JP6386419B2 - Optical transmission system and crosstalk measurement method - Google Patents

Description

  The present invention relates to an optical transmission system and a crosstalk measurement method.

  In order to dramatically increase the transmission capacity of optical transmission systems, multi-core optical transmission systems using multi-core fibers with multiple cores in the transmission path and multiple information in parallel using different modes of multi-mode fibers Development of multimode optical transmission systems for transmission is underway. Optical transmission using these multi-core fibers and multi-mode fibers for multiplexing optical signals is called space division multiplexing (SDM). In SDM optical transmission, a conventional single-mode fiber, that is, a standard single-mode fiber, is transmitted by propagating a Wavelength Division Multiplex (WDM) signal that transmits different information to different cores or modes. The transmission capacity can be dramatically increased as compared with the case of using a road.

  In a long-distance SDM optical transmission system, signal light is attenuated during transmission in the same manner as an optical transmission system using a standard single-mode fiber as a transmission line, and therefore an optical fiber amplifier for SDM that amplifies signal light with reduced intensity Is indispensable. In a multi-core fiber amplifier used in a multi-core optical transmission system, inter-core crosstalk occurs due to mode coupling between signal lights propagating through each core of an amplification multi-core fiber (Non-patent Document 1). In applying the multi-core fiber amplifier to the optical transmission system, it is necessary to measure the cross-talk value between the cores in the multi-core fiber amplifier and evaluate whether the cross-talk value between the cores is less than a specified value or a design value. (Non-Patent Documents 2 and 3).

K. Takenaga et al., "An Investigation on Crosstalk in Multi-Core Fibers by Introducing Random Fluctuation along Longitudinal Direction," IEICE TRANSACTIONS on Communications, 2011, Vol.E94-B, No.2, p.409-416 Y. Tsuchida et al., "Amplification Characteristics of a Multi-core Erbium-doped Fiber Amplifier," Optical Fiber Communication Conference and Exposition (OFC / NFOEC), 2012 and the National Fiber Optic Engineers Conference, 2012, p.1-3 H. Ono et al., "Inter-core crosstalk measurement in multi-core fiber amplifier using multiple intensity tones," Electronics Letters, 2014, Volume 50, Issue 14, p.1009-1010

  In view of the above circumstances, an object of the present invention is to provide an optical transmission system and a crosstalk measurement method that enable measurement of crosstalk between cores when a multicore fiber is used for transmission.

  One aspect of the present invention is an optical transmission system that performs transmission using a multi-core optical fiber, and generates signal light to be transmitted in each core of the multi-core optical fiber, and includes each core of the signal light in each frame. And a cross for determining whether or not crosstalk occurs between the cores based on a correlation between the signal light generation unit that arranges different synchronization signals and the synchronization signal included in the signal light transmitted through each of the cores. A crosstalk measuring section, and the crosstalk measuring section measures a crosstalk amount from the correlation when it is determined that crosstalk has occurred.

  According to another aspect of the present invention, in the optical transmission system, the crosstalk measurement unit calculates a correlation between the synchronization signal for each core and a signal included in the transmitted signal light, and the signal When the correlation between the synchronization signal arranged in the signal light generation unit and the synchronization signal different from that is equal to or greater than a predetermined threshold value, it is determined that crosstalk has occurred between the cores.

  According to another aspect of the present invention, in the optical transmission system, the signal light generation unit arranges the synchronization signal having a symmetrical waveform pattern in a frame of the signal light, and the crosstalk measurement unit In the signal included in the signal light, the autocorrelation is calculated as the correlation at an interval determined based on the symmetry, and the crosstalk measuring unit determines the correlation between the cores based on the peak of the autocorrelation. It is determined whether or not crosstalk has occurred.

  According to another aspect of the present invention, in the optical transmission system, the signal light generation unit includes a first waveform pattern serving as a reference, and a second waveform pattern obtained by inverting the first waveform pattern in the time direction. And a third waveform pattern that is phase conjugate with respect to the first waveform pattern and a fourth waveform pattern that is phase conjugate with respect to the second waveform pattern, Arranged in the optical frame, the crosstalk measurement unit calculates a delay correlation and a central symmetry correlation in the synchronization signal as the autocorrelation.

