Abstract:
A Faraday rotator mirror, including: a polarizing beam splitter, a polarizing beam converging device made from birefringent crystal, a Faraday rotator, a reflecting mirror. In operation, light rays from an optical coupling element pass through the polarizing beam splitter, the polarizing beam converging device, the Faraday rotator, and the reflecting mirror, in that order, and then return back through the Faraday rotator, the polarizing beam converging device, and the polarizing beam splitter, in that order, and are output from the optical coupling element in reverse to the original incidence path.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of International Patent Application No. PCT/CN2014/092355 with an international filing date of Nov. 27, 2014, designating the United States, now pending, and further claims priority benefits to Chinese Patent Application No. 201310646181.0 filed Dec. 4, 2013, Chinese Patent Application No. 201410539249.X filed Oct. 13, 2014, and Chinese Patent Application No. 201410657273.3 filed Nov. 18, 2014. The contents of all of the aforementioned applications, including any intervening amendments thereto, are incorporated herein by reference. Inquiries from the public to applicants or assignees concerning this document or the related applications should be directed to: Matthias Scholl P.C., Attn.: Dr. Matthias Scholl Esq., 245 First Street, 18th Floor, and Cambridge, Mass. 02142. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to the field of optical fiber sensing and optical fiber communication, and more particularly to a Faraday rotator mirror which is wavelength-independent and temperature-independent. 
         [0004]    2. Description of the Related Art 
         [0005]    The Faraday rotator mirror operates to eliminate polarization-induced signal fading in fiber-optic interferometers. Conventional Faraday rotator mirror is adversely influenced by factors such as wavelength and temperature, leading to a low signal-to-noise ratio. In general, birefringent elements and beam converging devices are introduced to solve the adverse effects caused by wavelength and temperature. This reduces the extinction ratio of the polarization apparatus, and increases the size and production costs thereof. 
       SUMMARY OF THE INVENTION 
       [0006]    In view of the above-described problems, it is one objective of the invention to provide a Faraday rotator mirror which is wavelength-independent and temperature-independent. The Faraday rotator mirror is capable of eliminating the polarized light components along the dispersion direction of the rotation angle after the rotation angle produced by the Faraday rotator deviates 90 degrees, so that lights having different wavelengths have the same linear polarization, thus eliminating the effects of wavelength and temperature on the rotation angle. Meanwhile, the extinction ratio of the Faraday rotator mirror is improved, and the insertion loss is reduced. The Faraday rotator mirror features proper size and low costs. 
         [0007]    To achieve the above objective, in accordance with one embodiment of the invention, there is provided a Faraday rotator mirror which is wavelength-independent and temperature-independent. Under the action of a beam converging device, two light beams radiating from a polarizing beam splitter have light paths thereof exchanged at a reflecting mirror and transmit reversely. Polarized lights experiencing twice rotation in a Faraday rotator are merged into an incident light path by a polarizing beam splitter, and meanwhile, polarized light components having 90 degrees&#39; deviation along the dispersion direction of the rotation angle deviate from the incident light path, thereby eliminating effects of wavelength and temperature on the Faraday rotator mirror. 
         [0008]    In a class of this embodiment, the function of beam convergence of the beam converging device is achieved by using any one of three kinds of optical elements: a refractive prism element which is isotropous and polarization-independent, such as Fresnel biprism made from glass; a refractive prism element which is anisotropic and polarization-dependent, such as Nomarski prism or Wollaston prism made from birefringent crystal; and beam reflective element made from reflective coating of optical interference or optical medium interface capable of total reflection. 
         [0009]    In a class of this embodiment, the Faraday rotator is an optical device adapted to rotate polarization directions of lights using a magneto-optical effect. The Faraday rotator comprises a non-reciprocal magneto-optic crystal and a permanent magnet adapted to provide a saturation magnetic field for crystals. The Faraday rotator comprises two Faraday magneto-optic crystals having arbitrary thicknesses, and a total rotation angle of one light beam after the light beam passes through the two Faraday magneto-optic crystals is 90 degrees. 
         [0010]    In a class of this embodiment, the polarizing beam splitter (PBS) is a coating element of optical interference, a birefringent crystal element, or a composite element of birefringent crystal, such as MacNeille Optical Coating PBS, Wollaston Prism PBS, Double Wollaston Prism PBS, Polarization Beam Displacer (PBD), Rochon Prism, Nicol Prism, Birefringent Crystal Wedge, Senarmont Prism, or Nomarski Prism. 
