JP5931759B2 - Optical transmission system and optical transmission method - Google Patents

Description

  The present invention relates to an optical transmission system and an optical transmission method for transmitting the same optical signal from an optical signal transmitter using a plurality of optical transmission paths or a plurality of wavelengths, and receiving the signals collectively by an optical signal receiver and combining the signals. .

  Due to the demand for higher capacity and higher speed in optical transmission, optical transmission systems exceeding 100 Gbps per wavelength are being studied.

  In order to realize this large-capacity and high-speed optical transmission system, a digital coherent technology combining a coherent optical communication technology and a digital signal processing technology is used. In digital coherent technology, a phase-modulated optical signal is transmitted on the transmission side, and an electrical signal is output on the reception side by coherent detection due to interference between the optical signal and the local light source. The digital signal is output by an AD (Analog to Digital) converter. After the conversion, the received waveform distorted in the transmission path is compensated in the digital domain by the digital signal processing unit. As described above, the digital coherent technology can correct the waveform distortion with a simple configuration. In addition, the waveform equalization processing in the digital signal processing unit enables the polarization separation of the optical signal at the same time as the compensation of the waveform distortion. Therefore, in the digital coherent transmission method, the polarization-multiplexed optical signal is generally used. It has been used to achieve large capacity and high speed (Non-patent Document 1).

S. J. Savory, “Digital filters for coherent optical receivers,” Optics Express, vol.16, no.2, pp.804-814, 2008. E. Yamazaki, et al., “Fast optical channel recovery in field demonstration of 100-Gbit / s Ethernet over OTN using real-time DSP,” Optics Express, vol.19, no.14, pp.13179-13184, 2011 . H. Y. Choi, T. Tsuritani, and I. Morita, “BER-adaptive flexible-format transmitter for elastic optical networks,” Optics Express, vol.20, no.17, pp.18652-18658, 2012. T. Tsuboi, M. Natori, and S. Mitachi, “Single-fiber optical protection ring architecture suitable for asymmetric traffic,” Global Telecommunications Conference, vol.7, pp.4044-4048, 2003.

  At present, a polarization multiplexing QPSK (Quadrature Phase Shift Keying) method is widely used as a modulation method for realizing optical transmission of 100 Gpbs per wavelength (Non-patent Document 2). Furthermore, in order to realize large capacity and high speed transmission, multi-level modulation signals such as polarization multiplexed 16QAM (Quadratuere Amplitude Modulation) are being studied. However, since the reception sensitivity is lowered due to the multi-level modulation signal, the transmission distance is limited (Non-Patent Document 3).

  In addition, in a conventional optical transmission system, there is a case in which a transmission line is redundant in advance to switch from a working line to a protection line when a failure occurs in the optical network in order to improve the reliability of the optical network. (Non-Patent Document 4), since it takes about several tens of ms to physically switch when a failure occurs, the optical signal is interrupted during that time.

  The present invention provides an optical transmission system and an optical transmission which improve transmission quality by improving reception sensitivity to facilitate large-capacity and high-speed transmission, and further improve stability by preventing optical signal interruption when an optical network failure occurs. It aims to provide a method.

An optical transmission system according to a first aspect of the present invention includes an optical signal transmitter that converts an electrical signal into which a signal for timing synchronization is inserted into an optical signal, branches the optical signal, and transmits the optical signal to a plurality of optical transmission paths, Each optical signal transmitted through the optical transmission path is coherently detected and converted into a plurality of electrical signals, the plurality of electrical signals are converted into digital signals, waveform equalization is performed, and a signal for timing synchronization is further obtained. and synchronizes the signal synthesizing timing using, and an optical signal receiver for demodulating optical signals receiver, an average power of a plurality of optical signals or coherent detection after the signals transmitted via the optical transmission path Is a configuration in which signals are synthesized by excluding signals below the threshold.

