1. A Scalable Design for a High Energy, High Repetition Rate, Diode-Pumped Solid State Laser (DPSSL) Amplifier
Paul Mason, Klaus Ertel, Saumyabrata Banerjee, Jonathan Phillips, Cristina Hernandez-Gomez, John Collier
Workshop on Petawatt Lasers at Hard X-Ray Light Sources
5-9th September 2011, Dresden, Germany
STFC Rutherford Appleton Laboratory,
Centre for Advanced Laser Technology and Applications
R Central Laser Facility, OX11 0QX, UK
+44 (0)

2. Motivation Next generation of high-energy PW-class lasers Exploitation
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
Beamline Facility
HELMHOLTZ- BEAMLINE

3. 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 potential
Minimise 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 cooling
Low (overall) aspect ratio
Minimise ASE
Heat flow parallel to beam
Minimise thermal lens

4. STFC Amplifier Concept
~175K
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)

5. Modelling Laser physics Thermal & fluid mechanics 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

6. Scalable Design HiPER HiLASE / ELI / XFEL Extractable energy ~ 1 kJ
Aperture
14 x 14 cm
200 cm2
5 x 5 cm
25 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.%
HiPER
HiLASE / ELI
Prototype
DiPOLE
Extractable energy
~ 1 kJ
~ 100 J
~ 10 J
Aperture
14 x 14 cm
200 cm2
5 x 5 cm
25 cm2
2 x 2 cm
4 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.%

7. DiPOLE Prototype Amplifier
Cr4+
Yb3+
Ceramic YAG disk with absorber cladding
Design sized for ~ 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
Diode pump laser

8. Optical Gain Material 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.%)
Cr4+
Yb3+
35 mm
55 mm
Pump 2 x 2
cm²
Fresnel limit ~84% PV
0.123
wave
940 nm
1030 nm

9. Amplifier Head Design Schematic CFD modelling Vacuum Disks
He flow
40 m3/hr ~ bar, 175 K
Pump
Vacuum
Disks
pressure
windows
vacuum
Uniform T across pumped region ~ 3K
He flow

10. Diode Pump Laser Built by Consortium Two systems supplied
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 absorption 175K
Measured
20 mm

11. DiPOLE Laboratory Cryo-cooling system Amplifier head
2 x 20 kW diode pump lasers
Amplifier head

12. Polarisation switching waveplate
Front-end Injection Seed
Free-space diode-pumped MOPA design
Built by Mathias Siebold’s 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 booster amplifier
6 or 8 pass configuration
Output energy ~ 100 mJ
nsec oscillator
Booster pump diode
Amplifier crystal
100 mJ
output
Polarisation switching waveplate
x3 or x4

13. 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
Pump
Seed
Amplified
beam

14. Spatial Beam Profiles @ 100K, 1 Hz
Gain  8
Gain  6
E = Hz

15. Pulse Energy v. Pump Pulse Duration
3 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

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

17. Thank you for your attention!
Any Questions?