Patent Publication Number: US-6666984-B2

Title: Chemical stripping apparatus and method

Description:
This is a divisional application of U.S. Ser. No. 09/804,761, filed Mar. 13, 2001, now U.S. Pat. No. 6,547,920. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a process and equipment for conveniently handling a filament in the form of an optical fiber during multiple processing operations that may be at least partially automated. More particularly the invention relates to compact handling of optical fibers during manufacturing operations to include Bragg gratings in at least a portion of their length via a series of manufacturing operations including mechanical stripping, acid stripping, Bragg grating writing, and optical fiber recoating and testing. 
     BACKGROUND OF THE INVENTION 
     Glass has been used for centuries as a material of choice in a variety of scientific and domestic applications. From the early use of prismatic glass for separating light into its component colors, glass has been widely used in optical devices that control or adjust the properties of light beams. A recent and rapidly expanding application of the light modifying properties of glass structures involves the drawing of fine filaments of highly purified glass, more commonly referred to as optical fibers, that direct light signals between light transmitting and receiving locations 
     During the late 1970s utilities began using optical fiber installations for internal communication, and by the early 1980s, a number of small optical fiber networks were installed. The use of such networks has been growing ever since to replace existing coaxial cable systems. Advantages provided by optical fiber communications networks include lower cost, the use of fewer signal repeaters to correct for signal distortion, and a higher signal carrying capacity than coaxial cable networks. 
     The capacity of fiber optic systems continues to increase. In 1980, the first systems could transmit 45 megabits per second. Current systems transmit up to 5 gigabits per second. So extensive is the modern use of optical fiber networks that fiber optic networks have essentially replaced all transcontinental copper cable networks and entirely new networks are being created continually. One prediction claims that every continent in the World will become part of a global fiber optic network. 
     A fiber optic system includes three main parts of transmit circuitry and light source, light detector and receiver circuitry, and fiber. The transmit circuitry converts electronic signals to modulate a light source that generates light signals for transmission. Connection from the light source to a length of optical fiber facilitates transmission of light signals for distances covered by the optical fiber. Attachment of light detector and receiver circuitry at the terminal end of a fiber produces a communication link The use of multiple communication links provides extended networks of transmitters and receivers. 
     Interconnection of fiber optic networks requires high precision devices in the form of optical connectors that join optical fibers to peripheral equipment and other optical fibers while maintaining adequate signal strength. In operation an optical connector centers the small fiber so that the light gathering core lies directly over and in alignment with a light transmitting source or another fiber. Sections of optical fiber may also be spliced together using mechanical splicing or fusion splicing techniques. 
     Special features may be built into selected, relatively short lengths of optical fibers to be spliced into fiber optic networks. A fiber Bragg grating represents a light-modifying feature that may be introduced or written into an optical fiber by simple exposure to ultraviolet light. The ability to write such gratings leads to a variety of devices. For example, Bragg gratings may be applied in telecommunications systems to control the wavelength of laser light, to introduce dispersion compensation, and, in the form of long period gratings, to modify the gain of optical fiber amplifiers. Fiber optic applications of fiber Bragg gratings, outside of telecommunications, include spectroscopy and remote sensing. 
     The process of introducing special features such as Bragg gratings into an optical fiber may include a number of steps requiring handling of relatively short lengths of optical fiber during a series of manufacturing operations. An optical fiber typically requires removal of protective coatings before changing the physical characteristics of the fiber to include a Bragg grating. After writing a Bragg grating, the fiber may be annealed and recoated. 
     Little has been revealed about the automation of processes to alter the characteristics of a fiber to provide it with a refractive index grating. Some evidence exists of individual processing steps but not of a type that may be readily incorporated in an automated sequence. Fiber loading for example is described in U.S. Pat. No. 5,988,556. This patent refers to automated winding of a continuous length of fiber from a fiber supply onto first and second sections of a shipping spool. The winder comprises a first device that collects a first portion of a continuous length of fiber and winds it onto the first section of the spool, and a second device for winding a second portion of the continuous fiber onto the second section of the spool. There is no evidence to show that the spooled fiber has a use other than as a shipping package. U.S. Pat. No. 6,027,062 describes an automated winder including fiber supply and collecting devices that move a fiber to a threading device that automatically threads the fiber onto a spool. This is similar to the goal of U.S. Pat. No. 4,511,095 to form of a coil of fiber wound onto a bobbin or similar structure. 
     The use of stackable cassettes for handling and organizing optical fibers is well known, particularly for storage of lengths of spliced fibers. Cassettes typically comprise shallow dish-shaped holders and enclosures for containment of loosely coiled optical fibers. Loose optical fiber coils do not have the same compact structure as spooled optical fibers. An intermediate form of coiled fiber, described in U.S. Pat. No. 5,894,540, may be produced using an assembly for holding a length of filamentary material in a wrapped configuration with a minimum bend radius. The filament or fiber may be wrapped around spools attached to a support plate. Adjustment of the spacing between spools removes slack from the fiber wrapped around them. Fiber cassettes and related fiber holding assemblies place loose fiber in a tidy condition for storage, usually following interconnection of lengths of optical fiber. U.S. Pat. No. 6,088,503 confirms the use of optical fiber cassettes as holders of optical fibers before, during and after splicing. The patent describes a clamping tool designed to align and hold a pair of fiber ends in preparation for optical fiber splicing. 
     Cassettes and related fiber organizing assemblies provide tidy storage for optical fibers around connected and spliced sections of optical fiber. There appears to be no evidence of such storage containers used for processing organized lengths of optical fiber during the manufacture of optical fiber devices. One manufacturing process requires the removal of protective buffers and coatings to reveal the bare surface of an optical fiber. Several processes are known for removing protective layers, such as buffers and coatings, from the surface of optical fibers. They include mechanical stripping, chemical stripping and thermal stripping. 
     Mechanical stripping of optical fibers and related coated filaments requires careful positioning of sharp tempered metal blades to expose a bare surface portion of a fiber without cutting or scratching or otherwise physically damaging the fiber surface. Known methods of mechanical stripping relate to cutting blade design and how a coating may be removed from the surface of a fiber. The predominant use of mechanical stripping involves the removal of protective layers from the ends of optical fibers, insulated wires and related filaments, prior to joining the filament ends together. U.S. Pat. No. 4,434,554 describes an optical fiber stripping device including a flat base having a number of fiber receiving channels of suitable depth to ensure only removal of a buffer coating from each fiber, when a blade penetrates the coating. The blade moves parallel to the axis of a fiber or group of fibers using a paring action to remove protective material. Channel size, based upon fiber diameter determines the selection of a flat base to provide a device that strips a fiber end without damaging the fiber itself. 
     One way to avoid damage to the bare surface of an optical fiber requires the use of blades designed to penetrate the protective buffer or fiber coating without reaching the fiber surface. Suitable blades have a substantially semicircular sharpened edge of a radius slightly larger than the radius of the bare optical fiber. Two opposing blades, penetrating the protective layer around the fiber, interfere with each other before the cutting edges reach the fiber surface. After penetrating a protective layer, close to the end of a fiber, movement of the blades parallel to the fiber axis displaces a section of the layer to provide a bare fiber end untouched by the blades. United States Patents, including U.S. Pat. No. Pat. No. 4,630,406, U.S. Pat. No. 5,269,206, U.S. Pat. No. 5,481,638, and U.S. Pat. No. 5,684,910, describe the manufacture and design of blades for cutting insulation from e.g. insulated electrical wires and optical fibers. Successful mechanical stripping using such blades may require additional treatments, including softening the protective layer as in U.S. Pat. No. 5,481,638 requiring a chemical filled chamber first to soften an encapsulating layer then to clean plastic material from the blades after stripping. U.S. Pat. No. 5,684,910 teaches an optical fiber having improved mechanical strippability. The improvement includes the use of a frangible boundary layer between a fiber coating and a buffer to facilitate separation from the bare fiber. Previous teachings include initial blade movement perpendicular to a filament axis, to penetrate a coating, followed by movement parallel to the filament axis to expose bare filament ends by displacement of protective layers. 
     Chemical stripping may be used as an alternative to mechanical stripping for preparing bare fiber ends. U.S. Pat. No. 4,865,411 and U.S. Pat. No. 4,976,596 deal with controlled removal of coating, by gradual withdrawal of a coated fiber from a chemical bath, to produce a contoured shallow taper adjacent to the bare glass fiber surface. A fixture, according to U.S. Pat. No. Pat. No. 5,451,294 provides support while dipping the end of a coated optical fiber into a chemical bath to dissolve coating from the end. Chemical stripping methods include common problems related to the handling of chemicals especially, as in this case, when the chemical strippers involve corrosive liquids. 
     Hot gas stripping may be used instead of mechanical or chemical stripping. One example of this process, described in U.S. Pat. No. Pat. No. 6,123,801, uses a hot inert gas to melt buffer coating and blow it from the surface of an optical fiber. The process requires high pressure gas streams and temperatures in the region of 800° C. to strip coating from the fiber. U.S. Pat. No. 5,939,136 describes a process for preparing optical fiber devices including thermal removal of a coating from an optical fiber, preferably performed using a heated gaseous stream. 
     A reason for removing protective buffers and related coatings from optical fibers is the need to change the characteristics of the fiber such as by writing of a refractive index grating, also known as a Bragg grating, in the core of an optical fiber. Refractive index changes occur during exposure of a bare fiber to radiation from an ultraviolet laser or similar exposure device. The majority of protective coatings for optical fibers absorb the fiber modifying radiation. This explains the need to remove the coatings before writing a refractive index grating. 
     Without further processing, an optical fiber including a refractive index grating also has a bare portion that requires application of protective coatings before use in an optical fiber device. The widely accepted method for recoating bare sections of optical fibers involves special coating molds. Methods similar to those used to coat drawn fibers, during their manufacture have also been described. 
     A recoating mold, described in U.S. Pat. No. 4,410,561, provides a coated optical fiber using a split mold die structure. The size and design of a cavity formed by the closed mold provides space that becomes filled during injection of curable, protective, fluid recoating compositions. It is desirable to avoid entrapment of air inside the mold since this could lead to a defective recoated fiber section. Complete filling of a mold cavity may involve intentional application of pressure. U.S. Pat. No. 5,022,735 uses a screw type plunger to pressurize recoating fluid injected into a conventional recoating mold. Some recoating molds include curing means to provide finished recoated sections of optical fibers. U.S. Pat. No. 4,662,307, for example, uses a split mold including an injection port and UV light port through which light passes to cure recoating compositions. The curing process requires multiple light sources. 
     Application of coatings to an optical fiber drawn from a pre-form typically places the emerging fiber in a vertical orientation. As it travels downward, the fiber may pass through a reservoir of coating fluid before exiting through an orifice sized to the desired external diameter of the coated fiber. It is possible to apply such a process to recoating of bare sections of optical fiber including a Bragg grating, as taught in U.S. Pat. No. 6,069,988. Upon exit from the orifice, the fiber moves past a source of curing radiation. The curing radiation differs from the radiation used for writing the Bragg grating so as not to destroy or change the characteristics of the grating. 
     There is evidence in Japanese Patents JP 60-122754 and JP 61-40846 for spraying protective plastic coatings on optical fibers exiting a draw tower. Coverage of the full circumference of the optical fiber requires the uses of either multiple spray heads or special spray containment shrouds. The use of multiple spray heads deposits only a fraction of the spray on the surface of the drawn fiber while the use of special shrouds involves complicated threading of a fiber. 
     Each point in the processes, of fiber stripping, modifying and recoating, requires care to prevent damaging the fragile optical fiber. Damage to optical fibers may occur by physical contact or exposure to tensile, torsional, twisting, and bending stresses. Excessive bending can change the optical characteristics of a fiber. Failure to meet required optical characteristics leads to rejection of an optical device and increases the expense of device manufacture. A need exists for improved means for handling optical fibers for post draw processing, to reduce incidence of damage thereby reducing the cost and increasing the yield of optical fiber devices. 
     SUMMARY OF THE INVENTION 
     The present invention satisfies the need for effective and compact handling of filamentary materials during manufacturing operations including process steps that produce structural and related changes in the filament. When applied to optical fibers, an article, also described herein as a filament organizer, provides compact containment of an optical fiber. The filament organizer allows relatively precise positioning of at least a portion of an optical fiber to facilitate processing of optical fibers related to optical couplers, fused couplers and tapered fiber devices and the like. Optical fiber modification may also refer to actions taken to change the inherent characteristics of an optical fiber or to incorporate an optical fiber into a functional assembly. The inherent characteristics of an optical fiber change with adjustment of its refractive index properties, as in the formation of a variety of fiber Bragg gratings. Incorporation of an optical fiber into a functional assembly provides useful devices such as temperature compensated fiber Bragg gratings. Refractive index changes and functional assembly production, according to the present invention, use a filament organizer that distributes an optical fiber between a lockable spool and a rotary spool to expose a central portion of a fiber to be modified. 
     A computer controlled, or otherwise programmed, fiber dispenser may be used to load a prescribed amount of a substantially twist-free optical fiber between a pair of spools mounted on a common axis. After fiber loading the spools are separated, with fiber extending between them, and mounted to a filament organizer for fiber storage and further processing. Use of computer controlled dispensing, combined with a filament organizer, allows accurate consistent loading and organization of a selected length of optical fiber within the boundaries of the filament organizer. Control of the loading process allows the production of numerous holders containing approximately equal lengths of fiber, organized in similar fashion. After successful loading of an optical fiber, a filament organizer provides a convenient article for handling the fiber through process operations required for the manufacture of optical fiber devices. Preferably the filament organizer includes means for applying a tension force between about 50 g to about 100 g to the filament held therein. 
     A variety of devices use optical fibers that have been structurally modified to include in-line optical waveguide refractive index gratings in at least a portion of their length. Physical property variation of gratings allows them to be tailored for specific applications. In one embodiment, the present invention provides a fiber Bragg grating obtained via a series of manufacturing operations including mechanical stripping of an optical fiber, acid stripping, pigtailing, optical fiber Bragg grating writing, annealing and optical measurement followed by recoating and testing. The final step of testing, including fiber proof testing, confirms attainment of performance requirements desired of an optical fiber Bragg grating. 
