Optical sensor based on total internal reflection diffraction grating

Abstract

The possibility of using a metal diffraction grating, working in total internal reflection, as an optical sensor is shown. Using the elementary theory of such gratings, the inverse problem is solved in Fraunhofer's approximation and the refractive-index change as a function of the diffraction-efficiency change is determined for small changes of the relative refractive index. The sensitivity of the total internal reflection diffraction grating sensor is much higher for the first diffraction order, provided that the two diffracted beams satisfy the condition for total internal reflection at the prism-investigated medium interface. It is shown that in gratings with large periods and critical angles of the order of 65° (quartz-water interface), a change in the refractive index of 5 × 10−5 can be detected if the diffraction efficiency is measured with an accuracy of ±1%. Experimentally, a refractive-index change of 3 × 10−4 has been measured and it is shown that in the vicinity of the critical angle, there is a satisfactory agreement between the experimental results and the elementary theory for s-polarization of the incident wave.