  In addition, according to one aspect of the present invention, in the optical transmission system, the signal light generation unit includes the first waveform pattern, the second waveform pattern, and the third waveform pattern in a different order for each core. And the fourth waveform pattern are arranged in each frame of the signal light.

  According to another aspect of the present invention, in the optical transmission system, the signal light generation unit is determined according to the different first waveform pattern determined for each of the cores and the first waveform pattern. The synchronization signal in which the second waveform pattern, the third waveform pattern, and the fourth waveform pattern are arranged is arranged in each frame of the signal light.

  According to another aspect of the present invention, in the above-described optical transmission system, frame extraction is performed by detecting a frame boundary from the signal light based on the synchronization signal arranged by the signal light generation unit. And a frame superimposing unit that superimposes the frames extracted by the frame extracting unit, wherein the crosstalk measuring unit is based on a correlation related to the synchronization signal included in the frames superimposed by the frame superimposing unit. Then, it is determined whether or not crosstalk occurs between the cores.

  Another embodiment of the present invention is a crosstalk measurement method in an optical transmission system that performs transmission using a multicore optical fiber, and generates signal light to be transmitted in each core included in the multicore optical fiber, and the signal light Cross-talk between the cores based on a correlation between the signal light generation step of arranging different synchronization signals for each of the cores in each frame and the synchronization signal included in the signal light transmitted through each of the cores A crosstalk measurement method comprising: a determination step for determining presence / absence of occurrence; and a measurement step for measuring a crosstalk amount from the correlation when it is determined that crosstalk has occurred.

  According to the present invention, it is possible to measure crosstalk between cores when a multicore fiber is used for transmission. Further, the data signal can be demodulated while suppressing the influence of crosstalk between the cores.

It is a block diagram which shows the structural example of the optical transmission system in 1st Embodiment. It is a block diagram which shows the structure of the signal processing part in 1st Embodiment. It is a figure which shows the outline | summary of the crosstalk detection in 1st Embodiment. It is a figure which shows the outline | summary of the signal waveform pattern of the synchronizing signal arrange | positioned at the signal light of each core of the optical transmission system in 2nd Embodiment. It is a figure which shows an example of a delay correlation and a center symmetrical correlation. It is a figure which shows the structural example of the signal waveform pattern of the synchronizing signal in 2nd Embodiment. It is a block diagram which shows the structural example of the signal processing part in 2nd Embodiment. It is a figure which shows the outline | summary of the process which the optical transmission system in 3rd Embodiment performs. It is a block diagram which shows the structural example of the signal processing part in 3rd Embodiment. It is a figure which shows the structural example of the signal processing part which applied the superimposition of the frame signal in the optical transmission system of 2nd Embodiment.

  Hereinafter, an optical transmission system and a crosstalk measurement method according to an embodiment of the present invention will be described with reference to the drawings. In the optical transmission system in each embodiment, a different signal is assigned to each core in a synchronization signal included in a frame transmitted by a multi-core fiber on the transmission side. On the receiving side, the presence / absence and intensity of crosstalk are measured based on the synchronization signal included in the transmitted frame for each core and the synchronization signal of the frame leaking from another core. In each embodiment described below, the same components are denoted by the same reference numerals, and redundant description is omitted.

(First embodiment)
FIG. 1 is a block diagram illustrating a configuration example of an optical transmission system according to the first embodiment. The optical transmission system includes a signal light transmitter 1, a multi-core transmitter 2, and a signal light receiver 3. The signal light transmission unit 1 includes a light source 11, an optical amplifier 12, an optical coupler 13, a drive signal generation unit 14, and N optical modulators 15. N is the number of signal lights transmitted in parallel in the multi-core transmission unit 2 of the optical transmission system.

  The light source 11 generates continuous light having one or a plurality of different wavelengths, and outputs the generated continuous light to the optical amplifier 12. The optical amplifier 12 amplifies the continuous light output from the light source 11 and outputs it to the optical coupler 13. The optical coupler 13 N-branches the continuous light output from the optical amplifier 12 and outputs the continuous light obtained by the branching to each of the optical modulators 15. The drive signal generation unit 14 inputs a data signal to be transmitted from the outside, and divides the input data signal into N streams. The drive signal generation unit 14 generates a signal waveform pattern corresponding to the data signal of each stream, and outputs each signal waveform pattern to the optical modulator 15.

