JP2777341B

In detecting a focus error for an optical pickup, an inverse image generator for generating inverse images onto a light beam reflected from an optical disk is used in a focus error detecting method and apparatus thereof. The inverse image generator is composed of a semi-transmissive plane for splitting a light beam into first and second light beams, first and second prisms for inverting the phase of the second light beam, and third and fourth prisms for synthesizing the phase-inverted second light beam with the first light beam. Also, the inverse image generator has two quadrant wave plates, a polarization plane and light synthesizing plane for preventing a light loss. Since the errors due to an optical axis variation is eliminated from the focus error signal detected from the synthetic light beam radiated by the inverse generator, the optical pickup can be driven with stability.