JP4621881B2 - Coherent optical transmission method - Google Patents

Description

  The present invention relates to a coherent optical communication system, and in particular, multi-value that is not affected by the phase fluctuation of a light source in principle by simultaneously transmitting two correlated signals through independent channels. The present invention relates to a coherent optical transmission method.

  Several methods using polarized light have been proposed as coherent light transmission that is not affected by the phase fluctuation of a light source in principle. In Non-Patent Document 1, first, an optical carrier wave is decomposed into two polarization components orthogonal to each other so as to have equal energy, one of the polarization components is subjected to binary phase modulation of 0 or π, and the other polarization component Are unmodulated reference light, and these are combined and transmitted simultaneously. At this time, the optical signal corresponds to one of two orthogonal polarization states. Here, generally, two orthogonal polarization states are horizontal linear polarization or vertical linear polarization, or right-handed circularly polarized light or left-handed circularly polarized light. The optical signal that has received the transmission signal is combined with the local oscillation light. After that, it is separated again into two orthogonally polarized components, and optical heterodyne detection is performed using common local oscillation light. Two electric signals obtained by optical heterodyne detection are mixed and stacked on each other. Only the low frequency components are extracted from the signal generated by this multiplication, and the original data is demodulated.

  Non-Patent Document 2 discloses a conventional method in which the modulation method of the conventional method is changed. First, the optical carrier wave is decomposed into two optical signals mainly including one of the two polarization components orthogonal to each other so as to have equal energy. One optical signal is subjected to binary phase modulation of 0 or π with one skipped data among the differentially encoded data. The other optical signal is subjected to binary phase modulation of 0 or π with a delay of half the bit time interval of the remaining data among the differentially encoded data in advance. These phase-modulated optical signals are combined and transmitted simultaneously. The received optical signal restores the original data in the same manner as in the conventional method.

  Since these are transmissions using two orthogonal polarization states, they are systems for only binary transmission.

Kuwano et al., "Polarization modulation / optical coherent detection method and its phase noise removal effect" IEICE Transactions BI, volJ75-B-I, No.2, pp.107-118, February 1992 Tsukamoto et al., “Differential optical phase modulator / heterodyne detection method using two polarization planes” IEICE Transactions BI, vol.J77-BI, No.10, pp.629-639, 1994 October

  In the conventional method proposed as coherent light transmission that is not affected by the phase fluctuation of the light source, the data to be transmitted is made to correspond to two orthogonal polarization states. Therefore, in principle, a binary signal is used. Therefore, there is a drawback that the throughput is not improved because only the transmission can be performed. For this reason, multilevel signal coherent optical transmission capable of supporting future high-speed and large-capacity communication has been desired.

  The present invention has been proposed in view of the above, and proposes a coherent optical transmission method that is not affected by the phase fluctuation of a light source in principle and can transmit a multilevel signal. .

  In the present invention, two signals having correlation can be simultaneously transmitted through independent channels, the phase fluctuations of the light source can be canceled, and the influence can be suppressed in principle. Further, it is possible to transmit a multilevel signal in which a reference code is transmitted on one of the optical carriers and a data signal transmitted on the reference code is superimposed on the other of the optical carriers. Furthermore, coherent optical transmission can be performed by performing optical heterodyne detection or optical self-homodyne detection using two optical carriers included in the received optical signal.

  Further, by branching and generating one optical carrier wave, two optical carrier waves having complete correlation can be obtained.

  Also, by dividing one optical carrier into two polarization components and using a polarization multiplexed optical path, the disturbances on the optical path applied to both optical carriers can be made equal.

  First, in order to clarify the essential part of the present invention, the electric field of the optical signal at each stage of the digital signal coherent optical transmission apparatus used in the present invention will be described using mathematical expressions. The coherent optical transmission device includes a first encoder that generates a multi-level modulation code string from a data string, and two code strings of a data code string 1 and a data code string 2 from the multi-level modulation code string and a reference code string. A transmission line for transmitting the second encoder for generating the data, the data code string 1 and the data code string 2 as two systems of optical signals superimposed on an optical carrier wave propagating through two close optical paths And the above two systems of optical signals were received and obtained from the received optical signals corresponding to the data code sequence 1 using the electrical signals obtained from the received optical signals corresponding to the data code sequence 2 as a reference. A second decoder that decodes a multilevel modulation code string obtained from an electrical signal, and a first decoder that decodes a data string from the decoded multilevel modulation code string.