         [0011]    Preferably, lights are input from an optical coupling element, pass through the polarizing beam splitter, the refractive beam converging device which is polarization-independent, the Faraday rotator, the reflecting mirror, in that order, return back through the Faraday rotator, the refractive beam converging device, the polarizing beam splitter, in that order, and get back to the optical coupling element; or the lights are input from the optical coupling element, pass through the polarizing beam splitter, the Faraday rotator, the refractive beam converging device, the reflecting mirror, in that order, return back through the refractive beam converging device, the Faraday rotator, the polarizing beam splitter, and get back to the optical coupling element. Two light beams separated by the polarizing beam splitter have light paths exchanged and transmit reversely after passing through the refractive beam converging device. Rotation angles of the two light beams after twice rotation of the Faraday rotator are nearly 90 degrees, and the two lights are merged into an incident light path after passing the polarizing beam splitter again, meanwhile, polarized light components of the two light beams along a dispersion direction of the rotation angles after 90 degrees deviation deviate from the incident light path, thereby eliminating effects of polarized light components having 90 degrees&#39; deviation along the dispersion direction of the rotation angle on states of polarization of lights on the incident light path. Thus lights having different wavelengths can output with the same state of polarization. 
         [0012]    In a class of this embodiment, the polarizing beam splitter (PBS) is a coating element of optical interference, a birefringent crystal element, or a composite element of birefringent crystal. The refractive beam converging device which is polarization-independent is a refractive element or a composite refractive element made from isotropic optical media, such as Fresnel biprism. 
         [0013]    In a class of this embodiment, the refractive beam converging device is polarization-independent. When two polarized light beams enter into the refractive beam converging device and are parallel, a part of the polarized light beams having overlapped light energy could not have light paths exchanged, and then returns back along the original light paths. For example, when the polarizing beam splitter is the Polarization Beam Displacer (PBD) and two polarized light beams that enter into the Fresnel biprism are parallel, the extinction ratio of refractive lights of the Fresnel biprism is relative to the space displacement of the two polarized light beams. To achieve a high extinction ratio, the space displacement is required to be more than two times of a mode field diameter based on Gaussian distribution. When the two polarized light beams are unparallel, a part of the polarized light beams having overlapped light energy could neither have light paths exchanged, nor returns back along the original light paths. No adverse impact is on the extinction ratio, however, light energy loses more. For example, when the polarizing beam splitter is the Wollaston Prism PBS, the extinction ratio of refractive lights is no longer dependent to the space displacement of the two polarized light beams 
         [0014]    Preferably, the lights are input from the optical coupling element, pass through the polarizing beam splitter, the refractive beam converging device, the Faraday rotator, the reflecting mirror, in that order, back through the Faraday rotator, the refractive beam converging device, the polarizing beam splitter, in that order, and are output reversely by the optical coupling element. 
         [0015]    Preferably, the lights are input from the optical coupling element, pass through the polarizing beam splitter, the Faraday rotator, the refractive beam converging device, the reflecting mirror, in that order, return back through the refractive beam converging device, the Faraday rotator, the polarizing beam splitter, in that order, and are output reversely by the optical coupling element. 
         [0016]    Preferably, the lights are input from the optical coupling element, pass through a first composite element of the polarizing beam splitter and a reflective beam converging device, the Faraday rotator, the reflecting mirror, in that order, return back through the Faraday rotator, the first composite element, and get back to the optical coupling element; two light beams separated by the first composite element meet, have light paths exchanged, and transmit reversely. Rotation angles of the two light beams after twice rotation of the Faraday rotator are nearly 90 degrees, and the two lights are merged into an incident light path after passing the first composite element for the second time, meanwhile, polarized light components of the two light beams along a dispersion direction of the rotation angles after 90 degrees deviation deviate from the incident light path, thereby eliminating effects of polarized light components having 90 degrees&#39; deviation along the dispersion direction of the rotation angle on states of polarization of lights on the incident light path. Thus lights having different wavelengths can output with the same state of polarization. 
