JP5871642B2 - Coherent transmission system and method - Google Patents

Description

  The present invention relates to a coherent transmission system and method for performing coherent optical communication, and more particularly to a coherent transmission system and method having good reception characteristics.

  In recent years, research and development of a digital coherent fiber transmission system in which a digital signal processing technique is applied to a coherent transmission method has progressed, and a part of the system has been introduced. The current optical communication system is premised on the wavelength multiplexing transmission system, and the coherent transmission system is also premised on the wavelength multiplexing transmission. However, in selecting the desired signal light, it is larger than the conventional direct detection system. There are advantages.

  Originally, the coherent detection method combines a laser beam having a frequency very close to the carrier frequency of a desired optical signal from among a large number of optical signals simultaneously received by a receiving unit, and receives the electrical signal by a photodiode. In contrast to the direct detection method, optical components for wavelength selection for extracting only the desired optical signal are not required in principle. [Non-Patent Document 1].

  Therefore, for example, if a digital coherent method is introduced into a ROADM (Reconfigurable Optical Drop Drop Multiplexer) system, a WADM (Wavelength Selective Switch) or an AWG (Wavelength Selective Wave) or AWG (Wavelength Selective Wave) or AWG (Wavelength Selective Wave) or AWG (Wavelength Selective Wave) It is considered that a device such as a variable filter becomes unnecessary, and cost reduction is realized. In the case of the direct detection method, if the performance of such wavelength selection components is insufficient, noise due to crosstalk due to non-selected wavelengths (channels) may occur. There is no. However, it is assumed that the receiver band is sufficiently smaller than the channel spacing (typically 50 GHz).

  As described above, in the coherent transmission method, it is possible in principle to receive light of all channels at once without using an optical filter or the like.

S. Ryu, “Coherent Lightwave Communication Systems”, Arttech House, 1995, pp. 1-4. S. Ryu, “Coherent Lightwave Communication Systems”, Arttech House, 1995, pp. 117-122. M.M. Seimetz, “High-Order Modulation for Optical Fiber Transmission,” Springer, 2009, pp. 79-84. Optical Internetworking Forum, Implementation Agreement for Integrated Dual Polarization Intradyne Coherent Receivers, IA # OIF-DPC-RX-01.0. Kikuchi, “New development of coherent optical fiber communication,” Applied Physics, Vol. 78, No. 9, pp. 856-861, 2009. Onaka, “Latest Technical Trends of Phase Modulation,” FOE-3, Fiber Optics EXPO Technical Seminar, 9th Optical Communication Technology Exhibition (FOE), January 23, 2009. Senoo, et al., “A Wide Dispersion Band 50 GHz Spacing 88 Channel Individual Compensation Variable Dispersion Compensator,” IEICE Technical Report, OPE 2011-82 (2011-08). K. S. Kim and M.M. E. Lines, “Temperature dependency of chromatographic dispersion in dispersion-shifted fibers: Expert and Applied Physics, Vol. 73, no. 5, pp. 2069-2074, 1993.

  As described above, in the coherent transmission system, in general, crosstalk due to a non-selected wavelength, that is, a different wavelength has not been originally considered. However, such recognition is not based on the premise of using an optical amplifier. In an existing optical fiber transmission system, an optical signal passes through the optical amplifier, so that ASE (Amplified Spontaneous Emission) generated from the optical amplifier is reduced. It will mix. For this reason, ASE is included in all received optical signals regardless of selection or non-selection, and the beat frequencies of the non-selection optical signal and the ASE signal included therein fall within the band of the receiver. , It becomes noise (beat noise). Therefore, even in the case of coherent reception, in reality, the effect of the non-selected optical signal becomes a problem, and the noise increases as the power of the non-selected optical signal and the number of channels increase. Occurs. Further, if there is intensity fluctuation in the non-selected optical signal, intensity noise due to the intensity fluctuation may also occur.

