Mark E. McDonald

Microholographic multilayer optical disk data storage

Micrometer-sized reflection holograms can be written into a rapidly rotating homogeneous photopolymer disk at the focus of a high-numerical-aperture beam and its retroreflection to implement high-capacity multilayer digital data storage. This retroreflection is generated by an optical system with positive unity magnification to ensure passive alignment of the counterpropagating beam. Analysis reveals that the storage capacity and transfer rate of this bit-based holographic storage system compare favorably with traditional page-based systems but at a fraction of the system complexity and cost. The analysis is experimentally validated at 532 nm by writing and reading 12 layers of microholograms in a 125-microm photopolymer disk continuously rotating at 3600 rpm. The experimental results predict a capacity limit of 140 Gbytes in a millimeter-thick disk or over 1 Tbyte with the wavelength and numerical aperture of Blu-Ray.

Error correction for increasing the usable capacity of photorefractive memories.

Conventional error-correction coding techniques can be used to reduce the minimum signal-to-noise ratio required for achievement an acceptable bit-error rate and can therefore be used to increase the maximum number of pages that can be stored in a photorefractive memory. Because error-correction bits will exact some cost per page in terms of memory capacity, we address the question of when the gain in capacity surpasses this cost. It is found that a factor-of-2 improvement in capacity can be readily achieved.

Technique for controlling cross-talk noise in volume holography

We study cross-talk noise in holographic memory and estimate storage limits. We examine the effects of reduced angular density and the use of an apodized reconstruction beam on capacity, cross-talk noise, and diffraction efficiency. Experimental Bragg-selectivity curves with and without an apodized reconstruction beam verify the expected reduction in cross talk.

Three-dimensional optical disk data storage via the localized alteration of a format hologram

Three-dimensional optical data storage is demonstrated in an initially homogenous volume by first recording a reflection grating in a holographic photopolymer. This causes the entire volume to be weakly reflecting to a confocal read/write head. Superposition of two or three such gratings with slightly different k-vectors creates a track and layer structure that specialized servo detection optics can use to lock the focus to these deeply-buried tracks. Writing is accomplished by locally modifying the reflectivity of the preexisting hologram. This modification can take the form of ablation, inelastic deformation via heating at the focus, or erasure via linear or two-photon continued polymerization in the previously unexposed fringes of the hologram. Storage by each method is demonstrated with up to eight data layers separated by as little as 12 microns.

Optical design for page access to volume optical media

4F lens designs are optimized for parallel access to volume holographic memories. Aberrations, diffraction, and component tolerancing are considered for their impact on parallelism, crystal information density, and overall system storage density. We find that a parallelism of ≥ 10(5) bits per page and a crystal information density of ≈2 Mbits/mm(3) are achievable with standard optical elements and that advanced designs offer significant improvements. Crystal surface tolerance measurements show that a diffraction-limited performance is achievable over apertures of 7.1 mm for LiNbO(3) and 1.5 mm for KNSBN(60). Lens-tolerancing simulations show that lens decenter degrades peak parallelism and peak crystal information density. Anew nonconfocal 4F system design with improved performance is presented.

Lens-design issues affecting parallel readout of optical disks

Lens designs are developed for parallel readout of data stored on optical disks. Optimizations of a single aspheric objective with standard wavelengths and numerical apertures are performed. These designs are evaluated for degree of parallelism achieved, improvement in aggregate data rate, source power requirements, and effect on head mass. We find that more than 225 channels and data rates of ~4 Gbits/s can be supported with low-cost readout optics.

Holographic Storage without Holography: Optical Data Storage by Localized Alteration of a Format Hologram

We propose and demonstrate multi-layer storage in holographic photopolymer by locally altering the reflectivity of a factory-written reflection hologram at the focus of a single objective lens. Linear, two-photon and thermal writing mechanisms are demonstrated.

Lens design issues impacting page access to volume optical media

Lens designs are discussed for page access to data stored in volume optical media. Optimizations of a 4F optical storage system are performed. These optimized designs are evaluated for degree of parallelism achieved, capacity, storage media volume, information storage density, and overall system volume. We find that the primary effect of aberrations is not to reduce memory capacity but to increase system volume resulting in lower overall storage density.

Micro-Holographic Multi-Layer Optical Disk Data Storage

We demonstrate 12-layer storage of 5.84 Gbits per square inch via micro-holograms written and read at 0.532 nm from a 125 micron photopolymer disk continuously rotating at 3600 RPM. Scaling predicts a potential TByte capacity.

Reference beam apodization in angularly multiplexed volume holography

Bragg selectivity in volume holography can be exploited to store many holographic pages within the same physical volume. The detailed character of this Bragg selectivity is also responsible for the crosstalk among stored pages and is governed by the 3D envelope function of the volume hologram. Other workers have discussed these crosstalk effects in detail. In this paper we discuss the effect of reference beam apodization on the angular selectivity of photorefractive volume holograms. Apodization using phase-only beam forming is studied and we focus on the use of apodization during hologram retrieval. The trade-offs among storage capacity, information density, and noise are discussed for both the apodized and unapodized configurations. The presence of conventional hologram crosstalk as well as effects due to material absorption and recording angle jitter are included.