The above noted Miller paper outlined a proposal for a miniature form of laser beam circuit or, more generally, for integrated circuit devices wherein the signal energy is transmitted at optical frequencies by coherent light. Such circuits are formed by fabricating regions in glass or semiconductor substrates having desired geometries and transmissive properties. It was, for example, noted that index of refraction changes of the order of 10.sup..sup.-2 or 10.sup.-.sup.3 in such a substrate allow guided laser beams of widths near 10 microns. It was proposed that photolithographic techniques would permit simultaneous construction of complex circuit patterns and devices.
The patent literature indicates various attempts at practical experimental realization of such devices. Such United States patent as U.S. Pat. No. 3,465,159 issued Sept. 2, 1969 to Stern; U.S. Pat. No. 3,610,727 issued Oct. 5, 1971 to Ulrich; U.S. Pat. No. 3,617,109 issued Nov. 2, 1971 to Tien; and U.S. Pat. No. 3,631,360 issued July 10, 1970 to Lehovec, are typical of circuits and devices of the type being considered.
The patent to Ulrich, for example, is primarily directed to an arrangement for coupling a laser beam into or out of a thin film light guide with high coupling efficiency. The patent broadly suggest that the thin film light guide, or waveguide, may have as a smooth extension thereof a "utilization circuit" for the light and alleges (without any supporting detailed disclosure) that with appropriate doping and/or appropriate electrodes (not shown) the utilization circuit can be made to fulfill any of a wide variety of useful functions "such as light modulation, amplification, frequency conversion or detection".
If an operative fabrication technique is found, the advantage of such a device wherein the utilization circuit is, for example, a detector, is that it is integrally coupled to the waveguide thus eliminating the usual coupling loss that occurs when a discrete detector is used. Also, such a device is compatible with other optical integrated circuit elements that can be formed in a semiconductor substrate such as light emitting diodes, modulators, and other waveguides.
The problem which in the past has prevented the actual fabrication of such proposed integrally coupled devices, is that any light which is transmitted unattenuated through a waveguide formed in a semiconductor substrate cannot be sensed by an integral detector made of the same semiconductor material. The only actual solution to this problem which in the past has been even vaguely suggested in Ulrich's proposal to dope a selected portion of the waveguide. In a gallium arsenide substrate such doping could presumably be achieved with substitutional atoms of such elements as indium or thallium which would cause a decrease in the band gap energy. The doped region is then, however, no longer the "same semiconductor material" since various intentionally provided impurities exist in a region the geometry of which is very difficult to control. Such doping could possibly be achieved by diffusion or epitaxial growth but it would obviously be very difficult to perform and does not lend itself to the fabrication of devices having very precisely controlled minute geometries.