Optical pick-up for DVD

The optical pick-up for DVD with CD (compact disc) compatibility is discussed. The difference of the substrate thickness between DVD and CD causes the different spherical aberration and prevents the laser beam from being focused into a diffraction limit spot size with only one objective lens. Several methods of reducing this aberration and possessing compatibility with CD are proposed. The twin lens type optical pick-up is one solution in overcoming this problem. It incorporates objective lenses for both DVD and CD. Each lens results in an optimum focused spot without the extra spherical aberration for each type of disc.