JP6211202B2 - Optical transmission method and optical transmission system - Google Patents

Description

  The present invention relates to an optical transmission method and an optical transmission system, and more particularly to an optical transmission method and an optical transmission system that realize long-distance large-capacity optical transmission using a wavelength multiplexing method.

  In order to perform long-distance and large-capacity transmission using an optical fiber, high-density signal multiplexing and overcoming the fiber nonlinear optical effect are problems.

  It is possible to increase the transmission capacity per optical fiber by performing high-density wavelength multiplexing by placing different information on a plurality of optical carriers or optical subcarriers (subcarriers). Here, the optical carrier wave and the optical subcarrier wave to be multiplexed are each called a channel. Also, the transmission capacity can be increased by making the modulation system multi-valued.

  As a conventional modulation method, on-off keying (OOK) in which a binary signal is assigned to the presence or absence of light and one bit is transmitted per symbol has been used. However, by increasing the number of signal points and increasing the number of transmission bits per symbol, such as quaternary phase modulation (Quaternary Phase-Shift Keying: QPSK) and 16 value quadrature amplitude modulation (Quadrature Amplitude Modulation: QAM), It is possible to increase the transmission capacity. In QPSK and 16QAM, signals are assigned to the same phase axis (In-Phase axis: I axis) and quadrature phase axis (Quadrature-Phase axis: Q axis) in the optical transmitter.

  Further, a method is known in which the number of transmission bits per symbol is doubled by using a polarization multiplexing method. In the polarization multiplexing method, signals can be independently assigned to two orthogonal polarization components (vertical polarization and horizontal polarization).

  For the demodulation of the OOK signal, a direct detection method that detects and identifies the presence or absence of an optical signal on the receiving side has been used. In addition, in the demodulation of differential binary phase modulation (Differential Phase-Shift Keying: DBPSK) signal, differential QPSK (DQPSK) signal, etc., direct detection is performed after delay interference of the optical signal, or (direct) delay detection. A scheme has been used.

  A digital coherent method that compensates for the electrical signal obtained by performing coherent detection, which detects signals by mixing interference between the local oscillation light source and the received signal at the receiving end, using digital signal processing. Is used (for example, see Non-Patent Documents 1 and 2).

  On the other hand, when performing long-distance optical transmission, a predetermined optical signal power-to-noise power ratio is required according to the bit rate, modulation method, and detection method in order to ensure signal quality at the receiving end. Therefore, it is necessary to perform signal transmission with high optical power. At this time, the waveform distortion caused by the nonlinear optical effect generated in the optical fiber degrades the signal quality.

  In order to reduce the degradation of signal quality due to the nonlinear optical effect, a method of increasing the local chromatic dispersion (CD) of the transmission line is conceivable. By adopting such a method, it is possible to avoid in-phase accumulation of waveform distortion caused by non-linearity and reduce transmission performance degradation (see, for example, Non-Patent Document 3).

  In the coherent optical transmission system, large-scale CD compensation (digital CD compensation) by digital signal processing is possible. Therefore, the coherent optical transmission system is preferably applied to a transmission line having a large local CD and no CD compensation in the transmission line.

  In a transmission path without CD compensation within the transmission path, CDs ranging from several tens of thousands ps / nm to several hundred thousand ps / nm may accumulate and remain in the receiving unit depending on the fiber type and transmission distance.

  When a fiber having a local CD of 20 ps / nm / km is transmitted through 10,000 km, the cumulative CD is 200,000 ps / nm. A baud rate 30 Gbaud class signal causes 1000 symbol class intersymbol interference. Intersymbol interference due to CD is expressed as a known linear waveform distortion. Therefore, ideally, the residual CD can be completely compensated by a waveform equalizer having a scale corresponding to the intersymbol interference length of the residual CD.

  However, in the coherent optical transmission system, it is known that the instantaneous frequency fluctuation caused by the phase noise due to the light source line width becomes phase noise (Equalization Enhanced Phase Noise: EEPN) expanded by large-scale digital CD compensation, resulting in performance degradation. (For example, refer nonpatent literature 4).

