Abstract:
A method and apparatus for applying a mid-IR graded microstructure to the end of an As2S3 optical fiber are presented herein. The method and apparatus transfer a microstructure from a negative imprint on a nickel shim to an As2S3 fiber tip with minimal shape distortion and minimal damage-threshold impact resulting in large gains in anti-reflective properties.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims the benefit of the filing date of U.S. Provisional Application No. 61/328,288 filed on Apr. 27, 2010, the entire contents of which are incorporated by reference hereto. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The claimed subject matter was reduced to practice with United States Government support under Contract No. N00173-05-C-6020 awarded by the United States Naval Research Laboratory. Accordingly, the United States Government has certain rights in the claimed subject matter. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosed subject matter relates to a method and apparatus for transferring a microstructure from a negative imprint on a nickel shim to an As2S3 optical fiber tip with minimal shape distortion and minimal damage-threshold impacts, thereby improving the anti-reflective properties of the As2S3 optical fiber. 
     However, the method and apparatus herein presented are not limited in application to As2S3 optical fibers. In fact, the apparatus and method herein presented can be practiced with other optical fiber materials having melting points below 600° C. 
     2. Brief Description of Related Art 
     Optical fibers can be used in a great number of applications in the Mid-IR wavelength region including sensing, imaging and processing. These optical fibers have large refractive indexes, ranging from about 2.3 to about 2.9. Air, on the other hand, has a refractive index of 1. This large difference in refractive index between the optical fibers and air leads to signal losses at the optical fiber/air interface. In some, applications these signal losses at the optical fiber/air interface can amount to 25% or more. 
     In order to prevent signal losses, some have turned to applying anti-reflective coatings to polished fiber tips. However, polishing can oftentimes lead to optical fiber fracture in delicate fiber materials. Furthermore, anti-reflective coatings often exhibit adhesion problems and rapid degradation as a result of exposure to high intensity signal radiation. 
     SUMMARY OF THE INVENTION 
     Therefore, a need exists for an improved method and apparatus for applying an anti-reflective treatment to mid-IR optical fibers that is reliable, efficient and does not damage the optical fibers. 
     In one embodiment, the disclosed subject matter relates to a method for preventing reflection losses in optical fibers, the method comprising the steps of heating an optical fiber tip to form a heated optical fiber tip, flattening the heated optical fiber tip to form a flattened optical fiber tip, heating the flattened optical fiber tip and imprinting a microstructure onto the flattened optical fiber tip. 
     In another embodiment, the disclosed subject matter relates to method for preventing reflection losses in properly terminated As2S3 fibers, the method comprising securing a properly terminated As2S3 fiber into a ferrule, so that a tip of the properly terminated As2S3 fiber protrudes about 1 mm to 2 mm from the ferrule, fastening the ferrule to a fixture, lowering the fixture onto a heating surface, such that the heating surface transfers heat to the properly terminated As2S3 fiber tip without touching the properly terminated As2S3 fiber tip, adjusting the orientation of the fixture with a hollow cylinder placed between the fixture and the heating surface to ensure perpendicularity of the properly terminated As2S3 fiber tip relative to the heating surface, lowering the fixture so that the properly terminated As2S3 fiber tip contacts the heating surface, replacing the heating surface with a hot imprinting surface, lowering the fixture onto the hot imprinting surface, such that the hot imprinting surface transfers heat to the properly terminated As2S3 fiber tip without touching the properly terminated As2S3 fiber tip and lowering the fixture so that the properly terminated As2S3 fiber tip contacts the hot imprinting surface. 
     In another embodiment the disclosed subject matter relates to an apparatus for creating an antireflective tip in an optical fiber, the apparatus comprising a fixture that is primarily capable of translational motion, with tip/tilt rotational fine-adjustment via goniometer or similar device, a heating element positioned along a translational axis of the fixture and a shaping member disposed on the heating element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will be better understood from the detailed description given below and by reference to the attached drawings in which: 
         FIG. 1A  is a flowchart depicting an embodiment of the method for preventing reflection losses in optical fibers. 
         FIG. 1B  is a flowchart depicting an embodiment of the method for preventing reflection losses in optical fibers. 
         FIG. 2  is a schematical view an embodiment of the apparatus for creating an antireflective tip in an optical fiber. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As used herein the term “properly terminated fiber,” or “appropriately terminated optical fiber,” or “properly terminated As2S3 fiber” means an optical fiber, including an As2S3 fiber and any other optical fiber having a melting point below 600° C., that has been previously cleaved and glued into a Zirconia Ferrule, such that the optical fiber tip protrudes out about one to two diameters. 
