PHOTONIC SECURITY SYSTEM USING SPATIAL CODES AND REMOTE CODED COHERENT OPTICAL COMMUNICATIONS

A novel photonic security system is described using 2-D spatial codes based on both optical phase and amplitude information. This security s ystem consists of an optical interferometric encoding subsystem that r apidly reads and encodes the 2-D complex-valued spatial code, forming a wideband frequency modulated optical beam and a colinear optical ref erence beam. After appropriate coherence coding of this beam pair, the light is launched into a low probability of intercept communication c hannel such as an optical fiber or a narrow beamwidth free-space optic al link. At the remote code receiving and data processing site, the re ceived light beam pair is first coherently decoded. Then, a high speed photodetector via optical heterodyne detection generates an encoded w ideband radio frequency signal that contains the original 2-D code. De coding is implemented in parallel via two independent systems. One dec oder uses a Fourier transforming lens to reconstruct an electronic ima ge interferogram of the complex-valued user code. This image interfero gram is sent to a high speed electronic image processor for verificati on purposes. The other decoder is a high speed coherent acousto-optic time integrating correlator that optically determines match-mismatch b etween the received encoded signal and the code signal generated by th e electronic database. Improved security to the overall communication network is added by using various keycodes such as a time varying keyc ode that determines the exact spatial beam scanning sequence required for both proper encoding and decoding of the 2-D code information. Thi s paper describes preliminary experiments using a simple 1-D amplitude modulated spatial code. (C) 1996 Society of Photo-Optical Instrumenta tion Engineers.