Abstract:
The invention provides an optical circulator having a plurality of ports, a non-reciprocal rotator, a beam shifter means in the form of at least one birefringent crystal, a polarization rotator, and a reflector. The plurality of ports is sequentially aligned at one end of the device, while the reflector is disposed at an opposite end. First and second lenses provide efficient coupling between the plurality of ports, in combination with the reflector. Conveniently, the beam shifter provides the beam displacement necessary to switch between successive ports, while simultaneously minimizing the size requirements of the other optical components.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to optical circulators for use in optical communication systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    An optical circulator is a passive, non-reciprocal device that directs the propagation of light in one direction among a plurality of input/output optical ports. For example, light launched into a first optical port is circulated to a second optical port, while light launched into the second optical port is circulated to a third optical port. In general, light propagation in a reverse direction from the second optical port back to the first optical port is inhibited.  
           [0003]    Several types of optical circulators have been developed. In general, the more successful designs include polarizing beamsplitters, non-reciprocal rotators such as Faraday rotators, and reciprocal rotators such as half waveplates.  
           [0004]    For example, one particularly successful design was proposed by Cheng in U.S. Pat. No. 5,991,076, incorporated herein by reference. The device taught by Cheng is a polarization insensitive three port optical circulator having two identical groups of optical elements disposed on either side of a pair of collimating/focussing lenses. Each group includes a polarizing beamsplitter in the form of a first birefringent crystal for separating an input beam of light launched into the first port into two orthogonally polarized sub-beams of light, at least one half waveplate for making the polarization of both sub-beams parallel, a Faraday rotator, and beam shifting means in the form of a second birefringent crystal for passing light launched from the first port to the second port and for directing light launched from the second port to the third port, wherein the first and third ports are on one side of the device and the second port is on an opposite side of the device.  
           [0005]    Advantageously, this optical arrangement minimizes the size of costly optical components such as the birefringent crystals and provides optimum coupling between ports. However, this symmetrical design has the disadvantage that it can be time consuming to align, is difficult to manufacture as a more than three port device, and requires a number of costly optical components. For example, the design typically requires two polarizing beamsplitters, two sets of reciprocal rotators, two non-reciprocal rotators and two beam shifting means.  
           [0006]    In U.S. Pat. No. 5,930,422, incorporated herein by reference, Cheng discloses a reflective optical circulator that uses about half the optical components disclosed in U.S. Pat. No. 5,991,076. Specifically, the optical circulator uses one polarizing beamsplitter, one Faraday rotator, one beam shifting means, one lens, and one reflector. The single lens is used as a light redirector, rather than a collimating/focussing element. Accordingly, the reflected light is disadvantageously incident on the circulating components with a nonzero angle of incidence. Moreover, the optical circulator disclosed therein requires very precise and time-consuming optical alignment to achieve optimal optical coupling.  
           [0007]    It is an object of the instant invention to provide an optical circulator that overcomes the above limitations.  
           [0008]    It is a further object of the instant invention to provide a compact optical circulator that has a folded configuration.  
         SUMMARY OF THE INVENTION  
         [0009]    The instant invention relates to a reflective optical circulator that includes a non-reciprocal rotator, beam shifting means, first and second lenses, a polarization rotator, and a reflector. The input/output ports of the optical circulator are sequentially aligned at one end of the device, while the reflector is disposed at an opposite end. The first and second lenses provide efficient coupling between the input and output ports, in combination with the reflector. More specifically, the first and second lenses provide an imaging system wherein the reflector is in the image plane of the input/output ports such that light transmitted to the reflector is folded directly back along substantially the same optical path. Conveniently, the beam shifting means provides the beam displacement necessary to switch between successive ports, while simultaneously minimizing the size requirements of the other optical components. Notably, the beam displacement is provided in one of the forward and backward propagating directions.  
