Memory Polymer Multiple Cavity Fiber Splicer

1. Field of the Invention

The present invention relates to splicing of optical fibers and more specifically to the splicing of at least three optical fiber ends by the use of a unique splicer block which is developed from memory polymers. Thus, the present invention is directed to splicers into which ends of optical fibers may be inserted and subsequently spliced in place into a plurality of cavities for permanent fiber optics transmission, and to methods of making the splicer as well as methods of using it.

2. Prior Art Statement

Hundreds of patents have issued which are directed to the art of fiber optics splicing and connecting. Most involve very complex mechanical connections and/or the use of optical adhesives or glues. Typical of the patents which show unique methods of splicing are the following:

U.S. Pat. No. 3,944,328 shows the simplistic approach of inline, butt-to-butt splicing of fiber optics utilizing a resinous block with aligned bores with mechanical retention inserts.

U.S. Pat. No. 4,178,067 involves the use of a mass of dimensionally unstable material in cylindrical form which is radially shrunk. After the fiber optics are inserted butt-to-butt, upon causing the unstable matter to expand radially, the presence of the outer sleeve forces the unstable matter to compress and cause co-linear alignment.

U.S. Pat. No. 4,261,644 involves the use of memory metals for mechanically splicing with application of heat.

U.S. Pat. No. 4,435,03B is directed to deformable material involving three integrally formed elongated members that are squeezed together to align optical fibers.

U.S. Pat. No. 4,597,632 is directed to temperature sensitive releasable optical connector which utilizes a shape memory effect metal to align and clamp ferrules.

U.S. Pat. No. 4,647,150 illustrates an arched alignment of optical fibers utilizing an optical adhesive as well as an innertube for butt-to-butt splicing.

U.S. Pat. No. 4,725,117 describes a complex optical fiber connection utilizing a heat-recoverable tube which is constructed of a memory material such as elastic or plastic memory metals, as well as an outer metal contact body. The basic idea of shrinking a memory plastic radially inward to achieve optical fiber alignment is taught in this patent.

U.S. Pat. No. 4,743,084 involves improvement in the use of deformable plastics or the use of shape memory materials as an integral part of a more complex mechanic structure.

U.S. Pat. No. 4,750,803 describes a connector which includes exit port means to allow air to escape during splicing.

In addition to the above, there were suggested many splicing methods using heat-shrinkable polymers (called also memory polymers, heat-recoverable polymers, etc.) to help align optical fibers. These polymers (described e.g. in U.S. Pat. Nos. 2,027,962; 3,086,242; 3,359,193; 3,370,112; 3,597,372 and 3,616,363) are either thermoplasts or post-crosslinked thermoplasts containing a crystalline polymer phase and/or amorphous polymer phase with relatively low glass-transition temperature due to either nature of the polymer or due to plastification effect. Such heat-shrinkable polymer can be forced into one shape and frozen in it; and shrunk by application of heat approximately into the original shape. Because such heat-shrinkable polymers consist of several polymer phases and/or a multitude of separate polymer chains, they do not have a precise shape which could be called "inherent" and they can return only approximately into a predetermined shape. In addition to that, the highly crystalline polymers must be heated above the melting point of their crystalline phase; the re-crystallization of the polymer causes significant volume contraction which is detrimental to the alignment. Because of that the heat-shrinkable component in itself cannot form a splicer with a low insertion loss. Such splicers require a combination of highly symmetric tubular shape, coupling gels or adhesives and elaborate support structures, which in turn cause a high cost of the device and of its installation. Typical of the patents which show the unique methods of splicing are the following: Great Britain Patent No. 1,588,227 describes a splicing method using heat-shrinkable crystalline thermoplastic sleeve to achieve fiber connection.

Notwithstanding the above prior art references, it should be noted that the present invention has not been anticipated or rendered obvious because the use of a unistructural memory polymer is neither disclosed nor suggested as a complete and simple but advanced structure in and of itself. Further, the unique steps of preparing the memory polymer utilized in the present invention splicer is also lacking in the prior art.

The present invention involves a multiple cavity optical fiber splice which is made of a unistructural mass of inherent shape memory polymer material. The unistructural mass has a longitudinal dimension with opposite ends which includes at least a first bore at one end and at least a second bore and a third bore at the other end wherein the bores go into the mass and at least two connect to each other. The unistructural mass has a first shape and a second shape and the first shape is a unique, recoverable, predetermined inherent shape wherein all of said bores each have a preset diameter to accomodate and tightly hold end segments of denuded optical fibers of predetermined diameter in a predetermined arrangement with one another. The second shape is a deformed shape which may be caused by solvent swelling and partial shrinking of the inherent memory polymer material such that all of the bores each have enlarged predetermined diameters which are greater than the diameters of the segments of the denuded optical fibers so as to loosely and freely receive the end segments of the denuded optical fibers. Thus, the unistructural mass of inherent shape memory polymer material may be initially formed in the first shape, also known as the inherent shape, and may be then swollen and partially shrunk to its second or deformed shape, wherein it is capable of being returned to its first shape by application of a non-mechanical stimulus thereto, such as heat. The invention is also directed to the method of preparing the optical splicer as well as the method of using the optical splicer to obtain a predetermined arrangement and splicing of the end segments of optical fibers.

