INFLPR

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



COMPRESSOR 200 J

COMPRESSOR >200 J

COMPRESSOR 200 J

COMPRESSOR >200 J

COMPRESSOR 200 J

COMPRESSOR >200 J



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


Rep rate 10 Hz

E > 300 J Compressible to < 20 fs and > 200 J


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

Documents

INFLPR