Active sub-Rayleigh alignment of parallel or antiparallel laser beams

摘要

We measure and stabilize the relative angle of parallel and antiparallel laser beams to 5 nrad/(Hz)~(1/2) resolution by comparing the phases of radio frequency beat notes on a quadrant photodetector. The absolute accuracy is 5.1 and 2.1 (mu)rad for antiparallel and parallel beams, respectively, which is more than 6 and 16 times below the Rayleigh criterion.