Patent ID: 7170600

Claim:
A spectrometer using a diffraction grating comprising: a light-incident portion including an incident-side optical waveguide emitting a light beam that includes a plurality of wavelength components and that approximates a Gaussian beam, and a collimating lens that is arranged on an emission side of the incident-side optical waveguide and that converts the light beam approximating a Gaussian beam that is emitted from the incident-side optical waveguide into a substantially collimated light beam; a diffraction grating having grooves on its surface, on which the light beam that has been converted into the substantially collimated light beam by the collimating lens is incident, the diffraction grating spectrally separating the light beam by emitting light beams whose emission direction depends on their wavelength; and a light-emitting portion having a plurality of focusing lenses that respectively condense the light beams that have been spectrally separated by the diffraction grating; wherein, when d is an effective diameter of the collimating lens and the focusing lenses, f is a focal length of the collimating lens and the focusing lenses, D is a physical outer diameter of the collimating lens and the focusing lenses, L is a distance between the diffraction grating and the focusing lenses, φ is an incident angle of the light beam with respect to the diffraction grating when the light beam is incident on the diffraction grating, NA is a numerical aperture of the incident-side optical waveguide, g V is a length of the diffraction grating along a direction perpendicular to the grooves, g P is a length of the diffraction grating along a direction parallel to the grooves, λ 1 and λ 2 are wavelengths of the adjacent incident light to be separated within the light beam incident on the diffraction grating, λ 0 is an average wavelength of the adjacent incident light of the wavelengths λ 1 and λ 2 , and Δψ (in radian) is an angular difference of the diffraction angles at the diffraction grating between the light of the wavelength λ 1 and the light of the wavelength λ 2 , then the following expressions are satisfied: d≧a g V ≧a /cos φ g P ≧a L≧D /Δψ where a is the larger value of 9λ 0 /(πΔψ) and 3f·NA.