Abstract:
A method of fabrication of a fiber bundle termination with reduced fiber-to-fiber pitch which includes reducing the outside diameter of fibers in the fiber bundle by etching and provides means for preventing breakage of the etched fibers during etching and assembly. A high yield fabrication and assembly process is described.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Modern information systems require a high speed data transmission. Fiber optical systems provide a high bandwidth and are widely use in communication systems. Fiber optic bundles are commonly utilized in fiber optical systems to increase the communication bandwidth. Many methods of creating fiber bundles exist. They include crimping, tie cables, insertion of fibers into ferrules, etc. Of particular interest are the efforts to reduce fiber-to-fiber pitch because many applications require or benefit from fiber bundles having a small fiber-to-fiber pitch. For example, a liquid crystal beam steering-based 1×6 fiber optical switch, which inherently utilizes a small steering angle, requires fiber bundles with fiber-to-fiber pitch of 50 μm or less to operate. In another example, in MEMS (Micro Electronic Mechanical System) micromirror-based switch, the beam deflection angle is proportional to micromirror&#39;s tilt angle. A fiber bundle with smaller fiber-to-fiber pitch requires a smaller micromirror tilt angle which would reduce driving voltage, thus increasing system&#39;s reliability and reducing its power consumption. 
         [0002]    Another example is a fiber coupler (such as power monitoring tap coupler), that couples light from one fiber to other fibers. In a fiber coupler assembly, a lens is usually used to couple the light into fibers. A fiber bundle with reduced fiber-to-fiber pitch results in fibers being closely packed at the optical axis of the lens which improves device&#39;s coupling efficiency. 
         [0003]    Circulators, variable optical attenuators, wavelength selective switches, reconfigurable optical add/drop modules, chromatic dispersion compensators, etc. also benefit from utilizing fiber bundles with reduced fiber-to-fiber pitch. 
         [0004]    Typical single mode fibers have an inner core (active area) diameter of about 9 micrometers (μm) and an outside diameter of 125 μm. In order to produce a fiber bundle with a reduced fiber-to-fiber pitch, the fiber outside diameter has to be reduced. 
         [0005]    A reduction of the outside diameter of the fibers also results in an increase of the packing fraction of the fiber bundle, which is defined as the ratio of the information-carrying cross-sectioned area of the fiber bundle (fiber cores) to the total cross-sectioned area of an endface of the fiber bundle. The increased packing fraction of the fiber bundle is beneficial for applications where increased density of light is important. 
         [0006]    A method of reducing fiber outside diameter is disclosed in the U.S. Pat. No. 3,912,362 to Hudson, hereinafter referred to as &#39;362 patent. The &#39;362 patent discloses fiber bundle termination where fibers are etched and inserted into a ferrule and secured to each other and the ferrule by adhesive. The &#39;362 patent discloses fibers with an inner active core having a large diameter. The inner core diameter of processed fibers disclosed in &#39;362 patent is 3.6 mils, which is equivalent to about 91.4 μm, and an outer diameter of those fibers equal to 5.5 mils (139.7 μm). The &#39;362 patent teaches etching the fibers to reduce the outer diameter to 3.8 mils (96.5 μm). Because the after-etch diameter of the fibers was still very large, fiber breakage was not a concern, and special means to prevent breakage of the fibers were not required. 
         [0007]    Accordingly, a need exists to create a fiber bundle with a fiber-to-fiber pitch smaller than 50 μm for fibers having small inner core diameter, typically in the range of about 3.5 μm to about 10 μm. That requires the after-etch fiber outside diameter of less than 50 μm. Fibers with small outside diameter are extremely fragile and are very difficult to handle. This problem is overcome by the hereinafter disclosed fabrication method that stabilizes the fibers and reduces the possibility of breakage associated with a small after-etch fiber outside diameter. 
       SUMMARY OF THE INVENTION 
       [0008]    A fiber bundle termination with reduced fiber-to-fiber pitch and the associated process capable of fabricating fiber bundles with a fiber-to-fiber pitch much smaller than the diameter of the original fiber is disclosed. The original fibers are first assembled into a holder that provides mechanical protection, and the outside diameters of the fibers protruding from the ferrule are subsequently reduced through a chemical etching process. The etched fibers are then assembled into a fiber bundle termination. The fibers are secured in the termination that packs the etched fiber tightly achieving a small fiber-to-fiber pitch. The termination also provides mechanical protection, and reduces a possibility of breakage of the fibers during the assembly process. Further, a high yield fabrication and assembly process is disclosed. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is an illustration showing unetched fibers secured by a holder into a fiber bundle. 
           [0010]      FIG. 2  is an illustration showing secured fiber bundle after the etch step. 
           [0011]      FIG. 2   a  is an illustration showing an expanded view of the etched fibers. 
           [0012]      FIG. 3  is an illustration showing the fibers protruded from the holder inserted into a collector. 
