Optimisation of a novel photopolymer for use in holographic data storage

Main Contact: Hosam Sherif

In 2002, it was estimated that people produced 5 exabytes (5 billion gigabytes) of data [1], the majority of which was in digital form. Since this figure is always growing and analogue media is constantly being converted to digital, new methods of storing this data are needed. Currently the two main storage methods i.e. magnetic and optical are just about keeping ahead of these needs; unfortunately this is not always going to be the case.

The limit of hard drive storage is expected to be reached within the next ten to fifteen years [2]. By then magnetic bit sizes will be so small that phenomenon such as the super paramagnetic effect (SPE) and shot noise will cause the polarity of the magnetic fields to flip randomly. Likewise, optical disks such as DVDs may reach their maximum capacity due to the diffraction limit of the laser light used.

One technology that may save us from an information overload is holographic data storage (HDS). Conceptually, holographic data storage has been around for several decades. In the early 70’s a basic storage system was built using mostly acousto-optic deflectors to record binary arrays, and optical detectors to read the data back [3]. However, it was not until the 90’s that real advances were made. The reason being that other technologies such as CCD detector arrays, mega pixel spatial light modulators, and inexpensive laser diodes had come of age.

Figure 1: (a) & (b) Image of data page and analogue image as they appeared on the SLM. (c) & (d) show the reconstructed holographic images which were produced with a combined beam exposure energy of 8 mJ/cm2 using the acrylamide-based photopolymer developed in the IEO.

So what advantages does HDS have over conventional storage methods? Firstly, data is stored in all three dimensions of the material and not just the surface. This 3D aspect allows for a phenomenon known as Bragg volume selectivity to be utilised, whereby many information-laden holograms can be multiplexed (i.e. superimposed) in the same volume of medium. It is necessary to 'Bragg detune each recorded hologram with respect its’ neighbours. This can be achieved in a number of ways, for example rotation of the media with respect to the recording object and reference beam, or changing the wavelength or phase of the recording laser beams with each new hologram recorded. A page of data can then be read out by illuminating the material with an appropriate reference beam. This makes for an inherently more efficient storage medium. Read/write speeds are also much higher since data is written and read out as 2D digital or analogue images in a 3D volume rather then a bit stream. An example of data pages that were written in the photopolymer developed in the IEO can be seen in figure 1. The bit rate error [4] is also decreased considerably because error causing factors such as scratches, dust, etc tend to affect the surface only and since a hologram is recorded as a complex interference pattern in the bulk of the material, these affects are limited.

The aim of this work is to test and optimise novel photopolymers developed in the IEO for use as holographic data storage medium. To facilitate this, an integrated optical set up has been constructed. The system is capable of functioning in two distinct ways. Firstly, it can be used as a method of characterising a photopolymer in terms of M/#, temporal stability, and angular selectivity. Secondly, with the insertion of a spatial light modulator the system along with relevant optics (collimator, polariser, CCD camera), can be used to record and read back page wise data.

Other research objectives include, developing improved algorithms for equalising multiplexed holograms, increased automation of data storage hardware/software, use of peristrophic (in-plane rotation) multiplexing in conjunction with angular multiplexing, add error correcting algorithms (CRC checks and Reed Solomon) to increase the fidelity of the recorded data, and optimise photopolymer parameters i.e. sensitivity, dynamic range, stability, shrinkage.

[1] Peter Lyman and Hal R. Varian, “how-much-info@sims.berkeley.edu”

[2] D. A. Thompson, J. S. Best, “The future of magnetic data storage technology ”, IBM J. Res. Develop. Vol. 44 No. 3 May 2000

[3] Langdon, R. M.(1970). A high capacity holographic memory. The Marconi Review, 33, 113-30.

[4] Hoffnagle J A and Jefferson CM 2000 Holographic Data Storage ed H J Coufal, D Psaltis and G T Sincerbox (New York: Springer) pp 91-100