JPH07231297A - Optical fiber polarized wave mode dispersion compensation equipment - Google Patents

Abstract

PURPOSE:To automatically obtain a state in which an equalization error signal is minimized by using a parameter control circuit to slightly change a parameter for a polarized wave mode dispersion equalization optical circuit thereby finding out the direction of decrease of the equalization error signal and repeating pameratemer control through a control loop. CONSTITUTION:A signal light (pulse signal) subject to polarized wave mode dispersion from an optical transmission line 1 is received by a photo detector 3 via an equalization (optical) circuit 2 and amplified by an AGC circuit 4 and it is identified by an identification circuit 6. The identified reception pulse signal is compared with a synchronization reception pulse signal before identification at a differential amplifier 9 to obtain a difference signal and it is rectified and integrated to generate the equalization error signal. Then a pulse control circuit 12 is used to change slightly a parameter (phik, thetak) of the polarized wave mode dispersion equalization circuit 2 thereby finding out a direction of a decreased equalization error signal and pulse control is repeated through the control loop to kick out the equalization error signal to be minimized automatically. Thus, PMD distortion is compensated.

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 for polarization mode dispersion characteristics which limits the 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 ultra-long-distance optical transmission system using an optical amplifier has been actively conducted for construction of a Pacific Ocean submarine optical cable system. Among them, the polarization mode dispersion (P
MD) There is a compensation method. Especially 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 the research and development of a non-regenerative repeater optical transmission system of 10000 km class.

Measures against this have just been started in research, and only one PMD compensation method at the laboratory stage has been reported. In the compensation method described in this report, polarization controllers are arranged at the transmitting end and the receiving end respectively, and every time the polarization controller at the transmitting end is changed, the polarization error controller causes the polarization error at the receiving end to change. 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 the condition for giving the minimum code error rate is continuously calculated.

However, in the above PMD compensation method, if it is adopted in an optical transmission system of 10000 km class, it is necessary to exchange control information between the transmitting end and the receiving end of 10000 km, which is extremely non-existent. It is realistic. Further, since the polarization controller used does not have wavelength dependence, it has a serious drawback that the polarization mode dispersion that can be equalized is limited to a special case.

[0005]

As described above, it has been conventionally required to compensate for transmission distortion due to polarization mode dispersion of an optical fiber in an ultrafast ultralong distance optical transmission system. However, no effective method has been devised to solve this problem.

The present invention has been made to solve the above problems, and an object thereof 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. To do.

[0007]

In order to achieve the above object, an optical fiber polarization mode dispersion compensating apparatus according to the present invention inputs a signal light in which polarization mode dispersion is performed from an optical transmission line, and an orthogonal optical wave. A first optical circuit that provides a constant group delay time difference with respect to the function, a function of varying the phase difference of orthogonal light waves according to a control signal, and a variable mutual conversion of an amplitude rotation connection angle of orthogonal light waves according to a control signal A second optical circuit having a function is repeatedly connected in N (N is an arbitrary natural number) stages in series,
An equalization optical circuit that variably controls a constant group delay time difference, a phase difference corresponding to a control signal, and an amplitude rotation connection angle by extracting orthogonal light waves of polarization-mode-dispersed signal light, and output from this equalization optical circuit. Optical receiving means for receiving a signal light, a code string identifying means for identifying a code string from the received signal obtained by this means, and a difference signal comparing the received signal before identification with the identifying signal of this means as a reference. Equalization error signal generating means for generating, rectifying and integrating to generate an equalization error signal, and phase difference and amplitude rotation connection to the optical equalization circuit so that the equalization error signal obtained by this means is minimized. And a parameter control means for generating and outputting a control signal for the corner.
It is a feature of.

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

[0009]

In the optical fiber polarization mode dispersion compensator according to the first characteristic configuration, the signal light subjected to the polarization mode dispersion is received through the equalization optical circuit, the code string is once identified, The difference signal is obtained by comparing the signals before and after the identification, the equalization error signal is generated by rectifying and integrating, and the equalization error signal is reduced by slightly changing the parameters of the equalization optical circuit. By finding and repeating the parameter control by the control loop, the equalization error signal is automatically driven to the minimum state.

In the equalizing optical circuit having the second characteristic configuration, a polarization maintaining fiber rotary connection type optical circuit is configured, and the physical characteristics of the polarization dependent fiber and the polarization maintaining fiber are used. In this way, the variable mutual conversion function of the amplitude rotation connection angle and the phase difference variable function are realized.

