JPH0795178B2 - Intermediate frequency stabilization method - Google Patents

Description

TECHNICAL FIELD The present invention relates to an intermediate frequency stabilizing method in an optical heterodyne receiving method used in an optical communication system.

(Prior Art) Optical heterodyne detection communication (coherent optical communication) has a higher reception sensitivity and higher frequency utilization efficiency than direct detection communication in which the intensity of light is modulated. Have. (Saito, Yamamoto, Kimura "Coherent Optical Fiber Transmission Modulation Technology-FSK Optical Heterodyne Detection", Denka Kogyo Practical Report, Vol. 31, No. 12, 1982)
In this coherent optical communication, the combined light of the signal light and the local oscillation light is received by a photodetector, a beat corresponding to the frequency difference between the signal light and the local oscillation light is obtained as an intermediate frequency signal, and this is demodulated to produce a base signal. A band signal is obtained.

By the way, in this method, the polarization state of the signal light and the locally oscillated light become inconsistent due to the polarization fluctuation of the signal light, and the reception sensitivity is deteriorated. Further, the intermediate frequency fluctuates due to the frequency fluctuations of the signal light and the local oscillation light, and the reception characteristics deteriorate. Therefore, in the optical heterodyne detection communication, it is necessary to compensate the polarization fluctuation of the signal light and stabilize the intermediate frequency.

In the optical heterodyne detection communication, the polarization diversity optical receiving method has been known as one of the methods for compensating for the polarization fluctuation of the signal light. This is because the combined light of the signal light and the locally oscillated light is separated into first and second combined light whose polarization states are orthogonal to each other in the polarization separation unit, and the light is received by individual light receivers to receive the first light.
And a second intermediate frequency signal, and the processing section demodulates and synthesizes the two intermediate frequency signals to obtain a stable baseband signal that does not depend on the polarization state of the signal light. As a method of demodulating and synthesizing signals in this processing unit, two intermediate frequency signals are made to match in phase and then synthesized, and a method of individually demodulating the two intermediate frequency signals and then synthesizing ( Baseband synthesis method). Among them, the latter baseband synthesis method has a simple structure because it is not necessary to adjust the phase of the intermediate frequency signal and can reduce the deterioration of reception sensitivity, and thus is actively researched. For example, Glance (B. Glance) has theoretically clarified that the sensitivity degradation in the polarization diversity optical receiving method using the baseband combining method that demodulates by PSK differential synchronous detection is 0.4 dB. (B.Glance, "Polarization independent coheren optical receiver"("Polarization independent coheren
t opticalreceiver ") Journal of Lightwave Technology 5th
Vol. 1987, p. 274) On the other hand, as another method of stabilizing the intermediate frequency, which is another problem of optical heterodyne detection communication, the intermediate frequency signal is input to the frequency discriminator, and the oscillation frequency of the local oscillation light source is output by the output. A control method or the like was used.

(Problems to be Solved by the Invention) However, in the polarization diversity optical receiving method using the baseband combining method, there are two systems of intermediate frequency signals,
Since the phase and intensity of each intermediate frequency signal fluctuate according to the polarization state of the signal light, if these two intermediate frequency signals are added together as they are, the two intermediate frequency signals will cancel depending on the polarization state of the signal light. There is a case that fits. Therefore, in order to stabilize the intermediate frequency, it is necessary to obtain the control signal by frequency-discriminating the two intermediate-frequency signals with the two frequency discriminators and then synthesizing the outputs thereof, resulting in an increase in the system scale. There was a problem.

Therefore, an object of the present invention is to provide an intermediate frequency stabilizing method having a simple structure, which is not affected by the polarization fluctuation of the signal light, in the polarization diversity optical receiving method using the baseband combining method.

(Means for Solving the Problems) In the present invention, a signal light transmitted from an optical transmission unit is separated into two polarization components orthogonal to each other by a polarization separation unit, and the polarization-separated signal light and a local component are separated. In the polarization diversity optical heterodyne reception method, a demodulated signal output is obtained by demodulating two intermediate frequency signals obtained by detecting a beat component with oscillating light with two photodetectors and then combining the two intermediate frequency signals, A part of the frequency signal is taken out, one of the intermediate frequency signals is delayed, the two are combined, the frequency fluctuation of the combined intermediate frequency signal is detected by a frequency discriminator, and the local oscillation is generated by the output of this frequency discriminator. It is an intermediate frequency stabilization method characterized by controlling the oscillation frequency of a light source.

(Operation) In the present invention, by delaying one of the two intermediate frequency signals as described above, the correlation between the two is reduced,
Even if they are combined, they will not cancel each other out. Therefore, by performing frequency discrimination using the synthesized intermediate frequency signal and feeding back the oscillation frequency of the local oscillation light source, the intermediate frequency can be stabilized regardless of the polarization state of the signal light.

(Embodiment) FIG. 1 is a block diagram of the first embodiment of the present invention, and FIG. 2 is an explanatory view of the operation of the present invention.

The embodiment shown in FIG. 1 is an application of the present invention to a 2 Gb / s PSK differential synchronous optical heterodyne detection receiver. The signal light 1 PSK-modulated at 2 Gb / s is multiplexed with the local oscillation light 3 by the optical multiplexer 2, then separated into two orthogonal polarization components by the polarization demultiplexing unit 4, and the photodetector 5 respectively. , 6 and the intermediate frequency signals 7 and 8 are output. In the processing unit 9, the intermediate frequency signals 7 and 8 are demodulated by the PSK differential synchronous optical heterodyne detection method and then combined, and a baseband signal 10 is output. This PSK differential synchronous optical heterodyne detection system is described in, for example, “400 Mb / s Optical D” by Emura.
PSK Heterodyne Detection Extractions Uses DBR Laser Diode with External Optical Feedback "(" 400Mb / s Optical DPSK
heterodyne detection experiments using DBR, laser
diode with external optikal feedback ") I OC OC-EC OC '85 (IOCC-ECO
C'85) A detailed explanation is given in the literature such as the technical digest and page 401.

