Optical head, optical element with diffraction grating, optical disc device and information processing device

An optical head, comprising:

• a first light source for emitting a first laser beam having a first wavelength λ1;
• a first planar optical element for transmitting and reflecting the first laser beam at a predetermined ratio;
• a first objective lens for converging the first laser beam onto an information recording surface of a 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 or an error signal;
• and
• a second photodetector for receiving transmitted light from the first planar optical element and creating an automatic power control signal for controlling an output of the first light source,
• wherein the first planar optical element 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, and
• a diffraction grating is formed on the second surface, and
• wherein the first information recording medium includes at least three information recording surfaces each having a light transmitting layer different in thickness,
• the optical head further comprises a spherical aberration correction unit for correcting a spherical aberration that occurs according to a thickness of the light transmitting layer of the first information recording medium, and
• the spherical aberration correction unit causes diverging light to enter the first objective lens upon recording or reproducing information to or from an information recording surface L0 with a thickest light transmitting layer, and causes converging light to enter the first objective lens upon recording or reproducing information to or from an information recording surface Ln with a thinnest light transmitting layer, and
• a grating depth d of the diffraction grating satisfies:
• (λ1)/6≦d≦(λ1)/4.

An optical head, comprising:

• a first light source for emitting a first laser beam having a first wavelength λ1;
• a second light source for emitting a second laser beam having a second wavelength λ2 that is larger than the first wavelength λ1;
• a first planar optical element for transmitting and reflecting the first laser beam at a predetermined ratio;
• a second planar optical element for transmitting and reflecting the second laser beam at a predetermined ratio and substantially transmitting the first laser beam;
• a first objective lens for converging the first laser beam onto an information recording surface of a first information recording medium;
• a second objective lens for converging the second laser beam onto an information recording surface of a second information recording medium of a type that is different from 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 or an error signal;
• and
• a second photodetector for receiving transmitted light from the first planar optical element and creating an automatic power control signal for controlling an output of the first light source,
• wherein the first planar optical element 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, and
• a diffraction grating is formed on the second surface.

An optical element with a diffraction grating for transmitting and reflecting, at a predetermined ratio, a laser beam having a predetermined wavelength λ, and guiding the laser beam to a photodetector that creates an automatic power control signal for controlling an output of a light source that emits the laser beam, the optical element comprising:

• a first surface which the laser beam enters;
• and
• a second surface facing the first surface,
• wherein the first surface and the second surface are mutually parallel,
• a diffraction grating is formed on the second surface,
• a grating depth d of the diffraction grating satisfies:
• λ/6≦d≦λ/4, and
• a pitch p of the diffraction grating satisfies:
• p>λ.