An optical fiber made of silica-based material includes a core and a
cladding formed around the core. The core is doped with germanium
dioxide, which increases refractive index and decreases acoustic-wave
velocity, and aluminum oxide, which increases both refractive index and
acoustic-wave velocity,
satisfying--2.814+0.594.times.W.sub.1.ltoreq.W.sub.2.ltoreq.54.100+0.218.-
times.W.sub.1, W.sub.1+W.sub.2.ltoreq.60, and
W.sub.2.gtoreq.56.63-2.04.times.W.sub.1, where W.sub.1 is doping amount
of germanium dioxide in weight percentage, which is larger than 4.74, and
W.sub.2 is doping amount of aluminum oxide in weight percentage. A
nonlinear coefficient of the optical fiber is equal to or larger than
2.6.times.10.sup.-9W.sup.-1.