Self-aligned plug connector for optical fibers

A casting is made of a corner having three mutually intersecting flat surfaces, one of which is perpendicular to the other two, while a fiber end is positioned against the corner in an orientation such that the end of the fiber butts against said one corner surface and the side surface of the fiber tangentially touches said other two corner surfaces. Two such castings, each carrying a fiber end, are aligned with each other in an alignment frame having two flat alignment surfaces which are oriented with respect to each other at the same orientation as the two corner surfaces which tangentially touch the fiber side surface. Since these orientations are the same, the castings fit precisely into the groove formed by the two flat alignment surfaces of the frame and can be moved toward each other until they touch. In this position the fiber ends carried by the castings are butt aligned.

TECHNICAL FIELD 
This invention relates to couplers for optical fibers and more specifically 
to self-aligned plug type connectors for optical fibers. 
BACKGROUND OF THE INVENTION 
Optically transparent glass and/or plastic fibers are being used 
advantageously in diverse areas of data communication and in other fields 
which require transfer of light energy between two locations. Such use has 
resulted in the need for convenient, reliable and efficient apparatus and 
techniques for coupling a pair of such fibers to each other. The 
difficulty in coupling optical fibers arises largely from their very small 
cross-section. In order to obtain highly efficient transfer of light 
energy from one optical fiber to another, the cores of the fibers must be 
positioned in axial alignment with each other and suitably close together. 
Since optical fiber cores typically have a diameter on the order of 50 
.mu.m, coupled optical fibers must be positioned with great precision. A 
further difficulty arises from the great fragility of such small hair-like 
fibers. 
Optical fiber coupling apparatus and techniques found in the prior art are 
reviewed by C. Kleekamp and B. Metcalf in "Designer's Guide to Fiber 
Optics--Part 4," Electronic Design News, pages 51-62 (Mar. 5, 1978). 
The coupling apparatus and techniques of the prior art may be divided into 
three types: the fiber splice; the alignment adjustable connector; and the 
self-aligned connector. A fiber splice is generally made by bringing two 
fibers into butted alignment through the use of a guiding structure. The 
two fibers and the guiding structure are then all permanently glued 
together in the aligned position with an index-matching adhesive. Guiding 
structures which have been used for splicing include V-shaped grooves, a 
square tube, and a bundle of three parallel rods which are 6.464 times 
larger in diameter than the fibers. The disadvantages of splices are that 
they form a permanent coupling of the fibers and that they are not 
generally convenient to install in the field. 
Optical fiber connectors have the advantage that a permanent coupling is 
not made. Connectors are taught, for example, in U.S. Pat. Nos. 3,936,143 
and 4,019,806. The disadvantages of adjustable connectors are their 
complexity and the inherent need to make a cumbersome manual adjustment 
which requires alignment monitoring apparatus. The need to make a manual 
alignment makes this type of connector very difficult to use in the field. 
Optical fiber connectors which are automatically aligned are, in principle, 
readily usable in the field. Most use cylinders and cones to automatically 
align and hold the fibers. One approach is to use a concentric sleeve and 
locking nuts to align and hold two ferrules. Each ferrule carries an 
optical fiber in concentric relationship therewith. A bundle of three or 
four rods within the ferrule has been used to center the fiber within the 
ferrule. Unfortunately, concentric sleeve connectors are complex and 
expensive. 
It is an object of this invention to provide a self-aligning optical fiber 
plug connector which has low optical energy loss even after many 
reconnection operations. 
It is another object to provide such a connector at low cost. 
Still another object is to provide a connector of this type which is simple 
to use and reliable, even in field use. 
A further object is to provide a durable self-aligning optical fiber 
connector which also protects the fiber end from deterioration while the 
plug connection is being made as well as before and after connection. 
It is also an object to provide a self-aligning optical fiber connector 
which avoids the use of reference surfaces which are rounded. 
