CA1179538A

CA1179538A - Coupling method and arrangement for fiber optic devices

Info

Publication number: CA1179538A
Application number: CA000402774A
Authority: CA
Inventors: Francis Sladen
Original assignee: Thomas and Betts Corp
Current assignee: ABB Installation Products Inc
Priority date: 1981-05-20
Filing date: 1982-05-12
Publication date: 1984-12-18
Also published as: SE8203088L; AU8360582A; DE3219374A1; DE8215017U1; IT1147980B; IT8248445A0; FR2506468A1; AU534194B2; JPS57198422A; GB2100465A; FR2506468B1; GB2100465B

Abstract

ABSTRACT OF THE INVENTION
A method and arrangement for optically coupling a fiber optic element to the end surface of an optical device utilizing a noncontact type coupling member. The fiber optic element includes an end face for optical coupling to the end surface of the optical device, and has an outer protective coating along a portion of its axial length adjacent the end face of the fiber optic element.
The outer diameter of the protective coating at an axial location spaced from the end face of the fiber optic element is of a first dimension. The noncontact type coupling member includes a first support surface for supporting the end face of an optical device in a first predetermined position, and a second support surface for supporting a contact portion of the protective coating of the fiber optic element in a second predetermined position axially spaced from the first predetermined position. The protective coating of the fiber optic element has a shape such that the contact portion thereof defines a surface which has an outer diameter less than the first dimension.
Also, a coating is provided on the end face of the fiber optic element so as to substantially cover the end of the core region thereof. Such covering coating has a prede-termined index of refraction which is substantially greater than the index of refraction of the medium between the end face of the fiber optic element and the end surface of the optical device when the fiber optic element and optic device are supported in spaced axial relation-ship,

Description

11~79538 A COUPLING METHOD AND ARRANGEMENT FOR
FIBER OPTIC DEVICES

FIELD OF THE INVENTION
The present invention relates to optical coupling of fiber optic elements and optical devices, and more particularly to a method and system for optically coupling a fiber optic element to an optical device utilizing a noncontact type coupling member so that the end faces of the fiber optic element and the optical device are axially spaced from one another. In this regard, as used in the present- application, the term ~optical device~ includes optical fibers as well as sources, detectors, mixers, filters, and other devices operating in the visible, infrared or ultraviolet regions of the electromagnetic spectrum and to which it may be desired to optically couple a fiber optic element for the transmission of electromagnetic energy therebetween. The present invention in particular relates to a method and system for physically and/or optically reducing the gap between the end face of a fiber optic element and the end surface of an optical device when same are coupled by means of a noncontact type coupling member so as to reduce I;ransmission losses between the fiber optic element and the optical device.

BACKGROUND OF THE INVENTION
Optical fiber communication systems, utilizing fi~ers that carry separate channels of information, require an efficient connecting means for joining individ-ual fibers and other devices so that light or other information bearing signals can be transmitted. This includes connection of fiber optic elements not only to '30 other fiber optic elements, but to light or signal sources, detectors, or various other types of devices such as mixers, and filters as well.

As can be appreciated, and as is known in the art, if excessive losses are encountered at the connector for coupling the fiber optic elements to optical devices, the entire system becomes impractical. Thus, of con-siderable relevance to the problem of developing practical ; fiber optic connectors is the level of transmission efficiency of the connectors. Various factors, including separation at the point of abuttment, and lateral separa-tion or axial misalignment are among the factors affecting 10 the light transfer efficiency of a connector. In this regard, as is known in the art, lateral or axial misalign-ment generally gives rise to substantially lower coupling efficiencies (results in greater losses) than axial , spacing between aligned fiber optic elements or between a 15 fiber optic element and an aligned optical device.
Numerous prior art methods and arrangements are known for optically coupling fiber optic elements to optical devices which have as their purpose minimization of transmission loss. One technique known in the prior art 20 has involved the use of connectors by which the ends of the fiber optic elements and the optical devices are presæed together so that the end faces of the fiber optic element and the optical device are in contact with one another. Examples of direct contact type connecting 25 arrangements are shown for instance in U.S. Patent Nos.
3,861,781; 4,158,476; and 3,734,594. Such arrangements tend to reduce transmission losses as a result of axial spacing between the end faces of the fiber optic element and the optical device, but have given rise to substantial 30 problems with respect to lateral or axial misalignments.
Further, direct contact type systems require complex polishing and breaking operations to form defect free end surfaces. Still further, such direct contact systems have resulted in scratching or marring of the end surfaces of 35 the fibers or devices in systems designed for repeatable . "

