October 30th, 2007 High Average Power Laser Program Workshop 1

Long lifetime optical coatings for 248 nm: development and testing

Presented by:Tom Lehecka

Penn State Electro-Optics Centertlehecka@eoc.psu.edu

Presented at:High Average Power Laser Program Workshop

Naval Research LaboratoryOctober 30-31, 2007

October 30th, 2007 High Average Power Laser Program Workshop 2

Outline

• Problem Definition• Optical specifications• Optical design• Test setup• Results to date• Near term goals

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Problem Definition

• High reflectivity (>99.5%) is required on optics after the final amplifier – direct impact on system efficiency

• High damage threshold (~10 J/cm2 goal, ~1 J/cm2 operation) optics required after final amplifier – direct impact on system cost and reliability

• Optical coatings and materials are known to “ darken” under continued UV exposure at 248 nm – leads to reduced reflectivity and perhaps lower damage threshold

• This problem is separate from final optic at target chamber

•Optics lifetime was considered a serious concern by the 2006 DOE review panel

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Optical Specifications

1. Reflectivity: Greater than 99.5% at 248 nm

2. Laser induced damage threshold: 10 J/cm2 for 2 ns pulses

3. Operational fluence: 1 J/cm2 for 2 ns pulses

4. Lifetime: Specified reflectivity and damage threshold maintained for one billion pulses at 1 J/cm2

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Test facility

•Lens position, along with laser energy, determines fluence on optic•DVR allows long term monitoring of surface condition – to date this has been sufficient for damage determination. •Review of DVR allows accurate determination of onset of damage.•Scattering detector can detect small changes in surface properties

GAM Laser200 cm f.l. lens

Output to digital video recorder

CCD Camera

Photodiode Scattering monitor

Diode laser(optional)

Optic under test

Power meter

KrF – 248 nm~100 mJ/pulse18 ns pulse length125 Hz max rep rate

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Laser Characterization

•Raw laser output provides a relatively flat top region for laser damage measurement•Energy in the bucket measurement provided absolute fluence

Power meter

Adjustable aperture at equivalent test piece location

Laser beam input

CCD camera image of the beam 50 cm from focusCamera located at location of optic under test

•3.4 X 1.9 mm beam dimension

•Provides 1.5 J/cm2 with 100 mJ input

Energy in the bucket measurement

Varying aperture size determines fluence

Lens

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Laser Fluence

Fluence ranging from <1 to >10 J/cm2 can be readily accessed with spot size > 1 mm

GAM Laser with 200 mm focal length lens

2.487

0.782

0.105

0.0450.024

0.0150.010 0.009

0.007

0.001

0.010

0.100

1.000

10.000

0 10 20 30 40 50 60 70 80 90

Focal Position (mm)

No

rma

lize

d F

lue

nc

e (

J/c

m^

2/m

J)

Focal Position (mm)

Beam x (mm)

Beam y (mm) Shape Area (cm^2)

Normalized Fluence (J/cm^2/mJ)

0 0.32 0.16 Ellipse 0.000402112 2.48710 0.475 0.343 Ellipse 0.001279572 0.78220 1.25 0.76 Rect. 0.0095 0.10530 1.92 1.15 Rect. 0.02208 0.04540 2.64 1.55 Rect. 0.04092 0.02450 3.39 1.95 Rect. 0.066105 0.01560 4.12 2.37 Rect. 0.097644 0.01070 4.5 2.47 Rect. 0.11115 0.00980 5.11 2.86 Rect. 0.146146 0.007

Normalized fluence ranges from 0.007 to 2.487 J/cm2/mJ

Due to beam nonuniformities near focus, only the range from 0.007 to 0.105 will be used for testing

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Optical Anatomy

Substrate

Dielectric coatinglayers (not to scale)Nominal layer thickness = /4

•Multi-layers of dielectric material with alternating indices of refraction are deposited on the substrate

•Fresnel reflections at each interface result in desired reflectivity

•Large variations between high and low index materials, or large numbers of layers required for high reflectivity

•Materials selection for deep UV are limited

Laser light in

Reflection out

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Optical Coating Issues

1. Materials selection for deep UV are limited

a. Oxides – SiO2, Al2O3, Sc2O3, HfO2

b. Fluorides – MgF2, AlF3, CaF2, Na3AlF6 (cryolite), LaF3, YF3, GdF3, NdF3

2. Oxides, particularly SiO2, are known from photolithography applications to

darken after continued 248 nm exposure

3. Fluorides do not provide a large index variation resulting in many layers required

for high reflectivity.

a. Many layers often result in crazing due to thermal stresses

b. Many layers can also increase scattering due to interaction with layers

• For HAPL development we have initially designed using fluorides. YF3, LaF3 and cryolite have been tried at Spectrum Thin Films.

• LaF3, NdF3, GdF3 and cryolite resulted in high damage threshold coatings (~27 J/cm2) for Japanese researchers in the 1990’s. Relatively low reflectivity ~97% with crazing observed for ~ 40 layers. (Izawa et. al, SPIE, 1990)

• COTS coatings are also being evaluated. These are typically oxide based.

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Results to Date (1)

Manufacturer Design Damage Threshold

Lifetime Notes

Thorlabs COTS, unknown 0.9 J/cm2 NA High quality optical surface, no pinholes

Alpine Research Optics

COTS, details unknown, oxide based

1.5 J/cm2 NA Moderate quality, some pinholes. Vendor recommended by GAM – survive 1e9 shots at low fluence.

Spectrum Thin Films

Custom

YF3/Na3AlF6

(cryolite)

3.5 J/cm2 NA Poor visible quality, many pinholes. 48 layers led to stress crazing

Spectrum Thin Films

Custom

LaF3/Na3AlF6

Not yet tested

NA 37 layers, lower reflectivity test. Some crazing and pinholes.

Due to lower than desired damage threshold, no lifetime testing has been started. Desire minimum of 5 J/cm2

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Results to Date (2)

One inch optic with multiple damage sites

Microscope image 1.5 J/cm2 fluence damage site, single shot

Microscope image 1.5 J/cm2 fluence damage site, 100 pulses

Initial damage is small pits which quickly grows to large damage zoneCCD camera easily detected the single shot damage

Damage result from Alpine Research Optics mirror

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Near Term Goals

1. Try a more standard coating, likely Al2O3/AlF3 to obtain

high damage threshold from Spectrum Thin Films, then lifetime test

2. Continue to work with Alpine Research Optics to develop their capability (they are doing this work gratis, so far)

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Summary

• Lifetime of high fluence high reflectivity optics for 248 nm is likely to be a challenge for a laser fusion system

•We have developed a simple test facility to assess damage threshold and lifetime of sample optics

•Tests to date have not yielded optics near the desired 10 J/cm2 laser induced damage threshold

•Alternatives are endless, funding is not