CONVERGENT DEVICE, OPTICAL HEAD, OPTICAL INFORMATION RECORDING/REPRODUCING AND OPTICAL INFORMATION RECORDING/REPRODUCING METHOD

摘要

An optical head comprises a light source generating light beams of two wavelengths. A converging element comprises a central region having numerical aperture NA1 and an outer region having numerical aperture between NA1 and NA2. The outer and central regions decrease aberration when light is converged through a transparent flat plate of thickness t1, and between t2 and 0.7 * t2, respectively. An optical system converges the first wavelength light beam through the converging element onto an information plane of an optical information recording medium of thickness t1, and converges the second wavelength light beam onto an information plane of an optical information recording medium of thickness t2 larger than t1. An optical element prevents reflection of the first wavelength light in a ring region corresponding to numerical aperture of the converging element between NA1 and 0.7 * NA1.