JP2659417B2 - Coherent optical communication system - Google Patents

Description

[Description of the Invention] [Table of contents] Overview Outline Industrial application field Conventional technology (Figs. 8 to 10) Problems to be solved by the invention Means for solving the problem (Fig. 1) FIG. 2 to FIG. 7) Effects of the Invention Outline Coherent optical communication system is caused by polarization fluctuation in an optical transmission line without using a polarization-maintaining optical fiber, a polarization diversity system, or a polarization active control system. In the transmission unit, the first and second transmission light sources whose polarization planes of the emitted light are not the same are oscillated at different frequencies, and the first and second transmission light sources are After directly or indirectly modulating the same signal and multiplexing the modulated light, the modulated light is transmitted to the optical transmission path, and in the receiving unit, the signal light transmitted by the optical transmission path,
The local light from the local light source oscillating at a frequency different from the oscillation frequency of the first and second transmission light sources is multiplexed with the local light, and the signal light and the combined light of the local light are subjected to optical-electrical conversion by the light receiver,
The first band-pass filter and the oscillation frequency of the second transmission light source and the oscillation light of the second transmission light source that pass an electric signal having a frequency that is the difference between the oscillation frequency of the first transmission light source and the oscillation frequency of the local light source. By a second band-pass filter that passes an electric signal having a frequency different from the oscillation frequency, the electric signal converted in the light receiver is separated, and the separated electric signals are combined after demodulation. Configure.

The present invention relates to a coherent optical communication system.

In recent years, as a next-generation optical communication system, a coherent optical communication system that treats light as a coherent wave has attracted attention, and has been actively studied by various communication research institutions and the like.
In the coherent optical communication system, the reception sensitivity is improved by 10 to 20 dB as compared with the intensity modulation direct detection system (IM-DD system) which has been practically used, so that long-distance transmission is possible. In addition, since optical frequency multiplexing transmission or multi-level transmission can be performed, there is a great merit such as a dramatic increase in transmission capacity, and as a communication method, compared to the IM-DD method in which light is simply flashed. , Basically have an advantage.

By the way, when implementing a coherent optical communication system,
If the oscillation frequency is slightly deviated from the transmitted light, or if the polarization states of the two lights are deviated when optical heterodyne or homodyne detection is performed using the local light that coincides, the power detected after optical-to-electric conversion is reduced. And the reception sensitivity deteriorates. When a normal single mode optical fiber is used as the optical transmission line, the polarization state of the transmission light fluctuates, and some improvement is required.

FIG. 8 is a block diagram showing a basic configuration of a coherent optical communication system. The signal light from the transmitting unit 111 is transmitted to the receiving unit via the optical transmission line 112. The signal light transmitted to the receiving unit is transmitted to the local light source 1 by an optical multiplexer 113 such as an optical coupler.
The local light is multiplexed with the local light from 14, and the multiplexed light is subjected to, for example, heterodyne detection by the receiver 115. The photodetector 116 such as a photodiode located at the first stage of the receiver 115 generates an electric signal having a frequency difference between the frequency of the signal light and the frequency of the local light due to its square detection characteristics. Are extracted by the IF circuit 117 and demodulated by the demodulation circuit 118 based on the intermediate frequency signal.

In such a system, the optical signal transmitted from the transmission unit 111 is obtained by directly or indirectly modulating a laser beam, and is generally linearly polarized light. However, when the optical transmission line 112 is a single mode optical fiber,
HE ₁₁ is a propagation mode due to its unavoidable non-axial symmetry
Since the mode degeneracy is released and the two modes HE _11x and HE _11y can be propagated, when the optical fiber is subjected to disturbances such as torsion, bending, external pressure, and temperature change, coupling between the two modes occurs. A phase difference occurs due to a difference in phase speed between modes (birefringence), and this causes a complicated change in the polarization state on the receiving side (the optical fiber emission end). Specifically, the emitted light becomes elliptically polarized light, or the polarization direction rotates. If such a fluctuation in the polarization state is large, the interference efficiency between the signal light and the local light decreases, and the receiving sensitivity deteriorates. In the worst state, reception becomes impossible.

 The means for addressing this problem are exemplified below.

(A) Use of polarization maintaining optical fiber A polarization maintaining optical fiber is used as an optical transmission line, and the polarization planes of the transmitting unit and the receiving unit are matched to maintain a constant polarization state of the signal light. Deterioration of the receiving sensitivity can be prevented.

