PISTIL interferometry (PISton and TILt interferometry), dedicated to the measurement of regular segmented wave surfaces such as segmented mirrors or the near-field output of coherent combined lasers, consists on making replicas of the central part of the wavefronts of each surface segment interfere with its nearest neighbors. The resulting interference pattern is made of an apposition of surfaces containing two-wave interference whose phase, frequency and orientation provide direct information on the respective heights and tilts of the segments in relation to each other. This technique has evolved into a new device called Full Light PISTIL. More compact and more efficient, this new version overcomes the drawbacks of previous PISTIL interferometer version by taking into account the whole surface of the incident wavefront. The performance obtained, estimated on a segmented mirror, is state of the art, and points the way towards a plug and play product adapted to the specific measurement needs of these surfaces, even in low light conditions.
The aim of this lecture is to show how the consideration of very specific measurement requirements, related to segmented surfaces and metasurfaces, has enabled a generalist multilateral shearing interferometer to evolve into new innovative devices. This has enabled state-of-the-art performance to be achieved, in particular by exploiting in the design a priori knowledge of the objects to be measured. Performance is illustrated here by a comparison of measurements on a canonical object, a PTT-111 deformable mirror from the company Iris AO, with a reference Phase Shifting Interferometer from the community.
Meta-optics allow the realization of new optical functions that are increasingly complex to realize and characterize locally. This is why we propose an interferometric method of systematic wavefront metrology of the meta-elements constituting a metasurface. This technique will allow the design of a library of nano-antennas, characterized in phase and amplitude. Once constituted, this library will allow the design of more complex optical functions. Tested for MIM (Metal-Isolating-Metal) metasurfaces, this technique can be applied to all metasurfaces.
Infrared focal plane arrays are widely used for astronomical observations and are constantly optimized to provide higher-quality images. The knowledge of the pixel spatial response becomes critical to extract ever more data from scientific missions. A specific cryogenic spot scan objective with an f-number of 1.2 has been developed in order to finely characterize 15 μm (or smaller) pixel pitch cooled infrared focal plane arrays in the spectral band between 3-5 μm. It will be operated in the MIRCOS test bench with different narrow-band filters. We present the first optical characterization of this objective obtained with a cryogenic wavefront analyzer – a quadrilateral shearing interferometer. The measured wavefront indicates a peak-to-valley amplitude of 2 μm, which corresponds to an optical quality of about λ/2 at 3.75 μm working wavelength. This value higher than expected might be due to a non-uniform cooling of the objective.
PISTIL (PISton and TILt) interferometry is a segmented wavefront metrology technique that can fulfill the role of being an independent phase analyzer for tiled laser arrays used in coherent beam combining (CBC). It presents a plug-and-play characteristics enabling others research or industrial applications such as metrology of segmented mirrors, MOEMS or measurement standards. It can operate onto complex optical benches. Alongside the PISTIL concept, we developed methods for phase extraction and meta-analysis, with best accuracy to rightfully address an end user needs in term of segmented wavefront diagnosis. We demonstrate those functionalities onto the HIBISCUS optical testbed equipped with a segmented mirror, specifically designed test data analysis pipelines and improve the control-command based on PISTIL wavefront analysis. In the current configuration, it can emulate CBC near field piston and tilt variations.
PISTIL (Piston and Tilt) is a recent interferometric system that computes the absolute piston and tip/tilt map of a segmented wavefront. Its high precision makes it usable as a metrology tool for wavefront sensing of coherently-combined laser arrays for example. This interferometer needs to correctly address high dynamic piston sensing, while dealing with fringes wrapping that leads to ambiguous phase estimations. We derived a mathematical combination for two measurements at different wavelengths and did a technical demonstration of it, using a IRIS-AO PTT111 Deformable Mirror as a segmented wavefront generator. We have verified that the loss of accuracy is slightly increased for a larger piston compared to a previous study, and we got a standard error of λ/160 with a Peak-to-valley of λ/50. This technique could be extended to a broader spectrum.
We use the chromaticism induced by the resonance of Metal-Insulator-Metal structure and the interaction surface which is greater than the geometrical surface of the nano-rod to combine locally different dimensions of nano-antennas in order to create two optical functions on the same surface. The purpose is to create a phase mask that spread a point source reflected by our system so that at 3 μm this point is horizontally duplicated and vertically at 5 μm. Thus our device changes color information into shape information which permitted another way to differentiate infrared signature of objects.
