Joël Deschamps

Free-standing subwavelength metallic gratings for snapshot multispectral imaging

A mosaic of ten spectral filters has been fabricated in a single 20 mm2 membrane drilled by nanoslits and coated by a gold layer. The nanostructured core-shell gratings exhibit 70% average maximum transmission efficiency in 15% aperture area, which represents a fivefold enhancement compared to the geometrical transmission. This mosaic of bandpass filters regularly spaced in the 3–5 μm wavelength range is used to demonstrate real-time spectral imaging in a multichannel camera.

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.

Infrared focal plane array with a built-in stationary Fourier-transform spectrometer: basic concepts.

A novel configuration of stationary Fourier transform infrared (FTIR) spectrometer is presented. Contrary to classic configurations, the interferometer is directly integrated in the focal plane array (FPA) during its process of fabrication. A first, to the best of our knowledge, demonstration of the spectrometric function has been achieved departing from a well-known structure of an HgCdTe photodetector. We show that the obtained FTIR-FPA can be described by intrinsic parameters such as an optical path difference and a so-called spectrometric efficiency. First experimental results are presented.

A Monte Carlo Study of

A simple Monte Carlo model is developed for understanding the multiplication process in HgCdTe infrared avalanche photodiodes and the impact of physical and technological parameters. A good agreement is achieved between simulations and experimental measurements of gain and excess noise factor. In both cases, an exponential gain and extremely low noise-F ~ 1 for multiplication gains up to 1000-were observed on 5.1-mum cutoff devices at 77 K, indicative of a single carrier impact ionization. A comparison study is presented to explain the effect of different combinations of scattering processes on the avalanche phenomenon in HgCdTe.

\hbox{Hg}_{0.7}\hbox{Cd}_{0.3}\hbox{Te}

We present what is believed to be a novel experimental method to measure the technological parameters (spectral response and quantum yield) of an infrared focal plane array. This method makes original use of a Fourier transform spectrometer, which allows us to simultaneously extract the spectral performances of all pixels from one single set of measurements. The methodology used and the principle of the experimental setup are detailed. A Fourier analysis is shown to provide various optogeometrical information on the detector microstructure. A demonstrator based on the HgCdTe technology was designed, and satisfactory experimental results were obtained.

e-APD

We present a new method to measure the modulation transfer function (MTF) beyond the Nyquist frequency of a multichannel imaging system for which all the channels have parallel optical axes. Such a multichannel optical system produces a set of undersampled subimages. If the subimages contain nonredundant information, high spatial frequencies are folded between low spatial frequencies, leading to the possible extraction of frequencies higher than the Nyquist frequency. The measurement of the MTF of the multichannel system leads to the estimation of the resolution enhancement of the final image that can be obtained by applying a postprocessing algorithm to the collection of undersampled subimages. Experimental images are presented to validate this method.

Single-scan extraction of two-dimensional parameters of infrared focal plane arrays utilizing a Fourier-transform spectrometer

