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…
• Fiber-laser sales: more
than
240 M$
in
CW
opération
and (USD)
modulated:
Single
2007mode fibre, up to 500W
Multimode fibre up to 50KW
• Expected to grow on
Optical
Conversion
Efficiency,
OPC
average
by 26%
per year
Up
to 75%
until
2011
Total efficiency : 25%
Power limitation due to Stimulated Raman Scattering (SRS)
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
• 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.
100
15
7
0
[from Manek-Honninger et al. , 2007]
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
ALU
Yb
Yb
1,7
Al
~3
Ge
<0.1
P
~1
4
3x10
Dm
µm
Dm2
µm2
Dc
µm
1025
7.6
57,4
5,4
Absorption
-1
Wavenumber (cm )
4
λc
(nm)
Composition
Weight % :
Type:
2x10
4
10
1100
1000
1000
900
900
2x10
)
)
800
-1
-1
4
3x10
1100
700
800
700
Absorption (m
Absorption (m
-1
Wavenumber (cm )
4
600
500
400
600
500
400
300
300
200
200
100
100
0
0
300
400
500
600
700
800
900
Wavelength (nm)
1000
1100
1200
275
300
325
350
375
400
425
Wavelength (nm)
450
475
500
Photo-Induced Absorption
3
-1
Wavenumber (10 cm )
26
24
22
20
18
16
14
12
1,0
0,9
P = 500mW
t = 300’
0,8
Normalized P.I.A.
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
400
450
500
550
600
650
700
Wavelength (nm)
750
800
850
900
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°
Fib. 1
Type:
ALU
Yb
Composition
weigth % :
Yb:
Al:
Ge:
P:
Yb:
Al:
Fib.2
ALU
Tm
Yb-Tm
Ge:
P
λc
(nm)
Dm
µm
1,7
~3
1025 7.5
<0.1
~1
1,7
Dm2
µm2
Dc
µm
57
5,4
65
5,6
~3
3.10-4 1043
8,0
<0.1
~1
purity materials 99.998% correspond to 340 ppbw
Upconverted emission spectra under 976 nm excitation
3
-1
Energy (10 cm )
35
30
25
20
15
1.2
1
1.1
3
3
( I6, P0)-> H6
1
3
1
D2-> H6
3
1
G4-> H6
3
G4-> F4
1.0
Normalized emission
0.9
3+
Yb
cooperative
emission
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
300
350
400
450
500
550
Wavelength (nm)
fibre 1,
fibre 2
600
650
Upconversion mechanims
P.I.A. time dependences for fiber 1 and fiber 2
15
-1
(P.I.A.) (m )
Yb
Yb-Tm
Experimental conditions:
10
• λexc = 976 nm
•λPIA = 440 nm
15
10
Excitation density:
10,8 W/mm2
5
5
0
0
10
20
30
40
0
0
250
500
750
1000
1250
1500
Times (min)
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
3
-1
Energy (10 cm )
35
30
25
20
15
500
-1
0,9
Glass matrix absorption (m )
1,0
Normalized emission
0,8
400
0,7
0,6
300
0,5
0,4
200
0,3
0,2
100
0,1
0,0
300
350
400
450
500
550
Wavelength(nm)
600
650
0
700
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
Intensitée lumineuse W.mm
Luminescence normalisée
1
10
0,1
-2
100
Longueur d'onde (nm)
300
360
475
487
500
515
0,01
10
100
Puissance @976 nm (mW)
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
0.552 J/cm² 10 ns
266nm
2 min
phosphate
1027 ions/ m3
12 % Yb2O3
Fibres
monomodes
Np photonics
stanford
X
400 mW
976 nm
10000 min
Préforme
aluminosilicate
0,2% atomique O
déduit
X
X
X
X
Transfert de charge
=>Yb2+=> centre colorés
ACREO
FIBERLAB
préforme
aluminosilicate
1% poids
~5mW/µm²
488nm
5h
26 jours at 160 bar
et50°C
Yb-O
ODC
Southampton
4 µm core
aluminosilicate
core abs. @ 976nm
1200dB/m
500mW/(4µm)²
977 nm
5-240min
543nm
Paires Yb2+-Yb3+
Mexique
Fibre Liekki
LMA DC
« commercial »
45 W / (22µm)²
976 nm
5-100 min
350nm 5 kHz 90µJ
5 minutes
Paires 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 Al
Si P
1,2% at.
X
X
X
X
c.f. 2007
ACREO
FIBERLAB