11'79~38 uncoupling and coupling of the fibers and devices. That is, in direct contact or abutting systems, the ends of the fibers or devices which are in abutting contact often become scratched or marred during repeated matings, thereby resulting in excessive light diffusion and increased losses at the abutting interface. Further still, such direct contact systems may also result in damage due to high ccntact pressures being applied when bringing the opposite ends of the fibers or devices together into 10 direct contact.
Another prior art technique which has been utilized, and which minimizes some of the adverse affects of direct contact type coupling, provides a noncontact coupling for the optical fibers and/or devices in which 15 the coupled fiber optic elements and/or devices are very preci~ely and accurately aligned laterally or axially, but in which the ends of the fibers or devices are axially spaced a small distance so as to provide a gap there-between. See for example, U.S. Patent Nos. 4,119,362;
20 4,186,998; 3,963,323; and 3,984,174. Such a noncontact technique avoids some of the problems of deterioration or damage to the end faces of the fiber optic elements or devices which are subject to repeated matings. Here it should be again noted that transmission losses resulting from axial separation of the end faces are generally less , than losses caused by the axial or lateral misalignment of - the ends of the fiber, and thus acceptable coupling arrangements are still provided.
More particularly, with many of these noncontac-t type coupling arrangements, the ends of the optical fibers or optical devices to be optically coupled are supported by a coupling member which is provided with support sur-faces or means which engage the outer peripheral edges of the optical fiber or devices so as not to mar or scratch the end face of the fiber or other device yet serve to accurately laterally or axially align the axes oi the 7~53~3 fiber optic elements or devices to be coupled. For instance, in the prior art systems of U.S. Patent Nos. 4,119,362 and 4,186,998, the connector members provide inwardly tapering openings into which the optical fibers or devices are inserted and pressed axially toward one another. In these arrangements, the end corners or edges of the fiber optic elements or devices (i.e., the edge between the end face and the outer peripheral surface of the fiber or device) engage the tapered walls of the openings to align the fibers or devices and position their end faces in close adjacency. The tapered support surfaces serve to provide a self-centering effect for the axis of the fiber optic elements or devices so that the fibers or devices will have aligned parallel axes. An index matching fluid may be provided between the end faces of the optically coupled fibers or devices (as in U.S. Patent No. 4,186,998), and or a lens surface may be provided (as in U.S. Patent No. 4,119,362).
A still further noncontact type coupling arrangement is disclosed in applicant's U.S. Patent No. 4,378,145, issued March 29, 1983. This patent application discloses a noncontact type coupling arrangement which employs a plurality of spherical balls or tapered cones which are in engagement with one another and which define first and second pluralities of discontinuous surfaces, each spaced or arranged in a circular locus and each circular locus having a diameter not greater than the diameter of the fiber or device which is to be supported or terminated thereby. The spherical balls or tapered cones also define an optical passageway therethrough which is aligned with the centers of the circular locus defined by the first and second pluralities of discontinuous surfaces. In this arrangement, the outer corner edges of the fiber engage and are supported by the first plurality of mg/l ~ _ 4 _ i~ S~8 discontinuous surfaces so that the axis of the fiber is aligned with the optical passageway and so that the axial end face of the fiber is spaced a small axial distance from the end face of an optical device to be coupled thereto and which is similarly supported by the second plurality of discontinuous surfaces lying in the circular - locus provided on the opposite side of the optical passage-way. In this manner, the discontinuous surfaces of the spheres or cones serve to accurately align the opposing lO fibers or devices to position the end faces in close adjacency to one another.
In such systems for supporting the end faces of optical fibers or devices in axial spaced alignment, it will be appreciated that the axial spacing or gap between 15 the end faces of the coupled fibers and optical devices is very sensitive to the diameter of the optical fiber or device. That is, if the diameter of the fiber or other optical device increases, such as for example as a result of manufacturing tolerances, the spacing or the axial gap 20 between the end faces also increases. This increase in ; axial spacing in turn increases the transmission loss provided by the coupling, and consequently, appreciably affects the efficiency of the coupling. While transmission losses resulting from axial spacing are generally less 25 than transmission losses resulting from lateral or axial misalignment, it is still desirable to minimize all contri-butions to transmission loss.
A further matter complicating the efficiency of optical coupling of fiber optic devices is the fact that in some known systems the outer surface of the fiber optic elements are coated with a suitable material, such as plastic, for protection against the environment and mechanical breakdown. The end of the fiber with coating is typically cleaved and polished prior to use, the protec-35 tive coating providing a contact area for engagement withthe support surface of the connector so that it is the ~1'7953~3 - 6 - T&B 876F

1 protective coating, and not the core or cladding of the fiber, which engages the support surface of the coupling member. Such an outer protective coating, since the coat-ing is provided on the peripheral surface of the fiber at the cleaved end thereof, has a significant effect on the axial spacing between the end faces of the optical fiber and on the control of the losses between the optical de-vices being coupled.
SUMMARY OF THE I~JVENTION
The present invention overcomes these and other disadvantages of the prior art in providing an arrangement and method for coupling of a fiber optic element to an optical device in a termination assembly so as to physi-cally or optically reduce or minimize the axial spacing between the end faces or surfaces of the fiber optic ele-ment and the optical device without adversely affecting the alignment characteristics or other advantages of such systems. More particularly, in accordance with one aspect of the present invention, there is provided a fiber optic termination assembly comprising an optical fiber, means terminating the fiber in an engaged relation and means for supporting the fiber in the engaged relation. The optical fiber has an end face and an outer surface defining an outer diametér at an axial location spaced from the end face. The fiber includes a contact surface between the outer surface and the end face, such contact surface having a diameter less than the diameter of the outer surface. The terminating means includes a support surface for engagement in abutting relation with the optical fiber contact surface. The support means is provided for sup-porting the optical fiber in the abutting relation with the support surface of the terminating means.
In accordance with a particular form of the in-vention, the optical fiber has a core and an optical member on the fiber end face substantially covering the end of the core. The optical member has a predetermined index of refraction. The optical fiber in the engaged relation 7'35;~8 - 7 - ~6s 876F

1 with the support surface defines with such support surface a cavity adjacent one surface of the optical member. An optical medium is disposed in the cavity in contact with the one surface of the optical member, the index of re-S fraction of the optical member being substantially greater than the index of refraction of the optical medium.
In accordance with a method of terminating an optical fiber, the fiber, provided with an end face and contact surface, is placed into contact with the support surface of the terminating means. Such fiber, in a par-ticular form, may be coated with a continuous optical material of predetermined index of refraction onto the fiber outer surface, contact surface and end face.
These and further features and characteristics of the present invention will be apparent from the fol-lowing detailed description in which reference is made to the enclosed drawings which illustrate preferred embodiments of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a sectional view of a fiber optic coupling apparatus for optically coupling a pair of fiber optic elements, in accordance with a preferred embodiment of the present invention.
Figure 2 i5 a schematic elevational view of a fiber optical element in abutting relationship with a noncontact type coupling member which may be utilized in accordance with the present invention.
Figure 3 is a schematic side view illustrating a known coupling arrangement for optically coupling a pair of fiber optic elements with the noncontact type coupling member of Figure 2.
Figure 4 is a schematic side view of a coupling arrangement in accordance with the present invention in which the protective coating of a fiber optic element is shaped so that the contact portion thereof defines a surface having a diameter which is less than the diameter of the coating at an axial position spaced from the end face of the fiber.

