JP2013182235A - Polarization mode dispersion generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarization mode dispersion generator which can variably generate DGD in a microsecond area, can be utilized in a wide wavelength band, and can dynamically imitate PMD by generating the PMD that varies similarly to the change of the PMD generated in an optical fiber.SOLUTION: First polarization mode dispersion is added in a first birefringence crystal 104 to input signal light 101 and it is outputted. For the output light, differential group delay of the input signal light is variably controlled in a mode mixer 105, and it is rotated centering on one axis of three orthogonal axes defining a Stokes space, and outputted. The output light is inputted to a birefringence phase canceller 106, a phase shift amount is adjusted so as to uniformize a phase difference between orthogonal polarization components within a wavelength band of the input signal light, and it is outputted. The output light is inputted to a second birefringence crystal 107, second polarization mode dispersion is added, and it is outputted.

Description

  The present invention relates to a polarization mode dispersion generating apparatus that can be used when evaluating the influence of polarization mode dispersion generated in an optical fiber transmission line on an optical transmission system.

  Polarization mode dispersion (PMD) is a phenomenon in which arrival time differences occur between orthogonal polarization mode components of signal light at the receiving end due to birefringence distributed in the optical fiber transmission line. This arrival time difference is called a differential group delay (DGD).

  In general, methods such as multi-level signal light intensity information and phase information, improvement of symbol rate, expansion of wavelength region, multiplexing of polarization space, and the like are applied to speed up optical transmission. When the symbol rate is increased, the bit period is shortened, so the influence of PMD appears significantly. In addition, the signal light affected by PMD has a different polarization state (SOP: State of Polarization) with respect to the wavelength. Therefore, in optical communication using polarization multiplexed signal light using orthogonal polarization space, the receiving side This has an adverse effect on the polarization separation processing to be performed.

  In recent years, in a coherent optical communication system using phase information in a practical stage, an optical stage compensation method and an electrical stage compensation method on the receiving side are known for PMD. The optical stage compensation method realizes a PMD having a reverse characteristic to the PMD of the optical fiber transmission line by an optical circuit configured by combining optical elements, and this method is a method for compensating the PMD of the optical fiber transmission line. There is also equalization.

  On the other hand, as an electrical stage compensation method, A / D (Analog to Digital) conversion is performed at twice the symbol rate, and digital signal processing is performed by a finite impulse response (FIR) filter to perform waveform equalization. Is also known. This is because a reverse transfer characteristic of a linear phenomenon can be calculated using an FIR filter on the receiving side, so that the polarization multiplexed signal can be separated and the PMD can be equalized. These digital signal processing systems have reported that polarization separation is possible in a processing time on the order of microseconds (μs), and the PMD equalization response speed generally performed electrically uses an optical PMD compensator. It is faster than if it were.

  It is necessary to evaluate PMD tolerance for optical transmission systems equipped with these polarization-dependent optical transmission lines. For this evaluation, a PMD generator that operates at a speed higher than the digital signal processing speed in the system. is necessary. Further, for example, when a PMD generator is used for evaluation of a wavelength division multiplexing optical transmission system, it is preferable that the PMD generator does not have wavelength dependency.

  On the other hand, when evaluating a coherent optical communication system in which the symbol rate of the optical signal is large compared to 1/2 of the Nyquist frequency of the A / D converter, PMD that exhibits the reverse transfer characteristics of PMD by optical means A compensation device is required. A PMD compensation device applied to such an ultra-wide wavelength band signal is required to ensure a high-speed response characteristic and a wide wavelength bandwidth sufficient for the signal bandwidth.

  The PMD compensator that realizes PMD compensation optically has the excellent feature that it does not depend on the modulation format and transmission bit rate of the optical pulse signal, and research to make it possible to cope with the expansion of the symbol rate that should be realized in the future. Is widely practiced. In addition, since the PMD emulator and the PMD generation device are required to have high-speed response characteristics, the number of setting parameters for these devices and the ease of setting are required.

  When a PMD emulator is used for PMD fluctuation tolerance evaluation of a polarization multiplexed optical transmission system, the PMD emulator is required to be able to generate a pseudo PMD that resembles a PMD change that occurs in an optical fiber transmission line. .

  For this reason, a PMD compensation device having a Michelson interferometer type configuration using a polarizing beam splitter has been disclosed (see Non-Patent Document 1). This PMD compensator generates DGD by changing the optical path length for one optical signal after the input signal light is separated into two optical paths by the polarization beam splitter. Since this PMD compensation device optically separates the input signal light into two paths, it is possible to generate a variable primary PMD having no wavelength dependency.

  The PMD generator disclosed in Patent Document 1 employs a technique for generating PMD by reciprocating a light path formed by using a reflecting mirror between a birefringent crystal and a polarization rotation element, and continuously increasing the speed. It is possible to generate PMD. However, the generated PMD vector is strongly wavelength dependent. Here, the PMD vector is a vector whose magnitude indicates the magnitude of DGD and whose direction is defined as a direction parallel to the unit Stokes vector of the main polarization state.

  In addition, a PMD compensation device configured by connecting a plurality of birefringent crystals of different sizes via a polarization rotation element is disclosed (see Non-Patent Document 2). This PMD compensator is configured by connecting multiple birefringent crystals of different sizes via polarization rotation elements, and the value of DGD generated in each birefringent crystal is a power of 2 Six birefringent crystals are used. And it is set as the structure which can set 64 types of DGD values by controlling the polarization | polarized-light rotation element installed between these birefringent crystals. As the polarization rotation element, an element having a polarization rotation axis orthogonal to the main polarization axis (PSP: Principal State of Polarization) of the birefringent crystal in Stokes space is used.

  According to this PMD compensator, the combination of fast axis and slow axis of these birefringent crystals can be selected by binary polarization rotation control of 0 degree and 90 degree of the polarization rotation element. Thus, a variable PMD can be realized, and a primary PMD with extremely low wavelength selectivity can be generated. The 90-degree polarization switch can be formed by using a switch using a magneto-optic effect, etc., and can generate PMD variably in a microsecond speed range.

Yoshihiro Kanda et al., "Highly Stable 160-Gb / s Field Transmission Employing Adaptive PMD Compensator with Ultra High Time-Resolution Variable DGD Generator", ECOC2008 We.3.E.6, September 2008 Lianshan Yan, et al., "Programmable Group-Delay Module Using Binary Polarization Switching", Journal of Lightwave Technology, Vol. 21, No. 7, pp. 1676-1684, July 2003

JP 2003-228026 A

  The evaluation of PMD resistance is an important evaluation item for an optical transmission system. In particular, since the SOP of an optical signal affected by PMD is not uniform in its spectrum, PMD adversely affects the operation performance of a polarization dependent system such as polarization separation of a polarization multiplexed signal. Furthermore, it is necessary to evaluate the reception characteristics when the magnitude of the PMD vector (DGD value) dynamically changes in the optical transmission system.

  In recent years, in a digital coherent system in a commercial stage, polarization separation in the microsecond region is possible by a digital signal processing circuit. As described above, in a digital coherent receiver, after converting an optical signal into an electrical signal, waveform equalization of a linear phenomenon including PMD is performed by an FIR filter that is an inverse transfer function of PMD. PMD equalization is performed by updating the tap coefficient of the adaptive FIR filter, but a high-speed PMD emulator is required to evaluate the processing speed.

