Polarization diversity optical receiver for coherent optical communication

Abstract: Disclosed is a polarized wave diversity optical receiver for coherent optical communication comprising: an optic­ al local oscillating circuit (7) for oscillating local oscillating light; a mixing circuit (8) for mixing sig­ nal light and the local oscillating light to obtain two polarized components; a detecting circuit (9) for detec­ ting the polarized components to output intermediate frequency signals (e S and e P ); and a frequency con­ trol circuit (11) for controlling, in accordance with the intermediate frequency signals (e S and e P ), the oscillating frequency of the optical local oscillating circuit (7). To ensure that the intermediate frequency is not disappeared, the frequency control circuit (11) outputs a combined signal of a sum and a difference of said intermediate frequency signals (e S and e P ).

Description: (1) Field of the Invention The present invention relates to a polarization wave diversity optical receiver for coherent heterodyne optical communication, and more particularly, to a polarization wave diversity optical receiver utilizing a baseband combining method. A coherent optical communication method utilizing the amplitude, frequency and phase, etc., of a light carrier having a high coherency to provide an optimum optical communication, has become widely used. A problem arising when realizing such a coherent optical communication method is the fluctuation of the state of the polarization wave, and as a countermeasure thereto, a polarization wave diversity receiving method has been proposed. In this polarization wave diversity receiving method, two polarized wave components are combined, and accordingly, in this method of combining intermediate frequencies and a method of combining basebands are considered. The intermediate frequency combining method is a method in which the phases of two polarization wave components are previously made to coincide and are then combined by adding; and the base band combining method is a method in which the two polarization wave components are demodulated and then combined by adding the two components together. In the intermediate frequency combining method, the matching of the phases is difficult, and thus the realization thereof is difficult, whereas, in the baseband combining method, although two detection circuits are necessary, the realization thereof is easy. (2) Description of the Related Art In a conventional coherent optical communication system, a polarization wave diversity receiving system is preferably employed to overcome the signal fading caused by the fluctuation of the state of the polarization wave. To realize the polarization wave diversity receiving system, the above-mentioned baseband combining method is conventionaly and preferably employed. In the conventional baseband combined method, two polarized components of intermediate frequency signals are simply combined and the combined signal is fed back to control a local oscillating circuit. Since the phases of the intermediate frequency signals, however, do not always match each other, the power of the combined signal may often become zero. If the power of the combined signal becomes zero, control of the local oscillating circuit is impossible. So, in the conventional baseband combining system, the stabilization of the intermediate frequency cannot always be guaranteed. US 4,723,315 (relating to some features of the preamble of claims 1, 7) describes mixing and splitting techniques to be used for detecting light coherently by heterodyne and homodyne differential detection. In an optical receiver a mixer mixes a local oscillating light from a local oscillator with the signal light and obtains two polarized components therefrom. A detection circuit formed by a beamsplitter receives and divides the two polarized components into two equal pairs of signals. In two different channels, the sum and the difference of these two signals are calculated and respectively processed through a square law detector. A frequency sensor is provided for evaluating the respective signal passed through the square law detectors for stripping off the data signal and allowing the intermediate frequency to be extracted. A control signal based on this intermediate frequency is used for controlling the local oscillator that outputs the local oscillating light. Furthermore, optical receivers are disclosed here, wherein signals based on the two different polarization components are derived and processed in an amplitude and phase detector that outputs control signals for some delay lines respectively provided for delaying the respective polarized components, before they are added in an adder that provides the output signal. In one further optical receiver disclosed here, a control signal from the amplitude and phase sensor is provided for controlling an optical phase shifter provided for shifting the phase of the received signal light, before it is input into the mixer. SUMMARY OF THE INVENTION An object of the present invention is to provide a polarized wave diversity optical receiver for coherent optical communication, in which a baseband combining method is used and an intermediate frequency signal for stabilizing an optical local oscillating circuit can be obtained regardless of the phase difference and the amplitude ratio of the optical detected signals. This object is solved by a polarized wave diversity optical receiver of claim 1. Furthermore, this object is solved by a polarized wave diversity optical receiver according to claim 7. The control of the local oscillating circuit is always possible and a stabilization of the intermediate frequency can be guaranteed when a phase or a frequency of either one of the polarized components of the local oscillating light from the optical local oscillating circuit is modulated for reducing the interrelationship of both intermediate frequency signals or when the phase or a frequency either of the intermediate frequency signals is modulated for reducing the interrelationship of both intermediate frequency signals. Further advantageous embodiments of the invention may be taken from the dependent claims. In one aspect of the invention, there is provided a polarized wave diversity optical receiver for coherent optical communication, comprising: an optical local oscillating circuit for oscillating local oscillating light; a mixing circuit for receiving signal light transmitted through an optical fiber and the local oscillating light from the optical local oscillating circuit, and for obtaining two polarized components; a detecting circuit for detecting the signals of the respective polarized components from the mixing circuit to output intermediate frequency signals; and local oscillating optical phase modulating means for modulating either one of the polarized components of the local oscillating light from the optical local oscillating circuit. In place of the phase modulating means, in the above-mentioned polarized wave diversity optical receiver, there may be provided, according to an aspect of the present invention, a frequency control circuit for controlling, in accordance with the intermediate frequency signals of the respective polarized components from the detecting circuit, the oscillating frequency of the optical local oscillating circuit. The frequency control circuit comprises means for obtaining an output signal which is a combined signal of a sum and a difference of the intermediate frequency signals. In place of the phase modulating means in the above-mentioned polarized optical receiver, there may also be provided, according to another aspect of the present invention, detected output signal modulating means for modulating either one of the detected outputs of the respective polarized wave components from the detecting circuit. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram showing a conventional coherent light communication system; Fig. 2 is a block diagram showing an example of a conventional coherent light communication polarization diversity light receiver; Figs. 3A to 3C are waveform diagrams explaining the function of the unit shown in Fig 2; Fig. 4 is a block diagram of a further example of a polarized wave diversity optical receiver; Figs. 5 to 7 are principal block diagrams of the present invention; Fig. 8 is a block diagram showing first structure of the optical receiver according to Fig. 4; Figs. 9A to 9D and Figs. 10A to 10D are waveform diagrams for explaining the function of the optical receiver of Fig. 8; Fig. 11 is a block diagram showing a second structure of the optical receiver of Fig. 4; Fig. 12 is a block diagram showing a third structure of the optical receiver of Fig. 4; Fig. 13 is a block diagram showing a fourth structure of the optical receiver of Fig. 4; Fig. 14 is a block diagram showing a first embodiment of the present invention; Fig. 15 is a block diagram showing a second embodiment of the present invention; Fig. 16 is a block diagram showing a third embodiment of the present invention; Fig. 17 is a block diagram showing an fourth embodiment of the present invention; Fig. 18 is a block diagram showing a fifth embodiment of the present invention; Fig. 19 is a block diagram showing a sixth embodiment of the present invention; and Fig. 20 is a block diagram showing a modification of the example in Fig. 8. DESCRIPTION OF THE PREFERRED EMBODIMENTS For better understanding of the present invention, conventional coherent optical communication will first be described with reference to Figs. 1, 2, and 3A and 3C. Figure 1 is a block diagram illustrating a conventional coherent optical communication system. In Fig. 1, 1' is a transmitter having a light source (LS) 2, an optical modulation circuit (MOD) 3, and a light source stabilizing unit (STAB) 4. A semiconductor laser, for example, capable of effecting single mode oscillating, is used as the light source 2. When the