Since the recent losses of several atmospheric instruments with good vertical sampling capabilities (SAGE II, SAGE III, GOMOS, SCIAMACHY, MIPAS), the scientific community is left with very few sounders delivering concentration profiles of key atmospheric species for understanding atmospheric processes and monitoring the Earth's radiative balance. The situation is so critical that, less than five such instruments will be on duty (most probably only 2 or 3) at the horizon 2020, whereas their number topped more than 15 in the years 2000. In parallel, recent inter-comparison exercises among the climate chemistry models (CCM) and instrument datasets have shown large differences in vertical distribution of constituents (SPARC CCMVal and Data Initiative), stressing the need for more accurate vertically-resolved data at all latitudes. In this frame, the Belgian Institute for Space Aeronomy (IASB-BIRA) proposed a small mission called ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere), which is currently in preliminary design phase (phase B according to ESA standards). Taking advantage of the good performances of the PROBA platform (PRoject for On-Board Autonomy) in terms of pointing precision and accuracy, onboard processing ressources, and agility, the ALTIUS concept relies on a hyperspectral imager observing limbscattered radiance and solar/stellar occultations every orbit. The objective is twofold: compared to scanning instruments, the imaging feature allows to better assess the tangent height of the sounded air masses (through easier star tracker information validation by recognition of scene details), while its spectral capabilities will be good enough to exploit the characteristic signatures of many molecular absorption cross-sections (O3, NO2, CH4, H2O, aerosols,⋯). The payload will be divided in three independent optical channels, associated to separated spectral ranges (UV: 250-450 nm, VIS: 440-800 nm, NIR: 900-1800 nm). This approach also offers better risk mitigation in case of failure of one channel. In each channel, the spectral filter will be an acousto-optical tunable filter (AOTF). Such devices offer reasonable étendue with good spectral resolution and excellent robustness and compactness and TeO2-based AOTF's have already been used in space missions towards Mars and Venus (MEX and VEX, ESA). While such TeO2 crystals are common in VIS-NIR applications, they are not transparent below 350 nm. Recent progresses towards UV AOTF's have been made with the advent of KH2PO4-based (KDP) filters. Through collaboration with the Moscow State University (MSU), several experiments were conducted on a KDP AOTF and gave confidence on this material. Here, we present the general concept of ALTIUS and its optical design with particular attention on the AOTF. Several results obtained with optical breadboards for the UV and VIS ranges will be exposed, such as the O3 and NO2 absorption cross-section measurements, or spectral images. These results illustrate the spectral and optical performances to be expected from an AOTF-based hyperspectral imager. Their implications for the ALTIUS mission will be discussed.@en

Diffraction of optical waves by an acoustic grating is a well-known phenomenon that enables the design of very versatile devices useful in photonic systems. For example, Acousto-Optic Tuneable Filters (AOTFs) can be dynamically tuned by radio-frequency signals. Among possible material choice, tellurium dioxide crystal is often used for practical applications due to its high efficiency. In such a birefringent material, the anisotropic configuration is often used. A feature of this configuration is the sensitivity to optical input polarisation: a selective coupling between polarized modes occurs. The incident must be polarised and the diffracted mode polarisation is orthogonal to the incident one. However, during the design process a very specific operation point can be found that ensures the simultaneous diffraction of both the ordinary and the extraordinary optical modes. In this presentation, we introduce the design of AOTF in birefringent crystals and present the main parameters that are subject to trade-off. Acousto-optic diffraction efficiency is sensitive to the so-called phase matching condition between optical wave and the ultrasonic wave. The offset from synchronicity is considered introducing a phase mismatch parameter. Diffraction efficiency evolution with respect to Bragg condition offset are illustrated. A custom device is finally presented that ensures simultaneous diffraction of both polarisation modes and compared to experimental results.@en

