Abstract:
The invention relates to an optical circulator for use as a front end for an optical device platform, which requires a pair of like-polarized sub-beams as input. A combined beam of light comprising a plurality of wavelength channels is input the optical circulator at an input port, and the pair of like-polarized sub-beams is launched via a pair of input/output ports into the optical device platform, wherein the sub-beams are modified. Within the optical device platform the combined beam of light is separated into individual channels, and one or more of the channels can be dropped or attenuated before the remaining channels are recombined into to modified sub-beams for output. The two modified sub-beams are launched back into the optical circulator, which directs the modified sub-beams to an output port remote from the input port. Any light reentering the optical circulator with the wrong polarization will neither exit the input nor the output port.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application claims priority from U.S. Patent Application 60/506,176 filed Dec. 20, 2003, which is incorporated herein by reference for all purposes. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to an optical circulator, and in particular to an optical circulator positioned at the front end of an optical device for launching an optical signal into the optical device and for redirecting the optical signal after modification by the optical device.  
         BACKGROUND OF THE INVENTION  
         [0003]    Conventional circulators, such as those disclosed in U.S. Pat. No. 5,574,596 issued Nov. 12, 1996 to Yihao Cheng; and U.S. Pat. No. 5,909,310 issued Jun. 1, 1999 to Wei-Zhong Li et al are three port devices that direct light entering a first port to a second port, and light entering the second port back to a third port. Generally, the light entering the first port is divided into two sub-beams, the polarization of which is manipulated to enable the sub-beams to pass through a polarization-dependent beam director along a first path, after which the sub-beams are recombined for output. Similarly, light entering the second port is divided into two sub-beams, but the polarization thereof is manipulated so that the sub-beams pass through the polarization-dependent beam director along a second path, different than the first, whereby the sub-beams are recombined at the third port.  
           [0004]    Unfortunately, these conventional circulators are designed to recombine the light beam traveling from the first port to the second port, and to receive a combined light beam at the second port for travel to the third port.  
           [0005]    An object of the present invention is to overcome the shortcomings of the prior art by providing an optical circulator that outputs and receives like-polarized sub-beams instead of a single combined beam at the second port.  
         SUMMARY OF THE INVENTION  
         [0006]    Accordingly, the present invention relates to a circulator device for launching on optical signal into an optical device and for outputting the optical signal after modification by the optical device comprising:  
           [0007]    an input port for inputting the optical signal;  
           [0008]    a polarization beam splitter optically coupled to the input port for separating the optical signal into first and second orthogonally polarized sub-beams;  
           [0009]    a first polarization rotator for rotating the polarization of at least one of the first and second orthogonally polarized sub-beams of the optical signal, so that both sub-beams of the optical signal have the same polarization;  
           [0010]    polarization beam directing means for directing optical sub-beams with a first polarization along one set of paths, and for directing optical sub-beams with a second polarization along another set of paths;  
           [0011]    non-reciprocal polarization rotating means for rotating the polarization of optical sub-beams traveling in a first direction therethrough, while having no overall effect on the polarization of optical sub-beams traveling in a second direction therethrough;  
           [0012]    a first input/output port for outputting the first sub-beam of the optical signal with the first polarization, and for inputting a first sub-beam of the modified optical signal with the first polarization, whereby the first sub-beam of the modified optical signal travels back through the non-reciprocal polarization rotating means and the polarization beam directing means;  
