How thulium impurities impact photodarkening effect in Yb 3+ -doped fibre laser?

1. How thulium impurities impact photodarkening effect in Yb3+-doped fibre laser? Peretti Romain1, Jurdyc Anne-Marie1, Jacquier Bernard1, Gonnet Cédric 2, Pastouret Alain 2, Burov Ekaterina 2, Cavani Olivier 2

2. Ytterbium fibre laser: status 1 • Ytterbium-doped MCVD silica fibres: • Jena (Ger) • Nlight (Leikki), Can/Fin • GSI/ JK lasers (UK) • fiber provider: Draka… • Fiber-laser sales: more than 240 M$ (USD) in 2007 • Expected to grow on average by 26% per year until 2011 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)

3. 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

4. Times in min. 100 15 7 0 Drawback and questions • Premature ageing of the lasers: power laser threshold increase with output power • Photon Induced Absorption (PIA) in the near UV and visible range • Photodarkening [from Manek-Honninger et al. , 2007]

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

6. 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)

7. Experimental set-up

8. Characteristics of the Yb-doped fiber Absorption

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

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

11. PIA time dependence changes with wavelength

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

13. Yb-doped and Yb:Tm doped fibres purity materials 99.998% correspond to 340 ppbw

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

15. Upconversion mechanims

16. P.I.A. time dependences for fiber 1 and fiber 2 • Experimental conditions: • λexc = 976 nm • λPIA = 440 nm • Excitation density: • 10,8 W/mm2 Clearly Tm ions are involved in the photodarkening process

17. 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?

18. Tm3+ fluorescence spectrum and host absorption

19. Discussion (2) → Upconversion process can bring 4f electron in high energy states of Tm3+ different mechanisms: Up conversion energy transfer from two Yb3+ to Tm3+ 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 ( P2, P3 and P4) 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

20. 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 Yb2+ 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)

21. 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

22. Supports: CNRS organisation Draka company

23. Thank you for your attention

24. Power dependences of the upconverted fluorescences