FIBRE REFLECTOR

Abstract: An air bubble (23) in the core (3) of an optical fibre (1, 1') constitutes a broad band reflector to light waves propagated in the fibre. A fibre end (3) is first stripped and cut in the conventional way, after which in the end surface, a pit (13) in the core region is produced by etching. The fibre end is therefore treated with e.g. diluted hydrofluoric acid, that etches the higher doped core (3) more rapidly than the cladding (5). The etched fibre end (1) is then welded to a standard fibre so that a splice (21) is obtained where the desired air bubble (23) is enclosed in the fibre core. The magnitude of the reflectance of the produced reflector can then be changed by repeated heating steps, in the same way as in welding of the splice (21) and/or by filling the bubble (23) with another medium than air or by a coating one of its walls with a suitable material, e.g. a metal.

Description: FIBRE REFLECTOR TECHNICAL FIELD The present invention relates to a method of producing an optical fibre reflector, a fibre reflector produced by means of the method and a use of the fibre reflector for verifying the function of an optical network. BACKGROUND Internal reflections in optical fibres have several applications, such as for sensor purposes and for verifying the function of a fibre optical network. Light transmitted in an optical fibre is reflected at the reflector and the reflected light can be detected. PRIOR ART Optical reflectors for generating internal reflections in optical fibres and methods for producing such reflectors are previously known from the patents US-A 4,892,388 and US-A 4,400,056. However, the prior methods are rather complicated and there is thus a need for providing simple production methods for reflectors in optical fibres. In the patent US-A 5,210,801 optical components are disclosed constructed of flat wave guides or wave guides having rectangular cross-sections, where a cavity is arranged located adjacent a wave guide core, however never so located that the material of the wave guide core is located directly at the cavity. The production of the components is made by means of the conventional, process technological, relatively complicated and costly methods which are used for manufacturing electronic integrated circuits and optical planar circuits. DESCRIPTION OF THE INVENTION It is an object of the invention to provide a simple method for producing internal reflections in optical fibres. It is also an object of the invention to provide an optical fibre provided with an internal reflector for obtaining internal reflections in the optical fibre. The objects mentioned above are achieved by the invention, the detailed characteristics of which appear of the appended claims. Thus, in a controlled way an air bubble, i.e. a small cavity, which in the main case is filled with air, is introduced in the core of an optical fibre by means of an etching or a welding procedure and thereby a broad band fibre reflector is obtained. The fibre end is first stripped, i.e. its exterior protective layer is removed, and is then cut in the conventional way for achieving an essentially flat end surface, that is located essentially perpendicularly to the longitudinal direction of the fibre end, after which, in the end surface thus formed, a pit or recess is made in the core region, i.e. in the region where the fibre core ends in the end surface. The pit can be produced by means of some mechanical method such as grinding or by treating the end surface with a laser beam for removing material at the suitable position. However, a chemical treatment is preferred, wherein the fibre end is etched with e.g. diluted hydrofluoric acid or a mixture thereof with similar substances such as ammonium fluoride. A recess is thereby produced, since the higher doped core is etched more rapidly than the cladding. The etched fibre end is then joined or spliced, in the preferred case welded, to a standard fibre having, in the preferred case, an essentially flat end surface, which is cut in the usual way and is located essentially perpendicularly to the longitudinal direction of this fibre end, what results in an air bubble enclosed in the core. The magnitude of the reflectance can be changed by repeated heating operations in the same way as in welding over the spliced or joint region and/or by filling the bubble with another medium than air or by coating one of its walls with a suitable material, e.g. a metal. Generally, an optical fibre has a fibre core and a cladding surrounding the core. In order to produce a reflector, a cavity is arranged in the optical fibre, which is in particular arranged in the fibre core. The cavity is generally completely surrounded by material of the fibre and is located so close to the core, that light which is introduced into and propagates along the fibre, will be disturbed significantly by the cavity, so