JP2011077935A - Optical receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-cost optical receiver for suppressing signal degradation due to temperature change without using a Faraday rotator. <P>SOLUTION: The optical receiver for demodulating an optical signal having been subjected to DQPSK modulation to a multilevel phase modulation signal includes: polarization conversion/demultiplexing means (501, 511) to convert linearly-polarized light (a) to two circularly-polarized light rays (a1, a2) perpendicular to each other; a delay means (521) to provide a one-bit part and a phase difference equivalent to predetermined inclination of a polarization surface between the two circularly-polarized light rays (a1, a2); a multiplexing/demultiplexing means (541) to multiplex the two circularly-polarized light rays to be branched into linearly-polarized light rays (a3, a4) corresponding to the phase difference; and a 1/2 wavelength plate (551) for rotating the one-side linearly-polarized light ray (a4) having passed through the multiplexing/demultiplexing means at a predetermined angle. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical receiver, and more particularly to an optical receiver that demodulates a DQPSK-modulated optical signal into a multilevel phase-modulated signal.

  In a next-generation long-distance large-capacity optical communication system that requires high speed and large capacity as communication traffic increases, the introduction of multilevel modulation / demodulation coding technology is being studied. One typical example is a differential quadrature phase shift keying (DQPSK modulation) system. In this method, compared with the conventional binary intensity modulation (OOK, On-Off keying) method, the signal band is narrow, the frequency utilization efficiency can be improved and the transmission distance can be increased, and higher sensitivity can be expected.

  First, in the quadrature phase shift keying (QPSK modulation) method, for each symbol “00”, “01”, “11”, and “10” composed of 2-bit data, “ “θ”, “θ + π / 2”, “θ + π”, and “θ + 3π / 2” are assigned. Here, “θ” is an arbitrary phase. Then, the receiver reproduces the transmission data by detecting the phase of the received signal. As a means for realizing the QPSK modulation method relatively easily, there is a DQPSK modulation method. In the DQPSK modulation, the phase change amount of the carrier wave between the value of the symbol transmitted first and the value of the symbol transmitted next (“0 ”,“ Π / 2 ”,“ π ”, and“ 3π / 2 ”) are associated with 2 bits of transmission information. Therefore, the receiver can recover the transmission data by detecting the phase difference between two adjacent symbols.

  As shown in Patent Document 1 or 2, in order to demodulate a DQPSK modulated optical signal, two delay interferometers for generating an I (In-phase) signal and generating a Q (Quadrature) signal as shown in FIG. (A delay interferometer using the waveguides 102 and 103 or a delay interferometer using the waveguides 104 and 105) is required, and the phase difference needs to be demodulated with high accuracy. In FIG. 1, the optical signal α is branched into two by the branching unit 101 in order to enter the two delay interferometers. However, the optical path length in the delay interferometer changes due to the influence of the ambient temperature of the optical receiver and the phase is not stable, so that it is difficult to perform highly accurate demodulation. In addition, there is a problem that which interferometer cannot identify which signal component is demodulated. In addition, it is necessary to optimally adjust the optical path length difference between the branching of the optical signal and introduction into the two interferometers, and the difference in optical path length constituting each interferometer, and the control system becomes extremely complicated. Was produced. The light waves a1 and a2 or the light waves a3 and a4 are incident on the respective balanced light receiving elements to obtain an I signal and a Q signal.

  On the other hand, in Patent Document 3, the present applicant has proposed an optical receiver that focuses on the plane of polarization as shown in FIG. Specifically, the DQPSK modulated light α is incident with the plane of polarization aligned in one direction, and is introduced into the 1-bit delay circuit. The 1-bit delay circuit includes a polarization maintaining fiber coupler (1, 2), and a half-wave plate 3 that rotates the plane of polarization by 90 degrees is disposed in one branched light path. As a result, the combined optical wave is composed of two signal lights delayed by 1 bit with each other in a state where the planes of polarization are orthogonal.

  Then, the signal is separated into four signal lights by using the polarization separation circuit 4. For example, the light waves b 1 and b 2 or the light waves b 3 and b 4 are incident on each balanced light receiving element to obtain an I signal or a Q signal. Can do. Thereby, the 1-bit delay circuit can be integrated into one, and it becomes possible to reduce the manufacturing cost and the complexity of adjusting the optical components.

