Apparatus for connecting optical fibers

A one-piece molded elastomer splicer for permanent or temporary coupling of guided light between two optical fibers. Two cleaved fibers to be spliced are inserted into opposite ends of an elastic capillary tube somewhat smaller in diameter than the fibers, and pushed in until they meet in the center. Symmetrical elastic restoring forces automatically align the two fiber axes along the axis of the tube. When desired, the splicer can be transparent so that the resulting splice quality can be checked with a low-power microscope. Manual insertion of the fibers is possible with care, but a simple mechanical insertion jig is suggested. An optical fiber connector with built-in fiber-to-connector splice means connects two optical fibers with very low insertion losses even after many connect/disconnect cycles, with a simple and reliable means for installation in the field. The connector has a central mating interface, fabricated, aligned, polished, and anti-reflection coated at the factory, but with a built-in splicer for attaching to fibers in the field by the user.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to methods of and apparatus for connecting optical 
fibers. In particular, it relates to elastic one-piece splicers for 
optical fibers, optical fiber connectors utilizing fiber-to-connector 
splice means, and to related methods. Accordingly, it is a general object 
of this invention to provide new and improved methods and apparatus of 
such character. 
2. Description of the Prior Art 
(a) Dual eccentric plugs have been used for connecting optical fibers. The 
fibers are epoxied into two cylindrical plugs, slightly off axis, and 
polished flat. The plugs are mounted with their axes parallel, but not 
co-linear. The plugs are rotated, with respect to each other, until the 
axes are co-linear, when maximum light throughput is achieved. 
Disadvantageously, such plugs required elaborate attachment of fiber and 
end preparation. Also, access was required to other ends of the fibers so 
that optical transmission could be monitored and optimized. 
(b) Micromanipulator-assisted epoxy or fused splice. The fibers are 
manipulated in air, preferably with five degrees of freedom, for optimal 
alignment. Then, a drop of epoxy is applied to the fibers and cured. 
Alternatively, an arc melts and fuses the fibers. The manipulators can 
then be removed. Disadvantageously, micromanipulators are expensive. Also, 
to optimize alignment, either optical transmission should be monitored as 
above, or the ends should be watched with a microscope from two different 
angles. 
(c) Alignment V-groove. Two fibers are brought together while being forced 
to the bottom of a groove, then either clamped or epoxied in place. 
Disadvantageously, precision parts and insertion techniques are required. 
It was common to end with fiber ends separated too far or overlapping. 
(d) Snug-fitting metal or glass capillary tube. Superficially similar to 
present invention, but requiring precision tolerances for alignment in a 
rigid tube rather than symmetric elastic forces. Disadvantageously, rigid 
capillary tube splicers require precision tolerance fits to a particular 
fiber size. When a fiber is too large, it does not fit in; when a fiber is 
too small, it does not hold on axis. Unfortunately, tolerances for axial 
alignment are generally somewhat tighter than manufacturing tolerances on 
fiber diameters; a tube that fits snugly on one fiber does not necessarily 
fit well enough on the other fiber to be spliced. 
(e) Most commerical optical fiber connectors have the following basic 
principle of their connector design philosphy in common: the ends of the 
fibers to be connected are themselves brought together and separated with 
each connect/disconnect cycle, with the connectors serving only to align 
them precisely each time and hold them together securely. Various types of 
interface configurations have been used when the ends were brought 
together; intimate glass-to-glass contact, precise tolerance air gap (less 
than one mil apart), index-matching fluids, lenses, and buffer membranes, 
for example. 
(f) Many successful connectors of the prior art, in terms of low insertion 
loss and simplicity of use, have been those that are installed onto the 
ends of a fiber at a factory, and cannot be installed by a user in the 
field. An example of this is a connector available from one manufacturer, 
in which a fiber end is epoxied into a machined plug, the fiber and plug 
are polished flat to a high optical quality, and the plug is then 
installed into a connector body with precise alignment of the fiber axis 
along the axis of the connector--all at the factory. The user makes the 
connection by screwing each connector end into a central axis-aligning 
section until the plugs meet. The precise factory machining and alignment 
account for the connector's low insertion loss and quick connecting means, 
but also results in extreme inflexibility when installing, changing, 
testing, or repairing an optical fiber system. Disadvantageously, such 
factory-installed connectors have been only obtainable attached to 
specific lengths of fiber, which cannot be altered or repaired in the 
field. 
