Patent ID: 7050379
Filing Date: 2006-05-23
Classification: G02B,G11B

Abstract:
1. An objective lens for an optical pickup, constituted of a first lens set including a diffractive type lens and a second lens set including a refractive type lens set in order from an object side, and having a numerical aperture not less than 0.8, and adapted for correcting the chromatic aberration at an image point on an optical axis for light with a wavelength within several nm about a reference wavelength which is not larger than 420 nm, wherein said first lens set has a compound surface constituted by adding a diffractive surface having a positive refractive power to an aspherical refractive surface having a negative refractive power; an amount of sag of the aspherical surface of said first lens set having a negative refractive power is described by a polynominal of an even order number with respect to a radius, as the cone coefficient of the aspherical coefficient (k) is set to −1; an amount of sag of the diffractive surface of the first lens set is described by a polynominal of an even order number with respect to a radius; the order number of the polynominal of the aspherical surface is equal to the order number of the polynominal of the diffractive surface; the coefficients of the same order numbers of the polynominal of the sag of the aspherical surface are equated to those of the polynominal of the sag of the diffractive surface in such a manner as to meet the equations:  where C the lens forming said second lens set is formed by a single lens including at least one aspherical surface; and an aperture is provided between said first and second lens sets.