Abstract:
An optical device for combining two orthogonally polarized beams or splitting a beam into two orthogonally polarized beams is provided that utilizes two collimating/focusing lenses and a thin film wire-grid polarizer. Because the thin film wire-grid polarizer can be fabricated in very thin profile, the provision of a thin film wire-grid polarizer allows the optical polarization beam combiner/splitter device to be highly integrated and simultaneously realize a number of performance advantages of a thin film wire-grid polarizer over other types of polarizers utilized in various prior art polarization beam combiner/splitters.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. patent application Ser. No. 10/158,025 entitled “Optical Polarization Beam Combiner/Splitter” in the name of inventors Anguel Nikolov and Stephen Chou filed May 30, 2002 under 35 U.S.C. 120 and incorporates this application by reference as if included herein in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention generally relates to optical polarization beam combiners/splitters and more particularly to packaged fiber-optic polarization beam combiners/splitters that utilize a thin film wire-grid polarizer.  
       BACKGROUND OF THE INVENTION  
       [0003]     Optical polarization beam combiners/splitters are used in many optical communications applications including those that require amplification of optical signals, preferred distribution of the polarization states, a combination of both, or separation of beam polarizations.  
         [0004]      FIGS. 1   a  and  1   b  illustrate an example of a prior art optical polarization beam combiner/splitter (“PBC/S”) device that utilizes a polarization beam splitter cube  7 . In the optical polarization beam splitter of  FIG. 1   a , an incident beam enters from a first source, such as optical fiber  1  located at the left side of the figure. The incoming beam from the first optical fiber  1  is collimated by a first collimating/focusing lens  4 , and then enters the polarization beam splitter cube  7 . The polarization beam splitter cube  7  is able to split an arbitrarily polarized light beam into two separated beams with orthogonally polarized directions. A first of the split beams exits to the right and is focused, for example, into a second optical fiber  2  through a second collimating/focusing lens  5 . A second of the split beams exits in an upward direction and is focused, for example, into a third optical fiber  3  through a third collimating/focusing lens  6 .  
         [0005]     In the optical polarization beam combiner of  FIG. 1   b , the propagation direction of the beams are reversed from those in  FIG. 1   a  so that second and third optical fibers  2 ′ and  3 ′ are now the beam input fibers and they carry polarized beams whose polarization states are well defined and orthogonal to each other. Also, because the second and third optical fibers  2 ′ and  3 ′ must carry polarized incident beams of defined polarization states, they must be polarization-maintaining fibers. The two incident beams are combined by the polarization beam splitter cube  7 ′ and exit through the first optical fiber  1 ′. The prior art device depicted in  FIGS. 1   a  and  1   b , have several drawbacks, such as a large overall device size necessitated by the need to employ orthogonally disposed beams, and a low extinction ratio, which is a characteristic of polarization beam splitter cubes.  
         [0006]     A second prior art optical PBC/S device is illustrated in  FIGS. 2   a  and  2   b  where a birefringent crystal  17  is the polarizer providing the beam combining and splitting function. In the optical polarization beam splitter of  FIG. 2   a , an incoming arbitrarily polarized light from a first source, such as, optical fiber  11 , is focused by a first collimating/focusing lens  14  and then split into two beams having orthogonal polarizations. Each of the polarized beams is focused, for example, by second and third collimating/focusing lenses  15  and  16  into second and third optical fibers  12  and  13  respectively.  
         [0007]     In the optical polarization beam combiner of  FIG. 2   b , two polarized incident beams are carried by second and third optical fibers  12 ′ and  13 ′. The two polarized incident beams must have orthogonal polarization states and they are focused onto the birefringent crystal  17 ′ by the second and third collimating/focusing lenses  15 ′ and  16 ′, respectively. The birefringent crystal  17 ′ combines the two incident beams into one output beam which is focused by the first collimating/focusing lens  14 ′ into the first optical fiber  11 ′.  
         [0008]     But, the device of  FIGS. 2   a  and  2   b  tends to be bulky. Because the second and third optical fibers  12  and  13  are on the same side of the birefringent crystal  17 , the birefringent crystal  17  must have a length sufficient to separate the two beams enough to accommodate the lenses  15  and  16 . Typically, lenses for such application have diameters of around 1.8 mm, requiring a minimum of 1.8 mm, separation between the two beams. This requires a birefringent crystal of about 18 mm in length. Another drawback of an optical PBC/S device employing a birefringent crystal is the relatively narrow range of incident angles the birefringent crystal can accommodate.  
         [0009]      FIG. 3  illustrates another prior art optical polarization beam combiner where a Wollaston prism  30  is disposed between collimating/focusing lenses  24  and  25 . The Wollaston prism  30  is the polarizer that provides the beam splitting and combining function. As in the prior art device depicted in  FIGS. 2   a  and  2   b , second and third optical fibers  22  and  23  are on the same side of the polarizing filter. An arbitrarily polarized incident beam from first source, such as optical fiber  21 , is split into component beams  27  and  28  by the Wollaston prism  30  and focused into the second and third optical fibers  22  and  23 . The first optical fiber  21  defines an optical axis  29  of the device and the lenses  24  and  25 , the Wollaston prism  30 , and the second and third optical fibers  22  and  23  are all aligned so that the component beams  27  and  28  leave the Wollaston prism  30  at angles symmetrical about the optical axis  29 . As a result, lens  25  focuses the component beams  27  and  28  into the optical fibers  22  and  23 , respectively, disposed symmetrically about optical axis  29 .  
         [0010]     By providing polarization-maintaining optical fibers for the second and third optical fibers  22  and  23 , the prior art device of  FIG. 3  can also be used as a polarization beam combiner that combines two orthogonally oriented polarized beams delivered via the second and the third optical fibers  22  and  23  into one composite output beam.  