- 1;L'~9538 Figure 5 is an enlarged schematic side view corresponding to a portion of the coupling arrangements shown in Figures 3 and 4, illustrating the differences in the positions that a fiber optic element is supported in accordance with the present invention and in accordance with the prior art, the end of a fiber optic element having an outer protective coating in accordance with the principles of the present invention being shown in full outline, and the end of a fiber optic element having an outer protective coating in accoxdance with the prior art being shown in dotted outline.
Figure 6 is a schematic side elevational view illustrating how light is refracted at the end face of a fiber optic element having a coating thereon which has an index of refraction which is su~sstantially greater than the index of refraction of the medium between the end faces of fiber optic elements coupled together in axial spaced relationship.
Figure 7 is a schematic side elevational view of a modified arrangement for coating the end face of a core of a fiber optic element with a coating material having an index of refraction higher than that of the medium between the end faces of optical devices which are coupled together.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings wherein like reference characters represent like elements, there is shown in Figure l an optical fib~sr connector assembly 10 employing a fiber optic element coupling arrangement in accordance with the present invention. The particular fiber optic coupling assembly 10 shown in Figure l is similar to that shown and described in U.S. Patent No. 4,378,145. As the coupling arrangement and method in accordance with the present mg/~ 8 -i~ 538 invention is particularly useful with the noncontact type coupling assembly 10 shown in U.S. Patent No. 4,378,145, the present invention will be described hereinbelow with reference to such a coupling assembly. However, it should be appreciated that the fiber optic element coupling arrangement and method in accordance with the present invention could also be utilized with other noncontact type connector assemblies in which the ends of the fiber optic elements or devices 12, 12' to be coupled are supported in axially spaced relationship (i.e., so that the end faces 14 of the fiber optic elements or devices 12, 12' being coupled do not contact one another). As noted hereinabove, such noncontact type coupling arrangements are desirable in order to minimize possible mechanical breakdown or deterioration of the optical characteristics of the fiber optic elements or devices 12, 12', such as might otherwise result from surface scratching due to contact, or cracking or chipping as a result of repeated matings of the fiber optic elements 12, 12' in the connector assembly 10.
Also, in the description hereinbelow, the coupling arrangement and method in accordance with the present invention will be described with reference to optically coupllng one fiber optic element 12 to a second fiber optic element 12' for transmission of light or other optical si~nals therebetween. However, it will be appreciated by persons skilled in the art that the coupling arrangement and method of the present invention could also be employed for coupling of a fiber optic element 12 to another type of optical device, such as for example, an optical source, an optical detector, mixers, filters, or other devices which when coupled to the fiber optic element will transmit light signals therebetween. In addition, the invention may also be used to terminate single fibers, such termination referring, for example, to the coupling of the fiber to an active device or to a dead-ending device preventing back mg/i~~ ~ _ g _ reflections of light. Consequently, the description of the preferred embodiment in which one fiber optic element 12 is optically coupled to another fiber optic element 12' should not be deemed to be limiting, but rather should be considered to be illustrative of the present invention.
The particular coupling assembly 10 shown in Figure 1 is designed to support a pair of optical fiber cables 16, 16' of a conventional construction in a manner so that the opposite end faces 14 of the fiber optic elements 12, 12' thereof are in axially spaced alignment, i.e., so that the axes 18, 18' of the respective fiber optic elements 12, 12' are axially aligned with one another but in which the end faces 14 are spacedly positioned a small distance apart. In this regard, as is known in the art, transmission losses associated with axial spacing of a pair of optical devices or fibers are generally less than those associated with lateral or axial offset of the respective fiber optic elements 12, 12'. However, even though losses resulting from an axial gap or spacing between the end faces 14 of the elements 12, 12' is of less concern with respect to the loss attributed to any particular connector assembly 10, it will of course be appreciated that any arrangement for minimizing the axial misalignment, which also minimizes the axial spacing, wlll serve to increase the efficiency of the connector assembly 10.
Advantageously, the coupling arrangement and method in accordance with the present invention is designed to reduce the effective spacing between the ends of the fiber optic elements or devices 12, 12' while still achieving the advantages of a particular coupling assembly 10 which results in very precise axial alignment of the fiber optic elements or devices 12, 12'.
The particular coupling assembly 10 shown in Figure 1 is more fully described in U.S. Patent No.
4,378,145 and thus reference should be made mg/~ 10 -5~38 thereto for a complete understanding of the structure and functioning of the connector assembly 10. Basically, the coupling assembly 10 employs a pair of cable subassemblies 20, 20' for supporting a pair of optical fiber cables 16, 16' and an elongated coupling housing 22 for terminating and connecting the cable subassemblies 20, 20' together.
As is conventional, the optical fiber cables 16, 16' each include a fiber optic element 12 surrounded by a buffer material or sheath (not shown), typically made of plastic 10 material, a plurality of strenthening members (not shown) such as strands of glass or plastic extending lengthwise along the cable 16, and an outer jacket 24, 24' made- of plastic material in order to provide protection for the fiber optic element 12 along its length. As is known, the ]5 fiber optic elements 12, 12' of the optical fiber cable 16, 16' generally include an optical core region 26, 26' which is surrounded by a cladding material 28, 28' (see Figure 3).
In preassembling the cables 16, 16' for termina-20 tion of the fiber optic elements 12, 12' thereof, the cable jacket 24, the strengthening members and the buffer materials are removed from the ends to be coupled. A
sleeve 30 may be placed over the end of the fiber optic elernent 12 and fixedly positioned, such as by crimping, in

