Optical recording and reproducing apparatus with improved focussing control

Claim 1

An optical information recording and reproducing apparatus for respectively recording and reproducing an information signal on and from a recording medium, having a recording track, comprising: beam generator means, including a laser, for generating a primary beam and first and second secondary beams which are spaced apart substantially in the direction along said track on said recording medium with said primary beam impinging on said recording medium between said first and second secondary beams; focussing lens means for focussing said primary beam and said first and second secondary beams on said recording medium; detecting means including a primary photodetector for detecting said primary beam reflected from said recording medium and first and second secondary photodetectors for detecting said first and second secondary beams reflected from said recording medium; beam intensity control means for controlling the intensity of at least said primary beam such that high and low intensities of said primary beam are sufficient to form indicia in said recording medium and to prevent the formation of said indicia, respectively, in accordance with said information signal when said apparatus is in a recording mode, and such that a substantially constant intensity is maintained in at least said primary beam to prevent indicia from being formed in said recording medium when said apparatus is in a reproducing mode; focussing control means for controlling the position of said focussing lens means in response to output signals of said first and second secondary photodetectors when said apparatus is in said recording mode, including difference signal generator means for generating a difference signal in accordance with a difference between said output signals, and control signal generator means responsive to said difference signal for generating a control signal to control the position of said focussing lens means; said primary beam and said first and second secondary beams tracing said track in a forward direction to selectively form said indicia along said track when said apparatus is in said recording mode, such that said first secondary beam is reflected from said recording medium at a location along said track ahead of said primary beam in said forward direction where no indicia have yet been formed, and said second secondary beam is reflected from said recording medium at a location along said track behind said primary beam where indicia have already been selectively formed and are in a stable state; and tracking control means for controlling the position of said primary beam in the transverse direction of said recording track in response to said difference signal when said apparatus is in said reproducing mode.