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

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 500WMultimode 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 2Recent route to reach very high power :

Large Mode Area fibre (LMA), using microstructured fibre

Theoretical profileMEB 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.

10015

70

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 fiberType:

Composition Weight % :

λc

(nm)

Dm

µm

Dm2

µm2

Dc

µm

ALUYb

Yb 1,7

1025 7.6 57,4 5,4Al ~3

Ge <0.1

P ~1

Absorption

Photo-Induced Absorption

400 450 500 550 600 650 700 750 800 850 9000,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,026 24 22 20 18 16 14 12

Wavenumber (103 cm-1)

Nor

malized

P.I.A.

Wavelength (nm)

P = 500mWt = 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:Compositionweigth % :

λc

(nm)

Dm

µm

Dm2

µm2

Dc

µm

Fib. 1ALUYb

Yb: 1,7

1025 7.5 57 5,4Al: ~3

Ge: <0.1

P: ~1

Fib.2ALU

Yb-Tm

Yb: 1,7

1043 8,0 65 5,6Al: ~3

Tm 3.10-4

Ge: <0.1P ~1

purity materials 99.998% correspond to 340 ppbw

Upconverted emission spectra under 976 nm excitation

300 350 400 450 500 550 600 6500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.235 30 25 20 15

Yb3+

cooperativeemission

1G4->3F

4

1G4->3H

6

1D2->3H

6(1I

6,3P

0)->3H

6

Norm

aliz

ed e

mis

sion

Wavelength (nm)

Energy (103cm-1)

fibre 1, fibre 2

Upconversion mechanims

P.I.A. time dependences for fiber 1 and fiber 2

0 250 500 750 1000 1250 15000

5

10

15

0 10 20 30 400

5

10

15

(P.I.

A.)

(m

-1)

Times (min)

Yb Yb-Tm

Experimental conditions:

• λexc = 976 nm•λPIA = 440 nm

Excitation density:10,8 W/mm2

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?

Tm3+ fluorescence spectrum and host absorption

300 350 400 450 500 550 600 650 7000,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,035 30 25 20 15

0

100

200

300

400

500

Norm

aliz

ed e

mis

sion

Wavelength(nm)

Gla

ss m

atr

ix a

bso

rptio

n (

m-1)

Energy (103cm-1)

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

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)

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

10 100

0,01

0,1

110 100

Longueur d'onde (nm) 300 360 475 487 500 515

Lum

inesc

ence

nor

mal

isée

Puissance @976 nm (mW)

Intensitée lumineuse W.mm-2

Type de fibre

Composition [Yb] PD puissance Lambda PD Temps de PD

Blanchiment Interprétation défauts affiliation

LMA DC ?Yb2O3

:0,3 and 0:43 mol%

2-8W/6µm² 976 3H X Yb2+/Yb-O/color center Liekki

Fibres monomodes

phosphate1027 ions/ m3

12 % Yb2O3

0.552 J/cm² 10 ns 266nm 2 min

XNp photonics

stanford400 mW 976 nm 10000 min

Préforme aluminosilicate0,2% atomique O

déduitX X X X

Transfert de charge =>Yb2+=> centre colorés

ACREOFIBERLAB

préforme aluminosilicate 1% poids ~5mW/µm² 488nm 5h26 jours at 160 bar

et50°CYb-OODC

Southampton

4 µm core aluminosilicatecore abs. @ 976nm

1200dB/m500mW/(4µm)² 977 nm 5-240min 543nm Paires Yb2+-Yb3+ Mexique

Fibre Liekki

LMA DC « commercial » 45 W / (22µm)² 976 nm 5-100 min350nm 5 kHz 90µJ

5 minutesPaires Yb3+-Yb3+ EOLITE

LMA DC Aluminosilicate 103 dB/m@915 300mW/ (17µm)² 976 nm 25 min Chauffe OFS laboratories

Fibre multi 0.5 mol% P2O5 and 4 mol% Al2O3

0.6 mol% Yb2O3 (N = 2.65 1026 m-3)

1 à 13 W 915 nm 500 min 13 à 1W@ 915 nm

Jena

Préforme Si AlSi P

1,2% at. X X X X c.f. 2007 ACREOFIBERLAB