1. Development of High Average Power Femtosecond Amplifiers with Ytterbium-doped crystals
Sandrine RICAUD
PhD supervisor: Frédéric DRUON
Thèse Cifre with Amplitude Sytèmes

2. Shorter pulses broader spectrum
Introduction
A femtosecond pulse or second?
Pulses are Fourier limited if:
.t = 0,315
Pulses with t = 100 fs   =12 nm centered at 1050 nm
Shorter pulses broader spectrum

3. Hot 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
Ds le domaine de lasers femto

4. 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
Matériaux ytterbium moins étudiés que néodyme car fonctionnement peu efficace en pompage flash: intérêt renait ac DL
Ideal candidate for diode-pumped
femtosecond laser

5. Ytterbium-doped materials
Y2O3
YAG
SFAP
KGW
LSO
YVO4
YSO
glass
BOYS
SYS
CaF2
SrF2
CALGO
YCOB
Sc2O3
GdCOB
KYW
Collaborations:
CIMAP LCMCP
Thermal conductivity (W/m/K)
GGG
Emission bandwidth (nm)
For High power
For Short pulses

6. Yb(2.6%):CaF2 grown by the Bridgman process
CaF2 interest
Exception to the rule: good spectroscopic and thermal properties
Well-known crystal (undoped), good growth control
Cubic structure (isotrop)
Yb3+:CaF2 Ca F
Optiques UV lithographie, IR (gamme très large)
Fluorine= CaF2
Non dopé= optique pour UV?
Yb(2.6%):CaF2 grown by the Bridgman process

7. Chirped Pulse Amplification
D. Strickland and G. Mourou, "Compression of Amplified Chirped Optical Pulses," Optics Comm. 56, 219 (1985).

8. Chirped Pulse Amplification
Yb:CALGO
15 nm, <100 fs
27 MHz
Yb:CaF2
regenerative amplifier
kHz
D. Strickland and G. Mourou, "Compression of Amplified Chirped Optical Pulses," Optics Comm. 56, 219 (1985).

9. 27 MHz, sub 100-fs, 15 nm bandwidth centered at 1043 nm
Yb:CALGO oscillator
27 MHz, sub 100-fs, 15 nm bandwidth centered at 1043 nm

10. Yb:CaF2 regenerative amplifier

11. 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

12. SHG FROG trace at 500 Hz 178 fs At 500 Hz repetition rate :
8.5 nm
15 nm
Measured Retrieved
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 %

13. 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, (July 2010)

14. Perspectives 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)

15. Thank you

16. V. Petit et al (Appl. Phys. B, 2004)
Spectroscopy
V. Petit et al (Appl. Phys. B, 2004)
Charge compensation
Yb3+
Ca2+
Clusters
Crystalline reorganization
Broad absorption and fluorescence spectra
Hexameric clusters : > 0.5% doped Yb:CaF2
Cluster= complex centers
Infrared luminescence of this system is dominated by one kind of active center : si dopage change tjs même allure de sections efficaces, tps fluo…
Smooth and wide optical bands
Relatively large cross sections
Diode pumping
Tunability / ultrashort pulses
Long emission lifetime (2.4 ms)
Hexameric cluster

17. Thermal conductivity (W.m-1.K-1) Thermo-optic coefficient (10-6 K-1)
Thermal properties
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
S-FAP
YAG
Y2O3
LSO
KGW
glass
YVO4
YSO
BOYS
SrF2
CaF2
CALGO
SYS
Favorable
directions
Thermal conductivity comparable to YAG undoped (10.7) but decreases faster than YAG because of higher difference of weight between Y/Yb3+ and Ca2+/Yb3+ (Y:yttrium)

