WO2013140970A1

WO2013140970A1 - Optical communication system and optical communication method having high phase noise resistance

Info

Publication number: WO2013140970A1
Application number: PCT/JP2013/055266
Authority: WO
Inventors: 大作小笠原; 林　和則
Original assignee: 日本電気株式会社; 国立大学法人京都大学
Priority date: 2012-03-22
Filing date: 2013-02-21
Publication date: 2013-09-26
Also published as: JPWO2013140970A1

Abstract

Provided is an optical communication system whereby phase noise resistance of an optical signal is improved. This optical communication system comprises an optical transmission apparatus which transmits an optical signal which is phase shifted at a prescribed symbol gap, and an optical receiving apparatus which receives the optical signal. The optical transmission apparatus further comprises a phase addition unit. The phase addition unit has a transmission characteristic in which an impulse response length is greater than or equal to the symbol gap, and carries out on a generated electrical signal a filtering process which applies a phase change upon the transmitted light of the optical transmission apparatus according to the frequency thereof. The light receiving apparatus further comprises a phase correction unit. The phase correction unit corrects the phase shift which is applied in the optical transmission apparatus and which is included in the phase shifted electrical signal.

Description

Optical communication system and optical communication method having high phase noise tolerance

The present invention relates to an optical communication system, and more particularly to an optical communication system having high phase noise tolerance.

