Abstract:
An extendable four-port circulator includes a middle birefringent crystal, a first birefringent crystal, a first non-reciprocal device, a second birefringent crystal, and a second non-reciprocal device. The first non-reciprocal device is coupled to the first birefringent crystal. The second non-reciprocal device is coupled to the second birefringent crystal. The middle birefringent crystal includes a first surface, a second surface, a third surface, and a fourth surface. The first surface is coupled to the first non-reciprocal device. The second surface is coupled to the second non-reciprocal device. The third surface defines first and second extension interfaces. The fourth surface defines third and fourth extension interfaces. A multi-port circulator includes at least one extendable four-port circulator.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    A circulator is often used with other optical devices to achieve certain optical functions. For example, a circulator can be used with a Brag Grating to extract an optical signal with a particular wavelength from a Wavelength Division Multiplexing (“WDM”) optical signal. FIG. 1 shows a four-port circulator  444  with four ports  1 ,  2 ,  3 , and  4 . An optical signal entering port  1  exits from port  2 , while an optical signal entering port  2  exits from port  3 , and an optical signal entering port  3  exits from port  4 .  
         SUMMARY OF THE INVENTION  
         [0002]    In one aspect, the invention provides an extendable four-port circulator. The extendable four-port circulator includes a middle birefringent crystal, a first birefringent crystal, a first non-reciprocal device, a second birefringent crystal, and a second non-reciprocal device. The first non-reciprocal device is coupled to the first birefringent crystal. The second non-reciprocal device is coupled to the second birefringent crystal. The middle birefringent crystal includes a first surface, a second surface, a third surface, and a fourth surface. The first surface is coupled to the first non-reciprocal device. The second surface is coupled to the second non-reciprocal device. The third surface defines a first and a second extension interface. The fourth surface defines a third and a fourth extension interface.  
           [0003]    In another aspect, the invention provides a multi-port circulator. The multi-port circulator includes a middle birefringent crystal, a first common non-reciprocal device, a second common non-reciprocal device, a first common birefringent crystal, and a second common birefringent crystal. The first common non-reciprocal device is coupled to the middle birefringent crystal. The second common non-reciprocal device is coupled to the middle birefringent crystal. The first common birefringent crystal is coupled to the first common non-reciprocal device. The second common birefringent crystal is coupled to the second common non-reciprocal device.  
           [0004]    In another aspect, the invention provides a multi-port circulator. The multi-port circulator includes a middle birefringent crystal, a first and a second common non-reciprocal device, a first and a third side birefringent crystal, and a second and a fourth side birefringent crystal. The first and the second common non-reciprocal devices each are coupled to the middle birefringent crystal. The first and the third side birefringent crystals each are coupled to the first common non-reciprocal device. The second and the fourth side birefringent crystals each are coupled to the second common non-reciprocal device.  
           [0005]    In another aspect, the invention provides a multi-port circulator. The multi-port circulator includes a middle birefringent crystal, a first and a third non-reciprocal device, a second and a fourth non-reciprocal device, a first side birefringent crystal, a second side birefringent crystal, a third side birefringent crystal, and a fourth side birefringent crystal. The first and the third non-reciprocal devices each are coupled to the middle birefringent crystal. The second and the fourth non-reciprocal devices each are coupled to the middle birefringent crystal. The first side birefringent crystal is coupled to the first non-reciprocal device. The second side birefringent crystal is coupled to the second non-reciprocal device. The third side birefringent crystal is coupled to the third non-reciprocal device. The fourth side birefringent crystal is coupled to the fourth non-reciprocal device.  
           [0006]    Aspects of the invention can include one or more of the following advantages. An extendable four-port circulator in an implementation of the instant invention may be cascaded to form a multi-port circulator. Other advantages will be readily apparent from the attached figures and the description below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 shows a four-port circulator.  
         [0008]    [0008]FIG. 2 a  illustrates a twelve-port circulator having ports  1001 - 1012 .  
         [0009]    [0009]FIG. 2 b  illustrates an extendable four-port circulator having four ports and four extension interfaces.  
         [0010]    [0010]FIG. 2 c  illustrates three extendable four-port circulators cascaded together to form a twelve-port circulator.  
         [0011]    [0011]FIG. 2 d  illustrates the position and orientation of the components of an extendable four port PM circulator shown in FIG. 2 c.    
         [0012]    [0012]FIG. 3 a ( 1 )-FIG. 3 a ( 3 ) illustrate that an optical signal introduced at port  1  is separated into two light beams that are recombined to exit from port  2 .  
         [0013]    [0013]FIG. 3 b ( 1 )-FIG. 3 b ( 3 ) illustrate that an optical signal introduced at port  2  is separated into two light beams that are recombined to exit from port  3 .  
         [0014]    [0014]FIG. 3 c ( 1 )-FIG. 3 c ( 3 ) illustrate that an optical signal introduced at port  3  is separated into two light beams that are recombined to exit from port  4 .  
