| 摘要 |
PURPOSE:To reduce the color aberration by decreasing a beam diameter of a focus servo laser made incident on the objective lens with a laser beam converging means and making the image forming position of an exposure laser beam the same regardless of the difference from a reflectance. CONSTITUTION:A focus servo laser beam from a laser oscillator 13 is made incident on a convex lens 21, where the beam is converted into a converging beam. Then the beam passes through a mirror 6 while being converged, is reflected in a reflection mirror 7 and made incident on an objective lens 8. Thus, the beam diameter for the focus servo laser is made smaller than the exposure laser beam diameter. Moreover, since the wavelength of the focus servo beam is longer than that of the exposure beam and the reflectance of the focus servo beam is smaller than that of the exposure beam, the image forming position by the lens 8 is made almost the same due to the reduced beam diameter and the color aberration is reduced. |
