Presented by: Andy Bayramian (for Jeff Bude)Lawrence Livermore National Laboratory
High Energy Class Diode Pumped Solid State Lasers Workshop
September 12, 2012
Granlibakken Conference Center, Lake Tahoe, CA
Improved performance for optical materials in high fluence applications
Improved performance for optical materials in high fluence applications
LLNL-PRES-582238
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The National Ignition Facility
Diode pumps high efficiency (18%)Helium cooled amps high repetition rate (16 Hz) with low stressNormal amp slabs compensated thermal birefringence, compact ampPassive switching performs at repetition rateLower output fluence less susceptible to optical damage
The LIFE laser beambox combines DPSSL technology with NIF-like architecture to provide for robust operations
3
16 J/cm2 14.7 J/cm2 3
Rotator /4Diodes Diodes
3 optical damage is an important design constraint for high fluence laser systems
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• Surface damage• Absorption of sub-
band gap light by damage precursors
• Surface damage sites initiate small
Damage initiation: 16J/cm2 3 on fused silica lens
30 Shots, 7 J/cm2, 10ns
• But, initiated sites can grow at low fluence
300 m
Growth ultimately limits an optic’s lifetime
30 m
We have made great progress understanding the physics of optical damage
Nano-scale precursors lead to macro-scale damage (initiation and growth)
• Formation of a solid-state absorption front (AF)
— Precursor heating— Bulk material becomes absorbing— Propagation of an AF
Initiated superheated cores grow in time with a constant AF velocity, VF
0.1 1
0.1
1
Abs
orpt
ion
Fron
t V
eloc
ity (u
m/n
s)
Intensity (GW/cm2)
Measurement Model with FE
HD Sim
Measured and Simulated AF Velocities agree well
• Identification of precursors • Eliminate them for LIFE conditions
surfaceprecursor
laser
Abs
orpt
ion
fron
t fo
rmat
ion
Depth below surface[um]Rad
ial d
ista
nce
on s
urfa
ce[u
m]
AF
5
3 optical damage can be mitigated using existing NIF technologies and fluence scaling
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Fused Silica Lenses
Tremendous progress has been made improving the damage resistance of
optics for NIF
0 5 10 15 20 25100
101
102
103
104
105
106
107
AfterMitigation
2009finish
1997finish
3w Fluence Distribution 1.8MJ NIF, 1000 Shots
2010 AMPTreatment
Initi
atio
ns p
er N
IF-s
ize
Opt
ic
3 3ns fluence (J/cm2)
Beam Contrast 10%
LIFE: average fluence reduced by >2X to add margin against modulation for
high average power
Fused Silica Lenses
0 5 10 15 20 25100
101
102
103
104
105
106
107
3w Fluence Distribution LIFE, 1e9 Shots
After Mitigation
2010 AMPTreatment
Initi
atio
ns p
er N
IF-s
ize
Opt
ic
3 3ns fluence (J/cm2)
Beam Contrast 10%KDP
Final optic
The average 3 fluence for LIFE was chosen to reduce damage initiation and growth to nearly zero
0 5 10 15 2010-6
10-5
10-4
10-3
10-2
10-1
100
101
Dam
age
Initi
atio
ns (m
m-2)
3 3nsGS Fluence (J/cm2)
AMP2 Silica
1/NIF optic
3.0J/cm2 LIFE
0 2 4 6 8 10 120.0
0.1
0.2
0.3
0.4 Output Surface Input Surface
Gro
wth
Coe
ffici
ent
3 3nsGS Fluence J/cm2
Silica
3.0J/cm2 LIFE
D = D0 exp(N ())
0 5 10 15 200
200
400
600
800
Inte
nsity
[a.u
.]
