Abstract:
The present invention relates to a device for use in a fiber optic system that may be a communication system, a sensing system or other system using guided-wave optical components. Reducing the number of lenses required to couple the waveguides and the free-space paths in the device offers the dual advantages of a reduced component count and simplified alignment. In an exemplary device having a first and second waveguides, a birefringent optical system defines bi-directional, polarization-dependent free-space paths. One of the bi-directional, polarization-dependent, free-space paths couples at least the first waveguide to the second waveguide. The birefringent optical system includes at least one prism for bending one of the polarization-dependent paths in a clockwise direction and one of the polarization-dependent paths in a counterclockwise direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional application of U.S. patent application Ser. No. 10/010,815, filed on Nov. 13, 2001, which issued as U.S. Pat. No. 6,741,764, and is incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention is directed generally to a fiber optic device, and more particularly to devices that can split or combine light signals according to the polarization of the light signals. 
   BACKGROUND  
   Optical fibers find many uses for directing beams of light between two points. Optical fibers have been developed to have low loss, low dispersion, polarization maintaining properties and can also act as amplifiers. As a result, optical fiber systems find widespread use, for example in optical communication applications and remote sensing. 
   Wavelength, optical power and polarization are important properties of the light signals propagating in a fiber optic system. Components within the system may modify the propagation of the signals by changing one or more of these properties. For example, multiple signals may be transmitted through a single fiber optic by combining the outputs from a plurality of laser transmitters, each transmitter having an output wavelength that is restricted to a unique spectral band. Amplitude and/or frequency modulation may be used to encode information on the transmitter outputs. The polarization property may be used for network operations that include the tuning, multiplexing, demultiplexing and switching of light signals, for example. 
   Systems that utilize the polarization property of light often require light signals to be separated or combined according to their polarization state. A single fiber optic device may be designed to carry out both processes, separating signals from a combined input that propagates through the device in a first direction and combining polarized signals that propagate through the device in the opposite direction. 
   Polarization beam separator/combiners for use in fiber optic systems may use non-guiding optical components to separate/combine the optical signals as they propagate through the device along free-space optical paths. Collimating lenses are typically used to couple the light propagating along the free-space optical paths to the input/output waveguides with a one-to-one correspondence between lenses and waveguides. Thus a polarization separator/combiner with three input/output waveguides typically incorporates three lenses that must be accurately aligned with respect to the waveguides and the free-space optical paths. 
   Conventional polarization separator/combiners share several common disadvantages that derive from the one-to-one correspondence between fibers and focusing optical systems. For example, the low-loss propagation of light is facilitated by the accurate alignment of the optical focusing assemblies to the optical fibers. Alignment tolerances may be of the order of one micron and must be maintained against both temperature variations and vibration during the operational lifetime of the device. Typically, the optical components are housed in a mechanical alignment and support assembly that increases in complexity, size and cost with the number optical coupling components. It is, therefore, disadvantageous to use a dedicated optical focusing assembly to couple each of the optical fibers 
   SUMMARY OF THE INVENTION 
   Generally, the present invention relates to a device for use in a fiber optic system that may be a communication system, a sensing system or other system using guided-wave optical components. 
   Reducing the number of lenses required to couple the waveguides and the free-space paths offers the dual advantages of a reduced component count and simplified alignment. It is, therefore, advantageous to provide a polarization splitter/combiner incorporating non-guiding optical components that interact with light propagating along free space paths, the free space paths coupled to a number, N, of input/output waveguides by a number, M, of focusing elements where M&lt;N. 
   One embodiment of the invention is directed to an optical device that includes a first waveguide, a second waveguide, and a birefringent optical system with bi-directional, polarization-dependent free-space paths. One of the bi-directional, polarization-dependent, free-space paths couples at least the first waveguide to the second waveguide, the birefringent optical system including at least one prism for bending one of the polarization-dependent paths in a clockwise direction and one of the polarization-dependent paths in a counterclockwise direction. 
   Another embodiment of the invention is directed to an optical device that includes a first waveguide, at least a second waveguide, and a folded optical system with bi-directional, polarization-dependent free-space paths that couple the first waveguide and the at least a second waveguide. The optical system includes a birefringent path separator that is traversed by light propagating along the free-space paths in a first direction and in a second direction approximately opposite to the first direction. 
   Another embodiment of the invention is directed to an optical device that includes a first waveguide, a second waveguide coupled to the first waveguide via a first bi-directional, polarization dependent path, and a third waveguide coupled to the first waveguide via a second bi-directional, polarization dependent path. A Wollaston prism is disposed on the first and second bi-directional, polarization dependent paths. The first and second bi-directional, polarization dependent paths overlap between the first waveguide and the Wollaston prism. A first converging optical subsystem is disposed to couple light between the second waveguide and the Wollaston prism and between the third waveguide and the Wollaston prism. The first converging optical subsystem includes at least one focusing element common to the first and the second bi-directional, polarization dependent paths. 
