JPH02120726A - Coherent light communication system - Google Patents

Abstract

PURPOSE:To prevent the receiving sensitivity from being deteriorated even if a polarized wave fluctuation of a transmission line is large by modulating light beams whose planes of polarization are different by the same signal and sending them out, converting them to electric signals on the receiving side and demodulating the separated electric signals, and thereafter, synthesizing them. CONSTITUTION:Light beams of first and second transmitting light sources 2, 3 in which planes of polarization of an emitted light are different are modulated 4 by the same signal, modulated light beams are multiplexed 5, and thereafter, sent out to an optical transmission line 6. Such signal light, and a local emitted light from a local light emission source 8 oscillated by a different frequency from that of the transmitting light sources 2, 3 are multiplexed, and the multiplexed light is brought to opto-electric conversion by a photodetector 10. Subsequently, an electric signal having a frequency of the difference between a frequency of a first transmitting light source 2 and a frequency of the local light emission source 8 is separated by a first band pass filter 11, and in the same way, an electric signal to a second light source 3 is separated by a second filter 12, the separated electric signals are demodulated 13, 14, and thereafter, multiplexed 15, therefore, the deterioration of receiving sensitivity can be prevented without using a polarization plane maintaining optical fiber, etc.

Description

[Detailed Description of the Invention] Table of Contents Overview Industrial Field of Application Prior Art (Figures 8 to 10) Means for Solving the Problems to be Solved by the Invention (Figure 1) Function Implementation Examples (Figures 2 to 7) Summary of the Effects of the Invention Regarding coherent optical communication systems, it is possible to improve polarization in optical transmission lines without using polarization-maintaining optical fibers, polarization diversity systems, or active polarization control systems. In order to prevent the deterioration of receiving sensitivity caused by wave fluctuation, the transmitting section oscillates first and second transmitting light sources whose emitted light planes of polarization are not the same at different frequencies. The two transmitting light sources are modulated directly or indirectly with the same signal, the modulated light is combined, and then sent out to the optical transmission line. In the receiving section, the signal light transmitted through the optical transmission line and the first signal are combined. and local light from a local light source that oscillates at a frequency different from the oscillation frequency of the second transmitting light source, and the combined light of the signal light and the local light is optical-to-electrically converted by a receiver, a first bandpass filter that passes an electrical signal having a frequency that is the difference between the oscillation frequency of the transmitting light source and the oscillating frequency of the local light source; and the oscillating frequency of the second transmitting light source and the oscillating frequency of the local light source. The electrical signals converted in the light receiver are separated by a second band-pass filter that passes electrical signals having frequencies different from each other, and the separated electrical signals are demodulated and then combined.

Industrial applications The present invention relates to a coherent optical communication system.

BACKGROUND ART In recent years, coherent optical communication systems that handle light as coherent waves have attracted attention as a next-generation optical communication system, and research is being actively conducted at various communication research institutes. In the coherent optical communication method, the reception sensitivity is improved by 10 to 20 dB compared to the intensity modulation direct detection method (IM-DD method) that has been put into practical use, so long-distance transmission is possible. In addition, since optical frequency multiplexing transmission or multilevel transmission can be performed, there are great advantages such as a dramatic increase in transmission capacity, and as a communication method, compared to the IM-DD method that simply blinks light. , basically has an advantage.

By the way, when implementing a coherent optical communication system, when performing optical heterodyne or homodyne detection using local light whose oscillation frequency is slightly different from or coincident with that of the transmitted light, it is possible to detect a difference in the polarization state of the two lights. , the power detected after optical-to-electrical conversion decreases, resulting in deterioration of reception sensitivity. When a normal single-mode optical fiber is used as an optical transmission line, the polarization state of the transmitted light changes, and some kind of improvement is required.

BACKGROUND ART FIG. 8 is a block diagram showing the basic configuration of a coherent optical communication system. Signal light from the transmitter 111 is transmitted to the receiver via an optical transmission line 112. The signal light transmitted to the receiving section is multiplexed with local light from a local light source 114 by an optical multiplexer 113 such as an optical coupler, and the combined light is subjected to, for example, hetero gain detection by a receiver 115. A light receiver 1 such as a photodiode located at the first stage of the receiver 115
16, due to its square law detection characteristic, an electrical signal having a frequency that is the difference between the frequency of the signal light and the frequency of the local light is generated, and a desired intermediate frequency signal of the signal is transmitted to the IF circuit 1.
17, and demodulation is performed by a demodulation circuit 118 based on the intermediate frequency signal.

