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The term ’’integrated optics’’ was coined, in analogy with integrated electronic circuits, to describe a network of optical devices (such as sources, modulators, switches, and detectors) on a single substrate joined by optical waveguides. Although much interesting and exciting research is in progress, we are far from realizing this concept today, even in the most rudimentary form. We are perhaps at the stage that integrated circuits were 15 or 20 years ago. Work is being done on suitable thin film and diffused waveguidematerials and on individual—unintegrated—devices in a planar waveguide geometry. Much of this work is summarized in a special issue of the I E E E T r a n s a c t i o n s o n M i c r o w a v e T h e o r y a n d T e c h n i q u e s (MTT‐23, January 1975).
In order to produce an optical waveguide, it is necessary that the guiding region (in the form of a transparent planar layer, a strip, or a fiber) be surrounded by a medium of lower refractive index. Then, as illustrated in Figs. 1 and 2, the optical energy will be guided along the high index path. Light may be injected into the end cross section of such a waveguide, or light may be coupled into a guide along its length by means of a grating or prism coupler (see Fig. 3) which perturbs the waveguide boundary. Typical substrates have dimensions of ∠1 cm, and to be of practical interest the waveguide losses should be less than ∠1 dB/cm.
The simplest planar waveguides are formed by depositing polymer films 1 to 10 μm thick on glass slides. Photolithographic or photochemical means are used to form strip guides which allow the light to be guided along a curved path in the substrate plane. Amorphous or polycrystalline films of other substances, including glass, have been formed by rf sputtering or evaporation. Fine gratings have been etched into such guides to act as narrow‐band optical filters. Most active devices require single‐crystal waveguides of materials that can serve as lasers, LED’s, detectors,modulators, or nonlinear optical devices. Good waveguiding layers have been produced in semiconductor materials, principally GaAlAs, by liquid phase epitaxy. Single‐crystal layers of oxides, such as LiNbO3 and LiTaO3, that serve as electro‐optic and acousto‐optic modulators have been grown by epitaxy from a melt and by sputttering. Magneto‐optic garnet films have also been grown by dipping a garnet substrate into a suitable melt. Diffusion has been found to be an especially simple and convenient means for establishing a waveguiding layer on the surface of crystals such as CdS, LiNbO3, and LiTaO3. A suitable impurity may be diffused into the surface, or for the latter two substances, Li may be diffused out of the surface to produce a waveguidingsurface gradient of refractive index. Diffusion depths of approximately 1 to 100 μm and index changes as large as 0.04 have been obtained. The various types of single‐crystal waveguides that have been studied are tabulated in Figs. 4 and 5.
In order to confine the optical beam in both transverse dimensions, a strip guide may be formed by etching a ridge on the guide surface or more simply by masking the diffusion into the surface of the crystal by standard photolithographic methods.
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