Abstract:
An optical device comprising a first combination of birefringent prisms with parallel optic axes for dividing an optical input beam into polarized beams, a second combination of birefringent prisms with parallel optic axes for combining polarized beams into an output beam, a polarization changer, and means for improving isolation in optical isolators, attenuators and switches and for improving material costs, light transmission, size and beam capacity in optical isolators, attenuators, switches and circulators.

Description:
TECHNICAL FIELD 
       [0001]    This invention is a continuation of patent application Ser. No. 10/553,132, filing date Apr. 16, 2004, now abandoned and relates to polarization independent optical isolators, attenuators, circulators and switches. 
     
    
     BACKGROUND ART 
       [0002]    Polarization independent optical isolators, attenuators, circulators and switches often use birefringent plates (also referred to as “walk-off” crystals) to divide an optical beam into parallel polarized beams and to combine parallel polarized beams into a single beam. 
         [0003]      FIG. 1A  shows a birefringent plate of yttrium orthovanadate in which an unpolarized beam of wavelength 1550 nm is so divided. The optic axis of the birefringent plate is in the plane of the drawing and oblique to the input face at an angle of 45 degrees giving an angular beam separation of 5.7 degrees. If separated beams pass through the plate in an opposite direction they may be combined into a single beam. 
         [0004]    U.S. Pat. No. 5,864,428 discloses means by which a beam may be divided into parallel polarized beams by use of birefringent prisms. In  FIG. 1B  a beam of wavelength 1550 nm passes through a 20 degree wedge of yttrium orthovanadate with its optic axis arranged normal to the plane of the drawing so that the beam is separated into polarized beams with an angular beam separation of 5.6 degrees. Component beams then pass through a similar prism to form parallel beams. 
         [0005]    As birefringent material is expensive, use of prisms may be advantageous in that the amount of birefringent material may be reduced, so reducing cost. Also, with a space between prisms, one or more polarization rotators may be arranged between prisms to increase optical isolation while maintaining an output free from polarization mode dispersion. 
         [0006]    It is therefore an object of this invention to provide a device such as an optical isolator, attenuator, circulator or switch which may be conservative in the use of birefringent material. 
         [0007]    It is also an object of the invention to provide a device such as an optical isolator, attenuator, circulator or switch which may be free from polarization mode dispersion and conservative in the use of birefringent material. 
         [0008]    It is a further object of the invention to provide a device such as an optical isolator, attenuator, circulator or switch with high optical isolation which may be free from polarization mode dispersion and conservative in the use of birefringent material. 
         [0009]    It is a further object of the invention to provide an improved optical isolator. 
         [0010]    It is a further object of the invention to provide an improved optical attenuator. 
         [0011]    It is a further object of the invention to provide an improved optical circulator. 
         [0012]    It is a further object of the invention to provide an improved optical switch. 
         [0013]    To this end in accordance with the invention the device may be characterised as an optical isolator, attenuator, circulator or switch with at least a combination of birefringent prisms with parallel optic axes for dividing an input beam into polarized beams or for combining polarized beams into an output beam. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1A  represents prior art, in which a birefringent plate of yttrium orthovanadate separates an unpolarized beam into polarized beams. 
           [0015]      FIG. 1B  represents prior art, in which birefringent prisms of yttrium orthovanadate separate an unpolarized beam into polarized beams. 
           [0016]      FIG. 2A  represents a non reciprocal polarization changer  3 , in which a beam passing from left to right has its plane of polarization rotated by 90 degrees. 
           [0017]      FIG. 2B  represents non reciprocal polarization changer  3 , in which a beam passing from right to left exits with its plane of polarization unchanged. 
           [0018]      FIG. 3A  represents an embodiment in accordance with the invention, being an optical isolator depicting a beam moving from a first port to a second port. 
           [0019]      FIG. 3B  represents an embodiment in accordance with the invention, being the optical isolator of  FIG. 3A  showing a beam moving from a second port toward a first port. 
