Patent ID: 6882777

Claim:
A diffraction grating device in which a diffraction grating based on perturbation of refractive index is formed through a predetermined range in a light guide direction in an optical waveguide and in which said diffraction grating selectively reflects light in a reflection band out of light guided through the optical waveguide, wherein, where a first band is defined as a wavelength band with a maximum bandwidth out of continuous wavelength bands for which the transmittance is not more than T 0 (in units of dB) and where a second band is defined as a wavelength band between a maximum wavelength and a minimum wavelength out of wavelengths for which the reflectance is R 0 (in units of dB), the perturbation of refractive index of said diffraction grating is of antero-posterior symmetry with respect to the center on a center position of said predetermined range, T 0 is not more than −20 dB, R 0 is not more than −20 dB, a ratio (B 1 /B 2 ) of a width B 1 of said first band (in units of nm) to a width B 2 of said second band (in units of nm) is not less than 0.3, and a maximum group delay difference (in units of ps) caused by reflection of light in said first band by said diffraction grating is not more than 0.011×exp(7.86×B 1 /B 2 −0.045×R 0 )/B 1 .