METHOD FOR PRODUCING OPTICAL FIBER ACCESS COUPLERS AND PRODUCT PRODUCED THEREBY

Abstract: An improved access coupler is produced from at least two optic fibers (22) in which the fibers are concurrently elongated under tension by a slide (14) moveable with respect to a fixed mount (12) and fused together by heat applied thereto along a combined length thereof by a heating coil (24). Improved coupling is obtained by a gradual taper of the fibers by moving the heat produced by the coil with respect to the fibers at a rate which is a percentage of the rate at which the fibers are elongated. A ratio of approximately four to one, in which the fibers are elongated about four times the movement of the fusing coil, has been found to produce the lowest loss access couplers.

Description: METHOD FOR PRODUCING OPTICAL FIBER ACCESS COUPLERS AND PRODUCT PRODUCED THEREBY 1 BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to fiber optic access couplers and, in particular, an improved coupler 5 structure and an improved method for producing such structure. 2. Description of the Prior Art and Background Considerations In U.S. Patent No. *.,291,9 2 10, an access coupler 10 and its method of production is described as comprising a pair of multi-mode optical fibers having a biconical taper section which are twisted around one another and fused together along a predetermined length to provide optical coupling between the fibers. Such an access 15 coupler is produced by applying heat at a stationary point in space to the twisted fibers and moving both ends of the fibers outwardly from the point of fusion, so that the fibers taper down from their ends towards their fused centers. 20 In this method, softened glass from either or both ends contributes to the tapers in unknown and uncontrollable proportions. In particular, It Is believed that the biconical tapers produced are steeper than what is desirable and, therefore, cause concentration f 1 of stress, prevent low tap ratios (e.g., 2% ) from being effectively produced, etc., a tap ratio being the measure¬ ment of percentages of the light transmitted to the fibers exiting from the coupler. Speci ically, it is 5 believed that the rate at which a fiber tapers or necks-down due to the combined elongation as heat is applied should be less than 505, the rate of taper being defined as the difference in diameters of the fiber as it exists prior to and after elongation, divided by the 10 . length of the taper between the unaf ected and elongated portions of the fiber. A quantative rate of taper is only approximate since the beginning and end points of the taper are not exactly determinable. Nevertheless, certain conclusions can be drawn therefrom in terms of 15 stress concentration in the flexibility of a fiber and the ability to produce different tap ratios. If the taper is too short and, therefore, too steep, a sudden change in the diameter of the fiber, or rate of taper, will result in a concentration of stresses 20 in the fused fiber. If the stress concentration is too great, the fused joint may facture. On the other hand, a small cross-section of a fiber is more flexible and, therefore, can relieve bending moments exerted on the cross-section. In addition, the length of the fused 25 portion and the rate of taper also affect the ability to obtain a desired tap ratio. In general, typical fused lengths range from a minimum of 0.010 inch to approxi¬ mately 0.100 inches. For a low tap ratio, e.g., 2%, a smaller fusion length is required, while for a larger 30 tap ratio, e.g., towards 50£, a larger fused length is used. Utilizing conventional fusion methods, a small tap ratio of 2% is provided when the fused length is about 0.005 Inches; however, while a short length may suf- 35 iciently ' couple the fibers to permit efficient transfer SUBSTITUTE SHEET of light (that Is, the losses are sufficiently low), the joint does not have enough physical strength to maintain the bond. On the other hand, if the fused length were extended to about 0.010 inch, the tap ratio would become considerably larger than 2% . Thus, since conventional fusion methods cannot resolve these problems, low tap ratios and reduced stress concentrations have not been obtainable. SUMMARY OF ' THE INVENTION The present invention avoids and overcomes these and other problems by better control of the taper and the rate of taper. It has been discovered that, if the heat, with respect to the combined fibers, were moved at a percentage of the movement by which the fibers * are elongated, the rate of taper could be made more gradual and less steep than heretofore obtained. Such a differential rate may be obtained In at least two illustrative ways. In a first method, one end of the bundle of fibers is held stationary. Then, while heat is applied to soften the fibers, the heat is moved with respect to the fibers in the same direction as the other end of the fibers is pulled, with the movement of the heat being at a percent of the pullin -exerted upon the fibers. In a second method, the heat may be held stationary while both ends of the bundle are moved away from each other, with one end being moved at a rate which is greater than that movement applied to the other end of the bundle. The preferred ratio of fiber elongation to relative movement between the heat and the fibers has been found to be approximately a 4 to 1 ratio. Ratios greater or lesser than approximately . to 1 have not produced the lowest loss in such couplers. sUDSTETUT SHEET Several advantages are derived therefrom. Better control of the steepness of the taper and the length of the fusion Is obtained. Small tap ratios with minimum stress concentrations are provided. Coupling losses are minimized. Other aims and advantages, as well as a more complete understanding of- the present invention, will •appear from the following explanation of exemplary embodiments and the accompanying drawings thereof. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates an optical fiber having a taper therein resulting from heating and elongation thereof; FIG. 2 is a first embodiment of a means for carrying out the Invention to form an access coupler of two or more fibers, with each fiber having a taper such as that illustrated in FIG. 1; and FIG. 3 Is a second embodiment of apparatus for carrying out the invention. DETAILED DESCRIPTION OF THE INVENTION As described above, the improved access coupler of the present invention may be described in terms of the rate of taper of each fiber fused together to form the coupler. It is to be understood, however, that such a discussion relating to a rate of taper Is based upon what is presently thought to be the reasons for the improved access coupler of the present invention. Further investigation, therefore, may show that this explanation Is In error or should be modified. With reference to FIG. 1, the rate of taper of an individual fiber may be defined according to the following equation: = the rate of taper, where SUBSTITUTE SHEET Φi is the diameter of the original fiber unaffected by elongation, . is the smaller fiber diameter which is reduced as a result of elongation, and L is the length of the taper between the terminations of the original and reduced fiber diameters. Such dimensions are illustrated in FIG. 1. Since the taper is not perfectly uniform and Its commencement and ending are not perfectly defined, any measurements in the length L of the taper, In particular, are only approximate. However, it Is believed that a rate of taper in excess of 0? Is too steep. Alternatively, rates of taper of less than 5 % contribute to the improved couplers of the present invention. FIG. 2 diagrammatically illustrates an actual laboratory setup which was used to obtain the improved access couplers of the present Invention. The entire apparatus was mounted on a fixed plate 10. Immovably secured to plate 10 Is a fixed mount 12. Spaced from mount 12 Is a slide 14 which is reciprocally received within a guide 16, in turn affixed to plate 10. Thus, < slide 14 has limited reciprocal motion towards and away from mount 12. Secured to mount 12 and slide 14 are a pair of rotatable spools 18 and 20 which are intended to support a plurality of optical fibers 22. The fibers are threaded through spool 18, extended through a coil 24 which acts as a source of heat, and finally threaded through spool 20. Initially, fibers 22 are generally parallel to one another as originally threaded through spools 18 and 20 and extended through coll 24. As is conventional in the production of access couplers, it is desired that the fibers be twisted to a desired extent. To obtain the preferred twisting, one or both spools 18 and 20 are twisted in opposite directions from one another, after which a pair of clamps 26 and 28, respectively on mount 12 and slide 14, are clamped about the fibers to maintain them in their prescribed twisted condition. The precise distance between slide 14 and mount 12 is preset by a micrometer screw 30, which is secured to guide 16 and contacts a stop 32 on slide 14. A second micrometer screw 3 is mounted on plate 10 and is adjusted to provide a distance from its tip 35 to stop 32 to set the length at which fibers 22 are to be elongated when fused together. A damper 36, secured to plate 10, Is attached to slide 14 by a connection 38 to ensure that, when slide 14 Initially moves, there is no sudden pull exerted upon the optical fibers which might otherwise fracture or separate them. Coil 24 is mounted on and supported by a second slide 40, which is received within rails or other guiding means which, in turn, are secured to plate 10. Slide 40 Is disposed to reciprocate parallelly and in the same direction with slide 14 so that coil 24 will move in parallel with fibers 22. As shown, slides 14 and 40 are 'mechanically interconnected by a pulley arrangement so that they will move In unison but at different rates. To accomplish this, a wire 42 and pulleys interconnect the slides. Wire 42 is secured to slide 14 at attachment 44 and extends about several pulleys 48, 50 and 52 to be attached at a tie-down 46 directly to plate 10. The wire extends about stationary pulleys.48 and 50, all of which are rotatably secured on plate 10. Another two pulleys 52 are rotatably secured to slide 40. Because of the mechanical arrange¬ ment of fixed pulleys 50 and moveable pulleys 52, there is a 4 to 1 mechanical advantage between slides 14 and 40. Specifically, slide 14 is capable of moving four times as fast as slide 40; therefore, optical fiber's 22 SUEΞTETUTE SHEET will be pulled by slide 14 four times as fast as coil 24 moves with respect to the fibers. The movement of the respective slides are shown by the arrows thereon denoted by indicia M-^ and M2 in the relationship of M]_ = M2- A spring is secured at its ends 5 to slide 40 and plate 10 to exert a common tensile force directly upon slide 40 and Indirectly through the pulley and wire arrangement upon slide 14. Thus, the same force acts upon both slides to move them. While the preferred ratio or rate of