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.
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°.
Historically LYNRED (created from the merger of SOFRADIR and ULIS in 2019) has used amorphous silicon materials (“a-Si”) as thermistor materials for its uncooled microbolometer products. If a-Si materials present several advantages that made the success of LYNRED’s products (easy to use and integrate in thermal camera), their intrinsic bolometric performances (i.e. TCR and 1/f noise) are still lower than the commonly used oxides thermistors[1] (i.e. VOx[2] and TiOx[3]). In order to stay in a leading position regarding sensor performances without any trade-off, LYNRED, with the support of its historical R&D partner the CEA-LETI, developed new materials. This strategy has led to new cutting edge products. At the end of 2020 a new 17 µm pixel pitch product (Pico640s[4]), with one of the highest sensor performance reported on the market (typical thermal sensitivity of 25 mK (f/1, 300K, 30Hz)), has been introduced in our product portfolio. We also launched our state of the art 12 µm product range with performances equivalent to our current 17µm product range. More generally, these developments open up new opportunities toward smaller pixel pitch. The symposium presentation and the associated article will present how we have increased the "Signal to Noise Ratio" (SNR) of our products while keeping all the elements which have been our hallmark. Special attention will be paid to NETD, stability of product characteristics during operation and manufacturing excellence. All these features were obtained only by hardware (at the pixel level) improvements without the need to use sophisticated algorithms or specific ROIC functions, in the spirit of LYNRED’s FPA products.
Thermal Image Sensor performances for high end applications can be compared using a Factor of Merit (FoM) based on NETD and thermal time constant (τth). This FoM is defined as FoM = NETD × τth and is expressed in mK.ms. Best bolometers FoM range from 500 to 720 mK.ms. When fast response is required from the detector due to short events or fast moving objects in the scene, bolometers can be limited by their τth , today around 12 ms for ULIS 17 μm components, leading to blurred images or poorer signal to noise ratio. Adjusting the bolometer τth can be achieved simply by lowering the pixel responsivity, keeping FoM globally unchanged and leading to unacceptable NETD performance. This paper discusses the bolometer key parameters and tradeoffs that can lead to improved FoM associated with drastically decreased thermal time constant. Thanks to a multi-parameter design optimization, ULIS has developed an improved 17μm pixel with an exceptional FoM of 125 mK.ms and thermal time constant below 3 ms, which is 4 times better than the state of the art. Prototypes characterization will be presented and the image quality will be compared with the one of a standard camera.
In order to evaluate the impact of technological evolutions on the spectral responsivity of microbolometer FPAs (Focal Plane Arrays) as well as to find out a way to estimate the mechanical stability of microbolometric pixel membranes, ULIS is proposing a new method to measuring the spectral response of the detector array over a large region (area of pixels) simultaneously. This is done by tweaking the standard protocol of a commercial FTIR (Fourier Transform InfraRed) spectrometer where the IR detector is replaced by the array to be measured. All the calculations (i.e. interferogram processing) are taken care of externally. We use this new set up to measure the angular spectral response of the detector array and to analyse the relationship between spectral response and mechanical behaviour of the pixel. Firstly the setup of this measurement is presented and some preliminary technical issues are outlined. Then we focus on the results obtained from the measurements on 17μm pitch pixels over a wide range of angles of incidence (from normal to 45° incidence). Finally, we share some theoretical insights on both those results and the inherent limitations of this protocol using a simple optical cavity model.
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.
