Abstract:
Embodiments of an optical assembly for use in polarization rotation applications are disclosed. In one aspect, an optical assembly includes a polarization beam splitter a rotational element and a path exchange mirror. The temperature, wavelength and manufacturing dependencies of polarization rotation of this optical assembly are minimal to nonexistent compared to conventional Faraday rotation assemblies as the optical fiber accepts only the desired rotation. As such these optical assemblies have no temperature and wavelength dependencies of the polarization rotation angle over broad temperature and wavelength ranges with minimal additional losses.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to an optical assembly for use in polarization rotation applications and, more particularly, to an optical assembly with minimal to no manufacturing variation in polarization rotation angle and minimal to no temperature and wavelength dependencies of the polarization rotation angle. 
       BACKGROUND 
       [0002]    Polarization rotation devices have been used for various purposes in optical systems, especially in fiber optic communication, optical image processing, and sensor applications particularly with the use of phase conjugate mirrors. The capability of these devices is demonstrated in  FIG. 1 . A phase conjugate mirror is desirable in fiber optical systems as a result of the signal distortion caused by fiber birefringence. Phase conjugation, by interchanging the bases of the incident beam polarization allows a signal to return through a system and experience the reverse distortion as opposed to additional distortion. Faraday rotation, or the Faraday effect, is one known method for creating a phase conjugation mirror. The Faraday effect allows for the realization of devices such as fiber optic isolators, circulators, and Faraday rotating mirrors. 
         [0003]    The Faraday rotation is determined by the following expression: θ=VBL, where θ is the angle of polarization rotation after a single pass through the rotator, V is the Verdet constant, B is the applied or internal magnetic field strength and L is the length of the rotator. The Verdet constant is a property inherent to a particular material and is highly dependent on both temperature and wavelength. This limits the use of systems employing Faraday rotation over broad temperature and wavelength ranges. In most applications the rotator is used in the magnetic saturation region so as to avoid variations due to the magnetic field. The length also presents precision rotation problems, as it cannot be exactly controlled under manufacturing conditions. With the use of thin film techniques and growth methods, the variations are slight but still present. 
         [0004]    The manufacturing tolerance as well as temperature and wavelength-dependent nature of conventional single crystals for polarization rotation limit the use of optical crystal devices in precision instruments as well as over broad temperature and wavelength ranges. These conventional means only provide the desired polarization rotation at a single wavelength and at a certain temperature, with that temperature and wavelength being dependent upon the manufacturing accuracy. Advances in optical communication, sensors, and image processing require broadband, multi-wavelength capacities such as WDM, CWDM, DWDM, in central offices and uncontrolled field environment. Therefore, there remains a need to develop optical assemblies with precise polarization rotation independent of other variables. 
       SUMMARY 
       [0005]    In one aspect, an optical assembly may include a polarization beam splitter, a Faraday rotating crystal and a path exchange mirror. The orientation of these optical elements will be such that the return light that re-enters the fiber optic will have undergone a polarization rotation of exactly 90°. Any return light incident on the plane of the fiber optic that has not undergone a 90° rotation will be scattered, thus insuring the precision of the polarization rotation. 
         [0006]    In some embodiments, the polarization beam splitter may include a beam displacing crystal. 
         [0007]    In some embodiments, the polarization beam splitter may include a birefringent crystal wedge. 
         [0008]    In some embodiments, the polarization beam splitter may include a reflective polarizer. 
         [0009]    In some embodiments, the polarization beam splitter may include a Glan-Thompson polarizer. 
         [0010]    In some embodiments, the Faraday rotating element may include a thin film garnet. 
         [0011]    In some embodiments, the Faraday rotating element may include a bismuth doped yttrium iron garnet. 
         [0012]    In some embodiments, the Faraday rotating element may include an yttrium iron garnet. 
         [0013]    In some embodiments, the Faraday rotating element may include a rare earth doped yttrium iron garnet. 
         [0014]    In some embodiments, the Faraday rotating element may produce a nominally 90° polarization rotation through one pass. 
         [0015]    In some embodiments, the path exchange mirror may include a 90° corner prism. 
         [0016]    In some embodiments, the path exchange mirror may include a corner prism at an angle of 90°-β, where β is the angle of separation of the two paths. 