           [0010]    In accordance with the instant invention there is provided an optical circulator having first, second, and third ports for transmitting light from the first port to the second port, and from the second port to a third port, circularly, comprising:  
           [0011]    first polarization means for converting light launched from one of the first and second ports into a beam of light having a predetermined polarization;  
           [0012]    beam shifting means for receiving the beam of light having the predetermined polarization and for providing one of a beam displacement and substantially no beam displacement in dependence upon a polarization state of the beam of light;  
           [0013]    second polarization means for rotating the polarization of the beam of light transmitted from the beam shifting means;  
           [0014]    reflecting means for redirecting the beam of light transmitted from the second polarization means back along a substantially same optical path; and  
           [0015]    imaging means for optically coupling the first, second, and third ports and the reflecting means, the imaging means having a first focal plane substantially at the first, second, and third ports and a second focal plane substantially at the reflecting means.  
           [0016]    For example, in one embodiment the first polarization means includes a polarization diversity unit having at least one birefringent crystal for converting the light launched from one of the input ports into polarized light. In another embodiment, the polarized light is provided with a plurality of polarization maintaining waveguides or a linear polarizer. Advantageously, embodiments including a polarization diversity unit are polarization insensitive.  
           [0017]    In accordance with the instant invention there is provided an optical circulator comprising:  
           [0018]    a plurality of ports including a first port for launching a first beam of light in a forward propagating direction, a second port for receiving the first beam of light in a backward propagating direction and launching a second beam of light in the forward propagating direction, and a third port for receiving the second beam of light in the backward propagating direction, the first and second ports and the second and third ports each separated by a distance d;  
           [0019]    a reflector optically coupled to the plurality of ports for redirecting the first and second beams of light propagating in the forward direction in the backward direction;  
           [0020]    a non-reciprocal rotator optically disposed between the plurality of ports and the reflector for rotating the polarization of the first and second beams of light in the forward and backwards propagating directions by a predetermined angle;  
           [0021]    a beam shifter optically disposed between the plurality of ports and the reflector for providing a beam displacement substantially equal to d for the first and second beams of light in one of the forward and backward propagating directions in dependence upon a polarization state thereof;  
           [0022]    a polarization rotator optically disposed between the beam shifter and the reflector for rotating the polarization of the first and second beams of light between the forward and backward propagating directions; and  
           [0023]    imaging means for providing collimating and focussing of the first and second beams of light.  
           [0024]    In accordance with the instant invention there is provided an optical circulator comprising:  
           [0025]    a plurality of ports disposed at a first end;  
           [0026]    a reflector disposed at a second end optically coupled to the plurality of ports;  
           [0027]    polarization diversity means optically coupled to the plurality of ports and the reflector for splitting a beam of light launched from a port of the plurality of ports into two forward propagating orthogonally polarized sub-beams of light, and for combining two backward propagating orthogonally polarized sub-beams of light into a single beam of light;  
           [0028]    a non-reciprocal rotator optically coupled to the polarization diversity means for rotating the polarization of each forward and backward propagating sub-beam of light transmitted therethrough by a predetermined angle,  
           [0029]    beam shifting means optically coupled to the non-reciprocal rotator for providing a beam displacement for each forward and backward propagating sub-beam of light transmitted therethrough in dependence upon the polarization thereof;  
           [0030]    a polarization rotator optically coupled to the beam shifting means for rotating the polarization of each forward and backward propagating sub-beam of light such that only one of the forward propagating sub-beams of light and the backward propagating sub-beams of light experiences the beam displacement; and  
           [0031]    imaging means optically coupled to the plurality of ports and the reflector, the imaging means having a first focal plane substantially at the plurality of ports and a second focal plane substantially at the reflector. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    Exemplary embodiments of the invention will now be described in conjunction with the drawings in which:  
         [0033]    [0033]FIG. 1 a  is a schematic diagram of an embodiment of an optical circulator in accordance with the instant invention viewed from the side;  
         [0034]    [0034]FIG. 1 b  is a top view of the optical circulator shown in FIG. 1 a;    