The objectives and advantages of the present invention will become more apparent and will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings wherein:

FIG. 1 shows an oblique frontal view of an optical fiber splicer of the present invention for simultaneous splicing of two sets of optical fibers;

FIG. 2 shows a cut sideview of an optical splicer of the present invention as shown in FIG. 1, having bores with the unistructural mass in its inherent shape;

FIG. 3 shows the side cut view of the optical fiber splicer of FIG. 2 which has been swollen and has mandrels inserted therein;

FIG. 4 shows a side cut view of the optical fiber splicer of FIG. 2 after it has been swollen and partially shrunk;

FIG. 5 shows a cut sideview of the optical fiber splicer of FIG. 2 wherein denuded end segments of two sets of two optical fibers have been inserted into the bores;

FIG. 6 shows the optical fiber splicer of FIG. 2, after the unistructural mass has been heated so as to return to its inherent shape, thereby aligning and holding the end segments of the two sets of optical fibers;

FIG. 7 shows a side cut view of an alternative optical fiber splicing device of the present invention adapted to receive three different optical fiber ends for splicing;

FIG. 8 shows the present invention splicing device of FIG. 7 with the optical fibers in spliced position;

FIG. 9 shows a cut end view of a present invention splicer for splicing five sets of optical fibers simultaneously; and,

FIG. 10 shows a perspective end view of a present invention splicing device adapted to splice two sets of optical fibers in parallel with each set having a stiffening splint to prevent radial stress deformation.

Splicing arrangements of optical fibers require high alignment precision. In butt-to-butt alignments, the fiber end faces are flat and perpendicular to the direction of propagation to optimize light transmission and there must be high mechanical integrity for fiber fixation. In multiple splicing butt-to-butt alignment, the parallelism of separate splicing bores becomes important to stability and proper fiber fixation of all fibers in a bundle. The outer diameter or cladding diameter of the majority of currently used glass optical fibers is standarized at 125 or 110 microns while their light guiding core range is between 4 microns for single mode fibers up to about 50 to 100 microns for multi-mode fibers. Thus, any misalignment of the spliced optical fibers, any defect at the faces of the fibers or on the surfaces or any gap between the fiber ends will contribute to the loss of the transmitted light and to a resulting loss of transmitted information. This transmission loss, referred to as "insertion loss" reduces the signal-to-noise level, and is expressed in decibels (dB). It is an object of all splicing techniques to minimize insertion losses.

The application of various splicing techniques such as those which are described in conjunction with the prior art patents set forth above under field conditions is very difficult and requires highly skilled and expensive personnel as well as splicing products which are fairly expensive. Thus, the splice as used for multi-mode applications may exhibit poor performance notwithstanding the effort put into reducing insertion losses. Further, the use of multi-mode applications in single mode fiber optic splicing also results in increased insertion losses. For example, a very common splicing technique involves fusing the two ends of the butt-to-butt optical fibers under a special stereomicroscope. This procedure provides low loss connections but due to the considerable set up time requires a substantial amount of expenditure and is extremely inconvenient for field applications. More portable or in-place splicing equipment seems to yield increased insertion losses based on their relative economics. In other words, the less expensive the technique, the more likely significant insertion losses will occur.

It is an objective of the present invention to achieve a splicer which would provide low cost, mass produced unistructural products which would be fast and convenient under field conditions and which would be suitable for use by personnel who do not require special technical skills or the use of special high performance equipment.

As indicated, the major problem with splicing and connecting optical fibers in most typical arrangements involves misalignment. However, the inherent memory polymer materials utilized in the present invention eliminate the problem due to accurate mutual positioning of the optical fibers.