[0011]

Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 shows the configuration of an optical fiber polarization mode dispersion compensator according to the present invention. Signal light (pulse signal) from an optical transmission line 1 exhibiting polarization mode dispersion is a polarization mode dispersion equalization optical circuit 2 Entered in. This polarization mode dispersion equalization optical circuit 2 is a first-stage orthogonal lightwave mutual conversion circuit (θ0
) 21 and a group delay time difference circuit (Ts) 22 between orthogonal light waves
N, the variable optical phase difference circuit (φk) 23N and the variable orthogonal lightwave mutual conversion circuit (θk) 24N, and the repeating optical circuit unit (where the number of repetitions is N), and the orthogonal lightwave group delay time difference circuit (Ts) as the final stage. ) 25 and.

Here, the first stage orthogonal lightwave mutual conversion circuit 21
Is an optical circuit that mutually converts the amplitude of orthogonal light waves by θ 0. Group delay time difference circuit between orthogonal light waves of repetitive optical circuit unit 2
The group delay time difference circuit 25 between the orthogonal light waves of 2N and the final stage is an optical circuit that gives a group delay time difference of Ts to the orthogonal light waves. Variable optical phase difference circuit (φk) for repetitive optical circuit
23N is φ based on a control signal for orthogonal light waves.
It is an optical circuit that gives a phase difference by k (optical phase shift amount). Variable orthogonal lightwave mutual conversion circuit 24N of repetitive optical circuit unit
Is an optical circuit that mutates the amplitude of the orthogonal light waves by θk (rotational connection angle) based on the control signal.

The signal light passing through the polarization mode dispersion equalization optical circuit 2 is photoelectrically converted by the photodetector 3 having no polarization dependence,
It is output as a reception pulse signal. This 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 (this Identify point A
And).

The clock reproducing circuit 5 reproduces the synchronous clock of the transmission signal from the input signal, and this reproduced clock is sent to the identifying circuit 6. The discrimination circuit 6 discriminates the code string in the received pulse signal at the timing of the reproduction clock. The discerned received pulse signal is the discrimination point A.
Is sent to the (+) input terminal of the differential amplifier 9 via the equalization filter (HEQ) 7 for generating the equalized waveform in the.

On the other hand, the received pulse signal at the identification point A is delayed by the delay circuit (DELAY) 8 and the identification circuit 6 and the equalization filter 7 are provided.
Is delayed by the time required for processing and is sent to the (−) input terminal of the differential amplifier 9. The differential amplifier 9 compares the received pulse signal supplied to the (-) input terminal with the identification pulse signal supplied to the (+) input terminal as a reference, and outputs the difference signal. This difference signal is a rectifier circuit (REC
T) rectified by 10 and low-pass filter (LPF) 11
Is integrated by 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 the equalization optical circuit 2 also receives the equalization error signal. The polarization mode dispersion is equalized and compensated by controlling the parameters {φk, θk} of the variable optical phase difference circuit 23N and the variable orthogonal lightwave mutual conversion circuit 24N.

That is, the compensator having the above-mentioned configuration receives the signal light (pulse signal), which has undergone the polarization mode dispersion from the optical transmission line 1, at the photodetector 3 via the equalization optical circuit 2, and A
After being amplified by the GC circuit 4, the identification circuit 6 identifies. By comparing the identified received pulse signal with the synchronized received pulse signal before identification by the differential amplifier 9, a difference signal is obtained and rectified and integrated to generate an equalization error signal.

Then, the parameter control circuit 12 slightly changes the parameters {φk, θk} of the polarization mode dispersion equalization optical circuit 2 to find the direction in which the equalization error signal decreases, and the parameter control is performed by the control loop described above. By repeating the process, the equalization error signal is automatically minimized. This makes it possible to compensate for PMD distortion.

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

FIG. 2 shows time waveforms before and after equalization when the signal light is subjected to a relatively large PMD distortion. FIG. 2 (a) shows the polarization mode dispersion characteristics given to the transmitted light by the optical fiber used in the optical transmission line 1, where the horizontal axis is the optical frequency [GHz] and the vertical axis is the fiber transfer function due to polarization mode dispersion. Matrix element | P11 (f) | ² (Amplitude squared characteristic)
Is shown. FIG. 2B shows the waveform of the reception wave b that has undergone PMD distortion by passing the transmission wave a through the optical transmission line 1. The horizontal axis represents time [ns] and the vertical axis represents light intensity. .
FIG. 2C shows the waveform of the received wave b'when the transmitted wave a is compensated by the equalization optical circuit 2, where the horizontal axis represents time [ns] and the vertical axis represents the light intensity. There is.

