Abstract:
Methods, systems, and apparatus for optical communications. One of the apparatuses comprises a birefringent crystal configured to separate an incoming light beam input at a first port into component light beams having orthogonal polarization directions and directing the component light beams on respective paths to exit locations on the birefringent crystal; and a Faraday rotator positioned between the birefringent crystal and a beam folding optic assembly, wherein the Faraday rotator is positioned such that light beams exiting the birefringent crystal along a first path from a first exit location pass through the Faraday rotator before being incident on the beam folding optic assembly and that light beams exiting the birefringent crystal along a second path from a second exit location pass directly to the beam folding optic assembly without being incident on the Faraday rotator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) of the filing date of U.S. patent application No. 61/907,889, for “Faraday Rotator Mirror,” which was filed on Nov. 22, 2013, and which is incorporated here by reference. 
    
    
     BACKGROUND 
     This specification relates to optical communications. 
     A conventional Faraday rotator is an optical device that rotates the polarization of light signals in the presence of a magnetic field. Faraday rotators are typically used in many different optical communications applications including fiber-optic Michelson Interferometers, laser amplifiers, sensors, and to compensate for induced birefringence in optical fibers. 
     SUMMARY 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in apparatuses that include a birefringent crystal configured to separate an incoming light beam input at a first port into component light beams having orthogonal polarization directions and directing the component light beams on respective paths to exit locations on the birefringent crystal; and a Faraday rotator positioned between the birefringent crystal and a beam folding optic assembly, wherein the Faraday rotator is positioned such that light beams exiting the birefringent crystal along a first path from a first exit location pass through the Faraday rotator before being incident on the beam folding optic assembly and that light beams exiting the birefringent crystal along a second path from a second exit location pass directly to the beam folding optic assembly without being incident on the Faraday rotator. 
     The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. The apparatus further includes a collimator optically coupled between the birefringent crystal and an optical fiber, wherein light beams enter and exit the apparatus through the optical fiber. The beam folding optic assembly folds light beams incident along the first path to the reverse of the second path and wherein the beam folding optic assembly folds light beams incident along the second path to the reverse of the first path. The Faraday rotator rotates incident light beams by substantially 90 degrees. The birefringent crystal provides Faraday rotation angle filtering to provide suppression of error in the Faraday rotator. 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in apparatuses that include a polarization beam splitter configured to separate an incoming light beam input at a first port into component light beams having orthogonal polarization directions and directing the component light beams to respective second and third ports of the polarization beam splitter; a Faraday rotator positioned between the polarization beam splitter and a first folding mirror such that light beams exiting the second port of the polarization beam splitter are directed through the Faraday rotator to the first folding mirror, and wherein the first folding mirror directs incident light beams from the Faraday rotator to a second folding mirror; and the second folding mirror positioned between the polarization beam splitter and the first folding mirror such that light beams exiting the third port of the polarization beam splitter are reflected by the second folding mirror to the first folding mirror, and wherein first folding mirror directs incident light beams from the second folding mirror to the Faraday rotator. 
     The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. The apparatus further includes a collimator optically coupled between the polarization beam splitter and an optical fiber, wherein light beams enter and exit the apparatus through the optical fiber. The first and second folding mirrors fold light beams incident along the first path from the second port of the polarization beam splitter to the reverse of the second path and wherein the first and second folding mirrors fold light beams incident along the second path from the third port of the polarization beam splitter to the reverse of the first path. The Faraday rotator rotates incident light beams by substantially 90 degrees. The of the polarization beam splitter provides Faraday rotation angle filtering to provide suppression of error in the Faraday rotator. 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in methods that include the actions of receiving a light beam, the components of the light beam having random polarization directions; separating the light beam into a first beam and a second beam, the first beam following a first path and the second beam following a second path, wherein the first beam and the second beam have orthogonal polarization directions; rotating the polarization direction of the first beam and then reflecting the first beam back along the second path; reflecting the second beam back along the first path and then rotating the polarization direction of the second beam; combining the first beam and the second beam such that the first beam and the second beam have orthogonal polarization directions, providing suppression of error in the rotation of the polarization direction of the first and second beams; and outputting the combined beam. 
     The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. The method can include collimating the received light beam before separating the light beam into a first beam and a second beam. The light separating is performed by passing the light beam through a birefringent crystal. The light separating is performed by passing the light beam through a polarization beam splitter. The rotating the polarization direction is performed by passing the first beam through a Faraday rotator. 
