| 摘要 |
In an optical head device, a birefringence correcting element 5a containing a material showing mono-axial refractive index anisotropy is located before an objective lens. The birefringence correcting element 5a is circumferentially divided into four regions by two straight lines passing through an optical axis and intersecting each other at right angle. Each of the four regions is radially divided into four sub-regions by three circles centered at the optical axis. The direction of optic axes of the sub-regions 11a to 14a and 11c to 14c is a direction of x-axis, while the direction of optic axes of the sub-regions 11b to 14b and 11d to 14d is a direction of y-axis. A phase difference of sub-regions between a polarized light component polarized in a direction parallel to the optic axis and another polarized light component polarized in a direction vertical to the optic axis increases in order of the sub-regions 11a to 11d, sub-regions 12a to 12d, sub-regions 13a to 13d and the sub-regions 14a to 14d.
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