(B) Polarization diversity scheme FIG. 9 is a block diagram showing the scheme. Optical transmission line 11
2 is separated by a polarization separator 121 into P-wave and S-wave components whose polarization planes are orthogonal to each other.
And an S-wave component. Each of the P-wave components is multiplexed by an optical multiplexer 123 and then demodulated by a receiver 125. Each of the S-wave components is multiplexed by an optical multiplexer 124 and then received by a receiver. 1
The signal is demodulated at 26 and both demodulated signals are combined at a combiner 127. According to this configuration, even when the polarization fluctuation in the optical transmission line is large, an output signal can be obtained from either one of the receivers 125 and 126, so that reception is not disabled.

(C) Polarization active control method FIG. 10 is a block diagram showing the same method. In this example, the polarization state detection circuit 133 detects the polarization state of the multiplexed light output from the optical multiplexer 132 that multiplexes the signal light and the local light, and based on the detection signal, the optical multiplexing of the optical transmission line 112 is performed. The polarization controller 131 provided near the wave filter 132 is driven to always obtain the maximum interference efficiency. In addition, polarization controller
131 may be provided between the local light source 114 and the optical multiplexer 132.

Problems to be Solved by the Invention Among the conventional techniques for preventing the deterioration of the receiving sensitivity due to the polarization fluctuation in the optical transmission line, the one using the polarization preserving optical fiber has a long length suitable for mass production. It is difficult to provide a polarization-maintaining optical fiber, and there is a problem when performing long-distance transmission. In addition, a great loss in industrial economy is expected in that an optical transmission line composed of a single mode optical fiber or the like conventionally laid cannot be used.

In the case of the polarization diversity system, since a receiving sequence is required for each of the P-polarized light and the S-polarized light, the configuration becomes complicated. For example, when the dual-balanced receiving system or the phase diversity system is used together, the optical components are not used. A large number is required, and the configuration is further complicated. Also, if there are two received sequences, for example, in a state where most of the power of the received light is being processed by one received sequence, the other received sequence functions only to generate almost noise, There is a disadvantage that the S / N ratio decreases.

In the active polarization control method, for example, four light receivers for detecting the polarization state of the signal light are required separately from the main light receiver in the reception sequence, and to configure a polarization controller. In addition, optical components corresponding to the quarter-wave plate and the half-wave plate are required, and the configuration becomes complicated. In addition, since the received signal light and the local light are distributed and used for polarization control, it is inefficient.

The present invention has been created in view of such circumstances,
An object of the present invention is to prevent deterioration of the receiving sensitivity due to polarization fluctuation in an optical transmission line without using a polarization-maintaining optical fiber, a polarization diversity method, and a polarization active control method.

Means for Solving the Problems The technical problem described above is solved by the configuration of (1) shown in FIG. 1A or the configuration of (2) shown in FIG.

(1) In the transmitter 1, the first and second transmission light sources 2 and 3 the plane of polarization of the emitted light is not the same to oscillate at different frequencies f _1, f _2, the first and second transmission sources 2, 3 is directly or indirectly modulated by the same signal by the modulator 4, the modulated light is multiplexed by the multiplexer 5, and then transmitted to the optical transmission line 6.

The receiving unit 7 includes a signal light transmitted by the optical transmission line 6, multiplexes the f ₁ and f ₂ different from the frequency f ₃ multiplexer 9 and local light from the local light source 8 oscillates at, the combined light light by the photodetector 10 - electrical conversion, frequency | passing electrical signals | f ₁ -f ₃ | of the first band-pass filter 11 and frequency for passing an electrical signal | f ₂ -f ₃ The second band-pass filter 12 separates the electric signal converted in the light receiver 10, and separates the separated electric signal into the demodulator 1
After being demodulated by 3 and 14, the signal is synthesized by the synthesizer 15.

