Method and apparatus for stripping coatings from optical fibers

Abstract: A method and apparatus are provided for removing protective coating material from a fiber optic cable including one or more optical fibers. A stream of hot inert gas is directed onto the cable to soften the protective coating material and blow it from the cable. The stream can be moved relative to the cable until the desired length of coating material has been removed.

Description: Field of the Invention The present invention relates to a method and apparatus for removing a coating at a desired position from a fiber optic cable comprising one or more optical fibers embedded in protective coating material. Background of the Invention Glass optical fibers have very small diameters and are susceptible to external influences such as mechanical stress and environmental conditions. To protect the fiber from such influences, it is provided with one or more coatings of a of protective material. Certain uses of optical fibers require that a portion of the coating be removed from an end of the fiber or a portion of the fiber that is remote from the ends. For example, to make a fiber optic coupler, coating is stripped from portions of at least two optical fibers, and the stripped portions are fused together in side-by-side relationship and stretched. In one type of coupler referred to as an overclad coupler, the stripped portions of the fibers are inserted into a tube which is collapsed onto the fibers and stretched. Regardless of how the coupler is made, it is inserted into a protective housing from which fiber pigtails extend for the purpose of connecting the coupler to other optical fibers. It is important that the stripped portions of the fibers do not become weakened during the stripping process since weakened fibers can fail during subsequent process steps or during handling of the coupler when tensile stress is applied to the pigtails. The process of fusion splicing together two optical fibers or two multi-fiber cables requires that the ends of each pair of fibers that are to be joined be stripped before they are fusion spliced. The spliced portions of the fibers are then recoated. It is increasingly necessary to make splices of reliably high strength. Therefore, the coating stripping process should not inordinately reduce fiber strength. Coating can be manually stripped from an optical fiber by placing the fiber within a hand-held tool, bringing blades of the tool into contact with opposite sides of the coating layer, and then moving the tool relative to the axis of the coated optical fiber. The bare portion of the fiber usually needs to be wiped with a cloth wetted with alcohol or the like to remove smudges and/or particles of coating that have been deposited on the bare portion of optical fiber by the coating removal process. This type of coating removal process has been built into equipment that performs the tasks of the technician, whereby the process is no longer manual. Some coating removal processes employ a solvent such as acetone to soften and swell the coating so that it is more easily removed. Alternatively, a heater within the coating removal apparatus can also cause softening and swelling of the coating. Coating can also be removed by contacting the coated fiber with materials such as sulfuric acid and hydrogen peroxide (to remove a polyester coating) or with sulfuric acid (to dissolve a polysyloxine coating). These processes have various disadvantages. Processes that are adapted to remove coating only from the end of a coated fiber are often not useful for removing coating from regions remote from the fiber ends, a requirement for making fiber optic couplers. Mechanical strippers can cause scoring or scratching of the fiber surface. For example, the fiber surface can be scratched as the bare glass fiber without its coating is removed from the mechanical stripping device. Some stripping methods do not result in a clean demarcation between the stripped coating and the coating remaining on the fiber. If the stripping process results in a ragged or uneven coating termination, the resultant sripped fiber may not be useful for its intended purpose. Mechanical stripping can also contaminate the fiber surface, thus requiring a subsequent cleaning step to remove particulates. The wiping step can cause more damage to the fiber than the mechanical stripping step. Mechanical stripping that is manually performed is highly operator dependent as it is typically performed with a hand held stripper. Chemical