  However, if a 1-bit delay circuit is made using, for example, a polarization maintaining fiber coupler, the polarization maintaining fiber coupler is very sensitive to disturbances such as temperature changes and external stress. This causes the problem of poor stability. Also, when using a spatial optical system such as a half mirror, the half mirror has a polarization dependency, so that an error is likely to occur in the optical path length of the branched light wave, and the time between the I component signal and the Q component signal is short. There is a high possibility that a gap will occur.

  Further, in order to solve such a problem, the present applicant provides an optical receiver that is independent of polarization in an optical receiver that demodulates a DQPSK-modulated optical signal into a multilevel phase-modulated signal. In patent document 4, the following optical receivers were proposed.

  In such an optical receiver, as shown in FIG. 3, in an optical receiver that demodulates a DQPSK-modulated optical signal α into a multilevel phase-modulated signal, the polarization plane of the DQPSK-modulated optical signal α is orthogonal. The polarization branching means (201) for branching into two light waves (A, B) and one of the split light A further split into two light waves (A1, A2) and passing through a +45 degree quarter wave plate Branch rotating means (211, 221) to be split, and branch rotating means (212, 222) to split the other split light B into two light waves (B 1, B 2) and pass through a −45 degree quarter wave plate Then, the four light waves (A1, A2, B1, B2) that have passed through each branch and rotation means are separated into two light waves whose polarization planes are orthogonal to each other, and one of the light waves is generated with a delay of 1 bit. 1 bit that multiplexes signals with their polarization planes orthogonal to each other The delay circuit means (231 to 254) are provided, and the two light waves (A1, B2) that have passed through the 1-bit delay circuit means pass through the quarter wave plates (261, 264) of -45 degrees, Further, a rotating / branching means that rotates the polarization plane by 22.5 degrees (270) and branches (281, 284) every two light waves ((A11, A12) or (B21, B22)) whose polarization planes are orthogonal to each other; The other two light waves (A2, B1) that have passed through the bit delay circuit means pass through the quarter wave plates (262, 263) of +45 degrees, and further rotate the plane of polarization by 22.5 degrees (270). ), A rotating / branching means for branching (282, 283) for every two light waves ((A21, A22) or (B11, B12)) having orthogonal planes of polarization, and four specific light waves (B11) that have passed through the rotating / branching means. , B12, B21, B22) +45 The eight light waves (A11, A12, A21, A22, B11, B12, B21, B22) obtained by passing through the half-wave plate (290) of the specific combination ((A11, B21), (A12, B22) , (A21, B11) or (A22, B12)), characterized in that it has multiplexing means (301-304) for multiplexing while maintaining the plane of polarization.

  When this optical receiver is configured by a spatial optical system, as shown in FIG. 4, a part of the optical receiver is configured by an optical system including a Faraday rotator (FR). In FIG. 4, the two light waves (A 1, A 2) in FIG. 3 are passed through a +45 degree quarter-wave plate (413) constituting a part of the branch rotation means (211), and polarized by the polarization beam splitter 414. The light waves are branched into two light waves whose surfaces are orthogonal to each other, reflected by the prism mirrors 416 and 417, respectively, incident again on the polarization beam splitter 414, and combined while maintaining the polarization surfaces in an orthogonal state.

  The installation positions of the prism mirrors 416 and 417 with respect to the polarization beam splitter 414 are set so that the optical path difference is 1 bit. For example, in order to adjust the optical path length, a light transmission medium 415 such as a polymer is used as a polarization beam. It arrange | positions between a splitter and a prism mirror.

  The light wave exiting the 1-bit delay circuit is guided to a −45 degree quarter-wave plate 420 or a +45 degree quarter-wave plate 421. The four light waves that have passed through the ± 45 degree quarter-wave plates (420, 421) are further configured to pass through a Faraday rotator (FR) that is a 22.5 degree polarization plane rotating means.

  However, when a Faraday rotator is used as means for rotating the polarization, there is a problem that the Faraday rotator has a temperature dependency that the rotation angle changes depending on the temperature, and the element itself becomes expensive. In particular, the temperature dependency is about 0.09 ° / ° C., and when the environment in which the optical receiver is used changes in the temperature range of about 0 to 70 ° C., the rotation angle of the linearly polarized wave is 5 from the desired value. This will cause a deviation of about 0 °, causing the signal accuracy to deteriorate significantly.