  When Pc, fc, and Φc (t) are optical carrier power, frequency, and phase noise, respectively, the optical carrier electric field from the light source is expressed by equation (1) in FIG. Here, it is assumed that there is phase noise represented by the equation (2) in FIG.

The electric field of the modulated optical signal is expressed by equation (3) in FIG. Here, it is assumed that a _xc (t) and a _xs (t) are discrete values depending on the data. Also, a _yc (t) and a _ys (t) are switched alternately. Here, there is a limitation of the equation (4) in FIG. 10, which eventually becomes the condition of the equation (5) in FIG.

The electric field of the transmitted optical signal is expressed by equation (6) in FIG. Here, the y-axis light is phase-shifted by π / 2 with respect to the x-axis light.
For received light, it is assumed that a rotation angle θ is added at the time of reception. Further, since the y-axis light is phase-shifted by π / 2, the electric field of the optical signal is expressed by equation (7) in FIG. Here, Pr is the power of the received optical signal, and uses the notation shown in equations (8) to (11) in FIG.

Next, heterodyne detection will be described.
The received light is expressed by the equation (12) in FIG. 12 of FIG. For local oscillation light, when Pl, fl, and Φl (t) are the power, frequency, and phase noise of local oscillation light, respectively, they are expressed by equations (13) and (14) in FIG.

  After passing through the low-pass filter, the photocurrent obtained by the photodetector becomes as shown in equation (15) in FIG. In this equation, the notation of equations (16) and (17) in FIG. 11 is used.

  When mixing the x-axis component and the y-axis component of equation (15) in FIG. 11, equation (18) is obtained. The latter half of equation (18) in FIG. 11 describes a signal that has passed through a low-pass filter that filters out 2ψ (t).

As apparent from the equation (18) in FIG. 11, the signal obtained by mixing is the coordinates of the encoded data signal in the signal space displaying the quadrature component and the in-phase component, a _yc (t) and a _ys. The inner product of (t) and the coordinates of the encoded reference signal sequence, a _xc (t) and a _xs (t). Thus, by allowing repeated 1,0 alternately a _xc and (t) a _xs and (t), can be cut into alternately a _xc (t) and a _xs (t). Alternatively, by outputting the inner product value only when a _xc (t) or a _xs (t) is 1, the coordinates of the encoded data signal, a _yc (t) and a _ys (t ) Can be obtained.

  Further, the inner product represented by the equation (18) in FIG. 11 is invariant with respect to the coordinate rotation around the origin of the signal space. For this reason, it is shown that both the coordinates of the data signal and the coordinates of the reference signal sequence do not change even when they are subjected to coordinate conversion by rotation (that is, when they undergo the same phase change). By detecting the coordinates of the data signal, it can be seen that it is not affected by phase noise.

  It can also be seen that the inner product can be obtained even if the encoded reference signal sequence and the encoded data signal are exchanged on the transmission side.

  Further, the inner product of the linear sum of the signal obtained by replacing the encoded reference signal sequence and the encoded data signal on the transmission side with the signal not replaced is changed according to the coefficient of the linear sum. Even if the above linear sum is unitarily transformed, the inner product is invariant. Therefore, it can be understood that even if the encoded reference signal sequence and the encoded data signal are mixed and transmitted, decoding can be performed correctly.

A case where homodyne detection is performed will be described below.
The electric field of the received optical signal is expressed by equation (19) in FIG. By passing this light through a polarization splitter tilted by −π / 4, the electric field of the light in the direction of the polarization axis is as shown in Expression (20) of FIG.

When the polarized light along the X component and the Y component in the equation (20) in FIG. 12 is photoelectrically converted and the high frequency component is removed using a low-pass filter, an electric signal in the equation (21) in FIG. 12 is obtained. Further, taking the difference between the X component and the Y component in equation (21) yields equation (21), and the coordinates of the encoded data signal, a _yc (t) and a _ys , as in the case of heterodyne detection. The inner product of (t) and the coordinates of the encoded reference signal sequence, a _xc (t) and a _xs (t). Therefore, it is clear that the original data can be restored from this signal in the same manner as in the heterodyne detection described above.

