JP6434712B2 - Optical communication system - Google Patents

Description

The present invention relates to an optical communication system in which a plurality of services are operated. In particular, the present invention relates to an optical communication system that can communicate even when OBI occurs .

  In the CATV system, an FTTH system using an optical fiber cable as a transmission path from a center to a subscriber is known. In particular, the FTTH CATV system constructed using RFTS-ONU (Radio Frequency over Glass Optical Network Unit) at the subscriber's house using CMTS and CM used in the existing CATV system as they are is called RFoG. is called.

  When operating a plurality of services using a plurality of CMTSs in the RFoG system, communication is controlled using different CMTSs for different services, and communication is not controlled between different services. For this reason, uplink signals of different services may be transmitted at the same time. If the upstream optical signals transmitted and overlapped at the same time are simultaneously input to the O / E converter on the center side, noise is generated according to the frequency difference between the two upstream optical signals, and the communication quality is deteriorated. Communication is interrupted. This noise is called optical beat noise (OBI; Optical Beat Interference).

  The occurrence of OBI becomes a problem particularly in telephone services. In data communication services and the like, it is possible to avoid communication failures even if OBI occurs by using techniques such as retransmission, but since such avoidance techniques cannot be used in telephone services, voice is interrupted due to the occurrence of OBI. The call is interrupted.

  As a technique for solving the problem of OBI, there is Patent Document 1. In Patent Document 1, by increasing the modulation degree of the upstream electrical signal and using a laser having a wide spectrum width as the laser in the R / ONU E / O converter, deterioration of the CN ratio is suppressed even if OBI occurs. Thus, it is possible to perform uplink communication normally.

JP 2012-169776 A

  However, in the method of Patent Document 1, when the transmission distance from the E / O converter of each R-ONU on the subscriber side to the O / E converter on the center side is long, second order distortion occurs and the signal There was a possibility that the quality would deteriorate. In the method of Patent Document 1, it is necessary to take the above measures for each R-ONU of each subscriber. When the number of subscribers is large, the number of R-ONUs increases accordingly, and it is difficult to reduce costs. The point was also a problem.

  Accordingly, an object of the present invention is to achieve higher communication quality and lower costs in an optical communication system that can communicate even when OBI occurs.

The present invention provides a plurality of CMTSs arranged on the center side, CMs arranged at subscriber's homes, and E / O conversions that convert the downlink electrical signals from the CMTSs and the CMTSs into downlink optical signals and output them. And an RFoG-ONU that is disposed at a subscriber's home, converts a downstream optical signal from the E / O converter into a downstream electrical signal, converts an upstream electrical signal from the CM into an upstream optical signal, and outputs the upstream optical signal, and RFoG -O / E converters that convert upstream optical signals from ONUs into upstream electrical signals and output to each CMTS, and downstream optical signals that are input from the E / O converters via optical transmission paths A wavelength multiplexing device that outputs to the RFoG-ONU via the path and outputs an upstream optical signal input from the RFoG-ONU via the optical transmission path to the O / E converter; And each CMTS is from an unsupported CM In an optical communication system in which the timing of uplink communication is not controlled, an optical transmission path between the wavelength multiplexing device and the O / E converter, and in the vicinity of the O / E converter , a polarization that makes the upstream optical signal non-polarized. An optical communication system is characterized in that a wave scrambler is inserted.

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  According to the present invention, since the upstream optical signal is made non-polarized and then converted to the upstream electrical signal by the O / E converter, the occurrence of OBI is suppressed, the deterioration of C / N is improved, and normal communication is performed. It can be performed. In addition, the number of polarization scramblers corresponding to the number of O / E converters on the center side may be provided, which is low in cost. Therefore, the present invention is suitable for an optical communication system that operates a plurality of communication services including a telephone service.

1 is a diagram illustrating a configuration of an RFoG system according to Embodiment 1. FIG. The figure which showed the structure of the spectrum width expansion apparatus 103. FIG. The figure explaining the relationship between the peak value of a frequency spectrum and a spectrum width. The graph which showed (alpha) and (xi) dependence of CN ratio.

  Specific examples of the present invention will be described below, but the present invention is not limited to the examples.