Inherent memory polymers are amorphous with moderate crosslinking density. They are substantially deformable above their softening temperature and have only one "inherent shape" in which all of the polymer segments are in their most probable conformation. The crosslinking density is such that each continuous piece of the inherent memory polymer is formed substantially (i.e. with exclusions of impurities such as residual solvents, monomers, oligomers, etc.) by one single giant macromolecule. Furthermore, each such macromolecule has substantially a uniform network density and substantially a single amorphous polymer phase. We refer to such structure as a "unistructural mass" to distinguish it from heat-shrinkable, shape-recovering or memory polymers having more than one polymer molecule and/or more than on polymer phase which lack the inherent shape memory. As it is obvious from review of the prior art, the typical heat-shrinkable polymer hitherto used for splicing are highly crystalline polymers, such as polyethylene, copolymers of ethylene-vinylacetate and the like. These polymers consist of two polymer phases of substantially different properties. For instance, polyethylene used in most of the applications has a crystalline phase with a melting point between about +105.degree. to 130.degree. C., and an amorphous phase with a glass-transition temperature between about -100.degree. C. and -70.degree. C. The amorphous crosslinked polymers used in the splicers according to our invention have an additional advantage in their high optical clarity. Because our splicers are typically clear, uniform plastic articles, the loss in transmitted light is immediately detectable by the light emitted at the fiber ends contact. This can be used, given proper instrumentation, for measuring the insertion loss directly and in a manner very suitable for field installations. The shape of such a polymer can be changed above its softening temperature into a deformed shape by the action of external forces but once the external forces are removed the internal forces of the polymer cause it to return to its inherent shape. The deformation of the inherent memory polymer is fully reversable and this allows complete recovery to its inherent shape. The return from deformed to inherent shape can be deferred by decreasing temperature on the deformed unistructural mass below its softening temperature. Its return to inherent shape can be then induced by some external stimuli such as an application of heat or other energy.

In general, the optical fiber splicer of the present invention is made from a mixture of monomers so as to create a unistructural mass which has a longitudal dimension with opposite ends which includes at least a first bore at one end and at least a second bore and a third bore at the other end. These bores enter the mass toward and in alignment with one another and, in fact, connect or intercept one another. It should be noted that the unistructural mass is defined as having three or more bores, although, preferably, in some embodiments, two or more single continuous openings from one end of the unistructural mass to the other are each formed by two bores and each might be characterized as a single bore. Thus, sets of two bores of the multiple bores in many embodiments of the present invention are equivalent to a continuous single bore.

A typical memory splice made from the inherent memory polymer material in the present invention may have a continuous lumen which is cylindrical and which has a diameter which is equivalent to or slightly less than the outer diameter of a bare optical fiber to be connected with another bare optical fiber. Prior to insertion of the fibers, the bore or lumen is deformed so that its inside diameter becomes larger than the outside diameter of the fibers to be spliced and the unistructural mass is "frozen" in this deformed state. As long as the memory polymers are frozen in their deformed state by exposure to temperatures below their glass transition temperatures, e.g. by solvent swelling and partial shrinking, they will remain frozen until the application of adequate heat to raise the temperature of the unistructural mass above the glass transition temperature, at which time the unistructural mass will return to its inherent shape. Therefore, the optical splicer of the present invention receives denuded segment ends of optical fibers which are easily inserted into the enlarged bores. Thereafter, the unistructural mass is exposed to a stimulus such as heat which causes the recovery of the inherent shape and the inherent aligning and tightening of the butt-to-butt optical fiber end segments. The splicer cools down to ambient temperature and forms a rigid glassy polymer which permanently holds the fiber ends safely, firmly and properly aligned.

In order to achieve a well defined inherent shape the polymer or mixture of polymers utilized has to have at least a minimum cross linking density. The shape of such polymer can be changed into a deformed shape by the action of external forces and can be returned to the inherent shape by the action of other external forces. It has been noted in general that the deformability of any polymer with inherent memory decreases with its cross linking density. To achieve a sufficient reversal of deformability the polymer should, in preferred embodiment, have maximum cross linking density. Thus, the inherent memory polymer materials used in the present invention have at least a minimum cross linking density and, ideally, have a maximum cross linking density.

Non-crystalline (i.e. amorphous) polymers have two types of behavior in two temperature ranges. The transition temperature between the two ranges is called Glass-Transition Temperature (T.sub.g). If temperature is lower than T.sub.g, the polymer is in glassy state in which its deformability is very low and modulus of elasticity is high. If temperature is higher than T.sub.g, then modulus of elasticity is low and deformability is high. The polymer in this temperature range has viscoelastic or rubbery behaviour, depending on crosslinking density and temperature.

The softening temperature T.sub.s is used in further description instead of T.sub.g and T.sub.m because it is readily measurable, e.g. by ASTM D569-48 or by a similar method, and because it is more closely related to the polymer performance with respect to the present invention.

Memory polymers useful in the present invention are those with T.sub.s higher than ambient temperature, and preferably higher than about 50.degree. C. The upper limit of T.sub.s is restricted only by temperature resistance of the memory polymer, optical fiber or other system components and means of the connector heating during installation. Therefore, there is no inherent upper limit on T.sub.s, but above practical consideration set the practical upper limit on T.sub.s to be about 200.degree. C., and preferably about 150.degree. C.