To achieve the above object, a multilevel coherent optical transmission method of the present invention includes a first encoder that generates a multilevel modulation code sequence from a data sequence,
A second encoder that generates two code strings of a data code string 1 and a data code string 2 from the multilevel modulation code string and the reference code string;
A transmission path for transmitting the data code string 1 and the data code string 2 as two optical signals superimposed on an optical carrier wave propagating through two close optical paths, and the two optical signals described above. A multi-level modulation code sequence obtained from an electrical signal obtained from an optical signal received corresponding to the data code sequence 1 using an electrical signal obtained from the received optical signal corresponding to the data code sequence 2 as a reference A second decoder for decoding
In a digital signal coherent optical transmission device comprising: a first decoder that decodes a data sequence from a decoded multilevel modulation code sequence;
The optical carrier is two optical carriers having correlation,
Data code sequence 1 is a multi-level modulation code sequence,
Data code string 2 is a reference code string,
The reference code string is a code string in which in-phase unit vectors and orthogonal unit vectors in the signal space of the multi-level modulation code string are arranged,
One of the two optical carriers having the above correlation is optically modulated with the data code string 1,
The other of the two optical carriers having the above correlation is optically modulated with the data code string 2, and
These two optically modulated optical carriers are transmitted in two optical paths,
Receiving the transmitted optical signal,
An electric signal corresponding to the data code string 1 is obtained by mixing each of the two received optical signals with each of the two local oscillation lights having correlation, performing optical heterodyne detection, and selecting an intermediate frequency band signal. And two electrical signals corresponding to the data code string 2 are obtained,
Multiplying the two electrical signals,
Select the low frequency band signal from the signal obtained by multiplication and restore the above multilevel modulation code,
The multi-level modulation code is decoded with respect to the first encoder to restore the data sequence.

The carrier used for the data code string 1 and the carrier used for the data code string 2 are signals having the same frequency,
When multiplying two electrical signals of an electrical signal corresponding to the data code string 1 and an electrical signal corresponding to the data code string 2 obtained by performing optical heterodyne detection, the phases of the two electrical signals are mutually They are stacked in a shifted state.

The carrier wave used for the data code string 2 is a signal having a frequency twice that of the carrier wave used for the data code string 1.
When multiplying two electric signals, that is, an electric signal corresponding to the data code string 1 and an electric signal corresponding to the data code string 2 obtained by performing optical heterodyne detection, the phases of the two electric signals are mutually changed. The products are stacked together.

The present invention also includes a first encoder that generates a multi-level modulation code sequence from a data sequence;
A second encoder that generates two code strings of a data code string 1 and a data code string 2 from the multilevel modulation code string and the reference code string;
A transmission path for transmitting the data code string 1 and the data code string 2 as two optical signals superimposed on an optical carrier wave propagating through two close optical paths, and the two optical signals described above. A multi-level modulation code sequence obtained from an electrical signal obtained from an optical signal received corresponding to the data code sequence 1 using an electrical signal obtained from the received optical signal corresponding to the data code sequence 2 as a reference A second decoder for decoding
In a digital signal coherent optical transmission device comprising: a first decoder that decodes a data sequence from a decoded multilevel modulation code sequence;
The optical carrier is two optical carriers having correlation,
Data code sequence 1 is a multi-level modulation code sequence,
Data code string 2 is a reference code string,
The reference code string is a code string in which in-phase unit vectors and orthogonal unit vectors in the signal space of the multi-level modulation code string are arranged,
One of the two optical carriers having the above correlation is optically modulated with the data code string 1,
The other of the two optical carriers having the above correlation is optically modulated with the data code string 2, and
These two optically modulated optical carriers are transmitted in two optical paths,
Receiving the transmitted optical signal,
Each of the received optical signals is split into two optical paths,
A first optical signal obtained by combining an optical signal on one optical path and an optical signal on the other optical path, an optical signal obtained by shifting the optical signal on one optical path by a half wavelength, and the other optical signal are combined. Generating a second optical signal;
Optical self-homodyne detection is performed for each of the combined first and second optical signals to generate an electrical signal,
Generate a difference signal for the two electrical signals obtained by the above optical self-homodyne detection, select the low frequency side component to restore the above multi-level modulation code,
The multi-level modulation code is decoded with respect to the first encoder to restore the data sequence.

  Also, two optical carriers having correlation are generated by branching one optical carrier.

  In addition, a method of branching and generating one optical carrier is to separate one optical carrier into two polarization components, and two optical paths that are close to each other use polarization multiplexed optical paths.

