Abstract:
The invention provides a compact optical circulator with three ports positioning between a dual-core collimator and a single-core collimator. The inventive circulator has a propagation director to couple a light from the dual-core collimator to one port of the circulator and another light form the other port of the circulator to the dual-core collimator. The invention further provides a compact optical circulator with three ports having a reflective compensator. The reflective compensator compensates the optical path length of two polarized beams. Furthermore, the invention eliminates the polarization mode dispersion by utilizing a reflective compensator. As well, the inventive circulator forgoes use of the reciprocal polarizing-rotating unit, thus reducing production costs.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to an optical circulator, and more particularly to an optical circulator with three ports for use in optical communication.  
           [0003]    2. Description of the Related Art  
           [0004]    In a three port optical circulator, an optical signal input at the first port will be transmitted to the second port. An optical signal input at the second port will be transmitted to the third port.  
           [0005]    With reference to FIG. 1, a compact circulator is disclosed in U.S. Pat. No. 5,204,771 by Koga. This circulator shows an improvement over the previous one since the two optical paths can be in close proximity, and the first and third ports  27 ,  28  are parallel.  
           [0006]    Unfortunately, this device still suffers from a disadvantage. If the first and third ports  27 ,  28  are very close together, the first and third fibers will have to share a common lens for collimating and focusing. It is impossible for a single lens to perform adequately for both fibers. The difficulty can be traced to the fact that the light beams coupled to the first and third ports are parallel, and a single lens cannot focus two parallel beams to two different points. Therefore, the circulator has a problem of coupling angle.  
         SUMMARY OF THE INVENTION  
         [0007]    The invention provides a compact optical circulator with three ports, wherein a propagation director is provided to solve the problem of coupling angle. The propagation director can focus two parallel beams appearing on the same side of the compact optical circulator to two different points (i.e. to two different fibers).  
           [0008]    It is an object of the present invention to provide a compact optical circulator, including a dual-core collimator, a propagation director, a first birefringent device, a first polarization rotator, a second birefringent device, a second polarization rotator, a third birefringent device and a single-core collimator.  
           [0009]    The invention further provides a compact optical circulator with three ports, wherein a reflective compensator is provided to eliminate the polarization mode dispersion (“PMD”).  
           [0010]    It is another object of the present invention to provide a compact optical circulator, including a dual-core collimator, a propagation director, a first birefringent device, a first polarization rotator, a second birefringent device, a second polarization rotator, a compensator, a third birefringent device and a single-core collimator.  
           [0011]    A feature of the invention is that one beam from the first port to the second port is normally incident on the first birefringent by passing through the propagation director. Thus, the compact optical circulator of the invention has an advantage of reduction of polarization dependent loss.  
           [0012]    Another feature of the invention is that the first and second polarization rotators are the non-reciprocal rotators respectively.  
           [0013]    The compact optical circulator of the invention has another advantage of reduction of production cost. A non-reciprocal rotator aligning with the optical axis of the birefringent device replaces the use of a reciprocal rotator and a non-reciprocal rotator, and thus the invention reduces production costs.  
           [0014]    The compact optical circulator of the invention has another advantage of elimination of the polarization mode dispersion utilizing a reflective compensator. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    This and other objections and features of the invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the drawings, in which:  
         [0016]    [0016]FIG. 1 schematically shows a conventional optical circulator;  
         [0017]    [0017]FIG. 2A is a top plane view showing a compact optical circulator of the first embodiment of the invention;  
         [0018]    [0018]FIG. 2B is a side view showing a compact optical circulator of the first embodiment of the invention;  
         [0019]    [0019]FIG. 3A schematically shows the spatial location and the polarizations of the light beam traveling from the first port to the second port in the compact optical circulator of the first example;  
         [0020]    [0020]FIG. 3B schematically shows the spatial location and the polarizations of the light beam traveling from the second port to the third port in the compact optical circulator of the first example;  
         [0021]    [0021]FIG. 4A schematically shows the spatial location and the polarizations of the light beam traveling from the first port to the second port in the compact optical circulator of the second example;  
         [0022]    [0022]FIG. 4B schematically shows the spatial location and the polarizations of the light beam traveling from the second port to the third port in the compact optical circulator of the second example;  
