JP3281162B2 - Optical fiber polarization mode 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 an optical fiber polarization mode dispersion compensator for compensating a polarization mode dispersion characteristic for limiting a transmission bandwidth of an optical fiber in an ultra-high speed ultra-long distance optical transmission system.

[0002]

2. Description of the Related Art As is well known, research and development of an ultra-high-speed and ultra-long-distance optical transmission system using an optical amplifier is actively conducted for the construction of an optical cable system on the Pacific Ocean. Among them, polarization mode dispersion (P
MD) There is a compensation method. In particular, with the practical use of optical amplifiers,
Transmission distortion due to polarization mode dispersion of an optical fiber, which has not been a problem in the past, is becoming a serious problem in research and development of a 10,000 km class non-regenerative repeater optical transmission system.

[0003] The countermeasure for this has just started in research, and only one PMD compensation method at the laboratory stage has been reported. In the compensation method described in this report, a polarization controller is arranged at each of the transmitting end and the receiving end, and the code error rate of the polarization controller at the receiving end is changed every time the polarization controller at the transmitting end is changed. By repeating the operation of controlling to the minimum while communicating with each other at the transmitting end and the receiving end,
This is a method in which conditions for giving the minimum bit error rate are continuously obtained.

However, if the above-described PMD compensation method is adopted in a 10,000 km-class optical transmission system, it is necessary to exchange control information between the transmitting end and the receiving end of up to 10,000 km, which is extremely non-existent. Realistic. Furthermore, since the polarization controller used does not have wavelength dependence, there is a great disadvantage that the polarization mode dispersion that can be equalized is limited to a special case.

[0005]

As described above, in the conventional ultra-high-speed ultra-long distance optical transmission system, it has been required to compensate for transmission distortion due to polarization mode dispersion of an optical fiber. However, no effective method has been devised to solve this problem.

An object of the present invention is to provide an optical fiber polarization mode dispersion compensator capable of compensating for transmission distortion due to polarization mode dispersion of an optical fiber. I do.

[0007]

Means for Solving the Problems An optical fiber polarization mode dispersion compensator according to the present invention in order to achieve the above object, a first optical circuit which gives a constant group delay time difference with respect to a straight interlinking lightwave, A second optical circuit having a function of varying the phase difference between orthogonal light waves according to a control signal and a function of variably and mutually converting an amplitude rotation connection angle of orthogonal light waves according to a control signal is N (N is an arbitrary natural number). ) Repeatedly connected in series,
Input the polarization mode dispersed signal light from the optical transmission line,
At each stage, take out the orthogonal light wave from the input signal light ,
While the first optical circuit gives a certain group delay time difference ,
An equalizing optical circuit for variably controlling a phase difference and an amplitude rotation connection angle corresponding to a control signal in the second optical circuit; an optical receiving means for receiving signal light output from the equalizing optical circuit; A code string identifying means for identifying a code string from the received signal obtained in step (a), and a difference signal generated by comparing the received signal before identification based on the identification signal obtained by this means , and rectified and integrated to obtain an equalization error An equalization error signal generating means for generating a signal, and generating and outputting a control signal for the phase difference and the amplitude rotation connection angle to the equalization optical circuit so that the equalization error signal obtained by this means is minimized. The first feature is that the apparatus is configured to include a parameter control unit that performs the control.

In particular, the second optical circuit realizes a variable mutual conversion function of an amplitude rotation connection angle by twisting a polarization dependent fiber, which can be regarded as having sufficiently small birefringence, at an angle corresponding to a control signal. A second feature is that a phase difference variable function is realized by partially applying a temperature change according to the control signal to the polarization maintaining fiber.

[0009]

In the optical fiber polarization mode dispersion compensator according to the first feature, the signal light subjected to the polarization mode dispersion is received via the equalizing optical circuit, and the code string is identified once. The difference signal is obtained by comparing the signals before and after the identification, and an equalization error signal is generated by rectifying and integrating, and the parameter of the equalization optical circuit is slightly changed to determine the direction in which the equalization error signal decreases. By finding and repeating the parameter control by the control loop, the state is automatically driven to a state where the equalization error signal is minimized.