2'~ engagement with the end portion of the buffer material j remaining on the fiber optic element 12 so as to be spaced from the end face 14 of the fiber optic element 12 to be ' coupled. The cable 16 is then inserted into an elongated housing 32 of the cable subassembly 20. The housing 32 30 includes a retainer ring 34 fixedly secured therewithin and having an opening through which the end of the fiber optic element 12 may pass. The retainer ring 34 also includes an aperture 36 formed essentially concentrically with the opening for receiving the crimped sleeve 30. A
35 split jacket ferrule 38, preferably having chamfered edges, and a compress1on nut 60 havin~ e~terior threads ,. --1~--are also provided on the cable jacket 24. The compression nut 40 is threadably secured to the end of the housing 32 so as to compress the ferrule 38 between the retainer ring 34 and the compression nut 40 to thereby secure the cable 16 in the end of the cable subassembly 20. ~he subassembly housing 32 also supports a fiber guide 42 having a tapered end surface 44 and a centrally disposed aperture 46 through which the exposed end of the fiber optic element 12 is inserted. The fiber guide 42 is arranged to move ]0 axially within the housing 32 under the influence of biasing means in the form of a spring 48. Also, a coupling nu-t 50 for connecting the subassembly 20 to the coupling ' housing 22 is provided on the end of the cable subassembly 20.
The pair of connector subassemblies 20, 20' having the cables 16, 16' secured thereto, are then inserted into the opposite ends of the coupling housing 22 and secured thereinplace by means of the coupling nuts 50.
More particularly, the coupling housing 22 includes 20 axially aligned entry apertures 52 in the opposite ends thereof for receipt of the coupling subassemblies 20, 20'.
'I'he cable subassemblies 20, 20' are assembled by inserting , , t,he fiber guides 42 thereof into the entry apertures 52 and threading the coupling nuts 50 onto the exteriorly 2~ threaded coupling housing 22. As the coupling nuts 50 are ,, threadably rotated, the housings 32 are drawn inwardly toward the coupling housing 22 and the fiber guides 42 of " each of the subassemblies 20, 20' inwardly toward a fiber engagement/abutment unit 54 fixedly supported in the 30 central bore in the coupling housing 22. A first seat 56 is provided in the fiber engagement/abutment unit 54 for receipt of the tapered end 44 of one of the fiber guides , 42, (i.e., for the right-hand cable subassembly 20' shown in Figure 1), and a second seat 58 is defined for the ', 35 other fiber guide 42 (i.e., the left-hand cable sub-assembly 2U shown in Figure 1) by an interiorly tapered 11~7~ 538 portion of the coupling housing 22. Upon seating of the fiber guides 42 in the respective seats 56, 58 and con-tinued threading of the coupling nuts 50, the fiber optic elements 12, 12' within the respective guides 42 extend through the guide apertures 46 as the springs 48 are i compressed, The connector assembly 10 is constructed so that upon engagement of the housings 32 of the cable sub-assemblies 20, 20' with the coupling housing 22, the 10 respective fiber optic elements 12, 12' abut the engagement/abutment unit 54 on both sides. The engage-ment/abutment unit 54 comprises an outer race member 60 having a plurality of spheres therein, each being indi-cated by the reference numeral 62, and a retaining ring 64 15 adapted to secure the spheres 62 within the outer race 60.
, The function of the engagement/abutment unit 54 is to axially align the fiber optic elements 12, 12' in each of the respective cable subassemblies 20, 20' so as to be in ~, precise axial alignment with one another, as more fully 20 described and explained hereinbelow.
Figures 2 and 3 illustrate how fiber optic elements (designated 12a, 12a') in general are known to be aligned and supported by the engagement/abutment unit 54.
The outer race 60 of the unit 54 is of a generally 25 circular configuration and is tangentially in contact with ; the outer surfaces of a plurality of spheres 62 housed ' therewithin at a common axial location, i.e., the spheres are arranged so that their centers 66 lie on a common plane (designated by the dashed line 68) which is yenerally perpendicular to the axis 18 of the fiber optic elements 12a, 12a' to be coupled thereby. In the particu-lar example shown, four spheres 62 are provided in the outer race 60, although three spheres or a greater number of spheres could be employed. Also, the spheres 62 are , arranged so as to provide an interstitial spacing or aperture 7~ therethrough which ls align-d with the axes '';

.
. ~ .

3~3 18, 18' of the fibers 12a, 12a' and which thus provides an optical passageway therethrough for aligned fiber optic elements 12a, 12a'. The surfaces of the spheres 62 provide support surfaces 72 for supporting the ends of the fiber optic elements 12a, 12a', the support surfaces 72 converging toward the optical passageway or aperture 70.
That is, the diameter of the support surfaces 72 on each side of the engagement/abutment unit 54 converges or decreases toward the axial center of the engagement/abut-ment unit 54 (i.e., toward the plane 68).
The end faces 14a, 14a' of the fiber optic elements 12a, 12a' to be optically coupled together are arranged to be in abutting relationship at essentially point areas 73 on the support surfaces 72 of the spheres l5 62 on opposite sides of the engagement/abutment unit 54, the point areas 73 for each fiber optic element 12a, 12a', being coincident with the plane of the end faces 14a, 14a' of the fiber optic elements 12a, 12a'. Abutment of the fiber optic elements 12a, 12a' with the point areas 73 of 2C) the spheres 62 in this manner is assured by preselection of the size of aperture or optical passageway 70 between the spheres 62 to be less than the outer fiber diameter such that the fiber optic elements 12a, 12a' are prevented from passing therethrough. Here, it should also be noted 2S that the term "abutting" refers to the contact of the peripheral edge surface 74 of the fiber optic elements 12a, 12a' with the spheres 62 of the engagement/abutment unit 54.
Accordingly, the multiple sphere 62 arrangement 30 provides an en~agement or support means which defines mutually discontinuous support surfaces 72 which are con-tacted at point areas 73 by portions of the end faces 14a, 14a' of the fiber optic elements 12a, 12a', with the discontinuous point areas 73 being spaced from one ano-ther 35 in a circular locus having a diameter which is equal to or less than the diameter of the fiber optic elements 12a, 12a'. As more fully described in U.S. Patent No. 4,378,145, this type of supporting arrangement provides for a self-centering of the fiber optic elements 12a, 12a' in which the fiber longitudinal axes 18, 18' are coincident with the center of the composite of the spheres 62 housed within the outer race 60.
The spheres 62 are desirable forms for such support/
alignment bodies in view of the convenience of manufacture and assembly. However, other shapes for the support means could be employed, such as for example tapered cones which are arranged within a race and which provide tapered surfaces so as to provide mutually discontinuous abutting point areas about an axis coinciding with the axis of the fibers to be terminated. If cones are employed, the surfaces of the cones would be selected such that the diameter of the circular locus of the abutting point areas is not greater than the diameter of the fiber optic elements to be terminated in order to ensure that the fiber optic elements do not pass through the interstitial spacing between the cone surfaces.
As noted hereinabove, with such an arrangement for supporting the end faces 14 of the fiber optic elements or optical devices 12a, 12a' to be coupled together, although very precise axial, concentric alignment will be provided, the end aces 14 of the coupled fibers or devices 12a, 12a' will be axially spaced. In this regard, it has been found that in this type of connection system, the axial spacing or size of the gap Ga between the end faces 14a, 14a' of the fiber optic elements or devices 12a, 12a' is very sensitive to the diameter of the fiber optic element or the devices 12a, 12a' being coupled. That is, a very small change in fiber diameter may result in a very large change in the axial spacing Ga between the end faces 14a, and hence result in an appreciable change in transmission loss at the interconnection.

