Experimental observation of reflected acoustic beam tilting with temperature in quasi-collinear acousto-optic filters

This paper is devoted to the experimental investigation of temperature influence on the acoustic beam reflection in quasi-collinear acousto-optic (AO) tunable filters, fabricated on the base of paratellurite crystal. Such AO devices apply acoustic beam reflection from the input optical facet in such a way that the reflected beam Poynting vector is collinear to the incident optical radiation wave vector. This allows achieving abnormally high AO interaction length and consequently both high diffraction efficiency and spectral resolution and to realize the multi-frequency diffraction applied for ultrashort laser pulse spectrum and phase control. Due to the acoustic power absorption, the significant heating of AO crystal near the reflecting facet is observed. The crystal heating affects the acoustic beam reflection condition through the ultrasound velocity variation and causes the reflected beam tilt from the initially chosen direction. The original method of reflected ultrasound beam structure measurement is proposed. The acoustic beams were visualized for various temperatures in two different quasi-collinear AO filters. The temperature variation range typical for the quasi-collinear AO filters in multi-frequency operation causes the beam tilt of about 1-2 degrees, depending on the reflection geometry. The beam tilt influence on the quasi- collinear AO filter transmission functions was also studied. It is shown that the observed tilt significantly affects both the AO diffraction efficiency and the transmission functions, changing their shape and bandwidth.