Patent ID: 7187630

Claim:
A focusing servo device, comprising: a driver for moving an objective lens along the optical axis of the objective lens, the objective lens irradiating a light beam onto a measurement surface and receives reflected light reflected by the measurement surface; a plurality of photo detectors mutually opposed on both sides of the optical axis of the objective lens as a symmetry axis for respectively detecting the reflected light from the objective lens; a position-shift detector for outputting a focus error signal indicating position shift of the objective lens from a focus position based on the reflected light respectively detected by the plurality of photo detectors; a position detector for detecting the position of the objective lens moved by the driver, the position detector outputting the position of the objective lens as a counted value; and a focal detector for recognizing the focus position based on the focus error signal from the position-shift detector and the position of the objective lens detected by the position detector and for actuating the driver to move the objective lens to the focus position, wherein the focal detector moves the objective lens to a position most adjacent to the measurement surface and to a position most remote from the measurement surface to detect a movable range of the objective lens, and while moving the objective lens within the movable range, the focal detector recognizes the counted value indicating a position of the objective lens when the focus error signal indicates a zero-cross point as the focus position, and after recognizing the counted value, the focal detector moves the objective lens to the previously passed focus position recognized as the counted value.