  On the other hand, if the coherent receiver using the balanced photodiode proposed for canceling the intensity noise due to local light is used, not only the intensity noise of the non-selected optical signal but also the non-selected optical signal and the ASE included therein. Although it is possible to cancel the influence of the generated beat noise, an actual receiver has imperfections defined by CMRR (Common Mode Rejection Ratio) [Non-Patent Document 2], and intensity noise or It is considered difficult to completely cancel the influence of beat noise.

The above will be described by taking the case of a reception system of the polarization multiplexing QPSK modulation method as a specific example. Hereinafter, the display of the electric field or the like is referred to [Non-Patent Document 3]. FIG. 1 shows a model diagram of the receiving system. Let E _s (t) be the electric field of the selected optical signal, and let E _i (t) be the signal optical field of the other channel that is not selected. Since these pass through an optical amplifier, for example, an EDFA (Erbium-Doped Fiber Amplifier), an ASE electric field of n _s (t) and ni (t) is finally added as noise. Here, n _s (t) is the ASE electric field at the selected optical signal frequency, and n _i (t) is the ASE electric field at the non-selected channel frequency. Each signal is divided into an X polarization and a Y polarization by a PBS (Polarization Beam Splitter) and is incident on a 2 × 4 90 degree hybrid. Since the following description is not directly related to polarization, attention is paid only to X polarization. The relationship between input and output to the 90-degree hybrid is expressed by equations (1) to (4).

式（５）の第２項は、選択光信号に付随するＡＳＥ成分であり、第３項は、非選択チャネル信号および非選択チャネル周波数におけるＡＳＥ成分の和である。 Here, E _s (t) and E _lo (t) are a signal electric field and a local light emission electric field, respectively. E _out1 (t) and E _out3 (t) are I-phase component electric fields, and E _out2 (t) and E _out4 (t) are Q-phase component electric fields. Each coefficient is ideally ½, but actually deviates slightly from ½, so strictly speaking, the coefficients a _{s1 to} a _s4 , a _l1 to, as in (1) to (4). a ₁₄ is used. Hereinafter, E _s (t) is replaced as shown in Equation (5) because it is assumed that multi-wavelength optical signals are collectively received.

The second term of equation (5) is the ASE component associated with the selected optical signal, and the third term is the sum of the unselected channel signal and the ASE component at the unselected channel frequency.

The output from the 90-degree hybrid is received by each photodiode. At this time, the current flowing through the photodiode is given by (6) to (9). Here, R _{1 to} R ₄ are the sensitivity of each photodiode. Also _{i sh1, i sh2, i sh3} , i sh4 is the shot noise current.

Next, CMRR is defined as (10) to (13) with respect to the signal light and the local light for the I phase and the Q phase, respectively. Here, the absolute values of these values correspond to the usual definition of CMRR [Non-Patent Document 4], but the essence is not changed.

ここで、

は、それぞれ選択光信号とＡＳＥとのビート成分（雑音）、非選択光信号とＡＳＥとのビート成分（雑音）、選択光信号と局発光とのビート成分（信号成分）、局発光とＡＳＥとのビート成分（雑音）である。 As described above, the output of the balanced photodiode is expressed as (14) and (15) with respect to the I phase and the Q phase. However, the phase fluctuation of the laser beam and shot noise were ignored.

here,

Are the beat component (noise) of the selected light signal and ASE, the beat component (noise) of the unselected light signal and ASE, the beat component (signal component) of the selected light signal and local light, and the local light and ASE, respectively. Beat component (noise).

On the other hand, in the case of an ideal receiving system, each CMRR is 0 and is expressed as (16) and (17). The first term is the beat signal between the selection signal light and the local light, and the second term is the beat noise between the local light and the ASE light.

  When CMRR is 0, the signal-to-noise ratio depends only on the second term, but when CMRR cannot be ignored due to imperfections due to manufacturing errors in coherent receivers using balanced photodiodes, various noise currents exist As a result, the signal-to-noise ratio is degraded. However, in the actual manufacture of the receiver, it is inevitable that an error occurs, and CMRR has some value, so that it is difficult to completely cancel various noise currents.

