A retracing free-space optical connection for reliable underwater optical communications

To establish a free-space underwater optical connection between a transmitter and a receiver suitable for operation in the single photon regime, techniques must be developed to compensate for: a) rotational and translational motions of both the transmitter and the receiver, b) their vibrations, and c) turbulence-induced deformations of the wavefront leading to beam wander, scintillation and beam spreading. The purpose of this work is to present and discuss experimental results obtained using a pseudo phase-conjugated mirror placed at the end of an underwater optical path disturbed by artificially induced turbulence. This configuration emulates a scenario whereby a modulated retroreflector is used to transmit the data back to the laser transmitter. The measurements obtained with the proposed retracing optical connection are found to be promising and pave the way for the use of this technique for quantum communications.