         [0011]     Another prior art optical PBC/S device utilizes prisms in a combination with a dielectric thin film. Such designs tend to be bulky, resulting in higher insertion loss. Another drawback for this type of device is the need for a matching index coating for the dielectric film. This is often implemented with an organic compound, which limits the overall power that the device can handle. Yet another prior art optical polarization beam combiner is a fused fiber wave guide. Fused fiber wave guides offer overall lowest insertion loss, but in most designs the two channels have different insertion losses and it is not easy to match them. Another distinct drawback for the fused fiber wave guide polarization beam combiner is the very narrow wavelength range of operation. Typically the range is a few nanometers and increasing the device bandwidth will result in increased insertion loss.  
         [0012]     Thus, there is a need for an optical PBC/S device that is compact, has relatively wide wavelength range of operation, is capable of handling high power beams, and is capable of handling more than one set of input/output beams with one set of collimating optical elements.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention provides a compact optical PBC/S device employing a subwavelength wire grid polarizing element that can be packaged into a highly integrated optical module. More particularly, a compact optical PBC/S device employing a thin film wire-grid polarizer is provided.  
         [0014]     In an embodiment where the optical device is a beam combiner, the optical PBC/S device comprises a first optical beam carrier, such as an optical fiber, that carries a first polarized incident beam, a second optical beam carrier that carries a second polarized incident beam polarized in an orthogonal orientation to the first polarized incident beam, and a third optical beam carrier that carries the device&#39;s depolarized output beam which is the composite of the first and the second polarized incident beams. As generally known in the art, the first and second optical beam carriers must be able to maintain the polarization of the incident beams to ensure that one of the incident beams has S polarization state and the other incident beam as P polarization state. Polarization-maintaining optical fibers are examples of such optical beam carriers. The third optical beam carrier can be a standard optical fiber since it carries the composite depolarized output beam.  
         [0015]     Two collimating/focusing lenses, each lenses having an inwardly-facing surface, an outwardly-facing surface, and an optical axis are oriented coaxially so that their optical axes align collinearly, defining the device&#39;s optical axis, and their inwardly-facing surfaces face each other. These two collimating/focusing lenses are positioned between the first and second optical beam carriers and the optical beam carriers are oriented so that the beams exiting or entering the beam carriers propagate parallel to the optical axis of the optical PBC/S device. For example, where the optical beam carriers are optical fibers, the optical fibers are configured so that their optical axes are parallel to the optical axis of the optical PBC/S device. The third optical beam carver for carrying the composite output beam is positioned on the same side of the two collimating/focusing lenses as the first optical beam carrier and also oriented so that the beam exiting or entering the beam carrier propagate parallel to the optical axis of the optical PBC/S device.  
         [0016]     Between the two collimating/focusing lenses is a thin film wire-grid polarizer that provides the beam combining/splitting functions. When the two polarized incident beams are collimated through the collimating/focusing lenses and encounters the thin film wire-grid polarizer, for a certain orientation of the wire grids with respect to the incoming polarizations, the S polarized incident beam will be reflected by the polarizer and the P polarized incident beam will transmit through the polarizer. The reflected S polarized beam and the transmitted P polarized beam combine into a composite depolarized output beam and exits through the third optical beam carrier. If the thin film wire-grid polarizer&#39;s orientation is rotated by 90 degrees, the P polarized light will be reflected and the S polarized light will be transmitted instead.  
         [0017]     In a typical application of this optical beam combiner embodiment, two incident beams of S and P polarizations are received into the device by the first and second optical beam carriers. If the S and P polarized incident beams are received by the first and second optical beam carriers respectively, the thin film wire-grid polarizer is appropriately oriented so that the S polarized incident beam is reflected by the thin film wire-grid polarizer and directed towards the third optical beam carrier. The P polarized incident beam, on the other hand, is transmitted through the thin film wire-grid polarizer and also directed towards the third optical beam carrier so that the transmitted beam combines with the reflected S polarized beam to form a composite output beam.  
         [0018]     In another embodiment of the optical beam combiner of the present invention, the optical PBC/S device is provided with a fourth optical beam carrier positioned on the same side of the two collimating/focusing lenses as the second optical beam carrier. The fourth optical beam carrier is positioned at a location that is the mirror image of the third optical beam carrier with respect to the plane defined by the thin film wire-grid polarizer. In this embodiment, the combined depolarized output beam can be directed to either the third optical beam carrier or the fourth optical beam carrier, as desired, by changing the orientation of the thin film wire-grid polarizer between two positions to control which of the two S and P polarized incident beams are reflected and transmitted. According to the generally known principles of optics involved with wire-grid polarizers, the two positions of the thin film wire-grid polarizer involved here differ by 90 degrees rotation of the thin film wire-grid polarizer about the optical axis of the optical PBC/S device. Furthermore, the composite depolarized output beam can be controllably apportioned between the third and the fourth optical beam carriers by positioning the thin film wire-grid polarizer between the two positions discussed above.  
         [0019]     In addition, the optical PBC/S device of the present invention can be used as a polarization beam splitter rather than a beam combiner by reversing the propagation direction of the light beams. In this application, an incident beam of arbitrary polarization is inputted into the third optical beam carrier and is decomposed into two beams of orthogonal polarization by the thin film wire-grid polarizer. The two output beams are then guided through the first and second optical beam carriers.  
         [0020]     The use of a thin film wire-grid polarizer, a very thin optical element, enables the overall optical device to be compact and simpler than the prior art optical PBC/S devices. This simplified design will provide reduced insertion loss through the device, smaller footprint, simpler assembly, improved manufacturing yields, and thus reduced overall optical packaging cost.  