           [0020]      FIG. 3C  represents an embodiment in accordance with the invention, being an optical isolator wherein reflective surfaces deflect diverging or converging beams. 
           [0021]      FIG. 3D  represents an embodiment in accordance with the invention wherein the amount of birefringent material is further reduced. 
           [0022]      FIGS. 3E and 3F  represent an embodiment in accordance with the invention, being an optical isolator with two degrees of isolation. 
           [0023]      FIGS. 3G and 3H  represent an embodiment in accordance with the invention, being an optical isolator with three degrees of isolation. 
           [0024]      FIG. 4A  represents an embodiment in accordance with the invention, being an optical attenuator depicting a beam moving from a first port to a second port. 
           [0025]      FIG. 4B  represents an embodiment in accordance with the invention, being the optical attenuator of  FIG. 4A  depicting a beam moving from a second port toward a first port. 
           [0026]      FIG. 5A  represents an embodiment in accordance with the invention, being a three port optical circulator showing a beam moving from a first port to a second port. 
           [0027]      FIG. 5B  represents an embodiment in accordance with the invention, being the three port optical circulator of  FIG. 5A  showing a beam moving from a second port to a third port. 
           [0028]      FIG. 5C  represents an embodiment in accordance with the invention, being a three port optical circulator wherein a central beam is deflected by a reflective surface. 
           [0029]      FIG. 5D  represents an embodiment in accordance with the invention, being a three port optical circulator in which outer beams are deflected by reflective surfaces. 
           [0030]      FIG. 6  represents an embodiment in accordance with the invention, being a four port optical circulator. 
           [0031]      FIG. 7A  represents an embodiment in accordance with the invention, being an optical switch showing beams moving between a first port and a second port. 
           [0032]      FIG. 7B  represents an embodiment in accordance with the invention, being the optical switch of  FIG. 7A  showing beams moving between a first port and a third port. 
           [0033]      FIG. 7C  represents an embodiment in accordance with the invention, being an optical switch in which a central beam is deflected by a reflective surface. 
           [0034]      FIG. 7D  represents an embodiment in accordance with the invention, being an optical switch in which outer beams are deflected by reflective surfaces. 
           [0035]      FIG. 7E  represents an embodiment in accordance with the invention, being an optical switch with two degrees of isolation. 
           [0036]      FIG. 7F  represents an embodiment in accordance with the invention, being an optical switch with three degrees of isolation. 
           [0037]      FIGS. 7G ,  7 H,  71  and  7 J represent the embodiment depicted in  FIG. 7F , showing how beams are isolated. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0038]      FIG. 2A  represents a non reciprocal polarization changer  3 , in which the plane of polarization of a beam entering from the left is rotated through an angle of 90 degrees. As observed from the left, beam  4  passes through Faraday rotator  6  and is rotated in a clockwise direction through an angle of 45 degrees. Beam  4  then passes through half waveplate  7 , which has its optic axis arranged so that the plane of polarization of beam  4  is rotated through an additional angle of 45 degrees in a clockwise direction. The plane of polarization of beam  4  is therefore rotated through a total angle of 90 degrees, and the plane of polarization depicted changes from a horizontal orientation to a vertical orientation. Similarly, the plane of polarization of beam  5  is changed from a vertical orientation to a horizontal orientation. 
         [0039]      FIG. 2B  represents non reciprocal polarization changer  3 , in which a beam entering from the right exits with its plane of polarization unchanged. As observed from the left, beam  4  passes through half waveplate  7  and is rotated in an anticlockwise direction through an angle of 45 degrees, and then passes through Faraday rotator  6  and is rotated in a clockwise direction through an angle of 45 degrees. The depicted plane of polarization of beam  4  remains in the horizontal plane before and after passing through non reciprocal polarization changer  3  and the plane of polarization of beam  5  remains in the vertical plane before and after passing through non reciprocal polarization changer  3 . 
         [0040]    Polarization changers are commonly used in optical isolators, attenuators, circulators and switches. 
