Patent ID: 8559290

Claim:
An optical information recording/reproducing optical system for recording information to and/or reproducing information from an optical disc by a laser beam impinging on a recording layer of the optical disc, comprising: a light source that emits the laser beam; an optical element that converts the laser beam emitted by the light source into a substantially collimated beam; and an objective lens that converges the laser beam emerging from the optical element onto the recording layer of the optical disc, wherein: a wavelength λ (unit: nm) of the laser beam falls within a range defined by a following condition: 390<λ<420 the optical element and the objective lens comprise same or different resin materials having a glass transition temperature Tg satisfying a condition: Tg>115° C. a first multilayer functional film, a second multilayer functional film and a third multilayer functional film are provided on an optical disc side optical surface of the objective lens, a light source side optical surface of the optical element, and an optical disc side optical surface of the optical element, respectively; a light source side optical surface of the objective lens opposite to the optical disc side optical surface of the objective lens is provided with a fourth multilayer functional film having four or more layers; the fourth multilayer functional film does not contain titanium (Ti); each of the first, second and third multilayer functional films includes a non-high refractive index layer comprising a composition containing one of silicon oxide, aluminum oxide, aluminum fluoride and magnesium fluoride or a mixture of at least two of silicon oxide, aluminum oxide, aluminum fluoride and magnesium fluoride; each of the first, second and third multilayer films does not have a layer containing a composition comprising at least one of elements titanium (Ti), tantalum (Ta), hafnium (Hf), zirconium (Zr), niobium (Nb), molybdenum (Mo) and chromium (Cr), a layer of the fourth multilayer functional film closest to a base material comprises the non-high refractive index layer; the fourth multilayer functional film is configured such that a wavelength λ max(2) at which reflectivity at vertical incidence of the fourth multilayer film has a maximum value and a wavelength λ min(2) at which reflectivity at vertical incidence of the fourth multilayer film has a minimum value respectively satisfy the following conditions: 350<λ max(2) <420 600<λ min(2) <750; and the fourth multilayer functional film is configured such that the reflectivity of the fourth multilayer functional film at vertical incidence decreases monotonously from the wavelength λ max(2) to the wavelength λ min(2) .