           [0013]    a second input/output port for outputting the second sub-beam of the optical signal with the first polarization, and for inputting a second sub-beam of the modified optical signal with the first polarization, whereby the second sub-beam of the modified optical signal travels back through the non-reciprocal polarization rotating means and the polarization beam directing means;  
           [0014]    a second polarization rotator for rotating the polarization of at least one of the first and second sub-beams of the modified optical signal, so that the sub-beams of the optical signal have orthogonal polarizations;  
           [0015]    a polarization beam combiner optically coupled to the output port for combining first and second orthogonally polarized sub-beams of the modified optical signal; and  
           [0016]    an output port for outputting the modified optical signal;  
           [0017]    whereby light entering the first and second input/output ports with the second polarization will be directed away from the input and output ports.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:  
         [0019]    [0019]FIG. 1 is a plan view of the circulator according to the present invention illustrating light traveling from a first port to a third and fourth ports;  
         [0020]    [0020]FIG. 2 is a plan view of the circular of FIG. 1 illustrating light traveling from the third and fourth ports to a second port;  
         [0021]    [0021]FIG. 3 is a side of the circulator of FIGS. 1 and 2 illustrating light traveling from the first port to the third and fourth ports, and from the third and fourth ports to the second port;  
         [0022]    [0022]FIG. 4 is a chart of the polarizations of sub-beams of light traveling through the various elements of the circulator of FIGS.  1  to  3 ;  
         [0023]    [0023]FIG. 5 is a schematic representation of an optical device platform utilizing the circulator of FIGS.  1  to  3 ;  
         [0024]    [0024]FIG. 6 is a plan view of an alternative embodiment of the present invention;  
         [0025]    [0025]FIG. 7 is a side view of the embodiment of FIG. 5;  
         [0026]    [0026]FIG. 8 is an exploded isometric view of the circulator of FIGS. 5 and 6 illustrating light traveling from the first port to the third and fourth ports;  
         [0027]    [0027]FIG. 9 is an exploded isometric view of the circulator of FIGS. 5, 6 and  7  illustrating light traveling from the third and fourth ports to the second port; and  
         [0028]    [0028]FIG. 10 is a chart of the polarizations of sub-beams of light traveling through the various elements of the circulator of FIGS.  5  to  8 . 
     
    
     DETAILED DESCRIPTION  
       [0029]    With reference to FIGS.  1  to  4 , the circulator  1  according to the present invention includes a first port  2  and a second port  3  adjacent each other on one end. The first port  2  launches a beam of light  4  into a first birefringent walk-off crystal  6  (Position A in FIG. 4), which splits the beam of light  4  into two orthogonally polarized sub-beams  7  and  8  (Position B). Sub-beam  7  travels through a first half-wave plate  9 , disposed with a birefringent axis at +22.5°, for rotating the polarization of the sub-beam  7  by +45°. The birefringent axis of a second half-wave plate  11  is oriented at −22.5° for rotating the polarization of the sub-beam  8  by −45°, whereby both sub-beams  7  and  8  have the same polarization (Position C). A first Faraday rotator  12  is positioned adjacent the first and second waveplates  9  and  11  for rotating the polarization of the sub-beams  7  and  8  by another +45°, whereby both sub-beams  7  and  8  are horizontally polarized (Position D). The horizontally polarized sub-beams  7  and  8  pass through a second birefringent walk-off crystal  13 , which directs the sub-beams according to their polarization. Since the sub-beams  7  and  8  are horizontally polarized, they travel straight through the second birefringent walk-off crystal  13  (Position E). A second Faraday rotator  14  rotates the polarization of both sub-beams  7  and  8  by +45° (Position F), and a third half-wave plate  16  rotates both of the sub-beams  7  and  8  by 45° (Position G), whereby the combination of the second Faraday rotator  14  and the third half-wave plate  16  have no cumulative effect on the polarization of the sub-beams  7  and  8 . A third port  17  is provided for outputting the sub-beam  7 , while a fourth port  18  is provided for outputting the sub-beam  8 .  
