On inherent spatio-spectral image distortion in AOTF-based imagers

Acousto-optical tunable filters (AOTFs) based on interaction of light and ultrasound in uniaxial birefringent crystals are widely used in imaging spectroscopy applications due to random spectral access, narrow controllable bandwidth, compactness, overal ease-of-use and image trasmittance capability. The spectral transmission of AOTF-based spectral imagers is usually characterized for paraxial light beam, but AOTFs inherently have non-uniform spatio-spectral transmission, so the central wavelength of the transmission window varies with the angle of incident light. We demonstrate that the spatio-spectral characteristics of acousto-optic (AO) interaction may be described either by the dependency of ultrasound frequency on the incident light angle for a given wavelength or by the dependency of wavelength on the incident angle for a given sound frequency. These dependencies are derived from the phase matching condition and are determined by the refractive indices, sound phase velocities and the AO diffraction geometry. We experimentally estimate the appearance of this specific spatio-spectral structure on the images acquired by AOTF-based imagers based on both collimating and confocal (telecentric) schemes and show that the variations of central wavelength and spectral bandwidth are noticeable for collimating setup even in the commonly used small field of view. The results of the study may be applied in design of AOTF-based spectral imagers and image processing algorithms.

Automated summary

Acousto-optical tunable filters (AOTFs) are widely used in imaging spectroscopy applications due to their random spectral access, narrow controllable bandwidth, compactness, ease of use, and image transmittance capability. These filters have non-uniform spatio-spectral transmission, resulting in variations of central wavelength with incident light angle, which can be described by the dependencies of ultrasound frequency on incident light angle or wavelength on incident angle.