to incorporate not only the spatial dimensions but also spectral
and polarization distribution. To realize spectral encoding
in the volume of a recording medium, researchers must use
a broadband source such as a supercontinuum source as the
excitation source.
Due to the nature of dispersion, the
focus of a broadband source will split
when it propagates through the dispersive medium. To maintain the intensity
distribution of the focus in the whole
volume of the recording medium and
remove the aberration effect, one must
engineer the point spread function of a
broadband source by providing spectroscopic compensation.
In addition, owing to their vectorial
properties, the electric field will depolarize into three components—Ex, Ey and
Ez —in the tight focus of an objective
with a large NA. The three electric-field
components of the point-spread function
can be engineered individually with a 3-D
polarizer. This will enable information to be encoded in the
polarization states of the writing beam; it will be added three-dimensionally inside the focus instead of only being multiplexed in the x-y plane. Therefore, this effort can significantly
expand the number of multiplexed channels.
Further, the point spread function can also be engineered
to split the single focus into multiple ones using phase modulation technology to engineer the input phase. This effort can
lead to an efficient solution to address the recording and reading speed associated with the bit-by-bit data storage.
An alternative approach to achieving a focus spot size
below the diffraction-limited barrier is to collect the evanes-
cent waves and propagate into a far-field region. This idea has
led to the superresolution achieved in materials of negative
refractive-index, which is called the superlens
effect. With metamaterials, researchers have
realized far-field superlenses for imaging sub-
diffraction objects. Designing and developing
such a far-field superlens for applications in
5-D optical data storage will be one of many
promising future explorations. The lens would
be capable of focusing a superresolution spot
in the far field.
For a 5-D optical
system, the point
spread function
must be engineered
to incorporate not
only the spatial
dimensions but also
spectral and polarization distribution.
Beyond five dimensions
Apart from the physical dimensions in the
spectral and polarization domains, an optical
beam also possesses orbital angular momen-
tum. This physical dimension can be adopted
to encode information. When successful, it
can provide the sixth dimension for optical
data storage, thereby resulting in a further expansion that will
help fill our insatiable need for data storage. t
The authors thank the Australian Research Council for its support, as
well as the current and previous team members for their contributions,
including Daniel Day, Dennis McPhail, Xiangping Li, Peter Zijlstra,
James Chon, Kyongsik Choi, Craig Bullen, Joel Van Embden , Shuhui
Wu and Richard Evans.
Min Gu ( mgu@swin.edu.au) and Xiangping Li are with the Centre for
Micro-Photonics, Faculty of Engineering and Industrial Sciences,
Swinburne University of Technology, Hawthorn, Victoria, Australia. Member
Breaking the diffraction-limited barrier
Although the capacity of 5-D optical data storage is potentially much larger than that of 3-D systems, the focus spot size
is still confined in the x-y-z spatial region by the diffractive
nature of light. For a given number of multiplexed channels,
the storage capacity is mainly restricted by the spatial resolution of the optical system. Therefore, breaking the diffraction
limitation to achieve a superresolution focus spot size is of significance to petabyte 5-D optical data storage. There have been
several approaches to break the diffraction-limited barrier of a
light beam, including far-field stimulated emission depletion
(STED) and superlens methods.
In a typical STED system, one beam can be used to
activate the recording, while another deactivates the recording. By spatially shaping the overlapping of the two beams,
one can achieve a superresolution focus spot size of l/20.
Combining the STED method with spectral and polarization
encoding techniques can significantly expand the current
storage capacity.
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