US Patent 7577077 Optical pickup apparatus and optical element

An optical pickup apparatus comprises first, second and third light sources to emit light fluxes of wavelength λ1, λ2 and λ3 for conducting recording and/or reproducing information for first, second and third optical information recording mediums having respective protective substrates of thickness t1, t2 and t3 and a diffractive optical element located on a common optical path for the first, second and third light sources. A converged-light spot is formed on the first optical information recording medium with m-th order diffracted-light ray of the wavelength λ1, on the second optical information recording medium with n-th order diffracted-light ray of the wavelength λ2, and on the third optical information recording medium with k-th order diffracted-light ray of the wavelength λ3 generated by the diffractive optical element respectively, wherein one of m, n and k is different from one of other two numbers.