Highly angle-tolerant spectral filter based on an etalon resonator incorporating a high index cavity

A high angular tolerance spectral filter was realized incorporating an etalon, which consists of a TIO2 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 TIO2 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 50°, the relative wavelength shift and transmission variation amounted to only ∼0.06 and ∼4%, respectively.