Abstract:
Optical isolators are used in optical communication systems, especially optical systems which employ semiconductor lasers. As transmission rates used in optical communications systems have increased, for example to several Gbits per second, the performance required of lasers used in such systems has also increased. It is well known that light reflected back from some parts of an optical communications system will adversely affect the operation of such a high performance laser leading to fluctuations in the spectrum, line width, or intrinsic noise of the laser. In this invention, a free space isolator is illustrated utilized to protect such high performance semiconductor lasers from these reflections by stabilizing an optical element within a base.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to the field of optical communications systems, and more particularly to systems and methods that employ semiconductor lasers in a free space environment. 
         [0003]    2. Background of the Invention 
         [0004]    An optical isolator, or optical diode, is an optical component which allows the transmission of light in only one direction. Isolators are typically used to prevent un-wanted feedback into an optical oscillator. The operation of the isolator depends on the Faraday Effect (which in turn is produced by magneto-optic effects), which is used, a Faraday rotator. A magnetic field, B, applied to the Faraday rotator causes a rotation in the polarization of the light due to the Faraday Effect. The angle of the rotation, β is given by, β=vBd, where v is the Vedet constant of the material that the rotator is made from, and d is the length of the rotator. 
         [0005]    A polarization dependent isolator is made of three parts, an input polarizer, a Faraday rotator, and an output polarizer, wherein the input polarizer is polarized vertically and the output polarizer is polarized at 45 degrees. Light traveling in the forward direction becomes polarized vertically by the input polarizer. The Faraday rotator rotates the polarization of the light by 45 degrees, enabling light to be transmitted through the isolator. Alternatively, light traveling in the reverse direction becomes polarized at 45 degrees by the analyzer wherein the Faraday rotator rotates the polarization by 45 degrees. In other words, the light is polarized horizontally and since the polarizer is vertically aligned, the light will be extinguished. 
         [0006]    An important optical element in an isolator is the Faraday rotator. The characteristics include a high Verdet constant, low absorption coefficient, low non-linear refractive index, and high damage threshold. The polarization rotation due to the Faraday rotator is always in the same relative direction. In the forward direction, the rotation is positive 45 degrees and, in the reverse direction, the rotation is negative 45 degrees. Therefore, when the light travels in the forward and reverse directions, a rotation of 90 degrees is achieved and higher isolation is comparable. 
         [0007]    Optical isolators are used in optical communication systems, and especially used with semiconductor lasers. As the transmission rates used in optical communications systems have increased, the performance required of lasers used in such systems has also increased. 
         [0008]    It is well known that light reflected back from some parts of an optical communications system will adversely affect the operation in high performance lasers. Such adverse affects include fluctuations in the spectrum, line width, or intrinsic noise of the laser. 
         [0009]    Therefore, what is desired is an optical isolator(s) that protects high performance semiconductor lasers from reflections and limit un-wanted positional changes of an optical isolator element within a free space isolator design by decreasing deficiencies from movements of the optical isolator element. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention discloses an apparatus and method that employ a free space isolator in a communication system. In various embodiments of the present invention, the sub-assembly of the free space isolator comprises an optical element and a glass base wherein the glass base is adjoined with the optical element by applying a small amount of epoxy to one side of the optical element. A fixture provides the angle of the optical isolator element and secures the optical element to the glass base, which blocks un-wanted reflective light from the system. The optical isolator element&#39;s movement is limited because a small amount of epoxy is applied and the glass base secures the optical isolator element thereby creating a more stable, reliable, and desirable free space isolator design. 
         [0011]    The use of epoxy is commonly used to connect components of an optical device. The effects that temperature and humidity have on epoxy are deleterious causing the epoxy to shrink or expand. This characteristic of epoxy may create un-wanted and un-desirable positional changes of the optical isolator element. In various embodiments of the present invention, a free space isolator design is provided to protect high performance semiconductor lasers from un-wanted light reflections. The sub-assembly prevents un-wanted and un-desirable movement in the optical isolator element by decreasing the amount of epoxy used to combine the sub-assembly. 
         [0012]    Although the features and advantages of the invention are generally described in this summary section and the following detailed description section in the context of embodiments, it shall be understood that the scope of the invention should not be limited to these particular embodiments. Many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims hereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Reference will be made to embodiments of the invention, examples of which may be illustrated in the accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments. 
           [0014]      FIG. 1A  illustrates an apparatus for securing an optical isolator element within a housing according to prior art. 
           [0015]      FIG. 1B  illustrates a housing with a tilt angle according to a prior art. 
           [0016]      FIG. 2A  illustrates an apparatus for securing an optical isolator element within a housing according to various embodiments of the present invention. 
           [0017]      FIG. 2B  illustrates an apparatus for securing an optical isolator element within a housing according to various embodiments of the present invention. 
           [0018]      FIG. 3  illustrates a method for securing an optical isolator element within an housing according to various embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The present invention discloses and apparatus and method to employ a free space isolator in communication systems. In various embodiments of the present invention, the sub-assembly of the free space isolator comprises an optical element and a glass base wherein the glass base is adjoined with the optical element by applying a small amount of epoxy to one side of the optical element. The glass base is a part of a cylinder metal housing which provides no tilt angle. The sub-assembly including the glass base and isolator element is bonded in a magnet ring providing a stable and secure platform for the isolator. A fixture provides the angle of the optical isolator element and secures the optical element to the glass base blocking un-wanted reflective light from the system. The optical isolator element&#39;s movement is reduced because a small amount of epoxy is applied to only one side and the glass base secures the epoxy side of the optical isolator element thereby creating a more stable, reliable, and desirable free space isolator design 
         [0020]    In the following description, for purposes of explanation, specific details are set forth in order to provide an understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these details. One skilled in the art will recognize that embodiments of the present invention, described below, may be performed in a variety of ways and using a variety of means. Those skilled in the art will also recognize additional modifications, applications, and embodiments are within the scope thereof, as are additional fields in which the invention may provide utility. Accordingly, the embodiments described below are illustrative of specific embodiments of the invention and are meant to avoid obscuring the invention. 
