Thermally induced transformation of “dark” precursors into laser-active centers: Hidden potential of bismuth-doped fibers

Abstract

This paper presents a comprehensive study of the transformation of “dark” (undetected with UV–IR spectroscopy) precursors into bismuth active centers (BACs) associated with Si atoms during thermal treatment, which can be considered as an assessment of the upper limit of Bi-doped fibers in terms of maximum achievable gain and as search for ways to improve their optical characteristics. This concept is based on the study of various bismuth-related forms in active media and the possible processes of their transformations, which, in essence, determines its “hidden” potential. Using Bi-doped low-GeO${}_2$-SiO₂ glass fibers it was established that the proposed concept allows us to give a generalized assessment of the prospects of fiber design, as well as to optimize the chemical composition and corresponding technological conditions for their manufacturing. We show that the fraction of Bi ions forming BACs can reach 90% in the best fibers after efficient conversion induced by thermal treatment, while the pristine fibers contain only 30% of Bi ions in BACs and $\sim$60% in “dark” precursors. It was experimentally found that physicochemical reaction of ”dark” precursors $\to$ BACs is characterized by an activation energy of $\sim$2.1 eV and a rate constant of 10${}^7$ s⁻¹. In addition, more information on Bi-related centers responsible for unsaturable loss was obtained. This approach showed that optical characterization of the initial state of the active medium, particularly the bismuth-doped GeO${}_2$-SiO₂ fibers, is not always decisive for determining its potential.