Type-II InAs/GaSb superlattice (T2SL) has recently matured into a commercially available technology addressing both MWIR and LWIR spectral domains. As the prerequisites such as Quantum Efficiency (QE) and dark current were met, more advanced figures of merits related to the ElectroOptic (EO) system as a whole can now be studied in order to position this technology. In this paper, we focus on modulation transfer function (MTF) measurements. Knowing the MTF of a detector is indeed of primary importance for the EO system designers, since spatial filtering affects the system range. We realized MTF measurements on a 320x256 MWIR T2SL FPA provided by IRnova, using a Continuously Self Imaging Grating (CSIG). The advantage of this experimental configuration is that no high performance projection optics is required. Indeed, the CSIG exploits the self-imaging property (known as Talbot effect) to project a pattern with known spatial frequencies on the photodetector. Such MTF measurements have never been done in Integrated Detector Dewar Cooler Assembly (IDDCA) configuration, so we had to study the effect of the vibrations induced by the cryocooler. Vibrations indeed affect the MTF measurement in the same way electrical diffusion would do. Using three accelerometers we optimized our experimental setup and extracted MTF measurements with reduced vibrations. The pixel size is 26μm for a pitch of 30μm.
Infrared focal plane arrays (IRFPA) are widely used to perform high quality measurements such as spectrum acquisition at high rate, ballistic missile defense, gas detection, and hyperspectral imaging. For these applications, the fixed pattern noise represents one of the major limiting factors of the array performance. This sensor imperfection refers to the nonuniformity between pixels, and is partially caused by disparities of the cut-off wavenumbers. In this work, we focus particularly on mercury cadmium telluride (HgCdTe), which is the most important material of IR cooled detector applications. Among the many advantages of this ternary alloy is the tunability of the bandgap energy with Cadmium composition, as well as the high quantum efficiency. In order to predict and understand spectral inhomogeneities of HgCdTe-based IRFPA, we propose a modeling approach based on the description of optical phenomena inside the pixels. The model considers the p-n junctions as a unique absorbent bulk layer, and derives the sensitivity of the global structure to both Cadmium composition and HgCdTe layer thickness. For this purpose, HgCdTe optical and material properties were necessary to be known at low temperature (80K), in our operating conditions. We therefore achieved the calculation of the real part of the refractive index using subtracti
Coherent beam combining of fiber amplifiers provides an attractive mean of reaching high power laser. In an interferometric phase measurement the beams issued for each fiber combined are imaged onto a sensor and interfere with a reference plane wave. This registration of interference patterns on a camera allows the measurement of the exact phase error of each fiber beam in a single shot. Therefore, this method is a promising candidate toward very large number of combined fibers. Based on this technique, several architectures can be proposed to coherently combine a high number of fibers. The first one based on digital holography transfers directly the image of the camera to spatial light modulator (SLM). The generated hologram is used to compensate the phase errors induced by the amplifiers. This architecture has therefore a collective phase measurement and correction. Unlike previous digital holography technique, the probe beams measuring the phase errors between the fibers are co-propagating with the phase-locked signal beams. This architecture is compatible with the use of multi-stage isolated amplifying fibers. In that case, only 20 pixels per fiber on the SLM are needed to obtain a residual phase shift error below λ/10rms. The second proposed architecture calculates the correction applied to each fiber channel by tracking the relative position of the interference finges. In this case, a phase modulator is placed on each channel. In that configuration, only 8 pixels per fiber on the camera is required for a stable close loop operation with a residual phase error of λ/20rms, which demonstrates the scalability of this concept.
Infrared Focal Plane Arrays (FPA) are increasingly used to measure multi- or hyperspectral images. Therefore, it is crucial to control and modelize their spectral response. The purpose of this paper is to propose a modeling approach, adjustable by experimental data, and applicable to the main cooled detector technologies. A physical model is presented, taking into account various optogeometrical properties of the detector, such as disparities of the pixels cut-off wavelengths. It describes the optical absorption phenomenon inside the pixel, by considering it as a stack of optical bulk layers. Then, an analytical model is proposed, based on the interference phenomenon occurring into the structure. This model considers only the three major waves interfering. It represents a good approximation of the physical model and a complementary understanding of the optical process inside the structure. This approach is applied to classical cooled FPAs as well as to specific instruments such as Microspoc (MICRO SPectrometer On Chip), a concept of miniaturized infrared Fourier transform spectrometer, integrated on a classical Mercury-Cadmium-Telluride FPA, and cooled by a cryostat.