         [0041]      FIG. 3A  represents an embodiment in accordance with the invention being an optical isolator  8 , comprising birefringent prisms  10  and  11 , Faraday rotator  12 , half waveplate  13  and birefringent prisms  14  and  15 . In this embodiment, birefringent prisms  10 ,  11 ,  14  and  15  are composed of yttrium orthovanadate and have their optic axes arranged normal to the plane of the drawing. Birefringent prism pairs  10  and  11 , and birefringent prism pairs  14  and  15  are optical devices which divide a beam into parallel, orthogonally polarized beams or combine parallel, orthogonally polarized beams into a single beam. 
         [0042]    For this embodiment, a beam  16 , entering optical isolator  8  through port  1 , passes through birefringent prism  10  to become orthogonally polarized beams  17  and  18 . Beams  17  and  18  then pass through birefringent prism  11  to become parallel beams. 
         [0043]    Beams  17  and  18  next pass through Faraday rotator  12  and half waveplate  13 , passing from left to right, and their planes of polarization are each rotated through an angle of 90 degrees, as shown in  FIG. 2A . Beam  17 , which was the ordinary beam in birefringent prisms  10  and  11 , becomes the extraordinary beam in birefringent prisms  14  and  15 . Beam  18  which was the extraordinary beam in birefringent prisms  10  and  11  becomes the ordinary beam in birefringent prisms  14  and  15 . Therefore beams  17  and  18  combine between birefringent prisms  14  and  15  to form a single beam which exits through port  2 . 
         [0044]      FIG. 3B  shows a beam  21  entering optical isolator  8  through port  2 , wherein beam  21  passes through birefringent prism  15  to become orthogonally polarized beams  19  and  20 , which separate and pass through birefringent prism  14  to become parallel beams. 
         [0045]    Beams  19  and  20  then pass through half waveplate  13  and Faraday rotator  12  from right to left, and in this direction the beams exit with their planes of polarization unchanged, as shown in  FIG. 2B . Beam  19 , which was the extraordinary beam in birefringent prisms  15  and  14 , remains the extraordinary beam in birefringent prism  11 . Beam  20 , which was the ordinary beam in birefringent prisms  15  and  14 , remains the ordinary beam in birefringent prism  11 . Therefore as beams  19  and  20  exit from birefringent prism  11  they continue to separate and do not pass into port  1 . Beams pass from port  1  to port  2 , but do not pass from port  2  to port  1 . 
         [0046]      FIG. 3C  represents an embodiment in accordance with the invention, being an optical isolator similar to isolator  8 , wherein reflective surfaces  22  and  23  are arranged between birefringent prisms to reduce isolator size. 
         [0047]      FIG. 3D  represents an embodiment in accordance with the invention, being an optical isolator similar to isolator  8 , wherein the amount of birefringent material is reduced by use of birefringent prisms  24 ,  25 ,  26  and  27 . 
         [0048]      FIGS. 3E and 3F  represent an embodiment in accordance with the invention, being an optical isolator with two degrees of isolation. 
         [0049]    By including a Faraday rotator  28  and a half waveplate  29  between prisms of the second birefringent prism pair, as shown, a second degree of isolation may be provided. Alternately, Faraday rotator  28  and half waveplate  29  may be places between prisms of the first birefringent prism pair. 
         [0050]      FIGS. 3G and 3H  represent an embodiment in accordance with the invention, being an optical isolator with three degrees of isolation. 
         [0051]      FIG. 4A  represents an embodiment in accordance with the invention being an optical attenuator  32 , which is optical isolator  8 , with additional element  33  placed between prisms  14  and  15 . The purpose of element  33  is to vary the amount of rotation applied to the planes of polarization of beams passing therethrough, so varying the intensity of the optical beam passing into port  2 . Element  33  may be, for example, a Faraday rotator or a liquid crystal cell with a variable controller. 
