We have accurately measured the nonlinear refractive index, n2, of the
glass found in silica-core and dispersion-shifted optical fibers. We
find that at 1.319 mum the n2 of silica-core and dispersion-shifted fi
bers is 2.36 x 10(-16) and 2.52 x 10(-16) cm2/W (+/- 5%), respectively
. We also estimate that the n2 Of germania is three times the n2 of pu
re silica. Because the wavelength dependence of n2 between 1.3 and 1.5
mum is known to be small, our measured values should also be valid at
1.55 mum.