         [0021]     Furthermore, the optical PBC/S device of the present invention also provides better optical performance over the prior art optical PBC/S because of the superior performance characteristics of thin film wire-grid polarizers. Thin film wire grid polarizers provide among other benefits, a broader wavelength range of operation and broader range of incidence angles.  
         [0022]     The thin film wire-grid polarizer is typically formed as a discrete device by fabricating thin film wire grid structures on an optically transparent substrate material, such as Si0 2 . But if desired, the thin film wire grid structures can be fabricated directly on the inwardly-facing surface of one of the collimating/focusing lenses, further integrating the optical PBC/S device.  
         [0023]     The thin film wire grid structures on the polarizer may preferably have nano-scale dimensions and thus allow very finely spaced subwavelength wire grids for use in high frequency applications such as combining or splitting beams in infrared, visible, or UV light range. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0024]     Understanding of the present invention will be facilitated by consideration of the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which like numerals refer to like parts:  
         [0025]      FIGS. 1   a  and  1   b  illustrate an example of a prior art optical PBC/S device;  
         [0026]      FIGS. 2   a  and  2   b  illustrate a second example of a prior art optical PBC/S device;  
         [0027]      FIG. 3  illustrates another example of a prior art optical PBC/S device;  
         [0028]      FIG. 4  illustrates a perspective schematic view of an embodiment of an optical PBC/S device incorporating a thin film wire-grid polarizer according to the present invention;  
         [0029]      FIG. 5  is an isolated view of the thin film wire-grid polarizer of  FIG. 4 ;  
         [0030]      FIG. 6  is a top-down view of the thin film wire-grid polarizer of  FIG. 5  illustrating the incidence angles of incident beams in a beam combiner application;  
         [0031]      FIG. 7  is a side view of the device of  FIG. 4  with the direction of beams illustrating the device in a beam combiner application;  
         [0032]      FIG. 8  illustrates a side view of another embodiment of the device of  FIG. 4  with the direction of beams illustrating the device in a beam splitter application;  
         [0033]      FIG. 9  illustrates an embodiment of the present invention where two optical PBC/S devices have been integrated into a single device;  
         [0034]      FIG. 10  illustrates another embodiment of the present invention;  
         [0035]      FIGS. 11   a - 11   c  illustrate an embodiment of the present invention where the thin film wire-grid polarizer is rotatably actuated;  
         [0036]      FIG. 12  is a plot graph of calculated transmittance and reflectance of polarized light through a metal wire-grid polarizer; and  
         [0037]      FIG. 13  illustrates an embodiment of the present invention where the optical fibers are provided in ferrules that hold the optical fibers in predetermined positions and orientation. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]     It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for the purpose of clarity, many other elements found in optical devices and methods of making the same. Those of ordinary skill in the art may recognize that other elements and/or steps are desirable and/or required in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. The disclosure herein is directed to all such variations and modifications to such elements and methods known to those skilled in the art.  
         [0039]      FIG. 4  illustrates a perspective view of an optical PBC/S device  100  according to an embodiment of the present invention. The optical PBC/S device according to the present invention operates equally well as a polarization beam combiner and a polarization beam splitter. The optical PBC/S device  100  will be first described in the context of its beam combining operational mode. The device  100  comprises a thin film wire-grid polarizer  150  provided between a pair of collimating/focusing lenses  110  and  120 . The thin film wire-grid polarizer  150  is illustrated in this example as comprising thin film wire grid structures  153  deposited on a substrate material  158 .  
         [0040]     Graded index lenses can be used for the collimating/focusing lenses  110  and  120 . The collimating/focusing lenses  110  and  120  are provided in a coaxial configuration so that their optical axes align to define the optical PBC/S device&#39;s optical axis  115 . In this configuration, the graded index lenses&#39; inwardly-facing surfaces  113  and  123  face each other and their outwardly-facing surfaces  112  and  122  face away from each other. On the outwardly-facing surface  112  side of the graded index lens  110  are two optical fibers oriented parallel to the device&#39;s optical axis  115 : a first input fiber  170  and an output fiber  190 . On the outwardly-facing surface  122  side of the collimating/focusing lens  120  is a second input fiber  180 . For the optical PBC/S device  100  to be used as a beam combiner, the first and second input fibers  170  and  180  must be polarization-maintaining fibers.  
         [0041]      FIGS. 5 and 6  illustrate the beam combining/splitting properties of the thin film wire-grid polarizer  150  of the optical PBC/S device  100  illustrated in  FIG. 4 . The thin film wire grid structures  153  deposited on one side of an optically transparent substrate  158  form an active area that combines or splits the beams. The substrate  158  is made of a glass or any material that will transmit the particular wavelengths of the beams involved. The thin film wire grid structures  153  will always reflect a beam of polarization parallel to the wire grids and transmit a beam of polarization orthogonal to the wire grids. Light with polarization parallel to the wire grids excites electron oscillations in the wire grids. The oscillating electrons radiate coherently and the combined electromagnetic field from the oscillating electrons forms the reflected beam. Both incoming and reflected beams in that polarization obey the laws for reflection from a solid metal surface.  
         [0042]     If the incident beam is of polarization perpendicular to the wire grids direction and the wire grids are sufficiently thin to not allow the incoming electromagnetic field of the incident beam to excite the electron oscillations, the incident beam will be transmitted through the wire grids without any alteration. For this polarization the wire grid acts like a dielectric. Therefore, the transmission optical axis  155  for the wire-grid structure  153  is perpendicular to the direction of the wire grids.  
