Abstract:
A non-reciprocal optical phase shifter comprises a plurality of switchable phase shifter stages. Each phase shifter stage is optically coupled to an adjacent phase shifter stage. At least one phase shifter stage produces a fixed Faraday rotation on the optical signal components. The plurality of switchable phase shifter stages is optically coupled to at least one phase shifter stage to produce a cumulative non-reciprocal phase shift that is the summation of the phase shifts of the plurality of switchable phase shifter stages and the at least one phase shifter stage.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention pertains to optical phase shifters, in general, and to optical non-reciprocal phase shifters, in particular.  
         BACKGROUND OF THE INVENTION  
         [0002]    A non-reciprocal phase shifter introduces a predetermined phase shift into an optical signal propagating in one direction and a different predetermined phase shift into an optical signal propagating in the opposite direction. In some instances, the magnitude of the phase shift in both directions is the same, but the shifts are of opposite sign.  
           [0003]    Non-reciprocal phase shift is based on the principle of Faraday rotation. With Faraday rotation, the angle of rotation is defined as θ=νBl. B is the magnetic flux density, ν is the constant of proportionality known as the Verdet constant, and l is the length of the crystal. The Verdet constant is a measure of a crystal&#39;s ability to rotate the plane of polarization of optical signals. The direction of rotation depends on whether light propagation is parallel or anti-parallel to the magnetic flux density.  
           [0004]    Applications of Faraday rotation include optical isolators and circulators. An optical isolator prevents or reduces the backward reflected light. A circulator directs light from one port to the next only one way. Both isolators and circulators are non-reciprocal devices. Most applications use 45 degrees rotation, which is achieved by using bulk crystals such as Yttrium Iron Garnet (YIG) or thin film crystals such as Bismuth Iron Garnet (BIG). The thickness, l, of a crystal is selected to provide 45 degrees rotation in a saturating magnetic field.  
           [0005]    Typical Faraday rotation of a crystal as a function of the magnetic field follows a hysteresis loop extending from −45 degrees to +45 degrees. With the crystal length, l, cut for 45 degrees rotation, the state of polarization is well defined when a saturating magnetic field is applied to the crystal in either direction. However, in a zero magnetic field, and at in between saturations, the rotation is not defined.  
           [0006]    Optical non-reciprocal phase shifters are useful in a variety of applications including telecommunications and optical gyroscopes. It is highly desirable to provide a non-reciprocal phase shifter that is easy to manufacture, small in size and inexpensive.  
         SUMMARY OF THE INVENTION  
         [0007]    In accordance with the principles of the invention, a non-reciprocal optical phase shifter comprises a plurality of switchable phase shifter stages. Each phase shifter stage is optically coupled to an adjacent phase shifter stage. At least one phase shifter stage produces a fixed Faraday rotation on the optical signal components. The plurality of switchable phase shifter stages is optically coupled to at least one phase shifter stage to produce a cumulative non-reciprocal phase shift that is the summation of the phase shifts of the plurality of switchable phase shifter stages and the at least one phase shifter stage.  
           [0008]    Each switchable phase shifter stages comprises a Faraday crystal and a corresponding electromagnet. At least one phase shifter stage comprises at least one Faraday crystal and a corresponding non-switchable magnet.  
           [0009]    In one embodiment of the invention, a non-reciprocal optical phase shifter, comprises a plurality of phase shifters arranged in pairs. Each phase shifter pair comprises: first and second Faraday rotation crystals; a non-switchable magnet proximate the first Faraday rotation crystal; and a switchable magnet proximate the second Faraday rotation crystal. The first and second Faraday rotation crystals are optically coupled. The plurality of phase shifters are optically coupled together whereby the output of said non-reciprocal phase shifter is the summation of phase shifts of the plurality of phase shifters.  
           [0010]    In accordance with one aspect of the invention, a non-reciprocal optical phase shifter, comprises a first phase shifter providing a fixed Faraday rotation to optical signals propagating there through; a plurality of second phase shifters optically coupled to the first phase shifter and to each other, each of the second phase shifters is operable to provide a cumulative switchable phase shift. The first phase shifter comprises a Faraday rotation crystal and a permanent magnet subjecting the Faraday rotation crystal to a fixed magnetic flux. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0011]    The invention will be better understood from a reading of the following detailed description in conjunction with the drawing figures in which like reference numerals are used to designate like elements, and in which:  
         [0012]    [0012]FIG. 1 is a cross-section of a non-reciprocal phase shifter in accordance with the Invention; and  
         [0013]    [0013]FIG. 2 is a cross-section of a second non-reciprocal phase shifter in accordance with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]    An embodiment of a non-reciprocal phase shifter (NRPS)  100  in accordance with the invention is shown in FIG. 1. NRPS  100  is a hermetically sealed unit that includes tubular aluminum housing  101 . Optical signals are coupled to and from the non-reciprocal phase shifter  100  via optical waveguides  121 ,  123 , which in the particular embodiment shown are optical fiber. However, in other embodiments, one or both of the waveguides  121 ,  123  may be waveguides formed on a substrate and the non-reciprocal phase shifter may be formed on the substrate also as an integrated optic device. Optical fiber  121  is coupled to collimator  129 . The particular manner in which fiber  121  is affixed to collimator  129  may be any of the various known methods of attaching optical fibers such as epoxy bonding. The particular details of how fiber  121  is attached to collimator  129  is not important to this invention. Similarly, optical fiber  123  is coupled to collimator  133 .  
