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

A double-wavelength light source unit including a double-wavelength light source emitting two light beams of linearly polarized light of different wavelengths λ1 and λ2 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 birefringent grating material forming a concavo-convex shaped grating and a filler filling the concavo-convex portion of the grating material, wherein an ordinary refractive index n0 of the birefringent grating material is substantially equal to a refractive index ns of the filler, and an extraordinary refractive index direction of the birefringent grating material is configured so that the first diffraction grating transmits the linearly polarized light beam of wavelength λ1 having a first polarization plane as an ordinarily polarized light beam without diffraction, and the first diffraction grating diffracts the linearly polarized light beam of wavelength λ2 having a second polarization plane perpendicular to the first polarization plane as an extraordinary polarized light beam and generates at least 0th and ±1st order diffraction light beams to be used as tracking beams;
• a second diffraction grating including a birefringent grating material forming a concavo-convex shaped grating and a filler filing the concavo-convex portion of the grating material, wherein an ordinary refractive index n0 of the birefringent grating material is substantially equal to a refractive index n0 of the filler, and an extraordinary refractive index direction of the birefringent grating material is configured to be perpendicular to the extraordinary refractive index direction of the birefringent grating material of the first diffraction grating, so that the second diffraction grating transmits a linearly polarized light beam of wavelength λ2 having the second polarization plane as an extraordinarily polarized light beam without diffraction, and the second diffraction grating diffracts the linearly polarized light beam of wavelength λ1 having the first polarization plane as an ordinarily polarized light beam and generates at least 0th and ±1st order diffraction light beams to be used as 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 λ1 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 λ2 so as to diffract an amount of the light beam of wavelength λ2 detectable by the photo detector;
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• a fourth diffraction grating configured to transmit without diffraction the linearly polarized light beam of wavelength λ1 having a first polarization plane, to diffract an amount of linearly polarized light beam of wavelength λ1 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 λ2 having a first polarization plane.

A double-wavelength light source unit including a double-wavelength light source emitting two light beams of linearly polarized light of different wavelengths λ1 and λ2 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 birefringent grating material forming a concavo-convex shaped grating and a filler filling the concavo-convex portion of the grating material, wherein an ordinary refractive index n0 of the birefringent grating material is substantially equal to a refractive index ns of the filler, and an extraordinary refractive index direction of the birefringent grating material is configured so that the first diffraction grating transmits the linearly polarized light beam of wavelength λ1 having a first polarization plane as ordinarily polarized light beam without diffraction, and the first diffraction grating diffracts the linearly polarized light beam of wavelength λ2 having a second polarization plane perpendicular to the first polarization plane as an extraordinary polarized light beam and generates at least 0th and ±1st order diffraction light beams to be used as tracking beams;
• a second diffraction grating including a birefringent grating material forming a concavo-convex shaped grating and a filler filing the concavo-convex portion of the grating material, wherein an ordinary refractive index n0 of the birefringent grating material is substantially equal to a refractive index ns of the filler, and an extraordinary refractive index direction of the birefringent grating material is configured to be perpendicular to the extraordinary refractive index direction of the birefringent grating material of the first diffraction grating, so that the second diffraction grating transmits the linearly polarized light beam of wavelength λ2 having the second polarization plane as an extraordinarily polarized light beam without diffraction, and the second diffraction grating diffracts the linearly polarized light beam of wavelength λ1 having the first polarization plane as an ordinarily polarized light beam and generates at least 0th and ±1st order diffraction light beams to be used as 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 λ1 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 λ2 so as to diffract an amount of the light beam detectable by the photo detector;
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• a fourth diffraction grating configured to transmit without diffraction the linearly polarized light beam of wavelength λ1 having a second polarization plane, to diffract an amount of a linearly polarized light beam of wavelength λ1 detectable by the photo detector and having a first polarization plane perpendicular to the second polarization plane, and to transmit without diffraction a linearly polarized light beam of wavelength λ2 having a second polarization plane;
• wherein a light-input side of the first diffraction grating and a light-output side of the second diffraction grating are each provided with a phase plate configured to generate a phase difference of an integer times of wavelength for the incident light of wavelength λ2, and to rotate the polarization plane of the light beam of wavelength λ1 by ninety degrees, respectively.

A double-wavelength light source unit including a double-wavelength light source emitting two light beams of linearly polarized light of different wavelengths from different emission points, a photo detector and a double-wavelength diffraction element for transmitting the two light beams of different wavelengths λ1 and λ2 , the double-wavelength diffraction element comprising:

• a first diffraction grating including a birefringent grating material forming a concavo-convex shaped grating and a filler filling the concavo-convex portion of the grating material, wherein an ordinary refractive index n0 of the birefringent grating material is substantially equal to a refractive index ns of the filler, and an extraordinary refractive index direction of the birefringent grating material is configured so that the first diffraction grating transmits the linearly polarized light beam of wavelength λ1 having a first polarization plane as ordinarily polarized light beam without diffraction, and the first diffraction grating diffracts the linearly polarized light beam of wavelength λ2 having a second polarization plane perpendicular to the first polarization plane as an extraordinary polarized light beam and generates at least 0th and ±1st order diffraction light beams to be used as tracking beams;
• a second diffraction grating including a birefringent grating material forming a concavo-convex shaped grating and a filler filing the concavo-convex portion of the grating material, wherein an ordinary refractive index n0 of the birefringent grating material is substantially equal to a refractive index ns of the filler, and an extraordinary refractive index direction of the birefringent grating material is configured to be perpendicular to the extraordinary refractive index direction of the birefringent grating material of the first diffraction grating, so that the second diffraction grating transmits the linearly polarized light beam of wavelength λ2 having the second polarization plane as an extraordinarily polarized light beam without diffraction, and the second diffraction grating diffracts the linearly polarized light beam of wavelength λ1 having the first polarization plane as an ordinarily polarized light beam and generates at least 0th and ±1st order diffraction light beams to be used as 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 λ1 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 λ2 so as to diffract an amount of the light beam detectable by the photo detector;
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• a fourth diffraction grating configured to transmit without diffraction the linearly polarized light beam of wavelength λ1 having a second polarization plane, to diffract an amount of a linearly polarized light beam of wavelength λ1 detectable by the photo detector and having a first polarization plane perpendicular to the second polarization plane, and to transmit without diffraction a linearly polarized light beam of wavelength λ2 having a second polarization plane;
• wherein a light-input side and a light-output side of a diffraction grating, constituted by the first diffraction grating and the second diffraction grating, are each provided with a phase plate configured to generate a phase difference of an integer times of wavelength for the incident light of wavelength λ2, and to rotate the polarization plane of the light beam of wavelength λ1, by ninety degrees, respectively.