| 摘要 |
PURPOSE:To detect with a high sensitivity a focal state extending over a wide range, and to enlarge the degree of freedom for arranging a photodetector by constituting an optical element for giving an astigmatism, of a lens for focusing a light in all directions and a cylindrical lens for diverging it in one direction. CONSTITUTION:A laser luminous flux is made incident on a recording medium 15 through an objective lens 14, and its reflected light is led to a photodetector 18. The luminous flux vertical to the paper surface forms a spot on a focal plane F1, and a focal plane F2 in the edge surface, by a convex lens 16 for focusing a light uniformly in all directions, and a concave cylindrical lens 17 for diverging a light in only one direction and the focusing action of the lens 16, respectively, and at its middle position, the photodetector 18 is placed. By a shape variation of this spot, a focused state is detected, and by using the lenses 16, 17, the astigmatism is enlarged, the detecting sensitivity is improved and the dynamic range is widened, a position of the focal plane can be set optionally by selecting a refractive power of the lens, and the degree of freedom for arranging and constituting the photodetector can be enlarged. |