The high level of accumulated expertise by ULIS and CEA/LETI on uncooled microbolometers made from amorphous
silicon enables ULIS to develop ¼ VGA IRFPA formats with 17μm pixel-pitch to enable the development of small power, small weight (SWAP) and high performance IR systems. ROIC architecture will be described where innovations
are widely on-chip implemented to enable an easier operation by the user. The detector configuration (integration time, windowing, gain, scanning direction…), is driven by a standard I²C link. Like most of the visible arrays, the detector adopts the HSYNC/VSYNC free-run mode of operation driven with only one master clock (MC) supplied to the ROIC which feeds back pixel, line and frame synchronizations. On-chip PROM memory for customer operational condition storage is available for detector characteristics. Low power consumption has been taken into account and less than 60 mW is possible in analog mode at 60 Hz and < 175 mW in digital mode (14 bits). A wide electrical dynamic range (2.4V) is maintained despite the use of advanced CMOS node. The specific appeal of this unit lies in the high uniformity and easy operation it provides. The reduction of the pixel-pitch turns this TEC-less ¼ VGA array into a product well adapted for high resolution and compact systems. NETD of 35 mK and thermal time constant of 10 ms have been measured leading to 350 mK.ms figure of merit. We insist on NETD trade-off with wide thermal dynamic range, as well as the high characteristics uniformity and pixel operability, achieved thanks to the mastering of the amorphous silicon technology coupled with the ROIC design. This technology node associated with advanced packaging technique, paves the way to compact low power system.
The high level of accumulated expertise by ULIS and CEA/LETI on uncooled microbolometers made from amorphous
silicon enables ULIS to develop ¼ VGA IRFPA formats with 17μm pixel-pitch to enable the development of small
power, small weight (SWAP) and high performance IR systems. ROIC architecture will be described where innovations
are widely on-chip implemented to enable an easier operation by the user. The detector configuration (integration time,
windowing, gain, scanning direction...), is driven by a standard I²C link. Like most of the visible arrays, the detector
adopts the HSYNC/VSYNC free-run mode of operation driven with only one master clock (MC) supplied to the ROIC
which feeds back pixel, line and frame synchronizations. On-chip PROM memory for customer operational condition
storage is available for detector characteristics.
Low power consumption has been taken into account and less than 60 mW is possible in analog mode at 60 Hz and <
175 mW in digital mode (14 bits). A wide electrical dynamic range (2.4V) is maintained despite the use of advanced
CMOS node. The specific appeal of this unit lies in the high uniformity and easy operation it provides. The reduction of
the pixel-pitch turns this TEC-less ¼ VGA array into a product well adapted for high resolution and compact systems.
NETD of 35 mK and thermal time constant of 10 ms have been measured leading to 350 mK.ms figure of merit. We
insist on NETD trade-off with wide thermal dynamic range, as well as the high characteristics uniformity and pixel
operability, achieved thanks to the mastering of the amorphous silicon technology coupled with the ROIC design. This
technology node associated with advanced packaging technique, paves the way to compact low power system.
This paper presents how to specify an ADC to digitalize the analog video of the uncooled infrared readout circuit. In a
first part the main features will be discussed to select the right resolution, SNR, THD and ENOB of the converter. In a
second part the characteristics more specifically sensitive for an ADC integrated in the readout circuit will be presented:
architecture, power consumption, electrical dynamic range, crosstalk issues. Indeed, the increasing demand for
integrated functions in uncooled readout circuits leads to on-chip ADC design as interface between the internal analog
core and the digital processing electronic. In addition this IP could be seen as an inescapable link to integrate also NUC,
BPR or all other processing functions on-chip. However specifying an on-chip ADC dedicated to focal plane array
raises many questions about its architecture and its performance requirements. We show two architectural approaches
are needed to cover the different sensor features in term of array size and frame speed. Finally we will conclude with a
trade-off between external or internal approach taking into account the application of the camera, the cost and the ADC
state of art.
The high level of accumulated expertise by ULIS and CEA/LETI on uncooled microbolometers made from amorphous silicon with 45, 35, and 25 μm enables ULIS to develop video graphics array (VGA) and extended graphics array (XGA) infrared focal plane array (IRFPA) formats with 17-μm pixel pitch to fulfill every application. These detectors keep all the recent innovations developed on the 25-μm pixel-pitch read out integrated circuit (ROIC) (detector configuration by serial link, low power consumption, and wide electrical dynamic range). The specific appeal of these units lies in the high spatial resolution it provides while keeping the small thermal time constant. The reduction of the pixel pitch turns the VGA array into a product well adapted for high-resolution and compact systems and the XGA a product well adapted for high-resolution imaging systems. High electro-optical performances have been demonstrated with noise equivalent temperature difference (NETD) < 50 mK. We insist on NETD and wide thermal dynamic range trade-off, and on the high characteristics uniformity achieved thanks to the mastering of the amorphous silicon technology as well as the ROIC design. This technology node paves the way to high-end products as well as low-end, compact, smaller formats, such as 320 × 240 and 160 × 120 or smaller.