         [0015]    [0015]FIG. 3 d ( 1 )-FIG. 3 d ( 3 ) illustrate that an optical signal introduced at port  4  is separated into two light beams that exit from two of the extension interfaces.  
         [0016]    [0016]FIG. 3 e ( 1 )-FIG. 3 e ( 3 ) illustrate that light beams entering two of the extension interfaces are combined to exit from port  1 .  
         [0017]    [0017]FIG. 4 a -FIG. 4 e  summarize the optical paths in the y-z plane traveled by the light beams respectively in FIG. 3 a -FIG. 3 e.    
         [0018]    [0018]FIG. 4 f  illustrates the optical paths in the x-z plane traveled by the light beams in FIG. 4 a -FIG. 4 e.    
         [0019]    [0019]FIG. 5 illustrates three extendable four-port circulators of FIG. 2 cascaded together to form a twelve-port circulator.  
         [0020]    [0020]FIG. 6 and FIG. 7 illustrate alternative implementations of the twelve-port circulator of FIG. 5.  
         [0021]    [0021]FIG. 8 a  and FIG. 8 b  illustrate an implementation of non-reciprocal device  120  having one half wave plate and two Faraday rotators.  
         [0022]    [0022]FIG. 9 a  and FIG. 9 b  illustrate an implementation of non-reciprocal device  120  having two half wave plates and one Faraday rotator.  
         [0023]    [0023]FIG. 10 a  and FIG. 10 b  illustrate an implementation of non-reciprocal device  160  having one half wave plate and two Faraday rotators.  
         [0024]    [0024]FIG. 11 a  and FIG. 11 b  illustrate an implementation of non-reciprocal device  160  having two half wave plates and one Faraday rotator.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    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 principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein.  
         [0026]    The present invention will be described in terms of an extendable four-port circulator and a twelve-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 this method and system will operate effectively for other components having similar properties, other configurations, and other relationships between components. In the instant application, the implementations of an extendable four-port circulator are described. Two extendable four-port circulators can be cascaded to form an eight-port circulator, and three extendable four-port circulators can be cascaded to form a twelve-port circulator.  
         [0027]    [0027]FIG. 2 a  shows a twelve-port circulator  1000  that includes twelve ports  1001 - 1012 . As shown in FIG. 2 a , an optical signal entering port  1001  will exit from port  1002 , and an optical signal entering port  1002  will exit from port  1003 . Similarly, an optical signal entering port  1003 ,  1004 ,  1005 ,  1006 ,  1007 ,  1008 ,  1009 ,  1010 , and  1011  will exit, respectively, from port  1004 ,  1005 ,  1006 ,  1007 ,  1008 ,  1009 ,  1010 ,  1011 , and  1012 .  
         [0028]    Twelve-port circulator  1000  can be constructed with a number of different methods. One possible implementation of a twelve-port circulator is to cascade three extendable four-port circulators. An extendable four-port circulator  555  described in the instant application is shown in FIG. 2 b . Extendable four-port circulator  555  includes four ports  1 ,  2 ,  3 , and  4 , along with extension interfaces  0   a ,  0   b ,  5   a , and  5   b . An optical signal entering port  1  will exit from port  2 , an optical signal entering port  2  will exit from port  3 , and an optical signal entering port  3  will exit from port  4 . In addition, an optical signals entering extension interfaces  0   a  and  0   b  will be merged and exit from port  1 , and an optical signal entering port  4  will be split into two optical signals exiting respectively from ports  5   a  and  5   b.    
         [0029]    [0029]FIG. 2 c  shows that three extendable four-port circulators  555 ′,  555 , and  555 ″ are cascaded together to form a twelve-port circulator  1000 . Interfaces  5   a ′ and  5   b ′ of circulator  555 ′ are respectively coupled to interfaces  0   a  and  0   b  of circulator  555 , and interfaces  5   a  and  5   b  of circulator  555  are respectively coupled to interfaces  0   a ″ and  0   b ″ of circulator  555 ″. Ports  1 ′,  2 ′,  3 ′, and  4 ′ of four-port circulator  555 ′ are respectively equivalent to ports  1001 ,  1002 ,  1003 , and  1004  of twelve-port circulator  1000 ; ports  1 ,  2 ,  3 , and  4  of four-port circulator  555  are respectively equivalent to ports  1005 ,  1006 ,  1007 , and  1008  of twelve-port circulator  1000 ; and ports  1 ″,  2 ″,  3 ″, and  4 ″ of four-port circulator  555 ″ are respectively equivalent to ports  1009 ,  1010 ,  1011 , and  1012  of twelve-port circulator  1000 .  
