Abstract:
The present invention provides a line of optical interleavers in which a novel beam-swapping element is utilized. The beam-swapping element of the present invention provides an effective and inexpensive alternative to polarization rotators and birefrigent elements employed in the prior art optical interleavers, hence rendering a simple and low-cost assembly to the optical interleavers of the present invention. The optical interleavers of the present invention further advantageously exploit a combination of two wavelength filters to cancel out wavelength-filter-induced-dispersion. Efforts are also painstakingly made in the optical interleavers of the present invention to substantially minimize other dispersion effects. As such, the optical interleavers of the present invention constitute the first kind in the art in which various dispersion effects are substantially minimized. Such characteristics would be highly desirable in fiber-optic networks. The optical interleavers of the present invention can be advantageously configured as multiplexers, de-multiplexers, or routers.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefits of Provisional Application 60/199,079 filed Apr. 20, 2000, which is herein incorporated by reference for all purposes. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to optical communications systems. More particularly, it provides a novel class of optical interleavers with substantially minimized dispersion for multiplexing or de-multiplexing of optical signals. 
     BACKGROUND ART 
     Wavelength division multiplexing (WDM) has emerged as the standard technique to transmit information in fiber-optic networks. This is because as the bandwidth of fiber data increases, electronic sorting becomes increasingly complex, while wavelength routing becomes ever more practical and elegant. 
     In a WDM system, each optical fiber simultaneously carries many different communications channels in light of respectively different wavelengths. Each channel is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology. 
     WDM systems use components generically referred to as optical interleavers to combine, split, or route optical signals of different channels. Interleavers typically fall into one of three categories, multiplexers, de-multiplexers and routers. A multiplexer takes optical signals of different channels from two or more different input ports and combines them so that they may be coupled to an output port for transmission over a single optical fiber. A de-multiplexer performs the opposite process, that is, it decomposes an optical signal containing two or more different channels according to their wavelength ranges and directs each channel to a different dedicated fiber. A router works much the same way as a de-multiplexer; however, a router can selectively direct each channel according to control signals to a desired coupling between an input channel and an output port. 
     FIG. 1 depicts a typical optical interleaver  999  of the prior art as described in U.S. Pat. No. 5,694,233, which is incorporated herein by reference. A WDM signal  500  containing two different spectral sets  501 ,  502  enters interleaver  999  at an input port  11 . AS used herein, the term “spectral set” refers to a particular range of wavelengths or frequencies that defines a unique information signal. A first birefringent element  30  spatially separates WDM signal  500  into horizontal and vertically polarized components  101  and  102  by a horizontal walk-off. Component signals  101  and  102  both carry the full frequency spectrum of the WDM signal  500 . 
     Components  101  and  102  are coupled to a polarization rotator  40 . The rotator  40  selectively rotates the polarization state of either signal  101  or  102  by a predefined amount. By way of example, in FIG. 1 signal  102  is rotated by 90° so that signals  103 ,  104  exiting rotator  40  are both horizontally polarized when they enter a wavelength filter  61 . 
     Wavelength filter  61  selectively rotates the polarization of wavelengths in either the first or second spectral set to produce filtered signals  105  and  106 . For example, wavelength filter  61  rotates wavelengths in the first spectral set  501  by 90° but does not rotate wavelengths in the second spectral set  502  at all. 
     The filtered signals  105  and  106  enter a second birefringent element  50  that vertically walks off the first spectral set into beams  107 ,  108 . The second spectral set forms beams  109 ,  110 . 
     A second wavelength filter  62  then selectively rotates the polarizations of signals  107  and  108 , but not signals  109  and  110 , thereby producing signals  111 ,  112 ,  113 ,  114  that have polarizations parallel to each other. A second polarization rotator  41  then rotates the polarizations of signals  111  and  113 , but not the polarizations of signals  112  and  114 . The resulting signals  115 ,  116 ,  117 , and  118  then enter a third birefringent element  70 . Note that second wavelength filter  62  may alternatively be replaced by a polarization rotator suitably configured to rotate the polarizations of signals  111  and  113 , but not  112  and  114 . 
     Third birefringent element  70  combines signals  115  and  116 , into the first spectral channel, which is coupled to output port  14 . Birefringent element  70  also combines signals  117  and  118  into the second spectral channel, which is coupled into output port  13 . 
     As described above, interleaver  999  operates as a de-multiplexer. By operating interleaver  999  in reverse, i.e., starting with spectral sets  501 ,  502  at ports  13  and  14  respectively, interleaver  999  operates as a multiplexer. Furthermore, by suitably controlling the polarization rotation induced by rotators  40  and  41 , interleaver  999  may be configured to operate as a router. 
     Interleaver  999  described above advantageously uses wavelength filters to separate an input WDM optical signal containing two spectral sets by way of different polarization modes and subsequently exploits the birefrigent walk-off effect to spatially separate different polarization modes, thereby de-multiplexing the input WDM optical signal. The use of the wavelength filters and birefrigent materials, however, inadvertently introduces various dispersion effects, which would degrade the performance of fiber-optic networks if uncompensated for. For instance, there is Polarization Mode Dispersion (PMD) known in the art, owing to the fact that different polarization modes traverse different optical path lengths in a birefrigent material. Moreover, since a wavelength filter is typically composed of a stacked plurality of birefrigent waveplates, different wavelengths of light undertake different polarizations in various constituent waveplates of a wavelength filter; and different polarizations subsequently lead to different optical path lengths. Hence, there is also Wavelength-Filter-Induced-Dispersion (WFID) that is both chromatic and polarization-related. Therefore, care must be taken to ensure that various dispersion effects are substantially minimized in an optical interleaver. 
     As fiber-optic systems rapidly spread as the backbone of modern communications networks, there is a need for optical interleavers in which dispersion effects are properly accounted for. The desired optical interleavers should also have a simple and low-cost assembly. 
     OBJECTS AND ADVANTAGES 
     Accordingly it is a principal object of the present invention to provide a line of optical interleavers in which a novel beam-swapping element is utilized. Moreover, efforts are painstakingly made in the optical interleavers of the present invention to minimize various dispersion effects. It is a further object of the present invention to provide methods for constructing these novel optical interleavers. 
     An advantage of the beam-swapping element of the present invention is that it provides an effective and inexpensive alternative to the second polarization rotator and wavelength filter employed in the prior art optical interleaver as shown in FIG. 1, hence rendering a simple and low-cost assembly to an optical interleaver of the present invention. The use of the beam-swapping element further avoids undesirable complications such as dispersion effects. Another significant advantage of the optical interleavers of the present invention is that they present the first kind in the art in which various dispersion effects are substantially minimized. Such characteristics are highly desirable in fiber-optic networks. 
     These and other objects and advantages will become apparent from the following description and accompanying drawings. 
