JP3230784B2 - Polarization dispersion compensator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization dispersion compensator for compensating for deterioration of transmission characteristics due to polarization dispersion of an optical transmission line such as an optical fiber through which an optical signal propagates and an optical amplifier repeater.

[0002]

2. Description of the Related Art As an optical transmission line used for large-capacity and long-distance transmission, a silica-based single-mode optical fiber having a small loss,
There is an optical amplifier repeater for compensating for the loss. However, the optical components constituting the optical fiber and the optical amplifier repeater have a property that the polarization dispersion, that is, the group velocity of the optical signal is different depending on two orthogonal polarization main axes. Distortion occurs in the signal waveform during propagation, causing signal degradation. Further, the polarization dispersion characteristic shows a temporal change due to a change in stress applied to the optical fiber, a change in temperature, and the like. Therefore, it is necessary to compensate for the deterioration of the transmission characteristics due to the polarization dispersion of the optical transmission line, and it is necessary to perform the compensation adaptively so as to follow the change with time.

The degradation of transmission characteristics due to polarization dispersion has already been formulated (CDPoole et al., IEEE Photon. Techn.
ol. Lett., vol.3, No.1, pp.68-70, 1991). According to this, the degradation amount p (dB) of the transmission characteristics due to polarization dispersion is determined by the input power ratio γ of the optical signal to the two polarization main axes of the optical transmission line.
(0 <γ <1) and the polarization dispersion value Δτ of the entire optical transmission line, and is expressed by p∝γ (1−γ) Δτ ² (1).

A conventional method for coping with such deterioration of transmission characteristics due to polarization dispersion controls a polarization state on the transmission side of an optical signal and sets an input power ratio γ to two polarization main axes of an optical transmission line to 0.
Or it was set to 1. The system configuration is described in the literature (T. Ono et al., OAA. '93, Technical Digest, vol. 1).
4, SuC-6, pp. 42-45, 1993), the main part of which is shown in FIG.

In the figure, a basic optical transmission system includes an optical transmitter 61 for transmitting an optical signal intensity-modulated by a transmission signal, an optical transmission line 62 for transmitting the optical signal, and a transmission signal based on the received optical signal. Is configured by an optical receiver 63 that detects Others are for preventing transmission characteristic deterioration due to polarization dispersion. Hereinafter, the configuration and operation will be described.

[0006] A modulation signal having a predetermined frequency f is input to an optical transmitter 61 to frequency-modulate a semiconductor laser serving as a light source.
Thereby, the frequency modulation is superimposed on the optical signal intensity-modulated by the transmission signal. A part of this optical signal is split on the receiving side via an optical coupler 71 and further split by a polarization beam splitter 7.
2 and received by the balanced receiver 73. When the polarization state of the optical signal incident on the optical transmission line 62 does not coincide with the polarization main axis of the optical transmission line 62 (γ ≠ 0, 1), the transmission characteristics are deteriorated based on the expression (1). The amount of degradation is detected as a change in intensity at the same frequency as the modulation frequency f superimposed on the optical signal in the balanced receiver 73. This detection signal is amplified by the amplifier 74, and the band-pass filter (B
The modulation frequency superimposed on the optical signal is cut out by the PF (75) and detected by the square detector 76.

The output signal of the square detector 76 becomes smaller as γ approaches 0 or 1, and the polarization state of the optical signal incident on the optical transmission line 62 coincides with the main polarization axis of the optical transmission line 62. At this time (γ = 0, 1), 0 is indicated. Therefore, the output signal of the square detector 76 is returned as an error signal to the polarization control circuit 77 provided on the transmission side of the optical signal via the transmission line 78, and the minimum value is set so that the error signal becomes zero. By performing control, transmission without deterioration of transmission characteristics due to polarization dispersion becomes possible.

The polarization control circuit 79 arranged before the polarization beam splitter 72 controls the polarization state so that the error signal is maximized. That is, the polarization of the optical signal is inclined by 45 ° with respect to the polarization beam splitter 72, and the maximum value of the intensity change at the same frequency as the modulation frequency f superimposed on the optical signal is obtained.