[0010] In the equalizing optical circuit having the configuration of the second feature, a polarization maintaining fiber rotation connection type optical circuit is formed, and the physical characteristics of the polarization dependent fiber and the polarization maintaining fiber are utilized. Thus, the function of changing the amplitude rotation connection angle and the function of changing the phase difference are realized.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a configuration of an optical fiber polarization mode dispersion compensator according to the present invention. A signal light (pulse signal) from an optical transmission line 1 showing polarization mode dispersion is applied to a polarization mode dispersion equalizing optical circuit 2. Is input to This polarization mode dispersion equalization optical circuit 2 is a first stage orthogonal light wave mutual conversion circuit (θ0
) 21 and a group delay time difference circuit (Ts) 22 between orthogonal lightwaves.
N, a variable optical phase difference circuit (φk) 23N and a variable orthogonal lightwave interconversion circuit (θk) 24N, a repetitive optical circuit section (the number of repetitions is N), and a group delay time difference circuit (Ts) between orthogonal lightwaves as the last stage. ) 25.

Here, the first-stage orthogonal light wave mutual conversion circuit 21
Is an optical circuit for mutually converting the amplitudes of orthogonal light waves by θ0. Group delay time difference circuit 2 between orthogonal lightwaves of repetitive optical circuit section
The 2N and final-stage orthogonal lightwave group delay time difference circuit 25 is an optical circuit that gives a group delay time difference of Ts to orthogonal lightwaves. Variable optical phase difference circuit (φk) for repetitive optical circuit
23N is φ based on the control signal for the orthogonal light wave.
This is an optical circuit that gives a phase difference by k (optical phase shift amount). Variable orthogonal light wave mutual conversion circuit 24N of repetitive optical circuit section
Is an optical circuit that variably and mutually converts the amplitude of orthogonal light waves by θk (rotation connection angle) based on a control signal.

The signal light having passed through the polarization mode dispersion equalizing optical circuit 2 is photoelectrically converted by a photodetector 3 having no polarization dependency, and
It is output as a reception pulse signal. The received pulse signal is amplified to a predetermined level by an AGC (automatic gain control) circuit 4 by a preamplifier 41 and a feedback resistor 42, and then sent to a clock recovery circuit (CLOCK-REG) 5 and an identification circuit (DECISION) 6. Identify point A
And).

The clock reproducing circuit 5 reproduces a synchronous clock of the transmission signal from the input signal, and the reproduced clock is sent to the identification circuit 6. The identification circuit 6 identifies the code string in the received pulse signal at the timing of the reproduction clock, and the identified received pulse signal is identified by an identification point A.
The signal is sent to the (+) input terminal of the differential amplifier 9 via an equalizing filter (HEQ) 7 for generating an equalized waveform at the step (a).

On the other hand, the received pulse signal at the discrimination point A is converted into a discrimination circuit 6 and an equalization filter 7 by a delay circuit (DELAY) 8.
And is sent to the (−) input terminal of the differential amplifier 9 after being delayed by the time required for the processing. The differential amplifier 9 compares the received pulse signal supplied to the (-) input terminal with reference to the identification pulse signal supplied to the (+) input terminal, and outputs the difference signal. This difference signal is supplied to the rectifier circuit (REC
T) Rectified by 10 and low-pass filter (LPF) 11
And is sent to the parameter control circuit (CONTROL) 12 as an equalization error signal.

The parameter control circuit 12 drives the first-stage quadrature optical phase difference circuit 21 of the polarization mode dispersion equalization optical circuit 2 so that the given equalization error signal is minimized, and also controls the equalization optical circuit 2. By controlling the parameters {φk, θk} of the variable optical phase difference circuit 23N and the variable orthogonal light wave mutual conversion circuit 24N, the polarization mode dispersion is equalized and compensated.

That is, the compensating device having the above configuration receives the signal light (pulse signal) subjected to the polarization mode dispersion from the optical transmission line 1 by the photodetector 3 via the equalizing optical circuit 2, and
After being amplified by the GC circuit 4, identification is performed by the identification circuit 6. The difference signal is obtained by comparing the identified reception pulse signal with the synchronized reception pulse signal before identification by the differential amplifier 9, and rectifies and integrates to generate an equalization error signal.

