1. Amplificarea pulsurilor laser ultrascurte. CPA in Ti:safir sau OPCPA? Solutii pentru laserul ELI-RO. (Partea I) R. Dabu Sectia Laseri, INFLPR

2. De ce aceasta prezentare? - Cunoasterea stadiului actual pe plan mondial in domeniul laserilor de mare putere in femtosecunde - Incercam sa dam un raspuns privind solutia tehnica potrivita pentru laserul ELI-RO - Ce putem face ca sa ne incadram in efortul stiintific necesar pentru realizarea acestui laser - Sa facem un pas mai departe in pregatirea unor specialisti in domeniul “laseri in femtosecunde de mare putere si directii de cercetare bazate pe acesti laseri” - Sa atragem in echipa de lucru tineri cu un background care sa le permita incadrarea rapida in acest domeniu

3. CUPRINS 1. Amplificarea pulsurilor laser cu deriva de frecventa (“chirped pulse amplification” - CPA) in Ti:safir. - Caractersiticile Ti:safir ca mediu amplificator laser. - Probleme legate de amplificarea pulsurilor de femtosecunde de mare energie. 2. Ce este amplificarea parametrica si, in particular, OPCPA. - Oscilatia, generarea si amplificarea parametrica ca fenomene in optica neliniara. - Relatiile care guverneaza fenomenele parametrice. - Castigul unui amplificator parametric, banda de frecventa. 3. Amplificare parametrica optica (OPA) de banda larga si de banda foarte larga. - Conditiile de obtinere a amplificarii parametrice de banda larga sau foarte larga. - Cum se calculeaza pentru un cristal dat parametrii de functionare in cele doua cazuri. - Potentialul aplicarii pentru laserii cu pulsuri ultrascurte de mare putere. - Amplificarea parametrica a pulsurilor largite cu deriva de frecventa – OPCPA. - Metode de obtinere a amplificarii de banda larga: la degenerescenta, amplificare necoliniara, folosirea mai multor laseri de pompaj. Exemple. - Metode de obtinere a amplificarii de banda foarte larga. Benzile de amplificare foarte larga in cristale BBO si DKDP pentru laserii din clasa PW. 4. Prezentarea unor sisteme laser amplificatoare in domeniul PW: - Laserul rusesc cu oscilator in fs la 1250 nm (Cr:forsterite) si amplificare in cristale DKDP. - Laserul englez (910 nm) cu amplificare de mare energie in DKDP. - Laserul german cu amplificare pe ~ 900 nm. - Laserul francez cu amplificare pe 800 nm in BBO si Ti:safir. - Comparatie intre diferite sisteme de amplificare (China, Korea, Japonia, Rusia, Franta, Germania si Anglia). OPCPA versus amplificare in Ti-safir: avantaje si dezavantaje. 5. Care ar fi cea mai buna solutie pentru laserul ELI-RO? Ce e de facut pentru realizarea la timp si la parametrii propusi a sistemului laser ELI-RO?

4. Nuclear Laser Facility Layout (as presented in the ELI Cz-Hu-Ro proposal) 2xFRONT END DPSSL-pumped OPCPA FE1: 10-20 mJ BW > 120 nm T CP = 50 ps 0.1-1 kHz C > 10^12 FE2: > 100 mJ BW > 80 nm T CP = 1-2 ns 10-100 Hz C > 10^12 TEST COMPRESSOR AMPLIFIERS Ti:Sapphire pumped by ns Nd:YAG & Nd:Glass lasers A1 + A2 BOOSTERS > 4 J, 10Hz DIAGNOSTICS TARGETS DIAGNOSTICS BW – Spectral bandwidth, C – intensity contrast, T CP - chirped pulse duration, T C – re-compressed pulse duration, Φ – focused laser beam diameter, I Σ – intensity on target Φ = 1-20 μm I Σ = 3 x 10 23 -24 W/cm 2 BEAM TRANSPORT IN VACUUM TARGETS A3 +A4+ A5 POWER AMPLIFIERS >300 J A3 +A4+ A5 POWER AMPLIFIERS >300 J A3 +A4+ A5 POWER AMPLIFIERS >300 J A1 + A2 BOOSTERS > 4 J, 10Hz COMPRESSOR 200 J COMPRESSOR >200 J COMPRESSOR 200 J COMPRESSOR >200 J COMPRESSOR 200 J COMPRESSOR >200 J BEAM TRANSPORT IN VACUUM