(2) in the transmission unit 21, modulates and distributed by the distributor 24 the modulated light by the modulator 23 to transmit light 22 which oscillates at a frequency f _1, directly or indirectly, polarization different from the polarization plane of the modulated light A specific polarization component is extracted from one of the distributed modulated lights by the first polarizer 25 that transmits the light, and the other distributed modulated light is transmitted to the nonlinear optical crystal 26 and transmitted to the transmitted light. oscillation frequency f ₁ which is different from the frequencies of the light sources 22
A _second polarizer that newly generates light of f2 and transmits light having a polarization plane different from the polarization plane of the modulated light and different from the polarization plane of the specific polarization component extracted by the first polarizer 25.
27, the specific polarization component is extracted from the transmitted light of the nonlinear optical crystal 26, and the specific polarization component extracted by the first and second polarizers 25 and 27 is multiplexed by the multiplexer 28, and then the light is transmitted to the optical transmission path 29. Send it out.

In the receiving unit 30, local light source oscillating at a signal light transmitted by the optical transmission line 29, f ₁ and f ₂ different from the frequency f ₃
A first band pass filter that multiplexes the local light from 31 with a multiplexer 32, converts the multiplexed light into electric light by an optical receiver 33, and passes an electric signal of a frequency | f ₁ −f ₃ | 34 and a second band-pass filter 35 for passing an electric signal of a frequency | f ₂ −f ₃ |, the electric signal converted in the photodetector 33 is separated, and the separated electric signal is demodulated by a demodulation circuit.
After being demodulated by 36 and 37, they are combined by a combiner 38.

In the specification of the present application, “polarized light” and “polarized wave” are used in the same meaning.

According to the configuration of the present invention, the polarization planes that are not the same, for example, the polarization planes orthogonal to each other are modulated by the same signal, transmitted to the optical transmission line, converted to electric signals on the receiving side, and demodulated respectively. Since the combining is performed later, a signal output can always be obtained even when the polarization fluctuation in the optical transmission line is large, and the deterioration of the receiving sensitivity can be prevented.

In the configuration of (1), the first and second transmission light sources are oscillated at different frequencies f ₁ and f ₂ because the first and second transmission light sources are oscillated at the same frequency. This is because the frequency control is easier than that of the above. In other words, in order to control the frequency so that the oscillation frequencies match, it is necessary to use a technology that is difficult to realize, such as an optical PLL.
Frequency control, for example, relative to different frequencies,
The control can be performed relatively easily by controlling the oscillation frequency to coincide with the peak value of the resonance spectrum of the AFC by the heterodyne detection and the Fabry-Perot resonator.

Also, in the configuration of (2), the reason why different frequencies are given to the distributed light of the single transmission light source by using the nonlinear optical effect is that, for example, a circularly polarized light output is given without giving different frequencies. Assuming that orthogonal polarization components cannot be taken out at the same time, it is necessary to apply a polarization scrambler method or the like having a limited bit rate.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 2 is a block diagram of a coherent optical communication system showing one embodiment of the present invention. In the transmission unit 40, reference numeral 41 denotes a transmission LD (semiconductor laser), which has a spectral line width such that phase noise does not affect a desired transmission bit rate by forming an external resonator or the like. The element temperature and the bias current are controlled so that the oscillation frequency becomes stable. Oscillation frequency of the transmitting LD41 is f _1, the emitted light is linear polarized light. 42
Is a transmission LD in which the spectral line width, element temperature and bias current are controlled similarly to the transmission LD 41, the oscillation frequency is f ₂ (≠ f ₁ ), and the polarization plane of the emitted light is the transmission LD.
It is set to be perpendicular to the plane of polarization of the light emitted from the LD 41.
In this embodiment, the transmission LDs 41 and 42 are directly modulated by a modulation signal based on transmission information, but may be indirectly externally modulated using an external modulator. Reference numeral 43 denotes an optical multiplexer such as an optical coupler, which is a P-wave and an S-wave whose polarization planes from the transmission LDs 41 and 42 are orthogonal to each other (for convenience, the polarization state of light emitted from the transmission LD 41 is a P-wave, and the transmission LD 42 (The polarization state of the outgoing light is S-wave)) and is transmitted to an optical transmission line 44 composed of, for example, a normal single-mode optical fiber.

Set the oscillation frequencies of the transmission LDs 41 and 42 to different frequencies
For stabilizing control to f ₁ and f ₂ , for example, the respective oscillation frequencies may be controlled so as to coincide with spectral peaks appearing at equal intervals on the frequency axis in a Fabry-Perot etalon or the like.

