Patent ID: 7839733
Filing Date: 2010-11-23
Classification: G11B

Abstract:
1. An optical device provided with an optical unit comprising: a focus-error detector for detecting an amount of focus error, with respect to the subject, of the light beam that is focused by the light focusing lens, by utilizing a diagonal-addition difference signal obtained by making a subtraction between addition signals each obtained by adding electric signals, for the light-receiving segments that are situated diagonally to each other, among electric signals, corresponding to light amounts detected at the four light-receiving segments, which are outputted from the optical unit, the astigmatism addition lens being configured with a reflected-light focusing lens for focusing the reflected light beam and a flat-plate optical element disposed slanted from the optical axis of the reflected beam that is focused by the reflected-light focusing lens, and the reflected light beam penetrating the flat-plate optical element, the optical device further comprising: a first control signal generator for detecting a first control signal, by making a subtraction between addition values each obtained by adding light amounts detected at the light-receiving segments, among the four light-receiving segments, which are adjacent to each other with respect to the first dividing line; and a second control signal generator for detecting a second control signal, by making a subtraction between addition values each obtained by adding light amounts detected at the light-receiving segments, among the four light-receiving segments, which are adjacent to each other with respect to the second dividing line, the first control signal and the second control signal being corrected by an offset component caused by asymmetry in an intensity distribution of the reflected light beam to which astigmatism has been added, and the diagonal-addition difference signal being corrected by a multiplication signal obtained by making a multiplication among the corrected first control signal, the corrected second control signal, and a predetermined constant.