Abstract:
A system and method for the passive alignment of a fiber using exterior surface absorption includes a fiber having a core, cladding and coating containing absorptive material. Absorptive material in the coating on the fiber expands and constricts depending on the amount of light that is exposed to the absorptive material. If a larger amount of light strikes one side of the fiber, that side of the fiber will constrict, and the areas that are not contacted by light will expand. This expansion and contraction process will continue until the position of the end of the fiber shifts a position where there is an equal amount of light on all sides of the end of the fiber. The use of absorptive material minimizes, or eliminates, the need for optical feedback and guarantees an accurate alignment of an optical fiber with an incoming light source, which is not offered by current fibers.

Description:
RELATED APPLICATION  
       [0001]    This application is a continuation in part of Provisional Application Serial No: 60/273,463 entitled “Passive Fiber Alignment Using Exterior Surface Absorption” filed Mar. 5, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to optical fibers. More specifically, the present invention pertains to methods of aligning optical fibers. The present invention is particularly, though not exclusively, useful for aligning optical fibers with converging light beams by using fibers with outside surfaces composed of absorption material which aligns the fiber with the light beam.  
         BACKGROUND OF THE INVENTION  
         [0003]    Over the past several decades, the use of optical fibers, or fiber optics, to transmit information on a light beam have become increasingly popular. In fact, much of the information which is transmitted today is done over optical fibers. The difficulty of aligning a light beam into an optical fiber typically requires optical feedback to make position corrections in either the light beam or the fiber. In some cases, particularly where single-mode (SM) fiber is used, it is necessary to continually monitor the optical feedback in order to maintain a proper alignment of the light beam on the fiber.  
           [0004]    Accordingly, it is an object of the present invention to provide a fiber capable of aligning itself so that its core aligns with the focal point of a light beam by incorporating an absorptive material into the outside surface of the flexible fiber.  
         SUMMARY OF THE PRESENT INVENTION  
         [0005]    In order to minimize or eliminate the need for optical feedback, the present invention utilizes the external surface of the fiber as a passive actuator to automatically center the fiber on an incoming light beam. Specifically, an absorptive material on the outside of the fiber to bend the fiber into alignment with the light beam. For example, a single light beam to be focused into a single-mode fiber can pass through a dual-focal length lens to create an actuator signal based on absorption on the outside of the fiber. Typically, the fiber core is concentric to the outside diameter within 0.5 microns; this concentricity is required for standard fiber optic connectors. By using the absorption of light on the outside surface of the fiber to align the fiber, the same level of accuracy may be achieved. Much like a sunflower bends its stalk according toward an incoming light source to optimize the light onto the flower, the fiber itself will change the position of its end in response to light striking the outside surface of the fiber.  
           [0006]    A light beam entering a fiber is usually gaussian in profile, being generated by another fiber or from a laser. Thus, most of the light is concentrated in the center of the beam, which is to be focused with an input lens and carefully aligned to the fiber core. A small amount of the light found in the external area of the beam can be focused with a concentric alignment lens, made in the form of a Fresnel lens, for example, or using diffractive optics. This lens would focus a ring of light onto the side of the fiber, which has been coated with absorptive material. The material property would be such that it shrinks when exposed to light. If there is any deviation in the alignment of the light beam, the ring of light strikes the surface of the fiber and causes the fiber to bend in the direction of the excess light, repositioning the fiber to be concentric to the incoming light beam, and maximizing the light which is received into the fiber core. One key advantage of this invention is that only the incident light beam, such as an incoming communication beam, is required, and any active optical feedback is minimized or wholly eliminated. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0007]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which like reference characters refer to similar parts, and in which:  
         [0008]    [0008]FIG. 1 is a side view of a preferred embodiment of the present invention showing a properly positioned optical fiber in relation to a converging light beam which provides an optimum optical signal into the core of the fiber;  
         [0009]    [0009]FIG. 2 is a side view of a preferred embodiment of the present invention showing an improperly positioned fiber receiving only a small portion of the incoming optical signal, and the alternative position, in which the optical fiber changes its position to center the converging beam into the center of the fiber&#39;s core;  
         [0010]    [0010]FIG. 3 is cross-sectional view of a preferred embodiment of the present invention taken along line  3 - 3  of FIG. 2, and showing the relative placement and dimensions of the core, cladding and exterior coating of the optical fiber;  
         [0011]    [0011]FIG. 4A is a side view of a preferred embodiment of the present invention showing an improperly aligned fiber in a larger converging beam; and  
         [0012]    [0012]FIG. 4B is a side view of a preferred embodiment of the present invention showing the fiber&#39;s adjustment and balanced position to the larger converging beam.  
