Patent ID: 8081558

Claim:
An optical system of an optical pick-up, comprising: a coupling lens that is made of resin and changes a degree of divergence of a light beam emitted by a light source; and an objective lens that is made of resin and has a numerical aperture NA larger than 0.8, the objective lens converging the light beam emerging from the coupling lens onto a recording surface of an optical disc, wherein: when β represents a magnification of an optical system formed by the coupling lens and the objective lens with respect to a use wavelength λ (unit: nm), the magnification β satisfies a condition: −0.13<β<−0.08 (1); when f (unit: mm) represents a focal length of the objective lens with respect to the use wavelength λ, the focal length f satisfies a condition: 15< f< 1.50 (2); the objective lens has an annular zone structure on at least one surface of the objective lens, the annular zone structure having a plurality of annular zones and having steps each of which extends in a direction of an optical axis of the objective lens and is formed at a boundary between adjacent ones of the plurality of annular zones to cause a predetermined optical path length difference between a light beam passing inside the boundary and a light beam passing outside the boundary; and when (Σφ 0.00-1.00 ) represents a sum of absolute values of heights of all the steps of the annular zone structure formed within an effective beam diameter, (Σφ 0.95-1.00 ) represents a sum of absolute values of heights of all the steps of the annular zone structure formed within a range of 95% to 100% of an effective beam radius with respect to the optical axis, n represents a refractive index of the objective lens at the use wavelength λ, and d 0 (unit: mm) represents a thickness of the objective lens defined on the optical axis, P ave represents an average of distances between adjacent ones of the steps formed within the range of 95% to 100% of the effective beam radius, θ represents a maximum angle of angles formed, within the range of 95% to 100% of the effective beam radius, between a normal to the at least one surface on which the annular zone structure is formed and a line parallel with the optical axis, L represents an absolute value of a height of a step formed at a position corresponding to the maximum angle, L 0 represents an absolute value of a height of a step formed at a position corresponding to the optical axis, and m represents a diffraction order at which a diffraction efficiency is maximized for the light beam having the use wavelength λ incident on the at least one surface on which the annular zone structure is formed, the optical system of an optical pick-up satisfies a condition: 0.065 < ( ( ∑ Φ 0.95 - 1.00 ) ( ∑ Φ 0.00 - 1.00 ) × d 0 × n f 2 × ( NA 0.85 ) 4 ) P ave + 1 L < 0.100 ⁢ ⁢ where ⁢ ⁢ L = L 0 + ( 1 - cos ⁢ ⁢ θ ) ⁢ ⁢ L 0 = m ⁢ ⁢ λ n - 1 . ( 3 )