Photonic Integrated Circuits (PICs) are chips with optical inputs and outputs that are linked by waveguides. They allow for a better control over the light propagation and an improved mechanical stability with respect to free space optical systems. In this article, a three-dimensional photonic integrated circuit (3D PIC) for hyperspectral imaging is proposed. We will also present our latest numerical and experimental results toward the fabrication of a hyperspectral imaging system using 3D PICs fabricated using an ultrafast femtosecond laser in a glass chip.
Optimizing freeform systems can encounter convergence difficulties due to the many degrees of freedom that these surfaces bring to optical systems. Moreover, the description of these freeform surfaces in a polynomial basis may impose prior knowledge on the shape of the surface. In this presentation, we will showcase a differential ray tracer with NURBS capabilities called FORMIDABLE. In contrast to available commercial optical design software, such as Zemax OpticStudio and Synopsys Code V, this library ican simulate and especially optimize Non-Uniform Rational B-Spline (NURBS) surfaces. The key advantage of NURBS lies in their ability to locally describe an optical surface, thereby minimizing preconceived notions about the surface shape, aside from the surface sampling determined by the density of the NURBS representation. The main drawback, however, is the significant increase in the degrees of freedom within the optical system, making the optimization of these surfaces a complex task with a conventional commercial optical design software. FORMIDABLE's implementation of differential ray-tracing capabilities allows faster convergence of systems described by many degrees of freedom and makes optimization with NURBS surfaces viable. The features of FORMIDABLE will first be described. Then its capabilities will be illustrated with the optimization of a classical non-reimaging Three-Mirror Anastigmat (TMA) by considering either a description of surfaces by NURBS or a description by the polynomial basis XY. Then, this optimized TMA will be compared with its equivalent optimized with Zemax OpticStudio. To enable this software comparison, we will use the same starting point and practically the same merit function. Standard metrics, such as Root Mean Square (RMS) spot size across the field of view (FOV) will be used to assess the imaging quality.
GRIN materials can help compensate chromatic aberrations and enhance athermalization in an optical system, leading to more lightweight and compact lens assemblies, often with a lower number of elements. For several years, Umicore and ISCR in Rennes have developed GRIN materials for systems, using several approaches to modify the index in a chalcogenide glass. We will mention two of the most promising paths in this review: partial and spatially controlled crystallization by fast heating of the perimeter of a glass rod and ionic exchange which modifies locally the glass composition, leading to local changes in refractive index.
A study of a compact reimaging freeform Three Mirror Anastigmat suitable for thermal infrared space imaging application in small satellites is performed, and shows that for such highly constrained systems, freeform optics must be used.
An automated method was developed to explore the landscape and find different congurations for fast freeform dioptric systems in the Long-Wave Infrared band for automotive application. It uses the Simultaneous Multiple Surfaces method.
KEYWORDS: Wavefronts, Optical surfaces, Modulation transfer functions, Imaging systems, Zemax, Etching, Digital signal processing, Diffractive optical elements, Design and modelling
We compare different methods to extend the depth of focus of a fast infrared imaging system. Instead of using a phase mask for wavefront coding, we place this element directly on an optical surface.
Freeform surfaces are a revolution in the field of spatial imaging because they allow the correction of optical aberrations in off-axis systems. Freeform surfaces are defined as non-rotationally symmetric surfaces, which also cannot be described as an off-axis part of a conicoid. The use of such surfaces can also enable to increase performances, such as the field of view, F-number or compactness of off-axis, fully reflective telescopes, and is thus interesting for nanosatellite imaging applications. In this paper, we will present a proof of concept for a fast, compact and well-corrected freeform Three Mirror Anastigmat (TMA) design suited for nanosatellite infrared thermal imaging using an uncooled micro-bolometer. The performance and tolerance analysis will be presented, along with an analysis of the mirrors’ shape and surface quality using an industrial surface characterisation tool. The consequences of the mirrors’ shape error on the optical quality will also be discussed, as well as a method to compensate for the loss in image quality induced by these shape defects.