     Each operation or step of the manufacturing process requires attachment of one or more filament organizers to one or more filament processors or apparatus designed specifically to accomplish a designated step. This requires that the size and shape of a filament organizer include aspects of design allowing convenient connection with several filament processors. As well as making suitable connection with several types of filament processors, an important requirement of a filament organizer is containment of a prescribed length of filament that may be up to several meters in length. Preferably, in the case of an optical fiber, a filament organizer holds most of the length of a filament on a pair of spools leaving a portion of filament available for processing. A spool holds two sections of optical fiber wound in the same direction on separate sides of a divided spool core. One section of fiber extends between a pair of spools while the other section of optical fiber provides a pigtail portion that may be readily unwound from each spool. There is a pigtail section at each end of a continuous length of optical fiber. 
     After winding a continuous length of optical fiber between a pair of spools and positioning the spools on a support board, fiber handling may proceed with reduced expectation of damage to the fiber. Also the use of a filament organizer allows ready access to a portion of fiber. Ready access to this portion of fiber allows it to be modified initially by removal of protective coatings from its surface and thereafter subjecting it to operations that change its physical and optical properties, as in the writing of a fiber Bragg grating into a bared portion of optical fiber. A filament organizer allows reproducible positioning of that portion of an optical fiber that will be modified. Reproducible positioning leads to predictable results of filament or optical fiber modification by operations that may be conducted using a process where at least several of the steps may be automated. 
     As indicated previously, a filament organizer provides a portion of filament or optical fiber suitably positioned for processing. Formation of an optical fiber Bragg grating according to the present invention requires that any polymeric protective coating, also referred to herein as a buffer coating, should be removed prior to the writing of the fiber Bragg grating. The coating may be removed using liquid or mechanical or thermal stripping. 
     An optical fiber covered with a single polymeric layer, referred to herein as a primary buffer, may require only liquid stripping using concentrated acid to remove the buffer. Removal of multiple protective coatings, including primary and secondary buffers according to the present invention, preferably uses a combination of mechanical stripping followed by acid stripping. Acid stripping herein refers to dissolving residual coating material in an acid medium with displacement of the acid using a water rinse and solvent wash applied to at least a portion of the fiber. Initial displacement of coating requires specially designed mechanical stripping equipment that cooperates with a filament organizer for precise positioning of the portion of an optical fiber from which protective coating will be displaced. Mechanical stripping equipment may be designed for conveniently processing one filament organizer or several combined in a single stacked configuration. This results in treatment of one or more fibers at a time depending on the number of filament organizers. Coating displacement, via mechanical stripping, creates gaps to the bare fiber through which acid may subsequently penetrate to more rapidly dissolve coating from the fiber portion. 
     Removal of coating by acid stripping preferably requires an apparatus that forms a loop of filament for each filament organizer included in a stacked configuration. The apparatus is constructed for formation of individual filament organizer loops having approximately the same size. The plane of each loop parallels that of its nearest neighbors. Acid stripping of one or more fiber loops occurs by immersing the arcuate portion of a loop into an acid bath. The depth of immersion of each loop into the acid bath controls the length of protective coating removed from a fiber to provide an optical fiber having a bare portion stripped to the silica surface of the fiber. Acid stripping provides a bare fiber surface that is substantially free from contaminants. 
     After all of the fibers in a stacked configuration have been mechanically stripped and acid stripped, the pigtail ends of each fiber are manually unwound and organized into groups using pigtail connectors. Pigtail ends trail about one meter from each end of a filament organizer. 
     As a further refinement, a filament organizer according to the present invention may include a conventional optical fiber connector for terminating optical fiber ends on the surface, and within the boundaries of the filament organizer. Optical connector termination of fibers reduces the length of pigtail portions of an optical fiber while still providing convenient points of attachment to external optical fiber devices. Compact fiber organization of this type distributes the length of an optical fiber on the surface of a filament organizer without any part of the fiber hanging over the edges of the organizer. Any of a variety of optical fiber device interconnects may be used to reduce the overall length of an optical fiber by shortening the pigtail ends. Reduction in the overall length of an optical fiber translates into cost savings associated with each filament organizer equipped with pigtail to optical fiber connector termination. 
     Following organization by grouping of pigtail ends each filament organizer in a stacked configuration provides a clean, dry, bare fiber portion ready for positioning in a fiber Bragg grating writing apparatus. After release of tension from a filament held by a filament organizer, the Bragg grating writing apparatus applies a selected tension to the portion of an optical fiber before it is modified to produce a Bragg grating. Production of multiple optical fiber Bragg gratings, having a substantially identical wavelength response, requires precise alignment and application of the same amount of tension to each optical fiber portion loaded into the fiber Bragg grating writing apparatus. Precise alignment of an optical fiber portion with the Bragg grating writing apparatus relies on features built into a filament organizer and the grating writing apparatus respectively for consistent relative positioning of one to the other. Consistent loading and fiber portion tensioning relies upon the use of a voice coil drive mechanism and air suspended bearings that facilitate accurate fine adjustment essentially free from drag. 
     After placing an appropriate portion of an optical fiber under tension in the fiber Bragg grating writing apparatus, the progress of Bragg grating writing may be monitored by observing a display of the wavelength envelope produced by the writing process. Signal information proceeds from an optical fiber to suitable monitoring equipment through connections between the equipment and pigtail ends of a fiber. This provides feedback of the quality of a grating at the time of writing and represents a convenient decision point for acceptance or rejection a fiber Bragg grating as it is written. 
     Annealing of fibers takes place in a thermal annealing apparatus and fulfills several requirements upon completion of writing of fiber Bragg gratings. This step of the process proceeds at a temperature of approximately 300° C. for a duration of more than about three minutes. The annealing process stabilizes the Bragg grating against wavelength drift for time periods exceeding about twenty to about twenty-five years. 
     After annealing and optical confirmation that the grating center wavelength meets requirements, the fibers and associated Bragg gratings are ready for recoating before final testing. The recoating operation uses equipment designed for a filament organizer or preferably a stacked configuration of filament organizers according to the present invention. It is possible to use in-mold recoating, spray recoating or an extrusion die coating process to recoat the previously stripped portion of each optical fiber. Injection die coating refers herein to conventional in-mold die recoating. Spray recoating uses multiple passes of an optical fiber between a spray head and a radiation curing source. The extrusion recoating process uses a split die that may be positioned around an optical fiber for application of a curable coating composition around the circumference of the fiber as the extrusion head traverses the length of an uncoated fiber portion. Preferably the die head includes a radiation source and the extruded coating cures by exposure to the radiation source. This allows application of recoating material followed immediately by curing. 
     Application of recoating material to protect a Bragg grating formed in an optical fiber represents the final processing operation for producing fiber Bragg gratings that may be used in telecommunications and related applications. A final check of the resulting product determines if it passes tensile strength and visual inspection requirements. After successfully meeting requirements, the spools holding a finished optical fiber Bragg grating may be detached from the filament organizer and used for conveniently holding, packaging and transporting the final product. A convenient form of packaging for transportation requires transfer of the full continuous length of a fiber Bragg grating to one spool after removing it from the filament organizer. The design of a spool provides a protective cover for the fiber Bragg grating element following transfer of the full length of optical fiber to one spool. 
     More particularly, the present invention provides a method for manufacturing an optical fiber refractive index grating. A suitable method comprises the steps of providing a substantially twist-free length of an optical fiber between a first spool and a second spool, for attachment of the first spool and the second spool to a support. The support has a first surface opposite a second surface, to provide a filament organizer including the first spool as a lockable spool and the second spool as a rotary spool. The filament organizer further comprises a tensioner coupled to the rotary spool to apply tension to at least a central portion of the length of an optical fiber disposed between the lockable spool and the rotary spool. Further processing of a fiber under tension includes removing at least a buffer coating from the central portion of an optical fiber before applying a controlled tension to the central portion of an optical fiber. A refractive index grating may then be written by changing the refractive index characteristics of the central portion during exposure of the central portion to an interference pattern of high intensity actinic radiation, to produce the refractive index grating. After formation the grating may be annealed and the resulting fiber device proof tested to confirm desired performance properties. 
     The method described previously uses a filament organizer, comprising a support having a first surface opposite a second surface and further including organizing mounts joined to said first surface and spacer blocks attached to said second surface. The filament organizer has a lockable spool adjacent to the first surface of the support, a rotary spool adjacent the first surface of the support, and a tensioner attached to the second surface of the support. The tensioner includes a tension wire for attachment to the rotary spool to apply tension thereto to transmit tension to a filament disposed between the lockable spool and the rotary spool. A tension relief assembly allows selective reduction of tension applied to a filament. The tension relief assembly includes the tension wire, providing connection between the tensioner and the rotary spool, a tension wire access, and at least one pulley for aligning the tension wire with the tension wire access. Other parts of the filament organizer include at least one mounting plate integrally formed with the support and extending outwardly therefrom, and at least one guide defining a filament path between the lockable spool and the rotary spool. Further the guide is rotationally mounted on the mounting plate, adjacent to the first surface of the support, to provide spacing of the filament path from the support. 
     During refractive index grating manufacture a mechanical stripping apparatus displaces resin from a resin covered filament, in the form of an optical fiber, by forming a removable sleeve portion between opposing filament ends. The mechanical stripping apparatus comprises a base that has a first clamp attached to the base to hold a filament at a first location. A second clamp is attached to the base and has a separation from the first clamp and is in axial alignment therewith for holding a filament at a second location. The apparatus includes a first set of cutting blades mounted on the base adjacent to the first clamp. The first set of cutting blades includes a first upper blade and a first lower blade. Each of the upper and lower blades includes an arcuate sharpened edge for cutting into resin around a resin covered filament proximate to the first location. A second set of cutting blades is mounted on the base adjacent to the second clamp such that a distance separates the first set of cutting blades from the second set of cutting blades. The distance between cutting blades is less than the separation between the clamps. The second set of cutting blades includes a second upper blade and a second lower blade with each blade including an arcuate knife edge for cutting into resin around a resin covered filament proximate to the second location. A blade actuator secured to the base, and coupled to the first set of cutting blades and the second set of cutting blades, moves the first upper blade and the first lower blade together. During this movement the sharpened edges penetrate resin around a resin covered filament proximate to the first location. The blade actuator also moves the second upper blade and the second lower blade together for the knife edges to penetrate resin around a resin covered filament proximate to the second location. A biasing component also on the base moves the first set of cutting blades and the second set of cutting blades towards each other during displacement of resin from a resin covered filament to form the removable sleeve portion. 
     The removable sleeve portion may be formed using a method for displacing resin from a resin covered optical fiber between opposing fiber ends. The method provides a mechanical stripping apparatus comprising a first clamp for holding an optical fiber at a first location, a second clamp having a separation from the first clamp and in axial alignment therewith for holding an optical fiber at a second location. A first set of cutting blades, of the mechanical stripping apparatus, is adjacent to the first clamp for cutting into resin around a resin covered optical fiber proximate to the first location. A second set of cutting blades is adjacent to the second clamp for cutting into resin around a resin covered optical fiber proximate to the second location. A distance separates the first set of cutting blades from the second set of cutting blades. The distance is less than the separation between the first and second clamps. The first set of cutting blades and the second set of cutting blades are adapted for movement towards each other during removal of resin from a resin covered optical fiber to form the removable sleeve portion. Resin displacement further includes clamping an optical fiber in the first clamp and clamping the optical fiber in the second clamp such that the optical fiber is under tension. Operating the first set of cutting blades and the second set of cutting blades, for cutting into the resin, produces the removable sleeve that has a gap at each end thereof. The gap at each end exposes a bare filament portion separating the removable sleeve portion from a tapered transition formed in the resin during cutting of the resin as the first and second-set of cutting blades move towards each other. 
     In another aspect according to the present invention an apparatus may be used to form a loop in a section of a filament prior to chemical stripping of resin from e.g. an optical fiber. The apparatus comprises a container including a front wall having a front guide slot formed therein and a rear wall having a rear guide slot formed therein coplanar and parallel to the front guide slot. The container further includes a floor containing at least one slit formed between and parallel to the front wall and the rear wall. A first filament gripper includes a stationary elastomer roller and a positionable cylinder holding a filament therebetween, at a first location thereof. The stationary elastomer roller is rotatably mounted from the front wall to the rear wall, so that the positionable cylinder is mounted, adjacent to the stationary elastomer roller, between the front guide slot and the rear guide slot for repositioning therein. A second filament gripper includes a movable elastomer roller and a movable cylinder holding the filament therebetween, at a second location. The second filament gripper has a separation from the first filament gripper and has substantially axial alignment therewith. The second filament gripper moves towards the first filament gripper to reduce the separation to bring the first location closer to the second location thereby producing a loop of filament between the first filament gripper and the second filament gripper. The loop of filament extends through a slit to below said floor of the container where it may be introduced into a reservoir having a solvent therein to surround at least a portion of the loop of filament to dissolve resin from the portion of the loop. A loop forming container according to the present invention may be sized to accommodate one or more filament organizers having a filament between a lockable spool and a rotary spool. Steps for forming one or more filament loops using a loop forming container may be included in a process for chemically stripping resin from a resin coated filament, preferably as an optical fiber. 
     Processing of a filament according to the present invention requires the use of a filament holding fixture comprising a gripper having an open position and a closed position. The gripper further comprises a lower jaw mount, and a lower jaw connected to the lower jaw mount, the lower jaw having a planar surface and an open-ended, V-shaped channel formed therein opening to the planar surface to receive at least a portion of a filament. The filament holding fixture also has an upper jaw mount, and an upper jaw assembly. The upper jaw assembly comprises a support flange attached to the upper jaw mount. The support flange includes a support surface, having a substantially conical recessed portion. A fiber clasp, included in the upper jaw assembly, has a contact face opposite a structured surface. The structured surface includes an open-ended groove of substantially rectangular cross section. There is a substantially conical depressed portion formed in the contact face of the fiber clasp. The open ended groove and the V-shaped channel are in longitudinal alignment to contact at least a portion of a filament when the gripper is in the closed position. A plurality of spring connectors hold the fiber clasp to the support flange. Also, an angular compensator is confined between the recessed portion of the support surface and the depressed portion of the contact face by force produced by the plurality of spring connectors. The angular compensator maintains separation of the support flange from the fiber clasp to allow them to move independently. This leads to fine adjustment of the fiber clasp for applying substantially equal force at points of contact of the open-ended groove and the V-shaped channel with a filament, preferably an optical fiber, held therebetween following movement of the gripper from the open to the closed position. 
