 is a combination of an AGC circuit 22, a full-wave rectifier circuit 23 that extracts a clock component included in a signal from an electrical waveform, and a detector 24 that converts the extracted clock amplitude component into a voltage. Clock extraction circuit.
[0016]
The polarization controller 5 is controlled by the control signal 10 from the frequency extractor 13 so that the extracted frequency component is maximized. As a result, of the orthogonal polarizations, the one with the higher propagation speed in the transmission path 3 is incident on the polarization axis with the lower propagation speed in the polarization maintaining fiber 6 and the one with the lower propagation speed is the polarization with the higher propagation speed. Incident on the axis. However, when the influence of PMD on the transmission path is zero, the orthogonally polarized waves are respectively coupled to one of the polarization axes in the polarization maintaining fiber 6.
[0017]
According to this method, the same control can be obtained irrespective of the magnitude relationship between the PMD of the transmission line 3 and the group delay difference of the polarization maintaining fiber 6, so that the horizontal polarization is output at the output end of the polarization maintaining coupler 8. Vertical polarization can be separated. Then, at the output end of the polarization maintaining fiber 6, only PMD corresponding to the difference between the PMD of the transmission line 3 and the group delay difference of the polarization maintaining fiber 6 remains. The above is the part corresponding to the optical compensator.
[0018]
Next, the horizontal polarization component and the vertical polarization component of the signal light output from the polarization maintaining fiber 6 are separated using the polarization beam splitter 11. The angles of the two orthogonal polarization axes at the output end of the compensating polarization maintaining fiber 6 and the polarization axis of the polarization beam splitter 11 are made to coincide with each other, and all optical components used between them are signals. It is configured using polarization maintaining components having a small group delay difference that does not affect the light. Subsequently, optical / electrical conversion is separately performed in the photodetectors 7b and 7c for each separated polarization component. The converted electric signal passes through the delay circuits 14a and 14b, respectively, and the phase difference of the signal branched on the output side of the delay circuits 14a and 14b is detected by the phase difference detector 17. The phase difference detector 17 can be composed of, for example, an autocorrelation detection circuit using a multiplication circuit and an integration circuit.
[0019]
The phase difference detector 17 controls the delay amounts Δt1 and Δt2 of the delay circuits 14a and 14b with the control signals 15a and 15b so that the correlation between the horizontal component and the vertical component of the converted electric signal is maximized. The delay circuits 14 a and 14 b only have to compensate for the residual PMD corresponding to the difference between the PMD of the transmission path and the group delay difference of the polarization maintaining fiber 6. Then, the discriminator 16 adds the output waveforms of the delay circuits 14a and 14b to complete the reception of the signal subjected to PMD compensation. The above is the portion corresponding to the electrical compensation unit.
[0020]
As described above, in the hybrid compensation method shown in FIG. 2, the polarization maintaining is performed while taking into consideration the PMD average value of the transmission line 3 and the PMD tolerance of the optical receiver 2 in a portion excluding the hybrid polarization compensation function. By setting the group delay difference of the fiber 6 to be relatively large and setting the delay amounts of the delay circuits 14a and 14b to be sufficiently small, the PMD of the transmission line 3 can be effectively compensated.
[0021]
FIG. 3 is a graph showing the PMD compensation effect by the hybrid compensation method using the configuration of FIG. The horizontal axis of this graph indicates the PMD size of the transmission path, and the vertical axis indicates the size of the penalty (PMD penalty) obtained by converting the waveform degradation level due to PMD received on the transmission path into degradation of reception sensitivity. Each curve on the graph shows PMD no compensation 18, electrical compensation method 19, optical compensation method 20 (configured using a single polarization controller and polarization maintaining fiber: N = 1), and this hybrid compensation Each of the cases according to the method 21 is shown.
[0022]
As shown in FIG. 3, in the case of the electrical compensation method 19, the compensation effect is large when the PMD is small, but the compensation effect is rapidly deteriorated when the PMD is large, so that a relatively small amount of PMD is compensated. Suitable to do. In addition, in the case of the optical compensation method 20 (N = 1), PMD larger than that in the case of the electrical compensation method 19 can be applied. However, as shown by A in FIG. Therefore, the compensation area cannot be made so large.
