Abstract

The spot size of an optical storage system is determined by diffraction to be approximately λ/2 N.A., where λ is the free space wavelength, and N.A. is the numerical aperture of the objective lens. For λ = 0.89 μm and N.A. = 0.55, the spot size is approximately 0.8 μm. To improve the definition, it is necessary to work at shorter wavelengths, typically in the blue, which yields a definition of about 0.4 μm, and/or to increase the numerical aperture. A further possibility is to employ near field optics in the manner described by Betzig.¹ He uses a tapered fiber covered with a metal film with a small pinhole at the end. The definition of the system is determined by the size of the pinhole, and can be 50 nm or less. The advantages of the system are its excellent definition and it polarization-preserving capability for magneto-optic storage. The disadvantages of the system are its poor light efficiency and the fact that it can only observe a single spot at a time, thus limiting its tracking ability.

© 1994 Optical Society of America