Acousto-Optical Tunable Filters (AOTFs) make use of the interaction between sound and light. To generate an ultrasound diffraction grating, a Radio-Frequency (RF) signal is applied to a piezoelectric transducer. The optical pass band can be controlled by adapting the properties of this transducer and the RF-signal. To achieve more precise control of the optical spectral bandwidth, we propose to use a multi-electrode array, consisting of five consecutive transducers. The Voltage Standing Wave Ratio (VSWR) is then used to evaluate the efficiency of the coupling between the RF-signal applied to the transducer and the acousto-optical crystal. A higher VSWR indicates that more power is being reflected back and less is being absorbed by the transducer, which in turn results in a decrease in optical performance. The aim of this paper is to develop an RF driving system capable of compensating for the VSWR behavior, linked to the individual impedance matching network of each transducer. For this, the five transducer setups were electrically characterized. These measurements allow the development of an RF power compensation system, leading to an increase of applied power at the level of each transducer. Hence, the absorbed power at transducer level increases, resulting in improved optical diffraction efficiency.@en

An instrument capable of imaging the field of NO2 in various open-air situations has been designed, manufactured, and tested. It is an improved version of the NO2 camera relying on an AOTF (acousto-optical tunable filter) which has demonstrated, amongst other things, its capability to quantify the NO2 released by power plant smokestacks. The improved version which is presented has a larger field of view, a higher frame rate, and better spectral registration performance. The working principle of the instrument has been preserved: by driving the AOTF with the appropriate acoustic frequency, a spectral image of the scene captured by the camera is recorded at a particular wavelength. The recording of a number of spectral images allows to form an hypercube: two spatial dimensions, and a spectral one. While the earlier instrument was relying on a handful of wavelengths to quantify the slant column density of NO2 observed in each pixel line of sight, the new instrument can now record "continuous" portions of the visible-light spectrum, typically between 440, and 460nm, where the NO2 exhibits some of its largest absorption lines. When the target is stable, like the air observed above a city skyline, the NO2 camera has enough time to build a large hypercube, and the spectrum measured in each pixel can be processed by the DOAS (differential optical absorption spectroscopy) method. This approach is better suited when NO2 is expected across the entire scene, not just in the plume of a smokestack for instance. The new instrument will be presented, and results of measurements performed in an urban context will be shown. The performance of the NO2 camera will be discussed based on the results of an intercomparison with the MAX-DOAS of Uccle, Brussels, and other air quality stations. @en

The AOTF-based NO2 camera is a remote sensing instrument primarily aimed at imaging and quantifying the NO2 field above cities or in industrial plumes. The measurement principle consists in acquiring a number of spectral images of the scene at selected wavelengths. Each pixel is therefore recording a discrete spectrum of the radiance collected in its acceptance cone, enabling the retrieval of the NO2 column density in its optical path by application of the DOAS method on the measured spectrum. The core element of the instrument principle is the acousto-optical tunable filter (AOTF). This device works under the principle of the acousto-optical interaction, the coupling of the light electric field with the modulation of the crystal lattice by a shear acoustic wave created by a transducer. The coupling takes place at a single wavelength, and diffracts that part of the spectrum into another direction. By blocking the undiffracted light beam, and imaging the diffracted order, one can capture a monochromatic image of the scene. We propose to expand the capabilities of the NO2 camera by exploiting another aspect of the acousto-optic interaction. The coupling between light and sound actually takes place in a birefringent crystal (TeO2), and one usually works with a single linear polarization of the incoming light (e-light, or o-light). The two polarization components are diffracted in different directions. If the current design is modified such that the two components can be imaged, then an information on the degree of linear polarization of the light can be obtained. In the atmosphere, the scattering of light by air (Rayleigh), and particles (Mie) is controlling the state of polarization of the scattered solar light. Hence, aerosols not only introduce a smooth spectral signatures, but also a change of the state of polarization. The proposed modification of the NO2 camera design can provide some sensitivity on this, potentially enhancing the scientific return of the instrument with aerosol retrievals capabilities. The new instrumental design will be presented, and vector radiative transfer simulations will be produced to estimate the benefit of this change.@en