     SUMMARY OF THE INVENTION 
     The present invention provides an optical interleaver comprising a first birefringent element that decomposes and spatially separates an input WDM signal carrying first and second spectral sets into first and second beams with orthogonal polarizations. The first and second spectral sets are substantially complementary. A first wavelength filter, optically coupled to receive the first and second beams, decomposes the first beam into third and fourth beams and the second beam into fifth and sixth beams, by preferentially rotating the polarization of the second (or the first) spectral set in each of the first and second beams by 90-degree. Upon emerging from the first wavelength filter, the third and fifth beams carry the first spectral set with orthogonal polarizations, and the fourth and sixth beams carry the second spectral set with orthogonal polarizations. A second birefringent element, optically coupled to the first wavelength filter, spatially separates the four beams by way of the birefrigent walk-off effect. Upon emerging from the second birefrigent element the four beams are spatially positioned such that they can be construed as travelling along the four corners of an imaginary “rectangular propagation pipe”, with the third and fifth beams carrying the first spectral set diagonally opposing each other, and the fourth and sixth beams carrying the second spectral set diagonally opposing each other. A beam-swapping element is optically coupled to receive the third and sixth beams, or the fourth and fifth beams, from the second birefrigent element. Upon passing through the beam-swapping element, the third and fifth beams become positioned such that they can be construed as falling on a first side-plane of the imaginary “rectangular propagation pipe” described above, and the fourth and sixth beams become positioned such that they can be construed as falling on a second side-plane of the imaginary “rectangular propagation pipe”, where the first and second side-planes are parallel to each other. The third and fifth beams are then combined into a first output signal carrying the first spectral set, and the fourth and sixth beams are combined into a second output signal carrying the second spectral set, by way of a third birefrigent element. The two output signals may be further directed to two output ports. 
     The beam-swapping element in the present invention can be in the form of a hexagon plate, or parallelogram plate, comprising first and second faces parallel to third and fourth faces respectively. The four faces are oriented such that when two parallel beams, e.g., the third and sixth beams (or the fourth and fifth beams) in the above embodiment, are incident on the first and second faces, they emerge from the third and fourth faces respectively, thereby “swapping” in position. The beam-swapping element can also be a Dove prism known in the art of optics, where two slanted, non-parallel faces are utilized. As such, when two parallel beams (e.g., the third and fifth beams in the above embodiment) are incident on the first slanted face of a Dove prism, they emerge from the second slanted face in such a way that the two beams remain parallel, however “swapped” in position. 
     The optical interleaver of the present invention further comprises a compensation assembly, for ensuring that upon being combined various dispersion effects in each and every beam have been substantially minimized. The compensation assembly utilizes various arrangements of optical elements to substantially equalize the optical path lengths of the beams upon being combined. The compensation assembly further advantageously exploits the use of a second wavelength filter to cancel out the dispersion effects the first wavelength filter has inflicted on the beams. 
     As such, the optical interleaver of the present invention constitutes the first kind in the art in which various dispersion effects are substantially minimized. These dispersion-minimized optical interleavers would be highly desirable in fiber-optic networks. A further advantage of the optical interleavers of the present invention is that routing is accomplished while conserving substantially all optical energy available in the input WDM signal. That is, both the horizontal and vertical polarized components are used and recombined to provide the output signals, resulting very few loss through the optical interleaver. 
     The optical interleaver of the present invention can be configured to operate as a multiplexer, a de-multiplexer, or a router, as depicted in the drawings and the detailed description that follow. 
     The novel features of this invention, as well as the invention itself, will be best understood from the following drawings and detailed description. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows an isometric representation of an optical interleaver according to the prior art; 
     FIGS. 2A-2D depict isometric representations of several exemplary embodiments of an optical interleaver according to the present invention; 
     FIGS. 3A-3D show several exemplary embodiments of a “beam-swapping” element according to the present invention; 
     FIGS. 4A-4C depict isometric representations of three exemplary embodiments of a dispersion-minimized optical interleaver according to the present invention. 
     FIG. 5 shows a model calculation of phase change as a function of wavelength for four polarization modes emerging from an exemplary embodiment of a wavelength filter; 
     FIG. 6 shows a model calculation of transmission as a function of wavelength for four polarization modes emerging from an exemplary embodiment of wavelength filter; and 
     FIG. 7 displays transmission as a function of wavelength obtained experimentally from an exemplary embodiment of an optical interleaver according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     Although the following detailed description contains many specific details for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the exemplary embodiment of the invention described below is set forth without any loss of generality to, and without imposing limitations upon, the claimed invention. 
     FIGS. 2A-2D depict isometric representations of several exemplary embodiments of an optical interleaver  200  according to the present invention. By way of example, FIG. 2A shows a first embodiment of optical interleaver  200  according to the present invention. A WDM signal  500  carrying two distinct and complementary spectral sets  501 ,  502  in its spectrum enters optical interleaver  200  at an input port  201 . As used herein, the term “spectral set” refers to a particular range of wavelengths or frequencies that defines a unique information signal. A first birefrigent element  202  spatially separates WDM signal  500  into horizontally and vertically polarized components, such that a vertically polarized beam  102  travels as an ordinary ray and passes through without changing course, while a horizontally polarized beam  101  travels as an extraordinary beam and consequently walks off from its original course. It should be noted that beams  101  and  102  both comprise the full spectrum of WDM signal  500 . A first compensation plate  209  is attached to one side of first birefrigent element  202 , so as to intercept second beam  102 . The thickness of first compensation plate  209  is selected such that upon traversing though first compensation plate  209 , first and second beams  101 ,  102  have substantially equalized optical path lengths. 
     A wavelength filter  203 , optically coupled to receive first and second beams  101 ,  102  from first birefrigent element  202 , decomposes first beam  101  into a third beam  103  with a horizontal polarization and a fourth beam  104  with a vertical polarization. A wavelength filter  203  also decomposes second beam  102  into a fifth beam  105  with a vertical polarization and a sixth beam  106  with a horizontal polarization. Note that third and fifth beams  103 ,  105  carry first spectral set  501 , whereas fourth and sixth beams  104 ,  106  carry second spectral set  502 . Thus, wavelength filter module  203  separates different spectral sets by way of different polarizations. 
     A second birefringent element  204  is optically coupled to wavelength filter  203  and spatially separates the four beams into four horizontally and vertically polarized components by way of the birefrigent walk-off effect. Second birefringent element  204  is configured such that vertically polarized beams  104 ,  105  walk off as extraordinary rays, while horizontally polarized beams  103 ,  106  pass through without changing course as ordinary rays. Note that upon emerging from second birefrigent element  204  the four beams are spatially positioned such that they can be construed as travelling along the four corners of an imaginary “rectangular propagation pipe”, with third and fifth beams  103 ,  105  carrying the first spectral set diagonally opposing each other, and fourth and sixth beams  104 ,  106  carrying the second spectral set diagonally opposing each other. The relative positions of the four beams can also be seen in panel  213 , which effectively provides a cross-sectional view of the imaginary “rectangular propagation pipe” described above. 
