Multiple-image encryption using phase jump gradient factors -based OAM multiplexing holography

Orbital angular momentum (OAM) has been widely implemented in information encryption as an independent degree of freedom. In this paper, we propose using phase gradient factor of vortex phase structure as an independent degree of freedom for further boosting OAM information carrying capacity. The orthogonality of different phase gradient factors is firstly demonstrated. Multiple images encoded in different phase gradients even with the same integral or fractional topological charge (TC) can be simultaneously encrypted free of crosstalk. Herein, the safety and the multiplexing capacity are greatly enhanced while keeping the fractional topological charge interval as small as 0.1. In the decryption process, the encoded information can only be reconstructed under the incident vortex beam illumination with inverse TC at correct focus distances and accurate phase jump gradient factors. The feasibility and the security of proposed method are both verified in the experiments and the numerical simulations. This opens up a new avenue for high security and practical information encryption systems.