In optical design, the designer's experience is critical. Indeed, an experienced optical designer will often choose a better starting point for optimization than an inexperienced one. Most of the time, lens design software use a local optimization algorithm, which is why the starting point is so important to get an excellent optical system. We present here an alternative to the classical optical design method and a solution to reduce the impact of the designer's experience. Our alternative couples the Simultaneous Multiple Surfaces (SMS) method, introduced by Benítez and Miñano with optimization in Zemax OpticStudio. The SMS method is a direct construction method of optical systems without optical aberrations for as many field points as the system contains surfaces. This method can deal with both aspheric and freeform optical systems depending on the dimension of the method implemented. Our implementation of the SMS method can design optical systems with three surfaces. We use the SMS method to define a freeform system with an F-number of 0.85. Then, we use this fast freeform system as a starting point to perform further optimization in Zemax OpticStudio. Finally, we achieve to design two diffraction-limited freeform systems, one over a square field of view of ±30° and another over a rectangular field of view of ±33° × ±26°.
An important challenge in infrared imaging today consists in addressing the SWaP problem (Size, Weight and Power), for example by simplifying as much as possible the optical system before the sensor. The work presented in this paper takes advantage of recent techniques in femtosecond laser direct writing to imprint optical systems. We want to simplify an infrared multispectral imaging system, which combines a lens array and a filter array. This work aims at merging a lens array with a filter array by writing gradient index lenses with a femtosecond laser inside a dedicated glass substrate. A classical gradient index requires a huge refractive index variation, which cannot be reached today with femtosecond laser processing (Δ𝑛𝑚𝑎𝑥 ~ 0.05). So, we decided to turn towards writing a gradient index Fresnel lens. A first-order Fresnel lens was designed with a Δ𝑛𝑚𝑎𝑥 < 0.05 discretized into 8 index levels to guarantee a diffraction efficiency of 85% on the overall spectral bandwidth of the filters. The multispectral design is made of an array of 2x2 Fresnel lenses in a landscape lens configuration. For a horizontal field of view of 40°, each GRIN Fresnel lens has 82 rings inside a total diameter of 3.8 mm. After characterizing the photosensitive response of the material to fs-laser writing, we started writing the first prototypes of graded index Fresnel lenses. For a first approach, a discretization on 4 index levels was chosen. A focal length measurement has been performed in order to compare it with our model.
Infrared cameras could serve automotive applications by delivering breakthrough perception systems for both in-cabin passengers monitoring and car surrounding. However, low-cost and high-throughput manufacturing methods are essential to sustain the growth in thermal imaging markets for automotive applications, and for other close-to-consumer applications, which have a fast growth potential. With the reduction of the pixel pitch of microbolometer detectors, their cost has decreased considerably and now the optical part represents a significant part of the system cost. Fast low cost infrared lenses suitable for microbolometers are already sold by companies like Umicore, Lightpath, FLIR… Chalcogenide glasses are widely used as materials for optics because they have many cost advantages, especially due to the possibility of mass molding the optics. However, with the reduction of the pixel pitch, it is more and more difficult to design high performance lenses with a limited number of optics. The possibility of molding the optics allows us to use many highly aspherical surfaces at affordable costs. However, Chalcogenide glasses have usually a lower refractive index than other more expensive infrared materials such as Germanium. Indeed, high refractive index materials are known to be effective in attenuating the amplitude of many geometric aberrations. In this presentation, we evaluate the interest of high index Chalcogenide lenses, especially TGG and TGS, to design optical systems meeting the needs of the automobile with a limited number of optics. TGG glass has an index of refraction of 3.396 at a wavelength of 10µm, i.e. its index of refraction is close to the Silicon one and was initially studied for space applications. TGS has a lower index of refraction (3.12@10µm) but can be used in a cost effective manufacturing process by using flash spark plasma sintering (SPS) on raw powder. Demonstrators with TGG glass have been made and their performance evaluated.
