Optical system for optical head
View Patent ↗An optical system for an optical head of an optical disc drive is capable of using first and second discs, protective layer of the second disc being thicker than that of the first disc, a data density of the second disc being lower than that of the first disc. A light source selectively emits first and second laser beams having first and second wavelengths, respectively. An objective lens converges the first and second beams on the first and second discs, respectively. The second beam is incident on the objective lens as diverging light, and the first beam is incident on the objective lens as a beam of which degree of divergence is smaller. A diffraction lens structure is formed in the high NA exclusive area, which compensates for aberrations of a portion of the first laser beam, and causes aberrations to a portion of the second laser beam passed therethrough.
1. An optical system for an optical head of an optical disc drive, the optical disc drive being capable of using at least a first disc and a second disc, a protective layer of the second disc being thicker than that of the first disc, a data density of the second disc being lower than that of the first disc, said optical system comprising:
a light source that selectively emits a first laser beam having a first wavelength and a second laser beam having a second wavelength, the second wavelength being longer than the first wavelength; and
an objective lens that converges the first beam and the second beam on the first disc and the second disc, respectively, the second beam being incident on said objective lens as a diverging light beam, the first beam being incident on said objective lens as a beam of which a degree of divergence is less than that of the second beam,
wherein one surface of said objective lens is divided into a common area at a central portion and a high NA exclusive area at a peripheral portion surrounding said common area, a diffraction lens structure including a plurality of concentric annular zones having minute steps being formed in said high NA exclusive area, said diffraction lens structure varies a spherical aberration depending on the wavelength of light passed therethrough, said diffraction lens structure compensating for aberrations of a portion of the first beam passed therethrough, said diffraction lens structure causes aberrations to a portion of the second laser beam passed therethrough,
the first laser beam passed through said common area and said high NA exclusive area being converged on the first disc to form a beam spot,
a portion of the second laser beam passed through said common area being converged on the second disc to form a beam spot,
the portion of the second laser beam passed through said high NA exclusive area being diffused on the second disc.
2. The optical system according to claim 1 , wherein said common area is configured to be a continuous surface without stepped portions.
3. The optical system according to claim 1 , wherein said common area is formed with a diffraction lens structure including a plurality of concentric annular zones having minute steps.
4. The optical system according to claim 3 , wherein said diffraction lens structure formed in said common area compensates for a chromatic aberration.
5. The optical system according to claim 1 , wherein the first laser beam incident on said objective lens is a parallel light beam.
6. The optical system according to claim 1 , wherein the first laser beam incident on said objective lens is a converging light beam.
7. An objective lens for an optical head of an optical disc drive, the optical disc drive being capable of using at least a first disc and a second disc, a protective layer of the second disc being thicker than that of the first disc, a data density of the second disc being lower than that of the first disc, a first laser beam having a first wavelength and a second laser beam having a second wavelength being selectively incident on said objective lens, the second wavelength being longer than the first wavelength, said objective lens converges the first beam and the second beam on the first disc and the second disc, respectively, said object lens comprising:
one surface that is divided into a common area at a central portion and a high NA exclusive area at a peripheral portion surrounding said common area, a diffraction lens structure including a plurality of concentric annular zones having minute steps being formed in said high NA exclusive area, said diffraction lens structure varies a spherical aberration depending on the wavelength of light passed therethrough, said diffraction lens structure compensating for aberrations of a portion of the first beam passed therethrough, said diffraction lens structure causes aberrations to a portion of the second laser beam passed therethrough,
the first laser beam passed through said common area and said high NA exclusive area being converged on the first disc to form a beam spot,
a portion of the second laser beam passed through said common area being converged on the second disc to form a beam spot,
the portion of the second laser beam passed through said high NA exclusive area being diffused on the second disc due to the aberration caused by the diffraction lens structure.
8. The objective lens according to claim 7 , wherein said common area is configured to be a continuous surface without stepped portions.
9. The objective lens according to claim 7 , wherein said common area is formed with a diffraction lens structure including a plurality of concentric annular zones having minute steps.
10. The objective lens according to claim 9 , wherein said diffraction lens structure formed in said common area compensates for a chromatic aberration.
11. The objective lens according to claim 7 , wherein the first laser beam incident on said objective lens is a parallel light beam.
12. The objective lens according to claim 7 , wherein the first laser beam incident on said objective lens is a converging light beam.