Optical fiber grating element, production method, and optical fiber filter

An optical fiber grating element, comprising:

• a multi-mode optical fiber having a cutoff wavelength regarding LP02-mode light on the longer wavelength side than a wavelength band in use, said multi-mode optical fiber having a first core region of a refractive index n1 extending along a predetermined axis, a second core region provided on a periphery of said first core region and having a refractive index n2 lower than that of said first core region, and a cladding region provided on a periphery of said second core region and having a refractive index n3 lower than that of said second core region;
• and
• a grating provided in a predetermined region of said first core region and selectively coupling fundamental LP01-mode light of a predetermined wavelength within the wavelength band in use to LP0m (m≧2)-mode light,
• wherein said optical fiber grating element satisfies the following relation:
• n1>Neff01>n2>Neff0m>n3
• where Neff01 is an effective refractive index with respect to the fundamental LP01-mode light and Neff0m an effective refractive index with respect to the LP0m (m≧2)-mode light.