CN1996076A - Compensation method for polarization mode dispersion by using high birefringence uniform fiber grating and structure thereof - Google Patents

Abstract

A method and structure for compensating polarization mode dispersion by using a high-birefringence uniform fiber grating, which belongs to the field of optical fiber communication technology, and is characterized in that it includes: a uniform fiber grating written into a polarization-maintaining fiber, a carrier whose width changes along the length direction Object, the carrier is fixed at one end and free at the other end. The optical fiber grating is cured on the carrier by ultraviolet glue or other methods, and the axial direction of the grating is consistent with the length direction of the carrier, thus forming a time-delay variable unit, and the carrier functions as a grating axial strain adjustment device. The structural unit is respectively connected with a polarization controller and a section of polarization-maintaining optical fiber or a birefringent crystal through a three-terminal circulator, thereby forming a simple and easy-to-implement fabrication process, low cost, smart structure, and easy continuous and dynamic differential group delay. tuned polarization mode dispersion compensator.

Description

Method and Structure for Compensating Polarization Mode Dispersion Using High Birefringence Uniform Fiber Bragg Grating

technical field

The present invention relates to a method and structure for compensating fiber polarization mode dispersion (PMD, Polarization Mode Dispersion) by using high birefringence uniform fiber grating and its preparation; The invention relates to PMD compensation; it belongs to the technical field of optical fiber communication.

Background technique

Due to the increasing demand for information, large-capacity and long-distance transmission will always be the goal of optical fiber communication technology. From 10Gb/s to 40Gb/s, especially above 40Gb/s, optical fiber PMD, which was not paid attention to by people, is gradually It has become the main obstacle restricting the upgrade and development of optical fiber communication system. Therefore, since the 1990s, PMD compensation technology has been widely concerned by researchers at home and abroad.

PMD compensation technology can be simply divided into two categories: electrical domain PMD compensation, optical domain PMD compensation. Optical domain PMD compensation has the advantages of no need for optical-to-optical conversion, transparent signal format, large bandwidth, and not limited by electronic bottlenecks. It is also one of the PMD compensation solutions that is expected to solve high-speed optical communication systems above 40Gbit/s. The compensation mechanism is to add a compensation unit at the receiving end of the transmission line that is equal to the PMD vector of the optical fiber line but opposite in direction, so that the PMD of the entire line from the signal input end to the signal receiving end is zero. The basic principle is shown in Figure 1 shown.

Because the PMD characteristic is different from other characteristics of optical fiber such as loss and dispersion, it is a random variable. Generally, a PMD compensator consists of four parts: polarization controller; variable delay line; feedback signal detection unit and control unit, as shown in the figure 2. The role of the polarization controller is to change the polarization state of the signal input to the variable delay line, or it can be understood as changing the PMD vector of the PMD compensator to make it opposite to the PMD vector in the line. The variable delay line is used to change the delay, that is, to change the modulus of the PMD vector of the compensator so that it is equal to the PMD in the line. The feedback signal detection unit is the quantity that describes the degree of PMD compensation, and the control unit is composed of two parts: the control algorithm (software part) and the control circuit (hardware part), which are used to make the PMD compensator make real-time adjustments according to the change of line PMD . One of the keys to the PMD compensator is the variable delay line. The basic requirements of a compensator with good compensation performance and fast response time for the delay line are: a large delay adjustment range, fast response time, small insertion loss and simple and compact structure.