         [0035]    [0035]FIG. 1 c  is a schematic diagram showing the operation of the optical circulator shown in FIG. 1 a;    
         [0036]    [0036]FIG. 2 a  is a schematic diagram of another embodiment of an optical circulator in accordance with the instant invention viewed from the side;  
         [0037]    [0037]FIG. 2 b  is a top view of the optical circulator shown in FIG. 2 a;    
         [0038]    [0038]FIG. 2 c  is a schematic diagram showing the operation of the optical circulator shown in FIG. 2 a;    
         [0039]    [0039]FIG. 3 a  is a schematic diagram of another embodiment of an optical circulator in accordance with the instant invention viewed from the side;  
         [0040]    [0040]FIG. 3 b  is a top view of the optical circulator shown in FIG. 3 a;    
         [0041]    [0041]FIG. 3 c  is a schematic diagram showing the operation of the optical circulator shown in FIG. 3 a;    
         [0042]    [0042]FIG. 4 a  is a schematic diagram of another embodiment of an optical circulator in accordance with the instant invention viewed from the side;  
         [0043]    [0043]FIG. 4 b  is a top view of the optical circulator shown in FIG. 4 a;    
         [0044]    [0044]FIG. 4 c  is a schematic diagram showing the operation of the optical circulator shown in FIG. 4 a;    
         [0045]    [0045]FIG. 5 a  is a schematic diagram of another embodiment of an optical circulator in accordance with the instant invention viewed from the side;  
         [0046]    [0046]FIG. 5 b  is a top view of the optical circulator shown in FIG. 5 a;    
         [0047]    [0047]FIG. 5 c  is a schematic diagram showing the operation of the optical circulator shown in FIG. 5 a ; and,  
         [0048]    [0048]FIG. 6 a  is a schematic diagram of yet another embodiment of an optical circulator in accordance with the instant invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0049]    Turning now to FIGS. 1 a  and  1   b  there is shown an optical circulator in accordance with an embodiment of the instant invention. The optical circulator  100  includes a tube  110  for housing a plurality of optical waveguides, conveniently shown as first  111 , second  112 , third  113 , and fourth  114  optical fibres. Optionally, each fibre has a thermally expanded core. Each optical fibre  111 ,  112 ,  113 ,  114  is optically coupled to a first polarizing beam splitter in the form of a birefringent crystal  120 , which splits a beam of light launched from one of the plurality of fibres  111 ,  112 ,  113 ,  114  into first and second sub-beams of light having orthogonal polarization states. A reciprocal polarization unit  130  is provided to ensure that both sub-beams of light have the same polarization state. For example, in this embodiment the reciprocal polarization unit  130  includes first  132  and second  134  orthogonally oriented half-waveplates for rotating the polarization of each sub-beam by −45° and +45°, respectively. Alternatively, the reciprocal polarization unit  130  includes a spacer (not shown) and a half waveplate (not shown) for rotating the polarization of each sub-beam by 0° and 90°, respectively. The first birefringent crystal  120  is optically coupled to a non-reciprocal polarization rotator  140 , such as a Faraday rotator, which rotates the polarization state of both sub-beams of light by about 45°. The non-reciprocal polarization rotator  140  is optically coupled to a second birefringent crystal  150 . Conveniently, the walk-off direction (i.e., the offset direction) of the second birefringent crystal  150  is parallel to a straight line coincident with each fibre end of the plurality of fibres  111 ,  112 ,  113 ,  114 . The walk-off direction of first birefringent crystal  120  is approximately 90° or −90° to the walk-off direction of the second birefringent crystal  150 . The second birefringent crystal  150  provides a beam displacement for each sub-beam of light passing therethrough in dependence upon its polarization state. Preferably, the first  120  and second  150  birefringent crystals are rutile, yttrium vanadate, magnesium fluoride, quartz, lithium niobate, or calcite crystals. An at least partially reflective surface  190 , such as a mirror, is provided to redirect light propagating in a forward direction from the plurality of fibres  111 ,  112 ,  113 , and  114  to light propagating in a backwards direction towards the plurality of fibres  111 ,  112 ,  113  and  114 , while a polarization rotator  180 , such as a quarter waveplate, or a second Faraday rotator, is provided for switching between orthogonal polarization states for the forward and backward propagating light. Conveniently, the reflective surface  190  is optionally coated on the polarization rotator  180 . A first lens  160  having a focal plane substantially at the plurality of fibres  111 ,  112 ,  113 , and  114  and second lens  170  having a focal plane substantially at the mirror  190 , are provided for focussing and collimating each sub-beam of light passing therethrough. For example, GRIN, spherical, and aspherical lenses are all suitable for providing the necessary collimating and focussing effects. More specifically, the first  160  and second  170  lenses provide an imaging system wherein the ends of optical fibres  111 ,  112 ,  113  and  114  are imaged onto the imaging plane coincident with the mirror  190 . Optionally, the first  160  and second  170  lenses provide different magnifications, however, it is preferred that the first  160  and second  170  lenses provide a one-to-one optical arrangement or imaging system.  
