Patent Publication Number: US-6215923-B1

Title: Optical interleaver

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of U.S. patent application Ser. No. 09/495,020 filed Jan. 31, 2000, which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to optical communications systems. More particularly, it relates to an optical interleaver for multiplexing or de-multiplexing optical signals. 
     BACKGROUND ART 
     Optical wavelength division multiplexing (WDM) has gradually become the standard backbone network for fiber optic communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal 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 referred to generically 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 divides an 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, issued to Wu et al. on Dec. 2, 1997, which is incorporated herein by reference for all purposes. A WDM signal  500  containing two different channels  501 ,  502  enters interleaver  999  at an input port  11 . 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 channel to produce filtered signals  105  and  106 . For example wavelength filter  61  rotates wavelengths in the first channel  501  by 90° but does not rotate wavelengths in the second channel  502  at all. 
     The filtered signals  105  and  106  enter a second birefringent element  50  that vertically walks off the first channel into beams  107 ,  108 . The second channel forms beams  109 ,  110 . 
     A second wavelength filter  62  then selectively rotates the polarizations of signals  107 ,  108  but not signals  109 ,  110  thereby producing signals  111 ,  112 ,  113 ,  114 , having polarizations that are parallel each other. A second polarization rotator  41  then rotates the polarizations of signals  111  and  113 , but not  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  41  suitably configured to rotate the polarizations of signals  111 ,  113  but not  112 ,  114 . 
     Third birefringent element  70  combines signals  115  and  116 , into the first channel, which is coupled to output port  14 . Birefringent element  70  also combines signals  117  and  118  into the second 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 channels  501 ,  502  at ports  13  and  14  respectively; interleaver 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  has certain drawbacks. First, each port requires its own collimator. Three collimators take up space and require a relatively large walk-off distance for the signals. Consequently, birefringent elements  30 ,  50  and  70  tend to be both long and wide. Second, the number of components, particularly birefringent elements, tends to make interleaver  999  bulky, expensive and more massive. Generally, the greater the mass of interleaver  999 , the more unstable its operation. Third, the coupling distance, i.e., the distance between port  11  and ports  13 ,  14 , tends to be long, which increases insertion losses in interleaver  999 . Furthermore, each of the ports  11 ,  13  and  14  requires a separate collimator to couple the signals into and out of optical fibers. This adds the complexity and expense of interleaver  999 . 
     There is a need, therefore, for an improved optical interleaver that overcomes the above difficulties. 
     OBJECTS AND ADVANTAGES 
     Accordingly, it is a primary object of the present invention to provide a compact optical interleaver that uses fewer parts. It is a further object of the invention to provide an interleaver with a single collimator for coupling optical signals to or from two or more optical fibers. 
     SUMMARY 
     These objects and advantages are attained by an optical interleaver having a compact design that allows the use of smaller birefringent elements. The interleaver generally comprises a first birefringent element optically coupled to at least two input/output ports. A first polarization rotator is optically coupled to the first birefringent element. A second polarization rotator element is optically coupled to the first polarization rotator element. A wavelength filter is optically coupled to the second polarization rotator element, and a second birefringent element is optically coupled to the wavelength filter. The interleaver may be configured to operate as a multiplexer, a de-multiplexer, or a router. 
     One embodiment of the invention includes a deflection plate disposed along an optical path between the two input/output ports. The deflection plate deflects the path of one or more optical signals so that a single collimator may be used to coupled the signals into two or more fibers coupled to at least two of the input/output ports. A typical collimator configuration includes a lens optically coupled to the two or more optical fibers. In this embodiment, the deflection plate typically includes at least one wedge-shaped portion configured to deflect a first of the two or more optical signals into the lens. 
     An alternative embodiment of the invention includes a reflector optically coupled to the third polarization rotator. The reflector reflects an optical signal that travels from the input/output ports through the elements of the interleaver back through these elements in reverse order back to the input/output ports. This configuration reduces the number of components required for the interleaver and allows the use of shorter birefringent elements. 
    
    
     Further advantages of the various embodiments of the invention are depicted in the drawings and the detailed description that follows. 
