Optical element and optical head apparatus including the same

An optical element having at least one surface divided into a plurality of regions, the optical element comprising:

• a first region including an optical axis and configured to converge light with a wavelength λ1 onto a storage surface of a first optical disc and converge light with a wavelength λ2 onto a storage surface of a second optical disc;
• and
• a second region formed around the outer circumference of the first region and configured to converge light with the wavelength λ1 onto the storage surface of the first optical disc and converge light with the wavelength λ2 onto the storage surface of the second optical disc, wherein
• the first region has a first diffraction structure cyclically formed on a first aspheric surface and having a step-like cross section, and
• the second region has a second diffraction structure cyclically formed on a second aspheric surface and having a step-like cross section, and
• the optical element satisfying the following conditions:
• −0.25<(Φ11+Φ12)−N1<0.25 (1)
• −0.25<(Φ21+Φ22)−N2<0.25 (2)
• where,
• Φ11 is a phase difference with reference to a phase at the first aspheric surface, Φ11 being given by a step in the first region, that is closest to the second region, for light with the wavelength λ1 to be converged onto the first optical disc,
• Φ12 is a phase difference with reference to a phase at the second aspheric surface, Φ12 being given by a step in the second region, that is closest to the first region, for light with the wavelength λ1 to be converged onto the first optical disc,
• Φ21 is a phase difference with reference to the phase at the first aspheric surface, Φ21 being given by the step in the first region, that is closest to the second region, for light with the wavelength λ2 to be converged onto the second optical disc,
• Φ22 is a phase difference with reference to the phase at the second aspheric surface, Φ22 being given by the step in the second region, that is closest to the first region, for light with the wavelength λ2 to be converged onto the second optical disc,
• N1 is an integer obtained by rounding the first decimal place of a numerical value given by Φ11+Φ12, and
• N2 is an integer obtained by rounding the first decimal place of a numerical value given by Φ21+Φ22.