Coaxial holographic encoding based on pure phase modulation

Abstract

We describe a simple technique for coaxial holographic image recording and reconstruction, employing a spatial light modulator (SLM) modified in pure phase mode. In the image encoding system, both the reference beam in the outside part and the signal beam in the inside part are displayed by an SLM based on the twisted nematic LCD. For a binary image, the part with amplitude of "1" is modulated with random phase, while the part with amplitude of "0" is modulated with constant phase. After blocking the dc component of the spatial frequencies, a Fourier transform (FT) hologram is recorded with a uniform intensity distribution. The amplitude image is reconstructed by illuminating the reference beam onto the hologram, which is much simpler than existing phase modulated FT holography techniques. The technique of coaxial holographic image encoding and recovering with pure phase modulation is demonstrated theoretically and experimentally in this paper. As the holograms are recorded without the high-intensity dc component, the storage density with volume medium may be increased with the increase of dynamic range. Such a simple modulation method will have potential applications in areas such as holographic encryption and high-density disk storage systems.

Recent advances in holographic data storage

“…In 1998, Sony Corporation presented a similar concept using shift multiplexing [24]. Following this, Optware Corporation and a number of others in the industry began developing systems based on these concepts [14,[25][26][27][28][29][30][31][32]. Collinear technologies have significant advantages compared with 2-axis holography: good write and read performance, uniform shift selectivity for both radial and tangential directions, and fairly large system tolerances have been experimentally reported [14,25,26].…”

“…The RL amplitude modulation avoids the Bragg degeneracy from point sources in the reference region to tighten the PSF; however, 80% of energy is wasted [32]. As images are binary amplitude data pages modulated with a SLM, the zero order spatial frequency (dc) component in the back focal plane of the Fourier lens takes a much higher percentage of the light intensity than any other spatial frequency components [28]. The high-intensity dc component will saturate the dynamic range of the recording medium, which consequently decreases the data storage density with shift multiplexing techniques [28].…”

“…As images are binary amplitude data pages modulated with a SLM, the zero order spatial frequency (dc) component in the back focal plane of the Fourier lens takes a much higher percentage of the light intensity than any other spatial frequency components [28]. The high-intensity dc component will saturate the dynamic range of the recording medium, which consequently decreases the data storage density with shift multiplexing techniques [28]. While no fully functioning system came out of Optware's work, collinear HDS techniques represented a tremendous step forward in the optical storage industry's understanding and participation in holographic storage, and stimulated advances aimed at leveraging traditional optical storage technologies in holography.…”

“…Early systems used shift multiplexing, but more recent systems have switched to phase correlation multiplexing [28,[32][33][34][35][36]. The phase structure is either embedded into the SLM, or imaged onto it, which homogenizes the Fourier transform of the data beam in addition to providing the phase information needed for phase correlation.…”

Novel modulation techniques for collinear holographic data encoding

“…In this section, we describe a novel method to achieve homogeneous collinear holographic data encoding. The reference beam and the object beam, which are collinearly aligned, are modulated by an SLM with pure phase modulation [21]. The amplitude image can be reconstructed through this pure phase modulated FT hologram directly without any additional techniques.…”