that it is partly reflected. It is obtained by the fact that the fibre core material extends up to the cavity and in particular the cavity can be essentially completely enclosed or completely surrounded by material in the core. For a production according to what has been discussed above the cavity will have substantially a lens-shape or essentially the shape of a flattened ellipsoidic body, having two opposite, large arched or curved surfaces. The fibre core material extends then up to at least the central part of these large surfaces and also to nearly all of these surfaces except possibly a small, exterior marginal area. In the cavity a substance can be provided that increases the reflective capability of the cavity to light waves propagating in the optical fibre. Such a substance can in particular have a reflective index different from the reflective index of the material surrounding the cavity and the optical fibre or it can be a metal material. The substance can, as for metal materials, have substantially solid form and then be deposited as a layer on only one surface in the cavity, on one of the large surfaces according to what has been said above, or generally only on one surface which is directed towards or is located at the wave guide core or only one surface which is substantially directed in one direction, in the longitudinal direction of the fibre. DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with reference to not limiting embodiments and the accompanying drawings in which: - Fig. 1 shows a fibre end having an etched end surface, - Fig. 2 shows a spliced fibre having an enclosed cavity, - Fig. 3 illustrates the treatment of the end surface of the fibre, - Fig. 4 shows schematically splicing of optical fibres, - Fig. 5 shows a section of a coated end of an optical fibre, - Fig. 6 shows a section of the coated end according to Fig. 5, where the coating has been partly removed, - Fig. 7 shows internal reflections in optical fibres used for verifying the function of an optical network. DESCRIPTION OF PREFERRED EMBODIMENTS In fig. 1 the end of an optical fibre 1 is shown. It comprises as conventional a fibre core 3, a cladding 5 surrounding the core and an exterior protective layer 7. For conventional quartz glass fibres used for telecommunication the core 3 and the cladding 5 consist of quartz glass having reflective indexes of different magnitudes, produced by suitable dopings, and the exterior protective layer is made of some polymer. In the production of an internal reflector first the exterior protective layer 7 is removed over a suitable distance from one end of the fibre. The end of the fibre is then submerged in a vessel 9, see Fig. 3, containing an etching liquid in the shape of hydrofluoric acid diluted with water, possibly mixed with ammonium fluoride, NH 4 F, during mixing by means of a mixer shown at 11. When the glass in the fibre is etched by means of one of the liquids mentioned, the higher doped fibre core 3 will be etched more rapidly than the cladding 5 which produces a pit or recess 13 in the end surface of the fibre, see Fig. 1. The pit 13 can be more or less well localized to the very fibre core 3, depending on the type of fibre. A continuous transition in the end surface between the region of the fibre core and the cladding region is however always obtained, also for step index fibres. The end surface of the fibre 1 comprising the pit or recess 13 within the core region is then spliced to an end surface of another optical fibre. The splicing process is advantageously made as a fusioned welding, where the heating is accomplished in some way, such as by means of an electrical arch, which will be described hereinafter, by means of a laser beam, hydrogen gas flame, etc. Also a completely mechanical splice is conceivable, such as by means of aligned V-grooves in a suitable fixture. The fibre 1 comprising the pit 13 in one end surface is thus inserted as one of the fibres in a fibre welding device, generally designed 15, some central parts of which being illustrated in Fig. 4. The fibre portion 1 is then attached in a guide in one chuck 17, which in the welding machine 15 is placed on some support 19. In the opposite chuck 17' a similar fibre 1' is placed, which in the conventional way is straightly cut but has not been treated for producing a pit 13. In the fibre welding machine 15 the electrodes 20 are activated by supplying high voltage thereto, whereby an electric arc is formed therebetween. It produces a heat development, so that the ends of the two fibres 1 and 1' melt, these ends being located, during the welding stage, rather close to each other. When the fibre ends have melted sufficiently, they can be pressed somewhat to each other, so that the melted material flows together and the fibre ends are welded to each other, after which the electric arch is turned off, by interrupting the voltage