Most of non-crosslinked polymers are soluble in one or more solvents. Such solvents are different for various polymers and referred to as "thermodynamically good solvents" (TGS). Solvent-polymer interactions are characterized in various ways, e.g. by Chi parameter in Flory-Huggins eqution. TGS have Chi <0.5 for a given polymer. (In other words, if Chi <0.5 for a given polymer-solvent pair, the non-crosslinked polymer is soluble in this solvent.)

Covalently crosslinked polymers cannot be dissolved without destruction of the network. Instead, they swell to certain equilibrium in the solvents with Chi <0.5 and become rubbery in the process. The swelling extent can be expressed in various ways, for instance as volume fraction of polymer v.sub.2 or solvent content B.sub.s in weight %. The swelling extent depends both on value of Chi and on crosslinking density. Because Chi value does not change with crosslinking appreciably and can be established independently of swelling (e.g. measured on linear model polymers), the swelling of crosslinked polymers in TGSs can be used as characteristic of crosslinking density.

Memory polymers useful in the present invention have minimum crosslinking density corresponding to v.sub.2 =0.05, and preferably v.sub.2 =0.1 in the TGS with Chi=0.3-0.4. The maximum cross linking density corresponds to v.sub.2 =0.5, and preferably to v.sub.2 =0.667 for the same TGS.

Swelling of the memory polymer has several effects important to the present invention:

1. Swelling in TGS to equilibrium decreases T.sub.s below ambient temperature so that swollen memory polymers are rubbery and readily deformable.

2. Swelling increases isotropically every inherent dimension of the memory polymer.

In the swollen state

where LD.sub.s and LDi are the swollen linear dimension in undeformed state and inherent linear dimension, respectively.

As long as the covalent network is isotropic, the above relation governs all linear dimensions, such as length, outside diameter, inside diameter of a cavity in the memory polymer, etc.

The polymers satisfying the essential requirements can have various chemical composition. Such inherent memory polymers can be crosslinked acrylates and methacrylates; N-substituted acryl and methacrylamides; crosslinked vinyl polymers, such as polystyrene, polyvinylpyrridine, polyvinylchloride, resins and the like. In other words, because the requirements are of physical nature, the function of the invention is independent of the specific composition of the memory polymer as long as the essential physical requirements are met.

Some specific memory polymer compositions are described in Examples set forth below.

The present invention consists in using inherent shape memory polymers of above described characteristics in conjunction with certain design of the connector of optical fiber and certain connecting o installation procedure. The designs and procedures described below assume that the optical fiber has circular crossection with coaxial core and jacket, because all current optical fibers are of such kind. It is not intended, however, to limit the invention to connecting the current optical fibers only. Should another type of fiber come to use later (e.g. with rectangular, triangular, octagonal or elliptical crossection or with asymmetric core) the splicer and connecting methods can be readily adapted to such new fiber as long as its shape is precisely defined.

The method of connector manufacturing and optical fiber connecting according to the present invention has several essential steps:

1. The article of inherent shape memory polymer is created which has bores which define a plurality of internal cavities of circular crossection in positions corresponding to intended positions of the optical fibers to be connected (for instance, a single cylindrical cavity for end-to-end connection or splicing). The inherent inner cavity diameter IDI is the same or smaller than the outside diameter of the optical fiber ODF (currently 125 microns for glass fibers). The cavity can be created in a number of ways, such as drilling or burning the memory polymer with laser beam or similarly. Particularly preferred method is to create the memory polymer around a Mandrel A of circular crossection and outside diameter ODMA. Mandrel A can have different dimension than the optical fiber if some conditions are met (defined below), but it is important that the shape of crossection of the Mandrel A of the optical fiber are the same and that the ratio ODMA/ODF has a certain preset value. This mandrel can be preferably the optical fiber itself, in which case ODMA=ODF and the conditions for Mandrel A are met automatically. The memory polymer unistructural mass can be created around such mandrel in a variety of ways. For instance, it can be created by polymerization of suitable mixture of monomers, including the crosslinking comonomer, in a mold with the mandrel inserted. Alternatively, it can be created by embedding the mandrel into a non-crosslinked polymer precursor of the memory polymer, and by post-crosslinking the composition by some known method depending on the chemistry of the memory polymer used (e.g. irradiation, heat, etc.).