         [0017]    In a class of this embodiment, the first composite element integrates functions of the polarizing beam splitter and the reflective beam converging device. The polarizing beam splitter is a coating element of optical interference, a birefringent crystal element, or a composite element of birefringent crystal. The reflective beam converging device is a beam reflective element made from reflective coating of optical interference or optical medium interface capable of total reflection. The first composite element is a lateral displacement polarizing splitter or a polarization beam displacer (PBD) having a high-reflectivity interface on a side surface. 
         [0018]    In a class of this embodiment, light reflection and refraction of the first composite element are merely dependent to an incident direction of a light, and the extinction ratio is not affected by overlapped light energy of two polarized light beams that are separated by the polarizing beam splitter. 
         [0019]    Preferably, the lights are input from the optical coupling element, pass through the first composite element of the polarizing beam splitter and the reflective beam converging device, the Faraday rotator, the reflecting mirror, in that order, return back through the Faraday rotator, the first composite element, and transmit reversely by the optical coupling element. 
         [0020]    Preferably, the lights are input from the optical coupling element, pass through the polarizing beam splitter, a polarizing beam converging device made from birefringent crystal, the Faraday rotator, the reflecting mirror, in that order, return back through the Faraday rotator, the polarizing beam converging device, and the polarizing beam splitter, in that order, and get back to the optical coupling element. Two light beams separated by the polarizing beam splitter have light paths exchanged and transmit reversely after passing through the polarizing beam converging device. Rotation angles of the two light beams after twice rotation of the Faraday rotator are nearly 90 degrees, and the two lights are merged into an incident light path after passing the polarizing beam splitter again, meanwhile, polarized light components of the two light beams along a dispersion direction of the rotation angles after 90 degrees deviation deviate from the incident light path, thereby eliminating effects of polarized light components having 90 degrees&#39; deviation along the dispersion direction of the rotation angle on states of polarization of lights on the incident light path. Thus lights having different wavelengths can output with the same state of polarization. 
         [0021]    In a class of this embodiment, the polarizing beam splitter is the coating element of optical interference, the birefringent crystal element, or the birefringent crystal composite element. The polarizing beam converging device is the birefringent crystal element or the composite element of birefringent crystal, such as Wollaston Prism, Double Wollaston Prism, Polarization Beam Displacer (PBD), Rochon Prism, Nicol Prism, Birefringent Crystal Wedge, Senarmont Prism, or Nomarski Prism; or, a second composite element integrating functions of the polarizing beam splitter and the polarizing beam converging device is also the birefringent crystal composite element, such as Nomarski Prism. 
         [0022]    In a class of this embodiment, the polarizing beam converging device is the birefringent crystal element or the composite element of birefringent crystal, and the birefringent crystal element and the composite element of birefringent crystal generate different light deviations only according to different states of polarization, thus the extinction ratio and light energy loss have nothing to do with overlapped light energy of two polarized light beams 
         [0023]    In a class of this embodiment, the Nomarski Prism is a composite crystal composed of two wedge-shaped birefringent crystals and features compact structure. The Nomarski Prism is mostly applied to differential interference contrast microscopy. The merging effect produced by the Nomarski Prism is dependent to polarization directions of two polarized light beams and has nothing to do with energy space distributions of the two polarized light beams Even if the energy spaces coincide, a high extinction ratio remains. The extinction ratio of the Nomarski Prism is only determined by the extinction ratio of the birefringent crystals, and the extinction ratio of the Faraday rotator mirror comprising the Nomarski Prism is not limited by energy space distributions of an ordinary light and an extraordinary light. 
         [0024]    In a class of this embodiment, when two polarized light beams enter into an optical lens (as a beam converging device) and are parallel, lights having overlapped light energy could not have light paths exchanged, and then return back along the original light path, which lowers the extinction ratio down. Thus the extinction ratio of the Faraday rotator mirror comprising the optical lens is dependent to the space distribution of light energy. To achieve a high extinction ratio, the space displacement of the two polarized light beams is required to be more than two times of a mode field diameter based on Gaussian distribution. The extinction ratio of the Faraday rotator mirror is dependent to the space displacement, and large-size birefringent materials lead to large space displacements of polarized lights and therefore, a high extinction ratio. 
         [0025]    Preferably, the light rays from an optical coupling element pass through the polarizing beam splitter, the polarizing beam converging device, the Faraday rotator, and the reflecting mirror, in that order, and then return back through the Faraday rotator, the polarizing beam converging device, and the polarizing beam splitter, in that order, and are output from the optical coupling element in reverse to an original incidence path. 
