Diode Pumped Cryogenic High EnergyYb-Doped Ceramic YAG Amplifier forUltra-High Intensity Applications

P. D. Mason, S. Banerjee, K. Ertel, P. J. Phillips, C.Hernandez-Gomez, J. Collier

ICUIL 2010 ConferenceSeptember 26th to October 1st 2010, Watkins Glen, NY, USA

paul.mason@stfc.ac.ukR1 2.62 Central Laser FacilitySTFC, Rutherford Appleton Laboratory, OX11 0QX, UK+44 (0)1235 778301

Motivation

• Next generation of high-energy PW-class lasers– Multi-Hz repetition rate

– Multi-% wall-plug efficiency

• Applications– Ultra-intense light-matter interactions

– Particle acceleration

– Intense X-ray generation

– Inertial confinement fusion

• High-energy DPSSL amplifiers needed– Pumping fs-OPCPA or Ti:S amplifiers

– Drive laser for ICF

BeamlineFacility

Amplifier Design Considerations

• Requirement– Pulses up to 1 kJ energy @ 10 Hz, few ns duration, overall > 10%

• Gain Medium

• Amplifier Geometry

Long fluorescence lifetimeHigher energy storage potentialMinimise number of diodes (cost)

Available in large size Handle high energies

Good thermo-mechanical properties Handle high average power

Sufficient gain cross section Efficient energy extraction

High surface-to-volume ratio Efficient cooling

Low (overall) aspect ratio Minimise ASE

Heat flow parallel to beam Minimise thermal lens

Amplifier Concept

• Ceramic Yb:YAG gain medium (slabs)– Best compromise to meet requirements

– Possibility of compound structures for ASE suppression

• Distributed face-cooling by stream of cold He gas – Heat flow along beam direction

– Low overall aspect ratio & high surface area

– Coolant compatible with cryo operation

• Operation at cryogenic temperatures– Reduced re-absorption, higher o-o efficiency

– Increased gain cross-section

– Better thermo-optical & thermo-mechanical properties

• Graded doping profile– Reduced overall thickness (up to factor of ~2)

• Lower B-integral

– Equalised heat load for slabs

Inputs

Pump intensity (each side) 5 kW/cm2

pump 5 nm FWHM

Pump duration 1 ms

Temperature 175 K

Results

Optimum doping x length 3.3 %cm

Storage efficiency50 %

5 J/cm2 stored

Small signal gain (G0) 3.8

Optimum Aspect ratio#

constant doping 0.78

graded doping 1.55

Amplifier Parameters

• Quasi-3 level model– 1D, time-dependent model

– Spectral dependence (abs.) included

– Assume Fmax = 5 J/cm2 forns pulses in YAG

• Results– Optimum doping x length

product maximum storage efficiency ~ 50%

– Optimum aspect ratio to ensure g0D 3 minimise risk of ASE

• Highly scalable concept– Just need to hit correct aspect

ratio & doping

Inputs

Pump intensity (each side) 5 kW/cm2

pump 5 nm FWHM

Pump duration 1 ms

Temperature 175 K

# Aperture / length

HiPERHiLASE /

ELIPrototypeDiPOLE

Extractable energy ~ 1 kJ ~ 100 J ~ 20 J

Aperture14 x 14 cm

200 cm2

4.5 x 4.5 cm20 cm2

2 x 2 cm4 cm2

Aspect ratio 1.4 1.3 1

No. of slabs 10 7 4

Slab thickness 1 cm 0.5 cm 0.5 cm

No. of doping levels 5 4 2

Average doping level

0.33 at.% 0.97 at.% 1.65 at.%

Amplifier Design Parameters

HiPERHiLASE /

ELI

Extractable energy ~ 1 kJ ~ 100 J

Aperture14 x 14 cm

200 cm2

4.5 x 4.5 cm20 cm2

Aspect ratio 1.4 1.3

No. of slabs 10 7

Slab thickness 1 cm 0.5 cm

No. of doping levels 5 4

Average doping level

0.33 at.% 0.97 at.%

DiPOLE Prototype

Cr4+

Yb3+

35

mm

55

mm Pump

2 x 2cm²

• Diode Pumped Optical Laser for Experiments– 10 to 20 Joule prototype laboratory test bed

• 4 x co-sintered ceramic Yb:YAG slabs– Circular 55 mm diameter x 5 mm thick

– Cr4+ cladding for ASE management

– Two doping concentrations 1.1 & 2.0 at.%

• Progress to date– Ceramic discs characterised

– Amplifier head designed & built• CFD modelling of He gas flow• Pressure testing

