Patent ID: 7385761

Claim:
A diffraction grating device that diffracts and reflects a light beam in a first band of wavelengths and that diffracts and reflects and thereby separates a plurality of light beams in a plurality of bands of wavelengths longer than the wavelengths of the first band, the plurality of light beams being incident from a direction in which the light beam in the first band of wavelengths is diffracted, wherein elevations and depressions on a diffraction grating have a first period in a first direction and a second period longer than the first period in a second direction perpendicular to the first direction, and wherein the following relationships are fulfilled: λ1 L<λ 1 U<λ 2 L<λ 2 U<λ 3 L<λ 3 U; n 2 <n 1·sin θ; φ≠0; 1 /[n 1·(1−sin 2 θ·sin 2 φ) 1/2 +n 1·sin θ·cos φ]≦Λ/λ3 U <Λ/λ2 L ≦1/[( n 2 2 −n 1 2 ·sin 2 θ·sin 2 φ) 1/2 +n 1·sin θ·cos φ]; and 1/[( n 2 2 −n 1 2 ·sin 2 θ·sin 2 φ) 1/2 +n 1·sin θ·cos φ]≦Λ/λ1 U <Λ/λ1 L ≦2 /[n 1·(1−sin 2 θ·sin 2 φ) 1/2 +n 1·sin θ·cos φ], where n1 represents a refractive index of a first medium present on a side of the diffraction grating that faces optical paths of the light beams; n2 represents a refractive index of a second medium present on a side of the diffraction grating opposite to the side thereof facing the optical paths of the light beams; θ represents an incidence angle at which a principal ray of the light beams is incident on the diffraction grating; φ represents an angle between a plane perpendicular to the diffraction grating and parallel to the first direction and an incidence plane of the principal ray of the light beams; Λ represents the first period of the elevations and depressions on the diffraction grating; λ1L represents a shortest wavelength of the first band of wavelengths; λ1U represents a longest wavelength of the first band of wavelengths; λ2L represents a shortest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, a band of shortest wavelengths; λ2U represents a longest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, a band of shortest wavelengths; λ3L represents a shortest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, a band of longest wavelengths; and λ3U represents a longest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, a band of longest wavelengths.