There is known from U.S. Patent 4,330,965 and U.S. Patent 4,384,431 apparatus for projecting an end of an optical fiber a predetermined distance beyond a reference surface such that the optical fiber engages a polishing medium and is reduced to a precise length by the polishing medium. A known apparatus for polishing an end of an optical fiber is disclosed in U.S. Patent 4,539,776, and comprises, a base having a front surface for movement over a polishing medium, a body for holding an assembly comprised of an optical fiber secured within a sleeve, and a spring for biasing the optical fiber of the assembly beyond the front surface of the base and against the polishing medium.

According to the known apparatus, the spring biases the assembly to engage against a conical socket. The optical fiber of the assembly protrudes beyond the socket and is biased by the spring against the polishing medium until the optical fiber is shortened by polishing. The same spring applies a spring force against the socket that duplicates the force which the assembly will experience when mounted in a socket of a connector. The same spring force urges the optical fiber against the polishing medium to assure that the optical fiber is shortened to a predetermined length beyond the socket.

According to the invention, a spring for biasing an optical fiber against a polishing medium is compressed when force is applied by the polishing medium against the optical fiber. Thereby the fiber retracts when the spring is compressed to limit the force applied by the polishing medium against the fiber.

The spring of the known apparatus duplicates the force which an assembly will experience when mounted in a connector. Accordingly the spring is part of the assembly and is designed for producing a high spring force in a connector. Such a sprin is not practical for allowing retraction of the optical fiber to limit a force applied by a polishing medium against the fiber.

According to the invention, apparatus is characterized in that the optical fiber is mounted for reciprocation along a passageway that intersects the front surface of the base, and the spring is mounted for compression between the body and a back portion of the base and is compressed by engagement of the optical fiber against the polishing medium to retract the optical fiber along the passageway to the front surface of the base when engaged by the polishing medium to limit the force of the polishing medium against the optical fiber.

According to a further aspect of the invention, the front surface of the base is convex to produce a convex polished end face on the fiber.

According to a further aspect of the invention, apparatus for polishing an optical fiber as defined above further includes a fastener on the body to which a coupling nut of the assembly is secured, and the assembly is comprised of an optical fiber, a sleeve encircling the fiber, a coupling nut and a connector spring for compression between the coupling nut and the sleeve, and the spring of the apparatus is lower in spring force than that of the connector spring. The invention will be described by way of example by reference to the accompanying drawings, in which;

FIGURE 1 is a fragmentary elevation view in section of an apparatus for polishing an optical fiber.

FIGURE 2 is a fragmentary elevation view in section of an assembly of a sleeve and an optical fiber.

FIGURE 3 is a view similar to Figure 1 and illustrating retraction of an optical fiber during a polishing operation.

FIGURE 4 is a fragmentary elevation view illustrating a portion of the apparatus shown in Figure 1 during a polishing operation.

FIGURE 5 is a view similar to Figure 2 and illustrating direct contact of polished ends of optical fibers. FIGURE 6 is an elevation view in section of an alternate base for the apparatus shown in Figure 1.

FIGURE 7 is a view similar to Figure 2 illustrating an optical fiber recessed in an encircling sleeve.

FIGURE 8 is a view similar to Figure 5 and illustrating mating together of two optical fibers the polished ends of which are recessed in encircling sleeves. FIGURE 1 illustrates a polishing bushing or apparatus 10 adapted to receive an optical fiber connector 20 which is comprised of an optical fiber 12 having an end face 14 to be polished as shown in Figure 2.

Optical fiber connector assembly 20 comprises a ceramic connector comprised of optical fiber 12 extending through and supported within a rigid ceramic sleeve 21 which protects the fiber and holds it tightly in position within the connector. Connector 20 further includes a connector body 22 from which rigid sleeve 21 extends; a connector nut 24, for releasably coupling connector 20 to a complementary optical fiber connector (not shown) , and a connector spring 26 for compression between the nut 24 and a flange 27 on body 22 from which sleeve 21 extends.

Polishing bushing 10 comprises an apparatus which includes a bushing base 31, a bushing body 32, a retainer 33 and a bushing spring 34. Bushing base 31 comprises a rigid member of steel or other suitable material and includes a spherical convex curved, front surface 36, a

back surface 37 and a passageway 38 extending therethrough from the front surface to the back surface. Base 31 also includes an annular sidewall portion 39 which extends rearwardly from back surface 37 and defines a cylindrical cavity 41. A portion of the outer surface of sidewall 39 is threaded as illustrated at 42.

