Patent ID: 8391120

Claim:
An optical head for recording or reproducing information on or from a first information recording medium including at least three information recording surfaces having different light transmitting layer thicknesses, comprising: a first light source for emitting first laser light having a first wavelength; a first mirror for reflecting the first laser light; a first objective lens for focusing the first laser light reflected by the first mirror on a specified information recording surface of the first information recording medium; a coupling lens arranged between the first light source and the first mirror; a second mirror in the form of a parallel plate arranged between the coupling lens and the first mirror; a spherical aberration corrector for correcting a third-order spherical aberration created according to the light transmitting layer thickness of the first information recording medium by moving the coupling lens in an optical axis direction; and a photodetector for receiving reflected light from the information recording surface of the first information recording medium; wherein the first objective lens is so designed as to be tilted in a radial direction of the first information recording medium, and satisfy a relationship of tc>(t 0 +tn)/2 when it is assumed that t 0 denotes the thickness of a light transmitting layer from the outer surface of the first information recording medium to an information recording surface L 0 having the largest light transmitting layer thickness, tn denotes the thickness of a light transmitting layer from the outer surface of the first information recording medium to an information recording surface Ln having the smallest light transmitting layer thickness and tc denotes the thickness of a virtual light transmitting layer at which the absolute value of a third-order spherical aberration is minimum when the first laser light is incident as parallel light on the first objective lens.