  Looking at (14) and (15), the noise current caused by the electric field of the optical signal of the other channel occupies a large proportion among various noise currents. However, in coherent communication, the optical signal is generally phase-modulated, There is little change. Therefore, the noise current is essentially considered to be a direct current component, and its influence is considered to be easily excluded electrically. However, in reality, since the optical signal propagates through the optical fiber that is the dispersion medium, the phase-modulated optical signal is converted into intensity-modulated light. Eventually, the noise due to the optical signal of the other channel becomes intensity noise and is selected. The influence on the optical signal is difficult to eliminate. FIG. 2 shows an experimental result showing that the penalty due to the influence of the non-selection signal varies depending on the propagation distance in the single mode fiber, that is, the amount of chromatic dispersion experienced by the optical signal.

  The present invention solves such a problem, and provides a coherent optical transmission system having a good collective reception characteristic by reducing the influence of wavelength dispersion on an optical transmission line regardless of the use of a balanced photodiode. The purpose is to do.

The coherent optical transmission system according to the present invention is a coherent optical transmission system that collectively receives signal light of a plurality of channels transmitted from a transmission unit by a coherent reception unit using a balanced photodiode, and receives the signal from the transmission unit. by compensating for by that waveform distortion of the converted intensity modulation component into an intensity-modulated signal from the phase modulation signal I by the chromatic dispersion of the optical transmission path to parts per channel, the intensity modulation produced in the optical transmission line the signal light does not contain a component Ru is received by the receiving unit.

  The coherent optical transmission system according to the present invention further includes a dispersion value calculation unit that determines a dispersion value of the optical transmission path for each channel using the length of the optical transmission path, and the transmission unit includes all channels. A multi-channel individual variable dispersion compensator that receives wave signal light and compensates waveform distortion due to wavelength dispersion for each channel using the dispersion value of the optical transmission line determined by the dispersion value calculation unit; Also good.

  The coherent optical transmission system according to the present invention further includes a dispersion value estimation unit that calculates a wavelength dispersion estimation value of the optical transmission path for each channel using the signal light of each channel received by the reception unit, and the transmission unit Is a multi-channel in which signal light combined with all channels is input, and waveform distortion compensation due to chromatic dispersion is compensated for each channel using the chromatic dispersion estimated value of the optical transmission path calculated by the dispersion value estimating unit An individual variable dispersion compensator may be provided.

  The coherent optical transmission system according to the present invention further includes a dispersion value calculation unit that determines a dispersion value of the optical transmission path for each channel using the length of the optical transmission path, and the reception unit includes: A multi-channel individual tunable dispersion compensator that performs waveform distortion compensation for each channel by using the dispersion value of the optical transmission line determined by the dispersion value calculation unit, which is input with transmitted signal light of a plurality of channels. You may prepare.

  The coherent optical transmission system according to the present invention further includes a dispersion value estimation unit that calculates a wavelength dispersion estimation value of the optical transmission path for each channel using the signal light of each channel received by the reception unit, and the reception unit Is a multi-channel that receives a plurality of channels of signal light transmitted from the transmitter and compensates for waveform distortion due to wavelength dispersion for each channel using the dispersion value of the optical transmission path calculated by the dispersion value estimator. An individual variable dispersion compensator may be provided.

The coherent optical transmission method according to the present invention is a coherent optical transmission method in which a plurality of channels of signal light transmitted from a transmission unit are collectively received by a coherent reception unit using a balanced photodiode, wherein the reception light is transmitted from the transmission unit. by compensating for by that waveform distortion of the converted intensity modulation component into an intensity-modulated signal from the phase modulation signal I by the chromatic dispersion of the optical transmission path to parts per channel, the intensity modulation produced in the optical transmission line a receiving procedure Ru is receiving the signal light does not contain a component in the receiving unit.