The measurement of NO 2 abundance in the air is of interest for monitoring pollution sources and air quality. A new instrumental concept has been recently introduced, which makes use of an acoustooptical tunable filter (AOTF) as the main component of a spectral imager. With the capability of resolving the absorption spectrum of NO 2 , and a high spatial and temporal sampling, this instrument can be seen as a NO 2 camera. In this instrument, the selection of different optical wavelengths by the AOTF is done by applying a radio frequency (RF) signal to the AOTF. The selected frequency and the applied amplitude are the key parameters for the quality of the measurements. The design and assembly of an RF chain, consisting of an RF generator and RF amplifier, will lead to an electronics bench capable of driving the NO 2 camera. In view of a potential spaceborne application of this instrumental concept, the design of the RF generator is only making use of space-qualified components. This paper focuses on the development of this part of the electronics bench. @en

The performance of an acousto-optical tunable filter (AOTF) depends on several key parameters, such as the diffraction efficiency and the applied RF signal at the external network of the transducer. The latter has a specific bandwidth in which its efficiency is high. The goal of this paper is to experimentally investigate the relationship between the electrically matched RF bandwidth and the achieved diffraction efficiency, especially at the edges of the RF frequency band. The experiment revealed that the impedance matching is not completely correlated to the diffraction efficiency. Temperature measurements on the AOTF under test show that, at one edge of the tuning range, the RF signal is converted into a noninteracting acoustic mode.@en

Auroras can be observed at the north and south pole of the Earth. At these locations, the Earth magnetic field enters the atmosphere, causing ionization of the atmospheric constituents. Due to this phenomenon, different optical wavelengths are emitted from these constituents, causing the typical spectacular displays. Several optical wavelengths are representative for the auroral emission. The purpose of the described instrument is to measure the degree of linear polarization of the northern lights, in order to detect and confirm previous observations in which only a low level of polarization was seen. The instrument is capable of resolving the polarization measured at any wavelength in the visible region, using two optical chains, each containing an acousto-optical tunable filter (AOTF). The latter is responsible for the selection of the desired optical wavelength and projects the two polarized beams onto an optical detection system. The driving of the AOTF, using a radio-frequency (RF) signal, is a key-element in establishing trustworthy results. The design of this double RF chain, consisting of a generator, an amplifier, and the necessary circumjacent electronics, is described in the study. In addition , the optical setup of the instrument, together with the detection system is explained. The instrument will be used in the harsh environment of the pole regions, so it must be able to withstand these weather conditions. Hence, a thermal compensation system is necessary and is also described. @en

Spectral imaging is a powerful tool for remote sensing applications. Among other valuable options, acousto-optical tunable filters (AOTF) offer interesting properties to this class of instruments. When sensitivity to ultra-violet (UV) light is required, KH2PO4 crystal (KDP) is a good choice for the acousto-optic (AO) interaction medium as its AO figure of merit is relatively large compared to other materials transparent in UV. However, it generally requires more acoustic power than well-known TeO2-based devices to achieve the highest diffraction efficiency. We report on measurements performed with a KDP-based AOTF (9-cut) driven with RF signals up to 3 W. It has been found that owing to various effects (Joules, acoustic wave attenuation,...) the heating of the crystal by a few degrees yields severe alteration of the AOTF spectral transmission function (STF): shift of the central wavelength, increase of the bandwidth and decrease of the peak filter efficiency have been observed. These three effects degrade the AOTF overall performance if not taken into account. This paper presents the experimental evidences and shows how a temperature-dependent AOTF model allows to derive the inner temperature of the device. Possible reasons for the flattening of the STF are also discussed.@en

