| 摘要 |
An optical head includes a laser source (11) irradiating the laser beam having a wavelength not longer than 500 nm, a collimator lens (13) for collimating the laser beam to the parallel rays, an optical beam shaping system (14, 15) that adjusts the cross sectional shape of the beam from the collimator lens, and an objective lens (16) that converges the beam on an optical information medium. The optical head has a relationship between a focal length fCL of the collimator lens and a magnification M of beam shaping satisfies the following equation:in which C is a constant in (lambda.mm)<1/2>, ASO is a maximum astigmatism per displacement of the collimator lens from the focal position, and n is a refractive index of a member of optical beam shaping system.
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