Abstract:
A laser beam is used to ablate the outer cladding of an extended portion of an optical fiber. The laser beam is focused at a tangential point on the outer cladding. The laser can be rotated around the optical fiber while the optical fiber is held stationary. Alternatively the optical fiber can be rotated while the focal point of the laser beam is kept at a constant position.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This Application is a divisional of U.S. utility application entitled, “EXTENDED OPTICAL FIBER AND METHOD,” having Ser. No. 09/133,731, filed Aug. 13, 1998, now abandoned which is entirely incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of optical waveguides, and more particularly, to the field of machining of optical waveguides using a laser. 
     BACKGROUND OF THE INVENTION 
     Currently, fiber optic technology is used in high speed communication systems. These systems facilitate the communication of video, data, and voice information through vast networks around the globe. Among the components which are used in such systems are various photodetectors which generally receive laser signals from optical fibers, converting them to electrical signals. 
     In general, photodetectors feature an active area or photo-sensitive surface which reacts to incident radiation, creating a corresponding electrical signal. In a typical configuration, an optical fiber is directed toward the active area of the photodetector so that laser radiation that propagates through the optical fiber falls on the active area. Accordingly, such photodetectors typically include input ports to receive and position optical fibers. 
     Some photodetectors such as various super high speed photodetectors employ narrow input ports or openings through which to receive the optical fiber. Whereas a typical single mode optical fiber may be 125 microns in diameter, these narrow input ports may range anywhere from approximately 10 microns to 50 microns in diameter. Consequently, there is a need for an optical fiber cable that will fit into such narrow input ports while maintaining proper propagation characteristics. 
     SUMMARY OF THE INVENTION 
     To address this need, the present invention entails an extended optical fiber having an extended portion and a normal portion. The extended portion is located at an end of the extended optical fiber and has a cladding of reduced diameter in relation with the cladding of the normal portion. A common core runs throughout the normal and extended portions. The thickness of the cladding in the extended portion is sufficient to ensure that the propagation characteristics of the extended optical fiber are unaffected through the extended portion. The extended portion provides the advantage of being easily inserted into a restrictive input port of a photodetector or other device. 
     The present invention may also be viewed as a method for producing the extended optical fiber. This method includes the steps of focusing a laser on a tangential point of the cladding material of an optical fiber resulting in the tangential ablation of the cladding material. Next, a reduced diameter section is created in the cladding material by rotating the optical fiber under the focus of the laser and moving the optical fiber in a linear direction into the focused laser. Finally, the optical fiber is cleaved at the reduced diameter section, resulting in an extended optical fiber according to the present invention. 
     Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. In the drawings, like reference numerals designate corresponding parts throughout the several views. 
     FIG. 1 is a drawing showing a conventional optical fiber; 
     FIG. 2 is a drawing showing an optical fiber with an extended portion according to an embodiment of the present invention; 
     FIG. 3 is a drawing showing the machining of the optical fiber of FIG. 1 using a laser according to an embodiment of the present invention; 
     FIG. 4 is a drawing showing the tangential ablation of the optical fiber of FIG. 3; 
     FIG. 5 is a drawing showing a side view of an optical fiber having a reduced diameter after machining as shown in FIG. 3; 
     FIG. 6 is a drawing showing a cleaved side of the machined optical fiber of FIG. 5; 
     FIG. 7A is a drawing showing the dipping of an optical fiber into a solution according to a method of another embodiment of the present invention; 
     FIG. 7B is a drawing showing the end of the optical fiber of FIG. 7A after dipping; and 
     FIG. 7C is a drawing showing the end of the optical fiber of FIG. 7B after the extended portion is cleaved. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning to FIG. 1, shown is a conventional optical fiber  50  having a core  55  surrounded by a cladding  60 . The core  55  has an index of refraction N 1  which is greater than the index of refraction N 2  of the cladding  60 . This relationship between the indexes of refraction where N 1 &gt;N 2  allows the propagation of light waves through the core  55  of the optical fiber  50 , reflecting off of the sides of the core  55  along the way. Such characteristics of an optical fiber are well known by those skilled in the art. By way of example, the diameter of the core  55  may be approximately 10 microns and the diameter of the cladding  60  may be approximately 125 microns for a single mode optical fiber, or the diameter of the core  55  may be approximately 60 microns with a cladding  60  of approximately 125 microns for a multimode fiber. 
