Abstract:
In another aspect, the invention provides a closed loop optical circulator including a first crystal for splitting an input light signal into two components, a second crystal for deflecting the two components received from the first crystal in a direction if the two components have a first polarization, a third crystal for deflecting the two components received from the second crystal in an opposite direction if the two components have the first polarization, and a fourth crystal for joining the two components received from the third crystal.

Description:
BACKGROUND OF THE INVENTION 
     An optical circulator is a multi-ported passive device designed to receive as an input an optical signal on one port and transmit the optical signal to another port. Conventional optical circulators are employed in systems that require the transmission of an optical signal in a particular direction. For example, U.S. Pat. No. 4,650,289 by Kuwahara describes a conventional optical circulator. FIG. 1 is a diagram of one such conventional optical circulator  10 . The conventional optical circulator  10  includes four ports, port A  12 , port B  24 , port C  32 , and port D  34 . The conventional optical circulator  10  also includes polarizer prisms  14  and  22 , mirrors  16  and  26 , Faraday rotators  18  and  28 , and optically active elements (e.g., half wave plates)  20  and  30 . Polarizer prisms  14  and  22  transmit light in different directions depending on the polarization of the light. Light polarized in a first direction is transmitted undeflected by the or polarizer prisms  14  and  22 . Light polarized in a second direction is transmitted at an angle of ninety degrees from the first direction. The mirrors  16  and  26  merely reflect light without a change in polarization. The Faraday rotators  18  and  28  rotate the direction of polarization of incident light by forty-five degrees in a particular direction regardless of the direction in which light traverses the Faraday rotators  18  and  28 . For example, the Faraday rotator  18  rotates the polarization of light from the mirror  16  in the same direction as light from the optically active element  20 . Optically active elements  20  and  30  rotate the polarization of incident light by forty-five degrees. However, the direction that the polarization is rotated depends upon the direction in which the light traverses the optically active elements  20  and  30  (i.e., optically active elements  20  and  30  are reciprocal devices). For example, optically active element  20  will rotate light from the Faraday rotator  18  by forty-five degrees in a particular direction. The optically active element  20  will rotate light from the polarizer prism  22  having the same polarization by forty-five degrees in the opposite direction. The Faraday rotator is an optically irreversible (i.e., non-reciprocal) element, that is, the rotation angle will double for light after a round trip through the Faraday rotator. Optically active elements  20  and  30  are reciprocal, that is, light after a round trip through these devices will not be rotated. 
     In operation, the Faraday rotator of  18  and optically active element  20  act as a function group that rotates polarization 90 degrees for light traveling from left to right (from  16  to  22 ) but doesn&#39;t rotate polarization for the light passing through from right to left (from  22  to  16 ). Similarly, optically active element  30  and faraday rotator  28  act as another function group with similar functionality. Input light has random polarization and includes two components. Polarizer prisms  14  or  22  reflect one component of the input light while another component passes through undeflected. For the purposes of this example, the first polarization P can be characterized as having a polarization that is in the incident plane (paper surface) and the second polarization S which polarization is perpendicular to the incident plane. The P components pass through polarizer prism  14  or  22 , but S components reflect 90 degrees at an intersection to the surface. More specifically, a light with random SOP (State of Polarization) input to port  1  and transmitted to prism  14 , divides into S and P components. The P components pass through to a second path (including components  12 ,  30 ,  28 ,  26  and  22 ), while the S components reflect to a first path (including components  14 ,  16 ,  18 ,  20  and  22 ). 
     For signal from port A to port B, the S components pass along the first path through the functional group of  18  and  20 , change polarization to be P, passes through polarizer prism  22  to port B. The P component from port A, passes through polarizer prism  14 , changes to be S polarization by functional group  30  and  28 , and then reflects at polarizer prism  22  to port B also. Accordingly, polarizer prism  14  acts as a splitter while polarizer prism  22  acts as a combiner, producing the full signal from port A to port B. 