         [0043]     For purposes of illustration, first and second polarized incident beams  162 ,  164  and a composite depolarized output beam  166  are depicted as collimated single beams. In this embodiment, the optical transmission axis  155  of the thin film wire-grid structure  153  is oriented in such a way as to reflect the first polarized incident beam  162  from the first input fiber  170 . In other words, the slow axis of the first input fiber  170 , designated as S in  FIG. 5 , is set to be orthogonal to the optical transmission axis  155  of the thin film wire grid structure  153 . On the other hand, the slow axis of the second polarization maintaining input fiber  180 , designated as P in  FIG. 5 , is set to be parallel to the optical transmission axis  155  so that the second polarized incident beam  164 , whose polarization state is orthogonal to that of the first polarized incident beam  162  transmits through the thin film wire-grid polarizer  150 . As a result, the reflected incident beam  162  and the transmitted incident beam  164  combine to form a composite depolarized output beam  166  having both polarization states.  
         [0044]      FIG. 6  is a top view of the thin film wire-grid polarizer  150  illustrating the angular relationship of the incident and the output beams to the optical axis  115  of the optical PBC/S device  100 . In this embodiment, the plane defined by the thin film wire-grid polarizer  150  is orthogonal to the optical axis  115 . The first polarized incident beam  162  having the S polarization state is reflected by the thin film wire-grid structure  153  so the incident angle  01  and the reflected angle  03  with respect to the optical axis  115  are equal. The second polarized incident beam  164  having the P polarization state transmits through the thin film wire-grid polarizer&#39;s substrate material  158  and the thin film wire grid structures  153  and continues. By setting the second incident beam&#39;s incident angle  02  equal to the first incident beam&#39;s incident angle  01 , the transmitted second incident beam will exit the thin film wire-grid structures  153  at the angle  03  and combine with the reflected first polarized incident beam  162  to form the composite depolarized output beam  166 . It is a typical feature of a thin film wire-grid polarizer that it can operate and maintain peak performance in a wide range of incidence angles.  
         [0045]     Because the thin film wire-grid structure  153  is provided on one side of the substrate material  158 , the wire-grid structure is not truly symmetrically positioned between the two collimating/focusing lenses  110  and  120 . But, because the thin film wire-grid polarizers can be fabricated on very thin, typically less than 0.5 mm thick, any aberration that may be introduced by the substrate material  158  can be significantly minimized.  
         [0046]     As with any metal grid polarizer, it is generally known in the art that a thin film wire grid&#39;s ability to reflect or transmit a beam of a given wavelength is dependent on the wire grid&#39;s dimensions and the choice of materials for the substrate. One can optimize the transmittance and reflectance of a wire grid polarizer by selecting appropriate grid dimensions and the materials for a given application. Subwavelength thin film wire-grid polarizers that will optimally operate in infrared to UV range should have nano-scale structures, i.e. structures having nanometer dimensions. For example, for an optimal operation in the infrared light range, the thin film wire grid&#39;s period is preferably between 150 rim.-250 nm with a fill ratio for the metal grating between 20-40%. For an optimal operation in the visible light range, the thin film wire grid&#39;s period is preferably on the order of 150 nm or less with a fill ratio of 50% or less. An example of a such wire grid polarizer optimized for visible light application is described in U.S. Pat. No. 6,288,840.  
         [0047]     Fabrication of subwavelength thin film wire-grid polarizers per se is generally known and need not be discussed in detail here. The material selection for the metal grid as well as the substrate material for the metal grid is also an important consideration. Appropriate material should be selected to minimize the absorption at the particular wavelength of operation. For example, in the UV range, most optical materials absorb a lot of light so one may need to fabricate the thin film wire grid structures on a low UV absorbing material such as fused silica. But it will be appreciated by one of ordinary skill in the art that by taking the above factors into consideration a thin film wire-grid polarizer can be optimized for operation in the infrared to the UV range.  
         [0048]      FIG. 7  is a cross-sectional illustration of the optical PBC/S device  100  and the operation of the device as a beam combiner will be described using the illustration. In a beam combining operational mode, two polarized incident beams  162  and  164 , each having a polarization state orthogonal to the polarization state of the other beam, are inputted by the two input fibers  170  and  180 , respectively. As mentioned above, these input fibers must be polarization-maintaining fibers. The first polarized incident beam  162  from the first input fiber  170  is collimated through the collimating/focusing lens  110  and exits at the inwardly-facing surface  113  of the collimating/focusing lens  110 . As discussed above in reference to  FIGS. 5 and 6 , the first input fiber  170  must be oriented so that the optical fiber&#39;s slow axis is orthogonal to the transmission optical axis  155  of the thin film wire-grid polarizer  150 . The resulting first polarized incident beam  162  is reflected by the thin film wire-grid polarizer  150  and reenters the collimating/focusing lens  110  and is focused into the output fiber  190 .  
         [0049]     The second polarized incident beam  164  from the second polarization maintaining optical fiber  180  is collimated through the collimating/focusing lens  120  and exits at the inwardly-facing surface  123  of the collimating/focusing lens  120 . The second polarized incident beam  164  then propagates through the collimating/focusing lens  120  and the thin film wire-grid polarizer  150  and couple with the reflected first polarized incident beam  162  to form the composite depolarized output beam  166 . But as discussed above in reference to  FIGS. 5 and 6 , in order for the second polarized incident beam  164  to transmit through the thin film wire-grid polarizer  150 , the second input fiber  180  must be oriented so that the optical fiber&#39;s slow axis is parallel to the transmission optical axis  155  of the thin film wire-grid polarizer  150 .  
