JP2004253931A - Orthogonal polarization multiplex transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an orthogonal polarization multiplex transmitter which realizes an orthogonal polarization multiplexing of a plurality of signal lights using the same non-polarization hold type optical components as the conventional ones, without complicating the constitution. <P>SOLUTION: The orthogonal polarization multiplex transmitter has an odd channel multiplexer 1 for multiplexing odd signal lights DATA 1, DATA 3 of a plurality of signal lights different in wavelength into a light DATA (2n-1) to generate a random polarization multiplex light 5, and an even channel multiplexer 2 for multiplexing even signal lights DATA 2, DATA 4 of the plurality of signal lights different in wavelength into a light DATA (2n) to generate a random polarization multiplex light 6. A multi-channel polarization controller 3a converts the multiplex light 5 to a linear polarization multiplex light 7 in the vertical direction, and a multi-channel polarization controller 3b converts the multiplex light 6 to a linear polarization multiplex light 8 in the horizontal direction. A polarization multiplexer 4 multiplexes the vertical polarization multiplex light 7 and the horizontal polarization multiplex light 8 while holding their polarized conditions to generate an orthogonal polarization multiplex light 9. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a wavelength division multiplexing transmission technique in the field of optical communication, and more particularly to an orthogonal polarization multiplexing transmission apparatus that controls the polarization state of transmission signal light to realize high-density wavelength division multiplexing transmission.
[0002]
[Prior art]
At present, in the field of optical communication, so-called wavelength multiplex transmission, in which a plurality of signal lights having different wavelengths are multiplexed on one optical fiber and transmitted, in order to realize long-distance, large-capacity transmission, has become mainstream. This makes it possible to easily increase the transmission capacity of the existing transmission line.
[0003]
However, in order to multiplex a large number of signal lights within a limited amplification band like a submarine repeater, the wavelength intervals of the signal lights have to be increased. In this case, interference called coherent crosstalk occurs between the leaked light from the adjacent wavelength signal and the signal light itself, causing signal degradation. In addition, a nonlinear phenomenon called four-wave mixing occurs during optical fiber transmission, which also causes signal degradation.
[0004]
As means for solving these problems, a so-called orthogonal polarization multiplexing method in which the polarization states of adjacent signal lights are orthogonal to each other during wavelength multiplexing is effective, and energetically studied. Generally, four-wave mixing occurs remarkably when the adjacent polarization states are parallel and adversely affects the transmission characteristics. However, when the adjacent polarization states are orthogonalized, they can be suppressed because there is no parallel component. In addition, if light can be received at the receiving side while maintaining orthogonality, it is possible to avoid coherent crosstalk and completely separate individual signals by combining polarization separation and wavelength separation.
[0005]
At present, the transmission rate (bit rate) per signal light wave is mainly 10 Gb / s, but it is expected that the transmission rate will be further increased in the future. That is, the optical spectrum of the signal light tends to expand, and the influence of coherent crosstalk tends to further increase. Therefore, orthogonal polarization multiplexing is an indispensable technology in high-density and high-speed wavelength multiplex transmission, and various technologies for providing a high-quality multiplexing device have been proposed.
[0006]
For example, Japanese Patent Laying-Open No. 2002-217832 (Patent Document 1) describes a polarization control device for independently controlling each polarization state and restoring orthogonality between adjacent channels. Specifically, after once separating the signal light wavelength for each channel using an optical demultiplexer, the polarization control is individually performed, and the wavelength is multiplexed again using an optical multiplexer. This controls the wavelength-division multiplexed light in an arbitrary polarization state to be orthogonally polarized.
[0007]
Japanese Patent Laying-Open No. 2001-298415 (Patent Document 2) discloses an optical multiplexing circuit that stabilizes polarization maintenance of multiplexed lights orthogonal to each other and improves transmission quality. Specifically, using a polarization-maintaining AWG (Arrayed Waveguide Grating), odd-channel multiplexed light and even-channel multiplexed light are generated from a plurality of signal lights having linearly polarized light. Polarization control is performed on one of the even-numbered channel multiplexed lights to generate odd-numbered and even-numbered channel multiplexed lights orthogonal to each other, and orthogonal polarization multiplexing is realized by multiplexing them. .