In the optical transmission system according to the second aspect of the present invention, the electrical signal into which the timing synchronization signal is inserted is converted into an optical signal having wavelengths λ1, λ2 (λ1 ≠ λ2) and wavelength-multiplexed, and the wavelength-multiplexed optical signal is branched. The optical signal transmitter for transmitting to a plurality of optical transmission lines and the wavelength multiplexed optical signal transmitted through the plurality of optical transmission lines are respectively demultiplexed, and the optical signal of wavelength λ1 and the optical signal of wavelength λ2 are respectively coherent. Light that is detected and converted into a plurality of electrical signals, the plurality of electrical signals are converted into digital signals, waveform equalization is performed, and timing is synchronized using a signal for timing synchronization, and the signal is synthesized and demodulated A signal receiver, and the optical signal receiver is configured to synthesize signals except for a wavelength multiplexed optical signal transmitted through a plurality of optical transmission paths or a signal whose average power after coherent detection is equal to or less than a threshold value. is there

The optical transmission method of the third invention is an optical signal transmitter that converts an electrical signal into which a signal for timing synchronization is inserted into an optical signal, branches the optical signal, and sends it to a plurality of optical transmission lines. An optical signal receiver coherently detects optical signals transmitted through multiple optical transmission paths and converts them to multiple electrical signals, and converts these multiple electrical signals into digital signals for waveform equalization. In addition, when synthesizing signals with synchronized timing using signals for timing synchronization , except for signals whose average power of optical signals transmitted through multiple optical transmission paths or signals after coherent detection is below a threshold value and signal combining Te, demodulate.

An optical transmission method according to a fourth aspect of the invention is an optical signal transmitter that converts an electrical signal into which a signal for timing synchronization is inserted into an optical signal having wavelengths λ1 and λ2 (λ1 ≠ λ2) and wavelength-multiplexes the optical signal. The multiplexed optical signal is branched and sent to a plurality of optical transmission lines, and the optical signal receiver demultiplexes the wavelength multiplexed optical signals transmitted through the plurality of optical transmission lines, respectively, and the optical signal having the wavelength λ1 and the wavelength Each optical signal of λ2 is coherently detected and converted into a plurality of electrical signals, the plurality of electrical signals are converted into digital signals, waveform equalization is performed, and timing is synchronized using a timing synchronization signal. At the time of signal synthesis, signal synthesis is performed except for a wavelength-multiplexed optical signal transmitted through a plurality of optical transmission paths or a signal whose average power after coherent detection is equal to or less than a threshold value , and demodulated.

  The optical transmission system and the optical transmission method of the present invention transmit the same optical signal from an optical signal transmitter using a plurality of optical transmission paths or wavelengths, and receive the signals collectively by an optical signal receiver to perform signal synthesis. As a result, the reception sensitivity can be improved. In particular, by synthesizing signals excluding signals whose average power is less than or equal to a threshold value, it is possible to easily realize large-capacity and high-speed transmission by preventing deterioration in transmission quality due to synthesis of noise components.

  Also, the optical transmission system and optical transmission method of the present invention combine and demodulate signals transmitted through a plurality of optical transmission paths, thereby preventing optical signal interruption when an optical network failure occurs. The stability of the transmission system can be increased.

It is a figure which shows the structural example of Example 1 of the optical transmission system of this invention. It is a figure which shows the structural example of Example 2 of the optical transmission system of this invention.

FIG. 1 shows a configuration example of Embodiment 1 of the optical transmission system of the present invention.
In FIG. 1, an optical signal transmitter 10 in the optical transmission system according to the first embodiment includes an electric signal generator 11 that outputs an electric signal of 100 Gbps, and an electric signal that is input from the electric signal generator 11 in a polarization multiplexing QPSK system. An optical signal generator 12 that converts and outputs a modulated optical signal and an optical splitter 13 that distributes the optical signal input from the optical signal generator 12 in three directions.

  Here, the optical signal transmitter 10 and the optical signal receiver 20 are connected via three optical transmission lines 31 to 33. The optical transmission lines 31 to 33 are single-core optical fibers, and the optical transmission lines 31 and 32 are composed of, for example, four sections with one section between the optical amplifiers, and the optical transmission path 33 is configured of, for example, six sections. To do.