Sysiphe is an airborne hyperspectral imaging system, result of a cooperation between France (Onera and DGA) and
Norway (NEO and FFI). It is a unique system by its spatial sampling -0.5m with a 500m swath at a ground height of
2000m- combined with its wide spectral coverage -from 0.4μm to 11.5μm in the atmospheric transmission bands. Its
infrared component, named Sieleters, consists in two high étendue imaging static Fourier transform spectrometers, one
for the midwave infrared and one for the longwave infrared. These two imaging spectrometers have very close design,
since both are made of a Michelson interferometer, a refractive imaging system, and a large IRFPA (1016x440 pixels).
Moreover, both are cryogenic and mounted on their own stabilization platform which allows at once to actively control
and independently measure the line of sigh. These data are useful to reconstruct and to georeference the spectral image
from the raw interferometric images. Sysiphe first flight occurred in September, 2013.
The multispectral imaging technique consists of imaging a given scene at various wavelengths of
interest, each one containing a different spectral information. By analyzing this spectral content,
the chemical species that are present can be localized on the image and identified by reconstructing
their spectral signature. In this way, following Ebbesen's seminal work in plasmonics
[1], purely metallic or hybrid metallodielectric structures [2, 3] seem to be ideal candidates to
perform spectral filtering due to their extraordinary transmission efficiency [4] and polarization
selectivity. Moreover, their compact feature makes it possible for them to gather in wide arrays
of filters that, once integrated into a cooled infrared camera, can achieve real-time multispectral
imaging [5].
As seen in Figure 1.d. the spectral signature reconstruction of a chemical species strongly
depends on the number of filters and their transmission spectra for the designed matrix. In
order to improve the multispectral camera, a complementary approach consists of changing the
filter design to realize a tunable filter whose spectral shape can be adjusted in real time according
to the imaged scene. We focused our attention on the superposition of subwavelength gratings
which seems to be a structure of great potential for multispectral imaging applications [6, 7].
In ground based astronomy, mainly all designs of sky survey telescopes are limited by the requirement
that the detecting surface is flat whereas the focal surface is curved. Two kinds of solution have been
investigated up to now. The first one consists in adding optical systems to flatten the image surface; however this
solution complicates the design and increases the system size. Somehow, this solution increases, in the same
time, the weight and price of the instrument. The second solution consists in curving artificially the focal surface
by using a mosaic of several detectors, which are positioned in a spherical shape. However, this attempt is
dedicated to low curvature and is limited by the technical difficulty to control the detectors alignment and tilt
between each others.
Today we would like to propose an ideal solution which is to curve the focal plane array in a spherical shape,
thanks to our monolithic process developed at CEA-LETI based on thinned silicon substrates which allows a
100% optical fill factor. Two infrared uncooled cameras have been performed, using 320 x 256 pixels and 25 μm
pitch micro-bolometer arrays curved at a bending radius of 80 mm. These two micro-cameras illustrate the
optical system simplification and miniaturization involved by curved focal plane arrays.
Moreover, the advantages of curved detectors on the optical performances (Point Spreading Function), as well as
on volume and cost savings have been highlighted by the simulation of the opto-mechanical architecture of the
spectrometer OptiMOS-EVE for the European Extremely Large Telescope (E-ELT).
Optical sub-wavelength structures allow to code space-varying complex transmittance functions that induce both amplitude and phase variations on a given wavefront at the micrometer scale. This paves the way to the miniaturization of optical devices based on the spatial coding of the complex transmittance. We describe here a dedicated setup in the infrared range (3-14,um) for the spatial and spectral characterization of such components. The setup combines (i) a quadri-wave lateral shearing interferometer, which enables a two dimensional measurement of phase and amplitude, and (ii) a Fourier Transform Infrared Interferometer for spectral resolution. We present both theoretical simulations and experimental results of the characterization of
subwavelength gratings.
Imaging static Fourier transform spectrometers are well-adapted instruments for remote sensing. However, the path from the raw images to the spectral images is quite long, thus multiplying the potential noise sources. In this article, we propose to review these perturbation sources. We first recall how detector or photon noise on the interferogram affects the estimated spectrum. Then, we focus on inhomogeneities of the focal plane array. Although these inhomogeneities are deterministic, they generate noise when they are ill-corrected. Finally, we describe defects due to errors that may occur during the image processing step. We also give some avenues to limit the impact of these noises.