[0009]

In the prior art, the receiving side detects an error signal caused by a mismatch between the polarization state of the optical signal incident on the optical transmission line and the polarization main axis of the optical transmission line. Then, the error signal is fed back to the transmission side, and the polarization state of the optical signal is adjusted to the polarization main axis of the optical transmission line on the transmission side, thereby preventing transmission characteristic deterioration due to polarization dispersion. . In addition, since the whole has a feedback control configuration,
It is possible to follow a change with time of transmission characteristic deterioration due to polarization dispersion. However, in the conventional configuration, the transmission path 78 for transmitting the error signal from the receiving side of the optical signal to the transmitting side was indispensable.

The present invention is capable of compensating transmission characteristic deterioration due to polarization dispersion of an optical transmission line only on the receiving side, and further compensating for polarization dispersion characteristics of the optical transmission line with time. The purpose is to provide a vessel.

[0011]

FIG. 1 shows a basic configuration of a polarization dispersion compensator according to the present invention. (1) is a configuration corresponding to claim 1, and (2) is a configuration corresponding to claim 2. The same components as those in the conventional configuration shown in FIG. 8 are denoted by the same reference numerals, and the same components in the respective basic configurations are denoted by the same reference numerals.

In FIG. 1A, an optical transmitter 61 receives a modulation signal of a predetermined frequency f, superimposes a frequency modulation on an optical signal intensity-modulated by a transmission signal, and sends out the optical signal to an optical transmission line 62. Including means. Output end of optical transmission line 62 and optical receiver 6
3, a polarization dispersion providing unit 10 for providing polarization dispersion having a characteristic opposite to the polarization dispersion of the optical transmission line 62 is provided. In addition, optical transmission
A polarization control circuit that controls the polarization state of an optical signal passing through the optical transmission line 62 according to a control signal between the output end of the transmission line 62 and the optical receiver 63 and before the polarization dispersion providing unit 10. 20 and
From the frequency modulation component of the output optical signal of the polarization control circuit 20,
The difference between the output polarization main axis of the system including the optical transmission line 62 and the polarization control circuit 20 and the polarization main axis of the polarization dispersion providing unit 10 is detected.
A transmission path polarization main axis detector 30 to be provided to the polarization control circuit 20 as a control signal is provided.

In FIG. 1B, an optical transmitter 61 receives a modulation signal of a predetermined frequency f, superimposes frequency modulation on an optical signal that has been intensity-modulated by a transmission signal, and transmits the signal to an optical transmission line 62. Including means. The optical receiver 40 includes means for detecting waveform deterioration of an optical signal and outputting a waveform deterioration signal. Further, between the output end of the optical transmission line 62 and the optical receiver 40, a polarization dispersion having a characteristic opposite to that of the optical transmission line 62 is given, and the polarization dispersion value changes according to the waveform deterioration signal. A variable polarization dispersion providing unit 50 is provided. Further, the output end of the optical transmission line 62 and
A polarization control circuit 20 for controlling a polarization state of an optical signal passing through the optical transmission line 62 in accordance with a control signal, between the optical receiver 40 and a stage preceding the variable polarization dispersion imparting section 50; From the frequency modulation component of the output optical signal of the circuit, the difference between the output polarization main axis of the system including the optical transmission line 62 and the polarization control circuit 20 and the polarization main axis of the variable polarization dispersion providing section 50 is detected. And a transmission line polarization main axis detector 30 provided to the polarization control circuit 20.

[0014]

The control signal outputted from the transmission line polarization main axis detector 30 is a signal between the output polarization main axis of the system composed of the optical transmission line 62 and the polarization control circuit 20 and the polarization main axis of the polarization dispersion imparting section 10. Show the difference. Therefore, the polarization control circuit 20 controls the polarization state of the optical signal that has passed through the optical transmission line 62 in accordance with the control signal. The slow axis and the fast axis of the output polarization main axis of the system composed of the optical transmission line 62 and the polarization control circuit 20 can be made to coincide with the slow axis of the axis and the slow axis of the group velocity. This allows
When the optical signal passes through the polarization dispersion providing unit 10, the optical transmission path 6
2 is given an inverse dispersion to the polarization dispersion of the optical transmission line 62.
, It is possible to suppress the deterioration of the transmission characteristics due to the polarization dispersion.