         [0050]     As discussed in reference to  FIG. 6 , in order for the two polarized incident beams  162  and  164  to properly couple and form the composite depolarized output beam  166 , the input fibers  170  and  180  must be located so that first and second incident beams  162  and  164  have equal incident angles  01  and  02  and the incident beams  162  and  164  meet at the same position on the thin film wire grid structures  153 .  
         [0051]     The incident angles  01  and  02  are determined by the exit angles of the incident beams  162  and  164  at the inwardly-facing surfaces  113  and  123  of the collimating/focusing lenses  110  and  120 , respectively. Because the collimating/focusing lenses  110  and  120  are graded index lenses, the exit angles are determined by the offset distances  172  and  182  of the input fibers  170  and  180  from the optical axis  115  of the collimating/focusing lenses  110  and  120 , respectively. The greater the offset distance, the greater the exit angle will be and the incident angles  01  and  02  will be increased accordingly. Hence, for a proper alignment between the input fiber  170  and the output fiber  190 , if the offset distance  172  is set high for the input fiber  170 , the offset distance  192  for the output fiber  190  must be correspondingly set to the same high value since the resulting incident angle  01  of the incident beam  162  and the reflected angle  03  of the reflected output beam  166  will be large. Then the symmetry in the arrangement requires that the offset distance  182 , of the second input fiber  180 , also should be set to match the offset distance  172  also so that the incident angle  02  of the second incident beam  164  will be appropriately adjusted for a proper coupling of the two incident beams  162  and  164  at the thin film wire-grid polarizer  150 .  
         [0052]     Persons of ordinary skill in the art will appreciate that by switching the input/output roles of the optical fibers  170 ,  180 , and  190 , and reversing the propagation directions of the beams involved, the same configuration of optical PBC/S device  100  functions as a beam splitter.  
         [0053]      FIG. 8  illustrates such an optical PBC/S device  200  depicting the propagation direction of the input and output beams involved in the beam splitter mode of operation. An incident beam  266  of arbitrary polarization is delivered through an input fiber  290 . The incident beam  266  is collimated through a first collimating/focusing lens  210  and exits at inwardly-facing surface  213  of the first collimating/focusing lens  210 . When the incident beam  266  encounters thin film wire-grid polarizer  250 , according to the optical principles described in reference to  FIGS. 5 and 6 , a component of the incident beam  266  having the S polarization state is reflected by the thin film wire-grid polarizer  250  as an output beam  262 . This polarized output beam  262  propagates through the first collimating/focusing lens  210  and is focused into a first output fiber  270  that is appropriately positioned with respect to the optical axis  215  to be in alignment with the focused output beam  262 . A component of the incident beam  266  having the P polarization state is transmitted through the thin film wire-grid polarizer  250  as an output beam  264 . This polarized output beam  264  propagates through a second collimating/focusing lens  220  and focused into an output fiber  280  that is appropriately positioned with respect to the optical axis  215  to be in alignment with the focused output beam  264 . For the beam splitter operation, the output fibers  270  and  280  need not be polarization-maintaining type fibers.  
         [0054]     Unlike the bulk optical elements, such as beam splitter cubes and Wollaston prisms, utilized in some prior art optical PBC/S devices, a thin film wire-grid polarizer typically can accommodate a wider range of incidence angles  01  and  02 . In turn, the input and output fibers can be positioned with relatively large offset distances  172 ,  182 , and  192 . In a thin film wire-grid polarizer that was fabricated with dimensions optimized for infrared application, the applicants were able to measure no significant degradation in performance of the polarizer for incidence angles up to about 20 degrees. One benefit of this aspect of the present invention is that more than one set of input and output fibers can be accommodated with only one set of optical elements, thus allowing multiplexing of optical signals with one set of optical elements.  
         [0055]      FIG. 9  illustrates an embodiment of the invention having two sets of input and output fibers. Optical fibers  370 ,  380 , and  390  represent one set of input/output fibers, and optical fibers  370   a ,  380   a , and  390   a  represent a second set of input/output fibers. Each set can be used independently to combine two orthogonally polarized beams or to split an arbitrarily polarized beam into two orthogonally polarized component beams according to the principles discussed above in reference to  FIGS. 4-8 . For example, an arbitrarily polarized incident beam received through the optical fiber  390   a  will be split by thin film wire-grid polarizer  350  into two polarized component beams and exit via the optical fibers  380   a  and  370   a . At the same time, two orthogonally polarized incident beams are received through the optical fibers  370  and  380 , which will be combined by the thin film wire-grid polarizer  350  into a composite beam and exit via the optical fiber  390 . As discussed above, for the beam combining operation, the two input fibers involved must be polarization-maintaining fibers. In effect, two functionally independent optical PBC/S devices are integrated into one package. Persons of ordinary skill in the art will appreciate that depending on the diameter of the collimating/focusing lenses  310  and  320 , multiple sets of input and output fibers can be provided thus allowing higher functional integration of packaged optical devices.  
         [0056]      FIG. 10  illustrates another configuration for the optical PBC/S device of  FIG. 9 . The optical PBC/S device  400  illustrated in  FIG. 10  is also provided with two sets of input/output fibers. But in this embodiment, the optical fibers are configured so that there are equal numbers of fibers on both sides of the device. Optical fibers  470 ,  480 , and  490  represent a first set of input/output fibers, and optical fibers  470   a ,  480   a , and  490   a  represent a second set of input/output fibers. As with the optical PBC/S device of  FIG. 9 , each set can be used independently to combine two orthogonally polarized beams or to split an arbitrarily polarized beam into two orthogonally polarized component beams.  