supply to the electrodes 20, and the splice produced is finally allowed to cool. A composite fibre is obtained, as is illustrated in Fig. 2, comprising the ends of the two fibres 1 and 1' welded to each other at the welding region 21. Owing to the pit 13 in the end of one of the fibres, after the welding a small air bubble or cavity 23 is formed in the fibre core in the splice-welded fibre. As has been possible to observe up to now, this bubble is completely enclosed in the fibre core and is in any case substantially always located within the core region, i.e. within the central region of the fibre where the reflected index varies and is different from the reflected index in the main portion of the fibre cladding 5. This air bubble 23 is a discontinuity in the fibre core and operates as a reflector or mirror for an electromagnetic wave transmitted in the optical fibre. The resulting bubble 23 must have such a size that it affects the radiation field of a light wave that propagates along the optical fibre. It means that its dimension generally should be of the same magnitude of order as the diameter of the fibre core and that its smallest cross-measure, usually equal to its width as seen in the longitudinal direction of the fibre, e.g. must exceed 1/5 or 1/10 of the core diameter. Further the cavity should not disturb the light wave to extensively, since in most cases it is desirable that the light wave also continues forwards in the fibre, even if it has a reduced intensity. The size of the resulting bubble 23 and thereby the magnitude of a reflection against the bubble 23 can to a certain extent be varied, after having welded the fibres 1 and 1* * to each other, by performing further heating steps to the melting state or near melting state of the welded splice, e.g. by means of an electrical arc in the welding device 15. In the further heating operations also the dimensions of the enclosed bubble is reduced more and more and then also the reflectance or alternatively the attenuation. An air bubble 23 produced in that manner at the core region causes reflections for light having different wave lengths. It has up to now been possible to vary the reflectance from -20dB and downwards by a variation of the production conditions. A method of increasing the reflectance owing to the enclosed air bubble 23 can be to fill, before splicing the fibre ends, the etched pit 13 with another substance than air. An example thereof is illustrated in Figs. 5 and 6. In Fig. 5 thus a section is shown of a fibre end having an etched pit 13, where a metal layer 25 is deposited all over the end of the fibre, i.e. in particular over the perpendicularly cut, flat end surface and over the pit 13. This layer 25 can be a metal layer which has been coated by means of some deposition method such as evaporation. Thereafter most of this layer 25 is removed over the end surface of the fibre, as is illustrated of the sectional view of Fig. 6. If the end surface of the fibre is thus polished in a suitable way, metal material will remain in the pit 13 but not on the other portions of the end surface. Material 27 that is left in the pit 13 will increase the reflections in the finished fibre, which is then, in the same way as above, obtained by welding the end of the fibre 1 to another optical fibre having no particularly treated end surface. In Fig. 7 it is schematically shown how an optical fibre 1, 1' produced in this way comprising an enclosed air bubble or discontinuity can be used for checking the operation of a fibre optical network, compare the International Patent Application having publication No. WO 90/06498. A control and monitoring unit 29, which e.g. can utilize OTDR (Optical Time Domain Reflectometry) , is in a suitable way coupled to a main line 31 in an optical fibre network. The main line 31 is further connected to an optical coupler 33, from which individual branch lines 39 extend. In each branch line 35 a welded assembly is connected consisting of spliced fibres 1, 1' comprising a reflector 21 made in the splice. The control and monitoring unit 29 emits light pulses into the main line 31, which are distributed further to the branch lines 35. The emitted light is reflected against the reflectors in the fibre assembles 1, l' and the control and monitoring unit 29 detects the returning light. If the reflectors 21 are located at different optical distances from the control and monitoring unit 29 the reflections from the different branch lines 35 can be distinguished. If an interrupt is obtained in some branch line 35, the earlier reflection from the reflector 21 in this branch line will cease and thereby it is possible to decide which one of the connected branch lines 35 is faulty or incorrect. In this case the control and monitoring unit 29 can output an alarm or suitable signal for indicating that the branch line in question has been detected to be in error.

