ARTICLE FOR AND METHOD OF JOINING OPTICAL FIBRES

Abstract: An article suitable for use in the adhesive joining of two optical fibers (22, 22') with low light loss comprises a tubular container (10) which is larger in cross-section at the ends than at a central region (13), the smaller central region being where the ends (23, 23') of the fibers are joined.

Description: Description Article for and method of joining optical fibres This invention relates to the joining of optical fibers, and particularly though not exclusively to the adhesive joining of optical fibers which have been optically aligned. In UK Patent Application Publication No. 2100463, the disclosure of which is incorporated herein by reference, there is disclosed a technique for the adhesive- joining of optical fibers. The fibers are butted end-to-end within a tube containing indexmatching adhesive, optically aligned by measuring the light intensity passing from one fiber to the other and maximizing that intensity, and the adhesive is then caused to harden. A preferred adhesive is a photopolymerizable adhesive, i.e. one that will harden on exposure to ultraviolet light. It has been discovered, however, that shrinkage and flow of the adhesive as it sets tend to cause a loss of orientation of the fiber'ends with respect to each other, which ray result in an unacceptably high loss at the joint. (Fusion joining can produce losses as low as less than 0.1 dB, and for telecommunication fibers a loss less than 0.3 dB and preferably as low as 0.1 dB is considered desirable.) This adverse effect is a consequence of the adhesive being in a tube: polymerization of a drop of adhesive between two fiber ends but otherwise unconstrained does not cause a problem. However, in a tube, the surface interface between the adhesive and the tube inner wall restricts the freedom of the adhesive to shrink without coming under stress. Eventually the stress is relieved by cracking or void formation in the adhesive, flow of the adhesive, or separation at an adhesive surface. Because the fibers are flexible, it is not believed possible to rely on simply holding the fibers to prevent their movement: although the parts which are held remain still, the forces exerted on the ends by the shrinking adhesive are quite sufficient to cause relative movement. One solution that has been proposed is to coat the inner wall of the tube with a release agent, or make at least the inner surface of the tube itself of an inherently releasing material, so that any separation which occurs takes place at the adhesive-tube Interface. This ls discussed in sore detail in US Patent Appllcatlon No. 465362, corresponding to copendlng and commonly assigned Application No. , filed (Myers/Moyer/Jases) (Case MPO849), the disclosure of which is incorporated herein by reference. It has been proposed in British Published Patent Application No. 2,039,379A, the disclosure of which is incorporated herein by reference, to "zone cure" a photopolymerizable adhesive about an optical fiber which is to be aligned with a lens. This zone curing comprises first curing the adhesive adjacent the lens and the end of the fiber, and then curing the remaining adhesive by gradually exposing it to light. The technique proposed in that Application relies on the ability to symmetrically cure the adhesive about the fiber end to avoid loss of alignment. U.S. Patent No. 4,201,618 (Lewis) disclosures an apparatus designed to achieve the symmetric curing of a photopolymerizable adhesive used to join the ends of optical fibers. The adhesive is contemplated as being unconstrained (not in a tube), and a uniformly stressed bonded connection is said to be formed. It is an object of the present invention to provide an article and a method whereby an improved joint between the ends of two optical fibres may be obtained. In accordance with one aspect of the present invention there is provided an article suitable for use in joining the ends of two optical fibers, which comprises: a generally tubular straight container which has two open ends, whose smallest nternal cross-sectlonal dimension is at least 150 microns and whose largest internal cross-sectional dimension is less than 3000 microns, and whose internal cross-sectlon is greater at the ends than at an intermediate region between the ends. In accordance with another aspect of the present invention, there is provided a method of adhesively joining two optical fibers, which comprises: (a) aligning the ends of the fibers within a quantity of an index-matching adhesive contained within an intermediate region of a generally tubular straight container which has two open ends, whose smallest cross-sectional dimension is at least 150 microns and whose largest internal cross-sectional dimension is less than 3000 microns, and whose internal cross-section is greater at the ends than at the intermediate region between the ends, and (b) causing the adhesive to harden, thereby joining the fibers. The interaçdLate region of the container may conveniently be located centrally thereof. The alignment technique employed in the method of the invention may be, for example, optical alignment, by which is meant the use of the technique disclosed in U.K. Patent Application Publication No. 2100463; or visual alignment, where the positions of the fiber ends are observed with a microscope and moved lnto axially aligned, abutting relatlonshlp. The preferred technique for joining the aligned fibers is by supplying a hardenable, fluid, index-matching adhesive to the ends of the aligned fibers. Preferably the adhesive is polymerizable (this term being used to include adhesives which can be further polymerized or cross-linked). The adhesive may be self-setting (e.g. a two-component epoxy or cyanoacrylate adhesive) but is preferably one which polymerizes only when exposed to selected