   Another embodiment of the invention is directed to an optical device that includes a first waveguide, a second waveguide, a third waveguide, and a converging optical system. A birefringent optical system defines a first polarized optical path between the first waveguide and the second waveguide and defines a second polarized optical path between the first waveguide and the third waveguide. The polarization of light propagating along the first polarized optical path is orthogonally polarized to the polarization of light propagating along the second polarized optical path. The converging optical system includes at least one focusing element disposed on both the first and second polarized optical paths where the first polarized optical path is spatially separated from the second polarized optical path. 
   Another embodiment of the invention is directed to an optical communications system that includes a transmitting unit, a receiving unit and an optical transport system coupled to carry optical information signals between the transmitting unit and the receiving unit. At least one of the transmitting unit, the receiving unit, and the optical transport system include an optical device for coupling a first light beam to a second polarized light beam and a first beam to an orthogonally polarized light beam. The optical device includes a first waveguide and a second waveguide, and a birefringent optical system with bi-directional, polarization-dependent free-space paths. One of the paths couples at least the first waveguide to the second waveguide. The birefringent optical system includes at least one prism for bending one of the polarization-dependent paths in a clockwise direction and bending one of the polarization-dependent paths in a counterclockwise direction. 
   Another embodiment of the invention is directed to an optical communications system that includes a transmitting unit, a receiving unit and an optical transport system coupled to carry optical information signals between the transmitting unit and the receiving unit. At least one of the transmitting unit, the receiving unit, and the optical transport includes an optical device for coupling a first light beam to a second polarized light beam. The optical device includes a first waveguide, a second waveguide and a folded optical system with bi-directional, polarization-dependent free-space paths that couple the first waveguide and at least the second waveguide. The folded optical system includes a birefringent path separator that is traversed by light propagating along the free-space paths in a first direction and second, approximately opposite direction. 
   Another embodiment of the invention is directed to an optical communications system that includes a transmitting unit, a receiving unit, and an optical transport system coupled to carry optical information signals between the transmitting unit and the receiving unit. At least one of the transmitting unit, the receiving unit, and the optical transport include an optical device for coupling a first light beam to a second polarized light beam. The optical device includes a first waveguide, a second waveguide coupled to the first waveguide via a first bi-directional, polarization dependent path, and a third waveguide coupled to the first waveguide via a second bi-directional, polarization dependent path. A Wollaston prism is disposed on the first and second bi-directional, polarization dependent paths, the first and second bi-directional, polarization dependent paths overlapping between the first waveguide and the Wollaston prism. A first converging optical subsystem couples light between the second waveguide and the Wollaston prism and between the third waveguide and the Wollaston prism. The first converging optical subsystem includes at least one focusing element common to the first and the second bi-directional, polarization dependent paths. 
   Another embodiment of the invention is directed to a method of coupling light propagating in a first waveguide to polarized light propagating in at least a second waveguide. The method includes propagating the light along first and second bi-directional, polarization-dependent free-space paths. The polarization of light propagating along the first bi-directional, polarization-dependent free-space path is orthogonal to the polarization of light propagating along the second bi-directional, polarization-dependent free-space path. The method also includes bending the first polarization-dependent path in a counterclockwise direction and the second polarization-dependent path in a clockwise direction with a prism. 
   Another embodiment of the invention is directed to a method of coupling light in a first waveguide to at least a second waveguide. The method includes propagating the light along bi-directional, polarization-dependent free-space paths. The paths include a first path for propagating polarized light and a second path for propagating light polarized orthogonally to polarization of light propagating along the first path. The method also includes traversing the light though a birefringent path separator in a first direction and in a second, approximately opposite direction. 
   Another embodiment of the invention is directed to a method of coupling light between a first waveguide and second and third waveguides. The method includes propagating the light along bi-directional, polarization-dependent free-space paths. This includes propagating polarized light along a first path between the first and second waveguides and propagating polarized light, polarized orthogonally relative to light propagating along the first path, along a second path between the first and third waveguides. The method also includes spatially separating and bending the first and second paths with a Wollaston prism. 
   Another embodiment of the invention is directed to a method of coupling between a first waveguide and second and third waveguides. The method includes interacting the light with a birefringent optical system along a first optical path between the first and second waveguides and a second optical path between the first and third waveguides. Light propagating along the second path has a polarization orthogonal to a polarization of light propagating along the first path where the first and second paths are spatially separated. The method also includes coupling the light between the birefringent optical system and the second and third waveguides with a converging optical subsystem having at least one focusing optical element common to the first and second paths where the first and second paths are spatially separated. 