  For example, in order to reduce performance degradation due to EEPN when performing digital CD compensation (received digital CD compensation) in the receiving unit, it is necessary to perform phase noise compensation of local oscillation light upstream of the received digital CD compensation. .

  The EEPN by the received digital CD compensation relates only to the phase noise of the local oscillation light source. On the other hand, when digital CD compensation (transmission digital CD compensation) is performed in the transmission unit, phase noise of the transmission light source is related.

  The phase noise of the received signal light and the phase noise of the local oscillation light cannot usually be distinguished. Therefore, a method is disclosed in which local oscillation phase noise estimation is performed by branching the local oscillation light into two and having a path for local oscillation light analysis separately from signal interference (see, for example, Patent Document 1). .

  In addition, in the case where light output from a single light source is modulated with a plurality of subcarriers, a method of performing differential analysis on the receiving side of pilot signals assigned to the plurality of subcarriers (see, for example, Patent Document 2) or specific modulation A method (for example, see Patent Document 3) that can track the phase noise change of the local oscillation light by using the method is disclosed.

  By these methods, the phase noise of the local oscillation light can be distinguished from the phase noise of the received signal light, and only the phase noise of the local oscillation light can be extracted and estimated and compensated. Thereafter, by performing reception digital CD compensation, performance degradation due to EEPN can be reduced.

US Patent Application Publication No. 2012/0213532 US Pat. No. 8,647,690 US Patent Application Publication No. 2013/0230312

Optical Internetworking Forum, “100G Ultra Long Haul DWDM Framework Document”, June 2009 E. Yamazaki, 27 others, “Fast optical channel recovery in field demonstration of 100-Gbit / s Ethernet over OTN using real-time DSP”, Optics Exp. 19, no. 14, pp. 13179-13184, 2011 P. Poggiolini, “The GN Model of Non-Linear Propagation in Uncompensated Coherent Optical Systems”, Journal of Lightwave Technology, vol. 30, no. 24, pp. 3857-3879, 2012 W. Shieh and Keang-Po Ho, “Equalization-enhanced phase noise for coherent detection systems using electronic digital signal processing”, Optics ex. 16, no. 20, pp. 15718-15727, 2008

  The methods of Patent Documents 1 to 3 described above are effective for reducing performance degradation caused by EEPN due to reception digital CD compensation. However, in the method of Patent Document 1, an optical component that divides the light output from the local oscillation light source into two parts, an electro-optical component for local oscillation light analysis, and a dedicated analog / digital converter for electrical digital signal processing However, there is a problem that the component configuration becomes complicated.

  Further, the methods of Patent Document 2 and Patent Document 3 have a problem in versatility in that it is necessary to perform subcarrier multiplexing and to use a specific modulation method.

  The present invention has been made to solve such a problem, and can estimate light source phase noise with a simple component configuration, reduce performance deterioration due to EEPN, and whether subcarrier multiplexing is present or not. It is an object of the present invention to obtain an optical transmission method and an optical transmission system that do not require the above-mentioned limitation or modulation scheme limitation.

  An optical transmission method according to the present invention is an optical transmission method using orthogonal polarization multiplexing and coherent detection, and includes a first cross-correlation that is a cross-correlation of optical phase noise between orthogonal polarizations in received signal light, and a local Using a correlation step exclusive of the second cross-correlation that is the cross-correlation of optical phase noise between orthogonal polarizations in the oscillation light, and the coherent detection, the phase noise of the received signal light and the phase noise of the local oscillation light Generating an orthogonal two-polarized electric signal, and estimating the phase noise of the received signal light and the phase noise of the local oscillation light, the delay difference of the first cross-correlation and the second mutual correlation By combining the estimation results of each phase noise in the estimation step under the condition that the correlation delay difference is maximized, either the phase noise of the received signal light or the phase noise of the local oscillation light is It possible to decrease, and has a compensation step of estimating and compensating distinguishes between phase noise of the phase noise and the local oscillator light of the received signal light.