In such a system, the optical signal sent out from the transmitter 111 is directly or indirectly modulated laser light, and is generally linearly polarized light. However, when the optical transmission line 112 is a single mode optical fiber, due to its unavoidable non-axial symmetry, the propagation mode H
E1, mode degeneracy solved, HE1, 1, HE1,...
The two modes can propagate, so when disturbances such as twisting, bending, external pressure, and temperature changes are applied to the optical fiber and coupling occurs between the two modes, a phase difference occurs due to the difference in phase velocity between the two modes. (birefringence), which causes the polarization state on the receiving side (output end of the optical fiber) to change in a complicated manner. Specifically, the emitted light becomes elliptically polarized light,
Alternatively, the polarization direction may be rotated. When such fluctuations in the polarization state are large, the interference efficiency between the signal light and the local light beam decreases, the reception sensitivity deteriorates, and in the worst case, reception becomes impossible.

Examples of means for dealing with this problem are given below.

(b) Use of polarization-maintaining optical fiber In addition to using a polarization-maintaining optical fiber as an optical transmission line,
By matching the planes of polarization in the transmitter and the receiver, the polarization state of the signal light can be kept constant and deterioration of reception sensitivity can be prevented.

(b) Polarization diversity method FIG. 9 is a block diagram showing the method. Optical transmission line 11
The signal light transmitted by 2 is separated into P wave and S wave components whose polarization planes are orthogonal to each other by a polarization separator 121.
The polarization separator 12 also applies to the local light from the local light source 114.
2 into P-wave and S-wave components, each P-wave component is multiplexed by an optical multiplexer 123 and demodulated by a receiver 125, and each S-wave component is multiplexed by an optical multiplexer 124.
After multiplexing, a receiver 126 demodulates the signals, and a rain post-modulation signal is synthesized by a combiner 127.

According to this configuration, even if the polarization fluctuation in the optical transmission path is large, an output signal can be obtained from either one of the receivers 125 and 126, so reception will not become impossible.

(c) Polarization active control method FIG. 10 is a block diagram showing the method. In this example, the polarization state of the combined light output from the optical multiplexer 132 that combines the signal light and the local light is detected by the polarization state detection circuit 133, and the polarization state of the optical transmission line 112 is detected based on the detected signal. A polarization controller 131 provided near the optical multiplexer 132 is driven to always obtain the maximum interference efficiency. still,
The polarization controller 131 includes a local light source 114 and an optical multiplexer 132.
It may be provided between.

Problems to be Solved by the Invention Among the conventional techniques for preventing deterioration of reception sensitivity due to polarization fluctuations in optical transmission lines, in the case of those using polarization-maintaining optical fibers, long lengths suitable for mass production have been proposed. However, it is difficult to provide polarization-maintaining optical fiber, which poses a problem for long-distance transmission.

Further, in that it is not possible to use optical transmission lines such as single-mode optical fibers that have been installed in the past, a great loss in terms of industrial economics is expected.

The polarization diversity method requires separate reception lines for P-polarized light and S-polarized light, which complicates the configuration. A large number of parts are required, making the configuration even more complex. Furthermore, if there are two reception streams, for example, if one reception stream is processing most of the power of the received light, the other reception stream will function almost exclusively to generate noise.
This results in the disadvantage that the S/N ratio decreases.

In the polarization active control method, for example, four receivers are required for detecting the polarization state of the signal light in addition to the main receiver in the reception chain, and four receivers are required to configure the polarization controller. In addition, optical components corresponding to a 1/4 wavelength plate and a 1/2 wavelength plate are required, which complicates the configuration. Furthermore, the received signal light and local light are distributed and used for polarization control, which is inefficient.

The present invention was created in view of these circumstances, and includes the use of a polarization-maintaining optical fiber, a polarization diversity method,
The purpose of this method is to prevent deterioration of reception sensitivity due to polarization fluctuations in an optical transmission line without using a polarization active control method.

Means for Solving the Problem The above-mentioned technical problem is solved by the configuration (1) shown in FIG. 1(a) or the configuration (2) shown in FIG. 1(b).