The high level of accumulated expertise by ULIS and CEA/LETI on uncooled microbolometers made from amorphous
silicon has enabled ULIS to develop VGA IRFPA formats with 17 μm pixel-pitch, hence building up the currently
available product catalog. This detector keeps all the innovations developed on the 25 μm pixel-pitch ROIC (detector
configuration by serial link, low power consumption and wide electrical dynamic range). The specific appeal of this unit
lies in the high spatial resolution it provides. The pixel-pitch reduction turns this TEC-less VGA array into a product
well adapted for high resolution and compact systems. Electro-optical performances of this IRFPA are presented
hereafter as well as recent performance improvement. We will focus on NETD trade-off with wide thermal dynamic
range, as well as the high characteristics uniformity and pixel operability, achieved thanks to the mastering of
amorphous silicon technology coupled with the ROIC design. Solar exposure is also taken into account and shows that
ULIS amorphous silicon is perfectly well suited to sustain high intensity exposure. This technology node associated
with advanced packaging technique paves the way to compact low power system.
The high level of accumulated expertise by ULIS and CEA/LETI on uncooled microbolometers made from amorphous
silicon with 45μm, 35μm and 25μm, enables ULIS to develop VGA and XGA IRFPA formats with 17μm pixel-pitch to
fulfill every applications. These detector keeps all the recent innovations developed on the 25μm pixel-pitch ROIC
(detector configuration by serial link, low power consumption and wide electrical dynamic range). The specific appeal
of these units lies in the high spatial resolution it provides while keeping the small thermal time constant. The reduction
of the pixel-pitch turns the TEC-less VGA array into a product well adapted for high resolution and compact systems
and the XGA a product well adapted for high resolution imaging systems. High electro-optical performances have been
demonstrated with NETD < 50mK. We insist on NETD and wide thermal dynamic range trade-off, and on the high
characteristics uniformity, achieved thanks to the mastering of the amorphous silicon technology as well as the ROIC
design. This technology node paves the way to high end products as well as low end compact smaller formats like 320 x
240 and 160 x 120 or smaller.
M. Dispenza, F. Brunetti, C.-S. Cojocaru, A. de Rossi, A. Di Carlo, D. Dolfi, A. Durand, A. Fiorello, A. Gohier, P. Guiset, M. Kotiranta, V. Krozer, P. Legagneux, R. Marchesin, S. Megtert, F. Bouamrane, M. Mineo, C. Paoloni, K. Pham, J. Schnell, A. Secchi, E. Tamburri, M. Terranova, G. Ulisse, V. Zhurbenko
Within the EC funded international project OPTHER (OPtically Driven TeraHertz AmplifiERs) a considerable
technological effort is being undertaken, in terms of technological development, THz device design and integration. The
ultimate goal is to develop a miniaturised THz amplifier based on vacuum-tube principles
The main target specifications of the OPTHER amplifier are the following:
- Operating frequency: in the band 0.3 to 2 THz
- Output power: > 10 mW ( 10 dBm )
- Gain: 10 to 20 dB.
The project is in the middle of its duration. Design and simulations have shown that these targets can be met with a
proper device configuration and careful optimization of the different parts of the amplifier. Two parallel schemes will be
employed for amplifier realisation: THz Drive Signal Amplifier and Optically Modulated Beam THz Amplifier.
The high level of accumulated expertise by ULIS and CEA/LETI on uncooled microbolometers made from amorphous
silicon enables ULIS to develop VGA IRFPA formats with 17μm pixel-pitch to build up the currently available product
catalog. This detector keeps all the innovations developed on the 25 μm pixel-pitch ROIC (detector configuration by
serial link, low power consumption and wide electrical dynamic range). The specific appeal of this unit lies in the high
spatial resolution it provides. The reduction of the pixel-pitch turns this TEC-less VGA array into a product well
adapted for high resolution and compact systems. In the last part of the paper, we will look more closely at the high
electro-optical performances of this IRFPA and the rapid performance enhancement. We will insist on NETD trade-off
with wide thermal dynamic range, as well as the high characteristics uniformity, achieved thanks to the mastering of the
amorphous silicon technology coupled with the ROIC design. This technology node paves the way to high end products
as well as low end compact smaller formats like 160 x 120 or smaller.