[0023]
On the other hand, in the case of the present hybrid compensation method 21, the influence by the residual PMD can be made almost zero by the electrical delay circuit, and further, the polarization maintaining fiber 6 used in the optical compensation circuit can be reduced. By adding the group delay difference, it can be applied to a transmission line having a larger PMD. Also, since the polarization direction control by optical compensation and the residual group delay difference control by electrical compensation can be performed independently, compensation control can be performed easily and stably. Furthermore, even if the group delay difference of the polarization maintaining fiber 6 is set smaller than the above method, the same stable control as described above can be performed. Further, by setting the delay amount of the delay circuit to be larger than that in the above method, it can be applied to a transmission line having a larger PMD.
[0024]
FIG. 4 is a diagram showing another embodiment of an optical receiver having a polarization mode dispersion compensation function according to the present invention. In this embodiment, the portion corresponding to the electrical compensator is the same as that of the embodiment of FIG. 2, but the portion corresponding to the optical compensator is different from the embodiment of FIG. It comprises a plurality of polarization maintaining fibers 6.
[0025]
That is, as shown in the figure, the optical receiver 2 ′ includes a plurality of polarization controllers 5 for controlling the input signal light to an arbitrary polarization state, and a plurality of polarization maintaining fibers having a certain amount of group delay difference. 6. Polarization maintaining coupler 8 that branches the optical power while maintaining the polarization state, polarization maintaining fiber 12 having a small group delay difference that does not affect the signal light, and polarization that separates the incident polarization component Beam splitter 11, photodetectors 7a, 7b and 7c for performing optical / electrical conversion, frequency extractor 13 for extracting frequency components from the received electrical waveform, delay circuits 14a and 14b for providing an electrical delay amount, and two received waveforms It comprises a phase difference detector 17 for detecting a phase difference, and a discriminator 16 for discriminating between “1” level and “0” level of the received waveform. Here, the orthogonal polarization axis at the output end of the compensating polarization maintaining fiber 6 needs to coincide with the horizontal axis and the vertical axis of the polarization beam splitter 11. The frequency extractor 13 is realized by a clock extraction circuit that extracts a clock component from the received electrical waveform, for example.
[0026]
In the present embodiment, first, signal light having an arbitrary polarization direction and group delay difference is input from the optical input unit 9 and transmitted through the plurality of polarization controllers 5 and the plurality of polarization maintaining fibers 6. , A part thereof is branched by the polarization maintaining optical coupler 8. The photodetector 7a optically / electrically converts the branched optical waveform. The frequency extractor 13 extracts a frequency component included in the signal from the converted electric waveform. Here, as the polarization controller 5, for example, a combination of a half-wave plate and a quarter-wave plate is used. FIG. 5 is a diagram showing an embodiment of the frequency extractor 13 according to the present invention. The frequency extractor 13 is a combination of an AGC circuit 22, a full-wave rectifier circuit 23 that extracts a clock component included in a signal from an electrical waveform, and a detector 24 that converts the extracted clock amplitude component into a voltage. Clock extraction circuit.
[0027]
Each polarization controller 5 is controlled by control signals 10a to 10d from the frequency extractor 13 so that the extracted frequency component is maximized. As a result, of the orthogonal polarizations, the one with the higher propagation speed in the transmission path 3 is incident on the polarization axis with the lower propagation speed in the polarization maintaining fiber 6 and the one with the lower propagation speed is the polarization with the higher propagation speed. Incident on the axis. However, when the influence of PMD on the transmission path is zero, the orthogonally polarized waves are respectively coupled to one of the polarization axes in the polarization maintaining fiber 6.
[0028]
According to this method, the same control can be obtained irrespective of the magnitude relationship between the PMD of the transmission line 3 and the group delay difference of the polarization maintaining fiber 6, so that the horizontal polarization is output at the output end of the polarization maintaining coupler 8. Vertical polarization can be separated. Then, at the output end of the polarization maintaining fiber 6, only PMD corresponding to the difference between the PMD of the transmission line 3 and the group delay difference of the polarization maintaining fiber 6 remains. The above is the part corresponding to the optical compensator.