         [0023]    [0023]FIG. 5A is a top plane view showing a compact optical circulator of the second embodiment of the invention;  
         [0024]    [0024]FIG. 5B is a side view showing a compact optical circulator of the second embodiment of the invention;  
         [0025]    [0025]FIG. 6A and 6B schematically show a reflective compensator of the second embodiment of the invention;  
         [0026]    [0026]FIG. 7A schematically shows the spatial location and the polarizations of the light beam traveling from the first port to the second port in the compact optical circulator of the third example;  
         [0027]    [0027]FIG. 7B schematically shows the spatial location and the polarizations of the light beam traveling from the second port to the third port in the compact optical circulator of the third example;  
         [0028]    [0028]FIG. 8A schematically shows the spatial location and the polarizations of the light beam traveling from the first port to the second port in the compact optical circulator of the fourth example;  
         [0029]    [0029]FIG. 8B schematically shows the spatial location and the polarizations of the light beam traveling from the second port to the third port in the compact optical circulator of the fourth example.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    First Embodiment  
         [0031]    [0031]FIG. 2A is a top plane view showing a compact optical circulator of the first embodiment of the invention, and FIG. 2B is a side view showing a compact optical circulator of the first embodiment of the invention. In the first embodiment of the invention, the compact optical circulator  1000  includes a dual-core collimator  1 , a propagation director  100 , a first birefringent device  101 , a first polarization rotator  102 , a second birefringent device  103 , a second polarization rotator  104 , a third birefringent device  105  and a single-core collimator  2 .  
         [0032]    The dual-core collimator  1  generally has a first fiber  11 , a third fiber  13  and a GRIN lens (not shown), wherein the first and third fibers  11 ,  13  are parallel. The compact optical circulator  1000  has three ports; wherein the first and third ports  10 ,  30  are located at the same side of the circulator  1000 , and the second port  20  is located at the other side of the circulator  1000 . The dual-core collimator  1  emits a light beam to the first port  10  of the circulator  1000 , and receives another light beam from the third port  30  of the circulator  1000 . In general, the light beam emitted from the first fiber  11  of the dual-core collimator  1  is refracted by travelling through the GRIN lens, and then must be coupled to the first port  10  of the circulator  1000 . Then, a propagation director  100  with a refracting plane is formed to direct the light beam from first fiber  11  to first port  10  by Snell&#39;s law. Another light beam emitted from the third port  30  of the circulator  1000  must be coupled to the third fiber  13  of the dual-core collimator  1 . Then, the propagation director  100  with another refracting plane is formed to direct the light beam from third port  30  to third fiber  13  by Snell&#39;S law. Therefore, the propagation director  100  can direct one light beam from first fiber  11  to first port  10  and another light beam from third port  30  to third fiber  13  at the same time.  
         [0033]    The first birefringent device  101  is a birefringent crystal, such as LiNbO 3 , YVO 4  etc, and has walk-off characteristics. While the first port  10  receives a light beam, the light beam is divided into an e-ray and o-ray by the first birefringent device  101 , wherein both the e-ray and o-ray have orthogonal polarizations. While a light beam is received from the second port  20  and divided into two polarized beams in the circulator  1000 , the two polarized beams are combined together by the first birefringent device  101 .  
         [0034]    The first and second polarization rotators  102 ,  104  respectively have a non-reciprocal polarizing-rotating crystal, such as Faraday rotator, and a reciprocal polarizing-rotating unit, such as a half-wave plate. As well, the first and second polarization rotators  102 ,  104  can further forgo the use of the reciprocal polarizing-rotating unit, and only include the non-reciprocal polarizing-rotating crystal.  
         [0035]    The second birefringent device  103  is also a birefringent crystal and has an optical axis. While a light beam is received from the first port  10  or second port  20  and divided into two polarized beams, the two polarized beams are shifted or not according to the optical axis.  
         [0036]    The third birefringent device  105  is also a birefringent crystal. While the second port  20  receives a light beam, the light beam is divided into an e-ray and o-ray by the third birefringent device  105 , wherein both the e-ray and o-ray have orthogonal polarizations. While a light beam is received from the first port  10  and divided into two polarized beams in the circulator  1000 , the two polarized beams are combined together by the third birefringent device  105 .  
         [0037]    First Example  
         [0038]    In the first example, the compact optical circulator with three ports includes: a dual-core collimator, a propagation director, a first birefringent crystal, a first polarization rotator, a second birefringent crystal, a second polarization rotator, a third birefringent crystal, and a single-core collimator. The first and second polarization rotators respectively have a Faraday rotator and a half-wave plate.  
