Formation mechanism of drawing-induced <i>E</i>′ centers in silica optical fibers

Abstract

The formation mechanism of drawing-induced E′ centers in silica optical fibers is clarified by analyzing the thermodynamic behavior of point defects during the drawing process. The E′ center concentration in the fibers is given by nd=np exp(−Ef/kTd−A/V1), where Td is the drawing temperature, V1 the drawing speed, and A a constant. The formation energy Ef is 3.8 eV while the precursor concentration np is 7×1022 g−1. The E′ center concentration at the fiber surface is greater than that at the center because of the transverse difference in the quenching rate during the drawing process. The annealing behavior of the E′ center concentration is expressed as nd(t)=nd(0)exp[−νt exp(−Ed/kTa)], where Ta is the annealing temperature. The activation energy for the motion of the E′ centers Ed is 0.2 eV while the frequency factor ν is 8×10−3 sec.−1.