         [0017]    In some embodiments, the path exchange mirror may include a 90° corner thin film coated prism. 
         [0018]    In some embodiments, the path exchange mirror may include a corner thin film coated prism at an angle of 90°-β. 
         [0019]    In some embodiments, the polarization beam splitter may include a polarization beam splitter with a polarization maintaining (PM) fiber at both outputs thereof and a non-PM fiber at an input thereof. Moreover, the Faraday rotating crystal may include a Faraday Rotator with a nominal rotation of 90° with PM fibers on both sides thereof to serve as a beam exchanger. 
         [0020]    In one aspect, a phase conjugate mirror comprising the optical assembly of the present disclosure is provided. 
         [0021]    Detailed description of various embodiments are provided below, with reference to the attached figures, to promote better understanding of the characteristics and benefits of the various embodiments of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a diagram of a phase conjugate mirror and a comparable conventional mirror. 
           [0023]      FIG. 2  is a diagram showing polarization rotation when light propagates through an optical assembly in accordance with one embodiment of the present disclosure. 
           [0024]      FIG. 3  is a diagram showing polarization rotation when light propagates through an optical assembly in accordance with another embodiment of the present disclosure. 
           [0025]      FIG. 4  is a diagram showing polarization rotation when light propagates through an optical assembly in accordance with another embodiment of the present disclosure. 
           [0026]      FIG. 5  is a diagram showing polarization rotation when light propagates through an optical assembly in accordance with yet another embodiment of the present disclosure. 
           [0027]      FIG. 6  is a diagram showing polarization rotation when light propagates through an optical assembly in accordance with yet another embodiment of the present disclosure. 
           [0028]      FIG. 7  is a diagram showing polarization rotation when light propagates through an optical assembly in accordance with yet another embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]    The present disclosure describes an optical assembly that has minimal to no variation of the polarization rotation angle over broad temperature and wavelength ranges for any given manufacturing tolerances. This may be realized by the utilization of 1) a polarization beam splitter 2) a Faraday rotation crystal and 3) a path exchange mirror. 
         [0030]    The above listed constituents work together in the following manner. The polarization beam splitter creates two paths with orthogonally oriented polarizations. One or both of these paths are incident upon a Faraday rotating crystal with a set internal or external magnetic field. Both paths then are incident upon the path exchange mirror which in one embodiment is a corner mirror. The now reversed paths are again incident on the Faraday rotating crystal. Again incident upon the polarization beam splitter, only the portion of the beam that is orthogonal to its initial state will propagate through the beam splitter in a way to be incident upon and propagate down the fiber. As such for any given amount of rotation accuracy the returned beam will be exactly perpendicular to its incoming state. 
       EMBODIMENT 1 
       [0031]    In Embodiment 1, an optical assembly includes a linear layout and parallel propagation of the two orthogonal beam paths. Referring to  FIG. 2 , the optical assembly  200  includes a first crystal  210  a second crystal  220  and a third prism  230 . In some embodiments, the optical assembly  200  may further include an optical waveguide. Crystal  220  is encompassed by an external magnet (not shown) having a magnetic field B axially aligned in a direction of propagation of light, or alternatively has a permanent internal magnetic field. A beam of light incident on the first crystal  210  is split into its constituent polarizations with displaced parallel paths. One of these beams is then incident upon crystal  220  and the polarization is rotated by 90° due to the Faraday effect. These beams are both then incident upon the 90° prism and by total internal reflection the beam paths are exchanged. The first crystal  210  may be any highly birefringent parallel crystal. The second crystal  220  may be any Faraday rotating garnet. Examples of the second crystal  220  include a bulk of single crystal of yttrium iron garnet (YIG), or a thin film bismuth doped iron garnet. The third prism  230  may be any crystal with sufficient index for total internal reflection at the incident angle. Examples of the third prism  230  include a quartz prism. The return path is similar and the beam will exit the device with a polarization entirely perpendicular to its incident state for the designed wavelengths and temperatures irrespective of manufacturing precision. The proposed design of rotating elements advantageously allows a desired value of rotation angle to be maintained across wider wavelength and temperature ranges for phase conjugate mirrors. 
