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
AMPLIFIERS
DPSSL-pumped
OPCPA
Ti:Sapphire pumped by ns
Nd:YAG & Nd:Glass lasers
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
A1 + A2
A3 +A4+ A5
BOOSTERS
> 4 J, 10Hz
POWER
AMPLIFIERS
>300 J
A1 + A2
COMPRESSOR
>200
200 JJ
BEAM
TRANSPORT
IN VACUUM
COMPRESSOR
>200
200 JJ
BEAM
TRANSPORT
IN VACUUM
COMPRESSOR
>200
200 JJ
BEAM
TRANSPORT
IN VACUUM
A3 +A4+ A5
BOOSTERS
> 4 J, 10Hz
POWER
AMPLIFIERS
>300 J
A1 + A2
A3 +A4+ A5
BOOSTERS
> 4 J, 10Hz
DIAGNOSTICS
POWER
AMPLIFIERS
>300 J
TARGETS
Φ = 1-20 μm
TEST
COMPRESSOR
TARGETS
DIAGNOSTICS
IΣ = 3 x 1023 -24 W/cm2
BW – Spectral bandwidth, C – intensity contrast, TCP- chirped pulse
duration, TC – re-compressed pulse duration, Φ – focused laser beam
diameter, IΣ – intensity on target
Time schedule for ELI-RO Laser
HIGH ENERGY
AMPLIFIERS,
COMPRESSOR,
BEAM TRANSPORT
AND FOCUSING
MEDIUM
ENERGY
AMPLIFIERS
FRONT-END
E ~ 200 mJ
E>4J
B ~ 100 nm
(compressible down to
15 fs)
Compressible down to
15 fs
Tstretched ~ 2 ns
Ns & ps contrast > 1012
Ns & ps contrast > 1012
Rep rate 10 Hz
Ns & ps contrast > 1012
Rep rate 0.1- 0.02 Hz
I FOCUSED ~ 1023-24 W/cm2
Rep rate ≥ 10 Hz
End of 2013
2010- Middle of 2012
2010
E > 300 J
Compressible to < 20 fs
and > 200 J
2011
2012
End of 2015
2013
2014
2015
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
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%
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 BucharestMagurele.
Principle of Chirped Pulse Amplification (CPA)
Oscillator
tp ~
tp
1

Stretcher
tp 
Amplification
0 . 441

- ultra-short pulse duration,
Compressor
for Gaussian temporal and spectral pulse profile

- 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
Definitions related to the broad-band ultrashort pulses
Ultrashort laser pulse is characterized by:
-Central frequency
 0 and corresponding wave-number k 0 
- Frequency spread   arround
2
0
n (0 )
 0 and corresponding spread in wave-number
k
Evolution in time of the pulse is related to:
2
1 d 
 d 
 (k )   0  
 ( k  k 0 )  
2
dk
2!  dk

0

c
Phase velocity V P 

k
n ( )
Group velocity
VG 
d
dk

c
n
dn
d
3

1d 
2
 (k  k 0 )  
3

3!  dk
0


 ( k  k 0 ) 3  ...

0
c
n
dn
d
If second, third order terms are negligible, the laser pulse travels undistorted in shape with the
goup velocity VG.
Definitions related to the broad-band ultrashort pulses
1
Group velocity mismatch GVM 

VG1
1
VG 2
 fs

 mm 
2

Group velocity dispersion GVD  d  dk   d  1   fs
mm 
d   d   d   V G  
Electric field of the laser pulse in the frequency domain: E ( )  A ( ) exp  j  ( ) 
where
2
1 d 
 d 
 ( )   ( 0 )  
 (   0 )  
2
d

2!  d 

0
Group delay
GD 
d
 fs 
d
d 
2
Group delay dispersion GDD 
d
TOD 
2
2
d 
d
VG
 fs 
3
Third order dispersion
3

1d  
2
 (   0 )  
 (   0 ) 3  ...
3 


3!  d   0
0
3
 fs 
3
 GD 


 L 
1
 GDD 
GVD  

 L 
L, medium length
1
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  2 . 8  10
P. F. Moulton, JOSA B, Vol. 3, 125 (1986)
 19
cm
2
Pulse amplification in Ti:sapphire
Energy gain:
G 
F out
F in




 F in 





ln 1   exp 

1
G

0 

F in 
F

 s 




Fs
Fs 
where Fin is the input pulse fluence, Foutis the output pulse
fluence,
h

L
 0 .9 J
cm
2
is the saturation fluence of Ti:sapphire, G 0  exp( n  l ), n is the inverted population, l is the medium length.
h  L  2 . 47  10
 19
J

 2 . 8  10
 19
cm
2
Very low input signal, Fin/Fs << 1, small signal gain:
High-level energy densities, Fin /Fs >> 1, saturated gain:
G  G 0  exp( n  l )
 F 
G  1   s  n  l
 F in 
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
W. Koechner, “Solid-State Laser Engineering”, Springer Verlag, Germany, 1996
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
(a)
(b)
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.
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
Lens
P1
2 mm
BaF2
Y
P2
P1, P2 – crossed polarizers
X
Energetic efficiency – 10-30%
Z
β angle
f
Fs nJ
Oscillator
Contrast improvement – 3-5
orders of magnitude
Peak intensity level
~ 3 x 10^12 W/cm^2
Ps
Stretcher
Double CPA PW laser
mJ
Amplifier
Fs
compressor
PW fs
pulses
High-energy fs
compressor
XPW
1-2 ns
Stretcher
High-energy tenhundred J
amplifier chain
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)
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