  An optical transmission system according to the present invention is an optical transmission system including an optical transmission unit and an optical reception unit, and the optical transmission unit includes a transmission light source that generates unmodulated light and a non-modulated light generated by the transmission light source. With respect to the modulated light, correlation control is performed so that the first cross-correlation that is the cross-correlation of the optical phase noise between the orthogonal polarization and the optical modulation unit that generates the orthogonal polarization multiplexed signal is maximized with the delay difference τ. And a correlation control unit that performs the correlation control by receiving the local oscillation light source that generates the local oscillation light as unmodulated light and the optical signal that is correlation-controlled by the correlation control unit as the received signal light. The second cross-correlation, which is the cross-correlation of optical phase noise between orthogonally polarized waves in the local oscillation light, is maximized with a delay difference of 0, and the received signal light and the local oscillation light are mixed and detected. , Photoelectric conversion unit for generating electrical signals, and photoelectric conversion Each of the phase noise of the received signal light and the phase noise of the local oscillation light included in the electrical signal generated by the above-described steps is performed, and the delay difference of the first cross-correlation and the delay difference of the second cross-correlation are maximized. It is possible to reduce either the phase noise of the received signal light or the phase noise of the local oscillation light by combining the estimation results of the respective phase noises under the following conditions. Noise compensation unit that estimates and compensates by distinguishing between phase oscillation noise and local oscillation phase noise, and a chromatic dispersion compensation unit that performs signal processing to compensate residual chromatic dispersion for the signal that has been phase noise compensated by the phase noise compensation unit Are provided.

  The present invention makes exclusive use of the cross-correlation of optical phase noise between orthogonally polarized waves in received signal light and the cross-correlation of optical phase noise between orthogonally polarized waves in locally oscillated light, so that signal processing in the receiving unit is performed. Thus, it is possible to distinguish and extract the phase noise of the received signal light and the local oscillation light. As a result, it is possible to estimate the light source phase noise with a simple component configuration, reduce the performance degradation due to EEPN, and not to limit the presence or absence of subcarrier multiplexing or the limitation of the modulation scheme, and An optical transmission system can be obtained.

It is a figure which shows an example of the optical transmission system using the optical transmission method which concerns on Embodiment 1 of this invention. It is a block diagram which shows an example of the internal structure of the optical transmission part which concerns on Embodiment 1 of this invention. It is a block diagram which shows an example of the internal structure of the optical receiver which concerns on Embodiment 1 of this invention. It is the conceptual diagram which showed an example of the correlation control result of the phase noise by the correlation control part which concerns on Embodiment 1 of this invention. It is the conceptual diagram which showed an example of the correlation of the phase noise of the local oscillation light which concerns on Embodiment 1 of this invention. It is a conceptual diagram which shows the phase of the local oscillation light estimated by the phase rotation part which concerns on Embodiment 1 of this invention. It is the figure which showed an example of the frequency fluctuation | variation estimation result of the local oscillation light source in the optical transmission system which concerns on Embodiment 1 of this invention. It is the figure which showed an example of the improvement effect by EEPN compensation in the optical transmission system which concerns on Embodiment 1 of this invention.

  Exemplary embodiments of an optical transmission method and an optical transmission system according to the present invention will be described below in detail with reference to the drawings.

Embodiment 1 FIG.
FIG. 1 is a diagram showing an example of an optical transmission system using the optical transmission method according to Embodiment 1 of the present invention. As shown in FIG. 1, the optical transmission system according to the first embodiment includes an optical transmission unit 1000, an optical transmission unit 2000 including an optical fiber, an optical repeater, and the like, and an optical reception unit 3000. The

  FIG. 2 is a block diagram showing an example of an internal configuration of the optical transmission unit 1000 according to Embodiment 1 of the present invention. The optical transmission unit 1000 includes a transmission light source 1100, an optical modulation unit 1200, and a correlation control unit 1300. Here, the optical modulation unit 1200 includes an optical branching unit 1201, an X-polarization light modulation unit 1202, a Y-polarization light modulation unit 1203, and a light combining unit 1204.

