How thulium impurities impact photodarkening effect in Yb 3+ -doped fibre laser? Peretti Romain 1, Jurdyc Anne-Marie 1, Jacquier Bernard 1, Gonnet Cédric 2, Pastouret Alain 2, Burov Ekaterina 2, Cavani Olivier 2

Ytterbium fibre laser: status 1 Ytterbium-doped MCVD silica fibres: Jena (Ger) Nlight (Leikki), Can/Fin GSI/ JK lasers (UK) fiber provider: Draka… CW opération and modulated: Single mode fibre, up to 500W Multimode fibre up to 50KW Optical Conversion Efficiency, OPC Up to 75% Total efficiency : 25% Power limitation due to Stimulated Raman Scattering (SRS) Fiber-laser sales: more than 240 M$ (USD) in 2007 Expected to grow on average by 26% per year until 2011

Ytterbium fibre laser, status 2 Recent route to reach very high power : Large Mode Area fibre (LMA), using microstructured fibre Theoretical profile MEB images (from XLIM) But still power limitation due to material

Drawback and questions [from Manek-Honninger et al., 2007] Premature ageing of the lasers: power laser threshold increase with output power Photon Induced Absorption (PIA) in the near UV and visible range Photodarkening Times in min

Photodarkening rate with excitation wavelength 1064 nm 633 nm [Manek-Honninger et al., 2007]).

What causes photodarkening? An open question → Attributed to defect centers such as color centers in the silica net : oxygen vacancies (Yoo & al 2007) existence of divalent ytterbium (Guzman Chávez et al. 2007, Engholm et al., 2007, Koponen et al. 2008) → physical mechanism is not clear yet: need of a near UV energy interaction (supported by UV excitation experiments) to create defect centers. An intermediate step is necessary : proposition of Yb3+ pairs or agregates (Suzuki et al. 2009)

Experimental set-up

Characteristics of the Yb-doped fiber Type: Composition Weight % : λ c (nm) D m µm D m 2 µm 2 D c µm ALU Yb 1, ,45,4 Al~3 Ge<0.1 P~1 Absorption

Photo-Induced Absorption P = 500mW t = 300’

P.I.A. spectrum as a function of irradiation time

PIA time dependence changes with wavelength

Blue-green fluorescence visible by naked eye from [Kir'yanov et al, 2007]

Yb-doped and Yb:Tm doped fibres N°Type: Composition weigth % : λ c (nm) D m µm D m 2 µm 2 D c µm Fib. 1 ALU Yb Yb:1, ,4 Al:~3 Ge:<0.1 P:~1 Fib.2 ALU Yb-Tm Yb:1, ,0 65 5,6 Al:~3 Tm Ge:<0.1 P~1 purity materials % correspond to 340 ppbw

Upconverted emission spectra under 976 nm excitation fibre 1, fibre 2

Upconversion mechanims

P.I.A. time dependences for fiber 1 and fiber 2 Experimental conditions: λ exc = 976 nm λ PIA = 440 nm Excitation density: 10,8 W/mm 2 Clearly Tm ions are involved in the photodarkening process

Discussion (1) → fluorescence detection of Tm ions in the ytterbium-doped fibre, as a residual impurity < 330 ppbw → by increasing Tm impurity (~300ppm) : photodarkening is increased as well as PIA time dependence is faster Thulium ions are involved in the photodarkening process The questions : by what physical mechanism? can we propose some ideas to improve the performances of high energy ytterbium fibre lasers?

Tm 3+ fluorescence spectrum and host absorption

Discussion (2) → Upconversion process can bring 4f electron in high energy states of Tm 3+ different mechanisms: Up conversion energy transfer from two Yb 3+ to Tm 3+ followed by several possible mechanisms involving: Excited State Absorption, or multistep Yb to Tm energy transfer… They all lead to high power dependences of the upconverted Tm fluorescence ( P 2, P 3 and P 4 ) This has been studied by several authors already, for instance in: G. Huber & al, Journal of Luminescence 72-74, 1 – 3 (1997) → Whatever the upconversion mechanism is, it brings population in the different upper excited states in resonance with lattice absorption due to either charge transfer band and to defect centers near the band gap then we understand the observation of an increased UV and visible absorption: Yb absorption + upconversion energy transfer to TM excited states → creation of traps

Agreement with other experimental observations from litterature From material point of view: Photodarkening is increasing with ytterbium contents ( Kitabayashi et al. 2006) Photodarkening is decreasing with increasing : → alumina contents (Kitabayashi et al 2006) → phosphorus ((LEE et al, 2008) Photodarkening is decreasing with erbium doping (Morasse et al 2007) Photodarkening is decreasing with heat treatment under oxygen atmosphere (Yoo et al, 2007 but Yb 2+ was already present) From spectroscopic arguments: PIA comparable for 980nm, visible and UV irradiation (Yoo et al, 2007 Morasse et al, 2007) Correlation with UV absorption and photodarkening efficiency (Engholm et al 2008) Recovering from photodarkening by specific UV radiation (Manek-Honninhger et al 2007) or infrared (Jetscke et al 2007)

Prospectives → decrease as much as possible thulium or other R.E impurities but experimental and cost limitations; nanostructuration of the materials to isolate Yb ions from other luminescent centers (see poster) → on the contrary, introduce impurity to quench the creation of defect centers: for instance : by doping with other ions to deplete population in Tm high energy states = under investigation (pattern) → reach limitations due to intrinsic break down of the materials physical process such as Stimulated Raman Scattering

Supports: CNRS organisation Draka company

Thank you for your attention

Power dependences of the upconverted fluorescences

Type de fibre Composition[Yb]PD puissanceLambda PDTemps de PD BlanchimentInterprétation défautsaffiliation LMA DC * * ? Yb 2 O 3 :0,3 and 0:43 mol% 2-8W/6µm²9763HXYb 2+ /Yb-O/color centerLiekki Fibres monomodes phosphate ions/ m 3 12 % Yb 2 O J/cm² 10 ns266nm2 min X Np photonics stanford 400 mW976 nm10000 min Préformealuminosilicate 0,2% atomique O déduit XXXX Transfert de charge =>Yb 2+ => centre colorés ACREO FIBERLAB préformealuminosilicate1% poids~5mW/µm²488nm5h 26 jours at 160 bar et50°C Yb-O ODC Southampton 4 µm corealuminosilicate core 976nm 1200dB/m 500mW/(4µm)²977 nm5-240min543nmPaires Yb 2+ -Yb 3+ Mexique Fibre Liekki LMA DC« commercial »45 W / (22µm)²976 nm5-100 min 350nm 5 kHz 90µJ 5 minutes Paires Yb 3+ -Yb 3+ EOLITE LMA DCAluminosilicate103 (17µm)²976 nm25 minChauffeOFS laboratories Fibre multi 0.5 mol% P 2 O 5 and 4 mol% Al 2 O mol% Yb 2 O 3 (N = m -3 ) 1 à 13 W915 nm500 min13 à 915 nm Jena PréformeSi Al Si P 1,2% at.XXXXc.f. 2007ACREO FIBERLAB