Abstract:
The invention is a non-reciprocal phase shifter that operates on optical signals. A Faraday rotator crystal is utilized in conjunction with a magnetic field source to produce non-reciprocal phase shifts in optical signals traversing the crystal in opposite directions.

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. 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  
         [0003]    In accordance with the principles of the invention, a non-reciprocal optical phase shifter, comprises a magneto-optic waveguide body of a material that, when subjected to magnetic fields, causes Faraday rotation effects on optical signals of a predetermined polarization. First and second waveguides are coupled to the magneto-optic waveguide body to couple optical signals thereto. A magnetic field source proximate the magneto-optic body, subjects the body to a magnetic field such that a non-reciprocal optical phase shift is produced in optical signals traversing said body in opposite directions.  
           [0004]    A first graded index lens couples the first waveguide to the magneto-optic body and a second graded index lens couples the second waveguide to the body.  
           [0005]    In the illustrative embodiment of the invention the magneto-optic body comprises a Faraday rotator crystal of yttrium iron garnet and the first and second waveguides are optical fibers.  
           [0006]    In accordance with one aspect of the invention the magnetic field source is an electromagnet. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0007]    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:  
         [0008]    [0008]FIG. 1 is a cross-section of a non-reciprocal phase shifter for single polarization in accordance with the invention; and  
         [0009]    [0009]FIG. 2 is a cross-section of a second polarization independent, non-reciprocal phase shifter in accordance with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0010]    [0010]FIG. 1 illustrates a first embodiment of a non-reciprocal phase shifter  100  in accordance with the invention. Optical signals are coupled to and from the non-reciprocal phase shifter  100  via optical waveguides  101 ,  103 , which in the particular embodiment shown are optical fiber. However, in other embodiments, one or both of the waveguides  101 ,  103  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. Non-reciprocal phase shifter  100  comprises a Faraday rotator crystal  105  which may be a crystal or thin-film device. A graded index lens  107  is attached to the end of optical fiber  101  and is attached to Faraday rotator crystal  105 . A second graded index lens  109  is coupled to optical fiber  103  and to Faraday rotator crystal  105 . Lenses  107 ,  109  are bonded to optical fibers  101 ,  103 , respectively and to Faraday rotator crystal  105  with an epoxy cement. Graded index lenses  101 ,  103  are each of a type known in the trade as Sel-Foc lenses.  
         [0011]    Faraday rotator crystal  105  may be any magneto-optic material that demonstrates Faraday rotation such as Yttrium Iron Garnet or Bismuth Iron Garnet.  
         [0012]    An electromagnet  125  disposed proximate Faraday rotator crystal  105  includes a coil assembly  113 . Electromagnet  125  provides a magnetic field indicated by field lines  135  when current flows through coil  113 . Non-reciprocal phase shifter  100  operates with optical waves of a single polarization. The polarization, i.e., TE or TM, is determined by the selected crystal orientation. Optical signals in one direction through non-reciprocal phase shifter  100  are designated as forward beam signals Ifw, and optical signals in the opposite direction are designated as backward beam signals Ibk. For forward beam signals Ifw, non-reciprocal phase shifter  100  provides a phase shift of ωt+Φ. For backward beam signals Ibw, non-reciprocal phase shifter  100  provides a reciprocal phase shift of ωt−Φ.  
         [0013]    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 of 1 dB or less, low cost and small size, i.e., under 1 inch in length.  
         [0014]    [0014]FIG. 2 illustrates a second non-reciprocal phase shifter  200  in accordance with the principles of the invention. Non-reciprocal phase shifter  200  differs in operation from non-reciprocal phase shifter  200  in that it is polarization independent. Non-reciprocal phase shifter  200  operates on TM and TE polarized signals, or signals with both TE and TM components. As with the structure of FIG. 1, optical signals are coupled to and from non-reciprocal phase shifter  200  via optical waveguides  201 ,  203 . As with non-reciprocal phase shifter  100 , waveguides  201 ,  203  are shown as optical fibers. However, one or both optical waveguides  201 ,  203  may be an optical waveguide carried on a substrate. Non-reciprocal phase shifter  200  may be formed on the same substrate with waveguides  201 ,  203  as an integrated optic device. Optical waveguides  201 ,  203  are coupled respectively to Sel-Foc lenses  207 ,  209 . Two Faraday rotators crystals  205 ,  206  are utilized. One Faraday rotator crystal  205  is used for TE polarization optical signals and the other Faraday rotator crystal  206  is used for TM polarization optical signals. Each Faraday rotator crystal  205 ,  206  is oriented so that the magnetic field produced by electromagnet  225  produces a phase shift. Each Sel-Foc lens  207 ,  209  is coupled to a corresponding polarization beam splitter  215 ,  217 . Beam splitters  215 ,  217  are in turn optically coupled to reflecting prisms  219 ,  221  to separate the TE and TM polarized optical signals. An electromagnet  225  disposed proximate Faraday rotator crystals  205 ,  206  includes a coil assembly  213 . Electromagnet  225  provides a magnetic field indicated by field lines  235  when current flows through coil  213 . With the arrangement shown in FIG. 2, two bi-directional optical paths can be traced through non-reciprocal phase shifter  200 .  
         [0015]    A first optical path for TE polarized wave components follows arrow  241 . Starting at the left end of non-reciprocal phase shifter  200 , TE polarized wave components on optical waveguide  203  are coupled to Sel-Foc lens  209 . Sel-Foc lens  209  couples the TE polarized wave components to polarization beam splitter  217 , which couples the TE polarized light to Faraday rotator crystal  205 . From Faraday rotator crystal  205 , the TE polarized wave components are coupled to polarization beam splitter  215 , and then to Sel-Foc lens  207  and to waveguide  201 .  
         [0016]    For forward propagating TE polarized wave components, Ifw, non-reciprocal phase shifter  100  provides a phase shift of ωt+Φ. For backward propagating TE polarized beam signals Ibw, non-reciprocal phase shifter  100  provides a reciprocal phase shift of ωt−Φ.  
         [0017]    A second optical path for TM polarized wave components follows arrow  251 . Starting at the left end of non-reciprocal phase shifter  200 , TM polarized light on optical waveguide  203  is coupled to Sel-Foc lense  209 . Sel-Foc lens  209  couples the TM polarized light to polarization beam splitter  217 , which couples the TM polarized light to reflecting prism  221 . The TM signals are coupled to Faraday rotator crystal  206 . From Faraday rotator crystal  206 , the TM polarized light is coupled to reflecting prism  219 . From reflecting prism  219 , the TM polarized light is coupled to polarization beam splitter  215 , and then to Sel-Foc lens  207  and to waveguide  201 .  
         [0018]    For forward propagating TM polarized wave components Ifw, non-reciprocal phase shifter  100  provides a phase shift of ωt+Φ. For backward propagating TM polarized beam signals Ibw, non-reciprocal phase shifter  100  provides a reciprocal phase shift of ωt−Φ. As with the non-reciprocal phase shifter of FIG. 1, non-reciprocal phase shifter  200  exhibits very low loss, 1 dB or less, is physically small and is of low cost.  
         [0019]    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.