  Furthermore, in order to follow the changes of SOP and PMD adaptively at the receiver, the PMD emulator used for PMD tolerance evaluation has a PMD that changes in the same way as the PMD changes that occur in the installed optical fiber. Is required (dynamic imitation of PMD). For this reason, the PMD emulator needs to have a function of changing the PMD continuously at a high speed, not discretely.

  Further, in ultra-high speed optical transmission with a symbol rate exceeding 100 Gb / s, a PMD compensation device that optically compensates for PMD is generally used. That is, with respect to transmission of an ultrahigh-speed optical signal, a PMD compensation device is used for the purpose of reducing intersymbol interference on the receiving side. In addition, optical signals affected by PMD are difficult to use with polarization dependent devices because SOP is non-uniform in the spectral domain. For this reason, it is necessary to execute PMD compensation by the PMD compensator before the polarization dependent device.

  The PMD emulator generates a PMD transfer function. The PMD compensator generates an inverse function of the transfer function of the PMD of the transmission line. Any of these devices can be appropriately formed by using a conventional well-known technique using a PMD generator of the same form for generating PMD. That is, if a PMD generator exists, the PMD compensator and the PMD emulator can be formed by appropriately combining elements such as a polarization plane controller with the PMD generator using a known technique. Therefore, the PMD generator will be described below.

  Considering application to wavelength division multiplexed signals including ultra-high-speed signals (ultra-wide wavelength band signals) and digital coherent systems, it is necessary to ensure the wide wavelength band of the PMD generator. In general, when a birefringent medium having a plurality of arbitrary PSPs is connected, the PMD spectrum has a strong wavelength dependence due to the influence of the birefringent phase. That is, in addition to the change in PSP indicating the direction of the polarization mode, higher-order PMD occurs. High-order PMD is a phenomenon in which the difference in propagation speed between a photoelectric field component parallel to the fast axis and a photoelectric field component parallel to the slow axis changes depending on the frequency (wavelength) of the optical carrier.

In view of the situation described above, a PMD generator that can be used when evaluating the PMD tolerance of an optical transmission system is required to have the following characteristics (1) to (3). That is,
(1) It is possible to variably generate DGD in the microsecond range.
(2) The wavelength dependency of the generated PMD is small and can be used in a wide wavelength band.
(3) To generate PMD that changes in the same way as PMD changes that occur in installed optical fibers, that is, to be able to imitate PMD dynamically.
Is requested.

  The PMD compensation device disclosed in Non-Patent Document 1 can generate uniform PMD in a wide wavelength band, but optically changes the optical path length between orthogonal polarization components by variable PMD operation. Therefore, the propagation time difference (DGD) between the orthogonal polarization components makes a round on the Poincare sphere with an optical carrier period (several femtoseconds). Therefore, compared to PMD generated in installed optical fiber, the amount of SOP change accompanying DGD change of several picoseconds is enormous, which is not suitable for evaluating polarization-dependent systems. In addition, since the change in DGD is mechanically realized using a stepping motor or the like, the operation speed is as low as about milliseconds. In addition, the PMD compensation device disclosed in Patent Document 1 can continuously change the PMD value and the amount of SOP fluctuation associated therewith, but has a large wavelength dependency.

  Similar to the PMD compensation apparatus disclosed in Non-Patent Document 1, the PMD compensation apparatus disclosed in Non-Patent Document 2 can generate a uniform PMD in a wide wavelength band. However, since the 90-degree polarization rotation element is controlled in accordance with the DGD variable operation, the SOP of the PMD additional light that is output is discrete. As described above, a PMD emulator needs to realize a continuous change in PMD in order to dynamically imitate PMD. However, in the PMD compensation device disclosed in Non-Patent Document 2, the output light This requirement is not satisfied because the SOP associated with the PMD value is discrete. In addition, the PMD compensation device disclosed in Non-Patent Document 2 requires many control elements in order to configure.

  Neither of the PMD generators constituting any of the PMD compensators described above can generate a uniform PMD in a wide wavelength band and generate a PMD continuously and rapidly at a high speed.

  In order to solve the above-mentioned problems, the inventor of this application can adjust the phase shift amount so that the phase difference between the orthogonal polarization components is uniform within the wavelength band of the signal light. A new mode mixer has been devised in which the DGD value between the orthogonal polarization components of the signal light can be variably controlled by controlling the canceller and one polarization rotation element. Then, the inventors came up with the idea that a PMD generator capable of solving the above-described problems can be constructed by appropriately configuring a PMD generator with the birefringence phase canceller and the mode mixer.

  Accordingly, an object of the present invention is to provide a PMD generator capable of obtaining the characteristics shown in the above (1) to (3).

  According to the first to third aspects of the present invention based on the above philosophy, the following PMD generator is provided.

  The PMD generator according to the first aspect of the present invention includes a first birefringent crystal, a mode mixer, a birefringent phase canceller, and a second birefringent crystal. The input signal light is input to the first birefringent crystal, added with the first PMD, and output. The output light output from the first birefringent crystal is input to the mode mixer, the SOP is variably controlled, and the SOP is orthogonal to the PMD vectors of the first birefringent crystal and the second birefringent crystal in the Stokes space. Rotated around one axis and output. The output light output from this mode mixer is input to the birefringence phase canceller, and the phase shift amount is adjusted and output so that the phase difference between the orthogonal polarization components is uniform within the wavelength band of this output light. . The output light output from the birefringent phase canceller is input to the second birefringent crystal, added with the second PMD, and output. The dynamic DGD operation of the PMD generator is obtained by SOP rotation of the mode mixer.

  The PMD generator according to the second aspect of the present invention includes an optical circulator, a birefringent crystal, a birefringent phase canceller, a mode mixer, and a reflecting mirror. The input signal light is input to the birefringent crystal via the optical circulator, added with PMD, and output. The output light output from the birefringent crystal is input to the birefringence phase canceller, and the phase shift amount is adjusted and output so that the phase difference between the orthogonal polarization components is uniform within the wavelength band of the output light. The The output light output from the birefringence phase canceller is input to the mode mixer, the SOP is variably controlled, and the SOP is one axis orthogonal to the PMD vectors of the first birefringent crystal and the second birefringent crystal in the Stokes space. Is rotated around the center and output. The output light output from the mode mixer is reflected by the reflecting mirror, passes through the mode mixer, the birefringent phase canceller, and the birefringent crystal again, and is output via the optical circulator. The dynamic DGD operation of the PMD generator is obtained by SOP rotation of the mode mixer.

  A PMD generator according to a third aspect of the present invention includes an optical circulator, a birefringence phase compensation difference group delay generator (hereinafter also referred to as a birefringence phase compensation DGD generator), a mode mixer, and a reflecting mirror. Composed. The input signal light is input to the birefringence phase compensation DGD generator via the optical circulator, PMD is added, and the phase difference with respect to one wavelength of the orthogonal polarization component of the input signal light is canceled, and this orthogonal Both polarization components are combined into a polarization combined signal and output. This polarization composite signal is input to the mode mixer, the SOP is variably controlled, and the SOP is rotated around one axis orthogonal to the PMD vector of the first birefringent crystal and the second birefringent crystal in the Stokes space. Is output. The output light output from the mode mixer is reflected by the reflecting mirror, passes through the mode mixer and the birefringence phase compensation DGD generator again, and is output through the optical circulator. The dynamic DGD operation of the PMD generator is obtained by SOP rotation of the mode mixer.