         [0039]    [0039]FIG. 3A schematically shows the spatial location and the polarizations of the light beam traveling from the first port to the second port in the compact optical circulator of the first example. Each arrow respectively shows the projecting component of the optical axis of each birefringent crystal in the x-y plane. While a light beam emitted from the first fiber  11  of the dual-core collimator  1  is received by the first port  10  of the circulator  1000 , the polarization of the light beam is random. When the light beam travels through the first birefringent crystal  101 , the light beam is split into two polarized beams according to the arrow representing the walk-off characteristic of the first birefringent crystal  101 . One is an e-ray polarized along the y-axis and the other is an o-ray polarized along the x-axis. Next, the e-ray and o-ray respectively travel through the Faraday rotators  102   a ,  102   b , the polarization of the e-ray rotates counterclockwise 45 degrees and the polarization of the o-ray rotates clockwise 45 degrees. Next, the two polarized beams travel through the half-wave plate  102   c , the two beams rotate clockwise 45 degrees. After the e-ray and o-ray travel through the first polarization rotator, the o-ray is changed into another e-ray and the two polarized beams pass the second birefringent crystal  103 . Next, the two polarized beams travel through the Faraday rotator  104   a ,  104   b  of the second polarization rotator  104 , one rotates clockwise 45 degrees and the other rotates counterclockwise 45 degrees. Next, the two polarized beams travel through the half-wave plate  104   c , and the two polarized beams rotate clockwise 45 degrees. When the e-ray travels through the first polarization rotator  102 , the second birefringent crystal  103  and the second polarization rotator  104 , the e-ray is changed into o-ray. When the o-ray travels through the first polarization rotator  102 , the second birefringent crystal  103  and the second polarization rotator  104 , the o-ray is changed into e-ray. When the e-ray and o-ray travel through the third birefringent crystal  105 , the two polarized beams (e-ray and o-ray) are combined together; wherein the e-ray is shifted toward the o-ray according to the arrow representing the walk-off characteristic of the third birefringent crystal  105 . Finally, the second port  20  of the circulator  1000  outputs the light beam into the single-core collimator  2 .  
         [0040]    [0040]FIG. 3B schematically shows the spatial location and the polarizations of the light beam traveling from the second port to the third port in the compact optical circulator of the first example. Each arrow respectively shows the projecting component of the optical axis of each birefringent crystal in the x-y plane. While a light beam emitted from the second fiber  22  of the single-core collimator  2  is received by the second port  20  of the circulator  1000 , the polarization of the light beam is random. When the light beam travels through the third birefringent crystal  105 , the light beam is split into two polarized beams according to the arrow representing the walk-off characteristic of the third birefringent crystal  105 . One is an e-ray polarized along the y-axis and the other is an o-ray polarized along the x-axis. Next, the two polarized beams travel through the half-wave plate  104   c , and the two beams rotate counterclockwise 45 degrees. Next, the e-ray and o-ray respectively travel through the Faraday rotators  104   a ,  104   b , the polarization of the e-ray rotates counterclockwise 45 degrees and the polarization of the o-ray rotates clockwise 45 degrees. Therefore, as the e-ray travels through the third polarization rotator  104 , the e-ray is changed into another o-ray. Next, the two polarized beams are shifted toward the minus x-axis according to the arrow representing the walk-off characteristic of the second birefringent crystal  103 . Next, the two polarized beams travel through the half-wave plate  102   c , and the two polarized beams rotate counterclockwise 45 degrees. Next, the two polarized beams travel through the Faraday rotator  102   a ,  102   b  of the first polarization rotator  102 , one rotates clockwise 45 degrees to form an o-ray and the other rotates counterclockwise 45 degrees to form an e-ray. When the e-ray travels through the second polarization rotator  104 , the second birefringent crystal  103  and the first polarization rotator  102 , the e-ray is changed into o-ray. When the o-ray travels through the second polarization rotator  104 , the second birefringent crystal  103  and the first polarization rotator  102 , the o-ray is changed into e-ray. When the e-ray and o-ray travel through the first birefringent crystal  101 , the two polarized beams (e-ray and o-ray) are combined together; wherein the e-ray is shifted toward the o-ray according to the arrow representing the walk-off characteristic of the first birefringent crystal  101 . Finally, the third port  30  of the circulator  1000  outputs the light beam into the dual-core collimator  1 .  