  FIG. 1 is a diagram illustrating a configuration of the RFoG system according to the first embodiment. As shown in FIG. 1, the RFoG system according to the first embodiment includes three CMTS (Cable Modem Termination System) 10A to 10C and an optical transceiver 11 arranged on the center side, and an RFoG-ONU 13 arranged on the subscriber side. And CMs (Cable Modem) 14A to C, an optical fiber cable 15 that connects the optical transceiver 11 and the RFoG-ONU 13, and an optical coupler 12 that branches the optical fiber cable 15.

  Hereinafter, each component of the RFoG system according to the first embodiment will be described in detail, and the operation thereof will also be described.

  The CMTSs 10 </ b> A to 10 </ b> C are arranged on the center side, and the downstream electrical signal output side of the CMTSs 10 </ b> A to 10 </ b> C is connected to the optical transceiver 11 via the multiplexer 16. The upstream electrical signal input side of the CMTSs 10 </ b> A to 10 </ b> C is connected to the optical transceiver 11 via the duplexer 18. The CMTSs 10A to 10C input and output RF band electrical signals and communicate with the CMs 14A to 14C on the subscriber side. This communication follows the DOCSIS protocol. Details will be described later. The electric signals output from the CMTSs 10A to 10C have different bands and perform three different communication services. For example, a data communication service such as the Internet, a VOD (Video On Demand) service, and a telephone service. Downstream electrical signals from the CMTSs 10 </ b> A to 10 </ b> C are frequency-multiplexed by the multiplexer 16 and input to the optical transceiver 11.

  The optical transceiver 11 includes an E / O converter 100 and an O / E converter 105 as shown in FIG. The downstream electrical signals input from the CMTSs 10A to 10C are converted into downstream optical signals having a predetermined wavelength by the E / O converter 100 and output. Then, after being branched by an optical coupler (not shown), amplified by an EDFA (erbium doped fiber amplifier) 101 and further branched by an optical coupler 12. The optical fiber cable 15 branched by the optical coupler 12 is connected to each subscriber's RFoG-ONU 13. The RFoG-ONU 13 is connected to the duplexer 17 via a coaxial cable, and the duplexer 17 is connected to each CM 14A to 14C. The RFoG-ONU 13 converts the downstream optical signal from the center side into a downstream electrical signal and outputs it to each CM 14A-C, and converts the upstream electrical signal from each CM 14A-C into an upstream optical signal and outputs it.

  A WDM device 102 is inserted into the optical fiber cable 15 between the EDFA 101 and each optical coupler 12. The WDM device 102 transmits the downstream optical signal from the EDFA 101 to the optical coupler 12 as it is, and the upstream optical signal from the optical coupler 12 is separated by the difference in wavelength from the downstream optical signal. The separated upstream optical signal is input to the spectral width expanding device 103 via the optical fiber cable 15.

  The spectrum width enlarging device 103 expands and outputs the spectrum width of the input upstream optical signal (the half-value width of the peak of the frequency spectrum, the same applies hereinafter). The spectral width expanding device 103 is inserted into the optical fiber cable 15 in the vicinity of the O / E converter 105.

  As shown in FIG. 2, the spectral width expanding device 103 includes a circulator 1030 and a CFBG (chirped fiber Bragg grating) 1031. The circulator 1030 has three ports, the first to third ports, outputs an optical signal input to the first port only from the second port, and outputs an optical signal input to the second port to the third port. Output only from. The first port of the circulator 1030 is connected to the optical fiber cable 15 on the subscriber side, and the upstream optical signal is input to the first port. A CFBG 1031 is connected to the second port. The third port is connected to the optical fiber cable 15 on the O / E converter 105 side, and the upstream optical signal reflected by the CFBG 1031 and input to the second port is output from the third port, The signal is input to the polarization scrambler 104 via the cable 15.

  The CFBG 1031 is a fiber whose core has a grating structure, and the grating period is different between the long wavelength side and the short wavelength side. Specifically, the grating period on one end side increases as it goes toward the other end side. The end of the CFBG 1031 with the smaller grating period is connected to the second port of the circulator 1030, and the upstream optical signal input from the second port to the CFBG 1031 is reflected from the short wavelength side toward the long wavelength side. Is reflected in the back to compress the pulse width, thereby expanding the spectral width of the upstream optical signal input from the CFBG 1031 to the second port.

  As the circulator 1030, those having various configurations known in the art can be used. For example, a circulator 1030 having a configuration using a Faraday rotator can be employed. However, those composed of passive elements are preferable to active elements. This is because it is advantageous in terms of maintainability.