   The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
       FIG. 1  schematically illustrates a polarization multiplexed optical communications system. 
       FIG. 2  schematically illustrates an optical amplifier pumped by polarized lasers. 
       FIG. 3  schematically illustrates a transmissive fiber optic polarization separator according to the prior art. 
       FIG. 4A  schematically illustrates a reflective fiber optic polarization separator/combiner according to the present invention. 
       FIG. 4B  schematically illustrates a transmissive fiber optic polarization separator/combiner according to the present invention. 
       FIG. 5  schematically illustrates a transmissive fiber optic polarization separator/combiner that includes a birefringent material and a prism. 
       FIG. 6  schematically illustrates a transmissive-fiber optic polarization separator/combiner that includes a Wollaston prism. 
       FIG. 7  schematically illustrates a reflective fiber optic polarization separator/combiner that includes a birefringent material and a polarization rotator. 
       FIG. 8  schematically illustrates a reflective fiber optic polarization separator/combiner that includes a birefringent material and a faceted reflector. 
       FIG. 9  schematically illustrates a faceted reflector formed from two prisms 
       FIG. 10  schematically illustrates a transmissive fiber optic polarization separator incorporating a faceted birefringent beam separator. 
   

   While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION 
   The present invention is applicable to optical fiber systems, and is believed to be particularly suited to combining and separating beams of polarized light. The approaches presented here may be simpler in construction, easier to align and lower in cost than conventional systems. 
   A typical fiber optic polarization combiner/separator is a three-port device that is designed to couple light propagating in a first waveguide with a combination of polarization states to polarized light with a first polarization direction propagating in a second waveguide and polarized light with a second orthogonal polarization direction in a third waveguide. If the beams propagate from the second and third waveguides to the first, the device acts as a polarization beam combiner. If the beams propagate in the opposite direction from the first waveguide to the second and third waveguides, the device acts as a polarization separator. 
   Polarization combiner/separators may find a number of applications in an optical fiber communications network. For example, the polarization-multiplexed system  100  of  FIG. 1  is designed to transport an optical signal with combined polarization from a transmitting unit  105  to a receiving unit  110 . The transmitting unit  105  includes two laser transmitters  115 ,  120  that transmit polarized optical signals. These signals may, for example, include a number of wavelength-multiplexed channels that are combined by the transmitters  115 ,  120  according to a dense wavelength division multiplexing (DWDM) standard. 
   In the system of  FIG. 1 , the optical signal  125  from the laser transmitter  115  may be polarized in a first direction and the optical signal  130  from the laser transmitter  120  may be polarized in a second, orthogonal direction. The optical signals  125 ,  130  from the laser transmitters  115 ,  120  propagate through polarization-maintaining optical fibers  135 ,  140  to the polarization combiner/separator  145 . This unit is configured to combine the polarized signals  125 ,  130  to form an output signal  150  with combined polarization states. The output signal  150  propagates through the transmitting unit output fiber  155  to the polarization-maintaining optical transport system  160 . The transport system  160  carries the polarization-multiplexed signal  150  to the receiving unit. input fiber  165 . In the receiving unit  110 , a second polarization separator/combiner  170  separates the two polarization components of the optical signal  150  and couples the orthogonally polarized optical signals  175 ,  180  to the optical fibers  185 ,  190 . The optical receivers  195 ,  200  recover the information from the optical signals  175 ,  180 . 
   Within an optical transport system, polarization separator/combiners may be used, for example, to combine the pump and the signal beams in a waveguide amplifier.  FIG. 2  schematically illustrates one particular embodiment of a waveguide amplifier unit  210  that increases the optical power of a polarized information signal  215 . The information signal  215 , which may have a wavelength of about 1.55 μm, is transported to the amplifier unit  210  by an optical fiber  220  and propagates from the amplifier unit  210  along the optical fiber  230 . The fibers  220 ,  230  may be polarization-maintaining. 
   The fiber amplifier  235 , which may be an erbium fiber amplifier or other suitable type of optical amplifier, is configured to transfer power from the pump beams  240 ,  245  obtained from one or more pump lasers, to the optical signal  215 . The polarization combiner  255  combines the polarized signal  215  propagating along the amplifier unit input fiber  220  with the orthogonally-polarized pump beam  240  that is transported to the polarization combiner  255  by the optical fiber  250 . The fiber amplifier input fiber  260  transports the combined signal and pump beams to the fiber amplifier  235 . 