An important challenge in infrared imaging today consists in addressing the SWaP problem (Size, Weight and Power), for example by simplifying as much as possible the optical system before the sensor. The work presented in this paper takes advantage of recent techniques in femtosecond laser direct writing to imprint optical systems. This work aims at merging a lens array with a filter array by writing gradient index lenses with a femtosecond laser inside a dedicated glass substrate. A classical gradient index lens requires a huge refractive index variation, which cannot be reached today with femtosecond laser processing (Δ𝑛𝑚𝑎𝑥 ~ 0,05). So, we decided to turn towards writing a gradient index Fresnel lens. A first-order Fresnel lens was designed with a Δ𝑛𝑚𝑎𝑥 < 5.10−2 discretised into 8 index levels to guarantee a diffraction efficiency of 85% on the overall spectral bandwidth of the filters. We studied a chalcogenide glass because of his good properties in mid-IR optics. We compared the different laser responses of the material: threshold of permanent changes, quantitative phase and index variation. Finally, we showed that reaching a phase variation of π radians with femtosecond laser writing is possible even for a huge scanning speed (0,01 mm/s up to 25 mm/s). By overlapping the written lines, we were able to implement a maximum phase variation of 8 radians (with only one written layer), demonstrating the possibility to write quickly a GRIN Fresnel lens with a diameter of some millimeters.
Infrared cameras could serve automotive applications by delivering breakthrough perception systems for both in-cabin passengers monitoring and car surrounding. However, low-cost and high-throughput manufacturing methods are essential to sustain the growth in thermal imaging markets for automotive applications, and for other close-to-consumer applications which have a fast growth potential. Fast low cost infrared lenses suitable for microbolometers are currently already sold by companies like Umicore, Lightpath, FLIR… They are either made of a single inverse meniscus Chalcogenide glass or of two Silicon optics. In this paper, we explore hybrid systems with a large field of view around 40° combining Chalcogenide and Silicon in order to take advantage of both materials. Both are compatible with wafer-level process. Silicon optics can be manufactured by photolithography process and are expected to be more cost-effective than Chalcogenide ones. However they are constrained in shape and sag height. On the other hand, Chalcogenide optics can be collectively molded and could have more free shapes. They are thus more suitable to reach high-demanding performance. So hybrid designs could be seen as a compromise between cost and performance. In this paper, we show that fast lenses with diameter constraints to few millimeters to make affordable wafer-level process lead to small size detectors. As a consequence, the pixel pitch reduction of microbolometers is a key point to maintain a good resolution. Finally, strategies to improve the production yield of hybrid lenses are explored.
The new degrees of freedom offered by the new family of freeform surfaces helps compensating for off axis aberrations in optical designs. We define a freeform surface as a non-rotationally symmetric surface which cannot be described as an off-axis part of a conicoid. The use of freeform surfaces allows an increase in performance such as on the field of view, the F-number and compactness of unobscured all reflective telescopes. Reflective designs have the advantage of being light, achromatic, and potentially low-cost due to cheaper materials involved. It could result in industrial production of compact panchromatic systems for various applications ranging from automotive sensors to imaging nanosatellites. In this paper we address the challenges posed by the use of these surfaces in optical design optimization, specifically the minimization of the surface counting, in order to simplify the manufacturing, assembly and integration phases. We focus on three mirror designs and work on the compactness with various flavors in the 3D space. We will discuss our results on a fast, unobscured, non reimaging and well corrected three mirror anastigmat (TMA) design for a nanosatellite thermal imaging payload using uncooled bolometers. Optimization method of a TMA under volume and straylight constraints is described. A tolerance analysis is performed and minimal alignment is required even for large tolerances.
