Analysis, design and realization of an RF driving system for space-based Acousto-Optical Tunable Filter applications

ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere) is a three-channel spaceborne spectral imager bound to fly aboard a PROBA satellite. The ALTIUS project (satellite and instrument) is developed under the supervision of the European Space Agency (ESA). The ALTIUS instrument will make hyperspectral images of the limb of the Earth. To do this, the instrument will use different techniques such as direct limb viewing, star occultation as well as sun occultation. It will measure in the visible, near-infrared and ultraviolet spectral domain. In the visible and near-infrared channel an AOTF (Acousto-Optical Tunable Filter) will be used for the selection of the appropriate optical wavelength. These AOTFs allow to scan at a fast rate through the complete spectral band. An AOTF is based on a birefringent crystal that gets an amplified Radio Frequency (RF) signal injected via a transducer. The latter converts the RF energy into soundwaves, creating an optical filter effect in the crystal. Based on different models found in literature, a theoretical transducer concept, together with a properly matched external impedance network, is calculated and simulated in this study. Also the relationship between the optical diffraction efficiency and electrical bandwidth of the impedance matching network is examined. Depending on the optical range (visible, near-infrared or ultraviolet), RF frequencies from 40 MHz up to 250 MHz are needed to drive the AOTFs. RF generation techniques available today for space applications, are not capable of generating frequencies exceeding 200 MHz. In this work, an in-depth trade-off study is performed of different RF generator techniques, taking into account the technical requirements for ALTIUS. A Phase-Locked Loop (PLL) based analog solution, a Direct Digital Synthesizer (DDS) integrated in an FPGA, and several others are discussed. The PLL-based RF generation technique is proposed for ALTIUS. It will be demonstrated that this technique is capable of generating the required high frequencies in a stable and accurate manner. For the visible and near-infrared channel, low-power space grade control electronics to drive the AOTFs are developed, each with their own specifications for resolution, sensitivity, frequency range, electrical and optical performance. For the ultraviolet channel a Fabry-Pérot system will be used to select the optical wavelengths instead of an AOTF. Nevertheless, the investigation on how to develop a high frequency RF chain in the ultraviolet for space flight is interesting as this could be useful for future (space) applications, and it is thus described in this work. All three RF systems are developed such that they can survive multiple years in a space environment (temperature, radiation, vibrations, EMI/EMC-demands). To achieve this, specific Electrical, Electronic and Electro-mechanical (EEE) components have been selected, the PCB-design was carried out in accordance to 'space qualified' ESA standards and extended test programs were executed. After the concept study and the design of the channels, also prototyping and on-ground testing were performed in preparation of building flight models and integration in the instrument in a later stage. During the design of the three RF chains, the focus was on low-power consumption, low-volume and survivability in space. The PLL RF generator module is first subjected to stand-alone tests, and is then used in integrated tests per channel (combination RF generator, RF amplifier and AOTF). All tests are discussed in this work. Besides ALTIUS, the designed PLL-based RF chains, and especially the UV RF chain, are also interesting for other (space) projects. In this work four additional applications are discussed, three of which can be transformed to space qualified instruments on short notice. The concept of using an AOTF in a spectrum imager in space applications is innovative. The development of suitable RF systems implies some technological challenges and/or difficulties such as high frequencies, broad frequency spectrum, limited power, stability and quality of the generated signal and temperature control. All these challenges have been tackled effectively and are discussed extensively in this work.status: publishe

Possible AOTF RF-Generator Solution Applicable for Space Missions

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Practical Driving Electronics for an AOTF-Based NO2 Camera

© 1963-2012 IEEE. 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.status: publishe

Investigation of a RF-generator applicable for integration into an ESA-space mission

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-instrument (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 power. This beam is focused onto a detector which performs the imaging.status: publishe

Testing of a possible RF-generator for a Space Based AOTF application in the Frame of an ESA Space Mission

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.status: publishe

Implementation of different RF-chains to drive Acoustic-Optical Tunable Filters in the frame of an ESA space mission

The work reported desribes different solutions to the problem of building a space-qualified RF chain for an acousto-optical tunable filter. This research is a part of the development of the ALTIUS space mission (atmospheric limb tracker for the investigation of the upcoming stratosphere), aiming at the measurement of atmospheric trace species (ozone, nitrogen dioxide, methane, water vapor,...) concentration profiles with a high spatial resolution.status: publishe

RF-driving of Acoustic-Optical Tunable Filters; design, realization and qualification of analog and digital modules for ESA

The ALTIUS-instrument is a three-channel spectral imager, measuring in the ultraviolet (UV) (250 nm to 450 nm), visible (440 nm to 800 nm) and near infrared (NIR) (900 nm to 1800 nm) wavelength domains, that is bound to fly aboard a PROBA-satellite. The ALTIUS-project (satellite and instrument) is developed under the supervision of ESA and with funding of Belgian Science Policy (BELSPO). The goal of the ALTIUS-instrument is to make hyper spectral images of the limb of the earth. To do this, the instrument will use different observation techniques such as “direct limb viewing”, ”stellar occultation” as well as “solar occultation”.status: publishe

VenSpec-H: an Infrared Spectrometer to Study Venus' Activity

International audienceIntroductionThe VenSpec-H instrument [1] is part of the EnVision M5 mission payload, which has been selected by ESA in June 2021 for launch in 2031. EnVision is a medium-class mission to determine the nature and current state of geological activity on Venus, and its relationship with the atmosphere, to understand how Venus and Earth could have evolved so differently. VenSpec-H will target different molecular species in nadir viewing geometry, as to better characterize the surface-atmosphere interaction. Scientific ObjectivesVenSpec-H is part of the VenSpec suite, including also an IR mapper and a UV spectrometer. The science objectives of this suite is to search for temporal variations in surface temperatures and tropospheric concentrations of gases indicative of volcanic eruptions; and study surface-atmosphere interactions and weathering by mapping surface emissivity and tropospheric gas abundances.To contribute to this investigation, VenSpec-H is designed to H2O and HDO contents in the first scale height of Venus atmosphere and to probe H2O, HDO, CO, OCS, SO2 in the 30 to 40km altitude range on the nightside of Venus and above the cloud top on the dayside. Instrument DesignVenSpec-H is a nadir pointing, high-resolution (R~8000) infrared spectrometer that will perform observations in different spectral windows between 1 and 2.5 microns.This instrument has direct heritage from NOMAD-LNO [2, 3] that is currently onboard ExoMars Trace Gas Orbiter, although a series of modifications have been introduced to address the specific scientific objectives of the EnVision mission. Studies were performed to optimize the instrument design, as well as to choose four spectral windows with sufficient SNR, to achieve the instruments scientific objectives [4]. AcknowledgementsThis work has been performed with the support of the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office