         [0050]    Referring to FIG. 1 c , the operation of the device is described in further detail. A beam of light launched from the first optical fibre  111  is passed through the first birefringent crystal  120 , which passes a first sub-beam corresponding to the ordinary component and provides a spatial displacement for a second sub-beam corresponding to the extraordinary component, as indicated in B.2. The sub-beam corresponding to the extraordinary component passes through half waveplate  132  where its polarization is rotated by +45°, while the sub-beam corresponding to the ordinary component passes through half waveplate  134  where its polarization is rotated by −45°, as indicated in C.3. Each sub-beam of light is transmitted through the Faraday rotator  140  where its polarization state is rotated by approximately 45° as shown in D.4. Since the second birefringent crystal  150  is oriented to pass light having the polarization indicated at D.4, each sub-beam passes through the second birefringent crystal  150  and lenses  160  and  170  with substantially no change or displacement. Each sub-beam of light passes through the quarter waveplate  180  where it becomes circularly polarized, and is reflected by mirror  190  such that it passes through the quarter waveplate  180  a second time. The net effect of the double pass is that the polarization states of both sub-beams of light are rotated by 90° as shown in F.7. Each sub-beam of light propagates through the second  170  and first  160  lenses and is incident on the second birefringent crystal  150 . Since the polarization state of each sub-beam was rotated by 90° by the quarter waveplate  180 , each sub-beam propagates along the extraordinary path of the polarizing beam splitter  150  such that a beam displacement dependent on the length of the crystal  150  is achieved. Conveniently, the length of the second birefringent crystal  150  is selected such that each sub-beam is shifted a distance substantially equal to the distance between the first  111  and second  112  optical fibres, as shown in D.9. For example, a beam displacement of about 125 μm is usually convenient. Each sub-beam propagates through the Faraday rotator  140 , the reciprocal polarization unit  130 , and finally the first birefringent crystal where they are combined and output the second fibre  112 . Similarly, an input optical signal launched from the second optical fibre  112  is coupled into the third optical fibre  113 , and an input optical signal launched from the third optical fibre  113  is coupled into the fourth optical fibre  114 .  
         [0051]    Referring to FIGS. 2 a  and  2   b  there is shown an alternate embodiment of an optical circulator in accordance with the instant invention. In this embodiment parts  210 ,  211 ,  212 ,  213 ,  214 ,  230 ,  240 ,  250 ,  260 ,  270 ,  280 , and  290  are similar to parts  110 ,  111 ,  112 ,  113 ,  114 ,  130 ,  140 ,  150 ,  160 ,  170 ,  180 , and  190  discussed above. However, in the instant embodiment a first birefringent plate  222 , a reciprocal rotator  224 , and a second birefringent plate  226  oppositely oriented from the first birefringent plate  222 , provide the two orthogonally polarized sub-beams of light, rather than the birefringent crystal  120  shown in FIGS. 1 a  and  1   b . More specifically, the first  222  and second  226  birefringent plates are oppositely oriented such that their respective walk-off directions are at 180°. Conveniently, the walk-off directions of the first  222  and second  226  birefringent plates are at a 90° angle to the walk-off direction of the second birefringent crystal  250 , which is parallel to a straight line coincident with each fibre end of the plurality of fibres  211 ,  212 ,  213 ,  214 . Preferably, the reciprocal rotator  224  is a half waveplate for rotating the polarization state of each sub-beam by about 90°. Optionally, tube  210 , first GRIN lens  260 , and second GRIN lens  270  are provided with slanted end faces to reduce backreflections. Furthermore, optional spacers  295  having a predetermined refractive index are provided to maintain beam alignment. Since the path length between the ordinary and extraordinary components is substantially equalized by this embodiment, polarization mode dispersion (PMD)) is significantly reduced.  
