| 摘要 |
PURPOSE:To detect an error signal at a high speed with a low cost by shifting relatively slightly light with a wave surface dividing means arranged in the optical path of form interference fringes and providing a space modulating means in the space of these interference fringes to form a moire and detecting the moire with a photoelectric detecting means. CONSTITUTION:The light incident from a beam splitter is shifted slightly by a diffraction grating 31, and interference fringes in the direction of an arrow are formed in accordance with shift. Though intervals of interference fringes are as shown in a figure (b) when an objective lens is focused on a recording medium, said intervals are made wider as shown in a figure (a) if the objective lens is too close, and they are made narrower as shown in a figure (c) if the objective lens is too distant. Spatial intensity modulation is given to the wave surface by a space mounting means 32 to form the moire. As a result, the moire of interference fringes and an amplitude grating 32 is projected on a photoelectric detecting means 33 as shown in figures (g), (h), and (h) correspondingly to states (b), (a), and (c) of interference fringes. That is, the moire is horizontal in the in-focus state and is rotated counterclockwise if the distance between the objective lens and the recording medium is too short and is rotated clockwise if this distance is too long. Thus, the device is operated at a high speed with a low cost. |
