METHOD AND APPARATUS FOR FABRICATING AN EXPANDED BEAM CYLINDRICALLY TERMINATED OPTICAL FIBER TAPER

Abstract: An apparatus and method are disclosed for fabricating an expanded beam cylindrically terminated optical fiber taper (36-38) from a reduced diameter preform rod (15). The present invention comprises mounting the reduced diameter preform rod in a first moving arrangement (11, 13, 16, 18, 19, 23) and a second moving arrangement (12, 14, 17, 25-29) and uniformly heating the circumference of a predetermined portion at an intermediate point of the preform rod with a heating arrangement (81) to a predetermined temperature, and then uniformly pulling the preform rod while continuing to heat additional material of the preform rod to produce a pair of expanded beam cylindrically terminated optical fiber tapers (37) connected by a predetermined length cylindrical central section of standard optical fiber (38) having a uniform diameter less than that of the original preform rod. The cylindrical central section is then severed midway between the two expanded beam optical fiber tapers to form two separate expanded beam cylindrically terminated optical fiber tapers comprised of (1) a predetermined length cylindrical end section of uniform diameter (36) equal to that of the original preform rod; (2) a reduced diameter optical fiber end section (38) suitable for splicing onto an optical fiber of similar dimensions; and (3) a tapered section (37), connecting the cylindrical end section to the optical fiber end section, having a gradually decreasing diameter that will avoid mode conversion of a lightwave signal propagating therethrough.

Description: Technical Field The present invention relates to a method and apparatus for fabricating optical fiber tapers and, more particularly, an optical fiber taper having expansion optics comprising a long uniform diameter section of a standard optical fiber gradually expanding through a tapered section into a cylindrical section of enlarged dimensions. Description of the Prior Art Optical fiber technology is a rapidly developing and maturing technology. Research in optical fiber technology has provided high quality optical fiber with improved transmission characteristics. This, in turn, has allowed the use of optical fiber in voice and data communication systems to continue to expand and the number of commercial applications of fiber optics to increase. Currently, optical fiber technology is being implemented in local area networks, long distance terrestrial systems and even longer distant submarine systems. As the area serviced by these communication systems expands the number of fiber connections also increases thus creating the need for improved fiber connectors and couplers having low loss and minimal distortion. Presently there are two categories of fiber to fiber connections: fiber splicing and demountable connectors. Fiber splicing involves forming a permanent joint between two individual optical fibers using fusion or mechanical techniques. Demountable fiber connectors allow for the connection and disconnection, with reproducible accurate fiber alignment, of two individual optical fibers. The types of demountable fiber connectors currently in use include the ferrule, biconical, ceramic capillary, double eccentric, and expanded beam connector. An expanded beam connector optically collimates the light beam on the launch side of a connector pair and then refocuses the beam into the receive fiber. Expanded beam optics have also been used to couple fibers to light sources and detectors. Connections and couplers that use expanded beam optics, compared to other techniques, make the achievement of lateral and axial alignment much less critical and the increased beam diameter reduces the effect of dust and dirt at the connection. Approaches used to implement expanded beam connectors and couplers have included forming spherical microlenses, glass bead lenses, and tapers on the end of a fiber. Optical fibers having a tapered expanded beam structure, in addition to the axial and lateral alignment improvements, also provide an angular alignment that is less stringent than that of lens structures. Various apparatus and methods for manufacturing optical fiber components are described in the prior art. U.S. Patent 4,565,558 issued to Keil et al. on January 21, 1986 and U.S. Patent 4,589,897 issued to Mathyssek et al. on May 20 1986, (similar to U.S. Patent 4,565,558) describe apparatus for making constrictions in or tapered ends with lenses in optical fibers. The apparatus comprises first and second holding means with parts in longitudinal alignment for holding opposite ends of an optical fiber. Heating means is provided for uniformly heating a circumference of a predetermined intermediate point of the fiber to above a predetermined temperature, and a single moving means is provided for moving the first holding means along its alignment axis at a predetermined uniform rate to uniformly pull the fiber longitudinally when the predetermined temperature is attained. A practical and novel expanded beam optical fiber taper is shown in Figure 1(c) in the article by N. Amitay