Cryogenic cameras are an innovative alternative in the design of miniaturized infrared cameras using cryogenic detectors. In this presentation, we will apply this technology to design a snapshot multispectral camera for gas leak detection. A UAV compatible demonstrator in a commercial Detector Dewar Cooler Assembly (DDCA), called SIMAGAZ, has been made and tested in the TOTAL Anomaly Detection Initiatives (TADI) platform and the Esperce site of ONERA. The TADI infrastructure manages monitored gas leaks at flowrates from 0.1 g/s to 300 g/s and hosts remote sensors to test them in three scenarios: crisis-management, safety monitoring, and environmental monitoring. In Esperce site, first UAV flights with SIMAGAZ were performed. We demonstrate the ability to detect and quantify in real time the origin of methane gas leak, the flowrate and the volume of the plume with SIMAGAZ on ground or from a UAV. The core camera weights around 1kg, for around 1L footprint and a power consumption of 10W at the cooling steady state. Results from TADI and Esperce campaigns will be presented.
Multi imaging snapshot systems are used for a wide range of applications in all the spectral ranges. We propose here a study and a realization of a multi-view snapshot system using a kaleidoscope in the Long-Wave Infrared (LWIR) and compatible with uncooled infrared detectors such as microbolometers. The optical system has a high numerical aperture, a wide range of fields of view and uses a single focal plane array. We will establish here the advantages of this technology on other design strategies and especially the kaleidoscope design will be compared with the TOMBO design. Then the optical conception rules for every subset of the kaleidoscope architecture will be described and the results of a first demonstrator will be presented. The features of this system will be compared with a TOMBO-based system with a front afocal.
Nanosats or CubeSats are emerging technologies corresponding to miniaturized satellites with a wet mass between 1 and 10kg. In this study, we explored the possibility of doing earth observation in the longwave infrared. The challenge is to integrate the longest focal length as possible in a 2U volume dedicated to the optical payload. Another challenge is to use a low cost, lightweight and low power consumption microbolometer which requires high apertures optics. For these volumes, there is a competition between refractive designs, often more compact when having high apertures, and reflective designs, having a lower mass and being easily athermalized. The choice is not obvious and we studied a telephoto refractive design and an off-axis Three Mirror Anastigmat (TMA) reflective design. The key technology for the telephoto design is the use of chalcogenide glasses whereas the key technology for the TMA is the use of freeform surfaces.
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.
KEYWORDS: Sensors, Modulation transfer functions, Signal to noise ratio, Astronomical imaging, Staring arrays, Point spread functions, Laser range finders, Astronomy, Fourier transforms, Charge-coupled devices
The intrapixel response is the signal detected by a single pixel illuminated by a Dirac distribution as a function of the position of this Dirac inside this pixel. It is also known as the pixel response function (PRF). This function measures the sensitivity variation at the subpixel scale and gives a spatial map of the sensitivity across a pixel.
Over the past years, a huge interest has grown in both the scientific and the industrial communities for miniaturized and functionalized cameras featuring new capabilities such as depth estimation or multispectral imaging. As a consequence, new optical architectures such as the plenoptic camera have been proposed and studied, primarily in the visible spectrum. These cameras usually include an optical element such as a microlens array or a prism array in order to obtain multiple sub-images of a same object point on the sensor, allowing for single snapshot image refocusing or depth estimation. In the meantime, recent developments in cooled infrared focal plane arrays technology have led to smaller pixel pitch and bigger formats, thus slowly reducing the resolution gap that existed between visible and infrared cameras. This gain in resolution enables the design of more functionalized infrared imaging systems, thus answering the critical need for more features in a limited volume, especially for military applications. However, the use of cooled infrared sensors brings an additional challenge to the design of such cameras because of the specific assembly in which the sensor has to be embedded called a dewar. In this paper we explain how we overcame these constraints to design and implement three different cooled infrared cameras with single focal plane array depth estimation capabilities. We then evaluate the performance of these cameras in terms of range and precision of the depth estimation and conclude on their potential applications.