At present, the variable delay line of the compensation unit in the PMD compensator generally has the following types: a, polarization maintaining fiber; b, optical device; c, birefringent LiNbO3 waveguide structure; d, high birefringence nonlinear chirped fiber grating, etc. . Polarization-maintaining fiber generally adopts temperature-adjusted differential group delay (DGD, Differential Group Delay), the response speed is relatively slow, the adjustment range is relatively small, and the operability is low. The optical device uses a polarization beam splitter to change the free space optical path difference to change the time delay. This structure is relatively complicated. Among them, fiber grating has the advantages of all-fiber structure, smart and compact, and DGD can be continuously adjusted in a large range, which has attracted the attention of researchers in recent years. Birefringent fibers have different time delay differences for different input polarization states, and the nonlinear chirp of the grating makes the grating DGD adjustable. Lee, S et al. (Photonics Technology Letters Photon Technology Letters, October, 1999, Volume 11, Issue 10, Pages 1277-1279) proposed for the first time a method for PMD compensation using birefringent nonlinear chirped fiber gratings. Later, their group Pan Z, Xie Y, Lee S and others optimized the connection structure (2000 OFC Conference, Vol. 3, pp. 113-115). The birefringent nonlinear chirped fiber grating can be written into the birefringent photosensitive fiber by ultraviolet using a nonlinear chirped mask, or the nonlinear chirp can be generated by controlling the UV exposure time of the birefringent photosensitive fiber by using a linear chirped mask. However, these methods either require expensive nonlinear chirped masks, or require precisely controlled exposure time, and the manufacturing technology has poor repeatability, so the yield is quite low. Moreover, different DGD adjustable ranges require different phase masks, which increases the cost of the product. Based on this, people such as Xu Kun (Opt.Comm.Optical communication, 2002, 202 period 297-302 pages) proposed a kind of adjustable PMD compensator based on sampling fiber grating structure, sampling grating can utilize uniform phase mask to make , but the production of sampling fiber grating is relatively difficult and the repeatability is poor. Subsequently, Xia Zhang et al. (Opt.Comm. Optical Communication, 2002, 214, pp. 123-127) published a design scheme for a linearly chirped fiber grating type adjustable PMD compensator, which uses a method of applying lateral stress to the grating. The method produces stress birefringence to adjust the DGD of the fiber Bragg grating. Due to the lateral pressure on the fiber Bragg grating, we know that the optical fiber, especially the fiber Bragg grating, is relatively fragile and easy to break when it is subjected to lateral force, so the reliability of the use is greatly improved. discount. However, the PMD compensators with the above structures are all based on linear chirped fiber gratings or nonlinear chirped gratings, which generally require expensive chirped phase masks to be fabricated.

Contents of the invention

The purpose of the present invention is to propose a method and a structure for compensating polarization mode dispersion using a high-birefringence uniform fiber grating. The PMD compensation device is simple to prepare and has a smart structure. There are congenital defects such as complex manufacturing process, high cost, poor repeatability and low reliability.

The purpose of the present invention is achieved as follows:

A method and structure for compensating PMD using a high birefringence uniform fiber grating, characterized in that it includes: the variable delay line 4 is composed of two parts: a high birefringence uniform fiber grating 1 and a width edge for adhesion Carriers 2 that vary in length. The carrier 2 has a uniform thickness, but its width is a varying function f(z) along the length z of the carrier. One end of the carrier 2 is fixed, while the other end is placed in a free state. It functions as a grating axial strain adjustment device. The fiber grating signal input end is connected to port b of a fiber optic circulator 6 with three ports, a polarization controller 5 is connected before port a of the circulator, and port c of the circulator is connected to a section of polarization-maintaining fiber or a birefringent crystal 7. Polarization-maintaining fiber fusion splicer or add a polarization rotator 11 to ensure that its PMD vector is opposite to the PMD vector direction of the fiber grating, thereby adjusting the DGD variation range of the compensator of the present invention, and then output the signal to the receiver through the coupler or beam splitter 10 end, part of the signal after beam splitting by the coupler enters the control unit 9 from the feedback signal detection unit 8, and adaptively adjusts the polarization controller 5 and the variable time delay line 4 based on the fiber Bragg grating to form a complete system based on high dual PMD compensator for refractive homogeneous fiber grating.