     A beam-swapping element  205 , in the form of a hexagon plate of a refractive material, is optically coupled to receive third and sixth beams  103 ,  106  from second birefrigent element  204 , as a way of example. Box  205 E provides a top view of beam-swapping element  205 , illustrating the underlying beam-swapping mechanism. Beam-swapping element  205  has first and second faces  205 A,  205 B parallel to third and fourth faces  205 C,  205 D respectively. Third and sixth beams  103 ,  106  are incident on and refracted at first and second faces  205 A,  205 B. Third and sixth beams  103 ,  106  are then refracted at and emerge from third and fourth faces  205 C,  205 D respectively, thereby swapping in position upon emerging. A second compensation plate  210  of a refractive material is optically coupled to receive fourth and fifth beams  104 ,  105  from second birefrigent element  204 . Box  210 C provides a top view of second compensation plate  210 , illustrating the respective passages of fourth and fifth beams  104 ,  105 . Fourth and fifth beams  104 ,  105  are incident on a first face  210 A and emerge from a second face  210 B of second compensation plate  210 , where faces  210 A,  210 B are parallel to each other. As such, upon emerging from beam-swapping element  205  and second compensation plate  210 , third and fifth beams  103 ,  105  become positioned such that they can be construed as falling on a first side-plane of the imaginary “rectangular propagation pipe”, and fourth and sixth beams  104 ,  106  become positioned such that they can be construed as falling on a second side-plane of the imaginary “rectangular propagation pipe”, where the first and second side-planes are parallel to each other. The spatial arrangement among the four beams at this point can also be seen in panel  214 . 
     Finally, a third birefirgent element  206  is optically coupled to receive third and sixth beams  103 ,  106  from beam-swapping element  205 , and fourth and fifth beams  104 ,  105  from second compensation plate  210 . Third birefirgent element  206  uses the birefrigent walk-off effect to recombine third and fifth beams  103 ,  105  into a first output signal  107  carrying the first spectral set  501 , and fourth and sixth beams  104 ,  106  into a second output signal  108  carrying the second spectral set  502 . The two output signals may be further directed to two output ports. 
     Panels  211 ,  212 ,  213 ,  214 , and  215  illustrate polarizations, relative positions, and changes in optical path lengths of the beams after passing through each optical element in optical interleaver  200  of FIG.  2 A. For instance, after passing through second birefrigent element  204 , fifth and fourth beams  105 ,  104  each acquires an additional optical path length of  6  relative to third and sixth beams  103 ,  106 , as shown in panel  213 . A length  210 L between first and second faces  210 A,  210 B of second compensation plate  210  is selected to be shorter than a length  215 L between first and third faces  205 A,  205 C (or between second and fourth faces  205 B,  205 D) of beam-swapping element  205 , such that each of fifth and fourth beams  105 ,  104  gains an extra optical path length of  26  in reference to third and sixth beams  103 ,  106 , as shown in panel  214 . Third birefirgent element  206  is configured in the same way as second birefirgent element  204 , such that each of fifth and fourth beams  105 ,  104  acquires another δ in optical path length, relative to third beam  103  and sixth beam  106 . Hence, upon being combined third and fifth beams  103 ,  105  have substantially equalized optical path lengths, so have fourth and sixth beams  104 ,  106 , as shown in panel  215 . 
     It should be noted hereinafter that in the above embodiment as well as in the succeeding embodiments of the present invention, “horizontal” and “vertical” terms, as conforming to their conventional definitions, are used as a way of example to describe the polarizations of the optical beams in the optical interleavers of the present invention. The use of these terms should not be construed as to limit the scope of the present invention by any measure. For instance, a rotation of optical interleaver  200  in the embodiment of FIG. 2A as a whole about an axis (e.g., the symmetry axis of the imaginary “rectangular propagation pipe” described above) parallel to the direction of propagation of the optical beams will not affect its functional performance, though the polarizations of the optical beams may no longer be described as being “horizontal” and “vertical” after the rotation. What remains being the case is that the four beams emerging from the second birefrigent element continue to be positioned such that they can be construed as travelling along the four corners of an imaginary “rectangular propagation pipe”, with the two beams in the same spectral set diagonally opposing each other. The two beams carrying the same spectral set later become positioned on the same side-plane of the imaginary “rectangular propagation pipe” by operation of the beam-swapping element. 
     FIG. 2B depicts a second embodiment of optical interleaver  200 . In this embodiment, first birefrigent element  202 , first compensation plate  209 , wavelength filter  203 , second birefrigent element  204 , beam-swapping element  205 , and third birefrigent element  206  remain functionally equivalent to those described in the embodiment of FIG.  2 A. This embodiment of optical interleaver  200  is substantially equivalent to the embodiment of FIG. 2A in operation. A second compensation plate  220  is configured to provide the same optical path length to fifth beam  105  or fourth beam  104  as beam-swapping element  205  would provide to third beam  103  or sixth beam  106 . A half-wave plate  227  is positioned to receive third and sixth beams  103 ,  106  from beam-swapping element  205 , and fourth and fifth beams  104 ,  105  from second compensation plate  220 . Half-wave plate  227  serves to rotate the polarization of each beam by 90-degree. Being vertically polarized, third and sixth beams  103 ,  106  then traverse as extraordinary rays in third birefrigent element  206  and consequently each acquire an additional optical path length of δ, relative to fifth and fourth beams  105 ,  104  now traversing as ordinary rays (for being horizontally polarized). As such, third and fifth beams  103 ,  105  have substantially equalized optical path lengths upon being combined, so have fourth and sixth beams  104 ,  106 . Panels  221 ,  222 ,  223 ,  224 ,  225  and  226  illustrate polarizations, relative positions, and changes in optical path lengths of the beams after passing through each optical element in this embodiment. 
     FIG. 2C shows a third embodiment of optical interleaver  200 . In this case, first birefrigent element  202 , wavelength filter  203 , second birefrigent element  204 , beam-swapping element  205 , and third birefrigent element  206  remain functionally equivalent to those illustrated in the embodiment of FIG.  2 A. This embodiment of optical interleaver  200  is substantially equivalent to the embodiment of FIG. 2A in operation. Note that there is no longer a compensation plate attached to first birefrigent element  202 . As shown in panel  231 , first beam  101  incurs an additional optical path length of δ′ relative to second beam  102  after passing through first birefrigent element  202 , owing to the birefrigent walk-off effect. This extra optical path length is subsequently passed onto third and fourth beams  103 ,  104 , as shown in panel  232 . As extraordinary rays in second birefrigent element  204 , fifth and fourth beams  105 ,  104  each acquire an extra optical path length of δ relative to third and sixth beams  103 ,  106 , as shown in panel  233 . A second compensation plate  230  is configured to provide an extra optical path length of (δ′−2δ) to each of fifth and fourth beams  105 ,  104 , in reference to what beam-swapping element  205  would provide to third and sixth beams  103 ,  106 , as shown in panel  234 . A first compensation plate  237  is implemented to receive sixth beam  106  from beam-swapping element  205  and serves to provide an additional optical path length of 2δ to sixth beam  106 , as shown in panel  235 . Upon passing through third birefrigent element  206 , each of fourth and fifth beams  104 ,  105  traversing as extraordinary rays acquires an additional optical path length of δ, relative to third and sixth beams  103 ,  106  traversing as ordinary rays. As such, third and fifth beams  103 ,  105  end up with substantially equalized optical path lengths upon being combined, so do fourth and sixth beams  104 ,  106 , as shown in panel  236 . 