semiconductor laser is used as mentioned above, the light source stabilizing unit 4 is provided with a temperature control unit for controlling the laser temperature to a constant value. Further, the optical modulation unit 3 is able to effect FSK (frequency shift keying) modulation, ASK (amplitude shift keying) modulation, and PSK (phase shift keying) or DPSK (differential phase shift keying) modulation of the input signal. Reference 5′ denotes an optical fiber used as the transmission medium and, for example, is usually a single mode optical fiber. Reference 6′ denotes a receiver which comprises an optical local oscillating circuit 7, a mixing circuit 8 for mixing the optical signal transmitted through the optical fiber 5' with the local oscillating light from the optical local oscillating circuit 7, a detection circuit 9 for detecting the mixed output of the mixing circuit 8, and a demodulating/discriminating circuit 10 for taking a signal corresponding to the above-mentioned input signal from the detected output of the detecting circuit 9. A semiconductor laser is also used as the optical local oscillating circuit 7, and the optical local oscillating circuit 7 is also maintained at a constant temperature by a temperature control circuit, so that the unit is stabilized. In principle, this construction allows a coherent optical communication to be conducted. A problem arising when realizing such a coherent optical communication is the above-mentioned fluctuation of the polarized state, and preferably, a polarization diversity receiving system is employed as a countermeasure thereto. Further, when realizing the polarization diversity receiving system, preferably the baseband combining method is utilized as mentioned before. Figure 2 is a block diagram of the polarization diversity optical receiver in which the polarization diversity optical receiving method utilizing the baseband combining method is used. In Fig. 2, 6 denotes a polarization diversity optical receiver comprising an optical local oscillating circuit 7 for oscillating a local oscillating light, a mixing circuit 8 for mixing the two types of polarized components of the signal light transmitted through an optical fiber 5 with the local oscillation light from the optical local oscillating circuit 7, a detecting circuit 9 for detecting the signals of the respective polarized components from the mixing circuit 8, a demodulating-adding-discriminating circuit 10 for taking out the signals corresponding to the transmitted input signal from the detected output of the detecting circuit 9, and an automatic frequency control circuit 11 for controlling oscillating signals of the local oscillating circuit 7 in accordance with the intermediate frequency signals e P and e S of the respective polarized components from the detecting circuit 9. The automatic frequency control circuit 11 comprises an adding circuit 11A for adding the intermediate frequency signals (detected signals) e S and e P of the respective polarized components from the detected circuit 9, and a control circuit 11B for controlling the oscillating frequency of the optical local oscillating circuit 7 in accordance with the output of the adding circuit 11A. The control circuit 11B comprises a frequency discriminating circuit for discriminating the frequency of the signal obtained by the adding circuit 11A, to output the corresponding voltage signal, and a drive circuit for converting the output of the frequency discriminating circuit into an electric current signal and input same to an oscillating frequency control input end of the optical local oscillating circuit 7. The intermediate frequency is stabilized by feeding back the detected output information as shown above. In a conventional polarization diversity optical receiver utilizing the baseband combining method as shown in Fig. 2, however, the phases of the two intermediate frequency signals e S and e P are not matched, and therefor, if added as they are, the control signal (the intermediate frequency signal for the AFC) applied to the optical local oscillating circuit 7 may be, in some cases, zero, so that the frequency discriminating circuit will not operate. Namely, when a heterodyne-type coherent optical communication system of FSK (or ASK) is considered, the above-mentioned detected outputs (intermediate frequency signals) e S and e P can be expressed as follows. e P = (a) 1/2 S(t)cos(wt) e S = (1-a) 1/2 S(t)cos(wt+) wherein a = (signal emitted light power)/(total signal emitted power), S(t) = S 0 m(t) where m(t) = 0 (in case of space) and m(t) = 1 (in case of mark), and  is a phase difference due to polarization. When both are combined as they are, the output signal e(t) is e(t) = e P (t)+e S (t) = S(t){(a) 1/2 cos(wt)+(1-a) 1/2 cos(wt+)} When the worst case (a = 1/2,  = π) is considered, e(t) = 0 will be obtained, and accordingly, the intermediate frequency signal for the AFC can not be obtained. This situation is illustrated in Figs. 3A to 3C. Figures 3A and 3B are examples of the waveforms of the intermediate frequencies e S and e P , respectively, and Fig. 3C is an example of the combined waveform. Next, when a heterodyne-type coherent optical communication system having PSK (or DPSK: differential phase shift keying) is considered, the above-mentioned detected outputs (intermediate frequency signals) e S and e P can be expressed as follows. e P = (a) 1/2 S 0 cos(wt+(t)) e S = (1-a) 1/2 S 0 cos(wt+(t)+) wherein (t) is πm(t) (wherein m(t) = 0 (in case of space) or 1 (in case of mark)), and when both are combined as they are, the output signal e(t) is e(t) = e P (t)+ℓ S (t) = S 0 {(a) 1/2 cos(wt+(t))+(1-a) 1/2 cos- (wt+(t)+)} When the worst case (a = 1/2,  = π) is considered, e(t) = 0 will be obtained, and accordingly, the intermediate frequency signal for the AFC cannot be obtained, as in the case of the heterodyne type coherent optical communication system having FSK (or ASK). Figures 5 to 7 are principal block diagram of the present invention. Before explaining these principles of the invention, first reference will be made to a further example of a polarized wave diversity optical receiver shown in Fig. 4. This polarization diversity optical receiver 6 comprises an optical local oscillating circuit 7, a mixing circuit 8, a detecting circuit 9, a demodulating-adding-discriminating circuit 10, and an automatic frequency control circuit 11. The optical local oscillating circuit 7 is used for oscillating a local oscillating light; the optical mixing circuit 8 is used for mixing the two types of polarized components of the signal light transmitted through the optical fiber 5 with the local oscillating light from the optical local oscillating circuit 7; the detecting circuit 9 is used for detecting the signals of the polarized components from the mixing circuit 8; and the demodulating-adding-discriminating circuit 10 is used for taking out a signal corresponding to the transmitted input signal from the detected output of the detecting circuit 9. Further, the automatic frequency control circuit 11 is used for controlling the oscillating frequency of the optical local oscillating circuit 7 in accordance with the intermediate frequency signals e S and e P of respective polarized components from the detecting circuit 9. The automatic frequency control circuit 11 comprises a first adder 111 for taking a sum of the intermediate frequency signals e S and e P , a subtracter 112 for taking a difference of the intermediate frequency signals e S and e P , a first frequency discriminating circuit 113 for discriminating a frequency with respect to the outputs from the first adder 111, a second frequency discriminating circuit 114 for discriminating a frequency with respect to the outputs from the subtracter 112, and a second adder 115 for adding the output from the first frequency discriminator 113 and the output from the second frequency discriminating 114, as well as power dividers 116 and 117 for inputting the intermediate frequency signals of the respective polarized components from the detecting circuit 9 to the first adder 111 and the subtracter 112, and automatic gain control circuits 118 and 119 for adjusting the output gain from the first adder 111 and the subtracter 112. Further, the automatic frequency control circuit 11 comprises a driver circuit 120 for converting the output of the second adder 115 to an electric current signal and inputting same to the oscillating frequency control input end of the optical local oscillating circuit 120. Now the principles of the invention are explained with reference to Fig. 5-7. A first principle of the inventive polarization diversity optical receiver 6 shown in Fig. 5 comprises an optical local oscillating circuit 7, a mixing circuit 8, a detecting circuit 9, a demodulating-adding-discriminating circuit 10, an automatic frequency control circuit 11, and a local oscillating optical phase modulation means 13. The mixing circuit 8 and the automatic frequency control circuit 11 of the polarization diversity optical receiver 6 shown in Fig. 5 are different from those of the polarization diversity optical receiver shown in Fig. 4. Namely, the mixing circuit 8 in Fig. 5 is used for dividing the signal light transmitted through the optical fiber 5 and the local oscillating light from the optical local oscillating circuit 7 into two types of polarized components respectively, and for mixing the respective polarized components. A local oscillating optical phase modulation means 13 is provided in the mixing circuit 8, and is used for modulating the phase of one of the polarized components divided from the local oscillating light from the optical local oscillating circuit 7. Note that this includes shifting the phase of one of the divided polarized components with respect to the phase of the other polarized components. Note that the automatic frequency control circuit 11 is provided, as in Fig. 2, with the adding circuit 11A and the control circuit 