With the spread of the Internet, the amount of traffic on the backbone network has increased rapidly. Therefore, development of an ultrahigh-speed long-distance optical communication system exceeding 100 Gbps per channel is strongly desired. As a technique for realizing such an ultra-high-speed long-distance optical communication system, attention is paid to an optical phase modulation method using a digital signal processing technique, a polarization demultiplexing technique, and the like.
The optical phase modulation method is a method in which data modulation is not performed only on the light intensity of the transmission laser light as in the conventional light intensity modulation method, but also on the optical phase of the transmission laser light. BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 8PSK (8 Phase Shift Keying), or QAM (Quadrature Amplitude Modulation) are known.
For example, in BPSK, 1 bit (for example, 0, 1) is assigned to each of two optical phases (for example, 90 degrees and 180 degrees). FIG. 1A is an example of a constellation diagram of BPSK. In QPSK, 2 bits (for example, 00, 01, 11, 10) are allocated to four optical phases (for example, 45 degrees, 135 degrees, 225 degrees, and 315 degrees). FIG. 1B is an example of a constellation diagram of QPSK. In QPSK, since 2 bits are allocated to one optical phase, the symbol rate of QPSK can be reduced to ½ of the symbol rate (that is, bit rate) of the light intensity modulation method. As described above, in the multi-level optical phase modulation method, since a symbol rate (baud rate) can be lowered by assigning a plurality of bits to one symbol, the operation speed of an electric device can be reduced. Reduction of device manufacturing cost can be expected. On the other hand, BPSK assigns 1 bit to one optical phase, and thus the above-mentioned effect cannot be obtained. However, since the distance between symbols is larger than other phase modulation methods, Since it has high tolerance against phase noise due to the nonlinear optical effect, it has a feature that it is suitable for a modulation system of an ultra-long distance optical communication system that transmits between continents.
In order to receive signal light to which an optical phase modulation method is applied, signal light and laser light (referred to as local oscillation light or local light) having substantially the same optical frequency as the signal light is called a 90-degree optical hybrid. An optical coherent method is used in which the output is received by a photodetector after being coupled by an optical element.
For the sake of understanding, assuming that the polarization state of the signal light and the local oscillation light is the same linear polarization, the AC component of the electrical signal output from the photodetector is the signal when the optical coherent method is used. It is a beat signal of light and local oscillation light, its amplitude is proportional to the light intensity of the signal light and local oscillation light, and if the phase is the same as the carrier frequency of the signal light and the optical frequency of the local oscillation light, This is the difference in optical phase between the signal light and the local light. If the optical phase of the local oscillation light is the same as the optical phase of the laser beam input to the optical modulator at the transmission end, the beat signal phase is the optical phase added to the laser beam at the transmission end. It is possible to demodulate transmission data by converting the phase of the beat signal into a bit string using mapping.
Actually, the value of the carrier frequency of the signal light and the value of the optical frequency of the local oscillation light do not completely match, and the optical phase of the local oscillation light at the receiver and the laser input to the optical modulator at the optical transmitter It does not match the optical phase of light. Therefore, it depends on the optical phase deviation which is the optical phase difference between the signal light and the local oscillation light input to the optical modulator in the optical transmitter, and the optical carrier frequency deviation which is the difference between the carrier frequency of the signal light and the optical frequency of the local light. The effect needs to be compensated.
On the other hand, polarization multiplexing / demultiplexing technology using digital signal processing is also attracting attention as one of the technologies for realizing an ultrahigh-speed optical communication system. The polarization demultiplexing technique multiplexes two independent optical signals whose carrier waves are arranged in the same frequency band and whose polarization states are orthogonal to each other in the optical transmitter, and in the optical receiver, the received signal is multiplexed from the received signal. This is a technology that realizes a double transmission speed by separating two independent optical signals. In other words, since the symbol rate (baud rate) of the optical signal can be halved, the operation speed of the electric device can be reduced and the apparatus cost can be reduced.
By combining the above-described optical phase modulation method and polarization multiplexing / demultiplexing technology, it is possible to realize an ultrahigh-speed long-distance optical communication system exceeding 100 Gbps per channel. By performing processing for compensating optical carrier frequency deviation and optical phase deviation and processing for separating into two independent optical signals (polarization separation processing) with a digital signal processing circuit implemented by LSI or the like, A technique for accurately demodulating has been proposed and is called an optical digital coherent communication system.
Next, transmission / reception processing in an ultrahigh-speed optical communication system using an optical digital coherent communication system will be described in detail with reference to the drawings.
FIG. 2 is an example of a block diagram showing an optical transmitter 600 in a polarization multiplexed optical communication system using an optical digital coherent communication system. The continuous light that is transmitted from the laser oscillator 101 at a predetermined optical frequency and branched into two is modulated by the drive signals transmitted from the drive signal transmission units 106-1 and 106-2 in the optical orthogonal modulators 102-1 and 102-2, respectively. . The drive signals are respectively generated by the signal generators 105-1 and 105-2 so as to be suitable for the optical phase modulation method from the transmission bit string. For example, in the case of BPSK or QPSK, a binary electric signal is generated, but when the multi-value number becomes large, such as 16QAM, it is necessary to generate a complex electric signal such as a quaternary value. The drive signal transmitters 106-1 and 106-2 can be configured with only an amplifier when using an optical phase modulation method with a small multi-value number such as BPSK or QPSK, but when using a QAM with a large multi-value number, etc. Needs to be composed of a combination of a digital analog converter (DAC) and an amplifier. In general, the driving signal transmission units 106-1 and 106-2 transmit a real axis signal on the phase plane and a driving signal of an imaginary axis signal, respectively. Thereafter, the output optical signal of the optical quadrature modulator 102-1 and the output optical signal of the optical quadrature modulator 102-2 are multiplexed in a state where the polarization states are orthogonal to each other in the polarization multiplexing unit 103. To output to the optical transmission line. Although the configuration of the optical transmitter described above is not described here, the configuration of the optical transmitter specialized for each optical phase modulation method other than the optical quadrature modulator can also be used.
FIG. 3 is an example of a block diagram showing a configuration of an optical receiver 700 in an optical communication system using an optical digital coherent communication system. The optical signal received from the optical transmission line is input to the 90-degree optical hybrid 201 together with the local oscillation light having an optical frequency substantially the same as the carrier frequency of the received optical signal. The 90-degree optical hybrid 201 outputs a total of four optical signals of a real part component and an imaginary part component of an optical signal having a polarization state parallel to each of two orthogonal polarization axes. These four optical signals are converted into analog electric signals by the optical detectors 202-1 to 202-1, and then converted into digital electric signals by the analog-digital converters (ADC) 203-1 to 20. These digital electrical signals are converted into digital electrical signals sampled at the symbol rate of the received optical signal by a resampling unit (not shown in FIG. 3), and then input to the chromatic dispersion compensation unit 209. The chromatic dispersion compensation unit 209 compensates for waveform distortion due to chromatic dispersion by setting the residual chromatic dispersion of received light to zero. The signal data in which the waveform distortion due to wavelength dispersion is compensated is input to the polarization separation unit 204. The polarization separation unit 204 extracts two independent optical signals that are polarization multiplexed based on the four input digital electrical signals. Each of the extracted optical signals is compensated by the optical carrier frequency deviation / optical phase deviation compensation units 205-1 and 205-2 for optical phase rotation due to optical carrier frequency deviation and optical phase deviation between the received optical signal and the local oscillation light. After that, the symbol identifying sections 206-1 and 206-2 demodulate the original transmission bit strings, respectively.
As described above, after combining optical phase modulation and polarization demultiplexing technology, the effects of optical carrier frequency deviation and optical phase deviation are compensated for each of the two independent optical signals that have undergone polarization separation. By doing so, it becomes possible to realize an ultra-high-speed optical communication system exceeding 100 Gbps per channel.
This type of optical communication system is also disclosed in

Patent Documents

1 and 2, for example. The optical receiver disclosed in Patent Document 1 performs fast Fourier transform on a time domain received signal, performs frequency equalization on a frequency domain component, and returns the time domain signal by inverse fast Fourier transform. . Further, the optical signal transmission device disclosed in Patent Document 2 performs chromatic dispersion pre-equalization on signal light. In Patent Document 3, in order to improve the bit error rate by removing the influence of intersymbol interference, fast Fourier transform is performed on the received signal in the time domain, frequency equalization is performed on the frequency domain component, A wireless terminal that returns to a time domain signal by inverse fast Fourier transform is disclosed.