treatment after the fibers have been aligned. Particularly useful are photopolymerizable adhesives which can be cured by exposure to UV light, for example a photo-initiated urethane/acrylate adhesive. Other hardenable adhesives can be used, however. The hardened adhesive will be transparent to radiation of at least one of the wavelengths used as a signal in optical fibers (typically 810-860, 1280-1320 and 1450-1550 nanometers). Where the fibers to be joined comprise both a core and a cladding up to their ends which are to be joined, the hardened adhesive will usually have a refractive index which is not substantially less than that of the core, i.e. at least 1.4, preferably at least 1.45. However, where plastic-clad fibers are to be joined and the optical cladding is stripped back from the ends, the refractive index of the adhesive will preferably be identical or very close to that of the cladding. The article may comprise means for supplying adhesive to the container. Preferably the article of the Invention comprises a quantity of index-matching adhesive. The index-matching adhesive may lie within the container, and may provide the only content of the container. The adhesive may be retained within the container by surface tension forces. The container is preferably a tube having a circular cross-section, but it can also be a tube having a different closed cross-sect ion, or an open trough or another form of container which will contain and retain the adhesive; all such containers are included by the term 1generally tubular container as used herein. If the container is stored with the adhesive therein, it can if desired be fitted with removable end caps which seal its open ends thus keeping the adhesive clean and, if necessary, preventing it from leaking out of the container. The smaller intermediate cross-section of the article of the invention offers two particular advantages. First, it reduces the volume of adhesive which is hardened near the fiber ends, thereby lessening the movement caused by polymer shrinkage and flow. Second, when the fibers are stripped of their buffers or claddings for a part of their length, it is desirable that the buffer extend into the adhesive to obtain a joint of maximum tensile strength between the fibers. However, it is also desirable that the fiber ends lie close to, but not touching, for example, 10 to 50 microns from, the inner surface of the container near the means for causing the adhesive to harden, to ensure that the adhesive hardens near the fiber ends first, locking thea into alignment before the ccst of the adhesive hardens. A container having a smaller central cross-section accommodates these apparently conflicting requirements. The container should be such as to allow adequate movement of the fibers during the alignment process and is preferably at least 1.2 times, e.g. 1.6 times, especially 3 times the external diameter of the fiber. Thus, for example, fibers having a 125 micron outer diameter (core plus cladding) and a 250 micron diameter buffer may conveniently be joined within a container having an internal diameter of 500 microns in the central region and 1000 microns at the ends. The internal cross-section of the central region is preferably 250 to 750 microns, especially about 400 to 600 microns. The internal cross-section of the ends is preferably from 1.5 to 10 times the internal cross-section of the central region, e.g. 500 to 3000 microns, preferably 750 to 2000 microns. The length of the smaller intermediate region should be sufficient to enable joining of the fiber ends, e.g. at least 1 mm, but is preferably less than about 5 -, especially in the case of a container for joining stripped fibers. The length of the container overall should be sufficient to allow a strong adhesive joint to be formed, for example at least 5 mm, and is preferably less than about 25 -, as an excessively long tube makes alignment of the fiber ends difficult. A preferred length range is 10 to 20 mm. The container may be composed of, for example, quartz, glass, metal, or a polymeric composition, provided that lt permits (and is not damaged by) the desired process for hardening the adhesive. When the preferred photo-polymerizable adhesive L5 used, the container should be W-tronsparent. The inner surface of the container is preferably coated with a release agent, and/or the material of the surface is inherently releasing, as has been discussed previously. The hardenable adhesive used preferably has a viscosity such that it is retained in the container at 23'C with the container horizontal, and more preferably under all other conditions likely to be encountered before the adhesive is hardened. On the other hand, its viscosity should not such that it is difficult to insert and move the optical fibers under the joining conditions. We prefer to use an adhesive having a viscosity of 750 to 3000 centipoise, particularly 1200 to 2500 centipoise, e.g. about 2200 centipoise, (viscosities referred to herein are measured at 23-C using a Model HBT Brookfield Viscometer, Spindle No. SC4-27 rotating at 100 rpm). The adhesive may alternatively be retained in, for example, a small polymeric bulb attached (or capable of attachment) to one of the ends of the container, and be dispensed into the container when the container is to be used. This offers particular advantage if a twocomponent adhesive is used, since a two-part bulb may be employed. An article for and method of joining two optical fibres, each in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawing, in which; Figure 1 shows the article in longitudinal sect ion; and Figure 2 shows the article of Figure 1 in use to make the joint. Referring to Figure 1, the container of the article, shown generally at 10, has two open ends 11 and 11' and end regions 12 and 12' which have a greater internal cross-section