Elastic property differences in a multi-mode optical fiber and the effect on predicted mechanical lifetime

Multi-mode optical fibers are used for high energy transmission. One configuration of such fibers has a fluorine-doped silica cladding, and a pure silica core. Nanoindentation measurements of elastic modulus and hardness in the core and cladding of such a fiber are presented, and compared to those in the preform. The cladding region is softer, and has a significantly lower elastic modulus than the core in both the fiber and preform. Ultrasonic measurements made on core and clad materials extracted from the preform confirm the observed elastic property differences in the fiber. Water-immersion mandrel wrap data for time to failure of up to ~ 1 year are analyzed to obtain allowable design stresses for 30 year lifetime for the fiber. Incorporating the lower clad elastic modulus leads to design stresses that are ~ 20–25% lower than those predicted without taking the correct modulus into account.