COHERENT HOMODYNE OPTICAL COMMUNICATION RECEIVERS WITH PHOTOREFRACTIVE OPTICAL BEAM COMBINERS

The principles of operation and the results of performance measurement s are reported of a new type of coherent optical receiver that used a dynamic volume index of refraction grating formed inside a photorefrac tive material to coherently combine signal and local oscillator light prior to photodetection. Because the refractive index grating is forme d by the interference pattern generated where mutually coherent optica l beams overlap, the receiver can automatically adjust to changes in a ngle of arrival or optical wavefront profiles which occur on time scal es longer than the grating formation time. The grating appears station ary to high-speed phase modulation imposed on the signal beam and cohe rently diffracts local oscillator light into the signal beam direction . Performance measurements are reported for a prototype system that us ed two independent Nd:YAG lasers at 1.064 mum, an iron-doped indium ph osphide photorefractive crystal, and a four-slot phase modulation sign al format. A receiver BER of 10(-6) was obtained at received signal po wers that corresponded to an average of 70 detected signal photons per bit at a source data rate of 50 Mb/s, 130 detected signal photons/bit at 220 Mb/s, and about 400 detected signal photons/bit at a 325 Mb/s source data rate. Quantum-limited operation corresponds to an average of six detected signal photons per transmitted information bit for thi s signal format.