WO1981003227A1

WO1981003227A1 - Method for precision junction of an optical fiber

Info

Publication number: WO1981003227A1
Application number: PCT/SE1981/000137
Authority: WO
Inventors: L Lidholt
Original assignee: L Lidholt
Priority date: 1980-05-09
Filing date: 1981-05-08
Publication date: 1981-11-12
Also published as: EP0051610A1; SE436453B; SE8003509L

Abstract

For precision connection of an optical fiber, the end of the fiber (3) is mounted centered and protruding from the end of a connector member (1), which has the shape of a truncated exterior conical surface (2). The centering is guaranteed by a precision tool (7) with a complementary interior conical surface (8) with a centered hole (9), and the fiber end is fixed with a thermosetting resin (10). It is suitable to use UV-setting resin and a material permeable to such radiation in the tool (7).

Description

Method for precision junction of an optical, fiber

The invention relates to a method of arranging precision junction of an optical fiber to another means, ^" e.g. another optical fiber, the fiber being arranged well centered in the axis of an exterior conical surface which is fitted against the other means which is also centered on the axis of an exterior conical surface, the two exterior conical surfaces being fitted into concentric, well-defined position against each other by means of a junction member provided with two interior conical surfaces which open in opposite directions and have coinciding axes.

In recent years the importance of optical fibers for transmitting information has grown. From being a technical curiosity they have been developed into practical signal bundles which appear to be taking over the function of telephone cables, thanks to their large capacity and small volume. On a trial basds at least, they have already come into use in the telephone system. Even in other fields in which signals are to be transmitted,, such fibers have proved useful, primarily due to the fact that the sensitivity to disturbance is substantially lower than in electrical wires, and that it is possible to make less expensive cables, especially when transferring high-frequencies where it has been usual to use coaxial cables. Coaxial cables are relatively bulky in relation to optical fibers, and connecting them presents problem. A precondition for the large-scale replacement of coaxial cables by optical fibers is the satisfactory solution of connection and junction problems. An optical fiber consists of a thin core of glass (often with a diameter of 50-200 micrometers) surrounded by a suitable jacket of glass or plastic. In one type of optical fibers there is an inner core surrounded by an outer portion, made in the same manufacturing step, with gradually decreasing index of refraction. Thus light introduced at one end of the fiber within a certain angle

o:/rpι to the axis of the fiber (numerical aperture) will be sent along the fiber by successive raflection; or^* viewed in another way, the fiber will act as a wave-guide for the light. Improvements have now made it possible to manufacture optical fibers with so little energy loss per unit of length that fiber, cables are now beginning to be economically competitive with other transfer media.

The joining of two optical fibers does not present insurmountable obstacles in the laboratory. It is sufficient to bring two ^"fiber ends together, end to end, and methods exist for fusing them together. It is also possible to bring together polished end sections, with or without liquid coupling medium. It is also possible to bring together two polished fiber ends while retaining a small air gap. The length, of the air gap is relatively insignificant as long as it is of the same size or less than the diameter of the fiber. The losses, however, increase markedly if the ends of the fibers are not directly opposite each other or if their axial directions differ.

One method of joining two fiber ends to each other is known which involves fusing the ends of the fibers. It requires, however, special relatively complicated equipment and th-e method cannot be used for connecting a fiber to a light-emitting diod or a photodetector, for example.

A detachable junction is known which functions as described in the introduction and in which the necessary precision is assured by conical surfaces. The fiber is connected via an exterior conical surface, functioning as a "male contact", in which the fiber is centered along an axis, by means of suitable centering means.

An example of this is a. connector member, in which the inner centering means consists of an arrangement with a number of-precision manufactured balls, e.g. three, which are pressed by a flat annular surface against an interior conical surface, concentric with the conical

OMPI exterior reference surface. The annular surface of the inner centering means pressed against the balls is in turn pressed via an intermediate annular means by an inwardly projecting shoulder from the rear piece. The rear piece has an external thread which^* fits into an internal thread in the front piece, and when put together the center means is pressed tightly by appropriate tightening of the screw means. It is locked by placing thermosetting resin in the thread. The fiber is fixed at the point of the connector with thermosett±ng resin, and in the rear piece by crimping ^• it about the protective jacket of the fiber which has been left on here. Then the connector with fiber is ground so as to produce a good junction surface in a well-defined axial pc -ition. However, this known method of connecting an optical fiber involves the use of relatively complicated means, resulting in cost and difficulty. Therefore it is a purpose of the invention to produce a simpler and less expensive junction. Another purpose if to achieve junction which is so simple that the optical fiber will be even more competitive in relation to electrical connections, especially as regards connections for computers, measuring instruments, instruments for industrial process control etc., where the available labour, due to the fact that their main duties are different, cannot receive the training received by those who work daily with telecommunication. In other words, something which, can be done relatively easily by a tele¬ communications worker dealing with optical fibers can often present great difficulty to laboratory or service personnel in a process industry or the like. It is therefore important that the connection of optical fibers be simplified, and in certain cases one might even accept greater power losses at the junctions if the signal paths are so short that losses in the fiber itself are negligible. ^* The above and other purposes and advantages are achieved according to the invention by a method of arrangin precision connection of optical fibers of the type describe in the introduction which is characterized in that the two interior conical surfaces in the junction member are made in a blank which is mounted for rotation in a machining apparatus about a first .axis whereafter a machining tool is advanced against the rotating blank along a second axis which forms an angle, with, and intersects the first axis, said angle corresponding to half of the intended cone angle, a cutting edge of the tool being moved in a plane coinciding with- the plane defined by said axes, both along a first line spaced from and parallel to the second axis and along a second line at the same distance from the secon axis but on the opposite side thereof, the two cutting edges each making one of the two interior conical surfaces. It is thereby possible to connect the exterior conical surfaces to each other with great precision.