     For signal from port B to port C, the S components arriving at port B are reflected to the second path, pass through functional group of  28  and  30 , maintain their S polarization, and are reflected at polarizer prism  14  to port C. The P component of the input light introduced at port B passes through polarizer prism  22  to the first path, passes through functional group of  18  and  20 , maintains the P polarization, and then passes through polarizer prism  14  to port C also. In this case, polarizer prism  22  is a splitter and polarizer prism  14  is a combiner. Thus the full signal from port B is received by port C. Similarly, the full signal from port C is delivered to port D and the full signal from port D to port A. 
     Optical circulators of this type are very difficult to manufacture. The difficulty arises in the perfectly parallel optical paths that must be maintained in the device (i.e., paths between polarizer prisms  14  and  22 ). At the present time, no such devices are commercially offered. 
     SUMMARY OF THE INVENTION 
     In one aspect the invention provides a closed loop optical circulator including a first port, a last port and means for establishing a last optical path where the last optical path provides a path from the last port to the first port The means for establishing includes two pairs of complementary crystals. Each crystal of a respective pair transmits an optical signal of one polarization without deflection and deflects an optical signal of another polarization. The first pair of complementary crystals deflects optical signals of a second polarization in a direction perpendicular to a plane of a page and receives an optical signal from the last port and transmits the optical signal to the first port. The second pair of complementary crystals operable deflects optical signals of a first polarization in a direction along the plane of the page and is disposed between the first pair of complementary crystals. The optical circulator includes two pairs of complementary half wave plate rotators. Each pair of complementary half wave plate rotators is disposed between a crystal of the first pair and a crystal of the second pair of complementary crystals. Each half wave plate rotator includes a pair of half wave plate rotator groups where it each group includes a half wave plate and a glass portion. The optical circulator includes a half wave plate and a Faraday rotator disposed between crystals of the second pair of complementary crystals. 
     In another aspect, the invention provides a closed loop optical circulator including a first port, a last port and a path between the two including two pairs of complementary crystals. Each crystal of a respective pair transmits an optical signal of one polarization without deflection and deflects an optical signal of another polarization. The first pair of complementary crystals deflects optical signals of a second polarization in a direction perpendicular to a plane of a page and receives an optical signal from the last port and transmits the optical signal to the first port. The second pair of complementary crystals deflects optical signals of a first polarization in a direction along the plane of the page and disposed between the first pair of complementary crystals. The optical circulator includes two pairs of complementary half wave plate rotators. Each pair of complementary half wave plate rotators is disposed between a crystal of the first pair and a crystal of the second pair of complementary crystals. Each half wave plate rotator includes a pair of half wave plate rotator groups where each group includes a half wave plate and a glass portion. The optical circulator includes a half wave plate and a Faraday rotator disposed between crystals of the second pair of complementary crystals. 
     In another aspect, the invention provides a closed loop optical circulator including a plurality of ports and a like plurality of paths. Each path couples a pair of ports, where light incident at a port is transmitted along a path to a next port in the closed loop circulator. The paths include a first crystal for splitting an input light signal into two components, a second crystal for deflecting the two components received from the first crystal in a direction if the two components have a first polarization, a third crystal for deflecting the two components received from the second crystal in an opposite direction if the two components have the first polarization, and a fourth crystal for joining the two components received from the third crystal. 
     Aspects of the invention can include one or more of the following features. The crystals can be constructed from birefringent material. The second and third crystals can be Yvo4 crystals. The first pair of complementary half wave plate rotators can include a first half wave rotator group having a half wave plate covering a second and third quadrants and a glass plate covering a first and fourth quadrants and a second half wave rotator group having a half wave plate covering a third and fourth quadrants and a glass plate covering a first and second quadrants. The second pair of complementary half wave plate rotators can include a first half wave rotator group having a half wave plate covering a third and fourth quadrants and a glass plate covering a first and second quadrants and a second half wave rotator group having a half wave plate covering a first and fourth quadrants and a glass plate covering a second and third quadrants. 
     In another aspect, the invention provides a closed loop optical circulator including a first crystal for splitting an input light signal into two components, a second crystal for deflecting the two components received from the first crystal in a direction if the two components have a first polarization, a third crystal for deflecting the two components received from the second crystal in an opposite direction if the two components have the first polarization, and a fourth crystal for joining the two components received from the third crystal. 