           [0013]      FIG. 4  is an illustration showing a preferred embodiment of the collector including an optional entry point for a bonding material injection. 
           [0014]      FIG. 5  illustrates a flow chart of the fiber bundle termination assembly process. 
           [0015]      FIG. 6  is an illustration showing a cross-section of the collector for one of the embodiments (with outside fiber diameter reduced to 50 μm). 
           [0016]      FIG. 7  illustrates a high-yield fabrication and assembly method. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    An improved fiber bundle termination and method of manufacturing is disclosed wherein fiber-to-fiber pitch is minimized by reducing the fiber outside diameter and then packing the fibers with reduced diameter tightly into a fiber bundle termination. 
         [0018]    Reduction of the outer diameter of the fibers may be achieved by etching the cladding of the fibers. Provision is also made to prevent breakage of the etched fibers. 
         [0019]    The fiber bundle termination with reduced fiber-to-fiber pitch may be used with fiber optical switches, fiber couplers, circulators, variable optical attenuators, wavelength selective switches, reconfigurable optical add/drop modules, chromatic dispersion compensators, etc. 
         [0020]    Referring now to  FIG. 1  and steps  52  and  54  of  FIG. 5 , an improved fiber bundle termination  10  is created by threading a plurality of fibers  11  through a holder  20  having a first endface  12 , second endface  13 , and inner cavity  19 . The fibers  11  are fixedly secured in the holder  20 . A portion  16  of the fiber bundle fibers protrudes from the second endface  13  of the holder. The 25 mm length was used for the holder  20 . The length of the protruding portion  16  of the fibers  11  is approximately 14 mm. A preferred range for the protruding portion  16  may be 10 to 20 mm. The fibers  11  may be fixedly secured, for example, by filling the holder  20  with a bonding material  21 , which can be an epoxy, glue, or any other suitable material, or by a surface tension between the fibers of the fiber bundle and walls of the inner cavity  19  of holder  20 . The filling of the holder  20  may be done prior, after, or during the threading of the fibers through the holder. A preferred bonding material  21  is an epoxy that has low shrinkage during curing, such as EPO-TEK 353ND. The curing requirements and process are well known to a person skilled in the art. 
         [0021]    In the next step, illustrated in  FIG. 2  and  FIG. 2   a  and outlined in step  56  of  FIG. 5 , a partial length  17  of the protruded portion  16  of fibers  11  is etched to achieve a reduction in the outside diameter of the fibers. To prevent fiber breakage at the second endface  13  of the holder  20 , it is not desirable to etch the whole length  16  of the protruded fibers. About 1 to 5 mm portion  15  of the protruded fibers  16  closest to the holder  20  is left unetched. The etched portion  17  and the unetched portion  15  together comprise the after-etch protruding fibers  14 . 
         [0022]    While outside diameter of fibers  11  may vary, commercially available fibers  11  typically have an outside fiber diameter of about 125 μm. The process disclosed hereinafter is currently capable of achieving the after-etch outside fiber diameter of 35 μm. Telecommunication single mode fibers have inner core diameters typically in the range of about 3.5 to about 10 μm. The fiber core diameter is chosen according to the intended use. 
         [0023]    Theoretically, it is possible to etch off substantially all the cladding of the fibers, but the smaller the after-etch outside diameter of the fibers, the more likely the breakage of the fibers during processing and assembly. In addition, when bonding material is to be applied, it is beneficial not to etch off all fiber cladding due to the fact that if all cladding is removed, bonding material will act as cladding and may degrade the performance of the fibers by, for example, creating absorption losses. 
         [0024]    Fibers  11  can be etched using known chemical processes to the value of the after-etch outside fiber diameter substantially equal to the fiber&#39;s inner core diameter. An etching solution can be chosen based on the type of material used for cladding. 
         [0025]    Some glass claddings, for example, may be etched with acids, such as BOE (Buffered Oxide Etch), hydrofluoric acid, phosphoric acids, or other suitable acids, while other glasses may be etched with alkaline solutions, such as sodium hydroxide, potassium hydroxide, or other suitable alkaline solutions. Ultrasonic agitation of the etching solution can be employed to improve uniformity of etching, or other methods can be used to stir the etching solution. A correlation between the properties of etching solutions (type, concentration, temperature, etc.) and the etch rate is well know in the art. The etch duration is related to the etch rate and the thickness of cladding that needs to be removed. The etch rate needs to be calibrated. Etching can also be performed by non liquid etches, such as, for example, plasma etching. 
