High performance optical storage medium with separate mechanical servo

Abstract

An optical storage medium incorporates stacked servo and data layers with the pattern of the servo layer being prepared prior to preparation of the data layer. The servo pattern comprises an undulation in a very thin film of high optical index material on the surface of the substrate of the medium. The undulating layer containing the servo pattern is separated from the active layer of the medium by a thin dielectric spacer film. The data layer is employed for formation of vesicular, or ablative or other marks forming surface discontinuities formed by any of the usual pit forming techniques. The preformatted servo information formed in the servo layer as buried undulations in the thin film is read out in amplitude or phase contrast. The thin undulating layer is of a significantly different index of refraction from the substrate and the next intervening layer. This layer can be transparent or semi-transparent. A smoothing layer is then applied over the phase-contrast layer to reduce any undulations at the top surface on which the active layer is formed. The amount of energy absorbed by the active layer of the structure is optimized for the data recording wavelength by choice of thickness of the smoothing layer.