The reduction of systematic effects is necessary to improve the accuracy in imaging and astrometry. For example, in Euclid Mission which aims at carrying out accurate measurements of dark energy and quantifying precisely its role in the evolution of the Universe, systematic effects need at be controlled to a level better than 10-7 (Euclid, Science Book). To achieve this goal, a high-level of knowledge of the system point spread function (PSF) is required. This paper follows the concept-paper presented at the last SPIE conference1 and gives the recent developments achieved in the design of the test bench for the intrapixel sensitivity measurements. The measurement technique we use is based on the projection of a high spatial resolution periodic pattern on the detector using the self-imaging property of a new class of diffractive objects named continuously self-imaging gratings (CSIG) and developed at ONERA. The principle combines the potential of global techniques, which make measurements at once on the whole FPA, and the accuracy of spot-scan-based techniques, which provide high local precision.
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.
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.
We present a prototype of an infrared cryogenic camera directly integrated inside an off-the-shelf SOFRADIR's 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.
Today huge efforts are made in the research and industrial areas to design compact and cheap uncooled infrared optical systems for low-cost imagery applications. Indeed, infrared cameras are currently too expensive to be widespread. If we manage to cut their cost, we expect to open new types of markets. In this paper, we will present the cheap broadband microimager we have designed. It operates in the long-wavelength infrared range and uses only one silicon lens at a minimal cost for the manufacturing process. Our concept is based on the use of a thin optics. Therefore inexpensive unconventional materials can be used because some absorption can be tolerated. Our imager uses a thin Fresnel lens. Up to now, Fresnel lenses have not been used for broadband imagery applications because of their disastrous chromatic properties. However, we show that working in a high diffraction order can significantly reduce chromatism. A prototype has been made and the performance of our camera will be discussed. Its characterization has been carried out in terms of modulation transfer function (MTF) and noise equivalent temperature difference (NETD). Finally, experimental images will be presented.
This paper is devoted to the presentation of a new technique of characterization of the Intra-Pixel Sensitivity Variations (IPSVs) of astronomical detectors. The IPSV is the spatial variation of the sensitivity within a pixel and it was demonstrated that this variation can contribute to the instrument global error. Then IPSV has not to be neglected especially in the case of under-sampled instruments for high quality imaging and accurate photometry. The common approaches to measure the IPSV consist in determining the pixel response function (PRF) by scanning an optical probe through the detector. These approaches require high-aperture optics, high precision mechanical devices and are time consuming. The original approach we will present in this paper consists in projecting high-resolution periodic patterns onto the whole sensor without classic optics but using the self-imaging property (the Talbot effect) of a Continuously Self Imaging Grating (CSIG) illuminated by a plane wave. This paper describes the test bench and its design rules. The methodology of the measurement is also presented. Two measurement procedures are available: global and local. In the global procedure, the mean PRF corresponding to the whole Focal Plane Array (FPA) or a sub-area of the FPA is evaluated. The results obtained applying this procedure on e2v CCD 204 are presented and discussed in detail. In the local procedure, a CSIG is moved in front of each pixel and a pixel PRF is reconstructed by resolving the inverse problem. The local procedure is presented and validated by simulations.
Recent advances in miniaturization of IR imaging technology have led to a burgeoning market for mini thermalimaging sensors. Seen in this context our development on smaller pixel pitch has opened the door to very compact products. When this competitive advantage is mixed with smaller coolers, thanks to HOT technology, we achieve valuable reductions in size, weight and power of the overall package. In the same time, we are moving towards a global offer based on digital interfaces that provides our customers lower power consumption and simplification on the IR system design process while freeing up more space. Additionally, we are also investigating new wafer level camera solution taking advantage of the progress in micro-optics. This paper discusses recent developments on hot and small pixel pitch technologies as well as efforts made on compact packaging solution developed by SOFRADIR in collaboration with CEA-LETI and ONERA.
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.
