Highly Angle-tolerant Spectral Filter Based on an Etalon Resonator Incorporating a High Index Cavity

Abstract

A high angular tolerance spectral filter was realized incorporating an etalon, which consists of a <TEX>$TiO_2$</TEX> cavity sandwiched between a pair of Ag/Ge mirrors. The effective angle was substantially extended thanks to the cavity's high refractive index. The device was created by embedding a 313-nm thick <TEX>$TiO_2$</TEX> film in 16-nm thick Ag/Ge films through sputtering, with the Ge layer alleviating the roughness and adhesion of the Ag layer. For normal incidence, the observed center wavelength and transmission were ~900 nm and ~60%, respectively; throughout the range of <TEX>$50^{\circ}$</TEX>, the relative wavelength shift and transmission variation amounted to only ~0.06 and ~4%, respectively.