     Referring next to FIG. 2, shown is an extended optical fiber  100  according to a first embodiment of the present invention. Although other methods may exist in which the extended optical fiber  100  may be produced, according to the preferred embodiment, the extended optical fiber  100  is created using a cladding ablation process applied to an optical fiber such as the optical fiber  50  of FIG.  1 . 
     The extended optical fiber  100  includes a normal portion  103  having cladding  60  of regular thickness and an extended portion  106 . The extended optical fiber  100  includes a core  55  which extends through both the normal portion  103  and the extended portion  106 . In the normal portion  103 , the core  55  is surrounded by the cladding  60  which is generally the cladding  60  (FIG. 1) of an optical fiber  50  before the cladding ablation process is applied. In the extended portion  106 , the core  55  is surrounded by a reduced cladding  116  having an identifiable reduced thickness resulting in a reduced diameter. 
     The reduced cladding  116  may vary in thickness, depending upon the application. In the preferred embodiment, the reduced cladding  116  is of residual thickness, providing only enough cladding material around the core  55  to maintain the propagation characteristics of the extended optical fiber  100  through the extended portion  106 . The amount of cladding material needed around the core  55  to maintain the propagation characteristics of the entire extended optical fiber  100  acts as a minimum threshold for the identifiable reduced thickness of the extended portion  106 . The reduced cladding  116  of the extended portion  106  provides an advantage in that its reduced overall diameter of the extended portion  106  may be inserted into input ports of photodetectors and other devices that will not accommodate the diameter of the normal portion  103  of the fiber optic cable  100 . The actual diameter of the extended portion  106  might be, for example, 20 microns where the thickness of the reduced cladding is 10 microns. 
     Turning to FIG. 3, shown is an optical lathe system  120  for creating an extended optical fiber  100  (FIG. 2) according to an embodiment of the present invention. The optical lathe system  120  is employed in a cladding ablation process to achieve the extended core optical fiber  100 . The optical lathe system  120  is comprised of a laser source  123  which generates a laser beam  126 . The laser beam is focused by a lens  129  onto a tangential point  133  on the surface of the cladding  60  (FIG. 1) of an optical fiber  50  (FIG.  1 ). A tangential region  136  around the point is ablated under the focus of the laser  126 . After initial contact is established between the laser  126  and the tangential point  133 , the optical fiber  50  is placed in a slow rotation shown generally by arrow  124  clockwise or counterclockwise so as to cause ablation around the entire cladding  60  of the optical fiber  50  at the depth of the tangential region. At the same time, the optical fiber  50  is placed in a slow linear motion shown by arrow  127  so that the focal point of the laser  126  reaches deeper into the cladding  60  of the optical fiber  50  with each rotation as indicated by arrow  124 , causing ablation of the cladding  60  of increasing depth. The linear motion depicted by arrow  127  is stopped when the cladding  60  has been ablated to an identifiable depth. Thus, the rotation as indicated by arrow  124  and the linear motion as indicated by arrow  127  result in a section of the optical fiber  50  with an identifiable reduced diameter. If desired, the optical fiber  50  may be moved longitudinally resulting in a corkscrew motion if a longer reduced diameter section is desired. This reduced diameter is generally less than the diameter of the cladding  60 , but greater than the diameter of the core  55 . Note that the rotation as indicated by arrow  124  may also be achieved by the rotation shown by arrow  125  of the laser source  123  and the laser  126  around the optical fiber  50 . The corresponding linear motion depicted by arrow  127  may be accomplished by moving the tangential point  136  in a linear direction toward the center of the optical fiber  50  during the rotation of arrow  125 . Whether the optical fiber  50  or the laser  126  are rotated, the ultimate result is the same. Thus, it is important to establish a rotational relationship between the optical fiber  50  and the laser  129  to achieve the ablation about the perimeter while at the same time moving the tangential point  136  at which the laser  126  is focused in a linear motion of arrow  127  toward the center of the optical fiber  50 . The linear motion as indicated by arrow  127  may also be termed a radial motion which is defined herein as movement in the direction of the tangential point  133  along the radius of the optical fiber  50  extending from the center of the optical fiber  50  to the tangential point  133 . 