(1) In the transmitter 1, the first and second transmitting light sources 2.3 whose output light planes of polarization are not the same are set to different frequencies f1.
, f, the first and second transmitting light sources 2.3 are directly or indirectly modulated with the same signal by the modulator 4, and the modulated light is multiplexed by the multiplexer 5, and then the optical transmission line 6 so that it is sent to

In the receiving section 7, the signal light transmitted through the optical transmission line 6 receives a frequency f different from f and f2.

A multiplexer 9 combines the local light from a local light source 8 which oscillates at
First bandpass filter 11 that passes an electrical signal with frequency |f+ fs'l and frequency |f2 fsl
The electrical signals converted in the photoreceiver 10 are separated by the second band-pass filter 12 that passes the electrical signals, and the separated electrical signals are demodulated by the demodulators 13 and 14, and then combined by the combiner 15. I try to do that.

(2) In the transmitter 21, the transmitting light source 22 that oscillates at a frequency f1 is directly or indirectly modulated by the modulator 23, the modulated light is distributed by the distributor 24, and the modulated light is distributed by the distributor 24, and A first polarizer 25 that transmits light extracts a specific polarized light component from one of the distributed modulated lights, transmits the other distributed modulated light to a nonlinear optical crystal 26, and transmits the transmitted light to a transmitting light source. 22 oscillation frequency f, and generates a new light having a frequency f2 different from the above, and has a polarization plane different from the polarization plane of the modulated light and different from the polarization plane of the specific polarized light component taken out by the first polarizer 25. A second polarizer 27 that transmits the light extracts a specific polarized light component from the light transmitted through the nonlinear optical crystal 26, and a multiplexer 28 combines the specific polarized light components extracted by the first and second polarizers 25 and 27. After the wave, the optical transmission line 29
so that it is sent to

In the receiving section 30, the signal light transmitted through the optical transmission path 29 and the local light from the local light source 31 that oscillates at a frequency f3 different from fl and f2 are combined by a multiplexer 32, and this combined light is is converted into electricity by a photoreceiver 33,
The electrical signal converted in the optical receiver 33 is separated by a first bandpass filter 34 that passes an electrical signal with a frequency of 1f+-f3l and a second bandpass filter 35 that passes an electrical signal with a frequency of |f2fsl. ,
The separated electrical signals are demodulated by demodulation circuits 36 and 37, respectively, and then combined by a combiner 38.

In addition, in this specification, "polarized light" and "polarized wave" are used with the same meaning.

According to the configuration of the present invention, lights whose polarization planes are not the same, for example, whose polarization planes are orthogonal to each other, are modulated with the same signal and sent out to the optical transmission line, and the receiving side converts them into electrical signals and demodulates them. Since the signals are synthesized after the signals are combined, a signal output can always be obtained even when polarization fluctuations in the optical transmission path are large, and deterioration of receiving sensitivity can be prevented.

In the configuration (1), the reason why the first and second transmitting light sources are oscillated at different frequencies f, , f2 is to control the frequency so that the oscillation frequencies of the first and second transmitting light sources match. This is because frequency control is easier compared to. In other words, in order to control the frequency so that the oscillation frequencies match, difficult-to-implement technologies such as optical PLL are required, whereas relative frequency control to different frequencies, for example, requires AFC using hetero gain detection, This can be done relatively easily by controlling the oscillation frequency to match the peak value of the resonance spectrum of the Fabry-Perot resonator.

In addition, in the configuration (2), by using a nonlinear optical effect, different frequencies are given to the distributed lights of a single transmitting light source. This is because, assuming that it is not possible to extract orthogonal polarization components at the same time, it will be necessary to apply a polarization scrambler method or the like that has a limit on the bit rate.

Example Embodiments of the present invention will be described below based on the drawings.

FIG. 2 is a block diagram of a coherent optical communication system showing one embodiment of the present invention. In the transmitter 40, 41
is a transmitting LD (semiconductor laser), which has a spectral linewidth that does not have the influence of phase noise on the desired transmission bit rate by forming an external resonator, etc. The element temperature and bias current are controlled so that the frequency is stable.Transmission LD
The oscillation frequency of 41 is f, and the emitted light is linearly polarized light. 42 is the spectral line width similar to the transmitting LD 41,
Transmitting LD with controlled element temperature and bias current
The oscillation frequency is f2 (≠f+), and the polarization plane of the emitted light is perpendicular to the polarization plane of the emitted light from the transmitting LD 41. Although the transmitting LDs 41 and 42 are directly modulated with modulation signals based on transmission information in this embodiment, they may be externally modulated indirectly using an external modulator. 43 is an optical multiplexer such as an optical coupler, and PIM and S waves whose polarization planes from the transmission LD 41 and 42 are orthogonal to each other (for convenience, the polarization state of the output light of the transmission LD 41 is the P wave, and the polarization state of the transmission LD 42 is It is assumed that the polarization state of the emitted light is S wave.) is multiplexed and sent to an optical transmission line 44 made of, for example, a normal single mode optical fiber.

