Serge Magli

Mid-infrared Shack-Hartmann wavefront sensor fully cryogenic using extended source for endoatmospheric applications

Adaptive optics provide real-time compensation for atmospheric turbulence. The correction quality relies on a key element: the wavefront sensor. We have designed an adaptive optics system in the mid-infrared range providing high spatial resolution for ground-to-air applications, integrating a Shack-Hartmann infrared wavefront sensor operating on an extended source. This paper describes and justifies the design of the infrared wavefront sensor, while defining and characterizing the Shack-Hartmann wavefront sensor camera. Performance and illustration of field tests are also reported.

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.

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.

OSMOSIS: a new joint laboratory between SOFRADIR and ONERA for the development of advanced DDCA with integrated optics

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 arrays (FPA) with micro-sized pixels, this miniaturization will become feasible with the integration of optical functions in the detector area. In the field of cooled infrared imaging systems, the detector area is the Detector-Dewar-Cooler Assembly (DDCA). SOFRADIR and ONERA have launched a new research and innovation partnership, called OSMOSIS, to develop disruptive technologies for DDCA to improve the performance and compactness of optronic systems. With this collaboration, we will break down the technological barriers of DDCA, a sealed and cooled environment dedicated to the infrared detectors, to explore Dewar-level integration of optics. This technological breakthrough will bring more compact multipurpose thermal imaging products, as well as new thermal capabilities such as 3D imagery or multispectral imagery. Previous developments will be recalled (SOIE and FISBI cameras) and new developments will be presented. In particular, we will focus on a dual-band MWIR-LWIR camera and a multichannel camera.

Shack-Hartmann wavefront sensor using IR extended source

Adaptive optics provides a real time compensation for atmospheric turbulence which severely limits the resolution of ground-based observation systems. The correction quality relies on a key component: the wavefront sensor. An adaptive optics system in the mid-IR providing high spatial resolution for ground-to-air applications has been designed at ONERA and is currently integrated. It includes an IR Shack-Hartmann wavefront sensor operating on an extended source. This paper describes and justifies the design of the IR wavefront sensor. First images and tests with the Shack-Hartmann wavefront sensor camera are presented.

Lightweight, compact, and affordable MW TV-format IR detectors

TV/4 format (320×256) detectors are today the most produced. They have demonstrated high thermal performance and are cost effective at the mass production level. However, in response to system requirements of very high resolution, miniaturization and cost reduction, SOFRADIR is now offering full TV format (640x512) detectors with graduated levels of performances regarding pitch size and cryogenics. In particular a TV format with 15 μm pitch is offered allowing the integration of highly miniaturized cryogenics dedicated to compact systems or, depending of the applications, the upgrade of existing systems by using the same miniature cryogenics as the ones used for TV/4 format. Regarding TV format (640x512) with 15μm pitch, technological adaptations were validated at CEA-LETI/LIR (France) at the beginning of 2002. With respect to the 20μm pitch TV formats whose purpose is to offer high resolution performances in slightly modified cryogenics, this new 15μm pitch MCT TV format exhibits even higher performances in very small size optimized cryogenics in order to fit the system requirements of miniaturization and cost reduction and to achieve a cost effective production level. In this way, this 15μm pitch 640x512 MWIR MCT detectors will become, in the coming years, the most affordable large format at production level.

Integrating a compact multichannel cryogenic infrared camera in an operational detector dewar cooler assembly

We present an ultra-compact infrared cryogenic camera integrated inside a standard SOFRADIR’s Detector Dewar Cooler Assembly (DDCA) and whose field of view is equal to 120°. The multichannel optical architecture produces four non-redundant images on a single SCORPIO detector with a pixel pitch of 15μm. This ultra-miniaturized optical system brings a very low additional optical and mechanical mass to be cooled in the DDCA: the cool-down time is comparable to the one of an equivalent DDCA without an imagery function. Limiting the number of channels is necessary to keep the highest number of resolved points in the final image. However, optical tolerances lead to irregular shifts between the channels. This paper discusses the limits of multichannel architectures. With an image processing algorithm, the four images produced by the camera are combined to process a single full-resolution image with an equivalent sampling pitch equal to 7.5μm. Experimental measurements on MTF and NETD show that this camera achieves good optical performances.

Exploring the imaging properties of thin lenses for cryogenic infrared cameras

Designing a cryogenic camera is a good strategy to miniaturize and simplify an infrared camera using a cooled detector. Indeed, the integration of optics inside the cold shield allows to simply athermalize the design, guarantees a cold pupil and releases the constraint on having a high back focal length for small focal length systems. By this way, cameras made of a single lens or two lenses are viable systems with good optical features and a good stability in image correction. However it involves a relatively significant additional optical mass inside the dewar and thus increases the cool down time of the camera. ONERA is currently exploring a minimalist strategy consisting in giving an imaging function to thin optical plates that are found in conventional dewars. By this way, we could make a cryogenic camera that has the same cool down time as a traditional dewar without an imagery function. Two examples will be presented: the first one is a camera using a dual-band infrared detector made of a lens outside the dewar and a lens inside the cold shield, the later having the main optical power of the system. We were able to design a cold plano-convex lens with a thickness lower than 1mm. The second example is an evolution of a former cryogenic camera called SOIE. We replaced the cold meniscus by a plano-convex Fresnel lens with a decrease of the optical thermal mass of 66%. The performances of both cameras will be compared.

Development of an infrared ultra-compact multichannel camera integrated in a SOFRADIR's detector Dewar cooler assembly

We present a prototype of an infrared cryogenic camera directly integrated inside an off-the-shelf SOFRADIRs Detector Dewar Cooler Assembly (DDCA) and whose field of view is equal to 120°. Based on the co-design principle between optical design and image processing, we have designed a multichannel camera which produces four non-redundant images on a single SCORPIO detector, with 640 × 512 pixels and a pixel pitch of 15 μm. This leads to an ultra-miniaturized optical system with a very low additional optical and mechanical mass to be cooled. By this way, the cool-down time of the camera is comparable to the one of an equivalent DDCA without an imagery function. Indeed, we obtain a cool-down time of 6 minutes with a THALES Cryogenics RM3. With a superresolution algorithm, the four images produced by the camera are combined to process a single full-resolution image with an equivalent sampling pitch equal to 7.5μm. The performances of this camera, assessed by experimental characterizations, are presented.

Dewar cooler integrated MWIR spectrometer for high rates and high dynamic range measurements

There is a need for compact, hand-held, spectrometers for the measurement of spectral signatures of chemicals or objects. To achieve this goal, a new concept of Fourier-transform interferometer (FTIR) directly integrated on the infrared focal plane array (FPA) has been developed at ONERA. The fundamental properties of this key element called MICROSPOC will be recalled and we will see how those properties can be exploited to get a snapshot, compact and cryogenic MWIR spectrometer. These design rules have been applied to develop a very compact device that combines the metrological properties of a FTIR-FPA of quantum HgCdTe technology with the radiometric performances of a last generation Sofradir detection block (Infrared Detector Dewar Cooler Assembly – IDDCA). The experimental performances of the prototype will be presented, in terms of spectral resolution, acquisition rate, dynamic range and noise equivalent spectral radiance. We will discuss at the end the potential of this technology to meet the requirements of different applications.