  The optical signal receiver 20 includes local light sources 21-1 to 21-3 that output local light having a wavelength equal to the optical signal wavelength, and an optical signal and local light transmitted via the three optical transmission paths 31 to 33. Are input to the coherent receivers 22-1 to 22-3, are subjected to coherent detection, converted into electrical signals, and output. The electrical signals are converted into digital signals by the AD converters 23-1 to 23-3, respectively, and polarization separation and waveform equalization are performed by the waveform equalizers 24-1 to 24-3, and the timing synchronizer 25- 1 to 25-3 synchronize timing with each other, and the signal synthesizer 26 synthesizes three digital signals whose timings are synchronized. The synthesized digital signal is input to the demodulator 27 and demodulated as a symbol string, and the error corrector 28 performs bit error correction.

  Table 1 shows transmission quality for each of the optical transmission lines 31 to 33 according to the configuration of the first embodiment. Here, after demodulating a waveform-equalized digital signal into a symbol string for the optical signals of the optical transmission lines 31 to 33, the bit error rate is calculated, and the Q value converted from the bit error rate is transmitted quality. It is a value indicating.

  Table 2 shows transmission quality after signal synthesis by the signal synthesizer 26. Here, the Q value calculated by the method in the same manner as in Table 1 according to the combination of the optical transmission lines 31 to 33 is shown.

  The Q value after the signal combination shown in Table 2 is larger than the Q value for each of the optical transmission lines 31 to 33 shown in Table 1 in all the combinations of the optical transmission lines 31 to 33. Can be confirmed. That is, the reception sensitivity is improved by signal synthesis, and the transmission distance can be extended.

  In the first embodiment, the same optical signal is transmitted from the optical signal transmitter 10 to the three optical transmission paths 31 to 33, and transmitted by the optical signal receiver 20 via the three optical transmission paths 31 to 33. Since the optical signals are detected and combined after waveform equalization and demodulated as one signal, an optical signal interruption at the receiving side will occur unless there are simultaneous failures in the three optical transmission lines 31-33. In addition, the stability of the optical transmission system can be improved.

  Here, when a failure occurs in any of the optical transmission lines 31 to 33, when the signal synthesizer 26 synthesizes the signal from the optical transmission line in which the failure has occurred, the noise component is signal-synthesized. Quality will deteriorate. Therefore, for example, the average power of the digital signals output from the AD converters 23-1 to 23-3 is monitored inside the waveform equalizers 24-1 to 24-3, and becomes equal to or less than a preset threshold value. In such a case, processing that does not synthesize the digital signal of the optical transmission line is performed. Alternatively, all the values of the digital signals from the optical transmission line whose average power is equal to or less than the threshold are combined as 0. Thereby, it is possible to prevent deterioration of transmission quality due to synthesis of noise components. Moreover, it is good also as a structure which monitors the average power of the optical signal input into coherent receiver 22-1 to 22-3, and performs the process similar to the above, when it becomes below a preset threshold value.

Hereinafter, modifications of the configuration of the first embodiment will be described.
In the optical signal transmitter 10, instead of the configuration using the optical splitter 13, the electric signal output from the electric signal generator 11 is branched into three and respectively input to the three optical signal generators 12 to be converted into optical signals. However, it may be configured to send out to each of the optical transmission lines 31 to 33.

  In the optical signal receiver 20, instead of the configuration using three local light sources 21, the local light output from one local light source 21 is branched into three and input to the coherent receivers 22-1 to 22-3. As good as

  Instead of the configuration using three optical transmission paths 31 to 33, generally two or more optical transmission paths are used, and the optical signal transmitter 10 and the optical signal receiver 20 are also configured according to the number of optical transmission paths. Also good.

  Instead of a configuration using three optical transmission lines 31 to 33 with single core optical fibers, a configuration using a multi-core fiber having three cores in one optical transmission path may be used. Further, a configuration using a multi-core fiber having two or more cores may be used instead of the configuration using two or more optical transmission lines. Furthermore, the configuration may be such that N multi-core fibers having the number of cores M are used, M × N parallel transmission is performed, and signal synthesis is performed.

  In the optical signal transmitter 10, a configuration for outputting an optical signal modulated by an arbitrary bit rate and modulation method instead of a configuration for outputting the optical signal modulated by the polarization multiplexing QPSK method of 100 Gbps by the optical signal generator 12. It may be. The same applies to Example 2 described below.

