Optical head for reading or writing data to a CD or DVD in which the numerical aperture varies according to the wavelength of light used so that an additional shutter is not needed to change between the different media

摘要

CDs and DVDs have different protection layer thicknesses and data densities and according to the invention a light source is provided that transmits a laser beam with one of two selectable wavelengths. An objective lens focuses the light beam on the recording medium surface. First and second laser beams are incident on the surface with different divergence. The divergence of the first beam is smaller than the divergence of the second beam. In a separate region of the objective a region of high numerical aperture is formed as a diffraction lens. This is structured so that for the first wavelength aberrations are compensated, while for the second wavelength aberrations are caused. An Independent claim is made for an objective lens for a read-write head for CDs and DVDs.