Threaded Front End Connector for Optical Fiber and Method of Making Same

The invention pertains to a connector for an optical fiber. A front end of the connector is a precision alignment surface constructed for alignment within a complimentary shaped cavity of an alignment sleeve or bushing to align the optical fiber along the alignment sleeve or bushing.

A known connector for an optical fiber is disclosed in U.S. Pat. No. 4,512,630. This known connector includes an alignment ferrule for encircling an optical fiber. The alignment ferrule has a unitary front end with a conical frustrum shape adapted for aligning an end of the fiber within a complimentary conical cavity of an alignment sleeve. To achieve accurate alignment, precise symmetry of the conical cavity and of the frustrum of the alignment ferrule are required. Symmetry has been achieved heretofore by first forming the conical surface of the cavity to an intermediate shape, followed by abrading the surface to a final shape according to the disclosure of U.S. Pat. No. 4,738,055. The alignment ferrule is first formed to a preliminary shape and then is ground to form a precise frustrum surface on the front end of the alignment ferrule, according to a process disclosed in U.S. Pat. No. 4,721,357.

Because these processes are costly, it is desirable to eliminate them. A previous attempt has been made to form a precisely defined alignment surface on an alignment ferrule without grinding. For example, according to U.S. Pat. No. 4,264,128, fluent material is molded over a rigid insert that partitions the mold and directs the flow of the material to provide precisely defined alignment surfaces.

An objective of the invention is to form a precisely defined alignment surface on an alignment ferrule without grinding. A further objective is to form a precisely defined alignment surface on an alignment ferrule without an imbedded insert to direct the flow of fluent molding material. A feature of the invention is directed to a separate portion of the ferrule constructed with a simple volumetric shape and a precisely defined alignment surface. The separate portion is assembled to a remainder of the alignment ferrule. Precise definition of the alignment surface is attained without grinding, because the separate portion is capable of being formed or otherwise constructed with a simple volumetric shape free from the intricacy and complexity associated with the remainder of the ferrule.

In addition, the separate portion is formed by molding a precision alignment surface of uniform and carefully controlled dimensions over an underlying wall having a uniform thickness extending all along the precision alignment surface. The uniform thickness promotes formation of the uniform and carefully controlled dimensions.

Another feature of the invention resides in molding a precision alignment surface of an alignment ferrule along a separate front portion of shorter axial dimension than the axial dimension of a separate, longer remainder portion of an alignment ferrule. The shorter axial dimension is formed by a corresponding short mold core pin that is easier to align in a corresponding mold than a longer core pin that is used to fabricate the remainder of the alignment ferrule.

According to another feature of the invention, a connector for an optical fiber comprises; an alignment ferrule for encircling an optical fiber, a precision alignment surface of the ferrule constructed for nonreformation upon alignment in a cavity having a shape complementary to that of the precision alignment surface, a first portion of said ferrule is constructed on a front end portion of said ferrule together with said precision alignment surface, and said first portion is constructed for assembly with a second separate portion of said ferrule, and said first portion is spaced from said precision alignment surface to isolate said precision alignment surface from distortion when said first portion is assembled with said separate second portion of said ferrule.

According to U.S. Pat. No. 4,440,469 it is known to assemble a nose portion on an alignment ferrule. The nose portion is fabricated of a resiliently compressible material. The nose portion radially compresses in the confines of the alignment bushing, and thereby becomes aligned along the axis of the bushing. Alignment in the bushing is accomplished by reforming the preliminary shape of the radially compressed material of the alignment ferrule, when the ferrule is compressed in the confines of the alignment bushing.

According to a feature of the invention, alignment of a ferrule is accomplished by a precise geometry of an alignment ferrule in an adjacent sleeve of precise geometry. This alignment technique is unrelated to purposely reforming the shape of an alignment ferrule. The alignment technique is recognized as a standard in Publication ZZZ: Sectional Specification For Fiber Optic Connector -Type BAM, of the International Electrotechnical Commission, Technical Committee Number 86: Fibre Optic Interconnecting Devices and Passive Components. According to the invention, reforming the shape of a precisely dimensional, symmetrical alignment ferrule is undesired. Reforming the shape of such an alignment surface would cause distorted alignment of such a surface in a symmetrical cavity. Thus, a feature of the invention resides in alignment of a nonreformed shape of a precisely symmetrical alignment ferrule against a symmetrical cavity.

These and other advantages, features and objectives of the invention are disclosed by way of example from the following detailed description and accompanying drawings.

FIG. 1 is a fragmentary perspective view of a connector, an optical cable, a crimp ferrule and a strain relief boot.

FIG. 2 is an enlarged longitudinal section view of the connector as shown in FIG. 1.

FIG. 3 is an enlarged perspective view of a portion of an alignment ferrule and the crimp ferrule.

FIG. 4 is an enlarged longitudinal section view of the connector assembled with the optical cable, the crimp ferrule and the strain relief boot.