Here, a part of the two intermediate frequency signals 7 and 8 is taken out to stabilize the frequency of the intermediate frequency signal, and after a delay of τ is given to one of the intermediate frequency signals by the delay circuit 11,
Both are combined by the combining circuit 12. The spectrum of the synthesized intermediate frequency signal 13 is shown in FIG. In this spectrum, valleys occur at a frequency interval of 1 / τ, but they occur because the intermediate frequency signals 7 and 8 cancel each other out. This valley becomes the deepest when the splitting ratio in the polarization splitting part of becomes 1: 1. However, by increasing the delay amount τ and sufficiently reducing the frequency interval of this valley,
The influence of this valley on the frequency stability can be made small enough to be ignored. In the present embodiment, a 6 m long coaxial cable is used as the delay circuit 11, the synthesized intermediate frequency signal 13 is frequency discriminated by a frequency discriminator 14, and a frequency control signal 15 is output. Feedback is applied to the oscillation frequency. In this embodiment, the frequency discriminator 14 including the delay line 17, the mixer 18, and the first low-pass filter 19 can stabilize the intermediate frequency to 4 GHz, and the frequency fluctuation amount is the polarization state fluctuation of the signal light. However, I was able to keep it below 10MHz.

FIG. 3 shows a block diagram of the second embodiment. In the second embodiment, in the frequency discriminator 14, the synthesized intermediate frequency signal
Frequency discrimination is performed by comparing the levels of 13 high-frequency components and low-frequency components. That is, the high-pass filter 20 and the low-pass filter 21 each having a cut-off frequency of 4 GHz extract the high-frequency component and the low-frequency component of the synthesized intermediate frequency signal 13 to detect the first power detector 22 and the second power detector. The signal level is detected by the device 23. The outputs of the first and second power detectors 22 and 23 are input to the adder 24 and the subtractor 25, and the output of the subtractor 25 is divided by the output of the adder 24 in the divider 26. It As a result, the output of the divider 26 does not depend on the intermediate frequency signal level but changes only in accordance with the frequency fluctuation of the intermediate frequency signal. Therefore, the oscillation frequency of the local oscillation light source 16 is controlled by using this as a control signal to control the frequency of the intermediate frequency signal. Could be stabilized at 4 GHz, and its frequency fluctuation could be suppressed to 5 MHz or less.

Although the two embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and it goes without saying that various modifications and changes can be made within the scope of the present invention.

For example, in the first and second embodiments, the signal modulation / demodulation method is 2 Gb /
Although the sPSK differential synchronous detection method is used, the present invention can be applied regardless of the signal modulation method and bit rate. It is also possible to use frequency discriminators other than those shown in the first and second embodiments for discriminating the frequency of the intermediate frequency signal. Further, in the first and second embodiments, the intermediate frequency signal is delayed by using the coaxial cable, but any delay method can be adopted as long as the necessary band for frequency discrimination and a sufficient delay amount are secured. I don't care.
For example, a delay line using a strip line, a broadband amplifier, or the like can be used as the delay circuit, and when the signal bit rate is low, a delay circuit using an LC circuit can be used.

(Effect of the Invention) As described above in detail, in the present invention, one of the two intermediate frequency signals is delayed and then combined, so that the phase difference between the two intermediate frequency signals changes due to the polarization fluctuation of the signal light. However, the two intermediate frequency signals do not cancel each other. Therefore, it is not necessary to separately discriminate the two intermediate frequency signals from each other, and the control signal for stabilizing the intermediate frequency can be obtained by discriminating the frequency of the combined intermediate frequency signal with one frequency discriminator. Therefore, according to the present invention, in the baseband combining type polarization diversity light receiving method, the intermediate frequency can be stabilized with a simple structure without depending on the polarization state of the signal light.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the first embodiment of the present invention, FIG. 2 is a diagram for explaining the operation of the present invention, and FIG. 3 is a diagram for explaining the second embodiment of the present invention. Is. In each figure, 1 ... Signal light, 2 ... Optical multiplexer, 3 ... Local oscillation light, 4
...... Polarization splitting unit, 5,6 …… Photodetector, 7,8 …… Intermediate frequency signal, 9 …… Processing unit, 10 …… Baseband signal, 11 ……
Delay circuit, 12 …… Synthesis circuit, 13 …… Synthesized intermediate frequency signal, 14 …… Frequency discriminator, 15 …… Intermediate frequency control signal, 16 …… Local oscillation light source, 17 …… Delay line, 18 …… Mixer, 19 ... First low-pass filter, 20 ... High-pass filter, 21 ... Second low-pass filter, 22 ... First power detector, 23 ... Second power detector, 24 ... Addition vessel,
95 ... Subtractor, 26 ... Divider.

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location H04B 10/04 10/06 10/142 10/152

Claims (1)

[Claims] 1. A signal light transmitted from an optical transmission unit is separated into two polarization components orthogonal to each other by a polarization separation unit, and the beat component of the polarization separated signal light and local oscillation light is divided into two. In the polarization diversity optical heterodyne receiving method in which two intermediate frequency signals detected by one photodetector are respectively demodulated and then combined to obtain a demodulated signal output, a part of the two intermediate frequency signals is taken out, After delaying one of the intermediate frequency signals, synthesizing them, detecting the frequency fluctuation of the synthesized intermediate frequency signal with a frequency discriminator, and controlling the oscillation frequency of the local oscillation light source by the output of this frequency discriminator. A method for stabilizing an intermediate frequency characterized by: