Optical Waveguide Structures for CO(2) Lasers

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-microm C0(2) laser. Results of Schottky barrier and Hall measurements show that the carrier concentration of some of these films is less than 10(12) cm(-3), and the resistivity can be as high as 1.4 x 10(5) ?-cm. Guided-wave modes were excited by means of a germanium prism or phase grating coupler. When the index difference Deltan between the undoped film and the substrate is approximately 0.3 (strong guide), a number of modes can be obtained with a typical angular full width at half maximum intensity Delta(theta) (1/2) of <0.5 degrees . For guides having thickness greater than 20 microm, the measured losses are <1 dB/cm for N <10(13) cm(-3) and 1.7 dB/cm for N = 4 x 10(15) cm(-3). Simultaneous excitation of two orthogonal guided-wave modes, i.e., TE(m) and TM(m) for m