Bushing body 32 comprises a generally cylindrical member having a front end 51, a back end 52 and an axial bore 53 extending therethrough from front end 51 to back end 52. Bore 53 includes a front bore portion 53a having a diameter substantially equal to passageway 38 in bushing base 31, and an enlarged diameter, bore portion 53b. Bushing body 32 is adapted to be positioned in cylindrical cavity 41 of base 31, as shown in Figure 1, such that bore portion 53a of body 32 is in alignment with passageway 38. Bushing body 32 includes coupling structure 56 of the bayonet type for engaging the bayonet coupling structure on connector nut 24 to releasably secure the connector 20 to bushing 10. In the illustrated embodiment, coupling structure 56 comprises a pair of pins or pegs adapted to extend into slots (not shown) on nut 24 as is known in the art. Bushing body 32 further includes an outwardly extending annular flange 57 having a back surface 58 which functions as a first bearing surface for bushing spring 34.

Bushing retainer 33 includes a circular plate portion 71 having an axial opening 72 for receiving bushing body 32, and a forwardly extending flange portion 73 having an internally threaded surface 74 for engaging externally threaded surface 44 on bushing base 31. The front, inner surface 76 of plate portion 71 functions as a second bearing surface for bushing spring 34.

Bushing spring 34 comprises a coil spring and is positioned around bushing body 32. The opposite ends of the spring bear against bearing surfaces 58 and 76 on the

bushing body 32 and retainer 33, respectively. As will be explained hereinafter, bushing spring 34 is substantially weaker than connector spring 26 and comprises resilient means for limiting the pressure applied to the end face 14 of optical fiber 12 during a polishing operation.

As best shown in Figure 2 optical fiber 12 includes a core 12a and a cladding layer 12b, and is retained within connector sleeve 21 by a layer of epoxy. The connector is manufactured such that a short length of fiber protrudes from the spherically convex curved end face 16 of sleeve 21. The radius 16' of end face 16 is greater than the radius of face 36. The protruding portion is typically covered by a bead of epoxy, and it is the end face of the protruding portion of the fiber which is to be polished by the polishing bushing 10.

To assemble polishing bushing 10, bushing body 32 having bushing spring 34 positioned therearound is inserted into cavity 41 of bushing base 31. Bushing retainer 33 is then threaded onto bushing base 31 to secure the components together. When assembled, the ends of bushing spring 34 will press against bearing surfaces 58 and 76 on bearing body 32 and retainer 33, respectively; and urge body 32 forwardly to maintain the front surface 51 of bearing body 32 in contact with and pressed against the rear surface 37 of bushing base 31.

Connector 20 is attached to bushing 10 by inserting the connector into enlarged bore portion 53b of bushing body 32 from the back end thereof, and securing the connector to the bushing via the coupling structure on connector nut 24 and bushing body 32.

When connector 20 is secured within bushing 10, rigid sleeve 21 extends through bore portion 53a in bushing body 32 and passageway 38 in bushing base 31 such that an end portion 81 of the rigid sleeve protrudes slightly beyond the front surface 36 of bushing base 31 as shown in Figure

1. The end face 14 of optical fiber 12 is then polished by moving bushing 10 over a polishing medium such as a polishing paper 82 illustrated in Figure 4. As known to those skilled in the art, polishing paper 82 is supported on a rigid plate 84, with a resilient pad 86 normally positioned between paper 82 and plate 84. Bushing 10 is moved over the polishing paper by hand, usually in a figure eight pattern, until the end face 14 of the fiber is polished. Bushing spring 34 comprises resilient means for permitting rigid sleeve 21 to move longitudinally within passageway 38 of bushing base 31 during a polishing operation to limit the pressure which can be applied against the end face 14 of optical fiber 12 by polishing paper 82 during polishing. In particular, spring 34 will be compressed permitting connector 20 to move rearwardly in bushing 10 and permitting rigid sleeve 21 to retract into passageway 38 of bushing base 31 to the position shown in ^' Figure 3. In this way, bushing 10 automatically prevents over polishing of optical fiber 12 and permits the end face of the fiber to be polished in a highly predictable, repeatable manner.

Bushing spring 34 is substantially weaker than connector spring 26. Accordingly, when force is applied during polishing, the connector as a whole moves rearwardly within the bushing, and the connector spring itself does not compress to any significant extent.

