Proceedings Article | 12 July 2023
Matthieu Porte, Elisa Baldit, Frédéric Bernard, Yann Ferrec, Nicolas Guérineau
KEYWORDS: Interferometers, Prisms, Fourier transform interferometers, Optical path differences, Wavefronts, Hyperspectral imaging, Visibility, Interferograms, Astronomical imaging, Staring arrays
Hyperspectral imaging, which consists in imaging a scene at a large number of wavelengths, has several applications, such as mineral identification, target detection, or gas concentration measurement. Most of the remote sensing missions would prefer to have compact instruments, and the ability to measure the information with a single acquisition (snapshot) may also be very interesting. Indeed, the information is not sensitive to the temporal variations of the scene: thus the acquisition of a three-dimensional (x,y,λ) hyperspectral data cubes of fast phenomenon (moving targets/gases) is possible. Furthermore, the hyperspectral image can be used in real time, and snapshot acquisition also reduces constraints on a scanning system. Among the concepts of "snapshot" hyperspectral imaging, the one proposed by Hirai (Hirai et al., Optical Review, 1, 205– 207 (1994)) is very interesting. It relies on the association of a microlens array and a Fourier transform interferometer. The latter can be birefringent (Kudenov et al., Optics express, 20(16), 17973-17986 (2012)), which makes the system more compact and less sensitive to vibration by avoiding the use of a beam splitter (as with a Michelson interferometer for instance). Several designs of birefringent interferometers are possible, although the most compact solution is to use a birefringent interferometer with a fringe localization plane at a finite distance and accessible without a relay lens. With such an interferometer, for example a Nomarski prism, the image plane and the plane of localization of the fringes can be easily superposed on a detector. In this paper, the principle and the parameters that define the spectral/spatial performances of this snapshot hyperspectral imaging design are described. Several scenarios from visible to longwave infrared are presented to highlight the trade-off between spectral and spatial resolution. We then present a study of the propagation of spherical wavefronts through a birefringent interferometer by using 3D simulation and 2D analytical calculation. This tool allows us to quantitatively estimate the impact of the interferometer on the spatial quality of the image (aberration, transverse/axial shift of each channel) and evaluate the real interference pattern and the fringe visibility, i.e. the spectral quality, for the whole field-of-view.