(g) A second manufacturer offers a commercially available, relatively 
easily field-installable connector. A fiber is cleaved and clamped into a 
connector end. This is then inserted into a central section where the 
fiber end is guided into a precisely molded dimple in a plastic disc. The 
dimple is filled with an optical gel which forms a gel lens whose two 
optical surfaces are the flat fiber end and the molded plastic surface. 
The two fibers, one on each side of the plastic disc, form two gel lenses 
which serve to image the light from one fiber into the other. 
Disadvantageously, such connectors have short sections of easily damaged 
bare fibers exposed when disconnected. The optical gel can trap dust 
particles and air bubbles, especially after a number of connect/disconnect 
cycles, which can cause significant light losses. Slight fiber cleaving 
irregularities alter the shape, and thus the performance, of the gel lens. 
(h) A third manufacturer offers a precision connector, similar to that 
discussed at (f) above, that can be installed in the field. 
Disadvantageously, the use of such connectors requires that field 
technicians learn the fine art of optical polishing, and perform it in a 
field environment, rather than a controlled optics shop with proper 
facilities. The reliability of such connections is questionable. 
SUMMARY OF THE INVENTION 
Another object of this invention is to provide a new and improved 
transparent, easily manufactured splicer. 
Still another object of this invention is to provide a new and improved 
rapid method of splicing. 
Yet another object of this invention is to provide a new and improved 
connector for connecting two optical fibers with very low insertion loss 
even after many connect/disconnect cycles, with a simple and reliable 
means for installing the connectors to the fibers in the field. 
In accordance with one aspect of one embodiment of the invention, a method 
for forming a temporary splice of two optical fibers includes inserting 
the fibers into opposite ends of an axial hole through a molded 
cylindrical piece of elastic material, the fibers being inserted until 
they meet approximately at the center of the piece. In one feature of the 
invention, index-matching fluid can be initially applied to the fibers. In 
accordance with another aspect of the invention, a method of forming a 
permanent splice of two optical fibers includes applying adhesive, uncured 
epoxy, or elastomer resin to the ends of the optical fibers, the applied 
substance having an appropriate matching refractive index when hardened. 
The fibers are inserted into opposite ends of an axial hole through a 
molded cylindrical piece of elastic material until they meet approximately 
at the center thereof. Though not a preferred mode of the invention, the 
molded cylindrical piece of elastic material with its axial hole can be 
formed by initially mixing uncured elastomer resin with its curing agent. 
The resultant mixture is poured into a cylindrical mold. A glass or metal 
fiber, of the desired hole diameter, is suspended in the center of the 
mold. The mold and fiber are then placed into an oven until curing occurs. 
Then, the fiber is removed from the cured elastomer, and the elastomer is 
released from the mold. In accordance with one feature of the invention, 
alignment is performed with the aid of magnifying means. 
In accordance with another embodiment of the invention, a combination 
includes two optical fibers and a molded cylindrical piece of elastic 
material having an axial hole therethrough. The two fibers are inserted 
into the axial hole at opposite ends thereof. Prior to insertion, the 
diameter of the axial hole is smaller than the diameter of either of the 
fibers. In accordance with certain features of the invention, the two 
fibers can have the same or different diameters. The molded piece can have 
its holes tapered to a larger diameter near the ends. The elastic material 
can include a urethane casting elastomer, and can be transparent. 