et al. "Optical Fiber Tapers - A Novel Approach to Self-Aligned Beam Expansion and Single-Mode Hardware," J. Lightwave Technol Vol. LT-5, No. 1, January 1987, at pages 70-76. The disclosed taper has a structure comprising a fiber pigtail and a tapered section followed by a cylindrical section of enlarged dimensions such that, for example, when a pair of these tapers are butted together to connect two fibers there is no mode conversion or coupling therethrough and the effects of lateral and axial misalignment are minimized. The problem remaining in the prior art is to provide an accurate, efficient, reproducible and inexpensive method and apparatus for fabricating the expanded beam optical fiber taper shown in figure 1(c) in the N. Amitay et al. article. According to the present invention there is provided a method as defined in claim 1, and an apparatus as defined in claim 6. Brief Description of the Drawing FIG. 1 is a perspective view of a preferred embodiment of an apparatus (10) for practicing a method in accordance with the present invention; FIG. 2 illustrates an expanded beam optical fiber taper including a gradually expanded cylindrical end fabricated in accordance with the present invention; FIG. 3 illustrates a reduced diameter optical preform rod including a core and cladding surrounding the core; FIG. 4 illustrates a final workpiece, fabricated using the apparatus 10 in FIG. 1, comprising a pair of expanded beam cylindrically terminated optical fiber tapers of FIG. 2 connected by a length of standard diameter optical fiber; FIG. 5 is a perspective view of apparatus 10 in FIG. 1 after heating and pulling the preform rod, but prior to removing the final workpiece; and FIG. 6 is a plot relating the radius to the length, of an exemplary expanded beam optical fiber taper, as shown in FIG. 2, from the standard single-mode optical fiber pigtail end section to the cylindrical end section. Detailed Description FIG. 1 shows a preferred embodiment of an apparatus 10 for fabricating an expanded beam cylindrically terminated optical fiber taper 35, as shown in FIG. 2, in accordance with the present invention. Apparatus 10 shown in FIG. 1 includes a first 11 and a second 12 holding means rotatably mounted on a first 13 and a second 14 movable carriage, respectively, for holding respective first and second opposite ends of a reduced diameter optical preform rod 15 from which the expanded beam cylindrically terminated optical fiber taper 35 will be made. The fabricated expanded beam cylindrically terminated optical fiber taper 35, in FIG. 2, comprises a cylindrical end section 36 corresponding in diameter to the diameter of the original reduced diameter optical preform rod 15, a standard diameter optical fiber pigtail end section 38 suitable for splicing to an optical fiber of similar dimensions, and a down-tapered section 37 connecting cylindrical end section 36 and optical fiber pigtail end section 38 that will avoid mode conversion of a lightwave signal propagating therein. As shown in FIG. 3, reduced diameter optical perform rod 15 comprises a core 41 surrounded by a cladding layer 42 with a first and a second uniform index of refraction, respectively, throughout. The reduced diameter optical preform rod 15 also includes an external diameter, equal to the desired diameter of cylindrical end section 36 of the expanded beam cylindrically terminated optical fiber taper 35 shown in FIG. 2, which is greater than the diameter of a standard optical fiber, e.g., 125 micrometers for a single-mode fiber, but is less than the diameter of a standard optical preform rod, e.g., 2.5 centimeters. Reduced diameter optical preform rod 15 can be derived by any suitable technique such as, for example, reducing the diameter of a standard diameter optical preform rod using a standard pulling tower. FIG. 1 shows each holding means 11 and 12 as exemplary chucks similar to those commonly used in a drill press or lathe to hold a tool or workpiece. First and second holding means 11 and 12 are coupled to a means capable of selectively rotating the reduced diameter optical preform rod 15 at a uniform rate and insuring that no torsion is applied to the reduced diameter optical preform rod 15 as it rotates. For example, in FIG. 1, the means shown for rotating the reduced diameter optical preform rod 15 comprises separate synchronous motors 16 and 17 coupled to holding means 11 and 12, respectively. It Is to be understood, however, that synchronous motors 16 and 17 are only for purposes of illustration and not for purposes of limitation and that any suitable arrangement for synchronously rotating the first 11 and second 12 holding means can be used, such as, for example, a single motor directly coupled to both holding means 11 and 12. First movable carriage 13 engages a drive on a first frame 18, for selectively moving first carriage 13 along the length of first frame 18 parallel to a longitudinal axis of the reduced diameter optical preform rod 15 at a first predetermined uniform rate. More particularly FIG. 1 shows first carriage 13 as including a plate 19 with a T-shaped extension 20 that slides within a