         [0052]    The plane of polarization of beam  17 , which was the extraordinary beam in prism  14 , after being partially rotated by element  33 , therefore has two polarization components when passing through prism  15 . The extraordinary component continues as in optical isolator  8 , while the ordinary component is refracted to exit from prism  15  as beam  35 . Similarly, the plane of polarization of beam  18 , which was the ordinary beam in prism  14 , after being partially rotated by element  33 , has two polarization components when passing into prism  15 . The ordinary component continues as in optical isolator  8 , while the extraordinary component is refracted to exit from prism  15  as beam  34 . Beams  34  and  35  disperse and do not enter port  2 . 
         [0053]    In a reverse direction, the plane of polarization of beam  19 , which was the extraordinary beam in prism  15 , after being partially rotated by element  33 , has two polarization components when passing into prism  14 . The extraordinary component continues as in optical isolator  8 , while the ordinary component is refracted to exit as beam  36 , as depicted in  FIG. 4B . Similarly, the plane of polarization of beam  20 , which was the ordinary beam in prism  15 , after being partially rotated by element  33 , has two polarization components when passing into prism  14 . The ordinary component continues as in optical isolator  8 , while the extraordinary component is refracted to exit as beam  37 , depicted. Beams  36  and  37  disperse and do not enter port  1 . 
         [0054]    In this embodiment attenuator  32  also acts as an optical isolator. It will be evident that two degrees of isolation can be obtained by including a Faraday rotator and a half waveplate between prisms  10  and  11 . 
         [0055]      FIG. 5A  represents an embodiment in accordance with the invention being an optical circulator  38 , comprising birefringent prisms  40  and  41 , Faraday rotator  42 , half waveplate  43 , birefringent prisms  44  and  45 , half waveplate  46  and birefringent prisms  47 ,  48  and  49 . In this embodiment birefringent prisms  40 ,  41 ,  44 ,  45 ,  47 ,  48  and  49  are composed of yttrium orthovanadate and have their optic axes arranged normal to the plane of the drawing, and birefringent prism pairs  40  and  41 , birefringent prism pairs  44  and  45 , and birefringent prisms  47 ,  48  and  49  are optical devices which divide a beam into parallel, orthogonally polarized beams or combine parallel, orthogonally polarized beams into a single beam. 
         [0056]    For this embodiment, a beam entering circulator  38  through port  1 , as beam  50 , passes through birefringent prism  49  to become orthogonally polarized beams  51  and  52 . Beams  51  and  52  then pass through prisms  47  and  48  to become parallel beams. Birefringent prisms  47  and  48  may be separate prisms or may be, for example, a single prism with a hole drilled through the center. Birefringent prisms  47  and  48  may also be a single prism wherein a central beam is deflected by a reflective surface  57  or outer beams are deflected by reflective surfaces  58 ,  59  and  60 , as shown in  FIGS. 5C and 5D . 
         [0057]    Beams  51  and  52  then pass through half waveplate  46  in which their planes of polarization are each rotated through an angle of 90 degrees. Beam  51 , which was the ordinary beam in prism  48 , becomes the extraordinary beam in prism  45 . Beam  52 , which was the extraordinary beam in prism  47 , becomes the ordinary beam in prism  45 . Therefore beams  51  and  52  pass between prisms  45  and  44  and partially combine. 
         [0058]    Beams  51  and  52  next pass through half waveplate  43  and Faraday rotator  42  from right to left, and in this direction beams  51  and  52  exit without any change in their planes of polarization, as shown in  FIG. 2B . Beams then pass through prisms  41  and  40  and continue to combine to exit as a single beam through port  2 . 
         [0059]      FIG. 5B  shows a beam entering circulator  38  through port  2 , as beam  53 , wherein beam  53  passes through prism  40  to become orthogonally polarized beams  54  and  55 , which separate and pass through prism  41  to become parallel beams. 
         [0060]    As beams  54  and  55  pass through Faraday rotator  42  and half waveplate  43  from left to right, their planes of polarization are rotated through an angle of 90 degrees, as depicted in  FIG. 2A . Beam  54 , which was the ordinary beam in prisms  40  and  41 , becomes the extraordinary beam in prisms  44  and  45 . Beam  55 , which was the extraordinary beam in prisms  40  and  41 , becomes the ordinary beam in prisms  44  and  45 . Therefore beams  54  and  55  combine between prisms  44  and  45  to become single beam  56 , which exits through port  3 . 