InfraRed (IR) sensor systems like night vision goggles, missile approach warning systems and telescopes have an increasing interest in decreasing their size and weight. At the same time optical aberrations are always more difficult to optimize with larger Focal Plane Arrays (FPAs) and larger field of view. Both challenges can now take advantage of a new optical parameter thanks to flexible microelectronics technologies: the FPA spherical curvature. This bio-inspired approach can correct optical aberrations and reduce the number of lenses in camera conception. Firstly, a new process to curve thin monolithic devices has been applied to uncooled microbolometers FPAs. A functional 256×320 25μm pitch (roughly 1cm2) uncooled FPA has been thinned and curved. Its electrical response showed no degradation after our process (variation of less than 2.3% on the response). Then a two lenses camera with a curved FPA is designed and characterized in comparison with a two lenses camera with a flat FPA. Their Modulation Transfer Functions (MTFs) show clearly an improvement in terms of beams dispersion. Secondly, a new process to fabricate monolithic cooled flip-chip MCT-IRCMOS FPAs was developed leading to the first spherical cooled IR FPA: with a radius of 550 mm. Other radii are achieved. A standard opto-electrical characterization at 80 K of the imager shows no additional short circuit and no mean response alteration compared to a standard IRCMOS shown in reference. Noise is also studied with a black body between 20 and 30°C.
Today, both military and civilian applications require miniaturized and cheap optical systems. One way to achieve this trend consists in decreasing the pixel pitch of focal plane arrays (FPA). In order to evaluate the performance of the overall optical systems, it is necessary to measure the modulation transfer function (MTF) of these pixels. However, small pixels lead to higher cut-off frequencies and therefore, original MTF measurements that are able to extract frequencies up to these high cut-off frequencies, are needed. In this paper, we will present a way to extract 1D MTF at high frequencies by projecting fringes on the FPA. The device uses a Lloyd mirror placed near and perpendicular to the focal plane array. Consequently, an interference pattern of fringes can be projected on the detector. By varying the angle of incidence of the light beam, we can tune the period of the interference fringes and, thus, explore a wide range of spatial frequencies, and mainly around the cut-off frequency of the pixel which is one of the most interesting area. Illustration of this method will be applied to a 640×480 microbolometer focal plane array with a pixel pitch of 17µm in the LWIR spectral region.
We present a compact real-time multispectral camera operating in the mid-infrared wavelength range. Multispectral images of a scene with two differently spectrally signed objects and of a burning solid propellant will be shown. Ability of real-time acquisition will thus be demonstrated and spectra of objects will be retrieved thanks to inversion algorithm applied on multispectral images.
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 has to be compatible with both cryogenic constraints and wafer-level fabrication. For this, we take advantage of the progress in micro-optics to design a multichannel optical architecture directly integrated on the detector. This wafer-level camera uses state of art microlenses with a high sag height. The additional mass of the optics is sufficiently small to be compatible with the cryogenic environment of the Dewar. The performance of this camera will be discussed. Its characterization has been carried out in terms of modulation transfer function and 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.
Guillaume Druart, Florence de la Barriere, Nicolas Guerineau, Gilles Lasfargues, Manuel Fendler, Nicolas Lhermet, Jean Taboury, Yann Reibel, Jean-Baptiste Moullec
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). A dewar is a sealed environment where the detector is cooled on a cold plate. We show in
this paper that wide field of view imagery functions can be simply added to the dewar. We investigate two ways
of integration and make two demonstrators. The first one called FISBI consists in replacing the window by a
fish-eye lens and in integrating a lens in the cold shield. This optical system has a field of view of 180°. The
second one, called IR-Cam-on-Chip, consists in integrating the optics directly on the focal plane array. This
optical system has a field of view of 120°. The additional mass of the optics is sufficiently small to be
compatible with the cryogenic environment of the DDCA. The performance of these cameras will be discussed
and several evolutions of these cameras will be introduced too.
An InfraRed (IR) cooled camera is generally composed by an optical block (warm lenses outside a dewar) and a
detection block (a cooled focal plane array inside the dewar). A minimalist approach to design a compact and robust
camera consists in giving the dewar an imaging function by replacing the cold pupil by a Diffractive Optical Element
(DOE). In this paper we present different DOE that can be used to design the camera. We present first a pinhole camera
that validates this approach but that is limited in radiometric performances and in angular resolution. We replace then the
pinhole by a continuously self-imaging DOE, such as the diffractive axicon, to improve both the radiometric
performances and the angular resolution. Finally, the MALDA is introduced to improve the performances of the axicon.