     The present invention further provides a filament tensioning apparatus for releasably securing a filament under tension. The tensioning apparatus comprises a tensioning holder and a pair of grippers. The tensioning holder includes at least one support bar, and a first carriage movably mounted at a first location on a support bar. The first carriage includes an upper surface having a first clamp and a voice coil mounted thereon for movement relative to a support bar. A second carriage is movably mounted at a second location on a support bar such that a separation exists between the first location and the second location. The second carriage includes an upper face having a second clamp and a load cell mounted thereon for movement relative to a support bar. The second clamp is in axial alignment with the first clamp to secure a measured filament portion including a bare portion thereof, located inside a first boundary and a second boundary, between the first clamp and the second clamp. A guide bar extends from the voice coil for contact with the load cell to adjust the separation of the first location from the second location, to change tension applied to the measured filament portion, during activation of the voice coil. The pair of grippers of the tensioning apparatus is in axial alignment with the fist clamp and the second clamp, to substantially immobilize the bare portion of the measured filament portion. A filament tensioning apparatus according to the present invention may include a coupling for attaching a filament organizer to position a filament, preferably an optical fiber, to be held between the first clamp and the second clamp. The filament organizer holds a filament between a lockable spool and a rotary spool. 
     A resin covered filament having had resin removed therefrom may require coating by a method that uses a filament recoating apparatus according to the present invention. Such a filament recoating apparatus comprises a frame for releasably securing a filament and a carriage mounted on the frame to oscillate between a first position and a second position. The recoating apparatus has a first filament holding fixture mounted on the carriage. A second filament holding fixture is also mounted on the carriage in axial alignment with the first filament holding fixture. The fixtures secure a measured filament portion including a bare portion thereof, located inside a first boundary and a second boundary, between the first filament holding fixture and the second filament holding fixture. At least one spray head is attached to the frame at the first position. A radiation source is attached to the frame at the second position. The measured filament portion moves between the spray head and the radiation source, during oscillation of the carriage between the first position and the second position to place the bare portion to receive a curable coating from the spray head. The spray head applies curable coating from the first boundary to the second boundary. Curing of the curable coating occurs by exposure to radiation from the radiation source. Droplets of curable coating composition may be deflected using a deflector, such as an air-knife, to selectively direct coating composition towards a plurality of bare filament portions of filaments, preferably optical fibers, grouped around a spray head. Different coating compositions may be applied to bare filament portions to provide recoated filaments using a first composition and overcoated filaments by application of a second coating composition over the first coating composition. The resulting filaments include a multilayer coating. 
     An alternative filament recoating apparatus, according to the present invention, comprises a planar surface and an extrusion coating assembly attached to the planar surface. The extrusion coating assembly comprises a first filament holding clamp and a second filament holding clamp opposite the first filament holding clamp. A measured filament portion including a bare portion thereof, located inside a first boundary and a second boundary, lies between the first filament holding clamp and the second filament holding clamp. A coating head, includes a die plate having formed therein an open ended channel including a wall having a fluid entry and a gas port formed therein adjacent a radiation source. The coating head further includes a cover die plate having formed therein an open ended elongate slot. The cover die plate has a hinged connection to the die plate for rotation of the cover die plate between an open position and a closed position. In the closed position the cover die plate lies adjacent to the die plate and the channel aligns with the elongate slot to form a tubular opening through the coating head to encircle a section of the bare portion. A linear transport mechanism adjacent to the coating head includes a guide rod and a carriage slidably mounted thereon for movement along the guide rod. A connecting rod from the carriage to the coating head provides linear displacement of the coating head during movement of the carriage to move the coating head from the first boundary to the second boundary. Curable fluid may be extruded from the fluid entry while energy from the radiation source cures the curable fluid to recoat the bare portion of a filament. 
     A method for extrusion coating a filament comprises the steps of providing a filament organizer having an extended filament between a fixed spool and a rotary spool to provide a measured filament portion and a bare filament portion of a filament, preferably an optical fiber. Recoating of the bare portion of a fiber follows attachment of the filament organizer to an extrusion coating fixture comprising a guide rod, a carriage movably mounted on the guide rod. A coating die, including a coating head and a radiation source, is joined to the carriage. The coating head has an opening for directing a curable coating composition to the bare filament portion positioned in a channel formed in the coating die and extending therethrough. A curable coating composition is applied to the bare filament portion to provide a recoated filament portion, followed by exposing the recoated filament portion to the radiation source for radiation curing of the curable coating composition applied to the bare portion. 
     Definitions 
     The terms “bare fiber,” or “bare fiber portion,” or “stripped fiber,” or phrases relating to such terms refer herein to the portion of an optical fiber from which protective coating has been removed to expose the silica surface of the fiber. 
     As used herein, the term “cladding” refers to the outer layer of an optical fiber, as drawn. 
     The term “buffer” or “primary buffer” refers herein to a polymer or resin layer next to a bare fiber. 
     A “coating” or “secondary buffer” is used herein to describe a polymer or resin layer next to a buffer or primary buffer. 
     The term “resin” as used herein is a general term describing polymer coverings for filaments particularly optical fibers. Materials used for previously defined buffers and coatings fall within the general term of resin. 
     The term “filament” herein refers to a fiber structure, preferably a “silica filament.” An optical fiber is a preferred form of filament according to the present invention. 
     A “tapered transition” describes the preferably graduated conical shape of the portion of buffer layers closest to a bare fiber portion after subjecting a coated optical fiber to mechanical stripping according to the present invention. 
     The term “ribbonizing” refers to the formation of a single layer of optical fibers, side by side, as a flat ribbon-like structure that facilitates the joining of ends of multiple fibers for insertion in one end of a fiber optic ribbon connector. 
     The term “angular compensator” or “ball joint leveler” as used herein means a self adjusting coupling inserted between parts of at least one jaw of a gripper to achieve optimum positional relationship between the contacting surface of the jaw and an object to apply even pressure over the surface of the object. 
     The use of a “non-contact” method for recoating bare portions of optical fibers means that no portion of the fiber touches any part of the recoating equipment. This is a benefit of suspending a fiber in a filament organizer that may be readily attached to the recoating apparatus with precise fiber to spray head alignment. 
     A “split sizing die” is a multi-part fiber recoating head that opens to receive an optical fiber, closes to extrude curable recoating material around the surface of a length of fiber and re-opens to release the coated fiber. 
     The term “shroud” refers to a shield over an ultrasonic spray head to direct a stream of inert gas to entrain and move a cloud of droplets of recoating composition towards a target surface, such as a bare portion of an optical fiber. 
     The present invention has been developed to provide a process and equipment for conveniently handling a filament in the form of an optical fiber during multiple processing operations that may be at least partially automated as a further benefit to the user. These enhancements and benefits are described in greater detail hereinbelow with respect to the several aspects and alternative embodiments of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a perspective view of a filament organizer according to the present invention. 
     FIG. 2 provides detail portions of a filament organizer according to the present invention in exploded perspective view 
     FIG. 3 shows a perspective view of an assembly for applying tension to a filament contained in a filament organizer according to the present invention. 
     FIG. 4 is a side elevation of a filament organizer. 
     FIG. 5 provides an perspective view of a plurality of filament organizers in a stacked configuration. 
     FIG. 6 is a cross sectional view through a dual coated optical fiber. 
     FIG. 7 shows a cross section of a dual coated optical fiber after acid stripping. 
     FIG. 8 provides a cross section of a dual coated optical fiber after mechanical stripping to provide a tapered transition. 
     FIG. 9 shows a side elevation of an optical fiber after mechanical stripping to produce a separated central buffer sleeve. 
     FIG. 10 is a cross sectional view showing a coated optical fiber positioned for mechanical stripping. 
     FIG. 11 is a cross sectional view showing a coated optical fiber after formation of a tapered transition 
     FIG. 12 shows a perspective view of a cutting blade according to the present invention for use during mechanical stripping of coating from an optical fiber. 
     FIG. 13 provides a detail view of a cutting edge of a cutting blade according to the present invention. 
     FIG. 14 is a detail view showing the relative positioning of a coated optical fiber and the cutting edge of a cutting blade according to the present invention. 
     FIG. 15 is a cross sectional view indicating the depth of cut of a cutting edge, and positioning of an upper blade that has penetrated a secondary buffer around an optical fiber. 
     FIG. 16 provides a cross sectional view showing depth of cut of an upper blade and a lower blade during mechanical stripping of secondary buffer from an optical fiber. 
     FIG. 17 is a diagrammatic representation of a side elevation showing the relative positioning of a filament organizer and mechanical stripping apparatus according to the present invention. 
     FIG. 18 provides a cross sectional view of a coated optical fiber loop during removal of coating by immersion in acid contained in an acid bath. 
     FIG. 19 is partial cross section showing the relative positioning of a filament organizer according to the present invention and an acid-containing bath before formation of an optical fiber loop. 
     FIG. 20 is partial cross section showing the relative positioning of a filament organizer according to the present invention including a loop suspended from the filament organizer for immersion of an arcuate portion of the loop below the acid surface. 
     FIG. 21 is a perspective view including a cutaway section to show internal detail of a loop former for a stacked configuration of filament organizers. 
     FIG. 22 provides a side elevational view of a filament organizer according to the present invention wherein pigtail portions of filament have been unwound from storage reels. 
     FIG. 23 is a diagrammatic side elevation of a filament organizer according to the present invention positioned in a voice-coil tensioning device during modification of a stripped portion of an optical fiber. 
     FIG. 24 provides a cross sectional view of a jaw used to prevent movement of a portion of an optical fiber while it undergoes modification. 
     FIG. 25 is a detailed cross sectional view of the structure of the jaw shown in FIG.  24 . 
     FIG. 26 is a perspective view of a jaw assembly according to the present invention. 
     FIG. 27 is a cross section taken through line  27 — 27  of FIG.  26 . 
     FIG. 28 is a diagrammatic side elevation of a filament organizer according to the present invention positioned in a spray recoating apparatus. 
     FIG. 29 is a diagrammatic side elevation of a filament organizer according to the present invention positioned in a split die recoating apparatus. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the Figures wherein like numbers refer to like parts throughout the several views. FIG. 1 is a perspective view of a filament organizer  10  including a substantially planar support board  12  that has opposing sides. The support board  12  has, on its first side  14 , also referred to herein as its upper side, points of attachment for a lockable spool  16  and a rotary spool  18 . Theses spools  16 ,  18  have a length of filament  20  wound around them and between them. The support board  12  may include mounting plates  22  for one or more guides  24  used to establish a preferred position for a portion of a filament  20  extending from the lockable spool  16  to the rotary spool  18 . Attachment of a guide  24  to a mounting plate  22  allows suitable movement of the guide in response to movement of the filament  20 . 
     A support board  12 , according to the present invention, optionally includes at least one opening  26  formed therein as a convenient location for holding a support board  12  by hand. An opening  26  preferably occupies a location sufficiently separated from an optical fiber  20  to prevent inadvertent touching of the surface, especially a bare surface of a filament  20 , such as an optical fiber. The positioning of an opening  26  has suitable separation from a filament when located between the lockable spool  16  and rotary spool  18  opposite an exposed portion of a filament  20 , as illustrated in FIG.  1 . 
     FIG. 2 uses a partially exploded view to show detail of an embodiment of a filament organizer  10  according to the present invention. This view depicts how a lockable spool  16 , a rotary spool  18 , and a pair of guides  24  may be attached to a substantially planar support board  12 . As suggested earlier, a computer controlled fiber dispenser may be used to load a prescribed amount of optical fiber between a pair of spools  16 ,  18 . A continuous length of fiber preferably has three sections including a main or central section, several meters in length separating opposite end or pigtail sections, each approximately one meter long. Typically the central section has a length of from about one meter to about six meters. Each of the spools  16 ,  18  provides storage for an optical fiber pigtail. Filament turns representing pigtail sections are wound on a spool  16 ,  18  in the same direction as the filament turns of the central section. The spools  16 ,  18  each include a core divider (see FIG. 4) to separate pigtails from the central section of an optical fiber  20  to facilitate unwinding of pigtails during filament processing. 
     After fiber loading, the spools  16 ,  18  are separated and mounted to a support board  12  for fiber storage with a length of filament stretching between the spools  16 ,  18 . One of the spools  16 ,  18  becomes a lockable spool  16  during sliding engagement between an axial channel  28  in the spool  16  and a post  30  secured to the support board  12 . Each face plate  32  of the lockable spool  16  includes at least a pair of openings  34  positioned on either side of the axial channel  28 . The openings  34  align for engagement with a pair of pegs  36  rigidly connected to the support board  12 . When the axial channel  28  and openings  34  seat over the post  30  and pegs  36  the lockable spool  16  cannot rotate since the pegs  36  restrict its movement. After mounting the lockable spool  16 , as described previously, a change in length of filament  20  between the lockable spool  16  and the rotary spool  18  requires adjustment of a filament  20  by rotating the rotary spool  18 . Rotation of the rotary spool  18  relies upon a bearing  38  held by friction in a hub orifice  40  formed in the support board  12  of the filament organizer  10 . The bearing  38  facilitates rotation of a spool hub  42  on one side and tensioning hub  44  on the other. The spool hub  42  has a spindle  46  and a pair of pins  48  for alignment with an axial opening  50  and receiving holes  52  in both face plates  54  of the rotary spool  18 . Although held in fixed relationship, the rotary spool  18  and spool hub  42  have rotational freedom provided by the bearing  38 . 
     The embodiment of the present invention illustrated in FIG. 2 includes a pair of guides  24  each taking the form of an idler wheel that rotates around an axle  56 . One end of the axle  56  connects to the first side  14  of the support board  12 . Filament  20  un-spooling, by rotation of the rotary spool  18 , produces sufficient filament  20  between the lockable spool  16  and the rotary spool  18  so that the path of the filament  20  from the lockable spool  16  to the rotary spool  18  passes around each of the idler wheels  24 . The resulting assembly, including the filament  20 , provides a preferred orientation, as shown in FIG. 1, when the filament  20  is an optical fiber. This orientation places an optical fiber  20  in a readily accessible, spatially precise position for processing, during the manufacture of a fiber Bragg grating, for example. 