Once the memory polymer is formed with the desired crosslinking density, the mandrel is removed. The removal can be carried out in various ways depending on the mandrel properties. For instance, the mandrel can be pulled out provided that it has appropriate deformability (e.g., Nylon 6), or can be melted, dissolved or etched away. The preferred process is to swell the memory polymer in a suitable solvent and pull the mandrel out from the enlarged cavity. After the polymerization and/or crosslinking process is finished, memory polymer can contain, in addition to the three-dimensional polymer network, work, an inert component, such as unreacted monomers or a diluent from the original mixture. If the volume fraction of such diluent is v.sub.d, then the inherent inner diameter of the cavity will be

2. The cavity is enlarged and a new mandrel is inserted which has outer diameter larger than ODF. This mandrel B can have crossection rather different from that of the optical fiber, but the optical fiber has to be readily insertable into the cavity of the size and shape of the Mandrel B. Any linear dimension of such cross-section has to be larger than ODF. If the Mandrel B has circular cross-section, its diameter is designated ODMB. The ratio ODMB/ODF will determine ease of optical fiber connection later.

In any case, it is necessary that ODMB/ODF >1, and preferably >1.1.

On the other hand, it is preferred that ODMB/ODF <2, because in opposite case two optical fibers could fit accidentally into a cavity designed for a single optical fiber. The preferred method of the cavity enlargement is the swelling of the inherent shape memory polymer. As a rule, for any polymer there are a number of good solvents which will swell the memory polymer so that the Mandrel B is comfortably inserted into the cavity. The necessary swelling can be readily calculated from equations (1) and (2) or determined by experiment.

Mandrel B can be made of miscellaneous materials, such as polymers, metals or glass. From the viewpoint of the latter, for removal of Mandrel B, the partly extensible polymers are preferred (e.g. polyamides, polyurethanes or polyolefins).

3. The inherent shape memory polymer is shrunk around the Mandrel B by removal of the solvent in which the memory polymer was swollen during the Mandrel B insertion. The solvent removal can be done by solvent extraction or evaporation.

After the solvent removal is finished, the inner cavity diameter is ODMB and the residual volume fraction of solvent in the polymer is v.sub.s.

The v.sub.s has to be such that

(a) T.sub.g of the memory polymer is higher than ambient temperature, and preferably higher than 50.degree. C.;

(b)

The residual solvent can be left in the memory polymer intentionally for various reasons, e.g., to decrease T.sub.g or other properties. If so, it is preferred to use in the previous step mixture of volatile and non-volatile solvent in such a proportion that the latter remains in concentration v.sub.s after the volatile solvent is evaporated completely.

4. Mandrel B is removed from the cavity. This removal can be carried out in various ways, such as etching the mandrel away by an agent which does not attack the memory polymer; dissolve the mandrel in a solvent which does not swell the memory polymer or does not decrease its T.sub.g below ambient temperature, or similarly. The preferred way is pulling Mandrel B made from a partly extensible polymer, out of the cavity.

After this operation the splicer is ready for installation of the optical fiber The Mandrel B removal can be carried out as a part of manufacturing process (i.e. prior to packaging the splicer) or just prior to the splice installation. The advantage of the latter method is increased resistance of the splicer to an accidental temperature increase during storage or shipment, particularly if the inherent shape memory polymer mass has a relatively low T.sub.s for convenient installation. The splicing method is very simple due to the autoaligning characteristics of the inherent shape memory polymer unistructural mass accordto the invention.

The installation procedure consists of few essential steps:

1. Optical fibers are denuded in proper lengths (i.e., plastic buffer is removed) and ends of fibers are prepared by the customary way (cleaving, polishing, etc.). This preparatory step is identical with other connecting and splicing methods.

2. Denuded and prepared ends of optical fibers are inserted into the cavities of the inherent shape memory polymer splicer. This step is very easy because the cavities have ID=ODMB which is larger than the received optical fiber OD and no aligning or accurate positioning of the fibers is necessary.

3. While fibers are held in place, the splicer is heated above T.sub.s for a time sufficient for release of the internal stresses and consequence contraction of the cavity to its final size and configuration, i.e. such that it goes from its deformed shape back to its inherent shape. The rate of such return can be very slow if the temperature is around glass transition temperature T.sub.g of the polymer. For this reason the polymer has to be heated to a temperature sufficiently above Ts but sufficiently below the temperature at which the components could be damaged. The cavity contracts around the fibers, as it tends to shrink to the diameter

The cavity containing the optical fibers cannot shrink to a smaller diameter than ODF, and holds the fibers by the force proportional to (ODF-IDC) difference.

The cavity contraction thus generates pressure forcing the fibers into the intended position.

The heating method is not particularly critical. The splicer may be heated be hot air, steam, electrically generated heat, etc.

4. The connector is cooled down to an ambient temperature and secured by a suitable support or protective system, if necessary.

Referring now to the drawings there is shown in FIG. 1 Optical Splicer 1 which is basically a mass of inherent memory polymer material which has been formed with four bores, namely 7,9,11 and 13. Bores 7 and 13 and bores 11 and 9 are at opposite ends 3 and 5 of its elongated dimension. As can be seen, bores 7 and 13 and bores 11 and 9 are, in fact, two single bores, each of a single diameter in total alignment in straight lines. Further, in this case, the two sets of bores are parallel to one another.