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 depicts an isometric view of an optical interleaver according to the prior art; 
     FIG. 2 depicts an isometric view of an optical interleaver according to a first embodiment of the present invention; 
     FIG. 3A depicts an isometric view of an optical interleaver configured as a multiplexer according to a second embodiment of the present invention; 
     FIG. 3B depicts cross sectional schematic views of the polarization of light at different points in the optical interleaver of FIG. 3A; 
     FIG. 4A depicts an isometric view of an optical interleaver configured as a de-multiplexer according to a third embodiment of the present invention; 
     FIG. 4B depicts cross sectional schematic views of the polarization of light at different points in the optical interleaver of FIG. 4A; 
     FIG. 5A depicts an embodiment of a collimator used with the embodiments of the interleaver of the present invention; 
     FIG. 5B depicts an alternative embodiment of the collimator of FIG.  5 A. 
    
    
     DETAILED DESCRIPTION 
     Although the following detailed description contains many specifics 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 following preferred embodiment of the invention is set forth without any loss of generality to, and without imposing limitations upon, the claimed invention. 
     A first embodiment of the optical interleaver is shown in FIG.  2 . The interleaver  100  is configured much the same way as interleaver  999  of FIG.  1  and operates in a similar fashion. However, interleaver  100  includes a deflection plate  120  disposed between birefringent element  50  and input/output ports  13  and  14 . Deflection plate  120  may be located between birefrignent element  50  and wavelength filter  62 , or between wavelength filter  62  and rotator  41 , or between rotator  41  and birefringent element  70 , or between birefringent element  70  and ports  13 ,  14 . For the sake of example, FIG. 2 depicts deflection plate  120  as being located between birefringent element  50  and wavelength filter  62  in the path of beams  109 ,  110 . 
     Examples of suitable materials for construction of birefringent elements include calcite, rutile, lithium niobate, YVO 4  based crystals, and the like. 
     Suitable polarization rotators include reciprocal rotators such as waveplates and non-reciprocal rotators such as magneto-optic based Faraday rotators. 
     In an alternative embodiment depicted in the inset of FIG. 2, a wedge shaped deflection plate  122  combined with a dual fiber collimator  124  couples two different beams  130  and  132  to two different fibers  140 ,  142  coupled to ports  13  and  14 . Initially beams  130  and  132  are parallel to each other. Beam  130  is enters collimator  124  at normal incidence. Consequently, the path of beam  130  travels substantially undeflected to optical fiber  140 . Wedge  122  deflects beam  132  into collimator  124 . Collimator  124  generally comprises a lens  126  and a capillary  128 , which holds fibers  140 ,  142 . Capillary  128  usually has a front surface  129  that is polished at an angle α with respect to a plane perpendicular to beam  130 . Angle α is typically between 8 and 10°. This configuration reduces the amount of light from beams  130 ,  132  reflected back into fibers  140 ,  142 . 
     Lens  126  is typically a graded refractive index (GRIN) lens. Beam  132  strikes lens  126  at an angle. As a result, beam  132  follows a curved path  127  that brings beam  132  parallel to beam  130  at fiber  142 . The advantage of this particular configuration is that only one collimator  124  is used and fibers  140  and  142  may be located close to each other, thereby saving space. Furthermore beams  107 ,  108 ,  109 , and  110  can be closer together. Therefore, birefringent element  50  can be smaller further reducing the cost, bulk, and mass of interleaver  100 . 
     FIG. 3 depicts an embodiment of an optical interleaver  200  that uses even fewer components than interleaver  100  does. Interleaver  200  generally comprises three input/output ports  211 ,  213 , and  214 , first and second birefringent elements  230 ,  250 , a compensation plate  220 , a wavelength filter  262 , polarization rotators  240 ,  244 ,  246  and a reflector  280 . 
     Input/output ports  211 ,  213 , and  214  are optically coupled to first birefringent element  230 . Each of ports  211 ,  213 , and  214  may be used for either input or output of optical signals depending on the desired application of interleaver  200 . The distinction between input and output in the context of the present description is that an input couples light into interleaver  200  and an output couples light out of interleaver  200 . Preferably, all three ports are located adjacent to each other on the same side of birefringent element  230 , i.e., on the same side of interleaver  200 . This configuration saves space and simplifies the design and construction of interleaver  200 . 