The purpose of this paper is to give an overview of the state of the art wavefront sensor detectors developments held in Europe for the last decade. The success of the next generation of instruments for 8 to 40-m class telescopes will depend on the ability of Adaptive Optics (AO) systems to provide excellent image quality and stability. This will be achieved by increasing the sampling, wavelength range and correction quality of the wave front error in both spatial and time domains. The modern generation of AO wavefront sensor detectors development started in the late nineties with the CCD50 detector fabricated by e2v technologies under ESO contract for the ESO NACO AO system. With a 128x128 pixels format, this 8 outputs CCD offered a 500 Hz frame rate with a readout noise of 7e-. A major breakthrough has been achieved with the recent development by e2v technologies of the CCD220. This 240x240 pixels 8 outputs EMCCD (CCD with internal multiplication) has been jointly funded by ESO and Europe under the FP6 programme. The CCD220 and the OCAM2 camera that operates the detector are now the most sensitive system in the world for advanced adaptive optics systems, offering less than 0.2 e readout noise at a frame rate of 1500 Hz with negligible dark current. Extremely easy to operate, OCAM2 only needs a 24 V power supply and a modest water cooling circuit. This system, commercialized by First Light Imaging, is extensively described in this paper. An upgrade of OCAM2 is foreseen to boost its frame rate to 2 kHz, opening the window of XAO wavefront sensing for the ELT using 4 synchronized cameras and pyramid wavefront sensing. Since this major success, new developments started in Europe. One is fully dedicated to Natural and Laser Guide Star AO for the E-ELT with ESO involvement. The spot elongation from a LGS Shack Hartman wavefront sensor necessitates an increase of the pixel format. Two detectors are currently developed by e2v. The NGSD will be a 880x840 pixels CMOS detector with a readout noise of 3 e (goal 1e) at 700 Hz frame rate. The LGSD is a scaling of the NGSD with 1760x1680 pixels and 3 e readout noise (goal 1e) at 700 Hz (goal 1000 Hz) frame rate. New technologies will be developed for that purpose: advanced CMOS pixel architecture, CMOS back thinned and back illuminated device for very high QE, full digital outputs with signal digital conversion on chip. In addition, the CMOS technology is extremely robust in a telescope environment. Both detectors will be used on the European ELT but also interest potentially all giant telescopes under development. Additional developments also started for wavefront sensing in the infrared based on a new technological breakthrough using ultra low noise Avalanche Photodiode (APD) arrays within the RAPID project. Developed by the SOFRADIR and CEA/LETI manufacturers, the latter will offer a 320x240 8 outputs 30 microns IR array, sensitive from 0.4 to 3.2 microns, with 2 e readout noise at 1500 Hz frame rate. The high QE response is almost flat over this wavelength range. Advanced packaging with miniature cryostat using liquid nitrogen free pulse tube cryocoolers is currently developed for this programme in order to allow use on this detector in any type of environment. First results of this project are detailed here. These programs are held with several partners, among them are the French astronomical laboratories (LAM, OHP, IPAG), the detector manufacturers (e2v technologies, Sofradir, CEA/LETI) and other partners (ESO, ONERA, IAC, GTC). Funding is: Opticon FP6 and FP7 from European Commission, ESO, CNRS and Université de Provence, Sofradir, ONERA, CEA/LETI and the French FUI (DGCIS).

Modulation transfer function measurement of a multichannel optical system

By measuring the spectral responses of infrared focal plane arrays (IRFPAs), one can extract at a given wavelength the cartography of the pixels responses, called the hyperspectral cartography. Recently, hyperspectral cartographies have been obtained from IRFPAs that exhibited small defects of substrate thickness. These defects produce Fizeau fringes across the FPA. By purposely amplifying this phenomenon during the process of realisation, one can easily generate a good approximation of a two-beam interferometer in the immediate neighbourhood of the FPA. Like a classic Michelson interferometer with tilted plane mirrors, this on-a-chip interferometer produces a spatially-modulated interferogram, the Fourier-transform of which yields the spectral content of the illuminating beam. A first prototype of this Fourier-transform microspectrometer on a chip (MICROSPOC) has been realised and tested. Experimental results will be presented and the potential of this approach will be discussed.

Advances in detector technologies for visible and infrared wavefront sensing

Today, both military and civilian applications require miniaturized optical systems in order to give an imagery function to vehicles with small payload capacity. After the development of megapixel focal plane array (FPA) with micro-sized pixels, this miniaturisation will become feasible with the integration of optical functions in the detector area. In the field of cooled infrared imaging system, the detector area is the Detector-Dewar-Cooler Assembly (DDCA). A dewar is a sealed environment where the detector is cooled on a cold plate. We show in this paper that an imagery function can be added to the dewar by simply integrating a single meniscus inside the cold shield. An infrared system with a wide field of view and high throughput is thus obtained without adding optics outside the dewar. The additional mass of the optic is sufficiently small to be compatible with the cryogenic environment of the DDCA. The temperature stabilization of the optic and the reduction of the background radiation are the main advantages of this system. The performance of this camera will be discussed and several evolutions of this camera will be introduced too.

Microspectrometer on a chip (MICROSPOC): first demonstration on a 320x240 LWIR HgCdTe focal plane array

There is an emerging demand for compact infrared instruments, imagers and/or spectrometers, integrated on ground or air vehicles for spatial and spectral data collection. To reach this goal, technological barriers have already been overcome, leading to the development of infrared focal plane arrays (IRFPAs) for high-performance applications (megapixel format, bispectral technology) but also for low-cost and high-volume manufacturing (technology of uncooled micro-bolometers). The next step is to reduce the optics and make it compatible with the successful IRFPAs fabrication technology. This paper presents MULTICAM, a small cryogenic infrared camera. This optical system is composed of multi-level arrays of microlenses integrated in the cryostat and inspired from invertebrate compound eyes. First experimental results will be presented.