Optical fiber and slanted fiber grating

A slanted fiber grating including a Bragg grating in at least a partial region in an optical fiber, the optical fiber made of silica-based glass and comprising:

• a core that includes a central axis of the optical fiber and that has a radius ra that is larger than 4.3 μm and smaller than or equal to 5.0 μm;
• a first cladding that is in contact with the core, that surrounds the core, that has an outer radius rb, and that has a refractive index lower than a refractive index of the core;
• and
• a second cladding that is in contact with the first cladding, that surrounds the first cladding, and that has a refractive index lower than the refractive index of the core and higher than the refractive index of the first cladding,
• wherein at least a part of the first cladding contains a photosensitive material whose refractive index increases by irradiation with light,
• wherein, at the radius ra, a derivative of a refractive index with respect to a distance from the central axis along a straight line perpendicular to the central axis is minimum,
• wherein, in a graph in which the straight line perpendicular to the central axis is a horizontal axis and the refractive index is a vertical axis, a difference between a maximum value of the refractive index of the core and a value that an extension of a straight line connecting a refractive index at a radius of (ra+1 μm) and a refractive index at a radius of (rb−1 μm) takes at the radius ra is in a range of 0.25% to 0.30% relative to a refractive index of 1.444024, and
• wherein the photosensitive material is GeO2.