[0015] In addition, the polarization amount in place of the polarization mode dispersion imparting section 10
A variable polarization dispersion imparting unit 50 capable of variably setting a scattering value,
The polarization dispersion value is changed according to the waveform deterioration signal detected by the optical receiver 40. As a result, in the variable polarization dispersion providing section 50, the optical signal is adaptively inversely dispersed with respect to the polarization dispersion of the optical transmission line 62, and the transmission characteristic deterioration due to the polarization dispersion of the optical transmission line 62 changes with time. And can be suppressed.

[0016]

FIG. 2 shows an embodiment of the first aspect of the present invention. Although the polarization dispersion compensator of the present invention includes a means for superimposing frequency modulation on an optical signal transmitted to an optical transmission line, it is the same as the conventional configuration shown in FIG. A polarization control circuit 20 corresponding to the basic configuration of (1),
An example configuration of the transmission line polarization main axis detector 30 and the polarization dispersion imparting unit 10 is shown.

In the figure, the polarization control circuit 20
Wave plate 21, 1/2 wave plate 22, and 1/4 wave plate 23
Are connected in cascade, and the polarization state of the optical signal exiting the optical transmission line 62 is controlled by a hill-climbing method using a control signal. The optical signal input from the polarization control circuit 20 to the transmission line polarization main axis detector 30 is split by a polarization independent beam splitter 31, one of which is split via a half-wave plate 32 into a polarization dispersion imparting unit 10. And the other is received by the balanced receiver 73 via the mirror 33 and the polarization beam splitter 72.
The optical path indicated by the broken line between the optical elements is a spatial transmission path so that the polarization state does not change.

The half-wave plate 32 is in an initial state.
It is used to align the slow axis and the fast axis of the polarization dispersion of the optical transmission line 62 with the fast axis and the slow axis of the polarization maintaining fiber 11 used as the polarization dispersion imparting unit 10, respectively.

The output signal of the balanced receiver 35 shows an intensity change at the same frequency as the modulation frequency f superimposed on the optical signal. The output signal is amplified by an amplifier 36, cuts out a modulation frequency superimposed on the optical signal by a band-pass filter (BPF) 37, is detected by a square detector 38, and is fed back to the polarization control circuit 20 as a control signal. You.

The magnitude of this control signal is such that when the power ratio between the P-polarized light and the S-polarized light at the input end of the transmission line polarization main axis detector 30 is sin ² θ: cos ² θ (2) γ (1−γ) sin ² 2θ (3) Therefore, by controlling the polarization control circuit 20 to the minimum value so that the control signal becomes 0, the polarization control circuit 20 includes the optical transmission line 62 and the polarization control circuit 20 on the fast axis and the slow axis of the polarization maintaining fiber 11. The slow axis and the fast axis of the output polarization main axis of the system can be matched with each other. As a result, transmission characteristic deterioration due to polarization dispersion can be suppressed to a value obtained by subtracting the polarization dispersion value of the polarization maintaining fiber 11 from the polarization dispersion value of the optical transmission line 62.

However, since the polarization dispersion value of the polarization maintaining fiber 11 is fixed, it is not possible to completely suppress the deterioration of transmission characteristics due to the polarization dispersion of the optical transmission line 62. But,
As shown in FIG. 3, by setting the polarization dispersion value of the polarization maintaining fiber 11 near the average of the distribution of the polarization dispersion values of the optical transmission line 62, efficient polarization dispersion compensation becomes possible. FIG.
The horizontal axis indicates the polarization dispersion value of the optical transmission line 62, the vertical axis indicates the transmission characteristic degradation amount p due to the polarization dispersion , and indicates that the transmission characteristic degradation is suppressed from A before compensation to B after compensation. Show.