In the receiving unit 45, 46 is for local oscillation whose oscillation spectrum line width and oscillation frequency are stabilized, similarly to the transmission LDs 41 and 42.
An LD, the oscillation frequency is f ₃ which is slightly deviated from the f _1, f _2. Although not shown, an AFC (automatic frequency tracking) device is provided so that a beat signal with light emitted from the transmission LDs 41 and 42 is stably maintained. Reference numeral 47 denotes an optical multiplexer such as an optical coupler, which multiplexes the signal light transmitted through the optical transmission line 44 and the local light from the local LD 46. Reference numeral 48 denotes a light receiver such as a photodiode for performing optical-to-electrical conversion of the multiplexed light from the optical multiplexing path 47, and functions as a square detector. The intermediate frequency signal having an intermediate frequency of, for example, several GHz generated in the light receiver 48 is sent to the band-pass filters 50 and 52 after being amplified by the IF amplifier 49. The band-pass filter 50 selectively passes an intermediate frequency signal having a frequency | f ₁ −f ₃ |, and the band-pass filter 52 selectively passes an intermediate frequency signal having a frequency | f ₂ −f ₃ |. The detection signal that has passed through the band-pass filter 50 is demodulated by the demodulator 51, and the detection signal that has passed through the band-pass filter 52 is demodulated by the demodulator 53. Each demodulated signal (baseband signal) is converted by a power combiner.
Combined (added) by 54. Then, by identifying the signal from the power combiner 54 by the identifier 55,
The transmission information is reproduced. The demodulators 51 and 53 are configured in accordance with a transmission-side modulation scheme such as a DPSK (differential phase shift keying) scheme and a CPFSK (phase continuous frequency shift keying) scheme.

FIG. 3 shows the polarization state and the level of the demodulated signal in each part when the polarization state on the receiving side of the optical transmission line 44 fluctuates to various states. 61 (61a~e) is the polarization state of the signal light of a frequency f ₁ to be input to the optical multiplexer 47,
62 (62a~e) is the polarization state of the signal light of a frequency f ₂ inputted to the optical multiplexer 47, 63 (63a~e) is polarized local light frequency f ₃ which is input to the optical multiplexer 47 State, 64 (64a-e) is the output signal level of demodulator 51, 65 (65a-e) is the output signal level of demodulator 53, and 66 (66a-e) is the power combiner 54.
Is the output signal level.

Since the polarization planes of the P-wave and S-wave from the transmission LDs 41 and 42 are orthogonal to each other on the transmission side, the polarization state of the signal light transmitted to the reception side is shown in FIGS. 3 (a) and 3 (b). As shown in
A combination of linearly polarized light, a combination of elliptically polarized light as shown in FIGS. 3C and 3D, a combination of circularly polarized light as shown in FIG. In any combination, a demodulated signal can be obtained from at least one of the demodulators 51 and 53, so that a decrease in interference efficiency due to polarization fluctuations is suppressed and a deterioration in reception sensitivity is prevented. Is done.

FIG. 4 is a block diagram of a receiving unit showing an embodiment combined with a dual balance type receiving system. The signal light transmitted by the optical transmission line is multiplexed by the local light from the local oscillation LD 70 and the optical multiplexer / demultiplexer 71 such as an optical coupler and output to two optical paths. Long receiver 7
Input to 2,73. The light receivers 72 and 73 are made of, for example, photodiodes having the same characteristics connected in series, and the signal extracted from the neutral point is amplified by the IF amplifier 74.
As in the previous embodiment, the signal is demodulated by a detection / demodulation circuit 75 including a band-pass filter, a demodulator, and a combiner, and is identified by an identifier 76.

According to such a dual balance type receiving method, the local light intensity noise in the same phase is canceled and the signal component in the opposite phase is added, so that the receiving sensitivity can be greatly improved.
When the dual balance type receiving system is applied, it is necessary to use, for example, a polarization diversity system in order to prevent the reception sensitivity from being deteriorated due to the polarization fluctuation in the optical transmission line without using the present invention. When the balanced reception system and the polarization diversity reception system are combined, for example, four light receivers are required, and in particular, the configuration of the optical system becomes complicated. On the other hand, according to the present embodiment, even when the receiver is used in combination with the dual balance type reception system, the number of the light receivers is two.
That is, it is possible to prevent the deterioration of the receiving sensitivity due to the polarization fluctuation in the optical transmission line without complicating the configuration.

