A METHOD OF MOUNTING AN OPTICAL FIBRE

Abstract: A D-fibre (2) having a flat surface (8) is mounted by pressing the fibre (2) into a heated thermoplastic substrate (6) by a former surface (14) until the flat surface (8) is flush with any adjacent surface (4) of the substrate (6). The former surface (14) may be non-planar to form an undulating flat surface of the fibre, for example. The method finds particular application in the production of half-coupler blocks as well as making components of other coupler and optical switches.

Description: This invention relates to a method of mounting an optical fibre of particular, but not exclusive, application to mounting D-fibres. D-fibres are optical fibres having a D-shaped cross-section with the core of the fibre near the linear portion of the cross-section such a fibre is described in US-A- 4,589,725. Fibres have been used as optical couplers by removing part of the cladding surrounding the fibre core of a pair of fibres and placing them in contact. The reduced cladding allows coupling between the cores due to their close proximity. One method of forming such a coupler is to make half-coupler blocks by embedding a gently curved fibre in a glass substrate and polishing it to remove a portion of the cladding. A pair of these half-coupler blocks can then be placed together to form an optical coupler. The surfaces of the substrates being flush with the exposed fibre surfaces hold the fibres in the desired position. Compare the process described in WO-A-82/04328. The manufacturing process is, however, time consuming and needs to be carried out with great care to ensure the correct amount of fibre cladding is removed. It is an object of the present invention to provide a method of mounting an optical fibre which largely overcomes these disadvantages. Accordingly, there is provided a method of mounting an optical fibre having a flat surface which includes the step of pressing the fibre into a heated thermoplastic substrate with a former surface until the flat surface of the fibre is flush with any adjacent surface of the substrate. By flat surface is meant the surface defined by the linear portion of the cross-sections of an optical fibre, as found for example with rectangular or D-shaped optical fibres, notwithstanding that the surface may be curved or twisted before, during or after mounting. The surface shape of the former will determine the final disposition of the flat surface whether it be curved, rippled or in a single plane. The invention is based on the fact first appreciated by the applicant that the resultant surface formed by the substrate and flat surface of the optical fibre can be made flat to about 0.4 microns which compares favourably with polished silica half couple blocks described above. The thermoplastic material is preferably a co-polymer of poly (2,6-dimethyl-p-phenylene oxide) and polystyrene, but may be of polypropylene or polymethylmethacrylate polystyrene or other suitable thermoplastic material. The invention also finds application in mounting an optical fibre for use in wavelength selective optical coupler where it is required to hold a D-fibre, for example, with the flat surface fixed in an undulatory form as described in the applicant's application GB8813667 titled "A Wavelength Selective Optical Coupler" filed on the 9th June 1988. This is achieved by preparing the former surface with the required undulations which are then impressed on the fibre as it is pressed into the substrate. According to a further aspect of the present invention there is provided an optical device which comprises an optical fibre having a flat surface embedded in a thermoplastic substrate with the flat surface flush with any adjacent surface of the substrate. This method of mounting an optical fibre and various applications of the method will now be described with reference to the accompanying drawings in which: Figure 1 is a perspective view illustrating a method of mounting an optical fibre according to the present invention; Figure 2 is a perspective view of a D-fibre being pressed to be held in an undulating formation; Figure 3 is a perspective view of an undulating optical fibre produce by the method illustrated by Figure 2; Figure 4 is a plan view of a D-fibre mounted in a substrate by the method of the present invention in which electrodes have been embedded in the substrate at the same time as the fibre; and Figure 5 is a perspective view illustrating a method of mounting of D-fibre in a non-continuous mount so as to allow the D-fibre to be deflectable. Referring to Figure 1, a D-fibre 2 is shown placed on the upper surface 4 of a thermoplastic substrate 6 with its flat surface 8 uppermost. The thermoplastic is the co-polymer of poly-(2,6-dimethyl-p-phenylene oxide) and polystyrene marketed by the General Electric Corporation under the trade name "Noryl" although other thermoplastics may be used. The substrate 6 is supported by a rigid support 10. The substrate is heated by a heater (not shown) until it is deformable by the fibre 2 when pressed by a former 12 having an optically flat, planar under-surface (former surface) 14, for example a glass microscope slide. The former 12 is pressed against the fibre with a pressure of 13,8-137,8 KPa (2 to 20 psi) while the substrate is maintained at 120°C is forced into the substrate until the flat surface 8 is flush with the top surface of the substrate. The former 12 is then removed and the substrate allowed to cool to room temperative over a period of about an hour. Referring now to Figure 2 there is shown a similar arrangement to that of Figure 1 except that a former 16 is used which has an undulatory former surface 18 which when pressed against the fibre 2 as in the method described above forms an undulating optical fibre as shown in Figure 3. This may then be placed in contact with a D-fibre held in substrate using the planar former 12 of Figure 1 to form a wavelength selective optical coupler as described in the applicant's patent application referred to earlier. An alternative method of using the former with a non-planar forming surface is to first press the former against the heated substrate to shape the top surface of the substrate in conformity with the former surface, remove the former, place the fibre on the substrate and then press the fibre into the substrate using the same former. This reduces the strain on the optical fibre during pressing. The former may be pressed against the fibre with a desired pressure before the substrate has started to be heated or before it has been heated sufficiently to allow the fibre to be pressed into it. In this case the substrate is heated or heated further after the pressure is applied until it is plastic enough to deform Referring now to Figure 4 there is shown a substrate 6 and D-fibre 2 in which electrodes 20 have been pressed into the substrate by the former. This can be done simultaneously or as a separate step. Such an arrangement, and similar arrangements, find application for example in electrostatically operated fibre switches where an adjacent fibre is attracted to the electrodes structure to come into coupling contact with the embedded fibre. Referring to Figure 5 a former 22 having an convex former surface 24 is shown about to be pressed against a D-fibre 2 supported at spaced apart portions 26 and 28 of a substrate 30 having a hollow centre. The former 22 is pressed the D-fibre 2 into the end portions whilst forcing the fibre 2 onto the hollow. This method produces a device in which the fibre can move up and down, for example in response to an electrostatic field produced by the electrodes of 20 of an adjacently placed device of Figure 4 thereby forming an electrostatic optical switch. The substrate 6 of Figure 4 may contain added high dielectric materials for example T i O₂ to enhance the electrostatic effect. It will be appreciated that other shapes of former and substrate can be employed with the method of the present invention to provide various configurations of supported optical fibre and that other forms of optical fibre having a flat surface extending along the fibre's length other than D-fibres may be used. The present invention provides a simple, method of forming a robust mount in which the flat surface of the optical fibre is flush with the surface of the supporting substrate to sub-micron accuracy more quickly and cheaply than previously possible. For example, a half coupler block can be made according to the present invention in less than 10 minutes, once a former has been prepared, at a cost considerably less than required for a polished half coupler block. The former is preferably of glass but other materials may be employed if they can be given the required surface finish.