  Note that the spectral width expanding apparatus 103 is not limited to the one described above, and an apparatus having an arbitrary configuration that expands the spectral width by chromatic dispersion can be used. For example, the spectral width can be expanded using a long wavelength dispersion fiber. However, it is preferable that the entire spectrum width expanding apparatus 103 is composed of passive elements in terms of maintainability. In particular, the spectral width expanding device 103 configured by the circulator 1030 and the CFBG 1031 used in the first embodiment can be configured by passive elements, and is advantageous in that it can be downsized. A device having polarization dependency may be used as the spectral width expanding device 103. However, the smaller the polarization coupling coefficient ξ (the coefficient indicating the coupling of the polarization planes of the two upstream optical signals), that is, the non-polarized light. Since the CN ratio deterioration due to OBI can be suppressed as close as possible to the (non-polarized) state, it is desirable to configure the spectral width expanding device 103 using elements that have no or little polarization dependency.

  The polarization scrambler 104 is inserted into the optical fiber cable 15 between the O / E converter 105 and the spectral width expanding device 103. The polarization scrambler 104 makes the polarization state of the input upstream optical signal non-polarized (non-polarized).

  The polarization scrambler 104 may be of any structure that can make the polarization state of the incoming upstream optical signal non-polarized. For example, a method of using a ferroelectric material such as lithium niobate as an optical waveguide and scrambling the polarization direction by controlling the voltage applied to the ferroelectric material can be used. The polarization scrambler 104 is preferably composed of a passive element from the viewpoint of maintainability.

  In the RFoG system according to the first embodiment, the polarization scrambler 104 and the spectrum width expanding device 103 are sequentially connected from the O / E converter 105 side. However, the reverse order, that is, the O / E converter is used. A configuration may be adopted in which the spectrum width expanding device 103 and the polarization scrambler 104 are connected in this order from the 105 side. In the case where a device having polarization dependency is used as the spectrum width expanding device 103, if the spectrum width is first expanded by the spectrum width expanding device 103 and then non-polarized by the polarization scrambler 104, CN by OBI is efficiently performed. The deterioration of the ratio can be suppressed.

  Further, in the RFoG system of the first embodiment, both the spectrum width expanding device 103 and the polarization scrambler 104 are used, but only the polarization scrambler 104 is used without using the spectrum width expanding device 103. Good.

  In addition, one or both of the spectral width expanding device 103 and the polarization scrambler 104 may be integrated into the optical transceiver 11 or may be configured separately.

  The input side of the O / E converter 105 is connected to the output side of the polarization scrambler 104. Further, the number of O / E converters 105 corresponding to the number of branches of the optical fiber cable 15 by the EDFA 101 is arranged. Thus, by converting the upstream optical signal to the upstream electrical signal using the plurality of O / E converters 105, the number of subordinate CMs 14 per one O / E converter 105 is reduced, and the upstream optical signal The probability of a collision is reduced, and as a result, the occurrence of OBI can be reduced. However, when the CN ratio deterioration due to OBI can be sufficiently suppressed by the spectral width expanding apparatus 103 and the polarization scrambler 104 of the present invention, the optical fiber cable 15 is joined by the optical coupler 12 and the O / E converter 105 The number may be reduced. In particular, when the CN ratio deterioration can be sufficiently suppressed, one O / E converter 105 may be provided. The O / E converter 105 converts the upstream optical signal from the polarization scrambler 104 into an upstream electrical signal and outputs it. Then, the output upstream electrical signal is demultiplexed by the demultiplexer 18 into upstream electrical signals having frequencies corresponding to the CMTSs 10A to 10C, and input to the CMTSs 10A to 10C.

  The number of spectrum width expanding devices 103 and polarization scramblers 104 is the same as the number of O / E converters 105. Therefore, the number of units is smaller and the cost is lower than the case where the OBI countermeasures are taken for each subscriber's RFoG-ONU 13.

  Next, the operation of the RFoG system of Example 1 will be described.

  In the RFoG system of the first embodiment, different communication services are operated for each of the CMTSs 10A to 10C, between the CMTS 10A and the plurality of CMs 14A, between the CMTS 10B and the plurality of CMs 14B, and between the CMTS 10C and the plurality of CMs 14C. In this case, the communication is controlled independently according to the DOCSIS protocol. Therefore, a plurality of CMs 14 under the same CMTS 10 (for example, a plurality of CMs 14A) are uplink-communicated in a time-sharing manner and controlled so that communication timing does not overlap.

