Method of Producing Optical Waveguides on a Substrate

This application claims the priority of U.S. patent application Ser. No. P 41 16 432.6, filed May 18, 1991, in the Federal Republic of Germany, the subject matter of which is incorporated herein by reference.

1. Field of the Invention

The present invention relates to a method of producing at least one optical waveguide in the form of a planar structure on a substrate.

2. Background Information

Such methods are known from the article entitled, "Bauelemente der integrierten Optik: eine Einfuhrung," (translation: Integrated Optical Devices: An Introduction), published in the periodical "Laser und Optoelektronik" (translation: Lasers and Optoelectronics), No. 4/1986, pages 323-326. In epitaxial methods, raised optical waveguide structures extending along the surface of the substrate are obtained in that a layer is initially applied to a monocrystalline substrate and is structured photolithographically in the form of the desired optical waveguides, that is, the layer is removed at the locations where the optical waveguide is to be produced. Then an epitaxy method is employed to produce the waveguide as a monocrystalline layer in the form of the structure. Because of the materials to be employed for the substrate, and light waves with substantially coinciding lattice constants which additionally must have different refractive indices, such epitaxial methods are limited at present to a few compounds. The monocrystalline optical waveguides produced by epitaxy generally have a cross section that is approximately adapted to a circular or semicircular shape so that the coupling in or out of light waves at the polished end faces poses relatively few problems.

In diffusion methods one obtains an optical waveguide cross section which is also a close approximation to a circular shape by diffusing doping materials into the substrate, that is, below the substrate surface. The coupling in and out of light waves at the polished end faces here again generally poses no particular problems.

If, however, only a coating method is employed such as, for example, the vapor deposition of a transparent layer and then the structuring of this layer by means of an etching process or by, for example, vapor-depositing a layer through a structured mask, optical waveguides are obtained which have an essentially rectangular cross section that is not well suited for coupling light waves into and out of, for example, an optical fiber.

It is therefore an object of the present invention to configure optical waveguides that have been produced by the application of a transparent layer so that it is easily possible to couple optical fibers at the ground or polished end faces of the waveguides.

This is accomplished by the method steps according to an embodiment of the invention which includes a method of producing at least one optical waveguide in the form of a planar structure on a substrate, comprising producing an optical waveguide structure of first and second superposed layers of transparent material on the substrate wherein the second layer is composed of a transparent material which has a greater refractive index and a lower melting point than the material of which the first layer is composed; performing a heating process at a temperature and for a duration so that only the material of the second layer melts and the surface of the molten second layer, when seen in cross section, changes to a shape that at least approximates the arc of a circle; and subsequent to performing a cooling process during which the material of the second layer solidifies in said shape that at least approximates the arc of a circle. With these measures, the optical waveguides receive a cross section that is significantly better suited for coupling to optical fibers. Moreover, a multitude of organic as well as inorganic compounds are available for this type of optical waveguide production. And it is not necessary to employ a monocrystalline material for the substrate. Rather, the substrate may be composed of any desired material. However, it should be selected so that its coefficient of thermal expansion at least approximately corresponds to that of the material from which the optical waveguide is formed.

Further advantageous details of the invention are defined will now be described with reference to embodiments that are illustrated in the drawing figures in which:

FIGS. 1, 2, 3, 4, 5, 6, 7, 8 and 9 depict the individual method steps if a double coating and removal process is employed;

FIGS. 10, 11, 12, 13 and 14 depict the individual method steps if a single removal process is employed; and

FIG. 15 is a sectional front view of an edge at the end face.

The reference numeral 1 identifies a substrate and reference numeral 2 its surface that serves as the carrier for optical waveguides or integrated optical circuits (FIGS. 1, 10). The substrate is preferably composed of glass or quartz, but other organic or inorganic materials and even metal may also be employed.

A first layer 3 of a transparent material is applied to substrate 1 so as to partially or completely cover its surface 2 (FIG. 2). Additionally, according to FIG. 3, a raised structure 4 is produced on the surface 2 of substrate 1 by covering a portion of the first layer of transparent material defining a structure 4 in the form of the desired optical waveguide--in the illustrated embodiment in the form of a Y branch--and subsequent removal of the non-covered remaining first layer 5 (FIG. 4). If a polymer is employed for first layer 3, the removal of remaining first layer 5 is preferably effected by plasma etching. However, other etching methods or the removal by means of suitable solvents are also conceivable.

