The effects of reflection phase shift on the optical properties of a micro-opto-electro-mechanical system Fabry–Perot tunable filter

Abstract

Unlike the conventional Fabry–Perot cavity, the gap length of a MEMS-based FP tunable filter is comparable with the wavelength. The phase shift on reflection from the top and bottom reflectors plays a crucial role in the determination of peak wavelength at low interference order. In this paper, the optical properties of filters with different mirrors including N, N+1/2 layer pairs dielectric mirrors and metal mirrors are analysed. By the transfer matrix method, the analytic expressions for phase shift on reflection near the central wavelength of mirrors are derived and an important parameter, the phase shift coefficient, that describes the phase shift dispersion is defined. Considering the phase shift, linear relations between the air gap and peak wavelength are presented in the tuning process. The refractive index of thin films and the number of layers constructing the upper and lower reflectors that determine the phase shift coefficients are analysed. Finally, the error of the linear relationship in a specified wavelength range is estimated.