5. FRONT-END 2010- Middle of 2012 MEDIUM ENERGY AMPLIFIERS HIGH ENERGY AMPLIFIERS, COMPRESSOR, BEAM TRANSPORT AND FOCUSING End of 2013End of 2015 E ~ 200 mJ B ~ 100 nm (compressible down to 15 fs) T stretched ~ 2 ns Ns & ps contrast > 10 12 Rep rate ≥ 10 Hz E > 4 J Compressible down to 15 fs Ns & ps contrast > 10 12 Rep rate 10 Hz E > 300 J Compressible to 200 J Ns & ps contrast > 10 12 Rep rate 0.1- 0.02 Hz I FOCUSED ~ 10 23-24 W/cm 2 201020112012201320142015 Time schedule for ELI-RO Laser

6. Two possible solutions for high energy femtosecond pulses amplification: Optical Parametric Chirped Pulse Amplification - OPCPA Ti:sapphire Chirped Pulse Amplification – TiS_CPA Amplifier media DKDP crystals - 20-30 cm diameter, already available No significant thermal problems Expected pulse duration: 5-15 fs Relatively cheap crystals Central wavelength of the amplified pulse: ~ 910 nm F20 cm Ti:S crystals – probably available in the next 1-2 years Efficient cooling required Transversal lasing problems Expected pulse duration: 15-25 fs More expensive crystals Central wavelength: ~ 800 nm Pump lasers Very precise synchronization Short pump pulse (2-3 ns) Conversion efficiency (pump to amplified signal radiation): 10-20% Non-critical synchronization Pump pulse duration non-critical in the nanosecond range (10-30 nsec) Conversion efficiency (from pump to amplified radiation): 30-40% 10 PW laser, a very difficult task (high risk project): X 50 more powerful than any existing femtosecond commercial laser X 20 more powerful than any existing femtosecond laboratory laser system X 500 more powerful than femtosecond TEWALAS laser at INFLPR Factors of (high) risk : - high energy (200-300 J/pulse) laser amplifier - re-compression of stretched amplified pulses and laser beam focusing - expected results of nuclear physics experiments

7. Selection criteria for ELI-RO laser system 1. Able to fulfill required specifications: - Peak pulse power ~ 10 PW per one amplifier chain - Pulse-width of the re-compressed amplified pulse < 20 fs - Rep-rate 1/10 – 1/60 Hz - Ns & ps contrast > 10 12 - Focused laser intensity 10 23-24 W/cm 2 (Laser beam focused near the diffraction limit) 2. Existing techniques proved by the long term laser facilities operation (200 TW Ti:sapphire CPA laser systems) 3. Existing laser components and devices necessary to reach 10 PW power (e.g. ~ 30 cm diameter DKDP crystals) 4. Required laser components and devices that could be probably developed in the next years (20-cm diameter Ti:S rods; Nd:YAG, Yb:YAG, Nd:glass, diode pump lasers; diffraction gratings, etc.) 5. Conditions of operation and expected laser system long-term stability 6. Costs of the whole laser system First target : 2012  Front-End able to satisfy the required laser specifications to be installed in Bucharest- Magurele.

8. Principle of Chirped Pulse Amplification (CPA) Amplification Oscillator Stretcher Compressor - ultra-short pulse duration, - phase-locked spectral band-width CPA technique involves the temporal stretching of ultra-short pulses with a large spectral bandwidth delivered by an oscillator. This way, the laser intensity is significantly reduced in order o avoid the damage of the optical components of the amplifiers and the temporal and spatial profile distortion by non-linear optical effects during the pulse propagation. After amplification, the laser pulse is compressed back to a pulse duration very closed to its initial value for Gaussian temporal and spectral pulse profile

9. Definitions related to the broad-band ultrashort pulses Ultrashort laser pulse is characterized by: -Central frequency and corresponding wave-number - Frequency spread arround and corresponding spread in wave-number Evolution in time of the pulse is related to: Phase velocity Group velocity If second, third order terms are negligible, the laser pulse travels undistorted in shape with the goup velocity V G.

