Double-pass acousto-optic modulator system

TLDR

Beam diffraction angle in acousto‑optic modulators depends on modulation frequency, but a double‑pass configuration compensates deflections and eliminates alignment problems. The study presents a compact, stable double‑pass AOM design achieving 75 % diffraction efficiency and a 68 MHz bandwidth for single‑mode fiber transmission. The system uses a second pass through the AOM with the beam’s polarization rotated 90°, causing it to counter‑propagate and be separated from the input by a polarizing beam splitter. The overall system efficiency, defined as the optical power out of the single‑mode fiber divided by the input power, is 60 %.

Abstract

A practical problem that arises when using acousto-optic modulators (AOMs) 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 configuration, which compensates for beam deflections. On a second pass through the AOM, the beam with its polarization rotated by 90° is deflected 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 efficiency and a tuning bandwidth of 68 MHz full width at half maximum for light transmitted through a single-mode fiber. The overall efficiency of the system (defined as the optical power out of the single-mode fiber divided by the optical power into the apparatus) is 60%.