The ALTIUS-instrument is a three-channel spectral imager, measuring in the ultraviolet, visible and near infrared wavelength domains, that is bound to fly aboard a PROBA-satellite. The goal of the ALTIUS-instrument is to make hyper spectral images of the limb of the earth. In each of ALTIUS' three channels an AOTF (Acousto-Optical Tunable Filter) will be used. For each channel an RF-generator will be developed and subjected to a suite of environmental tests such as thermal-vacuum, vibration, radiation, shock and Electromagnetic Compatibility (EMC). For the AOTF RF-generator different solutions are possible. Currently the design of an analog Hilbert transform and a high frequency phase-locked loop (PLL) are proposed as possible solutions. The choice of components, the PCB-design and the manufacturing of both designs has to be done in accordance to space qualified standards. This limits the choice of design and lifts the design challenge to a higher level. Because of the limited choice of space qualified components, general commercial approaches are not possible. For this reason specific designs have to be investigated and research has to be done to select the appropriate solution.@en

we examined acousto-optic interaction in the KH2PO4 crystal used in devices providing control of optical radiation parameters. The crystal was applied in a wide-angle tunable acousto-optic filter capable of image processing in the ultraviolet region of spectrum. The filter operated at different temperature conditions because of changes of environmental temperature and also due to heating of the crystal by driving electric and acoustic power. As found experimentally and proved theoretically, changes in the temperature of the instrument and appearance of thermal gradients in the crystal volume sufficiently influenced on operation characteristics of the acousto-optic device.@en

Wide aperture acousto-optical tunable filters (AOTF) appeared in the 70’s and found applications in various areas such as agriculture (crop stress monitoring), food industry (product quality), biology (fluorescence spectroscopy), etc. Their main advantages are the robustness, compactness, low power consumption, high filtering efficiency, tunability, potentially high spectral resolution (<1nm) and good image quality. In general, they offer interesting features to meet the needs in hyperspectral imaging applications. The physical process behind the wide aperture AOTF is the interaction of light and sound inside a birefringent crystal. For a given acoustic frequency, only photons of particular energy (i.e. wavelength) will couple with the acoustic wave, then leave the crystal in a slightly different direction than the rest of the light. By focusing the diverted beam onto a detector, one effectively performs a spectral image of the scene. Selecting another window of the light spectrum only requires the tuning of the sound frequency. A piezoelectric transducer bonded to the crystal is responsible for converting the electrical RF-signal into an acoustic wave. As most AOTFs operating in the visible and near-infrared domains are driven with frequencies ranging a few MHz to several hundreds of MHz, they do not necessitate particular electronics equipment. This statement does not hold when it comes to operating AOTFs in space environment: the RF-driving chain must be made from the limited catalogue of space-qualified parts. The problem gets even bigger when frequencies of several tens of MHz or higher must be generated. The work reported in this paper addresses different solutions to the problem of building a space-qualified RF-chain for an AOTF. This research was undertaken as part of the development of the ALTIUS space mission (atmospheric limb tracker for the investigation of the upcoming stratosphere) which aims at the measurement of atmospheric trace species (ozone, nitrogen dioxide, methane, water vapor,…) concentration profiles with a high spatial resolution. The measurement concept relies on the acquisition of spectral images of the bright atmospheric limb at well-chosen wavelengths. The imager concept allows to discope the need for scanning the atmosphere, as it was done by previous remote sensing missions. The instrument will be mounted onboard a PROBA-satellite (Project for On-Board Autonomy). The PROBA-satellite is a platform containing all the essential subsystems like a GPS, an attitude and orbit control system etc. to facilitate the payload which is placed on top of the platform. The complete project (platform and payload) is developed under the supervision of ESA (European Space Agency) and with funding of the Belgian Science Policy Office (BELSPO). The original ALTIUS concept made use of three independent spectral imagers (channels), each of them relying on an AOTF capable of isolating narrow passbands across the channel spectral range (ultraviolet (UV): from 250 to 400nm, Visible: from 440 to 800nm, near-infrared (NIR): from 900 to 1800nm). For the Visible and NIR channels, the two AOTFs will be made of a paratellurite crystal. For the UV channel, the AOTF would have been made from a KDP crystal (potassium dihydrogen phosphate). Unfortunately, the latter did not reach the necessary level of maturity for a space mission, and the KDP-based AOTF was replaced by a stack of Fabry-Pérot interferometers (FPI). Nevertheless, for technological interest, the AOTF approach will also be developed and matured for the UV-channel up to flight level. The focus of this paper is to design for each of the different channels a dedicated RF-chain, containing a RF-generator and a RF-amplifier. The output of the RF-amplifier is tunable to a specific frequency and a specific power level and is injected in the AOTF’s transducer via an impedance matching network. Different architectures are possible to generate the frequency range needed for the different wavelength domains. While the Hilbert Transform solution will be only summarized here and not further investigated, this paper focuses on the Phase Locked Loop (PLL) solution. PLL-solutions are breadboarded for the three wavelength domains. A detailed study is done on the achievable power levels in the Infrared, Visible and the Ultraviolet.@en