11B. A second principle of the inventive polarization diversity optical receiver 6 shown in Fig. 6 also comprises an optical local oscillating circuit 7, a mixing circuit 9, a demodulating-adding-discriminating circuit 10, an automatic frequency control circuit 11, and a local oscillating optical phase modulating means. The local oscillating optical phase modulating means is provided in the optical local oscillating circuit 7. Namely, the optical local oscillating circuit 7 in Fig. 6 comprises two semiconductor lasers 71 and 72 for outputting local oscillating lights having different polarization states to each other, and further, it is so constructed that the output phase of one of these semiconductor lasers 71 and 72 is modulated or one output frequency is shifted by a predetermined amount with respect to the other output frequency. Accordingly, in the local oscillating lights from the optical local oscillating circuit 7, the phase of one of the polarized components is modulated or the frequency of one polarized component is shifted by a predetermined amount with respect to the frequency of the other polarized component. A third principle of the inventive polarization diversity optical receiver 6 shown in Fig. 7 comprises an optical local oscillating circuit 7, a mixing circuit 8, a detecting circuit 9, a demodulating-adding-discriminating circuit 10, an automatic frequency control circuit 11, and a phase modulation means 14. The local oscillating optical phase modulating means 14 is used for modulating the phase of either of the detected outputs e S and e P of the respective polarized components from the detecting circuit 9. Note that this includes that for shifting the frequency of one of the detected outputs by a predetermined amount with respect to the frequency of the other detected output. Further, the phase modulating means 14 may be replaced by a time delay circuit for delaying one of the detected outputs by a predetermined time with respect to the other detected output. In Figs. 4 to 7, the same symbols represent substantially the same or similar parts. Due to the above construction of the polarization diversity optical receiver 6 shown in Fig. 4, when the intermediate frequency signals e S and e P are fed back, the sum of these intermediate frequency signals e S and e P is output from the adder 111 and the difference between these intermediate frequency signals e S and e P is output from the subtracter 112. Then, with respect to these sum signal e S + e P and the difference signal e S - e P , the frequencies are discriminated by the frequency discriminating circuit 113 and 114, respectively, and further, the outputs after the frequency discrimination are added and combined. Accordingly, neither the sum nor the difference of the intermediate frequency signals e S and e P is made zero, and thus the combined signal is not made zero. Therefore, at least one of the frequency discriminating circuits can be operated, and thus the intermediate frequency signal for stabilizing the optical local frequency oscillating circuit can be always obtained. Also, in the inventive polarization diversity optical receiver 6 shown in Fig. 5, the local oscillation optical phase modulating means 13 modulates one of the divided polarized components of the local oscillating light from the optical local oscillating circuit 7. Accordingly, the interrelationship between the detected output signals is reduced, and thus the combined signal is not made zero, resulting in a stabilization of the intermediate frequency. Further, in the polarization diversity optical receiver 6 shown in inventive Fig. 6, the local oscillating lights from the two semiconductor lasers 71 and 72 in the optical local oscillating circuit 7 have a different polarized states, and one of the output phases of the semiconductor lasers 71 and 72 is modulated, or one of the output frequency is shifted by a predetermined amount with respect to the other of the output frequency. Therefore, it is possible to modulate one of the phases of the polarized components of the local oscillating light from the optical local oscillating circuit 7 or to shift the frequency of one of the polarized components with respect to the frequency of the other polarized component by a predetermined amount, so that the interrelationship between both detected output signals is also reduced and the combined signal is not made zero, enabling a similar stabilization of the intermediate frequency. Still further, in the inventive polarization diversity optical receiver 6 shown in Fig. 7, since a phase modulation of either of the phases of the detected outputs e S and e P of the respective polarization components from the detection circuit 9 can be effected, the interrelationship between the output detected signals is reduced and the combined signal is not made zero, enabling a similar stabilization of the intermediate frequency. The same effects can be obtained when a frequency modulation or a delay is effected on either of the detected outputs e S and e P . (a) Description of a first structure of an optical receiver of Fig. 4 Figure 8 is a block diagram showing a heterodyne-type coherent optical communication system having FSK (or ASK). As shown in Fig. 8, 6 denotes a polarization diversity optical receiver as a receiver. The polarization diversity optical receiver 6 comprises an automatic frequency control circuit 11 in addition to the optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, and the decoding-adding-discriminating circuit 10. The optical local oscillating circuit 7 is used for oscillating local oscillating light P ℓ0 , and uses semiconductor lasers. The laser temperature is maintained at a constant value by a light source stabilizing unit 18 operating as a temperature control unit. The mixing circuit 8 is used for mixing the two polarized components of the signal light Psi transmitted through the optical fiber 5 with the local oscillating light P ℓ0 form the optical local oscillating circuit 7, respectively, and comprises a polarized wave separation optical fiber coupler 19 for separating the waves of the signal light Psi from the optical fiber 5 and the local oscillating light P ℓ0 from the optical local oscillating circuit 7, and a polarization maintaining optical fiber coupler 20 for maintaining the polarized state. The power distribution to the P polarized component of the signal light Psi is aPsi, and the power distribution to the S polarized component of the signal light Psi is (1-a) Psi; the power distribution to the P polarized component of the local oscillating light P ℓ0 is 1/2 P ℓ0 , and the power distribution to the S polarized component of the local oscillating light signal light P ℓ0 is 1/2 P ℓ0 . Note that a represents (signal emitting light power)/(total signal emitting light power of the P polarized component). The detecting circuit 9 is used for detecting the signals of the respective polarized components from the mixing circuit 3, and comprises light-electricity converting circuits 21P and 21S, which are constructed as double-balanced optical receivers, post amplifiers 22P and 22S, and bandpass filters 23P and 23S. The light electricity converting circuits 21P and 21S receive light signals and converting same into electric signals; the post amplifiers 22P and 22S amplify the electric outputs from the light-electricity converting circuits 21P and 21S; and the bandpass filters 23P and 23S filter only a desired frequency band and output the intermediate frequency signals e S and e P . The demodulating-adding-discriminating circuit 10 is provided with a demodulating function for taking out a signal corresponding to the signal transmitted from the detected outputs (intermediate frequency signals) e S and e P of the detection circuit 9, and comprises delay detection circuits 24P and 24S, an adding circuit 25, and a discriminating circuit 27. The delay detecting circuits 24P and 24S comprise delay circuits 24P-1 and 24S-1 and the delay times in the respective delay circuit 24P-1 and 24S-1 are set to be nearly in the case of FSK (ASK). The adding circuit 25 is used for combining by adding the outputs Vp and Vs of the respective delay detecting circuits 24P and 24S. The low pass filter 26 is used for eliminating unnecessary band noise, and the discriminating circuit 27 is used for discriminating "0" and "1" data from the signal, to take out a signal corresponding to the transmitted input signal. Note that the discriminating circuit 27 is able to extract timing signals for discriminating "0" and "1" from the input signals. The automatic frequency control circuit 11 is used for controlling the oscillating frequency of the optical local oscillating circuit 7 based on the intermediate frequency signals e S and e P of respective polarized components from the detecting circuit 9. The automatic frequency control circuit 11 comprises a first adder 111 for taking a sum of the intermediate frequency signals e S and e P , a subtracter 112 for taking a difference of the intermediate frequency signals e S and e P , a first frequency discriminating circuit 113 for discriminating a frequency with respect to the outputs from the first adder 111, a second frequency discriminating circuit 114 for discriminating a frequency with respect to the outputs from the subtracter 112, and a second adder 115 for adding the output from the first frequency discriminating 113 and the output from the second frequency discriminator 114, as well as power dividers 116 and 117 for inputting the intermediate frequency signals of the respective polarized components from the detecting circuit 9 to the first adder 111 and the subtracter 112, and automatic gain control circuits 118 and 119 for adjusting the output gain from the first adder 111 and the subtracter 112. The automatic frequency control circuit 11 further comprises a drive circuit 120 for converting the output of the second adder 115 to an electric current signal and inputting