Abstract:
A three-port circulator includes a non-reciprocal rotating element, a first birefringent wedge, and a second birefringent wedge. The non-reciprocal rotating element has a rotation axis and rotates the polarization of light passing through it by an predetermined angle. The first birefringent wedge has a first optical axis substantially perpendicular to the rotation axis. The second birefringent wedge has a second optical axis substantially perpendicular to both the rotation axis and the first optical axis. The second birefringent wedge is optically coupled between the non-reciprocal rotating element and the first birefringent wedge.

Description:
The present invention relates generally to optical technology. 
     BACKGROUND OF THE INVENTION 
     Optical circulators are commonly used in optical communication systems and optical measurement systems.  FIG. 1  shows a three-port circulator  10  that has ports  1 ,  2 , and  3 . Each of ports  1 ,  2 , and  3  can be coupled to a Polarization Maintenance (“PM”) fiber. As shown in  FIG. 1 , a polarized optical signal S 1  entering port  1  exits from port  2  as a polarized optical signal S 2 . A polarized optical signal S 2 ′ entering port  2  exits from port  3  as a polarized optical signal S 3 . 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention provides a three-port circulator. The three-port circulator includes a non-reciprocal rotating element, a first birefringent wedge, and a second birefringent wedge. The non-reciprocal rotating element has a rotation axis and is adapted to rotate a polarization of light passing therethrough by a predetermined angle. The first birefringent wedge has a first optical axis substantially perpendicular to the rotation axis. The second birefringent wedge has a second optical axis substantially perpendicular to both the rotation axis and the first optical axis. The second birefringent wedge is optically coupled between the non-reciprocal rotating element and the first birefringent wedge. The three-port circulator can also include a first lens optically coupled to the first wedge. The three-port circulator can also include a second lens optically coupled to the second wedge. The non-reciprocal rotating element in the three-port circulator can be a Faraday rotator. 
     In another aspect, the invention provides a method for constructing a three-port circulator having a first port, a second port and a third port. The method includes the step of providing a non-reciprocal rotating element for rotating a polarization of light passing therethrough by a first angle with respect to a rotation axis. The method includes the step of providing a first birefringent wedge having a first optical axis. The method includes the step of providing a second birefringent wedge having a second optical axis forming a second angle with the first optical axis. The method includes the step of directing an input polarized optical signal received from the first port to pass sequentially through the first birefringent wedge, the second birefringent wedge, and the non-reciprocal rotating element, to provide an output polarized optical signal directed to the second port. The method includes the step of directing an input polarized optical signal received from the second port to pass sequentially through the non-reciprocal rotating element, the second birefringent wedge and the first birefringent wedge, to provide an output polarized optical signal directed to the third port. 
     Aspects of the invention can include one or more of the following advantages. Implementations of the invention may provide a three-port circulator that has small insertion loss, compact size, and reduced manufacturing cost. Other advantages will be readily apparent from the attached figures and the description below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a three-port circulator  10  including ports  1 ,  2 , and  3 . 
         FIG. 2   a  illustrates an implementation of a three-port circulator. 
         FIG. 2   b  illustrates a specific configuration of the birefringent wedges and the Faraday rotator shown in  FIG. 2   a.    
         FIGS. 3   a  and  3   b  illustrate that a polarized optical signal S 1  exiting from PM fiber  201  enters PM fiber  202  as a polarized optical signal S 2 . 
         FIGS. 4   a  and  4   b  illustrate that a polarized optical signal S 2 ′ exiting from PM fiber  202  enters PM fiber  203  as a polarized optical signal S 3 . 
         FIGS. 5   a  and  5   b  illustrate that a polarized optical signal S 1  exiting from PM fiber  203  enters PM fiber  202  as a polarized optical signal S 2 . 
         FIGS. 6   a  and  6   b  illustrate that a polarized optical signal S 2 ′ exiting from PM fiber  202  enters PM fiber  201  as a polarized optical signal S 3 . 
         FIG. 7   a  illustrates an implementation of birefringent wedges  230  and  240  using birefringent crystal materials with n e  larger than n o . 
         FIG. 7   b  illustrates an implementation of birefringent wedges  230  and  240  using birefringent crystal materials with n e  smaller than n o . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to an improvement in optical technology. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the invention will be readily apparent to those skilled in the art and the generic principals herein may be applied to other implementations. Thus, the present invention is not intended to be limited to the implementations shown, but is to be accorded the widest scope consistent with the principals and features described herein. 
