Self-mix interferometer to measure transparent plates thickness and index of refraction

We present a new method to measure thickness and index of refraction of glass windows and slab, typically from a few micrometers to 1000 micrometers and over. We use a very simple setup yielding two optical phase shift readouts: one from a self-mixing interferometer based on the beam crossing the specimen, going to an external mirror and back to the laser retracing the path, and one based on a conventional, forward-going shear interferometer based on the double reflection at the slab walls. We show that, interestingly and unique to our method, the difference of the two phase shift readouts provides the thickness measurement, independent from refractive index. We then go back to one of the two readouts to solve for the index of refraction also. We present the experimental arrangement along with the theoretical derivation of the phase shift involved, and discuss the sources of errors and their impact on the measurement accuracy and precision. Experimental data show a repeatability of +/−1% in thickness and +/−0.1 in index of refraction, with a 825-nm laser diode emitting 5 mW. The method looks interesting as it requires very little components, is compact, and can be used with sources at different wavelength as well as state of polarization.