  A signal for timing synchronization is inserted in advance by the electric signal generator 11 of the optical signal transmitter 10, and signals for timing synchronization are detected by the timing synchronizers 25-1 to 25-3 of the optical signal receiver 20. It may be configured to perform timing synchronization of three equalized signals. Alternatively, in addition to inserting a timing synchronization signal, any timing synchronization method such as superimposing a timing synchronization pilot signal on an optical signal may be applied. The same applies to Example 2 described below.

FIG. 2 shows a configuration example of Embodiment 2 of the optical transmission system of the present invention.
In FIG. 2, the optical signal transmitter 40 in the optical transmission system according to the second embodiment includes an electric signal generator 41 that outputs an electric signal of 100 Gbps, for example, in two branches, and an electric signal input from the electric signal generator 41. Optical signal generators 42-1 and 42-2 that convert and output optical signals of wavelengths .lamda.1 and .lamda.2 respectively modulated by the wave multiplex QPSK system, and optical signals of wavelengths .lamda.1 and .lamda.2 that are input from each optical signal generator. It is constituted by a wavelength multiplexer 43 that multiplexes and outputs the wavelength.

  Here, the optical signal transmitter 40 and the optical signal receiver 50 are connected via one optical transmission line 31. The optical transmission line 31 is a single-core optical fiber, and the optical transmission line 31 is composed of, for example, four sections with one section between the optical amplifiers.

  The optical signal receiver 50 demultiplexes the wavelength-multiplexed optical signal input from the optical transmission path 31 into optical signals of the wavelengths λ1 and λ2 by the wavelength demultiplexer 51, and inputs the optical signals to the coherent receivers 52-1 and 52-2, respectively. To do. Here, the wavelength multiplexer 43 and the wavelength demultiplexer 51 can be configured using, for example, an AWG (Arrayed Waveguide Grating), an optical coupler, and a WSS (Wavelength Selective Switch).

  The coherent receiver 52-1 receives local light having a wavelength λ1 from the local light source 53-1 and coherently detects an optical signal having a wavelength λ1. Coherent detection is performed on an optical signal having a wavelength λ2 by inputting local light. Subsequent AD converters 54-1, 54-2, waveform equalizers 55-1, 55-2, timing synchronizers 56-1, 56-2, signal synthesizer 57, demodulator 58, and error corrector 59 The optical signals transmitted at a plurality of wavelengths λ1 and λ2 are combined by detection, waveform equalization, and demodulation and error correction.

  In the second embodiment, as the transmission performance of the optical signals having the wavelengths λ1 and λ2, the waveforms output from the waveform equalizers 55-1 and 55-2 of the optical signal receiver 50 are equalized as in the first embodiment. The Q value of the digital signal was calculated in the same manner as in Example 1. The Q value of the optical signal having the wavelength λ1 was 8.93 dB, and the Q value of the optical signal having the wavelength λ2 was 9.91 dB. The Q value of the digital signal after signal synthesis is 12.32 dB, confirming that transmission quality is improved by signal synthesis. That is, the reception sensitivity is improved by signal synthesis, and the transmission distance can be extended.

  In the second embodiment, one optical transmission line 31 is used. When a failure occurs in the optical transmission line 31, the signal is interrupted. Therefore, the wavelength multiplexed optical signal output from the optical signal transmitter 40 is transmitted through a plurality of optical transmission lines as in the first embodiment, and the optical signal that has been wavelength multiplexed by the optical signal receiver 50 is demultiplexed. However, by receiving and synthesizing the optical signals of each wavelength, optical signal disconnection does not occur on the receiving side unless all optical transmission lines fail simultaneously, and the stability of the optical transmission system Can be increased.

  Here, in the configuration using a plurality of optical transmission lines, the averaged power of the digital signal output from the AD converter is monitored inside the waveform equalizer as in the first embodiment, and is equal to or lower than a preset threshold value. In such a case, processing that does not synthesize the digital signal of the optical transmission path is performed. Alternatively, all the values of the digital signals from the optical transmission line whose average power is equal to or less than the threshold value may be combined. Moreover, it is good also as a structure which monitors the average power of the optical signal input into a coherent receiver, and performs a process similar to the above when it becomes below a preset threshold value.