Acousto-optical tunable filters (AOTFs) are still little known to the Earth atmosphere remote sensing community. The bulk of passive atmospheric remote sensing instruments remains divided into two families: those relying on interferometric techniques (mostly for the long-wave absorbing species), and those based on diffraction gratings (better suited for UV-VIS absorbing species). Still, AOTFs have some unique features which should deserve more attention, in particular their angular acceptance, and their polarization sensitivity. The first one because it allows to work in an imaging setup, the second because many atmospheric processes have a polarizing effect. In this paper, we will present different AOTF-based instrument concepts (or even prototypes) which take advantage of these features in order to improve the sate-of-the-art of measurement techniques in several fields of atmospheric science. We will first present the improved NO2 camera: its new capabilites, the subsystems which have been changed, and some preliminary results. Then, we will discuss two other potential applications: the study of the solar spectral irradiance variability in the UV, and the detection of auroral polarized emissions. For each concept, we will discuss the current challenges faced by the existing instruments, and analyze how the use of AOTFs could overcome them. A suggestion for the AOTF selection will be made, and the expected instrument performance will be estimated.@en

The abundance of NO2 in the boundary layer relates to air quality and pollution source monitoring. Observing the spatiotemporal distribution of NO2 above well-delimited (flue gas stacks, volcanoes, ships) or more extended sources (cities) allows for applications such as monitoring emission fluxes or studying the plume dynamic chemistry and its transport. So far, most attempts to map the NO2 field from the ground have been made with visible-light scanning grating spectrometers. Benefiting from a high retrieval accuracy, they only achieve a relatively low spatiotemporal resolution that hampers the detection of dynamic features. We present a new type of passive remote sensing instrument aiming at the measurement of the 2-D distributions of NO2 slant column densities (SCDs) with a high spatiotemporal resolution. The measurement principle has strong similarities with the popular filter-based SO2 camera as it relies on spectral images taken at wavelengths where the molecule absorption cross section is different. Contrary to the SO2 camera, the spectral selection is performed by an acousto-optical tunable filter (AOTF) capable of resolving the target molecule's spectral features. The NO2 camera capabilities are demonstrated by imaging the NO2 abundance in the plume of a coal-fired power plant. During this experiment, the 2-D distribution of the NO2 SCD was retrieved with a temporal resolution of 3 min and a spatial sampling of 50 cm (over a 250 × 250m2 area). The detection limit was close to 5 × 1016 molecules cm-2, with a maximum detected SCD of 4 × 1017molecules cm-2. Illustrating the added value of the NO2 camera measurements, the data reveal the dynamics of the NO to NO2 conversion in the early plume with an unprecedent resolution: from its release in the air, and for 100m upwards, the observed NO2 plume concentration increased at a rate of 0.75-1.25g s-1. In joint campaigns with SO2 cameras, the NO2 camera could also help in removing the bias introduced by the NO2 interference with the SO2 spectrum.@en

