Secure Optical 3D Probabilistic Shaping CAP System Based On Spherical Constellation Masking

A novel secure optical 3D probabilistic shaping carrier-less amplitude and phase (3D-PS-CAP) system based on spherical constellation masking is proposed in this letter. In order to improve the bit error rate (BER) performance of the system, probabilistic shaping (PS) is given full play. By employing different masking vectors in different cubes, masking for the constellation points is realized. We successfully conducted an experiment demonstrating 6.5 Gb/s 3D-PS-CAP-16 data transmission over 25 km standard single-mode fiber (SSMF). At BER of $1\times 10^{ {-3}}$ , the received sensitivity of 3D-PS-CAP-16 is improved by 1 dB. Moreover, with spherical constellation masking, a key space size of $9.5\times 10^{ {139}}$ is achieved to bring about high security and good encryption performance, suggesting broad application prospects in future short-range secure communications.