Information record

Abstract

A flat major surface of a disc-shaped substrate (e.g., of glass) is coated with a first light reflective layer (e.g., Rhodium) which is coated with a layer of a dielectric material (e.g., silicon dioxide) highly transparent of light of a frequency supplied by a playback laser, which transparent layer is coated with a second light reflective layer (e.g., Rhodium). The light output of a recording laser, which is intensity modulated in accordance with a signal to be recorded, is focused upon the coated surface of the disc as the disc is rotated. With the peak intensity of the focused light sufficient to at least cause melting of the second reflective layer, an information track is formed as a succession of spaced pits in which the first reflective layer is effectively exposed through the layer of dielectric material. For playback, light of a constant intensity is focused on the information track and the adjacent land areas as the disc is rotated. The focused light is of insufficient intensity to effect melting of the remaining portions of the second reflective layer, but is of a frequency at which the thickness of the dielectric layer exposed through the pits introduces a phase change of approximately (2K+1) .pi. radians between reflected portions of the focused light which fall on the pits and reflected portions of the focused light which fall on the undisturbed areas of the disc, where k is zero or any integer. A photodetector, positioned to receive light reflected from the information track as the pits pass through the path of the focused light, develops a signal representative of the recorded information.