DISCLOSURE OF INVENTION 
These and other objects and features of the present invention are achieved 
by casting a plug with flat surfaces onto fiber ends in order to provide 
flat reference surfaces for alignment within a cooperating frame. A mold 
is used to form a concave corner defined by three mutually intersecting 
flat surfaces, the third one of the defining surfaces being perpendicular 
to the line of intersection of the other two. A casting of the corner is 
then made while a fiber is positioned against the corner in an orientation 
such that the end of the fiber butts against the third defining surface 
while the first and second defining surfaces are both tangentially 
touching the side surface of the fiber, at least in the vicinity of the 
fiber end. The alignment frame has two flat alignment surfaces which are 
oriented to each other at the same angle as the first and second defining 
surfaces of the mold are oriented to each other. Since these angles are 
the same, a cast plug fits precisely into the groove formed by the two 
flat alignment surfaces of the frame. The cast reference surfaces of the 
plug which correspond to the first and second defining surfaces of the 
mold come into substantially uniform contact with the two flat alignment 
surfaces of the frame and precisely determine the position of the plug in 
two directions while allowing the plug still to be moved along the groove 
direction. Since the third defining surface of the mold is perpendicular 
to the line of intersection of the other two defining surfaces, the flat 
reference surface of the plug corresponding to the third defining surface 
is perpendicular to this direction. When two plugs are positioned in the 
same alignment frame, the third reference surfaces are parallel, so that 
the plugs may be positioned with the third reference surfaces facing and 
in contact with each other. In this position the optical fiber molded into 
the one plug becomes precisely aligned with and comes in butted contact 
with the optical fiber molded into the other plug facing it. The plugs may 
be mechanically held in this position by any suitable means, such as by 
springy clips.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates how an optical fiber end 10 may simultaneously butt 
against a reference plane 16 while it tangentially touches two other 
reference planes 12, 14. Reference planes 12 and 14 are preferably 
perpendicular to each other, but they may intersect each other at either a 
larger or a smaller angle and fiber end 10 may still be tangent to both. 
Reference plane 16 must be substantially parallel with the end surface 20 
of fiber end 10. Since optical fibers are most conveniently cleaved so 
that the end surface is perpendicular to the axis of the fiber and the 
axis of the fiber end is substantially parallel with the intersection 18 
of planes 12 and 14, reference plane 16 is preferably perpendicular to 
line 18. 
FIG. 2 shows a mold which may be used to cast an alignment plug onto an end 
of a fiber, the plug having reference surfaces corresponding to the 
reference planes of FIG. 1. Surfaces 22 and 24 correspond to reference 
planes 12 and 14 and define corresponding reference surfaces of a plug 
cast with the mold. Surfaces 22 and 24 also define an intersection 28 
therebetween. Fiber 30 having a jacket 31 extends through the end wall 32 
of the mold and an unjacketed end 10 thereof lies with the side surface 
thereof in tangential contact with surfaces 22 and 24. End wall 32 has two 
parts 34, 36, so that the fiber may be positioned with part 34 temporarily 
removed. It is also generally necessary to remove part 34 in order to 
remove a casting from the mold as will become more apparent. End surface 
20 of the fiber is brought into contact with the opposing end wall 38 
which has an inside surface 26 that corresponds to reference plane 16 and 
defines a corresponding reference surface of the casting. 
End wall 38 is illustrated as a separate element which is attached to a 
vee-shaped mold portion 40 via screws 42. End wall 38 and/or end wall 32 
or a part thereof alternatively may be formed integrally with portion 40. 
Mold portion 40 may also be fabricated in separate pieces. Preferably end 
wall 38 is optically transparent so that the position of fiber end 10 can 
be observed through end wall 38. It is possible to observe the butting of 
end surface 20 with surface 26 using interference techniques, for example. 
Fiber 30 either passes through end wall 32 at an acute angle to 
intersection line 28 or is redirected along this path (for example, by a 
rigid sleeve 43) so that the fiber must be bent back into an orientation 
parallel with line 28 through contact with surfaces 22,24. Fiber end 10 is 
held into contact with surfaces 22,24 by the elasticity of the fiber. 
After the fiber is seated within the mold as shown, the mold is filled with 
a solidifying liquid such as potting epoxy (e.g., Stycast 2850 epoxy, 
Stycast being a trademark of Emerson & Cuming, Inc. of Canton, Mass.) to 
form a cast plug. In order to remove the casting from the mold, part 34 of 
the mold is removed. The mold may be coated with mold release compound 
ahead of time or differential expansion may be used after casting to 
release the plug from the mold. 
Two such cast plugs 44,46 which may be identical are shown in FIG. 3 seated 
within an alignment frame 48. Alignment frame 48 has two alignment 
surfaces 52,54 which correspond with defining surfaces 22,24 of the mold. 
