Patent ID: 7633680

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 the following relationships are fulfilled: λ1L<λ1U<λ2L<λ2U<λ3L<λ3U; n 2 <n 1·sin θ; 1/( n 1+ n 1·sin θ)≦Λ/λ3 U<Λ/λ 2 L≦ 1/( n 2 +n 1·sin θ); 1/( n 2 +n 1·sin θ)≦Λ/λ1 U<Λ/λ 1 L≦ 2/( n 1+ n 1·sin θ); and Λ/λ3 L< 1/(2· n 1·sin θ)<Λ/λ2 U, where n1 represents a refractive index of a first medium present on a side of the diffraction grating that faces optical paths; n2represents a refractive index of a second medium present on a side of the diffraction grating opposite to the side thereof facing the optical paths; θ represents an incidence angle at which a principal ray of the light beams is incident on the diffraction grating; Λ represents a 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.