Multitone Photoconductive Sensors for Free-Space Optics

A multitone frequency-biasing technique is applied to an integrated photoconductive (PC) sensor for application to free-space optical (FSO) communication systems. The introduced technique and integrated device provide the required optical transmission, detection, and retroreflection capabilities for both passive uplink and active downlink operations. The physical structure incorporates three triangular PC switches, with 450- μm GaAs gaps, assembled in a corner-cube retroreflector architecture. The retroreflective nature of the element establishes a bidirectional passive uplink between the PC sensor and transmitter/source, whereas the three PC switches together provide unidirectional reception of incident optical signals during active downlink operation. At the same time, the threefold symmetry of the structure, together with the multitone biasing, provides a frequency-domain balancing mechanism to optimize the sensor alignment. The prototype is presented with both theoretical and experimental analyses, and proof-of-concept FSO communication is demonstrated over an indoor 5-m link length.