Patent ID: 8325581

Claim:
An optical head comprising: a light source; a coupling lens configured to make at least one of a degree of divergence and a degree of convergence of a light beam emitted from the light source variable; an objective lens configured to focus the light beam on an optical information recording medium having at least three information recording surfaces at which light-transmitting layers have different thicknesses; and a degree-of-divergence/convergence enlarging member formed by a single lens consisting of one component and configured to be fixed between the coupling lens and the objective lens, wherein a diameter D 1 of the light beam entering into the degree-of-divergence/convergence enlarging member and a diameter D 2 of the light beam emitted from the degree-of-divergence/convergence enlarging member satisfy the following equation (1): D1>D2 (1), wherein the degree-of-divergence/convergence enlarging member is an afocal lens in which the light beam emitted from the degree-of-divergence/convergence enlarging member becomes nearly a collimated beam when an incident beam to the degree-of-divergence/convergence enlarging member is nearly collimated beams, wherein, in the at least three information recording surfaces of the optical information recording medium, an inter-surface distance between an information recording surface of a thickest light-transmitting layer and an information recording surface of a thinnest light-transmitting layer exceeds 30 μm, wherein the degree-of-divergence/convergence enlarging member is fixed to a moving part of an actuator which drives the objective lens, and is driven integrally with the objective lens, wherein the degree-of-divergence/convergence enlarging member includes a chromatic aberration correcting function configured to correct a chromatic aberration of the objective lens generated as a result of a fluctuation in a wavelength of the light source, and wherein, in the degree-of-divergence/convergence enlarging member, a surface facing the coupling lens is a diffracting surface having a positive power and a surface facing the objective lens is a refracting surface having a negative power.