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: 10-20 mJ BW > 120 nm
TCP = 50 ps 0.1-1 kHz
C > 10^12
FE2: > 100 mJ BW > 80 nm
TCP= 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, TCP- chirped pulse duration, TC – re-compressed pulse duration, Φ – focused laser beam diameter, IΣ – intensity on target
Φ = 1-20 μm
IΣ = 3 x 1023 -24 W/cm2
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
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
BEAM TRANSPORT IN VACUUM
FRONT-END
2010- Middle of 2012
MEDIUM ENERGY
AMPLIFIERS
HIGH ENERGY AMPLIFIERS,
COMPRESSOR, BEAM TRANSPORT
AND FOCUSING
End of 2013 End of 2015
E ~ 200 mJ
B ~ 100 nm (compressible down to 15 fs)
Tstretched ~ 2 ns
Ns & ps contrast > 1012
Rep rate ≥ 10 Hz
E > 4 J
Compressible down to 15 fs
Ns & ps contrast > 1012
Rep rate 10 Hz
E > 300 J Compressible to < 20 fs and > 200 J
Ns & ps contrast > 1012
Rep rate 0.1- 0.02 Hz
I FOCUSED ~ 1023-24 W/cm2
2010 2011 2012 2013 2014 2015
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 - 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
20 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
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 > 1012
- Focused laser intensity 1023-24 W/cm2 (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)
AmplificationAmplificationOscillatorOscillator StretcherStretcher CompressorCompressor
1
~pt
- ultra-short pulse duration, - phase-locked spectral band-width pt
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
441.0pt 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:
0
0 k
...)(!3
1)(
!2
1)()( 3
0
03
32
0
02
2
00
0
kk
dk
dkk
dk
dkk
dk
dk
Phase velocity)(
n
c
kVP
Group velocity
d
dnn
c
d
dnn
c
dk
dVG
If second, third order terms are negligible, the laser pulse travels undistorted in shape with the goup velocity VG.
)(2
00
0 nk
Definitions related to the broad-band ultrashort pulses
Group velocity mismatch
Group velocity dispersion
mmfs
VVGVM
GG 21
11
mm
fsVd
d
d
dk
d
dGVD
G
21
Electric field of the laser pulse in the frequency domain:
where
)(exp)()( jAE
...)(!3
1)(
!2
1)()()( 3
0
03
32
0
02
2
00
0
d
d
d
d
d
d
Group delay fsd
dGD
Group delay dispersion 22
2
fsd
dGDD
Third order dispersion 33
3
fsd
dTOD
1
L
GDDGVD
L, medium length1
L
GDVG
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):
219108.2 cmP
P. F. Moulton, JOSA B, Vol. 3, 125 (1986)
Pulse amplification in Ti:sapphire
Energy gain:
where Fin is the input pulse fluence, Foutis the output pulse fluence,
01exp1ln G
F
F
F
F
F
FG
s
in
in
s
in
out
29.0cmJh
F Ls
is the saturation fluence of Ti:sapphire, , n is the inverted population, l is the medium length.
Very low input signal, Fin/Fs << 1, small signal gain:
)exp(0 lnG
)exp(0 lnGG
High-level energy densities, Fin /Fs >> 1, saturated gain: lnF
FG
in
s
1
Jh L191047.2 219108.2 cm
W. Koechner, “Solid-State Laser Engineering”, Springer Verlag, Germany, 1996
Damage threshold fluence at 532 nm, 10 ns pulse duration, 5-10 J/cm2
Conservative fluence at 532 nm, 10 ns pulse duration, 1-1.5 J/cm2
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: )3(
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 P1 Y2 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
fPeak intensity level ~ 3 x 10^12 W/cm^2
Nanosecond Contrast
Nanosecond Contrast @600mJ: 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 1010
Amplified pulse duration – expected not shorter than 15-20 fs
Required nanosecond and picosecond intensity contrast for a 10 PW laser (1023-24 W/cm2 focused peak power density) > 1012-13
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