A simple interferometric method for measuring severally the refractive index and the thickness of transparent plates

摘要

In this paper a new method for measuring simultaneously, thickness and refractive index of transparent plates is proposed. The method is based on a very simple lateral variable shear wavelength scanning interferometer. The transparent plate is made to rotate at controlled and constant angular speed. Radiation from a laser diode propagates though the sample and experiences an even number of reflections at the sample-air interfaces. The emerging wave fronts are laterally sheared with a shear depending from the actual angle of incidence. Varying the emission wavelength of the laser source a variation of the phase of interference signal is obtained. In this way, interferometric signals as function of incidence angle for each laser wavelength λ were obtained. Evaluating phase variations in correspondence of normal incidence, by a simple non-linear fit, it is possible to determine the optical thickness n.d. Then, retrieving the phase of the overall interference signal for all available incidence angles and employing the previously evaluated optical thickness, the refractive index value can be determined. Finally, geometrical thickness d was obtained straightforwardly as end result. The effectiveness and straightforwardness of the method has been demonstrated for a silicon sample and also for a z-cut Lithium Niobate sample.