same to the oscillating frequency control input end of the optical local oscillating circuit 120. In Fig. 8, semiconductor lasers for example, by which single mode oscillations are possible, are used as the light source 2. Also, as the light source stabilizing unit 4, a unit having, for example, a temperature control unit for controlling the temperatures of the lasers to a constant value, is used. Further, the amplifier 28 is used for amplifying the input signal (data) and supplying the current signal 1 AC to the oscillating frequency control end of the light source 2. A bias current 1 DC is also supplied to the oscillating frequency control end of the light source 2. And accordingly, the optical modulation is carried out so that the ASK modulation (pulse amplitude modulation) or FSK modulation (pulse frequency modulation) can be effected. Also, in Fig. 8, reference 5 denotes the optical fiber used as a transmission medium. According to the above constitution, when the intermediate frequency signals e S and e P are fed back, the power of the intermediate frequency signals e S and e P is divided into two parts which are input respectively to the first adder 111 and the subtracter 112, and then the sum data of the intermediate frequency signals e S and e P is output from the first adder 111 and the difference data of the intermediate frequency signals e S and e P is output from the subtracter 112. Then the output gain is adjusted by the AGC circuits 118 and 119. Subsequently, with respect to the sum data e S + e P and difference data e S - e P , the frequencies are discriminated by the frequency discriminating circuits 113 and 114, the respective outputs after the discrimination are added and combined by the second adder 115, and the thus-added and combined intermediate signal is input through the drive circuit 120 to the control input end of the optical local oscillating circuit 7. Accordingly, the sum and the difference of the intermediate frequency signals e S and e P are not made zero, and thus the combined signal is not made zero. Namely, as shown in Figs. 9A and 9B, when the phase difference of the intermediate frequency signals e S and e P is zero, the difference of the intermediate frequency signals e S and e P is made zero as shown in Fig. 9C, but the sum of the intermediate frequency signals e S and e P is not made zero as shown in Fig. 9D. Also, when the phase difference of the intermediate frequency signals e S and e P is π, as shown in Figs. 10A and 10B, the sum of the intermediate frequency signals e S and e P is made zero as shown in Fig. 10C, but the difference of the intermediate frequency signals e S and e P is not made zero, as shown in Fig. 10D. Therefore, either of the frequency discriminating circuits 114 and 113 can be operated, and thus the intermediate frequency signal for stabilizing the optical local oscillating circuit can be always obtained. Note that the gain adjusting ability of the AGC circuits 118 and 119 may be three times the gain adjusting ability usually available under an average power supply, when the mark ratio m = 1/2. This value is usually sufficient in a standard AGC circuit. Accordingly, an intermediate frequency signal having a desired frequency is obtained and the desired gain can be held, and thus the desired stabilization of the oscillating frequency (intermediate frequency) of the optical local oscillating circuit 7 can be realized. (b) Description of a Second structure of an optical receiver of Fig. 4 Figure 11 illustrates a heterodyne type coherent optical communication system having FSK (or ASK). As shown in Fig. 11, the polarization diversity optical receiver 6 also comprises an automatic frequency control circuit 11 in addition to the optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, and the decoding-adding-discriminating circuit 10. The constitutions and the functions of the optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, and the decoding-adding-discriminating circuit 10 are the same as those of the first embodiment, and therefore, a detailed description thereof will be omitted. In the optical receiver of Fig. 11, the constitution of the automatic frequency control circuit 11 is different from that in Fig. 8. Therefore, the constitution of the automatic frequency control circuit 11 is described as follows. Namely, the automatic frequency control circuit 11 comprises a first adder 111 for taking a sum of the intermediate frequency signals e S and e P , a subtracter 112 for taking a difference of the intermediate frequency signals e S and e P , a first frequency discriminating circuit 113 for discriminating a frequency with respect to the outputs from the first adder 111, a second frequency discriminating circuit 114 for discriminating a frequency with respect to the outputs from the subtracter 112, and a second adder 115 for adding the output from the first frequency discriminator 113 and the output from the second frequency discriminator 114, as well as power dividers 116 and 117 for inputting the intermediate frequency signals of the respective polarized components from the detecting circuit 9 to the first adder 111 and the subtracter 112, and automatic gain control circuits 118 and 119 for adjusting the output gain from the first adder 111 and the subtracter 112, and is further provided with a delay circuit 121 for delaying either of the intermediate frequency signals e S and e P of the respective polarized components from the bandpass filters 23P and 23S by, for example, one bit, before inputting same to the first adder 111 and the subtracter 112. As in Fig. 8, the automatic frequency control circuit 11 comprises a driver circuit 120 for converting the output of the second adder 115 to an electric current signal and for inputting same to the oscillating frequency control input end of the optical local oscillating circuit 120. In the above constitution, when the intermediate frequency signals e S and e P are fed back, one of the intermediate frequency signals e S and e P i.e., e S , is delayed, and the power of the delayed intermediate frequency signal e S and non-delayed intermediate frequency signal e P are respectively divided by the power dividers 116 and 117 and input to the first adder 111 and the subtracter 112, respectively. Subsequently, the sum data of the intermediate frequency signals e S and e P output from the first adder 111 and the difference data of the intermediate frequency signals e S and e P , is output from the subtracter 112, and then the output gain is adjusted by the AGC circuits 118 and 119. Then the frequencies of these sum signals e S + e P and the difference signal e S - e P are discriminating by the frequency discriminating circuits 113 and 114. The respective outputs after the frequency discrimination are added and combined by the second adder 115, and the adder and combined intermediate frequency signal is input through a drive circuit 120 to the control input end of the optical local oscillating circuit 7. Accordingly, the interrelationship between the intermediate frequency signals e S and e P can be further reduced than in the first embodiment, and therefore, the sum and the difference of the intermediate frequency signals e S and e P are not made zero, and as a result, the combined signal is not made zero. Accordingly, in this case also, since either of the frequency discriminating circuits 114 and 113 can be operated, the intermediate frequency signal for stabilizing the optical local oscillating circuit can be always obtained. Note that the gain adjusting ability of the AGC circuits 118 and 119 also is sufficient for a standard AGC circuit as in Fig. 8, and thus an intermediate frequency signal having a desired frequency can be always obtained and the desired gain can be held, whereby a required stabilization of the oscillating frequency (intermediate frequency) of the optical local oscillating circuit 7 can be obtained. (c) Description of a third structure of the optical receiver of Fig. 4 Figure 12 illustrates a heterodyne-type coherent optical communication system having DPSK. As shown in Fig. 12, 6 denotes a polarization diversity optical receiver which comprises an automatic frequency control circuit 11, in addition to the optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, and the demodulating-adding-discriminating circuit 10. In Fig. 12, the constitution of the automatic frequency control circuit 11 corresponds to that of Fig. 8, except for one portion thereof. Therefore, the constitution of the automatic frequency control circuit 11 will be described as follows. Namely, the automatic frequency control circuit 11 comprises, as in the first embodiment, a first adder 111 for taking a sum of the intermediate frequency signals e S and e P , a subtracter 112 for taking a difference of the intermediate frequency signals e S and e P , a first frequency discrimination circuit 113 for discriminating a frequency of the outputs from the first adder 111, a second frequency discriminating circuit 114 for discriminating a frequency of the outputs from the subtracter 112, and a second adder 115 for adding the output from the first frequency discriminator 113 and the output from the second frequency discriminator 114, as well as power dividers 116 and 117 for inputting the intermediate frequency signals of the respective polarized components from the detecting circuit 9 to the first adder 111 and the subtracter 112, and AGC circuits 118 and 119 for adjusting the output gains from the first adder 111 and the subtracter 112. But different from the first embodiment, the automatic frequency control circuit 11 comprises doublers 122 and 123 for doubling the respective output frequencies before the addition and the subtraction. As mentioned above, the difference between the automatic frequency control circuit 11 of the first embodiment resides in the provision of the doublers, by which the expansion of the spectrum due to the phase information