     In another aspect, the invention provides a closed loop optical circulator including first, second, third and fourth ports. The optical circulator includes a first crystal splitting an input light signal received at the first and third ports into two components respectively, and joining input light components received from each of the second and fourth ports respectively into output light signals. The optical circulator includes a second crystal deflecting the two components received from the first crystal in a direction for signals from the first port, while not reflecting signals from the third port, and deflecting the two components received from a third crystal in an opposite direction for signals from the fourth port, while not reflecting signals from the second port. The third crystal deflects the two components received from the second crystal in an opposite direction for signals from the first port while not reflecting signals from the third port, and deflects the two components received from a fourth crystal in an opposite direction for signals from the second port, while not reflecting signals from the fourth port. The fourth crystal splits an input light signal received at the second and fourth ports into two components respectively, and joins input light components received from each of the first and second ports respectively into output light signals. 
     Aspects of the invention can include one or more of the following advantages. The present invention provides an easily manufacturable optical circulator with a loop function such that an optical signal input at a last port is returned to a first port in the device. Other advantages will be readily apparent from the attached figures and the description below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a conventional optical circulator. 
     FIG. 2 depicts a perspective view of an optical circulator in accordance with the present invention. 
     FIG. 3 a  depicts the polarization of an optical signal traveling along a first optical path (from a first port to a second port) after passing through particular components. 
     FIG. 3 b  depicts the polarization after particular components when the optical signal travels along a second optical path (from the second port to a third port). 
     FIG. 3 c  depicts the polarization after particular components when the optical signal travels along a third optical path (from the third port to a fourth port). 
     FIG. 3 d  depicts the polarization after particular components when the optical signal travels along a fourth optical path (from the fourth port to the first port). 
    
    
     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 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. 
     The present invention will be described in terms of an optical circulator having specific components having a specific configuration. Similarly, the present invention will be described in terms of optical circulator 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. 
     Referring now to FIG. 2, depicting one implementation of an optical circulator  100  in accordance with the present invention. FIG. 2 shows a perspective view of the optical circulator  100 . The optical circulator  100  includes four ports, a first port  102 , a second port  128 , a third port  130  and fourth port  131 . The first port  102  is coupled to a first fiber (not shown) and is operable to receive and transmit optical signals. The second, third and fourth ports  128 ,  130  and  131  are coupled to second, third and fourth fibers (not shown), respectively, each of which are also operable to receive and transmit optical signals. The optical circulator  100  is configured such that an optical signal input to one port (e.g., first port  102 ) will be provided to a next port (e.g., second port  128 ) along an optical path (e.g., the first optical path). Accordingly, in the four-port design shown, four optical paths are provided. Optical circulator  100  is configured in a closed loop configuration such that optical signals from a last port (e.g., the fourth port  131 ) are transmitted to a first port (e.g., the first port  102 ). Optical circulator  100  is configured such that an optical signal input to the first port  102  will not be transmitted to the third port  130 . Similarly, an optical signal input to the second port  128  will not be provided to the first port  102  and optical signal inputted on the fourth port  131  will not be provided to the third port  130 . 
     In order to establish the four optical paths, the optical circulator  100  includes a first birefringent material (crystal)  108 , a first pair of half wave plate (HWP) rotators  112 , a first crystal  114 , a half wave plate  116 , a Faraday rotator  118 , a second crystal  120 , a second pair of HWP rotators  122 , and a second birefringent material  124 . 
     An optical signal input to a port is typically randomly polarized. The optical signal can be decomposed into two components with the state of polarization (SOP) of each orthogonal to each other and to the propagation direction. The two components are referred to as “o” and “e” rays. Thus, an input optical signal can be decomposed into a first portion having a first polarization and a second portion having a second polarization. Optical circulator  100  separates random SOP light into two components. The SOP of one of the components is rotated by 90 degrees, such that the two components have the same SOP. Accordingly, each component behaves the same along the path to the second optical port since each component along the path is polarization dependent. Just before traveling to the second port, the SOP of one component win be rotated 90 degrees back. Thereafter, a displacement element is used to combine the two components together into the second port. 