In order to stabilize and control the oscillation frequencies of the transmitting LDs 41 and 42 to different frequencies f+ and f2, for example, each oscillation frequency must be made to match the spectral peaks that appear at equal intervals on the frequency axis in the Fabry-Perot etalon, etc. You can control it like this.

In the receiving section 45, 46 is a local LD whose oscillation spectrum line width and oscillation frequency are stabilized like the transmitting LDs 41 and 42, and its oscillation frequency is f, which is slightly different from f1 and f2. . Although not shown, an AFC (automatic frequency tracking) device is provided so that the beat signal with the light emitted from the transmitting LDs 41 and 42 is kept stable.

47 is 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. 48 is a light receiver such as a photodiode that converts the white wave light from the optical multiplexer 47 into electricity, and functions as a square law detector. The intermediate frequency signal having an intermediate frequency of several GHz, for example, generated in the optical receiver 48 is transmitted to the IF
After being amplified by the amplifier 49, the bandpass filter 50
Sent to ゜52. The bandpass filter 50 has a frequency of 1
The bandpass filter 52 selectively passes the intermediate frequency signal of frequencies 1f2-f, 1, while selectively passing the intermediate frequency signal of fl-f31. The detected signal that has passed through the band-pass filter 50 is demodulated by a demodulator 51, the detected signal that has passed through the band-pass filter 52 is demodulated by a demodulator 53, and the respective demodulated signals (baseband signals) are combined by a power combiner 54. (added). Then, the transmitted information is reproduced by identifying the signal from the power combiner 54 using the discriminator 55. Note that the demodulators 51 and 53 are configured according to a transmitting side modulation method such as a DPSK (differential phase shift keying) method or a CPFSK (phase continuous frequency shift keying) method.

FIG. 3 shows the polarization state and demodulated signal level at each portion 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 with frequency f input to the optical multiplexer 47, 62 (62a-e) is the polarization state of the signal light with frequency f2 input to the optical multiplexer 47, and 63 (63a-e) is the polarization state of the signal light with frequency f2 input to the optical multiplexer 47.
e) is the polarization state of the local light of frequency f input to the optical multiplexer 47, 64 (64a-e) is the output signal level of the demodulator 51, and 65 (65a-e) is the output signal of the demodulator 53. Level 66 (66a-e) is the output signal level of power combiner 54.

Since the polarization planes of the P wave and S wave from the transmitting LD41.42 on the transmitting side are orthogonal to each other, the polarization state of the signal light transmitted to the receiving side is as shown in Fig. 3 (a) and (b). Combinations of linearly polarized lights as shown in Figure 3(C) and (d), combinations of elliptically polarized lights as shown in Figure 3(e), etc. Conceivable. In any combination, one of the demodulators 51, 53 (
Since a demodulated signal can be obtained from either one of the two, a decrease in interference efficiency due to polarization fluctuation is suppressed, and a deterioration in receiving sensitivity is prevented.

FIG. 4 is a block diagram of a receiving section showing an embodiment combined with a dual-balanced receiving system.

The signal light transmitted through the optical transmission line is combined with the local light from the local LD 70 and an optical multiplexer/demultiplexer 71 such as an optical coupler and output to two optical paths, and each optical output is transmitted to the same optical path. The light is input manually to the receivers 72 and 73. Light receiver 72
．． 73 consists of, for example, photodiodes with the same characteristics connected in series, and the signal taken out from the neutral point is connected to the IF
After being amplified by an amplifier 74, it is demodulated by a detection/demodulation circuit 75 consisting of a bandpass filter, a demodulator, and a combiner as in the previous embodiment, and then identified by a discriminator 76.