  According to the first to third aspects of the present invention, although described in detail later, a PMD generator having the characteristics shown in the above (1) to (3) is realized. In other words, a PMD generator capable of generating a uniform PMD spectrum over a wide wavelength band and generating PMD continuously and variably at high speed is realized.

  Further, according to the first aspect, variable control of the high-speed DGD of the input signal light can be easily realized by the mode mixer.

  According to the second aspect, since PMD generation is realized using only one birefringent crystal, an effect of being hardly influenced by the external environment can be obtained.

  According to the third aspect, similar to the PMD generator of the second aspect, the PMD generation is realized by using only one birefringent crystal, so that the effect of being hardly influenced by the external environment can be obtained. In addition, by using the birefringence phase compensation DGD generator, the required number of parts can be reduced.

FIG. 2 is a block configuration diagram showing a schematic configuration of a first PMD generation device. FIG. 3 is a block configuration diagram showing a schematic configuration of a birefringence phase canceller that is a component of the first PMD generation device, including a schematic configuration of a microdispersion generator. FIG. 2 is a block configuration diagram showing a schematic configuration of a mode mixer that is a component of the first PMD generation device. FIG. 2 is a block configuration diagram showing a schematic configuration of the entire device including schematic configurations of a mode mixer, a birefringence phase canceller, and a microdispersion generator of the first PMD generation device. It is a schematic three-dimensional block diagram of the conventional PMD generator of the same kind. It is a figure which shows the example of calculation of the PMD vector obtained by the conventional same type PMD generator. FIG. 10 is a diagram showing a calculation example in the case of the PMD vector obtained by the first PMD generation apparatus and having a sufficiently high resolution of the diffraction grating. FIG. 10 is a diagram showing a calculation example in the case of a PMD vector obtained by the first PMD generator and having a diffraction grating resolution of 10 GHz. It is a figure which shows the relationship of the magnitude | size of DGD controlled with respect to the phase difference which arises with a polarization rotation element. FIG. 5 is a block configuration diagram showing a schematic configuration of a second PMD generation device. It is a figure where it uses for description about the relationship between the optical axis of a mode mixer and the optical axis of a birefringent crystal. FIG. 5 is a block configuration diagram showing a schematic configuration of the entire device including a schematic configuration of a mode mixer, a birefringence phase canceller, and a microdispersion generator of the second PMD generation device. FIG. 10 is a diagram showing an example of calculation in the case of a PMD vector obtained by the second PMD generation apparatus, in which the resolution of the diffraction grating is sufficiently high and ideal. FIG. 5 is a block configuration diagram showing a schematic configuration of a third PMD generation device. FIG. 10 is a block configuration diagram showing a schematic configuration of the entire device including a schematic configuration of a birefringence phase compensation DGD generator, a mode mixer, and a minute dispersion generator 115 of a third PMD generation device. FIG. 10 is a diagram showing a calculation example in the case of the PMD vector obtained by the third PMD generation device and having an ideal case where the resolution of the diffraction grating is sufficiently high.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 1 to 4, FIG. 10 to FIG. 12, FIG. 14, and FIG. 15 illustrate one configuration example according to the present invention, and the arrangement relationship of each component is understood to the extent that the present invention can be understood. It is only schematically shown and the present invention is not limited to the illustrated example.

  In the following description, specific elements and operating conditions may be taken up. However, these elements and operating conditions are only one preferred example, and the present invention is not limited to these. In addition, when expressing a vector, it is common to add an arrow on the letter indicating the vector quantity or indicate it with bold letters, and the vector size itself is represented with ordinary letters. In the description, the vector quantity is indicated by ordinary characters except when the vector quantity is used in the mathematical expression.

<First PMD generator>
With reference to FIGS. 1 to 4, the configuration, the operation, and the effects obtained of the first PMD generator according to the embodiment of the present invention will be described.

(Constitution)
FIG. 1 is a schematic block diagram of the first PMD generator. The first PMD generator includes a first birefringent crystal 104, a mode mixer 105, a birefringent phase canceller 106, and a second birefringent crystal 107.

In FIG. 1, the first birefringent crystal 104 is displayed as “τ ₁ ”, the mode mixer 105 is displayed as “C ₀ ”, the birefringent phase canceller 106 is displayed as “C ₁ (f)”, and the second The birefringent crystal 107 is indicated as “τ ₂ ”.

  The input signal light 101 is input to the first birefringent crystal 104 via the first fiber collimator 102, and the first PMD is added and output. The output light output from the first birefringent crystal 104 is input to the mode mixer 105, the SOP is variably controlled, and the SOP is orthogonal to the PMD vectors of the first birefringent crystal and the second birefringent crystal in the Stokes space. It is rotated around one axis and output. The output light output from the mode mixer 105 is input to the birefringence phase canceller 106, and the phase shift amount is adjusted and output so that the phase difference between the orthogonal polarization components is uniform within the wavelength band of the output light. The The output light output from the birefringence phase canceller 106 is input to the second birefringent crystal 107, added with the second PMD, and output to the outside as the output signal light 109 via the second fiber collimator 108. Is done.

  FIG. 2 is a schematic block diagram of the birefringence phase canceller 106. As shown in FIG. The birefringence phase canceller 106 includes a polarizing beam splitter 110, a first quarter wavelength plate 111 (45 degree λ / 4 plate), a second quarter wavelength plate 113 (45 degree λ / 4 plate), a first A reflecting mirror 112 and a fine dispersion generator 115 are provided.

  The output light output from the mode mixer 105 is separated into two orthogonally polarized components by the polarization beam splitter 110.

  One of the separated polarization components passes through the first quarter-wave plate 111, is reflected by the first reflecting mirror 112, passes again through the first quarter-wave plate 111, and passes through the first quarter-wave plate 111, and then is polarized by the polarization beam splitter 110. It is reflected and output. The output light output from the birefringent phase canceller 106 is input to the second birefringent crystal 107.

  The other polarization component passes through the second quarter-wave plate 113, is input to the microdispersion generator 115 via the fiber collimator 114, and is output with the phase shift amount adjusted for each wavelength. This output light is again input to the second quarter-wave plate 113 via the fiber collimator 114, passes through this, passes through the polarization beam splitter 110, and is input to the second birefringent crystal 107.

  The fine dispersion generator 115 includes a collimator mirror 116, a diffraction grating 117, a lens 118, a phase shifter array 119, and a second reflecting mirror 120. The polarization component input to the minute dispersion generator 115 is reflected by the second reflecting mirror 120 through the collimator mirror 116, the diffraction grating 117, the lens 118, and the phase shifter array 119 sequentially. Thereafter, the light is output again through the phase shifter array 119, the lens 118, the diffraction grating 117, and the collimator mirror 116.

  The microdispersion generator 115 is an apparatus that can adjust the phase shift amount independently for each wavelength, and for example, a variable band spectrum shaper manufactured by OptQuest can be used as appropriate (a variable band spectrum shaper). For technical details, refer to JP 2008-310190 A). As shown in FIG. 2, the micro-dispersion generator 115 is configured to be able to variably change the phase shift amount for each wavelength by the phase shifter array 119 after being spectrally dispersed by the diffraction grating 117 and spectrally dispersed. . By arranging the micro-dispersion generator 115 in one optical path of the Michelson interferometer structure, the birefringence phase canceller 106 capable of controlling the SOP for each wavelength is configured.

