Microgratings that were designed and fabricated to generate IR absorption spectra of SF6 and NH3 on diffraction into a specific detection angle were tested by correlation spectroscopy. The micrograting diffraction provides a reference spectrum for a target molecule, and its cross‐correlation with the transmission spectrum of a gas cell is obtained by varying the diffraction angle. As our optical setup can measure the dispersive transmission spectrum and the correlation spectrum under the same conditions, the two kinds of spectra were compared directly in terms of signal‐to‐noise ratio (SNR). The SNR’s of the correlation spectra were a few times lower than those of the dispersed spectra; therefore, the correlation spectroscopy can hardly be placed above the dispersive spectroscopy with respect to the SNR. The merit of the correlation spectroscopy is that a rather small range of modulation wavelength is needed to identify the target. Therefore, the correlation spectroscopy would be more useful for such target molecules whose spectra consist of broad peaks spread throughout a wide wavelength range.

A miniaturized FTIR spectrometer based on lamellar grating interferometry is being developed for passive remote-sensing. Consisting of a pair of micro-mirror arrays, the lamellar grating can be fabricated using MEMS technology. This paper describes a method to compute the optical field in the interferometer to optimize the design parameters of the lamellar grating FTIR spectrometer. The lower limit of the micro-mirror width in the grating is related to the formation of a Talbot image in the near field and is estimated to be about 100 µm for the spectrometer to be used for the wavelength range of 7-14 µm. In calculating the far field at the detection window, the conventional Fraunhofer equation is inadequate for detection distance of our application, misleading the upper limit of the micro-mirror width to avoid interference from higher order diffractions. Instead, the far field is described by the unperturbed plane-wave combined with the boundary diffraction wave. As a result, the interference from the higher order diffractions turns out to be negligible as the micro-mirror width increases. Therefore, the upper limit of the micro-mirror width does not need to be set. Under this scheme, the interferometer patterns and their FT spectra are successfully generated.