     The present invention will be described in terms of a three-port circulator having specific components having specific configurations. Similarly, the present invention will be described in terms of components having specific relationships, such as distances or angles between components. However, one of ordinary skill in the art will readily recognize that the devices and systems described can include other components having similar properties, other configurations, and other relationships between components. 
       FIG. 2   a  illustrates an implementation of a three-port circulatior  200 . Three-port circulator  200  includes a birefringent wedge  230 , a birefringent wedge  240 , and a non□reciprocal rotating element such as Faraday rotator  250 . Three-port circulator  200  can includes lens  220  for optically coupling PM fibers  201  and  203  to birefringent wedge  230 , and lens  260  for optically coupling a PM fiber  202  to Faraday rotator  250 . The positions of PM fibers  201  and  203  can be fixed with a capillary  210 . The position of PM fiber  202  can be fixed with a capillary  270 . 
     Birefringent wedges  230  and  240  are in the form of tapered plates. Surface  232  of birefringent wedge  230  faces surface  242  of birefringent wedge  240 . In one implementation of three-port circulator  200 , surface  232  substantially parallels surface  242 . Surface  232  can contact surface  242  directly, or indirectly through other optical medias (including air). 
     In  FIGS. 2   a  and  2   b,  a coordinate system including the x-direction, the y-direction and the z-direction is illustrated. As shown in  FIG. 2   b,  the optical axis of birefringent wedge  230  is in the x-direction. The optical axis of birefringent wedge  240  is in the y-direction. Faraday rotator  250  is designed in such a way that, when light passes through Faraday rotator  250  either in the positive or the negative z-direction, the polarization of the light will be rotated 45 degrees with respect to the positive z-axis. Surface  262  of lens  260  faces Faraday rotator  250 . Three-port circulator  200  has a principal direction that is in the positive z-direction. Three-port circulator  200  also has a first-port direction that is in the z−αy direction, and a third-port direction that is in the −z−β y direction, where α and β are positive numbers. 
       FIGS. 3   a  and  3   b  illustrate that polarized optical signal S 1  exiting from PM fiber  201  enters PM fiber  202  as polarized optical signal S 2 . In more detail, polarized optical signal S 1  exits from PM fiber  201  as light  311 ( e ) with the x-polarization, and enters birefringent wedge  230  as an e-ray in an input direction (i.e., the z−αy direction). Light  311  ( e ) is refracted at surfaces  232  and  242 , and become light  312 ( o ) traveling in the principal direction (i.e., the positive z-direction). Light  312 ( o ) enters birefringent wedge  240  as an o-ray with the x-polarization, passes through Faraday rotator  250 , and become light  313  with the x+y polarization traveling in the principal direction. Light  313  enters PM fiber  202  through lens  260  as polarized optical signal S 2 . 
       FIGS. 4   a  and  4   b  illustrate that polarized optical signal S 2 ′ exiting from PM fiber  202  enters PM fiber  203  as polarized optical signal S 3 . In more detail, polarized optical signal S 2 ′ exits from PM fiber  202  as light  411  with the x+y polarization traveling in the reverse principal direction (i.e., the negative z-direction). Light  411  with the x+y polarization passes through Faraday rotator  250  and become light  412 ( e ) with the y-polarization. Light  412 ( e ) enters birefringent wedge  240  as an e-ray. Light  412 ( e ) is refracted at surfaces  242  and  232 , and become light  413 ( o ). Light  413 ( o ) enters birefringent wedge  230  as an o-ray with the y-polarization and exits from birefringent wedges  230  in an output direction (i.e., the −z−β y direction). Light  413 ( o ) enters PM fiber  203  through lens  220  as polarized optical signal S 3 . 
     In the implementation of three-port circulator in  FIG. 2   a,  Faraday rotator  250  rotates the polarization of the light by an angle of substantially positive 45 degrees. In an alternative implementation, a Faraday rotator that rotates the polarization of the light by an angle of substantially negative 45 degrees can also be used. 
     In the implementation of three-port circulator in  FIG. 2   a,  birefringent wedge  230  has the optical axis in the x-direction, and birefringent wedge  240  has the optical axis in the y-direction. In an alternative implementation, birefringent wedge  230  can have an optical axis in the cos(φ)x +sin(φ) y direction, and birefringent wedges  240  can have an optical axis in the cos(φ+90)x+sin(φ+90) y direction, where φ is any arbitrary angle. 
