Electrical bandwidth testing of an AOTF transducer as a function of the optical diffraction efficiency.

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.

Tunable acousto-optic spectral imager for atmospheric composition measurements in the visible spectral domain.

We describe a new spectral imaging instrument using a TeO(2) acousto-optical tunable filter (AOTF) operating in the visible domain (450-900 nm). It allows for fast (~1 second), monochromatic (FWHM ranges from 0.6 nm at 450 nm to 3.5 nm at 800 nm) picture acquisition with good spatial resolution. This instrument was designed as a breadboard of the visible channel of a new satellite-borne atmospheric limb spectral imager, named the Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere (ALTIUS), that is currently being developed. We tested its remote sensing capabilities by observing the dense, turbulent plume exhausted by a waste incinerator stack at two wavelengths sensitive to NO(2). An average value of 6.0±0.4×10(17) molecules cm(-2) has been obtained for the NO(2) slant column density within the plume, close to the stack outlet. Although this result was obtained with a rather low accuracy, it demonstrates the potential of spectral imaging by using AOTFs in remote sensing.

Optimal spectral inversion of atmospheric radiometric measurements in the near-UV to near-IR range: A case study.

WWe present a general analysis of the error budget in the spectral inversion of atmospheric radiometric measurements. By focussing on the case of an occultation experiment, we simplify the problem through a reduced number of absorbers in a linearized formalism. However, our analysis is quite general and applies to many other situations. For a spectrometer having an infinite spectral resolution, we discuss the origin of systematic and random errors. In particular, the difficult case of aerosols is investigated and several inversion techniques are compared. We underline the importance of carefully simulating the spectral inversion as a function of the target constituent to be retrieved, and the required accuracy level.

In-flight performance and calibration of SPICAV SOIR onboard Venus Express.

Solar occultation in the infrared, part of the Spectoscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) instrument onboard Venus Express, combines an echelle grating spectrometer with an acousto-optic tunable filter (AOTF). It performs solar occultation measurements in the IR region at high spectral resolution. The wavelength range probed allows a detailed chemical inventory of Venus's atmosphere above the cloud layer, highlighting the vertical distribution of gases. A general description of the instrument and its in-flight performance is given. Different calibrations and data corrections are investigated, in particular the dark current and thermal background, the nonlinearity and pixel-to-pixel variability of the detector, the sensitivity of the instrument, the AOTF properties, and the spectral calibration and resolution.

Optical and physical properties of stratospheric aerosols from balloon measurements in the visible and near-infrared domains. III. Presence of aerosols in the middle stratosphere.

The aerosol extinction measurements in the ultraviolet and visible wavelengths by the balloonborne spectrometer Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et NOx (SALOMON) show that aerosols are present in the middle stratosphere, above 25-km altitude. These observations are confirmed by the extinction measurements performed by a solar occultation radiometer. The balloonborne Laboratoire de Météorologie Dynamique (LMD) counter instrument also confirms the presence of aerosol around 30-km altitude, with an unrealistic excess of micronic particles assuming that only liquid sulfate aerosols are present. An unexpected spectral structure around 640-nm observed by SALOMON is also detectable in extinction measurements by the satellite instrument Stratospheric Aerosols and Gas Experiment III. This set of measurements could indicate that solid aerosols were detected at these altitude ranges. The amount of soot detected up to now in the lower stratosphere is too low to explain these measurements. Thus, the presence of interplanetary dust grains and micrometeorites may need to be invoked. Moreover, it seems that these grains fill the stratosphere in stratified layers.