Alignment surfaces 52 and 54 intersect each other at precisely the same 
angle that defining surfaces 22 and 24 intersect each other so that the 
reference surfaces of plugs 44,46 which were formed by defining surfaces 
22,24 broadly contact alignment surfaces 52,54 of the alignment frame. The 
plugs are oriented such that the reference surface of the plugs which were 
defined by end surface 26 of the mold contact each other. This brings the 
fiber end surfaces into a butted aligned relationship. Preferably the 
plugs are held into the alignment frame by force (indicated by arrows 50) 
generated in any convenient manner. 
Alignment frame 48 preferably has a groove 56 extending along and 
corresponding with the intersection of alignment surfaces 52,54. The 
purpose of this groove is to protect the fragile fiber ends. The fibers 
are exposed along a large part of edge 58 of the plug formed by the 
intersection of the reference surfaces. Groove 56 serves to prevent edge 
58 of the plugs from contacting either the alignment frame or any dirt 
which might collect within the frame. 
BEST MODE FOR CARRYING OUT THE INVENTION 
We prefer to cast the plugs using a novel magnetic casting apparatus which 
was conceived by two of us jointly with another individual. A patent 
application directed towards this casting apparatus was filed on Dec. 31, 
1979 simultaneously herewith and is entitled "Magnetic Fiber Optic Casting 
Apparatus," now U.S. Pat. No. 4,244,681. 
This novel casting apparatus is shown in FIG. 4. Gap 57 extends completely 
through the vee-shaped groove structure 40 to divide it into separate pole 
pieces 55,59. Pole pieces 55,59 are composed of a magnetic material (e.g., 
tool steel) and are supported by a frame 61 and end walls 32,38 which are 
not composed of a magnetic material. Wall 38 is preferably made of glass 
while wall parts 34,36 and frame 61 may be made of brass, for example. 
Pole pieces 55,59 cooperate with permanent magnets 64 and iron shunt 62 to 
concentrate magnetic flux across gap 57 so as to attract spheres 60 (which 
are also composed of magnetic material) towards gap 57 and into the 
vee-shaped groove. Spheres 60 forceably seat the fiber end 10 into contact 
with surfaces 22,24. The diameter of the spheres is preferably such that 
when they are in contact with one of the walls 22,24 and the seated fiber 
end 10, they are only very slightly spaced from the other of the surfaces 
22,24. Obviously the width of gap 57 must be less than the diameter of the 
fiber. The molding material preferably should have a viscosity such that 
it does not flow appreciably through gap 57. It should be apparent that 
gap 57 may contain a material which is not magnetic (i.e., a dielectric). 
Such material may serve as a spacer for positioning pole pieces 55,59. 
A bent strain relief tube 70 allows the fiber contained within it to be 
directed towards the intersection of surfaces 22,24 while still allowing 
the fiber to exit the molded plug in a direction perpendicular to end wall 
32. The strain relief tube 70 extends for a short distance outside of the 
mold to strain relieve the fiber. Shrink tubing 72 (shown in FIG. 5) joins 
the fiber with the strain relief tubing and further relieves strain. 
A practical alignment frame is illustrated in FIG. 5. The alignment frame 
has two frame parts 74,76 which slidably connect together by endwise 
inserting rails 78 into grooves 80. 
Element 82 supports one of the molded plugs 44 via a springy arm 84 and 
screw 86. Element 82 in turn is supported by frame part 74 via screws 88. 
The length of frame part 74 is such that the rails 78 must be partly 
inserted within grooves 80 before the end surface of the plug reaches 
frame part 76. Arm 84 furthermore holds plug 44 at a slightly inclined 
angle so that the forward portion of the plug edge 58 (which contains the 
fiber end portion) is above the intersection of surfaces 22,24, while the 
rear portion of the plug edge 58 is below this intersection. As a result, 
the fiber tip cannot come in contact with frame part 76 when the parts are 
slid together. At some point in the connection process, however, the 
reference surfaces of plug 44 will contact the leading edge of surfaces 
22,24 and cause plug 44 to be tipped back down into alignment with frame 
part 76. When the two parts 74,76 have been joined, fingers 92 of springy 
element 94 exert a down force on plug 44, which maintains plug 44 in a 
seated position within the alignment frame. Dimple 96 of element 94 
simultaneously exerts a down force on plug 46 for the same purpose. Screws 
98 mount element 94 to frame part 76. The upturned tips of fingers 92 
allow plug 44 to be pushed under fingers 92 during reconnection of the 
frame parts. It should be appreciated that frame part 74 furthermore 
protects the critical edge 58 of plug 44 even while the alignment parts 
74,76 are apart.