  Also, instead of a configuration in which two optical signals having different wavelengths obtained by modulating the same electrical signal are wavelength-multiplexed, two or more optical signals having different wavelengths are generally configured to be wavelength-multiplexed, and the optical signal transmitter 40 The optical signal receiver 50 may also be configured according to the number of wavelength multiplexing.

DESCRIPTION OF SYMBOLS 10 Optical signal transmitter 11 Electrical signal generator 12 Optical signal generator 13 Optical splitter 20 Optical signal receiver 21 Local light source 22 Coherent receiver 23 AD converter 24 Waveform equalizer 25 Timing synchronizer 26 Signal synthesizer 27 Demodulator 28 Error Corrector 31, 32, 33 Optical transmission line 40 Optical signal transmitter 41 Electrical signal generator 42 Optical signal generator 43 Wavelength multiplexer 50 Optical signal receiver 51 Wavelength demultiplexer 52 Coherent receiver 53 Local light source 54 AD converter 55 Waveform Equalizer 56 Timing synchronizer 57 Signal synthesizer 58 Demodulator 59 Error corrector

Claims (4)

An optical signal transmitter that converts an electrical signal into which a signal for timing synchronization is inserted into an optical signal, branches the optical signal, and sends it to a plurality of optical transmission lines;
The optical signals transmitted through the plurality of optical transmission paths are coherently detected and converted into a plurality of electrical signals, the plurality of electrical signals are converted into digital signals to perform waveform equalization, and the timing An optical signal receiver for synthesizing and demodulating signals with synchronized timing using a synchronization signal ,
The optical signal receiver is configured to synthesize signals except for signals whose average power of the optical signal transmitted through the plurality of optical transmission paths or the signal after coherent detection is equal to or less than a threshold value. Optical transmission system. An optical signal into which a signal for timing synchronization is inserted is converted into an optical signal having wavelengths λ1 and λ2 (λ1 ≠ λ2), wavelength-multiplexed, and the wavelength-multiplexed optical signal is branched and transmitted to a plurality of optical transmission lines. A signal transmitter;
Demultiplexing each of the wavelength-multiplexed optical signals transmitted through the plurality of optical transmission lines, coherently detecting the optical signal of the wavelength λ1 and the optical signal of the wavelength λ2, respectively, to convert into a plurality of electrical signals, An optical signal receiver that converts the plurality of electric signals into digital signals to perform waveform equalization, further synchronizes timing using the timing synchronization signal, and synthesizes and demodulates the signal ,
The optical signal receiver is configured to synthesize signals except for a signal whose average power of the wavelength-multiplexed optical signal transmitted through the plurality of optical transmission lines or the signal after the coherent detection is equal to or less than a threshold value. A characteristic optical transmission system. The optical signal transmitter converts an electrical signal into which a signal for timing synchronization is inserted into an optical signal, branches the optical signal, and sends it to a plurality of optical transmission lines.
The optical signal receiver performs coherent detection on each of the optical signals transmitted through the plurality of optical transmission paths to convert them into a plurality of electrical signals, and converts the plurality of electrical signals into digital signals for waveform equalization. Further, when synthesizing the signal by synchronizing the timing using the timing synchronization signal, the average power of the optical signal transmitted through the plurality of optical transmission lines or the signal after the coherent detection is An optical transmission method characterized in that signals are synthesized by removing signals below a threshold and demodulated. The optical signal transmitter converts the electrical signal into which the signal for timing synchronization is inserted into an optical signal having wavelengths λ1 and λ2 (λ1 ≠ λ2), and multiplexes the wavelength-multiplexed optical signal. Send it to the transmission line,
Optical signal receiver, the plurality of transmitted via the optical transmission path a said wavelength-multiplexed optical signals respectively demultiplexing the optical signal and the optical signal of the wavelength λ2 of the wavelength λ1 more and coherent detection respectively Converting the plurality of electrical signals into digital signals, performing waveform equalization, and further synthesizing the signals by synchronizing timing using the timing synchronization signals. An optical transmission method characterized in that signal synthesis is performed except for a signal whose average power of the wavelength-multiplexed optical signal or the signal after coherent detection transmitted through a path is not more than a threshold value , and demodulated.

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