stripping processes can be disadvantages in that they are difficult to control, and they may leave contaminants or residual coating on the fiber surface. Chemical stripping can also be disadvantages in that it utilizes dangerous caustic chemicals that need to be handled and disposed of. Moreover, chemicals can flow between the fiber and the coating that remains on the fiber just beyond the stripped region. It is particularly difficult to remove the coating from an array of optical fibers such as a ribbon cable or a circularly symmetric multi-fiber cable since coating material is situated between closely spaced fibers or between fibers and filamentary strength members in such cables. JP-A-60 055303 describes a method for removing coating material from a longitudinal portion of a fiber optic cable, the method having features set out in the preamble of claim 1 herein. The gas used, although hot, has a temperature of only about 200°C, and is used to melt a coating material of nylon or the like having a relatively low melting point. SU-A-1024861 also describes a method for removing coating material from a fiber optic cable, using a hot gas. No temperature is specified for the hot gas, apart from there being a requirement to heat the coating material to its melting point. A final removal of the coating material is carried out with a rotatable clamp. Summary of the Invention It is an object of the present invention to provide a clean, simple and inexpensive method and apparatus for removing coating from a coated optical fiber or a coated array of fibers. Another object is to provide a method of stripping coating from optical fibers in such a manner that the bare fiber surface is sufficiently clean that it does not have to be further wiped or treated before being used. A further object is to provide a method and apparatus capable of removing coating from the end of an optical fiber, a region of the fiber remote from its ends, or from an entire length of optical fiber. Briefly, this invention provides a method having the features set out in claim 1. In a preferred embodiment, relative movement is provided between the jet and the optical fiber. This can be done by moving the jet, moving the fiber, or moving both the jet and the fiber. If the jet is moved, it can be moved along the axis of the optical fiber and in a direction that is lateral to the axis of the fiber. It is advantageous to support the fiber such that it is not excessively vibrated by the gas jet. This invention further provides an apparatus for removing a predetermined length of coating material from a fiber optic cable, the apparatus having the features set out in claim 15 below. Brief Description of the Drawings Fig. 1 is a schematic illustration of an apparatus for removing coating material from a coated optical fiber. Figs. 2 and 12 schematically illustrate paths that can be followed by a hot gas jet as it removes coating material from a fiber optic cable. Figs. 3 and 4 are schematic illustrations of further fiber coating removing techniques. Figs. 5, 6 and 7 are cumulative Weibull probability plots for three different fibers. Fig. 5 pertains to a fiber stripped in accordance with the method of this invention. Fig. 6 pertains to a manually stripped and wiped fiber. Fig. 7 pertains to an unstripped fiber. Fig. 8 illustrates the removal of coating material from a ribbon cable. Figs. 9 and 11 shows apparatus for directing a plurality of jets of hot gas onto a fiber optic cable. Fig. 10 is a cross-sectional view of a fiber optic cable. Fig. 12 schematically illustrates a path that can be followed by a hot gas jet as it removes coating material from a fiber optic cable. Fig. 13 illustrates the removal of coating material from a plurality of individual optical fibers. Detailed Description Fig. 1 shows an apparatus for removing coating material from coated optical fiber 10 which comprises glass optical fiber 11 and coating 12. The coating composition must be such that it rapidly softens as its temperature is raised. To determine whether a given coating material meets this requirement, a fiber coated with the given material is simply subjected to the method of this invention. Obviously, some coatings, which are formulated so that they are resistant to high temperatures, are unsuitable for use in the present method. A source 16 of inert gas is supplied through filter 17 and flowmeter 18 to the inlet pipe 23 of a tube heater 20. Heater 20 may consist of an outer tube containing an axial heating element over which the inert gas