Characterization and analysis of photodarkening in double cladding ytterbium-doped silica fibers

Ytterbium-doped double cladding fibers have emerged as an excellent solution for power scaling of diffraction-limited high peak and average power laser sources. However, the photodarkening effect, which manifests as progressive growth of loss leading to degradation of laser performance, poses a very serious threat to the reliability of ytterbium-doped fiber lasers. This thesis is focused on developing methods to quantify the photodarkening process and associated thermal processes in ytterbium-doped aluminosilicate fibers. Inversion was found to be a key controlling variable in the forward photodarkening process. However, three thermal processes were observed that affect the photodarkening kinetics and therefore complicate the interpretation of kinetic data. A photodarkened sample was measured to exhibit further heat-induced darkening when subjected to elevated temperatures. In addition, photodarkening-induced absorption spectrum was found to exhibit heat-induced spectral broadening with the loss-change having a linear dependence on temperature proportional to |λ-λ0|. For the studied fiber, λ0 was found to be 612 nm. Thermal activation energy distribution of the photodarkening-induced defects was measured to be governed by a single defect with the peak located at 1.32 eV with a FWHM of 0.31 eV. Furthermore, temperature of the fiber was found to correlate with the pump power absorbed by the photodarkening-induced defects. The observed thermal processes are therefore progressively being activated during pumping, unless isothermal conditions are being maintained. Finally, a manifestation of the photodarkening rate dependence on inversion, i.e., transverse photodarkening-induced loss variation, was observed by comparing the near-field intensity distributions of a pristine and a photodarkened fiber. In addition to work related to photodarkening, this dissertation includes a theoretical study on the inversion, gain, and amplified spontaneous emission behaviour in a cladding-pumped erbium-doped fiber. Increasing the cladding area was found to result in a more uniform inversion distribution along the fiber length, therefore preventing short-wavelength gain and ASE build-up and improving the quantum conversion efficiency