Patent ID: 7050378

Claim:
An optical pickup for recording and reproducing an information signal by irradiating light along a track of a rotating optical disk, comprising: a light source; a beam expander for changing a parallelism of light emitted from the light source; and an objective lens for collecting the light emitted from the beam expander along the track of the optical disk and for irradiating the collected light thereonto, wherein the beam expander is comprised of a concave lens and a diffraction lens, and a focal distance f n of the diffraction lens exists in a range satisfying a following inequality: f 1 <f n <f 2 , wherein f 1 and f n are given by the following expressions: f 1 =((λ 2 /λ 1 )−1) f 0 v 0 f 2 =(1−(λ 2 /λ 1 )) b wherein partial dispersion v 0 and length b are given by: v 0 =( n 0 −1)/( n 1 −n 2 ) b =−f 0 ( f 0 +Δ)/Δ λ 0 , λ 1 and λ 2 are a designed reference wavelength, a designed lower limit wavelength and a designed upper limit wavelength, respectively (here, λ 0 −λ 1 =λ 2 −λ 0 ), n 0 , n 1 and n 2 are refractive indices of the objective lens, the indices corresponding to these wavelengths, respectively, f 0 is a focal distance of the objective lens, and Δ is a difference between focal distances corresponding to the wavelengths λ 1 and λ 2 .