In accordance with still another embodiment of this invention, an optical 
connector includes, in combination, a pair of connector bodies, each 
having an axial passageway therethrough and engaging means adapted to 
engage with each other. A first cylindrical plug is mounted within the 
axial passageway of the first connector body, the first plug having 
opposed first and second faces and having an axial orifice therethrough. A 
second cylindrical plug is mounted within the axial passageway of the 
second connector body, the second plug having opposed first and second 
faces and having an axial orifice therethrough. A first short section of 
optical fiber has a substantial portion thereof mounted within the first 
plug orifice; an axial position of the fiber extends from the second face 
of the first plug. In similar fashion, a second short section of optical 
fiber has a substantial portion thereof mounted within the second plug 
orifice; an axial portion of the fiber extends from the second face of the 
second plug. Individual splicing apparatus are mounted within the 
respective connector bodies for engaging with the axially extending 
portions of the short sections of optical fiber and optical fiber means to 
be connected. In accordance with certain features of the invention, at 
least one of the splicing apparatus includes a molded cylindrical piece of 
elastic material having an axial hole from one end to another. The 
material can be transparent and can include a urethane casting elastomer. 
The axial hole of the piece, prior to assembly, has a diameter dimensioned 
less than both the diameter of the short section of optical fiber and the 
diameter of the optical fiber means to be connected. A portion of each of 
the connector holes can be so formed, either transparent or cut-out, to 
permit visual inspection of fibers within the transparent material. Index 
matching material can be used to join a short section of optical fiber to 
optical fiber means to be connected. The first faces of the plugs can be 
ground and polished to high optical quality. An end of a section of 
optical fiber can be recessed from the first face of its associated plug 
such end being ground and polished to high optical quality.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An elastic splicer 11, in accordance with a preferred embodiment of the 
invention, consists of a molded cylindrical piece 12 of elastic material 
with a small axial cylindrical hole 13 therethrough, as shown in FIGS. 1 
and 2. The splicer 11 is an elastic capillary tube which has a slightly 
smaller inside diameter than the outside diameter of the smallest diameter 
fibers to be used. The splicer 11 is flexible enough to stretch to 
accommodate the largest fibers to be used. Preferably, the hole 13 is 
tapered (at 14, 14') to a larger diameter near the ends 16, 16' to 
facilitate insertion of optical fibers 17, 17'. 
Since both fibers 17, 17' to be spliced are larger in diameter than the 
tubular hole 13, when they are inserted in opposite ends 16, 16' of the 
splicer 11, they each stretch the tube wall. The stretched walls in turn 
exert elastic restoring forces on the fibers 17, 17'. Because of the 
cylindrical shape of both the fibers 17, 17' and the capillary tube 11, 
each fiber 17, 17' automatically centers along the original axis 18 of the 
tube by symmetrical radial restoring forces. 
The two fibers 17, 17' with different diameters each individually align 
along the common tube axis 18, i.e., their axes align with one another, as 
required for maximum optical coupling efficiency. 
The splicer 11 can be constructed of various elastomer materials, such as 
Conap TU-904 urethane casting elastomer. Such materials are transparent so 
that the resulting alignment can be evaluated by focussing on the fibers 
17, 17' through the elastomer with a microscope. In actual tests, the 
fiber 17, 17' axes were observed to be aligned to well within 2 
micrometers, near the resolution limit of the microscope. 
The manufacture of small quantities of such a splicer 11 can be performed, 
as an example, by the following method: Uncured elastomer resin is mixed 
with its curing agent, and the mixture is poured into a small cylindrical 
mold. A piece of glass fiber or metal wire of the desired tube diameter is 
suspended in the center of the mold, which is placed in an oven until 
cured. Then, the fiber or wire is pulled out of the cured elastomer, 
leaving a high-quality, smooth-walled capillary tube of the same size, and 
the elastomer is released from the mold. The splicer is then ready for 
use. For high-speed mass-production, techniques such as injection molding 
are preferred. 
In use, two bare glass fibers 17, 17' to be spliced are simply inserted 
into the splicer tube 11 and pushed in until they meet approximately in 
the center. For a temporary splice, that is all that is needed, since the 
forces of elasticity, adhesion, and friction prevent the fibers 17, 17' 
from moving apart or falling out on their own; a firm pull on the fibers 
17, 17' is generally required to pull them apart. 