18. Regenerative Amplifier
Diode-pumped CPA laser chain M2
Laser diode nm
Ø=200µm
Mirror R=300mm
Dichroic mirror
50 mm triplets PC
TFP M1
Grating stretcher
1600 l/mm
260 ps
Fs-oscillator
FWHM bandwidth:
15 nm
27 MHz FR M4 M3
λ/2
Grating compressor
TFP: Thin-Film Polarizer
FR: Faraday Rotator
PC: Pockels Cell
130 round trips
The regenerative amplifier contains a thin-film polarizer (TFP) and a BBO Pockels cell for polarization switching and hence injecting and extracting the oscillator pulses and the amplified pulses, respectively. The amplifier crystal is longitudinally pumped through a dichroic mirror using a 16-W 200-µm (N.A. 0.22)-fiber-coupled laser diode emitting at 980 nm. Thanks to the broad absorption band of the crystal, the emission wavelength of the laser diode does not need any stabilization with Bragg gratings. To optimize the overlap between the laser and the pump beams, the diode is collimated and focused by two 50 mm focal-length triplets to reduce optical aberrations. The cavity is designed in order to obtain diffraction limited laser beam at the output, with a cavity length of about 1.5 m. The Pockels cell is adjusted to act as a quarter wave plate at 45° in static state, i.e. without high voltage, and as a half wave plate with quarter wave voltage applied to the electrodes. Between the stretcher and the amplifier, a TFP, a Faraday rotator and a half-wave plate are placed in order to separate input and output beam. Finally, after a beam expander, the chirped pulses are compressed using two transmission gratings (1600 l/mm), with an overall efficiency of 45 %.
Yb:XxF2
Yb:CaF2 : 2.6-%-doped
5-mm-long
Yb:SrF2 : 2.9-%-doped
4-mm-long

19. 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

20. Spectroscopic properties at 77K
Gain pt signal du matériau plus important (3.1)
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)

21. Interest of cryogeny
10 300K
68 77K
G. A. Slack, "Thermal Conductivity of CaF2, MnF2, CoF2, and ZnF2 Crystals" Phys. Rev. 122, 1451–1461 (1961).

22. Experimental setup
OC: Output Coupler
P: Powermeter

23. Cw regime results 97 W ! Absorption : - 74 W( saturated) without laser
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
Efficacite abs/laser room temperature < 40%

24. Tunability curve Yb:CaF2 Laser diode 245 W @ 979 nm Ø=400µm 2% OC
Prism
2% OC P
Quantum defect 1.1% (992 nm)

25. Crystals with complex structure Crystals with simple structure
Crystal choice
Glass
(amorphous)
Crystals with complex structure
Crystals with simple structure
Emission bandwidth
Thermal conductivity
Materials
 (W m-1 K-1)
l (nm)
2 types de matériau qui résument la problématique
Yb:YAG
 = 10 9
Yb:Verre
 = 0,8 35

26. Crystals with complex structure Crystals with simple structure
Crystal choice
Glass
(amorphous)
Crystals with complex structure
Crystals with simple structure
Emission bandwidth
Thermal conductivity
Ideal crystal
Materials
 (W m-1 K-1)
l (nm)
Yb:YAG
 = 10 9
Yb:Verre
 = 0,8 35

27. 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

28. Outline Material properties High power laser Conclusion
- Yb:CaF2 interest
- Advantages of cryogenic temperature
Yb:CaF2 properties at 77K
High power laser
Experimental setup
Cw regime results
Conclusion

29. Choix des matériaux
Spectre d’émission large (lié à l’ion dopant et à la matrice)
Nd3+
Cr4+:forsterite
Cr4+:YAG
Ti3+:Saphir
Cr3+:LiSAF
Yb3+
Tm3+:verre
Er3+:verre
600
800
1000
1200
1400
1600
1800
2000 nm
Pompage avec des diodes laser de puissance
et 880 nm => ion dopant Néodyme
et 980 nm => ion dopant Ytterbium
DL rendement elec-opt 80%, compactes, fiables, forte puissance (W au kW)

30. 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 %-doped
ML oscillator: 99fs, 380mW
Regenerative amplifier:
mJ before compression
500Hz, 1.8mJ before compression
Multipass amplifier:
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)

31. Gain estimation Experimental small signal gain: Go=3.1
Inversion population estimated: β=0.4

32. Watch out for the doping
* R. Gaumé, et al. "A simple model for the prediction of thermal conductivity in pure and doped in saluting crystals," Appl. Phys. Let. 83, (2003).

34. 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).

35. 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, (2003).