Optical Pickup and Objective Optical System for Use in the Same

An optical pickup which is applicable to different kinds of optical information recording media having optical information recording surfaces and cover layers for protecting the optical information recording surfaces, respectively, and being different from each other in at least one of a wavelength of a laser beam corresponding to the optical information recording surface and a thickness of the cover layer, the optical pickup focusing a laser beam on a certain optical information recording surface corresponding thereto, and the optical pickup comprising:

• a light source capable of emitting laser beams of different wavelengths corresponding to the optical information recording surfaces, respectively;
• and
• an objective optical system for focusing any laser beam emitted from the light source on a certain optical information recording surface corresponding thereto, wherein
• the objective optical system includes at least two wavefront conversion surfaces for converting a wavefront of a laser beam entered therein into a different wavefront in response to the kind of the optical information recording medium corresponding to the laser beam and an objective lens for focusing the laser beam passed through the wavefront converting surfaces on the certain optical information recording surface and
• the optical pickup satisfies the following condition (1) for every wavelength of the corresponding laser beams:
• |SCMAX|<0.036 (1)
• wherein SCMAX is the maximum value of an offence against the sine condition of the objective optical system defined by the following formula (2):
• {(h/sin U)−f}/f (2)
• wherein h is a distance between the laser beam entering the objective lens and an optical axis of the objective lens,
• U is an angle formed between the optical axis and the travel direction of the laser beam entered the objective lens at the distance h from the optical axis and passed through the objective lens and
• f is a focal distance of the objective optical system.