         [0026]    Preferably, the light rays from an optical coupling element pass through the composite element of the polarizing beam splitter and the polarizing beam converging device, the Faraday rotator, and the reflecting mirror, in that order, and then return back through the Faraday rotator, the composite element of the polarizing beam splitter and the polarizing beam converging device, in that order, and are output from the optical coupling element in reverse to an original incidence path. 
         [0027]    The dispersion of the Faraday non-reciprocal magneto-optic crystal and adverse effects of temperature on the rotation angle are completed eliminated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The invention is described hereinbelow with reference to the accompanying drawings, in which: 
           [0029]      FIG. 1  is a curve diagram of relation of rotation angles and wavelengths of a Faraday crystal in accordance with one embodiment of the invention; 
           [0030]      FIG. 2  is a curve diagram of relation of rotations angles and temperatures of a Faraday crystal in accordance with one embodiment of the invention; 
           [0031]      FIG. 3  is a schematic diagram of a Nomarski Prism in accordance with one embodiment of the invention; 
           [0032]      FIGS. 4A-4B  are diagrams showing a Gaussian distribution of light intensities of two polarized light beams in accordance with one embodiment of the invention; 
           [0033]      FIG. 5  is a schematic diagram of a polarization beam displacer (PBD) having a high-reflectivity interface on a side surface in accordance with one embodiment of the invention; 
           [0034]      FIG. 6  is a schematic diagram of lateral displacement polarizing splitter in accordance with one embodiment of the invention; 
           [0035]      FIG. 7  is a schematic diagram of a Faraday rotator mirror and an optical path diagram in Example 1; 
           [0036]      FIGS. 8A-8B  are first diagrams showing states of polarization of transmitted light beams in Example 1; 
           [0037]      FIGS. 9A-9B  are second diagrams showing states of polarization of transmitted light beams in Example 1; 
           [0038]      FIGS. 10A-10B  are third diagrams showing states of polarization of transmitted light beams in Example 1; 
           [0039]      FIG. 11  is a schematic diagram of a Faraday rotator mirror and an optical path diagram in Example 2; 
           [0040]      FIG. 12  is a schematic diagram of a Faraday rotator mirror and an optical path diagram in Example 3; 
           [0041]      FIG. 13  is a schematic diagram of a Faraday rotator mirror and an optical path diagram in Example 4; and 
           [0042]      FIG. 14  is a schematic diagram of a Faraday rotator mirror and an optical path diagram in Example 5. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0043]    For further illustrating the invention, experiments detailing a Faraday rotator mirror which is wavelength-independent and temperature-independent are described below. It should be noted that the following examples are intended to describe and not to limit the invention. 
         [0044]      FIG. 1  shows that in a saturation magnetic field, rotation angles of linearly polarized lights by the non-reciprocal Faraday magneto-optic crystal are dependent to wavelengths; at certain temperature, the longer the wavelength is, the smaller the rotation angle is. 
         [0045]      FIG. 2  shows that in a saturation magnetic field, rotation angles of linearly polarized lights by the non-reciprocal Faraday magneto-optic crystal are relative to temperatures; for lights having a certain wavelength, the higher the temperature is, the smaller the rotation angle is. 
         [0046]      FIG. 3  is a schematic diagram of a Nomarski prism. The Nomarski Prism is a composite birefringent crystal comprising two wedge-shaped birefringent crystals. The arrows demonstrate two optical axis directions of the two crystals: one is parallel to the paper, and another is vertical to the paper. 
         [0047]      FIG. 4  is a diagram showing a Gaussian distribution of light intensities of two polarized light beams As shown in  FIG. 4 , light energies of two parallel lights partially overlap in the optical paths. 
         [0048]      FIG. 5  is a schematic diagram of a Polarization Beam Displacer (PBD) having a high-reflectivity interface on a side surface. Optionally, the high-reflectivity interface is a well-polished interface capable of total reflection, a coating or a metal coating of optical interference having high reflectivity. e is a Polarization Beam Displacer; f is the high-reflectivity interface; and X is an optical axis direction. 
         [0049]      FIG. 6  is a schematic diagram of Lateral Displacement Polarizing splitter having a high-reflectivity interface on a side surface. Optionally, the high-reflectivity interface is a well-polished total reflection interface, a coating or a metal coating of optical interference having high reflectivity. i is a hexahedral glass; j is a coating of optical interference (MacNeille coating); k is the high-reflectivity interface. j is not parallel to k and a certain angle is formed between j and k. 
