Holographic data storage is one of the solutions to store Terabyte-scale information in a 5-inch disc. Unlike the current optical data storage, such as CD, DVD, and Blu-ray Disc, holographic data storage stores a 2D data array as a hologram. In addition, a number of holograms are multiplexed on the same volume of a recording medium. Several multiplexing methods have been proposed and developed to realize huge storage capacity. Among them, multilayer recording enables the efficient use of the dynamic range of a thick medium, and thereby storage capacity is improved. However, a conventional multilayer recording method requires a movable stage to displace a recording medium along an optical axis. This makes a recording system large and complicated. In addition, the additional actuator leads to vibration in the recording system, which results in low quality retrieved data.
To realize multilayer recording without a movable stage, the use of a varifocal lens generated with a kinoform is presented. Kinoforms displayed on a spatial light modulator (SLM) allow the temporal generation of a 3D optical field. By using a kinoform as a varifocal lens, a focus plane can be displaced along an optical axis. This kinoform is superimposed onto a phase mask which is generally used for improving a recording performance. The proposed method, therefore, eliminates the need to use a movable stage for displacing a medium and allows stable multilayer recording in a simple setup. Experimental demonstration shows the feasibility of the proposed method.
In most of the conventional holographic storage systems, SLMs are used for just imprinting digital data onto a light. However, SLMs have the capability of generating arbitrary complex amplitude distribution by using digital holographic techniques, such as a computer-generated hologram and a kinoform. The use of digital holographic techniques opens the possibility of developing unique functionality and improving a recording performance in holographic data storage.
Nobukawa T, Nomura T.
A multilayer recording method using a varifocal lens generated with a kinoform is presented. In this recording method, a focus position is axially displaced by adding a defocus phase to a phase modulation pattern, which consists of a random phase mask and a computer-generated reference pattern. Shift selectivity and multiplexed recording are experimentally investigated in coaxial holographic data storage. Experimental results show that the proposed method allows the recording of holograms along an optical axis without any mechanical movement.
© 2015 Optical Society of America
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