FIG. 2 through 6 show cut side views of an inherent shape memory polymer material through its various stages of preparation and use. Thus, in FIG. 2, there is shown unistructural mass 21a which has longitudinal dimension ends 23 and 27 with bores 17a,19a,25a and 29a as shown. In FIG. 2, bores 25a and 29a have been formed to create a single cylindrical tunnel or lumen through mass 21a and bores 17a and 19a have likewise formed a second, single lumen through mass 21a, each of the two sets of bores creating a single, continuous opening. The diameter of bores 17a, 19a, 25a and 29 a are all the same and are greater than the diameters of denuded fiber optic ends to be spliced as discussed in conjunction with FIG. 5 below. The bores 17a,19a,25a and 29a may be formed by drilling, by other techniques which are discussed above, or by actually forming the unistructural inherent shape polymer material mass around two mandrels of equal or slightly smaller size than the size of the optical fibers to be spliced. Next, the unistructural mass 21a is swollen with solvent and, as shown in FIG. 3, the expanded mass, that is unistructual mass 21b has mandrels 41 and 43 inserted into it through the bores, as shown, and these bores 17b,19b,25b and 29b are of greater diameter than bores 17a,19a,25a and 29a due to the swelling. Additionally, mandrels 41 and 43 are of greater diameter than the diameters of the end segments of the optical fibers to be spliced. Unistructural mass 21b is subsequently shrunk partially and the mandrels are removed so as to create the unistructural mass 21c shown in FIG. 4. This unistructural mass 21c, with longitudinal ends 23 and 27 now have bores 17c,19c,25c and 29c. These bores are partially shrunk in diameter but are still of greater diameter than the end segments of the optical fibers to be spliced.

FIG. 5 shows unistructural mass 21c with ends 23 and 27 and bores 17c,19c,25c and 29c respectively. Here, optical fibers 33,37,51 and 53 have denuded end segments 31,35,47 and 39. Thus end segments 31,35,47 and 39 are of less diameter than bores 17c, 19c, 25c and 29c and are readily and freely inserted therein and contacted in butt-to-butt arrangements. Although not essential, various preparations for the tips or very ends of end segments 31,35,47 and 39 may be utilized prior to insertion and these techniques, such as cleaving are well-known in the art. In any event, once optical fiber end segments 31,35,47 and 39 have been inserted as shown in FIG. 5, unistructural mass 21c is returned to its original inherent shape shown in FIG. 2 by virtue of application of an external stimulus such as heat. This is shown in FIG. 6, after heat has been applied, and, as can be seen, end segments 31,35,47 and 39 are forced into excellent alignment and are held rigidly in place with permanent splicing using the single unistructural mass without any mechanical parts, without any adaptors or screws or any other of the devices which could create difficulties and/or minimize the useful life of the splice.

FIG. 7 shows a variation wherein unistructural mass 71 made of inherent shape memory polymer material has a longitudinal dimension with ends 73 and 75 opposite one another and bores 77,79 and 81 as shown, to create a coupler or junction for two-to-one splicing. The bores 77,79 and 81 are formed basically as described above except that the mandrels (not shown) are custom cut at desired angles to create the couplinq required. FIG. 8 shows the finished spliced product wherein optical fibers 85,89 and 93 have been efficiently and properly spliced at denuded, cut end segments 83,87 and 91. Such a device may be used for the injection of light from a remote source into a bus cable when used with fiber optic bundles or single multimode fibers with large core/clad ratios (e.g. 100/110 micron core/clad).

FIG. 9 shows an end cut view of a device of the present invention wherein unistructural mass 111 is formed as a cylinder or barrel with five sets of bores represented by bore openings 113,115,117,119 and 121. Unistructural mass 111 is formed in the same manner as unistructural mass 1 of FIG. 1 described above except that five mandrels are used instead of two.

FIG. 10 shows a side perspective view of a trapezoidal unistructural mass 211, which includes end 213 containing bores for optical fibers and for splints. More specifically, end 213 includes bore 205 (i.e. actually a double bore with one bore entering from each end to form a single channel) and bore 207 for receiving denuded optical fibers and bores 221 and 223 adapted to receive stiffening rods or splints 225 and 227 shown in place. In this particular figure, the device has been formed with two sets of bores having two different diameters and specifically with smaller bores 221 and 223 located contiguous to and parallel to larger bores 205 and 207. Partial shrinking of unistrucutural mass 211 has already been accomplished with splints 225 and 227 now tightly in place. Additionally, optional cement 215 and 217 used in some splicings are in place as shown and the device is ready for immediate insertion of four denuded fiber ends for a double splice. Splints 225 and 227 are structural members optionally included to assure that radially inward stresses do not break or distort the fiber ends.