     In the above embodiments of the present invention, the beam-swapping element can be in the form of a refractive hexagon plate, as exemplified in FIGS. 2A-2C, or a refractive parallelogram plate. FIGS. 3A-3B depict top views of a hexagon plate and a parallelogram plate respectively. Each plate has first and second faces  1 ,  2  parallel to third and fourth faces  3 ,  4  respectively, such that when two parallel beams λ 1 , λ 2  are incident on and refracted at faces  1 ,  2 , they are subsequently refracted at and emerge from faces  3 ,  4  respectively, thereby swapping in position. The beam-swapping plate can alternatively be a Dove prism known in the art of optics, as illustrated in FIG.  3 C. In this case, two slanted, non-parallel faces  1 ,  2  of a Dove prism are utilized, such that when two parallel beams λ 1 , λ 2  are incident on face  1  they emerge from face  2  and swap in position, due to a combination of refraction and internal reflection as shown in FIG.  3 C. Those skilled in the art will recognize that FIGS. 3A-3C provide only a few of many embodiments of a beam-swapping element of the present invention. A skilled artisan can devise a suitable beam-swapping element in accordance with the present invention for a given application. 
     It should be pointed out that in the embodiments of FIGS. 2A-2C, since the polarizations of third and sixth beams  103 ,  106  lie in their respective planes of incidence upon entering beam-swapping element  205 , it would be preferable for third and sixth beams  103 ,  106  to be incident at a Brewster angle on first and second faces  205 A,  205 B (see FIG. 2A) respectively by an appropriate arrangement of beam-swapping element  205 , thereby substantially eliminating light refection. In applications where an incidence at a Brewster angle cannot be attained, first and second faces  205 A,  205 B of beam-swapping element  205  can be coated with an anti-reflection layer, so as to reduce light reflection at these surfaces. 
     Those skilled in the art will also recognize that the embodiments of FIGS. 2A-2C provide only a few of many embodiments of an optical interleaver according to the present invention. Many alterations/substitutions can be implemented, without departing from the principle and scope of the present invention. For instance, the beam-swapping element can be alternatively coupled to receive fourth and fifth beams  104 ,  105 , and the second compensation plate optically coupled to receive third and fourth beams  103 ,  106  from second birefrigent element  204  in the embodiment of FIG. 2A, with the optical path lengths provided by the beam-swapping element and second compensation plate being exchanged as well (that is, lengths  215 L,  210 L exchange their respective values.) A similar exchange between the beam-swapping element and second compensation plate along with appropriate arrangements for compensating for optical path lengths can also be implemented in the embodiments of FIGS. 2B-2C. (Note that for the purpose of eliminating light reflection, it is preferable for the beam-swapping element to be optically coupled to the beams that are polarized in their planes of incidence, as explained above.) Moreover, first compensation element  209  in the embodiments of FIGS. 2A-2B can be alternatively attached to wavelength filter  203 , or second birefrigent element  204 , to provide additional optical path lengths to fifth and fourth beams  105 ,  104 , such that all four beams have substantially equalized optical path lengths upon emerging from wavelength filter  203  or second birefrigent element  204 . Additionally, the combined function of a plurality of compensation plates and/or a half-wave plate in the embodiments of FIG. 2A-2C can be provided by a single compensation element configured to provide different optical path lengths to different beams traversing through it. FIG. 3D illustrates an exemplary embodiment of such a compensation element, comprising first face  1  parallel to second and third faces  2 ,  3  respectively. A length L- 1  between first and second faces  1 ,  2  is configured to be different from a length L- 2  between first and third faces  1 ,  3 , so as to impart different optical path lengths to two beams λ 1 , λ 2  traversing through it. As a way of example, this exemplary compensation element can be implemented in the embodiment of FIG. 2A to substitute for the combined function of first and second compensation plates  209 ,  210  and optically coupled to receive fourth and fifth  104 ,  105  beams, whereas beam-swapping element  205  receives third and sixth beams  103 ,  106  from second birefrigent element  204 . In this case, L- 1  should he shorter than length  215 L of beam-swapping plate  205  (see FIG. 2A) such that sixth beam  106  acquires an additional optical path length of (2δ+δ′) relative to fourth beam  104 . Likewise, L- 2  should be shorter than length  215 L of beam-swapping plate  205 , such that third beam  103  acquires an additional optical path length of (2δ−δ′) relative to fifth beam  105 . As such, third and fifth beams  103 ,  105  end up with substantially equalized optical path lengths upon being combined, so do fourth and sixth beams  104 ,  106 , as in the previous embodiments. 
     FIG. 2D shows a fourth embodiment of optical interleaver  200 , illustrating yet another mechanism for beam-swapping and optical-path-length compensation. In this embodiment, first birefrigent element  202 , wavelength filter  203 , second an third birefrigent elements  204 ,  206  remain functionally equivalent to those illustrated in the embodiment of FIG.  2 A. This embodiment of optical interleaver  200  is substantially equivalent to the embodiment of FIG. 2A in operation. Note that there is no longer a compensation plate attached to first birefrigent element  202 . As in the embodiment of FIG. 2A, upon emerging from second birefrigent elements  204  third, fourth, fifth and sixth beams  103 ,  104 ,  105 ,  106  are spatially positioned such that they can be construed as travelling along the four corners of an imaginary “rectangular propagation pipe”, with third and fifth beams  103 ,  105  carrying the first spectral set diagonally opposing each other, and fourth and sixth beams  104 ,  106  carrying the second spectral set diagonally opposing each other. The relative positions of the four beams can also be seen in panel  243 , which effectively provides a cross-sectional view of the imaginary “rectangular propagation pipe” described above. A first half-wave plate module  248  is optically coupled to receive third, fourth, fifth, and sixth beams  103 ,  104 ,  105 ,  106  from second birefrigent elements  204  and selectively rotate the polarizations of third, fourth and fifth beam  103 ,  104 ,  105  by 90-degree respectively. A beam-swapping element  249 , in the form of a birefrigent plate, is optically coupled to receive third and sixth beams  103 ,  106  from half-wave plate module  248 , wherein third beam  103  walks through as an ordinary ray while sixth beam  106  walks off as an extraordinary ray. Hence, upon emerging from beam-swapping element  249 , sixth beam  106  has switched from one side to the other opposing side of third beam  103 . Box  249 A provides a top view of beam-swapping element  249 , illustrating how third and sixth beams  103 ,  106  swap in position. Contemporaneously, a compensation plate  250 , in the form of a refractive parallel plate, is optically coupled to receive fourth and fifth beams  104 ,  105  from second birefrigent elements  204 . Compensation plate  250  is positioned such that upon emerging fifth beam  105  becomes positioned on the same (first) side-plane of the imaginary “rectangular propagation pipe” as third beam  103 , and fourth beam  104  becomes positioned on the same (second) side-plane of the imaginary “rectangular propagation pipe” as sixth beam  106 , where the two (first and second) side-planes are parallel. (Note that at this point the imaginary “rectangular propagation pipe” construed above for describing the propagation of the four beams is shifted to some extent along a direction perpendicular to the direction of propagation of the beams.) Box  250 A provides a top view of compensation plate  250 , illustrating the respective passages of fourth and fifth beams  104 ,  105 . The relative (spatial) positions of the four beams at this point can also be seen in panel  245 . 
