Objective lens for optical recording medium and optical pickup device using it

An objective lens for converging with a first numerical aperture incident light of a first wavelength and for converging with a second numerical aperture incident light of a second wavelength, said objective lens comprising:

• an optical diffraction surface formed on a surface of the objective lens nearest a light source, said optical diffraction surface providing one of a diverging effect or a converging effect that is small for incident light of the first wavelength so that said light is converged at a first predetermined position by both a diffractive effect and the refractive power of the objective lens, and said optical diffraction surface providing a diverging effect or a converging effect that is large for incident light of the second wavelength so that said light is converged at a second predetermined position by both the diffractive effect and the refractive power of the objective lens;
• a peripheral portion for aperture adjustment that is established on the surface of the objective lens nearest a light source, said peripheral portion being structured so that, if the light source emits wavelengths equal to that of either the first wavelength or the second wavelength, the peripheral portion for aperture adjustment functions so as to reduce/eliminate a quantity of light at the periphery of a central light flux due to an interference effect for light having a wavelength λ1, and functions so as to maintain a quantity of light at a periphery of a central light flux for light having a wavelength λ2, the peripheral portion being provided with a step difference where light which passes through a position equivalent to the peripheral portion in the case when it is assumed that the peripheral portion does not exist, and a light which passes through a position where the peripheral portion exists, generate a phase difference so as to satisfy the following Conditions (1) and (2)
• Δλ1=(2 m+1)λ1/2+δ1 Condition (1)Δλ2=n λ2+δ2 Condition (2)
• and, the numerical aperture at the location of a boundary of the step difference has a value between the first numerical aperture and the second numerical aperture,
• where
• Δλ1 is the phase difference, for light of wavelength λ1, between light passing through a position equivalent to that of the peripheral portion in the case when it is assumed that the peripheral portion does not exist, and light passing through a position where the peripheral portion is formed;
• Δλ2 is the phase difference, for light of wavelength λ2, between light passing through a position equivalent to that of the peripheral portion in the case when it is assumed that the peripheral portion does not exist, and light passing through the position where the peripheral portion is formed;
• m and n are integers;
• δ1 is a phase shift amount at the wavelength λ1, with |δ1|<0.2 λ1;
• and,
• δ2 is a phase shift amount at the wavelength λ2, with |δ2|<0.2 λ2.