corresponding shaped groove 21 extending the length of first frame 18 to guide first carriage 13 as it moves along first frame 18. First frame 18 is positioned to permit the movement of first carriage 13, as it is guided by the T-shaped extension 20 and groove 21 arrangement, parallel to the longitudinal axis of the reduced diameter optical preform rod 15. The drive on first frame 18 is shown as a motor 22 which rotates a threaded rod 23 that meshes with a threaded sleeve (not shown) in the T-shaped extension 20 of the guiding arrangement of first carriage 13, to move first carriage 13 at the first predetermined uniform rate along first frame 18 in either a first or second opposite direction depending on the rotation of motor 22 and threaded rod 23. A selectively activated clutch 24 is used to couple motor 22 to threaded rod 23 and provide a means for insuring that a uniform torque is applied to, and removed from, threaded rod 23. Second carriage 14 also engages a drive on a second frame 26, for selectively moving second carriage 14 along the length of second frame 26 parallel to the longitudinal axis of the reduced diameter optical preform rod 15 at a second predetermined uniform rate. A guide arrangement 27 for second carriage 14, including a T-shaped extension (not shown) and a corresponding groove 25 similar to the T-shaped extension 20 and groove 21 used for guiding first carriage 13, guides second carriage 14 along second frame 26. Second frame 26 is positioned to permit the movement of second carriage 14 parallel to the longitudinal axis of the reduced diameter optical preform rod 15. The drive on second frame 26 is shown as a motor 28 which rotates a threaded rod 29 that meshes with a threaded sleeve (not shown) in the T-shaped extension (not shown) of the guide arrangement 27 of second carriage 14 to move the second carriage 14 at the second predetermined uniform rate along the second frame 26 in either a first or second opposite direction depending on the rotation of motor 28 and threaded rod 29. It is to be understood that both the drives and the means for guiding carriages 13 and 14 on first and second frames 18 and 26, respectively, can comprise any suitable arrangement for providing the functions described. A heat source 31, shown in FIG. 1 as an exemplary oxyhydrogen torch, is positioned by a support 32 adjacent to, and directed at, a predetermined portion 33 at an intermediate point of reduced diameter optical preform rod 15 where the expanded beam cylindrically terminated optical fiber tapers 35 are to be formed. To concurrently produce two expanded beam cylindrically terminated optical fiber tapers 35 from each reduced diameter optical preform rod 15, heat source 31 should provide a unform heat around a circumference of predetermined portion 33 of reduced diameter optical preform rod 15. It should be noted that a flame from an oxyhydrogen torch will not uniformly heat the circumference of a stationary reduced diameter optical preform rod 15. One technique for uniformly heating the reduced diameter optical preform rod 15 with the exemplary oxyhydrogen torch heat source 31, is to mount reduced diameter optical preform rod 15 within a shielding tube 34 in holding means 12 and then rotate both the reduced diameter optical preform rod 15 and shielding tube 34 using synchronous motors 16 and 17. Shielding tube 34 is preferably formed from a material, such as, for example, either silica or a ceramic, for providing both inductive heating to the reduced diameter optical preform rod 15 and a means for protecting reduced diameter optical preform rod 15 from being burned by the direct heat from the oxyhydrogen torch heat source 31. It is to be understood that the use of an oxyhydrogen torch for heat source 31 is for purposes of illustration and not for purposes of limitation, and that any other suitable heat source such as, for example, a CO₂ laser or an electric furnace could be substituted. When using a CO₂ laser as heat source 31, the reduced diameter optical preform rod 15 is rotated to provide uniform heating, but shielding tube 34 is not required since there is no danger of burning the reduced diameter optical preform rod 15. When an electric furnace is used for heat source 31, neither shielding tube 34 nor rotation of the reduced diameter optical preform rod 15 is necessary since the electric furnace provides uniform heating. A temperature sensing device 30, such as an exemplary optical pyrometer, is directed at predetermined portion 33 of reduced diameter optical preform rod 15 to detect when predetermined portion 33 has attained a predetermined temperature to permit the initiation of the operation of apparatus 10 as described herebelow and produce the pair of expanded beam cylindrically terminated optical fiber tapers 35. In operation, the first and second opposite ends of a reduced diameter optical preform rod 15 are mounted in the first and second holding means 11 and 12, respectively, within a shielding tube 34. With the heat source 31 deactivated, carriages 13 and 14 are moved along frames 18 and 26 using motors 22 and 28, respectively, to move the reduced diameter