| 摘要 |
PROBLEM TO BE SOLVED: To achieve highly accurate focus servo or the like for an information recording/reproducing medium that is capable of high-density recording by asymmetrically dividing the light reception region of a reflection light reception means into two portions, performing an operation to the sum of the output of each light reception region for obtaining first and second values, and then performing an operation to the difference. SOLUTION: At light reception regions A2-A5 of photodetectors PD2 and PD3, the difference between the sum of detection signals SA3 and SA4 being outputted from the light reception regions A3 and A4 that are located with a specific correspondence relationship and that of detection signals SA2 and SA5 being outputted from the light reception regions A2 and A5 is subjected to an operation to generate a focus error signal FE. The focus error signal FE indicates an amplitude change corresponding to each deflection amount in the front and rear pin states of an objective lens with a focusing state as a center, the characteristics of the amplitude are reversed between the front and rear pin states, thus quantitatively and accurately detecting the presence/ absence of the focusing of the objective lens and focal shift. |