of the intermediate frequencies can be cancelled. Namely, the modulated signal is cancelled and only the doubled frequency components of the intermediate frequency signals are output. Note that, as in Fig. 8, the automatic frequency control circuit 11 comprises a driver circuit 120 for converting the output of the second adder 115 to an electric current signal and inputting same to the oscillating frequency control input end of the optical local oscillating circuit 120. Also, the delay times in the delay circuits 24P-1 and 24S-1 in the delay detecting circuits 24P and 24S are set, in the case of DPSK, to one bit. Note that, in Fig. 12, 1 denotes a transmitter for DPSK. The transmitter 1 comprises a light source 2, an optical modulation circuit 3, a light source stabilizing circuit 30, an amplifier 31, a differential coding circuit 32, and a crystal oscillator 33. As the light source 2, for example, semiconductor lasers capable of single mode oscillation are used. When the semiconductor lasers are used as mentioned above, a unit having, for example, a temperature control unit for controlling the temperature of the lasers, is used. Further, the differential coding circuit 32 is used for receiving a reference signal from the crystal oscillator 33 and input signal (data) and for a differential coding of an output. The output of the differential coding circuit 32 is input through the amplifier 31 and the waveform shaping circuit 30 to the optical modulation circuit 3. Accordingly, optical modulation is carried out so that DPSK modulation of the input signal can be effected. Note that 5 denotes an optical fiber as a transmitting medium. In the above constitution, when the intermediate frequency signals e P and e S are fed back, the expansion of the spectrum due to the phase information of the intermediate frequency signals is cancelled in the doublers 122 and 123, and the power of the intermediate frequency signals e P and e S is divided into two parts by the power dividers 116 and 117, and input respectively to the first adder 111 and the subtracter 112. Subsequently, the sum data of the intermediate frequency signals e P and e S is output from the first adder 111 and the difference data of the intermediate frequency signals e P and e S is output from the output of the subtracter 112. Then, the output gains of these sum signal e S + e P and difference signal e S - e P are adjusted in the AGC circuits 118 and 119, and subsequently the frequencies of these sum signal e S + e P and difference signal e S - e P are discriminated by the frequency discriminating circuits 113 and 114. The respective outputs after the frequency discriminations are added and combined, and the added and combined intermediate frequency signals are input through the driving circuit 120 to the control input end of the optical local oscillating circuit 7. Accordingly, as in Fig. 8, the interrelationship between the intermediate frequency signals e S and e P can be reduced, and therefore, the sum and the difference of the intermediate frequency signals e S and e P are not made zero, and as a result, the combined signal is not made zero. Accordingly, in this case also, since either of the frequency discriminating circuits 114 and 113 can be operated, the intermediate frequency signal for stabilizing the optical local oscillating circuit can be always obtained. Note that the gain adjusting ability of the AGC circuits 118 and 119 is sufficient for a standard AGC circuit as in the first and the second embodiments, and thus an intermediate frequency signal having a desired frequency can be always obtained, and further, the desired gain can be held, so that the required stabilization of the oscillating frequency (intermediate frequency) of the optical local oscillating circuit 7 can be obtained, as in Fig. 8, 11. (d) Description of a fourth structure of an optical receiver of Fig. 4 Figure 13 illustrates a heterodyne-type coherent optical communication system having DPSK. The polarization diversity optical receiver 6 in Fig. 13 also comprises an automatic frequency control circuit 11, in addition to the optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, and the decoding-adding-discriminating circuit 10. The optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, and the decoding-adding-discriminating circuit 10 are the same as those of the first, second, and third embodiments, and therefore, a detailed description thereof is omitted. In Fig. 13, the constitution of the automatic frequency control circuit 11 corresponds to that of the second embodiment, except that it comprises doublers 122 and 123. The portion different from that in the above-mentioned third embodiment comprises a delay circuit for delaying one of the intermediate frequency signals e P and e S of the respective polarized components from the bandpass filters 23P and 23S by, for example, one bit, before inputting same to the first adder 111 and the subtracter 112. Therefore, according to Fig. 13, the interrelationship between the intermediate frequency signals e P and e S can be further reduced than in Fig. 12, and thus the sum and the difference of the intermediate frequency signals e P and e S are not made zero, and therefore, the combined signal is not made zero. As a result, either of the frequency discriminating circuits 114 and 113 can be operated, and accordingly, the intermediate frequency signal for stabilizing the optical local oscillating circuit can be always obtained. The gain adjusting ability of the AGC circuits 118 and 119 is also sufficient for a standard AGC circuit as in Fig. 8, 11, 12, and thus an intermediate frequency signal having a desired frequency can be always obtained, and therefore, the required stabilization of the oscillating frequency (intermediate frequency) of the optical local oscillating circuit 7 can be obtained. (e) Description of the first Embodiment Figure 14 is a block diagram showing a first embodiment of the present invention, and illustrates a heterodyne-type coherent optical communication system having FSK (or ASK). The polarization diversity optical receiver 6 shown in Fig. 14 comprises, in addition to the optical local oscillating circuit 7, a mixing circuit 8, a detecting circuit 9, and a demodulating-adding-discriminating circuit 10, an adding circuit llA and a control circuit 11B, which constitute an automatic frequency control circuit, and a local oscillating optical phase modulating means 13. The optical local oscillating circuit 7 provided with the light source stabilizing unit 18, the detecting circuit 9, and the demodulating-adding-discriminating circuit 10 is the same as in Fig. 8 and Figs. 11-1. The mixing circuit 8 is used for mixing the two polarized components of the signal light Psi transmitted through the optical fiber 5 with the local oscillating light P ℓ0 from the optical local oscillating circuit 7, respectively, and comprises a polarization wave separation optical fiber coupler 19 for separating the waves of the signal light Psi from the optical fiber 5 and the local oscillating light P ℓ0 from the optical local oscillating circuit 7, and a polarization maintaining optical fiber coupler 20 for maintaining the polarized state. The power distribution to the P polarized component of the signal light Psi is aPsi, and the power distribution to the S polarized component of the signal light Psi is (1-a) Psi; the power distribution to the P polarized component of the local oscillating light P ℓ0 is 1/2 P ℓ0 , and the power distribution to the S polarized component of the local oscillating light signal light P ℓ0 is 1/2 P ℓ0 . Note that a denotes (signal emitting light power)/(total signal emitting light power of the P polarized component). The local oscillating optical phase modulation means 13 is used for modulating the phase of the polarized component of the local oscillating light from the optical local oscillating circuit 7, and to this end, comprises an oscillator 131 and a phase modulator 132 for modulating the phase of the polarized component (light signal) of the local oscillating light from the optical local oscillating circuit 7, by using the signal from the oscillator 131. The adding circuit 11A is used for adding the detected outputs e S and e P of the respective polarized components from the bandpass filters 23P and 23S, and to this end, comprises an adder 111'. The adding circuit 11 comprises an automatic gain control circuit (AGC circuit) 15 for adjusting the output gain after the addition in the adder 111'. Further, the control circuit 11B is used for controlling the oscillating frequency of the optical local oscillating circuit 7 based on the output from the adding circuit 11A. To this end, the control circuit 11B is constructed as an automatic frequency control circuit (AFC circuit) comprising a frequency discriminating circuit 16 for discriminating the frequency of the signal obtained by the adding circuit 11A, to output the corresponding voltage output, and a drive circuit 17 for converting the output of the frequency discriminating circuit 16 into a current signal and inputting same to the oscillating frequency control input end of the optical local oscillating circuit 7. In the above construction, in the polarization diversity optical receiver 6 of this first embodiment, the phase of the split polarized component of the local oscillating light from the optical local oscillating circuit 7 is modulated by the local oscillating optical phase modulating means 13, and accordingly, the phase difference between the detected output signals e S and e P can be changed from zero to 2π as a function of time. As a result, the interrelationship between the detected output signals e S and e P can be reduced, and therefore, the combined signal is not made zero, and thus a desired intermediate frequency can be obtained for the AFC. Note that the output gain after the addition is adjusted by the automatic gain control circuit (AGC circuit) 