Florence de la Barrière

Design strategies to simplify and miniaturize imaging systems

We present the range of optical architectures for imaging systems based on a single optical component, an aperture stop, and a detector. Thanks to the formalism of third-order Seidel aberrations, several strategies of simplification and miniaturization of optical systems are examined. Figures of merit are also introduced to assess the basic optical properties and performance capabilities of such systems; by this way, we show the necessary trade-off between simplicity, miniaturization, and optical performance.

Modulation transfer function measurement of a multichannel optical system

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.

Compact infrared cryogenic wafer-level camera: design and experimental validation

We present a compact infrared cryogenic multichannel camera with a wide field of view equal to 120°. By merging the optics with the detector, the concept is compatible with both cryogenic constraints and wafer-level fabrication. The design strategy of such a camera is described, as well as its fabrication and integration process. Its characterization has been carried out in terms of the modulation transfer function and the noise equivalent temperature difference (NETD). The optical system is limited by the diffraction. By cooling the optics, we achieve a very low NETD equal to 15 mK compared with traditional infrared cameras. A postprocessing algorithm that aims at reconstructing a well-sampled image from the set of undersampled raw subimages produced by the camera is proposed and validated on experimental images.

Towards infrared DDCA with an imaging function

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.

Modulation Transfer Function Measurement of Infrared Focal-Plane Arrays with Small Fill Factors

This paper describes an original method to measure the modulation transfer function (MTF) of an infrared focal-plane array (IRFPA), based on a diffraction grating called a continuously self-imaging grating (CSIG). We give a general methodology to design the test bench, and we describe the data processing approach which has been developed to extract relevant information about the size of the photodiodes and filtering effects. The MTF measurement capability of this method is illustrated with a cooled IRFPA.

Integration of advanced optical functions on the focal plane array for very compact MCT-based micro cameras

Over the past decade, several technological breakthroughs have been achieved in the field of optical detection, in terms of spatial and thermal resolutions. The actual trend leads to the integration of new functions at the vicinity of the detector. This paper presents two types of integrated optics in the cryo-cooler, close to the MCT (CdHgTe) infrared detector array. The first one, for spectro-imaging applications, is a Fourier-transform microspectrometer on chip (MICROSPOC), developed for very fast acquisition of spectral signatures. Experimental results will be presented. The second one, for large field of view applications, illustrates the high potentiality of the integration of advanced optical functions in the Dewar of MCT detectors.

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.

Fabrication of concave and convex potassium bromide lens arrays by compression molding.

A new simple and cost-effective method has been developed for the fabrication of both plano-convex and plano-concave lens arrays with potentially important sag heights. The process is based on the use of potassium bromide (KBr) powder. At ambient temperature and under pressure, KBr powder is compressed on a molding die with the desired shape to form a solid lens array. The quality of the lens arrays has been assessed, and we present the first image produced by a converging KBr lens array.

Micro-camera and micro-spectrometer designs adapted to large infrared focal plane arrays

Todays infrared focal plane arrays concentrate in a small volume of typically 1 cm3 the results of three decades of research in microelectronics and packaging. Several technological breakthroughs have already been achieved leading to the development of infrared focal plane arrays (IRFPAs) for high-performances applications requiring spatial and thermal resolution, also for low-cost and high-manufacturing volumes (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 some methods and technologies we are exploring for high-performance and small infrared systems. These developments have led to a tool box of micro-concepts described by an optical function (imagery or spectrometry) integrated in the vicinity of the IRFPA. For this, old optical concepts have been revisited (pinhole optics, Talbot effect) and first demonstrations of original IRFPA-based micro-optical assemblies will be given.

Integration of advanced optical functions near the focal plane array: first steps toward the on-chip infrared camera

Today, both military and civilian applications require miniaturized and cheap optical systems. The miniaturization of imaging systems leads to breakthroughs in optical design. Multichannel systems, inspired by the compound eyes of insects, offer great opportunities as the principle is to divide the information contained in the whole scene into the different optical channels. An interesting approach is to take advantage of the infrared focal plane array technology and environment to integrate these systems near the detector, leading to very compact architectures. This paper presents a compact optical architecture based on a multichannel imaging system entirely integrated in the dewar used to cool the detector. This work gives encouraging results to prepare the next step in the miniaturization of optical systems, which is the integration of the imaging function directly on the focal plane array (wafer-level integration), leading to the design of an on-chip infrared camera.