Abstract
We demonstrate a distributed measurement technique to observe temperature changes along pumped Yb-doped fibers. This technique is based on an array of fiber Bragg gratings acting as a temperature sensor line. The Bragg gratings are inscribed directly into the Yb-doped fiber core using high-intensity ultrashort laser pulses and an interferometric setup. We studied the temperature evolution in differently co-doped Yb fibers during optical pumping and identified different effects contributing to the observed temperature increase. We found that preloading of fibers with hydrogen supports the formation of Yb 2þ during UV irradiation and has a large impact on fiber temperature during pumping. The proposed technique can be applied to investigate the homogeneity of pump absorption in active fibers and to support spatially resolved photodarkening measure- ments. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in
Highlights
Since fiber lasers and amplifiers have been scaled up to the kW-level,[1] they have attracted much attention in science and industry
We studied the temperature evolution in differently co-doped Yb fibers during optical pumping and identified different effects contributing to the observed temperature increase
An initial temperature step is assigned to an energy input through permanent loss of the fiber core, including the quantum defect (QD) that is the energy difference between pump and amplified spontaneous emission (ASE) light plus any additional absorption due to the background attenuation of the fiber
Summary
Since fiber lasers and amplifiers have been scaled up to the kW-level,[1] they have attracted much attention in science and industry. The fabrication of FBGs was first demonstrated by Hill et al.[2] The development of holographic inscription techniques[3] enables the realization of Bragg grating periods and therewith Bragg reflection wavelengths nearly independent of the laser source. Employed laser sources in holographic inscription setups are excimer and exciplex lasers emitting UV nanosecond pulses With such sources, FBGs in UVphotosensitive but mostly passive fibers have been realized, e.g., in Ge-doped (standard) fibers applied for temperature and strain sensing. Through FBG-inscription and in the latter case[17] can improve the spectral shape of the laser emission Another possibility to remove UV-induced losses is a subsequent low-power UV photobleaching[18] treatment, where the density of absorbing species can be reduced based on the existence of loss equilibrium states. We study the temperature evolution in differently co-doped fibers under pumping conditions, identifying various effects contributing to the observed temperature increase
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