     Particular embodiments of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. Faraday rotator mirror structures are provided that are temperature insensitive and provide a flat wavelength response. 
     The details of one or more embodiments of the subject matter of this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example Faraday rotator mirror. 
         FIG. 2  is a block diagram of another example Faraday rotator mirror. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     A Faraday rotator is intended to provide a specified rotation of a polarization direction of incident light beams. In many optical communications applications, there needs to be a very accurate Faraday rotation angle. For example, in a fiber-optic interferometer, Faraday rotator mirrors are used to eliminate interference signal fluctuations due to random polarization direction changes in the optical fibers. The exact rotation provided by a conventional Faraday rotator can vary due to temperature and wavelength changes. This specification describes different Faraday rotator mirror apparatuses that provide suppression of error in the Faraday rotator to provide a substantially consistent polarization output regardless of temperature or wavelength. 
       FIG. 1  is a block diagram of an example Faraday rotator mirror  100 . The Faraday rotator mirror  100  is coupled to an optical fiber  102 . Light beams, for example carrying one or more wavelengths used for optical communications, enter and exit the Faraday rotator mirror  100  through the optical fiber  102 . In particular, the optical fiber  102  is optically coupled to a first end of a collimator  104  of the Faraday rotator mirror  100 . The collimator  100  aligns an incoming light beam. A second end of the collimator  104  is optically coupled to a birefringent crystal  106 . The birefringent crystal  106  is optically coupled to a prism  110 . 
     The Faraday rotator mirror  100  also includes a Faraday rotator  108  positioned between the birefringent crystal  106  and the prism  110 . The Faraday rotator  108  is positioned between the birefringent crystal  106  and the prism  110  such that only one light path exiting from the birefringent crystal  106  is incident on the Faraday rotator  108  prior to entering the prism  110 . 
     The Faraday rotator  108  is also positioned within a magnetic field generating element  112 . The magnetic field generating element  112  can include, for example, a wire coil wound onto a circular iron core. An electrical current can be applied to the coil to generate a magnetic field. Alternatively, magnetic field generating element  112  can be a permanent circular shaped magnet. In some other implementations, any suitable magnetic field generating structure can be used. The applied magnetic field from the magnetic field generating element  112  causes the polarization of a light beam passing through the Faraday rotator  108  to be rotated by a specified amount. In particular, the Faraday rotator  112  can rotate the polarization of an incident light beam by substantially 90 degrees. 
     In operation, a light beam  10  is input from the optical fiber  102  to the collimator  104  of the Faraday rotator mirror  100 . The collimator  104  is configured to align incoming light to the same direction e.g., to form substantially parallel light. 
     The incoming light beam  10  can have random polarization directions. Light beams are formed of electromagnetic waves having varying electric and magnetic fields that oscillate in directions perpendicular to the direction of propagation. The polarization direction of a particular light wave in the beam corresponds to a direction parallel to the electrical field of the light wave. The incoming light beam is formed of many light waves having different polarization directions. Randomly polarized light can be described with respect to the component magnitude along orthogonal axes. In particular a Jones vector form can be used to describe the respective magnitude components of the electric field along the orthogonal axes as well as a phase component describing a phase retardation along the orthogonal axes. 
     The light beam  12  exiting the second end of the collimator  104  enters a first side of the birefringent crystal  106 . The birefringent crystal  106  separates the incoming light beam having random polarization directions into two separate light beams having orthogonal polarization directions relative to each other, commonly referred to as an “ordinary” beam  14  and an “extraordinary” beam  16 . In addition to separating the light beams by polarization direction, the two separate light beams also diverge due to the nature of the birefringent crystal  106 . 
     The ordinary beam  14  has a first polarization direction and follows a first path  1 A through the birefringent crystal  106 . The ordinary beam  14  exits the birefringent crystal  106  at a first exit location at a second end of the birefringent crystal  106 . The ordinary beam  14  is then incident on the Faraday rotator  108 . The polarization direction of the ordinary beam  14  is rotated such that the polarization direction is rotated by substantially 90 degrees upon exiting the Faraday rotator  108  to form rotated beam  18 . 
     The extraordinary beam  16  has a second polarization direction that is orthogonal to the first polarization direction of the ordinary beam  14 , and follows a second path  1 B through the birefringent crystal  106 . The extraordinary beam  16  exits the birefringent crystal  106  at a second exit location at the second end of the birefringent crystal  106  as beam  20 . 