  On the other hand, the communication timing is not controlled between CMs 14 under different CMTSs 10. Therefore, there is a possibility that an upstream optical signal from one subscriber collides with an upstream optical signal from another subscriber. For example, since CMTS 10A does not control the communication timing of CM 14B, and CMTS 10B does not control the communication timing of CM 14A, uplink communication collides between CM 14A of one subscriber and CM 14B of another subscriber. there is a possibility.

  When the collided upstream optical signal is converted into an upstream electrical signal by the O / E converter 105, OBI is generated according to the frequency difference between the two upstream optical signals. OBI is a wide band noise centered on the peak of the frequency difference between two upstream optical signals. OBI does not occur when the frequencies of the two upstream optical signals completely match, and OBI does not affect the communication quality when the frequencies are sufficiently separated. However, the wavelength of the laser used to generate the upstream optical signal in the RFoG-ONU 13 has a wavelength variation of about 2 nm due to environmental factors such as product error, temperature, signal data amount, and the like. For this reason, the frequency of the upstream optical signal of one RFoG-ONU 13 and the frequency of another RFoG-ONU 13 are different from each other to the extent that OBI occurs and affects the communication quality.

  When OBI occurs, the CN ratio deteriorates and communication failure occurs, and communication is interrupted in some cases. In the case of the RFoG system according to the first embodiment, two or three upstream optical signals collide with each other and may interfere to generate OBI.

ここで、ξは偏波結合係数、αは変調前後での上り光信号のスペクトルピークの下げ幅、Δνは変調前のレーザー光のスペクトル幅である。偏波結合係数ξは、上り光信号の偏波面のカップリングを示す係数であり、０から１までの値をとる。２つの偏波面が直交している場合は０、一致している場合には１である。この式（１）から、ＲＩＮ_beatはピークの下げ幅αと、偏波結合係数ξとに依存していることがわかる。 It is described in Non-Patent Document 1 that, when three upstream optical signals interfere with each other and OBI occurs, the peak value of RIN _beat and RIN _beat due to OBI are expressed by the following equation (1).

Here, ξ is the polarization coupling coefficient, α is the width of the spectrum peak of the upstream optical signal before and after modulation, and Δν is the spectrum width of the laser light before modulation. The polarization coupling coefficient ξ is a coefficient indicating the coupling of the polarization plane of the upstream optical signal, and takes a value from 0 to 1. It is 0 when the two planes of polarization are orthogonal, and 1 when they are coincident. From this equation (1), it can be seen that RIN _beat depends on the peak reduction width α and the polarization coupling coefficient ξ.

Non-Patent Document 1 describes that CNR is inversely proportional to RIN _beat when noise due to OBI is sufficiently larger than shot noise and thermal noise. Therefore, it can be seen that if the peak reduction width α is increased or the polarization coupling coefficient ξ is decreased, the value of RIN _beat is decreased and the CN ratio can be improved.

In order to increase the peak reduction width α, it is conceivable to increase the modulation degree of intensity modulation of laser light in each RFoG-ONU 13. However, in this method, it is necessary to control the degree of modulation for each RFoG-ONU 13, resulting in high cost. If the modulation degree exceeds 1, distortion occurs and communication quality deteriorates. Therefore, there is a limit in reducing the RIN _beat value in changing the modulation degree.

  In order to reduce the polarization coupling coefficient ξ, it is necessary to control the polarization state of each upstream optical signal before the collision. For example, it is necessary to control the polarization directions of two upstream optical signals to be orthogonal. However, such control is not realistic. First, since the polarization direction of the upstream optical signal changes randomly in time depending on conditions such as temperature and transmission distance, it is easy to control the polarization direction without significantly changing the intensity of the upstream optical signal. Not. Second, since the timing of uplink communication is not controlled between a CM 14 under one CMTS 10 and another CM 14 under another CMTS 10, it is impossible to predict which CM 14 will collide with an upstream optical signal. It is practically impossible to control the polarization directions of the two upstream optical signals to be orthogonal.

  Therefore, in the RFoG system of Example 1, the CN ratio deterioration due to OBI is suppressed by the following two methods.