  According to the present invention, in a coherent receiver using a balanced photodiode, even when the CMRR is large or a coherent receiver that does not use a balanced photodiode, The influence of intensity noise caused by chromatic dispersion in the optical transmission line can be reduced, and a coherent optical transmission system having good reception characteristics can be realized.

It is explanatory drawing of the model of the coherent receiving system of a polarization multiplexing QPSK modulation system using a balance type photodiode. This is the fiber length (chromatic dispersion amount) dependence of the Q value when propagating through a 1.3 μm zero-dispersion single mode fiber. It is explanatory drawing of the structural example of the conventional coherent transmission system. It is explanatory drawing of a structure of the receiver of a polarization multiplexing QPSK modulation system. It is explanatory drawing of the structural example of the coherent transmission system which has arrange | positioned the multichannel separate variable dispersion compensator in the transmission part. It is explanatory drawing of the structural example of the coherent transmission system which has arrange | positioned the multi-channel individual variable dispersion compensator in the receiving part. It is explanatory drawing of the structural example of a network management system.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

  An example of a conventional coherent optical transmission system is shown in FIG. In the coherent optical transmission system, a transmission unit 1 and a reception unit 2 are connected by an optical transmission path 3, and a plurality of channels of optical signals transmitted from the transmission unit 1 are received by the reception unit 2 using a coherent transmission method.

  The transmission unit 1 includes a plurality of PDM-QPSK (Polarization Division Multiplexed Quadrature Phase Shift Keying) transmitters 11, a multiplexer 12, a WSS (Wavelength Select Switch) 13, an ed i Prepare. The PDM-QPSK transmitter 11 generates an optical signal for each channel using a PDM-QPSK code. The multiplexer 12 multiplexes the optical signals from all the PDM-QPSK transmitters 11. The WSS 13 receives optical signals from a plurality of multiplexers (not shown) including the multiplexer 12, and selects and outputs the optical signals from at least one multiplexer. The EDFA 14 amplifies the optical signal from the WSS 13.

  The receiving unit 2 includes an EDFA 21, a coupler 22, and a PDM-QPSK receiver 23. The EDFA 21 amplifies the optical signal from the optical transmission path 3. The coupler 22 branches the optical signal amplified by the EDFA 21. The PDM-QPSK receiver 23 receives the optical signal of each channel branched by the coupler 22 using the polarization multiplexing QPSK modulation method.

  FIG. 4 shows a schematic diagram of the configuration of the PDM-QPSK receiver 23. The PDM-QPSK receiver includes a light receiving circuit 231 and a signal processing unit 232. The light receiving circuit 231 mixes the signal light and the local light, and converts the mixed light whose phase difference between the signal light and the local light is 0 °, 90 ° mixed light, 180 ° mixed light, and 270 ° mixed light to the X-polarized light. The light is separated for each wave and Y polarization. The signal processing unit 232 demodulates the optical signal of each channel using the signal from the light receiving circuit 231 and reproduces transmission data.

  Originally, in the coherent optical transmission system, chromatic dispersion is compensated by digital signal processing in the receiving unit 2 and chromatic dispersion compensation means is unnecessary [Non-Patent Document 5] [Non-Patent Document 6], but balanced photo Regardless of whether or not diodes are used, in order to reduce the effects of chromatic dispersion on the optical transmission line and to have good collective reception characteristics, the chromatic dispersion of the optical transmission line in the optical path of the optical signal of each channel must be compensated. That's fine. Therefore, in the coherent optical transmission system and the coherent optical transmission method according to the present embodiment, a dispersion compensator is disposed in the previous stage of receiving light in the receiving unit 2 so that the optical signal input to the receiving unit 2 does not include an intensity modulation component. Alternatively, a dispersion compensator for pre-equalizing the signal input to the optical transmission line 3 is arranged.