     The prism  110  is configured to fold the path of incident light beams back along the incident direction but spatially displaced. Other suitable folding optics, e.g., mirrors, can be used. In particular, each of the rotated beam  18  and the beam  20  are incident at positions on the prism  110  such that they are folded back along the path of each other. Thus, the rotated beam  18  exits the prism  110  at the location in which the beam  20  enters the prism  110  and the beam  20  exits the prism  110  at the location in which the rotated beam  18  enters the prism  110 . 
     After exiting the prism  110 , the beam  20  follows the reverse path of the rotated beam  18  through the Faraday rotator  108 . The polarization direction of the beam  20  is rotated such that the polarization direction is rotated by substantially 90 degrees upon exiting the Faraday rotator  108  to form rotated beam  22 . Rotated beam  22  then enters the birefringent crystal  106  at the first exit location at the second end of the birefringent crystal  106  and follows the first path  1 A through the birefringent crystal  106 . Since the rotated beam  22  has a rotated polarization direction that is now the same as the initial ordinary beam  14 , it directly follows the first path  1 A in the opposite direction as the ordinary beam  14 . 
     The rotated beam  18  follows the reverse path of the beam  20  after exiting the prism  110 . The rotated beam  18  enters the birefringent crystal  106  at the second exit location at the second end of the birefringent crystal  106  and follows the second path  1 B through the birefringent crystal  106 . In particular, since the rotated beam  18  has a rotated polarization direction that is now the same as the initial extraordinary beam  16 , it directly follows the second path  1 B in the opposite direction as the extraordinary beam  16 . 
     Thus, a loop is formed that routes light beams separated by the birefringent crystal  106  back along the path of each other. In particular, the ordinary beam  14  separated by the birefringent crystal  106  is looped back to the path of the extraordinary beam  16 . Similarly, the extraordinary beam  16  is looped back to the path of the ordinary beam  14 . Additionally, while the polarization directions remain orthogonal, the polarization directions of the ordinary beam  14  and the extraordinary beam  16  are switched upon their return to the birefringent crystal  106 . 
     A combined beam  24  exits the birefringent crystal  106  at the first location on the first side of the birefringent crystal  106 , passes through the collimator  104 , and exits the Faraday rotator mirror  100  though the optical fiber  102 . The combined beam  24  has orthogonal polarization directions. 
     The birefringent crystal  106  acts as a Faraday rotation angle filter. A Faraday rotator may not always rotate incoming light beams by exactly 90 degrees. In particular, many Faraday rotators exhibit temperature and wavelength dependence. For example, as temperature increases, the rotation angle can decrease. However, the operating temperature is typically not constant. Similarly, the rotation angle of a Faraday rotator is dependent upon a proportionality constant for the material of the rotator. However, this constant also varies with wavelength. The birefringent crystal  106 , however, only passes components of the light beams to the exit path that are rotated by 90 degrees. In particular, only components of light beams that have a polarization direction parallel to or perpendicular to the axis of the birefringent crystal  106 , depending the respective beam path, are passed along the exit path from the birefringent crystal  106  to the collimator  104 . For example, if a light beam has a polarization direction that is slightly off of perpendicular to the axis of the birefringent crystal, e.g., by 1 degree, due to error of the Faraday rotator, only the perpendicular component of the vector describing the direction of the electric field of the light beam is passed along the beam path to the exit. 
     Thus, the output light beam has orthogonal polarization directions regardless of any error in the rotation angle of the Faraday rotator caused by changes in temperature or wavelength. Depending on the error of birefringent crystal  106  in passing light beams having precise polarization directions, any error resulting from the Faraday rotator can be greatly suppressed. For example, if the error of the birefringent crystal  106  is 40 dB, a 10000× suppression of the Faraday rotation error can be achieved. 
       FIG. 2  is a block diagram of another example Faraday rotator mirror  200 . The Faraday rotator mirror  200  is coupled to an optical fiber  202 . Light beams, for example carrying one or more wavelengths used for optical communications, enter and exit the Faraday rotator mirror  200  through the optical fiber  202 . In particular, the optical fiber  202  is optically coupled to a first end of a collimator  204  of the Faraday rotator mirror  200 . A second end of the collimator  204  is optically coupled to a polarization beam splitter  206 . The polarization beam splitter  206  is optically coupled to folding mirrors  209  and  210 . 
