Abstract

Volumetric and areal storage densities represent key parameters of any storage system. Optical storage media can achieve high densities by focusing light to a small diffraction limited spot for reading and writing on the media. In the future as shorter wavelength recording technology becomes available, the areal density will improve quadratically but at each wavelength the areal density is still fundamentally limited by the optical diffraction limit of light. This assumption, however, is based on two-dimensional storage media and does not take advantage of a unique property of electromagnetic radiation which allows selective focusing of light at distinct levels throughout the volume of an optical storage medium. This selective focusing is possible because of the narrow depth of focus provided by the objective lens to allow selective reading and writing of data on each level by moving the focal position of the objective lens. Such an extension in the third dimension could obviously lead to substantial improvements of the volumetric storage capacities of an optical disk. In magnetic storage this extension is achieved using multiple low profile heads flying in between closely spaced disks.

© 1995 IEEE