Benchmarking and measuring photodarkening in Yb doped fibers

摘要

Photodarkening is a detrimental phenomenon known to affect ytterbium doped fibers. Methods to study the spectral and temporal properties of the photodarkening induced loss were developed. The spectral shape of the photodarkening loss measured from multiple aluminosilicate samples indicate that visible wavelength(s) could be used in benchmarking fibers for their PD induced loss. Two principal methods, core and cladding pumping, were introduced to induce a known and repeatable inversion to fiber samples. The photodarkening rate could be parameterized using a single variable, inversion. More generally, the photodarkening rate was found to follow a simple power law and to be proportional to [Yb~*]~(7±1) (the excited state Yb ion density). Two methods, stretched exponential and bi-exponential, were used to fit the rate measurements. Both fitting methods were found suitable, with the bi-exponential method having more potential in increasing the understanding of the mechanism(s) behind photodarkening. Coiling induced spatial changes in the inversion and subsequent photodarkening performance were demonstrated for a large-mode-area fiber laser.