   Optionally, a second pump beam  245  with mixed polarization states, propagating along optical fiber  270  may be coupled to the amplifier output fiber  275  by a WDM beam separator/combiner  280  that couples/separates beams according to wavelength. The WDM beam separator/combiner  280  also couples the optical signal  215  to the amplifier unit output fiber  230 . The pump beam  245  may include the output of two orthogonally-polarized pump beams  285 ,  290  that have been combined, for example, by a polarization combiner  295 . The optional pump beam  245  and the signal  215  propagate in opposite directions through the fiber amplifier  235  and the pump beam  240  and the signal  215  propagate in the same direction through the fiber amplifier  235 . 
   A fiber optic polarization separator/combiner unit may include bulk optical components and a plurality of focusing optical systems. The optical fibers that transport optical signals to and from a polarization separator/combiner unit are coupled to free space paths within the unit by focusing optical assemblies. The bulk optical components typically interact with the light propagating along the free-space paths, separating and/or combining the light according to polarization. There is typically a one-to-one correspondence between focusing optical assemblies and waveguides, the focusing optical assemblies being positioned to collimate the diverging light from input fibers and focus the light propagating along the free space paths into the output fibers 
   A conventional fiber optic polarization separator, for example as illustrated in  FIG. 3  as separator  300  typically includes three focusing lens assemblies  305 – 315  that couple the fibers  320 – 330  to the polarizing beam splitter  335  along the free space beam paths  340 – 350 . A combined polarization optical signal propagating along the waveguide  320  may be collimated by the focusing optical system  305  and directed to the polarization beam splitter  335  along the optical path  340 . The polarization beam splitter may, for example, be a cube polarizer. The beam splitter  335 , directs light having a first polarization direction along the optical path  350  and light polarized in the orthogonal direction along the optical path  345 . The focusing optical systems  310 ,  315  focus the light propagating along the free space optical paths  345 ,  350  into the output optical fibers  325 ,  330 . 
   Conventional polarization separator/combiners share several common disadvantages that derive from the one-to-one correspondence between fibers and focusing optical systems. In the polarization separator  300 , for example, the low-loss propagation of light is facilitated by the accurate alignment of the optical focusing assemblies  305 – 315  and the optical fibers  320 – 330  and the alignment optical of the focusing assemblies  305 – 315  and the free space paths  340 – 350 . Alignment tolerances may be of the order of one micron and must be maintained against both temperature variations and vibration during the operational lifetime of the polarization separator  300 . Typically, the optical components are housed in a mechanical alignment and support assembly that increases in complexity, size and cost with the number optical coupling components. It is, therefore, disadvantageous to use a dedicated optical focusing assembly to couple each of the optical fibers  320 – 325  to the free space paths  340 – 350 . 
   According to the present invention, the number of optical focusing assemblies required to couple a reflective or transmissive bulk optic polarization separator/combiner to a set of input/output fibers may be reduced through advantageous design of the bulk optic polarization separator/combiner and/or the coupling optical system. For example,  FIG. 4A  schematically illustrates a reflective fiber optic polarization separator/combiner  410  that utilizes an optical coupling module  415  to couple the optical fibers  420 – 430  to the free space optical paths  435 – 445 . The free space paths  435 – 445  are coupled according to the polarization state of the light propagating along the free space paths by the reflective free space optical system  450 . 
   The optical coupling module  415  is further detailed in U.S. Pat. No. 6,829,152, which is incorporated herein by reference. In one particular embodiment, an optical coupling module is an assembly that is couplable, for example, to the optical fibers  420 – 430 , comprising a first focusing element and a second focusing element, the first element positioned on a first optical axis to receive output light beams from the optical fibers and direct the light beams to intersect an optical axis at a first intersection position. The second focusing element is spaced apart from the first focusing element by a distance along the optical axis, the separation being selected to parallelize the light beams received from the first optical element. When compared to conventional fiber coupling methods, the coupling module method of  FIG. 4A  advantageously reduces the number of focusing optical assemblies. Corollary advantages may include a simplified alignment procedure and smaller transverse dimensions of a packaged fiber optic polarization separator/combiner. 
   The number of focusing optical assemblies included in a transmissive fiber optic polarization separator/combiner may also be reduced by modifying the bulk optical system in such a way that a single optical focusing assembly couples multiple optical fibers to their associated free space optical paths. For example,  FIG. 4B  schematically illustrates a transmissive fiber optic polarization separator  455  wherein the transmissive free space optical system  460  interacts with combined polarization light propagating along the free space optical path  497  from the fiber  495 , dividing the light into two polarized light beams that are directed along the non-parallel free space optical paths  465 ,  470 . The free space optical paths  465 ,  470  are coupled to the optical fibers  475 ,  480  by a first optical focusing assembly  485 . A second optical focusing assembly  493  couples the optical fiber  495  to the free space optical path  497 . 
