PURPOSE: A structure of a dispersion flattened fiber(DFF) having a large negative dispersion value and a fabrication method thereof are provided, which compensates for a dispersion more simply and accurately using an optical fiber having a flattened dispersion characteristics in a transmission wavelength band and a large negative dispersion value. CONSTITUTION: The dispersion flattened fiber(DFF) comprises a cladding(15), the first and the second ring type core(14,12), the first and the second low refractive index region(13,11) and a core(10). A polymer coating is formed on the most outer side of the optical fiber to protect the optical fiber. The cladding is formed on the inside of the polymer coating. And the first ring type core is formed in the inside of the cladding, and the first low refractive index region is formed in the inside of the first ring type core. And the second ring type core is formed in the inside of the first low refractive index region, and the second low refractive index region is formed in the inside of the second ring type core. At last, the core is formed on a center in the inside of the second low refractive index region.
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