[0008]
Japanese Patent Application Laid-Open No. Sho 59-210414 (Patent Document 3) discloses a polarization compensator that separates incident light into two orthogonal linearly polarized light components, matches one of the polarization directions with the other, and then synthesizes the components. Is disclosed.
[0009]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2002-217832 (specification paragraph numbers 0021 and 0025, FIG. 1)
[Patent Document 2]
JP 2001-298415 A (specification paragraph numbers 0015 to 0016, 0025, FIG. 1)
[Patent Document 3]
JP-A-59-210414 (1st to 18th lines in the upper right column on page 2; FIG. 1).
[0010]
[Problems to be solved by the invention]
However, in the polarization control device described in Patent Document 1, although each channel is monitored, there is an advantage that polarization control can be performed very strictly. However, a polarization controller, an optical tap, a polarization monitor, It is necessary to provide the same number of control units as the number of signal lights. Since the number of signal lights in the orthogonal polarization multiplexing method is considered to be around 100 waves or more than 100 waves, it is realistic to provide these components by the number of signals in consideration of the device cost and the mounting space of the components. It is considered difficult.
[0011]
The optical multiplexing circuit described in Patent Document 2 is a very effective means for realizing orthogonal polarization multiplexing. However, it is assumed that the signal light input to the polarization-maintaining AWG is linearly polarized light. It has become. Generally, the polarization extinction ratio of signal light deteriorates every time it propagates along a splice point between optical components and an optical connector connection between devices in addition to signal propagation of optical components. Therefore, it is considered that the state of linear polarization is considerably deteriorated before reaching the input part of the polarization maintaining AWG from the wavelength light source. In order to avoid this, for example, linear polarization control as described in Patent Document 1 is performed at all input ports of the polarization maintaining AWG, or all optical components from the wavelength light source to the AWG are controlled. Must be polarization-maintaining. In any case, it is necessary to use many components that are more expensive than the non-polarization maintaining optical components.
[0012]
The polarization compensator described in Patent Literature 3 is intended to adjust the polarization of single-channel light before being incident on an optical modulator, and does not adjust the polarization of modulated signal light. Absent.
[0013]
An object of the present invention is to provide an orthogonal polarization multiplexing transmission apparatus that can realize orthogonal polarization multiplexing of a plurality of signal lights without complicating the configuration using the same non-polarization maintaining type optical component as in the related art. It is in.
[0014]
[Means for Solving the Problems]
The orthogonal polarization multiplexing transmission apparatus according to the present invention is an apparatus that receives a plurality of signal lights having different wavelengths and multiplexes and transmits such that the polarization directions of adjacent signal lights are orthogonal to each other. First combining means for combining odd-numbered signal lights to generate a first multiplexed light, and second combining means for combining even-numbered signal lights in the plurality of signal lights to generate a second multiplexed light; First polarization control means for converting the first multiplexed light into linearly polarized light in a first direction to generate a first linearly polarized light multiplexed light, and converting the second multiplexed light in a second direction orthogonal to the first direction. A second polarization controller for converting the light into linearly polarized light to generate a second linearly polarized light multiplexed light, and multiplexing the first linearly polarized light multiplexed light and the second linearly polarized light multiplexed light while maintaining the respective polarization states; Polarization combining means for generating the orthogonal polarization multiplexed light, And butterflies.
[0015]
The first combining means and the first polarization control means are coupled by a non-polarization maintaining fiber, the first polarization control means and the polarization combining means are coupled by a polarization maintaining fiber, The second combining means and the second polarization control means are coupled by a non-polarization maintaining fiber, and the second polarization control means and the polarization combining means are coupled by a polarization maintaining fiber. It is characterized by.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a block diagram showing a configuration of an orthogonal polarization multiplexing transmission apparatus according to an embodiment of the present invention. Here, an example in which 2n-channel optical signals DATA1 to DATA (2n) having different wavelengths are wavelength-multiplexed will be described. The optical signals DATA1 to DATA (2n) are arranged in ascending order of the transmission wavelength, and are random polarized at this stage.
