COMPOUND LENS, OPTICAL HEAD DEVICE, OPTICAL DISK DEVICE, AND OPTICAL INFORMATION DEVICE

摘要

A compound objective lens is provided with a diffraction structure for diffracting light and a refraction surface for refracting light, the diffraction structure having a step-shaped cross-section with one cycle at an M level of M-1 (M being a natural number equal to or greater than 3) and a step (d1) adding an optical path difference longer than a wavelength 1 in relation to blue light and an optical path difference less than the wavelength 1 in relation to red light, each flat section of the diffraction structure being inclined upward.