         [0030]    [0030]FIG. 2 d  illustrates an implementation of an extendable four-port circulator  555 . Circulator  555  includes dual fiber collimator  100 , birefringent crystal  110 , non-reciprocal device  120 , wedge  130 , birefringent crystal  140 , wedge  150 , non-reciprocal device  160 , birefringent crystal  170 , and dual fiber collimator  200 . Each of dual fiber collimators  100  and  200  can be coupled to two fibers (not shown). Circulator  555  includes four ports  1 ,  2 ,  3 , and  4 , along with extension interfaces  0   a ,  0   b ,  5   a , and  5   b . Two couplings at collimator  100  constitute respectively ports  1  and  3 , and two couplings at collimator  200  constitute respectively ports  2  and  4 . Two areas on surface  142  of birefringent crystal  140  constitute extension interfaces  0   a  and  0   b , and two areas on surface  144  of birefringent crystal  140  constitute extension interfaces  5   a  and  5   b.    
         [0031]    A light beam may enter one of four regions of a given component in extendable four-port circulator  555 . The four regions are labeled as quadrant I, II, III, and IV, as shown in FIG. 2 d . The x-direction, y-direction and the z-direction are also shown in the figure. The positive z-direction is along the propagation direction of a light beam introduced at dual fiber collimator  100 .  
         [0032]    [0032]FIGS. 3 a ( 1 ),  3   a ( 2 ), and  3   a ( 3 ) illustrate that an optical signal introduced at port  1  is separated into light beams  12   a  and  12   b . Light beams  12   a  and  12   b  are recombined and exit from port  2 . FIG. 3 a ( 1 ) is a perspective view, FIG. 3 a ( 2 ) is a planar view on the y-z plane, and FIG. 3 a ( 3 ) is a planar view on the x-z plane.  
         [0033]    The optical signal introduced at port  1  is separated into light beam  12   a  with the y-polarization and light beam  12   b  with the x-polarization in birefringent crystal  110 . Light beam  12   a  is not deflected and exits from quadrant II of birefringent crystal  110 . Light beam  12   b  is deflected in the positive x-direction and exits from quadrant I of birefringent crystal  110 . After exiting from birefringent crystal  110 , light beams  12   a  and  12   b  travel in the positive z-direction and somewhat in the positive y-direction.  
         [0034]    Light beam  12   a  enters quadrant II of non-reciprocal device  120  with the y-polarization, and exits from quadrant II with the x-polarization. After passing through quadrant II of wedge  130 , light beam  12   a  is deflected to travel essentially in alignment with the positive z-direction. Light beam  12   a  then passes through quadrant II of birefringent crystal  140 , without being deflected. After passing through quadrant II of wedge  150 , light beam  12   a  is deflected to travel in the positive z-direction and somewhat in the negative y-direction (FIG. 3 a ( 2 )). Thereafter, light beam  12   a  enters quadrant II of non-reciprocal device  160  with the x-polarization and exits from quadrant II with the x-polarization. Finally, light beam  12   a  enters quadrant II of birefringent crystal  170 , is deflected in the positive x-direction, and enters dual fiber collimator  200  with the x-polarization.  
         [0035]    Light beam  12   b  enters quadrant I of non-reciprocal device  120  with the x-polarization, and exits from quadrant I with the x-polarization. After passing through quadrant I of wedge  130 , light beam  12   b  is deflected to travel essentially in alignment with the positive z-direction. Light beam  12   b  then passes through quadrant I of birefringent crystal  140 , without being deflected. After passing through quadrant I of wedge  150 , light beam  12   b  is deflected to travel in the positive z-direction and somewhat in the negative y-direction (FIG. 3 a ( 2 )). Thereafter, light beam  12   b  enters quadrant I of non-reciprocal device  160  with the x-polarization, and exits from quadrant I with the y-polarization. Finally, light beam  12   b  passes through quadrant I of birefringent crystal  170 , without being deflected, and enters dual fiber collimator  200  with the y-polarization.  
         [0036]    Light beams  12   a  and  12   b  enter collimator  200  respectively with the x-polarization and the y-polarization, are combined at collimator  200 , and exit from port  2 .  
         [0037]    [0037]FIGS. 3 b (l),  3   b ( 2 ), and  3   b ( 3 ) illustrate that an optical signal introduced at port  2  is separated into light beams  23   a  and  23   b . Light beams  23   a  and  23   b  are recombined and exit from port  3 . FIG. 3 b ( 1 ) is a perspective view, FIG. 3 b ( 2 ) is a planar view on the y-z plane, and FIG. 3 b ( 3 ) is a planar view on the x-z plane.  
         [0038]    The optical signal introduced at port  2  is separated into light beam  23   a  with the x-polarization and light beam  23   b  with the y-polarization. Light beam  23   a  is deflected in the negative x-direction and exits from quadrant II of birefringent crystal  170 . Light beam  23   b  is not deflected and exits from quadrant I of birefringent crystal  170 . After exiting from birefringent crystal  170 , light beams  23   a  and  23   b  travel in the negative z-direction and somewhat in the positive y-direction (FIG. 3 b ( 2 )).  