Layer 2 may be composed, for example, of a photopolymerizable material and structure 4 may be produced by photolithographic methods known from the production of printed circuits.

If glass is employed for first layer 3, the remaining layer may be removed by etching it away with hydrofluoric acid.

In principle, remaining first layer 5 or a boundary strip along structure 4 may also be removed by treating it with a laser beam or by other suitable methods.

In a further method step, structure 4 and at least a surrounding region of substrate surface 2 are covered with a second transparent layer 6 (FIG. 5). The material for this layer 6 is selected so that it has a lower melting point than the material of first layer 3 and so that, moreover, its refractive index is greater than that of first layer 3. Second layer 6 is advisably applied to a greater thickness than layer 3. For example, the layer thickness of the first layer 3 is about 0.5 to 5 .mu.m.

Then, according to FIGS. 6 and 7, a structure 4.1 is produced in second layer 6 according to the same or a different method as employed for the production of structure 4. This layer is identical to structure 4, possibly with the exception of its width 7. The width 7 of structure 4.1 may be about 70 to 100% of the width of structure 4.

In a subsequent heating process, structure 4.1 in second layer 6 is heated to such a temperature and for such a length of time until it begins to melt and its surface, due to the surface tension of the material, bulges in the form of a circular arc. Thus the cross section of the layer approaches a semi-circle or a circle segment. After cooling, structure 4.1 retains this shape and forms an optical waveguide 8 in the form of a stripline that is well suited for the conduction of light waves and into which or from which light waves can be coupled without problems, due to its favorable cross section (FIG. 8).

To improve or stabilize the optical characteristics of optical waveguide 8, a third transparent layer 9 may be applied in a further method step at least onto optical waveguide 8 (FIG. 9). The refractive index of this third layer is lower than that of the material of second layer 6.

Structures 4 and 4.1 in first layer 3 and second layer 6, respectively may also be produced in a single process step. This method is effected as follows with reference to FIGS. 10 to 14:

Initially, the substrate 1 shown in FIG. 10 is provided with the first layer 3 (FIG. 11). Then, a second layer 6 of a material that melts at a higher temperature and has a greater refractive index than the material of first layer 3 is applied to first layer 3 (FIG. 12). Thereafter, structures 4 and 4.1 are produced through suitable masks or, for example, by photolithographic methods in one method step, for example by plasma etching, laser beam treatment, etc., in that the remaining first and second layers 5 surrounding the structures are removed (FIGS. 13 and 14). Instead of just removing the entire remaining layer, a limited region adjacent to the structures may also be removed, for example, in the form of a channel or strip. The further treatment of this embodiment takes place as described with reference to FIGS. 8 and 9 by way of the heating and cooling process.

In order to realize good coupling of light waves, the end faces 10 of optical waveguide 8 are ground and polished. Advisably, the plane of end faces 10 is sloped, as shown in FIG. 15, by a small angle .alpha. at a substrate edge 11 relative to a line 13 that is perpendicular to the plane 12 of substrate 1 so as to avoid undesirable reflections. It should also be mentioned that with this method optical waveguides may be provided on both sides of substrate 1.

If a polymer is employed for the first layer 3 and the second layer 6, the material of the first layer 3, and also that of a possible third layer 9, may be composed of one or several polymers based on polyacrylate, polyimide, high-molecular polycarbonate or polyisocyanate, and the material of second layer 6 may be composed of at least one polymer based on polystyrene, polyurethane, low-molecular polycarbonate.

Inorganic materials employed with preference are glasses that are based on silicon dioxide. Advisably, if silicon dioxide is employed, its percentage is at least 30 weight percent, in particular, higher than 50 weight percent. The glasses may contain up to 30 weight percent lead and/or up to 30 weight percent zinc.

Within the scope of the invention, the first layer 3 may be a layer of an inorganic material and the second layer 6 as well as a possible third layer 9 may be composed of organic material.

The ratio of width to thickness of the structure of first layer 3 is advisably in the range of about 1:1 to 5:1 and that of the second layer 6 is selected to fall into a range of about 0.5:1 to about 2:1.

The width of structure 4.1 in second layer 6 may preferably be 0.7 to 10 .mu.m and the width of the structure 4 of the first layer is about 1 to 10 .mu.m. The thickness of structure 4.1 in second layer 6 is about 1 to 8 .mu.m before it is melted.

Care must be taken in the method that the upper side edges 13 of the structure 4 of the first layer 3 are made as sharp-edged as possible so as to form defined starting locations for the connection of the second layer 6 that melts onto it.