Then, the parameter control circuit 12 slightly changes the parameters {φk, θk} of the polarization mode dispersion equalizing optical circuit 2 to find a direction in which the equalization error signal decreases, and performs parameter control by the control loop described above. The repetition automatically drives the state where the equalization error signal is minimized. This makes it possible to compensate for the PMD distortion.

FIG. 2 and FIG. 3 both show simulation results of the compensating device having the above configuration. here,
As a PMD transmission line model, a section delay time difference of 30 ps,
The number of sections was 9. The equalizing optical circuit 2 has a two-stage configuration (N = 2)
And Ts = 60 ps.

FIG. 2 shows time waveforms before and after equalization when the signal light is subjected to relatively large PMD distortion. FIG. 2A shows the polarization mode dispersion characteristics given to the transmission light by the optical fiber used in the optical transmission line 1. The horizontal axis is the optical frequency [GHz], and the vertical axis is the fiber transfer function due to the polarization mode dispersion. Matrix component | P11 (f) | ² (amplitude square characteristic)
Is shown. FIG. 2B shows the waveform of the received wave b which has been subjected to PMD distortion by passing the transmitted wave a through the optical transmission line 1. The horizontal axis indicates time [ns], and the vertical axis indicates light intensity. .
FIG. 2C shows the waveform of the received wave b 'when the equalizing optical circuit 2 compensates for the transmitted wave a, in which the horizontal axis represents time [ns] and the vertical axis represents light intensity. I have.

On the other hand, FIG. 3 shows a PMD having a relatively small signal light.
FIG. 9 shows time waveforms before and after equalization for a case where distortion is received. FIGS. 3A to 3C respectively show FIGS.
This corresponds to (c), and the individual description is omitted. FIG.
3 (c) and FIG. 3 (c), in each case, the waveform distortion due to PMD is reduced by the equalization, and it can be confirmed that the compensating device having the above configuration is working effectively.

The compensation method of the compensator can realize not only an equalization characteristic suitable for the actual line but also a special code for controlling the equalization optical circuit by expanding the scale (number of stages) of the equalization optical circuit. Since it is not necessary to use such a method, it can be said that the method is extremely practical.

FIG. 4 shows a specific configuration of the PMD equalizing optical circuit 2, which is realized by a polarization maintaining fiber rotary connection type optical circuit. This configuration has a two-wavelength polarization control optical circuit (T
WPC). As the polarization maintaining fiber, for example, a so-called panda fiber (PANDA)
Is used.

That is, this configuration is similar to the transmission line fiber 3
A DSF (Polarization Dependent Fiber) 35, 36 of about 5 cm, which can be regarded as having a sufficiently small birefringence, is provided between the polarization maintaining fiber 32 and the polarization maintaining fibers 32 and 33, 33 and 34 connected in series. , 37, and D
A stable line connection is realized by equalizing by twisting at the SF.

In this configuration, the variable phase shift can be realized by partially heating the polarization maintaining fibers 32, 33. In the example of FIG. 4, the polarization maintaining fiber length is 2
0 m, and the power transistor 3
By mounting 8, 39 as a heat source, a quadrature light wave variable phase difference function is realized. The generation of the equalization error signal can be created using a normal gallium arsenide integrated circuit (GaAs IC).

In the experimental stage, the pulse generator uses 2.5 Gb
/ S pseudo random code is generated, an optical code train of 1.55 μm is obtained using an LN optical modulator, and a PMD waveform distortion generated by a simulated PMD generation optical circuit is added, thereby obtaining a PMD waveform distortion. A wave mode dispersion light wave was generated, and this was input to the compensator having the above configuration, and the PMD equalization optical circuit was manually controlled to minimize the equalization error signal.
It could be confirmed that almost equalization was possible.

Further, as a PMD estimation method on a real line,
The transmission path PMD was estimated by PMD equalization optical circuit synthesis of the transmission path from the received waveform samples, and the validity of the estimation was confirmed by comparing with the simulated PMD transmission path parameters. It is considered that this makes it possible to easily design a real line PMD equalizing optical circuit.

Also, it has been considered that the fully automatic equalization is difficult because the time constant of the variable phase shift unit of the PMD equalization optical circuit is long, so that convergence is slow. However, this control algorithm has been sufficiently confirmed by simulation. I have.

