Development of High Average Power Femtosecond Amplifiers with Ytterbium- doped crystals Sandrine...

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

2Journées de l’EDOM 7-8 mars 2011 www.elsa-laser.u-psud.fr/ 2

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


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


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


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


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


Chirped Pulse AmplificationChirped Pulse Amplification

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


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).


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


Yb:CaFYb:CaF22 regenerative regenerative amplifieramplifier


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


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


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)


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)


Thank you


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)


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


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


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


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


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


Experimental setupExperimental setup

OC: Output CouplerP: Powermeter


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


Tunability curveTunability curve

Quantum defect 1.1% (992 nm)

Laser diode245 W @ 979 nm

Ø=400µm

Yb:CaF2

Prism2% OC

P


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


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


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


OutlineOutline

• Material properties- Yb:CaF2 interest

- Advantages of cryogenic temperature

-Yb:CaF2 properties at 77K

• High power laser-Experimental setup-Cw regime results

• Conclusion


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)


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)


Gain estimationGain estimation

Experimental small signal gain: Go=3.1

Inversion population estimated: β=0.4


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).



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



* 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

Development of High Average Power Femtosecond Amplifiers with Ytterbium- doped crystals Sandrine...

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