Claims: CLAIMS 1. An optical fibre having a fibre core and a cladding surrounding it and comprising a reflector, c h a r a c t e r ¬ i z e d by a cavity in the optical fibre, in particular in the fibre core. 2. An optical fibre according to claim 1, c h a r a c t e r ¬ i z e d in that the cavity is located in such a way that light which propagates along the fibre is disturbed considerably by the cavity, so that it is partly reflected. 3. An optical fibre according to one of claims 1 - 2, c h a r ¬ a c t e r i z e d in that material in the fibre core extends up to the cavity. 4. An optical fibre according to one of claims 1 - 3, c h a r ¬ a c t e r i z e d in that the cavity is substantially completely enclosed by material of the fibre core. 5. An optical fibre according to claim 1, c h a r a c t e r ¬ i z e d in that the cavity has substantially a lens shape or substantially the shape of a flattened ellipsoidic body, comprising to opposite large curved or arched surfaces, and that material in the fibre core extends up to at least the central part of one of these large surfaces. 6. An optical fibre according to one of claims 1 - 5, c h a r ¬ a c t e r i z e d in that in the cavity a substance is provided increasing the reflective capability of the cavity for light waves propagating in the optical fibre, in particular a substance having a reflective index different from the reflective index of the material in the optical fibre enclosing the cavity or a metal material. 7. An optical fibre according to claim 6, c h a r a c t e r ¬ i z e d in that the substance has substantially a solid shape and is coated only on one surface in the cavity, in particular only on the surface in the cavity, which is directed towards a part or side of a fibre core, which is interrupted by the cavity. 8. A method of producing a reflector in an optical fibre comprising a core, c h a r a c t e r i z e d in - that in an end surface of a first piece of an optical fibre a recess is made in the end surface, - that the end surface is joined to, in particular is melted to or welded to, the end of a second piece of an optical fibre. 9. A. method according to claim 8, c h a r a c t e r i z e d in that the recess in the end surface is produced only within a region that corresponds to the end surface of the fibre core. 10. A method according to one of claims 8 - 9, c h a r a c ¬ t e r i z e d in that for producing the recess the end surface of the first piece is treated chemically, in particular is etched. 11. A method according to one of claims 8 - 9, c h a r a c ¬ t e r i z e d in that for producing the recess the end surface of the first piece is etched with a substance, that has different etching velocities for the material in the core in the first piece and the material outside the core in the first piece. 12. A method according to one of claims 8 - 9, c h a r a c ¬ t e r i z e d in that for producing the recess the end surface of the first piece is treated with hydrofluoric acid or a mixture thereof with ammonium fluoride. 13. A method according to one of claims 8 - 12, c h a r a c ¬ t e r i z e d in that in the recess a substance is introduced, that increases the reflective capability of the cavity to light waves propagating in the optical fibre, in particular a substance having a reflective index different from the reflective index of the material in the optical fibre surrounding the cavity or a metal material, before joining to the second fibre piece. 14. A method according to one of claims 8 - 12, c h a r a c t e r i z e d in that before joining to the second fibre piece over the end surface of the first piece a layer of a substance is coated, that increases the reflective capability of the cavity for light waves propagating in the optical fibre, in particular a substance having a reflective index different from the reflective index of the material in the optical fibre surrounding the cavity, preferably a metal material, after which the end surface is ground to a flat shape, so that the layer is removed everywhere on the end surface except in the recess. 15. A method according to one of claims 8 - 14 in the case where the joining of the fibre pieces is produced by melting their end surfaces to each other, c h a r a c t e r i z e d in that after joining to the second fibre piece the joint is heated to a temperature near the melt temperature of the materials in the fibre pieces for reducing the reflective capability of the produced reflector. 16. The use of an optical fibre according to one of claims 1 - 7 in an optical network comprising a main line and several branch lines connected thereto, c h a r a c t e r i z e d in - that a monitoring unit is placed connected to the main line, - that optical fibre pieces, each one comprising a reflector, is placed in each branch line, the reflectors being located at differently large optical distances from the monitoring unit, - that the monitoring unit is arranged to detect reflected light from the reflectors and to provide a signal or indicate in some other way, in the case where reflected light from a reflector is not detected. 17. An optical network comprising a main line and several branch lines connected thereto, c h a r a c t e r i z e d in - that a monitoring unit is connected to the main line, - that optical fibre pieces, each one comprising a reflector, are connected in each branch line, so that the reflectors are located at differently large optical distances from the monitoring unit, - that the monitoring unit is arranged to detect reflected light from the reflectors and to provide signal or indicate in some other way in the case where reflected light from reflector is not detected.

Inventors: CARLNAES TORBJOERN, KULL MARTIN, LJUNGQVIST GOERAN, CARLNAES, TORBJOERN, KULL, MARTIN, LJUNGQVIST, GOERAN
Applicants: CARLNAES TORBJOERN, ERICSSON TELEFON AB L M, KULL MARTIN, LJUNGQVIST GOERAN, CARLNAES, TORBJOERN, KULL, MARTIN, LJUNGQVIST, GOERAN, TELEFONAKTIEBOLAGET L M ERICSSON
Application year: 1995
Main classification: G02B6
IPC codes: G01M11, G02B6