Resolution and speed improvements of mid-infrared Fabry-Perot microspectrometers for the analysis of hydrocarbon gases

Micromachined tunable Fabry-Pérot filters (μFPF) are key elements in a new class of miniature spectrometers and analyzers. Different groups all over the world are working on μFPF for spectral ranges from the visible up to the long wave infrared. In order to achieve a large tuning range, the filters are normally operated in the first interference order. At the same time the spectral resolution is limited due to a limited effective finesse. A variety of applications demand for higher resolutions. This is particularly true for the multicomponent analysis of hydrocarbon gases, because the individual absorption bands are very similar and widely overlapping. In this paper μFPF in 3rd and 4th order configuration with a spectral resolution of about (20. . . 30) nm and a tuning range of (3.1. . . 3.7) μm are presented. For the measurement of additional gases in adjacent ranges (e.g. CO2 around 4.3 μm) a dualband configuration with simultaneous use of different orders is proposed. A largely reduced damping of the μFPF and the combination with a lead selenide photoresistor instead of a thermal detector allows for a fast acquisition of spectra.