optical preform rod 15 to a predetermined position so that, when the heat source 31 is activated, a predetermined portion 33 at an intermediate point of reduced diameter optical preform rod 15 will be heated by heat source 31. Motor 28 on second frame 26 is deactivated and motor 22 on first frame 18 is energized while clutch 24 remains deactivated to decouple motor 22 from the threaded rod 23. Synchronous motors 16 and 17 are then energized to rotate both the reduced diameter optical preform rod 15 and shielding tube 34, after which the oxyhydrogen torch forming heat source 31 is ignited to begin uniformly heating a circumference of the predetermined portion 33 of reduced diameter optical preform rod 15. When temperature sensing device 30 detects that a predetermined temperature has been attained at the heated predetermined portion 33 of reduced diameter optical preform rod 15, clutch 24 on first frame 18 is activated to couple motor 22 to threaded rod 23 allowing motor 22 to uniformly rotate threaded rod 23 in a direction to cause first carriage 13 to move along the first frame 18 away from second carriage 14 at the first predetermined uniform rate. This causes the heated predetermined portion 33 to elongate along the longitudinal axis of reduced diameter optical preform rod 15 to start the forming of a pair of expanded beam cylindrically terminated optical fiber tapers 35 in accordance with the present invention. As first carriage 13 continues its uniform movement away from second carriage 14, a point will be reached when a desired standard optical fiber diameter is attained and the continued drawing of such optical fiber diameter into a predetermined length cylindrical central section 51 of standard diameter optical fiber, as shown in FIG. 4, between the two down-tapered sections 37 of a pair of expanded beam cylindrically terminated optical fiber tapers 35 is desired. To provide continued material from reduced diameter optical preform rod 15 for heating and drawing such predetermined length of standard diameter optical fiber for the cylindrical central section 51, motor 28 is energized to uniformly rotate threaded rod 29 in a direction to cause second carriage 14 to travel along second frame 26 in the direction of first carriage 13 at the second predetermined uniform rate. Movement of second carriage 14 towards first carriage 13 causes the section of reduced diameter optical preform rod 15 that is uniformly heated by heat source 31 to slowly move from the predetermined portion 33 towards the second carriage 14. The drawing process continues until a desired final workpiece 50, shown in FIG. 4, is formed comprising a pair of oppositely directed expanded beam cylindrically terminated optical fiber tapers 35 connected by a predetermined length cylindrical central section 51 of standard diameter optical fiber. When the predetermined length cylindrical central section 51 of standard diameter optical fiber is attained, the heat source 31 is extinguished and clutch 24 and motors 16, 17, 22 and 28 are deactivated. FIG. 5 shows apparatus 10 at the completion of the drawing process before the final workpiece 50 is removed from the pair of holding means 11 and 12. The final workpiece 50 is severed near the center of the standard diameter optical fiber cylindrical central section 51 to create a pair of expanded beam cylindrically terminated optical fiber tapers 35, shown in FIG. 2, each having a cylindrical end section 36 corresponding in diameter to the diameter of the original reduced diameter optical preform rod 15, a standard diameter optical fiber pigtail end section 38 suitable for splicing to an optical fiber of similar dimensions, and a down-tapered section 37 connecting cylindrical end section 36 and optical fiber pigtail end section 38 that will maintain a mode or modes propagating therein. For example, an optical fiber pigtail end section 38, suitable for splicing to an exemplary single-mode optical fiber, comprises (1) a core having a diameter of, for example, approximately 5-10 micrometers; (2) a cladding, surrounding the core, having an external diameter of, for example, 120-125 micrometers; and (3) a length of, for example, approximately 27.94 to 33 cms (11-13 inches). FIG. 6 shows a plot that details the longitudinal cross-sectional shape of an exemplary expanded beam cylindrically terminated optical fiber taper 35 fabricated using the present method and apparatus 10. The plot relates the radius to the length, of an exemplary expanded beam cylindrically terminated optical fiber taper 35 from the standard single-mode optical fiber pigtail end section 38 to the cylindrical end section 36. When, for example, a pair of expanded beam cylindrically terminated optical fiber tapers 35 are spliced onto a pair of single-mode optical fibers to be used as a demountable connection between the single-mode optical fibers, the exposed cylindrical end sections 36 can be butted together and aligned using any suitable fiber connector. Because of the enlarged core diameter in the expanded cylindrical end sections 36, the resulting single-mode connector has a high tolerance to misalignment and the present