         [0057]     Another advantage realized by the optical PBC/S device of the present invention is illustrated by the optical PBC/S device  500  of  FIGS. 11   a  and  11   b . The optical PBC/S device  500  has a thin film wire-grid polarizer  550  positioned between two collimating/focusing lenses  510  and  520 . Three input/output optical fibers  570 ,  580 , and  590  are provided and configured such that the device can function as a beam splitter or a combiner according to the principles discussed in reference to the optical PBC/S devices  100  and  200  of  FIGS. 7 and 8  respectively. But in the optical PBC/S device  500 , a fourth optical fiber  593  has been added. The optical PBC/S device  500  is configured such that the position of the fourth optical fiber  593  is a mirror image of the optical fiber  590  with respect to the plane defined by the thin film wire grid on the thin film wire-grid polarizer  500 .  
         [0058]     By providing the fourth optical fiber  593 , additional functionality can be realized for the PBC/S device  500 . In this illustration, the optical PBC/S device  500  is set up as a basic beam combiner where two orthogonally polarized incident beams,  562  and  564  (S and P polarizations, respectively) are received through the input optical fibers  570  and  580 , respectively. The thin film wire-grid polarizer  550  is oriented in its first position, in which, it will reflect the S polarized incident beam  562  and transmit the P polarized incident beam  564 . Thus, as illustrated in  FIG. 11   a , the reflected and transmitted incident beams combine to form a composite depolarized output beam  566   a  that travels through the collimating/focusing lens  510  and is focused into the output fiber  590 .  
         [0059]     According to the present invention, by rotating the thin film wire-grid polarizer  550  about the optical axis  515  of the optical PBC/S device  500  from the basic beam combiner configuration, a part or all of the composite depolarized output beam can be redirected to the fourth optical fiber  593 .  FIG. 11   b  illustrates the optical PBC/S device  500  where the thin film wire-grid polarizer  550  has been rotated 90 degrees about the optical axis  515  from the basic beam combiner configuration of  FIG. 11   a  into its second position so that the optical transmission axis of the thin film wire-grid polarizer  550  is now rotated 90 degrees. According to the principles discussed in reference to  FIGS. 4-6 , this will switch the thin film wire-grid polarizer&#39;s ability to reflect and transmit the two polarized incident beams so that, now, the S polarized incident beam from the input fiber  570  is transmitted and the P polarized incident beam from the input fiber  580  is reflected. The resulting composite depolarized output beam  566   b  will now exit the optical PBC/S device  500  through the fourth optical fiber  593 . So the user can selectively direct the composite depolarized output beam between the two optical fibers  590  and  593  by rotating the thin film wire-grid polarizer between the first and the second positions. When desired, such operation can be automated by configuring the optical PBC/S device with an appropriate actuation mechanism that can rotate the thin film wire-grid polarizer  550  about the optical axis  515  of the device.  
         [0060]     Furthermore, in this embodiment, the output beam can be selectively apportioned between the output fibers  590  and  593  by positioning the thin film wire-grid polarizer  550  between the two positions discussed above. Unlike other types of polarizers, metal wire-grid polarizers, such as the thin film wire-grid polarizers discussed here, will partially reflect and transmit the remainder of a polarized incident beam when the wire grid&#39;s optical transmission axis is oriented somewhere between 0 and 90 degrees with respect to the polarization state of the incident beam. In other words, the thin film wire-grid polarizer&#39;s optical transmission axis is between the first and the second positions, discussed above, that define the optimal reflection and transmission of S and P polarization states.  FIG. 11   c  illustrates the PBC/S device  500  where the thin film wire-grid polarizer  550  is rotated by an angle δ from the basic beam-combiner configuration of  FIG. 11   a  where the rotation angle δ is somewhere between 0 and 90 degrees. Since the thin film wire-grid polarizer  550  was oriented to maximize the reflection of S polarization beam and the transmission of P polarization beam in the basic beam-combiner configuration; a deviation by angle δ between 0 and 90 degrees will result in partial reflection and transmission of the incident beams.  
         [0061]     As the angle δ changes between 0 and 90 degrees the proportion of the transmitted versus the reflected components of a given polarized incident beam changes continuously. When the rotation angle  8  of the thin film wire-grid polarizer  550  about the optical axis  515  of the optical PBC/S device  500  is between 0 and 90 degrees, the optical transmission axis of the wire grid polarizer  550  is no longer aligned to any of the polarization states of the two incident beams. This offsets the optical transmission axis of the wire grid polarizer  550  from the optimal condition for reflecting the S polarized incident beam from the fiber  570  and transmitting the P polarized incident beam from the fiber  580 . The result is that each incident beam is partially reflected and partially transmitted by the thin film wire-grid polarizer  550  and coupled to the output fibers  590  and  593 .  
         [0062]     In order to achieve the most efficient optical coupling from input fibers  570  and  580  to the two output fibers  590  and  593  simultaneously, the thin film wire-grid polarizer&#39;s wire-grid structures ideally must be positioned symmetrically with respect to the two collimating/focusing lenses  510  and  520  and the gap spacing between the two collimating/focusing lenses must be kept to a minimum. As previously discussed in reference to  FIGS. 4-6 , because thin film wire-grid polarizers can be fabricated on very thin substrate material, typically less than 0.5 mm, these conditions can be substantially satisfied by the optical PBC/S device of the present invention.  