  The optical modulator 15 as a signal light generation unit modulates the continuous light input from the optical coupler 13 based on the signal waveform pattern input from the drive signal generation unit 14. The optical modulator 15 outputs the signal light obtained by the modulation to the multi-core transmission unit 2. Each of the N signal lights output from the signal light transmission unit 1 to the multi-core transmission unit 2 is a signal corresponding to the data signals of N streams. The signal waveform pattern output from the drive signal generation unit 14 to each optical modulator 15 includes a signal waveform pattern corresponding to the synchronization signal for each frame. When the optical modulator 15 performs modulation based on the signal waveform pattern input from the drive signal generation unit 14, a synchronization signal is arranged in each frame of the signal light. The synchronization signal has a different pattern for each optical modulator 15, and a different synchronization signal is arranged for each of the N signal lights.

  The multi-core transmission unit 2 includes one or a plurality of multi-core optical fibers 21. The multi-core optical fiber 21 is an optical fiber including a plurality of cores. The core of the multi-core optical fiber 21 provided in the multi-core transmission unit 2 and the optical modulator 15 provided in the signal light transmission unit 1 are associated with each other on a one-to-one basis. The signal light output from the corresponding optical modulator 15 is input to the core of the multi-core optical fiber 21. N signal lights input from the signal light transmission unit 1 to the multi-core transmission unit 2 are transmitted to the signal light reception unit 3 through each core. In the multi-core transmission unit 2, signal light leaks (crosstalk) between neighboring cores, and part of the signal light passing through the cores leaks into the neighboring cores, thereby attenuating the intensity of the signal light. Further, signal light passing through another core leaks into signal light passing through the own core, thereby causing signal light crossing. FIG. 1 shows crosstalk between two cores for simplification of the drawing.

  The signal light receiving unit 3 includes an optical coupler 31, an optical filter 32, a photoelectric conversion unit 33, and a signal processing unit 34. The optical coupler 31 receives N signal lights from the multi-core transmission unit 2 and combines the input N signal lights into one signal light. The optical coupler 31 outputs the signal light obtained by collecting the signal lights to the optical filter 32. The optical filter 32 includes, for example, an OBPF (Optical Band Pass Filter) and the like, removes noise included in the signal light, and outputs the noise to the photoelectric conversion unit 33. The photoelectric conversion unit 33 includes, for example, a photodiode, converts input signal light into an electric signal, and outputs the electric signal to the signal processing unit 34.

  The signal processing unit 34 separates the input electric signal into signal components corresponding to each core, and further separates each signal component for each frame. The signal processing unit 34 measures the presence / absence and intensity of crosstalk between cores by determining whether or not a signal component of another core is included in the signal separated for each frame. The signal processing unit 34 demodulates the data signal included in each frame based on the measurement result of the inter-core crosstalk, and outputs the demodulated data signal to the outside.

  FIG. 2 is a block diagram illustrating a configuration of the signal processing unit 34 in the first embodiment. The signal processing unit 34 includes a reference signal storage unit 341, a frame extraction unit 342, a correlation calculation unit 343, a crosstalk measurement unit 344, and a demodulation unit 345. In the reference signal storage unit 341, a signal waveform pattern of a synchronization signal corresponding to each core of the multi-core transmission unit 2 is stored as a reference signal. That is, the signal waveform pattern of the synchronization signal assigned in advance to each core is shared between the signal light transmitter 1 and the signal light receiver 3.

  The frame extraction unit 342 calculates a correlation between each signal waveform pattern stored in the reference signal storage unit 341 and an electric signal input from the photoelectric conversion unit 33. The frame extraction unit 342 detects a point where the correlation is a peak as a frame boundary. The frame extraction unit 342 detects a frame boundary for each signal light transmitted by each core by using a different signal waveform pattern for each core. The frame extraction unit 342 extracts the electrical signal divided for each frame of the signal light as a frame signal, and outputs each frame signal to the correlation calculation unit 343 and the demodulation unit 345. The frame extraction unit 342 outputs a frame signal for each signal light transmitted by each core.

  The correlation calculation unit 343 calculates a correlation between each core frame signal and a signal waveform pattern corresponding to a core other than the core. Correlation calculation section 343 outputs, for each core, a correlation with the signal waveform pattern of each of the other cores in the frame signal of the core to crosstalk measurement section 344.