FIG. 5 is a block diagram of a receiving section showing an embodiment combined with a phase diversity receiving system. The signal light transmitted by the optical transmission line and the local light from the local oscillation LD 80 are multiplexed in the optical multiplexer / demultiplexer 81, and the multiplexed light is distributed into n-sequences and respectively transmitted to the light receivers 82-1 to 8n. Incident. Then, the signal from each photodetector is amplified by the IF amplifier 83 and demodulated by the detection / demodulation circuit 84, then combined by the combiner 85, and identified by the identifier 85. Optical multiplexer / demultiplexer 81
Has an optical configuration such that the phases of the output light of n series are shifted by 360 / n °, respectively. According to the phase diversity reception method, the frequency of the signal light and the frequency of the local light can be made extremely close to each other, so that only a reception band close to the homodyne detection method is required, and the bit rate can be easily increased. .

FIG. 6 shows that n = 3 and the frequency offset is the bit rate.
An example of an output waveform of each detection / demodulation circuit in the case of 1/18 is shown. Even when the output of any port becomes 0, the output of the other port does not become 0. Therefore, by combining these, transmission information can be reproduced well.

When the polarization diversity receiving method is combined with the phase diversity receiving method, the configuration of the optical system of the receiving unit is significantly complicated, as in the case where the polarization diversity receiving method and the dual balance type receiving method are combined. By combining the embodiment of the present invention with the phase diversity receiving method, it is possible to prevent the deterioration of the receiving sensitivity with a simple configuration.

FIG. 7 shows two transmission LDs as in the previous embodiments.
In this embodiment, a single transmission LD is used to obtain signal lights of different frequencies f ₁ and f ₂ , instead of using a single transmission LD. For transmission, for example, directly modulated according to transmission information
After the light emitted from the LD 91 (frequency f ₁ ) is collimated by the lens 92, it is distributed to two optical paths by the distributor 93 such as a half mirror. One distribution light is reflected by the reflector 94,
After passing through the polarizer 95, the light is focused by the lens 96 and is incident on the optical multiplexer 97. The other distributed light passes through the parametric oscillator 98 and the polarizer 99 in this order, is focused by the lens 100, and enters the optical multiplexer 97. Polarizer 95,99
Transmission polarization planes are orthogonal to each other, and the transmission LD91
The polarization plane of the outgoing light is formed at an angle of, for example, 45 ° with respect to the transmission polarization planes of the polarizers 95 and 99. Parametric oscillator 98 is provided with an optical resonator 98a and a nonlinear optical crystal 98b provided therein is constituted by the arrangement, the incident light of frequency f _1, is different from the frequency f ₁ Light of frequencies f ₂ and f ₄ can be newly generated. Here, between the frequencies f ₁ , f ₂ , and f ₄ , f ₁ = f ₂ + f ₄
The relationship holds.

According to such a configuration of the transmitting unit, it is possible to multiplex light having different polarization planes and different frequencies (f ₁ , f ₂ ) and to transmit the multiplexed light to the optical transmission line 44. By configuring the receiving unit in the same manner as in the embodiments described above, it is possible to prevent the deterioration of the receiving sensitivity due to the polarization fluctuation in the optical transmission line. Further, according to the present embodiment, since there is one transmission LD, the temperature control and the like of the LD are made easier by that amount,
The configuration of the transmission unit can be simplified.

In the above embodiments, as the most preferred embodiment of the present invention, has been described in which the polarization plane of light polarization and the frequency f ₂ of the optical frequency f ₁ are orthogonal, the present invention is limited to Not done. That is, polarization and frequency of the optical frequency f ₁
If the polarization plane of the light of f ₂ is not at least on the same plane,
Since the polarization components whose polarization planes are orthogonal to each other can be used, the present invention can be implemented even when the polarization planes of the two transmission lights are not orthogonal.

Effects of the Invention As described in detail above, according to the present invention, the use of a polarization-maintaining optical fiber, the polarization diversity scheme, and the polarization active control scheme do not result from the polarization fluctuation in the optical transmission line. There is an effect that deterioration of the receiving sensitivity can be prevented.

