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

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

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?

Nuclear Laser Facility Layout (as presented in the ELI Cz-Hu-Ro proposal) 2xFRONT END DPSSL-pumped OPCPA FE1: mJ BW > 120 nm T CP = 50 ps kHz C > 10^12 FE2: > 100 mJ BW > 80 nm T CP = 1-2 ns 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 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

FRONT-END 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 > Rep rate ≥ 10 Hz E > 4 J Compressible down to 15 fs Ns & ps contrast > Rep rate 10 Hz E > 300 J Compressible to 200 J Ns & ps contrast > Rep rate Hz I FOCUSED ~ W/cm Time schedule for ELI-RO Laser

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 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: 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 ( J/pulse) laser amplifier - re-compression of stretched amplified pulses and laser beam focusing - expected results of nuclear physics experiments

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 > Focused laser intensity 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.

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

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.

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

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)

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, J/cm 2

TEWALAS - schematic drawing of the laser system

TEWALAS - Laser system layout

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

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)

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

(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.

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

Nanosecond Contrast Nanosecond 8x10 -8

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 Amplified pulse duration – expected not shorter than fs Required nanosecond and picosecond intensity contrast for a 10 PW laser ( W/cm 2 focused peak power density) > 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