For more than 10 years now, uncooled sensors have given new opportunities in the IR field of
applications by being able to be produce in large volume. Compared to cooled technology, uncooled
detectors offer many interesting advantages: high reliability, lower cost ... whereas the performance
is high enough for a lot of applications. Thermography, building inspection, enhanced driver vision
and military (thermal weapon sight, low altitude UAV sensor) are applications which can be
provided with affordable IR focal plane arrays...
As uncooled IR sensors are mainly dedicated to these high volume applications, any uncooled
IRFPA technology has to be able to provide high performance sensors but also to be producible in
large volume at a minimum cost.
The high level of accumulated expertise by ULIS and CEA/LETI on uncooled microbolometers
made from amorphous silicon layer enables ULIS to develop a full range of IRFPA formats from
160x120 to 1024x768 pixels with 25μm and 17μm pixel-pitch, designed for high end and high
volume applications.
The detector ROIC designs rely on a simple architecture (detector configuration addressed by a
serial link for user defined amplifier gain, windowing capability...) which enables easier systems
upgrade and therefore a reduced system development non recurrent cost.
The packaging technique depends on the application environment and the production volume in
order to fit with the market expectation. Starting from metallic and ceramics package, very
advanced new technique is under development in order to reduce uncooled IRFPA production cost.
NETD in the range of 30mK (f/1, 300K, 60Hz) as well as operability higher than 99.99%, are
routinely achieved with amorphous silicon technology.
The high level of accumulated expertise by ULIS on uncooled microbolometers TEC-less operation enables ULIS to
develop 384 x 288 (1/4 VGA) IRFPA format with 25μm pixel-pitch especially designed for TEC-less application. This
detector, while keeping all the performances and all the innovations developed on previous ULIS ROIC (NETD
performance, detector configuration by serial link, low power consumption and wide electrical dynamic range ...), can
be operated on a wide range of ambient temperature, with constant settings.
We present in this paper the electro-optical performances and the TEC-less capability of this device. The thermal
behavior is described in detail.
The high level of accumulated expertise by ULIS and CEA/LETI on uncooled microbolometers made from amorphous
silicon enables ULIS to develop 1024 x 768 (XGA) IRFPA formats with 17μm pixel-pitch to address high end, high
performance applications.
This detector has kept all the innovations developed on the full TV format readout integrated circuit (ROIC): detector
configuration by serial link, two video outputs, low power consumption, wide electrical dynamic range... The specific
appeal of this unit lies in the high image resolution it provides. The reduction of the pixel-pitch turns this XGA array
into a product well adapted for high resolution yet compact systems. In the last part of the paper, we will look more
closely at the high electro-optical performances of this IRFPA and the rapid performance enhancement. We will insist
on NETD coupled with wide thermal dynamic range, as well as the outstanding uniformity and high pixel operability,
achieved thanks to the mastering of the amorphous silicon technology coupled with the ROIC design. This technology
node paves the way to high end VGA or 1/4VGA sensors as well as large diffusion compact smaller formats like
160 x 120 or smaller.
The Laboratoire Infrarouge (LIR) of the Electronics and Information Technology Laboratory (LETI) has been involved since 1993 in the development of subsequent bolometer technological process that aims at reducing the pitch of the amorphous silicon based uncooled microbolometer FPAs. These developments are primarily driven by cost reduction and system miniaturisation concerns. In this outlook, the LIR has recently developed a specific amorphous silicon technology for a 25 μm pitch IRFPA achievement.
