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Thin-film waveguide structures consisting of epitaxially grown low-carrier-concentration GaAs and having two distinct index profiles have been investigated with a 10.6-μm CO2 laser. Results of Schottky barrier and Hall measurements show that the carrier concentration of some of these films is less than 1012 cm−3, and the resistivity can be as high as 1.4 × 105 Ω-cm. Guided-wave modes were excited by means of a germanium prism or phase grating coupler. When the index difference Δn between the undoped film and the substrate is ≃0.3 (strong guide), a number of modes can be obtained with a typical angular full width at half maximum intensity Δθ1/2 of <0.5°. For guides having thickness greater than 20 μm, the measured losses are <1 dB/cm for N < 1013 cm−3 and 1.7 dB/cm for N = 4 × 1015 cm−3. Simultaneous excitation of two orthogonal guided-wave modes, i.e., TEm and TMm for m ≤ 2, is possible in this structure. For Δn ≃ 10−3 (weak guide) only one mode can be excited by means of a phase grating coupler with a typical Δθ1/2 of ≃3°. Measurements were also made of the transmission and cutoff characteristics of the TE and TM modes in weak guides as a function of the guide thickness that varied between 20 μ and 50 μ. Results indicate that optical transmission decreases rapidly as the thickness of the weak guide decreases toward the cutoff value.
© 1973 Optical Society of America
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