flows. Other types of gas heaters could be employed. For example, the gas could flow through a separate heating device prior to being delivered to tube 20. The gas emanates from a nozzle 25 that directs the appropriate gas flow onto the fiber. The nozzle can have a relatively small diameter as shown in Fig. 1 to increase gas pressure. Other nozzle designs can be employed to confine the flow to predetermined cross-sectional geometries. For example, a later discussed elongated nozzle directs an oblong cross-section flow onto a relatively long region of coated fiber. The inert gas is one that will not react with the fiber coating material. Examples are nitrogen, argon, helium, carbon dioxide and mixtures thereof. Such gases do not react with the coating layer; moreover, they retard the burning of the coating material. Tube heater 20 is mounted on a support apparatus 21 which is capable of rotating about axis 22 as indicated by arrows 26 and 27 and which is capable of moving along axis 22 as indicated by arrows 28 and 29. Axis 22 may be parallel to the axis of coated fiber 10, but it need not be so oriented. In one operational mode, coating material 12 is to be removed from coated fiber 10 between points a and b. To prevent hot gas jet 24 from excessively vibrating the fiber, a length of coated optical fiber is held between clamps 14 and 15. Tube heater 20 is positioned along axis 22 so that it rotates in a plane that includes point a along coated fiber 10, but its orientation about axis 22 is such that its output jet 24 is not directed at the fiber. The inert gas valve is opened, and the tube heater is then turned on. After the gas temperature has increased to the operating point, the coating removal process can begin. The tube heater is rotated about axis 22 in the direction of arrow 26 until the hot gas emanating from the tube heater nozzle is directed at point a of coated fiber 10. Tube heater 20 begins to traverse along axis 22 in the direction of arrow 29. After coating material has been removed between points a and b along coated fiber 10, tube heater 20 rotates about axis 22 in the direction of arrow 27 so that hot gas is no longer directed at the fiber. The exposed optical fiber 11 is sufficiently clean that it can be used in processes such as coupler manufacturing or splicing without further treatment. In an unsuccessful experiment, a jet of hot air was directed onto a coated fiber. Oxygen present in the gas jet reacted with the coating material and caused it to burn and adhere to the fiber surface. The hot gas preferably does not dwell on one region of the fiber for too long a time since this could weaken the fiber. For this reason the tube heater preferably begins to traverse along axis 22 as soon as it swings into the position where hot gas is directed onto the coated fiber. The path of the inert gas jet could be as illustrated in Fig. 2. The tube heater can simultaneously rotate (arrow 26) and move upwardly (arrow 29) until gas jet 31 is directed onto coated fiber 10. After the removal of sufficient coating material, it can rotate (arrow 27) while still moving upwardly (arrow 29). This generates a path 32 as shown in Fig. 2. The arrangement shown in Fig. 3 can be employed to remove coating material from the end of a coated fiber 40. As shown by arrow 41 the coated fiber is fed through a tube 42 having an inside diameter only slightly larger than the outside diameter of the coated fiber. The nozzle 43 of tube heater 45 is located near the point of exit of the fiber from tube 42 to minimize the amount of fiber vibration due to the gas jet 44 impinging on the fiber. The tube heater is rotatable as described in conjunction with Fig. 1 so that jet 44 can be directed away from fiber 50. Nozzle 43 is initially directed toward the axis of coated fiber 40. As the coated fiber emerges from tube 42, and into the path of jet 44, coating material 46 becomes softened by the hot gas, and small particles 48 of the coating material are blown from optical fiber 50 and into exhaust vent 49. A tail 47, which may remain after the stripping process, is sufficiently small that it does not interfere with subsequent processes in which the fiber is used. The speed with which relative motion occurs between the coated fiber and the gas jet depends upon parameters such as the gas temperature, the gas flow rate, the rate at which the gas jet traverses the coated fiber, and the softening characteristics of the particular coating material. These parameters are interdependent. If, for example, the