| 摘要 |
PURPOSE:To detect with a high sensitivity a focal state extending over a wide range, and to enlarge a degree of freedom for arranging a photodetector by constituting an optical element for giving an astigmatism, of two cylindrical lenses for refracting a light in each orthogonal direction. CONSTITUTION:A laser luminous flux is made incident on a recording medium 26 through a polarized prism 22, a 1/4 wavelength plate 24 and an objective lens 25, and its reflected light is led to a photodetector 29. By a diverging action of a cylindrical concave lens 28 for diverging a light in the direction on the paper surface, a focus is formed on a focal plane F1, and by a focusing action of a cylindrical convex lens 27 for focusing a light in the direction vertical to the paper surface, a focus is formed on a focal plane F2. Accordingly, by selecting a refractive power of the lenses 27, 28, an astigmatism being a distance of the focal planes F1, F2 can be set optionally, the sensitivity for detecting a focus is raised, the dynamic range can be widened, and the degree of freedom for arranging and constituting the photodetector 29 can be enlarged. |
