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Birefringent interferometers are often used for compact static Fourier transform spectrometers. In such devices, several uniaxial birefringent parallel or prismatic plates are stacked, with their optical axes set so that there is an efficient coupling from ordinary to extraordinary and extraordinary to ordinary eigenmodes of two successive plates. Such coupling, aside from few particular cases, is, however, not perfect, an effect that may adversely affect performance. In order to help the design and the tolerancing of these interferometers, we have developed a numerical modeling based on the propagation of plane waves inside and through the interface of birefringent media. This tool evaluates the traveled optical path length and the amplitude of the different polarization modes, enabling prediction of both the optical path differences on the interferometer outputs and the unwanted coupling strengths and related stray wave amplitudes. The tool behavior is illustrated on Savart and double-Wollaston interferometers and compared with experimental characterization of a calcite double-Wollaston prism.

We present the design and the realization of a compact and robust imaging spectrometer in the mid-infrared spectral range. This camera combines a small static Fourier transform birefringent interferometer and a cooled miniaturized infrared camera in order to build a robust and compact instrument that can be embedded in an unmanned aerial vehicle for hyperspectral imaging applications. This instrument has been tested during a gas detection measurement campaign. First results are presented.

Stationary Fourier transform spectrometry is an interesting concept for building reliable field or embedded spectroradiometers, especially for the mid- and far- IR. Here, a very compact configuration of a cryogenic stationary Fourier transform IR (FTIR) spectrometer is investigated, where the interferometer is directly integrated in the focal plane array (FPA). We present a theoretical analysis to explain and describe the fringe formation inside the FTIR-FPA structure when illuminated by an extended source positioned at a finite distance from the detection plane. The results are then exploited to propose a simple front lens design compatible with a handheld package.

One of the major limitations to the use of infrared focal plane arrays (IRFPAs) in stationary Fourier transform spectrometers (FTSs) comes from the spatial inhomogeneities of the pixel responses, where the inhomogeneities of the cut-off wavenumbers of the pixels can prevail. The hypothesis commonly assumed for FTSs that all the pixels are equivalent is thus inaccurate and results in a degradation of the estimated spectrum, even far from the cut-off wavenumbers. However, if the individual spectral responses of the pixels are measured beforehand, this a priori information can be used in the inversion process to produce reliable spectra. Thus, spatial inhomogeneities are not an obstacle for the use of infrared stationary FTS. This result is illustrated in this paper by numerical simulations, based on a realistic description of an IRFPA.

A high étendue static Fourier transform spectral imager has been developed for airborne use. This imaging spectrometer, based on a Michelson interferometer with rooftop mirrors, is compact and robust and benefits from a high collection efficiency. Experimental airborne images were acquired in the visible domain. The processing chain to convert raw images to hyperspectral data is described, and airborne spectral images are presented. These experimental results show that the spectral resolution is close to the one expected, but also that the signal to noise ratio is limited by various phenomena (jitter, elevation fluctuations, and one parasitic image). We discuss the origin of those limitations and suggest solutions to circumvent them.

Imaging lateral shearing interferometers are good candidates for airborne or spaceborne Fourier-transform spectral imaging. For such applications, compactness is one key parameter. In this article, we compare the size of four mirror-based interferometers, the Michelson interferometer with roof-top (or corner-cube) mirrors, and the cyclic interferometers with two, three, and four mirrors, focusing more particularly on the last two designs. We give the expression of the translation they induce between the two exiting rays. We then show that the cyclic interferometer with three mirrors can be made quite compact. Nevertheless, the Michelson interferometer is the most compact solution, especially for highly diverging beams.

The matched filter is a widely used detector in hyperspectral detection applications because of its simplicity and its efficiency in practical situations. We propose to estimate its performance with respect to the number of spectral bands. These spectral bands are selected thanks to a genetic algorithm in order to optimize the contrast between the target and the background in the detection plane. Our band selection method can be used to optimize not only the position but also the linewidth of the spectral bands. The optimized contrast always increases with the number of selected bands. However, in practical situations, the target spectral signature has to be estimated from the image. We show that in the presence of estimation error, the maximum number of bands may not always be the best choice in terms of detection performance.

A design of a miniaturized stationary Fourier transform IR spectrometer has been developed that produces a two-dimensional interferogram. The latter is disturbed by effects like parasitic interferences or disparities in the cutoff wavelength of the pixels. Thus, a simple Fourier transform cannot be used to estimate the spectrum of the scene. However, as these defects are deterministic, they can be measured and taken into account by inversion methods. A regularization term can also be added. The first experimental results prove the efficiency of this processing methodology.

We present a optical system with an extended point-spread function (PSF) for the localization of point sources in the visible and IR spectral ranges with a subpixel precision. This compact system involves a random phase mask (RPM) as its unique component. It exhibits original properties, because this RPM is used in a particular regime, called the “filamentation regime,” before the speckle region. The localization is performed by calculating the phase correlation between the PSF and the image obtained under off-axis illumination. Numerical simulations are presented to assess the basic optical properties of this RPM in the filamentation regime.

An imaging spectrometer in the 3-5 m wavelength range is presented. This wavelength range reveals important information about scenes such as gas or landmine detection, but the amount of light is usually low and signal to noise ratio is a real issue. We selected a Fourier transform (FT) configuration, expecting an advantage in signal to noise ratio in the presence of detector noise. Radiometric and noise models are summarized. A Michelson interferometer with its mirrors replaced by twin mirrors arranged at right angles in a hollow roof was chosen for its nearly straight equidistant fringes localized at infinity. Because in such FT-based spectral imagers, the interferogram is acquired over the whole field of the camera while the scene of interest scans the path difference range, vignetting should be strongly limited while keeping the size of the interferometer as small as possible for manufacturability and cost reasons. The key point for that purpose is to put the entrance pupil of the imaging lens inside the interferometer and to make careful trade-offs between lens F number and angular field of view. The resulting system has a spectral resolution of about 25cm-1that fulfils the requirement for most targeted applications. Examples of absorption bands detection are shown.

We have recently shown that dewar-level integration of optics is a promising way to develop compact IR cameras. Indeed, the integration of optics into the dewar leads to simple and entirely cooled optical architectures dedicated to imaging applications with large-field of view. Here, we review the optical elements we could add in those devices to make a hyper- or multispectral imager. Among them, we find specific focal-plane arrays with a built-in spectrometry function, plasmonic filters combined with a multichannel optical design, and birefringent interferometers. Several optical architectures will be detailed with first experimental results.

High-e´tendue Fourier transform imaging spectrometers can be useful tools for acquiring airborne hyperspectral images, since they do not need moving parts and, as Fourier transform spectrometers, they benefit from the Felgett advantage. The Sagnac interferometer and the Michelson interferometer with dihedrons can be used in such an instrument. These two interferometers act very similarly on the entering beam, and in particular they both work with fringes of equal inclination. This property enables them to benefit from the Jacquinot advantage, too. Nevertheless, to fully benefit from this advantage, it is necessary to maximize the geometrical e´tendue accepted by the interferometer. From this point of view, the Michelson interferometer with dihedrons appears to be much more interesting than the Sagnac interferometer. Indeed, calculation shows that, in the optimal symmetric configuration, and for the same beamsplitter diameter, the e´tendue accepted without vignetting by the Michelson interferometer is more than five times greater than the e´tendue accepted by the Sagnac interferometer. Even if, in a real instrument, the optical scheme cannot be exactly this optimal configuration, this result is a strong argument in support of the Michelson interferometer with dihedrons.