  FIG. 3 is a block diagram showing an example of an internal configuration of the optical receiving unit 3000 according to Embodiment 1 of the present invention. The optical receiving unit 3000 includes an analog front end unit 3100, a phase noise compensation unit 3200, a chromatic dispersion compensation unit 3300, and a carrier wave restoration unit 3400.

  Here, the analog front end unit 3100 includes a local oscillation light source 3101, a photoelectric conversion unit 3102, and an analog / digital conversion unit (A / D conversion unit) 3103. The phase noise compensation unit 3200 includes a signal pre-shaping unit 3201, a polarization separation unit 3202, a phase noise estimation unit 3203, a phase noise classification unit 3204, a signal adjustment unit 3205, and a phase rotation unit 3206. Is done.

  Hereinafter, the operation of the optical transmission system according to the first embodiment will be described in detail based on the configuration of FIGS.

  The transmission light source 1100 inside the optical transmission unit 1000 generates unmodulated light and outputs it to the optical modulation unit 1200. The center frequency of the unmodulated light generated by the transmission light source 1100 is, for example, a frequency that roughly matches one of the frequencies of the C-band ITU-T grid. Note that unmodulated light generated by the transmission light source 1100 includes phase noise. For this reason, the frequency spectrum has a certain spread, and the line width is generally in the order of kHz to MHz.

  The optical branching unit 1201 inside the optical modulation unit 1200 branches the unmodulated light generated by the transmission light source 1100 into two, and outputs them to the X-polarization light modulation unit 1202 and the Y-polarization light modulation unit 1203, respectively. Here, it is assumed that the X polarization and the Y polarization correspond to, for example, a horizontal polarization and a vertical polarization, respectively.

  The X-polarized light modulation unit 1202 modulates the unmodulated light branched by the light branching unit 1201 with an X-polarization modulation electric signal input from the outside (not shown), and outputs the modulated light to the light combining unit 1204. Here, the electric signal for X polarization is, for example, a QPSK modulation signal with a baud rate of 32 GBaud.

  On the other hand, the Y-polarized light modulation unit 1203 modulates the unmodulated light branched by the light branching unit 1201 with a Y-polarization modulation electric signal input from the outside (not shown), and outputs the modulated light to the light combining unit 1204. Here, the Y polarization electric signal is, for example, a QPSK modulation signal with a baud rate of 32 GBaud.

  The light combining unit 1204 performs orthogonal polarization combining of the signal light modulated by the X-polarized light modulation unit 1202 and the signal light modulated by the Y-polarized light modulation unit 1203 and outputs the result to the correlation control unit 1300.

  The correlation control unit 1300 controls the correlation between the X-polarized phase noise and the Y-polarized phase noise and outputs the signal light, which has been orthogonally polarized by the optical modulation unit 1200, to the optical transmission unit 2000. .

  Here, the correlation control performed by the correlation control unit 1300 is realized, for example, by giving a group delay difference τ between the X-polarized signal light and the Y-polarized signal light. As a result, the phase noise φx_tx [t] of the X-polarized signal light and the phase noise φy_tx [t] of the Y-polarized signal light have a maximum cross-correlation with the delay difference τ. FIG. 4 is a conceptual diagram showing an example of the phase noise correlation control result by the correlation control section 1300 according to Embodiment 1 of the present invention. The addition of the group delay difference between the orthogonal polarizations can be realized by a birefringent medium such as a polarization maintaining fiber, for example.

  The optical transmission unit 2000 transmits the optical signal input from the optical transmission unit 1000 and outputs the optical signal to the optical reception unit 3000. The optical transmission unit 2000 is specifically composed of an optical fiber, an optical repeater, an optical add / drop device, and the like.

  The analog front end unit 3100 inside the optical receiving unit 3000 generates unmodulated light in the local oscillation light source 3101 inside the optical receiving unit 3000 and outputs it to the photoelectric conversion unit 3102. Here, the center frequency of the unmodulated light output from the local oscillation light source 3101 is assumed to be a frequency that approximately matches the center frequency of the received signal light. Since the unmodulated light includes phase noise, the frequency spectrum has a certain spread and the line width is generally in the order of kHz to MHz.