To minimize optical throughput losses due to reflections from the fiber 
ends 19, 19' the fibers 17, 17' can be dipped into an index-matching fluid 
prior to insertion. The index-matching fluid tends to lubricate the fibers 
17, 17' thus facilitating insertion. 
By dipping the fiber ends 19, 19' into an adhesive or uncured epoxy or 
elastomer resin having the appropriate matching refractive index when 
hardened, the splice becomes permanent. 
Inserting a 5 mil glass fiber into a 4 mil hole is not ordinarily a 
simplified task. However, when the hole 13 diameter is tapered, as at 14, 
to a considerably larger diameter at the ends 16, 16', and when a 
lubricating fluid is used, the process is considerably facilitated. 
Preferably, it is desirable to monitor the process under a low-power 
magnifier or microscope, especially when the fibers 17, 17' are being 
brought into final contact (assuming, of course, that the splicer 11 is 
transparent). This also allows a final check of the fibers 17, 17' 
alignment, end separation, and freedom from light-blocking particles and 
bubbles between the fibers 17, 17', before the bond becomes permanent and 
installed in the system. 
In lieu of installing the fibers 17, 17' into the splicer 11 by hand, a 
simple jig could be used to hold the splicer 11 and the fibers 17, 17' 
firmly while slowly pushing in the fibers 17, 17' via a screw or 
micrometer means. 
The elastic connector 11, described hereinabove, provides numerous 
advantages: It is one-piece, easily manufactured, and can be made 
transparent, facilitating inspection. The entire splicing operation takes 
about one minute (plus curing time if required). Automatic fiber axis 
alignment eliminates need for micromanipulators, throughput monitoring, or 
assembling of precision parts. Symmetric elastic forces readily 
accommodate mismatched fiber diameters over a relatively wide range. 
Further, resiliant elastomer material protects the spliced fiber interface 
from rough handling and from exposure to the environment. 
The foregoing splicer 11 has numerous desirable features including the use 
of cylindrically symmetrical elastic restoring forces to automatically 
align two optical fibers along the same axis, and the use of a soft 
elastomer tube to accommodate different fiber diameters in a splice. 
No external parts, such as clamps, are required to hold the fibers in place 
for a temporary splice, or while permanent bonding material is setting. 
Different variations can be performed with the splicer, without departing 
from the spirit and scope of this invention. For example, more than one 
hole can be molded in a single-piece splicer for splicing several fibers 
in a very compact volume. Index-matching fluid can be injected into the 
capillary tube (either at the factory or in the field) before inserting 
the fibers 17, 17'. 
In addition to being used to splice two fibers 17, 17' at their ends, the 
splicer 11 can be incorporated in a number of different devices, such as 
bulkhead splicers, optical feedthroughs, LED or detector terminations, 
etc. The splicer 11 can be incorporated in an optical fiber connector, as 
described more fully below. 
An optical fiber connector using a simple, built-in fiber-to-connector 
splice means will now be described. This connector is suitable for 
connecting two optical fibers with very low insertion loss even after many 
connect/disconnect cycles, with a simple and reliable means for installing 
the connectors to the fibers in the field. 
Optical glass fibers are fragile: they break when bent beyond a critical 
curvature. They shatter when struck by hard objects, or when pressed 
against microscopic particles of dirt. Chips and microcracks are easily 
introduced, and often get worse in time, especially with repeated handling 
and flexing. 
Efficient coupling of light (visible, IR, etc.) energy from one fiber to 
another with prior art devices tends to be difficult. Optical 
discontinuities in a waveguide tend to convert guided modes to radiation 
modes, which result in energy escaping from the confines of the waveguide 
walls. For fiber connectors and splices, one approach is to butt the two 
fiber ends together in such a way as to optimally approximate a single, 
continuous waveguide. The parameters of principal concern for minimizing 
interface discontinuity losses are: 
(a) lateral misalignment of fiber axes, 
(b) angular misalignment of fiber axes, 
(c) longitudinal separation of fiber ends, 
(d) reflections from fiber ends, 
(e) optical wavefront distortions due to curved or angled end surfaces, and 
(f) optical scattering from chips, cracks, etc. 