10. Definitions related to the broad-band ultrashort pulses Group velocity mismatch Group velocity dispersion Electric field of the laser pulse in the frequency domain: where Group delay Group delay dispersion Third order dispersion L, medium length

11. Ti:sapphire amplification Polarized fluorescence spectra and calculated gain line for a optical c-axis normal cut Ti:sapphire rod; π – c-axis parallel polarized radiation; σ – c-axis normal polarization Stimulated emission cross section at 795 nm (c-axis parallel polarized radiation): P. F. Moulton, JOSA B, Vol. 3, 125 (1986)

12. Pulse amplification in Ti:sapphire Energy gain: where F in is the input pulse fluence, F out is the output pulse fluence, is the saturation fluence of Ti:sapphire,, n is the inverted population, l is the medium length. Very low input signal, F in /F s << 1, small signal gain: High-level energy densities, F in /F s >> 1, saturated gain: W. Koechner, “Solid-State Laser Engineering”, Springer Verlag, Germany, 1996 Damage threshold fluence at 532 nm, 10 ns pulse duration, 5-10 J/cm 2 Conservative fluence at 532 nm, 10 ns pulse duration, 1-1.5 J/cm 2

13. TEWALAS - schematic drawing of the laser system

14. TEWALAS - Laser system layout

15. Critical characteristics of Ti:sapphire amplifiers 1.Spectral band-width of the amplified pulses (re-compressed pulse duration) 2.Intensity contrast of femtosecond pulses versus amplified spontaneous emission (ASE) and nanosecond pre-pulses 3.Strehl ratio, focused spot

16. Pulse spectrum narrowing during Ti:S amplification – TEWALAS_INFLPR TEWALAS laser spectra: (a) without active Mazzler; (b) optimized by Mazzler. Mauve line – FEMTOLASERS oscillator; yellow line – after first multi-pass amplifier; after second multi-pass amplifier. (a) (b)

18. TEWALAS beam profiles (a) MP1, (b) MP2

19. (a) (b) (c) Pulse duration measurements using SPIDER. (a), (b) with Dazzler phase correction; (c) without phase correction. All cases: with spectrum correction by Mazzler.

21. ASE contrast improvement by cross-polarized wave (XPW) generation XPW generation – four-wave mixing process governed by the third–order nonlinearity: XPW generated wave has the same wavelength as the input pulse and a cubic dependence on the intensity A. Jullien et al, Opt. Lett. 30, 920 (2005); A. Jullien et al, Appl. Pys. B, 84, 409 (2006); L. Canova et al, Appl. Phys. B, 93, 443 (2008) Lens P1Y 2 mm BaF2 P2 X Z P1, P2 – crossed polarizers Energetic efficiency – 10-30% Contrast improvement – 3-5 orders of magnitude β angle Fs nJ Oscillator Ps Stretcher mJ Amplifier Fs compressor XPW 1-2 ns Stretcher High-energy ten- hundred J amplifier chain High-energy fs compressor PW fs pulses Double CPA PW laser f Peak intensity level ~ 3 x 10^12 W/cm^2

22. Nanosecond Contrast Nanosecond Contrast @600mJ: 8x10 -8

24. Problems of Ti:sapphire laser amplifiers for PW femtosecond laser facilities Gain narrowing due to the high factor amplification, 5 nJ → 250 J, M = 5 x 10 10 Amplified pulse duration – expected not shorter than 15-20 fs Required nanosecond and picosecond intensity contrast for a 10 PW laser (10 23-24 W/cm 2 focused peak power density) > 10 12-13 Thermal loading (532, 527 nm → 800 nm) Ti:sapphire rods, ~ 200 cm diameter required (currently available – 100 cm diameter) Transversal lasing in large diameter Ti:sapphire rods. Development of high energy, high repetition rate nanosecond green lasers, with smooth, uniform spatial intensity profile. Strehl ratio