Development of High Average Power
Femtosecond Amplifiers with Ytterbiumdoped crystals
Sandrine RICAUD
PhD supervisor: Frédéric DRUON
Thèse Cifre with Amplitude Sytèmes
Introduction
A femtosecond pulse or 10-15 second?
Pulses are Fourier limited if:
.t = 0,315
Pulses with t = 100 fs   =12 nm centered at 1050 nm
Shorter pulses
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broader spectrum
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Hot topics
• Diode-pumped solid-state laser
• High repetition rate, high energy
(high average power)
• Search for new materials, to generate ultrashort pulses ~ 100 fs
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Advantage of ytterbium
• Diode-pumped laser (980 nm)
• Large emission cross section
– tens of nm for Yb3+
– < 1 nm for Nd3+
• Simple structure
– No quenching even for closed
Yb3+ ions...
• Small quantum defect
Ideal candidate for diode-pumped
femtosecond laser
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Thermal conductivity (W/m/K)
Ytterbium-doped materials
Collaborations:
CIMAP
LCMCP
Sc2O3
Y2 O 3
YAG
CaF2
GGG
CALGO
SrF2
LSO
SFAP
YVO4
YSO
KGW
YCOB
BOYS
KYW glass
GdCOB
SYS
Emission bandwidth (nm)
For High power
For Short pulses
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CaF2 interest
• Exception to the rule: good spectroscopic and thermal
properties
• Well-known crystal (undoped), good growth control
• Cubic structure (isotrop)
Ca
Ca
F
F
Ca
Ca
FYb3+:CaF
F 2
Ca
Yb(2.6%):CaF2 grown by the
Bridgman process
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Chirped Pulse Amplification
D. Strickland and G. Mourou, "Compression
of Amplified Chirped Optical Pulses," Optics
Comm. 56, 219 (1985).
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Chirped Pulse Amplification
Yb:CALGO
15 nm, <100 fs
27 MHz
Yb:CaF2
regenerative amplifier
100-10 kHz
D. Strickland and G. Mourou, "Compression
of Amplified Chirped Optical Pulses," Optics
Comm. 56, 219 (1985).
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8
4
1,0
0.8
0,8
0.6
93 fs
0.4
0.2
0.0
Intensity (a.u.)
1.0
3
0,6
2
15 nm
0,4
1
0,2
Phase (rad)
Intensity (a.u.)
Yb:CALGO oscillator
0,0
-500 -400 -300 -200 -100
0
100 200 300 400 500
Time (fs)
1000
1020
1040
1060
0
1080
Wavelength (nm)
27 MHz, sub 100-fs, 15 nm bandwidth centered at 1043 nm
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Yb:CaF2 regenerative amplifier
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Yb:CaF2 amplifier
- Maximum energy plateau up
to 300 Hz : 1.6 mJ / 700 µJ
(uncompressed / compressed)
- Higher repetition rate : 10 kHz
1.4W / 0.6W
(uncompressed / compressed)
Beam profile :
Gaussian shape with M2 < 1.1
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SHG FROG trace at 500 Hz
178 fs
8.5 nm
15 nm
At 500 Hz repetition rate :
- pulse duration : 178 fs
- pulse energy : 1.4 mJ before compression
620 µJ after compression
- optical-to-optical efficiency : 4.5 %
Measured
Retrieved
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Conclusion
- Diode-pumped room-temperature regenerative Yb:CaF2
amplifier operating at low and high repetition rate.