14. Preferably, the gain adjusting ability of the AGC circuit 15 is set to cover, for example, a threefold gain adjustment of the average power. When the mark ratio m = 1/2, the threefold gain adjustment of the average power can be made by a standard AGC circuit 15. Accordingly, an intermediate frequency signal having a desired frequency is obtained and the desired gain can be held, and thus the required stabilization of the oscillating frequency (intermediate frequency) of the optical local oscillating circuit 7 can be obtained. Note that, as the local oscillating optical phase modulating means 13, the means for modulating the phase of the one polarized component (P or S) of the local oscillating light from the optical local oscillating circuit 7, can be replaced by the means for shifting the frequency of the polarized component (P or S) of the local oscillating light from the optical local oscillating circuit 7 by a desired amount with respect to the other polarized component. (f) Description of the second Embodiment Figure 15 is a block diagram showing the second embodiment of the present invention, and illustrates a heterodyne-type coherent optical communication system having FSK (or ASK). The polarization diversity optical receiver 6 comprises, in addition to the optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, and the demodulating-adding-discriminating circuit 10, an adding circuit 11A and a control circuit 11B, as well as a local oscillating optical phase modulating means for shifting the phase of the polarized component of the local oscillating light with respect to the phase of the other polarized component. The local oscillating optical phase modulation means is provided in the optical local oscillating circuit 7. Namely, the optical oscillating circuit 7 comprises a first semiconductor laser 71 for outputting local oscillating light having the P polarized component, and a second semiconductor laser 72 for outputting local oscillating light having the S polarized component. The difference between the output frequencies of the respective semiconductor lasers 71 and 72 is locked to a desired value of, for example, several mega hertz when the frequency of the semiconductor laser 71 or 72 is on the order of 2 × 10 4 hertz, by the automatic frequency control circuit (AFC circuit) 34. The AFC circuit 34 comprises a photo-electric converting circuit 35 constructed as a double-balanced optical receiver, a post amplifier 36, a bandpass filter 37, a frequency discriminating circuit 38, and a driver circuit 39. The photo-electric converting circuit 35 is used for receiving light signals from the respective semiconductor lasers 71 and 72 and converting same into electric signals; the post amplifier 36 is used for amplifying the electric output from the photo-electric converting circuit 35; the bandpass filter 37 is used for filtering and outputting only a desired frequency band; the frequency discriminating circuit 38 is used for discriminating the signals from the bandpass filter 37 and outputting the corresponding voltage signals; and the drive circuit 39 is used for converting the output of the frequency discriminating circuit 38 and inputting same to the oscillating frequency control input end of the optical local oscillating circuit 72. Note that the respective semiconductor lasers 71 and 72 are controlled by the light source stabilizing units 18A and 18B, whereby the laser temperature is kept to a constant value. Also, the mixing circuit 8, the detecting circuit 9, the demodulating-adding-discriminating circuit 10, and the control circuit 11B are the same as in the first embodiment, and therefore, a description thereof is omitted. In the above construction, in the polarization diversity optical receiver 6 shown in Fig. 15, since the respective local oscillating lights from the two semiconductor lasers 71 and 72 in the light local oscillating circuit 7 have different polarized states, and further, the frequency difference of the local oscillating lights is shifted by a predetermined amount, the interrelationship between the detected output signals is reduced and thus the combined signal is not made zero, and therefore the intermediate frequency is stabilized. Further, circuit for shifting the frequency of one output (polarized component P or S) of the semiconductor lasers 71 and 72 by a predetermined amount with respect to the frequency of the other output (polarized component S or P), can be replaced by a circuit for modulating the phase of one output (polarized component P or S) of the semiconductor lasers 71 and 72. (g) Description of the third Embodiment Figure 16 is a block diagram showing the third embodiment of the present invention, and illustrates a heterodyne type coherent optical communication system having FSK (or ASK). The polarization diversity optical receiver 6 shown in Fig. 16 comprises, in addition to the optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, the demodulating-adding-discriminating circuit 10, the adding circuit 11A and the control circuit 11B, as well as a detected output phase modulating means 14 for modulating the phase of either of the detected outputs e S and e P of the respective polarized components from the detecting circuit 9. The detected output phase modulating means 14 comprises an oscillator 141 and a phase modulator 142 for modulating, by the signal from the output of the oscillating circuit 141, the phase of one (electric signal) of the detected outputs e S and e P of the respective polarized components from the detecting circuit 9. The optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, and the demodulating-adding-discriminating circuit 10, and the control circuit 12 are the same as in the fifth embodiment, and therefore, a detailed description thereof is omitted. Note that the local oscillating optical phase modulating means 13 is not provided in the mixing circuit 8. In the above construction, in the polarization diversity optical receiver 6, either of the detected outputs e S and e P of the respective polarized components from the detecting circuit 9 is modulated by the detected output phase modulating means 14, and accordingly, the phase difference between, for example, the detected output signals e S and e P , can be changed from 0 to π as a function of time. As a result, the interrelationship between the detected output signals e S and e P can be reduced and thus the combined signal is not made zero, and therefore, a desired intermediate frequency signal for the AFC can be obtained whereby the stabilization of the intermediate frequency can be obtained in the same way as in the previous embodiments. Note that, as the detected output phase modulating means 14, the means for modulating the phase of either (e S or e P ) of the detected outputs e S and e P of the respective polarized components from the detecting circuit 9 can be replaced by a means for shifting the frequency of one (e S or e P ) of the detected outputs e S and e P of the respective polarized components from the detecting circuit 9 by a desired amount with respect to the frequency of the other polarized component (e P or e S ). (h) Description of the fourth Embodiment Figure 17 is a block diagram showing the fourth embodiment of the present invention, and illustrates a heterodyne-type coherent optical communication system having DPSK. As shown in Fig. 17, the polarization diversity optical receiver 6 comprises, in addition to the optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, the demodulating-adding-discriminating circuit 10, the adding circuit 11A and the control circuit 11B, as well as a local oscillating optical phase modulating means 13. The optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, the demodulating-adding-discriminating circuit 10, and the control circuit 11B are the same as in the first embodiment, and therefore, a detailed description thereof is omitted. Note that the adding circuit 11A is used for adding the detected outputs e S and e P of the respective polarized components from the bandpass filters 23P and 23S, and to this end, comprises an adder 111'. Also, doublers 29P and 29S are provided for doubling the output frequencies before the addition by the adder 111'. The output gains of the doublers 29P and 29S are adjusted by the automatic gain control circuit (AGC circuit) 15. The difference from the first embodiment shown in Fig. 14 resides in the provision of the doublers 29P and 29S, by which the expansion of the spectrum due to intermediate frequency signal can be cancelled. In the above construction, in the polarization diversity optical receiver 6 of this eighth embodiment, the phase of the split polarized component of the local oscillating light from the optical local oscillating circuit 7 is modulated by the local oscillating optical phase modulating means 13, and accordingly, the phase difference between, for example, the detected output signals e S and e P , can be changed from 0 to π as a function of time. As a result, the interrelationship between the detected output signals e S and e P can be reduced, and thus the combined signal is not made zero, whereby a desired intermediate frequency signal for the AFC can be obtained. Note that the gain of the output in which, by employing the doublers 29P and 29S before the addition, the expansion of the spectrum due to the phase information of the intermediate frequency signals is cancelled, is adjusted by the automatic gain control circuit (AGC circuit) 15. The gain adjusting ability of the AGC circuit 15 is obtained as long as the gain margin of the AGC circuit 15 is 7 dB when the mark ratio is 1/11 (minimum mark ratio) or 10/11 (maximum mark ratio). Such a value is easily realized by the standard AGC circuit 15. Accordingly, in the fourth embodiment also, an intermediate frequency signal having a desired frequency can be always obtained and a desired gain can be held, and thus the required stabilization of the oscillation