         [0041]    Second Example  
         [0042]    In the second example, the compact optical circulator with three ports includes: a dual-core collimator, a propagation director, a first birefringent crystal, a first polarization rotator, a second birefringent crystal, a second polarization rotator, a third birefringent crystal, and a single-core collimator. The first and second polarization rotators forgo the use of the reciprocal polarizing-rotating unit, such as a half-wave plate, and only include the non-reciprocal polarizing-rotating crystal, such as the Faraday rotator.  
         [0043]    [0043]FIG. 4A schematically shows the spatial location and the polarization of the light beam traveling from the first port to the second port in the compact optical circulator of the second example. Each arrow respectively shows the projecting component of the optical axis of each birefringent crystal in the x-y plane. While a light beam emitted from the first fiber  11  of the dual-core collimator  1  is received by the first port  10  of the circulator  1000 , the polarization of the light beam is random. When the light beam travels through the first birefringent crystal  101 , the light beam is split into two polarized beams according to the arrow representing the walk-off characteristic of the first birefringent crystal  101 . One is an e-ray polarized along the projecting component of the optical axis of the first birefringent crystal  101  and the other is an o-ray polarized perpendicular to the projecting component of the optical axis of the first birefringent crystal  101 . Next, the e-ray and o-ray respectively travel through the Faraday rotators  102   a ,  102   b , the polarization of the e-ray rotates counterclockwise 45 degrees and the polarization of the o-ray rotates clockwise 45 degrees. Next, the two polarized beams pass the second birefringent crystal  103 .  
         [0044]    Next, the two polarized beams travel through the Faraday rotator  104   a ,  104   b  of the second polarization rotator  104 , one rotates clockwise 45 degrees and the other rotates counterclockwise 45 degrees. When the e-ray travels through the first polarization rotator  102 , the second birefringent crystal  103  and the second polarization rotator  104 , the e-ray is changed into o-ray. When the o-ray travels through the first polarization rotator  102 , the second birefringent crystal  103  and the second polarization rotator  104 , the o-ray is changed into e-ray. When the e-ray and o-ray travel through the third birefringent crystal  105 , the two polarized beams (e-ray and o-ray) are combined together; wherein the e-ray is shifted toward the o-ray according to the arrow representing the walk-off characteristic of the third birefringent crystal  105 . Finally, the second port  20  of the circulator  1000  outputs the light beam into the single-core collimator  2 .  
         [0045]    [0045]FIG. 4B schematically shows the spatial location and the polarizations of the light beam traveling from the second port to the third port in the compact optical circulator of the second example. Each arrow respectively shows the projecting component of the optical axis of each birefringent crystal in the x-y plane. While a light beam emitted from the second fiber  22  of the single-core collimator  2  is received by the second port  20  of the circulator  1000 , the polarization of the light beam is random. When the light beam travels through the third birefringent crystal  105 , the light beam is split into two polarized beams according to the arrow representing the walk-off characteristic of the third birefringent crystal  105 . One is an e-ray polarized along the projecting component of the optical axis of the third birefringent crystal  105  and the other is an o-ray polarized perpendicular to the projecting component of the optical axis of the third birefringent crystal  105 . Next, the e-ray and o-ray respectively travel through the Faraday rotators  104   a ,  104   b , the polarization of the e-ray rotates clockwise 45 degrees and the polarization of the o-ray rotates counterclockwise 45 degrees. Next, the two polarized beams are shifted toward the minus x-axis according to the arrow representing the walk-off characteristic of the second birefringent crystal  103 . Next, the two polarized beams travel through the Faraday rotator  102   a ,  102   b  of the first polarization rotator  102 , one rotates clockwise 45 degrees to form an o-ray and the other rotates counterclockwise 45 degrees to form an e-ray. When the e-ray travels through the second polarization rotator  104 , the second birefringent crystal  103  and the first polarization rotator  102 , the e-ray is changed into o-ray. When the o-ray travels through the second polarization rotator  104 , the second birefringent crystal  103  and the first polarization rotator  102 , the o-ray is changed into e-ray. When the e-ray and o-ray travel through the first birefringent crystal  101 , the two polarized beams (e-ray and o-ray) are combined together; wherein the e-ray is shifted toward the o-ray according to the arrow representing the walk-off characteristic of the first birefringent crystal  101 . Finally, the third port  30  of the circulator  1000  outputs the light beam into the dual-core collimator  1 .  