         [0052]    Referring to FIG. 2 c , the operation of the device is described in further detail. A beam of light launched from the first optical fibre  211  is passed through the birefringent plate  222  where a first sub-beam corresponding to the ordinary component is transmitted straight through and a second sub-beam corresponding to the extraordinary component experiences a spatial walk-off, as indicated in B.2. Each sub-beam subsequently passes through the half waveplate  224  where its polarization state is rotated by 90°, and the second birefringent plate  226  where the first sub-beam experiences an opposite spatial walk-off and the second sub-beam passes straight through, as indicated in C.3 and D.4, respectively. The first sub-beam of light passes through half waveplate  234  where its polarization is rotated by +45°, while the second sub-beam passes through half waveplate  232  where its polarization is rotated by −45°, as indicated in E.5. Each sub-beam of light is transmitted through the Faraday rotator  240  where its polarization state is rotated by approximately 45° as shown in F.6. Since the second birefringent crystal  250  is oriented to pass light having the polarization indicated at F. 6 , each sub-beam passes through the second birefringent crystal  250  and lenses  260  and  270  with substantially no change. Each sub-beam of light passes through the quarter waveplate  280  where it becomes circularly polarized, and is reflected by mirror  290  such that it passes through the quarter waveplate  280  a second time. The net effect is that the polarization states of each of the first and second sub-beams of light is rotated by 90° as shown in G.8. Each sub-beam of light propagates through the second  270  and first  260  lenses and is incident on the second birefringent crystal  250 . Since the polarization state of each sub-beam was rotated by 90° by the quarter waveplate  280 , each sub-beam propagates along the extraordinary path of the birefringent crystal  250  such that a beam displacement dependent on the length of the crystal  250  is achieved. Conveniently, the length of the second birefringent crystal  250  is selected such that each sub-beam is shifted a distance substantially equal to the distance between the first  211  and second  212  optical fibres as shown F. 9 . For example, a beam displacement of about 125 μm is typical. Each sub-beam propagates through the Faraday rotator  240 , the reciprocal polarization unit  230 , and finally the second birefringent plate  226 , the reciprocal rotator  224 , and the first birefringent plate  222  where they are combined and output the second fibre  212 . Similarly, an input optical signal launched from the second optical fibre  212  is coupled into the third optical fibre  213 , and an input optical signal launched from the third optical fibre  213  is coupled into the fourth optical fibre  214 .  
         [0053]    Referring to FIGS. 3 a  and  3   b  there is shown another embodiment of the optical circulator. In this embodiment parts  310 ,  311 ,  312 ,  313 ,  314 ,  340 ,  350 ,  360 ,  370 ,  380 , and  390  are similar to parts  110 ,  111 ,  112 ,  113 ,  114 ,  140 ,  150 ,  160 ,  170 ,  180 , and  190  discussed above with respect to FIGS. 1 a  and  1   b . However, in the instant embodiment a first birefringent plate  321  and a second birefringent plate  323  oriented perpendicularly to the first birefringent plate  321 , provide two orthogonally polarized sub-beams of light, rather than the birefringent crystal  120  shown in FIGS. 1 a  and  1   b . More specifically, the first birefringent plate  321  has a walk-off direction that is perpendicular to the walk-off direction of the second birefringent plate  323 . The walk-off directions of the first  321  and second  323  birefringent plates are at a 45° angle to the walk-off direction of the second birefringent crystal  350 , which is parallel to a straight line coincident with each fibre end of the plurality of fibres  311 ,  312 ,  313 ,  314 . In this embodiment, the reciprocal polarization unit  330  includes half waveplate  332  and glass spacer  334 . Preferably, the glass spacer  334  has the same refractive index as the half waveplate  332 . Optionally, tube  310 , first GRIN lens  360 , and second GRIN lens  370  are provided with slanted end faces to reduce backreflections. Furthermore, optional spacers  395  are provided. Since the path length between the ordinary and extraordinary components is substantially equalized by this embodiment, polarization mode dispersion (PMD) is advantageously reduced.  