         [0039]    Light beam  23   a  enters quadrant II of non-reciprocal device  160  with the x-polarization, and exits from quadrant II with the y-polarization. After passing through quadrant II of wedge  150 , light beam  23   a  is deflected to travel essentially in alignment with the negative z-direction. Light beam  23   a  then enters quadrant II of birefringent crystal  140 , is deflected in the negative y-direction, and exits from quadrant III of birefringent crystal  140 . After passing through quadrant III of wedge  130 , light beam  23   a  is deflected to travel in the negative z-direction and somewhat in the positive y-direction. Thereafter, light beam  23   a  enters quadrant III of non-reciprocal device  120  with the y-polarization, and exits from quadrant III with the y-polarization. Finally, light beam  23   a  passes through quadrant III of birefringent crystal  110 , without being deflected, and enters dual fiber collimator  100  with the y-polarization.  
         [0040]    Light beam  23   b  enters quadrant I of non-reciprocal device  160  with the y-polarization, and exits from quadrant I with the y-polarization. After passing through quadrant I of wedge  150 , light beam  23   b  is deflected to travel essentially in alignment with the negative z-direction. Light beam  23   a  then enters quadrant I of birefringent crystal  140 , is deflected in the negative y-direction, and exits from quadrant IV of birefringent crystal  140 . After passing through quadrant IV of wedge  150 , light beam  23   b  is deflected to travel in the negative z-direction but leaning toward the positive y-direction. Thereafter, light beam  23   b  enters quadrant IV of non-reciprocal device  120  with the y-polarization, and exits the x-polarization. Finally, light beam  23   b  enters quadrant IV of birefringent crystal  110 , is deflected in the negative x-direction, and enters dual fiber collimator  100  with the x-polarization.  
         [0041]    Light beams  23   a  and  23   b  enter collimator  100  respectively with the y-polarization and the x-polarization, are combined at collimator  100 , and exit from port  3 .  
         [0042]    [0042]FIGS. 3 c ( 1 ),  3   c ( 2 ), and  3   c ( 3 ) illustrate that an optical signal introduced at port  3  is separated into light beams  34   a  and  34   b . Light beams  34   a  and  34   b  are recombined and exit from port  4 . FIG. 3 c ( 1 ) is a perspective view, FIG. 3 c ( 2 ) is a planar view on the y-z plane, and FIG. 3 c ( 3 ) is a planar view on the x-z plane.  
         [0043]    The optical signal introduced at port  3  is separated into light beam  34   a  with the y-polarization and light beam  34   b  with the x-polarization by birefringent crystal  110 . Light beam  34   a  is not deflected and exits from quadrant III of birefringent crystal  110 . Light beam  34   b  is deflected in the positive x-direction and exits from quadrant IV of birefringent crystal  110 . After exiting from birefringent crystal  110 , light beams  34   a  and  34   b  travel in the positive z-direction and somewhat in the negative y-direction (FIG. 3 c ( 2 )).  
         [0044]    Light beam  34   a  enters quadrant III of non-reciprocal device  120  with the y-polarization, and exits from quadrant III with the x-polarization. After passing through quadrant III of wedge  130 , light beam  34   a  is deflected to travel essentially in alignment with the positive z-direction. Light beam  34   a  then passes through quadrant III of birefringent crystal  140 , without being deflected. After passing through quadrant III of wedge  150 , light beam  34   a  is deflected to travel in the positive z-direction and somewhat in the positive y-direction. Thereafter, light beam  34   a  enters quadrant III of non-reciprocal device  160  with the x-polarization, and exits from quadrant III with the x-polarization. Finally, light beam  34   a  enters quadrant II of birefringent crystal  170 , is deflected in the positive x-direction, and enters dual fiber collimator  200  with the x-polarization.  
         [0045]    Light beam  34   b  enters quadrant IV of non-reciprocal device  120  with the x-polarization, and exits from quadrant I with the x-polarization. After passing through quadrant IV of wedge  130 , light beam  34   b  is deflected to travel essentially in alignment with the positive z-direction. Light beam  34   b  then passes through quadrant IV of birefringent crystal  140 , without being deflected. After passing through quadrant IV of wedge  150 , light beam  34   b  is deflected to travel in the positive z-direction and somewhat in the positive y-direction (FIG. 3 c ( 2 )). Thereafter, light beam  34   b  enters quadrant IV of non-reciprocal device  160  with the x-polarization, and exits from quadrant IV with the y-polarization. Finally, light beam  34   b  passes through quadrant IV of birefringent crystal  170 , and enters dual fiber collimator  200  with the y-polarization.  
         [0046]    Light beams  34   a  and  34   b  enter collimator  200  respectively with the x-polarization and the y-polarization, are combined at collimator  200 , and exit from port  4 .  
         [0047]    [0047]FIGS. 3 d ( 1 ),  3   d ( 2 ), and  3   d ( 3 ) illustrate that an optical signal introduced at port  4  is separated into light beams  45   a  and  45   b . Light beams  45   a  and  45   b  exit respectively from extension interfaces  5   a  and  5   b . FIG. 3 d ( 1 ) is a perspective view, FIG. 3 d ( 2 ) is a planar view on the y-z plane, and FIG. 3 d ( 3 ) is a planar view on the x-z plane.  