temperature of the jet were to increase from a first temperature to a second temperature, the relative motion between jet and fiber should increase from a first rate to a second rate to obtain a result similar to that obtained for the first temperature and first rate. Fig. 4 illustrates a technique for rapidly removing coating 55 from optical fiber 56. This technique might be employed for removing coating from a long length of fiber, perhaps an entire reel of fiber. Nozzle 57 of heater tube 58 is elongated so that it is capable of directing a stream 59 of hot inert gas is directed to a relatively long section of coating. An elongated nozzle, such as that shown in Fig. 4, could also be used to remove coating without providing any relative movement between nozzle and fiber. The length of the nozzle would be predetermined so that the jet would impinge upon the fiber and remove a predetermined length. It would probably be desirable to employ a relatively high temperature, high flow gas jet in the absence of relative jet/fiber movement. Example 1 - Removing Coating from Single Fiber The coated optical fiber employed in this example was a conventional silica-based single-mode optical fiber having an outside diameter of 125 µm. The optical fiber was provided with a urethane acyrlate coating having an outside diameter of 250 µm. A Convectronics Model 001-10002 tube heater was employed. The diameter of the outlet end of the nozzle was 1.76 mm. A thermocouple was temporarily located within the tube heater near the outlet end to calibrate the temperature of the inert gas exiting the tube as a function of voltage applied to the heating element. For the present example, an ac voltage of 126.7 volts was applied to the heater tube; this resulted in a measured gas temperature of about 820°C. The distance between the outlet end of tube heater 20 and the coated fiber during the fiber stripping operation was about 2.86 mm. Nitrogen flowed at a rate of 20.9 standard liters per minute into inlet pipe 23. The flow of nitrogen was initiated, and voltage was supplied to the heater tube. Following the procedure outlined in conjunction with Fig. 3, the coated optical fiber was fed from the delivery tube 42 at a rate of 1.75 cm/sec. Nozzle 43 was located about 2.5 mm below the bottom tip of the delivery tube. Coating material was softened and blown from the end of the fiber as it moved past the hot nitrogen jet emanating from the heater tube. After the coating was removed from a 25.4 mm length of the coated optical fiber, the jet was rotated so that it no longer impinged on the coating. The resultant stripped fiber was subjected to strength testing. The cumulative Weibull probability plot for the fiber is shown in Fig. 5. This fiber was stronger than manually stripped fiber which needed to be wiped with alcohol to remove from the stripped fiber particulates that remained from the stripping operation. A Weibull plot of manually stripped and wiped fiber is shown in Fig. 6. It is noted that manual stripping is the conventionally employed technique for removing coating material from optical fibers to prepare them for use in the overclad coupler manufacturing process. For comparison purposes, a Weibull plot for an unstripped (coated) optical fiber is shown in Fig. 7. The strength of a fiber stripped in accordance with this invention (Fig. 5) is much closer to the strength of an unstripped fiber (Fig. 7) than a manually stripped fiber (Fig. 6). Optical fiber stripped by the method of Example 1 can be employed to make a fiber optic coupler without any further cleaning step. When making overclad couplers, for example, a portion of the coating is removed from each of a plurality of optical fibers, and the uncoated or stripped portions of the fibers are inserted into the longitudinal bore of a glass tube such that the ends of each fiber extend beyond one or both of the ends of the tube. The midregion of the resultant coupler preform is heated to collapse it around the fibers, and the central portion of the midregion is stretched to elongate the fibers and reduce their diameter. Stretching is discontinued when the desired coupling has been achieved. If any coating material in the form of smears or particles remains on the stripped fiber when it is inserted into the glass tube, the coating material will burn or decompose during the tube collapse and stretch steps. Gas generated by this burning process forms seeds that cause unacceptable levels of excess loss in the resultant couplers. Couplers made from fibers stripped