         [0046]    Second Embodiment  
         [0047]    [0047]FIG. 5A is a top plane view showing a compact optical circulator of the second embodiment of the invention, and FIG. 5B is a side view showing a compact optical circulator of the second embodiment of the invention. In the second embodiment of the invention, the compact optical circulator  2000  includes a dual-core collimator  1 , a propagation director  100 , a first birefringent device  101 , a first polarization rotator  102 , a second birefringent device  103 , a second polarization rotator  104 , a reflective compensator  106 , a third birefringent device  105  and a single-core collimator  2 .  
         [0048]    The dual-core collimator  1  generally has a first fiber  11 , a second fiber  22  and a GRIN lens (not shown), wherein the first and second fibers  11 ,  22  are parallel. The compact optical circulator  2000  has three ports; wherein the first and second ports  10 ,  20  are located at the same side of the circulator  2000 , and the third port  30  is located at another side of the circulator  2000 . The dual-core collimator  1  emits a light beam to the first port  10  of the circulator  2000 , and receives another light beam from the second port  20  of the circulator  2000 . In general, the light beam emitted from the first fiber  11  of the dual-core collimator  1  is refracted by travelling through the GRIN lens, and then must be coupled to the first port  10  of the circulator  2000 . Then, forming a propagation director  100  with a refracting plane directs the light beam from first fiber  11  to first port  10  by Snell&#39;S law. Another light beam emitted from the second port  20  of the circulator  1000  must be coupled to the second fiber  22  of the dual-core collimator  1 . Then, forming the propagation director  100  with another refracting plane directs the light beam from second port  20  to second fiber  22  by Snell&#39;s law. Therefore, the propagation director  100  can direct one light beam from first fiber  11  to first port  10  and another light beam from second port  20  to second fiber  22  at the same time.  
         [0049]    The first birefringent device  101  is a birefringent crystal, such as LiNbO 3 , YVO 4  etc, and has walk-off characteristics. When the first port  10  receives a light beam, the light beam is divided into an e-ray and o-ray by the first birefringent device  101 , wherein both the e-ray and o-ray have orthogonal polarizations. When a light beam is received from the first port  10  and divided into two polarized beams in the circulator  2000 , the two polarized beams are combined together again by the first birefringent device  101 . When a light beam is received from the second port  20 , the light beam is divided into two polarized beams by the first birefringent device  101 .  
         [0050]    The first and second polarization rotators  102  and  104  respectively, have a non-reciprocal polarizing-rotating crystal, such as Faraday rotator, and a reciprocal polarizing-rotating unit, such as a half-wave plate. As well, the first and second polarization rotators  102 ,  104  can further forgo the use of the reciprocal polarizing-rotating unit, and only include the non-reciprocal polarizing-rotating crystal.  
         [0051]    The second birefringent device  103  is also a birefringent crystal and has an optical axis. While a light beam is received from the first port  10  or second port  20  and divided into two polarized beams, the two polarized beams are shifted or not according to the optical axis.  
         [0052]    The third birefringent device  105  is also a birefringent crystal. While the second port  20  receives a light beam and the light beam is divided into two polarized beams by the means mentioned above, the two polarized beams are combined together again by the third birefringent device  105 .  
         [0053]    As shown in FIG. 5A, when a light beam received from the first port  10  is transmitted in the circulator  2000 , the light beam is reflected to the second port  20  by a mirror. As the light beam is split into two polarized beams by the first and second birefringent devices  101 ,  103 , the mirror reflects the two polarized beams. Next, the two polarized beams are combined and output at the second port  20 . However, an optical path difference (“OPY”) is produced between the two polarized beams. For positive birefringent materials, the optical path length (“OPL”) of e-ray is longer than the optical path length of o-ray, so an optical path difference is produced between the e-ray and o-ray. Furthermore, polarization mode dispersion (“PMD”) is created at the second port  20 . In the circulator  2000  of the second embodiment of the invention, a reflective compensator  106  is provided to solve the problem of polarization mode dispersion.  
