Miniaturized optical pick-up with high sensitivity focusing error detecting device

摘要

Optical pick-up for reading information recorded on an optical card includes a light emitting diode, collimator lens, beam splitter and objective lens arranged so a de-focused illuminating light spot projects onto the card and reflects to a photodetector through the objective lens and beam splitter. The photodetector includes first light receiving regions arranged within a ring-shaped area where the luminance intensity of the beam does not substantially change even when a focus condition of the objective lens with respect to the card is varied, and second light receiving regions arranged outside the ring-shaped area. A focusing error signal is obtained by deriving a difference between a first sum of output signals of the first light receiving regions and a second sum of output signals of the second light receiving regions. The collimator lens may be arranged between the beam splitter and the objective lens and the photodetector arranged on a focal point of the collimator lens. Or, a sapphire ball may be directly secured to a light emitting diode to form a ring-shaped bright portion in an illuminating light beam emanating from the sapphire ball. Then the first and second light receiving regions of the photodetector are arranged on both sides of a caustic surface of an image of an illuminating light spot on the card. The focusing error signal can be obtained stably at a high sensitivity. Further the dynamic range of the focusing error signal can be widened.