First, the spectral width expanding device 103 is provided in the vicinity of the optical signal input side of the O / E converter 105 so that the spectral width is expanded before the upstream optical signal is input to the O / E converter 105. As shown in FIG. 3, since the spectrum width is expanded while maintaining the overall output, the peak value of the spectrum decreases. In this way, by expanding the spectrum width by the spectrum width expanding device 103, in addition to the peak value decrease due to modulation, the peak value is further decreased and the value of RIN _beat is decreased.

  Further, in the RFoG system of the first embodiment, the spectrum width expanding device 103 is disposed in the vicinity of the O / E converter 105. Therefore, the transmission distance by the optical fiber cable 15 from the O / E converter 105 to the spectrum width expanding device 103 can be short. That is, the secondary distortion generated in the upstream optical signal is small until the upstream optical signal output from the spectral width expanding apparatus 103 is input to the O / E converter 105. The deterioration of communication quality due to the provision is suppressed.

  Note that the above-mentioned second-order distortion can ignore the influence on the communication quality if the transmission distance is within 100 m. Therefore, the transmission distance from the O / E converter 105 to the spectrum width expanding device 103 is within 100 m. It is desirable to arrange so that.

  Further, the limit value of CNR capable of error correction is approximately 12 dB. Therefore, when ξ is 1.0, from FIG. 4, the peak reduction width α (a value obtained by combining the spectral peak reduction width due to modulation and the spectral peak reduction width associated with the spectral width expansion by the spectral width expansion device 103 is obtained. ) Needs to be 15 dB or more. The larger the value of α is, the higher the value of 15 dB, the better the CNR can be secured. However, in consideration of the CNR limit value of 16 dB that does not require error correction, when α is 0.5, When 9 is 13 to 13 dB and ξ is 1.0, and α is 15 to 19 dB, sufficient performance can be secured.

  As described above, in the RFoG system according to the first embodiment, since the spectrum width expanding device 103 is provided, the peak reduction width α can be increased without changing the modulation degree of the upstream optical signal. The CN ratio deterioration due to OBI can be suppressed, and good uplink communication can be realized.

Second, a polarization scrambler 104 is provided, and the upstream optical signal is made unpolarized before being input to the O / E converter 105. Thereby, the direction of the polarization plane of the upstream optical signal changes randomly in time, and the angle formed by the polarization plane of the other upstream optical signal with respect to the polarization plane of one upstream optical signal also changes randomly. Therefore, the polarization coupling coefficient ξ is approximately 0.5 on a time average, and the polarization coupling coefficient ξ can be reduced on average compared to the case where no polarization is made by the polarization scrambler 104, and RIN The value of _beat can be reduced.

  As described above, in the RFoG system of the first embodiment, the polarization coupling coefficient ξ can be easily reduced by providing the polarization scrambler 104 and making the upstream optical signal non-polarized. Deterioration can be suppressed and good uplink communication can be realized.

FIG. 4 is a graph showing a result of verifying the dependency of the peak reduction width α and the polarization coupling coefficient ξ on the CN ratio deterioration due to OBI. In the graph of FIG. 4, the horizontal axis is α (dB) and the vertical axis is CNR (dB). The polarization coupling coefficient ξ is set to four types of 0.16, 0.3, 0.5, and 1. The result of FIG. 4 is a case where OBI is generated by transmitting two upstream optical signals simultaneously. The modulation method of the upstream electrical signal is QPSK, and the modulation degree of the upstream optical signal is 0.3. When the modulation method is QPSK, the error-correctable bit error rate is 10 ⁻⁴ (corresponding to 12 dB in CNR). If the CNR is 16 dB or more, transmission without error correction is possible.

  As shown in FIG. 4, it can be seen that the CNR gradually increases as the peak reduction width α increases. When α is increased by the spectrum broadening device 103, the CNR can be improved by about 1.0 to 1.1 dB by increasing α by 1 dB.

  It can also be seen that the CNR increases even if the polarization coupling coefficient ξ is reduced. Since the two upstream optical signals are not polarized by the polarization scrambler 104, the polarization coupling coefficient ξ is 0.5 on average. From FIG. 4, it is possible to improve the CNR by about 6 dB as compared with the case where the polarization coupling coefficient ξ is 1 (when the polarization planes of the two upstream optical signals coincide with each other) by making no polarization.