   The transmissive fiber optic polarization separator  455  illustrated in  FIG. 4B  has fewer optical focusing assemblies than conventional polarization separators. Corollary advantages of the invention may include a simplified alignment procedure and smaller transverse dimensions of a packaged fiber optic polarization separator/combiner. 
     FIG. 5  illustrates a transmissive fiber-optic polarization separator  500  embodying features of the present invention. A light signal having combined polarization states is transported to the polarization separator  500  by an optical fiber  505 . The divergent light beam  510  exiting the fiber is approximately collimated by the first optical focusing assembly  515  that may be separated from the fiber end  520  by a distance, f 1 , that is approximately equal to the focal length of the optical focusing assembly  515 . The first optical focusing assembly  515  may be, for example, a single lens, or a combination of lenses. The collimated light beam from the optical focusing assembly  515  propagates along the free-space optical path  530  and interacts with the birefringent beam separator  535 . The birefringent beam separator  535  may, for example, be a birefringent crystal with its optical axis direction  540  oriented at an acute angle, α 1 , to the light propagation direction  545  of the free space path  530  in the plane of  FIG. 5 . 
   Within the birefringent beam separator  535 , the portion of the light that is polarized in the ordinary direction propagates along a first path  547  and experiences the ordinary index of refraction, n o , while the portion of the light polarized in the extraordinary direction experiences the extraordinary index of refraction, n e , and propagates along a second path  550  at an angle with respect to the first path  547 . Thus, portions of a light beam with combined polarization states that propagates towards the birefringent beam separator  535  along the optical path  530  may be separated into two polarized light beams that propagate through the birefringent beam separator  535  along separate optical paths  547 ,  550 . 
   The physical separation of the beams at the surface  538  typically increases with the absolute value of the difference between the ordinary and extraordinary indices of refraction, also known as the birefringence, and the length of the separator  535 . The separator length may be advantageously minimized by selecting a material with a large birefringence at the desired operating wavelength. The separator  535  may also be advantageously selected to have high transparency at the wavelength of interest, physical properties that are insensitive to temperature and humidity, and physical properties that facilitate optical polishing and coating. Materials that combine these properties at wavelengths between 1.5 μm and 1.65 μm include yttrium vanadate (YVO 4 ), rutile (TiO 2 ) and a-barium borate (α-BaB 2 O 4 ). 
   The polarized light beams leaving the birefringent crystal  535  propagate along free-space optical paths  555 ,  560  that are typically parallel and non-overlapping. A prism  565 , that may be a symmetric roof prism, with two facets tilted at acute angles γ, relative to the entrance surface  578 , bends the free space optical paths  555 ,  560  in clockwise and counterclockwise directions so that the free space optical paths  555 ,  560  intersect a plane containing the prism axis of symmetry  580  in a region  583  located between the prism and the focusing optical assembly  590 . In some embodiments, the optical paths  555 ,  560  may also intersect each other in the region  583 . 
   A focusing optical assembly  590 , which may comprise a single lens, or a combination of lenses, couples the optical paths  555 ,  560  to the optical fibers  593 ,  595 . Typically, the separation, f 2 , of the optical fibers  593 ,  595  from the focusing optical assembly  590  is approximately equal to the focal length of the focusing optical assembly  590 . The distance, L, between the focusing optical assembly  590  and the region  583  is typically greater than or equal to the focal length of the focusing optical assembly  590  and may be advantageously chosen to be equal to the distance, f 2 . 
   It may also be advantageous to join the birefringent beam separator  535  and the prism  565  by decreasing the distance, d, between the two beam separator  535  and the prism  565 . Optical contacting techniques, for example, may be used to join the two elements  535  and  565  and an antireflection (AR) coating may be applied to at least one of the surfaces,  538 ,  578  to minimize the reflected portions of light beams propagating along the paths  547 ,  550 ,  555  and  560 . Alternatively, the surfaces may be joined with an adhesive, for example, an ultraviolet-light-cured transparent optical epoxy. The adhesive may be applied to directly to the surfaces  538 ,  578  or to the edges of the birefringent beam separator  535  and the prism  565  that are adjacent to the surfaces  538 ,  578 . Reflection at the prism facets  570 ,  575  may also be minimized by applying AR coatings to the facets. 
   In the illustrated embodiment  500 , the first focusing optical assembly  515  and the second optical focusing assembly  590  may be Geltech 350140 lenses with a common focal distance, f=f 1 =f 2 . The prism may be a pentagon that is formed from K10 glass that is supplied by Schott Optical Glass Co. with acute angles γ that are equal to 9.9°. The birefringent beam separator may be manufactured from yttrium vanadate. 