Japanese Patent Publication No. 2010-057016 Japanese Patent Publication No. 7475 published in 1995 International Publication No. 2008/139624

However, related techniques including those shown in FIGS. 2 and 3 have a problem that transmission characteristics deteriorate due to the addition of phase noise to an optical signal.
As an example in which phase noise is added, fluctuations in the oscillation frequency of the local oscillation light add phase noise to the beat signal (received electrical signal) generated by the optical digital coherent communication method described above, so that the transmission characteristics of the received signal Cause deterioration. In addition, the phase noise increases as the magnitude (line width) of the oscillation frequency fluctuates, and the inter-symbol distance decreases as the multi-value number of the optical signal modulation method increases. Become.
As another example, phase noise is added to the optical signal due to the nonlinear optical effect during the transmission of the optical fiber, so that the transmission characteristic of the received signal is deteriorated.
Therefore, an object of the present invention is to provide an optical communication system having high phase noise tolerance.

According to the present invention, the optical transmitter includes an optical transmitter that modulates and transmits an optical signal at a predetermined symbol interval, and an optical receiver that receives the optical signal. The optical transmitter is an electrical signal corresponding to a bit string. And a signal processing unit that generates a signal having a transmission characteristic in which an impulse response length is equal to or greater than the symbol interval, and applies a phase change corresponding to a frequency to the transmission light of the optical transmitter to the generated electrical signal A phase adding unit to be implemented; a driving unit that generates a driving signal in accordance with the filtered electrical signal; a light source that outputs continuous light; and the continuous light is modulated based on the driving signal and transmitted. A modulation unit that outputs light, and the optical receiver is included in the photoelectric conversion unit that converts the optical signal transmitted from the optical transmitter into an electrical signal, and the converted electrical signal, The phase change given by the phase adder The optical communication system is obtained, characterized in that it comprises a phase compensation unit for amortization.
According to the present invention, there is provided an optical communication method executed between an optical transmitter that modulates an optical signal at a predetermined symbol interval and transmits the modulated optical signal, and an optical receiver that receives the optical signal. A signal generation step of generating an electrical signal corresponding to a bit string in the transmitter, and a transmission characteristic in which the impulse response length is equal to or greater than the symbol interval in the optical transmitter, and the transmission light of the optical transmitter is in accordance with the frequency A phase adding step for performing a filtering process for giving a phase change to the generated electric signal; a driving step for generating a driving signal in accordance with the electric signal subjected to the filtering process in the optical transmitter; and the optical transmission. A light output step of outputting continuous light from a light source in the optical device, a modulation step of modulating the continuous light based on the drive signal in the optical transmitter and outputting transmission light, and A photoelectric conversion step for converting the optical signal transmitted from the optical transmitter into an electric signal, and a phase change given in the phase addition step included in the converted electric signal in the optical receiver are compensated. And an optical communication method characterized by comprising a phase compensation step.

The optical communication system according to the present invention has improved phase noise tolerance of optical signals.

1A is a BPSK constellation diagram, and FIG. 1B is a QPSK constellation diagram.
FIG. 2 is a block diagram showing a configuration of an optical transmitter in an optical communication system using an optical digital coherent communication system according to a related technique.
FIG. 3 is a block diagram showing a configuration of an optical receiver in an optical communication system using an optical digital coherent communication system according to a related technique.
FIG. 4 is a block diagram showing the configuration of the optical transmitter in the optical communication system using the optical digital coherent communication system according to the first embodiment of the present invention.
FIG. 5 is a block diagram showing the configuration of the optical receiver in the optical communication system using the optical digital coherent communication system according to the first embodiment of the present invention.
6A is a QPSK constellation diagram of an optical communication system according to the related art, and FIG. 6B is a QPSK constellation diagram of an optical communication system according to the present invention.
FIG. 7 is a block diagram showing the configuration of the optical transmitter in the optical communication system using the optical digital coherent communication system according to the second embodiment of the present invention.
FIG. 8 is a block diagram showing the configuration of the optical receiver in the optical communication system using the optical digital coherent communication system according to the second embodiment of the present invention.

Hereinafter, an optical communication system according to the present invention will be described more specifically with reference to the drawings.
In the following description, the same or similar components as those in the related technology shown in FIGS. 2 and 3 are denoted by the same reference numerals and detailed description thereof is omitted.