According to such a dual-balance type reception system, in-phase local light intensity noise is canceled out and opposite-phase signal components are added, so that reception sensitivity can be greatly improved. When applying a dual-balanced reception method, it is necessary to use, for example, a polarization diapersity method in order to prevent deterioration of reception sensitivity due to polarization fluctuations in the optical transmission path, regardless of the present invention. Combining the dual balance reception method and the polarization diversity reception method requires, for example, four light receivers, which particularly complicates the configuration of the optical system. In comparison, according to this embodiment, even when used in combination with a dual-balanced receiving system, only two receivers are required, and the configuration can be easily corrected without complicating polarization fluctuations in the optical transmission path. It is possible to prevent the deterioration of reception sensitivity caused by this.

FIG. 5 is a block diagram of a receiving section showing an embodiment combined with a phase diversity receiving method.

The signal light transmitted through the optical transmission line and the local light from the local LD 80 are multiplexed in an optical multiplexer/demultiplexer 81, and the multiplexed light is distributed into n series of light receivers 82-1 to 82-1, respectively.
n. The signals from each light receiver are amplified by an IF amplifier 83 and demodulated by a detection/demodulation circuit 84, then combined by a combiner 85, and discriminated by a discriminator 86. The optical multiplexer/demultiplexer 81 has an optical configuration such that the phases of the n series of output lights are shifted by 360/n'. 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 a reception band similar to that of the homodyne detection method is required, and a high pit rate can be easily achieved. can.

In Figure 6, n=3 and the frequency offset is 1 of the bit rate.
An example of the output waveform of each detection/demodulation circuit in the case of /18 is shown. Even at the moment when the output of any port becomes 0, the output of other ports does not become 0, so by combining these, it is possible to reproduce the transmitted information well.

When the polarization diversity reception method is combined with the phase diversity reception method, the configuration of the optical system of the receiving section becomes significantly complicated, similar to when the polarization diversity reception method and the dual balance reception method are combined. However, by combining the embodiments of the present invention and the phase diversity reception method, it is possible to prevent the deterioration of reception sensitivity with a simple configuration.

FIG. 7 shows two transmitting LDs as in the previous embodiments.
This example shows an embodiment in which a single transmitting LD is used to obtain signal lights of different frequencies fI and f2, instead of using a single transmitting LD. The emitted light (frequency f+) of the transmitting LD 91, which is directly modulated according to the transmission information, is transmitted through the lens 9.
After being collimated by 2, the light is distributed into two optical paths by a distributor 93 such as a half mirror. One of the distributed lights is reflected by a reflecting mirror 94, passes through a polarizer 95, is focused by a lens 96, and enters an optical multiplexer 97. The other distributed light is transmitted through a parametric oscillator 98 and a polarizer 99.
in this order, is focused by the lens 100, and enters the optical multiplexer 97.

The transmitted polarization planes of the polarizers 95 and 99 are orthogonal to each other,
In addition, the polarization plane of the light emitted from the transmitting LD 91 is determined by the polarizer 95.
．． For example, it forms an angle of 45° with respect to the transmission polarization plane of 99. Parameters) The IJ hook shaker 98 is
It is configured to include an optical resonator 98a and a nonlinear optical crystal 98b provided inside the optical resonator 98a, and with this configuration,
For incident light of frequency f, it is possible to newly generate light of frequency f2f4 different from frequency f.

Here, frequencies f+, f2. Between fs, f emperor f,
The relationship 10f holds true.

According to such a configuration of the transmitting section, it is possible to combine the lights whose polarization planes are orthogonal to each other and whose frequencies are different (f+, fz) and send it out to the optical transmission line 44.
By configuring the receiving section in the same manner as in the previous embodiments, it is possible to prevent deterioration of receiving sensitivity due to polarization fluctuations in the optical transmission path. Further, according to this embodiment, since there is only one LD for transmission, temperature control of the LD, etc. can be made easier, and the configuration of the transmitter can be simplified.

In the above embodiment, as the most desirable embodiment of the present invention, the case where the plane of polarization of light with frequency f and the plane of polarization of light with frequency 12 are orthogonal has been described, but the present invention is not limited to this. . That is, unless the polarization plane of the light with frequency f and the polarization plane of light with frequency f2 are at least on the same plane, it is possible to use polarization components whose polarization planes are orthogonal to each other, so that the polarization planes of the two transmitted lights are The present invention can be practiced even when the two are not orthogonal.