   The transmissive fiber optic polarization beam separator  500  may also be operated as a transmissive fiber optic polarization beam combiner by reversing the direction of light propagation through the device. Polarized light beams propagating towards the optical focusing assembly  590  along the optical fibers  593 ,  595  may be combined by the polarization beam separator  500  to exit as a light beam with combined polarization states propagating away from the beam separator  500  along the optical fiber  505 . 
   The transmissive fiber optic polarization beam separator  500  may also be used to couple counterpropagating beams. For example, a polarized beam propagating towards the polarization beam separator  500  along the optical fiber  595  may be coupled to the fiber  505  as a beam that propagates from the beam separator  500 . Simultaneously, an orthogonally-polarized beam propagating towards the beam separator  500  along the fiber  505  may be coupled to the fiber  593  as a beam propagating away from the beam separator  500 . Alternatively, a beam with mixed polarization propagating towards the device along the fiber  505  may be separated into two orthogonally-polarized beams while polarized beams propagating towards the beam separator  500  along the fibers  593 ,  595  may be combined into a mixed polarization beam propagating away from the beam separator along the fiber  505 . 
     FIG. 10  illustrates another embodiment of a transmissive fiber-optic polarization separator  1000 . A light signal having combined polarization states is transported to the fiber optic polarization separator  1000  by an optical fiber  1005 . The divergent light beam  1010  exiting the fiber is approximately collimated by the first optical focusing assembly  1015  that may be separated from the fiber end  1020  by a distance, f 6 , that is approximately equal to the focal length of the optical focusing assembly  1015 . The optical focusing assembly  1015  may be, for example, a single lens, or a combination of lenses. The collimated light beam from the optical focusing assembly  1015  propagates along the free-space optical path  1030  and interacts with the faceted birefringent beam separator  1020 . 
   The faceted beam separator  1020  is formed from a birefringent material and oriented so that light with a combined polarization state propagating towards the birefringent beam separator  1020  is selected according to polarization state at the surface  1035 . For example, light that is polarized in the ordinary direction propagates along optical path  1050  and light polarized in the extraordinary direction propagates along the optical path  1045 . Light propagating along the optical path  1045  is coupled to the free space optical path  1065  at the facet  1055 . The facet  1055  may be AR-coated to reduce reflection losses and is tilted at an angle, δ 3 , with respect to the input surface  1035 . The angle, δ 3 , is selected to bend the light propagating along the path  1045  in a clockwise direction. Light propagating along the optical path  1050  is coupled to the free space optical path  1070  at the facet  1060 . The facet  1060  is tilted at an angle, δ 4 , with respect to the input surface  1035 . The angle, δ 4 , is selected to bend the light propagating along the path  1050  in a counterclockwise direction. The facet  1060  may also be AR-coated to minimize reflections. 
   The physical separation of the beams at the facets  1060 ,  1055  increases with the absolute value of the birefringence, and the length of the separator  1020 . The separator length may be advantageously minimized by selecting a material with a large birefringence at the desired operating wavelength. The separator  1020  may also be advantageously selected to have high transparency at the wavelength of interest, optical properties that are insensitive to temperature and humidity, and physical properties that facilitate optical polishing and coating. Materials that combine these properties at wavelengths near 1.5 μm include yttrium vanadate (YVO 4 ), rutile (TiO 2 ) and alpha barium borate (α-BaB 2 O 4 ). 
   A focusing optical assembly  1080  with an axis  1095  couples the free space paths  1065 ,  1070  to the optical fibers  1085 ,  1090 . The focusing optical assembly may comprise, for example, a single lens or group of lenses. Typically, the free-space paths  1065 ,  1070  cross a plane containing the axis  1095  in a beam crossing region  1075 . They may also intersect each other in the beam crossing region  1075 . The optical fibers  1085 ,  1090  may be disposed in a parallel configuration and separated from the optical assembly  1080  by a distance, f 7 , that may be equal to the focal length of the focusing optical assembly  1080 . Advantageously, the distance, X, between focusing assembly  1015  along the optical fiber  1005  the optical assembly  1080  and the beam crossing region  1075  may be approximately equal to the distance, f 7 . 
   The fiber optical polarization separator  1000  may be operated as a fiber optic polarization combiner by reversing the beam direction. Orthogonally-polarized light beams propagating towards the optical focusing assembly  1080  along the optical fibers  1085 ,  1090  will be combined by the polarization separator  1000  and propagate away from the optical focusing assembly  1015  along the optical fiber  1005 . Counterpropagating beams may also be combined and separated simultaneously. 