     A second half-wave plate module  251  is optically coupled to receive third and sixth beams  103 ,  106  from beam-swapping element  249  and fourth and fifth beams  104 ,  105  from compensation plate  250 , serving to selectively rotate the polarization of sixth beam  106  by 90-degree. Being vertically polarized, third and sixth beams  103 ,  106  subsequently walk off as extraordinary rays in third birefrigent elements  206 , whereas fourth and fifth beams  104 ,  105 , being horizontally polarized, walk through third birefrigent elements  206  as ordinary rays. As such, upon emerging from third birefrigent elements  206 , third and fifth beams  103 ,  105  are spatially combined into a first output signal  107 , and fourth and sixth beams  104 ,  106  are likewise combined into a second output signal  108 . 
     Panels  241 ,  242 ,  243 ,  244 ,  245   246 , and  247  illustrate polarizations, relative (spatial) positions, and changes in optical path lengths of the beams after passing through each optical element in the embodiment of FIG.  2 D. As shown in panel  241 , first beam  101  incurs an additional optical path length of δ′ relative to second beam  102  after passing through first birefrigent element  202 , owing to the birefrigent walk-off effect. This extra optical path length is subsequently passed onto third and fourth beams  103 ,  104 , as shown in panels  242 . After passing through second birefrigent element  204 , fourth and fifth beams  104 ,  105  each acquire an extra optical path length of δ relative to third and sixth beams  103 ,  106  respectively, as shown in panel  243 . Sixth beam  106  gains an extra optical path length of 2δ′, relative to third beam  103 , from traversing as an extraordinary ray through beam-swapping element  249 ; whereas compensation plate  250  is configured to provide an extra optical path length of δ′ to each of fourth and fifth beams  104 ,  105 , as shown in panel  245 . Upon passing through third birefrigent element  206 , each of third and sixth beams  103 ,  106  traversing as extraordinary rays acquires an additional optical path length of δ, in reference to fifth and fourth beams  105 ,  104  traversing as ordinary rays. As such, third and fifth beams  103 ,  105  have substantially equalized optical path lengths upon being combined, so have fourth and sixth beams  104 ,  106 . 
     It will be clear to those skilled in the art that there are a variety of ways to compensate for the optical path lengths of the optical beams in an optical interleaver of the present invention, A skilled artisan will know how to implement appropriate compensation elements to best suit a given application. 
     In the embodiments of FIGS. 2A-2D, each of first, second, and third birefrigent elements  202 ,  204 ,  206  generally comprises a birefrigent material, such as calcite, rutile, lithium niobate, or a YVO 4  based crystal. Wavelength filter  203  is typically made of a stacked plurality of birefrigent waveplates with each waveplate oriented in a predetermined direction with a predetermined length, so as to pass a selected set of wavelengths with a horizontal polarization and a complimentary set of wavelengths with a vertical polarization. For wavelength demultiplexing applications, the wavelength filter usually has a comb filter response curve with substantially flat top or square wave spectral response. 
     Because of the way a birefrigent wavelength filter is typically configured, different wavelengths of light undertake different polarizations in various constituent waveplates of a wavelength filter, and different polarizations subsequently lead to different optical path lengths, hence resulting in dispersion that is both chromatic and polarization-related. This wavelength-filter-induced-dispersion has not been accounted for in prior art optical interleavers, such as the optical interleaver described in FIG.  1 . The following presents a treatment of dispersion effects induced by a birefrigent wavelength filter by way of Jones matrix analysis. 
     Jones matrices are widely used to represent the effects of optical elements on polarized light. For a birefrigent waveplate with an optical axis oriented along x-axis, the representative Jones matrix is given by:                Jones                 Matrix     =            -   φ       ·     [                -   ϕ           0           0            ϕ           ]               (   1   )                                
     where          φ   =       k   0          z   ·     (         n   e     +     n   0       2     )           ,     ϕ   =       k   0          z   ·     (         n   e     -     n   0       2     )           ,       k   0     =         2      π     λ     .                              
     Here, n e  and n o  are refractive indices of e-ray and o-ray of the birefringent waveplate respectively, z is the thickness of the waveplate, and λ is the wavelength of light. 
     For a birefrigent waveplate with an optical axis oriented at an angle θ with respect to the x-axis, the corresponding Jones matrix can be expressed as:                      Jones                 Matrix     =                -   φ            [           cos                 θ             -   sin                   θ               sin                 θ           cos                 θ           ]            [                -   ϕ           0           0            ϕ           ]            [           cos                 θ           sin                 θ                 -   sin                   θ           cos                 θ           ]                   =            -   φ            [             cos                 ϕ     -     sin                 ϕcos2θ               -   sin                   2      θ                 sin                 ϕ                 -   sin                   2      θ                 sin                 ϕ             cos                 ϕ     +     sin                 ϕcos                 2      θ             ]                     (   2   )                                
     The above matrix can be generalized in the form of                Jones                 Matrix     =     [           a   -   ib           c   +   id                 -   c     +   id           a   +   ib           ]             (   3   )                                
     where a, b, c, and d are real numbers, each being a function of the wavelength of light, the polarization of light with respective to the optical axis of the waveplate, as well as the characteristics of the waveplate, as indicated in (2). 
     It can be further shown that a multiplication of two or more Jones matrices, each being of the form displayed in (3), yields a matrix of the same form as shown in (3). 
     Since a birefrigent wavelength filter typically comprises a plurality of multiple-order waveplates with their optical axes oriented at various angles, the Cones matrix of the entire wavelength filter is consequently a multiplication of the Jones matrices of the constituent waveplates. Hence, the Jones matrix of a wavelength filter is also of the form shown in (3). 
