| 摘要 |
PURPOSE:To improve the read or write density of an optical head by using a Fresnel lens as an objective lens and detecting a reflected light of high-order diffracted light. CONSTITUTION:An information recording medium 12 is positioned so as to be opposite to an objective lens 10 and a Fresnel lens is used as the objective lens 10. In this case, a primary diffracted light X1, a secondary diffracted light X2, a ternary diffracted light X3 with different focal distance are obtained. Then a recording face of an information recording medium 12 is positioned to the focus of the ternary diffracted light X3 forming a focus to a position closest to the objective lens 10 and the reflected light of the ternary diffracted light X3 is detected by a detector. Since the ternary diffracted light X3 has a large angle theta, an NA (rate of aperture) is increased and a small focus spot is formed to the recording face of the information recording medium 12. Thus, the read density is improved and the information recording medium 12 with a high recording density is used. |
