1Development of High Average Power Femtosecond Amplifiers with Ytterbium-doped crystals Sandrine RICAUDPhD supervisor: Frédéric DRUONThèse Cifre with Amplitude Sytèmes
2Shorter pulses broader spectrum IntroductionA femtosecond pulse or second?Pulses are Fourier limited if:.t = 0,315Pulses with t = 100 fs =12 nm centered at 1050 nmShorter pulses broader spectrum
3Hot topics Diode-pumped solid-state laser High repetition rate, high energy(high average power)Search for new materials, to generate ultra-short pulses ~ 100 fsDs le domaine de lasers femto
4Advantage of ytterbium Diode-pumped laser (980 nm)Large emission cross sectiontens of nm for Yb3+< 1 nm for Nd3+Simple structureNo quenching even for closed Yb3+ ions...Small quantum defectMatériaux ytterbium moins étudiés que néodyme car fonctionnement peu efficace en pompage flash: intérêt renait ac DLIdeal candidate for diode-pumpedfemtosecond laser
5Ytterbium-doped materials Y2O3YAGSFAPKGWLSOYVO4YSOglassBOYSSYSCaF2SrF2CALGOYCOBSc2O3GdCOBKYWCollaborations:CIMAP LCMCPThermal conductivity (W/m/K)GGGEmission bandwidth (nm)For High powerFor Short pulses
6Yb(2.6%):CaF2 grown by the Bridgman process CaF2 interestException to the rule: good spectroscopic and thermal propertiesWell-known crystal (undoped), good growth controlCubic structure (isotrop)Yb3+:CaF2CaFOptiques UV lithographie, IR (gamme très large)Fluorine= CaF2Non dopé= optique pour UV?Yb(2.6%):CaF2 grown by the Bridgman process
7Chirped Pulse Amplification D. Strickland and G. Mourou, "Compression of Amplified Chirped Optical Pulses," Optics Comm. 56, 219 (1985).
8Chirped Pulse Amplification Yb:CALGO15 nm, <100 fs27 MHzYb:CaF2regenerative amplifierkHzD. Strickland and G. Mourou, "Compression of Amplified Chirped Optical Pulses," Optics Comm. 56, 219 (1985).
927 MHz, sub 100-fs, 15 nm bandwidth centered at 1043 nm Yb:CALGO oscillator27 MHz, sub 100-fs, 15 nm bandwidth centered at 1043 nm
11Yb:CaF2 amplifierMaximum 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
12SHG FROG trace at 500 Hz 178 fs At 500 Hz repetition rate : 8.5 nm15 nmMeasured RetrievedAt 500 Hz repetition rate :- pulse duration : 178 fs- pulse energy : 1.4 mJ before compression620 µJ after compression- optical-to-optical efficiency : 4.5 %
13ConclusionDiode-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)
14Perspectives 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”, submittedThin-Disk technology(better cooling, pump recycling)
16V. Petit et al (Appl. Phys. B, 2004) SpectroscopyV. Petit et al (Appl. Phys. B, 2004)Charge compensationYb3+Ca2+ClustersCrystalline reorganizationBroad absorption and fluorescence spectraHexameric clusters : > 0.5% doped Yb:CaF2Cluster= complex centersInfrared 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 bandsRelatively large cross sectionsDiode pumpingTunability / ultrashort pulsesLong emission lifetime (2.4 ms)Hexameric cluster
17Thermal conductivity (W.m-1.K-1) Thermo-optic coefficient (10-6 K-1) Thermal propertiesUndoped crystal~ 2.7%-Yb-doped crystalThermal conductivity (W.m-1.K-1)9.76Thermo-optic coefficient (10-6 K-1)- 17.8- 11.3S-FAPYAGY2O3LSOKGWglassYVO4YSOBOYSSrF2CaF2CALGOSYSFavorabledirectionsThermal 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)
18Regenerative Amplifier Diode-pumped CPA laser chainM2Laser diodenmØ=200µmMirror R=300mmDichroic mirror50 mm tripletsPCTFPM1Grating stretcher1600 l/mm260 psFs-oscillatorFWHM bandwidth:15 nm27 MHzFRM4M3λ/2Grating compressorTFP: Thin-Film PolarizerFR: Faraday RotatorPC: Pockels Cell130 round tripsThe 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:XxF2Yb:CaF2 : 2.6-%-doped5-mm-longYb:SrF2 : 2.9-%-doped4-mm-long
19Advantages of cryogenic temperature Lower laser levels become less thermally populated: lower laser threshold, higher efficiencyBetter thermal properties (thermal conductivity, coefficient of thermal expansion)Emission and absorption cross sections increase: higher gain but more structuredHigher average power system
20Spectroscopic properties at 77K Gain pt signal du matériau plus important (3.1)Saturation intensity: 17 kW/cm2 compared to 33 kW/cm2 at room temperatureS. Ricaud et al., “Highly efficient, high-power, broadly tunable, cryogenically cooled and diode-pumped Yb:CaF2”, Opt. Lett. , vol. 35, p.3757 (2010)
21Interest of cryogeny10 300K68 77KG. A. Slack, "Thermal Conductivity of CaF2, MnF2, CoF2, and ZnF2 Crystals" Phys. Rev. 122, 1451–1461 (1961).
25Crystals with complex structure Crystals with simple structure Crystal choiceGlass(amorphous)Crystals with complex structureCrystals with simple structureEmission bandwidthThermal conductivityMaterials (W m-1 K-1)l (nm)2 types de matériau qui résument la problématiqueYb:YAG = 109Yb:Verre = 0,835
26Crystals with complex structure Crystals with simple structure Crystal choiceGlass(amorphous)Crystals with complex structureCrystals with simple structureEmission bandwidthThermal conductivityIdeal crystalMaterials (W m-1 K-1)l (nm)Yb:YAG = 109Yb:Verre = 0,835
27ConclusionFirst laser operation of a singly doped Yb:CaF2 at a cryogenic temperature and high power levelPromissing results at cryogenic temperature:Efficiency up to 70%Output power ~ 100WSmall signal gain: 3.1Broad laser wavelength tunabilityHigh gain at 992 nm
28Outline Material properties High power laser Conclusion - Yb:CaF2 interest- Advantages of cryogenic temperatureYb:CaF2 properties at 77KHigh power laserExperimental setupCw regime resultsConclusion
29Choix des matériauxSpectre d’émission large (lié à l’ion dopant et à la matrice)Nd3+Cr4+:forsteriteCr4+:YAGTi3+:SaphirCr3+:LiSAFYb3+Tm3+:verreEr3+:verre600800100012001400160018002000nmPompage avec des diodes laser de puissanceet 880 nm => ion dopant Néodymeet 980 nm => ion dopant YtterbiumDL rendement elec-opt 80%, compactes, fiables, forte puissance (W au kW)
30Yb:CaF2 background at room temperature Laser wavelength tunability: 50nmThermal behaviour: κ~9.7 W.m-1.K-1 undoped,κ~6 W.m-1.K %-dopedML oscillator: 99fs, 380mWRegenerative amplifier:mJ before compression500Hz, 1.8mJ before compressionMultipass amplifier:420mJ before compressionA. 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)
31Gain estimation Experimental small signal gain: Go=3.1 Inversion population estimated: β=0.4
32Watch 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).
34Thermal 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).
35Thermal propertiesusing 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).