[0017]
The orthogonal polarization multiplexing transmission apparatus according to the present embodiment is provided with an odd-numbered channel multiplexer 1 and an even-numbered channel multiplexer 2, and their input ports cannot pass unless a designated wavelength designated in advance is input. . The odd-numbered channel multiplexer 1 inputs the odd-numbered channel optical signals DATA1, DATA3,..., DATA (2n-1) at the corresponding input ports, and multiplexes them to output the odd-numbered channel multiplexed light 5. The even-channel multiplexer 2 receives the even-channel optical signals DATA2, DATA4,..., DATA (2n) at the corresponding input ports and multiplexes them to output the even-channel multiplexed light 6. The odd-numbered channel multiplexed light 5 and the even-numbered channel multiplexed light 6 remain in random polarization at this stage.
[0018]
The odd channel multiplexed light 5 and the even channel multiplexed light 6 are input to the multi-channel polarization controllers 3a and 3b, respectively. The multi-channel polarization controllers 3a and 3b convert these multiplexed lights 5 and 6 into odd-numbered channel vertical polarization multiplexed light 7 and even-numbered channel horizontal polarization multiplexed light 8 which are linearly polarized light orthogonal to each other. The output ports of the odd channel multiplexed light 5 and the even channel multiplexed light 6 are optically connected to predetermined input ports of the polarization combiner 4 by polarization maintaining fibers 10a and 10b, respectively. Accordingly, the odd-numbered channel vertical polarization multiplexed light 7 and the even-numbered channel horizontal polarization multiplexed light 8 are input to the polarization combiner 4 while maintaining the polarization direction.
[0019]
The polarization combiner 4 is an optical component for multiplexing while maintaining the polarization direction of the input multiplexed light. The polarization combiner 4 combines the odd channel vertical polarization multiplexed light 7 and the even channel horizontal polarization multiplexed light 8 to perform orthogonal polarization multiplexing. Light 9 is output.
[0020]
FIG. 2A is a block diagram showing the internal configuration of the multi-channel polarization controller 3a, and FIG. 2B is a block diagram showing the internal configuration of the multi-channel polarization controller 3b. Since the multi-channel polarization controllers 3a and 3b basically operate in the same manner and operate in the same manner except that the polarization directions are switched, the multi-channel polarization controller 3a shown in FIG. Will be described.
[0021]
As described above, the odd-channel multiplexed light 5 that is the random polarization is converted into the odd-channel vertical polarization multiplexed light 7 by the multi-channel polarization controller 3a. First, the odd-channel multiplexed light 5 is split into a multiplexed light 35a of a vertical polarization component and a multiplexed light 36a of a horizontal polarization component by a polarization separator 31a.
[0022]
The multiplexed light 36a of the horizontal polarization component propagating in one path is converted from the horizontal polarization into the multiplexed light 37a of the vertical polarization by the polarization rotation element 32a that rotates the linearly polarized light by 90 degrees, and is coupled through the delay element 33a. An input is made to one of the input ports 34a. The delay element 33a is a phase adjuster that corrects the optical path length so that interference with the other path does not occur when the signals are multiplexed by the coupler 34a. However, since the multiplexed light 36a in the present invention is composed of a plurality of signal lights (modulated lights), the phase adjustment by the delay element 33a is performed within a range where the signal pulse does not collapse.
[0023]
The multiplexed light 35a of the vertically polarized component propagating in the other path is input to the other input port of the coupler 34a through the attenuator 38a. The attenuator 38a is inserted in order to match the passage losses of the two paths in consideration of the passage loss of the polarization rotation element 32a and the delay element 33a and the loss difference between the ports of the polarization separator 31a and the coupler 34a. ing.
[0024]
The combiner 34a multiplexes the multiplexed lights input from the two paths, and outputs the odd-channel vertically polarized multiplexed light 7. As a result, even if the polarization state of the input multiplexed light 5 changes, the output channel power of the odd-numbered channel vertical polarization multiplexed light 7 becomes constant.
[0025]
Note that the basic configuration and operation of the multi-channel polarization controller 3b in FIG. 2B are the same, except that the polarization direction in FIG.
[0026]
As described above, the polarization maintaining type is required from the outputs of the polarization splitters 31a and 31b in the multi-channel polarization controllers 3a and 3b, and the multiplexed lights 5 and 6 remain in random polarization. Non-polarization-maintaining optical components can be used.
