development of high average power femtosecond amplifiers with ytterbium- doped crystals sandrine...
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Development of High Average Development of High Average
Power Femtosecond Amplifiers Power Femtosecond Amplifiers
with Ytterbium-doped crystalswith Ytterbium-doped crystals
Sandrine RICAUD
PhD supervisor: Frédéric DRUON
Thèse Cifre with Amplitude Sytèmes
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IntroductionIntroductionA 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 broader spectrum
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Hot topicsHot topics
• Diode-pumped solid-state laser
• High repetition rate, high energy
(high average power)
• Search for new materials, to generate ultra-short pulses ~ 100 fs
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Advantage of ytterbiumAdvantage 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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Ytterbium-doped materialsYtterbium-doped materials
Y2O3
YAG
SFAPKGW
LSO YVO4
YSOglass BOYS SYS
CaF2
SrF2
CALGO
YCOB
Sc2O3
GdCOBKYW
GGG
Th
erm
al c
on
du
ctiv
ity
(W/m
/K)
Emission bandwidth (nm)
For High powerFor Short pulses
Collaborations:CIMAP LCMCP
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CaFCaF22 interest interest
• Exception to the rule: good spectroscopic and thermal properties
• Well-known crystal (undoped), good growth control
• Cubic structure (isotrop)
Yb(2.6%):CaF2 grown by the Bridgman process
YbYb3+3+:CaF:CaF22
CaCaFF
CaCaCaCa
FF
FFFFCaCa
CaCa
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Chirped Pulse AmplificationChirped Pulse Amplification
D. Strickland and G. Mourou, "Compression of Amplified Chirped Optical Pulses," Optics Comm. 56, 219 (1985).
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Chirped Pulse AmplificationChirped Pulse Amplification
Yb:CALGO15 nm, <100 fs
27 MHz
Yb:CaF2
regenerative amplifier100-10 kHz
D. Strickland and G. Mourou, "Compression of Amplified Chirped Optical Pulses," Optics Comm. 56, 219 (1985).
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Yb:CALGO oscillatorYb:CALGO oscillator
-500 -400 -300 -200 -100 0 100 200 300 400 500
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(a.u
.)
Time (fs)
93 fs
1000 1020 1040 1060 1080
0,0
0,2
0,4
0,6
0,8
1,0
Wavelength (nm)In
tens
ity (a
.u.)
0
1
2
3
4
Phase (rad)
15 nm
27 MHz, sub 100-fs, 15 nm bandwidth centered at 1043 nm
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Yb:CaFYb:CaF22 regenerative regenerative amplifieramplifier
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Yb:CaFYb:CaF22 amplifier 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 HzSHG FROG trace at 500 Hz
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
178 fs8.5 nm
15 nm
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ConclusionConclusion
- 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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PerspectivesPerspectives
• Cooling crystals to cryogenic temperature
(better thermal and spectroscopic properties)S. Ricaud et al., “Highly efficient, high-power, broadly tunable, cryogenically cooled and diode-pumped 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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SpectroscopySpectroscopy
Broad absorption and fluorescence spectra
ClustersCrystalline reorganization
Charge compensation
YbYb3+3+CaCa2+2+
Hexameric cluster
V. Petit et al (Appl. Phys. B, 2004)
• Diode pumping• Tunability / ultrashort pulses• Long emission lifetime (2.4 ms)
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Thermal propertiesThermal properties
S-FAP
YAG
Y2O3
LSOKGW
glass
YVO4
YSOBOYS
SrF2
CaF2
CALGO
SYS
Favorabledirections
Undoped crystal~ 2.7%-Yb-doped
crystal
Thermal conductivity (W.m-
1.K-1)9.7 6
Thermo-optic coefficient (10-
6 K-1)- 17.8 - 11.3
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Regenerative AmplifierRegenerative Amplifier
Diode-pumped CPA laser chain
M2
Laser diode16 W @ 980 nm
Ø=200µm
Mirror R=300mm
Dichroic mirror
50 mm triplets
Mirror R=300mm
PC
TFP
M1
Grating stretcher1600 l/mm
260 ps
Fs-oscillatorFWHM bandwidth:
15 nm27 MHz
FR
TFP
M4
M3
λ/2
Grating compressor1600 l/mm
TFP: Thin-Film PolarizerFR: Faraday RotatorPC: Pockels Cell
Yb:CaF2 : 2.6-%-doped 5-mm-long
Yb:SrF2 : 2.9-%-doped 4-mm-long
Yb:XxF2
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Advantages of cryogenic Advantages of cryogenic temperaturetemperature
• 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 Spectroscopic properties at 77K77K
S. Ricaud et al., “Highly efficient, high-power, broadly tunable, cryogenically cooled and diode-pumped Yb:CaF2”, Opt. Lett. , vol. 35, p.3757 (2010)
Saturation intensity: 17 kW/cm2 compared to 33 kW/cm2 at room temperature
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Interest of cryogenyInterest of cryogeny
G. A. Slack, "Thermal Conductivity of CaF2, MnF2, CoF2, and ZnF2 Crystals" Phys. Rev. 122, 1451–1461 (1961).
10 W/m/K @ 300K
68 W/m/K @ 77K
(T) 2652
T 37
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Experimental setupExperimental setup
OC: Output CouplerP: Powermeter
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Cw regime resultsCw regime results
• 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
OC: 10%Maximal incident pump power: 212W
97 W !Absorption : - 74 W( saturated) without laser- Up to 150 W with laser
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Tunability curveTunability curve
Quantum defect 1.1% (992 nm)
Laser diode245 W @ 979 nm
Ø=400µm
Yb:CaF2
Prism2% OC
P
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Crystal choiceCrystal choice
Glass(amorphous)
Crystals with complex structure
Crystals with simple structure
Emission bandwidth
Thermal conductivity
Materials (W m-1 K-1)
Yb:YAG = 10
Yb:Verre = 0,8
(nm)
9
35
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Crystal choiceCrystal choice
Glass(amorphous)
Crystals with complex structure
Crystals with simple structure
Materials (W m-1 K-1)
Yb:YAG = 10
Yb:Verre = 0,8
Emission bandwidth
Thermal conductivity
Ideal crystal
(nm)
9
35
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ConclusionConclusion
• 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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OutlineOutline
• 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 matériauxChoix des matériaux
Cr4+ :fo
rste
rite
600 800 1000 1200 1400 1600 1800 2000
Cr3+ :L
iSAF
Cr4+ :Y
AG
Ti3+
:Sap
hir
Er3+
:ver
re
Tm3+
:ver
re
Yb3+
Nd3+
nm
• Pompage avec des diodes laser de puissance
-- 808 et 880 nm => ion dopant Néodyme
-- 940 et 980 nm => ion dopant Ytterbium
• Spectre d’émission large (lié à l’ion dopant et à la matrice)
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Yb:CaFYb:CaF22 background at room background at room temperaturetemperature
• 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 estimationGain estimation
Experimental small signal gain: Go=3.1
Inversion population estimated: β=0.4
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Watch out for the dopingWatch out for the doping
() 1
24ZNVm
32kB0
arctan 2
Vm4ZN
30
2kB
c ii
M i Mc iM i
i
2* 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 propertiesThermal properties
G. A. Slack, "Thermal Conductivity of CaF2, MnF2, CoF2, and ZnF2 Crystals" Phys. Rev. 122, 1451–1461 (1961).
10 W/m/K @ 300K
68 W/m/K @ 77K
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Thermal propertiesThermal properties
* 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).
using the Gaumé’s model [*] and assuming a sound velocity of 6000 m/s at 77 K