frequency (intermediate frequency) of the optical local oscillating circuit 7 can be obtained. Note that, as the local oscillating optical phase modulating means 13, as in the first embodiment, the means for modulating the phase of one polarized component (P or S) of the local oscillating light from the optical local oscillating circuit 7 can be replaced by a means for shifting the frequency of one polarized component (P or S) of the local oscillating light from the optical local oscillating circuit 7 by a desired amount with respect to the other polarized component. (i) Description of the fifth Embodiment Figure 18 is a block diagram showing the fifth embodiment of the present invention, and illustrates a heterodyne-type coherent optical communication system having PSK or DPSK. The polarization diversity optical receiver 6 is used for the heterodyne-type coherent optical communication system having DPSK and corresponds to the second embodiment shown in Fig. 15. Namely, the polarization diversity optical receiver 6 comprises, in addition to the optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, and the demodulating-adding-discriminating circuit 10, the adding circuit 11A to which the outputs from the doublers 29P and 29S are input, and the control circuit 11B, as well as a local oscillating optical phase modulating means for shifting the frequency of one polarized component of the local oscillating light with respect to the frequency of the other polarized component. The local oscillating optical phase modulating means is provided in the optical local oscillating circuit 7. Namely, the optical local oscillating circuit 7 comprises a first semiconductor laser 71 for outputting a local oscillating light having the P polarized component, and a second semiconductor laser 72 for outputting a local oscillating light having the S polarized component. The frequency difference between the respective semiconductor lasers 71 and 72 is locked to a desired value by an automatic frequency control circuit (AFC circuit) 34. The AFC circuit 34 comprises a photo-electric converting circuit 35 constructed as a double-balance optical receiver, a post amplifier 36, a bandpass filter 37, a frequency discriminating circuit 38, and a drive circuit 39. The photo-electric converting circuit 35, post amplifier 36, bandpass filter 37, frequency discriminating circuit 38, and drive circuit 39 are the same as used in the second embodiment, and therefore, a detailed description thereof is omitted. In this fifth embodiment also, the respective semiconductor lasers 71 and 72 are controlled by the light source stabilizing units 18A and 18B constructed as temperature control units, so that the laser temperature is kept to a constant value. Further, the mixing circuit 8, the detecting circuit 9, the demodulating-adding-discriminating circuit 10, the control circuit 11B, and the transmitter 1 (this transmitter 1 comprises a light source 2, an optical modulating circuit 3, a light source stabilizing unit 4, a waveform shaping circuit 30, an amplifier 31, a differential coding circuit 32, and a crystal oscillator 33) are the same as used in the fourth embodiment, and therefore, a detailed description thereof is omitted. In the above construction, in the polarization diversity optical receiver shown in Fig. 18, the respective local oscillating lights from the two semiconductor lasers 71 and 72 in the optical local oscillating circuit 7 are in the different polarized states, and the frequency difference between the local oscillating lights is shifted by a predetermined amount. Therefore, the interrelationship between the detected output signals is reduced and thus the combined signal is not made zero, and therefore, the required stabilization of the intermediate frequency is obtained as in the previous embodiments. Note that, in this embodiment also, the means for shifting the phase of one output (polarized components P or S) of the semiconductor lasers 71 and 72 by a desired amount with respect to the phase of the ether output (polarized component S or P) can be replaced by a means for modulating the phase of one output (polarized component P or S) of the semiconductor lasers 71 and 72. (j) Description of the sixth Embodiment Figure 19 is a block diagram showing the sixth embodiment of the present invention and illustrates heterodyne-type coherent optical communication system having PSK. The polarization diversity optical receiver 6 shown in Fig. 16 is used for the heterodyne-type coherent optical communication system having PSK and corresponds to seventh embodiment. Namely the polarization diversity optical receiver 6 comprises in addition to the optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, and the demodulating-adding-discriminating circuit 10, the adding circuit 11A, to which the outputs from the doublers 29P and 29S are input, and the control circuit 11B, as well as a detected output phase modulation means 14 for modulating either of the detected outputs e S and e P of the respective polarized components from the detecting circuit 9. The detected outputs phase modulating means 14 comprises an oscillator 141 and a phase modulator 142 for modulating, by the signal from the output of the oscillating circuit 141, the phase of one (electric signal) of the detected outputs e S and e P of the respective polarized components from the detecting circuit 9. Also, the optical local oscillating circuit 7, the mixing circuit 8, the detecting circuit 9, the demodulating adding discriminating circuit 10, and the control circuit 12 are the same as used in the eighth embodiment, and therefore, a detailed description thereof is omitted. Note that the local oscillating optical phase modulating means 13 is not provided in the mixing circuit 8. In the above construction, in the polarization diversity optical receiver 6 shown in Fig. 16, either of the detected outputs e S and e P of the respective polarized components from the detecting circuit 9 is modulated by the detected output phase modulating means 14, and accordingly, the phase difference between, for example, the detected output signals e S and e P , can be changed from 0 to π as a function of time. As a result, the interrelationship between the detected output signals e S and e P can be reduced and thus the combined signal is not made zero. Accordingly desired intermediate frequency signal for the AFC can be obtained, and thus the required stabilization of the intermediate frequency can be obtained as in the previous embodiments. Note that, as the detected output phase modulating means 14, the means for modulating the phase of either (e S and e P ) of the detected outputs e S and e P of the respective polarized components from the detecting circuit 9 can be replaced by a means for shifting the frequency of one (e S and e P ) of the detected outputs e S and e P of the respective polarized components from the detecting circuit 9 by a desired amount with respect to the frequency of the component (e S and e P ). Also, the number of bits for the delay by the delay circuit 14 in the automatic frequency control circuit 11 is not restricted to one bit, but may be 1 bit × N (N is usually an integer, but may not be an integer). Fig. 20 is a block diagram showing a modification of the optical receiver shown in Fig. 8 In Fig. 20, a mixing circuit 8a is different from the mixing circuit 8 shown in Fig. 8. The other portions are completely the same as those in Fig. 8. In the mixing circuit 8a, the signal light Psi transmitted through the optical fiber 5 is mixing with the local oscillating light P ℓ0 from the optical local oscillating circuit 7 by means of a polarized wave maintaining optical fiber coupler 20a, before splitting the signal light into the two polarized comments. After mixing, the mixed signal is split into two polarized components aPsi and (1-a) Psi. Each split and polarized component is further split by a polarized wave splitting optical fiber coupler 19a into two polarized components, one of which is supplied to the photo-electric converter 21P and the other of which supplied to the photo-electric converter 21S. This constitution also provides the same effects as in the first embodiment. The mixing circuit 8 shown in Figs. 11 to 19 may also be replaced by the mixing circuit 8a shown in Fig. 20, in a similar way as above. As described above, according to the coherent optical communication polarization diversity optical receiver of the present invention, when utilizing a baseband combining method, the sum and the difference of the intermediate frequency signals of two polarized components are frequency discriminated, and then combined, and therefore, the interrelationship of the intermediate frequency signals is reduced, and thus either of the first and the second frequency discriminating circuits can be operated. Accordingly, the intermediate frequency signal for stabilizing the optical local oscillating circuit can be always obtained, and as a result, an advantage is gained a required stabilization of the intermediate frequency. Also, according to the coherent optical communication polarization diversity optical receiver of the present invention, when utilizing a baseband combining method, by simply providing a local oscillating optical phase modulating means for modulating the phase of one polarized component of the local oscillating light from the optical local oscillating circuit or a detected output phase modulating means for modulating the phase of either of the detected outputs of the respective polarized components from the detecting circuit, the interrelationship of both intermediate frequency signals is reduced, and thus either of the first and the second frequency discriminating circuits can be operated, and the intermediate frequency signal for stabilizing the optical local oscillating circuit can be always obtained. As a result, an advantage is gained a required stabilization of the intermediate frequency.

