Ultra-security optical image encryption using speckles through a multi-mode fiber

To realize optical image encryption for long-distance transmission while considering its security performance, an optical image encryption scheme is proposed in this paper. In the scheme, the pixel information of the plaintext image is first encoded by orbital angular momentum (OAM) holograms; then, the information-coded OAM beam is transmitted through a 1 km multimode fiber to generate speckles as ciphertexts for encryption; and finally, a pre-trained deep learning model capable of learning the relationship between the output speckles and the input information-coded OAM modes is used for decryption. The proposed scheme is not only able to achieve high fidelity recovered image but also a remarkable level of security. The high security stems from the combined use of three keys: the order key, the model key, and the coder key, during optical encryption. The image can therefore only be decrypted by authorized users who simultaneously know the three keys. We have experimentally demonstrated the high fidelity and high security encryption/decryption capabilities. Our work can provide a promising avenue for further research into long-distance optical image transmission and encryption with ultra-high security.