Two Generations Away: Near-field recording for DVDs

[grazzhoppa]

CDRinfo.com reports that Phillips has created a laser lens that makes it possible to store 150 GB on a dual layer DVD sized disc. The "solid immersion lens" has been used in microscopes and with semiconductor production but not with optical storage yet. Near-field recording gets its name from the tiny, 25 nanometer, space between the head of the lens and the surface of the disc.

A little bit of info to put this in perspective:

As in the case of Blu-Ray, the fundumental 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 there will be 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 1.5-20! Through this, the capacity of a DVD medium can reach 150GB or more, on two layers.

A challenge for Phillips was to make sure the lens doesn't smash into the disc. When you measure things in micrometers and nanometers, small things become big problems. The surface of a disc isn't perfectly flat, it is a bit curved due its the manufacturing process. Supposedly the disc will undulate about 25 micrometers. Phillips put together a servo that keeps the head of the lends roughly 25 nanometers from the disc's surface.

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