[Brief description of the drawings]

1 is a principle diagram of the present invention, FIG. 2 is a block diagram of a coherent optical communication system showing an embodiment of the present invention, FIG. 3 is an operation explanatory diagram of the embodiment shown in FIG. 2, and FIG. FIG. 5 is a block diagram of a receiving unit showing an embodiment combined with a dual balance type receiving system. FIG. 5 is a block diagram of a receiving unit showing an embodiment combined with a phase diversity receiving system. FIG. FIG. 7, FIG. 7 is a block diagram of a transmission unit showing an embodiment in which one transmission LD is used, FIG. 8 is a block diagram of a coherent optical communication system, FIG. 9 is a block diagram showing a polarization diversity system, FIG. 10 is a block diagram showing the active polarization control method. 1,21 ... transmitting unit, 2 ... first transmitting light source, 3 ... second transmitting light source, 6,29 ... optical transmission path, 7,30 ... receiving unit, 8,31 ...
... Local light emitter, 10,33 ... Receiver, 11,34 ... First band pass filter, 12,35 ... Second band pass filter, 22 ...
... transmitted light, 25 ... first polarizer, 26 ... nonlinear optical crystal, 27 ... second polarizer.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshito Onoda 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (72) Inventor Takao Naito 1015 Kamikodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Fujitsu Limited

Claims (2)

(57) [Claims] A transmitting section for oscillating first and second transmitting light sources having different polarization planes of emitted light at different frequencies (f ₁ , f ₂ ); The first and second transmission light sources (2, 3) are directly or indirectly modulated with the same signal (4), the modulated light is multiplexed (5), and then transmitted to the optical transmission line (6). In 7), the signal light transmitted through the optical transmission line (6) and the oscillation frequencies (f ₁ , f ₂ ) of the first and second transmission light sources (2, 3) are used.
And (9) the local light from the local light source (8) that oscillates at a frequency (f ₃ ) different from that of the local light.
Electrical conversion, the difference between the oscillation frequency (f ₃₎ of the first transmission oscillation frequency of the light source (2) (f ₁₎ and the local light source (8) Frequency (| f ₁
−f ₃ |), the oscillation frequency (f ₂ ) of the first band-pass filter (11) and the second transmission light source (3), and the oscillation frequency (f) of the local light source (8). ₃ ) The electric signal converted in the photodetector (10) is separated by a second band-pass filter (12) that passes an electric signal having a frequency (| f ₂ −f ₃ |) different from the electric signal. A coherent optical communication system characterized in that the separated electric signals are demodulated (13, 14) and then combined (15). 2. A transmission section (21) which directly or indirectly modulates (23) a transmission light source (22) oscillating at a frequency (f ₁ ) and distributes (24) modulated light. The first polarizer (25) that transmits light having a polarization plane different from the polarization plane extracts a specific polarization component from the one of the distributed modulated lights, and converts the other distributed modulated light into a nonlinear optical crystal (26). ) To newly generate light having a frequency (f ₂ ) different from the oscillation frequency (f ₁ ) of the transmission light source (22) in the transmitted light, which is different from the polarization plane of the modulated light and the first light. Polarizer (2
The second polarizer (27) that transmits light having a polarization plane different from the polarization plane of the specific polarization component extracted in 5) extracts the specific polarization component from the transmitted light of the nonlinear optical crystal (26). The specific polarization components extracted by the first and second polarizers (25, 27) are combined (28) and then transmitted to an optical transmission path (29). In the receiving unit (30), the optical transmission path ( 29), the signal light having a frequency (f ₁ ) different from the oscillation frequency (f ₁ ) of the transmission light source (22) and the frequency (f ₂ ) of light newly generated in the transmitted light of the nonlinear optical crystal (26). f3) multiplexes (32) the local light emitted from the local light source (31) oscillating in ( ₃ ), and the signal light and the combined light of the local light are multiplexed by the photodetector (33).
Electrical conversion, the difference between the oscillation frequency (f ₃₎ of the oscillation frequency of the transmission light source (22) (f ₁₎ and the local light source (31) frequency (| f ₁ -f
₃ |) and the frequency (f ₂ ) of light newly generated in the transmitted light of the first band-pass filter (34) and the nonlinear optical crystal (26) that passes the electric signal of the local light source (31). The electric signal converted by the photodetector (33) is converted by the second band-pass filter (35) that passes an electric signal having a frequency (| f ₂ −f ₃ |) having a difference from the oscillation frequency (f ₃ ). A coherent optical communication system characterized in that the separated electric signals are demodulated (36, 37) and then combined (38).