Rotational-Doppler Fourier-Transform Absorption Spectroscopy

Optical absorption spectroscopy utilizes light to determine the molecular constituents of solid, liquid or gaseous samples. This is done by measuring and analysing the sample's absorption spectrum. Instead of using tunable single-frequency lasers, a broadband spectrum can be recorded in a single measurement using broadband light sources combined with Fourier-transform spectrometers [1]. Traditional Fourier-transform spectroscopy (FTS) is based on the well-known Michelson interferometer, where one measures the beating of a light field against a Doppler-shifted copy of the field. The beating is produced by combining the light fields from the two arms of the interferometer while scanning the length of one of the arms, which scanning results in a Doppler frequency shift for the optical frequency components that reflect from the translating mirror (see Fig. 1). Finally, a recording of the beating (the interferogram measured on a photodetector) is Fourier-transformed to yield the optical spectrum for further analysis.