Patent ID: 7274630

Claim:
A laser beam focus control apparatus comprising: an objective lens which converges an emitted laser beam and sends the laser beam to an information storage medium; a photodetector which receives reflected waves of the laser beam sent by the objective lens and outputs a corresponding detection signal; a generator which receives the detection signal from the photodetector and generates a focus sum signal and a focus error signal; a driver which supplies a driving signal for driving the objective lens in accordance with a supplied control signal; an actuator which receives the driving signal from the driver and properly moves the objective lens in accordance with the driving signal; and a controller configured to: (a) detect the focus error signal; (b) search and record timing of zero-crossing point of the focus error signal; (c) search and record peak timing of the focus sum signal that corresponds to the focus error signal on the time axis; (d) discriminate the focus error signal from the recording/reproducing layer surface and the focus error signal from the light-transmitting layer surface; (e) maintain the focus servo inactive when the zero-cross timing of the focus error signal and the peak timing of the focus sum signal are shifted from each other to prevent an erroneous focus operation on the light-transmitting layer surface; and (e) perform focus servo control operation only on the recording/reproducing layer surface wherein, assuming that a time from a timing at which the focus error signal exhibits a maximum value to a timing at which the focus error signal exhibits a minimum value is 1, when a time from the peak timing of the focus sum signal to the zero crossing timing of the focus error signal is not longer than 0.2, the controller generates the control signal, and supplies the control signal to the driver so as to properly move the objective lens and focus the laser beam on the information storage medium.