Time [ns]
3.9J/cm2 3 LIFE pulse
Fused silica Initiation Fused silica Growth
3.0J/cm2 3nsGaussian (GS) equivalent
0 2 4 6 8 10 12 14 16 18 20
0.1
1
10 Conditioned (SNL)
Dam
age
Den
sity
(mm
-3)
3 3nsGS Fluence (J/cm2)
KDP Bulk Damage
3.0J/cm2 LIFE
KDP Bulk Damage Initiation
pulse shape scaling
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Elimination of 3 damage for LIFE optics
Detect & repair
modulation sources
Decreasing damage initiation and growth to zero
Post-processing
Chemical treatment
Laser conditioning
Laser conditioning
Mitigation:Initiate remaining
precursors & remove
IR Laser Processing
Diamond turning
fs laser machining
Block what is left with up-
stream spatial blockers
Silica
KDP
Mirrors
Grind & polish
Polish & coat
Grow & cut
Optical fabrication
creates precursors
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Elimination of 3 damage for LIFE optics
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Expose to 2X LIFE average fluence (NIF levels)Initiated damage sites will be manageable (e.g., 10’s per lens)
Mitigate Initiations (robust to 2X LIFE fluence)RAM (silica), -machining (KDP), fs-machining (mirrors)
Assemble beam lines and burn in system (multi-shots)New initiations may occur due to modulation, contamination
If found, use spatial blockers
Begin long-term LIFE operation
Fabricate high damage resistant optics
Tremendous progress has been made in improving the damage resistance of fused silica optics
0 5 10 15 20 2510-5
10-4
10-3
10-2
10-1
100
101
2010AMP Treatment
2007 Quality Finish
Dam
age
Initi
atio
n D
ensi
ty (m
m-2)
3 3ns fluence (J/cm2)
1997 Quality Finish
Silica optic surface improvement from 1997 to the present
Optic Type
Expected damage sites per optic after 50 shots @
1 MJ 1.3 MJ 1.6 MJ 1.8 MJ
1997Finish 2,000 5,000 10,000 30,000
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Over the last several years, the main fluence limiting precursors on silica surface have been identified
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• Chemical impurities on the surface & in fractures• Precursor diagnostic (PD) and indentation
studies to isolate defects
Surface Layer modified by polishing
Physical model of laser damage precursors on polished fused silica
Fast luminescen
ce diagnostic
SEM Image
Fracture
5 m
Precursor Silica Indent Studies
Image after laser
damage
Laser damage location
Atomic-scale defects formed on fracture surfaces dominate at NIF fluence levels
Tremendous progress has been made in improving the damage resistance of fused silica optics
0 5 10 15 20 2510-5
10-4
10-3
10-2
10-1
100
101
2010AMP Treatment
2007 Quality Finish
Dam
age
Initi
atio
n D
ensi
ty (m
m-2)
3 3ns fluence (J/cm2)
1997 Quality Finish
Silica optic surface improvement from 1997 to the present
Optic Type
Expected damage sites per optic after 50 shots @
1 MJ 1.3 MJ 1.6 MJ 1.8 MJ
1997Finish 2,000 5,000 10,000 30,000
Optical Fabrication; Reduce scratches
2007Finish 17 54 128 258
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The Advanced Mitigation Process (AMP) is the result of astrongly-coupled S&T effort to identify & eliminate silica damage precursors
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The AMP post-finish process is designed to eliminate damage precursors
Microscope Image PD Image
Old process
Scratch from rogue particle during polishing
φTH ~5 J/cm2
New process (AMP1) φTH >15 J/cm2
50 μm
• Although advanced processing greatly reduced finishing flaws, they haven’t been completely eliminated
• AMP2 is a wet etch process designed to remove fracture defect layers
• Key is to eliminate defects without introducing new precursors (absorbing etch precipitates)
AMP2 dramatically reduces/eliminates damageinitiation from surface fractures on fused silica optics
• AMP2 can reduce scratches initiations by 100X for =12J/cm2
• 75% of NIF now contains AMP2 silica lenses
Scra
tche
s be
fore
lase
rA
fter l
aser
ex
posu
re*
AMP2 Etch
*Multiple 12 J/cm2
shots (351 nm, 3 ns)
250 m
No Etch
Optical micrographs of scratched fused silica
test samples
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Using mass transport model, AMP process has been optimized to minimize reaction product concentration left in the cracks of scratches
Calculated SiF62- concentration during AMP Process
Production AMP has started on new WFLs
from vendor
AMP process system
Tremendous progress has been made in improving the damage resistance of fused silica optics
0 5 10 15 20 2510-5
10-4
10-3
10-2
10-1
100
101