  FIG. 3 is a schematic block diagram of the mode mixer 105. As shown in FIG. The mode mixer 105 includes a third quarter wavelength plate 121 (45 degree λ / 4 plate), a polarization rotation element 122, and a fourth quarter wavelength plate (−45 degree λ / 4 plate) 123 in this order. It is set as the arrangement prepared.

  The polarization rotation element 122 is an element capable of continuously changing the phase difference between orthogonal polarization components by applying a voltage from the outside. With this polarization rotation element 122, the variable operation of the DGD is realized.

The optical axis of the polarization rotation element 122 is set to face the same direction as the optical axes of the first birefringent crystal 104 and the second birefringent crystal 107. A third quarter wavelength plate 121 (45 degree λ / 4 plate) and a fourth quarter wavelength plate 123 (-45 degree λ / 4 plate) are arranged on both sides of the polarization rotation element 122. Accordingly, the phase difference amount between the orthogonal polarization axis caused by the polarization rotation element 122, it is possible to rotate the SOP about the S ₃ axis of the Stokes space. The central axis of SOP rotation of the first birefringent crystal 104 and the second birefringent crystal 107 is the S ₁ axis of Stokes space, and the central axis of polarization rotation generated by the mode mixer 105 is the S ₃ axis of Stokes space. They are orthogonal to each other and the S ₁ axis and the S ₃ axis.

  Since the operation speed of the first PMD generator is determined by the operation characteristics of the polarization rotation element 122, it is preferable to use an element that can variably generate DGD at high speed for the polarization rotation element 122. . As a polarization rotation element that can operate in the microsecond range, for example, a polarization rotation element of Boston Applied Technology INC. (See Dynamic Polarization Rotator: http://www.bostonati.com/Products_PRM.html) is used as appropriate be able to.

(Operation)
The operation of the first PMD generator will be described with reference to FIG. FIG. 4 is a block configuration diagram showing a schematic overall configuration of the first PMD generator including the schematic configurations of the mode mixer 105, the birefringence phase canceller 106, and the minute dispersion generator 115. As shown in FIG.

The input signal light 101 is input to the first birefringent crystal 104 via the first fiber collimator 102. The first birefringent crystal 104 provides the input signal light 101 with a PMD vector τ ₁ having no wavelength dependency as the first PMD.

The signal light output from the first birefringent crystal 104 is input to the mode mixer 105. Mode mixer 105 rotates about the S ₃ axis of the Stokes space SOP of the input signal light. The signal light output from the mode mixer 105 is input to the birefringence phase canceller 106. The birefringence phase canceller 106 adjusts the phase shift amount so that the phase difference between the orthogonal polarization components is uniform within the wavelength band of the input signal light. The signal light output from the birefringent phase canceller 106 is input to the second birefringent crystal 107. The second birefringent crystal 107 gives a PMD vector τ ₂ having no wavelength dependency as the second PMD.

The PMD vector of the first birefringent crystal 104 is (τ ₁ , 0, 0) ^T , the PMD vector of the second birefringent crystal 107 is (τ ₂ , 0, 0) ^T , and the rotation matrix of the second birefringent crystal 107 is If R ₂ , the rotation matrix of the birefringence phase canceller 106 is C ₁ (f), and the rotation matrix of the mode mixer 105 is C ₀ , the rotation matrix of each element is given as follows. Here, φ is a birefringent phase generated in the second birefringent crystal 107, and is given by φ = 2πf | τ ₂ |. f is the frequency of the signal light. α is a phase difference between orthogonal polarization components generated by the polarization rotation element 122 in the mode mixer 105. θ is a phase generated in the phase shifter array 119 and is variable for each wavelength band dispersed by the diffraction grating 117.

  The multiplication of the rotation matrix of the second birefringent crystal 107 and the birefringent phase canceller 106 is given as follows.

Here, for simplicity, assuming that τ = τ ₁ = τ ₂ , the total PMD vector τ _all in the first PMD generator is given by the following equation (1) using the PMD connection law.

Here, the phase difference (birefringence phase) φ between the orthogonal polarization components generated by the second birefringent crystal 107 is given by 2πf | τ ₂ |, and any phase within the wavelength band of the optical signal applied in normal optical communication Regarding the wavelength, the birefringence phase canceller 106 is adjusted so that φ + θ = (constant value).

Since the size tau _all the PMD vector gives the DGD amount, with respect to the phase α caused by the polarization rotation element 122, a variable PMD operation is realized with no wavelength dependence in accordance with the following equation (2).

The wavelength dependency of the PMD vector is caused by the fact that the birefringence phase φ (= 2πf | τ ₂ |) differs depending on the wavelength. Birefringent phase canceller 106 is configured to cancel the wavelength dependence of the rotation matrix R _2. The setting condition for the fine dispersion generator 115 depends on the amount of DGD generated in the first birefringent crystal 104 and the second birefringent crystal 107. For example, when the amount of DGD of the first birefringent crystal 104 or the second birefringent crystal 107 is 10 ps (picoseconds), the birefringence phase φ becomes the same SOP with a frequency transition of 100 GHz.

  As described above, according to the first PMD generator, a PMD vector of a spectrum based on the equation (1) (a uniform PMD vector in a wide wavelength band) can be obtained, and continuously variable based on the equation (2). DGD is generated.

(effect)
Next, the effects obtained by the first PMD generator will be described with reference to FIGS. 5 to 8 in comparison with the effects obtained by the conventional PMD generator of the same type.

  FIG. 5 is a schematic three-dimensional configuration diagram of a conventional PMD generator of the same type. In the PMD generation device shown in FIG. 5, a mode mixer 405 is disposed after the birefringent crystal 404, and output light from the mode mixer 405 is reflected by a reflecting mirror 406. In the calculation of the PMD vector by the simulation shown below, the first birefringent crystal 104 and the second birefringent crystal constituting the first PMD generator are used as the birefringent crystal 404 constituting the conventional PMD generator of the same type. The same as 107 was used. The mode mixer 405 is the same as the mode mixer 105.

  FIG. 6 shows a calculation example of a PMD vector obtained by a conventional PMD generator of the same type. FIG. 7 shows a calculation example of the PMD vector obtained by the ideal first PMD generator in which the resolution of the diffraction grating 117 is sufficiently higher than 10 GHz. FIG. 8 shows a calculation example of the PMD vector obtained by the first PMD generator in which the resolution of the diffraction grating 117 is 10 GHz. Each of the calculation examples shown in FIGS. 6 to 8 calculates the DGD amount of the first birefringent crystal 104 and the second birefringent crystal 107 as 10 ps and the phase generation amount α of the polarization rotation element 122 as π / 4. It was obtained.

6 to 8, the horizontal axis indicates the value of 2πf | τ | in radians (rad), and the vertical axis indicates the Stokes parameters (s ₁ , s ₂ , s ₃ ) and DGD. Scaled in ps units. Each of the horizontal axes in FIGS. 6 to 8 corresponds to a 10 ps phase difference generated in the birefringent crystal, and in the expression using the wavelength band, the range from 0 to 2π corresponds to 0 to 100 GHz. Here, since the phase generation amount α of the polarization rotation element 122 is π / 4, the DGD value | τ _all | given by the equation (2) is 18.48 ps.

