Optical axis monitoring system

Claim 1

In an optical information recording apparatus for optically recording information on a recording surface of a record member comprising: a laser beam source providing a laser beam directed onto said surface, an electro-optical modulator positioned between said source and said surface and intercepting said beam to modulate said beam, the improvement comprising: a first beam splitter positioned in the path of said light beam downstream of said source and upstream of said modulator separating said beam into a main beam and a first sub-beam, a first monitoring light detector element positioned in the path of said sub-beam, a second beam splitter positioned in the path of said main beam downstream of said modulator further separating said main beam and forming a second sub-beam, a second monitoring light detecting element positioned in the path of said second sub-beam, a polarizing beam splitter positioned in the path of said main beam downstream of said second beam splitter and including a quarter-waveplate for rotating the main beam incident on said recording surface, a lens interposed between said polarizing beam splitter quarter-waveplate and said recording surface for reflecting light from said recording surface to said polarizing beam splitter through said quarter-waveplate, such that the reflected light is rotated by the beam splitter through a given angle, and wherein said polarizing beam splitter separates said reflected polarized light beam into a main reflected polarized light beam, and a polarized reflected light sub-beam, and a polarized light sub-beam light detecting element positioned in the path of said polarized reflected light sub-beam; whereby, an output signal is developed by the polarized light sub-beam detector element for controlling the position of the objective lens in a direction perpendicular to the recording surface so that the main recording light beam is prevented from being defocused by warpage of the recording surface under focus-servo operation, recording beam convergence is maintained on the recording surface at all time, and the optical axis of the main beam is maintained coincident with the designated optical axis by comparing the output signals of the two monitoring light detecting elements and permitting optical axis alignment with the designated optical adjustment solely by adjusting the laser beam source and without further adjustment of the balance of the components of the optical information recording apparatus.