Patent ID: 8289831

Claim:
An optical head, comprising: a first light source for emitting a first laser beam having a first wavelength Î»1; a second light source for emitting a second laser beam having a second wavelength Î»2 that is larger than the first wavelength Î»1; an objective lens for converging the first laser beam onto an information recording surface of a first information recording medium and converging the second laser beam onto an information recording surface of a second information recording medium; a planar reflective mirror for transmitting and reflecting the first laser beam and the second laser beam at a predetermined ratio so that the first laser beam and the second laser beam enter substantially perpendicularly the information recording surface of the first information recording medium; a first photodetector for receiving reflected light from the information recording surface of the first information recording medium and creating an information signal and/or an error signal; a spherical aberration correction unit for correcting a spherical aberration that occurs according to the thickness of a light transmitting layer of the first information recording medium; and a second photodetector for receiving transmitted light from the planar reflective mirror and creating an automatic power control signal for controlling an output of the first light source, wherein the planar reflective mirror includes a first surface which the first laser beam emitted from the first light source enters, and a second surface facing the first surface, the first surface and the second surface are mutually parallel, a reflective coat for transmitting and reflecting the first laser beam at a predetermined ratio is formed on the first surface, an antireflection coat having a reflectance R 1 for the first laser beam smaller than 0.01 is formed on the second surface, the first information recording medium includes at least three information recording surfaces each having a light transmitting layer different in thickness, and the spherical aberration correction unit causes diverging light to enter the objective lens upon recording or reproducing information to or from an information recording surface L 0 with the thickest light transmitting layer, and causes converging light to enter the objective lens upon recording or reproducing information to or from an information recording surface Ln with the thinnest light transmitting layer.