mg/ , - 15 -ill79538 This can be more readily understood with refer-ence to Figure 3 from which it will be readily appreciated that as the diameter of the fiber optic elements 12a, 12a' increases, the axial spacing or the size of the gap G
will likewise increase, thereby resulting in the end faces 14a, 14a' of the fiber optic elements 12a, 12a' being axially spaced or displaced from one another a greater distance. As the diameter of ~he fiber optic elements 12a, 12a' increases, the corner edge- 74 defined by the end 10 faces 14a and the cylindrical outer surface 76 of the fiber optic elements 12a of each fiber optic element 12a will contact the surfaces 72 of the spheres 62 at greater distances from the longitudinal axis 18 of the fiber optic elements 12a, 12a', the points of contact 73 in turn being located at a greater distance from the plane 68 of the centers of the spheres 62. This is iilustrated on a greatly enlarged scale in dotted outline in Figure 3 in which it can be seen that as the diameter of the fiber optic elements 12b, 12b' increases, the spacing between the end faces 14b and 14b' likewise increases.
In the prior art techniques, the diameter of the fi.ber optic elements 12a, 12a' at the point of contact is the same as the diameter of the fiber optic elements 12a, 12a' themselves. More particularly, in a known technique for preparing the fiber optic elements 12a, 12a' for optical coupling, it has been the common practice to simply cleave the end of the fiber optic element 12a to provide a very flat, smooth end face 14a which is perpen-dicular to the longitudinal axis 18 of the fiber optic ~0 element 12a. With such a technique, it will be appreciated that a very sharp peripheral edge 74 will be formed between the end face 14a and the cylindrical outer or peripheral surface 76 of the fiber optic element 12a, and it is this peripheral edge 74 which contacts the spheres 62 of the engagement/abutment unit 54.

~1'7~1538 -~7-This is also the case with those known arrange-ments in which a hard or impervious protective coating 78 defines the outer cylindrical surface 76 of the fiber optic element 12a. More particularly, as is known in the art, fiber optic elements 12a made of glass or plastic are very fragile. If such fiber optic elements 12a are sub-jected to repeated coupling and uncoupling, there is the possibility of damage or deterioration to the end face 14a thereof, which in turn may result in optical distortion 10 and losses when same are coupled in the connector assembly 10. Consequently, in some prior art coupling arrangements, a- thin, hard and impervious protective coating 78 is provided about the cylindrical surface 76 of the fiber optic element 12a in an effort to minimize the possibility 15 of such deterioration of the end face 14a. Generally, the thickness of the applied protective coating is on the order of 1-10 microns for fiber optic elements 12a having a diameter of 120-150 microns. Such a hard protective coating 78 is applied to the end of the fiber optic 20 element 12a by a dipping process which results in a very thin uniform coating being provided completely about the circumference of the fiber optic element 12a. Thereafter, upon curing of the coating, the fiber optic element 12a is cleaved so as to again provide a square type corner or 25 edge 74 between the end face 14a and the cylindrical surface 76 of the coated fiber optic element 12a. In such an arrangement, it is the outer diameter of the coating 78 which defines the diameter of the fiber optic element 12a, and thus, it is the corner or peripheral edge 74 of the 30 outer protective coating 78 which contacts the spheres 62.
The axial gap Ga between the end faces of the optical fiber elements 12a, 12a' is dependent upon the diameter of such optical fiber elements. Moreover, changes in the gap spacing are very sensitive to changes in the 35 optical fiber element diameter. For example, for typical commercially available optical iiber elements ~2a having a 11'79S38 diameter of 125 microns, the tolerance on the size of the diameter of the fiber optic elements 12a (i.e., the diameter of the fiber optic element 12a having the coating 78 thereon) is typically on the order of - 5 microns.
I~n a four ball connector designed to accommodate 125 micron fiber optic elements, it can be shown that a 10 micron change in outside diameter of the fiber optic element 12a results in an approximately 50 micron change in the size of the end gap G .
In order to reduce the size of the axial gap G
for given fiber diameters, in accordance with one aspect of the present invention as shown in Figures 4 and 5, the fiber optic elements 12, 12' are prepared so that the protective coating 80 provided thereon has a shape such 15 that the diameter of the contact areas 82 on the protec-tive coating 80 (i~e., the areas 82 which contact the supporting surface 72 of the connector spheres 62 in point areas 83), is less than the diameter of the coating 80 at a location along the axial length of the fiber optic 20 element 12 spaced from the end face 14 of the fiber optic element 12. This arrangement has the effect of reducing the diameter of the fiber optic elements 12, 12' at the location of contact with the support surface 72 of the connector assembly 10, i.e., the point surface areas 83 of 25 the spheres 62, such that the end faces 14, 14' of the fiber optic elements 12, 12' are brought closer toward the planè 68 of the centers 66 of the spheres 62, thereby reducing the size of the end gap G between the end faces 74 of the fiber optic elements 12, 12'. Here, it should be 30 noted that with the type of connector assembly 10 described hereinabove, because of the very precise lateral or axial alignment provided by the engagement/abutment unit 54, a major contributor to the loss in transmission is the end gap spacing G. Consequently, by reducing the 35 size of the axial gap G, the loss for the particular connector assembly 10 is also reduced.