in accordance with this invention exhibit low loss and no seeds. The method of this invention is not limited to those parameters which have been set forth in Example 1. There is a wide range of settings for gas flow, gas temperature, and relative movement between heater tube and optical fiber or fibers. The coating has been removed from an optical fiber with reasonable effectiveness at gas temperatures as low as about 550°C. If such lower gas temperatures are used, the relative movement between heater tube and optical fiber must decrease in order to achieve sufficient heating to soften the coating material. Alternatively, an elongated nozzle could be employed as shown in Fig. 4. However, gas flow must be sufficient to blow the softened coating material from the fiber. Fig. 8 illustrates the manner in which the coating can be removed from the optical fibers of a ribbon cable 65. Gas (arrow 68), which is fed to heater 66, issues from nozzle 67 as a stream 69 that encompasses the entire width of ribbon cable 65. Since nozzle 67 is rectangularly-shaped, the shape of stream 69 in a plane perpendicular to the direction of stream flow is oblong. As in Fig. 1, heater 66 (in a position lower than that illustrated) can move in the direction of arrow 70 until the hot inert gas stream 69 is directed onto cable 65 at region a. Heater 66 moves along cable 65 in the direction of arrow 72. After coating material has been removed between points a and b along cable 65, heater 66 retracts from cable 65 in the direction of arrow 71 until hot gas is no longer directed at the cable. The exposed optical fibers 64 are sufficiently clean that they can be subjected to further process steps without further treatment. Whereas Fig. 8 shows the removal of coating from a middle region of cable 65, the apparatus of Fig. 8 could also be employed for removing coating from the end of a ribbon cable. As shown in Fig. 9 heater tubes 75 can be used to direct a plurality of jets of hot gas onto a cable 76. This embodiment would be useful for removing coating from a ribbon cable the width of which is such that a large amount of heat would be required to soften its coating. The method of the present invention can also be employed to strip the coating from the end or a middle region of a circularly symmetric fiber optic cable such as that shown in Fig. 10. Cable 79 includes a tension member 80 surrounded by plastic coating material 81, a plurality of optical fibers 82 being equally spaced around member 80. Such cables are conventionally formed of a plurality of layers of coating material. Only a single mass 81 of coating material is illustrated for the sake of simplicity. One or more hot gas jets can be directed onto cable 79 to completely remove coating material from an end or middle region therefrom even though such material is located between closely spaced fibers as well as between the tension member and the fibers. Fig. 11 shows that two or more heater tubes 85 can be directed radially inwardly toward the longitudinal axis of optical transmission means 86 which may comprise a single coated optical fiber or a cable containing a plurality of fibers. Fig. 12 illustrates a path 90 that could be followed by a hot inert gas jet 88 as it removes coating material from an optical transmission means 89 which may comprise a coated fiber or a multi-fiber cable. In this embodiment the stream reciprocatingly moves across the cable while there is relative axial movement between the cable and the stream. The path of jet movement is as follows: (a) the jet is directed to one side of cable 89, (b) the jet is scanned across the cable, (c) the jet is moved along the cable in the direction of arrow 91, (d) the jet is scanned across the cable, and (e) steps (c) and (d) are repeated until the desired length of coating material is removed. The resultant path is represented by dashed line 90. Fig. 13 illustrates that coating material 93 can be simultaneously removed from a plurality of individual optical fibers 94 by directing nozzle 95 so that hot inert gas flows toward region a of the fibers. The nozzle is then moved toward region b (the fiber ends in this figure). This technique can be used to prepare a plurality of optical fibers for splicing or for use in a coupler. The coating material could also be removed from a central region of each of a plurality of fibers by this technique. Instead of arranging the fibers in a linear array as shown in Fig. 13, they could be grouped in a circular array or some other nonlinear arrangement.

