Optical homodyne 220-Mbps communication system using photorefractive beam combining

Abstract

Coherent optical communication systems that contain conventional optical beam splitters require complicated subsystems to maintain precise angular alignment and spatial-mode matching between signal and local oscillator laser light prior to photodetection. It has been shown that the volume index of refraction gratings formed inside photorefractive materials may also be used to coherently combine signal and local oscillator light.1 Systems that contain photorefractive beam combiners are much simpler to implement as the refractive index grating forms wherever the two optical beams overlap and adjusts its position automatically, on a time scale given by the grating formation time, to changes in spatial-mode profile or angle of arrival of either beam. The experimental performance is reported of an optical homodyne communication system that used a Nd:YAG unidirectional ring oscillator laser at 1064 nm, a phase modulation format equivalent to quaternary pulse position modulation at a source data rate of 220 Mbps, and an InP:Fe photorefractive beam combiner.