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 1mrad 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 f1 and f2. The power of f2 is adjusted relative to the power of f1 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.

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This method is mentioned briefly in
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Gehm
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Duke University
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6.

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).

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8.

One drawback of the acousto-optic deflector is that the sound velocity for the sheer mode in TeO2 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.

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