FIG. 5 is a longitudinal section view through mold dies and through a front end portion of an alignment ferrule of the connector shown in FIG. 1.

With reference to FIG. 1 a connector 1 for an optical fiber 2 of an optical fiber cable 3. The cable 3 is of known construction and includes a flexible buffer 4 concentrically encircling the fiber 2, multiple strands of strength members 5 extending in a direction axially of the buffer 4 and distributed around the periphery of the buffer 4, and an external jacket 6. The connector 1 is constructed for further assembly with a hollow, cylindrical, metal crimp ferrule 7 and an elongated, tubular, strain relief boot 8.

With reference to FIGS. 1 and 2 of the drawings, the connector 1 includes an elongated alignment ferrule 9 having a front end portion 10 and a second portion 11. The second portion 11 is encircled by, a coupling nut 12, a low friction, first washer 13 of low friction material, for example, polytetrafluoroethylene, a second, metal washer 14, and a coil spring 15. An internal radial inward flange 16 of the coupling nut 12 projects radially inward toward the second portion 11 of the alignment ferrule 9 and engages an external and radially outward projecting, unitary flange 17 of the second portion 11. A rear end 18 of the spring 15 engages the metal washer 14. The front end portion 10 is assembled by coupling means 19, for example, internal threads 19, to corresponding external threads 20 of the second portion 11. A front end 21 of the coil spring 15 engages a rear end 22 of the front end portion 10. The coil spring 15 is in moderate compression between the rear end 22 and the flange 16 of the coupling nut 12, as the front end portion 10 is threadably advanced along the external threads 20 of the second portion of the alignment ferrule 9. Advantageously, the above described parts are assembled without another part or a fixture being required to compress the spring 15 while the front end portion 10 is being assembled.

According to known practice, the coupling nut 12 of the connector 1 is assembled by hand onto an alignment means comprising a threaded sleeve or bushing, as disclosed in U.S. Pat. No. 4,512,630, to provide a threaded and disengageable coupling of the alignment ferrule 9 and the bushing. During assembly, the coupling nut 12 is rotatable with respect to the ferrule 9 and is slidable along the ferrule 9 to compress the coil spring 15. A precise alignment of the ferrule 9 in the bushing is desired. The ferrule 9 has a precision alignment surface 23 of the type constructed for nonreformation to a different shape when engaged against a complementary shaped cavity of the bushing, as disclosed in U.S. Pat. No. 4,512,630. Construction of the precision alignment surface 23 heretofore was accomplished by grinding the alignment ferrule 9, as disclosed in U.S. Pat. No. 4,721,357. The following description includes a precision alignment surface 23 constructed without grinding. The precision alignment surface 23 described below is always suitably shaped for precise alignment against the complementary shape of the cavity of the bushing, and does not require reformation to a different shape to accomplish the desired alignment.

With reference to FIG. 5, the front end portion 10 is fabricated of plastic material in a mold 24 having a first mold die 25, a second mold die 26 and a third mold die 27. The first mold die 25 has a cavity portion 28 shaped for receiving the plastic material and forming, on a first section 29 of the front end portion 10, the precision alignment surface 23 to a final shape, for example, a frustum conical shape. A front end 30 of the cavity portion 28 is recessed to form an integral, lump projection 31 extending axially and forward of a front end face 32 of the first section 29. The cavity portion 28 is shaped also for forming a unitary, enlarged, cylindrical second section 33 spaced axially to the rear of the precision alignment surface 23 on the first section 29.

A mold core pin 34 extends axially of the cavity portion 28 and has an external shape that forms an interior surface 35 of the front end portion 10, and that forms the front end portion 10 with an underlying wall 36 of uniform wall thickness all along the full circumferential extent of the precision alignment surface 23, and between the precision alignment surface 23 and the interior surface 35. The uniform thickness promotes formation of the uniform surface 23 and the carefully controlled dimensions. Further, the front end portion 10 is thus fabricated with a single volumetric shape.

The core pin 34 is shaped to form a curved funnel 37 recessed in the interior surface 35 toward the face 32 of the front end portion 10. The core pin 34 has a small diameter portion 38 extending through the face 32 of the front end portion 10 to form a small diameter axial passage 40 through the face 32 and centrally through the projection 31 and concentric with the precision alignment surface 23. The second mold die 26 and the third mold die 27 move toward and abut each other along a parting line 41 and close about the core pin 34.

The plastic material in a fluent state is introduced along a mold gate 42, which is a passage communicating with the cavity portion 28. The second mold die 26 and the third mold die 27 are against the first mold die 25 along another parting line 43 to form the rear end 22 of the front end portion 10. The fluent plastic material fills the cavity portion 28 and is solidified in the cavity portion 28 to form the front end portion 10, while the core pin 34 forms the interior surface 35 and the axially extending passage 40. The mold dies 25, 26, 27 separate from one other along the corresponding parting lines 41, 43. Thereby the core pin 34 is free to be withdrawn axially from the first mold die 25, and the molded front end portion 10 may be ejected from the mold 24. The second section 33 only is tapped to form the internal threads 19. Formation of the internal threads 19 along the interior surface 35 of the thin wall 36 would tend to cause undesired distortion of the front end portion 10 along the precision alignment surface 23.