FIG. 4 is a block diagram showing the configuration of the optical transmitter 100 in the first embodiment of the present invention. Referring to FIG. 4, an optical transmitter 100 is an optical transmitter in an optical communication system using an optical digital coherent communication system, and a laser oscillator 101 that transmits continuous light at a predetermined optical frequency, and phase modulation from a transmission bit string. A signal generation unit 105 that generates a drive signal so as to be suitable for the system, and a drive signal transmission unit 106 that transmits at least a drive signal based on a signal obtained from the signal generation unit 105 via a phase addition unit 107 described later. And an optical quadrature modulator 102 that QPSK-modulates the continuous light transmitted from the laser oscillator 101 with the drive signal from the drive signal transmission unit 106.
Although not shown, the optical transmitter 100 is further based on the chromatic dispersion that the optical signal output from the optical quadrature modulator 102 will receive during transmission with respect to the electrical signal generated by the signal generation unit 105. In addition, a chromatic dispersion pre-equalization unit that performs filter processing for adding a phase according to frequency may be included.
As shown in FIG. 4, the optical transmitter 100 in the present embodiment differs from the configuration of the optical transmitter 600 according to the related art shown in FIG. 2 in that a phase adding unit 107 that adds a phase change to a transmission optical signal And a first phase setting unit 108 for setting a phase change with respect to the phase adding unit 107.
Note that the first phase setting unit 108 is not necessarily provided.
FIG. 5 is a block diagram showing the configuration of the optical receiver 200 in the first embodiment of the present invention. Referring to FIG. 5, an optical receiver 200 is an optical receiver in an optical communication system using an optical digital coherent communication system, and an optical detector 202 that converts an optical signal received from an optical transmission path into an analog electric signal; Based on an analog-digital converter (ADC) 203 that converts an analog electrical signal transmitted from the optical detector 202 into a digital electrical signal, and a digital electrical signal that is input from the ADC 203 via a phase compensation unit 207 described later, residual chromatic dispersion is achieved. An optical carrier wave that compensates for optical phase rotation due to optical carrier frequency deviation and optical phase deviation between the received optical signal and the local oscillation light, from the chromatic dispersion compensation unit 209 that compensates, and the signal data from which the waveform distortion due to chromatic dispersion has been removed Frequency deviation / optical phase deviation compensation unit 205 and optical carrier frequency deviation / optical phase deviation compensation unit 2 The signal from 5, and a symbol identification unit 206 for demodulating the original transmission bit sequence.
The chromatic dispersion compensator 209 gives each wavelength component included in the input signal from the phase compensator 207 a different amplification factor and delay amount by a plurality of taps, and then combines the components to thereby combine the wavelengths. Waveform distortion due to chromatic dispersion is removed.
As shown in FIG. 5, the optical receiver 200 in the present embodiment is different from the configuration of the optical receiver 700 according to the related art shown in FIG. 3, and includes a phase compensation unit 207 that adds a phase change to the received electric signal. And a second phase setting unit 208 that sets a phase change with respect to the phase compensation unit 207.
Note that the second phase setting unit 208 is not necessarily provided.
Hereinafter, a method for generating a transmission signal using the optical transmitter 100 of the present invention will be described.
The phase adding unit 107 of the optical transmitter 100 performs a Fourier transform process on the time domain signal generated by the signal generating unit 105 and then performs a predetermined amount of phase change on each frequency component of the signal. After the addition, the signal is reconverted into a time domain signal by an inverse Fourier transform process. The signal to which the predetermined amount of phase change is added is supplied to the optical quadrature modulator 102 by the drive signal transmission unit 106 as in the related art described above. If the signal generated by the signal generation unit 105 is a signal in the frequency domain, the above-described Fourier transform process can be omitted.
Here, the magnitude of the phase change added to the signal in the phase adding unit 107 is the time spread (phase phase) of the signal output from the phase adding unit 107 when an impulse signal is used as the signal input to the phase adding unit 107. The impulse response length of the transfer characteristic of the adding unit 107 is required to be at least larger than the symbol interval of the optical signal. Thereby, since information can be distributed to a plurality of symbols, it is possible to reduce the probability that a bit error will occur due to phase noise added at an arbitrary time. As a result, the phase noise tolerance can be improved.
Furthermore, it is most desirable for improving the phase noise tolerance to increase the time spread of the signal output from the phase adding unit 107 (impulse response length of the transfer characteristic of the phase adding unit 107) as much as possible in a predetermined resource.
The finite resource includes, for example, the number of filter coefficients of the filters that constitute the phase adding unit 107. That is, the magnitude of the phase fluctuation is controlled so that the time spread of the signal output from the phase adding unit 107 is made as large as possible with respect to the number of filter coefficients of the filters constituting the phase adding unit 107.
For example, the following equation (1) can be used as the phase change amount that maximizes the impulse response length of the transfer characteristic of the phase adding unit 107.

In Equation (1), φk is the phase change amount of each frequency component, k is an integer of 0 to M, and M is the window size of the Fourier transform process or inverse Fourier transform process.
Equation (1) can be derived on condition that the entropy H defined by the following equation (2) is maximized.