JP 2006-295603 A JP 2007-158852 A Japanese Patent Application No. 2008-255528 (filed on September 30, 2008) Japanese Patent Application No. 2009-80319 (filed on Mar. 27, 2009)

  The problem to be solved by the present invention is to provide a low-cost optical receiver that solves the above-described problems, suppresses signal deterioration due to temperature changes, and does not use a Faraday rotator.

  In order to solve the above-described problem, the invention according to claim 1 is an optical receiver that demodulates a DQPSK-modulated optical signal into a multilevel phase-modulated signal, and two circularly polarized light (a1) orthogonal to linearly polarized light (a). , A2), a polarization converting / branching means for converting the two circularly polarized lights (a1, a2), a delay means for giving a phase difference corresponding to a predetermined inclination of the polarization plane, Combined and branched means for combining circularly polarized light and branching into linearly polarized light (a3, a4) corresponding to the phase difference, and one linearly polarized light (a4) passed through the combined branching means rotated by a predetermined angle And a two-wavelength plate.

  According to a second aspect of the present invention, in the optical receiver according to the first aspect, the polarization converting / branching means includes a quarter wavelength plate for converting linearly polarized light (a) into circularly polarized light, and the quarter wavelength. A branching means for splitting the light wave that has passed through the plate into two circularly polarized light (a1, a2) and a half-wave plate for converting one circularly polarized light (a2) into circularly polarized light of the opposite rotation. Features.

  According to a third aspect of the present invention, in the optical receiver according to the first aspect, the polarization conversion branching unit includes a branching unit that branches the linearly polarized light (a) into two linearly polarized light (a10, a20), and A quarter-wave plate that converts one linearly polarized light (a10) that has passed through the branching means into a predetermined circularly polarized light (a1), and another linearly polarized light (a20) that is orthogonal to the circularly polarized light (a1). It is characterized by being comprised from the quarter wavelength plate converted into.

  According to a fourth aspect of the present invention, in the optical receiver according to the second or third aspect, the branching unit and the combining / branching unit are configured by a single half mirror, and the optical path passing through the delay unit and the 1 The optical path passing through the / 2 wavelength plate is configured using the half mirror and two prism mirrors.

  According to a fifth aspect of the present invention, there is provided the optical receiver according to any one of the first to fourth aspects, wherein the DQPSK-modulated optical signal is branched into two linearly polarized lights (a, b) having orthogonal polarization planes. For each of the linearly polarized light, the polarization converting / branching means, the delaying means, the multiplexing / branching means, and the half-wave plate are provided for each of the linearly polarized lights. Two light waves (a31, a32, a41, a42, b31, b32, b41, b42) whose polarization planes are orthogonal to each other are the wave (a3, b3) and the linearly polarized wave (a4, b4) that has passed through the half-wave plate. ), And a part of the light waves (b31, b32, b41, b42) passing through the orthogonal polarization branching unit are passed through the half-wave plate, and the orthogonal polarization branching unit is Other light waves (a31, a32, a41, a42) passed through and a specific combination ( a31, b31), (a32, b32), characterized in that it has a multiplexing means for multiplexing while maintaining the polarization plane by (a41, b41) or (a42, b42)).

  According to the first aspect of the present invention, the polarization converting / branching means for converting the linearly polarized light (a) into the two circularly polarized lights (a1, a2) orthogonal to each other and the two circularly polarized lights (a1, a2) are provided for 1 bit. A delay unit that gives a phase difference corresponding to a predetermined inclination of the plane of polarization, and a multiplexing / branching unit that multiplexes two circularly polarized lights and branches the linearly polarized light (a3, a4) corresponding to the phase difference. By using circularly polarized light with a configuration having a half-wave plate that rotates one linearly polarized light (a4) that has passed through the multiplexing / branching means by a predetermined angle, without using a Faraday rotator, A light wave having a plane of polarization of a predetermined rotation angle corresponding to the Q signal component can be easily obtained. This makes it possible to provide a low-cost optical receiver that suppresses signal deterioration due to temperature changes.