         [0030]    With reference to FIG. 5, the present invention was designed for use as the circulator front end  1  for an optical device platform  22 , which can be used for various devices, e.g. a wavelength switch or a dynamic gain equalizer. Within the platform  22  the sub-beams  7  and  8  are directed at a focusing and redirecting element in the form of a concave mirror  23 , which reflects the sub-beams  7  and  8  towards a wavelength dispersive element  24 . The wavelength dispersive element  24  separates each sub-beam  7  and  8  into a set of distinct wavelength channel sub-beams  27   a  to  27   g  and  28   a  to  28   g , respectively, and directs the wavelength channel subbeams  27   a  to  27   g  and  28   a  to  28   g  towards the concave mirror  23  for a second refocusing and reflection. Subsequently, the wavelength channel sub-beams  27   a  to  27   g  and  28   a  to  28   g  are directed at a modifying element  29 , which can be in the form of a micro-electro-mechanical (MEMs) mirror array for the wavelength switch or an array of liquid crystal cells for the dynamic gain equalizer. The MEMs mirror array would redirect one or more pairs of the wavelength channel sub-beams, e.g.  27   b  and  28   b ,  27   c  and  28   c , along a different path, while the remaining wavelength channel sub-beams  27   a ,  28   a ,  27   d  to  27   g  and  28   d  to  28   g  are reflected back to the dispersive element  24  via the concave mirror  23  for recombination into sub-beams  7 ′ and  8 ′. The recombined sub-beams  7 ′ and  8 ′ are reflected via the concave mirror  23  to the front-end circulator  1 .  
         [0031]    Alternatively, if the modifying element  29  is an array of liquid crystal cells, the polarizations of the various wavelength channel sub-beams  27   a  to  27   g  and  28   a  to  28   g  are independently rotated by desired amounts, depending on the amount of attenuation that is required. Accordingly, when the wavelength channel sub-beams  27   a  to  27   g  and  28   a  to  28   g  pass through any polarization beam splitter an amount of unwanted light can be separated from the remaining beams. This separation can be done in the modifying element  29  or at the circulator front end  1 .  
         [0032]    With reference to FIGS. 2 and 3, the recombined sub-beams  7 ′ and  8 ′ re-enter the third and fourth ports  17  and  18 , respectively, (Position G′), and pass through the third half-wave plate  16  and the second Faraday rotator  14 . Since the third half-wave plate  16  is a reciprocal polarization rotator, the polarization of both sub-beams  7 ′ and  8 ′ is rotated by +45° (Position F′); however, since the second Faraday rotator  14  is a non-reciprocal polarization rotator, the polarization of both sub-beams  7 ′ and  8 ′ is rotated by another +45° for a combined total of +90°, e.g. from horizontally to vertically polarized (Position E′). As a result, the sub-beams  7 ′ and  8 ′ get walked of f by the second birefringent walk-off crystal  13 . Up until now the sub-beams  7 ,  8 ,  7 ′ and  8 ′ have traveled in the same plane, illustrated in FIG. 1 and the upper path in FIG. 3, through the circulator  1 ; however, because the birefringent axis of the second birefringent walk-off crystal  13  is perpendicular to the birefringent axis of the first birefringent walk-off crystal  6 , the second birefringent walk-off crystal  13  redirects the sub-beams  7 ′ and  8 ′ out of the original plane to a parallel plane, illustrated in FIG. 2 and the lower path in FIG. 3 (Position D′).  
         [0033]    The sub-beam  7 ′, traveling in the opposite direction to sub-beam  7 , passes through the first Faraday rotator  12  (+45°) and the first half-wave plate  9  (−45°), which results in no cumulative effect to the polarization thereof, e.g. stays vertically polarized (Position B′). The sub-beam  8 ′ passes through the Faraday rotator  12  (+45°) and the second half-wave plate  11  (+45°), which rotates the polarization of the sub-beam  8 ′ (+90°) from vertically to horizontally polarized (Position B′). Accordingly, the sub-beam  7 ′ is walked off by the first walk-off crystal  6 , towards the sub-beam  8 ′, which passes straight through the first walk-off crystal  6 . The recombined beam of light  4 ′ is then output the second port  3  (Position A′).  