In Expression (2), h _i is an impulse response of the phase adding unit 107.
The method for generating the transmission optical signal of the present invention using digital signal processing has been described above. However, if there is an optical device capable of adding the same phase change as the above-described phase change, the transmission optical signal of the present invention is thereby determined. Can also be generated.
Next, a method for demodulating a signal using the optical receiver 200 of the present invention will be described.
The phase compensator 207 of the optical receiver 200 performs Fourier transform processing on the received signal in the time domain, and then drives each frequency component of the received signal in the phase adder 107 of the optical transmitter 100. After adding a phase change amount having the same magnitude and a different sign from the phase change added to each frequency component of the signal, it is reconverted into a time domain received signal by inverse Fourier transform processing. The received signal to which a predetermined amount of phase change is added is supplied to the chromatic dispersion compensation unit 209.
As described above, since the phase change added by the optical transmitter 100 is compensated by the phase compensation unit 207, the subsequent demodulation processing can be performed in the same manner as a conventional optical receiver.
It should be noted that chromatic dispersion pre-equalization for adding chromatic dispersion in the optical transmitter can be performed simultaneously with the phase addition of the present invention by the phase adding unit 107, and the residual chromatic dispersion of the received light in the optical receiver. The phase compensation unit 207 can be implemented simultaneously with the phase compensation of the present invention. In this case, an effect of reducing power consumption can be obtained by reducing the circuit scale of the LSI realizing the present invention.
The effect of improving the phase noise tolerance by applying the present invention is shown in FIG. FIG. 6A is a constellation diagram of a QPSK signal received by adding phase noise in the related art transmitter / receiver 700 described above as a comparative example. On the other hand, FIG. 6B shows a case where the transceiver 200 of the present invention is used.
The constellation of the QPSK signal shown in FIG. 6A has a larger symbol spread in the phase direction due to phase noise than the symbol spread in the polar direction.
On the other hand, the constellation of the QPSK signal shown in FIG. 6B has substantially the same symbol spread in the polar and phase directions, and the present invention can reduce the symbol spread in the phase direction due to phase noise. I understand.