     The Faraday rotator mirror  200  also includes a Faraday rotator  208  positioned between a first output port of the polarization beam splitter  206  and the folding mirror  210 . The Faraday rotator  208  is also positioned within a magnetic field generating element  212 . The magnetic field generating element  212  can include, for example, a wire coil wound onto a circular iron core. An electrical current can be applied to the coil to generate a magnetic field. Alternatively, magnetic field generating element  212  can be a permanent circular shaped magnet. In some other implementations, any suitable magnetic field generating structure can be used. The applied magnetic field from the magnetic field generating element  212  causes the polarization of a light beam passing through the Faraday rotator  208  to be rotated by a specified amount. In particular, the Faraday rotator  212  can rotate the polarization of an incident light beam by 90 degrees. 
     In operation, a light beam  30  is input from the optical fiber  202  to the collimator  204  of the Faraday rotator mirror  200 . The collimator  204  is configured to align incoming light to the same direction e.g., to form substantially parallel light. The incoming light beam  30  can have random polarization directions. 
     The light beam  32  exiting the second end of the collimator  204  enters a first port of the polarization beam splitter (“PBS”)  206 . The PBS  206  passes light beams having a first polarization direction while reflecting beams having an orthogonal polarization direction. In particular, the PBS  206  can be formed of two prisms joined at an angle such that light beams having a first polarization direction pass directly through the PBS  206  while light beams having the orthogonal polarization direction are directed based on the angle at which the two prisms are joined. 
     In particular, the light beam  32  entering the PBS  206  at the first port is separated into component light beams having orthogonal polarizations. The component of the light beam  32  having a first polarization direction passes through the PBS  206  along path  2 A and exits the PBS  206  at a second port as light beam  34 . The component of the light beam  32  having a second polarization direction, orthogonal to the first polarization direction, is reflected along a second path  2 B in the PBS  206  and exits the PBS  206  at a third port as light beam  36 . 
     Light beam  34  passes through the Faraday rotator  208  and exits as rotated beam  38 . The rotated beam  38  has a polarization direction that has been rotated by substantially 90 degrees from the polarization direction of light beam  34 . The rotated beam  38  is reflected by folding mirror  210 , which directs the light beam  34  to folding mirror  209 . Folding mirror  209  reflects the rotated beam  38  to enter the third port of the PBS  206 . The rotated beam  38  follows path  2 B through the PBS  206 . Since the rotated beam  38  has had the polarization rotated by substantially 90 degrees, it is now substantially reflected by the PBS  206  along the path to the first port. This light path forms a loop through the Faraday rotator mirror  200  in a first direction. 
     Light beam  36  exiting from the third port of the PBS  206  is incident on the folding mirror  209 , which reflects the light beam  36  to the folding mirror  210 . The folding mirror  210  directs the light beam  34  to the Faraday rotator  208 . The Faraday rotator  208  rotates the polarization direction of the light beam  34 , which exits the Faraday rotator  208  as rotated beam  40 . Rotated beam  40  then enters the PBS  206  at the second port and follows path  2 A. Since the rotated beam  40  has had the polarization rotated by substantially 90 degrees, it now substantially passes through the PBS  208 , combining with the rotated beam  38  reflected by the PBS  206 , to exit the first port as combined beam  42 . This light path follows the same loop through the Faraday rotator mirror  200  in a second, opposite, direction. 
     The combined beam  42  has components having orthogonal polarization directions. The combined beam  42  passes back through the collimator  204  and exits the Faraday rotator mirror  200  through the optical fiber  202 . 
     The PBS  208  also acts as a Faraday rotation angle filter. The PBS  208  only passes components of the light beams to the exit path that are rotated by 90 degrees. In particular, only components of light beams that have a polarization direction parallel to or perpendicular, relative to the PBS  208  and the respective beam path, are passed out of the first port of the PBS  208  to the collimator  204 . For example, if a light beam entering the third port of the PBS  208  along path  2 B has a polarization direction that is slightly off of perpendicular, e.g., by 1 degree, due to error of the Faraday rotator, only the perpendicular component of the vector describing the direction of the electric field of the light beam is passed along the beam path to the first port of the PBS  208 . 
     Thus, the output light beam has orthogonal polarization directions regardless of any error in the rotation angle of the Faraday rotator caused by changes in temperature or wavelength. Depending on the error of the PBS  208  in passing light beams having precise polarization directions, any error resulting from the Faraday rotator can be greatly suppressed. For example, if the error of the PBS  208  is 40 dB, a 10000× suppression of the Faraday rotation error can be achieved. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system modules and components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. As one example, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous.