Connectors for coaxially coupling the end of a linear optical waveguide element for a receiving surface

For coupling the end of an optical fibre or other linear optical waveguide element to a small-area receiving surface (e.g. surface of photodetector diode) in such manner as to ensure that the fibre makes contact with the surface at right angles to and substantially centrally to the surface, the receiving surface is centrally arranged at the end of a cylindrical boss, and the end of the fibre or like element is supported in a coupling member comprising a body having an end portion in which the end of the element is centered and of which at least the end portion which in the coupled position is nearest to the receiving surface is enclosed in a cylindrical holder tube which extends beyond the end of the end portion to form a collar adapted to encircle the boss and providing a cylindrical bore for sliding co-operation with the cylindrical outer surface of the boss.

This invention relates to connectors for coupling the end of an optical 
fiber or other linear optical waveguide element to a small-area receiving 
surface, for example to the surface of a photodetector diode, in such a 
manner as to ensure that the fibre makes contact with the surface at right 
angles to and substantially centrally of the latter. 
It is an object of the invention to provide an improved, relatively 
inexpensive connector for coupling the end of an optical waveguide element 
to a small-area light-receiving surface, which comprises two connector 
members by means of which effective coupling can be readily and reliably 
achieved even in relatively inaccessible locations. 
According to the present invention there is provided a connector for 
coupling the end of an optical waveguide element to a light receiving 
surface mounted on a first connector member, in which the end of the 
optical waveguide element is supported in a second connector member, and 
in which one of the connector members includes a tubular part which 
slidingly receives at least part of the other connector member for 
accurately positioning the end of the waveguide on the receiving surface. 
The receiving surface may be centrally arranged at the end of a cylindrical 
boss, with the end of the fibre or like element being supported in the 
second connector member and the latter comprising a body having an end 
portion in which the end of the waveguide element is centred, and of which 
at least the end portion which in the coupled position of the coupling 
members is nearest to the receiving surface, is enclosed in a cylindrical 
holder tube which extends beyond the end of the said end portion to form a 
collar which is adapted to encircle the boss and which provides a 
cylindrical bore for sliding co-operation with the cylindrical outer 
surface of the boss. 
In order to ensure correct positioning of the end of the waveguide element 
in relation to the receiving surface, the said body is preferably provided 
with a longitudinal bore whose diameter only slightly exceeds that of the 
waveguide element so that the latter is a sliding fit therein, but which 
is flared at the outer end of the body to form a lead-in cone for the 
waveguide element. This bore is preferably formed while the body is cast 
in a mould which is partly constituted by the said holder tube, and 
through which a wire, of a material whose shrinkage is higher than that of 
the body, is extended along the axis of the holder tube under longitudinal 
tension, in a manner similar to the method of providing such bore 
described in our U.S. Pat. No. 3,944,328. 
When the invention is intended for use in cases in which the receiving 
surface is very small and liable to be eccentric to the boss sufficiently 
to involve the risk of unsatisfactory coupling with a waveguide element 
whose end is concentric with the boss, the cylindrical bore for sliding 
co-operation with the boss is, according to one form of the invention, 
provided in a collar member which is so mounted within the collar portion 
of the holder tube as to be adjustable therein along one of its diameters 
thus making it possible, when the collar member is so placed on the boss 
that the said diameter extends in the direction of the eccentricity of the 
receiving surface to align the said surface with the end of the fibre or 
other waveguide element.

Referring now first to FIG. 1, the connector member carrying the receiving 
surface is shown at 1, while 2 illustrates the connector member which 
supports the end of an optical fibre or other linear waveguide element. 
The surface-carrying member 1 comprises a boss-like cylindrical body or 
boss 3 which is carried at the end of a stem 4 to which one end face of 
the boss is attached, while a diode chip 5, constituting a photodetector 
diode is concentrically arranged on the opposite end face 3a of the boss 
3, with its receiving surface 6 parallel to the end face 3a of the boss. 
The other connector member 2 comprises a holder tube 7 which fits slidingly 
over the boss 3 of the surface-carrying member 1, and which encases a body 
8, which is firmly secured therein, preferably by having been cast in 
position, and in which a narrow tubular through-bore or passage 9 of a 
diameter only slightly exceeding that of the optical fibre or the like, 
which is to be coupled to the receiving surface 6 is arranged coaxially 
with the tube 7 to provide accurate sliding guidance for such fibre in 
coaxial alignment with the tube 7. The body 8 has two parallel end faces 
at right angles to its axis, and the tube 7 is arranged to extend beyond 
one of these end faces, hereinafter called the coupling face or the front 
end of the body 8 to form a collar 7a adapted to be slid on to the boss 3 
to permit substantial contact of the coupling face with the receiving 
surface when coupling is to be effected. At the opposite or rear end face 
of the body 8, the passage 9 ends with a flared lead-in portion 9a to 
facilitate the introduction of an optical fibre element. In order to 
couple such fibre element to the photodetector diode 5, the second 
connector member 2 is coupled to the first connector member 3 by first 
sliding the collar portion 7a of the holder tube 7 fully home on to the 
boss 3 of the member 1; then the optical fibre element is introduced into 
the passage 9 of the member 2 from the flared end of this passage, after 
some liquid whose index of refraction matches that of the optical fibre 
element, has been applied to the end of the fibre or to the receiving 
surface 6 or has been introduced into the passage 9. It will be readily 
appreciated that, provided the receiving surface 6 is arranged accurately 
at the centre of face 3a and at right angles to the axis of the boss 3, 
the use of the coupling member 2 will ensure accurate alignment of the end 
of the fibre with the receiving surface at accurately right angles to that 
surface. It will be obvious to those skilled in the art of fibre optics 
that the body 8, at least at the surface of the passage 9, should be of a 
material that will not be liable to transmit from the fibre to the body 8 
any substantial part of the radiation conveyed by the optical fibre which 
is to be coupled to the surface 6. 
FIG. 2 illustrates a casting jig that may be employed for the manufacture 
of the connector member 2 illustrated in FIG. 1. This device comprises a 
bracket plate 10 which is arranged to extend substantially horizontally, 
and which is provided at its lower side with a locating collar 11. The 
latter serves to locate the upper end of a length of tube which is 
intended to constitute the holding tube 7 of such connector member and has 
therefore been indicated by the reference number 7. Coaxially with this 
collar, the lower surface of the bracket plate 10 carries a projection 10a 
in the form of a truncated cone corresponding to the lead-in tapered 
portion 9a of the passage 9 in the body 8 of FIG. 1. Both the projection 
10a and the bracket plate 10 itself are penetrated by a bore 12 for 
accurate sliding guidance of a metal wire 13 for a purpose to be explained 
further below, and a spout 14 is arranged in the plate 10 between the 
projection 10 and the inner diameter of the tube 7 to permit the injection 
of suitable moulding material for the formation of the body 8 of the 
connector member to be produced. The front face of the body 8 is 
determined by a plug member 15, which is introduced into the other end of 
the tube 7 with a sliding fit and has an end flange 16 for supporting that 
end of the tube 7. This plug member is also provided with a coaxial bore 
19 whose diameter is equal to that of the bore 12, and which is preferably 
flared, as shown at 19a, at its end facing away from the body 8. The 
above-mentioned metal wire 13 is if necessary after the application of an 
anti-adhesive coating, threaded through both the aligned bores 12 and 16. 
Above the plate 10 it is held in a clamping block 17 which rests on the 
plate 10, while a clamping and tensioning member 18, supported against the 
outer end face 20 of the flanged plug member 15, applies a longitudinal 
pull to the wire 13. This ensures that the wire is straight and at the 
same time serves to hold the plug member 15 fully inserted into the tube 7 
and, through the flange 16, to hold the tube 7 in contact with the plate 
9. A suitable moulding material which will adhere firmly to the tube 7 but 
will, during its solification, shrink less than the wire 13, is now 
introduced through the spout 14 to fill the interior of the tube 7 between 
the plate 10 and the plug 15 and is then allowed to solidify. Then, after 
taking off the clamping-and-tensioning device 18, the tube 7 with the 
solidified body 8 and the plug member 15 can be pulled away from the 
bracket plate 10 and slipped off the wire 13. Finally the plug 15 is 
withdrawn out of the tube 7, thus completing the manufacture of a 
connector member 2 as shown in FIG. 1, and another similar connector 
member can then be similarly produced, employing another length of tube 7. 
While it is generally preferred to employ the body 8 itself to provide the 
wall surface of the passage 9, this wall surface may alternatively be 
provided by using a separate capillary tube, suitably flared to provide 
the lead-in taper 9a, and casting the body 8 round the capillary tube. In 
this case the bracket plate 10 may be made without a projecting cone 
portion such as that shown at 10a in FIG. 2. The capillary tube will be 
threaded on to the wire 13 above the plug 15, and a casting material is 
employed which will adhere firmly to the capillary tube as well as to the 
holding tube 7. 
While the connector member 2 as hereinbefore described will be found highly 
satisfactory for use with receiving surfaces which are accurately centred 
on the end face of the boss 3, it is sometimes desirable to permit the use 
of a coupling member of similar kind even when the receiving surface 
happens to be arranged slightly out of centre so that the degree of 
coupling achieved when the optical fibre or other similar light-guide 
element is placed against the centre of the end face of the boss 3, is 
unsatisfactory. 
A connector of modified form, which permits even in this case a 
satisfactory degree of coupling to be achieved is illustrated in FIG. 3. 
In this modified connector, a chip 5 presenting the receiving surface is, 
similarly as in the embodiment described with reference to FIG. 1, 
provided at the front face of a boss 3, but the body 8 of the second 
connector member 2A is cast in situ in a holding tube 21 -- an operation 
which may be carried out in the manner hereinabove described with 
reference to FIG. 2 -- This holding tube has an inside diameter which is 
larger than the diameter of the boss 3 by an amount sufficient to permit 
an auxiliary collar 22 to be accommodated with some radial play between 
the boss and the inner surface of the tube 21. This auxiliary collar 22 is 
a sliding fit in the boss 3 and is preferably provided with a flange 23 
whose upper side (as shown in the drawing) faces the adjacent end of the 
holding tube 21 and is adjustable along a diameter of the latter by means 
of a set screw 24. The latter is arranged in a suitably screw-threaded 
bore of the tube 21 and has a pointed end which engages a centering recess 
in the outer surface of the auxiliary collar 22. A pin 25 radially 
projecting from a point of this collar that is diametrically opposite to 
this recess, is slidably supported in a cross-bore of the tube 21 which is 
aligned with the bore carrying the set screw 24, and a spring collar 26 or 
equivalent resilient member is interposed at the latter side between the 
collar 22 and the holding tube 21. Owing to this arrangement, adjustment 
of the set screw 24 will move the collar 22 in the direction of the 
diameter determined by the centering recess and the pin 25, and it will be 
readily appreciated that, provided the connector member 2A is placed on 
the boss 3 with the diameter containing the screw 24 and pin 25 extending 
in the direction of eccentricity of the receiving surface, it is possible, 
by turning the screw 24 in the appropriate direction, to align the passage 
9, and thus the fibre element inserted into this passage, accurately with 
the receiving surface. 
Various details of the connector and its use may be modified within the 
scope of the invention. Thus the boss 3, instead of being carried on a 
stem 4 as shown in FIG. 1, may be provided on a plate member as shown at 
27 in FIG. 3, and while the index-matching liquid employed for improving 
the passage of optical wave energy from the optical fibre or the like to 
the receiving surface will in many cases be a liquid which will not change 
its character after its application, a liquid that will set after use may 
be used if the connection established is not required to be temporary.