JP3262444B2 - Automatic equalizer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic equalizer for compensating for transmission characteristic deterioration due to polarization dispersion or chromatic dispersion of an optical transmission line such as an optical fiber or an optical amplifier repeater through which an optical signal propagates.

[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, there are various kinds of dispersion in the optical fiber and the optical components constituting the optical amplification repeater. Mainly, there are chromatic dispersion in which the group velocity of the optical signal differs for each frequency component, and polarization dispersion that varies depending on the polarization state. Since these dispersions cause distortion of the signal waveform and deteriorate transmission characteristics, compensation is required.

[0003] Transmission characteristic degradation 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 represented by p∝γ (1−γ) Δτ ² .

FIG. 9 shows a prior application (Japanese Patent Application No. 5-273812).
2) shows a basic configuration of the polarization dispersion compensator. This is a configuration that compensates for transmission characteristic degradation due to polarization dispersion and can cope with polarization dispersion that fluctuates with time. That is, the optical transmitter 1 superimposes the frequency modulation (frequency f) on the optical signal and transmits the signal to the optical transmission line 3. On the receiving side, the transmission line polarization main axis detector 5 detects the polarization main axis at the output end of the system including the optical transmission line 3 and the polarization control circuit 4 using the frequency modulation signal. The control signal obtained here is fed back to the polarization control circuit 4 so that the two polarization main axes of the variable polarization dispersion providing unit 6 have a fast group velocity and a slow group velocity. The slow axis and the fast axis of the main polarization axis at the output end of the polarization control circuit 4 can be matched. Further, by controlling the polarization dispersion value of the variable polarization dispersion providing unit 6 using the signal-to-noise ratio in the optical receiver 2 and giving the inverse dispersion corresponding to the polarization dispersion of the optical transmission line 3, The transmission characteristic deterioration due to the polarization dispersion of the transmission path 3 is adaptively compensated.

On the other hand, conventional chromatic dispersion equalizers used for compensating chromatic dispersion include a grating type, a Fabry-Perot type, a multi-stage Mach-Zehnder type, and the like, but all have a fixed equalization characteristic.

[0006]

The polarization dispersion compensator of the prior application is configured to perform polarization dispersion compensation using a frequency modulation signal superimposed on a main signal, and has an effect on transmission characteristics of the main signal. Was inevitable.

Further, since the conventional chromatic dispersion equalizer has a fixed equalization characteristic, it cannot cope with a case where the transmission characteristic deterioration due to chromatic dispersion varies with time. Therefore, if the signal light wavelength shifts due to, for example, deterioration of the semiconductor laser serving as the light source or other factors, the chromatic dispersion value of the entire system including the optical transmission line and the chromatic dispersion equalizer deviates from zero dispersion, and the characteristic deterioration due to chromatic dispersion is reduced. Sometimes it grew.

According to the present invention, deterioration of transmission characteristics due to various dispersions of an optical transmission line can be compensated for only on the receiving side without manipulating a main signal propagating through the optical transmission line. It is an object of the present invention to provide an automatic equalizer that can automatically and adaptively compensate for changes over time.

[0009]

FIG. 1 shows the basic configuration of an automatic equalizer according to the present invention. In the figure, (1) is a basic configuration of an automatic equalizer that compensates for transmission characteristic deterioration due to polarization dispersion of an optical transmission line. This is a configuration in which the detecting means 50 is connected in cascade. (2) is a basic configuration of an automatic equalizer for compensating for transmission characteristic deterioration due to chromatic dispersion of the optical transmission line. The inverse chromatic dispersion imparting means 30 and the transmission characteristic deterioration detecting means 50 are cascaded at the output end of the optical transmission path 3. The configuration is connected to. (3) is a basic configuration of an automatic equalizer for compensating for transmission characteristic deterioration due to chromatic dispersion and polarization dispersion of the optical transmission line. The inverse dispersion imparting means 40 and the transmission characteristic deterioration detecting means are provided at the output end of the optical transmission line 3. Reference numeral 50 denotes a configuration connected in cascade.

