Double-wavelength light source unit and optical head device having four diffraction gratings

摘要

A double-wavelength light source unit capable of stable signal detection, is realized.;The double-wavelength light source unit comprises a double-wavelength light source, a photodetector and a double-wavelength diffraction element, wherein the double-wavelength diffraction element comprises a diffraction grating which transmits a light beam of λ₁ and diffracts a light beam of λ₂ among the light beams of wavelengths λ₁ and λ₂ (λ₁≠λ₂) emitted from two laser diode diodes as the double-wavelength light source, a diffraction grating which diffracts a light beam of λ₁ and transmits a light beam of λ₂, a diffraction grating constituted by a grating-material forming a grating and a filler which transmits a light beam of λ₁ since the refractive indices of both of these materials are equal at λ₁ and diffracts a light beam of λ₂ since the refractive indices are different at λ₂, and a diffraction grating which transmits a light beam of λ₁ having a first polarization plane and diffracts a light beam of λ₁ having a polarization plane perpendicular to the first polarization plane.