This new structure still relies on a single level microbridge arrangement and special cares have been taken in order to ensure noise reduction, thermal insulation increase with a special attention to low thermal time constant achievement. This paper presents a complete characterization of an advanced dedicated 320 x 240 IRCMOS circuit that takes advantage of this new 25 μm pitch bolometer process. Apart from NEDT (< 70 mK) histogram, the paper also puts emphasis on parameters that appear more and more as key points in IR system, like the thermal time constant and the residual fixed pattern noise.
Automotive security is an increasing concern which requires new visual and detection system. Infrared imaging is a very good technological tool to perform such security enhancement. LETI Infrared Laboratory, as a IR technology designer, is a partner in the studies and development of such system for vision enhancement or security system analysis. Thus, we have developed two behavioural models of infrared focal plane array detectors: one in the Short Wave IR and the other in the Long Wave IR band. These detector models are used either in simulation platform aimed at evaluating the impact and use of infrared sensors in automotive and aeronautic applications or in order to test image processing algorithm on virtual component up to come. This work is a part of European Commission projects.
The SWIR detector is calibrated with a 320x240 HgCdTe cooled FPA component from SOFRADIR and the LWIR one with an uncooled micro-bolometer array from ULIS (aSi:H microbolometer technology from LETI). The flexibility of the models permits to simulate cameras based on these component or new ones, using different read-out circuit or detector technologies.
In this paper we present the detector models and its achievement through electro-optical measurements and their applications.
To simulate an Enhanced Vision System (EVS), CEA/LETI Infrared Laboratory has developed two behavioural models of infrared focal plane arrays : one in the Short Wave IR and the other in the Long Wave IR band. These Infrared Focal Plane Arrays (IRFPAs) models will be implemented on simulation platform aimed at evaluating the impact and use of infrared sensors in automotive and aeronautic applications. To be realistic, model parameters are extracted from electro-optical characterization of real components. The SWIR detector is calibrated with a 320x256 HgCdTe cooled FPA component from SOFRADIR, and the LWIR one with an uncooled micro-bolometer array from ULIS (a_Si technology from LETI). The flexibility of the models allows to simulate cameras based on these components and to forecast future ones based on different read-out circuit or detector technologies.
In this paper we present the IRFPAs models, the main electro-optical characterization results and we compare some experimental measurements with simulations.
In this paper we present the first demonstration at LETI infrared laboratory of a megapixel HgCdTe MWIR focal plane array with a 15μm pitch. The detectors were interconnected by indium bumps to the CMOS readout circuit. The design of these interconnections has been adapted from the standard CEA-LETI process to achieve resolution and uniformity required by the reduced pitch: the indium bumps scale 12μm width and 7μm height. Because of the important mechanical constraints induced by the size of the component, specific developments were necessary in order to achieve the hybridization process with an extremely reduced amount of defaults. The readout circuit was designed in a 3.3V /0.35μm CMOS technology. Its main features were to allow the validation of the hybridization and technological processes. A Megapixel IRCMOS has been mounted in a dewar and fully characterized at 77K exhibiting excellent electro-optical performances and an operability greater than 99.8%.
A test bench has been developed at the ONERA in order to measure the spectral responses of infrared focal plane arrays. This test bench can deliver hyperspectral cartographies with rather good resolutions (better than 16 cm-1) on large spectral ranges (from 1.3 μm to 20 μm). The principle of this test bench will be described and experimental results obtained with a 320x240 uncooled microbolometer array will be presented. As a conclusion, the ability of uncooled microbolometer arrays to make spectral measurements will be discussed.
In this paper we present the first demonstration at LETI infrared laboratory of a megapixel HgCdTe MWIR focal plane array with a 15μm pitch. The detectors were interconnected by indium bumps to the CMOS readout circuit. The design of these interconnections has been adapted from the standard CEA-LETI process to achieve resolution and uniformity required by the reduced pitch. Because of the mismatch of thermal dilatation coefficients between the substrate and the HgCdTe, specific developments were necessary in order to achieve the hybridization process with an extremely reduced amount of defaults. The readout circuit was designed in a 3.3V/0.35μm CMOS technology. Its main features were to allow the validation of the hybridization and technological processes. A Megapixel IRCMOS has been fully characterized at 77K exhibiting excellent electro-optical performances and an operability greater than 99.8%.
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