[0010] The reverse polarization dispersion providing means 10 is configured to provide reverse dispersion to the polarization dispersion of the optical transmission line 3 and change its polarization dispersion characteristic according to a control signal. The inverse chromatic dispersion imparting means 30 is configured to give an inverse dispersion to the chromatic dispersion of the optical transmission line 3 and change its chromatic dispersion characteristic according to the control signal. The inverse dispersion imparting means 40 is configured to give an inverse dispersion to the polarization dispersion and the chromatic dispersion of the optical transmission line 3, and to change the polarization dispersion characteristics and the chromatic dispersion characteristics according to the control signal. The transmission characteristic deterioration detecting means 50 detects the transmission characteristic deterioration due to the dispersion characteristic of the optical transmission line 3 and the system including the reverse polarization dispersion providing means 10, the reverse chromatic dispersion providing means 30, and the reverse dispersion providing means 40, and responds accordingly. The configuration is such that a control signal is applied to the reverse polarization dispersion providing means 10, the reverse wavelength dispersion providing means 30, and the reverse dispersion providing means 40.

[0011]

The optical signal affected by the dispersion characteristics of the optical transmission line 3 passes through the inverse dispersion applying means 10, 30, 40 and is received by the transmission characteristic deterioration detecting means 50. Therefore, the optical signal received by the transmission characteristic deterioration detecting means 50 suffers from transmission characteristic deterioration due to dispersion characteristics determined by both the optical transmission line 3 and each of the inverse dispersion providing means 10, 30, and 40. The transmission characteristic deterioration detecting means 50 detects the transmission characteristic deterioration and feeds back a corresponding control signal to each of the inverse dispersion imparting means 10, 30, 40, and controls the transmission characteristic deterioration of the optical signal in a direction to reduce the deterioration. With this feedback system, even if each dispersion characteristic of the optical transmission line 3 changes with time, it is possible to automatically and adaptively compensate for the degradation of the transmission characteristic following the change.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An automatic equalizer according to the present invention comprises an inverse polarization dispersion applying means 10, an inverse chromatic dispersion applying means 30, and an inverse dispersion applying means 40.
And the transmission characteristic deterioration detecting means 50 are combined, and the embodiment will be described separately for each means.

FIGS. 2 and 3 show reverse polarization dispersion imparting means 1 for compensating for deterioration of transmission characteristics due to polarization dispersion of the optical transmission line 3.
0 is an example. FIG. 4 shows an inverse chromatic dispersion providing means 3 for compensating for deterioration of transmission characteristics due to chromatic dispersion of the optical transmission line 3.
0 is an example. FIG. 5 shows an embodiment of the inverse dispersion imparting means 40 for compensating for the deterioration of the transmission characteristics due to the chromatic dispersion and the polarization dispersion of the optical transmission line 3. 6 and 7 show an embodiment of the transmission characteristic deterioration detecting means 50.

First, a first embodiment and a second embodiment of the reverse polarization dispersion providing means 10 will be described with reference to FIGS. Reverse polarization dispersion providing means 10 of the first embodiment shown in FIG.
Is composed of a polarization control circuit 11, a polarization maintaining fiber 15 functioning as a polarization dispersion imparting unit, and a digital signal processing circuit (DSP) 16. Polarization control circuit 11
Is a configuration in which a quarter-wave plate 12, a half-wave plate 13, and a quarter-wave plate 14 are connected in cascade. The digital signal processing circuit 16 controls the angle of each of the wave plates 11, 12, and 13 in the direction of the optimum value by the “hill-climbing method” according to the control signal given from the transmission characteristic deterioration detecting means 50. The optical transmission line 3 and the polarization maintaining fiber 1 are
5 is changed to control the polarization dispersion characteristics of the system including the optical transmission line 3 and the reverse polarization dispersion imparting means 10.
By using a polarization dispersion value corresponding to the average value of the polarization dispersion values of the optical transmission line 3 as the polarization dispersion value of the polarization maintaining fiber 15, efficient polarization dispersion compensation can be performed (see Japanese Patent Application No. Hei. 5-273812).

