Ytterbium-Doping Related Stresses in Preforms for High-Power Fiber Lasers

Frozen-in mechanical stresses can significantly influence the optical and mechanical properties of optical fibers, especially in laser fibers for high-power operation. In the following, we will report on the polarimetric measurement of stresses induced by the spatially varying doping composition in fiber preforms. We investigated the effect of rare-earth laser ions and found that the dopant ytterbium generates higher stresses than other common dopants in lightwave technology such as phosphorus or aluminum. The stress-induced index change relevant for the guiding properties is derived from the stress data. Especially in large-mode-area laser fibers with low numerical apertures, such stresses can significantly modify the index profile and thereby influence the propagation behavior.