  The photoelectric conversion unit 3102 mixes and interferes with the received signal light input via the optical transmission unit 2000 and the unmodulated light input from the local oscillation light source 3101, and then converts the mixed signal into an electrical signal. The data is output to the conversion unit 3103. Here, as the photoelectric conversion unit 3102, for example, a polarization diversity coherent receiver may be used.

  Assuming that the phase disturbance in the optical transmission unit 2000 is within a negligible range, the phase noise between the X-polarized signal and the Y-polarized signal of the received signal light is the output end of the optical transmission unit 1000 shown in FIG. As with the phase noise feature in FIG. 4, the cross-correlation is maximized with the delay difference τ.

  On the other hand, in the photoelectric conversion unit 3102, correlation control between the X polarization and the Y polarization is not performed, and reception signal light and unmodulated light are mixed and interfered. Therefore, the phase noise φx_lo [t] of the X-polarized local oscillation light and the phase noise φy_lo [t] of the Y-polarized local oscillation light have a maximum cross-correlation with a delay difference of zero. FIG. 5 is a conceptual diagram showing an example of the correlation of the phase noise of the local oscillation light according to Embodiment 1 of the present invention.

  The analog / digital conversion unit 3103 performs analog / digital conversion on the electric signal input from the photoelectric conversion unit 3102 and outputs the converted signal to the phase noise compensation unit 3200.

  The signal pre-shaping unit 3201 inside the phase noise compensation unit 3200 performs signal shaping by digital signal processing on the digital signal A / D converted by the analog / digital conversion unit 3103, and outputs the signal to the polarization separation unit 3202. To do.

  The polarization separation unit 3202 separates the X-polarized signal and the Y-polarized signal from the digital signal shaped by the signal pre-shaping unit 3201, and outputs the separated X-polarized signal and Y-polarized signal. And output to both the phase noise estimation unit 3203 and the signal adjustment unit 3205.

  Here, the signal input to the polarization separation unit 3202 is generally a mixture of an X polarization signal and a Y polarization signal. When performing blind polarization separation without using a known signal such as a training signal, it is generally necessary to reduce the remaining CD value upstream of the polarization separation processing. For this reason, the signal pre-shaping unit 3201 needs to take measures such as reducing the residual CD in advance. On the other hand, when performing polarization separation using a known signal such as a training signal, it is not always necessary to reduce the remaining CD value upstream of the polarization separation processing.

  The phase noise estimation unit 3203 performs carrier wave phase recovery on the polarization-separated X-polarized signal and Y-polarized signal, respectively, so that the X-polarized phase noise φx [t] and the Y-polarized phase noise φy [T] is estimated, and each estimation result is output to the phase noise classification unit 3204.

  The phase noise φx [t] of the X polarization includes the synthesized phase noise φx_tx [t] of which correlation control is performed by the optical transmitter 1000 and the phase noise φx_lo [t] of the local oscillation light. Similarly, the phase noise φy [t] of the Y polarization includes a combination of the phase noise φy_tx [t] subjected to correlation control by the optical transmission unit 1000 and the phase noise φy_lo [t] of the local oscillation light. Yes.

  Based on the X-polarized phase noise φx [t] and the Y-polarized phase noise φy [t] estimated by the phase noise estimating unit 3203, the phase noise classification unit 3204 estimates the phase fluctuation component of the local oscillation light. The value Δφe_lo [t] is estimated and output to the phase rotation unit 3206.