All the percents in this and the following Examples are meant as percent by weight unless stated otherwise. Fifteen (15) cm long flint glass tube of I.D. 6 mm is closed by tightly fitting polyethylene caps on each end. In each cap, small holes are punched to allow insertion of optical fibers. Denuded and cleaned optical fibers (O.D. 110 microns) are inserted through the holes of the caps which is then fitted on the bottom end of the tube and sealed with an epoxy resin. Then two additional holes are punched into the other cap to allow for filling by a syringe, and the cap is installed on the top of the tube so that the optical fibers are protruding through their respective holes in the cap. The mold thus formed is then filled by the monomer mixture consisting of 98.75% of isopropylmethacrylate, 1.20% of tetraethyleneglycol methacrylate and 0.05% of dibenzoyl peroxide. The filled mold is then fixed into a rack assuring that the optical fibers going through the mold are tight and straight. The mold and rack are then put into a container filled with nitrogen, sealed and heated to 70 degrees C. in an oven for 14 hours. Afterwards, the mold is removed from the rack and the glass tube is broken carefully to extract the rod-shaped piece of the clear, rigid polymer with optical fibers embedded approximately evenly about the rod axis. The polymer rod is then swollen in methyl alcohol overnight so that it expands somewhat and becomes rubbery. The glass fibers are readily removed at this point. The rod is cut by a razor to 10 mm sections which are then swollen gradually in methylisobutylketone. After the complete swelling, the internal cavity diameter is approximately 175 microns so that the bores readily accept insertion of Nylon 6 fibers of a diameter of about 140 microns. With the Nylon mandrels in place, the splicers are deswollen in methanol and then dried first at 105 degrees C. for about 12 hours and cooled to an ambient temperature. After this time the polymer becomes again clear and rigid, and the Nylon fibers are readily extracted by moderate pull. The resulting splicer has two or more cavities of about 140 microns approximately distributed about the center. The polymer has Tg =81.3 degrees C. as determined by thermomechanical measurement. The splicers are used in splicing the optical fibers of larger outside diameter than the one used as mandrel A. Fiber ends are prepared by denuding in the customary way and cleaved with a commercial cleaver. Then the fiber ends are inserted into the splicer bores until they meet, and while held in place, the splicer is heated with a commercial heat-gun for several seconds and then left to cool to an ambient temperature (total time of heating and cooling is about 1 minute). Then the insertion loss of the spliced fibers is measured on an optical bench using a HeNe laser as a source and silicon photodiode as a detector. The loss is calculated from comparison between the spliced fibers and the original continuous fibers of the same length. It is found that the spliced fibers have a consistently lower insertion loss than 0.5 decibels (dB).

The same molds as in Example 1 are filled with a monomer mixture consisting of: 76.85% of isopropylmethacrylate, 20% of n-Butylmethacrylate, 3.1% of ethyleneglycol dimethacrylate and 0.05% of dibenzoyl peroxide. The mixture is polymerized under the conditions described in Example 1. After the swelling the polymer in methanol and extracting the optical fiber, the swollen rod is cut by a razor in parallel cuts at an angle about 45 degrees and dried in an oven until rigid. Then the angled faces are ground and polished to be uniformly smooth, and the splicers are reswollen in methanol. The rest of the process is identical with Example 1. The splicers have Tg about 65 degrees C and their angled, polished faces facilitate insertion of the optical fibers considerably, even though some splicers have off-centered bores. Single spliced fibers using this method (and using dry fibers without any coupling agents) have the following performance:

The splicing procedure including fiber preparation may take less than 5 minutes. This example shows that the splicers from the unistructural mass according to our invention can in themselves provide low loss splicing in spite of asymmetric shape and without using various auxiliary means (coupling gels, index-matching adhesives, mechanical supports, etc.) the other splicers needed for a comparable performance.

