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 system consists of an optical interferometric encoding sub-system that rapidly reads and encodes the 2-D complex-valued spatial code, forming a wideband frequency modulated optical beam and a colinear optical reference beam. After appropriate coherence coding of this beam pair, the light is launched into a low probability of intercept communication channel such as an optical fiber or a narrow beamwidth free-space optical link. At the remote code receiving and data processing site, the received light beam pair is first coherently decoded. Then, a high speed photodetector via optical heterodyne detection generates an encoded wideband radio frequency signal that contains the original 2-D code. Decoding is implemented in parallel via two independent systems. One decoder uses a Fourier transforming lens to reconstruct an electronic image interferogram of the complex-valued user code. This image interferogram is sent to a high speed electronic image processor for verification purposes. The other decoder is a high speed coherent acousto-optic time integrating correlator that optically determines match-mismatch between the received encoded signal and the code signal generated by the electronic database. Improved security to the overall communication network is added by using various keycodes such as a time varying keycode that determines the exact spatial beam scanning sequence required for both proper encoding and decoding of the 2-D code information. This paper describes preliminary experiments using a simple 1-D amplitude modulated spatial code.