Non-isothermal bleaching of photodarkened Yb-doped fibers

Abstract

We report on the thermal treatment of photodarkened Yb-doped fiber samples. The method of non-isothermal bleaching at different temperature ramp rates can be used to determine the thermal energy distribution of photodarkening induced color centers. A distributed activation energy with a mean value of about 1.3 eV and a FWHM of 0.5 eV was found. Spectral changes during thermal treatment were observed and could be interpreted, e.g. as an enhancement of the absorption cross section.

A physical model of the photo- and radiation-induced degradation of ytterbium-doped silica optical fibres

“…The kinetic analysis of this glow peak indicated that the thermal activation energy of the related trapped-hole center is about 1.3 eV. This estimate is in fair agreement with activation energies obtained by other methods for the thermal bleaching of PD, namely 1.3 eV [18] and 1.26-1.50 eV [19]. In view of these values, the PI of trapped holes should be most efficiently achieved by photons around 2.5 eV (or more).…”

“…Then, trapped-hole states responsible for the induced optical loss, i.e. acting as color centers, may spread from at least ∼1.27 eV (980 nm) above the VB (consistent with [18,19]). Based on these hypotheses, we propose to describe PD, RD and PRD by the model of Fig.…”

The three-electron bond =Si<O_2:∙Yb absorption center of pre-darkened ytterbium-doped silica

“…This opens for the possibility to directly relate observed change in absorption with activation energy. For pre-darkened fibers an unexpected increase in PD absorption is hereby observed for annealing ramps from room temperature to about 350-450 K followed by the expected bleaching that initiate at 470-600 K [11,12]. Thermally induced PD absorption is not observed for pristine fibers [12] while PD bleaching shows E A of ≈1.3 eV [12,13] to 1.5 eV [13] dependent on fiber composition and manufacturing condition.…”

“…The interpolated bond energies found hereby are along with Bose-Einstein phonon statistics applied to develop a Markov chain statistical model for annealing of pre-darkened fibers. This model gives good match to the annealing data from [11][12][13] for a distributed set of CC sites of individual TEB polarization.…”

“…In this section the kinetic model for annealing of PD material is applied on experimental data from thermal cycles [11], thermal ramps [12] and isothermal anneals [13].…”