OPTICAL HEAD AND OPTICAL DISC DEVICE

摘要

A small and low cost optical head by which stable compatible reproduction and compatible recording can be performed to different types of optical discs by using one objective lens. The order of diffraction light to have maximum diffraction efficiency of a diffraction element constituting the objective lens is set at third-order for blue light and second-order for red light and infrared light at an inner circumference section of the diffraction element, and is set at sixth-order for blue light, fourth-order for red light and third-order for infrared light at an intermediate circumference section and numerical aperture for infrared light is limited. At an outer circumference section, the order is set at an integer number other than multiples of 3 for blue light to limit the numerical aperture for red light.