US Patent 7209428 Optical system of optical pick-up

An optical system of an optical pick-up, which is provided with a light source that emits a light beam, and an objective lens that converges the light beam emitted by the light source onto a data recording layer of an optical disc. The optical system satisfies a condition: 0.75<d·(M−1)/(M·L)<1.0, where d (mm) represents a thickness of the objective lens, M represents magnification of the optical system, and L (mm) represents an O/I distance which is defined as a distance between an object and an image. A beam spot suitable for the optical disc is formed by the objective lens on the data recording layer of the objective lens without using a coupling lens.