l = 780 nm
NA = 0.45
l = 650 nm
NA = 0.6
4. 7 GB
l = 405 nm NA = 0.85Blue DVD
23. 5 GB
l = 900 nm NA = 1. 4 3-D CD
Evolution of optical data storage systems
Compact discs, the first-generation optical data storage devices,
emerged in the 1980s. They use a focused laser beam to induce a
localized change within a two-dimensional (2-D) layer near the
surface of a recording medium. The information occupies less
than 0.01 percent of the volume of a CD. Due to the limitation
of the recording wavelength and the numerical aperture
(NA) of the recording lens, the maximum
data capacity of the first-generation optical data storage systems is approximately
650 to 750 megabytes ( 1 MB = 1 million
bytes; 1 byte = 8 bits) for each CD.
The development of DVDs was aimed
at breaking this limit. DVDs marked the
beginning of the second generation of optical data storage technology. Because of the
diffraction-limited feature after the beam
passes through the objective, the resolution
of such a system can be written as r = 0.61
* l/NA, where l is the wavelength and NA
is the numerical aperture of the objective.
This technological development involved
the recording of information while reducing
the recording wavelength to 650 nm and
increasing the NA of the recording lens to 0.6.
The diffraction-limited resolution of the DVD system has
been improved 40 percent compared to the CD technique,
therefore greatly enhancing storage density to 4. 7 GB/disc.
Blue DVDs, or so-called Blu-rays, operate at a much shorter
wavelength of 405 nm. They also increase the NA to 0.85,
allowing more information to be stored in the same area with
a smaller focal spot size. Therefore storage capacity has been
greatly expanded to 23. 5 GB/disc.
of the volume of the optical disc has not been used. Researchers have pursued further research to explore the feasibility of
three-dimensional (3-D) optical data storage, such as double-layer DVDs and double-layer Blu-ray discs.
Some have tried adding an axial dimension, where informa-
tion is recorded in the x-y-z space. However, when a recording
beam is focused deep into the volume of an optical medium,
scattering loss occurs. The shorter the wavelength, the stronger
the loss. The energy cannot be delivered
efficiently for multilayer recording. In
addition, it is very difficult to manufacture
three-layer DVDs or Blu-Ray discs.
This challenge has spurred the revolutionary idea of two-photon (2P) excitation
by an infrared femtosecond (fs) pulsed
laser beam with a pulse width of 100 fs.
Due to its highly confined properties and
high efficiency of penetration into the volume of a recording medium using infrared
beams, the information can be recorded in
multiple layers of the recoding medium.
This approach is sometimes called 3-D
The 2P excitation technique has been
widely applied to a variety of materials
for high-density memories, including photopolymerizable
materials, photochromic materials, photorefractive materi-
als, photobleaching materials, void-fabricatable materials and
nanoparticles dispersed materials. The important milestones
that scientists have achieved toward the development of high-
density 3-D storage systems are summarized in the table on
the facing page.
In a 1989 study in the journal Science, researchers described
introducing the 2P excitation technique into 3-D optical
data storage with two orthogonal beams intersecting inside
the focuses of two objectives. The elegance of this approach
is highly localized recording and random access to data.
In a 2-D optical data
is recorded only
in one layer inside
the medium, 99.99
percent of the volume
of the optical disc has
not been used.
In a 2-D optical data storage system, where information is
recorded only in one layer inside the medium, 99.99 percent
30 | OPN Optics & Photonics News