Analysis of photodarkening effects in ytterbium-doped laser fibers

Abstract

Fiber lasers and amplifiers have become very important for many applications, such as telecommunications, materials processing, and medicine. Ytterbium (Yb) doped fibers are the preferred gain media in high peak and average power laser sources. However, the photodarkening effect, which manifests as a time dependent increase of loss in the doped core, is one of the main threats to the efficiency of the Yb-doped gain media leading to degradation of the laser performance. This thesis focused on developing accurate methods to characterize photodarkening and the associated thermal effects in Yb-doped silica fibers. In a first stage, a method for conducting repeatable photodarkening and thermal bleaching measurements over the same sample was developed. The activation energy associated with the thermal recovery of photodarkening-induced losses was determined. Moreover, thermal cycling studies revealed that a photodarkened sample exhibits a net increase of loss with increasing temperature prior to the initiation of thermal recovery. In addition, the photodarkening-induced absorption spectrum was found to broaden with increasing temperature, resulting in a non-permanent increase of loss in the near-infrared. At the same time, a method to quantify the fiber temperature during the course of photodarkening measurements was established. The fiber temperature was found to correlate with the pump power absorbed by the photodarkening-induced defects. Then, a thermal model was implemented to accurately parameterize the temperature increase of a photodarkened fiber as a function of the pump power. Furthermore, the photodarkening rate was found to follow a power law dependency of 4.5 on the inversion, in case the fiber temperature is properly controlled. Finally, the photodarkening-induced refractive index change, measured by a modal interference method, was found to be positive and in the order of 10^(-5) at around 1080 nm. Parallel to the experimental work on photodarkening, a new all vapor phase fabrication method was implemented to produce Yb-doped fibers by atomic layer deposition (ALD) in combination with the conventional modified chemical vapor deposition (MCVD) technique. A large mode area Yb-doped aluminosilicate fiber fabricated by this method showed uniform Yb-doping, high lasing efficiency (slope efficiency of 80%) and low background losses. This fabrication method can potentially result in an integrated ALD-MCVD system.