| 摘要 |
PURPOSE:To obtain a small-sized device at a low cost, also to detect stably a focus, and to derive an out-of-focus quantity of a good S/N by detecting a focus by utilizing an interference of each diffracted beam from an optical recording medium, in a state that an optical system of an optical head has been excited. CONSTITUTION:An optical system of an optical head 11 is excited, and by this excitation, a spot of a beam 14 crosses a recording track of an optical recording medium 16, a signal which has been formed by an interference of each diffracted beam from the optical recording medium 16 is synchronized with said crossing and detected, and by this detection, an out-of-focus quantity is derived. In this way, a focus is detected by utilizing an interference of each diffracted beam from the optical recording medium, therefore, a device is low in its cost and small in size. Also, the out-of-focus quantity is derived from a signal which is obtained in an excited state, therefore, even in a state that a tracking control is not being executed, a stable focus detection which can execute a focus control can be executed. |