         [0048]    The optical signal introduced at port  4  on dual fiber collimator  200  is separated into light beam  45   a  with the x-polarization and light beam  45   b  with the y-polarization by birefringent crystal  170 . Light beam  45   a  is deflected in the negative x-direction and exits from quadrant III of birefringent crystal  170 . Light beam  45   b  is not deflected and exits from quadrant IV of birefringent crystal  170 . After exiting from birefringent crystal  170 , light beams  45   a  and  45   b  travel in the negative z-direction and somewhat in the negative y-direction (FIG. 3 d ( 2 )).  
         [0049]    Light beam  45   a  enters quadrant III of non-reciprocal device  160  with the x-polarization, and exits from quadrant III with the y-polarization. After passing through quadrant III of wedge  150 , light beam  45   a  is deflected to travel essentially in alignment with the negative z-direction. Light beam  45   a  then enters quadrant III of birefringent crystal  140 , is deflected in the negative y-direction, and exits with the y-polarization from extension interface  5   a  on surface  144  of birefringent crystal  140 .  
         [0050]    Light beam  45   b  enters quadrant IV of non-reciprocal device  160  with the y-polarization, and exits from quadrant IV with the y-polarization. After passing through quadrant IV of wedge  150 , light beam  45   b  is deflected to travel essentially in alignment with the negative z-direction. Light beam  45   b  then enters quadrant IV of birefringent crystal  140 , is deflected in the negative y-direction, and exits from extension interface  5   b  on surface  144  of birefringent crystal  140  with the y-polarization.  
         [0051]    [0051]FIGS. 3 e ( 1 ),  3   e ( 2 ), and  3   e ( 3 ) illustrate that light beams  01   a  and  01   b  enter respectively extension interfaces  0   a  and  0   b  of extendable four-ports circulator  555 , are combined, and exit from port  1 . FIG. 3 e ( 1 ) is a perspective view, FIG. 3 e ( 2 ) is a planar view on the y-z plane, and FIG. 3 e ( 3 ) is a planar view on the x-z plane.  
         [0052]    Light beam  01   a  enters extension interface  0   a  on surface  142  of birefringent crystal  140  with the y-polarization, and exits from quadrant II of birefringent crystal  140  in a direction that is in alignment with the negative z-direction. After passing through quadrant II of wedge  130 , light beam  01   a  is deflected to travel in a direction that resembles the negative z-direction but leaning toward the negative y-direction (FIG. 3 e ( 2 )). Thereafter, light beam  01   a  enters quadrant II of non-reciprocal device  120  with the y-polarization, and exits from quadrant II with the y-polarization. Finally, light beam  01   b  passes through quadrant I of birefringent crystal  110 , without being deflected, and enters dual fiber collimator  100  with the y-polarization.  
         [0053]    Light beam  01   b  enters extension interface  0   b  on surface  142  of birefringent crystal  140  with the y-polarization, and exits from quadrant I of birefringent crystal  140  in the negative z-direction. After passing through quadrant I of wedge  130 , light beam  01   b  is deflected to travel in the negative z-direction and somewhat in the negative y-direction (FIG. 3 e ( 2 )). Thereafter, light beam  01   b  enters quadrant I of non-reciprocal device  120  with the y-polarization, and exits from quadrant I with the x-polarization. Finally, light beam  01   b  enters quadrant I of birefringent crystal  110 , is deflected in the negative x-direction, and enters dual fiber collimator  100  with the x-polarization.  
         [0054]    Light beams  01   a  and  01   b  enter collimator  100  respectively with the y-polarization and the x-polarization, are combined at collimator  100 , and exit from port  1 .  
         [0055]    [0055]FIGS. 4 a ,  4   b ,  4   c ,  4   d , and  4   e , shown in the y-z plane, show the optical paths traveled by the light beams respectively in FIGS. 3 a ( 3 ),  3   b ( 3 ),  3   c ( 3 ),  3   d ( 3 ), and  3   e ( 3 ). FIG. 4 f  shows the optical paths in the x-z plane. FIGS. 4 a - 4   e  show the paths traveled by light beams introduced at ports  1 ,  2 ,  3 , and  4 , and at extension interfaces  0   a  and  0   u , respectively. In each figure, the actual paths traveled by light beams are represented by arrow lines.  
         [0056]    [0056]FIG. 4 a  shows that an optical signal introduced at port  1  is separated into light beams  12   a  and  12   b , and light beams  12   a  and  12   b  are recombined to exit from port  2 . FIG. 4 b  shows that an optical signal introduced at port  2  is separated into light beams  23   a  and  23   b , and light beams  23   a  and  23   b  are recombined to exit from port  3 . FIG. 4 c  shows that an optical signal introduced at port  3  is separated into light beams  34   a  and  34   b , and light beams  34   a  and  34   b  are recombined to exit from port  4 . FIG. 4 d  shows that an optical signal introduced at port  4  is separated into light beams  45   a  and  45   b , and light beams  45   a  and  45   b  exit respectively from extension interfaces  5   a  and  5   b . FIG. 4 e  shows that light beams  01   a  and  01   b  enter respectively extension interfaces  0   a  and  0   b , and are combined to exit from port  1 .  