As for the configuration of the PMD equalizing optical circuit,
Although the configuration of the embodiment suitable for the basic confirmation has been described, in the practical stage, an equalizing optical circuit is mounted on the LiNbO ₃ optical IC by TE /
A method of configuring a TM converter, a phase shifter, or the like, or a two-parallel optical transversal filter (Two-parallel squence optical trans)
(a versal filter; presented at the Institute of Electronics, Information and Communication Engineers, 1993 Spring National Convention) on an optical circuit board can be used. It is considered that these configurations can sufficiently satisfy the requirements of the stability of the equalizing optical circuit and the high-speed response of the control. It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications can be made without departing from the spirit of the present invention.

[0030]

As described above, according to the present invention, it is possible to provide an optical fiber polarization mode dispersion compensator capable of compensating for transmission distortion due to polarization mode dispersion of an optical fiber.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of an optical fiber polarization mode dispersion compensator according to the present invention.

FIG. 2 is a diagram showing time waveforms before and after equalization for a comparatively large PMD distortion waveform as a simulation result of the embodiment.

FIG. 3 is a diagram showing time waveforms before and after equalization in a case of a relatively small PMD distortion waveform in the embodiment.

FIG. 4 is a diagram showing a specific configuration of a PMD equalizing optical circuit of the embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical transmission line, 2 ... Polarization mode dispersion equalization optical circuit, 21 ...
Orthogonal light wave mutual conversion circuit, 22N: orthogonal light wave group delay time difference circuit, 23N: variable optical phase difference circuit, 24N: variable orthogonal light wave mutual conversion circuit, 25: orthogonal light wave group delay time difference circuit (Ts), 4: AGC circuit , 41 ... preamplifier, 42 ...
Feedback resistor, 5: clock recovery circuit, 6: identification circuit, 7 ...
Equalization filter, 8 ... delay circuit, 9 ... differential amplifier, 10 ...
Rectifier circuit, 11 low-pass filter, 12 parameter control circuit, 31 transmission line fiber, 32-34 polarization maintaining fiber, 35-37 DSF, 38, 39 power transistor.

Continuation of the front page (56) References JP-A-5-130062 (JP, A) JP-A-6-284093 (JP, A) JP-A-7-131418 (JP, A) JP-A-7-221705 (JP) , A) European Patent Application Publication 578380 (EP, A 1) Natsuko Watanabe et al., 2-wavelength polarization control element, Proceedings of IEICE Autumn Conference 4, Japan, The Institute of Electronics, Information and Communication Engineers, August 15, 1993 , B-908, 4-149 (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08

Claims (2)

(57) [Claims] 1. A first optical circuit for providing a constant group delay time difference to orthogonal light waves, a function of varying a phase difference between orthogonal light waves according to a control signal, and controlling an amplitude rotation connection angle of orthogonal light waves. A second optical circuit having a function of variably interconverting according to a signal is repeatedly connected in series with N (N is an arbitrary natural number) stages, and converts a polarization mode-dispersed signal light from an optical transmission line.
Input, and at each stage, an orthogonal light wave is extracted from the input signal light , and a given group delay time difference is given by the first optical circuit.
A phase difference corresponding to a control signal in the second optical circuit ;
Equalizing optical circuit for variably controlling the amplitude rotation connection angle, optical receiving means for receiving signal light output from the equalizing optical circuit, and code string identifying means for identifying a code string from a received signal obtained by this means And an equalization error signal generation means for generating a difference signal by comparing the received signal before identification based on the identification signal obtained by this means, rectifying and integrating to generate an equalization error signal, An optical fiber polarization mode dispersion compensator comprising: a parameter control unit that generates and outputs a control signal for a phase difference and an amplitude rotation connection angle in the equalization optical circuit so that an obtained equalization error signal is minimized. 2. The second optical circuit realizes a variable mutual conversion function of an amplitude rotation connection angle by twisting a polarization dependent fiber that can be regarded as having sufficiently small birefringence by an angle corresponding to a control signal. 2. The optical fiber polarization mode dispersion compensator according to claim 1, wherein a phase difference variable function is realized by partially applying a temperature change according to the control signal to the polarization maintaining fiber.