Optical pickup device and focusing servo device employed therein

An optical pickup is adapted for recording and/or reproducing signals by radiating a light beam from a light source (11) on an optical recording medium (16) via an object lens (15) and includes a light receiving device (46,47) for receiving a return light from the optical recording medium (16) , a focusing error detecting unit (48, 25) for detecting an amount of deviation of the focal point along the optical axis, as a measure of a focusing error, based on signals from the light receiving device (46, 47), and an electrostriction device (20) arranged on a path of light outgoing from the light source (11) to the optical recording medium (16) and having a central section (23) and a peripheral section (24) exhibiting a phase difference from each other depending on an applied voltage. Focusing errors may be detected with high accuracy with the optical pickup. On the other hand, a focusing servo device is adapted for controlling the focal point along the optical axis of the optical pickup and includes an object lens driving unit (17) for moving the object lens (15) along the optical axis, a focusing error detecting unit (48, 25) for detecting an amount of deviation of the focal point along the optical axis, as a measure of a focusing error, based on signals from the light receiving device (46, 47), and an electrostriction device (20) arranged on a path of light outgoing from the light source (11) to the optical recording medium (16) and having a central section (23) and a peripheral section (24) exhibiting a phase difference from each other depending on an applied voltage. Depending on error detection signals from the focusing error detection unit (48, 25), rough servo is carried out by the object lens driving unit (17) up to the vicinity of the depth of focus, and fine servo is carried out by the electrostriction device (20) within the range of the depth of focus.