         [0026]    After etch, as illustrated in  FIG. 3  and outlined in steps  58  and  59  of  FIG. 5 , protruding fibers  14 , are threaded through a collector  30 , having a first endface  22 , a second endface  24 , an outside wall  29 , and a tapered cavity  26  therebetween, with the tapered cavity  26  having a larger diameter at the first endface  22  than at the second endface  24 . The protruding fibers  14  are inserted into the first endface  22 . The length of the collector  30  may be about 0.5 mm shorter than the length of protruding fibers  14 . The tapered cavity  26  may be circular, elliptical hexagonal, rectangular, or of any other geometrical configuration. The cavity is tapered from the first endface  22  to the second endface  24 . This taper arrangement allows the protruding fibers  14  to be easily fitted into a larger opening of the tapered cavity  26  at the first endface  22 . The tapering of the cavity compresses the fiber bundle  10  by channeling the fibers into a smaller cross sectional area thereby creating a fiber bundle  10  exhibiting a small fiber-to-fiber pitch at the second endface  24  of the collector  30 . As illustrated in  FIG. 4 , an entry point at the first endface  22  may optionally have a larger diameter initial non-tapered cavity to facilitate injection of a bonding material. The cross sectional area of the cavity at the second endface  24  of collector  30  accommodates a plurality of the fibers  11  in the fiber bundle  10  with the least cross sectional area possible. The collector  30  may be filled by a bonding material to fixedly secure the fibers. The bonding material can be an epoxy, glue, or any other suitable material. Filling of the collector  30  with the bonding material may be done prior, after, or during the initial treading of the fibers  11  through the collector  30 . It is preferred that the second endface surface  13  of holder  20  and the first endface surface  22  of collector  30  come into a contact with each other and that they be substantially parallel to each other. 
         [0027]    Fibers  14  protruding from the second endface  24  of collector  30  may be cut by cleaving, laser trimming, or by other suitable means. After that, it is necessary to polish the cut ends of the fibers at the second endface  24  of the collector  30  to make the end of the fiber bundle at the second endface  24  to be substantially in the same plane as the second endface  24 . To further protect fibers from damage during the polishing step, a bonding material  21 , such as an epoxy, can be applied over the protruded portion of the fibers  14 , forming a glob, and cured, if necessary, before a polishing step. This will further limit the mechanical disturbance of the fibers  11  due to the polishing step and prevent fiber cracking. 
         [0028]    The next step may be to deposit an antireflective coating  45  on the polished endfaces of the fibers that are in the same plane with the second endface  24  of collector  30 . The second endface  24  of collector  30  may also be coated simultaneously with the fibers. The process of applying antireflective coating is well known in the art. 
         [0029]      FIG. 6  illustrates a cross-section of the collector  30  for one of the embodiments (with outside fiber diameter reduced to 50 μm.) 
         [0030]    Holder  20  and collector  30  may be made from glass, ceramic, plastic, metal, or any other suitable material. 
         [0031]    To make the fiber bundle termination more robust, bonding material  21  may be applied between the second endface  13  of holder  20  and the first endface  22  of collector  30  and cured. 
         [0032]      FIG. 7  illustrates a preferred method of manufacturing a fiber bundle termination a self-aligned assembly that utilizes a guide  40 . The guide  40  can be either a reusable fixture or a part of integrated final assembly. Guide  40  is designed to receive holder  20  and collector  30 . The outer diameter of the holder  20  is approximately the same as the outer diameter of the collector  30 . Guide  40  is hollow with an interior diameter slightly larger than outside diameters of holder  20  and collector  30 . The inner diameter of the guide  40  is designed to allow holder  20  and collector  30  to slide into guide  40  with an application of a minimal force and also to prevent a voluntarily movement of the holder  20  and the collector  30  inside the guide  40 . 
         [0033]    As illustrated in  FIG. 7 , after the fiber bundle  11  is secured by the holder  20  and protruding fibers  16  etched to become after-etch reduced-diameter protruding fibers  14 , the holder  20  is inserted into the guide  40  part of the way so that said protruded fibers  14  remain inside the hollow of the guide  40 . In the next step, collector  30  may be lowered into the guide  40  until the first endface  22  of the collector  30  and the second endface  13  of holder  20  come into a contact. The collector  30  may be filled with the bonding material, where said filling can be performed prior, after, or during the insertion of collector  30  into holder  40 . A preferred method is to fill the cavity with the bonding material prior to the insertion of the fibers. A preferred bonding material is an epoxy selected from the group that has low shrinkage during curing, such as EPO-TEK 353ND. The curing requirements and process are well known to a person skilled in the art. 
         [0034]    The holder  20  and the collector  30  may be bonded together by applying the bonding material between the adjacent endface surfaces  13  and  22 . 
         [0035]    The guide  40  may be removed or it may be left in place to become a part of the final assembly. In one embodiment, the guide  40  may be bonded to holder  20  and/or collector  30  by applying a bonding material between the inside walls of the hollow of the guide  40  and the outside wall  18  of the holder  20  and/or the outside wall  29  of the collector  30 , and curing it. 
         [0036]    The holder  20 , the collector  30 , and the guide  40  may be made from glass, ceramic, plastic, metal, or any other suitable material.