Stray-light compensation in thin-film Fabry-Perot optical filters : application to an on-chip spectrometer

Abstract

This paper describes a method for significantly improving the performance of integrated Fabry- Perot- based optical spectral analysers, in terms of spectral selectivity and/or requirements on the input light beam conditioning. The method is based on compensation of the stray-light components induced by the non-idealities of both the incident light beam and a Fabry-Perot (F-P) resonator. The compensation structure consists of the same layer stack as used in the active F-P filter. The difference is that the optical length of the cavity is decreased below λ/10 (<40 nm if applied for measurements in the visible spectral range). This avoids any resonance inside the cavity for the spectral range of interest. However, the parasitic signal that is caused by stray-light transmittance is similar to that of the active channel and can be used for compensation. The method was applied to an array-type on-chip spectrometer operating at the visible spectral range. The spectral selectivity is improved by a factor 10 compared to the device without compensation. Moreover, applicability of the device is enlarged, as the requirements on the measured light beam (e.g. presence of stray-light, light beam divergence, background light levels, etc.) are less demanding.