taper configuration also avoids mode conversion therethrough. In the forgoing discussion, it is to be understood that the above-described embodiment and method of operation are simply illustrative of an apparatus and a method for fabricating an expanded beam cylindrically terminated optical fiber taper 35. Other suitable variations and modifications could be made to the method described, to apparatus 10 in FIG. 1, or in the dimensions in FIG. 6 and still remain within the scope of the present invention. For example, an alternative to using movable carriages 13 and 14 to position heat source 31 at predetermined portion 33 of the reduced diameter optical preform rod 15 is to provide a means for moving support 32 to position heat source 31 along the length of the reduced diameter optical preform rod 15. Additionally, the above-described method of operating apparatus 10 can be modified to comprise, as an alternative to activating heat source 31 prior to activating clutch 24, first activating clutch 24 to apply uniform tension upon the reduced diameter optical preform rod 15 and then activating heat source 31 to start the formation of the pair of expanded beam cylindrically terminated optical fiber tapers 35.

Claims: A method of fabricating expanded beam optical fiber tapers (37) having a structure comprising (1) cylindrical end sections (36) equal in diameter to a reduced diameter optical preform rod said reduced diameter optical preform rod having an external diameter that is greater than the external diameter of a standard optical fiber but is less than the external diameter of a standard optical preform rod; (2) a predetermined length cylindrical central section (51) having a diameter smaller than that of the cylindrical end sections and corresponding to that of conventional diameter optical fiber, and (3) a separate taper (37) coupling each cylindrical end section with the cylindrical central section where each taper includes a configuration for preventing mode conversion in a lightwave signal propagating therethrough; the method comprising the steps of: (a) holding opposite ends of said reduced diameter optical preform rod (15), where the optical preform rod comprises a core of predetermined uniform diameter and a cladding, surrounding the core; (b) uniformly heating (31) a circumference of a predetermined portion (33) at an intermediate point of the optical preform rod to a predetermined temperature; (c) uniformly drawing the reduced diameter optical preform rod longitudinally at a first predetermined uniform rate when the predetermined temperature is attained at the intermediate point to form a longitudinal elongation containing the tapered configurations connected substantially together by a point of reduced diameter relative to that of the rod, and (d) after achieving the longitudinally elongation in step (c), causing the predetermined portion being heated and one of the tapers to be moved towards each other at a second predetermined uniform rate, concurrent with the uniform pulling of the reduced diameter optical preform rod in step (c), to provide material from the reduced diameter optical preform rod for forming the predetermined length cylindrical central section of the resultant structure. A method according to claim 1, wherein step (b) is performed by: (b1) predisposing the reduced diameter optical preform rod within a shielding tube (34) to protect the reduced diameter optical preform rod from being burned when heated; (b2) uniformly heating the reduced diameter optical perform rod with a flame; and (b3) concurrent with step (b2), uniformly rotating (via 16,17) the reduced diameter optical preform rod and the shielding tube, with no torsion being applied to the reduced diameter optical preform rod, to provide uniform heating about the circumference of the predetermined portion at the intermediate point of the reduced diameter optical preform rod. A method according to claim 1, wherein step (b) is performed by: (b1)' uniformly heating the reduced diameter optical preform rod with a CO₂ laser; and (b2)' concurrent with step (b1)', uniformly rotating the reduced diameter optical preform rod, with no torsion being applied to the reduced diameter optical preform rod, to provide uniform heating about the circumference of the predetermined portion at the intermediate point of the reduced diameter optical preform rod. A method according to claim 1, wherein step (b) is performed by uniformly heating the reduced diameter optical preform rod with an electric furnace. A method according to claim 1, wherein the method comprises the further step of: (e) severing the resultant structure at the cylindrical central section to form two separate expanded beam optical fiber tapers. Apparatus for carrying out the method of claim 1, in order to fabricate expanded beam optical tapers having a structure comprising: (1) cylindrical end sections (36) equal in diameter to a reduced diameter optical preform rod, said reduced diameter optical preform rod having an external diameter that is greater than the external diameter of a standard optical fiber but is less than the external diameter of a