     Also note that the optical fiber  50  may be moved longitudinally along the axis of the optical fiber  50 , or the laser  126  may be moved along the axis of the optical fiber  50  thereby resulting in the ablation of the cladding  60  along the axis of the optical fiber  50 . Note such longitudinal movement when executed in conjunction with the forementioned rotational relationship results in ablation in a helical pattern. 
     Turning now, to FIG. 4, shown is a cross section of the optical fiber  50  and the laser of FIG.  3 . The laser  126  is focused by the lens  129  at a tangential point  133  on the surface of the cladding  60  so as to achieve ablation of the cladding material of a depth X. The optical fiber  50  undergoes rotation and linear motion as previously described until the cladding material has been ablated to an identifiable depth Y. It is understood that the actual value for the identifiable depth Y vary according to the specific application. Note that it is preferable that the laser  126  be a carbon dioxide laser for best results, however it is understood that other types of lasers may be employed to achieve the desired effects, such as for example, YAG lasers or excimer lasers. 
     Turning to FIG. 5, shown is a reduced diameter optical fiber  140  which has undergone the ablation process using the optical lathe system  120 . The reduced diameter optical fiber  140  includes a reduced diameter section  143  between normal sections  103 . By virtue of the ablation by the laser  126  (FIGS.  3  and  4 ), the reduced section  143  has a reduced cladding  116  resulting in the identifiable reduced diameter Z. The reduced diameter optical fiber  140  includes a core  55  that runs through the normal and reduced diameter sections  143  and  103 . Between the normal sections  103  and the reduced diameter section  143  are tapered sections  149 . The tapered sections  149  reflect the fact that the laser  126  does not cause ablation in perfect 90° angles as seen in the extended optical fiber  100  of FIG.  2 . Instead the ablation generally rolls off as the distance from the focus point  136  (FIG. 4) increases. 
     The reduced diameter optical fiber  140  is cleaved at an identifiable cleaving point  153  along the reduced diameter section  143 . Referring then, to FIG. 6, shown is the resulting extended optical fiber  100  after the cleaving operation. Note that the tapered section  149  exists between the normal portion  103  and the extended portion  106 . This tapered section  149  generally does not inhibit or otherwise effect the use of the extended optical fiber  100 . 
     Turning to FIGS. 7A through 7C, shown is a method for creating the extended optical fiber  100  (FIG. 2) of according to another embodiment of the present invention. As shown in FIG. 7A, an end of an optical fiber  50  is dipped into a solution  156  such as hydrochloric acid or its equivalent which dissolves the cladding material. The optical fiber  50  may be dipped into the solution  156  for a specified period of time or may be dipped numerous times of short duration. The result as seen in FIG. 7B is an optical fiber  159  having an extended portion  163  with a reduced cladding  166  and a core  55  that has been dissolved where exposed to the solution  156  (FIG.  7 A). In a final step, in FIG. 7C, the extended portion  163  is cleaved to provide a clean exit face  166  on the core  55  resulting in the extended optical fiber  100 . 
     Many variations and modifications may be made to the embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention, as defined by the following claims.