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The absorption and scattering losses at 6328 Å in MBBA thin-film waveguides were measured. The absorption loss was studied with a thermal-lens technique and was found to be highly dichroic. Thermal self-focusing was observed for the ordinary ray. The measured scattering losses were between 15 and 40 dB cm^-1, depending on the polarization and the liquid-crystal orientation. A modified bulk-scattering loss caused by the thermal fluctuation of the long-range-crystal ordering is calculated and compared with the measurements.

A simple and sensitive method of measuring the thermally induced index changes arising from absorption of a laser beam in a low-loss material is described. The sample is placed outside the laser cavity, but at the position of minimum radius of curvature of the wavefront, which is a confocal distance behind the waist. It is estimated that the method is sensitive enough to measure absorption coefficients of the order of 5 × 10^−6 cm^−1 and it is shown experimentally to have good accuracy on low-loss materials. A detailed comparison of sensitivity and accuracy estimates is given for the various published thermal-lens methods for measuring low absorption coefficients.Although the method is illustrated for the thermal effect, it is applicable to other nonlinear index changes induced by laser beams.