     
    
     DETAILED DESCRIPTION  
       [0013]    Referring initially to FIG. 1, a side view of the preferred embodiment of the Passive Fiber Alignment Using Exterior Surface Absorption of the present invention is shown and generally designated  100 . In FIG. 1, fiber  100  is properly aligned with converging light beam  200  such that the beam focusses on the core of the fiber. More specifically, fiber  100  includes coating  102  surrounding core  104  and cladding  106 , fiber end  108  and fiber shaft  110 . The core  104  and cladding  106  are often called the body of the fiber  100 .  
         [0014]    Converging light beam  200  results from light beam  202  passing though focusing lens  204 , which is shown as convex, for example. As converging beam  200  travels away from focusing lens  204 , converging beam  200  becomes more concentrated into a smaller area until it reaches focal point  206 . Focal point  206  of converging beam  200  strikes core  104  at fiber end  108  of a properly positioned fiber  100 . When the focal point  206  is precisely positioned on fiber end  108 , the light received into core  104  from light beam  202  is maximized.  
         [0015]    Although a converging light beam  200  has been described in conjunction with the present invention, it is to be appreciated that no limitation on the present invention is intended. Rather, the present invention may be used to align an optical fiber with virtually any light source, including but not limited to converging, diverging, or collimated, light sources.  
         [0016]    Turning now to FIG. 2, a side view of a preferred embodiment of the Passive Fiber Alignment Using Exterior Surface Absorption of the present invention is shown with an improper initial position of fiber  100  within converging beam  200 . In this position, the converging beam  200  is not centered on the fiber  100 , and results in a larger portion of the converging beam  200  striking the coating  102  on the outside surface of the fiber  100 . Due to the converging beam  200  striking the outside surface of the fiber  100 , fiber  100  changes its shape to an alternative position  112  (shown in dashed lines), which centers the core  104  of fiber  100  onto converging beam  200  to optimize the light received into the core  104 .  
         [0017]    As shown in FIG. 2, the initial position of fiber  100  does not allow light from converging beam  200  to contact core  104 . Rather, the converging light beam  200  is focussed onto focal point  206  which is located near cladding  106  and thus a majority of the incoming light beam  200  strikes the coating  102  on the outside surface of fiber  100 . Due to the improper position of fiber  100 , the light entering core  104  at fiber end  108  and traveling through fiber shaft  110  is decreased significantly. Unfortunately, this type of misalignment is a common problem in optical systems using currently available fibers. Also, because the ambient temperature within optical equipment may fluctuate significantly, the alignment of the optical fibers may change due to the particular thermal characteristics of the fiber. As a result, the misalignment of the optical fibers presents a challenge to the manufacturing of high quality optical devices, such as optical switches, and presents a particular problem in applications where single mode (SM) fiber having a smaller core diameter is used.  
         [0018]    In applications incorporating the present invention, coating  102  includes an absorptive material that expands and contracts depending on the quantity of light it receives. For example, the portion of coating  102  that receives a large amount of light constricts, while the portion that receives a smaller amount expands. The expanding and constricting of coating  102  on fiber  100  results in the movement of fiber  100  to alternative position  112  (shown in dashed lines). Coating  102  constricts where it is exposed to light, and expands where it is not exposed to light, until there is an equilibrium in coating  102 , or equal amount of light on all portions of coating  102 .  
         [0019]    [0019]FIG. 3 is a cross-sectional view of a preferred embodiment of the Passive Fiber Alignment Using Exterior Surface Absorption  100  of the present invention as taken along line  3 - 3  of FIG. 2, and shows the relative diameters of coating  102 , core  104  and cladding  106 . As shown, core  104  has diameter  120 , and is located in the center of fiber  100  and extends axially through fiber shaft  110  to fiber end  108 . The diameter  120  of core  104  can vary, depending on the type of fiber being used. For example, for a single mode (SM) fiber, diameter  120  of core  104  is approximately eight to ten microns (8-10 μm), whereas a multi-mode fiber may have a diameter  120  of core  104  approximately sixty two microns (62 μm). Regardless of whether the fiber  100  is single-mode or multi-mode, the diameter  122  of the cladding  106  is typically one hundred twenty five microns (125 μm).  
         [0020]    Coating  102  has diameter  124 , a thickness  126 , and surrounds cladding  106 . Coating  102  is composed of absorptive material and contracts or expands depending on the amount of light that the surface is exposed to. The flexible nature of core  104  and cladding  106  cooperate with coating  102  to provide for the movement of fiber  100  when coating  102  expands and constricts.  
         [0021]    While it is contemplated that coating  102  surrounds cladding  106  for the entire length of fiber  100 , it is also to be appreciated that the entire fiber  100  need not be coated with coating  102 . Instead, in an alternative embodiment, only the portion of fiber  100  which would be exposed to the converging light  200  can be coated with coating  102 . For example, the coating  102  may begin at end  108  of fiber  100 , and coat only a portion of the length of the fiber, such as a three millimeter (3 mm) length of fiber  100 .  