The second encoder is
1) Replace the in-phase component and the quadrature component of each of the data code string 1 and the data code string 2, or
2) The in-phase components or the orthogonal components of the data code sequence 1 and the data code sequence 2 are interchanged, or
3) The in-phase component of the data code string 1 and the quadrature component of the data code string 2 are exchanged, and the quadrature component of the data code string 1 and the in-phase component of the data code string 2 are exchanged, or
4) The in-phase component of the data code string 1 is multiplied by a constant and the in-phase component of the data code string 2 is multiplied by the reciprocal of the constant, or the orthogonal component of the data code string 1 is multiplied by the constant and the orthogonality of the data code string 2 The component is multiplied by the reciprocal of the constant.

Further, a method for restoring the multilevel modulation code from the selected low frequency component is as follows.
According to a signal obtained by restoring the carrier wave of the electrical signal corresponding to the data code string 2 included in the difference signal, the low frequency component selected from the difference signal is distributed to two lines, or
The low-frequency component selected from the difference signal is intermittently connected to the two lines in accordance with a signal obtained by restoring the carrier wave of the electrical signal corresponding to the data code string 2 included in the difference signal. The component and the orthogonal component are restored.

  The in-phase component or quadrature component of the restored multi-level modulation code is simultaneously input to the first decoder by holding or delaying the voltage or digital data corresponding to the in-phase component or quadrature component. It is.

Further, the present invention performs optical heterodyne detection by mixing each of the two received optical signals and each of the two local oscillation lights having correlation, and selects the intermediate frequency band signal to generate the data code string 1. When obtaining two electric signals, an electric signal corresponding to the electric signal corresponding to the data code string 2,
Each of the two received optical signals is mixed with each of two local oscillation lights having a phase delayed by a quarter wavelength, and two received optical signals whose phases are delayed by a quarter wavelength. Each of the two local oscillation lights having correlation with each other is generated, and each optical heterodyne detection is performed, and the intermediate frequency band signal is selected to correspond to the data code string 1 and mixed with the mixing 1 Obtaining a difference signal of electrical signals obtained from each of 2 and a difference signal of electrical signals corresponding to the data code string 2 and obtained from each of the mixing 1 and mixing 2 by selecting an intermediate frequency band signal, The difference signal between the two electrical signals is multiplied, a low frequency band signal is selected from the signals obtained by multiplication, and the multilevel modulation code is restored. For the encoder It is to restore the data string of the by performing reduction.

  In addition, when a transmission delay difference is generated in an adjacent optical path, by providing a circuit that compensates for the transmission delay difference, the light emitted from the light source is received at the same time, so that fluctuations in the light source can be easily offset.

  Hereinafter, embodiments of the present invention will be described. In the following description, devices having the same function or similar functions are denoted by the same reference numerals unless there is a special reason.

  As shown in FIG. 1, the correlated light generator 1 outputs two lights having a correlation. The correlated light generator 1 can achieve the object of the present invention as long as two correlated optical carriers are obtained. For example, as shown in FIG. 2A, the light split from the laser light from the laser light source 51, or the light from the laser light source 54 as shown in FIG. Using a wavelength-stabilized light source 57 that is phase-synchronized with light separated into wave components, or light from an oscillation wavelength-stabilized light source 56 that is a laser light source with a stabilized oscillation wavelength, as shown in FIG. Alternatively, as shown in FIG. 3B, a combination of light from the laser light source 51 and light obtained by wavelength conversion of the light may be used.

  FIG. 3B can also be used with light sources that output light of different wavelengths. FIG. 3B shows a case where the optical carrier wave converted by the wavelength converter 58 is used. In order to adjust the delay by the wavelength converter, the optical carrier wave on the other optical path is delayed by the delayer 59. When different wavelengths are used in this way, in heterodyne detection described later, it is desirable to adjust the wavelength of the local oscillation light so that the intermediate frequency signal becomes a signal in the same frequency band. Further, when performing self-heterodyne detection described later, it is desirable to reconvert so that the optical signal received on the receiving side has the same wavelength.

  In FIG. 1, these optical carriers are modulated using two optical modulators 2 and 3. The modulated signal is subjected to multi-level encoding in which a data sequence is encoded at a position in the signal space to generate a multi-level modulation code sequence (first encoding). Further, some operation is applied to the code (second encoding), and a modulated signal divided into the data code string 1 and the data code string 2 is used. As described above, the operation described above is an operation in which the inner product of the coordinates of the multi-level modulation code string and the coordinates of the reference signal string does not change.

  The optical carrier wave modulated in this way is transmitted to channel-1 and channel-2 in FIG. In this transmission, although not shown in the figure, it is desirable to provide an optical delay device that gives in advance a delay that compensates for the transmission delay difference between the respective channels so that they can be received simultaneously.