     FIG. 6  schematically illustrates another embodiment of a transmissive fiber optic beam separator  600  according to the present invention. Light with combined polarization states may be separated into polarized light beams by propagating light with combined polarization states along the optical fiber  605  from left to right. The optical fiber  605  is coupled to the free space optical path  610  by the focusing optical assembly  615 , that may comprise one or more lenses. Typically, the focusing optical assembly  615  is separated from the end of the optical fiber  605  by a distance that is approximately equal to the focal length, f 3 , of the optical focusing assembly  615 . 
   Light with combined polarization states propagating along the free-space optical path  610  interacts with a Wollaston prism  625  and is separated into orthogonally-polarized beams that are bent and coupled to the free space optical paths  635 ,  640 . The Wollaston prism  625  may be formed in a conventional fashion from two birefringent prisms having a common prism angle, β 1 . The center of the Wollaston prism  625  is separated by a distance, d 2 , from the focusing optical assembly,  615 . 
   The optical focusing assembly  645  couples the free space paths  635 ,  640  to the optical fibers  650 ,  655  that are typically separated from the optical focusing assembly  645  by a distance, f 4 , that is approximately equal to the focal length of the optical focusing assembly  645 . The optical focusing assembly  645  may also be positioned at a distance, d 2 , from the Wollaston prism  625 . Advantageously, the distances f 3  and f 4 , and the distances, d 2  and d 3 , may be equal to a common focal length, f 5 . 
   The Wollaston prism  625  may advantageously be fabricated from yttrium vanadate or rutile. For example, the focusing optical assemblies  615 ,  645  may be Geltech lenses with part number 350140 and the Wollaston prism may be formed from yttrium vanadate with a prism angle, β 1 , of 22.6°. Locating the components at distances, d 2  and d 3 , that are equal to the common focal length of the Geltech lenses provides a physical separation distance, d 5 , of the beams at the end of the polarized fibers  655 ,  650  that is equal to 250 μm. 
   The transmissive beam separator/combiner  600  may also be operated as a beam combiner. In this case, polarized light beams propagating towards the left in the optical fibers  650 ,  655  are combined into a single beam that is coupled into the optical fiber  605 . 
   An embodiment of a reflective fiber optic polarization beam separator/combiner  700  according to the present invention is illustrated schematically in  FIG. 7 . When operated as a beam separator, light with combined polarization states propagates through the optical fiber  705  to the beam separator/combiner  700 . A coupling module  710  as described in U.S. patent application Ser. No. 09/181,145 couples the optical fiber  705  to the free space optical path  715 . Light propagating along the optical path  715  is approximately collimated and interacts with the birefringent beam separator  720 . The beam separator may, for example, be formed from a birefringent material that is oriented with its optical axis direction  722  oriented at an acute angle, α 2 , with respect to the direction  726  of the free space optical path  715 . 
   Light with ordinary polarization propagates through the birefringent beam separator  720  in a first direction along the optical path  728 . Light polarized in the extraordinary direction propagates through the birefringent beam separator  720  in a second direction along the optical path  730 . At the beam separator surface  735  the optical paths  728 ,  730  are coupled to approximately parallel and separate bidirectional optical paths  738 ,  740 . 
   The polarized light beams propagating from the birefringent beam splitter  720  along the bidirectional optical paths  738 ,  740  interact with a polarization rotator  745  and are redirected to the left by the reflector  750 . As the polarized light beams propagate to the left from the reflector  750  to the birefringent beam separator  720  along the bidirectional optical paths  738 ,  740 , they interact a second time with the polarization rotator  745 . The polarization rotator and mirror are configured to rotate the polarization of the light beams propagating along the bidirectional paths  738 ,  740  by approximately 90° as they travel from the beam separator surface  735  to the reflector  750  and return to the beam separator surface  735 . The polarization separator may be, for example, a quarter wave retardation plate with its optical axis tilted at 45° with respect to the polarization directions of the light beams propagating along the free space paths  738 ,  740  at the beam separator surface  735 . 
   Light propagating towards the beam separator  720  along the beam path  738  travels through the beam separator  720  along the ordinary polarization beam path  755  and is coupled to the free space beam path  742 . Light propagating towards the beam separator  720  along the beam path  740  travels through the beam separator  720  along the extraordinary polarization beam path  760  and is coupled to the beam path  745 . The optical coupling module  710  couples the free space paths  742 ,  745  to the optical fibers  765 ,  770 . 