As shown in Figure 1, front surface 36 of bushing base 31 comprises a curved surface, preferably a spherically curved surface; and passageway 38 extends radially into base 31 from surface 36. By polishing optical fiber 12 with a bushing having a curved front surface, end face 14 of the fiber is polished to a convex curvature which protrudes slightly from the end face 16 of rigid sleeve 21 as shown in Figure 2. Because fiber 12 is

convexly curved and protrudes from rigid sleeve 21, when connector 20 is connected to a complementary connector 120 having a similarly polished fiber 112 supported in a rigid sleeve 121, as shown in Figure 5, the end faces of the mated fibers compress flat and are in virtual contact with one another over their entire end surfaces. Accordingly, there is essentially no gap or separation between mated fibers 12 and 112 thus minimizing insertion losses therebetween due to fiber to fiber separation and Fresnel reflections. The polishing bushing of the present invention, therefore, can be effectively used to polish optical fibers of the single mode or multimode type which require that insertion losses and reflection between connected fibers be minimized. With the polishing bushing of the present invention, optical fibers can be polished in a consistent, repeatable manner by relatively unskilled personnel. The bushing spring 34 prevents over polishing of fibers by preventing excessive forces from being applied to the end faces during polishing. In addition, the curved front surface of the bushing base ensures that the end faces of the fibers are always polished to a convex curved shape that protrudes from the end of the rigid sleeve by a consistent distance of less than about one micron. The bushing tends to wobble or tilt in a random manner during a polishing operation. If the extent of the wobble becomes excessive at any time during polishing, the end of the fiber will move out of contact with the polishing paper during such periods as shown in Figure 4, to prevent uneven polishing or damage to the fiber during such periods. With prior polishing bushings having flat surfaces, any wobble of the bushing during polishing often resulted in uneven polishing making it difficult to polish fibers in a consistent, repeatable manner. The polishing bushing surface 36 has a radius of curvature of 17.5 mm or

0.689 inches. The radius of curvature 16', shown in Figure 1, of the end face 16 of the sleeve 21 is 30 mm or 1.18 inches, and is greater than the radius of curvature of the polishing bushing surface 36. During a polishing operation, the end face of the optical fiber 12 protrudes from the end face 16 of the sleeve 21. The resilient pad undergoes resilient deflection in response to being compressed by the end face 16 of the sleeve 21 and the end face of the optical fiber, and conforms to the curvature of the end face 16. Since the polishing paper is flexible and conforms to the surface of the resilient pad, the polishing paper will conform also to the curvature of the end face 16 of the ceramic sleeve which compresses against the polishing paper and against the resilient pad under the polishing paper. The polishing paper also will conform to the curvature of the polishing bushing surface 36, even though of different radius of curvature than that of the ceramic sleeves 21. During the polishing operation, the described random wobble of the polishing bushing accompanying the movement of the end face of the optical iber against the polishing paper will present different portions of the end face against the polishing paper, and cause the paper to polish the end face 16 to a convex curved and nearly spherical bulbous shape with no sharp edges. The polishing paper will polish the surface of the end face 16 of the optical fiber 12 to a different radius of curvature than that of the polishing bushing surface 36, and more like the radius of curvature of the end face 16 of the ceramic sleeve. The bulbous shape of the end face of the optical fiber need not be precisely spherical or have a precise radius of curvature. During mating together of two optical fibers as shown in Figure 5, the bulbous end faces of two polished optical fibers 12,112 will press in contact against each other without sharp edges to be fractured, and with the direct contact

of the end faces preventing Fresnel reflection and other contributions to attenuation of optical signals transferred between the end faces.

Bushing bases 31 and 131 are interchangeable within bushing 10. Specifically, bushing 31 in Figure 1 can be easily removed from bushing 10 by simply unthreading it from bushing retainer 33, and bushing base 131 can then be threaded onto the retainer to provide polishing bushing 10 with a flat front surface. The bushing of the present invention can thus be sold in the form of a "kit" which includes both bushing bases 31 and 131 to provide convenience and flexibility to the user.

Although rigid sleeve 21 is referred to herein as a ceramic sleeve, sleeve 21 could also be formed of other suitable hard or soft materials, if desired. In addition, bushing spring 34 can comprise other types of springs rather than the coil spring described herein. Because the invention can take many forms, it should be recognized that the invention should be limited only insofar as is required by the scope of the following claims.