       EXAMPLE 1 
       [0050]    As shown in  FIG. 7 , a Faraday rotator mirror comprises a single-mode single-fiber collimator  11 , a Nomarski Prism  12 , a Faraday rotator  34 , and a plane mirror  15  made from optical coating. In a rectangular coordinate system, an optical axis X 1  of the Nomarski Prism is coincident with the x axis, and an optical axis X 2  thereof is in the y-z plane. 
         [0051]    A light  100  is emitted by the single-mode single-fiber collimator  11 , and radiates along the z axis. When passing through the Nomarski Prism  12 , the light is divided into two linearly polarized lights that are orthogonal: a first ordinary light  111  and a first extraordinary light  121 . The lights are converged after passing through an interface of two wedge blocks of the Nomarski Prism, and pass through the Faraday rotator  34  where vibration planes are rotated by nearly 45 degrees to be a first light beam  112  and a second light beam  122 . The first light beam  112  and the second light beam  122  meet at the plane mirror  15 , have optical paths exchanged, and become a first reflective light  113  and a second reflective light  123 , respectively. The reflective lights radiate along a negative direction of the z axis and pass through the Faraday rotator  34  for the second time. The vibration planes are again rotated by nearly 45 degrees (a total of 90 degrees) to be a third light beam  114  and a fourth light beam  124 . After entering into the Nomarski Prism, the fourth light beam  124  is divided to be a second extraordinary light  125  and a second ordinary light  126 . The second extraordinary light  125  is from the polarized light component of the fourth light beam  124  in the y-z plane. The third light beam  114  is divided to be a third ordinary light  115  and a third extraordinary light  116 . The third ordinary light  115  is from the polarized light component of the third light beam  114  in the x-axis direction. The third extraordinary light  116  and the second ordinary light  126  are merged to be a main light beam  199  after passing through the Nomarski Prism  12 . The third ordinary light  115  and the second extraordinary light  125  are separated from each other in a certain distance and deviate the main light beam  199 . Finally, the main light beam  199  is coupled to the single-mode single-fiber collimator  11 . 
         [0052]    The Nomarski Prism  12  is a composite element of a polarizing beam splitter and the polarizing beam converging device made from birefringent crystal and combines the functions of the two. The Faraday rotator mirror has a small size and a high extinction ratio. 
         [0053]    Amidst the light transmission, positions (A), (B), (C), (D), (E), and (F) are marked, and states of polarization at cross sections of the positions are shown correspondingly in  FIGS. 8A-10B . 
         [0054]    The Faraday rotator mirror is capable of eliminating the polarized light components along the dispersion direction of the rotation angle after 90 degrees&#39; deviation, thus eliminating the effects of wavelength and temperature on the rotation angle and on the Faraday rotator mirror. 
       EXAMPLE 2 
       [0055]    As shown in  FIG. 11 , a Faraday rotator mirror comprises a single-mode single-fiber collimator  11 , a polarizing beam splitter  22 , a Wollaston prism  23 , a Faraday rotator  34 , and a plane mirror  15  made from optical coating. In a rectangular coordinate system, an optical axis X 3  of the polarizing beam splitter is in the y-z plane, and an optical axis X 4  of the Wollaston prism is parallel to the x axis. An optical axis X 5  of the Wollaston prism is parallel to the y axis. 
         [0056]    A light  300  is emitted by the single-mode single-fiber collimator  11 , and radiates along the z axis. When passing through the polarizing beam splitter  22 , the light is divided into two linearly polarized lights that are orthogonal: a first ordinary light  321  and a first extraordinary light  311 . The lights are converged after passing through Wollaston prism  23 , and pass through the Faraday rotator  34  where vibration planes are rotated by nearly 45 degrees to be a first light beam  312  and a second light beam  322 . The first light beam  312  and the second light beam  322  meet at the plane mirror  15 , have optical paths exchanged, and become a first reflective light  313  and a second reflective light  323 , respectively. The reflective lights radiate along a negative direction of the z axis and pass through the Faraday rotator  34  for the second time. The vibration planes are again rotated by nearly 45 degrees (a total of 90 degrees) to be a third light beam  314  and a fourth light beam  324 . After passing through the Wollaston prism  23 , the fourth light beam  324  is divided by the polarizing beam splitter  22  to be a second ordinary light  325  and a second extraordinary light  326 . The second ordinary light  325  is from the polarized light component of the fourth light beam  324  in the x-axis direction. The third light beam  314  is divided by the polarizing beam splitter  22  to be a third extraordinary light  315  and a third ordinary light  316 . The third extraordinary light  315  is from the polarized light component of the third light beam  314  in the y-z plane. The third ordinary light  316  and the second extraordinary light  326  are merged to be a main light beam  399  after passing through the polarizing beam splitter  22 . The third extraordinary light  315  and the second ordinary light  325  are separated from each other in a certain distance and deviate the main light beam  399 . Finally, the main light beam  399  is coupled to the single-mode single-fiber collimator  11 . 
