| 摘要 |
PURPOSE:To obtain a focus error signal without using an external signal by a method wherein a light-path-length change means which changes the light path length of individual optical beams is installed between a multi-laser diode which radiate a plurality of optical beams and an optical beam irradiation object medium. CONSTITUTION:Optical beams radiated from individual beam emitting parts 11 to 13 at a three-beam laser diode 1 are incident on a stepped transparent plate 3. The individual optical beams which have been incident on the transparent plate 3 are passed through respectively different stepped faces 31 to 33, and their transmission light path lengths are changed. The individual optical beams which have been subjected to the changed action of the light path lengths by means of the transparent plate 3 are narrowed down to very small spots by using an object lens 4, and their images are formed on an optical recording medium 5. Reflected beams of light at the medium 5 are fed back to the light-emitting parts 11 to 13; composite resonance optical outputs on the basis of the feedback are detected by using a photodetector 2. Since the optical path lengths of the individual optical beams have been changed by using the transparent plate 3, their focal positions are different in the depth direction of the medium 5 at the individual optical beams. |