- Short pulses up to 1 kHz repetition rate (178 fs at 500
Hz).
- Maximum extracted energy : 1.6 mJ/0.7 mJ (before /
after compression).
- Highest average power : 1.4 W/0.6 W (before / after
compression).
- Optical efficiency ranging from 5 to 10%.
S. Ricaud et al., "Short pulse and high repetition rate diode-pumped Yb:CaF2 regenerative amplifier" Opt. Lett.
35, 2415-2417 (July 2010)
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Perspectives
• Cooling crystals to cryogenic temperature
(better thermal and spectroscopic
properties)
S. Ricaud et al., “Highly efficient, high-power, broadly tunable, cryogenically cooled and diodepumped Yb:CaF2”, Opt. Lett. , vol. 35, p.3757 (2010)
S. Ricaud et al., “High-power diode-pumped cryogenically-cooled Yb:CaF2 laser with extremely
low quantum defect”, submitted
• Thin-Disk technology
(better cooling, pump recycling)
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Thank you
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Spectroscopy
V. Petit et al (Appl. Phys. B, 2004)
Ca2+
Yb3+
Charge compensation
Crystalline reorganization
Clusters
Broad absorption and
fluorescence spectra
Hexameric cluster
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• Diode pumping
• Tunability / ultrashort pulses
• Long emission lifetime (2.4 ms)
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Thermal properties
Undoped crystal
~ 2.7%-Yb-doped crystal
Thermal conductivity (W.m-1.K-1)
9.7
6
Thermo-optic coefficient (10-6 K1)
- 17.8
- 11.3
Thermal conductivity
(undoped) (W.m-1.K-1)
18
Y2O3
16
Favorable
directions
14
12
YAG
10
CaF2
8
LSO
6
YVO4
YSO
KGW
4
S-FAP
2
SrF2
BOYS
CALGO
SYS
glass
0
0
10
20
30
40
50
60
70
80
90
Spectral bandwidth Δλ (FWHM) (nm)
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Regenerative Amplifier
Diode-pumped CPA laser chain
Grating compressor
1600 l/mm
Fs-oscillator
FWHM bandwidth:
15 nm
27 MHz
Grating
stretcher
1600 l/mm
260 ps
M4
FR
PC
M2
M3
λ/2
TFP
TFP
Mirror R=300mm
M1
TFP: Thin-Film Polarizer
FR: Faraday Rotator
PC: Pockels Cell
Laser diode
16 W @ 980 nm
Ø=200µm
Mirror R=300mm
Yb:XxF2
Yb:CaF2 : 2.6-%-doped
5-mm-long
Yb:SrF2 : 2.9-%-doped
4-mm-long
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50 mm triplets
Dichroic
mirror
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Advantages of cryogenic temperature
• Lower laser levels become less thermally
populated: lower laser threshold, higher
efficiency
• Better thermal properties (thermal conductivity,
coefficient of thermal expansion)
• Emission and absorption cross sections
increase: higher gain but more structured
Higher average power system
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Spectroscopic properties at 77K
Saturation intensity: 17 kW/cm2 compared to 33 kW/cm2 at room temperature
S. Ricaud et al., “Highly efficient, high-power, broadly tunable, cryogenically cooled and diode-pumped
Yb:CaF2”, Opt. Lett. , vol. 35, p.3757 (2010)
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Interest of cryogeny
68 W/m/K @ 77K
10 W/m/K @ 300K
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 (T) 
T  37