  The crosstalk measurement unit 344 determines whether there is a correlation equal to or greater than a predetermined threshold among the correlations of the respective cores input from the correlation calculation unit 343. The crosstalk measuring unit 344 has a signal waveform of another m-th core (m = 1, 2,..., N, m ≠ M) in a frame signal of a certain M-th core (M = 1, 2,..., N). If there is a portion having a high correlation with the pattern, it is determined that the m-th core signal light leaks into the M-th core signal light. Further, since the magnitude of the correlation depends on the intensity of the leaked signal light of the other core, the crosstalk measuring unit 344 measures the intensity of the crosstalk based on the correlation. In addition, the crosstalk measurement unit 344 specifies a combination of cores in which crosstalk has occurred based on the fact that the signal waveform pattern is different for each core. The crosstalk measuring unit 344 outputs a measurement result including the presence / absence of crosstalk, the combination of cores in which crosstalk is generated, and the intensity thereof to the demodulation unit 345.

  Based on the measurement result input from the crosstalk measurement unit 344, the demodulation unit 345 demodulates the frame signal for each core input from the frame extraction unit 342. The demodulator 345 converts the data signals of N streams obtained by demodulation into one or a plurality of series of data signals and outputs them to the outside.

  FIG. 3 is a diagram showing an outline of crosstalk detection in the first embodiment. In the example shown in the figure, signal waveform patterns corresponding to different synchronization signals (Sync_1, Sync_2,..., Sync_N) for each core are arranged at the head of the frame. The frame is provided with a payload portion following the synchronization signal, and carrier signals (P_1, P_2,...) Corresponding to the data signals are arranged in the payload portion. When the signal light of other cores (for example, the first core and the second core) leaks into the signal light of the M-th core, the main signals of the first core and the second core are included in the main signal component of the M-th core. The signal light on which the component is superimposed as a leakage signal component is input to the signal light receiving unit 3. At this time, in the electrical signal corresponding to the signal light, the main signal component of the other core is similarly superimposed as the leaked signal component.

  When the correlation between the signal waveform pattern (Ref_1, Ref_2,..., Ref_N) of the reference signal stored in the reference signal storage unit 341 and the electrical signal is sequentially calculated in the signal light receiving unit 3, the signal waveform pattern (Ref_1, Ref_2) is calculated. ,..., Ref_N), a correlation peak appears at a location (time or symbol position) where the same pattern exists. This peak position is the position of the signal waveform pattern (Sync_1, Sync_2,..., Sync_N) of the synchronization signal included in the signal light leaking from the other cores. At this time, by comparing the signal intensity of the reference signal signal waveform pattern (Ref_1, Ref_2, ..., Ref_N) and the detected sync signal signal waveform pattern (Sync_1, Sync_2, ..., Sync_N), Talk intensity (crosstalk amount) is measured.

  According to the optical transmission system of this embodiment, based on the correlation with the signal waveform patterns (Ref_1, Ref_2,..., Ref_N) of other m-th cores in the electrical signals or frame signals of the first to N-th cores. By measuring the presence / absence of crosstalk, detection of frame boundaries of other cores, and measurement of the amount of crosstalk, it becomes possible to measure crosstalk between cores.

(Second Embodiment)
In the optical transmission system according to the first embodiment, a signal waveform pattern of a synchronization signal that is different for each core is arranged on a frame boundary on the transmission side, and the presence or absence of occurrence of crosstalk is detected based on a known signal waveform pattern on the reception side. The configuration to be described has been described. In the second embodiment, instead of using a correlation with a known signal waveform pattern, by using a correlation (autocorrelation) between signals included in an electrical signal (frame signal), the presence or absence of occurrence of crosstalk, etc. Is detected.

FIG. 4 is a diagram illustrating an outline of a signal waveform pattern of a synchronization signal (Sync) arranged in the signal light of each core of the optical transmission system according to the second embodiment. In the figure, a case where a synchronization signal is arranged at the head of the frame is shown. The signal waveform pattern of the synchronization signal includes four signal patterns. The first signal pattern is a reference waveform pattern (S) serving as a reference. This reference waveform pattern is determined by an arbitrary method. The second signal pattern is a waveform pattern (S _C ) that is phase conjugate with respect to the reference waveform pattern. The third signal pattern is a waveform pattern (S _R ) obtained by inverting the reference waveform pattern in the time direction. The fourth signal pattern is a waveform pattern (S _CR ) that is phase conjugate with respect to the waveform pattern obtained by inverting the reference waveform pattern in the time direction.