  According to the invention of claim 2, the polarization converting / branching means includes a quarter wave plate that converts the linearly polarized light (a) into circularly polarized light, and a light wave that has passed through the quarter wave plate is converted into two circularly polarized lights (a1). , A2) and a half-wave plate for converting one circularly polarized light (a2) to the oppositely rotated circularly polarized light, the polarization converting / branching means can be configured with a simple configuration. A low-cost optical receiver can be provided.

  According to the invention of claim 3, the polarization conversion branching means includes a branching means for branching the linearly polarized light (a) into two linearly polarized lights (a10, a20), and one linearly polarized light (a10) that has passed through the branching means. Is composed of a quarter wave plate that converts the circularly polarized light (a1) into a circularly polarized light orthogonal to the circularly polarized light (a1). Therefore, since both means for converting into two circularly polarized light are quarter wave plates, it is possible to obtain an optical receiver with little temperature dependence.

  According to the invention of claim 4, since the branching unit and the combining and branching unit are configured using a single half mirror and two prism mirrors, it is possible to reduce the size of the spatial optical system related to the optical receiver. It becomes.

  According to the fifth aspect of the present invention, the DQPSK-modulated optical signal is split into two light waves whose polarization planes are orthogonal to each other, and as described above, the I signal component and the Q signal component using the polarization plane are generated in each light wave. After that, by extracting the corresponding light wave from each polarization plane and combining it with a specific combination, it becomes possible to obtain the I signal component and the Q signal component that are polarization independent.

It is a figure which shows the optical receiver of the DQPSK modulation comprised by the conventional two delay interferometers. It is a figure which shows the optical receiver of the DQPSK modulation using the polarization plane which the present applicant presented by the previous application. It is a figure explaining the basic concept concerning the optical receiver which the present applicant presented in the previous application. It is a top view for demonstrating a part of spatial optical system which concerns on the optical receiver of FIG. It is a figure explaining the basic concept which concerns on the optical receiver of this invention. It is a top view for demonstrating a part of spatial optical system which concerns on the optical receiver of FIG. It is a top view for demonstrating a part of other spatial optical system utilized for the optical receiver of this invention.

Hereinafter, the optical receiver of the present invention will be described in detail using preferred examples.
As shown in FIG. 5, in the optical receiver for demodulating a DQPSK modulated optical signal into a multi-level phase modulated signal, the present invention converts linearly polarized light (a) into two circularly polarized lights (a1, a2) orthogonal to each other. Polarization conversion branching means (501, 511) for conversion and delay means (521) for giving a phase difference corresponding to a predetermined inclination of the polarization plane between one circularly polarized light (a1, a2) and one bit And a multiplexing / branching means (541) for combining the two circularly polarized lights and branching them into linearly polarized light (a3, a4) corresponding to the phase difference, and one linearly polarized light (a4) having passed through the combining / branching means And a half-wave plate (551) that rotates the lens by a predetermined angle.

  As for the polarization converting / branching means, a quarter wave plate (501) for converting linearly polarized light (a) into circularly polarized light and a light wave that has passed through the quarter wave plate are converted into two circularly polarized lights (a1, a2). And a half-wave plate (531) for converting one circularly polarized light (a2) into a reversely polarized circularly polarized light.

  As another configuration of the polarization converting / branching means, as will be described later, a branching means for branching the linearly polarized light (a) into two linearly polarized lights (a10, a20) and one straight line that has passed through the branching means. A quarter-wave plate for converting the polarized light (a10) into a predetermined circularly polarized light (a1), and a quarter-wave plate for converting the other linearly polarized light (a20) into a circularly polarized light orthogonal to the circularly polarized light (a1). It is also possible to configure from In this case, since each optical system that generates circularly polarized waves (a1, a2) uses a quarter-wave plate, it is less affected by temperature changes, and light reception with less temperature dependence is achieved. Can be obtained.

  A feature of the present invention is that by converting linearly polarized light into circularly polarized light, without using a Faraday rotator, two half-wave plates (531, 551) or two quarter-wave plates are used. A light wave having a plane of polarization of a predetermined rotation angle corresponding to the I signal component (a3) and the Q signal component (a4) can be easily obtained. This eliminates signal degradation due to temperature changes when using a Faraday rotator, and provides a low-cost optical receiver because it uses a cheaper half-wave plate or quarter-wave plate than the Faraday rotator. It becomes possible to do.

