Sony has prototyped a player that is able to read an 60GB optical disc using the near field communication (NFC) technology, at "ODS 2006", an optical data storage topical international meeting held in Montreal, Canada.
In his speech titled "Technologies for Removability in a Near-Field Optical Disc System", Mr Tsutomu Ishimoto of Sony Corporation Japan presented the latest developments of Sony in the area of Near Field Communications, as they are applied in optical disc drives.
The Near-field optical Disc terminology refers to the extremely short distance between the read head of a player and the disc surface. The roughly 20-nm gap is directly comparable to the distance between the head and the disk surface in hard-disk assemblies. Sony said that it has already managed to reproduce a 60 GB, 12 cm single-layer disc and obtained an eye pattern (RF signal).
The fundamental for achieving large storage capacities on optical media 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.
NFC technology uses near field optics to read and write smaller marks than the resolution determined by the wavelength of the light (laser).
To use near field optics, however, a working distance that indicates the distance between a head and a disc needs to be reduced to less than 20 nm.
As a result, the challenge for Sony engineers was the development of a high NA lens system combined with a low-hovering recording head.
The answer is to use a low-wavelength laser such as the 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.
After having achieved a high NA, Sony had to make sure that the laser head would be able to accurately hover above the medium surface at distances that were roughly 20nm. This extremely small distance between the disc surface and the laser head would make difficult to insert and eject discs in a drive. And this because, compared to other devices like an HDD that encloses a disc inside, replaceable discs are prone to catch dusts and the distance between a disc due to surface fluctuation and eccentricity.
Sony adopted three approaches as technological efforts to demonstrate that NFC technology can be applied even in the case of replaceable optical discs. Firstly, the prototype drive featured a head cleaner to remove dust from the SIL (Solid Immersion Lens). Secondly, it also featured an air cleaner to remove dust from the environmental air before taking it into the drive using a filter. Thanks to this, the inside of the drive can be kept as clean as a class 10000 clean room. Thirdly, Sony succeeded in controlling the distance between an optical head and a disc under 3 nm by improving the servo structure. Specifically, the company boosted the cutoff frequency of a gap servo (so-called focus servo) controlling the distance between an optical head and a disc to 13 kHz, three times higher than the 4 kHz used for Blu-ray Disc servos.
The NFC optical technology could boost capacity to 150 gigabytes stored on two layers of an optical media. However, commercialization of the technology could be many years away.