         [0034]    With reference to the bottom row of FIG. 4, any vertically polarized light  7 ″ and  8 ″ re-entering the third and fourth ports  17  and  18 , respectively, (Position G″), will pass through the third half-wave plate  16  (+45°) and the second Faraday rotator  14  (+45°), resulting in a change in the state of polarization of +90°, e.g. vertically to horizontally polarized (Position E″). The horizontally polarized light  7 ″ and  8 ″ will not be walked off to the lower plane by the second birefringent walk-off crystal  13 , but will pass therethrough in the original (upper) plane (Position D″). The first Faraday rotator  12  (+45°) and the first half-wave plate  9  (−45°) have no cumulative effect on the polarization of the sub-beam  7 ″, which stays horizontally polarized (Position B″). The first Faraday rotator  12  (+45°) and the second half-wave plate  11  (+45°) rotate the polarization of the sub-beam  8 ″ by 90° from horizontally polarized to vertically polarized (Position B″). Accordingly, the sub-beam  7 ″ passes straight through the first birefringent walk-off crystal  6 , while the sub-beam  8 ″ gets walked off away from the sub-beam  7 ″. Neither sub-beam  7 ″ or  8 ″ gets directed to the input port  2  or the output port  3 , i.e. both sub-beam  7 ″ and  8 ″ are spilled off.  
         [0035]    An alternative embodiment, for use in collimated space, is illustrated in FIGS.  6  to  10 . An input port  31  comprises a ferrule  32  surrounding an end of an input fiber  33 , which launches a beam of light  34  into a GRIN lens  36  (Position I). A polarization beam splitter in the form of a first birefringent crystal  37  (Rutile or YVO 4 ) divides the beam  34  into first and second orthogonally polarized sub-beams  38  and  39  (Position II). A first half-wave plate  41 , with its birefringent axis oriented at +22.5°, rotates the polarization of the first sub-beam  38  by +45° (Position III), after which a first Faraday rotator  42  rotates the polarization of the first sub-beam  38  by an additional +45° (Position IV), whereby the polarization of the first sub-beam is rotated from horizontal to vertical. A second half-wave plate  43 , with its birefringent axis oriented at −22.5°, rotates the polarization of the second sub-beam  39  by 45° (Position III), after which the first Faraday rotator  42  rotates the polarization of the second sub-beam  38  by +45° (Position IV), whereby the polarization of the second sub-beam  38  remains the same, e.g. vertically polarized. A polarization beam director, in the form of a polarization beam splitting prism  44  with a pair of polarization beam splitting coating  46   a  and  46   b , redirects the vertically polarized first and second sub-beams  37  and  38  to a third half-wave plate  47  (Position V), with its birefringent axis oriented at +22.5°, which rotates the polarization of both sub-beams by +45° (Position VI). A second Faraday rotator  48  rotates the polarization of the first and second sub-beams  37  and  38  by a further +45°, whereby the first and second sub-beams are horizontally polarized for output the third and fourth ports  51  and  52  into the optical device platform (Position VII), as hereinbefore described.  
         [0036]    Modified sub-beams  37 ′ and  38 ′ re-entering the third and fourth ports  51  and  52 , respectively, (Position VIII) pass back through the second Faraday rotator  48 , which again rotates their polarization by +45° (Position IX). The third half-wave plate  47  negates the aforementioned polarization rotation by rotating the polarization of the modified sub-beams  37 ′ and  38 ′ by 45°, whereby the modified sub-beams  37 ′ and  38 ′ re-enter the polarization beam splitting cube  44  horizontally polarized (Position X), instead of vertically polarized, as before. Accordingly, the modified sub-beams  37 ′ and  38 ′ pass directly through the polarization beam splitting coating  46   b  to a third Faraday rotator  53  (Position XI), which rotates the polarization of both modified sub-beams  37 ′ and  38 ′ by −45° (Position XII). A fourth half-wave plate  54  rotates the polarization of the first modified sub-beam  37 ′ by a further −45°, whereby the first modified sub-beam  37 ′ becomes vertically polarized (Position XIII). A fifth half-wave plate  56  negates the effect of the third Faraday rotator  49  by rotating the polarization of the second modified sub-beam  38 ′ by +45°, whereby the second modified sub-beam  38 ′ remains horizontally polarized (Position XIII). A single magnet  57  is provided for use with all three Faraday rotators  42 ,  47  and  49 .  
         [0037]    A polarization beam combiner, in the form of a second birefringent walk-off crystal  58 , recombines the first and second modified sub-beams  37 ′ and  38 ′ into a single beam  34 ′ for output the output port  59 . A second GRIN lens  61  focuses the beam  34 ′ onto an output fiber  62 , which is encased in a ferrule  63 .