  FIG. 6 shows an example in which the optical system for obtaining the I signal component (a3) and the Q signal component (a4) from the linearly polarized light (a) is constituted by a spatial optical system. The linearly polarized light in which the I component signal and the Q component signal are on the same polarization plane is converted into circularly polarized light by the quarter wavelength plate (600). Reference numeral 601 denotes a half mirror (splitter) constituting a branching unit, which is different from the polarization beam splitter 414 of the spatial optical system shown in FIG.

  One (a1) of the circularly polarized light branched into two by the half mirror 601 is reflected toward the prism mirror 603 and returns to the half mirror 601. During this time, the circularly polarized light is delayed by a predetermined phase difference from 1 bit by the delay means. One bit is a delay amount necessary for demodulating a DQPSK-modulated optical signal. The predetermined phase difference is a phase difference when two circularly polarized lights are overlapped by a half mirror. It is used to set the angle of the polarization plane of (linearly polarized light) to a desired angle. However, although it is possible to configure the 1-bit delay and the delay of a predetermined phase difference by separate delay means, both can be shared by one member from the viewpoint of reducing the number of parts.

  The other circularly polarized light (a2) branched by the half mirror 601 is converted by the half-wave plate 604 into circularly polarized light of the opposite rotation. The circularly polarized light reflected by the prism mirror 605 is combined with the circularly polarized light that has passed through the delay means by the half mirror. Since the two light waves to be combined have opposite rotation directions and a predetermined phase difference, the combined light wave is linearly polarized light whose polarization plane is inclined by a predetermined angle. By setting the angle to 22.5 °, it is possible to obtain the same effect as the Faraday rotator.

  One linearly polarized light (a4) that has passed through the half mirror 601 serving as a multiplexing / branching means passes through another half-wave plate (606) and is further rotated by a predetermined angle, and then mirror 607 (or a prism mirror). Is emitted in the same direction as the light wave (a3) of the I component signal.

  A half-wave plate (604) for converting the circularly polarized light (a2) into circularly polarized light in the opposite direction is installed on the outgoing light side of the half mirror 601, but light waves are transmitted from the prism mirror (605) to the half mirror. It is also possible to install it on the returning optical path. Moreover, it is also possible to arrange | position on the optical path of the circularly polarized light (a1) which has arrange | positioned the delay means.

  As shown in FIG. 6, a spatial optical system can be configured using a single half mirror (601) and two prism mirrors (603, 605). It can be downsized.

  Instead of the spatial optical system shown in FIG. 6, it is possible to use the spatial optical system shown in FIG. In FIG. 7, in order to obtain two circularly polarized waves, the branching means (601) for branching the linearly polarized light (a) into two linearly polarized lights (a10, a20) and one linearly polarized light (a10) that has passed through the branching means. ) To a predetermined circularly polarized light (a1) and a quarter wavelength plate to convert the other linearly polarized light (a20) to a circularly polarized light orthogonal to the circularly polarized light (a1). (610). Thus, since both means for converting into two circularly polarized light are quarter wave plates, it is possible to obtain an optical receiver with little temperature dependency.

  In FIG. 5, not only the linearly polarized light (a) but also the linearly polarized light (b) is similarly applied to the ¼ wavelength plate (502), the branching means (512), the delaying means (522), and the ½ wavelength plate ( 532), multiplexing / branching means (542), and another half-wave plate (552). This is to provide an optical receiver that does not depend on polarization, as in Patent Document 4.

  In order to make the optical receiver of the present invention polarization independent, the received signal light α is split into two linearly polarized lights (a, b) whose planes of polarization are orthogonal using the polarization branching means (500). ing. A polarization beam splitter can be used as the polarization branching means. Similarly to Patent Document 4, in order to share the spatial optical system shown in FIG. 6 with two linearly polarized light (a, b), the linearly polarized light (a) is separated from the plane of FIG. The polarization branching means is arranged so that the linearly polarized light (b) travels in parallel.

  As for the linearly polarized light (b), similarly to the linearly polarized light (a), the quarter wavelength plate (502), the branching means (512), the delaying means (522), the half wavelength plate (532), and the multiplexing branch. Means (542) and another half-wave plate (552) are provided.