  When the optical delay device is not used, it is desirable to provide a transmission delay difference compensator to compensate the delay of each channel on the receiving side. The local oscillation light from the local oscillator 8 is combined with the light of each channel and mixed by the photodetectors 9 and 10. The intermediate frequency band of each electrical signal obtained from the photodetectors 9 and 10 is selected by the bandpass filter 11 or 12 and multiplied by the multiplier 13. The inner product is obtained by filtering the signal resulting from this multiplication by the low-pass filter 14. Since the signal corresponding to this inner product is a multilevel modulation code string, the carrier wave when the multilevel modulation code string is generated is extracted and separated into its quadrature component and in-phase component. The process from reception to here is the second decoding. In addition, a data string is restored from the quadrature component and the in-phase component (first decoding).

  The light from the correlated light generator 1 always undergoes phase fluctuations, and phase fluctuations due to fluctuations in the optical path length in the channel always occur. However, when the channels 1 and 2 are close to each other, the fluctuation of the optical path length has an in-phase component for the light of each channel, and the difference between the optical signal of the channel 1 and the optical signal of the channel 2 is obtained. Can be offset. By taking this difference, phase fluctuations originating from the light source can also be canceled out. In such cancellation, an encoded signal is extracted from the optical signal of channel 1 or 2 or both, and decoding is performed from the received signal based on the in-phase component and the quadrature component transmitted by the encoded signal. To be executed.

  FIG. 4 is an example in which self-homodyne detection is performed on the receiving side. Transmission is the same as in the above embodiment. On the receiving side, the optical carrier waves of channels 1 and 2 are branched by branching units 18 and 19, and multiplexed by a multiplexer 20 for optical detection to perform self-homodyne detection. Similarly, self-homodyne detection is performed by phase-shifting one of the lights branched by the branching device 18 or 19 by half a wavelength, combining the light by the multiplexer 21 and performing optical detection. By using the adder 23, the difference between these detection signals is taken and filtered by the low-pass filter 14, and as described above, the coordinates of the encoded data signal, the coordinates of the encoded reference signal sequence, and A signal corresponding to the inner product of can be obtained. Since the signal corresponding to this inner product is a multilevel modulation code string, the carrier wave when the multilevel modulation code string is generated is extracted and separated into its quadrature component and in-phase component. The process from reception to here is the second decoding. Further, the data string is restored from the quadrature component and the in-phase component (first decoding).

  As shown in FIG. 5A, first, the optical carrier wave from the light source 54 is separated into two orthogonal polarization components by the polarization separator 55. It is desirable that this separation be completely orthogonal, but the present invention can be applied if it is close to orthogonal. The x-axis polarization component is a digital value (for example, π / 4, 3π / 4, 5π / 4, or 7π / 4 as shown in FIG. 6A) by the optical phase modulator 3a in FIG. ) And the light intensity modulator 3b modulates the light intensity with a digital value (for example, 1, 1/3, 1/5 as shown in FIG. 7C). Here, FIG. 5B is a diagram for explaining the optical modulator 3 in more detail. At this time, the order of the phase modulation and intensity modulation of the x-axis polarization component may be reversed. The phase of the y-axis polarization component is modulated with a digital value (for example, either 0 or π / 2) by the optical phase modulator 3. The digital value of the phase modulation of the y-axis polarization component is set so as to be a reference when determining the digital value of the phase modulation of the x-axis polarization component. These x-axis polarization component and y-axis polarization component are combined again by the polarization multiplexer 60, transmitted through the quarter-wave plate 60 after giving a phase difference, and transmitted. The operation for giving this phase difference is an operation for making the transmitted light circularly polarized light.

  Here, the quarter-wave plate 61 can be omitted by adjusting the phase before passing through the polarization multiplexer 60 so that the transmitted light becomes circularly polarized light. For example, when the optical modulator is of the Mach-Zehnder interferometer type, the phase can be adjusted by adjusting the offset voltage.

  In general, since polarization axis mismatch occurs between transmission and reception, it is assumed that there is a rotational offset θ between the (x, y) coordinate system on the transmission side and the (X, Y) coordinate system on the reception side. However, it is assumed that there is no polarization disturbance other than the polarization axis mismatch in the transmission path, that is, the x-axis polarization component and the y-axis polarization component are transmitted independently.