     First and second birefringent materials  108  and  124  are displacement elements. The birefringent materials treat components having a first polarization state differently from components having a second polarization state. First birefringent material  108  decomposes light received on a first port into two components (o and e rays) whose SOP are perpendicular to each other and perpendicular to propagation direction. The first birefringent material  108  transmits a first component having a first polarization state undeflected (i.e., o rays having a vertical SOP are un-deflected). The first birefringent material  108  transmits a second component having a second polarization state with a deflection, shown in FIG. 3 a  (i.e., e rays having a horizontal SOP are deflected). The deflection is shown as being horizontal and substantially perpendicular to the direction of propagation. Two components of a received optical signal are separated a predefined distance due to the deflection. For example, FIG. 3 a  shows optical circulator  100  and includes four quadrants (Q 1 -Q 4 ). First port  102  is coupled to a first quadrant. At the first birefringent material  108 , o rays associated with optical signals received on the first port  102  pass through the first birefringent material  108  and remain in the first quadrant Q 1 . E rays are deflected to the second quadrant Q 2 . 
     The second birefringent material  124  is complementary, providing a deflection that is a same predefined distance. For example, at the second birefringent material  124 , o rays associated with optical signals received on the first port  102  pass through the second birefringent material  108  and remain in the second quadrant Q 1 . E rays are deflected from the first quadrant to the second quadrant Q 2 , thereby combining the components in the second quadrant that in turn is coupled to the second port  128 . 
     The first pair of HWP rotators  112  includes HWP rotator group  111  and HWP rotator group  113 . HWP rotator group  111  includes a half wave plate and a bare glass plate where the half wave plate only covers quadrants Q 2  and Q 3  while the bare glass plate covers quadrants Q 1  and Q 4 . The half wave plate rotates the SOP of a component to a mirror position against its optical axis. The orientation of the optical axis for the half wave plate is 45 degrees against the crystal edge. HWP rotator group  113  includes a half wave plate covering quadrants Q 3  and Q 4 , and bare glass plate covering quadrant Q 1  and Q 2 . 
     The second pair of HWP rotators  122  includes HWP rotator group  121  and HWP rotator group  123 . HWP rotator group  121  includes a half wave plate and a bare glass plate where the half wave plate only covers quadrants Q 3  and Q 4  while the bare glass plate covers quadrants Q 1  and Q 2 . The orientation of the optical axis for the half wave plate is 45 degrees against the crystal edge. HWP rotator group  124  includes a half wave plate covering quadrants Q 1  and Q 4 , and bare glass plate covering quadrant Q 2  and Q 3 . 
     First and second crystals  114  and  120  are similar to first and second birefringent crystals  108  and  124  in that they both deflect light of one polarization while transmitting light undeflected of another polarization. First and second crystals  114  and  120  operate to deflect light of the first polarization (having a vertical SOP) and transmit light of the second polarization (having a horizontal SOP). In addition, first and second crystals  114  and  120  operate to deflect light of the first polarization in a direction that is along the plane of the page. First and second crystals  114  and  120  each include an axis that is substantially parallel to the axis of the Faraday rotator. For example, light of a first polarization state and traveling towards the second port  128  is deflected approximately along the plane of the page (in the −y direction) by the first crystal  114 . Similarly, the second crystal  120  deflects light of the first polarization state and traveling to the second port  128  along the plane of the page (in the +y direction). In one implementation, each of first and second crystals  114  and  120  are YVo4 crystals. 
     Half wave plate  116  covers all four quadrants. The optical axis of half wave plate  116  is at 22.5 degrees to the vertical edge, pointing to Q 2 . For example, the half wave plate  116  rotates the SOP of the two components received from crystal  114  in quadrants Q 3  and Q 4  to a mirror position against its optical axis, so the SOP of the two components becomes 45 degrees (10:30 O&#39;clock). 
     Faraday rotator  118  rotates the SOP components 45 degrees clockwise. For example, the SOP of the two components received from half wave plate  116  in quadrants Q 3  and Q 4  transitions to vertical again after passing through the faraday rotator  118 . 
