Spectrally tunable illumination system based on acousto-optic diffraction of light

Abstract

Spectrally tunable illumination is widely used for spectral imaging, spectroscopy, spectral-domain optical coherence tomography and other applications. Most of swept-wavelength light sources are based on mechanical change of narrow-band filters or on a supercontinuum laser coupled with a tunable filter. Such commercially available sources suffer either from a limited number of spectral channels or from a spatial noise caused by parasitic interferences. Here, we describe an alternative solution, which is free from the mentioned disadvantages and provides speckle-free uniform illumination. It is based on acousto-optic (AO) diffraction of a wide-band light. AO tunable filters (AOTFs) are fully software-controlled solid-state devices and do not require mechanical scanning. The selected wavelength of light is determined by the period of the volume phase grating induced in the crystal by an ultrasound wave, i.e. by the frequency of the electric signal applied to the piezo-transducer that excites ultrasound wave. Due to high spectral resolution, small entrance pupil diameter and small angular aperture of AOTFs, its utilisation together with wide-band light sources is problematic even with a specific optical coupling to optimize the light-energy and aberration parameters of the filtered light. In this paper, we show that efficiency of this approach may be radically improved by acoustic frequency modulation.