Patent ID: 7203399

Claim:
A fiber grating type filter comprising: an optical fiber mainly composed of silica glass, said optical fiber comprising: a core region extending along a predetermined axis, and not containing a photosensitive dopant which contributes to predetermined wavelength light photosensitivity as a glass property; and a cladding region provided on an outer periphery of said core region, and having at least a doped part with the photosensitive dopant; and a grating provided in a predetermined region of the doped part of said cladding region, which is doped with the photosensitive dopant, in a longitudinal direction of said optical fiber, said grating having a grating plane slanted with respect to the predetermined axis, wherein an angle of slant defined between the normal to the optical axis and the grating plane of said grating is greater than 1 degree but less than 2 degrees, wherein said cladding region comprises an inner cladding provided on and in direct contact with an outer periphery of said core region, said inner cladding containing the photosensitive dopant; and an outer cladding provided on and outer periphery of said inner cladding, wherein said optical fiber has a cutoff wavelength shorter than 1.3 μm but longer than 1.1 μm, and wherein a relative refractive index difference between said core region and the doped part of said cladding region is 0.30% or more but 0.40% or less.