Under the condition of constant temperature, the axial stress ε(z) is applied to the uniform fiber grating (z represents the axial position of the grating), then due to the elastic effect of the fiber, the Bragg wavelength of the grating at z can be expressed as:

λ(z)＝2n _eff [Λ ₀ +Λ ₀ (1-ρ _e )ε(z)] (1)

In the formula, the fiber refractive index n _eff , ρ _e is the elastic-optic coefficient, and Λ ₀ is the period of the uniform fiber grating. It can be seen directly from the above formula that the reflection Bragg wavelength shift of the fiber grating is directly related to the stress. If a gradient stress is applied to the grating, the Bragg wavelength will change linearly along the length of the grating due to the change of the effective period of the grating, so that the uniform fiber grating becomes a linearly chirped fiber grating. However, if the applied stress is a nonlinear function along the axis z direction, it can be seen that the uniform fiber grating will evolve into a nonlinear chirped fiber grating at this time. Therefore, if a nonlinear stress is applied to a high birefringence uniform fiber grating in its axial direction, the grating evolves into a high birefringence nonlinear chirped fiber grating. After the signal is reflected by the grating, a variable time delay is introduced between the two fast and slow polarization main axes of the grating. The nonlinear relationship between reflection wavelength and reflection position determines the group velocity dispersion and DGD properties of the grating, so that the grating has DGD adjustment capability. Therefore, by designing a suitable grating stress distribution field, desired grating characteristics can be obtained.

The variable delay line structure of the present invention mainly includes two parts: high birefringence uniform fiber grating 1 and a carrier 2 whose width varies along the length direction. One end of the carrier is fixed and the other end is in a free state. The grating can be cured to the carrier with UV glue or other glues. Assuming that the length of the grating is l, the length of the carrier is L, the thickness is t, and the function of the width changing with the length of the carrier is expressed as w(z). Applying an offset Y to the free end of the carrier 2 generates an axially nonlinearly distributed stress field, thereby converting the birefringent uniform fiber grating 1 into a birefringent nonlinear chirped fiber grating 3. When the offset is applied There are two situations, one is to produce extrusion strain on the grating, so that the Bragg period of the grating in the axial direction becomes smaller, and the other is to produce a stretching effect on the grating, so that the Bragg period of the grating in the axial direction becomes larger . An important feature of the grating is that its DGD is adjustable, because by changing the offset of the free end of the carrier, the axial nonlinear stress distribution ε(z) is generated, and then the DGD of the grating is dynamically adjusted.

In a complete PMD compensator structure based on high birefringence uniform fiber grating, the feedback control structure composed of feedback signal detection unit, control unit and polarization controller is a method widely recognized by researchers. The content of the present invention focuses on the variable delay line structure 4 based on a uniform fiber grating. Another content of this invention is: a section of polarization-maintaining fiber or birefringent crystal 7 with a specific DGD value τ _fix is connected to the c-end of the circulator 6, and a polarization-maintaining fiber fusion splicer or an additional polarization rotator 11 is used for connection Make sure its PMD vector is in the opposite direction to the fiber grating PMD vector. Its purpose is to change the DGD adjustment range of the variable delay line structure 4 , that is, the minimum value and the maximum value. The birefringent uniform fiber grating 1 has an initial value DGD value τ ₀ , and the nonlinear stress determines the adjustment range Δτ of DGD. It can be known that the adjustment range of the variable delay line DGD is (τ ₀ -τ _fix , τ ₀ -τ _fix +Δτ), if τ _fix =τ ₀ , then the variable delay line structure 4 can be realized in (0 , Δτ) range is dynamically and continuously adjustable.

The realization method of the PMD compensator based on the high birefringence uniform fiber grating comprises the following steps:

The first choice is ordinary polarization-maintaining fiber (which can be Panda-type, bow-tie-type, D-type, etc.) or germanium-doped photosensitive polarization-maintaining fiber with hydrogen loading. When selecting an optical fiber, the main parameter is based on the birefringence or beat length of the polarization-maintaining fiber, because the larger (smaller) the birefringence, the smaller (larger) the beat length, the larger the initial DGD value of the fiber grating written on it ( Smaller), the corresponding adjustment range is larger (smaller). Utilizing the ultraviolet writing technology, the polarization-maintaining fiber is passed through a uniform phase mask to write the grating in ultraviolet to form a birefringent uniform fiber grating 1 .

The second is to adhere the grating to a carrier 2 whose width varies along the length direction by using curing glue such as ultraviolet glue or other methods. One end of the carrier 2 is fixed, and the other end is in a free state. The adjustment of the free end offset can be electronically controlled by stepping, servo motors or materials with voltage-strain response characteristics, such as piezoelectric ceramics, so as to adjust the axial strain of the grating.