FIG. 4 shows the configuration of the first embodiment of the present invention. Although the polarization dispersion compensator of the present invention includes a means for superimposing frequency modulation on an optical signal transmitted to an optical transmission line, it is the same as the conventional configuration shown in FIG. Polarization control circuit 2 corresponding to the basic configuration of (2)
0, transmission line polarization main axis detector 30, variable polarization dispersion imparting section 5
0 and an example configuration of the optical receiver 40a. However,
The configuration and operation of the polarization control circuit 20 and the transmission line polarization main axis detector 30 are the same as those of the embodiment shown in FIG.

In the figure, an optical signal input from a transmission line polarization main axis detector 30 to a variable polarization dispersion imparting section 50 is split by a polarization beam splitter 51 corresponding to two polarization main axes. Mirror 52, movable corner cube 53
, And the other passes through a mirror 54 and is combined at a polarization beam splitter 55. The propagation delay difference between the two optical paths is controlled by moving the movable corner cube 53 based on the waveform deterioration signal (signal-to-noise ratio in this embodiment) output from the optical receiver 40. This propagation delay difference gives the polarization dispersion value in the variable polarization dispersion giving section 50.

The variable polarization dispersion providing section 50 transmits the light to the optical receiver 40.
The optical signal input to a is received by the photoelectric converter 41, and the baseband transmission signal (main signal) is amplified by the amplifier 42 and extracted. The frequency f _{1 of} this main signal is cut out by a band-pass filter (BPF1) 43 ₁ ,
Baseband power W ₁ multiplication and detector 44 ₁ is detected. This main signal is supplied to a band-pass filter (BPF).
2) The frequency f ₂ is cut out at 43 ₂ and the square detector 44
Baseband power W _{2 2} is detected. Dividing circuit 45 performs calculation of W ₂ / W _1, and outputs the waveform degradation signal by detecting a signal-to-noise ratio. FIG. 6 shows the principle that this signal-to-noise ratio becomes a waveform-degraded signal.

Deterioration of a signal waveform due to polarization dispersion appears as deterioration of a high-frequency component. Therefore, by taking the ratio of the components of the _two frequencies f ₁ and f ₂ and controlling the polarization dispersion value set by the variable polarization dispersion imparting unit 50 so that the ratio is minimized, the polarization of the optical transmission line 62 can be improved. It is possible to suppress deterioration of transmission characteristics due to dispersion following a change with time. Further, this configuration, since the same polarization dispersion value <br/> the polarization dispersion value of the optical transmission line 62 can be imparted, to completely suppress the transmission characteristics degradation due principle polarization dispersion it can.

FIG. 5 shows the configuration of a second embodiment of the present invention. Note that the polarization dispersion compensator of the present invention is similar to that of FIG.
Is replaced by an optical receiver 40b having a different configuration in place of the optical receiver 40a having the configuration of the first embodiment. Other configurations and operations are the same as those of the first embodiment shown in FIG.

The variable polarization dispersion providing section 50 transmits the signal to the optical receiver 40.
The optical signal input to b is received by the photoelectric converter 41 via the optical amplifier 46 with an automatic level control mechanism. The output signal is amplified by an amplifier 42 and then output to a capacitor 47.
And the DC component is cut off, and is detected by the square detector 48. FIG. 7 shows the principle that the detection output by this configuration becomes a waveform degraded signal.

Deterioration of the signal waveform due to polarization dispersion is manifested as deterioration of the eye opening degree, but by performing square detection, the waveform is degraded as a decrease in power as shown in FIGS. 7 (1) and (2). A signal can be obtained. That is, the variable polarization dispersion providing unit 50 sets the output of the square wave detector 48 to the maximum.
By controlling the polarization dispersion value set in (1), deterioration of transmission characteristics due to polarization dispersion of the optical transmission line 62 can follow time-dependent changes and be completely suppressed as in the first embodiment.

[0029]

As described above, the polarization dispersion compensator of the present invention can compensate for the deterioration of the transmission characteristics due to the polarization dispersion of the optical transmission line by processing only on the receiving side. Furthermore, even with respect to the temporal change of the polarization dispersion characteristic of the optical transmission line, the transmission characteristic deterioration can be compensated by adaptive control on the receiving side.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a basic configuration of a polarization dispersion compensator of the present invention.

