A Scalable Design for a High Energy, High Repetition Rate, Diode-Pumped Solid State Laser (DPSSL) AmplifierPaul Mason, Klaus Ertel, Saumyabrata Banerjee, Jonathan Phillips, Cristina Hernandez-Gomez, John Collier

Workshop on Petawatt Lasers at Hard X-Ray Light Sources5-9th September 2011, Dresden, Germany

paul.mason@stfc.ac.ukSTFC Rutherford Appleton Laboratory,Centre for Advanced Laser Technology and ApplicationsR1 2.62 Central Laser Facility, OX11 0QX, UK+44 (0)1235 778301

Motivation• Next generation of high-energy PW-class lasers

– Multi-J to kJ pulse energy– Multi-Hz repetition rate– Multi-% wall-plug efficiency

• Exploitation– Ultra-intense light-matter interactions– Particle acceleration – Inertial confinement fusion

• High-energy DPSSL amplifiers needed– Pumping fs-OPCPA or Ti:S amplifiers– Drive laser for ICF– Pump technology for HELMHOLTZ-BEAMLINE

BeamlineFacility

HELMHOLTZ- BEAMLINE

Amplifier Design Considerations• Requirements

– Pulses from 10’s J to 1 kJ, 1 to 10 Hz, few ns duration, efficiency 1 to 10%

• Gain Medium

– Ceramic Yb:YAG down-selected as medium of choice

• Amplifier Geometry

Long fluorescence lifetime Higher 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

Low quantum defect Increased efficiency & reduced heat load

High surface-to-volume ratio Efficient coolingLow (overall) aspect ratio Minimise ASEHeat flow parallel to beam Minimise thermal lens

STFC Amplifier Concept• Diode-pumped multi-slab amplifier

– Ceramic Yb:YAG gain medium– Co-sintered absorber cladding for ASE suppression

• Distributed face-cooling by stream of cold He gas – Heat flow along beam direction– Low overall aspect ratio & high surface area

• Operation at cryogenic temperatures– Higher o-o efficiency – reduction of re-absorption– Increased gain cross-section– Better thermo-optical & thermo-mechanical properties

• Graded doping profile– Equalised heat load in each slab– Reduces overall thickness (up to factor of ~2)

~175K

Modelling• Laser physics

– Assumptions• Target output fluence 5 J/cm²• Pump 940 nm, laser 1030 nm

– Efficiency & gain• Optimum doping x length product

for maximum storage ~ 50%• Optimum aspect ratio to minimise

risk of ASE (g0D < 3) of ~1.5– Extraction

• Extraction efficiency ~ 50%

• Thermal & fluid mechanics– Temperature distribution – Stress analysis– Optimised He flow conditions

50%

3.8

pump region

Cr4+:YAG

Yb:YAG

HiPER HiLASE / ELI

PrototypeDiPOLE

Extractable energy ~ 1 kJ ~ 100 J ~ 10 J

Aperture 14 x 14 cm200 cm2

5 x 5 cm25 cm2

2 x 2 cm4 cm2

Aspect ratio 1.4 1.2 1

No. of slabs 10 6 4

Slab thickness 1 cm 0.7 cm 0.5 cm

No. of doping levels 5 3 2

Average doping level 0.33 at.% 0.79 at.% 1.65 at.%

HiPER HiLASE / ELI / XFEL

Extractable energy ~ 1 kJ ~ 100 J

Aperture 14 x 14 cm200 cm2

5 x 5 cm25 cm2

Aspect ratio 1.4 1.2

No. of slabs 10 6

Slab thickness 1 cm 0.7 cm

No. of doping levels 5 3

Average doping level 0.33 at.% 0.79 at.%

Scalable Design

DiPOLE Prototype Amplifier

• Design sized for ~ 10 J @ 10 Hz

• Aims– Validate & calibrate numerical models– Quantify ASE losses– Test cryogenic gas-cooling technology– Test (other) ceramic gain media– Demonstrate viability of concept

• Progress to date– Cryogenic gas-cooling system commissioned– Amplifier head, diode pump lasers & front-end

installed – Full multi-pass relay-imaging extraction

architecture under construction– Initial pulse amplification tests underway

Cr4+

Yb3+

Ceramic YAG disk with absorber cladding

Diode pump laser

• 4 x co-sintered ceramic Yb:YAG disks– Circular 55 mm diameter x 5 mm thick– Cr4+ absorbing cladding– Two doping concentrations (1.1 & 2.0 at.%)

Optical Gain MaterialCr4+

Yb3+

35 m

m

55 m

m Pump2 x 2cm²

PV0.123wave

Fresnel limit ~84%

940

nm

1030

nm

Amplifier Head Design

• Schematic • CFD modelling

Uniform T across pumped region ~ 3K

He flow

He flow40 m3/hr ~ 25 m/s @ 10 bar, 175 K

PumpPumpVa

cuum

Disks

pressurewindowsvacuum

windows

Diode Pump Laser• Built by Consortium

– Ingeneric, Amtron & Jenoptic

• Two systems supplied– 0 = 939 nm, FWHM < 6 nm– Peak power 20 kW, 0.1 to 10 Hz– Pulse duration 0.2 to 1.2 ms– Uniform square intensity profile

– Steep well defined edges– ~ 80 % spectral power within 3 nm

– Good match to Yb:YAG absorptionspectrum @ 175K

Measured

20 m

m

20 mm

DiPOLE Laboratory

Cryo-cooling system

2 x 20 kW diode pump lasers

Amplifier head

Front-end Injection Seed• Free-space diode-pumped MOPA design

– Built by Mathias Siebold’s team @ HZDR Germany

• Cavity-dumped Yb:glass oscillator– Tuneable 1020 to 1040 nm

• ~ 0.2 nm– Fixed temporal profile

• Duration 5 to 10 ns– PRF up to 10 Hz– Output energy up to 300 µJ

• Multi-pass Yb:YAG boosteramplifier– 6 or 8 pass configuration– Output energy ~ 100 mJ

nsecoscillator

Booster pump diode

Amplifier crystal

100 mJoutput

Polarisation switching waveplate

x3 or x4

Initial Pulse Amplification Results

• Simple bow tie extraction architecture– 1, 2 or 3 passes– Limited by diffraction effects

• Injection seed– Gaussian beam expanded to overfill pump region– Energy ~ 60 mJ

PumpPump

SeedAmplifiedbeam

Spatial Beam Profiles @ 100K, 1 Hz

E = 2.6 J @ 10 Hz

Gain 8

Gain 6

Pulse Energy v. Pump Pulse Duration• 3 passes @ 1 Hz

• Relay-imaging multi (6 to 8) pass extraction architecture is required to allow >10 J energy extraction at 175K

5.9 J

Onset of ASE loss

Conclusions• Cryogenic gas cooled Yb:YAG amplifier offers potential for

efficient, high energy, high repetition rate operation– At least 25% optical-to-optical efficiency predicted

• Proposed multi-slab architecture should be scalable toat least 1 kJ generating ns pulses at up to 10 Hz– Limit to scaling is acceptable B-integral

• DiPOLE prototype amplifier shows very promising results– Installation of relay-imaging multi-pass should deliver 10 J @ 10 Hz

• Strong candidate pump technology for generating high energy, ns pulses at ~ 1 Hz for HELMHOLTZ-BEAMLINE

Thank you for your attention!

Any Questions?