Michel Tauvy

Modulation transfer function measurement of an infrared focal plane array by use of the self-imaging property of a canted periodic target

We present a new technique for measuring the modulation transfer function (MTF) of a focal plane array (FPA). The main idea is to project a periodic pattern of thin lines that are canted with respect to the sensors columns. Practically, one aims the projection by using the self-imaging property of a periodic target. The technique, called the canted periodic target test, has been validated experimentally on a specific infrared FPA, leading to MTF evaluation to as great as five times the Nyquist frequency.

Room-temperature electroluminescence in the mid-infrared (2-3 μm) from bulk chromium-doped ZnSe

Electroluminescence associated with impact excitation or ionization of deep Cr(2+) impurity centers in bulk ZnSe is reported. A broad signal of mid-infrared luminescence between 2 and 3 microm is observed once the biased bulk ZnSe device runs into a nonlinear conduction regime. Optical powers in the nanowatt range have been measured at room temperature. The different mechanisms involved in this intracenter infrared light emission are discussed.

Ultimate performance of quantum well infrared photodetectors in the tunneling regime

Thanks to their wavelength diversity and to their excellent uniformity, Quantum well infrared photodetectors (QWIP) emerge as potential candidates for astronomical or defense applications in the very long wavelength infrared (VLWIR) spectral domain. However, these applications deal with very low backgrounds and are very stringent on dark current requirements. In this paper, we present the full electro-optical characterization of a 15 μm QWIP, with emphasis on the dark current measurements. Data exhibit striking features, such as a plateau regime in the I(V) curves at low temperature (4–25 K). We show that present theories fail to describe this phenomenon and establish the need for a fully microscopic approach.

In situ optical testing of infrared lenses for high-performance cameras.

We propose to evaluate infrared lenses with a dedicated analyzer having the same mechanical interface as the usual cameras. The proposed analysis is based on a wavefront measurement and allows a diagnostic of possible internal defects of the analyzed lens. The infrared lens analyzer described is constituted with a quadriwave lateral shearing interferometer and works with a blackbody light. We describe the response of this interferometer and an innovative method to obtain the wavefront under test. We finally present the experimental analysis of long-wavelength infrared lenses and the particular case of a modified lens that generates a large spherical aberration.

MULTICAM: a miniature cryogenic camera for infrared detection

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.

MICROCARD : a micro-camera based on a circular diffraction grating for MWIR and LWIR imagery

Circular diffraction gratings (also called diffractive axicons) are optical components producing achromatic non-diffracting beams. They thus produce a focal line rather than a focal point for classical lenses. We have recently shown in the visible spectral range that this property can be used to design a simple imaging system with a long depth of focus and a linear variable zoom by using and translating a diffractive axicon as the only component. We have then adapted this principle for the mid-wavelength infrared (MWIR) spectral range and the long-wavelength infrared (LWIR) spectral range. A LWIR low-cost micro-camera, called MICROCARD, has been designed and realized. First images from this camera will be shown. Moreover a way to design a compact MWIR micro-camera with moveable parts integrated directly into the cryostat will be presented.

Van Hove singularities in intersubband transitions in multiquantum well photodetectors

Photocurrent spectra of quantum well infrared photodetector (QWIP) devices have been studied over more than three orders of magnitude, revealing features which have been largely overlooked before. Electric field assisted tunneling and, more surprisingly, Van Hove singularities at the miniband edges are shown to play an important role in the low and high energy parts of the QWIP photocurrent spectra, respectively. The photoresponse of QWIPs away from their peak responsivity is found to be non-negligible (>1% in the 3–5μm for a 8–12μm detector), which has to be taken into consideration when optimizing multispectral devices.

Experimental study and modelization of oblique incidence effect on QWIP's spectral response

As far as calibrated radiometric imaging is concerned, a complete prediction of oblique incidence effect on the FPA pixels’ response is required. Since a light coupling scheme needs to be used in QWIP detectors, this effect is particularly complicated to understand. This article presents two complementary test benches which allow to quantify the effect of oblique incidence on cooled infrared detectors issued from different technologies. The first test bench performs measurements over a wide angular range with low background emission levels, but gives spectrally integrated measurements. The second one delivers spectrally resolved responses for incident angles lower than 30°. In order to validate both experimental concepts, we studied QWIPs equipped with 2D periodic gratings. Relatively large pixels (100x100μm2) were chosen to ease comparison with models. Calculations based on the modal expansion method reveal that diffraction off an infinite grating does not account very well for the observed spectral responses.

Effect of oblique incidence on the spectral response of quantum well infrared photodetectors

A full understanding of the effect of oblique incidence on the response of the pixels of a focal plane array is required for calibrated radiometric imaging. Measuring the angular response of infrared detectors requires an extremely complex experimental setup. This paper presents two complementary experimental setups that enable us to study the effect of the oblique incidence of light on the response of cooled infrared phototdetectors based on different technologies. The angular response measurement setup provides spectrally integrated measurements over a wide angular range (−80°–+80°) with a low background level and a low stray light. In contrast, the spectral response measurement setup provides the spectrally resolved response for incidence angles lower than 30°. Both experimental setups were used to characterize single-quantum well infrared photodetectors with crossed gratings. Relatively large pixel arrays (100×100μm2) were used to reduce finite-size effects. However, the comparison of our experimental...

Experimental evaluation of the modulation transfer function of an infrared focal plane array using the Talbot effect

A test bench for measuring the Modulation Transfer Function of an InfraRed Focal Plane Array (IRFPA) is described. The system is based on the use of the self-imaging property of a periodic target made of thin slits for a direct projection of a high-resolution pattern directly on to the tested sensor. The test bench has furnished two different types of experimental results on a specific IRFPA. On one hand, a global method called Canted Periodic Target Test has been developed and applied leading to MTF evaluation up to five times the Nyquist frequency. On the other hand, a local line scanning of every pixel has been made.