The second embodiment of the present invention differs from the first embodiment in that a polarization multiplexing / demultiplexing technique is further applied to the optical digital coherent communication system according to the present invention.
FIG. 7 is a block diagram showing the configuration of the optical transmitter 300 in the second embodiment of the present invention. Referring to FIG. 7, an optical transmitter 300 is an optical transmitter in a polarization multiplexed optical communication system using an optical digital coherent communication system, as in the related art shown in FIG. 2, and is continuously transmitted at a predetermined optical frequency. The laser oscillator 101 that transmits light, the signal generation units 105-1 and 10-2 that generate drive signals from the transmission bit string so as to be suitable for the phase modulation method, and at least an amplifier, are described below from the signal generation units 105-1 and 105-2. Driving signal transmitting units 106-1 to 106-2 for transmitting a driving signal based on signals obtained through the phase adding units 107-1 to 107-2, and continuous light branched from the laser oscillator 101. Optical quadrature modulators 102-1 and 102-2 that perform QPSK modulation using the drive signals from the signal transmitters 106-1 and 106-2, respectively, and states in which their polarization states are orthogonal to each other After having multiplexed, and a polarization multiplexing unit 103 to be output to the optical transmission path.
Although not shown, the optical transmitter 300 further transmits the optical signals output from the optical quadrature modulators 102-1 and 102-2 to the electrical signals generated by the signal generators 105-1 and 105-2. You may have a wavelength dispersion pre-equalization part which performs the filter process which adds the phase according to a frequency based on the wavelength dispersion which will be received.
As shown in FIG. 7, the optical transmitter 300 according to the present embodiment is different from the configuration of the optical transmitter 600 according to the related art shown in FIG. 1 and 2 and a first phase setting unit 108 for setting a phase change for the phase adding units 107-1 and 107-2.
Note that the first phase setting unit 108 is not necessarily provided.
FIG. 8 is a block diagram showing the configuration of the optical receiver 400 in the embodiment of the present invention. Referring to FIG. 8, an optical receiver 400 is an optical receiver in a polarization multiplexed optical communication system using an optical digital coherent communication system, as in the related art shown in FIG. 3, and is received from an optical transmission line. Real and imaginary part components of an optical signal in which an optical signal is input together with local oscillation light having an optical frequency substantially the same as the carrier frequency of the received optical signal and has a polarization state parallel to each of two orthogonal polarization axes A 90-degree optical hybrid 201 that outputs a total of four optical signals, optical detectors 202-1 to 202-4 that convert the four optical signals transmitted from the 90-degree optical hybrid 201 into analog electrical signals, and an optical detector, respectively. Analog-to-digital converter (ADC) 2 for converting the four analog electric signals sent from 202-1 to 202-4 into digital electric signals, respectively. 3-1 to 4, chromatic dispersion compensation unit 209 that compensates for residual chromatic dispersion based on four digital electric signals input from ADCs 203-1 to 207 through phase compensation unit 207, which will be described later, and chromatic dispersion A polarization separation unit 204 that extracts two independent optical signals polarization-multiplexed from the signal data from which the waveform distortion has been removed, and an optical carrier frequency between the received optical signal and the local oscillation light, respectively. Optical carrier frequency deviation / optical phase deviation compensation units 205-1 and 20-2 that compensate for optical phase rotation due to deviation and optical phase deviation, and signals from optical carrier frequency deviation and optical phase deviation compensation units 205-1 and 205-2, Symbol identifying units 206-1 and 206-2 that demodulate the original transmission bit string.
The chromatic dispersion compensator 209 gives each wavelength component included in the input signal from the phase compensator 207 a different amplification factor and delay amount by a plurality of taps, and then combines the components to thereby combine the wavelengths. Waveform distortion due to chromatic dispersion is removed.
As shown in FIG. 8, the optical receiver 400 in this embodiment differs from the configuration of the optical receiver 700 according to the related art shown in FIG. 3, and includes a phase compensation unit 207 that adds a phase change to the received electric signal. And a second phase setting unit 208 that sets a phase change with respect to the phase compensation unit 207.
Note that the second phase setting unit 208 is not necessarily provided.
Hereinafter, a method for generating a transmission signal using the optical transmitter 300 of the present invention will be described.
In the phase addition units 107-1 and 107-2 of the optical transmitter 300, the Fourier transform processing is performed on the time domain signals generated by the signal generation units 105-1 and 105-2, and then each frequency component of the signal is converted. On the other hand, after a predetermined amount of phase change is added, the signal is reconverted into a time domain signal by inverse Fourier transform processing. Signals to which a predetermined amount of phase change is added are supplied to the optical quadrature modulators 102-1 and 102-2 by the drive signal transmission units 106-1 and 106-2, as in the related art described above. If the signals generated by the signal generators 105-1 to 105-2 are signals in the frequency domain, the above-described Fourier transform process can be omitted.
Here, the magnitude of the phase change added to the signal in the phase adding units 107-1 and 107-2 is the same as that in the case where an impulse signal is used as the signal input to the phase adding units 107-1 and 107-2. The time spread of the signal output from ~ 2 (impulse response length of the transfer characteristic of the phase adding units 107-1 and 107-2) needs to be larger than at least the symbol interval of the optical signal. Thereby, since information can be distributed to a plurality of symbols, it is possible to reduce the probability that a bit error will occur due to phase noise added at an arbitrary time. As a result, the phase noise tolerance can be improved.
Further, it is possible to increase the time spread of the signals output from the phase adding units 107-1 and 2-2 (impulse response length of the transfer characteristics of the phase adding units 107-1 and 10-2) as much as possible in a predetermined resource. Most desirable to improve yield strength.
The finite resource includes, for example, the number of filter coefficients of the filters constituting the phase adding units 107-1 and 107-2. That is, the magnitude of the phase fluctuation is large so that the time spread of the signals output from the phase addition units 107-1 and 107-2 is as large as possible with respect to the number of filter coefficients of the filters constituting the phase addition units 107-1 and 107-2. To control.
For example, the following equation (3) can be used as the phase change amount that maximizes the impulse response length of the transfer characteristics of the phase adding units 107-1 and 107-2.

In Expression (3), φk is a phase change amount of each frequency component, k is an integer of 0 or more and M or less, and M is a window size of the Fourier transform process or the inverse Fourier transform process.
Equation (3) can be derived on condition that the entropy H defined by the following equation (4) is maximized.

In Expression (4), h _i is an impulse response of the phase adding units 107-1 and 107-2.
The method for generating the transmission optical signal of the present invention using digital signal processing has been described above. However, if there is an optical device capable of adding the same phase change as the above-described phase change, the transmission optical signal of the present invention is thereby determined. Can also be generated.
Next, a method for demodulating a signal using the optical receiver 400 of the present invention will be described.
In the phase compensation unit 207 of the optical receiver 400, the Fourier transform processing is performed on the received signal in the time domain, and then the phase adding unit 107-1 of the optical transmitter 300 is applied to each frequency component of the received signal. After adding a phase change amount having the same magnitude and different sign from the phase change added to each frequency component of the drive signal in ˜2, it is reconverted into a time domain received signal by inverse Fourier transform processing. The received signal to which the predetermined amount of phase change is added is supplied to the polarization separation unit 204 as in the related art described above.
As described above, since the phase change added by the optical transmitter 300 is compensated by the phase compensation unit 207, the demodulation processing after the polarization separation unit 204 can be performed in the same manner as a conventional optical receiver.
Note that chromatic dispersion pre-equalization for adding chromatic dispersion in the optical transmitter can be performed simultaneously with the phase addition of the present invention by the phase adding units 107-1 and 107-2. Residual chromatic dispersion can be performed simultaneously with the phase compensation of the present invention by the phase compensation unit 207. In this case, an effect of reducing power consumption can be obtained by reducing the circuit scale of the LSI realizing the present invention.