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.
In 2010, the existence of a zero of transmission at high wavelengths different from the Rayleigh Wood anomaly
was highlighted when two identical subwavelength gratings are brought close enough. We recently revealed
the origin of this transmission extinction and the mechanism is recalled here. Furthermore, we present an
experimental setup to measure the amplitude of this extinction and study its spectral behavior when changing
geometrical parameters which represent a real technological challenge. This way a new generation of tunable
filters in the mid-infrared with perfect spectral shape control and high rejection efficiency can be designed with
practical use to gas sensing applications.
We will present a brief overview of the interest in subwavelength gratings for spectral filtering in the mid-infrared wavelength range. Guided-mode, plasmonic and dipolar resonances will be considered. We will particularly focus on components fabricated in our laboratories, achieving band-pass or cut-band filtering. Optical characterization will be shown, revealing resonances with high quality factors. Multispectral camera has been realized by integrating our components into a cooled infrared focal plane array.
In this paper, we present the design of a very precise collimated fiber array that meets requirements for beam combining. Calculations permit to determine the tolerances toward key parameters and specify the components to manufacture. Thus, the collimated fiber array is composed of a high quality commercial microlens array and an especially dedicated fiber holder that we design and realize experimentally. Manufacture techniques for both the microlens and the holder are chosen to be collective and then compatible with a high number of fibers. With the collimated fiber array hence obtained, the individual beam quality was measured to be λ/10 and the pointing accuracy is under 0.6 mrad.
The traditional design of optical systems is severely complicated by the curved shape of the image surface which
has to be recorded on a planar retina. This constraint decreases the image quality; optical elements are then
added to avoid aberrations and lead to increase the dimensions of the system. However, miniaturization could be
achieved, without decreasing resolution and sensibility, by recording the image surface on a curved retina. The
optical advantages of curved sensors have been demonstrated; the simplification leads to scale down the entire
system. Moreover, the hemispherical shape increases the field of view (FOV).
In this paper the advantages of curved focal plane will be detailed through two applications: spectrometry and
large FOV telescopes. In astronomy, large FOV and miniaturization with good resolution can only be achieved
by curving the focal plane; the difficulty is to curve in a hemispherical shape large detectors. The advantages are
highlighted by the European Extremely Large Telescope (E-ELT) project.
Despite this high interest in curved detectors, only few articles are dedicated to this hemispherical shape
technology. Some solutions exist, which mainly consist in structuring the die in sub-devices. We propose a
solution to curve an IR sensor with a fill factor equal to 100%. To do so, we developed a dedicated bonding
process which allows curving silicon using its mechanical properties. A curved uncooled infrared detector has
been performed without mechanical and electrical damage.
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.
KEYWORDS: Long wavelength infrared, Mercury cadmium telluride, Avalanche photodetectors, Monte Carlo methods, Ionization, Mid-IR, Avalanche photodiodes, Signal to noise ratio, Temperature metrology, Mercury
We report the performances of LWIR (λc = 9.0 μm at 80K) HgCdTe electron injected
avalanche photodiodes (e-APD). In these devices, the exponential gain curve, up to gains
equal to 23 at 100K, and the low excess noise factor close to unity (F ~ 1-1.25) are indicative
of a single carrier multiplication process, which is electron impact ionization. The dark
current is mainly due to a diffusion current at low reverse bias and tunneling currents at high
reverse bias. A Monte Carlo model has been developed for understanding the multiplication
process in Hg1-xCdxTe e-APDs. We find a good agreement between first simulation results
and experimental measurements of the gain and the excess noise factor in both MWIR (x =
0.3) and LWIR (x = 0.235) e-APDs at 80K. Furthermore, simulations do not show any heavy
hole impact ionization. This model which enables to perform phenomenological studies aims
at identifying the main physical and technological parameters that influence the gain and the
excess noise. In the present work, it is used to study the influence of the thickness of the ndoped
region on the gain and the excess noise factor. We found that F still decreases while the
thickness of the n- layer decreases. However, an optimum thickness of the n- layer exists
around 1μm in terms of gain-voltage characteristic.