         [0063]     The graph of  FIG. 12  illustrates this partial reflectance/transmittance effect. In the graph, the calculated power levels of reflected and transmitted beams through a thin film wire-grid structure are plotted as functions of the rotation angle δ. The Y-axis represents the normalized power level of an output beam and the X-axis represents the angle between the polarization orientation of the incident beam and the wire grid direction (which is δ for a S polarized incident beam and (90−δ) for P polarized incident beam). As illustrated in the graph, for a given angle δ, each of the P and S polarized incident beams will be split into two component output beams (a reflected beam and a transmitted beam) by a metal wire-grid polarizer in inverse proportions. The reflected output beam&#39;s power level follows the basic cosine 2  function and the transmitted output beam&#39;s power level follows the basic sine 2  function. Thus, notwithstanding some negligible power loss through the thin film wire-grid polarizer, the power levels for the two component output beams at any given angle δ will add up to 1.0 on the normalized scale. And, as long as the two incident beams are of equal power, the composite depolarized output beams at the fibers  590  and  593  will always be equally balanced between S and P polarizations.  
         [0064]     Another practical implication of this partial reflectance/transmittance effect is that, by keeping the rotation angle δ of the thin film wire-grid polarizer  550  small, the fourth optical fiber  593  can be used as a tapping port to tap a small fraction of the output beam of the beam combiner to monitor the power level. When the rotation angle δ is small, while a majority of the S polarized incident beam from the fiber  570  is reflected, a small fraction of the S polarized incident beam is transmitted through the thin film wire-grid polarizer  550  and focused into the fourth optical fiber  593 . Similarly, while the majority of the P polarized incident beam from the fiber  580  is transmitted, the same small fraction of the P polarized incident beam is reflected by the thin film wire-grid polarizer  550  and also focused into the fourth optical fiber  593 . Thus, the output beam focused into the fourth optical fiber  593  is also a composite beam composed of the same proportion of S and P polarized component beams as the main output beam observed at the output fiber  590  and the power level of the combined output beam observed at the fourth optical fiber  593  is proportional to the power level of the main output beam according to the graph illustrated in  FIG. 12 . Thus, the fourth optical fiber  593  can be used to tap a small fraction of the combined output beam to monitor its power level. The tapped output beam can be diverted to an optical detector for this purpose. Although tapping the output beams is commonly practiced with prior art optical beam combiners to monitor power level of the output beam, the prior art optical beam combiners require additional beam splitting hardware to tap the output beam. Thus, the optical PBC/S device of the present invention significantly improves integration of packaged optical PBC/S optical device by eliminating the additional hardware to tap the output beam.  
         [0065]     Persons of ordinary skill in the art would appreciate that the thin film wire-grid polarizer  550  in the optical PBC/S device  500  can be permanently configured to provide a tapping port that taps a fixed fraction of the main combined output beam or, alternatively, configured with a rotating actuation mechanism. By attaching the thin film wire-grid polarizer  550  to a rotating actuation mechanism, the power level of the output beams at either of the output fibers  590  and  593  can be tuned to a desired level. This ability to provide variable output power avoids the need to use an additional variable optical attenuator where control of the combined beam output power is desired.  
         [0066]     As illustrated in the graph of  FIG. 12 , when the thin film wire-grid polarizer&#39;s rotational angle δ is 45 degrees, the polarized incident beam is equally split into two component output beams. Thus, by inputting two orthogonally polarized incident beams  562  and  564  of equal power level, two depolarized output beams having the same output power can be obtained. This is very useful in many applications for optical amplifiers. In optical amplifiers such as distributed gain Raman amplifiers, a pumping scheme with two depolarized light of equal power is essential to reduce nose and optimize amplification.  
         [0067]     In another application of the optical PBC/S device of the present invention, the optical PBC/S device, in conjunction with a Fiber Bragg grating, can be used to simultaneously lock two pump lasers. Using the optical PBC/S device  500  of the present invention, orthogonally polarized beams from two pump lasers are combined into a depolarized beam. In this application, the optical PBC/S device  500  is configured to tap a small fraction of the output beam through the fourth optical fiber  593  and coupled to a Fiber Bragg grating. The Fiber Bragg grating will then reflect a narrow band of the spectrum back into the optical PBC/S device which is routed back to the two pump lasers by the optical PBC/S device. This optical feedback serves to lock the two pump lasers&#39; wavelength to the central wavelength of the Fiber Bragg grating.  
         [0068]     Another advantage of providing the fourth optical fiber  593  is realized when the optical PBC/S device  500 , of  FIGS. 11   a - 11   c  is used in a beam splitter mode. In this mode, the incident beam can be received through either one of the optical fibers  590  or  593  and one can select which of the two optical fibers  570  and  580  should output S or P polarized output beam by rotating the thin film wire-grid polarizer  550  between two positions that are 90 degrees apart. For example, where the incident beam is received through the optical fiber  590 , by orienting the thin film wire-grid polarizer  550  so that its optical transmission axis is orthogonal to the polarization orientation of an S polarized beam, the thin film wire-grid polarizer  550  will reflect the S polarized component of the incident beam and direct it towards the optical fiber  570 . At the same time, the thin film wire-grid polarizer  550  will transmit the P polarized component of the incident beam and direct it towards the optical fiber  580 . The output location of the S and P polarized beams can be switched between the optical fibers  570  and  580  by rotating the thin film wire-grid polarizer  550  by 90 degrees.  