Claims: A method for removing coating material from a longitudinal portion of a fiber optical cable (10; 40; 55, 56; 65; 79; 89; 94) including at least one glass optical fiber (11, 50, 56, 64, 82, 94) surrounded by protective coating material (12, 46, 55, 81, 93), said method comprising directing a stream of hot gas (24, 44, 59, 69, 88) onto that portion of said coating material that is to be removed, the composition of said gas being such that it does not react with said coating material, characterized in that said hot gas stream is at a temperature of at least about 550°C to soften the coating material and of sufficient velocity to remove the entire thickness of said softened coating material from said portion of said fiber optic cable (10; 40; 55, 56; 65; 79; 89; 94) whereby the outer surface of said glass fiber is exposed. A method in accordance with claim 1, wherein said method further comprises the step of providing relative movement between said stream (24, 44, 59, 69, 88) and said cable (10; 40; 55, 56; 65; 89). A method in accordance with claim 2, wherein the step of providing relative movement comprises moving said stream (24) along the axis of said cable (10) and in a direction that is lateral to the axis of said cable. A method in accordance with claim 2, wherein the step of providing relative movement comprises delivering said cable (40) from a tube (42) and positioning said stream (44) such that it impinges upon said cable as it emanates from said tube. A method in accordance with claim 2, wherein the step of providing relative movement comprises maintaining said stream stationary and moving said cable with respect to said stream. A method in accordance with claim 1, further comprising the step of supporting said cable (10, 40) in such a manner that it resists vibrations caused by said stream (24, 44) impinging thereon. A method in accordance with claim 6, wherein said coating is to be removed from a predetermined region of said cable (10) and said cable is fixedly supported at opposite sides of predetermined region. A method in accordance with claim 1, wherein the step of directing a stream comprises directing a stream having a circularly symmetrical cross-section. A method in accordance with claim 1, wherein the step of directing a stream comprises directing a stream (59, 69) having an oblong cross-section in a plane perpendicular to the direction of stream flow. A method in accordance with claim 1, wherein the step of directing a stream comprises directing a plurality of jets of said hot gas onto said cable (76, 86). A method in accordance with claim 1, wherein the step of directing a stream comprises directing a plurality of jets of said hot gas onto said cable (86) from different directions. A method in accordance with claim 1, wherein said stream (88) reciprocatingly moves across said cable (89) while there is relative axial movement between said cable and said stream. A method in accordance with claim 1, wherein said cable (65, 79) includes a plurality of optical fibers (64, 82). A method in accordance with claim 1, wherein said coating material (93) is simultaneously removed from a plurality of individual optical fibers (94) disposed in side-by-side relationship. Apparatus for removing a predetermined length of coating material from a fiber optic cable (10; 40; 55, 56; 65; 79; 89; 94) including at least one glass optical fiber (11, 50, 56, 64, 82, 94) surrounded by protective coating material (12, 45, 55, 81, 93), said apparatus comprising a source of heated inert gas (16-24) the composition of which is such that it does not react with said coating material, and means (20, 25; 45, 53; 58, 57; 66, 67; 75; 85; 95) for directing a stream of said heated gas onto said predetermined length of cable, characterized in that the stream directing means is operable to direct a hot gas stream onto said predetermined length of cable at a temperature of at least about 550°C to soften the coating material and of sufficient velocity to remove the entire thickness of said softened coating material from said portion of fiber optic cable, whereby the outer surface of said glass fiber is exposed. Apparatus in accordance with claim 15, further comprising a device (14, 15; 42) for inhibiting the vibration of said cable. Apparatus in accordance with claim 15, wherein said device for inhibiting comprises a first clamp (14) affixed to said cable and a second clamp (15) affixed to said cable and spaced from said first clamp. Apparatus in accordance with claim 16, wherein said device for inhibiting comprises a tube (42) having an outlet end adjacent said stream, and means for delivering said cable from said tube. Apparatus in accordance with claim 15, further comprising means for moving said gas stream (24, 44, 59, 69, 88) with respect to said cable (10; 40; 55, 56; 65; 89). Apparatus in accordance with claim 15, further comprising means for moving said gas stream (24) along the longitudinal axis of said cable (10) and means for moving said stream laterally with respect to said cable. Apparatus in accordance with claim 15, wherein said means for directing a stream comprises a heater tube (20, 45, 58, 66, 75, 85) having a nozzle (25, 43, 57, 67, 95) from which said stream emanates. Apparatus in accordance with claim 15, wherein said cable (10; 40; 55, 56) is a single optical fiber (11; 50; 56) having a layer of coating material (12, 45, 55) thereon. Apparatus in accordance with claim 15, wherein said cable (65, 79) includes a plurality of optical fibers (64, 82). Apparatus in accordance with claim 17, wherein said optical fibers are disposed in a planar array.

Inventors: MILLER WILLIAM JAMES, MILLER, WILLIAM JAMES
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 1995
Main classification: G02B6
IPC codes: G02B6, H02G1