120912 bude improved optical performance€¦ · diode pumps high efficiency ... sem image fracture...

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

2

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

4

• 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

6

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


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

7

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

8


9

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)


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

10

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


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)


1997 Quality Finish


Optic Type


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

13

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

14

15

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


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)


1997 Quality Finish


Optic Type


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

17

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


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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

21

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

22

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


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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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120912 bude improved optical performance€¦ · diode pumps high efficiency ... sem image fracture...

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