Influence of driving power on the performance of UV KDP-based acousto-optical tunable filters

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 degrees-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.