         [0057]    [0057]FIG. 5 shows three extendable four-port circulators  555 ′,  555 , and  555 ″ cascaded together to form twelve-port circulator  1000 . Extension interfaces  5   a ′ and  5   b ′ of circulator  555 ′ (not shown) are respectively coupled to extension interfaces  0   a  and  0   b  of circulator  555  (not shown) by directly contacting surface  144 ′ of circulator  555 ′ with surface  142  of circulator  555 . In one implementation, extension interfaces  5   a  and  5   b  of circulator  555  (not shown) are respectively coupled to extension interfaces  0   a ″ and  0   b ″ of circulator  555 ″ (not shown) by directly contacting surface  144  of circulator  555  with surface  142 ″ of circulator  555 ″. Extension interfaces  0   a ′ and  0   b ′ on surface  142 ′ (not shown) of circulator  555 ′ and extension interfaces  5   a ″ and  5   b ″ on surface  144 ″ (not shown) of circulator  555 ″ are not used and therefore need not to be implemented. Birefringent crystal  140 ′,  140 , and  144 ″ can be replaced with a single birefringent crystal  410 .  
         [0058]    [0058]FIG. 6 shows an alternative implementation of twelve-port circulator  1000 . Twelve-port circulator  1000  includes four pair of reflectors: reflectors  311  and  312 , reflectors  321  and  322 , reflectors  511  and  512 , and reflectors  521  and  522 . Optical paths traveling in dual fiber collimator  100 ′, birefringent crystal  110 ′, non-reciprocal device  120 ′, and wedge  130 ′ are shifted together in the positive y-direction using reflectors  311  and  312 . Optical paths traveling in wedge  150 ′, nonreciprocal device  160 ′, birefringent crystal  170 ′, and dual fiber collimator  200  are shifted together in the positive y-direction using reflectors  321  and  322 . Optical paths traveling in dual fiber collimator  100 ″, birefringent crystal  110 ″, non-reciprocal device  120 ″, and wedge  130 ″ are shifted together in the negative y-direction using reflectors  511  and  512 . Finally, Optical paths traveling in wedge  150 ″, non-reciprocal device  160 ″, birefringent crystal  170 ″, and dual fiber collimator  200 ″ are shifted together in the negative y-direction using reflectors  521  and  522 .  
         [0059]    Alternative implementations of twelve-port circulator  1000  can also use one pair, two pair, or three pairs of reflectors, instead of four pairs. Further, a single reflector can be used to replace a pair of reflectors. For example, if only reflector  312  is used and reflector  311  is eliminated, it is possible to rotate together by 90 degrees the orientations of dual fiber collimator  100 ′, birefringent crystal  110 ′, non-reciprocal device  120 ′, and wedge  130 ′, such that the propagation direction of a light beam introduced at dual fiber collimator  100  is initially in the negative y-direction. In the implementations of twelve-port circulator  1000 , reflectors, pairs of reflectors, or wedges may be generally referred to as path-conditioning components.  
         [0060]    [0060]FIG. 7 shows another implementation of twelve-port circulator  1000  including at least one common component for replacing a group of individual components in FIG. 5. For example, birefringent crystals  110 ′,  110 , and  110 ″ in FIG. 5 can be replaced with common birefringent crystal  101  in FIG. 7. Similarly, birefringent crystals  170 ′,  170 , and  170 ″ can be replaced with common birefringent crystal  710 . Non-reciprocal devices  120 ′,  120 , and  120 ″ can be replaced with common non-reciprocal device  210 . Non-reciprocal devices  160 ′,  160 , and  160 ″ can be replaced with common non-reciprocal device  610 .  
         [0061]    As described above, the functions of each component in extendable four-port circulator  555  (FIG. 2) may depend on both the direction and the quadrant that a light beam enters. The construction of each component in extendable four-port circulator  555  of FIG. 2 is described below. The functions of each component, as a light beam travels in the positive z-direction, are described with respect to FIGS. 3 a  and  3   c . Likewise, the function of each component, as a light beam travels in the negative z-direction, are described with respect to FIGS. 3 b ,  3   d , and  3   e.    
         [0062]    Birefringent crystal  110  is constructed and orientated in such a way to perform the following functions: (1) light passing through birefringent crystal  110  in the positive z-direction with the y-polarization will not be deflected, and light with the x-polarization will be deflected in the positive x-direction; (2) light passing through birefringent crystal  110  in the negative z-direction with the y-polarization will not be deflected, and light beam with the x-polarization will be deflected in the negative x-direction. Accordingly, birefringent crystal  110  splits or joins light beams in accordance with their respective polarizations. The polarization of the o-ray in birefringent crystal  110  is in the y-direction.  