In order for this to provide full precision in the fiber junction, it is suitable to center the fiber in the axis of the exterior conical surface by means of a pre¬ cision piece fitting into the exterior conical surface and which has a centered hole holding the fiber, so that the fiber can be centered and fixed, e.g. with thermosetting resin.

In accordance with a preferred embodiment of the method, a UV-setting resin is used. Such resins are well- known to the plastics expert and are available on the market. Furthermore it is suitable to make the centered precision piece of UV-translucent material. It is possible to use quartz here, but for the present a thermosetting ^• resin is used for manufacturing reasons, and it can be of the same UV-setting type as that mentioned previously. In certain cases where tolerances are greater, a rubber- like plastic-material of a known type per se is used which guides the fiber elastically into its desired center position. It is also possible to set the UV-setting resin with UV-light supplied via the fiber itself, preferably from the fiber end at the junction.

According to a preferred embodiment the precision requirements are^* fulfilled in the connection by the two interior conical surfaces of the junction member being made in a blank which is mounted in a machining apparatus, preferably a automatic lathe, so that the blank is rotatable about a first axis. It is preferable to first make a hole along the first axis with a diameter somewhat exceeding the diameter of the intersection of the two intended interior conical surfaces. The two conical surfaces are cut with an individual edge or the like, said edges being moved parallel with each other equal— distant from a second axis which forms with the first axis an angle corresponding to half of the intended cone angle. Furthermore the two edges must be moved in a specific plane which coincides with the plane defined by the first and the second axes which must therefore intersect. It is important for precision that the two cones be made in one set-up, so that the two interior cones will be coaxial to said first axis and will each be cut out by one of the two cutting edges. Moving these cutting edges in the same direction results in cone angles of the same size. It is preferable that the two edges be arranged in the same tool and be in effect during the same work step, but it is also possible to let the two conical surfaces be cut in separate steps, depending on available machines. The invention will now be described in more detail with ref-er"ence to non-limited examples. Fig. 1 shows schematically and in longitudinal section a precision junction according to the invention. Fig. 2 shows schematically a work step. Fig. 3 shows, also schematical¬ ly, another work step. Two junction members, 1 and 1', are shown put together in Fig. 1 so that their fibers 3 have coinciding axes at least at their outer ends, which is the purpose of the The fibers coincide with the axes of the exterior cones 2 and 2* in the connector members. The two connector members are held in position by a junction member 20 shown schematically. Fixing members, which -are not shown, hold the conical surfaces against each other and are suit¬ ably in the form of threaded sleeves screwed on from either side which act on the connector members via shoulder portions 21 and 21 ' . Since such means for holding the members together can be designed without difficulty in a number of ways by a .person having understood the principle of the invention, they have not been shown here so as not to unnecessarily complicate the Figure making it more difficult to understand the principle of the invention Fig. 2 shows how a fiber 3 is arranged in a connector member 1. An optical fiber usually has a protective jacket which must be removed at the end. In the present case there is an outer layer 30, an intermediate layer 31 and an elastic layer 32 of an almost shapeless elastomer material. These materials are successively eliminated as shown in Fig. 2. The end of the fiber 3 is then inserted in the tubular connector member 1, the interior hole of which should preferably become narrower in steps in relation to the design of the protective covering.

At the point of the connector member, in the center of its exterior conical portion, the hole narrows to a diameter only somewhat larger than the diameter of the optical fiber. A small amount of thermosetting resin is inserted here, and a precision tool 7 is applied which has an interior conical surface 8 fitting the conical surface 2, and a hole 9 which is well-centered and fits the fiber. When put in place the fiber 3 will be centered. The resin is then cured, and according to a preferred embodiment the resin is UV-setting. It is thus suitable that the tool 7 be transparent so as to permit UV- illumination.11. Such curing usually only takes a few seconds. The tool 7 can then be removed and the fiber cut off and polished to a good surface, preferably in the

OMPI ° plane of the surface 6. The fiber is fastened with its protective cover in the connector member, preferably by crimping around the jacket 30.

