Gas Sensing through Mixed Polarization in Birefringent Porous Silicon Thin Film

Abstract

The interrogation of a sensor based on a highly birefringent film irradiated by an incident beam whose polarization is oriented 45o with respect to the normal film axes is shown to display exceptional features and resolve some outstanding issues in interferometric sensing. The measured pattern as a function of wavelength is complicated displaying a superposition of Fabri-Perot and birefringence oscillations. By means of appropriate optical signal processing based on Fourier analysis, three different periodicity patterns can be elucidated when sensing a single event, improving the sensing process in several ways: it allows sensing at several sensitivity scales, and it clears out the problems of phase and direction ambiguities. A novel effect is observed in which the phase displacement of different patterns move in opposite directions as sensing takes place