FIG. 2 is a block diagram showing the configuration of an embodiment of the invention described in claim 1;

FIG. 3 is a diagram showing an example of improvement of transmission characteristic deterioration in the embodiment of FIG. 2;

FIG. 4 is a block diagram showing the configuration of a first embodiment of the invention described in claim 2;

FIG. 5 is a block diagram showing a configuration of a second embodiment of the invention described in claim 2;

FIG. 6 is a view for explaining the principle that the signal-to-noise ratio in the embodiment shown in FIG. 4 becomes a waveform control signal.

FIG. 7 is a view for explaining the principle that the square detection output in the embodiment shown in FIG. 5 becomes a waveform-degraded signal;

FIG. 8 is a block diagram showing a conventional polarization dispersion suppressing system.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Polarization dispersion | distribution provision part 11 Polarization holding fiber 20 Polarization control circuit 21, 23 1/4 wavelength plate 22 1/2 wavelength plate 30 Transmission line polarization main axis detector 31 Polarization-independent beam splitter 32 1/2 wavelength Plate 33 mirror 40 optical receiver 41 photoelectric converter 42 amplifier 43 band-pass filter 44 square detector 45 division circuit 46 optical amplifier with automatic level control mechanism 47 capacitor 48 square detector 50 variable polarization dispersion imparting section 51 , 55 Polarization beam splitter 52, 54 Mirror 53 Movable corner cube 61 Optical transmitter 62 Optical transmission line 63 Optical receiver 71 Optical coupler 72 Polarization beam splitter 73 Balanced receiver 74 Amplifier 75 Bandpass filter 76 Square detection 77, 79 Polarization control circuit 78 Transmission line

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08

Claims (2)

(57) [Claims] 1. A polarization dispersion compensator for an optical transmission system comprising: an optical transmitter for transmitting an optical signal; an optical transmission line for transmitting the optical signal; and an optical receiver for receiving the optical signal. The transmitter includes means for superimposing frequency modulation on the optical signal, and between the output end of the optical transmission line and the optical receiver, a polarization dispersion having a characteristic opposite to the polarization dispersion of the optical transmission line. A polarization dispersion providing unit for controlling the polarization state of the optical signal passing through the optical transmission line between the output end of the optical transmission line and the optical receiver and before the polarization dispersion providing unit. A polarization control circuit for controlling according to a signal,
From the frequency modulation component of the output optical signal of the polarization control circuit, the difference between the output polarization main axis of the system including the optical transmission line and the polarization control circuit and the polarization main axis of the polarization dispersion imparting unit is detected and controlled. 1. A polarization dispersion compensator, comprising: a transmission line polarization main axis detector for providing a signal to a polarization control circuit as a signal. 2. A polarization dispersion compensator for an optical transmission system comprising: an optical transmitter for transmitting an optical signal; an optical transmission line for transmitting the optical signal; and an optical receiver for receiving the optical signal. The transmitter includes a unit that superimposes frequency modulation on the optical signal, The optical receiver includes a unit that detects a waveform deterioration of the optical signal and outputs a waveform deterioration signal, and includes an output terminal of the optical transmission line. Between the optical receiver and the optical receiver, a variable polarization dispersion providing unit that provides a polarization dispersion of the opposite characteristic to the polarization dispersion of the optical transmission line, and changes the polarization dispersion value according to the waveform deterioration signal Between the output end of the optical transmission line and the optical receiver and before the variable polarization dispersion providing unit, a polarization control unit that controls the polarization state of the optical signal passing through the optical transmission line according to a control signal. From the wave control circuit and the frequency modulation component of the output optical signal of the polarization control circuit,
The difference between the output polarization main axis of the system including the optical transmission line and the polarization control circuit and the polarization main axis of the variable polarization dispersion providing unit is detected.
A polarization dispersion compensator, comprising: a transmission path polarization main axis detector to be provided to a polarization control circuit as a control signal.