The third is to connect the grating structure with a three-terminal circulator 6 port b, and connect a polarization controller before the circulator 6 port a, and use a polarization maintaining fusion splicer to connect a section of polarization maintaining fiber or birefringence with a suitable DGD value at the c end Crystal 7, used for DGD variation range adjustment. When connecting, use a polarization-maintaining fiber fusion splicer or add a polarization rotator 11 to ensure that its PMD vector is opposite to that of the fiber grating.

Fourthly, select the feedback signal, design the software and hardware parts of the control unit, and carry out packaging processing to form the PMD compensator based on the high birefringence uniform fiber grating described in this invention.

The main advantage of the present invention is that it adopts a high-birefringence uniform fiber grating that is simple to manufacture, high in yield, and low in price, instead of requiring expensive nonlinear phase masks, complex in fabrication, and high in process requirements. Chirped fiber grating. The offset of the free end of the carrier can be controlled by stepping motors, servo motors or piezoelectric ceramic materials with voltage-strain response characteristics.

Description of drawings

Fig. 1 is the schematic diagram of PMD compensation system;

Fig. 2 is the structural representation of PMD compensator;

Fig. 3 is a schematic diagram of the structure of the variable delay line based on the high birefringence uniform fiber grating of the present invention (a), and a schematic diagram of the structure of the high birefringence uniform fiber Bragg grating (b): the signal is reflected by the grating and introduced between the two fast and slow polarization principal axes. Variable delay amount;

Fig. 4 is the structural representation of the PMD compensator based on the high birefringence uniform fiber grating of the present invention;

Fig. 5 is a top view of a variable delay line structure based on a high-birefringence uniform fiber grating according to the present invention;

Figure 6 is a top view effect diagram of nonlinear stress applied to the fiber grating when the free end is offset by Y distance: (a) extrusion method, (b) stretching method;

Fig. 7 is a schematic diagram of time delay curves of birefringent fiber gratings under different offsets;

Fig. 8 is a schematic diagram of the relationship between DGD and offset;

Figure 9 shows the variable delay line structure based on high birefringence uniform fiber grating when the function of the change of the width of the carrier along the length direction is linear decrease (a), nonlinear increase (b) or nonlinear decrease (c) schematic diagram.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

When designing the PMD compensator based on high birefringence uniform fiber grating according to the present invention, it is necessary to know in advance the compensation channel wavelength of the PMD compensator, the range of PMD variation in the actual optical fiber line, etc., so as to determine the birefringence of the selected polarization-maintaining fiber The size, the central Bragg wavelength of the grating, the length of the grating and the carrier, and the variation function of the width of the carrier along the axis of the grating and the carrier. The specific design thinking proposed according to this inventive concept can be briefly described as follows:

First, measure and count the PMD distribution in the actual optical fiber line, thereby determine the DGD adjustment range of the PMD compensator, and design the change function of the length and width of the carrier along the length direction, which is one of the important parts of the present invention.

Secondly, according to the compensated channel wavelength, select a suitable phase mask, and use ultraviolet writing technology to write a fiber grating with Bragg wavelength consistent with the channel wavelength in the polarization-maintaining fiber.

Then, it is necessary to determine the DGD size of a section of polarization-maintaining fiber or birefringent crystal used for adjusting the DGD variation range. Generally, the PMD compensator requires the adjustment range of the variable delay unit to be (0, Δτ), so the direction of the PMD vector of the polarization-maintaining fiber or the birefringent crystal is opposite to that of the initial PMD vector of the fiber grating (that is, when there is no strain), equal in size.

Finally, the signal input end of the fiber grating 1 shown in Fig. 3 is connected to the port b of a fiber circulator 6 with three ports, the polarization controller 5 is connected before the port a of the circulator 6, and the port c of the circulator 6 is connected to a section of protection Polarized optical fiber or birefringent crystal 7, adopt polarization maintaining optical fiber fusion splicer or add a polarization rotator 11 during connection to ensure that its PMD vector is opposite to the PMD vector direction of fiber grating, thereby adjust the DGD variation range of the compensator of the present invention, then through coupling The signal is output to the receiving end by the device or the beam splitter 10, and part of the signal enters the control unit 9 from the feedback signal detection unit 8 after the beam splitting by the coupler 10, and the polarization controller 5 and the variable time delay line 4 based on the fiber grating By performing adaptive adjustment, the PMD compensator based on the high birefringence uniform fiber grating according to the present invention can be designed and realized, as shown in FIG. 4 .