         [0022]    In a preferred embodiment, coating  102  may be made of a heat or light absorbing material which constricts when exposed to heat or light. For example, these materials may include a black, heat and light absorbing oxide or paint, and may also include metals or organic polymers having these constricting properties. In a preferred embodiment, coating  102  includes titanium nitride (TiNi). This material may be evaporatively coated onto cladding  106  of fiber  100 , or fiber  100  may be dipped into the material to coat the outer surface of cladding  106 . Other suitable materials for use in coating  102  include FLEXINOL, DYNALLOY, and NITINOL, which all have the constricting characteristics necessary for use within the present invention.  
         [0023]    One method of manufacturing the fiber of the present invention includes the dipping the end portion of fiber  100  into a coating material. This would result in core  104  being covered with the coating material  102 . Once coated, end  108  may be polished by any method known in the art to remove coating material  102  from end  108 , thereby exposing the core  104  for receiving the incoming light  202 .  
         [0024]    Fiber  100  may also be manufactured with coating  102  being integral to cladding  106 . In this alternative embodiment, fiber  100  is formed with a core  104  and a cladding  106  which has been manufactured to include a coating material  102 , thus eliminating the need for an external coating  102 . This coating materials is shown in FIG. 3 as particles included in the outer surface of the cladding. This fiber with an integral coating material facilitates the use of fiber  100  by eliminating any hazard of scratching the coating during the installation of the fiber  100  in optical equipment.  
         [0025]    Referring to FIG. 4A, a side view of a preferred embodiment of the Passive Fiber Alignment Using Exterior Surface Absorption of the present invention shows fiber  100  improperly aligned with larger converging beam  210 . Focal point  212  of larger converging beam  210  contacts fiber end  108  in the area of cladding  106 , resulting in a lack of light traveling into and through core  104 . Due to the misalignment of the end  108  of fiber  100  with converging beam  210 , a larger amount of light strikes coating  102  on left side  132  of fiber  100 , and a smaller amount of light strikes coating  102  on the right side  130  of fiber  100 .  
         [0026]    The imbalance of light between left side  132  and right side  130  of fiber  100  causes left side  132  to constrict while right side  130  begins to expand. Coating  102  will continue this process of expanding and constricting until there is an equal amount of light around fiber end  108 . The process will stop when fiber  100  is in an equilibrium or balanced position where the light striking left side  132  equals the light striking right side  130 . This process is best summarized as the self-alignment of the optical fiber  100  with an incoming converging light beam  200 .  
         [0027]    Referring now to FIG. 4B, a side view of the preferred embodiment of the Passive Fiber Alignment Using Exterior Surface Absorption of the present invention showing the adjustment of fiber  100  to a proper, balanced position where the larger converging beam  210  focuses on core  104  of fiber  100 . As shown, in response to the imbalanced light striking coating  102  on the left side  132  and right side  130  of fiber  100 , the end  108  of fiber  100  has adjusted its position to equalize the light striking sides  130  and  132 . This adjusted position places the location of the core  104  on end  108  so that the focal point  212  of the converging beam  210  will now contact core  104  at fiber end  108 .  
         [0028]    The bending of fiber  100  to position the core  104  at focal point  212  is caused by the constricting of coating  102  on the left side  132  of fiber  100 , and the expanding of the coating  102  on right side  130 . In the new, adjusted position, the light from converging beam  210  is balanced on sides  130  and  132  which produces an equal amount of constriction of coating  102  on the left side  132  and right side  130 . This equilibrium, or stabilized position, places core  104  of fiber  100  in its optimal position to allow converging light  210  to strike core  104  and enter fiber  100 .  
         [0029]    [0029]FIG. 4A also shows that only a length of fiber  100  need be coated with coating  102 . Specifically, coating  102  is shown covering only a portion of cladding  106 , with exposed cladding (designated  103 ) depicting an un-coated portion of fiber  102 . The length  105  of the coated portion may vary depending on the application, including the magnitude of positional correction necessary, and the architecture of the optical component.  
         [0030]    While the coating discussed herein is described as a coating on an optical fiber, it is to be appreciated that the absorptive material may be coated on the exterior surface of the fiber. Alternatively, the absorptive material may be impregnated into the fiber cladding (as shown in FIG. 3), or into an exterior surface of the fiber itself. As yet another alternative, the absorptive material may be added to the exterior surface of, or coated on, the buffer sleeve (not shown) in order to properly position the fiber contained within the sleeve.  
         [0031]    For clarity of the discussion, the present invention has been discussed in conjunction with a converging light beam striking the left side  132  and right side  130  of fiber  100 . However, it is to be appreciated that the present invention operates to adjust the end  108  of fiber  100  in three dimensions, thereby providing a solution to problems in optical systems where an optical fiber must be aligned with an incoming light source, such as converging light beam  210 .  
         [0032]    While the methods and apparatus for the passive fiber alignment of the present invention as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of preferred embodiments of the invention and that no limitations are intended to the details of the method, construction or design herein shown other than as described in the appended claims.