         [0015]    In accordance with the principles of the invention, a plurality, N, of non-reciprocal phase shifter stages are provided. The phase shifter stages are disposed within the optical signal path between collimators  129 ,  133 . In the embodiment of FIG. 1, the phase shifter stages are arranged in pairs  105 . Each phase shifter pair  105  includes two Faraday rotation crystals  113 ,  115 . Faraday rotation crystals are known in the art. In the embodiment shown, A plurality of permanent magnets  141  is provided. Each magnet  141  is ring shaped and each is positioned concentric with a corresponding one Faraday rotation crystal  115 . In addition, a plurality of electromagnets  143  is provided. Each electromagnet is positioned concentric with a corresponding one Faraday rotation crystal  113 . Each electromagnet is a wire coil or solenoid.  
         [0016]    The magnetic field for each electromagnet  143  produces a magnetic flux density B in its corresponding crystal  113 . The magnetic field produced by each electromagnet  143  is that of a long solenoid and is characterized by B=μ 0  i n, where μ 0  is permeability of free space, i is the current through the coil, and n is the number of turns per unit length.  
         [0017]    The switching time constant for a coil is τ, with τ=L/R, where L is the inductance and R is the resistance of the coil. The inductance L is determined by L=μ 0  n 2  l A, where n is the number of turns per unit length, l is the length of the coil, and A is the cross-sectional area of the coil. The resistance, R, is determined from R+ρl/A, where p is the resistivity of the material used for the wire, typically copper, l is the length of the wire, A is the cross-sectional area of the wire. Time constant τ, is thus proportional to n, the number of turns per unit length. To speed up the switching time the number of turns is reduced. By reducing the number of turns, the magnetic flux density is decreased. With a decreased magnetic flux density, the thickness of each Faraday rotation crystal needs to be reduced to provide for a hysteresis loop that saturates at the decreased magnetic flux density.  
         [0018]    In accordance with the principles of the invention a higher speed non-reciprocal phase shifter is obtained by providing a plurality of optically coupled non-reciprocal phase shifters  105 , each phase shifter providing a portion of the total phase rotation. With N phase shifters, and a desired maximum phase shift of 90 degrees, each phase shifter must provide fixed and switchable rotations of 90/N, where N is the number of phase shifters. For 10 phase shifters arranged as 5 phase shifter pairs  105 , each permanent magnet produces a fixed rotation of θ=90/N, and each switchable magnet produces a switchable magnetic field to switch rotation such that θ=+/−90/N. With five phase shifter pairs  105 , to produce an angle of rotation of 90 degrees requires that each phase shifter pair  105  produces a rotation of 0 to 18 degrees. To produce such a rotation, each permanent magnet and each switchable magnet must provide a Faraday rotation of θ=90/N=9 degrees  
         [0019]    In operation, each crystal  115  is subjected to a flux by its corresponding permanent magnet  141  to provide a fixed predetermined rotation angle, 9 degrees for this embodiment, and each crystal  113  is subjected to a flux from its corresponding electromagnet  143 . The flux from each electromagnet  143  is to produce a flux that switches. Each electromagnet  143  is configured to switches the magnetic polarity of the magnetic flux to switch the Faraday rotation in its corresponding crystal  113  between +9 degrees and −9 degrees. Each pair  105  of phase shifter produces a 0 to 18 degree phase shift and the combined result of the five phase shifter pairs is additive. Thus, by switching all of electromagnets  143 , the combined phase shift produced is 0 to 90 degrees.  
         [0020]    The non-reciprocal phase shifter  100  of FIG. 1 is simply assembled, with construction similar to that of optical isolators. Advantageously, non-reciprocal phase shifter  100  provides low insertion loss, low cost and small size.  
         [0021]    In a second embodiment of the invention, shown in FIG. 2, a non-reciprocal phase shifter  200  includes ten phase shifter stages to also produce a switchable non-reciprocal phase shift of 0 to 90 degrees. In non-reciprocal phase shifter  200 , a single permanent magnet  141  is used, and a plurality,  9 , of electromagnets  143  is utilized. The permanent magnet in combination with Faraday crystal  115  produces a fixed phase shift of +/−45 degrees. Each of the remaining phase shifter stages utilizes an electromagnet in conjunction with a corresponding Faraday crystal  113  to produce a phase shift of +/−5 degrees. With nine stages producing Faraday rotation of +/−5 degrees, the total phase shift produced by the nine Faraday crystals  113  is 9×(+/−5) =+/−45 degrees. The combined total phase shift of the ten phase shifter stages is 0 to +/−90 degrees.  
         [0022]    In both embodiments, the Faraday rotation crystals are thin film crystals that in the illustrative embodiments are Bismuth Iron Garnet (BIG). In other embodiments, other Faraday rotation crystals such as Yttrium Iron Garnet (YIG) may be utilized.  
         [0023]    As will be appreciated by those skilled in the art, various modifications can be made to the embodiments shown in the various drawing figures and described above without departing from the spirit or scope of the invention. In addition, reference is made to various directions in the above description. It will be understood that the directional orientations are with reference to the particular drawing layout and are not intended to be limiting or restrictive. It is not intended that the invention be limited to the illustrative embodiments shown and described. It is intended that the invention be limited in scope only by the claims appended hereto.