Philips Research has announced at ISOM 2006, the International Symposium on Optical Materials conference held this past week in Japan, important new developments involving near-field recording technology.
Philips researchers (C. A. Verschuren, D. M. Bruls, B. Yin, J. M. A. van den Eerenbeemd, F. Zijp (Philips Research Laboratories,
Netherlands)) have made significant progress in advancing a new storage technology called near-field optical recording. The Philips Research announcement notes that it has successfully recorded
75GB of data on a 120mm diameter disc, using near-field techniques.
The Near-field optical recording terminology refers to the extremely short distance between the read/write head and the disc surface. The roughly 25-nm gap is directly comparable to the distance between the head and the disk surface in hard-disk assemblies. The technology has been extensively studied over the past years for use in various optical recording applications, because it allows much smaller bits to be recorded. However, numerous technical problems have prevented its use in commercially practical applications. New results from Philips give promise that some of these problems may soon be overcome.
As in the case of Blu-Ray, the fundamental for achieving large storage capacities is to increase the data density recorded on the medium. But the data density of an optical recording medium depends on the focused laser beam spot size, which is limited by diffraction. The beam spot size can be reduced by using a shorter wavelength laser or a larger Numerical Aperture (NA) objective lens. In the case of CD media, we are talking about a 780nm laser and 0.45 NA (0.7GB), for DVD we have 650nm and 0.6 NA (4.7GB) and for the Blu-Ray it is a "blue" 405nm laser, 0.85 NA (25GB).
Philips' approach for high density recording under the near field recording concept uses a 405 nm laser beam, focused by a pair of special lenses that offer a NA of 1.45 (!). Through this, the capacity of a DVD medium can reach 150GB or more, on two layers.
The challenge for Philips engineers was the development of a high NA lens system combined with a low-hovering recording head.
The answer is to use a blue laser to write and read data through a "solid immersion lens" (SIL). This type of optics is already used in microscopes and in lithography equipment for semiconductor production. The SIL uses the different refractive index of glass and air to achieve a high numerical aperture. The SIL optical head is composed of a hemisphere which is made from high refractive index glass and high NA focusing objective lens.
Philips? current experiments has a numerical aperture of 1.45 and
a refraction index of 1.58. The track pitch is 210nm (nanometers), and the channel bit-length is 38nm. A spacing layer just three micrometers thick (!) separates the SIL from the recordable or recorded surface of the disc
medium. According to Philips, the raw error rate is 6 x 10 to the minus 5th power. By raising
the numerical aperture to 1.61, a single layer can accommodate 125GB on a
120mm diameter disc, and as many as four layers can be used to create a
single disc capacity of 500GB. The recordable material used in the Philips
experiments is based on a CuSi (copper-silicon) inorganic formulation.
After having achieved the high NA, Philips had to make sure that the laser head would be able to accurately hover above the medium surface at distances that were roughly 25nm. A central ingredient of Philips' new technology is a servo that controls the position of the read/write head. For its tests on a Near field recording set-up, Philips mounted the lens in advanced 3D actuators, used for focusing and tracking.
Philips expects to be able to demonstrate two-layer near-field recording capabilities in 2007. Commercialization, however, of this research project is several years away.
Sony has also been active in near-field studies for many years, and its
results were first reported at ODS 2004. This report revealed recording
densities of 80.6Gbits/square inch and one-layer capacities of about
112GB. Blue lasers were used. The numerical aperture of the Sony SIL was
1.84, and it was separated from the recording surface by a gap of 20nm,
with no cover layer employed.