     Formation of an optical fiber Bragg grating frequently requires application of tension to a filament  20 , i.e. optical fiber, held by a filament organizer  10  according to the present invention. Reference to FIG. 3 indicates one means to apply tension to a filament  20  using a tensioner  58  attached to the lower side  60  of a support board  12 . The tensioner  58  applies a constant force to a filament  20  or optical fiber to maintain the filament  20  under slight tension. Force from the tensioner  58  may be transmitted to the filament  20  through a series of components including a tension wire  62  connected between the tensioner  58  and the tensioning hub  44 . The tensioning hub  44  acts upon the spool hub  42  because of a direct connection between the two. Movement of the spool hub  42  causes corresponding turning movement of the rotary spool  18  which is centered on the spindle  46  and driven by the movement of pins  48  attached to the spool hub  42 . Movement of the rotary spool  18  produces tension in the filament  20  or optical fiber proportional to the constant force produced by the tensioner  58   
     The tension in the tensioner  58  acts on the tension wire  62  with a force of from 20 g to 200 g. Force distribution through the tension wire  62  and the tensioning hub  44  leads to a resultant force of about 50 g to about 100 g of tension in a filament  20  held in a filament organizer  10 . A tensioning hub  44  is typically about half the diameter of a rotary spool  18 . 
     The tension wire  62  may pass unimpeded between the tensioner  58  and the tensioning hub  44 . Preferably, however, the location of a section of the tension wire  62  allows access and positional adjustment of the tension wire  62  to remove tension from the filament  20 . Removal of the effect of the constant force tensioner  58  uses a force reduction assembly comprising a pair of pulleys  64  on either side of a notch  66  formed in an edge of the support board  12 . The tension wire  62 , passing around the pulleys  64  and across the notch  66 , may be grasped in the vicinity of the notch  66  and extended slightly parallel to the edge of the board  12 , and toward the nearest guide  24 . Movement of about 1.0 mm to about 2.0 mm releases the tension force applied to a filament  20  by the tensioner  58 . Release of tension relaxes a filament  20 , in the form of an optical fiber, in preparation to re-tension the fiber. Relaxation and re-tensioning of an optical fiber  20  applies a known and repeatable amount of tension required by the Bragg grating writing process so that resulting optical fiber Bragg gratings will exhibit substantially the same wavelength response. 
     After installing a filament  20  under tension on a filament organizer  10 , a length of filament several meters long may be conveniently carried, in a protected spooled condition, between handling stations, during filament processing. Earlier methods for filament processing required an operator to hand-carry lengths of filament measuring in excess of six meters. Care was required to avoid contact of trailing filament with the floor and other surfaces that could cause contamination and reduction in the yield of manufactured filament devices. A filament organizer  10  according to the present invention may be handheld using the opening  26  in the support board  12 . The position of the opening  26  minimizes undesirable inadvertent hand contact with any exposed portion of the filament  20 . 
     FIG. 4 shows a side elevation of a filament organizer  10  according to the present invention to indicate the relative positioning of components on the upper  14  and lower  60  side of a support board  12 . The figure shows the tensioner  58 , spacer blocks  70 , tensioning hub  44 , tension wire  62  and a pulley  64  on the lower side of the support board  12 . The upper side of the support board  12  provides a surface for attachment of organizing mounts  72 , a lockable spool  16  and a rotary spool  18  having a filament or optical fiber  20  held under slight tension between them, as described previously. The spools  16 ,  18  used to store a length of optical fiber  20  are divided spools since the lockable spool  16  includes a dividing wall  15  and the rotary spool has a partitioning wall  17 . Part of an optical fiber  20  lying between the spools  16 ,  18  occupies a lower core  19  of a spool  16 ,  18  below each of the dividing wall  15  and the partitioning wall  17 . This leaves the upper core portion  21  of each spool, i.e. above the dividing wall  15  and the partitioning wall  17  to receive a pigtail end of a continuous length of optical fiber  20 . Separation of a central length from the ends or pigtails of an optical fiber  20  using lower  19  and upper  21  core portions of storage spools  16 ,  18  facilitates unwinding of only the pigtail portions of a fiber  20  during fiber processing. 
     With suitable design, two or more filament organizers  10  may be combined to form an assemblage of filament organizers  10 . The term, stacked configuration  68 , describes herein an assemblage of filament organizers  10 , as illustrated in FIG.  5 . Design considerations include the placing of stabilizing spacer blocks  70  on the lower surface  60  of a support board  12  for mating registration with organizer mounts  72  positioned on the upper surface  14  of a support board  12 . The spacer blocks  70  may have insufficient height to hold a tensioner  58  clear of a planar surface upon which a filament organizer  10  may be placed prior to forming a stacked configuration  68  thereon. This problem may be overcome using a suitably contoured spacer between the lower surface  60  of a support board  12  and the planar surface. Correct stack  68  formation requires addition of filament organizers  10  one on top of another with suitable alignment of downward facing spacer blocks  70  and upward facing organizer mounts  72  to produce a stable stacked configuration  68  by mating registration between filament organizers  10 . The combined height of spacer blocks  70  and organizer mounts  72  provides sufficient spacing between filament organizers  10 . Alignment of spacer blocks  70  and organizer mounts  72  produces a stacked configuration  68  neatly organizing a number of filaments  20 , corresponding to the number of filament organizers  10 , in a pre-determined relationship. This relationship facilitates optimum orientation of a stacked configuration  68  with filament processing equipment. The spacing between filaments  20  in a stacked configuration  68  is from about 12.5 mm (0.5 inch) to about 27.5 mm (1.2 inches) preferably about 18 mm (0.7 inch) to about 23 mm (0.9 inch). 
     Spacer blocks  70  and organizer mounts  72  may be viewed as primary components for aligning one filament organizer  10  relative to its nearest neighbor. Actual fiber  20  positioning and spacing depends upon the location and relative height of the spacer blocks  70  and the organizer mounts  72  on a support board  12 . This means that the design of a support board  12  determines the position of a length of filament  20  so that it may be readily located in a stacked configuration  68 . In addition to this, a filament  20  occupies a known position on a support board  12  with consistent spacing of the fiber  20  from other portions of the support board  12 , such as the mounting plates  22  and upper side  14  and lower side  60  of a support board  12 . These features provide reference points for uniting filament organizers  10 ,  68  to various pieces of apparatus with one or more filaments  10  positioned ready for processing. This provides a filament organizer  10  and a stacked configuration  68  suitable for use in automated filament processing without operator intervention. 
     Means for handling multiple filament organizers  10  in a stacked configuration  68  includes installation of a stacking connector  74  that optionally includes a carrying handle  76 . Preferably a stacking connector  74  comprises one or more rods  78  inserted into through-holes  80  (see FIG. 1) included in a support board  12 . A rod  78  may include a flange as a support for the lowest filament organizer  10  in a stacked configuration  68 . Alternatively, when a stacking connector  74  comprises additional rods  78  threaded through multiple filament organizers  10 , a bracket may be used to connect the ends of rods  78  adjacent to the lower face  60  of the lowest filament organizer  10  in the stack  68 . A carrying handle  76  may be attached to a stacking connector  74  by any of a variety of joining methods. Rods  78  protruding from a stacked configuration  68  may include threaded end portions that may be received in tubular openings (not visible) in a carrying handle  76 . A threaded nut or similar retainer  82  may secure the handle  76  to the stacking connector  74 . This provides a stable stacked configuration  68  of multiple filament organizers  10  that may be carried with ease between stations for processing a plurality of filaments  20  in a single batch or automated operation. 
     A filament  20  in the form of an optical fiber, once installed in a filament organizer  10 , requires a series of steps to produce refractive index modifying features in a selected portion of the optical fiber  20 . Optical fiber manufacture, using a draw tower, typically includes the application of protective coatings over the length of the fiber. Identification of protective coatings for optical fibers uses a variety of terms including buffer and coating. The term buffer usually identifies a material coated directly on a bare optical fiber. A coating usually designates a protective material coated over a buffer layer. 
     FIG. 6 is a cross sectional view of an optical fiber showing layers of protective coatings. As used herein the term primary buffer refers to a buffer coating  100  and the term secondary buffer  102  refers to a coating applied to the primary buffer  100 . Optical fiber Bragg grating manufacture requires the removal of both the primary buffer  100  and the secondary buffer  102  from a central portion of an optical fiber  20  stored on a filament organizer  10 . One method for stripping a buffer coat requires dipping the fiber in a hot concentrated sulfuric acid bath. Preferably the sulfuric acid concentration is at least 95% before heating and stripping the buffer coating  100 ,  102  from an optical fiber. Acid stripping occurs at a temperature above about 150° C. Damage to a glass core  106  is less likely to occur with acid stripping than with other methods used to remove buffer coats  100 ,  102 , since glass is resistant to acid. 
     A hot acid bath provides an effective medium for removing a single buffer coat, but some types of optical fiber have multiple coatings that may dissolve at different rates. These types of optical fiber may include a relatively insoluble, hard secondary buffer coating  102  over a softer protective primary buffer coating  100 , as illustrated in FIG.  6  and FIG.  7 . During hot acid stripping, the softer primary buffer coating  100  may dissolve faster than the secondary buffer coating  102 . The process of dissolving polymer layers from an bare optical fiber  106  may be accompanied by decomposition due to depolymerizing sulfonation caused by the attack of the concentrated sulfuric acid. Polymer decomposition products may impair the appearance and performance of a modified optical fiber according to the present invention. 
     Placement of the primary buffer coating  100  under the secondary buffer coating  102  can result in preferential removal by acid of the primary buffer coating  100 . Preferential removal of the primary buffer coating  100  produces an undercut  104  below the secondary buffer coating  102  that can collapse inward towards the bare optical fiber  106 . As the secondary buffer coating  102  collapses it can trap acid or air bubbles next to the bare optical fiber  106 . Entrapment of material including acid, and other liquids or gases, can produce conditions leading to premature failure of an optical fiber  20  for intended applications. 
     A contributor to premature failure, as indicated previously, may be the existence of decomposed polymer species after strong acid treatment. This situation may be avoided using an intermediate, mechanical stripping method to provide a cut tapered transition section  108 , as shown in FIG. 8, between the primary coating  100  and an bare optical fiber  106 . The tapered transition  108  prevents the collapse of a secondary buffer coating  102 , as previously described. When mechanical stripping, according to the present invention, precedes acid stripping the formation of a tapered end  108  can present a preferred geometry at the junction between the primary buffer coating  100  and the bare surface of an optical fiber  106 . 
     FIG. 9 shows how an intermediate, central portion of an optical fiber  20  may be stripped from two points along its length using a pair of cutting blades. The intermediate portion to be stripped preferably resides in a filament organizer  10  according to the present invention. A mechanical optical fiber stripping apparatus accommodates a filament organizer  10 , providing correct orientation for stripping buffer coatings  100 ,  102  from an optical fiber  20 . The apparatus controls blades (not shown) that cut into the secondary buffer coating  102  to produce a separated buffer sleeve  110  between a pair of tapered transitions  108  that define the length of bare optical fiber when the primary  100  and secondary  102  buffer coatings have been removed. Each end of the buffer sleeve  110  includes a peeled-back collar  112  that provides a gap for access to the bare surface of the optical fiber  106 . 
     FIG.  10  and FIG. 11 provide clarification of the basic components and steps required for stripping a central portion of a coated optical fiber  20 . A first clamp  120  holds one end of a portion of a coated optical fiber  20 . The coated optical fiber  20  comprises a fiber core  106  overcoated with one or more protective resin layers  100 ,  102 . A second clamp  122  holds the other end of the portion of the coated optical fiber  20 . Both clamps  120 ,  122  grip the outer surface of a relatively hard secondary buffer coating  102 . This prevents damage to the underlying optical fiber core  106 . Preferably the clamps  120 ,  122  include frictional gripping surfaces such as rubber or elastomer gripping surfaces that resist fiber movement during mechanical stripping. 
     The immobilized coated optical fiber exists under slight tension, preferably of about 50 g. Typical separation between the first clamp  120  and the second clamp  122  is from about 50.0 mm (2.0 inches) to about 100 mm (4.0 inches) preferably 75.0 mm (3.0 inches) to about 90 mm (3.5 inches). After limiting optical fiber  20  movement between a pair of clamps  120 ,  122  at least one set of cutting blades  124  may be placed abutting the coated optical fiber  20  with the sharp edge of an upper cutting blade  126  resting against the surface of the coating  102  surrounding the optical fiber  20 . The desired position is shown by the location of a first set of cutting blades  124  relative to the clamped, coated optical fiber  20 . A second set of cutting blades  130  is shown in FIG. 10 in a position, adopted by the cutting blades  130 , after penetration of the secondary buffer  102  of an optical fiber  20 . Each set of cutting blades includes an upper blade  126  and a lower blade  128 . The sharp edge of each cutting blade  126 ,  128  includes at least one notch having a radius in common with any primary buffer coat  100  applied to an bare optical fiber  106 . During fiber stripping, the upper  126  and lower  128  blades move inwards, as shown for the second set of cutting blades  130 , cutting through the secondary buffer  102  until they touch one another, before penetrating the primary buffer  100 . The distance between the first set of cutting blades  124  and the second set of cutting blades  130 , at this point, is typically between about 30 mm (1.2 inches) and about 40 mm (1.5 inches). After cutting through an outer or secondary buffer coating  102 , application of suitable force moves the second set of cutting blades  130  closer to the first set of cutting blades  124  and parallel to the axis of the optical fiber  20 . This transverse movement of the second set of blades  130  generates a stripping action that results in a gap  132  in the coating around the optical fiber core  106 . The stripping action exposes a bare fiber portion  106  in the gap  132 . One side of this gap  132  has a contoured, tapered transition  108 , in the primary buffer  100 , from the bare optical fiber  106  to the secondary buffer coating  102 . The other side of the gap includes a compressed, peeled-back collar  112  of stripped coating  100 ,  102 . When the cutting and stripping operations have been completed at one end of the coated optical fiber portion, the opposite end of the fiber  20  may be stripped by initiation of cutting action of the first set of cutting blades  124 . This produces a second gap  134  in the coating around the bare fiber  106 , as illustrated in FIG.  9 . The second gap  134  includes a similar tapered transition  108  to that produced by the cutting action of the second set of cutting blades  130 . 
     Application of acid stripping to a mechanically stripped fiber, as in FIG. 9, preferably exposes only the buffer sleeve  110  to acid attack. As long as the tapered ends  108 , also referred to herein as tapered transitions, remain out of the strong aqueous acid they remain free from attack and chemically unchanged. In this condition the tapered transitions  108  have a surface energy more compatible with recoating compositions. This allows the recoating compositions to readily wet the surface of the tapered transitions  108  following modification of the central portion of a filament. Surface compatibility and ready wetting by recoating compositions produces defect free junctions between previously coated and recoated sections of optical fibers. The existence of defects, e.g. air bubbles, in transition areas of an optical fiber may adversely affect the mechanical strength and light transmission characteristics of an optical fiber device, rendering it unsuitable for its intended use. 
     FIG. 12 shows the design of blade components used for cutting the secondary buffer coating  102  and displacing the primary buffer coating  100  to cause separation of the primary buffer coating  100  from a bare optical fiber  106 . As illustrated in FIG. 12, a blade  140  provides detail of features that may be included in both sets of cutting blades  124 ,  130 . The blade includes provision for stripping several optical fibers  20  simultaneously. It will be appreciated that the same blade  140  may be used to strip single or multiple fibers  20  depending on the number of filament organizers  10  inserted into the stripping apparatus (see FIG.  17 ). 