         [0054]    [0054]FIG. 6A schematically shows a reflective compensator of the second embodiment of the invention. As shown in FIG. 6A, the reflective compensator  106   a  is made of transparent material  41 , such as glass, and has a first reflecting layer  42 , an anti-reflecting layer  43  and a second reflecting layer  44 ; wherein the first reflecting layer  41  and the anti-reflecting layer  42  forms on the same surface of the reflective compensator  106   a , and the second reflecting layer  44  forms on the opposite surface parallel to the first reflecting layer  42  and anti-reflecting layer  43 . In the second embodiment, the total area of the first reflecting layer  42  and anti-reflecting layer  43  is equal to the area of the second reflecting layer  44 , and the area of the second reflecting layer  44  is half of the cross-sectional area of the optical circulator  2000 . As a light beam received from the first port  10  is split into two polarized beams, one polarized beam having longer OPL is reflected by the first reflecting layer  42 , and the other polarized beam having shorter OPL passes the anti-reflecting layer  43  and is reflected to leave the reflective compensator  106   a  by the second reflecting layer  44 . The OPL of the polarized beam having shorter OPL is increased by traveling and reflecting in the reflective compensator  106   a , the two polarized beams have the same OPL. In other words, the double distance between the anti-reflecting layer  43  and second reflecting layer  44  increases the OPL of one polarized beam having shorter OPL. Accordingly, the reflective compensator  106   a  of the second embodiment of the invention can solve the problem of OPD. As well, referring to FIG. 5A, 5B and  6 A, the area of the reflective compensator  106   a  is half of the cross-sectional area of the optical circulator  2000 , and then the light beam received from the second port  20  can travel through the reflective compensator  106   a.    
         [0055]    [0055]FIG. 6B schematically shows another reflective compensator of the second embodiment of the invention. As shown in FIG. 6B, the reflective compensator  106   b  has a reflecting substrate  46  and a transparent material  45 , such as glass, disposed on the reflecting substrate  46 . As a light beam received from the first port  10  is split into two polarized beams, one polarized beam having longer OPL is reflected by the reflecting substrate  46 , and the other polarized beam having shorter OPL enters the transparent material  45  and is reflected to leave the reflective compensator  106   b  by the reflecting substrate  46 . The OPL of the polarized beam having shorter OPL is increased by traveling and reflecting in the reflective compensator  106   b , the two polarized beams have the same OPL. In other words, the thickness of the transparent material  45  increases the OPL of one polarized beam having shorter OPL. Accordingly, the reflective compensator  106   b  of the second embodiment of the invention can solve the problem of OPD. As well, referring to FIG. 5A, 5B and  6 B, the area of the reflective compensator  106   b  is half of the cross-sectional area of the optical circulator  2000 , and then the light beam received from the second port  20  can travel through the reflective compensator  106   b.    
         [0056]    Third Example  
         [0057]    In the third example, the compact optical circulator with three ports includes: a dual-core collimator, a propagation director, a first birefringent crystal, a first polarization rotator, a second birefringent crystal, a second polarization rotator, a reflective compensator, a third birefringent crystal, and a single-core collimator. The first and second polarization rotators respectively have a Faraday rotator and a half-wave plate, and the reflective compensator is positioned between the Faraday rotator and the half-wave plate of the second polarization rotator.  
         [0058]    [0058]FIG. 7A schematically shows the spatial location and the polarization of the light beam traveling from the first port to the second port in the compact optical circulator of the third example. Each arrow respectively shows the projecting component of the optical axis of each birefringent crystal in the x-y plane. While a light beam emitted from the first fiber  11  of the dual-core collimator  1  is received by the first port  10  of the circulator  2000 , the polarization of the light beam is random. When the light beam travels through the first birefringent crystal  101 , the light beam is split into two polarized beams according to the arrow representing the walk-off characteristic of the first birefringent crystal  101 ; wherein one is an e-ray polarized along the y-axis and the other is an o-ray polarized along the x-axis. Next, the e-ray and o-ray respectively travel through the Faraday rotators  102   a ,  102   b , the polarization of the e-ray rotates counterclockwise 45 degrees and the polarization of the o-ray rotates clockwise 45 degrees. Next, the two polarized beams travel through the half-wave plate  102   c , the two beams rotate clockwise 45 degrees. After the two polarized beams travel through the first polarization rotator  102 , the polarizations of the two polarized beams represent o-ray for the second birefringent crystal  103 . Thus, the two polarized beams pass the second birefringent crystal  103  without shifting. Next, the two polarized beams travel through the Faraday rotators  104   a ,  104   b  of the second polarization rotator  104 , one rotates clockwise 45 degrees and the other rotates counterclockwise 45 degrees. Next, the two polarized beams are respectively reflected onto the Faraday rotators  104   a ,  104   b  by the reflective compensator  106 . Next, the two polarized beams travel through the Faraday rotators  104   a ,  104   b  of the second polarization rotator  104  again, one rotates clockwise 45 degrees and the other rotates counterclockwise 45 degrees. After the two polarized beams travel through the second polarization rotator  104 , the polarizations of the two polarized beams represent e-ray for the second birefringent crystal  103 . Thus, the two polarized beams are shifted toward the minus x-axis according to the arrow representing the walk-off characteristic of the second birefringent crystal  103 . Next, the two polarized beams travel through the half-wave plate  102   c , and the two polarized beams rotate counterclockwise 45 degrees. Next, the two polarized beams travel through the Faraday rotators  102   a ,  102   b , one rotates counterclockwise 45 degrees and the other rotates clockwise 45 degrees. When the e-ray and o-ray travel through the first birefringent crystal  101 , the two polarized beams (e-ray and o-ray) are combined together; wherein the e-ray is shifted toward the o-ray according to the arrow representing the walk-off characteristic of the first birefringent crystal  101 . Finally, the second port  20  of the circulator  2000  outputs the light beam into the second fiber  22  of the dual-core collimator  1 .  