     Referring to  FIG. 1A , a preferred embodiment of the claimed method is depicted in flow chart  100 . In step  103 , using standard optic practices, a flat shaping member is cleaned, first using acetone and subsequently using methanol. In step  106 , ionized air is applied to the flat shaping member in order to remove any excess acetone and/or methanol left over from step  103 . In step  109 , a flat shaping member is inspected for cleanliness under a high power, long focal length digital microscope, having at least 80× magnification capabilities. 
     In order to ensure proper cleanliness during this stage of the process, if microscopic inspection in step  109  reveals that further cleaning is necessary, the cleaning procedure should be repeated beginning with step  103 , until microscopic inspection in step  109  reveals that the flat shaping member is clean. 
     In step  112 , using standard optic cleaning practices, an imprinting member is cleaned using methanol. In step  115 , ionized air is applied to the imprinting member in order to remove any excess methanol left over from step  112 . The imprinting member is then cleaned using an Argon Plasma Cleaner (such as “Plasma Preen” made by Terra Universal, Inc) (step  118 ), and inspected for cleanliness (step  121 ) under a high power, long focal length digital microscope, having at least 80× magnification capabilities. 
     In order to ensure proper cleanliness during this stage of the process, if microscopic inspection in step  121  reveals that further cleaning is necessary, the cleaning procedure should be repeated beginning with step  112 , until microscopic inspection in step  121  reveals that the flat shaping member is clean. 
     In step  124 , the flat shaping member is placed on a temperature adjustable heating unit, such as a hot plate. While the shaping member is heating up (in step  124 ), in step  127  a properly terminated As2S3 fiber is inserted into a fastening fixture, usually made of steel. The As2S3 fiber ferrule is inserted into the fixture such that the Zirconia-clad tip protrudes about 1 mm to about 2 mm from the end of the fastening fixture. The fastening fixture holds the Zirconia ferrule parallel to the primary axis of translation and features a larger perpendicular surface which can be used as a reference surface in the next step (the hollow cylinder adjustment). The fixture is capable of translational motion, so that as the fixture moves down, the fixture moves toward the shaping member positioned directly below the fixture and as the fixture moves up, the fixture moves away from the shaping member positioned directly below the fixture. 
     Once the ferrule and optic fiber are fastened in the fixture (step  127 ), in step  130 , a hollow cylinder with flat, parallel ends is placed on top of the flat shaping member and the fixture is lowered so as to touch the opposite end of the cylinder. The tip/tilt adjustment feature is used (step  133 ) to make sure the fixture is flush with the cylinder end (adjusted by eye). This ensures that the ferrule/optic fiber assembly&#39;s translational axis is normal to the surface of the shaping member. Further, because the cylinder is hollow, only the outer surface of the fixture contacts the ring, while the optic fiber tip remains untouched within the hollow space. 
     In step  133 , goniometers are adjusted to ensure that the ferrule/optic fiber assembly is normal (along the axis of translational motion) to the top surface of the shaping member. Thus once the ring is removed the bottom surface of the ferrule and the optic fiber tip contained within it are nominally parallel to the flat shaping member (to within the tolerances of the assemblies). 
     Referring to  FIG. 1B , in step  136 , the temperature of the flat shaping member is allowed to stabilize with the temperature of a hot plate surface, to a temperature range of about 170° C. to about 270° C. It is important to ensure the flattening member stabilizes within the above-mentioned temperature range because a lower temperature may result in defective flattening, while a higher temperature could result in optic fiber damage. 
     Once the flattening member has stabilized at the desired temperature (step  136 ), in step  139  the fixture is lowered bringing the ferrule/optic fiber assembly toward the heated flattening member. Since the optic fiber tip protrudes from the ferrule, as the fixture moves toward the heated flattening member, the optic fiber tip is closer to the heated flattening member than any portion of the ferrule. The fixture should move down toward the heated flat shaping member until the optic fiber tip is about 100 μm to about 200 μm from the heated flat shaping member. Once the optic fiber tip is within this desired range, the fixture is held in place for about 60 seconds. This permits the tip to be heated radiatively and by air convection due to its close proximity to the heated surface. 
     At the end of the 60 seconds of step  139 , the fixture again moves down toward the heated flat shaping member until the optic fiber tip contacts the heated surface of the flat shaping member (step  142 ). To ensure appropriate flattening of the optic fiber tip, in step  142  a prescribed pressure of about 3,000 PSI to about 144,000 PSI is applied on the optic fiber tip against the flat surface of the heated flat shaping member. The period of time during which contact and pressure are applied can vary. In fact, contact and pressure can be maintained for a period of about 10 seconds to about 300 seconds (Note: time, temperature and pressure are co-dependent variables—reducing one quantity can often be made up by increasing another). 
