18-07-2011, 10:45 AM
holographic data storage.pdf (Size: 682.09 KB / Downloads: 158)
INTRODUCTION
1.1 History
Digital data are ubiquitous in modern life. The capabilities of current storage
technologies are continually being challenged by applications as far ranging
as the distribution of content, digital video, interactive multimedia, small
personal data storage devices, archiving of valuable digital assets, and downloading
over high-speed networks . Current optical data storage technologies,
such as the compact disk (CD), digital versatile disk (DVD), and Bluray disk
(BD) , have been widely adopted because of the ability to provide random access
to data, the availability of inexpensive removable media, and the ability
to rapidly replicate content (video, for example)[1].
Traditional optical storage technologies, including CD, DVD and BD,
stream data one bit at a time , and record the data on the surface of the
disk-shaped media. In these technologies, the data are read back by detecting
changes in the reflectivity of the small marks made on the surface of the
media during recording. The traditional path for increasing optical recording
density is to record smaller marks, closer together. These improvements in
characteristic mark sizes and track spacing have yielded storage densities
for CD, DVD, and BD of approximately 0.66, 3.2, and 17 Gb in m−2 ,
respectively.
To further increase storage capacities, multi-layer disk recording is possible,
but signal to noise losses, and reduced media manufacturing yields,
make using significantly more than two layers impractical. Considerable
drive technology changes, such as homodyne detection and dynamic spher-
CHAPTER 1. INTRODUCTION 3
ical aberration compensation servo techniques, have been proposed to deal
with the signal to noise losses inherent in multiple layers[2].
1.2 Holographic Data Storage
Holog aphic data storage (HDS) breaks through the density limitations of
conventional storage technologies by going beyond two-dimensional layered
approaches, to write data in three dimensions. Before discussing page-based
HDS, here an alternate approach; bitwise holographic storage .
In bitwise holographic storage, multiple layers of small localized holograms
are recorded at the focus of two counter-propagating beams. Each
of these holograms represents a single bit that is subsequently read out by
monitoring the reflectance of a single focused beam . racking the hologram
locations through the volume in three dimens ions is typically accomplished
using a reference surface or part of the holograms themselves. Bitwise holographic
storage is appealing because the drive technology and components
are similar to traditional optical storage , and because the media is homogenous
and hence easy to manufacture. However, there are several serious
drawbacks. First, it is difficult to achieve fast transfer rates. Also, it requires
the invention of a material that is optically nonlinear. The technique also
requires a complex servo system because the two recording beams must be
dynamically focused into the same volume. Finally, the multiple layers of
micro holograms cause distortion in the optical beams, which significantly
limits the achievable density.
Page-wise HDS has demonstrated the highest storage densities (712 Gb
in m−2 ) of any removable technology, and has a theoretically achievable
density of around 40 Tb in m−2. High storage densities , fast transfer rates
and random access, combined with durable, reliable , low cost media, make
page-wise holography a compelling choice for next-generation storage and
content distribution applications. The flexibility of the technology allows
the development of a wide variety of holographic storage products, ranging
from handheld devices for consumers to storage products for the enterprise
market.
Figure 1.1 shows the highlihts in holographic storage developments over
the last 15years. The right-hand side of the figure shows technical advances
made by Bell Laboratories and InPhase Technologies, while those of other
companies and institutions are shown on the left-hand side of the figure[