Self-referential holography and its applications to data storage and phase-to-intensity conversion

Common-path angular-multiplexing holographic data storage based on computer-generated holography

An unconventional angular-multiplexed recording technique is proposed for holographic data storage based on a computer-generated hologram (CGH) technique. While general angular-multiplexed recording techniques require a Mach-Zehnder interferometer to record data pages as volume holograms, the proposed method records ones with a common-path configuration with the help of a CGH technique, which prevents the optical setup from being bulky. In the proposed method, the CGH reconstructs signal and reference beams simultaneously, and these beams interfere in a recording medium. By changing the diffraction angle of the reference beam from the CGH, angular multiplexing is accomplished with a common-path optical setup without additional optical elements. Multiplexed recording of four data pages is demonstrated in a proof-of-principle experiment, which indicates the feasibility of the proposed method.

Computer-generated-hologram-based holographic data storage using common-path off-axis digital holography

A reconstruction method for multilevel complex encoded data-pages is proposed to increase the recording density of computer-generated-hologram-based holographic data storage by using off-axis digital holography. Although the detection process is based on off-axis digital holography, the proposed method keeps the optical setup a simple and common-path configuration owing to the computer-generated holography. Five-level complex encoded data-pages can be experimentally reconstructed.

Correlation-Based Multiplexing of Complex Amplitude Data Pages in a Holographic Storage System Using Digital Holographic Techniques

Holographic recording media can store the amplitude and the phase, or the complex amplitude, of a beam on the basis of holography. Owing to this characteristic, digital data can be encoded onto the complex amplitude of a signal beam in holographic data storage. However, most of conventional holographic storage systems encode digital data onto the amplitude alone because there are difficulties for modulating and detecting the phase. To solve the difficulties, a holographic storage system using digital holographic techniques has been proposed. With the help of digital holographic techniques, it is possible to modulate and detect the complex amplitude of a signal beam. Moreover, the proposed system can modulate the complex amplitude of a reference beam. In this paper, by making use of the capability, a correlation-based multiplexing with uncorrelated reference beams is demonstrated in the proposed system. Multiple holograms can be recorded in the same volume of a recording medium with no need for mechanical movements. Experimental results show that the proposed system with a correlation-based multiplexing can improve the storage capacity and can utilize the full potential of a recording medium without crosstalk noise stem from the optical setup.

Linear phase encoding for holographic data storage with a single phase-only spatial light modulator

A linear phase encoding is presented for realizing a compact and simple holographic data storage system with a single spatial light modulator (SLM). This encoding method makes it possible to modulate a complex amplitude distribution with a single phase-only SLM in a holographic storage system. In addition, an undesired light due to the imperfection of an SLM can be removed by spatial frequency filtering with a Nyquist aperture. The linear phase encoding is introduced to coaxial holographic data storage. The generation of a signal beam using linear phase encoding is experimentally verified in an interferometer. In a coaxial holographic data storage system, single data recording, shift selectivity, and shift multiplexed recording are experimentally demonstrated.

Recording procedures for high-quality signal readout in self-referential holographic data storage

Two methods are proposed to improve the readout quality of signals in self-referential holographic data storage (SR-HDS), in which no reference beam is required to record and read digital data holographically: the off-the-focus (OtF) method and the oversampling additional pattern (OsAP) method. The focal point is located outside of the recording medium in the OtF method, and the signal pattern is recorded with an additional pattern that possesses a higher spatial frequency than the signal pattern in the OsAP method. Experimental results show that both methods are effective for drastically improving the signal-to-noise ratio (SNR). In addition, through numerical simulation, it can be observed that the readout quality is improved by the achievement of homogeneous hologram distribution.

Shift-multiplexed self-referential holographic data storage

The feasibility and the properties of shift-multiplexed self-referential holographic data storage (SR-HDS) were investigated. Although SR-HDS has attractive features as typified by referenceless holographic recording, its multiplexing properties, which are consummately important for holographic data storage, have not been clarified until now. The results of numerical and experimental evaluations of medium shift dependence in SR-HDS clarified that the shift selectivity is almost the same as in collinear holography. Furthermore, 25 datapages were successfully shift-multiplexed with the shift pitch of 8.3 μm by the numerical simulation.

Design method of input phase mask to improve light use efficiency and reconstructed image quality for holographic memory

A design method of an input phase mask for holographic memory is proposed. In the method, a modification of a design procedure and another restraint condition are applied to our conventional design method. The light use efficiency and the quality of a reconstructed image are improved. The performance of an input phase mask designed by the method is confirmed by numerical simulations. Finally, a suitable design condition of an input phase mask is determined from simulation results.

Spatial cross modulation method using a random diffuser and phase-only spatial light modulator for constructing arbitrary complex fields

We propose a spatial cross modulation method using a random diffuser and a phase-only spatial light modulator (SLM), by which arbitrary complex-amplitude fields can be generated with higher spatial resolution and diffraction efficiency than off-axis and double-phase computer-generated holograms. Our method encodes the original complex object as a phase-only diffusion image by scattering the complex object using a random diffuser. In addition, all incoming light to the SLM is consumed for a single diffraction order, making a diffraction efficiency of more than 90% possible. This method can be applied for holographic data storage, three-dimensional displays, and other such applications.