     In step  145 , the pressure on the optic fiber tip is removed and the fixture is moved, away from the heated surface of the flat shaping member. The optic fiber tip at the end of step  145  should be flat, consistent with the surface of the heated flat shaping member. 
     In step  148 , the flat shaping member is removed from hot plate and replaced with the imprinting member. The imprinting member is allowed to stabilize to a temperature range of about 170° C. to about 270° C. The surface of the imprinting member closest to the optic fiber tip contains a negative imprint of the microstructure that will later be contact transferred to the optic fiber tip on step  154  below. The microstructure on the imprinting member can consists of any desired pattern arrangement, but usually contains a plurality of protrusions and recesses. In one embodiment, the microstructure used is manufactured by TelAztec LLC of Burlington Mass. 01803. 
     Once the imprinting member&#39;s temperature is stabilized (step  148 ), in step  151  the fixture is lowered bringing the ferrule/optic fiber assembly toward the heated imprinting member. The fixture should move down toward the heated imprinting member until the optic fiber tip is about 100 μm to about 200 μm from the heated imprinting member. Once the optic fiber tip is within this desired range, the fixture is held in place for about 60 seconds. This permits the tip to be heated radiatively and by air convection due to its close proximity to the heated surface. 
     At the end of the 60 seconds of step  151 , the fixture again moves toward the heated imprinting member until the optic fiber tip contacts the heated imprinting member (step  154 ). To ensure appropriate imprinting of the optic fiber tip, a prescribed pressure of about 3,000 PSI to about 80,000 PSI is be applied on the optic fiber tip against the surface of the imprinting member. Contact and pressure should be maintained for a period of about 30 seconds. The period of time during which contact and pressure are applied can vary. In fact, contact and pressure can be maintained for a period of about 10 seconds to about 300 seconds (Note: time, temperature and pressure are co-dependent variables—reducing one quantity can often be made up by increasing another). 
     At the end of the 30 second period, in step  157  the pressure is removed and the fixture is moved away from the imprinting member. The optic fiber tip at the end of step  157  should have a microstructure, consistent with the surface of the imprinting member. 
     In step  160 , the ferrule/optic fiber assembly is removed from the fixture and is inspected under appropriate magnification to ensure proper microstructure transfer. 
     Referring to  FIG. 2 , a preferred embodiment of the apparatus for creating an antireflective tip in an optical fiber  200  (hereinafter “the apparatus”) is depicted. The apparatus  200  has a fixture  227  that is capable of rotational motion. In one embodiment the fixture  227  is capable of rotational motion along a y-axis that is perpendicular to a work bench  203 . However, in other embodiments the fixture  227  may be capable of translational motion along the x-axis or z-axis. 
     The apparatus  200  also has a heating element  218  that is positioned along the axis of translational motion of the fixture  227 . The heating element  218  has a temperature sensing means  221  disposed on it. The temperature sensing means  221  can consist of a thermistor, a thermometer or any other temperature sensing device commercially available. 
     The axis of translational motion of the fixture  227  can vary. For instance, in one embodiment, where the axis of translational motion corresponds to an axis of gravitational acceleration, in this embodiment the heating element  218  can be positioned below the fixture  227 , which is disposed on a work bench  203 . In an alternate embodiment, the heating element  218  can be position above the fixture  227 . Further, in other embodiments the axis of translational motion of the fixture  227  can be perpendicular to the axis of gravitational acceleration, in this embodiment the heating element  218  can be positioned at any number of positions along the axis of translational motion of the fixture  227 . The heating element  218  can be any commercially available hot plate having either variable temperature of variable power settings. 
     Positioned on the heating element  218 , there is a shaping member  224 . The shaping member  224  is positioned so that at least a portion of the shaping member  224  intersects the axis of translational motion of the fixture  227 . The shaping member  224  can have different patterns. For instance, in one embodiment the shaping member  224  has a substantially flat shape. In an alternate embodiment, the shaping member  224  can have a negative imprinting region having an irregular shape consisting of a plurality of protrusions and recessions. 
     Further, the shaping member  224  can have either single shaping pattern or a multiple shaping patterns. For instance, in one embodiment the shaping member  224  can have a single substantially flat shaping pattern. In a different embodiment, the shaping member  224  can have a single negative imprinting region consisting of an irregular shape pattern having a plurality of protrusions and recessions. In other embodiments, the shaping member  224  can have combinations of patterns located on different regions of the shaping member  224 . For instance in one embodiment the shaping member  224  can have a first region that is substantially flat and a second region that is substantially irregular. 
