Design and Construction of a High Energy, High Average ......5 Slab Amplifier Specifications Must amplify infrared radiation at 1053 nm to be doubled to 527 nm Green pump should have

7/19/2006

Design and Construction of a High Energy, High Average

Power Nd:Glass Slab Amplifier

Dale MartzDepartment of Electrical & Computer

Engineering

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OutlineIntroductionNd:Glass Slab

Nd:Glass Material PropertiesSlab vs. Rod Geometry

Discussion of the Rod GeometryAdvantages of the Slab

Design of the Amplifier Head and Support SystemsMountingCoolingDesign of the Pulse Forming Network (PFN)

Design Specifications & ConsiderationsSimmering of Lamps

ResultsSingle Pass Gain

ConclusionsFuture Work

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IntroductionEUV lasers require high energy pumping by optical lasers

Ex: 13.2 nm Ni-Like Cd Laser*1 µW @ 5 Hz300-350 mJ 120 ps pre-pulse1 J 8 ps heating pump pulse

Current Ti-Sapphire (800 nm) pump laser has 3 stages of amplification3rd stage is pumped by a Nd:YAG laser

Infrared radiation (1064 nm) up-converted to 2nd harmonic5 J of green light (532 nm)Repetition rate of 5 Hz

*Weith et al. - Opt. Lett. 31, 1994-1996 (2006)

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Replacement Pump Laser Schematic

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Slab Amplifier SpecificationsMust amplify infrared radiation at 1053 nm to be doubled to 527 nm

Green pump should have 14-18 J per arm (both sides of the Ti-Sapph should be pumped)

Infrared output for one arm should have 20-25 J of energy

A gain of 3.3x per pass has been shown to produce the desired energy after eight passes*Input beam to the slab will be 12 cm x 8 mm

60 mJ with a 20 ns pulse width FWHM at 1053 nm and a 1 nm bandwidth

Must be able to operate at > 1 Hz

*Dane et al. – Journal of Quantum Electronics 31, 148-163 (1995)

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Nd:Glass

Similar to Nd:YAGFour level system

Has an amorphous structureNot crystalline

Glass has high energy storage potential

Large VolumeSmall stimulated emission cross-section3.5x10-20 cm2

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Why a Slab Geometry?Generation of good beam quality at high energies and high average power

Due to increased repetition rate of pumping, temperature gradients across the cross-sectional area of gain medium develop

Thermal focusing

Stressed induced biaxial focusing

Stressed induced birefringence resulting in depolarization

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Thermal Loading in a RodTemperature Distribution

T(K)Temperature Gradient

G(104 K/m)

A rod with 14 cm2 cross-sectional area, with 1250 W heat loading and cooling to 35 oC on the outer surface – Temperature differential of 485 oC

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Change in the Index of Refraction Due to Temperature Change

Given the temperature distribution modeled for a 1250 W heat load the change in the index of refraction can be calculated

))((*)( oo TxTdTdNnxn −+=

Where is the temperature coefficient of refractive index

For the glass that is used,

dTdN

CdTdN o/10*2 6−≈

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Thermal Focusing (Lensing)Given the temperature distribution modeled for a 1250 W heat load the change in the index of refraction can be calculated

Using this index profile, Hecht gives an expression to find the focal length of the thermal lens

))((*)( oo TxTdTdNnxn −+=

* Approximation assuming focal length much larger than the rod length

The resulting focal length that fits this profile is:f = 52 cm from the end of the rod (92 cm from the beginning of the rod)

dffrnrn /)()( 22max −+−=

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Thermal Focusing (Lensing) - continued

Because the resulting focal length of 52 cm is on the order of the length of the rod (40 cm), ray tracing was employed to verify the exampleThe focal length was found to be:f = 45 cm from the end of the rod (85 cm total) - lensing is severe

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Thermal Loading in a SlabA slab with 14 cm2 cross-sectional area

1250 W heat loadingequal to the heat loading on the rod

Cooling to 35 oC on the pump faces

Temperature differential of 38 oC

Pump Faces

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Discussion of Phase-Front Aberrations

