Experimental Study of the Influence of the Transmission Function of Acousto-Optic Tuneable Filter on the Characteristics of the Interference Pattern in Off-Axis Digital Holography

Abstract

Acousto-optic tuneable filters (AOTFs) are used in digital holography (DH) to obtain interference images at different wavelengths, which expands the possibilities of studying technical and biological objects. However, the width of the spectral bands selected by AOTF is quite large, which can lead to a decrease in the size of the high-contrast region of the interference pattern in off-axis DH schemes and reduce the quality of the obtained holograms. In this work, the effect of the geometry of acousto-optic (AO) interaction and the power of the driving signal on the spectral transmission function of the AOTF, the visibility of the interference pattern, and the width of the effective field of view (FOV) is experimentally studied. For this, a setup with a broadband radiation source, Mach-Zehnder interferometer, and a spectrometer was used. The interference patterns were recorded and the transmission spectra were measured for several values of the angle of incidence on the entrance face of the AOTF in the frequency tuning range corresponding to the visible spectrum. We evaluated the dependence of the coherence length on the rotation angle of the AOTF and the central transmission wavelength and compared the results obtained by the interference method and those calculated from the spectra measured by the spectrometer. It is shown that when the AOTF is rotated through the angles from 5° to –15° relative to the wide-aperture AO interaction geometry, it is possible to increase the coherence length and the width of the effective FOV by a factor of 2.5. It was established that the width of the FOV can significantly decrease with an increase in the driving power. The results obtained can be used to certify the AOTF, optimize the characteristics of DH setups with AOTF, and determine the optimal parameters of their operation.