  In order to ensure sufficient communication quality, the CNR is desirably 16 dB or more. To that end, from FIG. 4, when the polarization coupling coefficient ξ is 0.16, α is 3 dB or more, when ξ is 0.3, α is 9 dB or more, and when ξ is 0.5, α Is 13 dB or more and ξ is 1, if α is 20 dB or more, the CNR may be 16 dB or more.

  As in the first embodiment, when both the spectral width expanding device 103 and the polarization scrambler 104 are used, the polarization coupling coefficient ξ is averaged by depolarizing the two upstream optical signals by the polarization scrambler 104. Therefore, in order to set the CNR to 16 dB or more, the spectral width expanding device 103 may set α to 13 dB or more. In addition, when only the spectral width expanding device 103 is used and the polarization scrambler 104 is not used, the polarization coupling coefficient ξ may be 1, so that α is preferably 20 dB or more.

  As described above, in the RFoG system according to the first embodiment, the spectrum width expansion device 103 is provided to increase the spectrum width of the upstream optical signal, and the upstream optical signal is made non-polarized by the polarization scrambler 104. Even so, the deterioration of the CN ratio is suppressed, and the communication state can be maintained well. Further, since the spectrum width expanding device 103 and the polarization scrambler 104 are arranged in the vicinity of the O / E converter 105, deterioration of communication quality is suppressed, and the spectrum width expanding device 103 and the polarization scrambler 104 are The introduction cost is low because the number is small.

  In the first embodiment, an example is shown in which three services are operated in the RFoG system. However, the present invention is applicable to the case where two or more services are operated. In particular, the RFoG system of the present invention is suitable when a telephone service is included as one of a plurality of services. In data communication services, etc., if a communication error occurs, it can be avoided by techniques such as retransmission, but such a technique cannot be used in the telephone service, and communication errors will interrupt the call. means. In the RFoG system of the present invention, by improving the CN ratio, it is possible to greatly reduce the possibility of occurrence of a call interruption due to a communication error in a telephone service.

  The RFoG system of the first embodiment has a double star network topology in which a plurality of optical fiber cables 15 extending from the center are further branched by the optical coupler 12, but may be a star, tree, ring, or other network topology. Good.

  Further, in the RFoG system of the first embodiment, one spectrum width expanding device 103 and one polarization scrambler 104 are provided, but a configuration in which a plurality of spectrum width expanding devices 103 are connected in series may be used. Alternatively, a configuration may be adopted in which the polarization scramblers 104 are connected in series.

  The present invention can be applied to an RFoG system that operates a plurality of services such as telephone service and data communication.

10A-C: CMTS
11: Optical transceiver 12: Optical coupler 13: RFoG-ONU
14: CM
15: optical fiber cable 16: multiplexer 17, 18: demultiplexer 100: E / O converter 101: EDFA
102: WDM apparatus 103: Spectrum width expansion apparatus 104: Polarization scrambler 105: O / E converter 1030: Circulator 1031: CFBG

Claims (2)

A plurality of CMTSs arranged on the center side, CMs arranged at the subscriber's homes, CMs corresponding to the CMTSs, and E / O converters that convert downstream electrical signals from the CMTSs into downstream optical signals and output them An RFoG-ONU that is arranged at a subscriber's home, converts a downstream optical signal from the E / O converter into a downstream electrical signal, converts an upstream electrical signal from the CM into an upstream optical signal, and outputs the upstream optical signal; An O / E converter that converts an upstream optical signal from the RFoG-ONU into an upstream electrical signal and outputs it to each CMTS, and a downstream optical signal that is input from the E / O converter via an optical transmission line, Output to the RFoG-ONU via an optical transmission line, and output an upstream optical signal input from the RFoG-ONU via the optical transmission line to the O / E converter via the optical transmission line A wavelength multiplexing device, and each of the above MTS is the timing of the uplink communication from the CM that do not correspond in an optical communication system that is not controlled,
A polarization scrambler that makes the upstream optical signal unpolarized is inserted in the vicinity of the O / E converter in the optical transmission path between the wavelength multiplexing device and the O / E converter.
An optical communication system. The optical communication system according to claim 1, wherein a plurality of the O / E converters are provided, and a plurality of the polarization scramblers are provided for each of the O / E converters.

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