In the frame of an ESA-mission (European Space Agency) a RF-generator which can withstand the harsh space environment is being developed. This RF-generator is a part of the ALTIUS-instrument1 (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere). The goal of the mission is to make hyper spectral images of the limb of the earth in different wavelength domains. For this, a three-channel spectral imager is implemented, measuring in the ultraviolet (250 nm to 450 nm), visible (440 nm to 800 nm) and near infrared (900 nm to 1800 nm) wavelength domains, that is bound to fly aboard a PROBA-satellite. The RF-generator will apply a RF-signal to an AOTF (Acousto-Optical Tunable Filter). The AOTF manages to convert RF-power into acoustic power which implies a diffracted beam at a different angle at the output of the AOTF as a function of the applied frequency and power2. This beam is focused onto a detector which performs the imaging. In this paper a high frequency RF-generator is discussed based on a PLL (Phase Locked Loop) for the three channels (Figure). Other solutions applicable for space were investigated by the author as well3, 4. A merged PCB (Printed Circuit Board) is designed which houses the frequency synthesizer (the ADF4208), an in-house designed loop filter, a VCO (Voltage Controlled Oscillator) and a divide-by-8 prescaler. The selected components are compatible with the ESA standards for space-qualified components. Tests are carried out on the setup for the UV-channel because for this a relatively high frequency has to be generated in comparison with the available RF-generating techniques for space applications. Different design parameters were checked such as the spectral output, spectral resolution, output power, frequency stability and phase noise. The levels of these parameters are based on the harsh environment which the electronics has to withstand. Based on the results it can be stated that the spectral output (130 – 260 MHz) as well as the spectral resolution of 5 kHz is achieved within the requested tolerances. The output power level varies between +2.5 dBm and +3.3 dBm depending on the applied frequency. A long term frequency stability of 5 kHz within 100 seconds is achieved as well. The phase noise at 10 kHz offset is -101 dBc/Hz which is well within specification. Currently only the UV-channel setup is breadboarded as a merged PCB. For the other channels an additional prescaler is needed (Figure). This will have an impact on the performance of the RF-generator and is under investigation.@en

In the frame of a mission of the European Space Agency (ESA) a space qualified RF-generator needs to be developed to drive an AOTF (Acousto-Optical Tunable Filter) to make hyper-spectral images of the limb of the Earth. Custom off-the-shelf electronics can not fulfil the requirements for this scientific mission, hence alternative solutions for the RF-generation are investigated. This paper describes one of the possible solutions.@en

The latest generation of nadir-looking air pollution monitoring instruments (e.g. Sentinel-5P) deliver daily global maps of tropospheric NO2 columns with an unprecedented spatial resolution and accuracy. Still, the ground pixel size makes the product unusable for applications targeting the assessment of personal exposure, or other street-level effects. While a lot of research is dedicated to upscaling the satellite products by the synergistic use of air quality monitoring stations and pollution dispersion models, there is a need for innovative instrumentation capable of bridging the street-level needs with the city-wide sampling of satellite NO2 maps. An innovative passive optical instrument has been developed, which is capable of imaging the field of NO2 above the skyline of a city. The measurement principle consists in recording a multispectral cube of the scene at several wavelengths providing a good sensitivity to NO2. A DOAS-like retrieval algorithm fits the abundance of NO2 seen by every pixel of the image. The product is a high resolution map of the NO2 above a city, with a spatial and temporal resolution unseen so far. We present the instrumental concept, and measurement approach. Data acquired above Brussels, Belgium, have been validated with a reference instrument, a MAX-DOAS spectrometer driven in an unusual way in order to sample the NO2 camera field of view. Excellent consistency between the two instruments was found, opening the way for further applications of this concept.@en