         [0057]    The Wollaston prism  23  functions as a polarizing beam converging device made from birefringent crystal. 
         [0058]    The Faraday rotator mirror is capable of eliminating the polarized light components along the dispersion direction of the rotation angle after 90 degrees&#39; deviation, thus eliminating the effects of wavelength and temperature on the rotation angle and on the Faraday rotator mirror. 
       EXAMPLE 3 
       [0059]    As shown in  FIG. 12 , a Faraday rotator mirror comprises a single-mode single-fiber collimator  11 , a Wollaston prism  42 , a Fresnel biprism  13 , a Faraday rotator  34 , and a plane mirror  15  made from optical coating. In a rectangular coordinate system, an optical axis X 4  of the Wollaston prism is parallel to the y axis, and an optical axis X 5  of the Wollaston prism is parallel to the x axis. An edge of the Fresnel biprism  13  is parallel to the x axis. 
         [0060]    A light  400  is emitted by the single-mode single-fiber collimator  11 , and radiates along the z axis. When passing through the Wollaston prism  42 , the light is divided into two linearly polarized lights that are orthogonal: a first extraordinary light  411  and a first ordinary light  421 . The lights are converged after passing through the Fresnel biprism  13 , and pass through the Faraday rotator  34  where vibration planes are rotated by nearly 45 degrees to be a first light beam  412  and a second light beam  422 . The first light beam  412  and the second light beam  422  meet at the plane mirror  15 , have optical paths exchanged, and become a first reflective light  413  and a second reflective light  423 , respectively. The reflective lights radiate along a negative direction of the z axis and pass through the Faraday rotator  34  and the Fresnel biprism  13  for the second time. The vibration planes are again rotated by nearly 45 degrees (a total of 90 degrees) to be a third light beam  414  and a fourth light beam  424 . After entering into the Wollaston prism  42 , the fourth light beam  424  is divided to be a second ordinary light  425  and a second extraordinary light  426 . The second ordinary light  425  is from the polarized light component of the fourth light beam  424  in the x-axis direction. The third light beam  414  is divided to be a third extraordinary light  415  and a third ordinary light  416 . The third extraordinary light  415  is from the polarized light component of the third light beam  414  in the y-z plane. The third ordinary light  416  and the second extraordinary light  426  are merged to be a main light beam  499  after passing through the Wollaston prism  42 . The third extraordinary light  415  and the second ordinary light  425  are separated from each other in a certain distance and deviate the main light beam  499 . Finally, the main light beam  499  is coupled to the single-mode single-fiber collimator  11 . 
         [0061]    The Fresnel biprism  13  functions as a polarization-independent refractive beam converging device. 
         [0062]    The Faraday rotator mirror is capable of eliminating the polarized light components along the dispersion direction of the rotation angle after 90 degrees&#39; deviation, thus eliminating the effects of wavelength and temperature on the rotation angle and on the Faraday rotator mirror. 
       EXAMPLE 4 
       [0063]    As shown in  FIG. 13 , a Faraday rotator mirror comprises a single-mode single-fiber collimator  11 , a polarization beam displacer(PBD)  62  having a high-reflectivity interface on a side surface, a Faraday rotator  34 , and a plane mirror  15  made from optical coating. In a rectangular coordinate system, an optical axis X 9  of the polarization beam displacer is in the y-z plane. 
