Sidelobe considerations in AOTF imaging

An acousto-optic tunable filter (AOTF) is an all solid-state robust device with no-moving parts that has been used in the development of hyperspectral imagers from the ultraviolet to the longwave infrared. Such a device is fabricated by bonding a piezoelectric transducer on a specially cut prism in a birefringent crystal. When a radio frequency (RF) signal is applied to the transducer, a diffraction grating is set up in the crystal with grating period determined by the applied RF and the diffracted wavelength depends upon the applied RF and the prism parameters. When broadband white light is incident on the prism input facet, two orthogonally polarized diffracted beams at a specific wavelength with a narrow bandpass are transmitted for each applied RF. The transmitted wavelength can be tuned by varying the applied RF and this is used in a hyperspectral imager design. The spectral response of the transmitted intensity from an AOTF with a rectangular transducer has a sinc² distribution with a main lobe and a large number of sidelobes. In general, in hyperspectral imaging applications the light leakage from the sidelobes is not taken into account. When imaging is carried out for a scene with a laser or an intense source of light, the spectral images are affected by the light transmitted through these sidelobes away from the main lobe. We have carried out a detailed study of light transmitted through the sidelobes of a TeO₂ AOTF with a rectangular transducer operating in the shortwave infrared (SWIR) region from 0.9 to 1.7 μm. The AOTF imaging system used a telecentric confocal optics that compensates for AOTF aberrations, which are severe at high sidelobe operation. We used a 16-channel RF driver with independent amplitude and frequency control. By switching off specific RF signals applied to the AOTF, we measured the detailed sidelobe structure for the transmitted intensity by analyzing the spectral images and compared it with theoretical prediction. Here we present our experimental setup and results.