Dispersion-controlled optical fiber

A dispersion-controlled optical fiber comprising:

• a center core having a refractive index N1 for forming a passageway for transmitting optical signals;
• a cladding having a refractive index N0 for enclosing said center core;
• an upper core surrounding said center core and having a refractive index distribution increased starting from a refractive index N2 at its outer circumference to the refractive index N1 at its internal circumference;
• and,
• a minutely depressed refractive index region having a reflective index N3 and interposed between said upper core and cladding, wherein the refractive index N3 is lower than the refractive index N0, wherein the internal radius a and external radius b of said upper core, and the internal radius c of said cladding meet with the relationships:
• 0.06≦a/b≦0.8 and 0.02≦a/c≦0.9.