movement between slides 14 and 40 is 4 to 1, which has proven to be most advantageous in producing the lowest loss couplers over other experimental ratios of 2 to 1, 3 to 1, and 5 to 1, it is recognized that other arrange- ments of pulleys, in particular, different numbers of pulleys 50 and 52, will provide different ratios. For example, a third fixed pulley 50 may be utilized so that fixed point 46 of wire 42 will be on slide 40, rather than to plate 10, to vary the ratio, if that is what is desired. It Is further possible to adjust the ratio or rate of relative movement between the coil and the optical fibers and the lengthening or stretching of the fibers by use of the modification illustrated in FIG. 3- This modification has been devised in part to prevent absolute movement of coil 24 because wires attached thereto are of large gauge and may cause undesired drag upon the slide, such as on slide 40 of FIG. 2. Therefore, In FIG. 3, one end of optical fiber bundle 22 is affixed to a moveable plate 60 by a fixture 62. Plate 60 Is adapted to move linearly In the direction of arrow 2- Reciprocally received on plate 60 Is a slide 64 which moves according to the arrow denoted by indicium Mj. Optical fibers 22 secured to their respective plate and slide by spools 18 and 20 which may be the same as those depicted in FIG. 2. Respective slidable motions of plate 60 and slide 64 are effected by Individual mechanisms 66 and 68, which comprise conventional motors and gearing. Slide actuating mechanisms 66 and 68 are adapted, either through gearing or variable stepping motors, to cause plate 60 to slide in the direction of arrow M2 at a rate which Is different from that of slide 64, moving in the direction of arrow Mι_. According to the principles of the Invention, M]_ moves faster than M2 to produce the same relative elongation of fibers 22 and movement of fibers 22 with respect to coil 24. Thus, the embodiment of FIG. 3 produces the same relative movements as that provided by the mechanism depicted in FIG. 2. In the embodiment of FIG. 3_ however, it is easier to adjust the relative ratios of M to M2 in order to provide the most optimum relative movements, for example, having such ratios as 4.2 to 1 and 3-7 to 1, If these are found to be even more advantageous in producing even lower loss couplers. n. order to detect the amount of heat generated by coll 24, a heat sensor 70, see FIG. 2, is positioned adjacent the coil. Although the invention has been described with reference to particular embodiments thereof, it should be realized that various changes and modifications may be made therein without departing from the spirit and scope of the invention. SUBSTITUTE SH ET

Claims: CLAIMS What Is Claimed is: 1. In an access coupler having two or more optic fibers fused together at a length thereof, with tapered sections bounding said fused length, the improvement in which said tapered sections have a rate of taper of less than 0.50.. 2. The improvement according to Claim 1 In which said fused length Is in excess of 0.005 inches and said fused optic fibers have a tap ratio of less than 25?. 3. In an access coupler having two or more optic fibers fused together at a length thereof with tapered sections bounding said fused length, the Improvement in which said tapered sections have a rate of taper defined by the equation . Φ...i- ""■—Φ=?., where Φ and Φ are the L diameters of each fiber respectively prior to and followin one of said tapered sections and L is the length of said one tapered section. 4. The improvement according to Claims 1 or 3 in which said fused lengths Include said fibers being twisted about one another. 5. In a method for producing an access coupler from at least two optic fibers in which the fibers are concurrently elongated under tension and fused together by heat applied thereto along a combined length thereof, the Improvement comprising the step of moving the heat with respect to the fibers at a rate which is different from the rate at which the fibers are elongated. 6. The improvement according to Claim 5 further comprising the steps of securing the combined fiber length at its respective ends, positioning the heat intermediate the secured ends, and placing the tension on only a first of the ends while holding the second of the ends stationary, said moving step comprising the step of moving the heat along the fiber length towards the first end. 7- The improvement according to Claim 5 further comprising the steps of securing the combined fiber length at Its respective ends and positioning the heat intermediate the secured ends, said moving step comprising the step of moving the ends away from each other at the differential rate. 8. The improvement according to Claims 5 » 6 or 7 in which the differential rate is about 25J > so that the elongation causes the fibers at one of their ends to move about four times faster than the heat moves with respect .to the fibers. 9. In a method for producing an access coupler from at least two optic fibers in which the fibers are concurrently elongated and bonded together by heat to form a fused length bounded by tapers, the Improvement wherein movement between the heat and the respective tapers is at a ratio greater than one to one. 10. The improvement according to Claim 9 further comprising the step of setting the ratio at approximately four to one. 4 - - i E OMPI

Inventors: CLARK KENNETH M, CLARK, KENNETH, M.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1983
Main classification: C03B37
IPC codes: C03B37, G02B6