Near-Field Method to Extend Optical Recording
John Baliga, Associate Editor -- Semiconductor International, 9/1/2000
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Unlike CDs and DVDs, this recording method is not for removable media. It relies on fly heights comparable to HDDs, which means the head and media must be assembled in a cleanroom the same way an HDD is assembled.
The slider is essentially a VCSEL fabricated on a GaAs substrate (Figure). The VCSEL is defined within a large die, which becomes the slider.
The keys to the high density are the small aperture of the VCSEL and small fly height of the slider. The VCSEL is fabricated with an aperture much smaller than a wavelength, and the distance to the medium is half the aperture size. Both of these conditions ensure near-field operation, which means the recorded bit size is not determined or limited by the wavelength of the laser. Current state-of-the-art hard disk drives have fly heights on the order of 25 nm, so the fly height does not limit the bit size.
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In addition to the distributed Bragg reflector on the output, there is a metal mirror layer, in which the small aperture is made using a focused ion beam or similar technique. Since there is a high optical field near the metal layer, the output through the aperture also is high. The company has developed proprietary techniques to minimize optical losses, including those due to the metal layer. Apertures as small as 50 nm have been fabricated.1 With the medium so close to the laser aperture, reflection normally would be seen as a problem, since VCSELs and other semiconductor lasers can be very sensitive to the beam being reflected back into the cavity. This is actually used as a bonus to make the laser double as a read head. When operated at constant current above threshold, the voltage changes when the beam is reflected back into the cavity.2 Different storage states in the medium can have different reflectivities, which the laser can detect.
Optical phase change materials, such as those used on rewritable DVDs, typically have modest crystallization speeds on the order of 50 nsec. Siros is developing an approach using proprietary materials to deliver data rates suited to hard disk drive applications. Also, since writing and erasing changes the optical properties of the medium, rather than storing a magnetization on a section of the medium, optical recording is not susceptible to the superparamagnetic effect that is the ultimate limitation in current magnetic recording technology. •
REFERENCES
1. A. Partovi, "Optical Near-Field Aperture Storage Technique (ONFAST) for High Density, High Performance Data Storage Applications," Joint International Symposium on Optical Memory and Optical Data Storage 1999, SPIE, Koloa, Hawaii, July 1999.
2. A. Partovi, D. Peale, M. Wlttig, C. Murray, G. Zydzik, L. Hopkins, K. Baldwin, W. Hobson, J. Wynn, J. Lopata, L. Dhar, R. Chichester, J. Yeh, "High-Power Laser Light Source for Near-Field Optics and its Application to High-Denstiy Optical Data Storage," Applied Physics Letters, volume 75, number 11, Sept. 13, 1999, pp. 1515-1517.