     Referring to FIGS. 3 a  and  3   b , to further illustrate the optical circulator  100  in accordance with the present invention, an optical signal traversing the first optical path  300  (from the first port  102  to the second port  128 ) and an optical signal traversing the second optical path  325  (from the second port  128  to the third port  130 ) is shown. For clarity, specific rotations of light. polarized in the first and second states will be discussed. However, as discussed above, different polarization rotations can be used. 
     First, an optical signal traveling along the first optical path  300  is discussed. As discussed above, the optical signal input to the first port  102  can be considered to have a random polarization. 
     FIG. 3 a  depicts the polarizations after passing through particular elements as the optical. signal travels along a first optical path  300 , from the first port  102  to the second port  128 . The first polarization state is depicted as vertical in FIG. 3 a , while the second polarization state is horizontal. The polarizations are labeled consistently with their respective elements. For example, the polarizations of the first and second portions of the optical signal after transmission by the first birefringent material  108  (FIG. 2) are labeled  108  in FIG. 3 a . In the first optical path  300 , the components are not affected by transmission through HWP rotator group  113  (FIG. 2) and HWP rotator group  121  (FIG.  2 ), and as such, have not been shown. 
     Referring now to FIGS. 2 and 3 a , the optical signal is provided from a fiber collimator for the first port  102  to the first birefringent material  108  in quadrant Q 1 . The first birefringent material  108  transmits a first component having a first polarization state undeflected in quadrant Q 1  (i.e., o rays having a vertical SOP are un-deflected). The first birefringent material  108  transmits a second component having a second polarization state with a deflection from quadrant Q 1  to Q 2 , shown in FIG. 3 a  (i.e., e rays having a horizontal SOP are deflected). The first and second components of the optical signal are then provided in quadrants Q 1  and Q 2  to the first HWP rotators  112 . 
     The half wave plate in HWP rotator group  111  rotates the horizontal SOP of an e ray received from the first birefringent material  108  (covered by the HWP) to mirror the position of its optical axis (i.e., the SOP of the e ray becomes vertical). The SOP of the o ray received from the first birefringent material  108  remains unchanged (i.e., vertical) because it just passes through the bare glass plate. After traversing HWP rotator group  111  , the SOP of the two components in quadrants Q 1  and Q 2  (e and o rays) are the same. 
     The components received from the HWP rotator group  113  in quadrants Q 1  and Q 2  are moved to quadrants Q 3  and Q 4  as they pass through crystal  114 . Half wave plate  116  rotates the SOP of the two components received from crystal  114  to a mirror position against its optical axis, so the SOP of the two components becomes 45 degrees (10:30 O&#39;clock). Faraday rotator  118  rotates the SOP of the two components 45 degrees clockwise. More specifically, the SOP of the two components received from half wave plate  116  in quadrants Q 3  and Q 4  transitions to vertical again after passing through the faraday rotator  118 . 
     The second pair of HWP rotators  122  includes HWP rotator group  121  and HWP rotator group  123 . HWP rotator group  123  maintains the component received in quadrant Q 2  from HWP rotator group  121  (i.e., maintains its vertical SOP as it passes through the bare glass plate), and the rotates to horizontal the component in quadrant Q 1  received from HWP rotator group  121  that is covered by a half wave plate. The SOP of two components becomes orthogonal again and is ready to be recombined by second birefringent material  124 . At the input to the second birefringent material  124 , the first portion of the optical signal has the second polarization state, while the second portion of the optical signal has the first polarization state. When the first and second portions of the optical signal are transmitted through the second birefringent material  124 , the first portion of the optical signal is deflected in the −x direction from quadrant Q 1  to Q 2 , while the second portion of the optical signal is transmitted undeflected in quadrant Q 2 . Consequently, the first and second portions of the optical signal are recombined in quadrant Q 2 . The optical signal can then be output by the second port  128 . 
     The optical circulator  100  functions similarly when an optical signal is input to the second port  128 . The second optical path  325 , traversed when the optical signal is input to the second port  128 , is discussed with reference to FIG. 3 b . Again, for clarity, specific rotations of light polarized in the first and second states will be discussed. However, as discussed above, different polarization rotations can be used. 