As shown in FIG. 6, according to the conventional PMD generator of the same type, the direction of the PMD vector is largely rotated with respect to the wavelength. This can be read from FIG. 6 because the values of the Stokes parameters (s ₁ , s ₂ , s ₃ ) vary greatly. On the other hand, according to the first PMD generator, it can be seen that the transition of the change in the PMD vector with respect to the wavelength can be sufficiently suppressed, and a highly uniform PMD generation characteristic is obtained in a wide wavelength band. . This is because the values of the Stokes parameters (s ₁ , s ₂ , s ₃ ) are not changed in FIG. 7, and the values of the Stokes parameters (s ₁ , s ₂ , s ₃ ) are the same as those in FIG. It is understood that it does not fluctuate as much. As shown in FIGS. 6 to 8, the higher the resolution of the diffraction grating 117, the smaller the variation in the value of the Stokes parameters (s ₁ , s ₂ , s ₃ ).

  In addition, once the condition setting for the minute dispersion generator 115 is performed, it is not necessary to change the condition after that. Therefore, the DGD variable operation can be realized only by modulating the voltage applied to the polarization rotation element 122.

  As is clear from the above equation (2), the range of the phase shift amount to be generated by the polarization rotation element 122 required for DGD control from the maximum DGD to the minimum DGD is 0 to πrad. With reference to FIG. 9, the magnitude of the DGD controlled with respect to the phase difference generated in the polarization rotation element 122 will be described. In FIG. 9, the horizontal axis indicates the phase difference generated by the polarization rotation element 122 in radians, and the vertical axis indicates the DGD amount in units of ps. The magnitude of DGD when the phase difference generated in the polarization rotation element 122 is 0 is 20 ps, and the magnitude of DGD when the phase difference is πrad is 0 ps. That is, DGD control from the maximum DGD (20 ps) to the minimum DGD (0 ps) is realized when the phase shift amount to be generated by the polarization rotation element 122 is in the range of 0 to πrad.

  If an element capable of continuous phase change in the microsecond region is used as the polarization rotation element 122, high-speed variable DGD can be generated in the microsecond region. Furthermore, the phase difference between the orthogonal polarization components necessary for the variable control from the minimum DGD to the maximum DGD is π, and the fluctuation of the SOP associated therewith is sufficiently small. Although the calculation results when the phase shift amount α is π / 4 are shown with reference to FIGS. 7 to 9, the wavelength spread of the PMD vector is generally sufficiently suppressed regardless of the value of α.

  From the above, according to the first PMD generator, the characteristics shown in the above (1) to (3) to be provided by the PMD generator can be realized.

<Second PMD generator>
With reference to FIGS. 10 to 13, the configuration of the second PMD generator according to the embodiment of the present invention, the operation thereof, and the obtained effects will be described. Elements having the same functions as those of the first PMD generator described above are denoted by the same reference numerals, and redundant description thereof may be omitted. In addition, regarding the same components as those of the first PMD generation device described above, redundant description of the operations and effects thereof will be omitted.

(Constitution)
FIG. 10 is a schematic block configuration diagram of the second PMD generator. The second PMD generator includes an optical circulator 202, a birefringent crystal 204, a birefringent phase canceller 106, a mode mixer 205, and a reflecting mirror 206. The birefringent crystal 204 is the same element as the first or second birefringent crystal (104, 107) included in the first PMD generator, and the birefringent phase canceller 106 is also the first PMD generator. This is the same element as the birefringent phase canceller.

  The input signal light 201 is input to the birefringent crystal 204 through the optical circulator 202 and the fiber collimator 203, added with PMD, and output from the birefringent crystal 204. The output light output from the birefringent crystal 204 is input to the birefringent phase canceller 106, and the phase shift amount is adjusted so that the phase difference between the orthogonal polarization components is uniform within the wavelength band of the output light. Is output. The output light output from the birefringence phase canceller 106 is input to the mode mixer 205 so that the SOP is variably controlled, and the SOP is orthogonal to the PMD vectors of the first birefringent crystal and the second birefringent crystal in the Stokes space. It is rotated around one axis and output. The output light output from the mode mixer 205 is reflected by the reflecting mirror 206, passes through the mode mixer 205, the birefringent phase canceller 106, and the birefringent crystal 204 again, and is output through the optical circulator 202. ing.

In FIG. 10, the birefringent crystal 204 is indicated as “τ ₁ ”, the birefringent phase canceller 106 is indicated as “C ₁ (f)”, and the mode mixer 205 is indicated as “C ₀₂ ”.

  With reference to FIGS. 11A and 11B, the relationship between the optical axis of the mode mixer 205 and the optical axis of the birefringent crystal 204 will be described. FIG. 11A is a diagram showing the positional relationship between the mode mixer 205 and the birefringent crystal 204, and FIG. 11B is a diagram for explaining the direction of the mode mixer 205.

The mode mixer 205 uses a polarization rotation element having the same configuration and function as the mode mixer 105 included in the first PMD generator. The optical axis of the polarization rotator 122 is disposed so as to be inclined by 45 degrees with respect to the optical axis of the birefringent crystal 204. The mode mixer 205 by such a structure, the mode mixer 205 is able to rotate the SOP about the S ₂ axis of the Stokes space. The central axis of SOP rotation of the birefringent crystal 204 is the S ₁ axis of Stokes space, and the central axis of polarization rotation generated by the mode mixer 205 is the S ₂ axis of Stokes space. These S ₁ axis and S ₂ axis are They are orthogonal to each other.

  With reference to FIG. 12, the birefringence phase canceller 106, the mode mixer 205, and the minute dispersion generator 115 included in the second PMD generator will be described. The birefringent phase canceller 106 also has the same structure as the element provided in the first PMD generator.

  The output light output from the birefringent crystal 204 is separated into two orthogonally polarized components by the polarization beam splitter 110.

  One polarization component passes through the first quarter-wave plate 111, is reflected by the first reflecting mirror 112, passes again through the first quarter-wave plate 111, and is reflected by the polarization beam splitter 110. Is input to the mode mixer 205, is output from the mode mixer 205, is reflected by the reflecting mirror 206, passes through the mode mixer 205 again, is reflected by the polarization beam splitter 110, and is reflected by the first quarter wave plate 111, the first 1 reciprocates through the reflecting mirror 112, passes through the polarizing beam splitter 110 again, and is input to the birefringent crystal 204.

  The other polarization component passes through the second quarter wave plate 113 and is input to the microdispersion generator 115 via the fiber collimator 114, and the phase shift amount is adjusted for each wavelength of the other polarization component. Is output. This output light is again input to the second quarter-wave plate 113 via the fiber collimator 114, passes through this, is reflected by the polarization beam splitter 110, and is input to the birefringent crystal 204. ing.

  The fine dispersion generator 115 includes a collimator mirror 116, a diffraction grating 117, a lens 118, a phase shifter array 119, and a second reflecting mirror 120.