[0029]
Next, the horizontal polarization component and the vertical polarization component of the signal light output from the polarization maintaining fiber 6 are separated using the polarization beam splitter 11. The angles of the two orthogonal polarization axes at the output end of the compensating polarization maintaining fiber 6 and the polarization axis of the polarization beam splitter 11 are made to coincide with each other, and all optical components used between them are signals. It is configured using polarization maintaining components having a small group delay difference that does not affect the light. Subsequently, optical / electrical conversion is separately performed in the photodetectors 7b and 7c for each separated polarization component. The converted electric signal passes through the delay circuits 14a and 14b, respectively, and the phase difference of the signal branched on the output side of the delay circuits 14a and 14b is detected by the phase difference detector 17. The phase difference detector 17 can be composed of, for example, an autocorrelation detection circuit using a multiplication circuit and an integration circuit.
[0030]
The phase difference detector 17 controls the delay amounts Δt1 and Δt2 of the delay circuits 14a and 14b with the control signals 15a and 15b so that the correlation between the horizontal component and the vertical component of the converted electric signal is maximized. The delay circuits 14 a and 14 b only have to compensate for the residual PMD corresponding to the difference between the PMD of the transmission path and the group delay difference of the polarization maintaining fiber 6. Then, the discriminator 16 adds the output waveforms of the delay circuits 14a and 14b to complete the reception of the signal subjected to PMD compensation. The above is the portion corresponding to the electrical compensation unit.
[0031]
As described above, in the hybrid compensation method shown in FIG. 4, when the PMD range of the transmission line to be applied is the same, the polarization controller 5 and the polarization maintaining fiber 6 are used by the delay circuit of the electrical compensation method. The number of stages N of the optical compensation method can be reduced, and the control can be further stabilized.
[0032]
As described above, the present invention adopts a compensation method that is a hybrid of the optical compensation method and the electrical compensation method, thereby controlling the polarization direction by the optical compensation method and the residual group delay difference by the electrical compensation method. Can be performed independently, whereby control can be simplified and stabilized, and the range of application to PMD generated in the transmission path can be expanded.
[0033]
【The invention's effect】
According to the present invention, it is possible to obtain a polarization mode dispersion compensation method and an optical receiver that can perform compensation control simply and stably.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an example of an optical transmission system to which a polarization mode dispersion compensation method according to the present invention is applied.
FIG. 2 is a diagram showing an embodiment of an optical receiver having a polarization mode dispersion compensation function according to the present invention.
FIG. 3 is a graph showing a PMD compensation effect by the present hybrid compensation method.
FIG. 4 is a diagram showing another embodiment of an optical receiver having a polarization mode dispersion compensation function according to the present invention.
FIG. 5 is a diagram showing an embodiment of a frequency extractor according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Optical transmitter 2, 2 'Optical receiver 3 Transmission path 4 Optical amplifier 5 Polarization controller 6, 12 Polarization maintaining fiber 7a, 7b, 7c Photo detector 8 Polarization maintaining coupler 9 Optical input part 10a-10d, 15a , 15b Control signal 11 Polarization beam splitter 13 Frequency extractors 14a, 14b Delay circuit 16 Discriminator 17 Phase difference detector 18 PMD penalty without compensation 19 PMD penalty 20 by electrical compensation method 20 PMD penalty 21 by optical compensation method PMD penalty by hybrid compensation method 22 AGC circuit 23 Full-wave rectifier circuit 24 Detector

Claims (6)

A step of optically compensating the polarization direction of the input light, a step of branching the optical power while maintaining the polarization state by the step of compensating , and a group delay difference between the signal lights branched in the branching step And a method of compensating for polarization mode dispersion.   2. The polarization mode dispersion compensation method according to claim 1, wherein a part of the group delay difference is electrically compensated using a frequency extractor and a delay circuit.   3. The polarization mode dispersion compensation method according to claim 2, further comprising a clock extraction circuit as the frequency extractor.   An optical compensator that optically compensates for the polarization direction of the input light and a part of the group delay difference; an optical coupler that branches the optical power while maintaining the polarization state by the optical compensator; and An optical receiver comprising: an electrical compensator that electrically compensates for a residual group delay difference; and a polarization maintaining fiber positioned between the optical coupler and the electrical compensator.   The optical receiver according to claim 4, wherein the electrical compensation unit includes a frequency extractor and a delay circuit.   6. The optical receiver according to claim 5, further comprising a clock extraction circuit as the frequency extractor.

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