1. Diode Pumped Cryogenic High Energy Yb-Doped Ceramic YAG Amplifier for Ultra-High Intensity Applications
P. D. Mason, S. Banerjee, K. Ertel, P. J. Phillips, C.Hernandez-Gomez, J. Collier
ICUIL 2010 Conference
September 26th to October 1st 2010, Watkins Glen, NY, USA
R Central Laser Facility
STFC, Rutherford Appleton Laboratory, OX11 0QX, UK
+44 (0)

2. Motivation Next generation of high-energy PW-class lasers Applications
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
Beamline Facility

3. Amplifier Design Considerations
Requirement
Pulses up to 1 kJ 10 Hz, few ns duration, overall > 10%
Gain Medium
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
High surface-to-volume ratio
Efficient cooling
Low (overall) aspect ratio
Minimise ASE
Heat flow parallel to beam
Minimise thermal lens

4. 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

5. Amplifier Parameters Quasi-3 level model Results
1D, time-dependent model
Spectral dependence (abs.) included
Assume Fmax = 5 J/cm2 for ns 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
Dlpump
5 nm FWHM
Pump duration
1 ms
Temperature
175 K
Results
Optimum doping x length
3.3 %cm
Storage efficiency
50 %
5 J/cm2 stored
Small signal gain (G0)
3.8
Optimum Aspect ratio#
constant doping
0.78
graded doping
1.55
Inputs
Pump intensity (each side)
5 kW/cm2
Dlpump
5 nm FWHM
Pump duration
1 ms
Temperature
175 K
# Aperture / length

6. Amplifier Design Parameters
HiPER
HiLASE / ELI
Extractable energy
~ 1 kJ
~ 100 J
Aperture
14 x 14 cm
200 cm2
4.5 x 4.5 cm
20 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.%
HiPER
HiLASE / ELI
Prototype
DiPOLE
Extractable energy
~ 1 kJ
~ 100 J
~ 20 J
Aperture
14 x 14 cm
200 cm2
4.5 x 4.5 cm
20 cm2
2 x 2 cm
4 cm2
Aspect ratio
1.4
1.3 1
No. of slabs 10 7 4
Slab thickness
1 cm
0.5 cm
No. of doping levels 5 2
Average doping level
0.33 at.%
0.97 at.%
1.65 at.%

7. DiPOLE Prototype Progress to date
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
Cr4+
Yb3+
35 mm
55 mm
Pump 2 x 2
cm²

8. Ceramic Yb:YAG Discs Transmission spectra Uncoated, room temperature
Transmitted wavefront
Fresnel limit ~84% PV
0.123
wave
940 nm
1030 nm

9. Amplifier Head Head layout CFD modelling
Predicted temperature gradient in Yb:YAG amplifier disc
He flow
2 cm
2.0%
1.1%
Pump
Vacuum
vessel
Uniform T across pumped region ~ 3K
He flow

10. Vacuum insulated transfer
Cryo-cooling System
Vacuum insulated transfer
lines
Cryostat
Amplifier head
Helium cooling circuit

11. Diode Pump Laser Built by Consortium Specifications
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 to 10 Hz
Other specs. independent of PRF

12. Diode Pump Laser Beam profile specification Demonstrated performance
Spatial profiles (Modelled)
Near Field
Far Field
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
Preliminary measurement

13. Lab Layout LN2 tank Cryo-cooling system Optical tables Amplifier
Floor area ~30 m2
Amplifier

14. Next Steps Short-term (3 to 6 months) Long-term (6 to 12 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

15. Any Questions ?

16. Yb-doped Materials Parameter (at RT) Glass S-FAP YAG CaF2 Wavelengths
(pump/emission in nm)
/ 1030
900 /
1047
940 /
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
Non-linear index (n2 x esu)
0.1 to 1.2
1.5
2.7
0.43
Bandwidth
> 50 nm
OK?
Availability of large aperture
Good
Limited
OK (Ceramic)
(Ceramic)
Thermal properties
(K in Wm-1K-1)
Poor
1.0 OK
2.0
10.5
6.1

17. Yb:YAG Energy Level Diagrams
Room temperature (300K)
Cryogenic cooling (175K)
940 nm
Yb3+
2F7/2
1030 nm
4 Level-like
2F5/2
Significantly reduced re-absorption loss
f13=0.64%
940 nm
Yb3+
2F7/2
1030 nm
Quasi-3 Level
2F5/2
Re-absorption
loss
Low quantum defect (QD) p/ las~ 91%
f13=4.6%

18. Temperature Dependence
Pump Fluence (J/cm²)
Storage Efficiency (%)
Small Signal Gain
T=175K
T=300K
Operating fluence

19. Doping Profile (1 kJ Amplifier)

20. Pump Absorption Efficiency vs. lpump Absorption + Pump Spectra 175 K
175 K, 10 kW/cm2
300 K, 20 kW/cm2
Efficiency vs. lpump
Absorption + Pump Spectra
175 K
300 K
Pump, FWHM = 5nm

21. Absorption Spectra
Factor of 2
940 nm
1030 nm

22. Ceramic YAG with Absorber Cladding
Sample of Co-Sintered YAG
(Konoshima)
Reflection at Interface?
Camera
Laser
Cr4+:YAG a b c
Yb:YAG ?
Yb:YAG
Cr4+:YAG a b c
Nothing!

23. 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
Extraction
Amp 1
Amp 2
Injection
Amp1 + 2
Reverser 1
Reverser 2
Extraction
Losses:
17.4 %
(distributed)
10 %