Multiple focus lens, an optical head apparatus and an optical information recording-reproducing apparatus

DETAILED DESCRIPTION In order to solve these and other problems, this invention aims to provide a multiple focus lens designed to substantially standardize the conditions for correction of coma for disks (recording media or substrates) which are different each other in thickness, an optical head apparatus and an optical information recording-reproducing apparatus in which the lenses are used.
For this purpose, a multiple focus lens of this invention comprises a diffracting means whose coma is corrected so that light beams having different diffraction orders will respectively focus on plural substrates having different thicknesses.
The whole lens is tilted to correct its axial coma, and the tilt angles are substantially the same toward the substrates.
When an optical head apparatus is assembled, the whole lens is tilted to correct its axial coma.
If the tilt angles of the respective lenses are predetermined substantially the same toward plural information recording media (optical disks) having different substrate thicknesses, excellent focusing spots are obtained on plural information recording media by using one objective lens.
Therefore, information can be recorded or reproduced with stabilized performance.
Coma can occur on axis because of errors in the manufacturing process, and such aberrations should be corrected by tilting the whole objective lens.
According to this invention, specifically, the correction angle is maintained at a certain value if the thickness of the disks is changed.
Therefore, it is not necessary to re-adjust the tilt angle of the objective lens for each disk.
In the above-mentioned configuration, the 0th order diffracted light is focused on the information recording surface of a first substrate, while the +1st order diffracted light of the diffracting means is focused on the information recording surface of a second substrate whose thickness is different from that of the first substrate, so that the configuration meets the requirement of Formula (1).
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