We present the application of Quadri-Wave Lateral Shearing Interferometry (QWLSI), a wave front sensing technique,
to characterize optical beams at infrared wavelengths from 2 to 16μm with a single instrument. This technique can be
used to quantify the quality of optical systems (like thermal infrared lenses) by measuring their aberrations. It can also
evaluate laser sources in the infrared range like some gas lasers (HeNe laser or CO2 laser), infrared Optical Parametric
Oscillator laser sources or Quantum Cascade Laser sources. In all cases, QWLSI offers the crucial advantage that it
yields an analyzed wave front without the use of a reference arm and consequent time consuming alignment. In this
paper, we first present the single interferometer which can be used with wavelength within 2 and 16μm, covering this
way the MWIR (λ within 3 and 5μm) and LWIR (λ within 8 and 14μm) ranges. We then present the characterization of
two gas lasers: an infrared HeNe lasers (λ=3.39μm) and a CO2 laser (λ=10.6μm) with this instrument. We finally show
the experimental analysis of an infrared lens at two different wavelengths, one in the MWIR range (λ=3.39μm) and the
other in the LWIR range (λ=10.6μm). λ
We present the application of Quadri-Wave Lateral Shearing Interferometry (QWLSI), a wave front sensing technique, to characterize thermal infrared lenses for wavelengths within 8 and 14μm. Wave front sensing is not only a tool to quantify optical quality, but also to map the local (dust, scratches) or global possible defects. This method offers the crucial advantage that it yields an analyzed wave front without the use of a reference arm and consequent time consuming alignment. Moreover thanks to the acceptance of QWLSI to high numerical aperture beams, no additional optics is required. This makes lens characterization convenient and very fast. We particularly show the experimental characterization of single Germanium lens and finally present the characterization of complex optical imaging systems for high-performance infrared cameras. The analysis is made in conditions that are very close to the usual conditions of the camera use; that is to say, directly in the convergent beam and in polychromatic (black body) light.
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.
Sub-wavelength gratings allow to code complex transmittance functions that introduce both amplitude and
phase variations in the propagation of a given wavefront. These micro-structures are a promising technique
to miniaturize optical functions such as light polarizing, light confinement, spectral filtering... Realizations in
the visible and the infrared domain have been fulfilled: for example micro-lenses, anti-reflection coatings or
sinusoidal-transmittance can easily be coded. This technique is all the more advantageous in the mid-wavelength
infrared (MWIR) or long-wavelength infrared (LWIR) spectral range since there are only a few materials available
in this spectral range. However the characterization of these structures is problematical, since it involves phase
and amplitude measurements. It is even more complicated in the far infrared domain (8 - 14 μm), as will be
detailed. Besides, the finite size of the gratings introduces phase steps, which is well-known to be a problematic
issue. We describe here a dedicated bench to characterize sub-wavelength gratings in the LWIR spectral range.
The core of the bench is a quadri-wave lateral shearing interferometer based on a diffraction grating, which allows
a complete two-dimensional characterization of both phase and amplitude in a single measurement. We present
here theoretical and experimental results of a characterization of such a sub-wavelength grating.
Lateral shearing interferometers (LSIs) are efficient tools for optical analysis. They allow classical optical wave-front
aberrations measurements as well as the precise evaluation of abrupt steps. The basic element of an LSI
is the transmittance grating, which diffracts a number of orders (two in the case of a mono-dimensional LSI,
ideally three or four non coplanar orders in the case of bi-dimensional LSI). This brings the need for specifically
designed transmittance gratings. For instance, a mono-dimensional LSI needs a sinusoidal-shaped transmittance,
since its Fourier transform carries exactly 2 orders. Such transmittances are however either impossible or at least
extremely costly to design using classical macroscopic techniques, mainly because the usual thin film deposition
techniques require several technological steps, in order to get the desired light filtering effect.
Given these constraints, we made use of sub-wavelength structures in order to build a new class of LSI. They
are made of sub-wavelength lamellar metallic gratings specifically designed for the mid-infrared, and allow the
precise coding of the desired transmission shape all over the LSI grating.
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.
