On-line monitoring technique of the refractive-index profile in the VAD process

A nondestructive on-line monitoring method to estimate the refractive-index profile in vapor-phase axial deposition (VAD) processing has been developed. This technique is developed on the basis of the deposition properties of a SiO
2
-GeO
2
glass particles in the flame hydrolysis reaction. The profile monitoring accuracy is
\pm 7 \times 10^{-5}
in refractive-index difference. The estimated profiles during the porous preform fabrication are well coincided with those of the consolidated preform. The average bandwidth properties of graded-index fibers produced by adjusting the fabrication conditions, to be optimized with using present method, is 2.5 GHz . km
^0.9
at 1.3-μm wavelength.