     To understand the operation of interleaver  200  as a multiplexer, it is best to refer to FIGS. 3A and 3B simultaneously. Each of cross sections A through N depicted in FIG. 3B shows a view looking input/output ports  211 ,  213 ,  214  at different points in interleaver  200 . In the following discussion ports  211  and  214  are inputs and port  213  is an output. Signals  501  and  502  enter interleaver  200  at ports  211 ,  214  as shown at A in FIG.  3 B. Each of signals  501  and  502  carries a channel distinguished by a characteristic wavelength range. First birefringent element  230  spatially separates signals  501  into two polarized components  201 ,  203 , and signal  502  into two polarized components  203  and  204  by a horizontal walk-off effect as shown at B in FIG.  3 B. 
     Ordinary components  201  and  202  pass through compensation plate  220  while extraordinary components  203  and  204  do not. Compensation plate  220  thus compensates for the difference in phase resulting from the longer optical paths for the extraordinary components. 
     Components  201 ,  202 ,  203 , and  204  are coupled to first polarization rotator  240 . Rotator  240  is divided into first and second portions  241 ,  242  that rotate the polarization of light in opposite senses as shown at C′ in FIG. 3B. A first portion  241  of rotator  240  selectively rotates the polarization of components  201  and  202  by 45° in a counter-clockwise sense. A second portion  242  of rotator  240  rotates the polarization of components  203 , and  204  by 45° in a clockwise sense so components  201 ,  202 ,  203 , and  204  are all polarized parallel to each other as shown at C in FIG.  3 B. Preferably both portions of rotator  240  are reciprocal rotators, such as waveplates. A reciprocal rotator rotates the polarization of light in one sense when the light travels through it in a forward direction and in an opposite sense when light travels through it in the reverse direction. 
     All four components  201 ,  202 ,  203 , and  204  are coupled to second polarization rotator  244 . Preferably, rotator  244  is a non-reciprocal rotator such as a Faraday rotator. Rotator  244  rotates the polarization of all four components  201 ,  202 ,  203 , and  204  by 45° counter-clockwise so that they are all horizontal as shown at D in FIG.  3 B. The four components are then optically coupled to wavelength filter  262 . 
     Wavelength filter  262  selectively rotates the polarization of wavelengths in either the first or second channel. For example filter  262  rotates the polarization of wavelengths corresponding to signal  502  by 90°. As a result components  202  and  204  are rotated by 90° while components  201  and  203  are not. Thus, components  201  and  203  of signal  501  are horizontally polarized but components  202  and  204  of signal  502  are vertically polarized as shown at E in FIG.  3 B. 
     All four components  201 ,  202 ,  203 , and  204  are coupled to second birefringent element  250 . Birefringent element  250  has an optic axis  251  configured such that vertically polarized components  202 ,  204  are extraordinary rays and horizontally polarized components  201 ,  203  are ordinary rays. Second birefringent element  250  therefore vertically walks components  202  and  204  towards components  201 ,  203  as shown at F in FIG.  3 B. 
     To allow a shorter length of second birefringent element  250 , components  201 ,  202 ,  203 , and  204  are coupled to third rotator  246  and reflector  280 . Reflector  280  reflects components  201 ,  202 ,  203 , and  204  back through the optical elements of interleaver  200  in reverse order. Consequently, interleaver  200  requires only one wavelength filter and two birefringent crystals. Therefore, interleaver  200  uses fewer components, occupies less space, weighs less, and costs less to manufacture than prior interleavers. Rotator  246  is preferably a non-reciprocal rotator like rotator  244 . Rotator  246  rotates the polarizations of components  201 ,  202 ,  203 , and  204  by 45° clockwise as shown at G in FIG.  3 B. Upon reflection by reflector  280  the polarizations of components  201 ,  202 ,  203 , and  204  are configured as shown at H in FIG.  3 B. For cross sections H-N, the view is now along the direction of propagation, i.e., towards left in FIG.  3 A. 
     After reflection by reflector  280 , rotator  246  again rotates components  201 ,  202 ,  203  and  204 . Since rotator  246  is non-reciprocal, the components are again rotated clock-wise by 45° so that components  202 ,  204  are polarized horizontally and components  201 ,  203  are polarized vertically as shown at I in FIG.  3 B. As such, components  202 ,  204  are ordinary rays and components  201 ,  203  are extraordinary rays on the return trip through birefringent element  250 . Therefore, birefringent element  250  walks beams  201 ,  203  towards beams  202 ,  204 . Beam  201  combines with beam  202  to form beam  205  and beam  203  combines with beam  204  to form beam  205  as shown at J in FIG.  3 B. 