In the example shown in FIG. 4, in the signal waveform pattern of the synchronization signal, the reference waveform pattern (S) is arranged at the head portion, the waveform pattern (S _C ) is arranged after the reference waveform pattern (S), and the waveform is arranged at the tail. A pattern (S _CR ) is arranged, and a waveform pattern (S _R ) is arranged immediately before the waveform pattern (S _CR ). Although spacing between the waveform pattern (S _C) and the waveform pattern in FIG. (S _R) is provided, or without providing the gap.

In the signal waveform pattern of the synchronization signal configured as described above, when the correlation is obtained by sequentially superimposing the interval from the beginning of the reference waveform pattern (S) and the beginning of the waveform pattern (S _R ), both waveform patterns are obtained. Delay correlation is obtained from (S, S _R ). Also, toward the head from the tail of the waveform pattern (S _C), towards the tail from the head of the waveform pattern (S _R), when the correlation by superimposing the signals in order, both waveform patterns (S _C, S _{Since R 1} ) has a waveform that is temporally inverted, a central symmetry correlation using central symmetry is obtained.

  FIG. 5 is a diagram illustrating an example of the delay correlation and the central symmetric correlation. In the waveforms shown in FIGS. 5A, 5 </ b> B, and 5 </ b> C, the horizontal axis indicates time, and the vertical axis indicates the strength of correlation. As shown in FIG. 5A, the delay correlation has a mountain-shaped waveform that is symmetric at the peak portion in the signal waveform pattern of the synchronization signal. As shown in FIG. 5B, the central symmetric correlation has a waveform having a plurality of peaks that are symmetric at the highest peak. A peak waveform as shown in FIG. 5C can be obtained by multiplying the delayed correlation and the central symmetric correlation and superimposing them. By using the peak waveform shown in FIG. 5C, the position of the signal waveform pattern of the synchronization signal can be specified. Also, the crosstalk amount can be estimated from the correlation peak intensity. Further, since the peak waveform obtained by multiplying the delay correlation and the central symmetry correlation is very sharp, the position of the signal waveform pattern of the synchronization signal can be specified with high accuracy.

FIG. 6 is a diagram illustrating a configuration example of a signal waveform pattern of a synchronization signal according to the second embodiment. The configuration example shown in FIG. 6A uses a common reference waveform pattern (S) in the synchronization signals of the first to N-th cores, but the reference waveform pattern (S) and the waveform patterns (S _C , S _R , S _CR ), different synchronization signals are generated. For example, the synchronization signal arranged in each frame of the signal light transmitted by the first core is configured by a combination of waveform patterns [S, S _C , S _R , S _CR ] in order, and each of the signal lights transmitted by the second core The synchronization signal arranged in the frame is composed of a combination of waveform patterns [S, S _R , S _C , S _CR ] in order. When the signal waveform pattern of the synchronization signal shown in FIG. 6A is used, since the peak positions of the delay correlation and the central symmetry correlation are different for each core, the signal waveform pattern of the detected synchronization signal is in any core. Whether it is a signal waveform pattern can be specified.

The configuration example shown in FIG. 6B uses different reference waveform patterns (S1, S2,..., SN) in the synchronization signals of the first core to the Nth core, but the reference waveform pattern (SM) and waveform are used. By applying the same arrangement relationship to the patterns (SM _C , SM _R , SM _CR ) (M = 1, 2,..., N), different synchronization signals are generated. Also in this configuration example, since the peak positions of the delay correlation and the central symmetry correlation are different for each core, it is possible to specify which core is the signal waveform pattern of the detected synchronization signal.

  FIG. 7 is a block diagram illustrating a configuration example of the signal processing unit 35 in the second embodiment. The optical transmission system according to the second embodiment includes a signal processing unit 35 instead of the signal processing unit 34 of the optical transmission system according to the first embodiment. The signal processing unit 35 includes a reference signal storage unit 341, a frame extraction unit 342, a correlation calculation unit 353, a crosstalk measurement unit 354, and a demodulation unit 345. The reference signal storage unit 341, the frame extraction unit 342, and the demodulation unit 345 have the same configuration as that of the first embodiment, and thus redundant description is omitted.