  The other polarization component output from the birefringent crystal 204 and separated into two orthogonal polarization components by the polarization beam splitter 110, and the polarization component that has passed through the second quarter-wave plate 113 is collimated by the collimator mirror 116. The grating 117, the lens 118, and the phase shifter array 119 are sequentially passed through and reflected by the second reflecting mirror 120. The phase shifter array 119, the lens 118, and the diffraction grating 117 are passed again in this order and reflected by the collimator mirror 116. It is configured to be returned to the two-quarter wavelength plate 113.

(Operation)
The operation of the second PMD generator will be described with reference to FIG. FIG. 12 is a block diagram showing the overall configuration of the second PMD generator including the schematic configurations of the mode mixer 205, the birefringence phase canceller 106, and the microdispersion generator 115.

The input signal light 201 is input to the birefringent crystal 204 via the optical circulator 202 and the fiber collimator 203, and is given a PMD vector τ ₁ having no wavelength dependency. The output light output from the birefringent crystal 204 is input to the birefringent phase canceller 106 and adjusted so that the phase difference between the orthogonal polarization components becomes equal to the wavelength. The output light from the birefringence phase canceller 106 has its SOP rotated around the S ₂ axis of the Stokes space by the mode mixer 205. The output light from the mode mixer 205 is reflected by the reflecting mirror 206 and follows the path of the mode mixer 205, the birefringent phase canceller 106, and the birefringent crystal 204. The output signal light 207 given PMD is output from the output end of the optical circulator 202 to the outside.

  The PMD vector generated in the birefringent crystal 204 and the rotation matrix that defines the birefringent phase canceller 106 are the same as those for the corresponding elements provided in the first PMD generator.

When the rotation matrix of the birefringent crystal 204 is R ₁ and the rotation matrix of the mode mixer 205 is C ₀₂ , the respective rotation matrices are given as follows.

Here, φ is a birefringent phase generated in the birefringent crystal 204, and is given by φ = 2πf | τ |. f is the frequency of light. α is a phase difference between orthogonal polarization components generated by the polarization rotation element in the mode mixer 205. The total PMD vector τ _all in the second PMD generator is given by the following equation (3) using the PMD connection law.

  Here, the phase difference (birefringence phase) φ between orthogonal polarization components generated by the birefringent crystal 204 is given by 2πf | τ |, but for any wavelength within the wavelength band of an optical signal applied in normal optical communication. , Φ + θ = (constant value), the birefringence phase canceller 106 is adjusted.

  Regarding the phase α generated in the mode mixer 205, since the second PMD generator is configured to be a reflection type, it passes through the mode mixer 205 twice. Therefore, in the second PMD generator, the mode mixer 205 With respect to the generated phase α, a variable PMD operation without wavelength dependency according to the following equation (4) is realized.

(effect)
Next, effects obtained by the second PMD generator will be described with reference to FIG. FIG. 13 shows a calculation example of the PMD vector obtained by the ideal second PMD generator whose resolution of the diffraction grating 117 is sufficiently higher than 10 GHz. The calculation example shown in FIG. 13 is obtained by calculating the DGD amount of the birefringent crystal 204 as 10 ps and the phase generation amount α of the mode mixer 205 as π / 4.

The horizontal axis of FIG. 13 shows the value of 2πf | τ | in radians, and the vertical axis shows the Stokes parameters (s ₁ , s ₂ , s ₃ ) and DGD in units of ps. The horizontal axis in FIG. 13 corresponds to the 10 ps phase difference generated in the birefringent crystal 204, and in the expression using the wavelength band, the range from 0 to 2π corresponds to 0 to 100 GHz. Here, since the phase generation amount α generated in the mode mixer 205 is π / 4, the DGD value | τ _all | given by the equation (4) is 14.14 ps.

According to the second PMD generation device, it can be seen that the transition of the change in the PMD vector with respect to the wavelength can be sufficiently suppressed, and PMD generation characteristics with high uniformity can be obtained in a wide wavelength band. This can be read from FIG. 13 because the values of the Stokes parameters (s ₁ , s ₂ , s ₃ ) do not change.

  It was shown that a PMD with a flat wavelength dependence can be generated even with a reflective configuration. In addition, the reflection type configuration requires only one birefringent crystal and is not easily affected by the external environment. The value of α may be half that of the first PMD generator, and there is an advantage that the voltage applied to the polarization rotator constituting the mode mixer 205 can be lowered.

<Third PMD generator>
With reference to FIGS. 14 to 16, the configuration of the third PMD generating apparatus according to the embodiment of the present invention, the operation thereof, and the obtained effects will be described. Elements having the same functions as those of the first and second PMD generators described above may be denoted by the same reference numerals, and redundant description thereof may be omitted. In addition, regarding the same components as those of the first and second PMD generators described above, redundant description of the operations and effects will be omitted.

  FIG. 14 is a schematic block diagram of a third PMD generator. The third PMD generation device includes an optical circulator 202, a birefringence phase compensation DGD generator 305, a mode mixer 306, and a reflecting mirror 206.

  The input signal light 301 is input to the birefringence phase compensation DGD generator 305 via the optical circulator 202, and PMD is added to cancel the phase difference of one of the orthogonal polarization components of the input signal light with respect to one wavelength. Thus, both of the orthogonal polarization components are combined with each other as a polarization combined signal and output.

  The polarization synthesis signal is input to the mode mixer 306, the SOP is variably controlled, and the SOP is rotated around one axis orthogonal to the PMD vectors of the first and second birefringent crystals in the Stokes space. Is output.

  The output light output from the mode mixer 306 is reflected by the reflecting mirror 206, passes through the mode mixer 306 and the birefringence phase compensation DGD generator 305 again, and is output via the optical circulator 202. Yes.

In FIG. 14, the birefringence phase compensation DGD generator 305 is indicated as “τ ₁ & C ₁ (f)”, and the mode mixer 306 is indicated as “C ₀₂ ”.

  With reference to FIG. 15, the birefringence phase compensation DGD generator 305, the mode mixer 306, and the minute dispersion generator 115 included in the third PMD generator will be described.

  The birefringence phase compensation DGD generator 305 includes a polarizing beam splitter 110, a first quarter wavelength plate 111, a second quarter wavelength plate 113, a first reflecting mirror 112, and a minute dispersion generator 115. Yes. The minute dispersion generator 115 includes a collimator mirror 116, a diffraction grating 117, a lens 118, a phase shifter array 119, and a second reflecting mirror 120.

  The input signal light 301 input from the optical circulator 202 to the birefringence phase compensation DGD generator 305 via the fiber collimator 203 is separated into two orthogonally polarized components by the polarization beam splitter 110.

  One of the polarization components passes through the first quarter-wave plate 111, is reflected by the first reflecting mirror 112, passes through the first quarter-wave plate 111 again, and is reflected by the polarization beam splitter 110. Is input to the mode mixer 306, is output from the mode mixer 306, is reflected by the reflecting mirror 206, passes again through the mode mixer 306, and is reflected by the polarization beam splitter 110, and the first quarter wave plate 111, The light travels back and forth through the first reflecting mirror 112 and is input again to the polarizing beam splitter 110, passes through the polarizing beam splitter 110, and is output via the optical circulator 202.