[0027]
In addition, polarization splitters 31a and 31b divide all polarized light into linearly polarized light of a horizontal component and a vertical component, and re-multiplex the polarized light after the polarization control so that the passage losses of both paths are matched. Unlike extracting a specific linearly polarized light component, the transmitted light power does not vary depending on the incident polarization state. As long as the polarization rotation elements 32a and 32b have a signal bandwidth used in submarine communication, the wavelength dependence of the polarization rotation angle can be sufficiently ignored. Can be. Therefore, the linear polarization control has almost no wavelength dependence, and can be treated as a multi-channel polarization controller having a sufficient polarization extinction ratio. Further, the polarization controller does not require electric control, and can be treated as a passive optical component.
[0028]
(Example)
The 2n-wave signal light input to the orthogonal polarization multiplex transmission apparatus of the present invention has a wavelength band of 1539 nm to 1565 nm, and assumes a wavelength interval of 25 GHz (about 0.2 nm) obtained by dividing an ITU-T grid at 100 GHz intervals into four equal parts. are doing. Although the maximum number of signal lights is 130, this time, a part of the above-mentioned wavelength band is used, and the number of signal lights is set to 32 (thus n = 16).
[0029]
As the odd-numbered channel multiplexer 1 and the even-numbered channel multiplexer 2, a 16-port, 50 GHz array waveguide grating (AWG) effective for multiplexing multiple channels with low loss was used. Thus, by inputting a predetermined signal wavelength to the input port, 16-wave odd-channel multiplexed light 5 and 16-wave even-channel multiplexed light 6 were generated.
[0030]
The odd channel multiplexed light 5 and the even channel multiplexed light 6 are converted into orthogonal linearly polarized 16-wave channel vertical multiplex light 7 and 16-wave channel horizontal multiplex light 8 by orthogonal polarization linear controllers 3a and 3b, respectively. The light propagates through a polarization maintaining fiber (PMF; Polarization Maintaining Fiber) 10 a and 10 b and is input to the polarization combiner 4.
[0031]
As the polarization combiner 4, a polarization beam coupler (PBC) was used. The PBC multiplexes with a loss of about 1 dB while maintaining the respective polarization directions by inputting the vertical polarization and the horizontal polarization to predetermined input ports to obtain 32 orthogonal polarization multiplexed light 9. Was completed.
[0032]
In the multi-channel polarization controllers 3a and 3b, a polarization beam splitter (PBS; Polarization Beam Splitter) was used as the polarization separators 31a and 31b, and the random polarization was split into two linear polarizations of a vertical component and a horizontal component.
[0033]
As the polarization rotating elements 32a and 32b that rotate the linearly polarized light to a predetermined angle, a λ / 2 plate that can easily rotate the polarization angle by 90 degrees by optimizing the film thickness of the elements was used.
[0034]
As the attenuators 38a and 38b, a fixed-attenuation type polarization maintaining type was used. A 2-input port polarization maintaining type 3 dB optical coupler was used for the couplers 34a and 34b. Delay elements 33a and 33b for adjusting the optical path lengths of both paths are a delay device that adjusts the length (PMF length) of a polarization maintaining fiber connecting between a λ / 2 plate and a polarization maintaining type 3dB optical coupler; Alternatively, it is realized by an optical delay line capable of freely adjusting the optical path difference using a movable mirror (prism). This prevents the optical signal pulse from deteriorating after the multiplexing. Note that the passage loss of the multi-channel polarization controllers 3a and 3b in this example was 5.4 dB.
[0035]
In order to confirm the basic characteristics of the multi-channel polarization controllers 3a and 3b, the output power was compared before and after the polarization controllers were inserted.
[0036]
FIG. 3A is a schematic diagram of an output power measurement system before a polarization controller is inserted, and FIG. 3B is a schematic diagram of an output power measurement system with a polarization controller inserted. FIG. 4 is a graph showing the results of measuring the output power before and after inserting the polarization controller.
[0037]
First, as shown in FIG. 3A, the output of the linearly polarized LD light source was changed by a polarization controller (PC), and the optical power after passing through the deflector was monitored. The change of the normalized output power at this time is plotted as “before insertion” in FIG. According to this, it is understood that the amount of rejection is 30 dB or more when polarized light perpendicular to the transmission direction of the deflector is incident.
