Patent ID: 7548359

Claim:
A double-wavelength light source unit including a double-wavelength light source emitting two light beams of linearly polarized light of different wavelengths λ 1 and λ 2 from different emission points, a photo detector and a double-wavelength diffraction element for transmitting the two light beams of different wavelengths, the double-wavelength diffraction element comprising: a first diffraction grating including a grating material forming a concavo-convex shaped grating and a filler filling the concavo-convex portion of the grating material, the refractive indices of the grating material and the filler of the first diffraction grating are equal for the incident light beam of wavelength λ 1 so as to transmit the light beam without diffracting and the refractive indices of the grating material and the filler of the first diffraction grating are different for the incident light beam of wavelength λ 2 so as to diffract the light beam to generate at least 0 th and ±1 st order diffraction light beams as the tracking beams; a second diffraction grating including a grating material forming a concavo-convex shaped grating and a filler filling the concavo-convex portion of the grating material, and the refractive indices of the grating material and the filler of the second diffraction grating are equal for the incident light beam of wavelength λ 2 , so as to transmit the light beam without diffracting and the refractive indices of the grating material and the filler of the second diffraction grating are different for the incident light beam of wavelength λ 1 so as to diffract the light beam to generate at least 0 th and ±1 st order diffraction light beams as the tracking beams; a third diffraction grating including a grating material forming a concavo-convex shaped grating and a filler filling the concavo-convex portion of the grating material, wherein the refractive indices of the grating material and the filler for the light beam of wavelength λ 1 are equal so as to transmit the light beam without diffracting and the refractive indices of the grating material and the filler are different for the light beam of wavelength of λ 2 so as to diffract an amount of the light beam of wavelength λ 2 detectable by the photo detector; and a fourth diffraction grating configured to transmit without diffraction a linearly polarized light beam of wavelength λ 1 having a first polarization plane, to diffract an amount of a linearly polarized light beam of wavelength λ 1 detectable by the photo detector and having a second polarization plane perpendicular to the first polarization plane, and to transmit without diffraction a linearly polarized light beam of wavelength λ 2 having the first polarization plane.