     As discussed above, the optical signal input to the second port  128  can be considered to have a random polarization. FIG. 3 b  depicts the polarizations after passing through particular elements as the optical signal travels along a second optical path  325 , from the second port  128  to the third port  130 . The polarizations are labeled consistently with their respective elements. The components are not affected by transmission through the HWP rotator group  121  (FIG. 2) and the first crystal  114  (FIG. 2) in the second optical path  325 , and as such, have not been shown. 
     Referring now to FIGS. 2 and 3 b , the optical signal is provided from a fiber collimator for the second port  128  to the second birefringent material  124 . The second birefringent material  124  splits the optical signal into a first component and a second component. The first component has the first polarization state, while the second component has the second polarization state. As discussed above, the first component traverses the second birefringent material  124  undeflected in the quadrant Q 2 . However, the second component, having the second polarization state, is deflected in a horizontal, (+x) direction to quadrant Q 1 . The first and second components of the optical signal are then provided in quadrants Q 1  and Q 2  respectively, to the second HWP rotators  122 . 
     HWP rotator group  123  rotates the polarization of the second component of the optical signal received in quadrant Q 1  such that both components have the same polarization (vertical) when they pass from the second HWP rotators  122  to crystal  120 . Both components are deflected in a vertical direction (−y direction) in the second crystal  120  from quadrants Q 1  and Q 2  to Q 3  and Q 4 , respectively. The polarization of the first and second components of the optical signal are then rotated by each of Faraday rotator  118  and half wave plate  116  and then are provided (including passing through the first crystal  114  since both have a second polarization state) to the first HWP rotators  112 . HWP rotator group  113  rotates the polarizations of both the first and second components in quadrants Q 3  and Q 4 , while the HWP group rotator  111  rotates the polarization of the first component of the optical signal in quadrant Q 3 . When the first and second components of the optical signal are transmitted through the first birefringent material  108 , the first component of the optical signal is deflected in the +x direction from quadrant Q 3  to Q 4 , while the second portion of the optical signal is transmitted undeflected in quadrant Q 4 . Consequently, the first and second portions of the optical signal are recombined in quadrant Q 4 . The optical signal can then be output by the third port  130 . 
     The optical circulator  100  functions similarly when an optical signal is input to the third port  130 . The third optical path  350 , traversed when the optical signal is input to the third port  130 , is discussed with reference to FIG. 3 c . Again, for clarity, specific rotations of light polarized in the first and second states will be discussed. However, as discussed above, different polarization rotations can be used. 
     As discussed above, the optical signal input to the third port  130  can be considered to have a random polarization. FIG. 3 c  depicts the polarizations after passing through particular elements as the optical signal travels along a third optical path  350 , from the third port  130  to the fourth port  131 . The components are not affected by transmission through the first and second crystals  114  and  120  (FIG. 2) in the third optical path  350 , and as such, have not been shown. 
     Referring now to FIGS. 2 and 3 c , the optical signal is provided from a fiber collimator for the third port  130  to the first birefringent material  108 . The first birefringent material  108  transmits a first component having a first polarization state undeflected in quadrant Q 4  (i.e., o rays having a vertical SOP are un-deflected). The first birefringent material  108  transmits a second component having a second polarization state with a deflection from quadrant Q 4  to Q 3 , shown in FIG. 3 c  (i.e., e rays having a horizontal SOP are deflected). The first and second components of the optical signal are then provided in quadrants Q 3  and Q 4  to the first HWP rotators  112 . 
     The half wave plate in HWP rotator group  111  rotates the horizontal SOP of an e ray received from the first birefringent material  108  (covered by the HWP) to mirror the position of its optical axis (i.e., the SOP of the e ray becomes vertical). The SOP of the o ray received from the first birefringent material  108  remains unchanged (i.e., vertical) because it just passes through the bare glass plate. After traversing HWP rotator group  111 , the SOP of the two components in quadrants Q 3  and Q 4  (e and o rays) are the same. 