11'~9538 In accordance with the preferred embodiment of -the present invention, the protective coating 80 which is shaped to provide a contact area 82 having a diameter which is less than the outer diameter of the protective coating 80 at an axially spaced position along the fiber optic element 12 is achieved by initially cleaving the end of an uncoated fiber optic element, similar to the cleaving method employed in the prior art, and thereafter applying a coating material 80 to the cleaved end of the 10 fiber optic element 12 and curing same so as to provide the desired shape for the protective coating 80. Advan-tageously, the coating 80 may be applied by dipping the cleaved end of the uncoated fiber optic element into a suitable coating solution and then withdrawing same to 15 cure or dry the coating 80 on the end of the fiber optic element 12. The phenomena of surface tension tends to ensure that there is an even thickness of coating 80 around the entire end of the fiber optic element 12, thus maintaining the concentricity of the core 26 and cladding 20 28 of the fiber optic element 12 within the coating 80 which is important in the particular alignment technique employed in accordance with which the preferred embodiment of the present invention.
In addition, the coating 80 on the periphery of 25 the fiber optic element 12 covers the end face 14 of the fiber optic element 12 and forms a small radius at the edge 82 which thus defines the contact area 82 of the protective coating 80. This can be best seen in Figures 4 and 5. The provision of this radius at the peripheral 30 corner or edge 82 of the fiber optic element 12 is important in providing the desired shape for the coating in which the outer diameter of the contact area 82 (i.e., the portion of the protective coating which con-tacts the spheres 62) is less than the outer diameter of 35 the coating 80 along the axial length of the element 12 adjacent to the end thereof. Since the outer diameter of the protective coating 80 at the point contacts 83 is less il'~9S38 than the diameter of the fiber optic element 12, the effect is that the end face 14 of the fiber optic element 12 is moved forwardly toward the plane 68 of the centers 66 of the spheres 62, thereby reducing the axial end gap G
between the end faces 14 of the fiber optic elements 12,' 12' which are coupled together. This effect can best be seen in Figure 5 which illustrates the differences between the positions of the end faces 14 of the fiber optic eléments 1~, 12' having a rounded corner (shown in fuil ]o outline) and the end faces 14a of the fiber optic elements 12a, 12a' having .a square peripheral edge 74 (shown in dotted outline).
Here, it should be noted that the small radius provided at the peripheral edge 82 of the fiber optic 15 element 12 naturally or inherently results when the protec-tive coating 80 is applied by dipping the end of a cleaved, uncoated fiber optic element into an appropriate coating solution, and then withdrawing same from the coating solution and drying or curing the coating material 2~ 80. It of course should be appreciated however that different shapes for the protective coating 80 could' be achieved by simply controlling the application 'and drying or curing processes in any well known manner, such as for example by the use of drying jets, by mechanical means, or 2.S by moving the coated fiber 12 in a specified manner while the coating solution 80 cures. Also, appropriate shaping operations could be performed on the end of the fiber optic element 12 after the coating 80 has been applied and cured in order to reduce the diameter of the contact areas ~o 82 of the fiber optic element 12.
Still further, the protective coating 80 may be provided by a very thin sheath of suitable material which is placed over the end of the fiber optic element 12 and which serves to surround the end of the fiber optic' 7~5 element 12. The sheath may have a closed bottom surface with the end face 14 of the fiber optic element 12 abutting same, or the sheath could simply have an open 9S3~3 bottom. The sheath would provide the contact area 82 for engaging the supporti.ng surface 72 of the spheres 62 and, in accordance with the present invention, would define a surface 82 having an outer diameter which is less than the outer diameter of the sheath at an axially spaced location along the fiber optic element 12.
Here it should also be noted that the advantages of physically reducing the size of the axial gap G can also be obtained if no protective coating 80 were provided 10 on the fiber optic element 12 by shaping the end of the fiber optic element to provide a contact area which is of a lesser dimension than the diameter of the fiber optic element 12 at a location axially spaced from the end face ].4, such as by shaping the end of the cladding 28. In 15 summary, in order to achieve the advantages in accordance with the present invention,. the contact portion or portions 82 of the fiber optic element 12 which contact the support surface 72 of the connector assembly 10 are shaped or formed so as to have a diameter about the end 20 face 14 of the fiber optic element 12 which is less than the outside diameter of the fiber optic element 12 at an axially spaced location along the fiber optic el.ement 12 adjacent the end of thereof. In this manner, the end face ~4 of the fiber optic element 12 will be moved forwardly 25 when it contacts the support surface 72 of the engage-ment/abutment unit 54.
Here it should be noted that it has been found in experiments that the technique in accordance with the preferred embodiment of the present invention, in which a 30 cleaved fiber optic element is coated with a suitable coating material 80 and which is then simply dryed or cured without any external shaping operations being per-formed (thus inherently providing a small radius of curva-ture at the peripheral edge 82 of the fiber optic element 12) has consistently produced a reduction in the axial end gap G on the order of 18-20 microns for a 125 micron 11~79538 nominal fiber optic element 12 utilizing a four sphere arrangement which has been optimized for a 125 micron fiber. Such a reduction was achieved in comparison to conventional prior art techniques in which the end of an optical fiber 12a is initially coated and then simply cleaved to provide a square end face 14. Thus, these experiments indicate that the small radius for the contact surface 82 of the protective coating 80, believed to be inherently obtained simply by a dipping technique after 10 cleaving, is sufficient to provide a significant reduction ' in the size of the end gap G. However~, for other types of arrangements, it may be desired to provide a greater reduction in diameter, in which case a different shape for the protective coating 80 could be provided, such as for ' 15 example a bevelled edge or a concave edge.
These experiments have also shown that the technique in accordance with the present invention reduces the loss of a four sphere connector 10 typically by .2db over that using fiber ends prepared using the conventional technique of simply coating the ends of the fiber optic element 12a and then cleaving same to provide a substan-tially square end face 14a. Here it should be noted that in noncontact type coupling arrangements such as utilized in accordance with the present invention, the major contri-butor to the loss, since the axial or lateral alignment of the fiber optic element 12 is very precise, is the spacing ~, of the end gap G. Consequently, improvements on the order of .2db over the prior art techniques for such fiber optic connector assemblies 10 can be of prime importance in optical fiber transmission systems for the long wavelength region of high silicate fibers, amounting to an increase , in spacing of repeaters of many hundreds of meters. As can be appreciated, this can significantly increase the economics of such systems.
It is preferred that the thickness of the protec-tive coating ao provided on the end of the fi~er rptic element 12 be very small, for example on the order of l-lo microns for fiber optic elements 12 having a nominal diameter (including the coating 80) of 125 microns. Such a small thickness coating 80 is all that is required to protect the end of the fiber optic element 12 when same is assembled in the connector assembly 10. However, thicker coatings 80 could be utilized, and applied using a single ; dip process or even a multiple dip process t and still achieve the advantages in accordance with the present 10 invention. In this regard, the thickness of the coating on the end face should, in general, be not greater than the thickness of the buffer material surrounding the optical cladding 28.
Also, since the end face 14 of the fiber optic 15 element 12 is also coated in the preferred embodiment, the coating 80 is to be of a transparent material so as to permit the transmission of light signals therethrough.
Typical examples of coating materials which may be used for providing a thin protective coating 80 on the fiber 20 optic element 12 and which are transparent include various plastic resins and polymer mixtures, such as for example, Eastman 398-3, which is a cellulose acetate lacquer having an index of refraction of about 1.475.
In accordance with another aspect of the present ~5 invention, the performance o the coupling arrangement can be further improved by providing a coating on the end face 14 of the fiber optic element or device 12 which has an index of refraction which is substantially greater than that of the medium between the ends of the fiber optic 30 elements or devices 12, 12a, thereby resulting in the effective end face 14 of the fiber optic elements 12, 12' being moved optically toward the center of the connector assembly 10, i.e., toward the plane 68 in which the centers 66 of the spheres 62 are located.
More particularly, in accordancè with the noncon-tact type coupling arrangements with which the present ~1~7~3S38 invention is utilized, a gap or space G is provided between the end faces 14 of the fiber optic elements 12, 12' being coupled together. The index of refraction of the medium between the end faces 14 of the fiber optic elements 12, 12' will either be that of air ~when no further material is introduced into the gap G, as in the aforementioned U.S. Patent No. 4,378,145) or that of a fluid or other material which is physically located between the end faces 14 of the fiber optic elements 12, 12' (as in U.S. Patent No.