     Beams  205  and  206  then pass back through wavelength filter  262  where the horizontally polarized components corresponding to wavelengths in signal  502  are rotated by 90° such that all wavelengths in signals  205  and  206  are vertically polarized as shown at K in FIG.  3 B. Signals  205  and  206  then pass back through rotator  244 , which rotates their respective polarizations by 45° counter-clockwise as shown at L in FIG.  3 B. Rotator  240  then rotates the polarization of signal  205  counter-clockwise and the polarization of signal  206  clockwise as shown at M in FIG.  3 B. Beam  205  is therefore an extraordinary ray in birefringent element  230 . Vertically polarized beam  206  is an ordinary ray in birefringent element  230 . Beam  206  passes through compensation plate  220 , but beam  205  does not. Compensation plate  220  compensates for a phase difference induced by the longer optical path length for extraordinary beam  206 . Birefringent element  230  walks beam  205  towards  206  and combines them into output signal  503  as shown at N in FIG.  3 B. Output signal  503  is then coupled to output port  213 . 
     As described above, interleaver  200  operates as a multiplexer. By operating interleaver  200  in reverse, i.e. starting with a multiplexed signal containing two or more channels at input/output ports  213  interleaver  200  may be configured to operate as a de-multiplexer. 
     FIG. 4A depicts an embodiment of optical interleaver  200  configured to operate as a de-multiplexer. FIG. 4B depicts the operation of each element of interleaver  200  in this configuration. In this embodiment, port  213  serves as an input while ports  211  and  214  serve as outputs. WDM signal  500  containing channels  501 ′ and  502 ′ enters interleaver  200  at port  213  as shown at A in FIG.  4 B. Channel  501 ′ includes a vertically polarized component  201  and a horizontally polarized component  203 . Similarly channel  502 ′ includes a vertically polarized component  202  and a horizontally polarized component  204 . First birefringent element  230  spatially separates WDM signal  500  into horizontally and vertically polarized components  508 ,  504  by a horizontal walk-off effect as shown at B in FIG.  4 B. 
     Ordinary component  508  passes through compensation plate  220  while extraordinary component  504  does not. Compensation plate  220  thus compensates for the difference in phase resulting from the longer optical paths for extraordinary component  504 . 
     Components  508 , and  504  are respectively coupled to first and second portions  241 ,  242  of polarization rotator  240 . Portions  241 ,  242  rotate the polarization of light in opposite senses as shown a C′ in FIG.  4 B. First portion  241  of rotator  240  selectively rotates the polarization of component  508  by 45° in a counter-clockwise sense. Second portion  242  of rotator  240  rotates the polarization of component  504  by 45° in a clockwise sense so that components  508  and  504  are all polarized parallel to each other as shown at C in FIG.  4 B. Both portions  241 ,  242  of rotator  240  are preferably reciprocal rotators as described above. 
     Both components  508 , and  504  are coupled to non-reciprocal polarization rotator  244 . Unlike the multiplexer case described above, rotator  244  rotates the polarization of both components  508 , and  504  by 45° clockwise so that they are both vertical as shown at D in FIG.  4 B. The two components are then optically coupled to wavelength filter  262 . 
     Wavelength filter  262  selectively rotates the polarization of wavelengths in either the first or second channel. For example filter  262  rotates the polarization of wavelengths corresponding to signal  502 ′ by 90°. As a result components  202  and  204  are rotated by 90° while components  201  and  203  are not. Thus, components  201  and  203  of signal  501 ′ are vertically polarized but components  202  and  204  of signal  502 ′ are horizontally polarized as shown at E in FIG.  4 B. 
     Components  503  and  504 , i.e., components  201 ,  202 ,  203 , and  204 , are coupled to second birefringent element  250 . Birefringent element  250  has an optic axis  251  configured such that components  201  and  203  vertically walk off from components  202 ,  204  as shown at F in FIG.  4 B. 
     Components  201 ,  202 ,  203 , and  204  are coupled to non-reciprocal rotator  246  and reflector  280 . Rotator  246  rotates the polarizations of components  201 ,  202 ,  203 , and  204  by 45° clockwise as shown at G in FIG.  4 B. Reflector  280  reflects components  201 ,  202 ,  203 , and  204  back through the optical elements of interleaver  200  in reverse order. Upon reflection by reflector  280  the polarizations of components  201 ,  202 ,  203 , and  204  are configured as shown at H in FIG.  4 B. In cross-sections H-N, the view along the direction of propagation is now towards left in FIG.  4 A. 