     In  FIGS. 3   a  and  3   b,  polarized optical signal S 1  exiting from PM fiber  201  enters birefringent wedge  230  as an e-ray; in  FIGS. 4   a  and  4   b,  light exits from birefringent wedge  230  as an o-ray and enters PM fiber  203  as polarized optical signal S 3 . In an alternative implementation, polarized optical signal S 1  exiting from PM fiber  203  can enter birefringent wedge  230  as an o-ray; light can exit from birefringent wedge  230  as an e-ray and enter PM fiber  201  as polarized optical signal S 3 . 
       FIGS. 5   a  and  5   b  illustrate that polarized optical signal S 1  exiting from PM fiber  203  enters PM fiber  202  as polarized optical signal S 2 . In more detail, polarized optical signal S 1  exits from PM fiber  203  as light  511 ( o ) with the y-polarization, and enters birefringent wedge  230  as an o-ray in an input direction (i.e., the z+βy direction). Light  511 ( o ) is refracted at surfaces  232  and  242 , and become light  512 ( e ) traveling in the principal direction (i.e., the positive z-direction). Light  512 ( e ) enters birefringent wedge  240  as an e-ray with the y-polarization, passes through Faraday rotator  250 , and become light  513  with the x-y polarization traveling in the principal direction. Light  513  enters PM fiber  202  through lens  260  as polarized optical signal S 2 . 
       FIGS. 6   a  and  6   b  illustrate that polarized optical signal S 2 ′ exiting from PM fiber  202  enters PM fiber  201  as polarized optical signal S 3 . In more detail, polarized optical signal S 2 ′ exits from PM fiber  202  as light  611  with the x-y polarization traveling in the reverse principal direction (i.e., the negative z-direction). Light  611  with the x-y polarization passes through Faraday rotator  250  and become light  612 ( o ) with the x-polarization. Light  612 ( o ) enters birefringent wedge  240  as an o-ray. Light  612 ( o ) is refracted at surfaces  242  and  232 , and become light  613 ( e ). Light  613 ( e ) enters birefringent wedge  230  as an e-ray with the x-polarization and exits from birefringent wedge  230  in an output direction (i.e., the −z+αy direction). Light  613 ( e ) enters PM fiber  201  through lens  220  as polarized optical signal S 3 . 
     Birefringent wedges  230  and  240  can be constructed from birefringent crystal materials, such as, calcite, rutile, lithium niobate or yttrium orthvanadate. 
     A birefringent crystal material in general has refractive indexes n e  for e-rays and n o  for o-rays. Birefringent wedges  230  and  240  can be constructed using birefringent crystal materials with indexes n e  larger than n o , or birefringent crystal materials with indexes n e  smaller than n o    
       FIGS. 7   a  and  7   b  illustrate implementations of birefringent wedges  230  and  240  in the form of tapered plate. Surface  232  of birefringent wedge  230  substantially parallels surface  242  of birefringent wedge  240 . The tapering angle of birefringent wedges  230  and  240  is  102 . 
       FIG. 7   a  illustrates an implementation of birefringent wedges  230  and  240  using birefringent crystal materials with indexes n e  larger than n o .  FIG. 7   a  also illustrates the paths traveled by e-ray  311 ( e ) and o-ray  413 ( o ). E-ray  311 ( e ) is incident upon surface  232  of birefringent wedge  230  in the cos(θ e ) z−sin(θ e ) y direction and exits from birefringent wedge  240  in the positive z-direction. Here θ e  satisfies equation n e  sin( χ −θ e )=n o  sin( χ ). O-ray  413 ( o ) enters wedge  230  from surface  232  in the −cos(θ e ) z−sin(θ o ) y direction. Here θ o  satisfies equation n o  sin( χ +θ o )=n e  sin( 102 ). 
       FIG. 7   b  illustrates an implementation of birefringent wedges  230  and  240  using birefringent crystal materials with indexes n e  smaller than no  FIG. 7   b  also illustrates the paths traveled bye-ray  311 ( e)  and o-ray  413 ( o ). E-ray  311 ( e ) is incident upon surface  232  of birefringent wedge  230  in the cos(θ e )z−sin(θ e ) y direction and exits from birefringent wedge  230  from surface  232  in the positive z-direction. Here θ e  satisfies equation n e  sin( 102 +θ e )=n o  sin( χ ). O-ray  413 ( o ) enters wedge  240  in the −cos(θ o )z−sin(θ o ) y direction. Here θ o  satisfies equation n o  sin( χ −θ o )=n e  sin( χ ). 
     A method and system has been disclosed for providing three-port circulators. Although the present invention has been described in accordance with the implementations shown, one of ordinary skill in the art will readily recognize that there could be variations to the implementations and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.