| 摘要 |
PROBLEM TO BE SOLVED: To eliminate inverse influences to a focusing serbo and a high-speed seek by suppressing influences such as abberation due to the using of a small- sized single lens and so forth as much as possible even though the single lens is used for the condensing lens of a signal detection system to reduce a T/F crosstalk. SOLUTION: A hologram plate 27' imparts a tilt component and positive and negative defocus components on an incident light luminous flux. Then, a hologram 27 in which shapes and intensity distributions of left and right spots are made symmetry is formed by adding a limit while throttling the diffration directions of the plate 27' and by suppressing the occurence amount of abberation due to a condensing lens 28 while making the numerical aperture small. Then, a vertically long area H is provided by dividing the area of plate 27 into three parts. Moreover, since fist-order diffracted lights respectively diffracted in areas Ib, Jb are unnecessary as signals for servos as to a divided luminous flux B, and exert inverse influences on respective signals for servos, light receiving elements for servos 32a, 32b are constituted to be squares so as not detect these±1st-order diffracted lights to be distinguished from other oblong light receiving elements for data 31a, 31b, 33a, 33b.
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