A Monte Carlo model is developed for understanding the multiplication process in HgCdTe infrared avalanche
photodiodes (APD). A good agreement is achieved between simulations and experimental measurements of gain and
excess noise factor on midwave infrared electron injected Hg0.7 Cd0.3Te APD manufactured at CEA/LETI. In both cases,
an exponential gain and a low excess noise factor - close to unity out to gains greater than 1000 - were observed on 5.1-μm cut-off devices at 77K. These properties are indicative of a single ionizing carrier multiplication process that is to say
in our case the electron. Simulations also predict that holes do not achieve enough energy to impact ionize and to
contribute to the gain, which confirms the previous observation. A comparison study is presented to explain the effect of
different combinations of scattering processes on the avalanche phenomenon in HgCdTe. We find that alloy scattering
with random scattering angle increases multiplication gain and reduces excess noise factor compared to the case
including impact ionization only. It also appears that, in the more complete scattering environment, optical phonon
scattering delays significantly the onset of avalanche.
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.
The need for segmented wave front measurements has been rocketing for several years. The applications are
various: thickness of metallic masks, diffracting elements, phasing of the primary segmented mirrors of telescopes,
such as the Keck telescope, laser beam coherent recombination... Lateral shearing interferometers are common
wave front sensors, used with success to test classical optical components. This technique does not require a
reference wave, which is a major advantage. The lateral shearing interferometry has also proved successful to
analyze segmented wave front; results of such a measurement by a diffraction-grating based interferometer are
presented and analyzed. We dwell upon quadri-wave lateral shearing interferometers (QWLSI), which offer the
possibility to characterize two-dimensionally the wave front, in a single measurement. This technique combines
accuracy and qualities such as compactness and simplicity. Moreover, a chromatic regime of lateral shearing
interferometers based on diffraction grating can be pointed out; this allows a two-color analysis to greatly extend
the dynamic range. In the first parts we will present general considerations on QWLSI and segmented surface;
then a technique to increase the dynamic range is investigated both theoretically and experimentally.
We propose a novel interpretation of stray light, which is modeled using a scattering process. For this, a description and simulations based on the bidirectional scattering distribution function (BSDF) are developed. We focus on the particular example of a window closing an optical cavity. Such a window is known to introduce a stray light component that is experimentally very subtle to handle. Our idea is motivated by the uncomfortable observation that for years, the usual estimation of this parasitic contribution (be it for absolute or relative measurements, for visible or IR applications, for field or laboratory experiments, etc.) was based on the rather imprecise and obviously incorrect assumption (nevertheless today a widely used rule of thumb) that the optical window can be approximated by a graybody emitting in its transparency range. This statement obviously goes against Kirchhoff's law of radiation, and can thus not be physically sound. Various typical experimental situations are explored with our model, and a comparison is made with results given by the rule of thumb. This shows some cases of relative concordance. However, the scatter-based model should provide an individually more accurate estimation, as shown by some examples.
Based on multi-lateral shearing interferometry, a powerful technique, called the Quadri-Wave Lateral Shearing Interferometer (QWLSI) is used to evaluate the wavefront in an accurate and precise way. Our device can be used for the characterization of complex and very aberrant optical devices, the control of optical components and also for laser beam evaluation. This communication will detail the response of the QWLSI and its metrological performances, such as its high resolution, its adjustable sensitivity and dynamic. It will then be focused on two innovative applications of the QWLSI. The first application concerns the evaluation of infrared lenses dedicated to high-performance cameras. We will present experimental results recently completed by our prototype dedicated to the LWIR domain (λ=8-14μm). In a second part, we will study the possibility to analyze wave-fronts with discontinuities. Such wave-fronts can be produced by segmented mirrors, diffractive components or also bundle of single-mode fibers. We will finally present simulation results for this latter application.
Subwavelength metallic structures are used to design gratings with a great variety of transmittance levels. Such gratings can answer growing needs for complex transmittance devices, particularly useful for wave-front analysis applications. Having in mind the conception of a perfectly sinusoidal transmittance for the mid-infrared, we have decided to test the ability of subwavelength lamellar gratings to code the transmittance with several levels. In order to calibrate gratings transmission, as a function of the fill factor, we have designed, realized and measured samples made of six 2mm x 2mm gratings, with transmittance ranging from 10% to 95%. Experimental results for TM- and TE-polarized light are reported and analysed.
Based on lateral shearing interferometry, a powerful technique, called the Quadri-Wave Lateral Shearing Interferometer (QWLSI), is used to evaluate the wave-front distortions with a high accuracy. Our device can be used for the characterization of complex and very aberrant optical devices, the testing of optical components, the control of adaptive optics and also for laser beam evaluation. The particular design of the QWLSI provides him interesting properties for wave-front metrology such as a high transverse resolution, a tunable sensitivity or also an evaluation of the measurement error. An innovative interferogram analysis allows also an increasing accuracy of the measurement. After dealing with these aspects, we will conclude by presenting an experimental wave-front analysis of a femtosecond laser pulse and an analysis in the far infrared spectral range [λ=8-14 μm].