Effects of the Invention As detailed above, according to the present invention, it is possible to eliminate polarization fluctuations caused by polarization fluctuations in an optical transmission line without using a polarization-maintaining optical fiber, a polarization diversity method, or an active polarization control method. This has the effect of making it possible to prevent deterioration of reception sensitivity.

[Brief explanation of drawings]

Fig. 1 is a diagram of the principle of the present invention, Fig. 2 is a block diagram of a coherent optical communication system showing an embodiment of the invention, Fig. 3 is an explanatory diagram of the operation of the embodiment shown in Fig. 2, and Fig. 4 is a diagram illustrating the operation of the embodiment shown in Fig. 2. FIG. 5 is a block diagram of a receiving section showing an embodiment combined with a dual-balanced reception method. FIG. 5 is a block diagram of a receiving section showing an embodiment combined with a phase diversity reception method. FIG. Diagram for explaining operation, Figure 7 is a block diagram of a transmitting unit showing an embodiment with one transmitting LD, Figure 8 is a block diagram of a coherent optical communication system, and Figure 9 is a block diagram showing a polarization diversity system. FIG. 10 is a block diagram showing the polarization active control method. 1.21... Transmitting section, 2... First transmitting light source, 3... Second transmitting light source, 6.29... Optical transmission line, 7.30... Receiving section, 8. 31... Local light source, 10.33... Light receiver, 11.34... First band pass filter, 12.35
... second bandpass filter, 22 ... transmission light source,
25... First polarizer, 26... Nonlinear optical crystal, 27... Second polarizer.

Claims (2)

[Claims] (1) In the transmitter (1), the first and second transmitting light sources (with which the plane of polarization of the emitted light is not the same) are used.
2, 3) are oscillated at different frequencies (f_1, f_2), the first and second transmitting light sources (2, 3) are modulated directly or indirectly with the same signal (4), and the modulated light is combined. (5) After that, it is sent to the optical transmission line (6), and in the receiving section (7), the signal light transmitted by the optical transmission line (6) and the first and second transmitting light sources (2, 3) oscillation frequency (f_1
. and pass an electrical signal having a frequency (|f_1-f_3|) that is the difference between the oscillation frequency (f_1) of the first transmitting light source (2) and the oscillation frequency (f_3) of the local light source (8). The first bandpass filter (11) and the second transmission light source (
The frequency of the difference between the oscillation frequency (f_2) of 3) and the oscillation frequency (f_3) of the local light source (8) (|f_2-f_3
A second bandpass filter ( | ) that passes the electrical signal of
12), the electrical signals converted in the light receiver (10) are separated, and the separated electrical signals are demodulated (13, 14) and then combined (15). Optical communication method. (2) In the transmitting section (21), the transmitting light source (22) that oscillates at the frequency (f_1) is directly or indirectly modulated (23), the modulated light is distributed (24), and the polarization plane of the modulated light is A first polarizer (25) that transmits light of different polarization planes extracts a specific polarized light component from one of the distributed modulated lights, and the other distributed modulated light is transferred to a nonlinear optical crystal (26).
to transmit the transmitted light to the transmitted light to generate a new light having a frequency (f_2) different from the oscillation frequency (f_1) of the transmitting light source (22), which is different from the polarization plane of the modulated light and the first polarizer ( 2
A second polarizer (27) that transmits light with a plane of polarization different from the plane of polarization of the specific polarization component extracted by 5) extracts the specific polarization component from the light transmitted through the nonlinear optical crystal (26). After combining (28) the specific polarized light components taken out by the first and second polarizers (25, 27), the optical transmission line (
29), and in the receiving section (30), the signal light transmitted by the optical transmission line (29), the oscillation frequency (f_1) of the transmitting light source (22) and the nonlinear optical crystal (26) are combined. The frequency of the newly generated light (f_2) and the local light from the local light source (31) oscillating at a different frequency (f_3) are combined (32) into the transmitted light, and a combined light of the signal light and the local light is generated. is optical-to-electrically converted by the light receiver (33), and the frequency (|f_1-f_3| ) and the nonlinear optical crystal (2).
A second electrical signal having a frequency (|f_2-f_3|) that is the difference between the frequency (f_2) of the light newly generated in the transmitted light of 6) and the oscillation frequency (f_3) of the local light source (31). The bandpass filter (35) allows the light receiver (3
A coherent optical communication system characterized in that the electrical signals converted in step 3) are separated, and the separated electrical signals are demodulated (36, 37) and then combined (38).