Performance analysis of phase-code multiplexed holographic memory

Double-frequency grating shearing interferometer with built-in phase-shifting function for robust multi-level phase retrieval

In this paper, we propose and demonstrate a novel interferometer and signal process to retrieve two-dimensional signals with multilevel phases. The interferometer is based on a shearing interferometry with double-frequency grating, and phase-shifting interferometry is derived as a built-in function of the lateral displacement of the grating. The interferometer not only retrieves the multilevel phase signals but also eliminates slow-varying phase errors wherever they occur. Owing to the common path algorithm, the new interferometer is more robust in dynamic circumstances for optical testing and data processing. We propose a pre-integral signal process for two-dimensional (2D) data processing to prevent post-phase-integral due to shearing interferometry. The simulation and experiment showed that the proposed interferometer with a pre-integral process has various advantages in signal processing for multilevel phase retrieval, and will be useful for higher data rates in optical data storage and free-space communication.

Frequency expanded non-interferometric phase retrieval for holographic data storage

A novel frequency expanded non-interferometric phase retrieval method for holographic data storage is proposed. The limitation of twice Nyquist frequency for phase retrieval with high fidelity is broken through. Only 1 times Nyquist size frequency of the hologram is recorded, reconstructed and detected in our method which is the smallest range for complex multi-level phase retrieval in the current holographic data storage report. Required high-order frequencies for quick phase retrieval are generated artificially by expanding 1 times Nyquist frequency to high-order frequencies with utilizations of frequency periodicity. Therefore, our proposed method can increase storage density due to reducing the recording consumption of media and decreasing the recording area by 4 times compared with twice Nyquist frequency recording.

Fast non-interferometric iterative phase retrieval for holographic data storage

Fast non-interferometric phase retrieval is a very important technique for phase-encoded holographic data storage and other phase based applications due to its advantage of easy implementation, simple system setup, and robust noise tolerance. Here we present an iterative non-interferometric phase retrieval for 4-level phase encoded holographic data storage based on an iterative Fourier transform algorithm and known portion of the encoded data, which increases the storage code rate to two-times that of an amplitude based method. Only a single image at the Fourier plane of the beam is captured for the iterative reconstruction. Since beam intensity at the Fourier plane of the reconstructed beam is more concentrated than the reconstructed beam itself, the requirement of diffractive efficiency of the recording media is reduced, which will improve the dynamic range of recording media significantly. The phase retrieval only requires 10 iterations to achieve a less than 5% phase data error rate, which is successfully demonstrated by recording and reconstructing a test image data experimentally. We believe our method will further advance the holographic data storage technique in the era of big data.