Objective   In order to improve the capacity and security of the optical encryption system, a new method based on OAM holography and frequency shift is proposed.

  Methods   Since the orbital angular momentum eigenstates mathematically constitute a complete set of orthogonal basis vectors, the orbital angular momentum can be used to realize the encoding of multiple image information. This approach achieves parallel encryption of multiple images information using Fourier transform frequency-shift and OAM holography techniques, which eliminates the crosstalk between multiple image information. Firstly, several original images information are sampled, and the sampling constants is determined by the spatial frequency of the helical phase with different helical mode indexes, which is defined as the pixel width corresponding to 30% of the maximum amplitude modulus (Fig.3). Through this process, we obtained multiple different sampled images. The orbital angular momentum-preserving holograms are then generated by the coherent superposition of multiple sampled image information modulated by random phase, Fourier transform and frequency shift phase (Fig.4). Finally, the helical phases of different helical mode indexes are encoded into two orbital angular momentum preserving holograms, and two OAM selective holograms are obtained, they are superposed coherently to form the final OAM composite selected hologram (Fig.5). In decrypted process, the orbital angular momentum compound choice hologram is loaded onto the spatial light modulator, which is illuminated by a vortex beam containing a specific helical mode index and passes through a Fourier lens, a receiving device on the rear focal plane of the lens can receive a plurality of decrypted images (Fig.6).

  Results and Discussions   Compared with other multi-image encryption algorithms, the proposed method can encrypt multiple image information of different sizes and types into a single hologram through two encryption processes, and no original information is displayed in the ciphertext image. The correlation of ciphertext image information in all directions is very low, which effectively reduces the statistical characteristics of highly correlated original image information, and can effectively resist statistical attacks based on pixel correlation, with high security (Tab.1, Tab.2). This encryption system has high encryption flexibility and great capacity. It can not only design different frequency shift factors to encrypt a group of multiple images information in parallel under the same helical mode index, but also has the advantages of high encryption flexibility and high encryption capacity, several groups of image information can also be encrypted by using different helical mode index.

  Conclusions   In this method, the infinite OAM mode of the vortex beam are set as a new optical key, which greatly improves the security of the encryption system. In addition, due to the frequency-shift phase modulation, the size of the image to be encrypted is not limited by the number of pixels in the Spatial light modulator, which greatly improves the feasibility and effectiveness of optical realization of information encryption. The simulation results show that the proposed method has high safety, anti-noise and anti-shear capability (Fig.11, Fig.12).