To provide an optical fiber grating which is increased the confinement index of light power by increasing the ratio of the light power propagating in a core, can be easily produced, and can be handled in the same manner as for ordinary light.
The constitution of the optical fiber has the core having the max. refractive index n1 in the central part, has a first clad having a refractive index n2 on the outer side thereof and, further, has a second clad having a refractive index n3 on the outer side thereof. The respective refractive indices are n1>n3>n2. The normalization frequency V expressed by V=2πa/λx(n12-n22)1/2 when a core radius is (a) and a use wavelength is λ is ≥2.405. The refractive index n3 of the second clad in the optical fiber constitution described above is made larger than the effective refractive index N2 of a secondary mode and is made smaller than the effective refractive index N1 of a base mode, by which the loss of a short wavelength band is suppressed.
ITO MASUMI
NISHIMURA MASAYUKI
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