Diffraction effects and Talbot effect under polychromatic light are exposed for such DOE and two different design rules
are derived from those effects to allow the design of a compact camera with dimensions compatible with the size of an
industrial dewar. Experimental prototypes are presented and radiometric performances are compared and show the best
performances for the MALDA.
MCT technologies under development in France address strategic operational needs. This includes better identification
range as well as lighter weight requirement, operation at higher detector temperature and cost reduction issues.
This paper describes the status of MCT IR technology in France at Leti and Sofradir. A focus will be made on hot
detector technology for SWAP applications.
Solutions for high performance detectors such as dual bands or megapixels will be discussed. In the meantime, the
development of avalanche photodiodes, integrated optics, or TV format with digital interface is key to bring customers
cutting-edge functionalities.
Today, both military and civilian applications require miniaturized and cheap optical systems. To reduce their size and their mass, imaging systems have to be as simple as possible, which means that they have to involve a minimal number of optical elements. The simplest system can be defined as a system which is composed of only three elements: a single optical component, an aperture stop and a detector. However, these elements can be complex if needed: for instance, curved detector, optics with aspheric surfaces or diffractive optical elements, microlens array with a complex shape... This paper aims at presenting the range of optical architectures available for a simple system. Thanks to the formalism of third-order Seidel aberrations, several strategies of simplification and miniaturization of optical systems are examined. This approach leads to a classification of existing miniaturized imaging systems which are described in literature (such as multichannel systems). Figures of merit are also introduced to assess the performance capabilities of such systems, showing the necessary trade-off between simplicity, miniaturization, and optical performance.
Guillaume Druart, Florence de la Barriere, Nicolas Guérineau, Joel Deschamps, Manuel Fendler, Nicolas Lhermet, Jacques Rulliere, Serge Magli, Yann Reibel, Jean-Baptiste Moullec
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.
We have recently shown that dewar-level integration of optics is a promising way to develop compact IR cameras.
Indeed, the integration of optics into the dewar leads to simple and entirely cooled optical architectures dedicated to
imaging applications with large-field of view. Here, we review the optical elements we could add in those devices to
make a hyper- or multispectral imager. Among them, we find specific focal-plane arrays with a built-in spectrometry
function, plasmonic filters combined with a multichannel optical design, and birefringent interferometers. Several optical
architectures will be detailed with first experimental results.
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.
Today's 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 (IRFPA's) 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 IRFPA's 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.
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.
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.
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.
When illuminated by a plane wave, continuously self-imaging gratings (CSIGs) produce a field whose intensity
profile is a propagation- and wavelength-invariant biperiodic array of bright spots. In the case of an extended
and incoherent source, we show that CSIGs produce multiple images of the source. The fundamental properties
of these gratings will be derived. In particular, methods to assess the image quality in angle of CSIGs will be
introduced. It turns out that this new type of pinhole-array camera works on the same principle as diffractive
axicons, which are known to produce wavelength-invariant nondiffracting beams. The formalism developed for
CSIGs will be also extended to axicons. CSIGs and axicons both produce focal lines and can be robust in field, in
compensation of a trade-off with the resolution. They also offer interesting properties in terms of compactness,
achromaticity and long depth of focus for imaging systems. However, compared to classical imaging systems,
they produce degraded images and an image processing is necessary to restore these images. Experimental images
obtained with these components in the visible and infrared spectral ranges will be presented.
We describe an experiment that allows distant users to perform a labwork using Erbium Doped Fiber Amplifier (EDFA) in order to understand the basics physics and engineering involved. The EDFA and the measurement instrumentation are specially designed so as to allow for remote control through the web. The purpose of the project can then be distant learning for students from developing countries which cannot afford this kind of high-cost equipment.
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