| 摘要 |
PROBLEM TO BE SOLVED: To improve an S/N ratio by setting light quantity efficiency to a maximum for two wavelengths in an optical pickup device that utilizes two optical wavelengths. SOLUTION: The device is provided with a laser light source having a wavelength λ1, a laser light source having a wavelength λ2 that is longer than the wavelength λ1, an optial system, which guides the light beams generated by the light sources onto the surfaces of optical recording media 15 and 15', and a light receiving element which photoelectrically converts the reflected light beams from the surfaces of the media 15 and 15'. A step shaped phase diffraction grating element 13 is provided in the optical system above. In the element 13, the number of steps is set to N1+1 or N2+1 where N1 is a maximum integer that does not exceed a rational number N=1/((λ2/λ1)-1) and N2 is a minimum integer that exceeds N and the amount of a step difference in one step is set to λ1/(n-1) where n is a refractive index. |
