Optical head and recording reproduction method

An optical head, comprising:

• a first light source for emitting light with a first wavelength;
• a second light source for emitting light with a second wavelength longer than the first wavelength;
• a third light source for emitting light with a third wavelength longer than the second wavelength;
• and
• an objective lens for focusing the respective lights with the first, second, and third wavelengths on information recording surfaces of information recording media corresponding to the lights with the respective wavelengths,
• wherein the objective lens is designed so as to reduce aberration with respect to the light with the second wavelength, and
• optical-path lengths from the respective second and third light sources to the objective lens are shorter than an optical-path length from the first light source to the objective lens so that aberration caused by the objective lens due to chromatic dispersion caused by a material of the objective lens is reduced with respect to the respective lights with the first and third wavelengths.

A recording and reproduction method, in which with respect to an information recording medium having an information recording surface and a protective layer thereon, information is recorded on and/or reproduced from the information recording surface by projecting light on the information recording medium from a side of the protective layer,

• wherein an optical head is used, which comprises:
• a first light source for emitting light with a first wavelength;
• a second light source for emitting light with a second wavelength longer than the first wavelength;
• and an objective lens for focusing the light with the first wavelength and the light with the second wavelength on information recording surfaces of information recording media corresponding to the lights with respective wavelengths, the objective lens being designed so as to reduce aberration with respect to the light with the second wavelength,
• a thickness of a protective layer on one of the information recording surfaces, which corresponds to the light with the first wavelength, is allowed to be different from that of a protective layer on the other information recording surface corresponding to the light with the second wavelength so that aberration caused by the objective lens due to chromatic dispersion caused by a material of the objective lens is reduced with respect to the light with the first wavelength,
• the lights with the first and second wavelengths entering the objective lens are substantially parallel lights,
• the protective layer on the information recording surface corresponding to the light with the second wavelength has a thickness in a range of 0.55 mm to 0.65 mm, and
• the protective layer on the information recording surface corresponding to the light with the first wavelength has a thickness in a range of 0.65 mm to 0.75 mm.