         [0054]    Referring to FIG. 3 c , the operation of the device is described in further detail. A beam of light launched from the first optical fibre  311  is passed through the birefringent plate  321  where a first sub-beam is walked off in a first direction while a second sub-beam is transmitted straight through, as indicated in B.2. When the first and second sub-beams pass through the second birefringentt plate  323 , the first sub-beam is passed straight through while the second sub-beam is walked off in a second direction perpendicular to the first, as indicated in C.3. The first sub-beam of light passes through half waveplate  132  where its polarization is rotated by 90°, while the second sub-beam passes through spacer  134  where its polarization not rotated, as indicated in D.4. Each sub-beam of light is transmitted through the Faraday rotator  340  where its polarization state is rotated by approximately 45° as shown in E.5. Since the second birefringent crystal  350  is oriented to pass light having the polarization indicated at E.5, each sub-beam passes through the second birefringent crystal  350  and lenses  360  and  370  with substantially no change. Each sub-beam of light passes through the quarter waveplate  380  where it becomes circularly polarized, and is reflected by mirror  390  such that it passes through the quarter waveplate  380  a second time. The net effect is that the polarization states of each of the first and second sub-beams of light are rotated by 90° as shown in F.7. Each sub-beam of light propagates through the second  370  and first  360  lenses and is incident on the second birefringent crystal  350 . Since the polarization state of each sub-beam was rotated by 90° by the quarter waveplate  380 , each sub-beam propagates along the extraordinary path of the birefringent crystal  350  such that a beam displacement dependent on the length of the crystal  350  is achieved. Conveniently, the length of the second birefringent crystal  350  is selected such that each sub-beam is shifted a distance substantially equal to the distance between the first  311  and second  312  optical fibres. Notably, the spatial displacement provided by the first  321  and second  323  birefringent plates, which is preferably the same, does not necessarily correspond to the beam displacement provided by the second birefringent crystal  350 . For example, a beam displacement of about 125 μm is typically provided by the second birefringent crystal, while a spatial displacement provided by the first  321  and second  323  birefringent plates could range from 100 to 400 μm. Each sub-beam propagates through the Faraday rotator  340 , the reciprocal polarization unit  330 , and finally the first  321  and second  323  birefringent plates, where they are combined and output the second fibre  312 . Similarly, an input optical signal launched from the second optical fibre  312  is coupled into the third optical fibre  313 , and an input optical signal launched from the third optical fibre  313  is coupled into the fourth optical fibre  314 .  
         [0055]    Referring to FIGS. 4 a  and  4   b  there is shown yet another embodiment of the optical circulator. In this embodiment parts  410 ,  411 ,  412 ,  413 ,  414 ,  440 ,  460 ,  470 ,  480 , and  490  are similar to parts  110 ,  111 ,  112 ,  113 ,  114 ,  140 ,  160 ,  170 ,  180 , and  190  discussed above with respect to FIGS. 1 a  and  1   b . However, in the instant embodiment a first birefringent plate  421  and a second birefringent plate  423  oriented perpendicularly to the first birefringent plate  421 , provide two orthogonally polarized sub-beams of light, rather than the birefringent crystal  120  shown in FIGS. 1 a  and  1   b . Similarly, first  452  and second  454  birefringent crystals replace the birefringent crystal  150  shown in FIGS. 1 a  and  1   b . Preferably, the walk-off directions of the first  421  and second  423  birefringent plates are perpendicular to one another, the walk-off directions of the first  452  and second  454  birefringent crystals are opposite one another, and the walk-off directions of the first  421  and second  423  birefringent plates are at a 45° or 135° angle to the walk-off directions of the first  452  second  454  birefringent crystals. Optionally, tube  410 , first GRIN lens  460 , and second GRIN lens  470  are provided with slanted end faces to reduce backreflections. Furthermore, optional spacers  495  are provided. Since the path length between the ordinary and extraordinary components is substantially equalized by this embodiment, polarization mode dispersion (PMD) is significantly reduced. Advantageously, this embodiment further obviates the need for a reciprocal polarization unit (e.g.,  330  shown in FIGS. 3 a  and  3   b ).  