As described above, with respect to the received signal light, the cross-correlation between the X-polarized phase noise and the Y-polarized phase noise is maximized by the delay difference τ, and with respect to the local oscillation light, the X-polarized phase noise and the Y-polarized light The cross-correlation of the wave phase noise is maximized with a delay difference of zero. By utilizing such a feature, the phase fluctuation component of the local oscillation light can be estimated by the following equation (1).
Δφe_lo [t]
= Φx [t + τ / 2] −φy [t−τ / 2] −φxy [t] (1)

This corresponds to a process of canceling a component having a maximum cross-correlation with the delay difference τ. Here, φxy [t] in the above equation (1) is a fixed phase difference between φx [t] and φy [t], and is obtained, for example, as in the following equation (2). Can do.
φx [t + τ / 4] −φy [t−τ / 4] (2)

  The signal adjustment unit 3205 performs a process for returning the pre-shaping performed by the signal pre-shaping unit 3201 to the digital signal separated by the polarization separating unit 3202, and outputs the digital signal to the phase rotating unit 3206.

  Specifically, for example, when the residual CD is reduced in advance in the signal pre-shaping unit 3201, the signal adjustment unit 3205 performs a process of returning the reduced CD value to the original value.

  The phase rotation unit 3206 integrates the phase fluctuation component estimated value Δφe_lo [t] of the local oscillation light estimated by the phase noise sorting unit 3204 to obtain the estimated value φe_lo [t] of the local oscillation light phase. Further, the phase rotation unit 3206 gives a phase rotation of an angle −φe_lo [t] to the digital signal input from the signal adjustment unit 3205 based on the estimated local oscillation light phase value φe_lo [t] obtained. And output to the chromatic dispersion compensation unit 3300.

  FIG. 6 is a conceptual diagram showing the phase φe_lo [t] of the local oscillation light estimated by the phase rotation unit 3206 according to Embodiment 1 of the present invention. As long as the fluctuation component can be compensated for, deterioration due to EEPN can be reduced. For this reason, although φx_lo [t] and φy_lo [t] usually have an offset, there is no problem.

  The chromatic dispersion compensation unit 3300 performs CD compensation by digital signal processing on the digital signal that has been subjected to phase noise compensation by the phase rotation unit 3206 and outputs the result to the carrier wave restoration unit 3400.

  The carrier recovery unit 3400 performs carrier recovery on the digital signal that has been subjected to CD compensation by the chromatic dispersion compensation unit 3300, and is output to the outside (not shown) and decoded.

Next, simulation results representing the effects of the present invention are shown. A 128 Gbit / s polarization multiplexed QPSK signal shaped by a Root Raised Cosine type low-pass filter with a roll-off factor of 0.1 was simulated under the following conditions.
The optical transmission unit 1000 adds a delay difference τ = 1000 symbol between the X / Y polarized waves.
A CD of 20,000 ps / nm is provided by the optical transmission unit 2000.
-Digital CD compensation is performed collectively by the optical receiver 3000.

  FIG. 7 is a diagram illustrating an example of a frequency fluctuation estimation result of the local oscillation light source in the optical transmission system according to Embodiment 1 of the present invention. More specifically, both the transmission light source 1100 and the local oscillation light source 3101 are diagrams showing an example in which the phase fluctuation of the local oscillation light is estimated with the light source line width set to 500 kHz. From the result of FIG. 7, it can be seen that the phase fluctuation of the local oscillation light can be estimated within an error range of about 0.1π with respect to the true value.

  FIG. 8 is a diagram showing an example of the improvement effect by the EEPN compensation in the optical transmission system according to the first embodiment of the present invention. More specifically, it is a diagram showing the light source line width dependence of the Q value indicating the signal quality. In FIG. 8, the optical signal power to noise power ratio is 15 dB (noise bandwidth: 0.1 nm), the polarization state is changed randomly, the bit error rate is obtained, and the average value of the bit error rate is obtained. The result of conversion to a Q value is shown later.

  Under the condition of a line width of 500 kHz, performance degradation of 1.3 dB occurred when the optical transmission system of the first embodiment was not used. On the other hand, by using the optical transmission system of the first embodiment, as shown in FIG. 8, the performance deterioration amount was reduced by 0.35 dB, and the effectiveness could be confirmed. The effect of EEPN is more conspicuous as the multi-level of the modulation scheme is higher. For this reason, there is a possibility that the amount of improvement increases in a multi-level modulation scheme such as 16QAM.