  FIG. 8 shows the configuration of the receiving side. On the receiving side, after excess light is removed by the band pass filter 62, the light passes through the quarter-wave plate 63 again. Thereafter, it is mixed with the local oscillation light from the local oscillation light source 64 and separated into the X-axis polarization component and the Y-polarization component orthogonal by the polarization separator 2. The respective polarization components are optically heterodyne detected independently by the optical detectors 9 and 10, and only the intermediate frequency (IF) band components are extracted by the bandpass filters 11 and 12. The IF band component signals corresponding to the X-axis polarization component and the Y-axis polarization component are multiplied by the multiplier 13, and only the low-frequency component signal is extracted from the multiplier output by the low-pass filter 14. Thus, a signal corresponding to the inner product of the coordinates of the encoded data signal and the coordinates of the encoded reference signal sequence can be obtained. Since the signal corresponding to this inner product is a multilevel modulation code string, the carrier wave when the multilevel modulation code string is generated is extracted and separated into its quadrature component and in-phase component. The process from reception to here is the second decoding. Further, the data string is restored from the quadrature component and the in-phase component (first decoding).

  FIG. 9 shows a configuration on the receiving side using optical self-homodyne detection. The signal received here may be a signal transmitted in the third embodiment. First, excess light is removed by the band-pass filter 62, and then the quarter-wave plate 63 is passed again. By this operation, an optical signal in which linearly polarized light having two orthogonal polarization planes is overlapped with circularly polarized light is obtained. This light is passed through the polarization separator 2 and branched into two optical signals. Here, the mutual inclination is adjusted so that the polarization plane of the output light of the polarization separator 2 and the polarization plane of the output light of the polarization separator 2 have an inclination of π / 4. This tilt angle does not necessarily have to be kept strictly, but is preferably close to π / 4. With such a setting, each of the two linearly polarized lights input to the photodetector polarization separator 2 is branched into two linearly polarized lights, for a total of four linearly polarized lights. However, the two have the same plane of polarization and the other two have different planes of polarization. These are combined and pass through the polarization separator 2, one of which has the same plane of polarization is input to the photodetector 9 and the other is input to the photodetector 10. In the photodetector 11, each of the received two linearly polarized light is mixed and optical self-homodyne detection is performed. The photo detector 12 also performs optical self-homodyne detection. The electric signals obtained by the homodyne detection are added and filtered to become a signal corresponding to the inner product of the coordinates of the encoded data signal and the coordinates of the encoded reference signal sequence. Since the signal corresponding to this inner product is a multilevel modulation code string, the carrier wave when the multilevel modulation code string is generated is extracted and separated into its quadrature component and in-phase component. The process from reception to here is the second decoding. Further, the data string is restored from the quadrature component and the in-phase component (first decoding).

  The second encoding can be used as long as the inner product of the coordinates of the encoded data signal and the coordinates of the encoded reference signal sequence does not change. For example, the signal corresponding to the base axis of quadrature modulation may be rotated around the optical path, or it may change with time. In the second encoding, conversion may be performed so as to generate a signal obtained by unitarily converting the linear sum of the coordinates of the encoded data signal and the coordinates of the encoded reference signal sequence. Further, the receiving unit extracts a second encoded signal from the received signal. At the time of this extraction, it is necessary to know based on what processing is performed and transmitted, but it is possible to transmit correctly by performing such encoding.

  Since the present invention is devised so that light can be handled in the same way as radio waves, it can be widely applied to fields centered on optical wireless technology. In particular, there are the following applications expected for large-capacity transmission that was difficult with conventional flicker transmission alone. For example, when performing communications in outer space such as low-orbit satellites, geostationary orbit satellites, planetary exploration satellites, space shuttles, and space stations, and communications between stratospheric platforms, the present invention of optical heterodyne detection system can be applied. Therefore, it becomes possible to lay an optical network having a large capacity that has never been achieved. In addition, optical communication between buildings has been put to practical use as an application on the ground, but the multi-value technology of the present invention can be expected to greatly increase the transmission capacity, which has been difficult with conventional binary transmission. The reception sensitivity can be greatly improved by heterodyne detection. Furthermore, application to indoor communication and the like for home appliances, robots, office equipment, etc. is also possible, and in this case, the present invention of an optical self-homodyne detection system with a simpler configuration will be effective. In addition to this, application as a large capacity bus system of optical interconnection is also conceivable.

