Hervé Sauer

Experimental results from an airborne static Fourier transform imaging spectrometer

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.

Demonstration of image-zooming capability for diffractive axicons

Diffractive axicons are optical components producing achromatic nondiffracting beams. They thus produce a focal line rather than a focal point for classical lenses. This gives the interesting property of a long focal depth. We show that this property can be used to design a simple imaging system with a linear variable zoom by using and translating a diffractive axicon as the only optical component.

Fully tunable active polarization imager for contrast enhancement and partial polarimetry

We present the design and the practical implementation of a polarimetric imaging system based on liquid-crystal modulators that allows generation and analysis of any polarization state on the Poincaré sphere. This system is more versatile than standard Mueller imagers that are based on optimized, but limited, sets of illumination and analysis states. Examples of benefits brought by these extra degrees of freedom are illustrated on two different applications: contrast enhancement and extraction of partial polarimetric properties of a scene.

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

Extra-thin infrared camera for low-cost surveillance applications

We designed a cheap broadband uncooled microimager operating in the long-wavelength infrared range using only one lens at a minimal cost for the manufacturing process. The approach is based on thin optics where the device volume is small and therefore inexpensive materials can be used because some absorption can be tolerated. We have used a Fresnel lens on a thin silicon substrate. Up to now, Fresnel lenses have not been used for broadband imaging because of their chromatic properties. However, working in a relatively high diffraction order can significantly reduce chromatism. A prototype has been made for short range or indoor low-cost surveillance applications like people counting, and experimental images are presented.

Optimization of a silicon-on-sapphire waveguide device for optical bistable operation

A silicon-on-sapphire waveguide is studied theoretically and experimentally in a pulsed regime to obtain bistable operation. Very fast switchings can be observed and are explained as a result of the competition between electronic and thermal nonlinear effects. Pseudobistable operation is demonstrated when the electronic effects are favored, owing to an increase in the silicon photogenerated electron–hole pair lifetime or to a reduction in the incident pulse duration. Other factors such as the grating-coupler efficiency and the sample thermalization can, likewise, improve the device operation.

Diffractive optical elements in hybrid lenses: modeling and design by zone decomposition

We propose to model hybrid optical systems (i.e., lenses with conventional and diffractive optical elements) as multiaperture systems in which the images formed by each zone of the diffractive optical element should be summed up coherently. This new zone decomposition concept is explained and compared with the standard diffraction-order expansion with the help of a hybrid triplet example.

Reduction of material mass of optical component in cryogenic camera by using high-order Fresnel lens on a thin germanium substrate.

We designed a compact infrared cryogenic camera using only one lens mounted inside the detector area. In the field of cooled infrared imaging systems, the maximal detector area is determined by the dewar. It is generally a sealed and cooled environment dedicated to the infrared quantum detector. By integrating an optical function inside it, we improve the compactness of the camera as well as its performances. The originality of our approach is to use a thin integrated optics which is a high quality Fresnel lens on a thin germanium substrate. The aim is to reduce the additional mass of the optical part integrated inside the dewar to obtain almost the same cool down time as a conventional dewar with no imaging function. A prototype has been made and its characterization has been carried out.

Compactness of lateral shearing interferometers

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.

Surface plasmon resonance imaging instrumentation and data handling for biochips: review and perspectives

Sensors based on surface plasmon resonance have the potential to provide information on the binding of biological molecules on adequate substrates over typically thousand channels in parallel, without the need for any marker and in real time compared to the scale of biochemical reactions. The need to optimize selectivity and sensitivity has triggered continued research efforts. We review those related with optics and image processing, at the same time identifying some aspects that deserve further investigation before the potential of the technique is fully utilized.