2010AMP Treatment
2007 Quality Finish
Dam
age
Initi
atio
n D
ensi
ty (m
m-2)
3 3ns fluence (J/cm2)
1997 Quality Finish
Silica optic surface improvement from 1997 to the present
Optic Type
Expected damage sites per optic after 50 shots @
1 MJ 1.3 MJ 1.6 MJ 1.8 MJ
1997Finish 2,000 5,000 10,000 30,000
Optical Fabrication; Reduce scratches
AMP surface treatment;Make scratches benign
• Progress continues to reach higher levels of damage resistance
• Near zero flaw polishing• Next generation AMP
• Scaling to full aperture
2007Finish 17 54 128 258
2010AMP2 0 0 2 2
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Advanced processing for mirrors continues to improve damage resistance of 3 mirrors
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• Omega (LLE) is demonstrating porous electron-beam thin-film technologies which show a 4X improvement for small beam damage tests
• These mirrors routinely withstand 3.9J/cm2 fluences (70% contrast)
Laser conditioning advanced 3 mirrors will lead to even higher damage resistance
Process optimization at LLE has increased 3 damage fluence to 12J/cm2 at 0.5ns
Pulse-length scaling to LIFE pulse-shapes should increase this further
3 mirror process optimization
Elimination of 3 damage for LIFE optics
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Expose to 2X LIFE average fluence (NIF levels)Initiated damage sites will be manageable (e.g., 10’s per lens)
Mitigate Initiations (robust to 2X LIFE fluence)RAM (silica), -machining (KDP), fs-machining (mirrors)
Assemble beam lines and burn in system (multi-shots)New initiations may occur due to modulation, contamination
If found, use spatial blockers
Begin long-term LIFE operation
Fabricate high damage resistant optics
Scan pattern of circles with varying radii
12.5 s laser pulse
Damage withsub-surface cracks
Scanning laser spot
Flaws on fused silica are mitigated with asmall-beam CO2 laser operating at 10.6-μm
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• RAM (Rapid Ablation Mitigation) protocol
• Raster-scan CO2 laser to remove damaged region through ablation
Damage and sub-surface cracks ablated
Cone in surfaceDownstream modulation controlled
Cone of damaged material removed
Fuse
d si
lica
air~2cm
ablation plume
CO2laser
RAM mitigation eliminates damage sitere-initiation up to 12J/cm2
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Damage growth below 5J/cm2, 3ns, 3
Damage site before mitigation
550 m
475 m
Damage site after mitigation
No damage growth below 12J/cm2, 3ns, 3
2.1 mm
Precise shape control eliminates damage due to down-stream modulation up to 12J/cm2
Extending new mitigation strategies for 1 mirrors to 3
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1 mm
Femtosecond laser machined sites
The fs laser machining process moves growth fluences to 30J/cm2 at 1
1 mm
• Femto-second laser machining is being developed at LLNL to mitigate initiated sites on dielectric mirrors
• Process removes affected “disks” of material• Non-thermal removal of damaged material is a key for thin-film stacks
Elimination of 3 damage for LIFE optics
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Expose to 2X LIFE average fluence (NIF levels)Initiated damage sites will be manageable (e.g., 10’s per lens)
Mitigate Initiations (robust to 2X LIFE fluence)RAM (silica), -machining (KDP), fs-machining (mirrors)
Assemble beam lines and burn in system (multi-shots)New initiations may occur due to modulation, contamination
If found, use spatial blockers
Begin long-term LIFE operation
Fabricate high damage resistant optics
Main amplifier
Power ampSF4
Final Optics Assembly
Transport spatial filter
Pre-AmplifierModule (PAM)
Cavity spatial filter
372 mm beam
18 mm beam
Programmable blockers are introduced in the 48 PAMs
We want to produceshadowregions here
Blockers introduced upstream in the NIF beampathshadow defects downstream in the Final Optics Assembly
NIF-1210-20678s2.ppt
After LIFE beam-line burn-in,
block regions affected by
modulation or contamination
(0.25% of beam area)
Conclusions
• The LIFE system is designed to eliminate the issue of single shotoptical damage
— Technologies already developed for NIF and the NIF optics recycle loop can eliminate damage initiation and growth for silica lenses and conversion crystals
— Similar suppression of damage in 3 mirrors seems likely based on advances in fabrication and mitigation
— Programmable blockers— Manage contamination and beam quality
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