Claims: A polarized wave diversity optical receiver for coherent cptical communication comprising: an optical local oscillating circuit (7) for oscillating local oscillating light; a mixing circuit (8) for receiving signal light transmitted through an optical fiber (5) and said local oscillating light from said optical local oscillating circuit (7), and for obtaining two polarized components; a detecting circuit (9) for detecting the signals of the respective polarized components from said mixing circuit (8) and to output intermediate frequency signals (e S and e P ); an adding circuit (11A) for adding the respective polarized components of the intermediate frequency signals (e S and e P ) from said detecting circuit (9); and a control circuit (11B) for controlling said optical local oscillating circuit (7) depending on the added output of said adding circuit (11A); characterized by a local oscillating optical signal modulating means (13; 131, 132; 71, 72) for modulating a phase or a frequency of either one of the polarized components of the local oscillating light from said optical local oscillating circuit (7) such that the interrelationship of both intermediate frequency signals is reduced. A. polarized wave diversity optical receiver for coherent optical communication according to claim 1, wherein said mixing circuit (8) is adapted to split the signal light transmitted through said optical fiber (5) and said local oscillating light from said optical local oscillating circuit (7) into two types of polarized components, respectively, and then to mix the same polarized components to output. polarized wave diversity optical receiver for coherent optical communication according to claim 2, further comprising wave dividing means (19) for dividing said local oscillating light from said optical local oscillating circuit (7) into two types of polarized components, said local oscillating optical phase modulating means (13) modulating the phase of either of the polarized components of said local oscillating light from said wave dividing means (19). A polarized wave diversity optical receiver for coherent optical communication according to claim 2, further comprising wave dividing means (19) for dividing said local oscillating light from said optical local oscillating circuit (7) into two types of polarized components, and said local oscillating optical phase modulating means (13) being one for shifting the frequency of one of the polarized components of said local oscillating light from said wave dividing means (19) by a desired amount with respect to the frequency of the other polarized component. A polarized wave diversity optical receiver for coherent optical communication according to claim 1, wherein said optical local oscillating circuit (7) comprises two semiconductor lasers (71 and 72) for outputting local oscillating lights of different polarized wave components, the output phase cr the output frequency of either of the outputs of said semiconductor lasers (71 and 72) being modulated. A polarized wave diversity optical receiver for coherent optical communication according to claim 5, wherein the difference between the output frequencies of said two semiconductor lasers (71 and 72) being locked to a desired value through the frequency control circuit (34). A polarized wave diversity optical receiver for coherent optical communication comprising: an optical local oscillating circuit (7) for oscillating local oscillating light; a mixing circuit (8) for receiving signal light transmitted through an optical fiber (5) and said local oscillating light from said optical local oscillating circuit (7), and for obtaining two polarized components; a detecting circuit (9) for detecting the signals of the respective polarized components from said mixing circuit (8) and to output intermediate frequency signals (e S and e P ); an adding circuit (11A) for adding the respective polarized components of the intermediate frequency signals (e S and e P ) from said detecting circuit (9); and a control circuit (11B) for controlling said optical local oscillating circuit (7) depending on the added output of said adding circuit (11A); characterized by a detecting output signal modulating means (14; 141, 142) for modulating a phase or a frequency of either of the intermediate frequency signals from said detecting circuit (9) such that the interrelationship of both intermediate frequency signals is reduced, and for inputting the modulated intermediate frequency signals into said adding circuit (11A). A polarized wave diversity optical receiver for coherent optical communication according to claim 7, wherein said mixing circuit (8) is adapted to split the signal light transmitted through said optical fiber (5) and said local oscillating light from said optical local oscillating circuit (7) into two types of polarized components, respectively, and then to mix the same polarized components to output. A polarized wave diversity optical receiver for coherent optical communication according to claims 1 or 7, wherein said mixing circuit (8) is adapted to mix the signal light transmitted through said optical fiber (5) and said local oscillating light from said optical local oscillating circuit (7), and then to split the mixed signal into two types of polarized components to output. A polarized wave diversity optical receiver for coherent optical communication according to claims 1 to 7, wherein the power of said signal light is assumed to be 1, the power distribution of the polarized components is expressed in such a way that the power of one of the polarized components is a and the other is (1-a), and the power distribution of the two types of polarized wave components of said local oscillating light are the same (each being 1/2). A polarized wave diversity optical receiver for coherent optical communication according to claims 3 or 7, wherein the phase modulating by means of said phase modulating means (13) is carried out by changing in time. A polarized wave diversity optical receiver for coherent optical communication according to claims 2, 5 or 7, wherein said signal light is FSK modulated signal or ASK modulated signal. A polarized wave diversity optical receiver for coherent optical communication according to claim 8, further comprising doublers for doubling the frequencies of the signals before inputting into said adding circuit to cancel the phase information. A polarized wave diversity optical receiver for coherent optical communication according to claims 3, 4, 5, 7 or 8, further comprising doublers for doubling the frequencies of the signals before inputting into said adding circuit to cancel the phase information. A polarized wave diversity optical receiver for coherent optical communication according to claim 13, wherein said signal light is PSK modulated signal or DPSK modulated signal.

Inventors: CHIKAMA TERUMI, KIYONAGA TETSUYA, KUWAHARA HIDEO, MIYATA HIDEYUKI, NAITO TAKAO, ONAKA HIROSHI, ONODA YOSHIHITO, WATANABE SHIGEKI, CHIKAMA, TERUMI, KIYONAGA, TETSUYA, KUWAHARA, HIDEO, MIYATA, HIDEYUKI, NAITO, TAKAO, ONAKA, HIROSHI, ONODA, YOSHIHITO, WATANABE, SHIGEKI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: H04B10
IPC codes: H04B10, G02F2