  The other polarization component passes through the second quarter-wave plate 113 and is input to the microdispersion generator 115, and the phase shift amount is adjusted and output for each wavelength of the other polarization component. The polarization component input to the micro-dispersion generator 115 that passes through the second quarter-wave plate 113 and is input to the polarization beam splitter 110 and reflected by the polarization beam splitter 110 is phase-shifted for each wavelength. The amount is adjusted and output.

  The mode mixer 306 is the same element as the mode mixer 205 provided in the second PMD generator described above, and includes a third quarter-wave plate 121, a polarization rotation element 122, and a fourth quarter-wave plate 123. Are arranged in this order.

  The birefringence phase compensation DGD generator 305 is also a Michelson interferometer type DGD generator having the same configuration as the birefringence phase canceller 106 provided in the first and second PMD generators described above. A minute dispersion generator 115 capable of adjusting the phase shift amount with respect to the wavelength of one optical path is disposed. The time difference resulting from the optical path difference between the reciprocating optical path between the polarizing beam splitter 110 and the first reflecting mirror 112 and the reciprocating optical path between the polarizing beam splitter 110 and the second reflecting mirror 120 corresponds to DGD. That is, the DGD generated here corresponds to the DGD generated in the first and second birefringent crystals included in the first PMD generator, and is generated in the birefringent crystal included in the second PMD generator. Equivalent to DGD.

  The phase difference φ between orthogonal polarization modes generated by the DGD is given by the following equation (5) using the PMD vector τ when the minute dispersion generator 115 is not involved. Here, f is the frequency of the signal light.

The rotation matrix R _c of the birefringence phase compensation DGD generator 305 is given by the following equation.

(Operation)
Refer to FIG. 15, which is a block diagram showing the overall schematic configuration of the third PMD generator including the schematic configurations of the birefringence phase compensation DGD generator 305, the mode mixer 306, and the microdispersion generator 115. Now, the operation of the third PMD generator will be described.

The two polarization components separated into the first and second polarization components by the polarization beam splitter 110 are again subjected to polarization synthesis by the polarization beam splitter 110 and generated as a polarization synthesis signal. This polarization synthesis signal is input to the mode mixer 306. Polarization-signal input to the mode mixer 306 is reflected by the reflecting mirror 206 and the polarization axis is rotated by α as the center of rotation of the S ₂ axis of the Stokes space. The reflected polarization composite signal passes through the mode mixer 306, is input to the polarization beam splitter 110, is reflected, and is finally output as output signal light 307 via the fiber collimator 203 and the optical circulator 202. The

The total PMD vector τ _all in the third PMD generator is given by the following equation (6) using the PMD connection law.

  Here, the minute dispersion generator 115 is adjusted so that the phase difference (birefringence phase) between the orthogonal polarization components becomes φ + θ = (constant value) at each central wavelength of the diffraction grating 117.

  Regarding the phase α generated in the mode mixer 306, since the third PMD generator is configured in a reflective manner like the second PMD generator described above, it passes through the mode mixer 306 twice. In the PMD generator 3, a variable PMD operation without wavelength dependency according to the following equation (7) is realized for the phase α generated in the mode mixer 306.

(effect)
Next, effects obtained by the third PMD generator will be described with reference to FIG. FIG. 16 shows a calculation example of a PMD vector obtained by an ideal third PMD generator whose resolution of the diffraction grating 117 is sufficiently higher than 10 GHz. The calculation example shown in FIG. 16 is obtained by calculating the amount of DGD generated in the birefringence phase compensation DGD generator 305 as 10 ps and the phase generation amount α of the mode mixer 306 as π / 4.

The horizontal axis of FIG. 16 shows the value of 2πf | τ | in radians, and the vertical axis shows the Stokes parameters and DGD in units of ps. Since the phase generation amount α generated in the mode mixer 306 is π / 4, the DGD value | τ _all | given by the equation (6) is 14.14 ps.

According to the third PMD generation device, it can be seen that the transition of the change in the PMD vector with respect to the wavelength can be sufficiently suppressed, and PMD generation characteristics with high uniformity can be obtained in a wide wavelength band. This can be read from FIG. 16 because the values of the Stokes parameters (s ₁ , s ₂ , s ₃ ) do not change.

  According to the third PMD generator, in addition to the special effects obtained by the second PMD generator described above, the DGD generator is realized by the birefringence phase compensation DGD generator of the Michelson interferometer type. Therefore, it is possible to reduce the number of parts required for the configuration of the apparatus.

<Application example of PMD generator>
The first to third PMD generation devices are devices whose main purpose is to generate PMD. If these PMD generators are used, a PMD compensation device can be formed by disposing a polarization plane controller for arbitrarily adjusting the SOP of the input signal light in the previous stage of the device.

  Further, by reading the phase shift amount generated by the mode mixer 105, 205 or 306 in a state where the PMD of the optical transmission path is compensated by a device configured by connecting the polarization plane controller and the PMD compensation device described above, The size of the DGD can be determined by the following equation (8) when using the first PMD generator, and by the following equation (9) when using the second or third PMD generator. It can also be used as a device.

  In the first to third PMD generators, the polarization rotator is used to configure the mode mixer 105, 205, or 306, but a half-wave plate can be used instead of the polarization rotator. It is. It is also possible to use a birefringent medium such as a polarization maintaining optical fiber as the birefringent crystal used in the first and second PMD generators.

101, 201, 301: Input signal light
102: 1st fiber collimator
104: First birefringent crystal
105, 205, 306, 405: Mode mixer
106: Birefringence phase canceller
107: Second birefringent crystal
108: Second fiber collimator
109, 207, 307: Output signal light
110: Polarizing beam splitter
111: First quarter wave plate (45 degree λ / 2 plate)
112: First reflector
113: Second quarter wave plate (45 degree λ / 2 plate)
114, 203: Fiber collimator
115: Fine dispersion generator
116: Collimator mirror
117: Diffraction grating
118: Lens
119: Phase shifter array
120: Second reflector
121: Third quarter wave plate (45 degree λ / 2 plate)
122: Polarization rotating element
123: Fourth quarter wave plate (-45 degrees λ / 2 plate)
202: Optical circulator
204, 404: Birefringent crystal
206, 406: Reflector
305: Birefringence phase compensation differential group delay generator (birefringence phase compensation DGD generator)

Claims (12)