     Fastened to the fixture  227 , there is a ferrule  233 . The ferrule  233  can be any type of commercially available and appropriate ferrule, including steel ferrules as long as the process of inserting the fiber into the ferrule  233  does not damage it. The bare optical fiber  245  is glued into the ferrule  233  in advance. The bare optical fiber  245  and ferrule  233  assembly can have additional secondary ferrules, fixture adapters or armor surrounding the ferrule-terminated fiber such that the assembly fits snugly into the fixture  227 . In a preferred embodiment, the bare optical fiber  245  is an As2S3 fiber. Further, the bare optical fiber  245  is terminated with a Zirconia ferrule nested inside a steel adapter placed in the ferrule  233 , so that at least of a portion of the bare optical fiber  245  (a fiber tip) protrudes through the ferrule  233  assembly. 
     Upon fastening to the fixture  227  the ferrule  233  and bare optical fiber  245  moves along the translational axis of motion of the fixture  227 , either toward or away from the shaping member  224 . Guiding the translational motion there can be a rail  230  that is operatively connected to the fixture  227 , so that the fixture  227  can move along the rail  230 . A connecting member  236  can be used to connect the rail  230  to z-axis goniometer  239  and x-axis goniometer  242 , which allow for two-dimensional rotational adjustments of the fixture  227  along the z-axis and x-axis, respectively. In general, the goniometers&#39;  239   242  axes are set up perpendicular to one-another as well as perpendicular to the primary axis of translation (y-axis in this example). 
     In one embodiment, there can be platform  251  connected to the x-axis goniometer  242  and also operatively connected via a connecting member  236  to translational stage  254 , thereby providing translational motion to the fixture  227 . In an alternate embodiment, the platform  251  can be connected to the z-axis goniometer  239  and also be operatively connected via a connecting member  236  to translational stage  254 . Translational stage  254  provides translational motion to the fixture  227  along any desired axis depending on the desired set up. For instance in one embodiment, translational stage  254  may provide translational motion along an axis normal (i.e. along a y-axis) to a work bench  203 . In other embodiments, the translational stage  254  may provide translational motion along an axis parallel (i.e. along an x-axis) to a work bench  203 . 
     Additional translational members may be provided in order to facilitate two or three dimensional adjustment capabilities. For instance, in one embodiment a z-axis translational stage  257  can be operatively connected to translational stage  254  to provide overall control of both y-axis and z-axis motion, respectively. Further, an x-axis translational stage  260  can be operatively connected to z-axis translational stage  254  to provide overall control of y-axis, z-axis and x-axis, respectively. 
     The apparatus  200  also has a pressure exerting member  248 . The pressure exerting member  248  exerts pressure along the translational axis of rotation of the fixture  227 , and more particularly along the length of the bare optical fiber  245 . In one embodiment the pressure exerting member  248  can be a weight. In this embodiment, the weight can be disposed on platform  251 , where it transfers pressure to the fixture  227  by gravity. In other embodiments, the pressure exerting member  248  can be a hydraulic device, mechanical screw, or other pressure exerting device operatively connected to the fixture  227 . The pressure exerting member  248  is capable of exerting a pressure in the range of about 3,000 PSI to about 144,000 PSI. In the case where weights are used, the applied pressure is inferred by dividing the weight by the bare optical fiber  245  surface area. In other embodiments, a scale or force-feedback sensor can be used to measure the applied force for pressure calculations. 
     A microscope  209  can be positioned within line of sight of the fixture  227  in order to facilitate visual inspection of the bare optical fiber  245  while the apparatus  200  is in use. In one embodiment, the microscope  209  has a long focal length and has at least 80× magnification capabilities along with electronic video capabilities. The microscope can be positioned on a microscope stand  206  and can be optionally connected via a video-signal cable  212  to a monitor  215  that displays the images captured by the microscope  209 , thereby facilitation inspection of the bare optical fiber  245 . 
     It is to be understood, that the above-described arrangements are intended solely to illustrate the application of the principles of the disclosed subject matter. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the disclosed subject matter in the present Application. Accordingly, the appended claims are intended to cover such modifications and alternative arrangements. Thus, while the disclosed subject matter of the present Application has been described above with particularity and detail in connection with what is presently deemed to be the most practical and preferred embodiments, it will be apparent to those skilled in the art that numerous modifications, including, but not limited to, variations in size, materials, shape, form, function and manner of operation, assembly and use may be made without departing from the principles and concepts set forth herein.