  Furthermore, the optical signal of each channel passes through various optical paths, and the amount of dispersion to be compensated differs for each channel. If a failure or the like occurs, it is necessary to immediately delete the operating optical path and set a new optical path, and the amount of dispersion to be compensated changes. Therefore, in order to perform dispersion compensation individually and dynamically, a multi-channel individual variable dispersion compensator is arranged immediately after the multiplexing of all channels in the transmission unit 1 or a multi-channel individual variable in the reception unit 2 immediately before the receiver. A dispersion compensator may be disposed. That is, in a transmission procedure in which the transmission unit 1 transmits signal light of a plurality of channels or a reception procedure in which the reception unit 2 receives signal light of a plurality of channels, waveform distortion due to wavelength dispersion of the optical transmission path 3 is determined for each channel. Compensation is sufficient.

  FIG. 5 shows an example of a coherent optical transmission system according to the present embodiment. In the transmission unit 1, a multi-channel individual variable dispersion compensator (indicated as TDC (Tunable Dispersion Compensator) in the figure) 50 is connected between the multiplexer 12 and the WSS 13, and the multi-channel individual variable dispersion compensator is immediately after combining all the channels. A dispersion compensator 50 is provided. The multi-channel individual tunable dispersion compensator 50 performs waveform distortion compensation due to chromatic dispersion for each channel. As the multi-channel individual variable dispersion compensator 50, there is a combination of circular AWG (Arrayed Waveguide Grating), a bulk diffraction grating, and LCOS (Liquid Crystal On Silicon) [Non-Patent Document 7].

  FIG. 6 shows a case where the multi-channel individual variable dispersion compensator 50 is arranged immediately before the PDM-QPSK receiver 23. The multi-channel individual tunable dispersion compensator 50 receives the optical signal from the coupler 22, performs waveform distortion compensation due to chromatic dispersion for each channel, and outputs it to the PDM-QPSK receiver 23.

  When setting the dispersion value of each channel in the multi-channel individual variable dispersion compensator 50, information on the dispersion value of the optical transmission line 3 is necessary. There are several methods. The first method is a method of determining the dispersion value of the optical transmission line 3 from the length of the optical path and reflecting the determined value in the dispersion setting value of the multi-channel individual variable dispersion compensator 50. Is a method of reflecting the dispersion estimated value obtained by digital signal processing in the dispersion estimated value of the multi-channel individual variable dispersion compensator 50.

  In the case of the first method, for example, a network management system shown in FIG. 7 is used. The network management system shown in FIG. 7 includes a network management unit 42 connected to each node 41 constituting the network, and the network management unit 42 forms a link together with the device configuration and link information of each node 41. The length of the optical transmission line 3 between 41 is managed. The node 41 has the functions of the transmission unit 1 and the reception unit 2 according to this embodiment. The network management unit 42 sets the optical path in each node 41 by sending a command for notifying the length of the optical transmission line 3 to the multi-channel individual variable dispersion compensator 50 of the transmission unit 1 or the reception unit 2. In this case, a dispersion value calculation unit (not shown) provided in the transmission unit 1 or the reception unit 2 uses the length of the optical transmission line 3 received from the network management system to calculate the chromatic dispersion value of the optical transmission line 3 for each channel. The dispersion value of each channel is set in the multi-channel individual variable dispersion compensator 50.

  In the case of the second method, for example, when the multi-channel individual variable dispersion compensator 50 is in the receiving unit 2, dispersion estimation value information may be transmitted to the multi-channel individual dispersion compensator 50 through communication within the apparatus. When the multi-channel individual variable dispersion compensator 50 is in the transmission unit 1, the dispersion information is sent from the reception unit 2 to the transmission unit 1 via another path, or the information is once sent to the network management system, and the transmission unit 1 The dispersion value information may be sent to. Thereby, the transmission unit 1 can set the dispersion value of each channel in the multi-channel individual variable dispersion compensator 50.