   The fiber optical polarization separator  700  may also operate as a beam combiner if orthogonally polarized light beams propagate towards the separator  700  along the optical fibers  755 ,  760 . In this case, a beam with combined polarization is transported away from the polarization separator  700  by the optical fiber  705 . Simultaneous beam separation and combination is also possible with bidirectionally propagating beams. 
   The mechanical complexity of the polarization separator  700  may be advantageously decreased by combining the reflector  750  and polarization rotator  745  into a single unit. This may be accomplished, for example, by joining the reflector  750  and polarization rotator  745  or by coating the reflector  750  directly on the surface  775  of the polarization rotator  745 . The mechanical complexity of the polarization separator  700  may be also be decreased by joining the polarization rotator  745  and the birefringent beam separator  720 . Additional mechanical simplification may be accomplished either by coating the reflector  750  directly on the joined polarization rotator  745  and beam separator  720  or by additionally joining the polarization rotator  745 , beam separator  720  and reflector  750  to form a single mechanical assembly. 
     FIG. 8  illustrates another embodiment of a reflective fiber optic beam separator/coupler  800  according to the invention. In polarization separating operation, light that may have a combined polarization state propagates towards the separator/coupler  800  along the optical fiber  805 . Orthogonally-polarized light beams propagate to the left along the optical fibers  810 ,  815 . The coupling module  820  couples the optical fiber  805  to the free space path  835 , the optical fiber  810  to the free space path  830  and the optical fiber  815  to the free space path  825 . The operation and design of the coupling module is described in U.S. patent application Ser. No. 09/181,145. 
   Light with combined polarization states from the fiber  805  propagates to the right along the free space path  835  and interacts with the birefringent beam separator  840 . The birefringent beam separator  840  may be formed from a birefringent crystal with large birefringence and high transparency at the wavelength of the light propagating through the device. 
   Light propagating along the free space path  835  propagates through the birefringent beam selector along one of two optical paths according to the polarization state of the light. Light polarized in the extraordinary direction propagates along the optical path  855  and light polarized in the ordinary direction propagates along the optical path  860 . At the beam separator surface  862 , the optical path  855  is coupled to the free space optical path  865  and the optical path  860  is coupled to the free space optical path  870 . Typically, the length of the birefringent beam separator  840  is chosen to completely separate the optical paths  855  and  860  at the surface  862 . 
   Light propagating along the free space optical paths  865 ,  870  is redirected towards the birefringent beam separator  840  by the faceted reflector  875  that may have an axis of symmetry  878 . Light propagating to the right along the optical path  870  is redirected to the left along optical path  882  and light propagating to the right along the optical path  865  is redirected to the left along the optical path  880 . The faceted reflector is typically positioned to symmetrically dispose the beam path  870  and the beam path  882  on opposite sides of the symmetry axis  878  and to symmetrically dispose the beam path  865  and the beam path  880  on opposite sides of the symmetry axis  878 . 
   The birefringent beam separator extraordinary polarization optical path  885  couples the free space path  880  to the free space  830  and the ordinary polarization optical path  887  couples the free space path  882  to the free space path  825 . The optical coupling module couples the free space paths  825 ,  830  and the optical fibers  810 ,  815  in such a way the that light polarized in a first direction and propagating to the left along the optical path  830  is transported from the polarization separator  800  by the optical fiber  810  and orthogonally-polarized light propagating along the free space path  835  is transported from the polarization separator  800  by the optical fiber  805 . 
   The fiber optic polarization separator  800  may be operated as a fiber optic polarization combiner by reversing the direction of light propagation. Orthogonally-polarized beams propagating towards the coupling module  820  along the optical fibers  805 ,  810  may be combined by the polarization separator  800  into a beam with combined polarization states propagating away from the optical coupler  820  on the optical fiber  815 . Simultaneous combining and separating operation may also be possible with counterpropagating beams. 
   A faceted reflector may alternatively be an asymmetric assembly that is formed, for example, from two right angle prisms  905 ,  910 . The faceted reflector assembly  900  that is illustrated in  FIG. 9 , for example comprises two right angle prisms that are optically coupled along the plane  910 . Light propagating towards the prism  905  along the input free space optical path  915  is coupled by the path  925  to light propagating away from the prism  910  along the output free space optical path  940 . Light propagating towards the prism  905  along the input optical path  920  is similarly coupled to the light propagating away from the prism  910  along the optical path  935  by the optical path  930 . While the faceted reflector lacks a symmetry axis, the beam paths are symmetric with respect to the axis  950 . For example, the beam paths  915  and  940  are symmetrically disposed on either side of the axis  950  and the beam paths  920  and  935  are symmetrically disposed on either side of the axis  950 . The separation between symmetrically disposed beam paths may be adjusted by changing the displacement, k, between the prism surfaces. 
   The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.