In light of the above considerations, an optical fiber connector in 
accordance with the invention is mechanically rugged, field-qualified, 
well-protected, and precision constructed. 
An optical fiber connector 20, in accordance with a preferred embodiment of 
the invention, is depicted in a perspective view in FIG. 3 and in 
sectional view in FIG. 4. 
Connector bodies 21 and 22-23 are shown as two hollow tubes with standard 
(BNC-type) bayonet/pin connection means 24,26. 
Precision fiber-holding plugs 27, 28 are fabricated and installed in the 
connector bodies 21 and 22-23 at the factory. The plugs 27, 28 permanently 
hold short sections of optical fiber 29, 31 precisely along the connector 
axis 32. Prior to installing the plugs 27, 28 in the bodies 21 and 22-23, 
the mating plug ends 33, 34 are ground and polished to high optical 
quality, including the fibers 29, 31. The polished fiber 29, 31 ends 36, 
37 are recessed from the respective plug surface 33, 34 by a few 
micrometers, thus maintaining a precise spacing between the fiber ends 36, 
37 when the plugs 27, 28 are brought together. The other end 38, 39 of the 
fiber 29, 31 in each plug 27, 28 is cleaved a short distance (e.g., about 
1 cm) from the unpolished end 41, 42 of the respective plug 27, 28. 
For optimum optical transmission through the main connector interface, the 
fibers 29, 31 are given an anti-reflection coating on the polished end 36, 
37. Standard vacuum coating techniques can be used, and a large number of 
fiber/plugs can be placed in a coating machine for production. Because the 
fiber surface 36 is nearly flush with the surrounding plug surface 33, the 
coating is free of "edge effects" which would prevent coating a 
free-standing fiber. 
It is noted that such commercially available optical connectors which are 
attached at a factory with a long length (i.e., up to several km) of 
optical fiber cable could not be economically anti-reflection coated 
because (a) the cabling materials must all be vacuum-compatible and (b) 
only a few connectors could be coated at a time because of the space 
requirements for placing attached spools of cable in a vacuum chamber 
along with the connector ends. 
The fiber 29/plug 27 and the fiber 31/plug 28 are installed and centered in 
the connector bodies 21 and 22-23, respectively, as shown in FIG. 4, such 
that when they are brought together to be connected, the fiber axes 32 are 
aligned by the close fit of the plug 28 within the inner diameter of the 
connector body 21. The plug faces 33, 34 contact and are held together by 
the bayonet spring force; the connector body 21, 23 faces 43, 44 do not 
contact, and thus need not be precision machined. 
Optical fiber splicing devices 111, 211 are also installed inside the 
connector bodies 21, 23 at the factory, such that the fiber sections 29, 
31 extending from the plugs 27, 28 are inserted into the splicers 111, 
211, respectively, and are ready for splicing in the field to external 
fibers 117, 217. Shown in FIG. 4, for example, are capillary tube splicing 
means 111, 211 similar to the elastic capillary splicer 11 described 
hereinabove, see FIGS. 1 and 2. Although this is the preferred splicing 
mode contemplated, any compact, easy-to-use splicing apparatus that yields 
repeatably high transmission efficiency can be used along with the 
appropriate connector housing modification required to accommodate it. 