Aluminum mold is prepared in the following way: two rectangular holes (about 90.times.20 mm) are cut into a rectangular aluminum plate 100.times.50 mm large and about 5 mm thick so that about a 2 mm wide strip is left between both apertures. Short lengths (about 50 mm) of denuded optical fiber (O.D. 110 microns) are then laid across the width of the plate and glued to it by epoxide glue on each end. Since the fibers are relatively short and supported in the middle by the 2 mm aluminum strip, no external mechanism is needed to straighten them or stretch them. This part is then put into a flat-bottomed aluminum weighing boat and filled with the same monomer mixture as in Example 2. The volume of the monomer mixture is selected so that the fibers are approximately in the middle of the layer. Then the mold is flushed with nitrogen and covered by an aluminum weighing boat of the same size which floats freely on the monomer. The mold is put into a nitrogen-filled container and polymerized as in previous examples. After the polymerization is finished, both boats are stripped from the frame plus polymer, and the polymer is let to swell in methanol. The swollen polymer is then taken from the frame, the sections of the fiber are pulled out and the strips of polymer are cut in parallel fashion so that the bores left behind by the optical fibers are roughly in the middle between and parallel to the cuts. The sections are then cut again by a razor to a shape approximating a trapezoid, and dried. The dry pieces are then ground and polished into the shape of a trapezoid having a larger rectangular face of about 10.times.4 mm opposite smaller rectangular face of about 2.times.2 mm and height of about 4mm. The bore axis is about 2 mm above the largest face and length of about 6mm. The splicer precursors are reswollen in methanol, and then swelled consecutively in ethanol, isopropanol and acetone (in the order of increasing swelling). Once the maximum swelling in acetone is achieved, the bore has a diameter over 170 microns so that the bores readily accept a Nylon mandrel of a diameter of about 140 microns. The polymer over the mandrel is then carefully deswelled using above swelling solvents in opposite order, and dried as in the previous examples. The clear, homogeneous splicers of the trapezoidal shape are stored with the Nylon mandrel inside to protect the bore from contamination. The mandrel is readily pulled out immediately prior splicing. The splicers are more convenient to use than the rod-shaped splicers described in the previous Examples; and in spite of utter lack of symmetry, they have a similar low insertion loss without using coupling agents or auxiliary support structures. The above examples specifically illustrate some of the preferred embodiments of the splicer of the present invention, including its preparation and its use. Obviously, variations may be made by the artisan without exceeding the scope of the present invention. The following examples illustrate testing and Polymer variations, as follows:

The procedure to analyze the performance of the splicer is to measure the optical loss suffered by a continuous fiber after the fiber has been cut and then reconnected by the splicer. The experimental set-up consists of a source, mechanical apparatus to hold the fiber and align it with the source, and a power meter to measure the amount of light propagated through the fiber.

The source used is a 5mW HeNe laser chosen for its ease of beam manipulation. The output of the HeNe laser is first steered through a light baffle to insure that diffuse scattering of the beam from some of the optical components used (primarily the input coupler) will not be reinjected into the laser cavity. A 0.5D neutral density filter (32% transmission) is used as the primary element in the baffle. The output of the baffle is steered into the fiber optic coupler. The coupler focuses the beam into the input end of a 200 m spool of fiber. The fiber now serves as the sole propagation link of the system. The output end of the fiber is held by the output collimator which serves to mechanically hold the fiber and collimate the light emerging from it to illuminate an optical detector. The optical detector consists of a silicon planar diffused PIN photodiode whose output is fed into a digital power meter for visual display.

The procedure for measuring the performance with this equipment is as follows:

1. The input coupler is adjusted to achieve the maximum power through the fiber (0.05-0.1 mW). This power is then recorded (I=).

2. The fiber is then cut approximately 3 m before the detector and the cut ends prepared by removing 50 mm of buffer and cleaving the fiber ends to a length of 10 mm.

3. The cut ends are then inserted into a splicer which is heated to effect the memory action.

4. After the splice is heated by a commercial heat gun and allowed to cool (total time approximately 60 seconds), the resultant power through the fiber is then recorded (Io).

5. Loss is calculated in dB: ##EQU1## Insertion losses measured on candidate splices averages 0.34dB as discussed in conjunction with Example 2 above. It is believed that the splice performance can be further improved to the point of an average loss of 0.2 dB with a much narrower standard deviation. The performance achieved compares quite favorably with the performance of other multimode splices. Thus, the performance work of 0.2 dB insertion loss may be achieved with the present invention in the field whereas this is usually not achieved with the prior art techniques. The only mechanical splice reported to have similar performance without a coupling cement or gel is the GTE elastomeric splice which is very expensive due to the amount of precision molding necessary to produce the splice. This splice makes use of a compliant material in which a precision cavity is molded or machined. The material deforms to allow insertion of the fibers and then relies on elastic forces to align and fix the fibers performance data from present invention type spliced optical fibers is shown in Table 1:

Table 2 shows various glass transition temperatures calculated (T.sub.g) for formulations 6-0 through 12-0 and Table 3 illustrates the activity of various solvents on crosslinked IPMA with 1% TEGDMA. These merely illustrate the types of variations which may occur regarding T.sub.g based on changing polymer component percentages and based on solvent variations. Table 4 shows swelling changes for solvent and component mix changes simultaneously.

The time taken to make a splice, as outlined in a previous section, is dominated by the time it takes to sufficiently swell the buffer in dichloromethane in order to remove it from the fiber. In actual field applications, this procedure can be easily shortened by using commercially available mechanical strippers

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.