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 </= 2, is possible in this structure. For Deltan approximately 10(-3) (weak guide) only one mode can be excited by means of a phase grating coupler with a typical Deltatheta((1/2)) of approximately degrees . 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 micro and 50 micro Results indicate that optical transmission decreases rapidly as the thickness of the weak guide decreases toward the cutoff value.