The effect of doping trace amounts of germania Into the core of silica core fibers was investigated by fabricating a set of preforms by MCVD with varying amounts of core dopant along the length of each, then comparing the loss performance achieved by the corresponding fibers. Pure silica core fibers sometimes exhibit anomalously high broad-background losses, as seen in Fig. 1. Such losses are eliminated by the addition of 0.04-mol % germania to the core, permitting the achievement of minimum losses as low as 0.16 dB/km at 1.57 μm,1,2 as shown in Figs. 1 and 2. The same effect is observed when the core is doped with trace amounts of phosphorus pentoxide instead of germania. Although the mechanism affecting minimum attainable loss in silica core fibers Is not yet understood, it appears that anomalously high losses are only observed in silica core fibers exposed to processing temperatures as high as 2300°C during manufacture, while those made using lower temperatures usually do not exhibit such losses.3 It Is speculated that the germania dopant required during MCVD reduces either scattering or absorption losses by eliminating the defects found In pure silica, perhaps preventing formation of an oxygen vacancy.
© 1987 Optical Society of AmericaPDF Article