Embodiment one:

As shown in Figure 3, the variable time delay unit 4 of the PMD compensator based on the high birefringence uniform fiber grating of the present invention design is made up of two parts: a high birefringence uniform fiber grating 1 and a width for adhesion Carrier 2 varying along the length direction. The carrier 2 has a uniform thickness, but its width is a varying function f(z) along the length z of the carrier. One end of the carrier 2 is fixed, while the other end is placed in a free state. It functions as a grating axial strain adjustment device. Fig. 5 is a top view of a variable delay line structure based on a high-birefringence uniform fiber grating. Choose a carrier with a wedge-shaped structure, the width of the carrier increases linearly with the length, which can be described mathematically as follows:

$\mathrm{<span\; class="notranslate">\; w</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; w</span>}}_{\mathrm{<span\; class="notranslate">\; fix</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; z</span>}}{\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; w</span>}}_{\mathrm{<span\; class="notranslate">\; free</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; w</span>}}_{\mathrm{<span\; class="notranslate">\; fix</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

where w _fix and w _free are the widths of the fixed end and free end of the carrier, respectively. If an offset Y is applied to the free end along the y direction, as shown in Figure 6(a) (extrusion mode), according to the bending moment curvature equation, it can be deduced that the stress generated along the axis z direction has a separate function:

$\mathrm{\&epsiv;}\left(z\right)=\frac{t}{2}R\left(z\right),$ $R\left(z\right)=\frac{-(L-z)\&Center\; Dot;\mathrm{\&Delta;w}\&Center\; Dot;Y}{L^{3}w\left(z\right)[(\frac{\mathrm{\&Delta;w}}{2w_{\mathrm{free}}}-\frac{w_{\mathrm{free}}}{\mathrm{\&Delta;w}})-{(1-\frac{w_{\mathrm{fix}}}{\mathrm{\&Delta;w}})}^2\ln (1-\frac{\mathrm{\&Delta;w}}{w_{\mathrm{fix}}})]},$ Δw＝w _free -w _fix

(3)

In the formula, R(z) is the radius of curvature of the carrier at point z in the axial direction. Fig. 7 is a schematic diagram of changes in time delay curves of birefringent fiber gratings under different offsets Y ₁ and Y ₂ (Y ₁ <Y ₂ ). It can be seen that under different offsets, the slope of the delay curve of the birefringent fiber grating is different at a certain wavelength, thereby changing the DGD value. When Y ₁ <Y ₂ , the corresponding DGD relationship is Δτ ₁ <Δτ ₂ . Figure 8 shows a schematic diagram of the relationship between the DGD value of the fiber grating and the offset at a specific wavelength, that is, the channel wavelength. Δτ ₀ refers to the DGD value of the grating when there is no stress, so we select a section of polarization-maintaining optical fiber or birefringent crystal with a DGD value of Δτ ₀ , connect the devices as shown in Figure 6, and package them to realize the uniform fiber grating based on the present invention PMD compensator structure.

Embodiment two:

When the offset is applied, the offset is applied along the -y direction. This situation is equivalent to applying a tensile strain to the grating, so that the Bragg period of the grating in the entire axial direction becomes longer. The schematic diagram of the effect is shown in Figure 6(b ) shown. Other steps are then the same as embodiment one.

Embodiment three:

The function of the variation of the width of the carrier along the length direction used in the above two embodiments is linearly increasing. In practical applications, it can also decrease linearly, increase nonlinearly or decrease nonlinearly. The purpose achieved is the same, even if the value of the variable delay line DGD is dynamically adjustable. The schematic diagrams of the carrier in these three cases are shown in Fig. 9(a)(b)(c) respectively.