         [0056]    Referring to FIG. 4 c , the operation of the device is described in further detail. A beam of light launched from the first optical fibre  411  is passed through the birefringent plate  421  where a first sub-beam is walked off in a first direction while a second sub-beam is transmitted straight through, as indicated in B.2. When the first and second sub-beams of light pass through the second orthogonally oriented birefringent plate  423 , the first sub-beam is passed straight through while the second sub-beam is walked off in a second direction perpendicular to the first, as indicated in C.3. Each sub-beam of light is transmitted through the Faraday rotator  440  where its polarization state is rotated by approximately 45° as shown in D.4. The first sub-beam of light passes through the first birefringent crystal  452  where it is walked off in a third direction, while the second sub-beam of light passes through the second birefringent crystal  454  where it is passed straight through with substantially no walk-off, as shown in E.5. Each sub-beam of light passes through the quarter waveplate  480  where it becomes circularly polarized, and is reflected by mirror  490  such that it passes through the quarter waveplate  480  a second time. The net effect is that the polarization states of each of the first and second sub-beams of light are rotated by 90° as shown in E.6, before they are passed through the second  470  and first  460  lenses. Since the polarization state of each sub-beam was rotated by 90° by the quarter waveplate  480 , the first sub-beam passes straight through the first birefringent crystal  452 , while the second birefringent crystal  454  provides a walk-off for the second sub-beam in the third direction, as shown in D.7. Each sub-beam propagates through the Faraday rotator  440 , the first birefringent plate  421 , and the second birefringent plate  423 , where they are combined and output the second fibre  412 . Similarly, an input optical signal launched from the second optical fibre  412  is coupled into the third optical fibre  413 , and an input optical signal launched from the third optical fibre  413  is coupled into the fourth optical fibre  414 .  
         [0057]    In each of the four embodiments described heretofore, a first birefringent crystal ( 120 ) or pair of crystals ( 222 / 226 ,  321 / 323 , or  412 / 423 ) provide the necessary polarized light. However, other methods of providing polarized light are also within the scope of the invention. For example, a linear polarizer (not shown) or polarization maintaining fibre both provide light having a predetermined polarization.  
         [0058]    Referring to FIGS. 5 a  and  5   b , there is shown a fifth embodiment of the invention wherein the optical circulator  500  includes a tube  510  for housing a first  511 , second  512 , third  513 , and fourth  514  polarization maintaining (PM) optical fibres. Optionally, each fibre has a thermally expanded core. Each optical fibre  511 ,  512 ,  513 ,  514  is optically coupled to a non-reciprocal polarization rotator  540 , such as a Faraday rotator, which rotates the polarization state of a polarized beam of light launched from the first fibre  511  by about 45°. The non-reciprocal polarization rotator  540  is optically coupled to a birefringent crystal  550 , which provides a beam displacement for each beam of light passing therethrough in dependence upon its polarization state. An at least partially reflective surface  590 , such as a mirror, is provided to convert light propagating in a forward direction from the plurality of fibres  511 , 512 , 513 , and  514  to light propagating in a backwards direction towards the plurality of fibres  511 , 112 , 113  and  514 , while a polarization rotator  580 , such as a quarter waveplate or a second Faraday rotator, is provided for rotating the polarizations between orthogonal polarization states for the forward and backward propagating light. A first  560  lens having a focal plane substantially at the plurality of fibres  511 ,  512 ,  513 , and  514  and second  570  lens having a focal plane substantially at the mirror  590 , are provided for focussing and collimating each beam of light passing therethrough. For example, GRIN, spherical, and aspherical lenses are all suitable for providing the necessary collimating and focussing effects. More specifically, the first  560  and second  570  lenses provide an imaging system wherein the ends of optical fibres  511 ,  512 ,  513  and  514  are imaged onto the imaging plane coincident with the mirror  590 . Optionally, the first  560  and second  570  lenses provide different magnifications, however, it is preferred that the first  560  and second  570  lenses provide a one-to-one optical arrangement or imaging system.  
