Patent ID: 7020055

Claim:
An optical pickup apparatus comprising: a light source generating and emitting a light beam; an objective lens condensing the light beam emitted from the light source to form a light spot on a recording medium; an optical path changing device changing a proceeding path of the light beam; a detection unit detecting the light beam reflected/diffracted by the recording medium and passing through the objective lens and the optical path changing device by dividing the light beam into a plurality of light areas to generate light area detection signals; and a signal processor obtaining focus error signal from the light area detection signals output from the detection unit, wherein the focus error signal comprises a first signal, a second signal, and an S curve obtained therefrom, the signal processor generating a maximum value (a) and an absolute value (b) of maximum and minimum points of the S-curve, respectively, with respect to a predetermined reference level, and obtaining a calculation value of (a−b)/(a+b), and outputting a thickness signal of the recording medium based on the obtained calculation value.