     As a way of example, let an incident beam polarized in the x direction be represented by a vector          [         1           0         ]     ,                          
     where the incident beam comprises two wavelengths λ 1  and λ 2 . Upon passing through a birefringent wavelength filter, the output beam is given by                  [             a   t     -     ib   t               c   t     +     id   t                   -     c   t       +     id   t               a   t     +     ib   t             ]     ·     [         1           0         ]       =       [             a   t     -     ib   t                   -     c   t       +     id   t             ]     =     [         Hx           Hy         ]               (   4   )                                
     where the matrix on the left-hand side represents the Jones matrix for the entire wavelength filter (hence the subscript “t” is employed to denote this effect), given by a multiplication of the Jones matrices of the constituent waveplates of the wavelength filter. Suppose that the wavelength filter is configured to selectively rotate the polarization of the second wavelength λ 2  by 90-degree and leave the polarization of the first wavelength λ 1  unchanged. In the output beam, therefore, the horizontally polarized component Hx carries the first wavelength λ 1  and the vertically polarized component He carries the second wavelength λ 2 . 
     Similarly, let an incident light seam polarized in the y direction be represented by a vector          [         0           1         ]     ,                          
     where the incident beam comprise two wavelengths λ 1  and λ 2 . Upon passing through the same birefrigent wavelength filter presented above, the output beam is given by                  [             a   t     -     ib   t               c   t     +     id   t                   -     c   t       +     id   t               a   t     +     ib   t             ]     ·     [         0           1         ]       =       [             c   t     +     id   t                   a   t     +     ib   t             ]     =     [         Vx           Vy         ]               (   5   )                                
     In this case, the horizontally polarized component Vx carries the second wavelength λ 2 , and the vertically polarized component Vy carries the first wavelength λ 1.    
     An important result from the above analysis is that the phase of Hx is given by [−tan −1 (b t /a t )], whereas the phase of Vy is tan −1 (b t /a t ). And the amplitude of these two components is the same, given by {square root over (a t   2 +L +b t   2 +L )}. That is to say that upon emerging from the wavelength filter, the two components carrying the first wavelength λ 1  (whose polarizations are unaltered with respect to their respective incident beams) incur additional phases (with respect to their respective incident beams) that are opposite in sign and equal in magnitude, while retaining the same amplitude. Likewise, the two components carrying the second wavelength λ 2 i.e., He and Vx (whose polarizations are rotated by 90-degree with respect to their respective incident beams), also incur additional phases (with respect to their respective incident beams) that are opposite in sign and equal in magnitude, while retaining the same amplitude. 
     As a way of example, FIG. 5 displays the phase of each of Hx, He, Vx, and Vy as a function of wavelength, and FIG. 6 displays the transmission (proportional to the (amplitude) 2 ) of each of Hx, He, Vx, and Vy as a function of wavelength, calculated for an exemplary embodiment of a wavelength filter using the Jones matrix analysis presented above. The exemplary wavelength filter comprises two waveplates, where the first waveplate is about 900 th  order with its optical axis oriented at 45 degrees with respect to the x-axis and the second waveplate is about 1800 th  order with its optical axis oriented at (−15) degrees with respect to the x-axis. These graphs clearly demonstrate that the phases of the two components carrying the same wavelength, either Hx and Vy, or He and Vx, are opposite in sign and equal in magnitude, while retaining the same amplitude (and therefore transmission) within the wavelength range being considered. 
     Based on the analysis along with the model calculation described above, one can infer that if a beam of light characterized by a wavelength λ enters a first wavelength filter with a horizontal polarization and subsequently enters a second wavelength filter with a vertical polarization, where the first and second wavelength filters are configured to be functionally equivalent, the net phase change the light beam would incur from passing through the two wavelength filters would be zero. That is, the operation of the second wavelength filter in this case effectively cancels out the phase change the first wavelength filter has imparted on the beam. This is the case, irrespective of whether the polarization of the beam is being rotated by the wavelength filters. This finding can also be applied to a beam of light carrying a particular range of wavelengths, where the phase change each component of wavelength incurs from the first wavelength filter is subsequently cancelled out by the working of the second wavelength filter. 
     As used herein in this specification and appending claims, the phase change (or time delay) a light beam incurs upon passing through a wavelength filter is termed “Wavelength-Filter-Induced-Dispersion” (WFID). 
     In light of the above discussion, efforts must be made to compensate for WFID arising from wavelength filters employed in an optical interleaver, in addition to compensating for the dispersion effects resulted from traversing different optical path lengths in other optical elements of the optical interleaver. It should be noted in the prior art optical interleaver shown in FIG. 1, as disclosed in U.S. Pat. No. 5,694,233, each of two polarization modes  107 ,  108  in the first spectral set ends up with a net zero of WFID from passing through wavelength filters  61 ,  62 , since they enter first wavelength filter  61  as being horizontally polarized while subsequently entering second wavelength filter  62  as being vertically polarized. However, each of two polarization modes  109 ,  110  in the second spectral set nonetheless gains a non-zero WFID upon passing through wavelength filters  61 ,  62 , since they enter both first and second wavelength filter  61 ,  62  as being horizontally polarized. And this non-zero WFID remains being uncompensated for when the beams being combined. 
     FIGS. 4A-4C show several exemplary embodiments of an optical interleaver  400  of the present invention, in which efforts are painstakingly made to compensate for various dispersion effects. By way of example, FIG. 4A shows a first embodiment of optical interleaver  400  according to the present invention. A WDM signal  500  carrying two distinct spectral sets  501 ,  502  in its spectrum enters optical interleaver  400  at an input port  401 . As used above, the term “spectral set” refers to a particular range of wavelengths or frequencies that defines a unique information signal. A first birefrigent element  402  spatially separates WDM signal  500  into horizontally and vertically polarized components, such that a vertically polarized component  102  travels as an ordinary ray and passes through without changing course, while a horizontally polarized component  101  travels as an extraordinary ray and consequently walks off from its original course. It should be noted that first and second beams  101  and  102  both comprise the full spectrum of WDM signal  500 . A first compensation plate  409  is attached to one side of first birefrigent element  402 , so as to intercept second beam  102 . The thickness of first compensation plate  409  is selected such that upon passing through first birefrigent element  402  along with first compensation plate  409 , first and second beams  101 ,  102  have substantially equalized optical path lengths. 
     A first wavelength filter  403  is optically coupled to receive first and second beams  101 ,  102  from first birefrigent element  402  along with first compensation plate  409 . First wavelength filter  403  decomposes first beam  101  into a third beam  103  with a horizontal polarization and a fourth beam  104  with a vertical polarization, and second beam  102  into a fifth beam  105  with a vertical polarization and a sixth beam  106  with a horizontal polarization. Note that the end effect of first wavelength filter  403  is to change the polarization of the second spectral set signal in first beam  101  from being horizontal to vertical and the polarization of the second spectral set signal in second beam  102  from being vertical to horizontal, while leaving the polarizations of the first spectral set signals in both first and second beams  101 ,  102  unaltered. As such, third and fifth beams  103 ,  105  carry first spectral set  501 , while fourth and sixth beams  104 ,  106  carry second spectral set  502 . 
