Photodarkening in Yb doped fibers: experimental evidence of equilibrium states depending on the pump power

Abstract

Photodarkening in Yb doped fibers was examined at 633 nm in-situ during cladding pumping at 915 nm with varying pump powers and with no indication of an onset threshold. For the first time, the partial bleaching of the photodarkening loss by the pump power itself was observed. We found the relaxation to well-defined equilibrium states of the core excess loss, depending on the Yb inversion. From the dependence of the measured rate constant on the density of excited Yb ions we conclude, that on average 3 to 4 excited Yb ions create or bleach one color center responsible for the core excess loss.

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

“…Because of the presence of a PB mechanism competing with PD and RD, simulated PD and PRD levels always reach a stationary value at long irradiation time, in agreement with data (see [3] for pure PD and the Duchez's article in this proceeding for PRD). A detailed analysis of the equations shows that solutions for pure RD (absence of pump: x inv = 0) are determined by the three following ratios: r = N/H, α = β/γ, θ = δ/γ, representative of the relative concentration of Yb 3+ ions with respect to that of hole traps, of the competition between trapping and recombination of holes and between the formation and recombination of Yb 2+ ions due to the ionizing radiation, respectively.…”

“…Both properties have been experimentally established outside of any saturation of the PD level (the PDEL results from equilibrium between PD and PB and not from a saturation effect. It was indeed shown to increase with inversion [3]). We thus deal with situations where traps are not saturated.…”

“…The point was to highlight whether the model is capable of reproducing the well-known experimental features of PD, as reported for instance by Jetschke et al [3,5]. Two main properties have to be accounted for: (i) the linear dependence of the PD equilibrium level (PDEL) on x inv [3], and (ii) the power-law dependence on x inv of the rate constant extracted from the fit of a stretched exponential function to the PD and PB kinetics [3,5]. Both properties have been experimentally established outside of any saturation of the PD level (the PDEL results from equilibrium between PD and PB and not from a saturation effect.…”

“…These findings are consistent with proposals by other teams [2,[15][16][17], notably the Engholm's [15][16][17] according to which PD is due to an Yb 3+ to Yb 2+ conversion, again with hole trapping, upon absorption of a UV photon (resulting from the deexcitation of several excited Yb 3+ ions) into the charge transfer (CT) bands of Yb 3+ ions. Darkening can be bleached by photons at 3.49 eV (355 nm) [1], 2.28 eV (543 nm) [2], 1.36 eV (915 nm) [3], and 1,27 eV (980 nm, Duchez's work). If photo-bleaching (PB) results from the photo-ionization (PI) of trapped holes, this variety of energies can suggest that PI takes place between localized trapped-hole states in the glass band-gap and a continuum, namely the valence band (VB).…”

“…Ytterbium-doped silica optical fibers (YDF) can be damaged by pump photons (photo-darkening, PD) [1][2][3][4][5][6] and possibly by external ionizing radiation (radiation-induced darkening, RD) [7][8][9]. In both cases, darkening results from a large excess optical absorption that develops from ultra-violet up to near infra-red ranges.…”