The reverse polarization dispersion providing means 10 of the second embodiment shown in FIG. 3 has a repetitive structure in which a similar polarization control circuit 11 and a polarization delay line 21 are connected in cascade in n stages. The polarization control circuit 11 includes a 波長 wavelength plate 12, a 波長 wavelength plate 13, 1 /
This is a structure in which four wavelength plates 14 are connected in cascade. The polarization delay line 21 is composed of a polarization beam splitter 22, an optical delay line 23, and a polarization beam splitter 24. The polarization delay line 21 gives different delay times to the two polarized waves separated by the polarization beam splitter 22, and re-polarizes. In this structure, the beam is split by the beam splitter 24. The angle of each wave plate of the polarization control circuit 11 and the delay time T of the optical delay line 23 of the polarization delay line 21 are set by a digital signal processing circuit (DSP) 16. That is, the digital signal processing circuit 16 applies a perturbation in the alphabetical order shown in the figure according to the control signal given from the transmission characteristic deterioration detecting means 50, and
2, 13, 14 and the delay time T of the optical delay line 23
Is controlled in the direction of the optimum value by the “hill climbing method”.

The modeling of polarization dispersion is treated as a case where a large number of optical fibers having birefringence are coupled with random angles of axes representing the birefringence. The repetition structure of the inverse dispersion imparting means 10 of this embodiment has exactly the same configuration as the modeling of the polarization dispersion, and if the number of repetitions n is sufficiently large, the transmission characteristic degradation due to the polarization dispersion is in principle possible. Can be completely suppressed.

Next, an embodiment of the reverse wavelength dispersion providing means 30 will be described with reference to FIG. The inverse chromatic dispersion imparting means 30 of the present embodiment is a chromatic dispersion equalizer having a configuration in which Mach-Zehnder interferometers each including an optical coupler 31 and an optical delay line 32 are connected in cascade by n stages. This is the collection of lecture papers, B-956, 4th volume, p.108). The delay time T of the optical delay line 32 at each stage is determined by the transmission characteristic deterioration detecting means 50.
Are set uniformly in accordance with the control signal given from the controller, and the amount of chromatic dispersion changes in accordance with the change in the delay time T. In other words, the inverse chromatic dispersion imparting means 30 of the present embodiment changes the delay time T of the optical delay line 32 at each stage according to the control signal obtained by the transmission characteristic deterioration detecting means 50, thereby obtaining the transmission characteristic due to chromatic dispersion. Deterioration can be compensated automatically and adaptively.

Next, with reference to FIG. 5, a description will be given of an embodiment of the inverse dispersion imparting means 40 for compensating the transmission characteristic deterioration due to the chromatic dispersion and the polarization dispersion of the optical transmission line 3. The inverse dispersion imparting means 40 of the present embodiment comprises an inverse wavelength dispersion imparting means 3 in which the optical coupler 31 and the optical delay line 32 shown in FIG.
0, the polarization control circuit 11 and the polarization delay line 2 shown in FIG.
1 is connected in cascade with reverse polarization dispersion imparting means 10 in which n stages are connected in cascade. Further, the digital signal processing circuit (DSP) 16 perturbs in the alphabetical order shown in the figure according to the control signal given from the transmission characteristic deterioration detecting means 50, and the optical delay line of each stage of the inverse chromatic dispersion providing means 30. 32, the angle of each of the wave plates 12, 13, and 14 at each stage of the reverse polarization dispersion imparting means 10, and the optical delay line 2.
3 is controlled in the direction of the optimum value by the “hill-climbing method”.

With this configuration, the optical signal output from the optical transmission line 3 is compensated for chromatic dispersion by the inverse chromatic dispersion imparting means 30 and is further compensated for polarization dispersion by the inverse polarization dispersion imparting means 10. Can be. That is, it is possible to simultaneously compensate for transmission characteristic degradation due to chromatic dispersion and polarization dispersion of the optical transmission line 3.

Next, a first embodiment and a second embodiment of the transmission characteristic deterioration detecting means 50 will be described with reference to FIGS. In the following description, the reverse polarization dispersion providing means 1
0, the inverse chromatic dispersion imparting means 30, and the inverse dispersion imparting means 40 are collectively described as an inverse dispersion imparting means 60.

Since the STM (synchronous transfer mode) signal contains information for detecting the bit error rate in the overhead portion, the bit error rate of the optical signal can be known by detecting this. Therefore, assuming that the signal format is an STM signal, the transmission characteristic deterioration detecting means 50 can be provided with the bit error rate detecting function of the optical receiver 2 as shown in FIG. The detected bit error rate is fed back to the inverse dispersion providing means 60 as a control signal, and the minimum value is controlled and the optical transmission line 3 is controlled.
Is controlled so as to be canceled by the dispersion characteristic of the inverse dispersion imparting means 60. As a result, transmission characteristic deterioration due to various dispersion characteristics of the optical transmission line 3 is suppressed, and transmission with a good bit error rate characteristic can be performed.

