Fiber optic connector

An optical fiber connector of the watch bearing jewel type. To prevent "growing out" of a fiber end to which the connector is terminated, a thin protective window is placed against the bearing jewel and supported by a fiber stub mounted in a second watch bearing jewel. The fiber and the fiber stub are optically coupled via the window.

BACKGROUND OF THE INVENTION 
This invention relates to optical fiber couplings and, in particular, to 
fiber splices and connectors of the type in which watch bearing jewels are 
employed to align two fibers to be coupled. 
Optical fiber connectors or splices of the type in which one or more watch 
bearing jewels are employed to align a pair of fibers are increasingly in 
common use in the construction of fiber optic transmission systems. A well 
known problem with such connectors or splices, when used with plastic 
coated, clad silica fibers, is the phenomenon of "growing out" of the 
fiber ends. After the fiber end has been stripped of its coating and 
fitted to the connector or splice, the coating immediately adjacent the 
fiber end tends to relax and force the fiber end further into the 
connector. Such movement of the fiber can cause damage to the fiber end 
and/or the connector itself. 
We have previously designed a termination for a plastic coated optical 
fiber having a bared end portion. The termination includes a body having a 
cylindrical outer reference surface, a coaxial inner bore for receiving 
the coated fiber and alignment means for locating the fiber end coaxial 
with the inner bore. A transparent flat protection window is arranged 
adjacent and normal to the fiber end. 
The purpose of the window is to protect the fiber end from damage when 
coupling to a similar connector member. The window introduces only a 
relatively small coupling loss. 
In certain applications it is advantageous to reduce even the small 
coupling loss introduced by such a protection window to a minimum. This 
may be done by reducing the thickness of the window so that, when the 
connector member is mated with a similar member, the two fibers being 
coupled are virtually in contact, being separated only by two very thin 
windows. However, it has been found that such reduced thickness windows 
are fragile and tend to be fractured by the fiber itself which, when 
"growing out" occurs, exerts a considerable pressure on the window. 
SUMMARY OF THE INVENTION 
According to the principal aspect of the invention there is provided an 
optical fiber termination for a plastic coated fiber. The termination 
comprises a tubular body member having alignment means for receiving and 
aligning the fiber coaxial with the body member. An optical fiber stub is 
mounted in the bore of a watch bearing jewel positioned adjacent to and in 
front of the alignment means. A transparent protective window is disposed 
between the fiber alignment means and the bearing jewel against which the 
fiber and the fiber stub abut, whereby the fiber and the fiber stub are 
coupled optically via the window.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawing in detail, the optical fiber connector member 
10 depicted therein includes a tubular e.g. metal ferrule 11 and the outer 
surface of which provides a reference surface for mating with a similar 
connector member. The ferrule 11 has an accurately machined coaxial bore 
12 into one of which a watch bearing jewel 13 is fitted. The bore 14 of 
the jewel 13 is of such a size as to receive and locate the bared end 15 
of a plastic coated optical fiber 16. A relatively thin flat transparent 
window 17 is placed adjacent to and in front of the jewel 13 and is held 
in place by a second jewel 18 fitted into the ferrule bore 12. The jewels 
13 and 18 are preferably of the single oil cup type and are arranged 
back-to-back, i.e. with their respective oil cups 19 and 20 facing away 
from one another. 
The jewel 18 is provided with a clad optical fiber stub 21 located in the 
bore 22 of the jewel and secured to the jewel by a quantity of a cured 
epoxy resin adhesive material 23. The epoxy resin is polished flat on both 
the front and back face of the jewel 18 so that the fiber stub 21, which 
is supported by the resin, is flush with the front and back resin 
surfaces. In some applications the window may be affixed to the jewel 18 
by means of the resin 23 thus providing a reference surface for the fiber 
stub 21 and eliminating one polishing step. In order that the resin 23 
should be sufficiently hard to polish readily and resist wear when in use, 
it is preferred to load the resin with a powdered filler material. 
The jewel 18 together with its fiber stub 21 is pressed into the end of the 
ferrule bore such that the window 17 firmly abuts the jewel 13, the front 
face 24 of the resin coated jewel 18 being flush with or slightly 
protruding from the end of the ferrule 11. The face 24 of the resin coated 
jewel 18 then provides a mating surface for coupling to a similar 
connector member. The jewel 18 together with the fiber stub 21 provide a 
support for the window 17 and thus prevent punch-through of the fiber end 
15 arising from axial pressure on that fiber end 
The optical fiber 16 to which the connector assembly is to be terminated 
may be secured within the ferrule bore by a body of an adhesive material, 
not shown. Alternatively the ferrule 11 may be crimped onto the fiber 16. 
A typical assembly sequence of the connector shown in the drawing is as 
follows. 
The jewel 13 is first pressed into the ferrule and the assembly is then 
fitted to the prepared end of the fiber 15. This fiber may be of the 
plastic, e.g. polypropylene, coated silicone resin clad silica type and is 
prepared by cleaving the fiber perpendicular to its axis and then removing 
the plastic coating adjacent to the cleaved end. The cladding is not 
removed from the fiber. The window 17 is then inserted adjacent the jewel 
13 and the second jewel 18 is pressed into the ferrule thereby locating 
the window 17. A cleaved and bored fiber end is inserted into the bore 22 
of the jewel 18 and is retained therein with an epoxy resin material. When 
the resin has cured, it is polished flush with the ferrule end, at the 
same line forming the fiber stub 21. 
By way of example, and not by limitation, the bore 14 of the jewel 13 is 
chosen to have a clearance of about 2-10 microns on the bared fiber end 15 
and the bore 22 of the jewel 18 has a like clearance on the fiber stub 21. 
The window 17 may be formed of silica having a refractive index of 1.48 or 
sapphire having a refractive index of 1.78. Sapphire is preferred as it is 
harder than silica and is easier to polish in thin sections. Typically, 
the window thickness is 50 microns. The fiber stub 2 has a similar core 
diameter to that of fiber 16 and has an equal or greater numerical 
aperture. Some optimization of connector loss can be achieved by making 
the core diameter of stub 21 greater than the core diameter of fiber 15 by 
an amount corresponding to the alignment tolerances in the connector 
member. 
Various modifications of the connector arrangement will be apparent to 
those skilled in the art. Thus, the jewels 13 and 18 may be replaced by 
plain bearing jewels, i.e. of the type not provided with an oil cup, the 
consequent greater length of the bore of such jewels providing additional 
alignment of the fiber end 15 and the fiber stub 21. In a further 
application the window 17 may comprise a central light guiding portion, 
e.g. a very short fiber stub, surrounded by a support region of a 
relatively low refractive index, not shown. In a further modification the 
jewel 18 with its fiber stub 21 may be prepared prior to insertion in the 
ferrule, i.e. the stub is resin potted and the arrangement is polished, 
prior to insertion into the ferrule. The jewel 18 may then be fitted with 
its oil cup facing inwardly. In this way a very accurate alignment of the 
fiber stub 21 at the front face of the connector member is provided. 
The connector member described herein is intended for use with fibers of 
the plastic coated resin clad silica type such as are employed in the 
construction of optical communication systems. The connector may of course 
also be used with other types of clad optical fibers. 
In a further embodiment the double jewel and window technique may be 
applied to the construction of an optical fiber splice. In such an 
arrangement the fiber stub 21 of the jewel 18 is replaced by a second 
optical fiber (not shown) the arrangement being sealed in a suitable 
protective housing.