[0038]
On the other hand, as shown in FIG. 3B, when the polarization controller of the present invention was inserted, the amount of rejection was reduced to 1.4 dB. This indicates that the component in the transmission direction of the polarizer is transmitted almost uniformly, that is, the component is output as linearly polarized light regardless of the incident polarization state.
[0039]
Although the evaluation was performed at a transmission wavelength of 1565 nm, it was confirmed that the characteristics did not change even at 1539 nm. Further, the output power measurement system is configured by individually combining optical components. However, since the output power measurement system is similar to the known internal configuration of the optical isolator, the size can be reduced.
[0040]
The number of input ports and the wavelength interval of the AWG used in the present embodiment depend on the applied system and the device configuration, and are not limited to this example.
[0041]
In addition, since the signal light wavelengths are generally arranged at equal intervals, a periodic AWG is effectively applied to the channel multiplexers 1 and 2. In the case of handling signal lights at non-equidistant intervals, a filter type multiplexer combining transmission and reflection of a plurality of bandpass filters can be used.
[0042]
Further, instead of the PBC of the polarization combiner 4, a polarization maintaining interleaver may be used. The PBC combines two orthogonal linear polarizations with low loss, while the interleaver removes inter-signal noise at a predesigned input signal wavelength (odd-wavelength side and even-wavelength side) while removing low noise. It has the function of multiplexing due to loss. For this reason, when the isolation of the AWG (the amount of blocking of an adjacent port) is insufficient, the interleaver is an optical component often used to improve transmission characteristics.
[0043]
(Other embodiments)
FIG. 5 is a block diagram showing the internal configuration of the multi-channel polarization controller 3b of the orthogonal polarization multiplex transmission apparatus according to another embodiment of the present invention. As described above, the other multi-channel polarization controller 3a has the same configuration, and a description thereof will be omitted.
[0044]
As shown in FIG. 5, a variable voltage type can be used in order to accurately adjust optical components used in the multi-channel polarization controller 3b. For example, a variable optical attenuator (VOA) can be used for the attenuator 38_Vb, and a Faraday rotation device whose polarization plane rotates by an applied voltage to the polarization rotation element 32_Vb can be used. Furthermore, by using an adjustable optical delay line using a movable mirror (prism) as the delay element 33b, it is possible to perform appropriate phase adjustment that does not break the signal pulse.
[0045]
【The invention's effect】
As described above, according to the present invention, the multi-channel polarization controller is provided immediately before the input of the polarization combiner for determining the orthogonal polarization multiplexing, and the linear polarization control is performed. There is no need to consider the polarization state of the signal light before the device. For this reason, all the optical components and the device configuration from the transmission wavelength light source to the input section of the polarization controller can be made conventional non-polarization maintaining type, and expensive polarization maintaining type optical components become unnecessary. No need to use. Also, there is no need to consider measures against degradation of the polarization extinction ratio due to the passage of a splice, an optical connector connection, or an optical component. Furthermore, since random polarized light is converted into linear polarized light collectively for each of the two wavelength groups of odd / even numbers, the number of components is much smaller than that of individually controlling the polarization.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating a configuration of an orthogonal polarization multiplex transmission device according to an embodiment of the present invention.
FIG. 2A is a block diagram illustrating an internal configuration of a multi-channel polarization controller 3a, and FIG. 2B is a block diagram illustrating an internal configuration of a multi-channel polarization controller 3b.
FIG. 3A is a schematic diagram of an output power measurement system before a polarization controller is inserted, and FIG. 3B is a schematic diagram of an output power measurement system with a polarization controller inserted.
FIG. 4 is a graph showing results of measuring output power before and after inserting a polarization controller.