         [0063]    Non-reciprocal device  120  is constructed to perform the following functions: (1) light passing through non-reciprocal device  120  in the positive z-direction and entering device  120  through quadrant I or IV with the x-polarization remains as light with the x-polarization, and light entering device  120  through quadrant II or III with the y-polarization becomes light with the x-polarization; (2) light passing through non-reciprocal device  120  in the negative z-direction and entering device  120  through quadrant I or IV with the y-polarization will become light with the x-polarization, and light entering device  120  through quadrant II or III with the y-polarization remains as light with the y-polarization.  
         [0064]    Birefringent crystal  140  is constructed and orientated in such a way to perform the following functions: (1) light passing through birefringent crystal  140  in the positive z-direction with the x-polarization will not be deflected; (2) light passing through birefringent crystal  140  in the negative z-direction with the y-polarization will be deflected in the negative y-direction. The polarization of the o-ray in birefringent crystal  140  is in the x-direction.  
         [0065]    Non-reciprocal device  160  is constructed to perform the following functions: (1) light passing through non-reciprocal device  160  in the positive z-direction and entering device  160  through quadrant I or IV with the x-polarization will become light with the y-polarization, and light entering device  160  through quadrant II or III with the x-polarization remains as light with the x-polarization; (2) light passing through non-reciprocal device  160  in the negative z-direction and entering device  160  through quadrant I or IV with the y-polarization remain as light with the y-polarization, and light entering device  160  through quadrant II or III with the x-polarization will become light with the y-polarization.  
         [0066]    One implementation of non-reciprocal device  120 , as shown in FIGS. 8 a  and  8   b , includes half wave plate  122  and Faraday rotators  125   a  and  125   b . In one implementation, the optical axis of half wave plate  122  is in the direction of a vector rotated +22.5 degrees from the positive x-direction. When a light beam passes through Faraday rotator  125   a , in either the positive or the negative z-directions, the polarization of the light beam will be rotated by +45 degrees with respect to the positive z-axis. When a light beam passes through Faraday rotator  125   b , either in the positive or the negative z-directions, the polarization of the light beam will be rotated by −45 degrees with respect to the positive z-axis.  
         [0067]    As shown in FIG. 8 a , after passing through half wave plate  122  in the positive z-direction, a light beam with the x-polarization becomes a light beam with the x+y polarization, and a light beam with the y-polarization becomes a light beam with the x-y polarization. After passing through Faraday rotator  125   a , a light beam with the x-y polarization, is rotated +45 degrees, and becomes a light beam with the x-polarization. Likewise, after passing through Faraday rotator  125   b , a light beam with the x+y polarization, is rotated −45 degrees, and becomes a light beam with the x-polarization.  
         [0068]    As shown in FIG. 8 b , after passing through Faraday rotator  125   a  in the negative z-direction, a light beam with the y-polarization, is rotated +45 degrees, and becomes a light beam with the x-y polarization. A light beam with the x-y polarization, after passing through half wave plate  122 , becomes a light beam with the y-polarization. After passing through Faraday rotator  125   b  in the negative z-direction, a light beam with the y-polarization, is rotated −45 degrees, and becomes a light beam with the x+y polarization. A light beam with the x+y polarization, after passing through half wave plate  122 , becomes a light beam with the x-polarization.  
         [0069]    In the implementations of FIGS. 8 a  and  8   b , when the position of half wave plate  122  is exchanged with the position of Faraday rotators  125   a  and  125   b , the functions of non-reciprocal device  120  remain the same. It is also possible to choose other optical axes for half wave plate  122  and other rotation directions for Faraday rotators  125   a  and  125   b.    
         [0070]    Another implementations of non-reciprocal device  120 , as shown in FIGS. 9 a  and  9   b , include half wave plates  122   a  and  122   b  and Faraday rotator  125 . The optical axis of half wave plate  122   a  is in the direction of a vector rotated +22.5 degrees from the positive x-direction. The optical axis of half wave plate  122   b  is in the direction of a vector rotated −22.5 degrees from the positive x-direction. When a light beam passes through Faraday rotator  125 , in either the positive or the negative z-directions, the polarization of the light beam will be rotated by +45 degrees.  
         [0071]    As shown in FIG. 9 a , a light beam with the y-polarization, after passing through half wave plate  122   a  in the positive z-direction becomes a light beam with the x+y polarization. Likewise, a light beam with the x-polarization, after passing through half wave plate  122   b  in the positive z-direction, becomes a light beam with the x-y polarization. The light beams with the x-y polarization, after passing through Faraday rotators  125 , are rotated +45 degrees, become light beams with the x-polarization.  
         [0072]    As shown in FIG. 9 b , a light beam with the y-polarization, after passing through Faraday rotators  125  in the negative z-direction, is rotated +45 degrees and becomes light beams with the x-y polarization. A light beam with the x-y polarization, after passing through half wave plate  122 a, becomes a light beam with the y-polarization. A light beam with the x-y polarization, after passing through half wave plate  122 b, becomes a light beam with the y-polarization.  