The tool 7 is preferably made by casting on a precision- turned molding piece, an exterior conical surface and an end stub, corresponding to the hole-_#9, being turned in the same set-up. This should be made very slightly larger than the upper tolerance measurement of the fiber 3. Especially for somewhat larger fibers (e.g. 100 micrometers) the precision requirements are such that it is possible to make the tool 7 of an elastic thermosetting plastic, so that the fiber 3^' is elastically held and pulled towards the center of the cone. It is then possible to make the hole 9 for example.5 micrometers smaller than the diameter of the fiber, since the plastic will be sufficiently flexible.

Fig, 3 shows schematically how the two cones 4 and 5 in the junction member can be made, so that their axes coincide. It is not possible with ordinary machineshop methods to make the cones from either side so that their axes coincide properly both as regards direction and position.

A blank 12 is set up in a lathe chuck for example so that it can rotate about a first axis 13. A hole 17 is then drilled of which only a short cylinder surface remains in the Figure, as a result of the subsequent machining. The top slide o ,the lathe is then turned at an angle corresponding to half of the cone angle so that its move¬ ment follows a second axis 14. A tool with two edges is fixed therein and is then advanced by means of the top slide movement so that the edges follow the lines 15 and 16 on either side of the second axis 14 which intersects the axis 13.

It is suitable to use a tool which is rotationally symmetrical about the axis 14. As can be seen in the schematic cross section in Fig. 3, the two edges will each cut one cone surface 4, 5. Series production will make it possible to achieve a completely interchangeable system;

OMPI

/j. wi^po one should see to it that the conical surfaces of the connector members are dimensioned so that the annular pla surfaces 6 (see Fig. 2). cannot be abutted against each other but form a gap of about 20 micrometers. The invention provides an inexpensive and simple method of connecting optical fibers. It is also possible to make multiple connector arrangements, with several connector- members collected in a common male connector member which, can be coupled together with another connecto member with an intermediate piece between them provided with the same number of junction members 20. The inventio is not limited to the connection of optical fibers to each other; rather it can also be used for coupling the fiber ends of light sources (e.g. light diodes) , light detectors (e.g. photo diodes), with the light sources or light detectors being mounted to corresponding conical surfaces with sufficient centering.

Claims

1. Method of arranging precision connection of an optical fiber (31 to another means,, e.g. another optical fiber, the fiber being arranged well-centered in the axis

5 of an exterior conical surface (2) which is fitted against the other means which is also centered on the axis of an ■ exterior conical surface, the two exterior conical surfaces being fitted into concentric position against each other by means of a junction.member (20) provided with two 0 interior conical surfaces (4,5} which open in opposite directions and have coinciding axes, characterized in that the two interior conical surfaces (4,5) in the junction member are made in a blank (12) which is mounted for rotation in a machining apparatus about a first axis (13) , -5 whereafter a machining tool is advanced against the rotating blank along a second axis (14) which forms an angle with and intersects the first axis (13) , said angle corresponding to half of the intended cone angle, a cutting edge of the tool being moved in a plane coinciding with 0 the plane defined by said axes, both along a first line

(15) spaced from and parallel to the second axis (14) and along a second line (16) at the same distance from the second axis but on the opposite side thereof, the two cutting edges^* thereby each making one of the two interior 5 conical surfaces.

2. Method according to claim 1, characterized in that the two interior conical surfaces are each made with an individual cutting edge, said cutting edges being arranged symmetrically on either side of the tool. 0

3. Method according to claim 1 or 2, characterized in that prior to said machining, a hole (17) is drilled in the blank along said first axis.

4. Method according to claim 3, characterized in that the hole is drilled with a diameter somewhat exceeding 5 the diameter of the intersection of the two intended conical surfaces.

5. Method according to anyone of the preceding claims characterized in that the freed end of the optical fiber (3) is inserted into a tubular connector member (1) which is provided at one end with a truncated exterior conical surface (2) the axis of which coincide with the longitudin direction of the tube, and with a circular end surface (6) in the center of which the tube hole ends, the fiber end being inserted in the opposite end to the conical surface so that the extreme end of ^'the fiber protrudes through the hole of the end surface, that a precision tool (7) is applied to the connector member (1) , said tool having a truncated interior conical surface (8) which is comple¬ mentary to the exterior conical surface (21 of the connect member, and a hole (91 arranged with-great precision in the center of th.e cone axis, said hole having a diameter adapted to the diameter of the fiber so that the fiber end is thereby guided with great precision to the axis of the connector member, that a thermosetting resin (10) is introduced into the hole of the connector member, that the thermosetting resin is cured so that the position of the fiber end is fixed, that the fiber end, the protective jacket of which, is left on at the end of the connector member opposite to the exterior conical end, is permanentl fixed to the connector member, preferably by crimping the connector member about the protective jacket of the fiber.

6. Method according to claim 5, characterized in that the thermosetting resin is cured by means of UV-light (11)

7. Method according to claim 6, characterized in that the precision tool is made of a material which is trans¬ lucent to UV-light. and that the curing is done by illuminating the thermosetting resin through the tool.

8. Method according to claim 6, characterized in that the curing is effected by supplying UV-light via the optical fiber.

9. Method according to anyone of claims 5-8, characterized in that the precision tool is made of an elastic material which is thereby capable of absorbing variations in the diameter of the optical fiber.