Stationary Fourier transform spectrometry is a well-known concept to build reliable field or embedded spectroradiometers, especially for the mid- and far- infrared. However, the quality of the interference pattern imaged on the focal plane array is crucial to obtain a good spectrum by Fourier transform. We describe here an accurate modeling of the interferometer behavior that takes into account the instrument aberrations and field of view in order to quantitatively predict, at each wavelength and for a spatially extended uniform incoherent source, the real interferogram defects, namely, fringe distortion, fringe blurring and illumination non-uniformity. To investigate these effects, we first derive the properties of the elementary interferograms built by each source point. For this purpose, we use ray-tracing to extract optical path and vignetting information with the help of a commercial optical design software package, and we reconstruct from them the two wavefronts that hit the detector using general numerical methods with the help of standard computing tools. The whole interferogram being formed by the incoherent superposition of all elementary interferograms, we next, compute the relevant quantities by appropriate numerical quadratures. We illustrate this method with two potential layouts of a Fourier transform spectrometer that we are currently designing for accurate radiometric measurements in the 2.9μm-9.6μm range with a spectral resolution better than 8cm-1 on a 4.5°x0.6° field of view.
A powerful and achromatic interferometric technique based on recent advances in the technology of non-diffracting arrays is used to evaluate wave-front distortions with a high transverse resolution and an easily tunable dynamic range. The device presented here belongs to the family of Multiple Wave Lateral Shearing Interferometers (MWLSI) and has the natural capability to measure simultaneously four wave-front derivatives. In this communication, we propose to exploit all the information de facto included in the interferogram; in this perspective we suggest a new method of reconstruction of the wave-front knowing its derivatives. We will show that our device is already successful in the field of laser metrology in the near infrared domain and that it is promising for the far infrared domain with the presentation of a prototype dedicated to measurements at 10.6µm.
We present a quality criterion for telescopes based on the fulfillment of observation needs as defined by a client. It is intended for the pre-conception and broad control level. The criterion is built from the fidelity measure by limiting the spatial scales taken into account to the scales useful to the proper imaging of the detail of interest. By construction this mono-dimensional criterion allows trade-off between spatial and radiometric resolution. The comparison of different design strategies is also possible, for example between undersampled large aperture telescopes and well sampled smaller telescopes. It can also be used to predict the usefulness of each available telescope for a given observational purpose. Being global, the criterion requires only high-level specifications, thus allowing the client to exercise a greater degree of control over the instrument definition. We present here a pre-calibration of the mission quality criteria enabling to give an absolute quality value for the telescope and to determine whether the observation mission is fulfilled. A comparison of test-case telescopes is then made by varying several design parameters.
An infrared spectroradiometer is being developed at the ONERA to make field measurements of radiance and emissivity of terrestrial surface materials (MISTERE). The instrument, entirely cooled, is a stationary Fourier spectrometer. The principle of measurement based on the sampling of a canted interferogram by a 2D array of detectors will be detailed and the expected performances in terms of resolution, free spectral range, signal-to-noise ratio and scan rate will be presented.
This paper describes a new test bench for measuring the modulation transfer function of an infrared focal plane array. The system is based on the use of a plane target made with a continuously self-imaging grating (CSIG) that projects in polychromatic light a biperiodic pattern of small and non-diffracting spots called a nondiffracting array.
KEYWORDS: Imaging systems, Image quality, Modulation transfer functions, Signal to noise ratio, Visual system, Image analysis, Image processing, Systems modeling, Signal processing, Visual process modeling
To ensure that an imaging system supplies images that meet the needs of the observation mission, it is important to assess the imaging system performance. In this paper we propose a new quality criterion for imaging system assessment. This method is based on three parts: end-to-end analysis of the imaging system, standardized expression of the mission by an objective definition of the observation tasks and 'a priori' knowledge of the properties of the objects to be observed. This quality criterion can also be used as a tool to aid in the design of observation systems based on the properties of the objects to be observed.