4,186,998 and U.S. Patent No. 4,119,362). In accordance with this aspect of the present invention, the index of refraction of the coating 80 on the end faces 14, 14' of the fiber optic elements 12, 12' is substantially greater than the index of refraction of the medium. By substantially greater, it is meant that the index of refraction is at least 20% greater than the index of refraction of the medium.
Although not meant to be bound as to the theory of this aspect of the present invention, it is believed that the coating 80 having the substantially higher index of refraction than that of the medium serves to optically move the effective end face of the fiber optic elements 12, 12' closer together. More particularly, Figure 6 shows a greatly enlarged cross section of the fiber optic element 12 having a coating 80 provided thereon overlying the end face 14 of the core region 26 of the fiber optic element 12. In this figure, the cone 90 defined by the limiting light ray 92 for propogation of light through the fiber optic element 12, at an angle ~1 with respect to the normal, exits from the core region 26 of the fiber optic element 12 and defines a cone 94 in the coating region 80 in which the light ray 96 is at an angle ~2 with respect to the normal, angle ~2 being less than angle ~1 Upon leaving the coating region 80, the light ray 98 deining the cone 100 is refracted away from the normal at an angle mg/~ t-~ - 24 -11~795~8 as a result of the index of refraction of the coa-ting being greater than the index of refraction of the medium 102 it is entering, i.e., the medium 102 in the gap between the end faces 14 of the fiber optic elements 12, 12'. Thus the cone 100 of light exiting from the coating 80 appears to be emanating from the fiber optic element 12 at a new effective end face 104 which is defined by projecting the refracted rays 98 defining the cone 100 rearwardly along a straight line to determine the inter-10 section, i.e., at the location P in Figure 6. This neweffective end face 102 of the fiber optic element 12 çorresponds to the plane of the virtual image of the cone of light 90 exiting from the core region 28 of tne fiber optic element 12. Thus, the end face 14 of the fiber optic 15 element 12 appears optically to be much closer or to have been moved toward the center plane 68 of the connector assembly 10. This phenomena is similar to the phenemona of viewing an object under water in which the distance of the object from the surface of the water appears to be much 20 less than the actual distance of the object below the surface of the water. As can be appreciated, the greater the differences between the index of refraction of the coating B0 and of the separating medium 102, the more dvantageous the effect provided by this aspect of the ~, present invention--namely, the closer the effective end face 104 of the fiber optic element 12 appears to be to the center plane 68 of the connector assembly 10, and thus to the end face 14 of the opposite fiber optic element (not shown).
In accordance with the preferred embodiment of the present invention, the coatiny 80 on the end face 14 of the core region 26 of the fiber optic element 12 rnay advantageously be provided by suitably choosing an appro-priate material having an index of refraction higher than ~S that of the medium 102 for the the protective coating 80 which is applied to the end of the fiber optic element 12 795~13 in accordance with the first aspect of the present inven-tion (i.e., to provide a protective coating 80 having a shape such that the diameter of the contact area 82 of the protective coating 80 is less than the diameter of the protective coating 80 on the surface 76 of the fiber optic e]ement 12 at an axial location spaced from the end face 1~ of the fiber optic element 12). In this manner, the coating 80 not only provides the feature of optically moving the end face 14 of the fiber optic elements 12, 12' 10 closer to the center of the connector assembly 10 but at the same time provides a smaller diameter for the contact area 82. This is advantageous since both features for optically and physically reducing the axial gap G between the end faces 14 of the fiber optic elements 12, 12'- may 15 be provided by simply coating the end of the fiber optic element 12 in a single operation.
Preferably, the differences between the indices of refraction of the coating and of the medium are on the order of at least 20% in order to achieve an appreciable 20 effect on the loss characteristics for the coupling arrangement. In this regard, if the separating medium 102 comprises air which has an index of refraction of about 1.0, the index of refraction of the coating 80 should be on the order of 1.20 or greater. Most transparent 2'8 materials have an index of refraction above 1.35 and thus virtually any such material could be utilized for the coating 80. Here it should be noted that the index of refraction of the coating 80 on the end face 14 of the core region 26 may be greater than the index of refraction 30 of the core region 26 (which is generally on the brder of 1.45-1.5), or may be smaller than that of the core 26, and still provide the advantageous effects in accordance with the present invention. In the preferred embodiment, as noted hereinabove, the particular material for the protec-35 tive coating 80 may comprise any suitable transparentmaterial such as most plastic resins or polymer mixtures 11~795;~

