OPTICAL ELEMENT

PROBLEM TO BE SOLVED: To enable stable recording/reproducing by condensing a light beam on three kinds of recording surfaces of optical recording mediums using a light source with a plurality of wavelengths and single plastic objective lens and correcting spherical aberration due to temperature change without loss of light use efficiency. SOLUTION: Material characteristic and phase step shape are properly set to obtain an appropriate amount of spherical aberration on the surface of a phase shifter 503. When satisfying an equation (1) where a variation of a refraction index of a spherical aberration correcting element 501 is (dn/dT) when a variation of oscillating wavelength of the light source due to the temperature variation is (dλ/dT), an expansion coefficient of the step difference h in the optical axis direction on the surface of the phase shifter 503 is (dh/dT), and the number of stages is m, the appropriate amount of the spherical aberration is generated on the surface of the phase shifter 503, to satisfactorily condense the spot on the recording surface of a BD optical recording medium 107, and the spherical aberration is 0.07 λrms or less when the equation (1) is satisfied. COPYRIGHT: (C)2010,JPO&INPIT