     A stripping blade  140  according to the present invention includes at least one bevel  142  as a portion of the blade  140  that includes several channels  144  machined into its surface. The channels  144  open to an edge  146  of a bevel  142  as sharpened notches  148  having approximately circular cross-section when viewed from the side opposite the bevel  142 , as in FIG. 14. A detail view, shown in FIG. 13 provides clarification of the structure of the bevel  142  including the channels  144  machined therein. A coated optical fiber  20  is included in FIG. 14 to indicate its preferred position before penetration of the secondary buffer  102  by a sharpened notch  148  of a cutting blade  140 . The knife-edge of a sharpened notch  148  preferably reaches only towards the outer surface of the primary coating  100  of an optical fiber  20  without cutting into it. When used for stripping coating from an optical fiber  20 , the notches  148  cut a circular path around an optical fiber core  106  as shown in FIG.  15  and FIG.  16 . In the illustration of FIG. 15 a sharpened notch  148  appears as it would after an upper cutting blade  126  penetrates the secondary buffer  102  of an optical fiber  20 . This relates to the position of the second set of cutting blades  130  as shown in FIG.  10 . The relationship between the upper blade  126  and lower blade  128  of the second set of cutting blades  130  appears in the diagrammatic representation shown in FIG.  16 . The sharpened notches  148  of the upper blade  126  and lower blade  128  have penetrated the secondary buffer coating  102  without reaching the surface of the primary buffer coating  100 . Since the advancing edges  146  of the blades  126 ,  128  have made contact there can be no further advancement of either blade  126 ,  128 . 
     Stripping blades  126 ,  128  according to the present invention perform biaxial movement. Initial movement of a blade  126 , towards a fiber core, produces a cut as a blade penetrates the secondary buffer coating  102  of the fiber  20 . After traveling the thickness of the secondary buffer coating  102 , the blade begins to move toward the center of the optical fiber  20 , parallel to its axis. This movement disrupts the coating  100 ,  102 , producing a taper  108  clearly visible in the softer primary buffer coating  100 . In certain cases, the taper may also extend into the secondary buffer layer as shown in FIG.  11 . The taper  108  provides a conical boundary separating the bare optical fiber  106  from the overlying buffer structure  100 , 102 . 
     As described, the mechanical stripping apparatus includes two sets of vertically opening and closing cutting blades  124 ,  130  adapted for vertical, then horizontal movement either independently or simultaneously. A pair of clamps  120 ,  122 , on either side of the cutting blades  124 ,  130 , holds a strippable filament in a taught condition during the stripping process. Another embodiment of a mechanical stripping apparatus alters the angle of the incision during the cutting process to modify the shape of a tapered transition  108 . As the blades  124 ,  130  close towards the coating  100 ,  102  around a clamped fiber  20  an angled surface or biasing cam surface deflects the path of the blades to a prescribed entry angle into the coating  102  so as to provide a controlled tapered transition. This produces an intentionally angled cut by moving the blades  124 ,  130  diagonally into the coating. Any change in the angle of the cam surface produces a corresponding change in the angle of a tapered transition  108  to allow consistently reproducible contours of a coating  100 ,  102  abutting either side of a bare portion of an optical fiber. Suitable selection of the cam angle produces tapered transitions  108  having contours and dimensions facilitating essentially defect-free recoating of bare optical fiber portions. Successful mechanical stripping to provide a tapered transition may proceed under ambient conditions, as indicated previously. With some buffers, however, the modulus of the buffer resin is in a range that complicates the formation of a tapered transition. In such cases, it may be necessary to soften the resin by heat or chemical action before attempting the mechanical stripping process to produce the desired tapered transition. 
     Completion of the mechanical process of fiber stripping leaves a central portion of an optical fiber  20  having a central sleeve  110  of protective buffer coating  100 ,  102  that has been separated from the remainder of the buffer coating  100 ,  102  over the optical fiber core  106 . Opposing gaps  132 ,  134  between the sleeve  110  and remainder of the buffer coating  100 ,  102  provide points for hot acid to penetrate under the central sleeve  110  to facilitate removal of the sleeve  110 , which dissolves in hot acid. Preferably the acid does not reach the tapered transitions  108  of a previously mechanically stripped coated optical fiber  20 . Removal of the central sleeve  110 , as a solution in hot acid, followed by rinsing in water and alcohol, leaves a clean, bare portion of an optical fiber  106  in suitable condition for further processing. Depending on the effectiveness of mechanical stripping, much of the disrupted buffer sleeve  110  may be lifted from the bare fiber portion. This reduces the amount of buffer coating to be dissolved from a fiber  20  during acid stripping. 
     Stripping of protective buffer coating from an optical fiber  20  may be conducted as an automated or semi-automated process using equipment suitably designed for the task. Preferably design of the equipment allows processing of multiple fibers  20  in a single operation. FIG. 17 shows the positioning of a filament organizer  10  relative to a mechanical stripping apparatus  150 . A mechanical stripping apparatus  150  according to the present invention includes a base  152  as a mounting platform for optical fiber clamps  120 ,  122  and sets of cutting blades  124 ,  130 . Optical fiber clamps  120 ,  122  may either move relative to the base  152  or be secured thereto. Optional securing of the clamps  120 ,  122  facilitates mechanical stripping with a fiber  20  either at its original tension, set by the filament organizer  10 , or under tension produced by moveable clamps  120 ,  122 . 
     The sets of cutting blades  124 ,  130  slidably engage the surface of the base  152 . Slidable engagement of the sets of cutting blades  124 ,  130  facilitates the axial movement of the blades  124 ,  130  to form a tapered transition  108 , as previously described. A filament organizer  10  may be suspended by any suitable method relative to the mechanical stripping apparatus  150  provided that the filament  20 , clamps  120 ,  122  and cutting blades  124 , 130  have alignment on a common axis. 
     After the preliminary step of mechanically stripping the central portion of a fiber  20 , acid stripping may require formation of a loop, suspended from a filament organizer  10 . The suspended loop may be submerged in hot concentrated sulfuric acid. An acid bath is a convenient and clean method to remove the buffer coat from acid-resistant glass. 
     A known method uses acid to remove protective coatings from optical fibers. The method requires handling of fibers each individually as much as six to eight meters long. Handling of such optical fibers requires caution because of the small diameter and transparency of the filamentary structure. If the optical fiber snags an object during handling, the glass fiber core could fracture without showing immediate evidence of damage. 
     A manual method for loop formation includes extending an optical fiber over two blocks having a distance of separation of about six inches. The folding of a first block  160  over a second block  162  produces a loop  164 , shown by the diagram of FIG.  18 . This figure also includes an acid bath  166  with capability to suspend the blocks  160 , 162  and loop  164  in suitable position for acid  168 , preferably sulfuric acid, to dissolve protective buffer coatings  100 ,  102  from the U-shaped loop  164 . The length of coating  100 ,  102  removed from an optical fiber  20  will depend upon the depth to which the optical fiber loop  164  extends below the surface  170  of the acid. 
     Loops having substantially a desired size and shape form relatively easily, but individual fibers need careful handling to avoid damage to exposed glass surfaces. The “U” shaped loop  164  of fiber  20  lies on one side of a pair of blocks  160 , 162  with relatively long trailing fiber ends  172  extending from the opposite side of the blocks  160 , 162 . In this arrangement the loop  164  and the fiber ends  172  are at risk for breakage or related damage. Damage occurs in different ways including inadvertent contact or impact during fiber processing operations including acid removal of buffer coating  100 ,  102  from the fiber  20 , fiber Bragg grating writing, fiber annealing, fiber recoating and the like. This problem may now be avoided using filament organizers  10  according to the present invention. Filament in the form of optical fiber  20  is readily loaded onto filament organizers  10  without damage. Also the design of a filament organizer  10  allows stacking of multiple organizers to increase process throughput. A stacked configuration  68  of filament organizers places filaments, i.e. optical fibers  20 , in a suitably spaced-apart relationship for processing from fiber stripping to fiber Bragg grating recoating, as further described below. 
     FIG. 19 indicates arrangement of a filament organizer  10  for acid stripping using an apparatus that will reposition an optical fiber  20  in a filament organizer  10  to produce a suspended optical fiber loop similar to the previously described loop  164  formation between blocks  160 ,  162 . The apparatus grips the coated optical fiber  20  at two points along its length as indicated in FIG. 19. A loop forms when these two points move toward each other, as shown in FIG.  20 . The fiber is held at one point between a first pair of rolls  180  and at a second point by a second pair of rolls  182 . A layer of elastomer covers a lower roll  184 ,  188  of each pair of rolls  180 ,  182 . Each roll diameter is based on the minimum advisable fiber bend diameter to avoid inducing damaging bending stress during optical fiber manipulation to form a loop. The elastomer provides compliance to lower contact stress, reduce fiber slip, and hold multiple fibers simultaneously. 
     Each pair of rolls  180 , 182  converges to pinch the fiber  20 . Next, the upper roll  186 ,  190  of each pair rotates toward each other, inducing a shallow bend in the fiber  20 . The shallow bend establishes a plane to be occupied by a machine-formed loop when the pairs of rolls  180 ,  182  move toward each other. The looping method works with the fiber tray  10  by using two removable upper rolls  186 ,  190  and two non-removable lower rolls  184 ,  188 . 
     The present invention, in one of its embodiments, facilitates the process of acid stripping of multiple fibers in a single operation. Successful processing of multiple fibers requires several features that are possible using filament organizers according to the present invention. Of particular importance is the use of a tool that shapes optical fiber into loops while minimizing the possibility of fiber damage. A suitable loop-shaping tool produces loops with repeatable size and shape. Once formed, loops preferably do not bend out of plane. This latter feature is important to the processing of multiple fibers that would tend to interfere with each other if out-of-plane bending occurred. Also, loops formed using stacked configurations of multiple fibers possess substantially the same size and shape, as required for automated and/or semi-automated processing. 
     FIG. 21 shows a perspective view of a loop former  200  according to the present invention including detail of the position of a stacked configuration  68  of filament organizers  10  inside a loop forming container  202 . The loop-forming container  202  is essentially a closed box including a floor  204 , a front wall (not shown), a rear wall  206  and first  208  and second  210  sidewalls. Inside the container  202 , a first ledge  212  occupies a position adjacent the junction between the first sidewall  208  and the floor  204 . A second ledge  214  occupies a similar position adjacent the junction of the second sidewall  210  and the floor  204 . The floor  204  includes a plurality of longitudinal slits  216  disposed orthogonally towards the first  212  and second  214  ledges. Each of the front wall and the rear wall  206  includes a generally U-shaped guide slot  218  that is indicated in dotted line form in FIG.  21 . Preferably the guide slot  218  includes a horizontal slot  220  joined to an angled slot  222 , at one end, and an opposed angled slot  224  at the other. A seated roller  226  includes an axle  228  engaging one end of the horizontal slot  220  in the front and rear  206  walls of the loop-forming container  202 . The seated roller  226  preferably has a covering of an elastomeric material. A movable roller  230  includes axle ends  232  extending into the horizontal slot  220  at the front and rear  206  walls. The movable roller  230  is repositionable along the length of the horizontal slot  220  to facilitate formation of multiple extended loops  234  from a stacked configuration  68  of filament organizers  10  positioned in the loop-forming container  202 . 
     The loop former  200  provides accommodation for a stacked configuration  68  of filament organizers  10 . Installation of a stacked configuration  68  inside a loop-forming container  202  requires orientation of the stacked configuration  68  to provide alignment of filaments  20  with the plurality of slits  216  in the floor  204  of the container  202 . This may be accomplished by holding a stacked configuration  68  by the openings  26  in support boards  12  followed by lowering the stacked configuration  68  into the loop-forming container  202  until the mounting plates  22  of the filament organizers  10  rest on the first  212  and second  214  ledges, as shown in FIG.  21 . The stacked configuration  68  is installed with the seated roller  226  and movable roller  230  at opposite ends of the horizontal slot  220 . Correct positioning of the stacked configuration  68  relative to the seated  226  and movable  230  rollers provides gentle contact between each filament  20  and the surfaces of the rollers  226 ,  230 . In this condition, the filaments  20  should still be under tension and free from bends. 
     Having positioned the stacked configuration  68  in the loop-forming container  202  with the filaments  20  touching the fully separated rollers  226 ,  230  a first rod  236  inserted through the angled slot  222 , of the front wall, extends across the container  202  to enter the angled slot  222  in the rear wall  206  of the loop forming container  202 . After insertion, the first rod  236  occupies a step  238  of the angled slot  222 . A second rod  240  similarly positioned, in a notch  242  of the opposed angled slot  224 , completes formation of the first pair  180  and second pair  182  of rolls. Movement of the first  236  and second  240  rods to follow the contours of the angled slot  222  and the opposed angled slot  224  initially establishes contact between the rods  236 ,  240  and the filaments  20 . An elastomer band stretched over the ends of each pair of rolls  180 ,  182  draws the rolls together to increase their gripping force on the filaments  20 . With continued gentle urging, the rods  236 , 240  continue movement towards the horizontal slot  220 . This movement places the rods  236 , 240  side by side with the corresponding rollers  230 ,  226  at each end of the horizontal slot  220 . During this movement the filaments  20  begin to wrap around the rollers  226 ,  230  in response to downward force applied by the rods  236 , 240 . The applied force also draws filament from each rotary spool  18  with the resulting formation of preformed loops having a height approximately equal to the diameter of the rods  236 ,  240 . Preformed loops become extended loops  234  of greater height through movement of the first pair of rolls  180  towards the second pair of rolls  182 . This creates an exposed loop  234  protruding through each slit  216  in the floor  204  of the loop-forming container  202 . By design, the slits  216  limit the amount of out-of-plane bending by the filaments  20 . Design features of each slit  216  include a loop entry  244  about 15. 0mm (0.625 inch) wide, and a narrower loop station  246 , having a width of about 1.6 mm (0.064 inch). The loop entry  244  is wider than the loop station  246  to prevent contact of the mechanically stripped fiber portion of an optical fiber  20  with the sides or other parts of a slit  216  during the initial stages of exposed, extended loop  234  formation. As the height of the loop  234  increases, the mechanically stripped portion of the fiber emerges below the floor  204  of the loop-forming container  202 . Any optical fiber  20  residing in a slit  216 , at this point, has a covering of buffer coating to protect the optical fiber from contact with any of the slit  216  surfaces. Thus, protected, the looped optical fiber  234  enters the narrow loop station  246  where it will stay during removal of residual primary and secondary buffer coatings by acid stripping. The loop entry  244  opens to the upper and lower surfaces of the floor  204  of the loop-forming container  202 . Preferably the opening to the upper surface of the floor  204  is wider (15.0 mm) than the opening (12.5 mm) to the lower surface of the floor  204 . This description indicates that the loop entry  244  of each slit  216  has a somewhat V-shaped cross section to assist in controlling the spatial positioning of each exposed, extended loop  234 . Loop control provided by each narrow loop station  246  counteracts torsional stresses introduced during manufacture of filaments  20  in the form of optical fibers. It will be readily appreciated that each extended loop  234  hangs below the floor  204  of the loop-forming container  202  ready for immersion in an acid-containing bath  166  as indicated in FIG.  20 . 
     FIG. 22 shows the condition of an optical fiber  20  after the process steps of mechanical stripping, and acid stripping to provide a length of an optical fiber  20  having a buffer-free bare central portion  250  suitably prepared for refractive index modification associated with the writing of a Bragg grating. Before the actual writing of a grating occurs, the unspooling of an end section of fiber provides an optical fiber pigtail  252  suitable for the formation of splices or connections between optical fibers  20 . At this point in the process, a pigtail  252  at each end of the optical fiber, carried on the filament organizer  10 , provides a point of connection so that the progress and accuracy of grating writing may be optically monitored during the writing process. 