         [0073]    In the implementation of FIGS. 9 a  and  9   b , when the position of half wave plates  122   a  and  122   b  is exchanged with the position of Faraday rotators  125 , the functions of non-reciprocal device  120  remain the same. Other optical axes for half wave plate  122   a  and  122   b  and other rotation directions for Faraday rotator  125  can be selected.  
         [0074]    Similar to non-reciprocal device  120 , common non-reciprocal device  210  (in FIG. 7) can be constructed using one half wave plate in combination with two Faraday rotators, or using two half wave plates in combination with one Faraday rotator.  
         [0075]    Non-reciprocal device  160  can be implemented in a number of different ways. One implementation of non-reciprocal device  160 , as shown in FIGS. 10 a  and  10   b , includes half wave plate  162  and Faraday rotators  165   a  and  165   b . The optical axis of half wave plate  162  is in the direction of a vector rotated +22.5 degrees from the positive x-direction. When a light beam passes Faraday rotators  165   a , in either the positive or the negative z-directions, the polarization of the light beam will be rotated by −45 degrees. When a light beam passes Faraday rotators  165   b , either in the positive or the negative z-directions, the polarization of the light beam will be rotated by +45 degrees.  
         [0076]    As shown in FIG. 10 a , light beams with the x-polarization, after passing through half wave plate  162  in the positive z-direction, become light beams with the x+y polarization. A light beam with the x+y polarization, after passing through Faraday rotator  165 a, is rotated −45 degrees, and becomes a light beam with the x-polarization. A light beam with the x+y polarization, after passing through Faraday rotator  165   b , is rotated +45 degrees, and becomes a light beam with the y-polarization.  
         [0077]    As shown in FIG. 10 b , after passing through Faraday rotators  165   a  in the negative z-direction, a light beam with the x-polarization, rotated −45 degrees, become a light beam with the x-y polarization. Likewise, after passing through Faraday rotators  165   b  in the negative z-direction, a light beam with the y-polarization, rotated +45 degrees, become a light beam with the x-y polarization. The light beams with the x-y polarization, after passing through half wave plate  162 , become light beams with the y-polarization.  
         [0078]    In the implementation of FIGS. 10 a  and  10   b , when the position of half wave plate  162  is exchanged with the position of Faraday rotators  165   a  and  165   b , the functions of non-reciprocal device  160  remain the same. Other optical axes for half wave plate  162  and other rotation directions for Faraday rotators  165   a  and  165   b.    
         [0079]    Another implementation of non-reciprocal device  160 , as shown in FIGS. 11 a  and FIG. 11 b , includes half wave plates  162   a  and  162   b  and Faraday rotator  165 . The optical axis of half wave plate  162   a  is in the direction of a vector rotated −22.5 degrees from the positive x-direction. The optical axis of half wave plate  162   b  is in the direction of a vector rotated +22.5 degrees from the positive x-direction. When a light beam passes through Faraday rotators  165 , in either the positive or the negative z-directions, the polarization of the light beam will be rotated by +45 degrees.  
         [0080]    As shown in FIG. 11 a , a light beam with the x-polarization, after passing through half wave plate  162   a  in the positive z-direction, becomes a. light beam with the x-y polarization. A light beam with the x-polarization, after passing through half wave plate  162   b  in the positive z-direction, becomes a light beam with the x+y polarization. After passing through Faraday rotator  165  and being rotated +45 degrees, a light beam with. the x-y polarization becomes a light beam with the x-polarization, and a light beam with the x+y polarization becomes a light beam with the y-polarization.  
         [0081]    As shown in FIG. 11 b , after passing through Faraday rotator  165  in the negative z-direction and being rotated +45 degrees, a light beam with the x-polarization becomes a light beam with the x+y polarization, and a light beam with the y-polarization becomes a light beam with the x-y polarization. After passing through half wave plate  162   a , a light beam with the x+y polarization becomes a light beam with the y-polarization. Likewise, after passing through half wave plate  162   b , a light beam with the x-y polarization becomes a light beam with the y-polarization.  
         [0082]    In the implementation of FIGS. 11 a  and  11   b , the position of half wave plates  162   a  and  162   b  can be exchanged with the position of Faraday rotator  165 , and the functions of non-reciprocal device  160  remain unchanged. Other optical axes for half wave plate  162   a  and  162   b  and other rotation directions for Faraday rotator  165  can be selected.  
         [0083]    Similar to non-reciprocal device  160 , common non-reciprocal device  610  (in FIG. 7) can be constructed using one half wave plate in combination with two Faraday rotators, or using two half wave plates in combination with one Faraday rotator.  
         [0084]    A method and system has been disclosed for providing extendable four-port circulators, which may be cascaded or combined to form a twelve-port circulator. Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. For example, two extendable four-port circulators can be cascaded or combined to form an eight-port circulator, s and four can be cascaded or combined to form a sixteen-port circulator. In general, an integer number N of extendable four-port circulators can be cascaded or combined to form a  4 N-port circulator. 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.