  In the second method, the reception unit 2 includes a dispersion value estimation unit (not shown) that calculates the chromatic dispersion estimation value of the optical transmission path 3 for each channel using the received optical signal of each channel. For example, the dispersion value is estimated by adding a reverse characteristic of the assumed chromatic dispersion characteristic to a predetermined signal for each channel, and transmitting from the transmission unit 1. The dispersion value estimation unit of the reception unit 2 estimates the chromatic dispersion value of the optical transmission line 3 for each channel by comparing the received signal waveform with a predetermined signal waveform.

  Also, in order to cope with environmental fluctuations, particularly dispersion fluctuations of the optical transmission line 3 due to temperature fluctuations shown in [Non-Patent Document 8], distributed information is appropriately acquired from the network management system during operation, and multi-channel If the dispersion setting value of the individual variable dispersion compensator 50 is updated, the transmission quality can always be kept high.

  The present invention is applied to a wavelength division multiplexing optical transmission system, and is particularly useful for a wavelength division multiplexing optical network having an optical add / drop function.

1: transmitter 2: receiver 3: optical transmission line 11: PDM-QPSK transmitter 12: multiplexer 13: WSS
14: EDFA
21: EDFA
22: Coupler 23: PDM-QPSK receiver 231: Light receiving circuit 232: Signal processing unit 31: Fiber 32: EDFA
41: Node 42: Network management unit 50: TDC

Claims (6)

A coherent optical transmission system that collectively receives signal light of a plurality of channels transmitted from a transmitter by a coherent receiver using a balanced photodiode,
By compensating for by that waveform distortion of the converted intensity modulation component into an intensity-modulated signal from the phase modulation signal I by the chromatic dispersion of the optical transmission path from the transmitting unit to the receiving unit for each channel, the optical transmission coherent optical transmission system characterized in Rukoto is received resulting signal lights including no intensity modulation component in the road to the receiver. A dispersion value calculation unit that determines a dispersion value of the optical transmission line for each channel using the length of the optical transmission line;
The transmission unit receives signal light combined with all channels, and uses the dispersion value of the optical transmission path determined by the dispersion value calculation unit to perform waveform distortion compensation by wavelength dispersion for each channel. The coherent optical transmission system according to claim 1, further comprising a channel individual variable dispersion compensator. A dispersion value estimation unit that calculates a chromatic dispersion estimation value of the optical transmission line for each channel using the signal light of each channel received by the reception unit;
The transmission unit receives signal light obtained by combining all channels, and uses the chromatic dispersion estimation value of the optical transmission path calculated by the dispersion value estimation unit to compensate for waveform distortion due to chromatic dispersion for each channel. The coherent optical transmission system according to claim 1, further comprising a multi-channel individual variable dispersion compensator for performing the operation. A dispersion value calculation unit that determines a dispersion value of the optical transmission line for each channel using the length of the optical transmission line;
The receiving unit receives a plurality of channels of signal light transmitted from the transmitting unit, and uses the dispersion value of the optical transmission path determined by the dispersion value calculating unit to compensate for waveform distortion due to wavelength dispersion for each channel. The coherent optical transmission system according to claim 1, further comprising a multi-channel individual variable dispersion compensator for performing the operation. A dispersion value estimation unit that calculates a chromatic dispersion estimation value of the optical transmission line for each channel using the signal light of each channel received by the reception unit;
The reception unit receives a plurality of channels of signal light transmitted from the transmission unit, and compensates for waveform distortion due to wavelength dispersion for each channel using the dispersion value of the optical transmission path calculated by the dispersion value estimation unit. The coherent optical transmission system according to claim 1, further comprising a multi-channel individual variable dispersion compensator for performing the operation. A coherent optical transmission method for collectively receiving signal light of a plurality of channels transmitted from a transmission unit with a coherent reception unit using a balanced photodiode,
By compensating for by that waveform distortion of the converted intensity modulation component into an intensity-modulated signal from the phase modulation signal I by the chromatic dispersion of the optical transmission path from the transmitting unit to the receiving unit for each channel, the optical transmission coherent optical transmission method characterized by having a receiving procedure signal light which does not include the resulting intensity modulation component Ru is received by the receiving unit in the road.

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