Claims: 1. A method of mounting an optical fibre having a flat surface which includes the step of pressing the fibre into a heated thermoplastic substrate with a former surface until the flat surface of the fibre is flush with any adjacent surface of the substrate. 2. A method according to claim 1 in which the optical fibre is a D-fibre. 3. A method according to either of claims 1 and 2 in which the former surface is planar. 4. A method according to any one of claims 1 and 2 in which the former surface is undulatory. 5. A method according to claim 4 in which the former surface is pressed against the heated thermoplastic substrate before pressing the fibre into it. 6. A method according to any preceding claim in which a non-fibre component is also pressed by the former surface into the heated thermoplastic substrate. 7. A method as claimed in any preceding claim in which the substrate is a co-polymer of poly(2,6-dimethyl-p-phenylene oxide) and polystyrene. 8. A method as claimed on any preceding claim in which the former surface is a glass surface. 9. An optical device comprising an optical fibre having a flat surface embedded in a thermoplastic substrate with the flat surface flush with any adjacent surface of the substrate. 10. A device as claimed in claim 9 in which the flat surface of the optical fibre is undulating. 11. A device as claimed in claim 9 in which the fibre has an unsupported portion between two portions supported by the substrate. 12. A device as claimed in claim 11 in which the unsupported portion of the optical fibre is free to move relative to the substrate. 13. A device as claimed in any one of claim 9 to 12 in which the optical fibre is a D-fibre. 14. An optical device as claimed in any one of claims 9 to 13 in which the substrate is a co-polymer of poly(2,6-dimethyl-p-phenylene oxide) and polystyrene.

Inventors: KHAVAND, FARZIN MOHAMMAD HOSSAIN
Applicants: BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1989
Main classification: B29C67
IPC codes: G02B6, B29C70