Focusing apparatus and optical apparatus using the same

摘要

An optical disc (100) prepared by bonding two substrates each having a thickness "h" is used as a recording medium. At least one of the substrates is provided at the back surface with a signal mark or a reflective layer capable of providing a signal mark. A laser beam (210) is radiated onto the optical disc (100) through an objective lens (203), and the light reflected from the reflective layer of the substrate is detected by an optical detecting device (213). One output of the optical detecting device (213) is supplied to a signal reading device (215), and the other output is supplied to a focusing apparatus (216) for detecting a focus error of the objective lens (203) and for controlling a position of the objective lens (203) with respect to the optical disc (100). In the optical disc apparatus, a half-width DELTA lambda of the laser beam source satisfies the inequality: $where\; lambda\; 0\; is\; a\; principal\; wavelength\; of\; the\; laser\; beam\; source\; (206),\; DELTA\; lambda\; is\; a\; half-width\; of\; laser\; beam\; source,\; NA\; is\; a\; numerical\; aperture\; of\; the\; objective\; lens,\; f\; is\; a\; focal\; length\; of\; the\; objective\; lens,\; n2\; is\; a\; refractive\; index\; of\; substrate\; material,\; dmax\; is\; a\; maximum\; diameter\; of\; laser\; spot\; beam\; radiated\; to\; a\; signal\; pit,\; and\; g\; is\; a\; general\; functional\; formula\; for\; obtaining\; the\; maximum\; half-width\; DELTA\; lambda\; based\; on\; lambda\; 0,\; NA,\; f\; and\; dmax.\mathop{\mathrm{image}}$