The transmission characteristic deterioration detecting means 50 of the second embodiment shown in FIG. 7 includes an optical coupler 51, an opto-electrical converter 52,
It comprises an amplifier 53, a band-pass filter (BPF) 54, a square detector 55, and a low-pass filter (LPF) 56. The optical coupler 51 is inserted before the optical receiver 2 and splits an optical signal that has passed through a system including the optical transmission line 3 and the inverse dispersion providing means 60. The optical signal split by the optical coupler 51 is converted into an electric signal by an opto-electric converter 52 having a band equal to or more than k (0 <k <1) times the bit rate. After the electric signal is amplified by the amplifier 53, the electric signal is passed through a band-pass filter 54 whose center frequency is k times the bit rate, further detected by a square detector 55 and passed through a low-pass filter 56. The baseband signal obtained here is fed back to the inverse dispersion providing means 60 as a control signal. With such a configuration, eye opening deterioration due to dispersion is detected as shown in FIG. That is, the control signal (baseband signal)
Of the transmission characteristic can be detected from the change in the spectrum of the signal.

The magnitude of the control signal can be calculated by branching the component of the center frequency ω of the band-pass filter 54 in the baseband signal at the power branching ratio γ and giving a delay Δτ thereto. As a result, the magnitude of the control signal is obtained in proportion to 1−γ (1−γ) ω ² Δτ ² . Here, ω is the bandpass filter 5
The center frequency of No. 4 is represented by an angular frequency. The parameters Δτ and γ are the dispersion value of the system composed of the optical transmission line 3 and the inverse dispersion imparting means 60 and the branching ratio of the optical power incident on the incident-side polarization main axis. Therefore, when such a control signal is controlled to the maximum value, the value of the parameter γ (1−γ) Δτ ² representing the eye opening deterioration becomes small, and the deterioration of the transmission characteristics due to the dispersion of the optical transmission line 3 can be compensated.

[0024]

As described above, the automatic equalizer according to the present invention compensates for the polarization dispersion and chromatic dispersion of the optical transmission line and the deterioration of the transmission characteristics due to the polarization dispersion and the chromatic dispersion by processing only on the receiving side. can do. Further, even with respect to the time-dependent change in the polarization dispersion characteristic of the optical transmission line and the time-dependent change in the wavelength dispersion characteristic due to the time-dependent change in the wavelength of the light source, 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 an automatic equalizer according to the present invention.

FIG. 2 is a block diagram showing a first embodiment of a reverse polarization dispersion providing means 10;

FIG. 3 is a block diagram showing a second embodiment of the reverse polarization dispersion providing means 10;

FIG. 4 is a block diagram showing an embodiment of an inverse chromatic dispersion applying unit 30;

FIG. 5 is a block diagram showing an embodiment of an inverse dispersion imparting means 40 for compensating for transmission characteristic deterioration due to chromatic dispersion and polarization dispersion of the optical transmission line 3.

FIG. 6 is a block diagram showing a first embodiment of the transmission characteristic deterioration detecting means 50;

FIG. 7 is a block diagram showing a second embodiment of the transmission characteristic deterioration detecting means 50;

FIG. 8 is a view for explaining the effect of waveform deterioration caused by dispersion on a baseband signal.

FIG. 9 is a block diagram showing a basic configuration of a polarization dispersion compensator of the prior application (Japanese Patent Application No. 5-273812).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical transmitter 2 Optical receiver 3 Optical transmission line 4 Polarization control circuit 5 Transmission line polarization main axis detector 6 Variable polarization dispersion giving part 10 Reverse polarization dispersion giving means 11 Polarization control circuit 12, 14 1/4 Wave plate 13 1/2 wavelength plate 15 Polarization maintaining fiber 16 Digital signal processing circuit (DSP) 21 Polarization delay line 22, 24 Polarization beam splitter 23 Optical delay line 30 Reverse wavelength dispersion imparting means 31 Optical coupler 32 Optical delay line 40 Inverse dispersion imparting means 50 Transmission characteristic degradation detecting means 51 Optical coupler 52 Opto-electric converter 53 Amplifier 54 Band pass filter (BPF) 55 Square detector 56 Low pass filter (LPF) 60 Inverse dispersion imparting means