         [0059]    [0059]FIG. 7B schematically shows the spatial location and the polarizations of the light beam traveling from the second port to the third port in the compact optical circulator of the third example. Each arrow respectively shows the projecting component of the optical axis of each birefringent crystal in the x-y plane. While a light beam emitted from the second fiber  22  of the dual-core collimator  1  is received by the second port  20  of the circulator  2000 , the polarization of the light beam is random. When the light beam travels through the first birefringent crystal  101 , the light beam is split into two polarized beams according to the arrow representing the walk-off characteristic of the first birefringent crystal  101 . One is an e-ray polarized along the y-axis and the other is an o-ray polarized along the x-axis. Next, the e-ray and o-ray respectively travel through the Faraday rotators  102   a ,  102   b , the polarization of the e-ray rotates counterclockwise 45 degrees and the polarization of the o-ray rotates clockwise 45 degrees. Next, the two polarized beams travel through the half-wave plate  102   c , and the two beams rotate clockwise 45 degrees. After the e-ray and o-ray travel through the first polarization rotator  102 , the polarizations of the two polarized beams represent e-ray for the second birefringent crystal  103 . The two polarized beams pass the second birefringent crystal  103  without shifting. Next, the two polarized beams travel through the Faraday rotator  104   a ,  104   b  of the second polarization rotator  104 , one rotates clockwise 45 degrees and the other rotates counterclockwise 45 degrees. Next, the two polarized beams travel through the half-wave plate  104   c , and the two polarized beams rotate clockwise 45 degrees. For the third birefringent crystal  105 , as the e-ray travels through the first polarization rotator  102 , the second birefringent crystal  103  and the second polarization rotator  104 , the e-ray is changed into o-ray. For the third birefringent crystal  105 , as the o-ray travels through the first polarization rotator  102 , the second birefringent crystal  103  and the second polarization rotator  104 , the o-ray is changed into e-ray. When the e-ray and o-ray travel through the third birefringent crystal  105 , the two polarized beams (e-ray and o-ray) are combined together; wherein the e-ray is shifted toward the o-ray according to the arrow representing the walk-off characteristic of the third birefringent crystal  105 . Finally, the third port  30  of the circulator  2000  outputs the light beam from the third port  30  to the single-core collimator  1 .  
         [0060]    Fourth Example  
         [0061]    In the fourth example, the compact optical circulator with three ports includes: a dual-core collimator, a propagation director, a first birefringent crystal, a first polarization rotator, a second birefringent crystal, a second polarization rotator, a reflective compensator, a third birefringent crystal, and a single-core collimator. The first and second polarization rotators forgo the use of the reciprocal polarizing-rotating unit, such as a half-wave plate, and only include the non-reciprocal polarizing-rotating crystal, such as the Faraday rotator. The reflective compensator is positioned between the second polarization rotator and the third birefringent crystal.  