         [0061]    Beam  56  may pass through half waveplate  46 . Half waveplate  46  may also have a hole drilled through the center or be two separate elements, one on either side of the path of beam  56 . Also, if prisms  47 ,  48  and  49  have their optic axes oriented vertically in the plane of the drawing, waveplate  46  may be removed altogether. 
         [0062]    A beam which enters the device through port  1  exits through port  2  and a beam which enters the device through port  2  exits through port  3 , the device being a 3 port optical circulator. 
         [0063]    A Faraday rotator and a half waveplate may also be placed between prisms  47 ,  48  and  49  so that beam  50  passes from port  1  to port  2  as before, but so that port  1  is further isolated from port  2 . 
         [0064]    By adding elements as shown in  FIG. 6 , a 4 port optical circulator  62  may be formed, in which a beam entering through port  1  exits through port  2 , a beam entering through port  2  exits through port  3 , a beam entering through port  3  exits through port  4 , and a beam entering through port  4  exits through port  1 . 
         [0065]      FIGS. 7A and 7B  represent an embodiment in accordance with the invention being an optical switch  68 , comprising birefringent prisms  70  and  71 , reciprocal polarization changer  72 , birefringent prisms  74  and  75 , half waveplate  76  and birefringent prisms  77 ,  78  and  79 . In this embodiment birefringent prisms  70 ,  71 ,  74   75 ,  77 ,  78  and  79  are composed of yttrium orthovanadate with their optic axes arranged normal to the plane of the drawing, and birefringent prism pairs  70  and  71 , birefringent prism pairs  74  and  75 , and birefringent prisms  77 ,  78  and  79  divide a beam into parallel, orthogonally polarized beams or combine parallel, orthogonally polarized beams into a single beam. 
         [0066]    Polarization changer  72  may be, for example, a liquid crystal cell or a Faraday rotator with a switching controller. In this embodiment polarization changer  72 , when in a first state (hereinafter referred to as the “OFF” state), allows polarized beams to travel in either direction with their planes of polarization unchanged, and when in a second state (hereinafter referred to as the “ON” state), causes the planes of polarization of beams travelling in either direction to be rotated by an angle of 90 degrees. 
         [0067]    For this embodiment, a beam entering optical switch  68  through port  1 , as beam  83 , passes through birefringent prism  70  to become orthogonally polarized beams  84  and  85 . Beams  84  and  85  then pass through prism  71  to become parallel beams. 
         [0068]    In the OFF state, beams  84  and  85  pass through polarization changer  72  with their planes of polarization unchanged. Beam  84 , which was the ordinary beam in prisms  70  and  71 , remains the ordinary beam in prisms  74  and  75 . Beam  85 , which was the extraordinary beam in prisms  70  and  71 , remains the extraordinary beam in prisms  74  and  75 . Therefore beams  84  and  85  pass between prisms  74  and  75  and continue to separate. 
         [0069]    Beams  84  and  85  next pass through half waveplate  76  wherein their planes of polarization are rotated through an angle of 90 degrees. Beam  84 , which was the ordinary beam in prism  75 , becomes the extraordinary beam in prism  77 . Beam  85 , which was the extraordinary beam in prism  75 , becomes the ordinary beam in prism  78 . Beams  84  and  85  then combine between prisms  77 ,  78  and  79  to exit through prism  79  as single beam  80 . Beam  80  leaves through port  2 . 
         [0070]    Birefringent prisms  77  and  78  may be separate prisms or may be, for example, a single prism with a hole drilled through the center. Birefringent prisms  77  and  78  may also be a single prism, wherein a central beam is deflected by a reflective surface  87 , as shown in  FIG. 7C , or by reflective surfaces  88 ,  89  and  90 , as shown in  FIG. 7D . 