       EMBODIMENT 2 
       [0032]    Embodiment 2 differs from embodiment 1 in the placement of the Faraday rotating crystal. Referring to  FIG. 3 , the optical assembly  300  includes a first crystal  310  a second crystal  320  and a third prism  330 . In some embodiments, the optical assembly  300  may further include an optical waveguide. Crystal  320  is positioned such that only one path is incident upon it. Crystal  320  is encompassed by an external magnet (not shown) having a magnetic field B axially aligned in a direction of propagation of light, or alternatively has a permanent internal magnetic field. A beam of light incident on the first crystal  310  is split into its constituent polarizations with displaced parallel paths. One of these beams is then incident on the second crystal  320  and is rotated 90° due to the Faraday effect. These beams are both then incident upon the 90° prism and by total internal reflection the beam paths are exchanged. The first crystal  310  may be any highly birefringent parallel crystal. The second crystal  320  may be any Faraday rotating garnet. Examples of the second crystal  320  include a bulk of single crystal of yttrium iron garnet (YIG), or a thin film bismuth doped iron garnet. The third prism  330  may be any optically transparent material. Examples of the third prism  330  include a quartz prism. The return path is similar and the beam will exit the device with a polarization entirely perpendicular to its incident state for the designed wavelengths and temperatures irrespective of manufacturing precision. The proposed design of rotating elements advantageously allows a desired value of rotation angle to be maintained across wider wavelength and temperature ranges for phase conjugate mirrors. 
       EMBODIMENT 3 
       [0033]    Embodiment 3 differs from Embodiment 1 in the addition of a thin film reflecting coating to the path exchange mirror. Referring to  FIG. 4 , the optical assembly  400  includes a first crystal  410  a second crystal  420  and a third prism  430  with a thin film metallic coating. In some embodiments, the optical assembly  400  may further include an optical waveguide. Crystal  420  is positioned such that only one path is incident upon it. Crystal  420  is encompassed by an external magnet (not shown) having a magnetic field B axially aligned in a direction of propagation of light, or alternatively has a permanent internal magnetic field. A beam of light incident on the first crystal  410  is split into its constituent polarizations with displaced parallel paths. One of these beams is then incident on the second crystal  420  and is rotated 90° due to the Faraday effect. These beams are both then incident upon the 90° prism and by reflection from a metallic surface the beam paths are exchanged. The first crystal  410  may be any highly birefringent parallel crystal. The second crystal  420  may be any Faraday rotating garnet. Examples of the second crystal  420  include a bulk of single crystal of yttrium iron garnet (YIG), or a thin film bismuth doped iron garnet. The third prism  430  may be any optically transparent crystal with a metallic coating. Examples of the third prism  430  include a quartz prism with a thin film gold coating. The return path is similar and the beam will exit the device with a polarization entirely perpendicular to its incident state for the designed wavelengths and temperatures irrespective of manufacturing precision. The proposed design of rotating elements advantageously allows a desired value of rotation angle to be maintained across wider wavelength and temperature ranges for phase conjugate mirrors. 
       EMBODIMENT 4 
       [0034]    Embodiment 4 differs from Embodiment 3 in that the first crystal is not a parallelepiped, but rather a birefringent wedge, creating non-parallel paths with a angle separation of β and requiring the third prism to be of an angle 90°-β. 
         [0035]    Referring to  FIG. 5 , an optical assembly  500  includes a first crystal  510 , which is a birefringent wedge, a second crystal  520 , which is a Faraday rotator with a nominal rotation of 90°, and a third prism  530  which is an angle of 90°-β. In some embodiments, the optical assembly  500  may further include an optical waveguide. The second crystal  520  is encompassed within an external magnet (not shown) having a magnetic field B axially aligned in a direction of propagation of light, or alternatively has an internal permanent magnetic field of similar alignment. A beam of light incident on the first crystal  510  is split into its constituent polarizations with displaced nonparallel paths. One of these beams is then incident on the second crystal  520  and is rotated 90° due to the Faraday effect. These beams are then incident upon the prism and by metallic reflection the beam paths are exchanged. The first crystal  510  may be any highly birefringent wedge crystal. The second crystal  520  may be any Faraday rotating garnet. Examples of the second crystal  520  include a bulk of single crystal of yttrium iron garnet (YIG), or a thin film bismuth doped iron garnet. The third prism  530  may be any crystal with a metallic thing film coating. Examples of the third prism  530  include a gold thin film on a quartz prism. The return path is similar and the beam will exit the device with a polarization entirely perpendicular to its incident state for the designed wavelengths and temperatures irrespective of manufacturing precision. The proposed design of rotating elements advantageously allows a desired value of rotation angle to be maintained across wider wavelength and temperature ranges for phase conjugate mirrors. 
