Objective lens for optical disk system and optical head using the same

An optical head comprising:

• an objective lens for focusing a light from a light source on an information recording medium;
• light separator means for separating a light reflected from the information recording medium and passed through the objective lens from the light from the light source;
• and photo-detecting means for detecting the separated reflected light, the objective lens having opposite first and second aspherical surfaces each providing a positive refraction for focusing the light through a transparent substrate of the recording medium and satisfying substantially the following condition:
• ##EQU16## where, f.sub.L :
• focal length of the lens,
• d.sub.c :
• thickness of the transparent substrate,
• n.sub.c :
• refractive index of the transparent substrate,
• .lambda.:
• center wavelength of the light.
• NA:
• numerical of appertures of the lens,
• .nu..sub.L :
• local dispersion of the lens, and
• .nu..sub.c :
• local dispersion of the transparent substrate, the local dispersion being expressed by:
• ##EQU17## where n.sub..lambda. is a refractive index at a center wavelength .lambda. of the light, n.sub..lambda.+5 is a refractive index at a wavelength 5 nm longer than the center wavelength of the light, and n.sub..lambda.-5 is a refractive index at a wavelength 5 nm shorter than the center wavelength of the light.