         [0062]    [0062]FIG. 8A schematically shows the spatial location and the polarizations of the light beam traveling from the first port to the second port in the compact optical circulator of the fourth example. Each arrow respectively shows the projecting component of the optical axis of each birefringent crystal in the x-y plane. While a light beam emitted from the first fiber  11  of the dual-core collimator  1  is received by the first port  10  of the circulator  2000 , the polarization of the light beam is random. When the light beam travels through the first birefringent crystal  101 , the light beam is split into two polarized beams according to the arrow representing the walk-off characteristic of the first birefringent crystal  101 . One is an e-ray polarized along the projecting component of the optical axis of the first birefringent crystal  101  and the other is an o-ray polarized perpendicular to the projecting component of the optical axis of the first birefringent crystal  101 . Next, the e-ray and o-ray respectively travel through the Faraday rotators  102   a ,  102   b , the polarization of the e-ray rotates counterclockwise 45 degrees and the polarization of the o-ray rotates clockwise 45 degrees. After the e-ray and o-ray travel through the two Faraday rotators  102   a ,  102   b , the polarizations of the two polarized beams represent o-ray for the second birefringent crystal  103 . Next, the two polarized beams travel through the second birefringent crystal  103  without shifting. Next, the two polarized beams travel through the Faraday rotators  104   a ,  104   b  of the second polarization rotator  104 , one rotates counterclockwise 45 degrees and the other rotates clockwise 45 degrees. Next, the two polarized beams are respectively reflected onto the Faraday rotators  104   a ,  104   b  by the reflective compensator  106 . Next, the two polarized beams travel through the Faraday rotators  104   a ,  104   b  of the second polarization rotator  104  again, one rotates counterclockwise 45 degrees and the other rotates clockwise 45 degrees. After the two polarized beams travel through the second polarization rotator  104 , the polarizations of the two polarized beams represent e-ray for the second birefringent crystal  103 . Thus, the two polarized beams are shifted toward the minus x-axis according to the arrow representing the walk-off characteristic of the second birefringent crystal  103 . Next, the two polarized beams travel through the Faraday rotators  102   a ,  102   b , one rotates counterclockwise 45 degrees and the other rotates clockwise 45 degrees. Next, the two polarized beams travel through the first birefringent crystal  101 , the two polarized beams (e-ray and o-ray) are combined together; wherein the e-ray is shifted toward the o-ray according to the arrow representing the walk-off characteristic of the first birefringent crystal  101 . Finally, the second port  20  of the circulator  2000  outputs the light beam into the second fiber  22  of the dual-core collimator  1 .  
         [0063]    [0063]FIG. 8B schematically shows the spatial location and the polarizations of the light beam traveling from the second port to the third port in the compact optical circulator of the fourth example. Each arrow respectively shows the projecting component of the optical axis of each birefringent crystal in the x-y plane. While a light beam emitted from the second fiber  22  of the dual-core collimator  1  is received by the second port  20  of the circulator  2000 , the polarization of the light beam is random. When the light beam travels through the first birefringent crystal  101 , the light beam is split into two polarized beams according to the arrow representing the walk-off characteristic of the first birefringent crystal  101 . One is an e-ray polarized along the projecting component of the optical axis of the first birefringent crystal  101  and the other is an o-ray polarized perpendicular to the projecting component of the optical axis of the first birefringent crystal  101 . Next, the e-ray and o-ray respectively travel through the Faraday rotators  102   a ,  102   b , the polarization of the e-ray rotates counterclockwise 45 degrees and the polarization of the o-ray rotates clockwise 45 degrees. After the e-ray and o-ray travel through the two Faraday rotators  102   a ,  102   b , the polarizations of the two polarized beams represent o-ray for the second birefringent crystal  103 . Next, the two polarized beams travel through the second birefringent crystal  103  without shifting. Next, the two polarized beams travel through the Faraday rotators  104   a ,  104   b  of the second polarization rotator  104 , one rotates counterclockwise 45 degrees and the other rotates clockwise 45 degrees. When the e-ray travels through the first polarization rotator  102 , the second birefringent crystal  103  and the second polarization rotator  104 , the e-ray is changed into o-ray. When the o-ray travels through the first polarization rotator  102 , the second birefringent crystal  103  and the second polarization rotator  104 , the o-ray is changed into e-ray. When the e-ray and o-ray travel through the third birefringent crystal  105 , the two polarized beams (e-ray and o-ray) are combined together; wherein the e-ray is shifted toward the o-ray according to the arrow representing the walk-off characteristic of the third birefringent crystal  105 . Finally, the third port  30  of the circulator  2000  outputs the light beam into the single-core collimator  2 .  
         [0064]    In the invention, the birefringent devices mentioned are selected from the group consisting of LiNbO 3 , YVO 4 , Calcite, Ti 0   2 , and others.  
         [0065]    While the preferred embodiment of the present invention has been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. The scope of the invention, therefore, is to be determined solely by the following claims.