| 摘要 |
PROBLEM TO BE SOLVED: To provide an optical system for an optical head that the focal distance of an objective lens is reduced while the operating distance is secured and that a numerical aperture can be properly set without preparing an independent diaphragm. SOLUTION: In the case of using an optical disk D1 having a thin protective layer, a laser beam with a short wavelength emitted from a first laser module 21 is made incident on the objective lens 10 as a parallel light beam through a collimator lens 24 and converged on a recording surface. In the case of using an optical disk D2 having a thick protective layer, a laser beam with a long wavelength emitted from a second laser module 22 is made incident on the objective lens 10 as a divergent light beam and converged on the recording surface. A diffraction lens structure formed on the high NA dedicated region RE of the objective lens 10 becomes the large numerical aperture, for the purpose of properly compensating the spherical aberration, when the laser beam with a short wavelength is converged on the optical disk D1, and becomes the small numerical aperture, for the purpose of producing the spherical aberration, when the laser beam with a long wavelength is converged on the optical disk D2. |