     The half wave plate in HWP rotator group  113  rotates the SOP of both components in quadrants Q 3  and Q 4  to horizontal. The components are passed (undeflected through crystal  114 ) to half wave plate  116 . Half wave plate  116  and Faraday rotator  118  each rotate the components in quadrants Q 3  and Q 4 . More specifically, the SOP of the two components received from half wave plate  116  in quadrants Q 3  and Q 4  transitions to horizontal again after passing through the faraday rotator  118 . 
     HWP rotator group  123  maintains the component received in quadrant Q 3  from HWP rotator group  121  (i.e., maintains its vertical SOP as it passes through the bare glass plate), and then rotates to horizontal the component in quadrant Q 4  that is covered by a half wave plate. The SOP of two components becomes orthogonal again and is ready to be recombined by second birefringent material  124 . When the first and second components of the optical signal are transmitted through the second birefringent material  124 , the first component of the optical signal received in quadrant Q 4  is deflected in the −x direction from quadrant Q 4  to Q 3 , while the second portion of the optical signal is transmitted undeflected in quadrant Q 3 . Consequently, the first and second portions of the optical signal are recombined in quadrant Q 3 . The optical signal can then be output by the fourth port  131 . 
     The optical circulator  100  functions similarly when an optical signal is input to the fourth port  131 . The fourth optical path  375 , traversed when the optical signal is input to the fourth port  131 , is discussed with reference to FIG. 3 d . Again, for clarity, specific rotations of light polarized in the first and second states will be discussed. However, as discussed above, different polarization rotations can be used. 
     FIG. 3 d  depicts the polarizations after passing through particular elements as the optical signal travels along a fourth optical path  375 , from the fourth port  131  to the first port  102 . The first polarization state is depicted as vertical in FIG. 3 d , while the second polarization state is horizontal. The polarizations are labeled consistently with their respective elements. For example, the polarizations of the first and second portions of the optical signal after transmission by the second birefringent material  124  (FIG. 2) are labeled  124  in FIG. 3 d . The components are not affected by transmission through HWP rotator group  113  (FIG. 2) and the second crystal  120  (FIG. 2) in the fourth optical path  375 , and as such, have not been shown. 
     Referring now to FIGS. 2 and 3 d , the optical signal is provided from a fiber collimator for the fourth port  131  to the second birefringent material  124 . The second birefringent material  124  splits the optical signal into a first component and a second component. The first component has the first polarization state, while the second component has the second polarization state. As discussed above, the first component traverses the second birefringent material  124  undeflected in the quadrant Q 3 . However, the second component, having the second polarization state, is deflected in a horizontal, (+x) direction to quadrant Q 4 . The first and second components of the optical signal are then provided in quadrants Q 3  and Q 4  respectively, to the second HWP rotators  122 . 
     HWP rotator group  123  rotates the polarization of the second component of the optical signal received in quadrant Q 4  such that both components have the same polarization (vertical) when they pass from the second HWP rotators  122  to HWP rotator group  121 . HWP rotator group  121  rotates the polarization of both components of the optical signal received in quadrants Q 3  and Q 4  such that both components have a horizontal polarization (and as such pass through crystal  120  (FIG.  2 )). The polarization of the first and second components of the optical signal are then rotated by each of Faraday rotator  118  and half wave plate  116  and then are provided to crystal  114 . Crystal  114  deflects the components having a vertical SOP in quadrants Q 3  and Q 4  in a direction along the plane of the page to quadrants Q 1  and Q 2 , respectively. The signals are then presented to first HWP rotators  112  in quadrants Q 1  and Q 2 . HWP rotator group  111  rotates the polarization of the first component of the optical signal in quadrant Q 2 . When the first and second components of the optical signal are transmitted through the first birefringent material  108 , the first component of the optical signal is deflected in the −x direction from quadrant Q 2  to Q 1 , while the second portion of the optical signal is transmitted undeflected in quadrant Q 1 . Consequently, the first and second portions of the optical signal are recombined in quadrant Q 1 . The optical signal can then be output by the first port  102 . 
     A method and system has been disclosed for providing an optical circulator, which may have low losses and be low in cost to manufacture. 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. 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.