  The correlation calculation unit 353 calculates a delay correlation based on the combination interval for calculating the delay correlation for each of the core frame signals. Further, the correlation calculation unit 353 calculates the central symmetric correlation based on the combination interval for calculating the central symmetric correlation for each of the core frame signals. The combination interval for calculating the delay correlation and the combination interval for calculating the central symmetric correlation are uniquely determined as shown in FIG. 4 based on the arrangement relationship of the four waveform patterns in the signal waveform pattern of the synchronization signal. The correlation calculation unit 353 outputs the delay correlation and the central symmetry correlation calculated for each core frame signal to the crosstalk measurement unit 354.

  The crosstalk measuring unit 354 multiplies the delayed correlation and the central symmetric correlation input from the correlation calculating unit 343. The crosstalk measuring unit 354 determines whether or not crosstalk has occurred based on the correlation obtained by the multiplication. When crosstalk has occurred, the crosstalk measurement unit 354 determines which other core signal light leaks based on the waveform of the delay correlation and central symmetry correlation and the appearance of the peak position. Is identified. In addition, the crosstalk measuring unit 354 estimates the amount of crosstalk based on the peak values of the delay correlation and the central symmetry correlation. The crosstalk measurement unit 354 outputs a measurement result including the presence / absence of crosstalk, the combination of cores in which crosstalk is generated, and the intensity thereof to the demodulation unit 345.

  According to the optical transmission system of this embodiment, based on the autocorrelation (delay correlation and central symmetric correlation) in the electrical signal or frame signal of each of the first to Nth cores, the presence or absence of crosstalk, the other cores By detecting the frame boundary and measuring the amount of crosstalk, the crosstalk between cores can be measured.

(Third embodiment)
In the optical transmission system according to the first embodiment, the reception side calculates the correlation of the signal waveform patterns (Ref_1, Ref_2,..., Ref_N) of other cores for each frame and detects the presence or absence of occurrence of crosstalk. explained. When crosstalk occurs, the signal waveform pattern of the other core appears superimposed on the synchronization signal of the main signal component and the carrier signal of the payload portion. Therefore, depending on the value of the carrier signal, the signal waveform pattern of the other core The detection accuracy may be reduced due to erroneous detection or cancellation. Therefore, in the optical transmission system according to the third embodiment, before the correlation is calculated, the influence by the value of the carrier signal is averaged by superimposing in units of frame signals, thereby improving the detection accuracy.

FIG. 8 is a diagram illustrating an outline of processing performed by the optical transmission system according to the third embodiment. In FIG. 8, f _t represents a synchronization signal of the main signal component, and f _XT1 and f _XT2 represent synchronization signals of other cores, that is, synchronization signals of leaking signal light. FIG. 8A is a diagram illustrating the influence that the synchronization signals (f _XT1 , f _XT2 ) of other cores included in the frame signal receive from the carrier signal in the first and second embodiments. FIG. 8B is a diagram illustrating the influence of the synchronization signals (f _XT1 , f _XT2 ) included in the frame signal from the carrier signal in the third embodiment. By performing superimposition in units of frame signals, as shown in FIG. 8B, the frame signals cancel each other out or are averaged, so that the synchronization signals (f _XT1 , f of other cores). _XT2 ) is _less affected by the carrier signal component. Further, since the positions of the synchronization signals (f _XT1 , f _XT2 ) of other cores included in the frame signal are substantially constant, the synchronization signals (f _XT1 , f _XT1 , Since f _XT2 ) is emphasized, the influence received from the component of the carrier signal is further suppressed.

  FIG. 9 is a block diagram illustrating a configuration example of the signal processing unit 36 according to the third embodiment. The optical transmission system according to the third embodiment includes a signal processing unit 36 instead of the signal processing unit 34 of the optical transmission system according to the first embodiment. The signal processing unit 36 includes a reference signal storage unit 341, a frame extraction unit 342, a frame superimposition unit 365, a correlation calculation unit 343, a crosstalk measurement unit 344, and a demodulation unit 345. Since the reference signal storage unit 341, the frame extraction unit 342, the correlation calculation unit 343, the crosstalk measurement unit 344, and the demodulation unit 345 have the same configuration as that of the first embodiment, redundant description is omitted.

