Method of Forming a Lens at the End of a Light Guide

The invention relates to a method to couple a light guide to a semiconductor radiator such as a laser, and more particularly to a light guide of tiny diameter, e.g., in the 100 .mu.m range.

Known methods to couple light guide filaments to semiconductor radiators have many difficulties associated therewith due to the small diameter of the cooperating opto-electronic elements. The light guides made in accordance with this method have only mediocre efficiency.

"Applied Optics", Vol. 14, No. 12, Dec. 1975, pp. 2815 to 2816 describes methods to couple optical fibers to semiconductor lasers. The ends of the light guide are melted so that, instead of a flat end portion, a semispherical lens dome ought to result. Couplings for light guide filaments made according to this method do not, however, have the necessary semispherical shape of the core necessary for low-loss coupling. Rather, upon melting, the cladding or jacket material either becomes plastic before the core material so that it pulls in, forming a pupil-like orifice, or the core material becomes soft before the cladding material and, under the influence of surface tension, only a flat lens with long focal length will result at the end of the fiber. The desired, approximately semispherical shape of the end of the fiber with a spherical radius which corresponds approximately to the radius of the core was obtained only in a single case under special experimental conditions which were not transferable to routine manufacture of other light guide fibers.

It is an object to provide a light guide and a method to make a light guide which has a lensatic end capable of being coupled to a radiator, and more particularly to a semiconductor radiator such as a semiconductor laser which light guide has a high degree of coupling efficiency, and thus has an end portion which is essentially semispherical throughout the diameter of the light guiding element of the light guide itself.

Briefly, a lens made of a material having a lower melting point than that of the light guide core and/or jacket material is adhered to the end of the light guide. The diameter of the light guide is first reduced in the region of its end portion so that the lens which is attached to the light guide, and of essentially semispherical shape, can be directly adhered to the end thereof.

In accordance with a feature of the invention, the reduction of the diameter of the light guide is obtained by etching the light guide. The lens element may be attached to the light guide by melting it on, or, if he lens is a transparent epoxy, by curing a drop thereof at the end of the light guide. Preferably, the index of refraction of the material of the attached semispherical lens corresponds essentially to the index of refraction of the core. If the light guide is of glass, the lens may, for example, be an epoxy resin; if the light guide is of quartz glass, the lens may be a low melting point glass, or glass mixture. The lower melting point of the glass permits contacting the melt by the light guide fiber, e.g., from above, and adhering a drop, without danger that the cladding of the fiber itself will melt and deform. The method, and the light guides made in accordance with the method of the invention, have the advantage, with respect to those of the prior art, that the desired shape of the end of the core filament can be obtained with all customary light guide filaments and the results obtained are reproducible with high reliability.

Drawings, illustrating an example:

FIG. 1 illustrates a coupling end of a core fiber with a thick jacket made in accordance with the above-described prior art method;

FIG. 2 is a fiber with thin jacket having a melting point lower than that of the core, and likewise made in accordance with the prior art; and

FIG. 3 illustrates a coupling end of a quartz-core jacketed fiber made in accordance with the method of the invention.

A jacketed core fiber (FIG. 1) has a relatively thin light-conducting core 1 within a relatively thick cladding or jacket 2. The indices of refraction between the core and the jacket are different with respect to each other so that, at the interface surface between core 1 and jacket 2, total reflection will result for light rays guided in the core under a suitable angle. Upon softening the end portion of the fiber to melting temperature to obtain a lensatic dome 3, a semispherical shape of the overall light guide is obtained in the best case. However, the core 1 itself will have only a relatively flat spherical end portion 4 thereon which is a minor portion of an entire semisphere.

FIG. 2 illustrates a light guide fiber with a thin jacket 6 surrounding the core 5. Upon softening the end of the fiber to melting temperature, jacket 6 draws itself around the opening of the core in diaphragm, or pupil, or orifice shape due to the prevailing surface tension relationships, so that only a small entrance pupil of small diameter will remain free. The efficiency of coupling is very low. The spherical portion towards the entrance opening, also in this case, is comparatively flat. This is undesirable.

It is difficult to mechanically work the light guide fibers due to their small dimension, typically an 80 .mu.m core diameter and a 130 .mu.m overall diameter. Therefore, operation by melting of the fiber end has been deemed to be advantageous.

The invention is described with reference to FIG. 3. A cladded, or jacketed core fiber, similar to FIG. 1, contains a core 11 surrounded by a jacket 12. To avoid that configurations corresponding to FIGS. 1 and 2 result, upon melting of the fiber end, the jacket or cladding 12 is almost completely removed in the vicinity of the end of the fiber over a predetermined distance. This can be done, for example, by etching. In case of step-index fibers, care must be taken to prevent complete removal of the jacket since, otherwise, the interface 13 between the core 11 and the jacket 12, necessary for total reflection, will be lost. After etching, the end of the fiber is plane-finished as preliminary preparation step to attach a separate semispherical lens element 15. If it were attempted to give the end of the fiber the desired shape by simply melting then, due to surface tensions, a shape similar to that shown in FIG. 2 will result since the portion of the jacket which remains will draw itself partially around the end face opening of the jacket 11. In accordance with a feature of the invention, the plane-finished flat fiber end 14 has a semispherical lens 15 adhered thereto by melting it unto the end 14 of the fiber. The lens is made of a material having a lower melting point than that of the core or of the jacket of the light guide 11,12 itself, so that the cladding will not draw around the fiber as in FIG. 2. This semispherical lens 15, preferably, has the same index of refraction as the core material so that, upon transition of light between the lens 15 and the core 11, no reflection or light losses will result.

In case a gradient light guide is used in which the refraction index decreases from the center to the edge in accordance with a predetermined function, the preparation is done in the same manner as above described. Here, also, the end of the fiber is etched off up to the desired diameter and the so prepared plane-ground end has a semispherical lens attached thereto, e.g. by melting on. The lens is made of a material having a lower melting point, which is transparent, and having nearly the same index of refraction as the core of the guide. A simple melting process of the fiber, as in the prior art, would also lead to the result that melted portions from the edge regions and having a different index of refraction penetrate forwardly up to approximately the center of the entrance opening, and degrade the optical properties (FIG. 2). If the light guide fiber is of glass, the semispherical lens to be adhered may consist, for example, of an epoxy resin; when using a quartz glass fiber, the lens can consist of a glass or glass mixture having a lower melting point.

Lens element 15 which is made of epoxy resin, which is transparent, can either be melted-on, or adhered by the adhesive quality of the epoxy itself. To attach the lens element 15, the diameter of the light guide is reduced as described, for example by etching, but leaving the interface 13 if the light guide is of the step-index type, thus leaving sufficient material to provide the sharp gradient for reflection. The end 14 is finished as above-described. A flowable, or still plastic mass of epoxy, that is, resin plus curing agent or hardener, already mixed, and which will cure to a transparent material is prepared, and the flat end of the glass fiber is touched, from above, with the fluid mass of epoxy. A drop will adhere to the end face 14 which is lifted off the surface of the fluid epoxy mass. This drop will draw together into semispherical shape, as shown in FIG. 3 at 15. The result, again, will be as essentially semi-spherical transparent lens having a sphere diameter corresponding essentially to the diameter of the fiber in the region of the interface 14.

Various changes and modifications may be made within the scope of the inventive concept.