FIG. 5 is a block diagram showing an internal configuration of a multi-channel polarization controller 3b of the orthogonal polarization multiplex transmission device according to another embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Odd-channel multiplexer 2 Even-channel multiplexer 3a Multi-channel polarization controller 3b Multi-channel polarization controller 4 Polarization combiner 5 Odd-channel multiplexed light 6 Even-channel multiplexed light 7 Odd-channel vertical-polarized multiplexed light 8 Even-numbered channel horizontal polarization multiplexed light 9 orthogonal polarization multiplexed light 10a polarization-maintaining fiber 10b polarization-maintaining fiber

Claims (8)

In a device for inputting a plurality of signal lights having different wavelengths and multiplexing and transmitting such that the polarization directions of adjacent signal lights are orthogonal to each other,
First combining means for combining odd-numbered signal lights in the plurality of signal lights to generate a first multiplexed light;
Second combining means for combining the even-numbered signal lights in the plurality of signal lights to generate a second multiplexed light;
First polarization control means for converting the first multiplexed light into linearly polarized light in a first direction to generate a first linearly polarized light multiplexed light;
Second polarization control means for converting the second multiplexed light into linearly polarized light in a second direction orthogonal to the first direction to generate a second linearly polarized light multiplexed light;
Polarization combining means for combining the first linearly polarized light multiplexed light and the second linearly polarized light multiplexed light while maintaining respective polarization states to generate the orthogonally polarized light multiplexed light;
An orthogonal polarization multiplex transmission device comprising: The first combining means and the first polarization control means are coupled by a non-polarization maintaining fiber, the first polarization control means and the polarization combining means are coupled by a polarization maintaining fiber,
The second combining means and the second polarization control means are coupled by a non-polarization maintaining fiber, and the second polarization control means and the polarization combining means are coupled by a polarization maintaining fiber. ,
The orthogonal polarization multiplex transmission device according to claim 1, wherein: The first polarization control means includes:
First polarization separation means for separating the first multiplexed light into multiplexed lights of a first polarization direction component and a second polarization direction component orthogonal to each other, and outputting the multiplexed lights to a first path and a second path, respectively;
First polarization rotating means for rotating the polarization direction of the multiplexed light of the second path by 90 degrees to generate third multiplexed light in the same direction as the first polarization direction;
First optical path length correcting means for correcting an optical path length difference between the first path and the second path;
First attenuation means for correcting a path loss difference between the first path and the second path;
A first coupling for generating the first linearly polarized light multiplexed light by multiplexing the multiplexed lights of the first path and the second path with the optical path length difference and the path loss difference corrected while maintaining linear polarization; Means,
Has,
The second polarization control means includes:
Second polarization separation means for separating the second multiplexed light into multiplexed lights of the first polarization direction component and the second polarization direction component orthogonal to each other, and outputting the multiplexed lights to a third path and a fourth path, respectively;
Second polarization rotation means for rotating the polarization direction of the multiplexed light of the third path by 90 degrees to generate fourth multiplexed light in the same direction as the second polarization direction;
Second optical path length correction means for correcting the optical path length difference between the third path and the fourth path;
Second damping means for correcting a path loss difference between the third path and the fourth path;
A second coupling that combines the multiplexed lights of the third path and the fourth path, in which the optical path length difference and the path loss difference are corrected, while maintaining linear polarization, and generates the second linearly polarized light multiplexed light; Means,
2. The orthogonal polarization multiplex transmission apparatus according to claim 1, comprising: The first attenuation means matches the path loss of the first path with the path loss of the second path, and the second attenuation means matches the path loss of the third path with the path loss of the fourth path. The orthogonal polarization multiplex transmission apparatus according to claim 3, wherein the transmission is performed. The first optical path length correcting means is a delay for adjusting a phase so as not to cause interference between two multiplexed lights having the same polarization direction and input to the first coupling means, and not to break a signal pulse. Consisting of elements,
The second optical path length correction means is a delay for adjusting a phase so as not to cause interference between two multiplexed lights having the same polarization direction and input to the second coupling means, and not to destroy a signal pulse. Consisting of elements,
4. The orthogonal polarization multiplex transmission device according to claim 3, wherein: 5. The orthogonal polarization multiplex transmission apparatus according to claim 4, wherein said first attenuation means and said second attenuation means each have variable attenuation. 6. The orthogonal polarization multiplex transmission apparatus according to claim 5, wherein the first optical path length correction unit and the second optical path length correction unit each have a variable delay amount. 4. The orthogonal polarization multiplex transmission apparatus according to claim 3, wherein each of said first polarization rotation means and said second polarization rotation means has a variable polarization rotation angle.

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