  The frame superimposing unit 365 performs a process of superimposing and superimposing the frame signal input from the frame extracting unit 342 on a per-frame signal basis. The frame superimposing unit 365 outputs the frame signal of each core obtained by performing the predetermined number of overlappings to the correlation calculating unit 343. In the third embodiment, since the frame signal of each core input to the correlation calculation unit 343 is reduced by the number of times of superposition, the calculation amount for calculating the correlation with respect to the frame signal is reduced. On the other hand, as shown in FIG. 8, since the influence of the signal waveform pattern of the other core included in the frame signal from the carrier signal component is suppressed, the signal waveform pattern of the other core is detected with high sensitivity. And the accuracy of the crosstalk measurement result can be improved.

  Note that the improvement in crosstalk measurement accuracy by superimposing frame signals can also be applied to the optical transmission system in the second embodiment. FIG. 10 is a diagram illustrating a configuration example of the signal processing unit 37 to which frame signal superposition is applied in the optical transmission system according to the second embodiment. The optical transmission system in this case includes a signal processing unit 37 instead of the signal processing unit 35 of the optical transmission system in the second embodiment. The signal processing unit 37 includes a reference signal storage unit 341, a frame extraction unit 342, a frame superimposition unit 365, a correlation calculation unit 353, a crosstalk measurement unit 354, and a demodulation unit 345. Like the signal processing unit 36 shown in FIG. 9, the frame superimposing unit 365 is provided between the frame extracting unit 342 and the correlation calculating unit 353, so that the accuracy of the crosstalk measurement result can be improved.

  According to the optical transmission system of this embodiment, the crosstalk measurement based on the correlation or autocorrelation with the signal waveform pattern of the other m-th core in the electrical signals or frame signals of the first to N-th cores is performed. It becomes possible to improve the accuracy of the measurement result.

  As described above, according to the optical transmission system in each embodiment, the presence or absence of inter-core crosstalk, the combination of cores in which crosstalk occurs, and the intensity of signal light leaking from other cores are measured. be able to. Further, the optical signal light receiving unit can demodulate the data signal from each frame based on the measurement result, thereby suppressing the influence of the inter-core crosstalk. As a result, it is possible to maintain the quality of the transmitted signal, extend the transmission distance, and improve the transmission capacity.

  In the optical transmission system in each embodiment, the configuration in which the signal light transmitted by each core of the multi-core transmission unit 2 is collected into one signal light by the optical coupler 31 has been described. However, without providing the optical coupler 31 in the signal light receiving unit 3, an optical filter 32, a photoelectric conversion unit 33, and a signal processing unit 34 are provided for each signal light transmitted by each core, and crosstalk measurement and The data signal may be demodulated. In this case, the data signals output from each signal processing unit 34 may be combined into one series of data signals by a process reverse to the process of dividing into N streams in the drive signal generation unit 14.

  In the optical transmission system in each embodiment, the configuration in which the signal light transmitter 1 includes one light source 11 has been described. However, a light source 11 and an optical amplifier 12 may be provided in the signal light transmitter 1 for each signal light transmitted in the multi-core transmission unit 2, that is, for each optical modulator 15. In this case, the optical coupler 13 is not necessary.

  In the optical transmission system in each embodiment, the configuration in which the synchronization signal is arranged at the head of the frame has been described. However, the synchronization signal may be arranged at the end of the frame, as long as it is arranged in a region adjacent to the boundary of the frame.

  You may make it implement | achieve all or one part of the signal processing part in embodiment mentioned above with a computer. For example, it is realized by recording a program for realizing each component included in the signal processing unit on a computer-readable recording medium, causing the computer system to read and execute the program recorded on the recording medium. Also good. Here, the “computer system” includes an OS and hardware such as peripheral devices. The “computer-readable recording medium” refers to a storage device such as a flexible medium, a magneto-optical disk, a portable medium such as a ROM and a CD-ROM, and a hard disk incorporated in a computer system. Further, the “computer-readable recording medium” is a program that dynamically holds a program for a short time, like a communication line when a program is transmitted via a network such as the Internet or a communication line such as a telephone line. In this case, a volatile memory inside a computer system serving as a server or a client in that case may be included and a program held for a certain period of time. Further, the program may be for realizing a part of the above-described constituent elements, and may be realized by combining the above-described constituent elements with a program already recorded in a computer system. It may be realized by using hardware such as PLD (Programmable Logic Device) or FPGA (Field Programmable Gate Array).