         [0059]    Referring to FIG. 5 c , the operation of the device is described in further detail. A beam of polarized light launched from the first PM optical fibre  511  is passed through the Faraday rotator  540  where its polarization state is rotated by approximately 45° as shown in B.2. Since the birefringent crystal  550  is oriented to pass light having the polarization indicated at B.2, the beam of light passes through the birefringent crystal  550  with substantially no change or displacement. The beam of light passes through the lenses  560  and  570  and quarter waveplate  580 , where it becomes circularly polarized and is reflected by mirror  590  such that it passes through the quarter waveplate  580  a second time. The net effect of the double pass is that the polarization state of the beam of light is rotated by 90° as shown in D.5. The beam of light propagates through the second  570  and first  560  lenses again and is incident on the birefringent crystal  550 . Since the polarization state of the beam of light was rotated by 90° by the quarter waveplate  580 , it propagates along the extraordinary path of the polarizing beam splitter  550  in the return path, such that it experiences a displacement dependent on the length of the crystal  550 . Conveniently, the length and orientation of the birefringent crystal  550  is selected such the backward propagating beam of light aligns with the second fibre  512  as shown in B.7. For example, a beam displacement of about 125 μm is usually convenient. The beam of light subsequently propagates through the Faraday rotator  540  and is output the second PM fibre  512 . Similarly, an input optical signal launched from the second PM optical fibre  512  is coupled into the third PM optical fibre  513 , and an input optical signal launched from the third PM optical fibre  513  is coupled into the fourth PM optical fibre  514 .  
         [0060]    Advantageously, the reflective design shown in each of the above embodiments requires only one Faraday rotator and the material for one beam shifting birefringent crystal. This is in contrast to the prior art circulator taught in U.S. Pat. No. 5,991,076, which requires two Faraday rotators and two beam shifting birefringent crystals. This reduction in material represents a reduction in the size of the device and a significant decrease in the over all material cost.  
         [0061]    Moreover, the dual lens arrangement provides improved alignment yields since each input beam of light returns to the same plane regardless of its position on the lens. More specifically, the dual lens arrangement provides a retro-reflective system, i.e., the reflected return path is essentially the same as the incident path on the mirror. Accordingly, small movements of the components during the alignment do not cause significant misalignment errors. This is in contrast to prior art circulators taught in U.S. Pat. Nos. 5,991,076 and 5,930,422 that experience lateral displacement errors if the optical components are secured in place with epoxy with even minimal misalignment. Advantageously, an optical circulator in accordance with the instant invention is easily tuned by aligning the reflector during final assembly.  
         [0062]    Furthermore, the retro-reflective arrangement provided by the dual lens reduces the size requirements for the optical circulating components. Moreover, the instant invention is easily manufactured as a three, four, or higher port non-reciprocal optical circulator.  
         [0063]    Numerous other embodiments can be envisaged without departing from the spirit and scope of the invention. For example, although the first and second lenses should be optically disposed between the plurality of fibres and the reflector, it is not necessary for them to be arranged as shown in FIGS. 1 a ,  1   b ,  1   c  through to FIGS. 5 a ,  5   b , and  5   c . Referring to FIG. 6, there is shown another embodiment of a circulator in accordance with the instant invention, wherein the beam shifting birefringent crystal  650  is disposed between the second lens  670  and the quarter waveplate  680 . Notably, this embodiment substantially equalizes the distance between the first lens  660  and the plurality of fibres  611 ,  612 ,  613 ,  614 , and the second lens  670  and the reflector  690 . Moreover, this embodiment advantageously reduces the distance between the plurality of fibres  611 ,  612 ,  613 ,  614  and the first lens  660 . Preferably, each of the polarizing beam splitter  620 , the reciprocal polarization unit  630 , the non-reciprocal polarization rotator  640 , the second birefringent crystal  650 , and the polarization rotator  680  are disposed in one of object and image space, and not collimated space. Of course, other arrangements are also possible and within the scope of the invention.