In the present invention, the phase adding unit 107 in the optical transmitter 100 of the first embodiment and the optical transmitter 300 of the second embodiment can be configured using an FIR filter (Finite Impulse Response filter).
That is, it is possible to generate a drive signal by filtering a time domain signal using an FIR filter having a filter coefficient as a value obtained by performing an inverse Fourier transform process on a phase change in the frequency domain.
As described above, according to the present invention, an optical communication system with high phase noise tolerance can be realized.
Further, some or all of the embodiments described above may be described as in the following supplementary notes, but are not limited to the following.
(Appendix 1)
An optical transmitter that modulates and transmits an optical signal at a predetermined symbol interval; and an optical receiver that receives the optical signal;
The optical transmitter is
A signal generator for generating an electrical signal corresponding to the bit string;
A phase adding unit that has a transmission characteristic in which an impulse response length is equal to or greater than the symbol interval, and that performs a filtering process on the generated electric signal to apply a phase change corresponding to a frequency to the transmission light of the optical transmitter;
A drive unit that generates a drive signal according to the electrical signal subjected to the filtering process;
A light source that outputs continuous light;
A modulator that modulates the continuous light based on the drive signal and outputs transmission light;
The optical receiver is:
A photoelectric converter that converts the optical signal transmitted from the optical transmitter into an electrical signal;
An optical communication system comprising: a phase compensation unit that compensates for a phase change provided by the phase addition unit included in the converted electrical signal.
(Appendix 2)
The phase addition unit generates a filter process for adding a phase corresponding to the frequency so as to maximize the impulse response length with respect to the number of filter coefficients of the phase addition unit that performs the filter process. The optical communication system according to appendix 1, which is performed on the electrical signal.
(Appendix 3)
The phase adding unit is configured to maximize entropy defined by a value obtained by adding a negative sign to a sum of products of amplitude squares of coefficients of the impulse response and a logarithmic value based on 2 of the square of the amplitude. The optical communication system according to appendix 1, wherein filtering processing for adding a phase corresponding to the frequency to the generated electrical signal is performed.
(Appendix 4)
The optical communication system according to any one of appendices 1 to 3, wherein the phase compensation unit further compensates for chromatic dispersion remaining in the received light.
(Appendix 5)
The optical communication system according to appendix 4, wherein the phase adding unit performs a filtering process for further adding a phase corresponding to a frequency based on chromatic dispersion that the optical signal receives during transmission to the generated electric signal.
(Appendix 6)
The optical transmitter further includes a first phase setting unit that sets a phase change with respect to the phase adding unit,
6. The optical communication system according to claim 1, wherein the optical receiver further includes a second phase setting unit that sets a phase change with respect to the phase compensation unit.
(Appendix 7)
An optical communication method executed between an optical transmitter for modulating and transmitting an optical signal at a predetermined symbol interval and an optical receiver for receiving the optical signal,
A signal generation step of generating an electrical signal corresponding to a bit string in the optical transmitter;
A phase in which the generated electric signal is subjected to a filtering process that has a transfer characteristic in which the impulse response length is equal to or greater than the symbol interval in the optical transmitter, and applies a phase change corresponding to the frequency to the transmission light of the optical transmitter Additional process;
A drive step of generating a drive signal in accordance with the filtered electrical signal in the optical transmitter;
A light output step of outputting continuous light from a light source in the optical transmitter;
A modulation step of modulating the continuous light based on the drive signal in the optical transmitter and outputting transmission light;
A photoelectric conversion step of converting the optical signal transmitted from the optical transmitter into an electric signal in the optical receiver;
A phase compensation step of compensating for a phase change given in the phase addition step included in the converted electrical signal in the optical receiver.
(Appendix 8)
Note that in the phase addition step, a filter process for adding a phase corresponding to the frequency so as to maximize the impulse response length with respect to the number of filter coefficients of the filter process is performed on the generated electric signal. 7. Optical communication method of 7.
(Appendix 9)
In the phase addition step, the entropy defined by the sum of the square of the amplitude of each coefficient of the impulse response and the logarithmic product based on 2 of the square of the amplitude is added to a maximum value so that the entropy is maximized. The optical communication method according to appendix 7, wherein filtering processing for adding a phase corresponding to the frequency to the generated electric signal is performed on the generated electric signal.
(Appendix 10)
The optical communication method according to any one of appendices 7 to 9, wherein in the phase compensation step, chromatic dispersion remaining in the received light is further compensated.
(Appendix 11)
The optical communication method according to appendix 10, wherein, in the phase addition step, a filter process is further performed for the generated electrical signal to further add a phase corresponding to a frequency based on chromatic dispersion that the optical signal receives during transmission.
(Appendix 12)
A first phase setting step for setting a phase change added in the phase adding step in the optical transmitter;
The optical communication method according to any one of appendices 7 to 11, further comprising a second phase setting step of setting a phase change compensated in the phase compensation step in the optical receiver.