     Optical fiber Bragg gratings may be written in a plurality of optical fibers  20  each having a bare central portion  250 . Convenient handling of these fibers  20  uses filament organizers  10  in a stacked configuration  68  according to the present invention. The pigtails  252  of each optical fiber  20  require positioning using fixtures to permit accurate alignment of fiber ends with equipment that monitors the progress of fiber Bragg grating writing. The fixture is an optical fiber connector including a central body with opposing fiber receiving ends. The monitoring equipment may use optical fiber or non-contact coupling to the pigtail ends to complete the optical circuit. Connection to fibers  20  from each filament organizer  10  in a stacked configuration  68  uses pigtail ends adapted to plug into the connector. A connector may accommodate pigtail ends of a single optical fiber  20  or a plurality of fibers  20  having pigtail ends previously terminated to the requirements of a multi-fiber connector. The term “ribbonized” has been applied to one form of termination wherein the ends of pigtail sections of fiber lie side by side to form a single layer of fibers having a flat ribbon-like appearance. Positioning of the pigtail ends allows them to mate with a multi-fiber connector. 
     FIG. 23 indicates a filament tensioning apparatus  260  used for further processing of an optical fiber  20  held in a filament organizer  10  according to the present invention. After placement of an optical fiber  20  on a support board  12 , as discussed previously, the portion located between the mounting plates  22  of the filament organizer  10  exists in a condition of tension applied by a tensioner  58 . In preparation for the writing of a grating, the tension wire  62 , of a selected filament organizer  10  (see FIG.  3 ), may be grasped in the vicinity of the notch  66  and extended slightly parallel to the edge of the board  12 , and toward the nearest guide  24 . Movement of about 1.0 mm to about 2.0 mm releases the tension force applied to a filament  20  by the tensioner  58 . Preferably the filament organizer  10  is one of a stacked configuration  68  positioned on a platform of an indexing unit. The indexing unit raises and lowers the stacked configuration  68  using any one of a variety of mechanical and hydraulic structures. In one embodiment, sliding engagement of the platform with one or more vertical posts or beams allows upward and downward movement of the platform and stacked configuration it supports. The platform may include organizing mounts for mating engagement with spacer blocks on the lowest filament organizer so that the stacked configuration  68  is suitably aligned with the optical fiber  20 , of each filament organizer, accessible to a fiber tensioning apparatus  260 . 
     The indexing unit remains stationary during the approach of a fiber tensioning apparatus  260  to apply clamps and grippers to a central portion of each optical fiber  20  in a stacked configuration  68 . Approach of the fiber tensioning apparatus  260  may be facilitated by an alignment mechanism for optimum positioning between an optical fiber  20  and clamps  262 ,  264  of a fiber tensioning apparatus. Optimum alignment does not necessarily require attachment of the indexing unit to a fiber tensioning apparatus  260 . 
     Clamps  262 ,  264  attached at each end of the central load-free fiber portion  20  retain the central portion in its load-free condition, before subjecting the optical fiber to a selected tension force. The clamps comprise components of a fiber Bragg grating tensioning holder  266  used to stretch the fiber  20  under a prescribed load during fiber Bragg grating writing. A tensioning holder  266  according to the present invention comprises a voice coil  268  as a load applicator to a load cell  270  that measures the load applied between the pair of clamps  262 ,  264  holding the central portion of the fiber  20 . After precise application of a desired selected tension, a pair of grippers  272 ,  274  isolates a measured fiber portion  276 , between the clamps  262 ,  264 , setting up a tension zone independent of outside tension variations. This maintains a prescribed load on the measured fiber portion  276  and prevents any fiber slippage relative to the grippers  272 ,  274  and hence the clamps  262 , 264 . The fiber Bragg grating may be written into the bare portion  250 , of the isolated, measured fiber portion  276  held between the pair of grippers  272 ,  274 . Tension applied to a clamped optical fiber  20  anticipates shrinkage that will occur, changing the separation between grating features after a grating has been written and after a piece of axially strained measured fiber portion  276  has been released from the pair of grippers  272 , 274 . 
     A voice coil driven tensioning holder  266  is favored over any of several possible load applying units including a DC servo motor and encoder combination, a precision pneumatic cylinder, a high precision micro-positioning linear stepper motor and a mechanical balance beam. A precision pneumatic cylinder, for example, provides insufficient fiber tension and fine pressure control to accurately apply a prescribed amount of tension to an optical fiber. A high precision micro-positioning linear stepper motor is equally incapable of providing required precise tension adjustment. Problems associated with the use of a mechanical balance beam include the fact that it is primarily a manual process not particularly conducive to automation. 
     Voice coil activated clamping structures are known. For example, U.S. Pat. No. 4,653,681 describes a voice coil activated fine wire clamp, used in wire bonding applications. Clamp jaws may be moved to an open position from a normally closed position using a voice coil motor under microprocessor control. A voice coil programmable wire tensioner, described in U.S. Pat. No. 5,114,066 also facilitates wire bonding. This shows that it&#39;s known to use a voice coil in wire bonding applications. However, it appears that the use of a computer controlled, voice coil motor has not been used to apply repeatable, precise amounts, of tension to optical fibers for consistent production of optical fiber Bragg gratings having essentially the same wavelength response. 
     The advantageous use of a voice coil actuator  268  provides a linear output force corresponding to an input current that may be finely controlled. A high precision power supply with a voice coil actuator  268  produces a stable signal leading to an output force that is remarkably constant. This allows selection of a wide range of output force, limited only by the magnitude of energy transfer between a coil and a magnet. The output force of the actuator  268  is proportional to the input current, similar to a DC motor. A tensioning method based upon a voice coil actuator  268  occurs in response to bearing-free passage of energy between a coil and a magnet. Tension adjustment using this method offers significant advantages over prior methods that used addition and removal of static weights to increase or decrease tension on a fiber. 
     FIG. 23 shows that the mounts  278 ,  280  for the voice coil and load cell include air bushing carriages  282 ,  284  for minimal friction relative to a support bar  286 . Air bushing carriages  282 ,  284  reduce static friction in the system to a low, almost insignificant level. Reduction of friction in the bushings  282 ,  284  allows accurate application of fiber tension corresponding to the force acting on the load cell  270 . This results in improved control of the force generated by the voice coil actuator  268  and more consistent application of tension to an optical fiber  20 . Each carriage  282 ,  284  includes a clamp  262 ,  264  for attachment to a central portion of an optical fiber  20 . The separation between the clamps  262 ,  264  identifies the central portion of the optical fiber  20  to be tensioned. An extending guide rod  288  attached to the moving coil of the voice coil actuator  268  pushes against the load cell  270  increasing separation between the two carriages  282 ,  284 . Increasing separation between the carriages  282 ,  284  operates through the clamps  262 , 264  to move them away from each other to add strain to the optical fiber  20 . Upon attainment of a desired strain a pair of grippers  272 ,  274  grasp an inner measured fiber portion  276  of the optical fiber  20 . The measured fiber portion  276  is somewhat shorter than the central portion between the clamps  262 ,  264 . Accurate maintenance of force at selected levels allows the writing of acceptable fiber Bragg gratings. Force selection and control relates to the use of a high precision load cell  270  to measure and display the tension initially applied to the fiber  20  and maintained during the writing process. The load cell  270  may also provide feedback during computer controlled automated fiber Bragg grating writing. 
     An important aspect of writing a fiber grating is the need to hold a measured fiber portion  276  in a fixed, immobile condition throughout the process. This requires the use of a jaw assembly  290  attached particularly to the grippers  272 ,  274  for removably securing a measured fiber portion  276  in the desired immobilized condition. Clamps  262 ,  264  attached to the filament tensioning apparatus  260  may use the same jaw assembly  290  or another providing adequate clamping of a central portion of a fiber  20 . 
     FIG. 24 illustrates a fiber portion gripper  272  with an attached jaw assembly  290  of suitable design. The jaw assembly  290  comprises a lower jaw  292  attached at the end of a gripper  272  and an upper jaw  294  for engagement with the lower jaw  292  to grip and immobilize a measured fiber portion  276 , shown in cross section in FIG.  24 . 
     FIG. 25 provides a detail drawing of a V-shaped channel  296  formed in the upper surface of the lower jaw  292  and a rectangular cross section groove  298  in the lower surface of the upper jaw  294 . The sizing of each of the channel  296  and groove  298  of the jaw assembly  290  corresponds to the diameter of the measured fiber portion  276  that is held in an immobilized condition. 
     A jaw assembly  290  design requires matching of the dimensions of a fiber  20  with those of a V-shaped channel  296  and a rectangular groove  298 . Dimensional matching allows the application of substantially equal pressure at contact points around the circumference of a measured fiber portion  276  held immobile for the writing of a Bragg grating according to the present invention. Preferably a fiber  20  is held between the V-shaped channel  296  and the rectangular groove  298  with equal pressure applied at points of contact around its circumference. This is indicated in FIG. 25 by the fact that the two points of contact of the fiber  20  with the V-shaped channel  296  and the fiber&#39;s point of contact with the groove  298  are equidistant from the bare optical fiber  106 . Uniform application of pressure leads to even distribution of stresses across the diameter of a measured fiber portion  276  to reduce fiber damage to a minimum, considering the amount of pressure required for the grippers  272 ,  274  to hold the measured fiber portion  276  in an immobile condition. V-groove chucks are known for clamping portions of optical fibers, as taught by U.S. Pat. No. 4,623,156 and U.S. Pat. No. 5,340,371. It does not appear in either case that consideration is given to equalizing the amount of pressure applied to points around the circumference of a fiber. 
     In a preferred embodiment of the present invention, pressure equalization around the circumference of an optical fiber  20  requires the use of a floating upper jaw assembly  295 , as shown in FIG.  26 . The self-adjusting, floating upper jaw assembly  295  comprises a fiber clasp  300 , a support flange  302 , and an angular compensator  304  (see FIG. 27) separating the fiber clasp  300  from the support flange  302 . A fiber clasp  300  may also be referred to herein as a filament clasp. Each gripper  272 ,  274  includes, in this case, an upper jaw mount  306  and a lower jaw mount  308 . A lower jaw  292  attaches to the lower jaw mount  308  and an upper jaw assembly  295  attaches to the upper jaw mount  306  by the support flange  302 . Suspension of a fiber clasp  300  from a support flange  302  preferably uses a spring-loaded connector  310 . Spring tension operating between the fiber clasp  300  and support flange  302  retains an angular compensator  304  between them. During capture of a filament  20  between the lower jaw  292  and upper jaw  294  of a gripper  272 ,  274 , the use of a floating upper jaw assembly  295  allows application of gripping force to a filament  20  substantially without displacement or rotation of the filament  20 . The clamps  262 ,  264  may also include a floating jaw assembly. 
     FIG. 27 shows the result of gripping a filament  20  between a floating jaw assembly  295  and a lower jaw  292 . As the floating jaw assembly  295  moves towards the lower jaw  292 , the rectangular groove  298  of the filament gripper  292 ,  294  makes contact with a filament positioned in the V-shaped channel  296  of the lower jaw  292 . As contact occurs, the filament clasp  300  may adjust slightly to apply gripping force uniformly to the filament  20 , without disturbing it. Adjustment of the filament clasp  300  relies upon its independent movement due to the angular compensator  304  that separates it from the support flange  302 . A preferred angular compensator  304  according to the present invention comprises a spherical element that prevents contact between the filament clasp  300  and the support flange  302 . Preferably the angular compensator  304  seats between a substantially conical shaped depressed portion  301  in the fiber clasp  300  and a substantially conical recess  303  in the support flange  302 . The angular compensator  304  maintains separation of the support flange  302  from the filament clasp  300  to allow them to move independently. Also, the spherical structure of the angular compensator  304  allows effective change of angle around the perimeter of the filament clasp  300 . 
     The previous discussion provided a description of positioning, clamping and gripping a single optical fiber  20  using an apparatus  260  including a tensioning holder  266  to tension the fiber  20  during writing of a Bragg grating. The description involves the relative positioning between a filament organizer  10  and a tensioning holder  266 . When a filament organizer  10  represents one of a number of organizers  10  in a stacked configuration  68  the writing of a Bragg grating may be accomplished in a variety of ways. For example, fiber optic Bragg gratings may be written one at a time using a step and repeat process to move a fiber  20  carried in a selected filament organizer  10  into the correct position, relative to the tensioning holder  266  to execute writing of a Bragg grating. The wavelength response of an optical fiber  20  may be monitored, as described previously, during Bragg grating writing. An alternative to sequential writing of Bragg gratings may be to use a bank of tensioning holders  266  and related writing devices for producing a plurality of Bragg gratings simultaneously. 
     The step and repeat process using an indexer to reposition a stacked configuration  68 , e.g. preferably by up or down directional movement, present s a new fiber to the Bragg grating writing device. The stacked configuration  68  fits into the platform of an indexer adapted to provide mating with a known positional relationship between a stacked configuration  68  and the platform using alignment between the spacer blocks  70  and organizer mounts  72 . Having established the preferred placement of the stacked configuration  68  relative to the indexer, and having made connection of the fiber pigtails  252  to the optical detection system, a scan of each fiber verifies the existence of reliable optical connections. 
     The placement of a stacked configuration  68  in an indexer with fiber optic connection to an optical detection system precedes seriate Bragg grating writing process in which the indexer initially uses an optical sensor to scan the filament organizers  10 , counting the number in the stacked configuration  68 . This process designates the first filament organizer  10  in the stacked configuration  68 . A sequence of operations modifies the optical fiber  20  held in this first filament organizer  10 . Before modifying the optical fiber itself, removal of the effect of the constant force tensioner  58 , as described previously, uses a force reduction assembly comprising a pair of pulleys  64  on either side of a notch  66  formed in an edge of the support board  12 . The tension wire  62 , passing around the pulleys  64  and across the notch  66 , may be grasped in the vicinity of the notch  66  and extended slightly, parallel to the edge of the board  12 , and toward the nearest guide  24 . This releases the tension on the rotary spool  18  of the filament organizer  10 , thereby releasing the tension in the filament or optical fiber  20 . 
     Preparation for modifying a filament  20 , in the form of an optical fiber, requires securing the tension-free portion of the optical fiber using an apparatus that combines a filament tensioning apparatus  260  and an interference pattern generator (not shown). Each of the filament tensioning apparatus  260  and the interference pattern generator may be moved separately initially to secure and position an optical fiber  20 , as discussed previously, and then to modify the fiber&#39;s structure. 
     The filament tensioning apparatus  260  grips the fiber  20 , as described above, using a first clamp  262  and second clamp  264 . Force operating between the two clamps  262 ,  264  applies tension to the portion of optical fiber  20  between them. The force may be generated using a voice coil actuator  268 . The amount of tension is predetermined and measured using a load cell  270 . At this point the optical detection system provides a reference scan of the optical fiber  20 , including the portion held under tension between the clamps  262 ,  264 . 
