  Diffraction beams generated by an acousto-optic modulator (AOM) are widely
used in various optical experiments, some of which require high angular
stability with the temporal modulation of optical power. Usually, it is
difficult to realize both angular stability and high-power modulation in a
passive setup without a servo system of radio-frequency compensation. Here, we
present a method to suppress the angular drift and pointing noise only with the
thermal management of the AOM crystal. We analyze the dependence of the angular
drift on the refractive index variation, and find that the angular drift is
very sensitivity to the temperature gradient which could induce the refractive
index gradient inside the AOM crystal. It reminds us such angular drift could
be significantly suppressed by carefully overlapping the zero temperature
gradient area with the position of the acousto-optic interaction zone. We
implement a water-cooling setup, and find that the angular drift of an AOM is
reduced over 100 times during the thermal transient, and the angular noise is
also suppressed to 1/3 of the non-cooled case. It should be emphasized that
this thermal control method is a general to suppress the beam drift in both the
diffraction and the perpendicular-to-diffraction directions. The refractive
index thermal coefficient of tellurium dioxide crystal at 1064 nm determined by
this angular drift-temperature model is 16$\times$10$^{-6}$ K$^{-1}$ consistent
with previous studies. This thermal control technique provides potential
applications for optical trapping and remote sensoring that demand for
intensity ramps.