         [0021]    Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, characteristic, or function described in connection with the embodiment is included in at least one embodiment of the invention. The appearance of the phrase “in one embodiment,” “in an embodiment,” or the like in various places in the specification are not necessarily all referring to the same embodiment. 
         [0022]      FIGS. 1A and 1B  illustrate a prior design of a free space isolator  100  apparatus. The free space isolator  100  comprises an optical element  150 , which further comprises a first polarizer  155 , a second polarizer  165 , and a Faraday rotator  160 , a magnet ring  130 , a cylinder metal housing  110  with a tilt angle  115  within the metal housing  110 , and epoxy  120 . The Faraday rotator  160  and first  155  and second  165  polarizer&#39;s are plate shaped and the surfaces are filled with epoxy  120 . The optical element  150  comprises the first polarizer  155  surrounded by epoxy  120  coupled to the Faraday rotator  160  surrounded by epoxy  120  coupled to the second polarizer  165  surrounded by epoxy. 
         [0023]    The optical isolator element  150  is bonded into the magnet ring  130 . The optical isolator element  150  is surrounded by a ring of epoxy  120  and placed inside the magnet ring  130  bonding the optical isolator element  150  to the magnet ring  130 . A magnetic flux is generated by the magnet ring  130  and is parallel to the ring axis. The magnet ring  130  is then surrounded by the epoxy ring  120  and is put into the metal housing  110 .  FIG. 1B  is an illustration of the inside of the metal housing  110  in which the inside of the metal housing  110  is tilted  115  at an angle compared with the outside of the surface. 
         [0024]    In an embodiment of the present invention,  FIG. 2  illustrates a free space isolator design.  FIG. 2  comprises an optical isolator element  245 ; which further comprises Faraday rotator  250 , a first polarizer  260 , and a second polarizer  270 , a magnet ring  225 , a cylinder metal housing  205 , a glass base  235 , a metal ring  215 , and epoxy as an adhesive. 
         [0025]    One side of the optical isolator element  245  is bonded with the glass base  235  using epoxy. The epoxy side of the optical isolator element  245  is affixed to glass base  235  and the glass base  235  adjoins the optical isolator element  245  within the magnet ring  225 . This sub-assembly provides that the optical isolator element  245  is directly or approximately centered within the cylinder metal housing  205 . The angle  255  of the optical isolator element  245  is created when the optical isolator element  245  is affixed to the glass base  235 . The angle provided is fixed and centralized according to the fixtures provided. 
         [0026]    The metal housing  205  secures the magnetic ring  225  at a fixed angle with epoxy. The magnetic ring, in turn, secures the glass base  235  at a fixed angle with epoxy. This bonding provides a stable, more secure fixture for the isolator element  245 . A first side of the optical isolator element  245  is bonded with the glass base  235  by applying epoxy to the bonding side of the isolator element  245 . The metal ring  215  ensures that un-wanted light reflections are thwarted by it. 
         [0027]    Therefore, in this invention the amount of epoxy used to fill gaps between the optical isolator element  245  and the magnet ring  225  is reduced and a fixture is implemented to secure the optical isolator element  245  into the glass base  235 , which in turn is secures to the magnet ring  225 . The reduction of epoxy applied to the isolator  245  and the use of a fixture to create the angle within the isolator  245  provides a more stable and reliable free space isolator 
         [0028]    When temperature and humidity changes occur, the use of less epoxy and a more stable, secure fixture allows fewer positional changes in the optical isolator element  245 . In prior isolators, which employ more epoxy to adjoin the sub-assemblies to the main housing, as temperature and humidity changes occur, the epoxy expands or shrinks causing various positional changes in an optical isolator. These positional changes create a more un-stable and non-reliable free space isolator. Thus, in various embodiments of the present invention, the reduction of epoxy and the inclusion of a fixed glass base that provides a fixed angle for light refraction allows for a more reliable, stable, secure, and efficient free space optical isolator element. Alternatively, the free space isolator element may comprise multiple stages wherein a plurality of rotator&#39;s and polarizer&#39;s are bonded together with epoxy employing an identical sequencing as expressed in throughout this invention. 
         [0029]    In another embodiment of the present invention,  FIG. 3  illustrates a method to secure a sub-assembly to a housing creating a more reliable free space isolator design. The method comprising the steps of tilting an optical isolator element at an angle within a fixture thereby blocking un-wanted light reflection from an optical system  305 . Bonding the optical isolator element to a glass base by applying epoxy to only one side of the optical isolator element  315 . Adjoining the glass base to a magnet ring by applying epoxy to the entire glass base and affixing the glass base into the inner side of the magnet ring  325 . Bonding the magnet ring with a metal housing by applying epoxy to the outer surface of the magnet ring and placing magnet ring inside the metal housing  335 . 
         [0030]    While the invention is susceptible to various modifications and alternative forms, a specific example thereof has been shown in the drawings and is herein described in detail. It should be understood, however, that the invention is not to be limited to the particular form disclosed, but to the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.