         [0071]    For a beam passing through optical switch  68  in a reverse direction, as beam  80 , beam  80  passes through birefringent prism  79  and divides into orthogonally polarised beams  81  and  82 . Beams  81  and  82  then retrace the paths of beams  84  and  85  to exit as single beam  83 . Beam  83  leaves through port  1 . 
         [0072]    In the ON state, beams  84  and  85  pass through polarization changer  72  where their planes of polarization are rotated through an angle of 90 degrees. Beam  84 , which was the ordinary beam in prisms  70  and  71 , becomes the extraordinary beam in prisms  74  and  75 . Beam  85 , which was the extraordinary beam in prisms  70  and  71 , becomes the ordinary beam in prisms  74  and  75 . Beams  84  and  85  therefore pass between prisms  74  and  75  and combine to exit as beam  86 . Beam  86  leaves through port  3 . 
         [0073]    Beam  86  may pass through half waveplate  76 . Half waveplate  76  may also have a hole drilled through the center or be two separate elements, one on either side of the path of beam  86 . 
         [0074]    Also, if prisms  77 ,  78  and  79  have their optic axes oriented vertically in the plane of the drawing, waveplate  76  may be removed altogether. 
         [0075]    For a beam passing through optical switch  68  in a reverse directions, beam  86  retraces the paths of beams  84  and  85  to exit as single beam  83 . Beam  83  leaves through port  1 . 
         [0076]    Two way communication between ports  1  and  2  can therefore be switched to two way communication between ports  1  and  3  by changing the state of reciprocal polarization changer  72  from the OFF state to the ON state, or from the ON state to the OFF state. 
         [0077]      FIG. 7E  represents an embodiment in accordance with the invention, being an optical switch with two degrees of isolation. Element  93  is a half waveplate as in the previous embodiment and elements  91  and  92  are reciprocal rotators. When optical beams pass between ports  1  and  2 , and polarization rotator  92  is in the ON state, residual beams may be disrupted from passing between ports  1  and  3 . Similarly, when optical beams pass between ports  1  and  3 , and polarization rotator  91  is in the ON state, residual beams may be disrupted from passing between ports  1  and  2 . 
         [0078]      FIG. 7F  represents an embodiment in accordance with the invention, being an optical switch  101  with three degrees of isolation. Element  95  is a half waveplate as in the previous embodiment and elements  100 ,  96 ,  97 ,  98  and  99  are reciprocal rotators. When optical beams pass between ports  1  and  2 , residual beams may be disrupted from passing between ports  1  and  3 . When optical beams pass between ports  1  and  3 , residual beams may be disrupted from passing between ports  1  and  2 . 
         [0079]      FIGS. 7G ,  7 H,  71  and  7 J represent optical switch  101 , showing how beams are disrupted. 
         [0080]      FIG. 7G  represents optical switch  101 , wherein reciprocal rotators are in the following states:  100  OFF,  96  OFF,  99  OFF,  97  ON,  98  ON. 
         [0081]      FIG. 7H  represents optical switch  101 , wherein reciprocal rotators are in the following states:  100  OFF,  96  OFF,  99  OFF,  97  ON,  98  ON. 
         [0082]      FIG. 7I  represents optical switch  101 , wherein reciprocal rotators are in the following states:  100  ON,  97  OFF,  98  OFF,  96  ON,  99  ON. 
         [0083]      FIG. 7J  represents optical switch  101 , wherein reciprocal rotators are in the following states:  100  ON,  97  OFF,  98  OFF,  96  ON,  99  ON. 
         [0084]    The devices described herein are free from polarization mode dispersion, may accommodate various frequencies or signals and may be composed of various materials. By way of example, prisms may be composed of yttrium orthovanadate, rutile, calcite, alpha-barium borate or lithium niobate and Faraday rotators may be composed of various magneto-optic materials as described, for example, in U.S. Pat. No. 5,608,570 by Brandle et al. 
         [0085]    Modifications and variations to the described embodiments will be apparent to those skilled in the art and all such modifications and variations should be considered as within the scope of the present invention.