Patent ID: 8339925

Claim:
An optical information recording/reproducing optical system for recording information to and/or reproducing information from an optical disc by letting a laser beam impinge 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: 400<λ<410 (1), the optical element and the objective lens are made of same resin materials or different resin materials having a glass transition temperature Tg satisfying a condition: Tg>115° C. (2), each of optical surfaces of the optical element and the objective lens is configured not to have an optical thin film which contains at least one of elements of titanium (Ti), tantalum (Ta), hafnium (Hf), zirconium (Zr), niobium (Nb), molybdenum (Mo) and chromium (Cr), each of optical surfaces of the optical element is provided with an antireflection film which is made of 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, when optical surfaces from an optical surface to which the laser beam emerging from the light source enters to an exit surface of the objective lens are defined as i-th surfaces (i=1, 2, 3, . . . , n), a reflectivity defined when light having a wavelength λ UV (λ UV =365 nm) is incident perpendicularly on i-th optical surface is represented as R UVi (unit: %) and a reflectivity defined when light having a wavelength λ BL (λ BL =405 nm) is incident perpendicularly on i-th optical surface is represented as R BLi (unit: %), the optical information recording/reproducing optical system satisfies a condition: ∏ i = 1 n - 1 ⁢ ( 1 - R ( BL ) ⁢ i 100 ) - ∏ i = 1 n - 1 ⁢ ( 1 - R ( UV ) ⁢ i 100 ) > 0.05 . ( 3 )