G. A. Slack, "Thermal Conductivity of CaF2, MnF2, CoF2, and
ZnF2 Crystals" Phys. Rev. 122, 1451–1461 (1961).
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Experimental setup
OC: Output Coupler
P: Powermeter
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Cw regime results
OC: 10%
Maximal incident pump
power: 212W
97 W !
Absorption :
- 74 W( saturated) without
laser
- Up to 150 W with laser
• High pump power: 245W
• High efficiency > 60%
• Good beam quality maintained
• Measured thermo-optic coefficient around -11 x10-6 K-1 (theory -3.1
x10-6 K-1 )
• Small signal gain estimation: 3.1
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Tunability curve
Laser diode
245 W @ 979 nm
Ø=400µm
Yb:CaF2
2% OC
Prism
P
Quantum defect
1.1% (992 nm)
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Crystal choice
Glass
(amorphous)
Crystals with complex
structure
Crystals with simple
structure
Emission
bandwidth
Thermal
conductivity
 (W m-1 K-1)
l(nm)
Yb:YAG
 = 10
9
Yb:Verre
 = 0,8
35
Materials
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Crystal choice
Glass
(amorphous)
Crystals with complex
structure
Crystals with simple
structure
Emission
bandwidth
Thermal
conductivity
Ideal crystal
 (W m-1 K-1)
l(nm)
Yb:YAG
 = 10
9
Yb:Verre
 = 0,8
35
Materials
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Conclusion
• First laser operation of a singly doped Yb:CaF2
at a cryogenic temperature and high power level
• Promissing results at cryogenic temperature:
–
–
–
–
–
Efficiency up to 70%
Output power ~ 100W
Small signal gain: 3.1
Broad laser wavelength tunability
High gain at 992 nm
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Outline
• Material properties
- Yb:CaF2 interest
- Advantages of cryogenic temperature
- Yb:CaF2 properties at 77K
• High power laser
- Experimental setup
- Cw regime results
• Conclusion
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Choix des matériaux
• Spectre d’émission large (lié à l’ion dopant et à la
matrice)
600
800
1000
1200
1400
1600
1800
2000
nm
• Pompage avec des diodes laser de puissance
-- 808 et 880 nm => ion dopant Néodyme
-- 940 et 980 nm => ion dopant Ytterbium
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Yb:CaF2 background at room temperature
• Laser wavelength tunability: 50nm
• Thermal behaviour: κ~9.7 W.m-1.K-1 undoped,
κ~6 W.m-1.K-1 2.7%-doped
• ML oscillator: 99fs, 380mW
• Regenerative amplifier:
215fs @1Hz, 17.3 mJ before compression
178fs @ 500Hz, 1.8mJ before compression
• Multipass amplifier:
192fs @1Hz, 420mJ before compression
A. Lucca et al., “High-power tunable diode-pumped Yb3+:CaF2 laser ”, Opt. Lett., vol. 29, p.1879 (2004)
J. Boudeile et al., “Thermal behaviour of ytterbium-doped fluorite crystals under high power pumping ”, Opt. Exp., vol. 16 (2008)
F. Friebel et al., “Diode-pumped 99fs Yb:CaF2 oscillator”, Opt. Lett., vol. 34, p.1474 (2009)
S. Ricaud et al., “Short-pulse and high-repetition-rate diode-pumped Yb:CaF2 regenerative amplifier”, Opt. Lett., vol. 35 (2010)
M. Siebold et al., “Broad-band regenerative laser amplification in ytterbium-doped calcium fluoride (Yb:CaF2) ”, Ap. Phys. B 89 (2007)
M. Siebold et al., “Terawatt diode-pumped Yb:CaF2 laser”, Opt. Lett., vol. 33, p.2770 (2008)
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Gain estimation
Experimental small signal gain: Go=3.1
Inversion population estimated: β=0.4
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Watch out for the doping


1 4ZN 2kB 0
V


m
0
3
 ( ) 
arctan2
  3

2
Vm

4ZN 2kB 


2
M  M 
   c i  i

c i M i 

i

www.elsa-laser.u-psud.fr/
 i

* R. Gaumé, et al. "A simple model for the prediction of thermal conductivity in pure
and doped in saluting crystals," Appl. Phys. Let. 83, 1355-1357 (2003).
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Thermal properties
68 W/m/K @ 77K
10 W/m/K @ 300K
G. A. Slack, "Thermal Conductivity of CaF2, MnF2, CoF2, and ZnF2 Crystals" Phys.
Rev. 122, 1451–1461 (1961).
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Thermal properties
using the Gaumé’s model [*] and assuming a sound velocity of 6000
m/s at 77 K
* R. Gaumé, et al. "A simple model for the prediction of thermal conductivity in pure and doped in saluting
crystals," Appl. Phys. Let. 83, 1355-1357 (2003).
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