Spectral losses in unclad optical fibers have been measured in the region between 0.5 and 1.12 µm with the filtered light of a xenon-arc lamp. Total losses were obtained nondestructively by measuring the light refracted from the fiber when it was immersed in a higher-index liquid. The radial irradiance distribution of the refracted light was used as a measure of the mode spectrum. Because scattering losses could be shown to approach the Rayleigh-scattering limit, and total losses were between 6 and 8 dB/km, i.e., 75 to 84% per kilometer, for some of the highest-purity synthetic vitreous silicas, we tentatively conclude that the spectral losses of unclad fibers represent a good approximation to the spectral losses of the bulk material.
P. Kaiser and H. W. Astle, "Measurement of spectral total and scattering losses in unclad optical fibers," J. Opt. Soc. Am. 64, 469-474 (1974)