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes designs and the like that do not depart from the gist of the present invention.

  When multi-core fiber is used for transmission, it can be applied to applications where measurement of crosstalk between cores is indispensable.

  DESCRIPTION OF SYMBOLS 1 ... Signal light transmission part, 2 ... Multi-core transmission part, 3 ... Signal light reception part, 11 ... Light source, 12 ... Optical amplifier, 13 ... Optical coupler, 14 ... Drive signal generation part, 15 ... Optical modulator, 21 ... Multi-core Optical fiber, 31 ... Optical coupler, 32 ... Optical filter, 33 ... Photoelectric conversion unit, 34, 35, 36, 37 ... Signal processing unit, 341 ... Reference signal storage unit, 342 ... Frame extraction unit, 343,353 ... Correlation calculation , 344, 354... Crosstalk measuring unit, 345... Demodulator, 365.

Claims (8)

An optical transmission system that performs transmission using a multi-core optical fiber,
A signal light generator that generates signal light to be transmitted in each core of the multi-core optical fiber, and arranges a different synchronization signal for each core in each frame of the signal light; and
A crosstalk measuring unit that determines whether or not crosstalk occurs between the cores based on a correlation related to the synchronization signal included in the signal light transmitted through each of the cores;
With
When the crosstalk measuring unit determines that crosstalk has occurred, the crosstalk measuring unit measures a crosstalk amount from the correlation.
Optical transmission system. The crosstalk measurement unit calculates a correlation between the synchronization signal for each core and a signal included in the transmitted signal light, and a synchronization signal different from the synchronization signal arranged in the signal light generation unit in the signal It is determined that crosstalk has occurred between the cores when the correlation with
The optical transmission system according to claim 1. The signal light generation unit arranges the synchronization signal composed of a symmetrical waveform pattern in a frame of the signal light,
The crosstalk measurement unit calculates an autocorrelation as the correlation at an interval determined based on the symmetry in the signal included in the signal light,
The crosstalk measuring unit determines whether or not crosstalk occurs between the cores based on the autocorrelation peak;
The optical transmission system according to claim 1. The signal light generation unit includes a first waveform pattern serving as a reference, a second waveform pattern obtained by inverting the first waveform pattern in a time direction, and a phase conjugate with respect to the first waveform pattern. 3 is arranged in the frame of the signal light, the synchronization signal combining the waveform pattern of 3 and the fourth waveform pattern that is phase conjugate with respect to the second waveform pattern,
The crosstalk measuring unit calculates a delay correlation and a central symmetric correlation in the synchronization signal as the autocorrelation.
The optical transmission system according to claim 3. The signal light generation unit outputs the synchronization signal in which the first waveform pattern, the second waveform pattern, the third waveform pattern, and the fourth waveform pattern are arranged in a different order for each core. Arranged in each frame of the signal light,
The optical transmission system according to claim 4. The signal light generation unit includes the first waveform pattern that is different for each core, the second waveform pattern that is determined according to the first waveform pattern, the third waveform pattern, and the fourth waveform pattern. The synchronization signal in which the waveform pattern is arranged is arranged in each frame of the signal light,
The optical transmission system according to claim 4. A frame extraction unit that extracts a frame by detecting a frame boundary from the signal light based on the synchronization signal arranged by the signal light generation unit;
A frame superimposing unit that superimposes the frames extracted by the frame extracting unit;
Further comprising
The crosstalk measuring unit determines whether or not crosstalk occurs between the cores based on a correlation related to the synchronization signal included in the frame superimposed by the frame superimposing unit;
The optical transmission system according to any one of claims 1 to 6. A crosstalk measurement method in an optical transmission system that performs transmission using a multi-core optical fiber,
A signal light generation step of generating signal light to be transmitted in each core of the multi-core optical fiber, and arranging different synchronization signals for each of the cores in each frame of the signal light;
A determination step of determining presence or absence of occurrence of crosstalk between the cores based on a correlation related to the synchronization signal included in the signal light transmitted through each of the cores;
A measurement step of measuring the amount of crosstalk from the correlation when it is determined that crosstalk has occurred;
Crosstalk measurement method including

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