Patent ID: 7151737

Claim:
An optical head comprising: a laser source for irradiating a laser beam having a wavelength not longer than 500 nm; a collimator lens for collimating the laser beam to parallel rays; an optical beam shaping system for adjusting a cross sectional shape of the laser beam from the collimator lens; and an objective lens for converging the laser beam on an optical information medium, wherein a relationship between a focal length f CL of the collimator lens and a magnification M of beam shaping satisfies the following equation, 6 ≥ ( C f CL ) 2 ⁢ ⁢ arcsin ⁡ [ ( M - 1 ) ( n 2 ⁢ M - 1 ) ] wherein C is a constant not smaller than 29 and not greater than 33 in (λ·mm) 1/2 , and n is a refractive index of a member of the optical beam shaping system.