Development of a Miniature Static Imaging Fourier Transform Spectrometer (SIFTS)

This aim of this project is to develop a miniaturised Fourier transform spectrometer, known as a Static Imaging Fourier Transform Spectrometer (SIFTS). This instrument utilises a simple optical arrangement to split and then recombine light to form a complex modulated interference pattern (known as an interferogram). The frequency spectrum of the source radiation is obtained by applying a Fourier transform to the recorded interferogram. The static optical configuration is arranged such that the interferogram is dispersed spatially along a focal plane, where a detector can be positioned. In this configuration the interferogram is at discrete locations determined by the architecture of the pixel elements within the detector array. Unlike conventional Michelson FTIR spectrometers, the SIFTS design is compact (50 mm by 50 mm by 30 mm) and lightweight (~0.4 kg). As the optical configuration is static and there is no need to use a scanning mirror to obtain an interferogram, the acquisition time is limited only by the frame rate of the detector array (~1 x 10-4 s-1), which gives the SIFTS a significant temporal sampling advantage over traditional FTIR spectrometers. A breadboard version of the SIFTS has been built at RAL, which has successfully demonstrated the principles behind the technology. The breadboard instrument operates in the ultra-violet and visible spectral region, using a commercially available charged coupled device (CCD) detector array. By coupling a tungsten halogen light source to the input of the SIFTS instrument via fibre optics, it has been possible to record transmission and reflectance spectra from various targets in the spectral region 400 to 1100 nm. The fundamental rotation-vibration absorption bands associated with gas molecules are, however, located in the near and mid infrared part of the spectrum. Therefore it is intended to develop an infrared version of the SIFTS instrument to maximise the sensitivity of trace gas measurements. This project will adapt this proven technique for use in the near infrared, so that the instrument is spectrally sensitive in the region where key gases are active. This will involve optimising the existing design for use in the infrared, by incorporating optical elements and a detector array that are sensitive at longer wavelengths.