Correction elements to lower light intensity around an optical axis of an optical head with a plurality of focal points

DETAILED DESCRIPTION An object of the present invention is to provide a light head apparatus which can read information more stably by decreasing disadvantages of higher optical intensity at the outside of a light beam while converging the light beam on information media or optical disks having different thicknesses.
Another object of the present invention is to provide an improved optical head apparatus which can converge a light beam of diffraction limit on information media or optical disks having different thicknesses by using a hologram lens having sufficient intensity of transmission light.
A further object of the present invention is to provide a simple fabrication method of a correction element.
In one aspect of the invention of an optical head apparatus, a lens converges a light beam into a micro-spot on an information medium.
One feature of the invention is that the lens has a plurality of focal points having different numerical apertures at the same side with respect to the lens.
The lens comprises a combination of an object lens which refracts light and a hologram lens.
Another feature of the invention is that a correction element changes optical intensity of the light beam.
The correction element is provided in an optical path from the light source to the lens, and it has a portion distant from the optical axis where the optical intensity is lowered.
The optical intensity is changed by providing a distribution of, for example, transmittance or zeroth order diffraction efficiency.
The transmittance represents a ratio of light transmitting without being reflected or absorbed, while the zeroth order diffraction efficiency represents a ratio of light transmitting without being diffracted.
The correction element has a grating in the portion thereof distant from the optical axis.
Preferably, hypothetical contour curves along which the transmittance or the zeroth order diffraction efficiency have the same values of the correction element are convex with respect to a radial direction from the optical axis to an outer peripheral of the correction element