Double-pass acousto-optic modulator system

A practical problem that arises when using acousto-optic modulators sAOMsd to scan the laser frequency is the dependence of the beam diffraction angle on the modulation frequency. Alignment problems with AOM-modulated laser beams can be effectively eliminated by using the AOM in the double-pass con®guration, which compensates for beam de¯ections. On a second pass through the AOM, the beam with its polarization rotated by 90° is de¯ected back such that it counterpropagates the incident laser beam and it can be separated from the input beam with a polarizing beam splitter. Here we present our design for a compact, stable, double-pass AOM with 75% double-pass diffraction ef®ciency and a tuning bandwidth of 68 MHz full width at half maximum for light transmitted through a single-mode ®ber. The overall ef®ciency of the system sde®ned as the optical power out of the single-mode ®ber divided by the optical power into the apparatusd is 60%.