standard optical preform rod; (2) a predetermined length cylindrical central section (51) having a diameter smaller than that of the cylindrical end sections and corresponding to that of conventional diameter optical fiber, and (3) a separate taper (37) coupling each cylindrical end section with the cylindrical central section where each taper includes a configuration for preventing mode conversion in a lightwave signal propagating therethrough; the apparatus comprising: first and second holding means (11,12), each holding means including a longitudinal axis which is aligned with the longitudinal axis of the other holding means for holding opposite ends of the optical preform rod (15); heating means (31) for uniformly heating a circumference of a predetermined intermediate point of the optical preform rod to above a predetermined temperature; first moving means (13) for moving the first holding means along its longitudinal axis at a first predetermined uniform rate to uniformly pull the optical preform rod being held by the first and second holding means longitudinally when the predetermined temperature is attained; and second moving means (14) for moving the second holding means (12) and the heating means towards relative to each other in a direction along the longitudinal axis of the second holding means, at a second predetermined uniform rate, concurrent with the movement of the first moving means, when the longitudinal elongation has occurred to provide material from the reduced diameter optical preform rod extending the length of the cylindrical central section of the resultant structure to a second longer predetermined length. Apparatus according to claim 6, wherein the apparatus further comprises: means (32) for positioning and directing the heating means at the predetermined portion of the reduced diameter optical preform rod; and means (30) for sensing the temperature at the predetermined portion of the reduced diameter optical preform rod to determine when the predetermined temperature is attained. Apparatus according to claim 6, wherein the heating means comprises: a device (31) for emitting a flame; means (34) for protecting the reduced diameter optical preform rod from being burned by the flame; and means (16,17) for uniformly rotating the reduced diameter optical preform rod, with no torsion being applied to the reduced diameter optical preform rod, to provide uniform heating about the circumference of the predetermined portion at the intermediate point in the reduced diameter optical preform rod. Apparatus according to claim 6, wherein the heating means comprises: a CO₂ laser for heating the predetermined portion of the reduced diameter optical preform rod; and means (16,17) for uniformly rotating the reduced diameter optical preform rod, with no torsion being applied to the reduced diameter optical preform rod, to provide uniform heating about the circumference of the predetermined portion at the intermediate point of the reduced diameter optical preform rod. Apparatus according to claim 6, wherein the heating means comprises an electric furnace. Apparatus according to claim 6, wherein the first moving means comprises: a frame (18); a carriage (13), including the first holding means; and driving means (22,24) for moving the carriage along the frame at the first predetermined uniform rate, parallel to a longitudinal axis of the reduced diameter optical preform rod, away from the second holding means to pull the reduced diameter optical preform rod with the first holding means. Apparatus according to claim 6, wherein the second moving means comprises: a frame (26); and a carriage (14), including the second holding means; and driving means (28) for moving the carriage along the frame at the second predetermined uniform rate, parallel to a longitudinal axis of the reduced diameter optical preform rod, towards the first holding means to provide additional material of the reduced diameter optical preform rod, from an area of the reduced diameter optical preform rod adjacent to the predetermined portion of the reduced diameter optical preform rod in the direction of the end of the reduced diameter optical preform rod held by the second holding means, to the uniform heating means. Apparatus according to claim 11, CHARACTERISED IN THAT the second moving means comprises: a frame (26); a carriage (14), including the second holding means; and driving means (28) for moving the second carriage along the second frame at the second predetermined uniform rate, parallel to the longitudinal axis of the reduced diameter optical preform rod, towards the first holding means to provide additional material of the reduced diameter optical preform rod, from a area of the reduced diameter optical preform rod adjacent to the predetermined portion of the reduced diameter optical preform rod in the direction of the end of the reduced diameter optical preform rod held by the second holding means, to the uniform heating means.

Inventors: PRESBY, HERMAN MELVIN
Applicants: AMERICAN TELEPHONE AND TELEGRAPH COMPANY
Application year: 1988
Main classification: C03B37
IPC codes: G02B6, C03B23, C03B37, C03B20