     After reflection by reflector  280 , components  201 ,  202 ,  203 , and  204  are again rotated 45° clockwise by rotator  246  so that components  202 ,  204  are polarized vertically and components  201 ,  203  are polarized horizontally as shown at I in FIG.  4 B. As such, components  201 ,  203  are ordinary rays and components  202 ,  204  are extraordinary rays on the return trip through birefringent element  250 . Therefore, birefringent element  250  walks beams  202 ,  204  away from beams  201   203  as shown at J in FIG.  4 B. 
     Beams  201 ,  202 ,  203  and  204  then pass back through wavelength filter  262  where the horizontally polarized components  202 ,  204  corresponding to wavelengths in signal  502 ′ are rotated by 90°. Consequently, all wavelengths in signals  201 ,  202 ,  203  and  204  are horizontally polarized as shown at K in FIG.  4 B. Beams  201 ,  202 ,  203  and  204  then pass back through rotator  244 , which rotates their respective polarizations by 45° clockwise as shown a L in FIG.  4 B. Rotator  240  then rotates the polarizations of beams  201 ,  202  counter-clockwise and the polarizations of beams  203 ,  204  clockwise as shown at M in FIG.  4 B. Beams  201 ,  202  are therefore extraordinary rays in birefringent element  230  while beams  203 ,  204  are ordinary rays. Therefore, birefringent element  230  walks beams  201 ,  202  towards beams  203 ,  204 . Ordinary beams  203 ,  204  pass through compensation plate  220  but extraordinary beams  201 ,  202  do not, thereby compensating for phase differences. Beam  201  combines with beam  203  to form output signal  501  and beam  202  combines with beam  204  to form output beam  502  as shown at N in FIG.  4 B. Output signals  501 ′,  502 ′ are then coupled to output port  211 ,  214  respectively. 
     Those skilled in the art will recognize that interleaver  200  may operate as both a multiplexer or demultiplexer whether or not the sense of rotation of rotator  244  is reversed. This may be accomplished, for example, by reversing the direction of the magnetic field if rotator  244  is a Faraday rotator. Those skilled in the art will also recognize that, by suitably controlling the polarization rotation induced by rotators  240  and  244 , interleaver  200  may be configured to operate as a router. 
     In alternative embodiments depicted in FIGS. 5A and 5B input/output ports  211 ,  213  and  214  can be combined with a wedge shaped deflection plate  320  and a triple fiber collimator  324  to couple three different beams  331 ,  332 , and  333  to three different fibers  341 ,  342 , and  343 . Initially beams  331 ,  332  and  333  are parallel to each other. As depicted in FIG. 5A, deflection plate  320  includes two wedge-shaped portions  322  and a central portion  323  having parallel front and back sides. Beam  332  enters central portion  323  deflection plate  320  at normal incidence as shown in FIG.  5 A. Beam  332  is, therefore, undeflected and travels substantially undeflected to optical fiber  342 . Wedge shaped portions  322  deflect beams  331  and  333  into collimator  324 . Alternatively, as depicted in FIG. 5B, a deflection plate  321  may include two wedge-shaped portions  322  with a central gap  325  in between. beam  332  passes undeflected through gap  325 . Wedge shaped portions  322  deflect beams  331  and  333  into collimator  324 . Deflection plate  320  or  321  may be made in a single piece or multiple pieces. Furthermore, deflection plate  320  or  321  may be axially symmetric about an axis defined, for example, by the path of central beam  332 . 
     Collimator  324  generally comprises a lens  326  and a capillary  328 . Lens  326  is typically a graded refractive index (GRIN) lens. Capillary  328  holds fibers  341 ,  342 ,  343 . Beam  332  strikes a central portion of lens  326  at normal incidence. As a result, beam  332  proceeds undeflected to fiber  342 . Beams  331  and  333  strike lens  326  at an angle. As a result, beams  331  and  333  follow curved paths  327 ,  329  that bring beams  331  and  333  parallel to beam  332  at fibers  341  and  343 . Although FIGS. 5A and 5B depict apparatus for three beams to three fibers, the basic configuration may be readily adapted to any number of beams and fibers. 
     It will be clear to one skilled in the art that the above embodiment may be altered in many ways without departing from the scope of the invention. For example, the deflection plate may be located anywhere along the optical path between an input and the corresponding output. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.