In order to illustrate the realization of the present invention, the above-mentioned embodiment has been described, but other changes and modifications of the present invention are obvious to those skilled in the art, and the present invention is not limited to the described specific implementation, therefore, disclosed in the present invention Any modification or equivalent transformation within the scope of the true essence and basic principles of the content shall belong to the protection scope of the claims of the present invention.

Claims (5)

1, a kind of structure of utilizing high birefringence uniform fiber grating compensation PMD is characterized in that: the object carrier (2) that a high birefringence uniform fiber grating (1) and a width that is used to adhere to change along its length; Object carrier (2) has homogeneous thickness, but its width is the function f (z) of a variation along object carrier length z direction; Object carrier (2) one ends are fixed, and the other end is free state; As optical grating axial strain adjusted device; The port (b) that fiber grating signal input part and has the optical fiber circulator (6) of three ports links to each other, at the preceding connection Polarization Controller of circulator port (a) (5), circulator port (c) connects one section polarization maintaining optical fibre or birefringece crystal (7), pass through coupling mechanism or beam splitter (10) then signal is outputed to receiving end, enter control module (9) through coupling mechanism beam splitting rear section signal by feedback signal probe unit (8), carry out the self-adaptation adjusting to Polarization Controller (5) and this variable time delay line (4), constitute a complete PMD compensator based on the high birefringence uniform fiber grating based on fiber grating.

2, a kind of structure of utilizing high birefringence uniform fiber grating compensation PMD as claimed in claim 1, it is characterized in that: based on the PMD compensator of high birefringence uniform fiber grating, object carrier (2) width that is used for the optical grating axial strain adjusted is a function that changes along its length direction, its change curve can be linear increase or reduce, also can be non-linear increase or reduce, and that object carrier (2) thickness keeps is evenly constant.

3, a kind of structure of utilizing high birefringence uniform fiber grating compensation PMD as claimed in claim 1, it is characterized in that: based on the PMD compensator of high birefringence uniform fiber grating, be used for optical grating axial strain adjusted device, be placement, offset manner and the offset direction of object carrier (2): object carrier (2) one ends are fixed, and the other end is free state; The free end side-play amount is regulated can be by stepping, raise the clothes motor or have voltage one strain-responsive characteristic material, carries out automatically controlled as piezoelectric ceramics etc.; The offset direction is along the object carrier thickness direction, thereby generation is pushed or tensile strain to optical grating axial.

4, a kind of structure of utilizing high birefringence uniform fiber grating compensation PMD as claimed in claim 1, it is characterized in that: based on the PMD compensator of high birefringence uniform fiber grating, one section polarization maintaining optical fibre or birefringece crystal (7) that three port circulator (6) ports (c) are connected with specific DGD size, adopt a Polarization Maintaining Optical Fiber Fusion Splicer or an additional polarization rotator (11) to guarantee that its PMD vector is opposite with the PMD direction vector of fiber grating during connection, thus the DGD variation range of regulating compensator of the present invention.

5, a kind of implementation method of the PMD compensator based on the high birefringence uniform fiber grating is characterized in that comprising following steps:

(1) carries out ultraviolet by even phase mask plate on the polarization maintaining optical fibre after carrying processing and write grating through hydrogen, the form dielectric grid uniform fiber grating adopts curing glue such as ultraviolet glue or alternate manner that this grating is adhered on the object carrier that a width changes along its length; Object carrier one end is fixed, and an end is free state; The free end side-play amount is regulated can be by stepping, raise the clothes motor or have voltage one strain-responsive characteristic material, carry out as piezoelectric ceramics etc. automatically controlled, thereby strain is regulated to optical grating axial;

(2) this optical grating construction is linked to each other with one or three end circulator (6) ports (b), and at the preceding connection Polarization Controller of circulator (6) port (a) (5), (c) end connects one section polarization maintaining optical fibre or birefringece crystal (7) with suitable DGD value, adopt a Polarization Maintaining Optical Fiber Fusion Splicer or an additional polarization rotator (11) to guarantee that its PMD vector is opposite with the PMD direction vector of fiber grating, is used for the adjustment of DGD variation range during connection;

(3) select feedback signal, the software and hardware part of design control module is carried out encapsulation process, can constitute described PMD compensator based on the high birefringence uniform fiber grating.

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