We report on a scheme to improve the pointing stability of the first order beam diffracted by an acousto-optic modulator (AOM). Due to thermal effects inside the crystal, the angular position of the beam can change by as much as when the radio-frequency power in the AOM is reduced to decrease the first order beam intensity. This is done, for example, to perform forced evaporative cooling in ultracold atom experiments using far-off-resonant optical traps. We solve this problem by driving the AOM with two radio frequencies and . The power of is adjusted relative to the power of to keep the total power constant. Using this, the beam displacement is decreased by a factor of 20. The method is simple to implement in existing experimental setups, without any modification of the optics.
We obtain the AOM bandwidth by measuring the reflected power as a function of the radio frequency using a directional coupler (Mini-Circuits ZDC-10-1).
One drawback of the acousto-optic deflector is that the sound velocity for the sheer mode in is significantly smaller (by a factor of 5) than for the longitudinal mode, leading to longer rise times. Using the two-frequency method with a longitudinal mode AOM allows one to keep fast rise times.