  As described above, according to the first embodiment, the optical transmitter 1000 controls the cross-correlation of phase noise between the X polarization and the Y polarization. As a result, the cross-correlation between the optical phase noises between the orthogonal polarizations in the received signal light and the cross-correlation between the optical phase noises between the orthogonal polarizations in the local oscillation light can be made exclusive. Furthermore, in the optical receiving unit 3000, the phase noise of the received signal light and the phase noise of the local oscillation light can be distinguished and estimated.

  In addition, there is no need for an optical component that splits the light output from the local oscillation light source into two parts, an electro-optical component for local oscillation light analysis, or a dedicated analog / digital converter for electrical digital signal processing. The light source phase noise can be estimated with a simple component configuration, and the performance deterioration caused by the EEPN can be reduced.

  Furthermore, the present invention is general-purpose without limiting the presence or absence of subcarrier multiplexing and the modulation method. In this way, an optical transmission method and an optical transmission system useful for long-distance large-capacity optical transmission can be realized.

Claims (3)

An optical transmission method using orthogonal polarization multiplexing and coherent detection,
The first cross-correlation that is the cross-correlation of the optical phase noise between the orthogonal polarizations in the received signal light and the second cross-correlation that is the cross-correlation of the optical phase noise between the orthogonal polarizations in the local oscillation light are exclusive. And a correlation step
Using the coherent detection, an orthogonal two-polarized electric signal in which the phase noise of the reception signal light and the phase noise of the local oscillation light are mixed is generated, and the phase noise of the reception signal light and the phase noise of the local oscillation light are generated. An estimation step for estimating each of
By combining the estimation results of the respective phase noises in the estimation step under the condition that the delay difference of the first cross-correlation and the delay difference of the second cross-correlation are maximized, the phase noise of the received signal light Or a compensation step that makes it possible to reduce any one of the phase noises of the local oscillation light, and to estimate and compensate the phase noise of the received signal light and the phase noise of the local oscillation light separately. Optical transmission method. The correlation step includes
In the optical transmission unit, by performing correlation control, a correlation first step for giving a correlation between orthogonal polarizations in the received signal light so that the first cross-correlation is maximized with a delay difference τ;
And a correlation second step of causing a correlation between orthogonal polarizations in the local oscillation light so that the second cross-correlation is maximized at a delay difference of 0 without performing correlation control in the optical receiver. 2. The optical transmission method according to 1. An optical transmission system comprising an optical transmitter and an optical receiver,
The optical transmitter is
A transmission light source that generates unmodulated light;
Regarding the unmodulated light generated by the transmission light source, an optical modulation unit that generates an orthogonal polarization multiplexed signal;
A correlation control unit that performs correlation control so that a first cross-correlation that is a cross-correlation of optical phase noise between orthogonal polarizations is maximized with a delay difference τ;
The optical receiver is
A local oscillation light source that generates local oscillation light as unmodulated light;
A second cross-correlation that is a cross-correlation of optical phase noise between orthogonal polarizations in the local oscillation light by receiving the optical signal that has been subjected to correlation control in the correlation control unit as reception signal light and performing no correlation control. Is a maximum at a delay difference of 0, and the received signal light and the local oscillation light are mixed and interfered with each other to detect and generate an electrical signal,
Estimating each of phase noise of the received signal light and phase noise of the local oscillation light included in the electrical signal generated by the photoelectric conversion unit, the delay difference of the first cross-correlation and the first The phase noise of the received signal light or the phase noise of the local oscillation light is reduced by combining the estimation results of the respective phase noises under the condition that the delay difference of the cross-correlation between the two is maximized A phase noise compensator that distinguishes and estimates and compensates for the phase noise of the received signal light and the phase noise of the local oscillation light,
An optical transmission system comprising: a chromatic dispersion compensation unit that performs signal processing for compensating residual chromatic dispersion on a signal that has been phase noise compensated by the phase noise compensation unit.

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