Attaching the external fibers 117, 217 to the connector 20 via the splicing 
devices 111, 211, is analogous to attaching external cable wires to an 
electrical connector by soldering or crimping. Freed from the restrictions 
of quick connect/disconnect capability, the optical splice can utilize an 
index matching material (e.g., an epoxy) at the interface to essentially 
eliminate losses due to end reflections and fiber end imperfections due to 
field cleaving methods (polishing not required). The connector 20 is 
completely fabricated at a factory except for the final splicing of the 
external fibers 117, 217. The installation technique is therefore entirely 
dependent on the splicing method chosen, and can require that one or more 
additional parts be attached by the user. In the preferred embodiment of 
an elastic splicing capillary tube 11 built into the connector 20, the 
installation is particularly simple. After cleaving the fibers 117, 217 
and injecting some index-matching material into the capillary, the fibers 
117, 217 are simply inserted into the respective capillary and pushed in 
until they contact the built-in corresponding fiber section 29, 31. With a 
cutout 46, 47 provided in the connector body 21, 23, respectively, 
opposite the splice interface (FIG. 3), the resulting splice can be 
inspected visually through the transparent capillary tube with the aid of 
a magnifier or low-power microscope before the bond sets permanently. 
With non-setting index matching fluid used in the splicer 111, 211, 
temporary splices result and the connector 20 can be disassembled by 
pulling the fibers 117, 217 out of the splicers 111, 211. The inward 
radial restoring forces of the stretched elastic capillary tubes 111, 211 
holds the fibers 117, 217 in against moderate tugs on the fibers; for more 
strength, additional strain-relief clamping means on the connector 20 can 
be provided. 
After installation, the connector 20 is used just like any analogous 
electrical connector (like a BNC connector, for the version described 
above and in FIGS. 3 and 4). Simply bring the male and female connector 
halves together, push firmly, and turn the bayonet lock. Disconnecting 
them is the reverse procedure. 
The connector 20, described hereinabove in accordance with the invention, 
has numerous desirable features: 
(i) Simple, rugged construction, few moving parts. 
(ii) Simple, reliable, field installation method. 
(iii) Fiber completely protected from environment and abuse; only the end 
surfaces are exposed when the connector bodies are disconnected. 
(iv) Bare fibers are not to be handled, flexed, or clamped over and over 
with each connect/disconnect cycle. 
(v) Fiber ends are ground and polished optically flat and perpendicular to 
the fiber axis. 
(vi) The fiber ends are anti-reflection coated. 
(vii) Connecting and disconnecting is as easy and reliable as electrical 
connectors. 
(viii) Very low insertion loss. 
Various modifications will suggest themselves to those ordinarily skilled 
in the art without departing from the spirit and scope of this invention. 
For example, a multi-fiber optical cable connector could be made in 
essentially the same manner as the single-fiber version described above. 
Several fiber sections could be installed in the plug at the factory with 
similar precise alignment techniques, and ground, polished, and coated as 
before. Likewise, a multi-fiber splicing means can be built into the 
connector with each of the plug fiber ends inserted into the splicing 
means ready for splicing to external cable fibers. Further, a single-piece 
elastomer splicer can be molded with an array of several capillary tubes 
in the same manner that a single tube is molded. 
As another example, instead of the plug faces 33, 34 contacting when the 
connector bodies 21 and 22-23 are brought together, the connector body 
faces 43, 44, suitably machined, can be the contacting parts. The plugs 
27, 28 should in that case be positioned in the connector bodies 21, 22-23 
such that when the faces 43, 44 are in contact (and held together by the 
bayonet spring) the plug faces 33, 34 would be held apart by a precise air 
gap. It would then not be necessary for the fibers 29, 31 to be recessed 
from the plug surfaces 33, 34 to protect them from butting contact with 
each other; the surfaces would be polished uniformly flat. 
In still another example, although a simple bayonet securing means was 
described for the connector 20, a wide variety of alternative securing 
means could be used instead. For example, the body 22 could include a 
rotating threaded sleeve which screws onto mating threads on the body 21; 
tightening the sleeve when the connector mating surfaces contact would 
replace the spring-loaded bayonet. As another example, two identical 
connectors similar to the one on the right in FIG. 4 could be designed to 
be inserted into a central union section. Numerous other suitable securing 
means are well known in the electrical connector art, and can be readily 
adapted to the fiber connector. 
Thus, it is desired that this invention be construed broadly, and that is 
be limited solely by the scope of the allowed claims.