On the other hand, FIG. 3 shows a PMD whose signal light is relatively small.
The time waveform before and after equalization for the case of being distorted is shown. FIGS. 3A to 3C are shown in FIGS.
This corresponds to (c), and the individual description is omitted. Figure 2
As is clear from (c) and FIG. 3 (c), waveform distortion due to PMD is reduced in both cases, and it can be confirmed that the compensator having the above-described configuration is effectively operating.

The compensating method of the compensating device described above not only realizes equalization characteristics suitable for the actual line by expanding the scale (number of stages) of the equalizing optical circuit, but also uses a special code for controlling the equalizing optical circuit. It can be said that it is extremely practical because it is not necessary to use

FIG. 4 shows a concrete structure of the PMD equalization optical circuit 2, which is realized by a polarization maintaining fiber rotation connection type optical circuit. This configuration has a dual wavelength polarization control optical circuit (T
It has a proven structure as WPC). The polarization maintaining fiber is, for example, a so-called panda fiber (PANDA).
Is used.

That is, this configuration is used in the transmission line fiber 3
1 and the polarization-maintaining fiber 32, and between the polarization-maintaining fibers 32 and 33 and 33 and 34 connected in series, the DSFs (polarization-dependent fibers) 35 and 36 of about 5 cm can be regarded as having sufficiently small birefringence. , 37 through fusion, D
By twisting at the SF part, a stable line connection is realized by equalization.

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

At the experimental stage, the pulse generator was 2.5 Gb.
/ S pseudo-random code is generated, an optical code string of 1.55 μm is obtained by using the LN optical modulator, and the PMD waveform distortion generated by the simulated PMD generation optical circuit is added to obtain the polarization having the PMD waveform distortion. A wave mode dispersion light wave is generated, input to the compensator having the above-mentioned configuration, and the PMD equalization optical circuit is manually controlled so as to minimize the equalization error signal.
It was confirmed that the equalization was possible almost completely.

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

Also, the fully automatic equalization was considered to be slow and difficult to converge because the time constant of the variable phase shift section of the PMD equalization optical circuit was long, but this control algorithm was sufficiently confirmed by simulation. There is.

Regarding the configuration of the PMD equalization optical circuit,
Although the configuration of the embodiment suitable for basic confirmation is shown, in the practical stage, an equalizing optical circuit is provided on the LiNbO ₃ optical IC with TE / TE.
A method of configuring a TM converter, a phase shifter, or a two-parallel squence optical transversal filter.
versal filter; announced at IEICE, National Assembly Spring 1993) can be used on the optical circuit board. It is considered that these configurations can sufficiently meet the requirements for stability of the equalization optical circuit and high-speed control response. Needless to say, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the present invention.

[0030]

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

[Brief description of drawings]

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

FIG. 2 is a diagram showing a time waveform before and after equalization for a case of a relatively large PMD distortion waveform as a simulation result of the example.

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

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

[Explanation of symbols]

1 ... Optical transmission line, 2 ... Polarization mode dispersion equalization optical circuit, 21 ...
Orthogonal lightwave mutual conversion circuit, 22N ... Orthogonal lightwave group delay time difference circuit, 23N ... Variable optical phase difference circuit, 24N ... Variable orthogonal lightwave mutual conversion circuit, 25 ... Orthogonal lightwave 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.

Claims (2)

[Claims] 1. A first optical circuit for inputting signal light in which polarization modes are dispersed from an optical transmission line and giving a constant group delay time difference to orthogonal light waves, and a control signal for controlling a phase difference between orthogonal light waves. A second optical circuit having a function of varying in accordance with a control signal and a function of performing variable mutual conversion of an amplitude rotation connection angle of orthogonal light waves in response to a control signal, N (N is an arbitrary natural number) stages are repeatedly connected in series, An equalization optical circuit for variably controlling a constant group delay time difference, a phase difference corresponding to a control signal, and an amplitude rotation connection angle by extracting orthogonal light waves of mode-dispersed signal light, and a signal output from this equalization optical circuit Light receiving means for receiving light, code string identifying means for identifying a code string from the received signal obtained by this means, and a difference signal is generated by comparing the received signal before identification with the identification signal of this means as a reference. , Rectify, integrate and generate equalization error signal And an parameter control means for generating and outputting a control signal for the phase difference and the amplitude rotation connection angle to the optical equalization circuit so that the equalization error signal obtained by this means is minimized. Optical fiber polarization mode dispersion compensator. 2. 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 by an angle according to a control signal. 2. The optical fiber polarization mode dispersion compensator according to claim 1, wherein the phase difference variable function is realized by partially changing the temperature of the polarization maintaining fiber according to the control signal.