Optical information carrier including standard and high density layers

Abstract

A record carrier having standard and high-density information layers. The standard density layer, e.g. in a standard CD format, is positioned at the bottom of a substrate, and is read out through the substrate with a laser beam. The thickness of the substrate is within the CD specification, e.g. 1.2 mm. At least one high-density layer is formed by a material which is (nearly) transparent for the laser wavelength of standard CD players (.lambda.=780 nm), but is (at least partially) reflective for shorter wavelengths used for high-density discs (.lambda.=635 nm). To achieve a high density, a smaller track pitch and smaller pits are used in the high-density layer(s). To achieve better reading signals from both layers, the high-density layer is positioned closer to the side of entrance of the laser beam in the substrate. A higher numeric aperture of the focusing lens and, therefore, a smaller spot for the shorter wavelength results. The high-density layer is preferably positioned midway in the substrate, e.g. at 0.6 mm. Such high-density layer configuration is compatible with two-sided high-density discs which have a total thickness of 1.2 mm. The information content of the standard layer may be duplicated on the high-density layer, or it may be represented by a different coding scheme.