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.HernandezGomez, J. Collier
ICUIL 2010 Conference
September 26th to October 1st 2010, Watkins Glen, NY, USA
paul.mason@stfc.ac.uk
R1 2.62 Central Laser Facility
STFC, 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
Beamline
Facility
Amplifier Design Considerations
• Requirement
– Pulses up to 1 kJ energy @ 10 Hz, few ns duration, overall > 10%
• Gain Medium
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
• Amplifier Geometry
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
Amplifier Parameters
• Quasi-3 level model
– 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)
Dlpump
5 kW/cm2
5 nm FWHM
Pump duration
1 ms
Temperature
175 K
Results
Optimum doping x length
Storage efficiency
Small signal gain (G0)
Optimum
Aspect
ratio#
#
3.3 %cm
50 %
5 J/cm2 stored
3.8
constant doping
0.78
graded doping
1.55
Aperture / length
Amplifier Design Parameters
HiPER
HiLASE /
ELI
Prototype
DiPOLE
~ 1 kJ
~ 100 J
~ 20 J
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
1 cm
0.5 cm
0.5 cm
5
4
2
0.33 at.%
0.97 at.%
1.65 at.%
Extractable energy
Aperture
Slab thickness
No. of doping levels
Average doping
level
DiPOLE Prototype
• Diode Pumped Optical Laser for Experiments
– 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
Pump
2x2
cm²
Yb3+
35 mm
• 4 x co-sintered ceramic Yb:YAG slabs
55 mm
– 10 to 20 Joule prototype laboratory test bed
Cr4+
Ceramic Yb:YAG Discs
• Transmission spectra
• Uncoated, room temperature
1030 nm
940 nm
Fresnel limit ~84%
• Transmitted wavefront
PV
0.123
wave
Amplifier Head
• Head layout
• CFD modelling
•
Vacuum
vessel
2 cm
1.1%
Pump
Pump
2.0%
He flow
Predicted temperature gradient in
Yb:YAG amplifier disc
Uniform DT across pumped
region ~ 3K
Cryo-cooling System
Vacuum insulated transfer
lines
Amplifier
head
Helium
cooling
circuit
Cryostat
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
– l0 = 940 nm, DlFWHM < 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
Spatial profiles (Modelled)
Near Field
Far Field
– 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
Lab Layout
LN2 tank
Cryo-cooling
system
Optical
tables
Floor area
~30 m2
Amplifier
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
940-980 /
1030
900 /
1047
940 /
1030
940-980 /
1030
~ 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
0.1 to
1.2
1.5
2.7
0.43
High
> 50 nm
Low
OK?
High
> 50 nm
Availability of large
aperture
Good
Limited
OK
(Ceramic)
Limited
(Ceramic)
Thermal properties
(K in Wm-1K-1)
Poor
1.0
OK
2.0
Good
10.5
OK
6.1
Wavelengths
(pump/emission in nm)
Fluorescence lifetime
(msec)
Non-linear index
(n2 x 10-13 esu)
Bandwidth
Yb:YAG Energy Level Diagrams
• Room temperature (300K)
• Cryogenic cooling (175K)
Quasi-3 Level
f13=4.6%
7/2
Yb3+
940 nm
2F
1030 nm
5/2
Re-absorption
loss
1030 nm
Low quantum
defect (QD)
lp/ llas~ 91%
2F
2F
5/2
940 nm
2F
4 Level-like
Significantly reduced
re-absorption loss
f13=0.64%
7/2
Yb3+
Temperature Dependence
Operating fluence
T=300K
Pump Fluence (J/cm²)
Small Signal Gain
Storage Efficiency (%)
T=175K
Doping Profile (1 kJ Amplifier)
Pump Absorption
Absorption + Pump Spectra
Efficiency vs. lpump
175 K
300 K
Pump, FWHM = 5nm
175 K, 10 kW/cm2
300 K, 20 kW/cm2
Absorption Spectra
1030 nm
940 nm
Factor
of 2
Ceramic YAG with Absorber Cladding
Sample of Co-Sintered YAG
(Konoshima)
Reflection at Interface?
Laser
Cr4+:YAG
Camera
a
b
c
?
Yb:YAG
Yb:YAG
Cr4+:YAG
a
c
b
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
Amp 2
Amp 1
Extraction
Beam
Transport
Reverser 1
Reverser 2
Injection
Injection
Losses:
Amp1 + 2
17.4 %
(distributed)
Reverser 1
Amp1 + 2
10 %
17.4 %
(distributed)
Reverser 2
10 %
Amp1 + 2
17.4 %
(distributed)
Reverser 1
10 %
Amp1 + 2
17.4 %
(distributed)
Extraction
10 %