     To reproducibly modify an optical fiber  20 , preferably a measured portion  276  of the fiber  20  remains in a fixed condition held by a first gripper  272  and second gripper  274  that grip the fiber and hold it. Once the measured portion  276  of the optical fiber has been immobilized, an interference pattern generator moves into close proximity to the measured portion  276  of the optical fiber  20 . Light, from a contained laser source, passes through an opened shutter, and an optical system, including the interference pattern generator to produce an interference pattern. The proximity of the interference pattern generator to the optical fiber  20  provides sufficient energy to reproduce the line characteristics of the interference pattern or interferogram in the core  106  of the optical fiber  20 , preferably within the measured fiber portion  276 . Impingement of actinic radiation, produced by an ultraviolet laser, produces an optical fiber Bragg grating as a result of changes in refractive index in parts of the optical fiber core  106  affected by the radiation. Refractive index modulation corresponds to the pattern of the interferogram, produced by the interference pattern generator. Progress in reproduction of an interferogram in the core of an optical fiber may be monitored using an optical detection system for data acquisition. Data acquisition follows changes in the transmission spectrum produced by a developing Bragg grating with time. Upon sensing the desired transmission spectrum, the optical detection system closes the shutter to prevent further exposure of the optical fiber to laser light. 
     Following completion of optical fiber modification and removal of the interference pattern generator from the vicinity of the measured fiber portion, the grippers  272 ,  274  and clamps  262 ,  264  retract from the fiber  20  to allow the filament tensioning apparatus to move to the next filament organizer  10  in the stacked configuration  68 . Once separation of filament organizer  10  from the Bragg grating writing equipment occurs, the force reduction assembly releases the optical fiber placing it once again under the tension generated by the tensioner  58  of the filament organizer  10 . This completes the modification of a given optical fiber so that the indexer can readjust to align the optical fiber  20  in the next filament organizer  10 , in a stacked configuration  68 , with the filament tensioning apparatus and the interference pattern generator before repetition of the Bragg grating writing cycle. 
     Annealing using an annealing oven at 300° C. for 10 minutes provides stabilization for a Bragg grating produced by refractive index alteration of an optical fiber. An annealed Bragg grating may require protection by recoating the central portion of the optical fiber, which was previously stripped of protective coating. Any of a number of methods may be used for protective recoating of optical fiber Bragg gratings including in-mold application, extrusion coating and spray coating a fiber with a curable liquid coating. Equipment is commercially available for in-mold application of liquid recoat formulations. The quality of in-mold optical fiber section recoating varies with the skill of an operator to carefully position a fiber in a mold cavity. Also, product yields have been reduced because of coating defects and fiber strength issues associated with fiber handling and sectional recoating. As alternatives, either spray coating or extrusion coating may be used for recoating optical fibers that include Bragg gratings according to the present invention. 
     A filament organizer  10  according to the present invention may be used to advantage for positioning uncoated portions  250  of and optical fiber  20  in a fiber recoating mold. Since the filament organizer  10  also applies tension to the optical fiber  20 , an alignment plate attached to a mold recoater of the type supplied by Vytran Corporation of Morganville, N.J. is the only requirement for correct fiber positioning within a groove such as a semicircular or V-groove of the split mold apparatus. The alignment plate may use strategically positioned studs to engage the through holes  80  of the planar support  12  of a filament organizer  10 . This eliminates the need for mold positioning using micromanipulator platforms and the like. The effective diameter of the groove is somewhat greater than that of the remaining coated portions of the fiber. Due to pre-tensioning of the fiber by the filament organizer the common need for external tensioning is eliminated. Once the vulnerable uncoated portions  250  of the fiber  20  have been suspended clear of the groove surface, the hinged mold is closed and recoating material is injected into the groove until it extends to the coated portion of the fiber. The molding material is then cured yielding a recoated section with dimensional characteristics essentially identical to those of the original coated fiber. 
     Fiber recoaters of the type described briefly above include a split steel mold, each portion of which contains a matching semicircular groove to accommodate the fiber. The grooves, when clamped together, formed a cylindrical bore slightly larger than the coated fiber OD to permit escape of air during injection of the coating material. The original coating in this arrangement keeps the uncoated section suspended in the bore. A short uncoated length of fiber, typically no more than half an inch, minimizes the possibility of damage through contact with the bore. Also, a series of clamps, attached on either side of a central fiber portion, prevent the uncoated portion from touching the bore. Before injecting recoating fluid, the upper half of the mold is clamped in position to form the cylindrical bore. The curable recoating fluid may be a room temperature curing epoxy resin or other resin that cures either at elevated temperature or in response to suitable radiant energy such as ultraviolet radiation. 
     FIG. 28 illustrates the use of a filament organizer  10  to store an optical fiber  20  having a bare portion  250  that has been modified to include a Bragg grating. An exposed Bragg grating may be recoated after positioning the filament organizer  10  in a suitable spray recoating apparatus  320 . For correct positioning of a filament organizer relative to a spray recoating apparatus  320  the bare portion of an optical fiber  250  lies in the path of spray ejected from a recoating spray head  322 . Such correct positioning is achievable by any of a variety of methods and devices. One such method uses a plate suitably positioned relative to a spray recoating apparatus and including alignment studs to engage through holes in a filament organizer  10  to place the bare portion  250  of an optical fiber  20  in the optimum position for application of recoating spray. 
     A spray recoating apparatus  320  comprises at least one recoating spray head  322  and a radiation source  324 . A filament organizer  10  is adapted for oscillatory movement of the bare portion  250  of an optical fiber between the recoating spray head  322  and the radiation source  324 . Preferably, the position of the recoating spray head  322  is from about 1 cm to about 2 cm from the fiber  20 , preventing contact between the spray head  322  and a deposited coating. The spray recoating method provides controlled sectional recoat that achieves performance characteristics not obtainable from conventional in-mold recoating processes. It is a non-contact method since the optical fiber  20 , including the bare portion  250 , does not touch any part of the recoating equipment. This represents another benefit of suspending a fiber  20  in a filament organizer  10  that may be readily attached to the recoating apparatus with precise fiber  20  to spray head  322  alignment. Another benefit of spray recoating involves over coating one recoating composition with another exhibiting different properties to produce a multilayer buffer structure, around a fiber, including layers that differ in properties such as modulus and durability or hardness. 
     The use of a spray recoating process allows flexible placement of a single filament or multiple filaments in the path of spray or mist from a recoating spray head  322 . Where a filament organizer  10  provides the preferred means for handling an optical fiber  20 , several filament organizers  10  may be closely located with variable orientation to place a plurality of fibers in the path of a single spray or directed mist. An additional advantage of spray recoating versus conventional cavity-mold recoating is the provision of a recoating spray head  322  that may be adjusted or translated to differing lengths of bared optical fiber portions  250 . 
     As the bared portion  250  of a fiber  20  traverses the location of the recoating spray head  322 , one side of the bare fiber portion  250  receives a light deposit of droplets from a mist of a curable recoating composition. Movement of the filament organizer then places the deposit of droplets in the illumination path of the radiation source  324 . The radiation cures the layer of recoating composition. Returning to the location of the recoating spray head  322 , the filament organizer  10  flips over to expose the opposite side of the previously bare fiber portion  250  to the spray of curable recoating composition. This allows application of a fine mist of recoating composition to the exposed optical fiber surface. This layer may be cured as described previously. Repeated processing by coating and curing with oscillation and flipping of the filament organizer  10  protects the fiber with multiple layers of recoating composition. The recoated fiber surface has a matte appearance resulting from the build up of successive layers of coating material. Coating topography up to about 15 μm was revealed on the surface of a microscope slide by surface scanning with an ELFA STEP mechanical stylus profilometer available from Tencor Corporation. 
     Approximately fifty applications of recoating composition followed by curing, after each pass, provide a layer having a thickness over the recoated length similar to that of the original buffer coatings on other parts of an optical fiber  20 . This technique allows layers of recoating composition to be applied to the surface of an optical fiber to build a protective recoat having a thickness of from about 10 microns to about 100 microns on a bare fiber  106 . The diameters of spray-recoated optical fibers may be measured using a microscope and a QUADRA-CHEK 2000, from Metronics Inc., Bedford, N.H. Coating thickness may be varied depending on the application. 
     Another embodiment of the present invention provides a second recoating spray head  326  and optionally a second radiation source  328  positioned opposite the previously discussed recoating spray head  322  and radiation source  324 . The description of multiple spray heads  326  and radiation sources  328  as occupying opposing or staggered opposing positions includes alignment of positions but is not limited thereto. Any number of spray heads, positioned strategically, may be used in a fiber recoating process. Placement of a spray head and radiation source on both sides of an optical fiber  20  facilitates recoating of both sides of the bare fiber portion  250 , while eliminating the need to flip the filament organizer through 180° . As indicated previously, the use of additional radiation sources  328  is optional since the beam from a single radiation source  328  may be directed to effect curing around the circumference of a recoated fiber. 
     The contours of a deposit of droplets applied to a bare fiber  106  will reflect the size and shape of the droplet cloud issuing from a spray head  322 . If required, a means for shaping the droplet cloud could produce a desired pattern of droplets on the surface of a fiber  20 . Suitable shaping means include stencils, other types of masking devices, and stream deflectors such as air knives. 
     A preferred recoating process according to the present invention uses an air knife to direct an atomized stream at various angles of contact with an optical fiber  20 . Air knife  12  adjustment of the shape of a droplet cloud, and its angle of impingement with an optical fiber  20 , may allow the use of a minimum of spray heads  322 ,  326  to achieve optimum fiber recoat uniformity and concentricity. Also, the use of air knife deflection of small volumes of recoating compositions provides an advantage when compared to the control of diverging streams of relatively high volume spray heads described in Japanese patent JP 60-122754. U.S Pat. No. 5,219,120 teaches the use of an air horn that provides a moving sheet of air to entrain a substantially uniform linear dispersion of atomized fluid moving above and extending substantially across the width of the air horn. The air horn spreads the dispersion of atomized fluid to a width suitable for spraying the flat surface of a circuit board. Such extensive spreading of a cloud of droplets does not apply directly to narrow curved surfaces such as those of an optical fiber. Also, the air horn described in U.S. Pat. No. 5,219,120 is a separate structure from the fluid atomizer. 
     Preferably air knife deflection according to the present invention occurs through the use of an air knife attachment that fits over the exit nozzle of a spray head. The air knife attachment includes a pair of receiving chambers, at least one on either side of the spray head, into which air may be directed. Each receiving chamber has an air entry at one end connected to an air reservoir. The opposite end of each chamber includes an air knife slit that exits from the chamber at an angle to the axis of the spray head. Air issuing from an air knife slit deflects the spray cloud, generated e.g. by an ultrasonic atomizing spray head, at an angle corresponding to the angle formed between the slit and the axis of the spray head. Independent operation of each air knife, described above, causes selective deflection of a spray cloud at an angle that directs the droplet cloud towards an uncoated portion of an optical fiber. Selective deflection of a droplet cloud allows positioning of a number of optical fibers around a spray head nozzle. Impingement of air from exit slots of air receiving chambers deflects atomized spray at various angles for sequential recoating of the number of optical fibers held around the spray head using filament organizers  10  according to the present invention. The use of air deflection preferably requires that the recoating composition is not oxygen inhibited. This does not prevent the use of oxygen inhibited recoating fluids providing an inert gas is connected to the receiving chambers of the air knife attachment. 
     The process of recoating a bared portion  250  of an optical fiber  20  may use spray heads  322 ,  326  based upon either ink jet or ultrasonic atomization technology. Preferably, the application of curable recoating composition, to an optical fiber  20 , uses ultrasonic atomization technology to dispense small particles (&lt;50 μm) of a fluid, having a viscosity from about 40 to about 900 centipoises, preferably 40 centipoises to about 400 centipoises, over a bared portion  250  of the fiber  20 . Viscosity measurements were made at 25° C., using a BOHLIN CS-50 rheometer. Other requirements for a coating composition for recoating optical fibers according to the present invention depend upon the intended use of a recoated optical fiber device, such as a Bragg grating. Example 1 of Table 1 provides a load bearing coating, preferably having a high modulus, high glass transition temperature (Tg), and temperature stability above the upper operating temperature for a selected application. Examples 2 and 3 produce cured coatings that flex and bend with a recoated portion of a fiber. Preferably coating compositions, in this case, possess thermomechanical properties similar to undisturbed buffer coating, originally applied to the fiber. Immediate curing of such a coating reduces undesirable agglomeration, which could result in beading or poor concentricity. 
     An ultrasonic atomization processes differs from a spray atomization process that, requires air velocity to break up a sprayable composition into droplets. Droplet size of a spray atomization process is larger (50 to 100 microns diameter) and the spray velocity, at its lowest pressure of 20 psi, propels the droplets with a force causing the droplets to spread upon impact with a fiber surface. Being relatively high, the impact force of an air atomized spray against a fiber causes build-up of agglomerated droplet beads accompanied by formation of a non-concentric coating. 
     The ultrasonic atomization process generates volumes of coating composition that are extremely small, in the range from about 0.001 ml/min to about 0.010 ml/min using a 2.0 cc glass syringe available from Popper &amp; Sons. The flow rate for dispensing a substantially non-directional cloud of droplets less than 50 microns in diameter depends upon the speed at which the fiber is scanned in front of the atomizer head. A low velocity flow of nitrogen, or other inert carrying gas directs the cloud of ultrafine droplets of recoating composition towards a target surface. The low cloud volume and extremely small droplet size cause the formation of a textured discontinuous covering of the fiber surface. Although coatings are low enough in viscosity for spray application, preferred coating compositions exhibit minimal flow, after application, prior to coating. Flow and droplet agglomeration is further limited because the recoating composition, immediately after application, undergoes exposure to curing radiation from the radiation source  324 ,  328 . Repeated application of recoating composition builds up a protective coating over a bared optical fiber portion  250 . A recoated optical fiber preferably has a relatively smooth appearance bubble-free appearance. This requirement guides the selection of materials used to prepare recoating compositions according to the present invention. 
     Suitable recoating compositions include low molecular weight, low viscosity epoxy functional, 100% solids resins that photocrosslink preferably via an ionic mechanism initiated by a cationic photoinitiator, especially an iodonium salt photoinitiator. Such coatings have good adhesion to the unstripped buffer coats on a fiber as well as to the bare surface of the fiber. Ionic curing occurs without exclusion of oxygen. Radical curing recoating compositions may also be used in an inert environment. Suitable radiation sources for photocrosslinking include those having wavelength emission in the blue/visible and ultraviolet wavelength regions of the spectrum. Cured coatings according to the present invention. 
     A typical cured recoating composition has an elongation at least equal to and preferably greater than that of glass, i.e. more than 7%. Also, a cured recoating composition has toughness and sufficient adhesion to glass to withstand accidental rubbing or contact with other objects during handling of a recoated fiber. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Filament Coating Formulations 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Material* 
                 Example 1 
                 Example 2 
                 Example 3 
               