This paper describes a new test bench for measuring the modulation transfer function of an infrared focal plane array. The system is based on the use of a plane target made of eight gratings that projects in polychromatic light a biperiodic pattern of small and nondiffracting spots called a nondiffracting array.
After six years of existence, Achromatic Three-Wave Lateral Shearing Interferometer (ATWLSI) is currently in use in wave-front analysis of high-intensity ultra-short laser pulses. The purpose of this paper is to compare ATWLSI to the well-known Hartmann-Shack wave-front sensor (HSWFS) for different generic applications: optical testing, atmospheric turbulence and ultra-short laser pulses analysis and correction. This qualitative comparison is made on the basis of a description of HSWFS, seen as a lateral shearing interferometer.
A test bench for measuring the Modulation Transfer Function of an InfraRed Focal Plane Array (IRFPA) is described. The system is based on the use of the self-imaging property of a periodic target made of thin slits for a direct projection of a high-resolution pattern directly on to the tested sensor. The test bench has furnished two different types of experimental results on a specific IRFPA. On one hand, a global method called Canted Periodic Target Test has been developed and applied leading to MTF evaluation up to five times the Nyquist frequency. On the other hand, a local line scanning of every pixel has been made.
An optimal imaging system must fulfill its specifications without an expensive and undue quality. However the translation of high-level human requirements into low-level system demands is not easy. As human specifications deal only with objects or scenes to be sen, the knowledge of these objects and its properties relevant to the information transfer through the imaging system is critical. As many imaging system quality criteria are based on the knowledge of second order statistical properties of scenes or objects to be imaged, the goal of this paper is to show that it is possible to extract these properties from high-level mission requirements.
The Modulation Transfer Function (MTF) is of fundamental importance in the testing of imaging systems as it is used to characterize the transfer of the spatial frequencies for the observed object. Different techniques based on the use of periodic targets made of unresolved lines or points have been proposed to assess this figure of merit for sampled imaging systems. The main potential problem in implementation of these methods is the fact that it is often difficult to insure a good balance in intensity between the individual lines or points belonging to the target. In a recent paper, we describe an analytical model allowing a first estimation of the importance of this problem. The purpose of this paper is to present this model and to apply it to the specific case of the 2D characterization of the MTF of an imaging system.
The line spread function (LSF) is a classical figure of merit used to derive the modulation transfer function (MTF) of an imaging system. However this test cannot be directly applied to cameras based on an infrared focal plane array (IRFPA), as these sampled systems exhibit aliasing effects. ONERA has proposed a new technique called generalized line spread function allowing MTF assessment for sampled imaging systems. A testing of spatial frequency filtering becomes possible until at least twice the Nyquist frequency. The purpose of this paper is to show that this method is well-adapted to IRFPA cameras. In particular, the effects of bad a priori estimation of the characteristics of the camera are discussed. Secondly, we have carried on a laboratory experiment with a widespread infrared imaging system. The purpose is to demonstrate experimentally the advantages of this new method, not only in terms of performances but also in terms of simplicity of implementation and robustness. The infrared imaging system tested is a 512 by 512 Si:Pt IR5120C Mitsubishi camera equipped with a F/1.2 Angenieux lens.
The three-wave lateral shearing interferometer is an interferometer specially designed for optical testing. It determines three non-collinear phase gradients from one single fringe pattern. From these quantities, two orthogonal derivatives and the measurement error are estimated, allowing the reconstruction of the aberrated wavefront. This new interferometer has several benefits; among them that its sensitivity and dynamics can be easily adjusted to the analyzed aberrations.
Deconvolution from wavefront sensing is a new technique of high-resolution imaging through atmospheric turbulence developed at ONERA. The capabilities of the method have been demonstrated during first laboratory experiments. A deconvolution experimental set-up was installed on the 4.2 m William Herschel telescope on La Palma in November 1990. The results obtained during these observations are presented: the astronomical sources were observed at optical wavelengths; the resolution improvement reached a factor greater than 10.
Atmospheric turbulence is a major limitation to optical high resolution imaging systems observing through the lower atmosphere. In most conditions, it limits the angular resolution to about one arcsecond, which is the resolution of a small 10 cm telescope operating in the visible. From a theoretical standpoint, its effects on beam propagation can be rather well predicted with the knowledge of atmospheric conditions and the assumption of the Kolmogorov turbulence law. Then, the practical problem turns out to be the characterization of propagation conditions and the compensation of turbulence effects.
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