An apparatus for the measurement of attenuation of light in thin-film waveguides is described. It involves a prism that rides on a liquid film along the guide and couples the light out of the guide. A spatial resolution of ~0.5 mm and an accuracy of 0.02 dB/cm were achieved.
H. P. Weber, F. A. Dunn, and W. N. Leibolt, "Loss Measurements in Thin-Film Optical Waveguides," Appl. Opt. 12, 755-757 (1973)