The present invention is not limited to the embodiments described above, and it goes without saying that various modifications are possible within the technical scope described in the claims. For example, in the above-described embodiment, the example in which the present invention is applied to the digital coherent optical communication system that is currently being researched and developed widely has been described. However, the phase change addition means and the compensation means are not necessarily digital signal processing. Need not be. The present invention is also applicable to an optical communication system that uses a single-polarized optical signal that is not polarization multiplexed.
In addition, this application claims its benefit on the basis of priority from Japanese Patent Application No. 2012-066543 filed on March 22, 2012, the disclosure of which is hereby incorporated herein in its entirety Incorporated as a reference.

Claims

An optical transmitter for modulating and transmitting an optical signal at a predetermined symbol interval; and an optical receiver for receiving the optical signal;
The optical transmitter is
A signal generator for generating an electrical signal corresponding to the bit string;
A phase adding unit that has a transmission characteristic in which an impulse response length is equal to or greater than the symbol interval, and that performs a filtering process on the generated electric signal to apply a phase change corresponding to a frequency to the transmission light of the optical transmitter;
A drive unit that generates a drive signal according to the electrical signal subjected to the filtering process;
A light source that outputs continuous light;
A modulator that modulates the continuous light based on the drive signal and outputs transmission light;
The optical receiver is:
A photoelectric converter that converts the optical signal transmitted from the optical transmitter into an electrical signal;
An optical communication system, comprising: a phase compensation unit that compensates for a phase change provided by the phase addition unit included in the converted electrical signal.
The phase adding unit generates a filter process for adding a phase corresponding to the frequency so as to maximize the impulse response length with respect to the number of filter coefficients of the phase adding unit that performs the filter process. The optical communication system according to claim 1, wherein the optical communication system is applied to the electrical signal.
The phase adding unit maximizes the entropy defined by a value obtained by adding a negative sign to the sum of products of the square of the amplitude of each coefficient of the impulse response and a logarithmic value based on 2 of the square of the amplitude. The optical communication system according to claim 1, wherein a filter process for adding a phase corresponding to the frequency to the generated electric signal is performed.
The optical communication system according to any one of claims 1 to 3, wherein the phase compensation unit further compensates chromatic dispersion remaining in the received light.
5. The optical communication according to claim 4, wherein the phase addition unit performs a filter process for further adding a phase corresponding to a frequency based on chromatic dispersion that the optical signal receives during transmission to the generated electrical signal. system.
The optical transmitter further includes a first phase setting unit that sets a phase change with respect to the phase adding unit,
The optical communication system according to any one of claims 1 to 5, wherein the optical receiver further includes a second phase setting unit that sets a phase change with respect to the phase compensation unit.
An optical communication method executed between an optical transmitter that modulates an optical signal at a predetermined symbol interval and transmits the optical signal and an optical receiver that receives the optical signal,
A signal generation step of generating an electrical signal corresponding to a bit string in the optical transmitter;
A phase in which the generated electric signal is subjected to a filtering process that has a transfer characteristic in which the impulse response length is equal to or greater than the symbol interval in the optical transmitter, and applies a phase change corresponding to the frequency to the transmission light of the optical transmitter Additional process;
A drive step of generating a drive signal in accordance with the filtered electrical signal in the optical transmitter;
A light output step of outputting continuous light from a light source in the optical transmitter;
A modulation step of modulating the continuous light based on the drive signal in the optical transmitter and outputting transmission light;
A photoelectric conversion step of converting the optical signal transmitted from the optical transmitter into an electrical signal in the optical receiver;
A phase compensation step of compensating for a phase change given in the phase addition step included in the converted electrical signal in the optical receiver.
The said phase addition process implements the filter process which adds the phase according to the said frequency so that the said impulse response length may be maximized with respect to the number of the filter coefficients of the said filter process to the produced | generated said electric signal. Item 8. The optical communication method according to Item 7.
In the phase addition step, the entropy defined by the sum of the square of the amplitude of each coefficient of the impulse response and the logarithmic value based on 2 of the square of the amplitude is added to a maximum value so as to be maximized. The optical communication method according to claim 7, wherein filtering processing for adding a phase corresponding to the frequency to the generated electric signal is performed on the generated electric signal.
The optical communication method according to any one of claims 7 to 9, wherein in the phase compensation step, chromatic dispersion remaining in the received light is further compensated.