Yann Ferrec

Optimal geometry for Sagnac and Michelson interferometers used as spectral imagers

High-etendue 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 etendue 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 etendue accepted without vignetting by the Michelson interferometer is more than five times greater than the etendue 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 interfero-meter with dihedrons

Influence of band selection and target estimation error on the performance of the matched filter in hyperspectral imaging.

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.

Miniature and cooled hyperspectral camera for outdoor surveillance applications in the mid-infrared

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.

Angular acceptance analysis of an infrared focal plane array with a built-in stationary Fourier transform spectrometer

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.

Adaptive band selection snapshot multispectral imaging in the VIS/NIR domain

Hyperspectral imaging has proven its efficiency for target detection applications but the acquisition mode and the data rate are major issues when dealing with real-time detection applications. It can be useful to use snapshot spectral imagers able to acquire all the spectral channels simultaneously on a single image sensor. Such snapshot spectral imagers suffer from the lack of spectral resolution. It is then mandatory to carefully select the spectral content of the acquired image with respect to the proposed application. We present a novel approach of hyperspectral band selection for target detection which maximizes the contrast between the background and the target by proper optimization of positions and linewidths of a limited number of filters. Based on a set of tunable band-pass filters such as Fabry-Perot filters, the device should be able to adapt itself to the current scene and the target looked for. Simulations based on real hyperspectral images show that such snapshot imagers could compete well against hyperspectral imagers in terms of detection efficiency while allowing snapshot acquisition, and real-time detection.

Inverse problem approaches for stationary Fourier transform spectrometers

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.

Relevance of an inverse problem approach to overcome cut-off wavenumbers disparities in infrared stationary Fourier transform spectrometers.

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.

Compact designs of hyper- or multispectral imagers compatible with the detector dewar

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.

Accurate modeling of optical system aberrations applied to the design of a stationary Fourier transform spectroradiometer

Stationary Fourier transform spectrometry is a well-known concept to build reliable field or embedded spectroradiometers, especially for the mid- and far- infrared. However, the quality of the interference pattern imaged on the focal plane array is crucial to obtain a good spectrum by Fourier transform. We describe here an accurate modeling of the interferometer behavior that takes into account the instrument aberrations and field of view in order to quantitatively predict, at each wavelength and for a spatially extended uniform incoherent source, the real interferogram defects, namely, fringe distortion, fringe blurring and illumination non-uniformity. To investigate these effects, we first derive the properties of the elementary interferograms built by each source point. For this purpose, we use ray-tracing to extract optical path and vignetting information with the help of a commercial optical design software package, and we reconstruct from them the two wavefronts that hit the detector using general numerical methods with the help of standard computing tools. The whole interferogram being formed by the incoherent superposition of all elementary interferograms, we next, compute the relevant quantities by appropriate numerical quadratures. We illustrate this method with two potential layouts of a Fourier transform spectrometer that we are currently designing for accurate radiometric measurements in the 2.9μm-9.6μm range with a spectral resolution better than 8cm-1 on a 4.5°x0.6° field of view.

Compact high-resolution micro-spectrometer on chip: spectral calibration and first spectrum

Compact and hand-held spectrometers may be very interesting for the measurement of spectral signatures of chemicals or objects. To achieve this goal, ONERA and IPAG have developed a new on chip Fourier Transform Spectrometer operating in the visible spectral range with a high spectral resolution (near 2 cm-1), named visible HR SPOC (visible High Resolution Spectrometer On Chip). It is directly inspired from the MICROSPOC infrared spectrometer, studied at ONERA in the past years. This spectrometer is made of a stair-step two-wave interferometer directly glued on a CMOS detector making it a very compact prototype. After calibrating the optical path difference, measurements of experimental spectra are presented.