  Next, the linearly polarized wave (a3, b3) passed through the combining / branching means (541, 542) and the linearly polarized wave (a4, b4) passed through the other half-wave plates (551, 552) are converted into a polarized beam. The light is branched into two light waves (a31, a32, a41, a42, b31, b32, b41, b42) having orthogonal polarization planes by means of orthogonal polarization branching means such as a splitter.

  Part of the light waves (b31, b32, b41, b42) that have passed through the orthogonal polarization branching unit are passed through the half-wave plates (571-574), and the other light waves (a31, a32, a41, a42) and a specific combination ((a31, b31), (a32, b32), (a41, b41) or (a42, b42)) as shown in FIG. To do. A polarization beam splitter can be used as the multiplexing means.

  When the optical system shown in FIG. 6 or FIG. 7 is shared by two linearly polarized light (a, b), or an unnecessary phase difference is generated between the optical paths due to a phase change when passing through various optical components. There is. Since such a phase difference affects the demodulation accuracy of the signal, it is preferable to appropriately correct so as not to cause an unnecessary optical path difference between the light waves.

  As described above, according to the present invention, it is possible to provide a low-cost optical receiver that suppresses signal deterioration due to a temperature change without using a Faraday rotator.

201,231-234,251-254,281-284,301-304,561-564,581-584 Polarizing beam splitter 211,212 Branch means (polarizing beam splitter)
221, 222, 261 to 264, 501, 502, 610, 611 ¼ wave plate 270 22.5 degree polarization plane rotating means 290, 521, 522, 551, 552, 571 to 574 ½ wave plate 511, 512 Branching means (half mirror)
541, 542 Combined branching means (half mirror)

Claims (5)

In an optical receiver that demodulates a DQPSK modulated optical signal into a multi-level phase modulated signal,
Polarization conversion branching means for converting linearly polarized light (a) into two circularly polarized lights (a1, a2) orthogonal to each other;
A delay means for providing a phase difference corresponding to a predetermined inclination of a plane of polarization and one bit between two circularly polarized lights (a1, a2);
A combining / branching means for combining two circularly polarized lights and branching them into linearly polarized light (a3, a4) corresponding to the phase difference;
An optical receiver comprising: a half-wave plate that rotates one linearly polarized light (a4) that has passed through the multiplexing and branching means by a predetermined angle. The optical receiver according to claim 1.
The polarization conversion branching means includes:
A quarter wave plate that converts linearly polarized light (a) into circularly polarized light;
Branching means for branching the light wave that has passed through the quarter-wave plate into two circularly polarized lights (a1, a2);
An optical receiver comprising: a half-wave plate that converts one circularly polarized light (a2) into circularly polarized light that rotates in the opposite direction. The optical receiver according to claim 1.
The polarization conversion branching means includes:
Branching means for branching the linearly polarized light (a) into two linearly polarized lights (a10, a20);
A quarter-wave plate for converting one linearly polarized light (a10) that has passed through the branching means into a predetermined circularly polarized light (a1);
An optical receiver comprising a quarter wave plate for converting the other linearly polarized light (a20) into circularly polarized light orthogonal to the circularly polarized light (a1).   4. The optical receiver according to claim 2, wherein the branching unit and the combining / branching unit are configured by a single half mirror, and an optical path passing through the delay unit and an optical path passing through the half-wave plate. Is configured using the half mirror and two prism mirrors.   5. The optical receiver according to claim 1, wherein the DQPSK-modulated optical signal is branched into two linearly polarized lights (a, b) having orthogonal polarization planes, and the linearly polarized light Are provided with the polarization conversion branching means, the delay means, the multiplexing branching means, and the half-wave plate, and the linearly polarized waves (a3, b3) that have passed through the multiplexing and branching means and the Orthogonal polarization that branches the linearly polarized wave (a4, b4) that has passed through the half-wave plate into two light waves (a31, a32, a41, a42, b31, b32, b41, b42) whose polarization planes are orthogonal to each other. The branching means and a part of the light waves (b31, b32, b41, b42) that have passed through the orthogonal polarization branching means are passed through the half-wave plate, and the other light waves (a31, a32) that have passed through the orthogonal polarization branching means. , A41, a42) and a specific combination ((a31, b31), ( 32, b32), (a41, b41) or (a42, b42)) optical receiver, characterized in that it comprises a multiplexing means for multiplexing while maintaining the polarization plane in.

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