High-resolution multimodulation Fourier-transform spectroscopy

High-resolution double-modulation Fourier-transform (FT) spectroscopy (FTS) is demonstrated for what is, to our knowledge, the first time. Two high-re solution FT interferograms are simultaneously recorded. The first one is no nselective and contains all the spectral information from the observed sour ce, and the other one is made of the samples that are sensitive to only a s pecific source modulation. General formulations and practical recording pro cedures are given for phase- and intensity-modulated spectra. The advantage of selectivity is illustrated by velocity-modulated emission spectra of th e Delta nu = 1 sequence of the Doppler-shifted ArH+ ion. It is also shown t hat for a source perturbation of small amplitude, only the product of inten sity x shift can be retrieved from the selective line shapes obtained in a phase-modulated laser or FT spectra. Thanks to the multimodulation FTS appr oach, the intensity and the shift of the transitions are measured in a sing le experiment. (C) 1999 Optical Society of America. OCIS codes: 300.6300, 3 00.6320, 300.350, 300.6380.