With reference to FIG. 3, the second portion 11 has been fabricated by a mold, not shown, according to a process as described in conjunction with the mold 24. The second portion 11 includes a conical frustrum front end 44 that is axially aligned with and unitary with an elongated cylindrical portion 45, in turn, axially aligned with and unitary with the enlarged diameter flange 17. The second portion 11 includes a unitary rear end portion 46 of stepped diameter, providing a radially extending shoulder 47.

An exterior of the rear end portion 46 is roughened by molding a textured rough surface 48. As shown in FIGS. 3 and 4, an axial passage 49 extends entirely along the second portion 11, and has a funnel 50 tapered toward and communicating with the front end 44. Subsequent to molding, the external threads 20 on the cylindrical portion 45 are formed, together with an external, radially extending shoulder 51 facing forwardly.

The internal threads 19 of the front end portion 10 are threadably advanced along the external threads 20 until the rear end engages the shoulder 51, thereby fixing the position of the front end portion 10 with respect to the second portion 11, and providing a clearance 52 between the wall 36 along the precision alignment surface 23 and the second portion 11. The second portion 11 projects forwardly of both of the threads 19, 20, and faces the untapped interior surface 35 without exerting pressure on the interior surface 35. The presence of pressure would tend to cause undesired distortion of the wall 36 and the precision alignment surface 23. Because the first section 29 is separate from the internal threads 19 on the second section 33, the precision alignment surface 23 on the first section 29 is separated from a coupling formed by the interengaged threads 19, 20, and from stress transferred across the coupling, during threaded assembly of the front end portion 10 and during use.

The front end portion 10 has a shorter axial dimension than that of the second portion 11. The shorter axial dimension promotes accurate alignment of the corresponding shorter mold core pin 34 that is easier to align in a corresponding mold 24 than to align a longer core pin in a corresponding mold, not shown.

The connector 1 and the associated crimp ferrule 7 and strain relief boot 8 are delivered to a customer as shown in FIGS. 1 and 2. The customer assembles the connector 1 to the optical cable 3 shown in FIG. 1. With reference to FIG. 3, the connector 1 is assembled by assembling the optical cable 3 successively through the strain relief boot 8 and through the crimp ferrule 7. The buffer covered fiber 2 is assembled into and along the passage 49 from a rear of the passage 49. The funnel 50 of the passage 49 is axially aligned with and spaced by a clearance from the funnel 37 of the passage 40. The funnels 50, 37 guide the optical fiber 2 of the cable 3 from the second portion 11 and into the passage 40 of the front end portion 10. The optical fiber 2 projects through the projection 31 and is positioned for being polished flush with the projection 31. Polishing may be accomplished by any known manner.

According to FIG. 4, a quantity of adhesive 53, for example epoxy, may be applied to the passage 49 prior to the introduction of the buffer covered fiber 2. A hypodermic syringe, not shown, is used to inject the adhesive 53 deeply into the passage 49 adjacent to the front end 44. Such use of a hypodermic syringe is disclosed in U.S. patent application No. 159,151, (filed Feb. 23, 1988) pending. The adhesive 53 is injected until a quantity of the adhesive 53 emerges from the passage 49 and is deposited in the clearance 52. The adhesive 53 bridges the clearance 52, and upon solidification, forms a joint that bonds together the front end portion 10 and the second portion 11 into a unitary structure. The unitary structure avoids potential breaking of the optical fiber 2, by preventing movement of the front end portion 10 relative to the remainder of the alignment ferrule 9. Further, the unitary structure is provided of disparate parts comprising, the front end portion 10 having a precise overall geometry, and the remainder of the alignment ferrule 9 having a less precise geometry. The adhesive forms a joint without compression against the front end portion 10, leaving the front end portion free of consequent distortion when the joint is formed. The adhesive 53 will coat and adhere the optical fiber 2 and the buffer 4 and will adhere to walls of the passages 40, 49 to secure the optical fiber 2 and the buffer 4 in the passages 40, 49. The strength members 5 are clamped between the rear end portion 46 of the alignment ferrule 9 and the crimp ferrule 7 which has been moved axially along the cable 3 to a forward position, removed from the cable jacket 6, engaged against the shoulder 47 and encircling the rear end portion 46. The knurling 48 provides increased friction to retain the crimp ferrule 7 in place. The strain relief boot 8 is moved axially along the cable 3 until the boot 8 encircles and frictionally engages the crimp ferrule 7 and an adjacent end of the jacket 6.

The invention has each of the previously discussed advantages and features. Each of the advantages or features exists independently of the others, and each contributes to the use and importance of the invention with or without the presence of the others.