     A second birefringent element  404  is optically coupled to first wavelength filter  403  and spatially separates the four beams into four horizontally and vertically polarized components by way of the birefrigent walk-off effect. It is configured such that vertically polarized beams  104 ,  105  walk off as extraordinary rays while horizontally polarized beams  103 ,  106  pass through without changing course as ordinary rays. Note that upon emerging from second birefrigent element  404  the four beams are spatially positioned such that they can be construed as travelling along the four corners of an imaginary “rectangular propagation pipe”, with third and fifth beams  103 ,  105  carrying the first spectral set diagonally opposing each other, and fourth and sixth beams  104 ,  106  carrying the second spectral set diagonally opposing each other. The relative positions of the four beams can also be seen in panel  415 , which effectively provides a cross-sectional view of the imaginary “rectangular propagation pipe” described above. 
     A beam-swapping element  405 , in the form of a refractive hexagon plate, is optically coupled to receive third and sixth beams  103 ,  106  from second birefrigent element  404 , as a way of example. Box  405 E provides a top view of beam-swapping element  405 , illustrating the underlying beam-swapping mechanism. Beam-swapping element  405  has first and second faces  405 A,  405 B parallel to third and fourth faces  405 C,  405 D respectively. Third and sixth beams  103 ,  106  are incident on and refracted at first and second faces  405 A,  405 B. Third and sixth beams  103 ,  106  are subsequently refracted at and emerge from third and fourth faces  405 C,  405 D respectively, thereby swapping in position upon emerging. A second refractive compensation plate  410  is optically coupled to receive fourth and fifth beams  104 ,  105  from second birefrigent element  404 . Box  410 C provides a top view of second compensation plate  410 , illustrating the respective passages of fourth and fifth beams  104 ,  105 . Fourth and fifth beams  104 ,  105  are incident on a first face  410 A and emerge from a second face  410 B of second compensation plate  410 , where faces  410 A,  410 B are parallel to each other. As such, upon emerging from beam-swapping element  405  and second compensation plate  410 , third and fifth beams  103 ,  105  become positioned such that they can be construed as falling on a first side-plane of the imaginary “rectangular propagation pipe”, and fourth and sixth beams  104 ,  106  become positioned such that they can be construed as falling on a second side-plane of the imaginary “rectangular propagation pipe”, where the first and second side-planes are parallel to each other. The spatial arrangement among the four beams at this point can also be seen in panel  416 . Note that in this case, second compensation plate  410  is configured to provide the same optical path length to each of fifth and fourth beams  105 ,  104  as beam-swapping element  405  would provide to each of third and sixth beams  103 ,  106 . 
     A half-wave plate module  406  is optically coupled to receive third and sixth beams  103 ,  106  from beam-swapping element  405 , and fourth and fifth beams  104 ,  105  from second compensation plate  410 . Half-wave plate module  406  is configured to selectively rotate the polarizations of third and fifth beams  103 ,  105  by 90-degree respectively and leave the polarizations of fourth and sixth beams  104 ,  106  unchanged. Hence, upon emerging from half-wave plate module  406 , third and fourth beams  103 ,  104 , now diagonally opposing each other in position, are vertically polarized; and sixth and fifth beams  106 ,  105 , diagonally opposing each other in position, are horizontally polarized, as shown in panel  417 . 
     A second wavelength filter  407  is optically coupled to receive third, fourth, fifth and sixth beams  103 ,  104 ,  105 ,  106  from half-wave plate module  406 . Second wavelength filter  407  is configured in the same way as first wavelength filter  403 , serving to rotate the polarizations of fourth and sixth beams  104 ,  106  by 90-degree respectively, while leaving the polarizations of third and fifth beams  103 ,  105  unchanged. 
     Since fourth and sixth beams  104 ,  106  enter first wavelength filter  403  as being horizontally and vertically polarized respectively, and subsequently enter second wavelength filter  407  as being vertically and horizontally polarized respectively by contrast, each of fourth and sixth beams  104 ,  106  ends up with a net zero of WFID after passing through second wavelength filter  407 . Moreover, because of the work of half-wave plate module  406 , third and fifth beams  103 ,  105  enter second wavelength filter  407  being vertically and horizontally polarized respectively, as opposed to being horizontally and vertically polarized upon entering first wavelength filter  403 . Hence, each of third and fifth beams  103 ,  105  incurs no net WFID after passing through second wavelength filter  407 , either. As such, the operation of second wavelength filter  407  effectively “undoes” whatever dispersion effects first wavelength filter  403  has inflicted onto the beams, thereby canceling out any WFID these beams have incurred from traversing through first wavelength filter  403 . 
     Finally, a third birefrigent element  408  is optically coupled to receive third, fourth, fifth and sixth beams  103 ,  104 ,  105 ,  106  from second wavelength filter  407 . Third birefrigent element  408  spatially combined third and fifth beams  103 ,  105  into a first output signal  107  carrying first spectral set  501 , and fourth and sixth beams  104 ,  106  into a second output signal  108  carrying second spectral set  502 . Note that third and sixth beams  103 ,  106  now walk off as extraordinary rays in third birefrigent element  408 , in contrast to being ordinary rays in second birefrigent element  404 . Hence, by configuring second and third birefrigent elements  404 ,  408  in a functionally equivalent way, third and fifth beams  103 ,  105  end up with substantially equalized optical path lengths, so do fourth and sixth beams  104 ,  106 , upon being combined. First and second output signals  107 ,  108  are further directed to an output port  411  by way of a roof prism  412 . 
     Panels  413 ,  414 ,  415 ,  416 ,  417 ,  418 , and  419  depict the polarizations and relative (spatial) positions of the light beams after passing through each optical element in the embodiment of FIG.  4 A. 
     FIG. 4B shows a second embodiment of optical interleaver  400 . In this embodiment, first birefrigent element  402 , first compensation plate  409 , first wavelength filter  403 , second birefrigent element  404 , beam-swapping element  405 , second compensation plate  410 , second wavelength filter  407 , and third birefrigent element  408  remain functionally equivalent to those described in the embodiment of FIG. 4A. A half-wave plate assembly, comprising two half-wave plates  451 ,  452  diagonally opposing each other in position, is attached to second birefrigent element  404  and serves to rotate the polarizations of third and fifth beams  103 ,  105  by 90-degree respectively. After emerging from second birefrigent element  404  along with half-wave plates  451 ,  452 , third and sixth beams  103 ,  106  are subsequently swapped in position by way of beam-swapping element  405 , while fourth and fifth beam pass through second compensation plate  410  in their original courses. As such, upon entering second wavelength filter  403 , the polarizations and relative (spatial) positions of third, fourth, fifth and sixth beams  103 ,  104 ,  105 ,  106  become the same as in the embodiment of FIG. 4A, so is the rest of operation. 