whi,ch have indices of refraction which range between approximately 1.35-1.60.
Although in the preferred embodiment in accord-ance with the present invention the protective coating 80 r> provided on the end of the fiber optic element 12 not only serves to physically reduce the size of the gap G between the end faces 14 of the fiber optic elements 12, 12' but also serves to optically reduce the size of the gap G, it ~ill of course be appreciated that it is not necessary 10 that the protective coating 80 extend completely across the end face 14 of the fiber optic element 12 in order to achieve the advantageous effect of physically reducing the si7,e of the axial gap G. Instead, the axial gap G could be physically reduced by an arrangement in which only the l5 peripheral corner edges 82' of the fiber optic element 12"
have a coating 80' thereon for contacting the support surfaces 72 of the spheres 62, as for example shown in Figure 7. Then a second coating material 106 could be provided over the core region 26 having a desired index of 20 refraction which is different from that of the protective coating 80'. In Figure 7, the coating 106 of the end face 14 of the core region 28 serves to optically reduce the gap C between the end faces 14 of the fiber op-tic elements 12, ]2' (by having an index of refraction substantially 2r~ greater than the index of refraction of the medium 102 ir t;he gap G), whereas the protective coating 80' provided on the periphery of the fiber optic element 12" provides the desired shape (i.e., radius) for reducing the outer diameter of the contact portions 82' thereof which will ~S0 contact the spheres 62 of the connector assembly 10. In this instance, the protective coating 80' could be of any ;index of refraction.
It should also be noted that althouyh in the preferred embodiment a particular connector assembly 10 3'j has been illustrated as comprising a plurality of spherical bal]s or elements 62 which provide -the support -2~-surfaces 72 for the fiber optic element 12l a continuous frustoconical or tapered surface could be utilized for providing the support surface for the peripheral edges of t,he fiber optic element, such as for example that provided by the coupling members shown in U.S. Patent Nos.
4,186,998 and 4,119,362.
Therefore, in accordance with one aspect of the present invention, there is provided a fiber optic coupling system and method for coupling of an optical ]0 device 12' having an end surface 14' to a fiber optic element 12 having an end face 14. The fiber optic element 12 has an outer protective coating 80 along a portion of its axial length adjacent the end face 14 thereof, which protective coating 80 has an outer diameter of a first 15 dimension at an axial location spaced from the end face 14 of the fiber optic element 12. The fiber optic element 12 is adapted to be supported by a noncontact type coupling member 10 for optically coupling the fiber optic element lZ with the optical device 12'. The coupling member 10 20 serves to support the end surface 14 of the optical device ~2' in a first predetermined position and further includes .~ support surface 72 for supporting a contact portion 82 ~f the protective coating 80 of the fiber optic element 12 in a second predetermined position which is axially spaced 2r, ~rorn the first predetermined position. The pro-tective coating 80 of the fiber optic element 12 has a shape such that the contact portion 82 of the protective coating 80 defines a surface which has an outer diameter less than the first dimension. In this manner, the axial gap or 30 space G between the end faces 14 of the fiber optic element 12 and the optical device 12' may be minimized, thereby providing a connector system having a high efficiency for transmission of light signals.
In accordance with another aspect of the present invention, the fiber optic element 12 includes a core 28 and a coating 80 on the end face 14 of the core 26 which substantially covers the end of the core 26. The coating --~`` 11'79S38 80 on the end face 14 of the core 26 has a predetermined index of refraction. Coupling means 10 are provided for supporting the fiber optic element 12 and an optical device 12' in alignment with one another with the end face ', 14 of the fiber optic element 12 being axially spaced from the end surface 14' of the optical device 12'. The coupling means 10 provides a medium between the end face 14 of the fiber optiG element 12 and the end surface 14' of the optical device 12' which has an index of refraction 10 which is substantially less than the predetermined index of refraction of the coating 80. In this manner, the - effective end face 104 of the fiber optic element 12 is moved optically closer toward the end surface 14 of the optical device 12', thereby serving to reduce or minimize 15 the transmission loss of the coupling system.
Advantageously, the coating on the end face 14 of the core 26 and the protective coating ,on the fiber optic element 12 in accordance with the present invention may be provided by applying a protective coating material 80 to the end of the fiber optic element 12, including the end face 14 thereof, and curing the protective,coating 80 so as to provide a shape in which the contact portion 82 of the protective coating 80, for .contacting the support surface 72 of the coupling device 10, has a diameter which is less than the diameter of the protective coating 80 at an axially spaced position from the end face 14 of the fiber optic element 12, the index of refraction of the coating 80 being substantially greater than the index of refraction of the medium between the end face 14 of the fiber optic element 12 and the end surface 14 of the ' optical device 12'.
While the preferred embodiments of the present invention has been shown and described, it will be under-stood that such are merely illustrative and that changes 35 may be made without departing from the scope, of the invention as claimed.

Claims

-30- T&B 876F

CLAIMS:

1. A fiber optic termination assembly comprising:
an optical fiber having an end face, an outer surface defining an outer diameter at an axial location spaced from said end face and a contact surface between said outer surface and said end face having a diameter less than said outer diameter;
terminating means including a support surface for engaging said contact surface of said optical fiber in abutting relation therewith; and means for supporting said optical fiber in said abutting relation with said support surface of said ter-minating means.

2. The fiber optic termination assembly of claim 1 wherein said support surface of said terminating means defines mutually discontinuous surfaces spaced from each other in a plane substantially parallel to said end face in a circular locus having a diameter less than the diam-eter of said contacting surface of said optical fiber.

3. The fiber optic termination assembly of claim 1 wherein said contact surface of said optical fiber de-fines a peripheral edge curved about the circumference of said optical fiber.

4. The fiber optic termination assembly of claim 1, wherein said optical fiber has a core and an optical member on the end face of said optical fiber substan-tially covering the end of said core, said optical member having a predetermined index of refraction; wherein said fiber in the engaged relation with said support surface defines with said support surface a cavity adjacent one surface of said optical member; and further including an optical medium within said cavity in contact with said one surface of said optical member, the index of refrac-tion of said optical member being substantially greater than the index of refraction of said optical medium.

5. The fiber optic termination assembly of claim 4 wherein said optical member is a transparent coating.

-31- T&B 876F

6. The fiber optic termination assembly of claim 4 wherein said optical medium comprises air.

7. A method of optically terminating an optical fiber in the termination assembly of claim 1, comprising the steps of:
providing the optical fiber to include said end face and said contact surface;
providing said support surface on said termi-nating means for engagement with the contact surface of said optical fiber; and placing said contact surface of said optical-fiber into-engagement with said support surface.

8. The method of claim 7 wherein the step of providing an optical fiber comprises applying a continuous coating of optical material having a predetermined index of refraction onto said outer surface, said contact surface and said end face.

9. The method of claim 8 further including the step of providing an optical medium adjacent to and in contact with the coating on said end face.

10. The method of claim 9 wherein the index of refraction of said optical coating is selected to be substantially greater than the index of refraction of said optical medium.