         [0069]      FIG. 13  illustrates yet another embodiment of the present invention in which the benefit of a thin film wire-grid polarizer&#39;s ability to accommodate a wider range of incidence angles is utilized in optical PBC/S device  600 . The optical PBC/S device  600  is configured to accommodate a non-symmetric positioning of the input and output fibers. The device  600  is provided with a basic set of optical fibers  670 ,  680 , and  690 . When the device  600  is operated as a beam combiner, the optical fibers  670  and  680  are used as the input fibers and the optical fiber  690  is used as the output fiber. When the device  600  is operated as a beam splitter, the three optical fibers switch their input/output roles. In the embodiments of the present invention previously discussed, the thin film wire-grid polarizer is always orthogonally positioned with respect to the optical axis of the optical PBC/S device and the optical fibers are symmetrically positioned about the optical axis of the optical PBC/S device to ensure that the beams align and focus properly into the appropriate fibers. The orthogonal orientation of the thin film wire-grid polarizer also ensures, that the gap between the two collimating/focusing lenses in which the thin film wire-grid polarizer sits is kept as small as possible to achieve the most compact configuration for the optical PBC/S device. In this embodiment, however, the thin film wire-grid polarizer is allowed to deviate from its orthogonal orientation with respect to the optical axis of the optical PBC/S device to accommodate non-symmetric arrangement of the optical fibers  670 ,  680 , and  690 .  
         [0070]     As illustrated in  FIG. 13 , the optical fibers  670 ,  680 , and  690  are not symmetrically positioned about the optical axis  615 . The offset distances  672  and  692  of the optical fibers  670  and  690 , respectively, are not equal. This non-symmetric positioning of the optical fibers, however, is compensated by tilting the thin film wire-grid polarizer  650  by a tilt angle a. Because the thin film wire-grid polarizer  650  is relatively thin (typically less than 1.0 mm) the polarizer can be tilted without necessarily increasing the gap spacing between the two collimating/focusing lenses  610  and  620  substantially. Persons of ordinary skill in the art will appreciate that actual dimensions of the thin film wire-grid polarizer and the collimating/focusing lenses will determine the gap spacing required between the two collimating/focusing lenses to accommodate a given tilt angle a. But, because thin film wire-grid polarizers are much thinner than the polarizers employed in prior art optical PBC/S devices, the optical PBC/S device of the present invention can better accommodate non-symmetrically positioned input/output optical fibers without compromising the overall compactness of the optical PBC/S device.  
         [0071]     Because the offset distance  672  is smaller than the offset distance  692 , if the thin film wire-grid polarizer  650  were positioned orthogonally, the exit angle of the incident beam  662  at the inwardly-facing surface  613  of the collimating/focusing lens  610  will be too shallow and the reflected output beam  666  will not focus into the output fiber  690 . But, by adjusting the tilt angle a of the thin film wire-grid polarizer  650 , the incident beam  662  can be reflected back into the collimating/focusing lens  610  at a proper angle and focused into the output fiber  690 . This aspect of the invention provides the flexibility to use the optical PBC/S device in an application that may not allow symmetrical placement of the optical fibers.  
         [0072]     It is generally known in the art that in optical PBC/S devices, the optical fibers can be secured in ferrules in predetermined configuration for easier handling of the optical fibers. By securing the optical fibers in ferrules, properly aligning the fibers with respect to the optical axis of the PBC/S device is simplified. The various embodiments of the PCB/S device according to the present invention discussed herein also can be readily configured with such ferrule-mounted optical fibers as input/output optical beam carriers. Each ferrule can be configured to hold at least one pair of optical fibers where one of the optical fibers is a polarization-maintaining fiber and the other optical fiber is a standard optical fiber. That will allow at least one polarization-maintaining fibers on each end of the PBC/S device that can be used as input/output optical beam carriers depending on whether the PBC/S device is used as a beam combiner or a splitter. The polarization-maintaining fibers can be secured in their respective ferrules in predetermined orientation for ease of use, so that their optical axes are orthogonal to each other when mounted onto the PBC/S device.  
         [0073]     Compared to prior art optical PBC/S devices that utilize bulk optical elements, the optical PBC/S device of the present invention provides, among other benefits, polarization functionality in a compact format because the thin film wire-grid polarizers can be readily fabricated to have a thickness in sub-millimeter range rather than millimeter dimensions of the bulk optical elements. Furthermore, utilizing such recently developed techniques such as nanoimprinting lithography, thin film wire grid structures of submicron dimensions can be fabricated cost efficiently compared to alternative lithographic methods. Fabrication of such submicron scale wire grid structures is described in ZHAONING YU, PARU DESHPANDE, WEI WU, JIAN WANG, AND STEPHEN Y. CHOU,  Reflective Polarizer Based on a Stacked Double - Layer Subwavelength Metal Grating Structure Fabricated Using Nanoimprint Lithography , APPL. PHYS. LETT. Vol. 77, No. 7, 927 (Aug. 14, 2000).  
         [0074]     Applying the nanoimprinting lithography, thin film wire-grid polarizers on substrates of thickness in the range of about 200-1000 pin is readily achieved. Thus, the optical PBC/S device of the invention requires much smaller separation between the two collimating/focusing lenses compared to prior art devices utilizing bulk optical elements whose dimensions are in millimeters. For a thin film wire-grid polarizer that is 500 gm thick the gap between the collimating/focusing lenses can be kept as small as 500 pm. The result is that, according to the present invention, a very compact optical PBC/S device that is significantly smaller than the prior art packaged optical PBC/S devices can be provided.  
         [0075]     Furthermore, a performance advantage may also be realized. A smaller gap between the collimating/focusing lenses lowers the device&#39;s insertion loss caused by the diffraction between the collimating/focusing lenses. In that regard, even further improvement can be achieved by providing the thin film wire grid structures directly on the inwardly-facing surface of one of the collimating/focusing lenses. This eliminates the need for a separate substrate material for the thin film wire grid structures and can reduce the spacing between the collimating/focusing lenses down to the thickness of the thin film wire grid structures which are 1 gm or less.  
         [0076]     Those of ordinary skill in the art may recognize that many modifications and variations of the present invention may be implemented without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.