       EMBODIMENT 5 
       [0036]    In Embodiment 5 an optical assembly includes a nonlinear layout and perpendicularly propagating beams. Referring to  FIG. 6 , an optical assembly  600  includes a first crystal  610 , which is a reflective linear polarizer, a second crystal  620 , which is a Faraday rotator with a nominal rotation of 90°, and a third crystal  630  which is a rectangular thin film coated reflector. In some embodiments, the optical assembly  600  may further include an optical waveguide. The second crystal  620  is encompassed within an external magnet (not shown) having a magnetic field B axially aligned in a direction of propagation of light, or alternatively has an internal permanent magnetic field of similar alignment. A beam of light incident on the first crystal  610  is split into its constituent polarizations with perpendicular propagation paths. One beam is then incident on the second crystal  620  and has a polarization rotation of 90° due to the Faraday effect. These beams are then incident upon the prism and by metallic reflection the beam paths are exchanged. The first crystal  610  may be any polarization splitting crystal. The second crystal  620  may be any Faraday rotating garnet. Examples of the second crystal  620  include a bulk of single crystal of yttrium iron garnet (YIG), or a thin film bismuth doped iron garnet. The third crystal  630  may be any crystal with a thin film metallic coating. Examples of the third crystal  630  include a quartz cube with a gold thin film. The return path is similar and the beam will exit the device with a polarization entirely perpendicular to its incident state for the designed wavelengths and temperatures irrespective of manufacturing precision. The proposed design of rotating elements advantageously allows a desired value of rotation angle to be maintained across wider wavelength and temperature ranges for phase conjugate mirrors. 
       EMBODIMENT 6 
       [0037]    In Embodiment 6 an optical assembly includes a nonlinear layout and fiber integration. Referring to  FIG. 7 , an optical assembly  700  includes a first element  710 , which is a polarization beam splitter with a polarization maintaining (PM) fiber at both outputs thereof and a non-PM fiber at the input thereof, a second crystal  720 , which is a Faraday rotator with a nominal rotation of 90° with PM fibers on both sides thereof to serve as a beam exchanger, and a third path exchange fiber  730  which is a PM fiber with collimators on both ends. In some embodiments, the optical assembly  700  may further include an optical waveguide. The second crystal  720  is encompassed within an external magnet (not shown) having a magnetic field B axially aligned in a direction of propagation of light, or alternatively has an internal permanent magnetic field of similar alignment. A beam of light incident on the first element  710  is split into its constituent polarizations with propagation paths along the two output fibers. The two beams propagate the path exchange fiber  730  and are incident upon the second crystal  720  and undergo polarization rotation of 90° due to the Faraday effect. The first crystal  710  may be any in-line polarization splitting device. The second crystal  720  may be any Faraday rotating garnet. Examples of the second crystal  720  include a bulk of single crystal of yttrium iron garnet (YIG), or a thin film bismuth doped iron garnet. The third path exchange fiber  730  may be any length of PM fiber with any method of collimation at the fiber end. Examples of the collimators include convex lenses aligned to the pigtail. The return path is similar and the beam will exit the device with a polarization entirely perpendicular to its incident state for the designed wavelengths and temperatures irrespective of manufacturing precision. The proposed design of rotating elements advantageously allows a desired value of rotation angle to be maintained across wider wavelength and temperature ranges for phase conjugate mirrors. 
         [0038]    These embodiments may be used together, individually or with Polarization Splitting/Combining elements, Polarization Rotation elements, and Path Exchanging elements interchanged to create a phase conjugate mirror employing Faraday rotation. These assemblies allow a precise value of polarization rotation across wider wavelength and temperature ranges as well as an independence of manufacturing capabilities.