     Panels  453 ,  454 ,  455 ,  456 ,  457 ,  458 , and  459  illustrate the polarizations and relative positions of the light beams after passing through each optical element in the embodiment of FIG.  4 B. 
     FIG. 4C shows a third embodiment of optical interleaver  400 . In this embodiment, first birefrigent element  402 , first compensation plate  409 , first wavelength filter  403 , second birefrigent element  404 , beam-swapping element  405 , second compensation plate  410 , and third birefrigent element  408  remain operationally equivalent to those described in the embodiment of FIG. 4A. A half-wave assembly, comprising two half-wave plates  451 ,  452  diagonally opposing each other, is also attached to second birefrigent element  404  and serves to rotate the polarizations of third and fifth beams  103 ,  105  by  90 -degree respectively. Second wavelength filter  407  in this case is optically coupled to receive third, fourth, fifth and sixth beams  103 ,  104 ,  105 ,  106  from second birefrigent element  404  along with half-wave plates  451 ,  452 . Second wavelength filter  407  selectively rotates the polarizations of fourth and sixth beams  104 ,  106  by 90-degree respectively, while leaving the polarizations of third and fifth beams  103 ,  105  unaltered. Notice that because of the way half-wave plates  451 ,  452  are spatially arranged, third, fourth, fifth and sixth beams  103 ,  104 ,  105 ,  106  enter second wavelength filter  407  with the same polarizations as they do respectively in the embodiment of FIG.  4 A. Hence, the operation of second wavelength filter  407  also effectively “undoes” what first wavelength filter  403  has inflicted onto these beams, thereby canceling out any WFID these beams have incurred from passing through first wavelength filter  403 . 
     Beam-swapping element  405  is optically coupled to receive fourth and fifth beams  104 ,  105  from second wavelength filter  407  and swap the two beams in position. Second compensation plate  410  is optically coupled to receive third and sixth beams  103 ,  106  from second wavelength filter  407 . Note that by optically coupling beam-swapping element  405  to horizontally polarized fourth and fifth beams  104 ,  105 , an incidence at a Brewster angle can be advantageously exploited for each of fourth and fifth beams  104 ,  105 , so as to reduce slight reflection at the surfaces of beam-swapping element  405 . Further note that all four beams incur no relative change in optical path lengths from passing through beam-swapping element  405  and second compensation plate  410 , as in the embodiment of FIG.  4 A. As such, upon entering third birefrigent element  408 , the polarizations, the relative (spatial) positions, and the relative optical path lengths of the four beams become the same as in the embodiment of FIG. 4A or FIG. 4B, so is the remaining operation. 
     Panels  473 ,  474 ,  475 ,  476 ,  477 ,  478 , and  479  depict the polarizations and relative positions of the light beams after passing through each optical element in the embodiment of FIG.  4 C. 
     In the embodiments of FIGS. 4A-4C, each of first, second, and third birefrigent elements  402 ,  404 ,  408  typically comprises a birefrigent material, such as calcite, rutile, lithium niobate, or a YVO 4  based crystal. First and second wavelength filters  403 ,  407 , each generally comprising a stacked plurality of birefrigent waveplates, are configured to be functionally equivalent, for the purposes of minimizing WFID as explained above. The beam-swapping plate can be in the form of a refractive hexagon plate, as exemplified in FIGS. 4A-4C, or in one of other embodiments illustrated in FIGS. 3B-3D. A birefrigent beam-swapping element along with an appropriate compensation plate, analogous to beam-swapping element  249  and compensation plate  250  described in the embodiment of FIG. 2D, can also be implemented with suitable compensation mechanisms designed for equalizing optical path lengths of the beams. A skilled artisan can devise a suitable beam-swapping element in accordance with the present invention for a given application. 
     As such, the optical interleaver thus described presents the first kind in the art in which various dispersion effects are substantially minimized. That is, not only the two polarization modes in each spectral set have substantially equalized optical path lengths upon being combined, each polarization mode ends up with a net zero of WFID upon exiting the interleaver. Such characteristics are highly desirable in fiber-optic networks. A further advantage of the optical interleavers of the present invention is that routing is accomplished while conserving substantially all optical energy available in the input WDM signal. That is, both the horizontal and vertical polarized components are used and recombined to provide the output signals, resulting very few loss through the interleaver. 
     To demonstrate the functionality and performance of the optical interleavers of the present invention, FIG. 7 displays a plot of transmission as a function of wavelength obtained experimentally from an exemplary optical interleaver configured according to the embodiment of FIG. 4A of the present invention. Each wavelength filter in this exemplary interleaver comprises two waveplates made of YVO 4 , where the first waveplate is about 1860 th  order with its optical axis oriented at 45 degrees with respect to the x-axis and the second waveplate is about 3720 th  order with its optical axis oriented at (−15) degrees with respect to the x-axis. The corresponding channel spacing is about 50 GHz. The experimental data shown in FIG. 7 demonstrate the superior performance of the optical interleaver of the present invention, in contrast to the performance of the prior art optical interleavers, such as the one shown FIG.  1 . 
     Optical interleavers as exemplified in the above embodiments operate as de-multiplexers. Multiplexers can also be provided by operating these interleavers in reverse. Furthermore, by suitably controlling the polarization rotation induced by the wavelength filters, these interleavers can be configured to operate as optical routers. 
     Those skilled in the art will recognize that the exemplary embodiments of optical interleavers depicted above are provided for the illustration purposes, to elucidate the principle and the utility of the present invention. Various alterations and substitutions can be made with departing from the principle and the scope of the present invention. For instance, the first birefrigent element in the above embodiments can be configured such that the first and second beams traverse as ordinary and extraordinary rays respectively. The wavelength filters can be designed to selectively rotate the polarizations of the third and fifth beams carrying the first spectral set by 90-degree respectively, while leaving the polarizations of the fourth and sixth beams carrying the second spectral set unchanged. Moreover, the second birefrigent element can be configured such that the third and sixth beams walk off as extraordinary rays, whereas the fourth and fifth beams walk through as ordinary rays. A similar arrangement can be made accordingly in the third birefrigent element, and so on. Those skilled in the art will appreciate the utility and versatility of the present invention, and design an optical interleaver in accordance with the present invention to best suit a given application. 
     Those skilled in the art will also recognize that although in the above exemplary embodiments an input WDM signal  500  comprising two spectral sets  501 ,  502  is used as a way of example to illustrate the functionality and operation of the optical interleavers of the present invention, it should not be construed in any way to limit the utility of the present inventions. That is, the optical interleavers of the present invention can be used to de-multiplex a WDM signal comprising more than two spectral sets, or multiplex two or more spectral sets into a WDM signal. A skilled artisan will know how to can design an optical interleaver in accordance with the present invention suitable for a given application. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alternations can be made herein without departing from the principle and the scope of the invention. Accordingly, the scope of the present invention should be determined by the following claims and their legal equivalents.