Holographic data storage (HDS) has the characteristics of large storage capacity and fast data transmission rate, which makes it a sustainable development technology in the era of big data. Phase-coded holographic data storage systems have attracted great attention because of such advantages as the high efficiency of light energy, stable structure, and easy modulation by a phase spatial light modulator (SLM). An orthogonal-phase cryptographic collinear holographic storage system is a new optical encryption technique that uses the optical interference principle of laser beams to write and read data and orthogonal phase encryption to ensure data security. Currently, orthogonal phase encryption techniques are often used in off-axis holography. Off-axis holographic storage systems are less resistant to environmental interference, whereby the jitter of the system causes the positions of the phase-key pixels used for encryption and decryption to fail to match properly, leading to information leakage. Therefore, in this study, we attempt to apply and explore the possibility of using orthogonal phase encryption with collinear holographic storage systems. Orthogonal phase encryption can protect the stored data from illegal access and theft to ensure data security. This is critical for maintaining the confidentiality of sensitive information involving personal privacy, trade secrets, and similar information.

Based on Kogelnik’s coupled wave theory, the propagation of phase during the formation and reproduction of holograms is accurately described by analyzing the propagation process of light. The recording key and the reading key are in an orthogonal state, that is, the number of units with phase difference of 0 is equal to the number of units with phase difference of π, so that the reconstruction intensity of the two parts of the signal light is cancelled through the phase orthogonality. In the system investigated in this study, the collinear system stores the signal light in the pagination encryption by modulating the reference wave with different encoding units. In this study, we propose two encryption modes for reference wave coding. The sector orthogonal phase-coding reference wave mode (SOPCR) divides the reference wave into eight units per sector, while the random orthogonal phase-coding reference wave mode (ROPCR) divides the reference wave into eight units randomly. The orthogonal phase code is generated from the Hadamard matrix and uploaded to the reference wave unit via the SLM in the form of phase keys. The feasibility of orthogonal phase encryption in a collinear holographic storage system is verified by recording and reading the encrypted images. The safety of SOPCR and ROPCR is evaluated by correlation coefficient (Corr) and mean square error (MSE) analyses.

Experimental results show that both methods can complete the encryption and decryption process using orthogonal phase coding. As shown in Fig. 8, the designed signal light [Fig.6(a)] is stored in the material and can be read out by the orthogonal phase key to complete the encryption and decryption function of the phase key. Due to the presence of crosstalk noise between pages, the reconstructed hologram [Fig.8(a)] cannot achieve the ideal reconstruction effect, and there is a crosstalk hologram. Because the diffraction efficiency of the material itself cannot reach 100%, the reconstructed hologram [Fig.8(b)] cannot achieve the ideal contrast and reconstruction effect. The correlation coefficient and mean-square error are used to evaluate the encryption performance of the two methods. The original reconstructed holograms of SOPCR and ROPCR are used as references for correlation coefficient and MSE since neither hologram achieves 100% diffraction efficiency. The correlation coefficients (Table 2) of SOPCR and ROPCR are 0.57 and 0.02, and the mean square errors (Table 3) of SOPCR and ROPCR are 537 and 1872, respectively. The correlation coefficients obtained by SOPCR are all higher than those obtained by ROPCR, while the mean square errors are all lower than those obtained by ROPCR. The crosstalk of the reconstructed holograms obtained by SOPCR is larger and the security of ROPCR is higher.

Orthogonal phase coding can perform the basic functions of encryption and decryption in collinear holographic storage. Systems with random orthogonal reference wave modes are more secure, as shown by correlation coefficient and mean square error tables. Orthogonal phase coding techniques have certain security properties and can enable information encryption. It has been shown that orthogonal phase encryption can be applied in collinear holographic storage systems. Moreover, there is a crosstalk problem in the sector orthogonal phase-coding mode and a low contrast problem in the random orthogonal phase-coding mode. Future work will also focus on the causes of these problems to improve the security and stability of orthogonal phase encrypted holographic storage systems.