– Cryo-cooling system completed

– Diode pump lasers being assembled

– Lab. refit near completion

Ceramic Yb:YAG Discs

• Transmission spectra• Uncoated, room temperature

Fresnel limit ~84%

94

0 n

m 10

30

nm

• Transmitted wavefront

PV0.123wave

• Head layout

Amplifier Head

• CFD modelling• Predicted temperature gradient in

Yb:YAG amplifier disc

He flow

2 cm

2.0%

1.1%

PumpPump

Vacuumvessel

Uniform T across pumped region ~ 3K

He flow

Cryostat

Amplifierhead

Helium cooling circuit

Cryo-cooling SystemVacuum insulated transfer

lines

Diode Pump Laser

• Built by Consortium– Ingeneric: Opto-mechanical design & build

– Amtron: Power supplies & control system

– Jenoptic: Laser diode modules

• Specifications– 2 pump units – left & right handed 0 = 940 nm, FWHM < 6 nm

– Peak power 20 kW

– Pulse duration 0.2 to 1.2 ms

– Pulse repetition rate variable 0.1 to 10 Hz

• Other specs. independent of PRF

Diode Pump Laser

• Beam profile specification– Uniform square profile

– Steep profile edges

– Low (<10°) symmetrical divergence

• Demonstrated performance– Square beam shape

– Low-level intensity modulations

– Steep edge profiles

– 20 kW peak output power

• High confidence that other specifications will be demonstrated shortly

Spatial profiles (Modelled)

Near Field Far Field

Preliminary measurement

Lab Layout

Optical tables

Amplifier

LN2 tank

Floor area ~30 m2

Cryo-coolingsystem

Next Steps

• Short-term (3 to 6 months)– Complete lab. refit

– Install & test cryo-cooler & diode pump lasers

– Characterise amplifier over range of temperature & flow conditions• Spectral measurements (absorption, fluorescence)• Thermo-optical distortions (aberrations, thermal lensing etc.)• Opto-mechanical stability• Small signal gain & ASE assessment

• Long-term (6 to 12 months)– Specify and build front-end system

• Shaped seed oscillator & regen. amplifier

– Complete design of multi-pass extraction architecture (8 passes)

– Amplify pulses• Demonstrate >10 J, 10 Hz, >25 % o-o efficiency

Any Questions ?

Yb-doped Materials

Parameter (at RT) Glass S-FAP YAG CaF2

Wavelengths(pump/emission in nm)

940-980 / 1030

900 /1047

940 /1030

940-980 / 1030

Fluorescence lifetime(msec)

~ 2.0 ~ 1.3 ~ 1.0 ~ 2.4

Emission cross-section(peak x10-20 cm2)

0.7 6.2 3.3 0.5

Gain Low High Medium Low

Non-linear index(n2 x 10-13 esu)

0.1 to1.2

1.5 2.7 0.43

BandwidthHigh

> 50 nmLow OK?

High> 50 nm

Availability of large aperture

Good LimitedOK

(Ceramic)Limited

(Ceramic)

Thermal properties(K in Wm-1K-1)

Poor1.0

OK2.0

Good10.5

OK6.1

Yb:YAG Energy Level Diagrams

• Room temperature (300K)

940

nm

Yb3+

2F7/2

1030

nm

Quasi-3 Level

2F5/2

Re-

abso

rpti

on

loss

Low quantum defect (QD)

p/ las~ 91%

f13=4.6%

• Cryogenic cooling (175K)

940

nm

Yb3+

2F7/2

1030

nm

4 Level-like

2F5/2

Significantly reduced re-absorption loss

f13=0.64%

Temperature Dependence

Pump Fluence (J/cm²)

Sto

rage

Effi

cien

cy (

%)

Sm

all S

igna

l Gai

n

T=175K

T=300K

Operating fluence

Doping Profile (1 kJ Amplifier)

Absorption + Pump Spectra

175 K300 KPump, FWHM = 5nm

175 K, 10 kW/cm2

300 K, 20 kW/cm2

Efficiency vs. pump

Pump Absorption

Absorption Spectra

940

nm

103

0 n

m

Factor of 2

Ceramic YAG with Absorber Cladding

Cr4+:YAG

Yb:YAG

LaserCamera

?

Cr4+:YAG

Yb:YAG

Sample of Co-Sintered YAG(Konoshima)

Reflection at Interface?

a cb

a b c

Nothing!

Beamline Efficiency Modelling• Beamline parameters

– 2 amplifiers, 4-passes

– 1% loss between slabs, 10% loss after each pass (reverser & extraction)

– Losses in pump optics ignored

Beam Transport

Reverser 1 Reverser 2

Injection

ExtractionAmp 1Amp 2

Injection Amp1 + 2 Reverser 1 Amp1 + 2 Reverser 2 Amp1 + 2Reverser 1 ExtractionAmp1 + 2

Losses: 17.4 %(distributed)

10 % 10 %10 % 10 %17.4 %(distributed)

17.4 %(distributed)

17.4 %(distributed)