               
                   
                   
               
               
                   
                 Weight % Epoxy A 
                 57.0 
                 — 
                 — 
               
               
                   
                 Weight % Epoxy B 
                 38.0 
                 — 
                 — 
               
               
                   
                 Weight % Epoxy C 
                 — 
                 67.0 
                 66.5 
               
               
                   
                 Weight % Epoxy D 
                 — 
                 25.1 
                 28.5 
               
               
                   
                 Weight % Polyether Glycol 
                 — 
                  2.9 
                 — 
               
               
                   
                 Weight % Iodonium Salt 
                  5.0 
                  5.0 
                  5.0 
               
               
                   
                 Solution 
               
               
                   
                   
               
               
                   
                 *Key:  
               
               
                   
                 Epoxy A is CYRACURE UVR-6105 available from Union Carbide Corporation.  
               
               
                   
                 Epoxy B is HELOXY 107 available from Resolution Performance Products.  
               
               
                   
                 Epoxy C is EPONEX 1510 available from Resolution Performance Products.  
               
               
                   
                 Epoxy D is HELOXY 7 available from Resolution Performance Products.  
               
               
                   
                 Polyether Glycol is TERATHANE 650 available from E.I. du Pont de Nemours and Company.  
               
               
                   
                 Iodonium salt solution is UV 9380C available from General Electric Company.  
               
            
           
         
       
     
     Measurement of Coating Composition Viscosity 
     A Bohlin Model CS-50 controlled stress rheometer was used to measure the viscosities of coating compositions, for recoating filaments according to the present invention. The test method uses parallel plate geometry and “stress viscometry” mode. Viscosity measurement begins with placement of a coating composition on the base surface of the parallel plate geometry. A second surface, mounted to rotate on a spindle, is lowered into contact with the coating composition until a specified gap exists between the surfaces of the parallel plate geometry. Rotation of the spindle raises the rate of rotation to a number of revolutions per minute to produce a predefined stress (torque). The calculation of viscosity values includes consideration of the geometry of the surfaces, the torque and the gap. Viscosities cited herein were obtained at 25° C. using a surface diameter of 20 mm, a gap between surfaces of 0.3 mm, and a stress of 93.8 Pascals. 
     A spray head that included an ultrasonic atomizer was used to apply curable recoating formulations, shown in Table 1, to the bare surfaces of several samples of silica fiber, each having a diameter of about 125 microns. Each formulation was dispensed via the tip of the atomizing horn of an ultrasonic atomizer available from Sono-Tek. The power supply of the ultrasonic atomizer was set to a level of 5.4 watts. Successful atomization of recoating formulations, having viscosities in the range from about 40 centipoises to about 400 centipoises was achieved using a micro-bore fluid delivery tube through the center of the nozzle body of the ultrasonic atomizer. Most preferably the coating composition has a viscosity of about 200 centipoises. Recoating formulations were supplied to the micro-bore tube at a syringe pump delivery rate of 0.015 ml/min. A preferred method uses a 21.5 gauge micro-bore tube available from Small Parts Inc., Miami, Fla. This provides precise control of small volumes of recoating composition delivered to the point of atomization. 
     Ultrasonic atomization as described previously produces a non-directional mist of coating composition that needs to be entrained in a directional gas stream. Preferably the directional gas stream comprises an inert gas, e.g. nitrogen gas, under the control of a shroud around the micro-bore tube. A nitrogen gas stream flowing through the shroud around the atomizer head at a rate of 1.0 liter/min yields a suitably controlled atomized mist of recoating formulation. Adjustment of the air shroud alters the contours of the gas stream thereby modifying the size, shape and coverage of a stream of droplets of curable recoating formulation impinging on a selected surface. A continuous coating may be formed on a surface using as few as about 4 to about 6 applications of a coating formulation. However, depending upon process conditions, application of coating formulation may need to be repeated form about 40 to about 60 times to build a coating thickness of up to 250 microns on a selected surface. 
     A filament recoating formulation was shown to produce a suitable stream of material for application using an ink jet printing/spray head as follows: 
     Example 4 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Epoxy A 
                 76.0 weight % 
               
               
                   
                 Epoxy B 
                 19.0 weight % 
               
               
                   
                 Photoinitiator soluton 
                  5.0 weight % 
               
               
                   
                   
               
            
           
         
       
     
     The photoinitiator solution contains 40 parts or iodonium methide; 60 parts of decyl alcohol and 4 parts of isopropylthioxanthone. 
     The ink jet printing/spray head operated at a head temperature of 70° C. A ink jet printing/spray head, available from Trident International Inc., Brookfield, Conn. was selected to apply recoating composition to several samples of silica fiber, each having a diameter of 125 microns. The print head has 64 nozzles, each 50 microns in diameter. Use of a filament organizer mounted on a filament recoating apparatus provided suitable alignment of a fiber with an ink jet printing/spray head prior to application of recoating composition. Particles of the composition were jetted over a I cm length, on one side of a filament of each of five samples of silica fiber. An EFOS ULTRACURE radiation source (EFOS Inc., of Mississauga, Ontario, Canada), with an ultraviolet radiation wand, was used to direct energy to the coated sample to initiate cure. Repeated passes under the recoating spray head, followed by ultraviolet radiation curing, produced adequate coverage of the bare optical fiber. 
     FIG. 29 provides a diagrammatic representation of a preferred split die extrusion coating method that uses an recoating fluid extrusion apparatus  330  having a die head assembly  332  encircling an optical fiber  20  to recoat a bared optical fiber portion  250  that contains a Bragg grating. A study, reported in Electronics Letters Vol. 34, No. 12, Jun. 11, 1998, investigated a split die recoating process to apply a solution of polyimide to a bare portion of an optical fiber. The process involved drawing a fiber through the fluid filled split die, then driving off solvent at 70° C. followed by baking the polyimide recoated section at 300° C. 
     Split die extrusion coating according to the present invention offers improvements for fiber recoating including controlled application and relatively low temperature curing of recoating compositions as follows. The die head assembly mentioned above comprises a split sizing die  334  and an in-line radiation cure chamber  336  that is closed around the optical fiber  20 . Accurate fiber  20  positioning, for recoating and protection of the Bragg grating occurs during engagement of a filament organizer  10  with the recoating fluid extrusion apparatus  330 . Any one of a variety of methods may be used for engagement between a filament organizer  10  and an extrusion apparatus  330  provided that the die head assembly  332  has movable alignment to deposit a substantially uniform layer around the fiber portion  250  that needs recoating. During recoating, the bared fiber portion  250  of an optical fiber  20  remains stationary between fiber positioners. The split sizing die  334  lies adjacent to one end of the bare fiber portion  250  from which curable recoating composition will be applied across the remainder of the bare portion  250 . Photocurable coatings extrude from the leading edge of the sizing die  334  as it traverses the length of the bared optical fiber portion  250 . The radiation cure chamber  336  moves with the sizing die  334  following behind it to initiate curing of the photocurable recoating composition immediately after its deposition on the surface of the optical fiber  20 . The recoating composition curing reaction preferably requires an inert atmosphere. For this purpose an inert gas delivery tube  338  directs a flow of nitrogen into the radiation cure chamber  336  that is illuminated using a suitable source of radiation, preferably ultraviolet radiation. 
     A linear transport mechanism  350  adjacent to the coating head  332  includes a guide rod  352  and a carriage  354  slidably mounted on the guide rod  352  for movement along the guide rod  352 . A connecting rod  356  from the carriage  354  to the coating head  332  provides linear displacement of the coating head assembly  332  during movement of the carriage  354  to move the coating head  332  from the first boundary to the second boundary of a bare portion  250  of an optical fiber  20 . Curable fluid may be extruded from the sizing die  334  and energy from the radiation source  336  used to cure the fluid during recoating of the bare portion  250  of an optical fiber  20 . 
     During its motion, the split die  334  applies a substantially uniform thickness of recoating composition along a length of fiber  20  that includes the bare fiber portion  250  and margins at each end that overlap the original secondary buffer  102 . Uniform coverage of an optical fiber  20  with a concentric layer of a recoating composition relies upon the accuracy of positioning a filament organizer  10  to preferably place the fiber  20  coaxial with the sizing die  334 . The radiation cure chamber  336  has a size such that its internal surfaces do not touch the layer of recoating composition, either before or after curing. When coating concentrically, the bare fiber portion  250  will only come into contact with the recoating composition. The split configuration of the sizing die  334  and the radiation cure chamber  336  allows easy positioning of a fiber  20  in a recoating fluid extrusion apparatus  330 . Correct fiber positioning, as mentioned previously, is a result of accurate engagement of a filament organizer  10  with a recoating fluid extrusion apparatus  330 . Upon re-opening the die head assembly  332 , after completing the recoating and curing process, a gap between a recoated fiber  20  and the internal surfaces of the radiation cure chamber  336  allows clean removal of the fiber  20  from the assembly  332 . 
     Changes in the length of bared fiber portions  250  may be accommodated by adjustment of the distance that a die head assembly  332  may travel while extruding recoating composition. The surface tension of the recoating composition tends to smooth out any irregularities in the coating before it reaches the radiation cure chamber  336 , even though the die head assembly  332  has a length of only about 6.0 mm to about 7.5 mm. A benefit of this short length is avoidance of contamination by recoating composition. Also small amounts of residual recoating composition may be relatively easily cleaned from inside the assembly  332 . 
     Although a bare fiber portion  250  has a horizontal orientation during application of protective recoating composition, the moving extrusion die  334  produces similar results to coating heads operating vertically during fiber draw coating processes. The relative motion between the sizing die  334  and the fiber  20  simulates the draw process. This eliminates mold recoating defects such as flash, gate marks, sinks, and coating delamination caused by coating adhering to the surface of a mold. 
     The extrusion of terminal margins, at each end of the bare fiber portion  250 , means that initial deposit of material by extrusion occurs at a region of the fiber  20  that is still protected by the original coating  100 ,  102 . This substantially prevents optical fiber strength losses generally associated with loading the fiber  20  into a traditional recoating mold. Bared fiber portions  250  recoated by split die extrusion according to the present invention provided evidence of strength retention by surviving Vitran proof testing to levels exceeding 800 kpsi. 
     A process for manufacturing an optical fiber Bragg grating has been described to show how a compact filament organizer  10  may be used to handle and transport optical fibers  20  between various types of processing equipment. Each piece of processing equipment may include a pair of mounting pins for alignment and insertion in through holes  80  of a filament organizer  10  for correct positioning of a central portion of an optical fiber  02  relative to the selected piece of apparatus. Such easy positioning also facilitates automation of at least parts of the Bragg grating manufacturing process unlike previous similar processes that rely upon operator skill for correct fiber positioning. It will be appreciated that engagement between mounting pins and through holes is only one of a number of methods for aligning an optical fiber for processing. 
     As required, details of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention.