A high-speed modulated retro-reflector for lasers

We have used an acousto-optic modulator (AOM) to impose an amplitude-modulation signal on an incident laser beam. The amplitude-modulated beam is retroreflected through the AOM. This beam is diffracted again by the AOM so that it overlaps the incident beam and is frequency shifted with respect to it. The return beam is also orthogonally polarized with respect to the incident beam by a wave plate. These features allow us to detect the amplitude-modulated signal with high signal to noise ratio using heterodyne detection. Since the optical setup is simple and can be made very compact, this device may be ideal for certain forms of high-speed, free-space optical communication. We have used a 60 MHz AOM to demonstrate a 1 MHz communication rate, and studied the performance limitations of this device. Finally, we discuss the realization of a communication rate approaching 1 GHz using this method.