Comprising a first birefringent crystal, a mode mixer, a birefringent phase canceller, and a second birefringent crystal;
The first birefringent crystal is inputted with input signal light, added with first polarization mode dispersion, and outputted,
The mode mixer receives the output light output from the first birefringent crystal, variably controls the polarization state, and the polarization state in the Stokes space, the first birefringent crystal and the second birefringent crystal Rotate and output around one axis orthogonal to the polarization mode dispersion vector of
The birefringence phase canceller receives the output light output from the mode mixer, and adjusts and outputs the phase shift amount so that the phase difference between the orthogonal polarization components is uniform within the wavelength band of the output light. ,
The polarization mode dispersion generating apparatus, wherein the second birefringent crystal receives the output light output from the birefringence phase canceller, adds the second polarization mode dispersion, and outputs the added light. The birefringence phase canceller is
A polarizing beam splitter, a first 1/4 wavelength plate, a second 1/4 wavelength plate, a first reflecting mirror, and a microdispersion generator,
The polarization beam splitter separates the output light output from the mode mixer into two orthogonally polarized components,
One polarization component passes through the first quarter-wave plate, is reflected by the first reflecting mirror, passes again through the first quarter-wave plate, is reflected by the polarization beam splitter, and is reflected by the polarization beam splitter. Input to the second birefringent crystal,
The other polarization component passes through the second quarter-wave plate and is input to the micro-dispersion generator, the phase shift amount is adjusted for each wavelength of the other polarization component, and the output light is output. 2. The polarization according to claim 1, wherein the polarization is again input to the second birefringent crystal through the second quarter-wave plate, through the polarization beam splitter. Mode dispersion generator. The mode mixer is configured to include a third quarter-wave plate, a polarization rotation element, and a fourth quarter-wave plate arranged in this order,
2. The polarization mode according to claim 1, wherein an optical axis of the polarization rotation element is disposed so as to face the same direction as the optical axes of the first birefringent crystal and the second birefringent crystal. Distributed generator. The fine dispersion generator is
A collimator mirror, a diffraction grating, a lens, a phase shifter array, and a second reflecting mirror,
The other polarization component output from the mode mixer and separated into two orthogonal polarization components by the polarization beam splitter, and the polarization component that has passed through the second quarter-wave plate is the collimator mirror, the diffraction It passes through the grating, the lens, and the shifter array in order and is reflected by the second reflecting mirror, passes again through the phase shifter array, the lens, and the diffraction grating in this order, and is reflected by the collimator mirror, and 3. The polarization mode dispersion generator according to claim 2, wherein the polarization mode dispersion generator is configured to be returned to the ¼ wavelength plate. An optical circulator, a birefringent crystal, a birefringent phase canceller, a mode mixer, and a reflecting mirror;
Input signal light is input to the birefringent crystal through the optical circulator,
The birefringent crystal adds polarization mode dispersion to the input signal light and outputs it,
The birefringent phase canceller receives the output light output from the birefringent crystal and adjusts the phase shift amount so that the phase difference between the orthogonal polarization components is uniform within the wavelength band of the output light. And
The mode mixer receives the output light output from the birefringence phase canceller, variably controls the polarization state, and the polarization state in the Stokes space, the first birefringent crystal and the second birefringent crystal Rotate and output around one axis orthogonal to the polarization mode dispersion vector of
The output light output from the mode mixer is reflected by the reflecting mirror, and again passes through the mode mixer, the birefringent phase canceller, and the birefringent crystal and is output via the optical circulator. A polarization mode dispersion generator characterized by comprising: The birefringence phase canceller is
A polarizing beam splitter, a first 1/4 wavelength plate, a second 1/4 wavelength plate, a first reflecting mirror, and a microdispersion generator,
The polarizing beam splitter separates the output light output from the birefringent crystal into two orthogonally polarized components,
One polarization component passes through the first quarter-wave plate, is reflected by the first reflecting mirror, passes again through the first quarter-wave plate, and is reflected by the polarizing beam splitter. Input to the mode mixer, output from the mode mixer, reflected by the reflecting mirror, again passed through the mode mixer and reflected by the polarization beam splitter, and the first quarter-wave plate, the first 1 Reciprocating through the reflecting mirror and passing through the polarizing beam splitter again and input to the birefringent crystal,
The other polarization component passes through the second quarter-wave plate and is input to the microdispersion generator, and the phase shift amount is adjusted and output for each wavelength of the other polarization component. 6. The polarization mode according to claim 5, wherein the polarization mode is configured to pass through the second quarter-wave plate again and be reflected by the polarization beam splitter and input to the birefringent crystal. Distributed generator. The mode mixer is configured to include a third quarter-wave plate, a polarization rotation element, and a fourth quarter-wave plate arranged in this order,
6. The polarization mode dispersion generating apparatus according to claim 5, wherein an optical axis of the polarization rotation element is arranged to be inclined by 45 degrees with respect to an optical axis of the birefringent crystal. The fine dispersion generator is
A collimator mirror, a diffraction grating, a lens, a phase shifter array, and a second reflecting mirror,
The other polarization component output from the birefringent crystal and separated into two polarization components orthogonal to each other by the polarization beam splitter, and the polarization component that has passed through the second quarter-wave plate is the collimator mirror, Passing sequentially through the diffraction grating, the lens, and the shifter array, and reflected by the second reflecting mirror, and again passing through the shifter array, the lens, and the diffraction grating, and then reflected by the collimator mirror, 7. The polarization mode dispersion generating apparatus according to claim 6, wherein the polarization mode dispersion generating apparatus is configured to be returned to the two-quarter wavelength plate. An optical circulator, a birefringence phase compensation difference group delay generator, a mode mixer, and a reflecting mirror,
Input signal light is input to the birefringence phase compensation difference group delay generator via the optical circulator,
The birefringence phase compensation difference group delay generator adds polarization mode dispersion to the input signal light, cancels a phase difference with respect to one wavelength of orthogonal polarization components of the input signal light, and Both components are combined and output as a combined signal.
The mode mixer receives the polarization composite signal and variably controls the polarization state, and the polarization state is orthogonal to the polarization mode dispersion vector of the first birefringent crystal and the second birefringent crystal in Stokes space. Rotate around one axis to output,
The output light output from the mode mixer is reflected by the reflecting mirror, passes again through the mode mixer and the birefringence phase compensation difference group delay generator, and is output through the optical circulator. A polarization mode dispersion generating device characterized by the above. The birefringence phase compensation difference group delay generator is
A polarizing beam splitter, a first 1/4 wavelength plate, a second 1/4 wavelength plate, a first reflecting mirror, and a microdispersion generator,
The polarization beam splitter is inputted with the input signal light through the optical circulator, and separates the input signal light into two orthogonally polarized components and outputs them,
One polarization component passes through the first quarter-wave plate, is reflected by the first reflecting mirror, passes again through the first quarter-wave plate, and is reflected by the polarizing beam splitter. Input to the mode mixer, output from the mode mixer, reflected by the reflecting mirror, again passed through the mode mixer and reflected by the polarization beam splitter, and the first quarter-wave plate, the first 1 Reciprocating through the reflecting mirror and again passing through the polarizing beam splitter and input to the optical circulator,
The other polarization component passes through the second quarter-wave plate and is input to the microdispersion generator, and the phase shift amount is adjusted and output for each wavelength of the other polarization component. 10. The polarization mode dispersion according to claim 9, wherein the polarization mode dispersion is configured to pass through the second quarter-wave plate again and be reflected by the polarization beam splitter and input to the optical circulator. Generator. 10. The mode mixer according to claim 9, wherein the mode mixer includes a third quarter-wave plate, a polarization rotation element, and a fourth quarter-wave plate arranged in this order. Polarization mode dispersion generator. The fine dispersion generator is
A collimator mirror, a diffraction grating, a lens, a phase shifter array, and a second reflecting mirror,
The other polarization component in which the input signal light output from the optical circulator is separated into two orthogonal polarization components by the polarization beam splitter, and the polarization component that has passed through the second quarter-wave plate is , Sequentially passing through the collimator mirror, the diffraction grating, the lens, and the shifter array, reflected by the second reflecting mirror, and again passing through the shifter array, the lens, and the diffraction grating in this order and reflecting by the collimator mirror. 11. The polarization mode dispersion generator according to claim 10, wherein the polarization mode dispersion generator is configured to be returned to the second quarter-wave plate.

Images