Jae-ho Lee

Residual stress change of optical fiber for Ge dopant variation

As GeO2 concentration increased, residual stress increased. For the same tension, axial stress induced by drawing is linearly proportional to GeO2-dopant concentration. The residual stress of the fiber (about axial direction) is originated from phase distribution caused by photo-elastic effect: A phenomena that a fiber becomes birefringent through the nonisotropic stresses. The photo-elastic effect makes phase shift such that intensity difference of light is appeared. The phase profile is obtained by measuring the intensity difference. And phase profile can be converted to the stress profile by Abel transformation. The magnitude of stress induced by drawing tension, is different as glass material constitution. As material is the more viscous, induces the more stress. Generally the glass for which dopants such as Ge, B, P and F are added has lower viscosity value compared with pure silica glass. And as dopant concentration increased, viscosity decreased. We investigated the relation between GeO2-dopant concentration and the magnitude of residual stress. Perform core was deposited with Ge-Si ratio having parabolic increment about radial direction. GeO2 concentration of core varied from 0 to 21.67mol%. That was also described to index change. The index difference (delta N) of core and clad was 0.026. The preform was drawn for fixed tension. Residual stress of optical fiber was measured after drawing. And fiber was fully annealed by flame. After then stress was measured one more. We obtained stress as the difference these two data: before and after anneal process. In fiber drawing process, tension was differently applied for each point of core region so that the measured stress about radial direction was not correct. To compensate the error, stress of sample was referred to pure-silica rod of that.