Abstract: An optical recording and reproducing disc for recording and/or reproducing information by the application of laser beams having a wavelength .lambda. in the range of 0.7 to 1.0 .mu.m, the disc being constituted by a disc-shaped base having at least one spiral guide track thereon with top surfaces parallel to the surface of the base at a height above the base different from the top surfaces of the portions of the base between the spires of the track, which height is in the range of .lambda./12 to .lambda./6, the guide track having a width approximately equal to the effective diameter of the laser beams; and a uniform thickness coating of a light recording material on the top surface of the guide track and the portions of the base between the spires of the guide track, the light recording material being a material the transmittance and/or the reflectance of which is varied by irradiation by the laser beams.

Abstract: A servo control system includes an error amplifier for deriving an error signal representing the displacement of a focus point of laser beam from different planes of an optical record disc, a differentiator for differentiating the error signal to detect its rate of variation, and a comparator for detecting the maximum peak of the differentiated signal which corresponds to reflections from the plane on which information is recorded. The focus point is moved in a direction toward the disc in an open loop mode until the maximum peak of the differentiated signal is detected, whereupon the focus point is controlled in response to the error signal in a closed loop mode to keep track of the recording plane.

Abstract: In an optical recording device in which the writing laser light beam is focused to a small spot on a disk which is coated with a photosensitive medium and spins at a predetermined velocity so that the data are recorded in terms of variable density or surface irregularities, the forward DC current applied to a semiconductor laser is varied depending upon the radius position of the writing laser beam spot on the disk but the AC current which is superposed on the DC current and rises or drops above or below the DC current level by a predetermined amplitude depending upon the data to be recorded is maintained constant regardless of the radius position of the writing laser beam spot, whereby the duty ratio can be maintained constant regardless of the distance from the center of the disk of a selected recording track.