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-6-284093 (JP, A) JP-A-7-177088 (JP, A) JP-A-5-316051 (JP, A) JP-A-6-284 61948 (JP, A) JP-A 7-131418 (JP, A) Ken Koseki, group delay dispersion equalization optical circuit, IEICE Spring Conference 1992, Japan, The Institute of Electronics, Information and Communication Engineers, Japan 4-volume, 4-108 (58) Fields surveyed (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08

Claims (7)

(57) [Claims] 1. An automatic equalizer for compensating for transmission characteristic deterioration due to polarization dispersion characteristics of an optical transmission line, wherein an inverse end of the polarization dispersion of the optical transmission line is given to an output end of the optical transmission line, and connect the inverse polarization dispersion imparting means polarization dispersion characteristic is changed in accordance with the control signal, the optical transmission characteristics degradation of the optical main signal due to the dispersion characteristics of the system consisting of the opposite polarization dispersion imparting means and said optical transmission path Direct from main signal
An automatic equalizer, comprising: transmission characteristic deterioration detecting means for detecting contact and providing a control signal according to the contact to the reverse polarization dispersion providing means. 2. The automatic equalizer according to claim 1, wherein the reverse polarization dispersion applying means controls a polarization state of the optical signal in accordance with the control signal, and a predetermined polarization dispersion control circuit. A polarization dispersion imparting unit having a value is connected in cascade, so that an optical signal passing through the polarization dispersion imparting unit is given a polarization dispersion having a characteristic opposite to the polarization dispersion of the optical transmission path, An automatic equalizer characterized in that the polarization state of an optical signal input to a polarization dispersion imparting unit is controlled according to the control signal. 3. The automatic equalizer according to claim 1, wherein the reverse polarization dispersion providing means controls a polarization state of the optical signal according to the control signal, and a polarization control circuit according to the control signal. A plurality of variable polarization dispersion imparting units whose polarization dispersion values change are connected in cascade, and the polarization dispersion of the optical transmission line is totally reduced with respect to the optical signal passing through the variable polarization dispersion imparting units of the respective stages. As the polarization dispersion of the inverse characteristic is given, the polarization state of the optical signal input to the variable polarization dispersion imparting unit of each stage and the polarization dispersion value of the variable polarization dispersion imparting unit of each stage are included in the control signal. An automatic equalizer, characterized in that the automatic equalizer is configured to control according to the condition. 4. An automatic equalizer for compensating for transmission characteristics deterioration due to polarization dispersion characteristics and chromatic dispersion characteristics of an optical transmission line, comprising: an output terminal of the optical transmission line; An inverse dispersion providing means for providing an inverse dispersion and changing its polarization dispersion characteristic and chromatic dispersion characteristic according to a control signal is connected, and an optical main unit based on a dispersion characteristic of a system including the optical transmission line and the inverse dispersion imparting means is connected. An automatic equalizer comprising a transmission characteristic deterioration detecting means for directly detecting signal transmission characteristic deterioration from an optical main signal and providing a control signal corresponding thereto to the inverse dispersion providing means. 5. The automatic equalizer according to claim 4, wherein the inverse dispersion imparting means includes a plurality of cascaded chromatic dispersion equalizers having a Mach-Zehnder interferometer structure whose chromatic dispersion characteristics change according to a control signal. An automatic equalizer characterized in that it has a configuration in which the inverse chromatic dispersion providing means having a connected configuration and the reverse polarization dispersion providing means according to claim 3 are connected in cascade. 6. The automatic equalizer according to claim 1, wherein said transmission characteristic deterioration detecting means is configured to generate a control signal using a bit error rate detected by an optical receiver. An automatic equalizer characterized by the following. 7. The automatic equalizer according to claim 1, wherein the transmission characteristic deterioration detecting means converts an optical signal passing through a system including an optical transmission line and an inverse dispersion providing means into an electric signal. An automatic equalizer characterized by generating a control signal according to a change in the spectrum of the baseband signal.