It is a block diagram which shows the 1st Example which performs an optical heterodyne detection using a correlation light generator. It is a block diagram which shows the example of a correlation light generator. It is a block diagram which shows the example of a correlation light generator. It is a block diagram which shows the 2nd Example which performs a self homodyne detection by the receiving side. It is a block diagram which shows the example of the transmission part which performs polarization multiplexing. It is a figure explaining polarization multiplexing. It is a figure explaining polarization multiplexing. It is a block diagram which shows the 3rd Example which performs polarization multiplexing and optical heterodyne detection. It is a block diagram which shows the 3rd Example which performs polarization multiplexing and optical self-homodyne detection. It is a figure which shows the numerical formula for demonstrating the principle of operation of this invention. It is a figure which shows the continuation of the numerical formula for demonstrating the operating principle of this invention. It is a figure which shows the continuation of the numerical formula for demonstrating the operating principle of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Correlated light generator 2 Optical modulator 3 Optical modulator 3a Phase modulator 3b Intensity modulator 4 First encoder 5 Second encoder 6, 7 Transflective mirror 8 Local oscillator 9, 10 Optical detector 11, 12 Band Filter 13 Multiplier 14 Low-pass filter 15 Separator 16 Extractor 17 First decoders 18 and 19 Branching devices 20 and 21 Multiplexer 22 Phase shifter 23 Adder 24 Second decoder 51 Laser light source 52 Semi-transparent mirror 53 Reflector 54 Laser light source 55 Polarization separator 56 Oscillation wavelength stabilization light source 57 Wavelength stabilization light source 58 Wavelength converter 59 Delay device 60 Polarization multiplexer 61 Quarter wave plate 62 Band pass filter 63 Quarter wave plate 64 Local oscillation light source

Claims (11)