than a central region 13. The inner surface 14 of the container may comprise a release agent and/or the material comprising the container itself may be self-releasing. A quantity of index-matching adhesive 15 is shown within the container. Figure 2 illustrates the container of Figure 1 as used to make an adhesive joint between two optical fibers. The buffers 21 and 21' of two optical fibers, shown generally at 20 and 20', have been cut back, exposing the glass fibers 22 and 22'. The fibers are inserted into the container so that the ends 23 and 23' of the fibers are almost abutting within the central region 13 of the container 10. The fibers are then aligned (by means not illustrated here), and the indexmatching adhesive 15 is caused to harden to make the joint. EXAMPLE An apparatus generally as described in U.K. Patent Application Publication No. 2100463 was used to join two identical optical fibers. The fibers (which are available from Corning Glass) consisted of a glass core of diameter 50 microns, a glass cladding of outside diameter 125 microns, and a polyacrylate buffer of outer diameter 250 microns. The buffer was stripped from the fiber ends and the stripped fiber ends cleaved to provide clean end surfaces about 3 se from the end of the buffer. The fibers were then inserted into an article according to this invention. The article, made of poly (methyl pentene) had an overall length of 12.7 mm, and a narrow central region length of 3.2 se. The inner diameter of the end regions was 1500 microns and of the central region was S00 microns, with the wall thickness being 500 microns. The article contained a UV-polymerizable adhesive having a refractive index (when cured) of about 1.49, which adhesive comprised a mixture of about 3 parts by weight of trimethylolpropane triacrylate and 1 part by weight of a urethane/acrylate oligomer (Uvethane 788 from Thiokol) with a small quantity of a photoinitiator. The adhesive had a viscosity (uncured) of about 2200 centipoise. The fibers were inserted into the adhesive-filled container such that the stripped ends almost touched at the center of the tube near the wall nearest a UV lamp, and were then precisely aligned. The adhesive was then cured in a two-stage zone cure; in which (1) a length of 1 mm about the ends of the fibers was exposed to the lamp, and (2) the rest of the container was exposed. An excellent splice could be reproducibly prepared with a loss less than 0.1 dB.

Claims: CLAIMS: 1. An article suitable for use in joining the ends of two optical fibers, which comprises: a generally tubular straight container which has two open ends, whose smallest internal cross-sectional dimension is at least 150 microns and whose largest internal cross-sectional dimension is less than 3000 microns, and whose internal cross-section is greater at the ends than at an intermediate region between the ends. 2. An article according to claim 1 wherein the container is composed of glass or polymer. 1. The article of claim 1 wherein the inner surface of the container is coated with a release agent, and/or is composed of an inherently releasing material. 4. An article according to any preceding claim, wherein the length of the container is between 5 and 25 millimeters. 5. An article according to any preceding claim, wherein the length of the intermediate region is between 1 and 5 millimeters. 6. An article acording to any preceding claim, wherein the smallest internal cross-sectional dimension of the intermediate region is between 250 and 750 microns. 7. An article according to any preceding claim, wherein the ratio of the smallest internal cross-section to the largest internal cross-section is between 1:1.5 and 1:10. 8. An article according to any preceding claim, wherein the container has a smallest internal radius of curvature of at least 125 microns. 9. An article according to any preceding claim, which has a circular cross-section. 10. An article according to any preceding claim, comprising a quantity of index-matching adhesive. 11. An article according to claim 10, wherein the quantity of index-matching adhesive lies within the container, provides the only content of the container, and is retained in the container by surface-tension forces. 12. An article according to claim 10 or 11, wherein the adhesive is photopolymerizable. 13. An article according to any preceding claim, wherein the intermediate region is located substantially centrally of the container. 14. A method of adhesively joining two optical fibers, which comprises: (a) aligning the ends of the fibers within a quantity of an index-matching adhesive contained within an intermediate region of a generally tubular straight container which has two open ends, whose smallest cross-sectional dimension is at least 150 microns and whose largest internal cross-sectional dimension is less than 3000 microns, and whose internal cross-section is greater at the ends than at the intermediate region between the ends, and (b) causing the adhesive to harden, thereby joining the fibers. 15. A method according to claim 14, wherein the adhesive is photopolymerizable and step (b) comprises exposing the adhesive to ultraviolet light. 16. A method according to claim 14 or 15, wherein step (b) comprises: (i) causing the adhesive immediately adjacent the fiber ends to harden; and (ii) subsequently causing the remainder of the adhesive to harden. 17. A method according to any of claims 14 to 16, wherein the ends of the optical fibers are aligned in step (a) at a position close to the inner surface of the container. 18. A joint between two aligned optical fibers which has been made by the method of any of claims 14 to 17.

Inventors: ARRINGTON, JOHN PRESTON, SHEN, NELSON MU-CHING
Applicants: RAYCHEM CORPORATION (A DELAWARE CORPORATION)
Application year: 1984
Main classification: G02B6
IPC codes: G02B6