Abstract

This paper proposes a high-security Orthogonal chirp division multiplexing-Orbital angular momentum (OCDM-OAM) optical transmission system based on small-scale dataset gated recurrent unit (GRU) chaotic-enhanced encryption.The system uses a hyper-chaotic model to interfere with bit streams, sub-carriers, symbols, and constellation angles. The GRU neural network trains and learns chaotic models with different initial values to generate chaotic sequences with chaos enhancement. A small-scale dataset is also taken to reduce the algorithm's complexity, and the final output is an encrypted sequence of time-domain chirps symbol perturbation with dynamic enhancement, substantially improving the system's security. The encrypted orthogonal chirp multiplexed signal is sent to the orbital angular momentum-based free space optical (FSO) communication system to increase spectrum efficiency and system robustness. The experimental research focuses on the security and higher-order modulation of the OCDM-OAM system, using spatial light modulators (SLM) to generate a phase diagram to achieve OAM modulation and demodulation of Gaussian light, reducing inter-channel interference and realizing light transmission in space. The OAM optical transmission platform verifies the high-security performance of the proposed encryption scheme. It is demonstrated that the higher-order OAM has a small impact on the bit error rate (BER) of the OAM system within the forward error correction (FEC) threshold. The experimental results show that the proposed encryption scheme achieves a large key space of 10³²² and 6.82 Gb/s transmission of valid information, which significantly improves the security performance of the OCDM-OAM spatial optical transmission system. It is a potential approach to the next wave of transmission systems for high-security OAM high-order multiplexing.