A first encoder that generates a multi-level modulation code sequence from the data sequence;
A second encoder that generates two code strings of a data code string 1 and a data code string 2 from the multilevel modulation code string and the reference code string;
A transmission path for transmitting the data code string 1 and the data code string 2 as two optical signals superimposed on an optical carrier wave propagating through two close optical paths, and the two optical signals described above. A multi-level modulation code sequence obtained from an electrical signal obtained from an optical signal received corresponding to the data code sequence 1 using an electrical signal obtained from the received optical signal corresponding to the data code sequence 2 as a reference A second decoder for decoding
In a digital signal coherent optical transmission device comprising: a first decoder that decodes a data sequence from a decoded multilevel modulation code sequence;
The optical carrier is two optical carriers having correlation,
Data code sequence 1 is a multi-level modulation code sequence,
Data code string 2 is a reference code string,
The reference code string is a code string in which in-phase unit vectors and orthogonal unit vectors in the signal space of the multi-level modulation code string are arranged,
One of the two optical carriers having the above correlation is optically modulated with the data code string 1,
The other of the two optical carriers having the above correlation is optically modulated with the data code string 2, and
These two optically modulated optical carriers are transmitted in two optical paths,
Receiving the transmitted optical signal,
An electric signal corresponding to the data code string 1 is obtained by mixing each of the two received optical signals with each of the two local oscillation lights having correlation, performing optical heterodyne detection, and selecting an intermediate frequency band signal. And two electrical signals corresponding to the data code string 2 are obtained,
Multiplying the two electrical signals,
Select the low frequency band signal from the signal obtained by multiplication and restore the above multilevel modulation code,
A coherent optical transmission method characterized in that the data string is restored by decoding the multi-level modulation code with respect to the first encoder.   The clock of the multi-level modulation code string and the clock of the reference code string have the same frequency, and an electric signal corresponding to the data code string 1 and an electric signal corresponding to the data code string 2 obtained by performing optical heterodyne detection 2. The coherent optical transmission method according to claim 1, wherein when the two electric signals are multiplied, the two electric signals are multiplied in a state shifted from each other.   The clock of the reference code string is an integer multiple of the clock of the multi-level modulation code string, and an electric signal corresponding to the data code string 1 and an electric signal corresponding to the data code string 2 obtained by performing the optical heterodyne detection. 2. The coherent optical transmission method according to claim 1, wherein when the two electric signals are multiplied, the two electric signals are multiplied in a phase-matched state. A first encoder that generates a multi-level modulation code sequence from the data sequence;
A second encoder that generates two code strings of a data code string 1 and a data code string 2 from the multilevel modulation code string and the reference code string;
A transmission path for transmitting the data code string 1 and the data code string 2 as two optical signals superimposed on an optical carrier wave propagating through two close optical paths, and the two optical signals described above. A multi-level modulation code sequence obtained from an electrical signal obtained from an optical signal received corresponding to the data code sequence 1 using an electrical signal obtained from the received optical signal corresponding to the data code sequence 2 as a reference A second decoder for decoding
In a digital signal coherent optical transmission device comprising: a first decoder that decodes a data sequence from a decoded multilevel modulation code sequence;
The optical carrier is two optical carriers having correlation,
Data code sequence 1 is a multi-level modulation code sequence,
Data code string 2 is a reference code string,
The reference code string is a code string in which in-phase unit vectors and orthogonal unit vectors in the signal space of the multi-level modulation code string are arranged,
One of the two optical carriers having the above correlation is optically modulated with the data code string 1,
The other of the two optical carriers having the above correlation is optically modulated with the data code string 2, and
These two optically modulated optical carriers are transmitted in two optical paths,
Receiving the transmitted optical signal,
Each of the received optical signals is split into two optical paths,
A first optical signal obtained by combining an optical signal on one optical path and an optical signal on the other optical path, an optical signal obtained by shifting the optical signal on one optical path by a half wavelength, and the other optical signal are combined. Generating a second optical signal;
Optical self-homodyne detection is performed for each of the combined first and second optical signals to generate an electrical signal,
Generate a difference signal for the two electrical signals obtained by the above optical self-homodyne detection, select the low frequency side component to restore the above multi-level modulation code,
A coherent optical transmission method characterized in that the data string is restored by decoding the multi-level modulation code with respect to the first encoder.   5. The coherent optical transmission method according to claim 1, wherein two optical carriers having correlation are generated by branching one optical carrier.   The method of branching and generating one optical carrier is to separate one optical carrier into two polarization components, and two optical paths that are close to each other use polarization multiplexed optical paths. Item 6. The coherent optical transmission method according to Item 5. The second encoder is
1) Replace the in-phase component and the quadrature component of each of the data code string 1 and the data code string 2, or
2) The in-phase components or the orthogonal components of the data code sequence 1 and the data code sequence 2 are interchanged, or
3) The in-phase component of the data code string 1 and the quadrature component of the data code string 2 are exchanged, and the quadrature component of the data code string 1 and the in-phase component of the data code string 2 are exchanged, or
4) The in-phase component of the data code string 1 is multiplied by a constant and the in-phase component of the data code string 2 is multiplied by the reciprocal of the constant, or the orthogonal component of the data code string 1 is multiplied by the constant and the orthogonality of the data code string 2 5. The coherent optical transmission method according to claim 1, wherein the component is multiplied by an inverse of the constant. A method for recovering the multilevel modulation code from the selected low-frequency component is as follows.
According to a signal obtained by restoring the carrier wave of the electrical signal corresponding to the data code string 2 included in the difference signal, the low frequency component selected from the difference signal is distributed to two lines, or
The low-frequency component selected from the difference signal is intermittently connected to the two lines in accordance with a signal obtained by restoring the carrier wave of the electrical signal corresponding to the data code string 2 included in the difference signal. The coherent optical transmission method according to claim 1, wherein the component and the orthogonal component are restored.   The in-phase component or quadrature component of the restored multilevel modulation code is simultaneously input to the first decoder by holding or delaying the voltage or digital data corresponding to the in-phase component or quadrature component. A coherent light transmission method according to claim 1. Each of the two received optical signals is mixed with each of the two locally oscillating lights having a correlation, optical heterodyne detection is performed, and an intermediate frequency band signal is selected to select an electric signal and data corresponding to the data code string 1 When obtaining two electrical signals with an electrical signal corresponding to code string 2,
Each of the two received optical signals is mixed with each of two local oscillation lights having a phase delayed by a quarter wavelength, and two received optical signals whose phases are delayed by a quarter wavelength. Each of the two locally oscillated lights having correlation with each other is generated, and each optical heterodyne detection is performed.
By selecting the intermediate frequency band signal, the difference signal of the electrical signal corresponding to the data code string 1 and obtained from each of the mixture 1 and the mixture 2;
By selecting the intermediate frequency band signal, it obtains a difference signal between the electric signals corresponding to the data code string 2 and obtained from each of the mixture 1 and the mixture 2, and multiplies the difference signal between the two electric signals,
Select the low frequency band signal from the signal obtained by multiplication and restore the above multilevel modulation code,
4. The coherent optical transmission method according to claim 1, wherein the multi-level modulation code is decoded with respect to a first encoder to restore the data sequence.   The coherent optical transmission method according to any one of claims 1 to 6, further comprising a circuit that compensates for the transmission delay difference when a transmission delay difference occurs in an adjacent optical path.

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