         [0064]    A light  600  is emitted by the single-mode single-fiber collimator  11 , and radiates along the z axis. When passing through the PBD  62 , the light is divided into two linearly polarized lights that are orthogonal: a first extraordinary light  611  and a first ordinary light  621 . The extraordinary light  611  is reflected by high-reflectivity interface of the PBD  62 , and converges with the first ordinary light  621 . The lights pass through the Faraday rotator  34  where vibration planes thereof are rotated by nearly 45 degrees to be a first light beam  612  and a second light beam  622 . The first light beam  612  and the second light beam  622  meet at the plane mirror  15 , have optical paths exchanged, and become a first reflective light  613  and a second reflective light  623 , respectively. The reflective lights radiate along a negative direction of the z axis and pass through the Faraday rotator  34  for the second time. The vibration planes are again rotated by nearly 45 degrees (a total of 90 degrees) to be a third light beam  614  and a fourth light beam  624 . After entering into the PBD  62 , the fourth light beam  624  is refracted, reflected, and divided to be a second ordinary light  625  and a second extraordinary light  626 . The second ordinary light  625  is from the polarized light component of the fourth light beam  624  in the y-z plane. The third light beam  614  is divided to be a third extraordinary light  615  and a third ordinary light  616 . The third extraordinary light  615  is from the polarized light component of the third light beam  614  in the y-z plane. The third ordinary light  616  and the second extraordinary light  626  are merged to be a main light beam  699  after passing through the PBD  62 . The third extraordinary light  615  and the second ordinary light  625  are separated from each other in a certain distance and deviate the main light beam  699 . Finally, the main light beam  699  is coupled to the single-mode single-fiber collimator  11 . 
         [0065]    The PBD  62  is a composite element of a polarizing beam splitter and a refractive beam converging device and combines the functions of the two. 
         [0066]    The Faraday rotator mirror is capable of eliminating the polarized light components along the dispersion direction of the rotation angle after 90 degrees&#39; deviation, thus eliminating the effects of wavelength and temperature on the rotation angle and on the Faraday rotator mirror. 
       EXAMPLE 5 
       [0067]    As shown in  FIG. 14 , a Faraday rotator mirror comprises a single-mode single-fiber collimator  11 , a lateral displacement polarizing splitter  72 , a Faraday rotator  34 , and a plane mirror  15  made from optical coating. 
         [0068]    In the following description, light s in the electric field direction of the polarized light is parallel to the x axis and light p is vertical to the x axis. 
         [0069]    A light  700  is emitted by the single-mode single-fiber collimator  11 , and radiates along the z axis. When passing through the lateral displacement polarizing splitter  72 , the light is divided into two linearly polarized lights that are orthogonal: a first s light  711  and a second p light  721 . The first s light  711  is reflected by a reflective interface of the Lateral Displacement Polarizing splitter  72 , and converges with the second p light  721 . The lights pass through the Faraday rotator  34  where vibration planes thereof are rotated by nearly 45 degrees to be a first light beam  712  and a second light beam  722 . The first light beam  712  and the second light beam  722  meet at the plane mirror  15 , have optical paths exchanged, and become a first reflective light  713  and a second reflective light  723 , respectively. The reflective lights radiate along a negative direction of the z axis and pass through the Faraday rotator  34  for the second time. The vibration planes are again rotated by nearly 45 degrees (a total of 90 degrees) to be a third light beam  714  and a fourth light beam  724 . After entering into the Lateral Displacement Polarizing splitter  72 , the fourth light beam  724  is reflected and divided to be a second p light  725  and a second s light  726 . The second p light  725  is from the polarized light component of the fourth light beam  724  vertical to the x axis. The third light beam  714  is divided to be a third s light  715  and a third p light  716 . The third s light  715  is from the polarized light component of the third light beam  714  parallel to the x axis. The third p light  716  and the second s light  726  are merged to be a main light beam  799  after passing through the lateral displacement polarizing splitter  72 . The third s light  715  and the second p light  725  are separated from each other in a certain distance and deviate the main light beam  799 . Finally, the main light beam  799  is coupled to the single-mode single-fiber collimator  11 . 
         [0070]    The lateral displacement polarizing splitter  72  is a composite element of a polarizing beam splitter and the refractive beam converging device and combines the functions of the two. 
         [0071]    The Faraday rotator mirror is capable of eliminating the polarized light components along the dispersion direction of the rotation angle after 90 degrees&#39; deviation, thus eliminating the effects of wavelength and temperature on the rotation angle and on the Faraday rotator mirror. 
         [0072]    Unless otherwise indicated, the numerical ranges involved in the invention include the end values. While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.