As the rod caused spherical focusing, the slab will cause cylindrical focusingTo avoid aberrations and focusing effects, the beam’s phase-front should pass through an averaged (non-uniform) temperature/stress environmentThe slab geometry allows this by letting the beam take a zig-zagoptical path through the gain medium

Pump Faces

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APG-1 from Schott North AmericaAPG-1 is an advanced phosphate based laser glass

40 cm x 14 cm x 1 cm

Upper-level lifetime = 361 µs

Lower-level lifetime = 192-380 ps

Emission peak at 1053.9 nm

Emission bandwidth of 27.8 nm

Index of refraction n = 1.526

The doping of the glass is 2.7% by weight of Nd3-3.5 x1020 Nd atoms/cm3

Thermal Cond. = 0.78 W/m*K

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Use of Flashlamps to Pump

Emission spectrum of the flashlamp

(Fenix Tech.)

Absorption spectrum of the Nd:Glass slab

(Schott N.A.)

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ConclusionFuture Work

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Frame and Mounts for SlabFrame, window seals and mounts are made from titaniumWindows form thin 2.5 mm cooling channelsBrass seals do not touch slab, but provide a method to seal with an o-ringBottom & Top seals are made from Delrin plastic

Water enters and exits through these pieces

*Designed by Dave Alessi

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Reflector CavityCavity

Made from Delrin plastic

Defuse ReflectorMade from SpectralonTM, resistant to deionized water

Flow tubesProvides cooling channel for flashlampsDoped with cerium to block UV light

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Parasitic OscillationsAbsorbing Glass

Fits inside titanium mountAbsorbing glass

n = 1.5243 mm thickHA-30 (Hoya Corp.)

Attached via index of refraction matched elastomer

Also used to attach the titanium braces to the slab

Allows for a mechanical cushion between the metal and the glass

Entrance faces are parallel but tilted

1.5 degree wedge

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Nd:Glass Slab Amplifier Head

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Cooling SpecificationsUse of deionized water as a coolant

cp= 4.184 J/g oC - specific heatµ = 0.00764 g/cm*s - dynamic viscosity

Chosen flow rate of (mp) 3 L/s (kg/s) or 180 L/minCorresponds to the removal of 1250 W of heat

Should keep the temperature differential across the 14 cm dimension of the slab close to 0.1 oC

Reynolds number – ratio of the inertia and viscous forcesTurbulent if > 2300Our channel is 10,870 – sufficient

Re = 4*mp/(µ*wp) - wp = 2*(0.25 + 36) = 72.5 cm

tcmP pp ∆= **

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Fluid Flow

reflectorcavityleft side

reflectorcavityright side

Pump

2 m of 1’’ tubepsip 50.3=∆

Slab cooling channelspsip 18.0≈∆

90° smooth bend and splitter

smooth bend

3 m of 1’’ tubepsip 25.5=∆

psip 11.1≈∆

psip 28.0≈∆

Lamp water jacketspsip 75.1≈∆

)"1/(18 tubingwpsiptotal ≈∆

smooth bendpsip 28.0≈∆

Small entranceexpansion

Entrance, expansion losspsip 28.0≈∆

Heat Exchangerpsip 4=∆

psip 58.0≈∆

90 degree sharp turn at entrancepsip 22.0≈∆

90° smooth bend

90° smooth bend

90° smooth bend

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Cooling Test

Cooling of a un-doped glass slab installed in the amplifier head (2.5 kJ/shot)

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Pulse Forming Network (PFN)A simple RLC circuitPulse requirements

270 µs current pulse2.5 kJ/pulse electrical energy per pulse to all four lampsCritically damped to ensure minimization of ringing

The amplifier has four lamps to pump both sides of the slabTwo PFN units for one amplifier head (one PFN unit for two lamps in series)

Over designing the PFN unit: 2 kJ/pulse (4 kJ/pulse total)For a 145.16 µF Capacitor, 5.3 kV charge voltage corresponds to 2 kJ

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Variable ResistanceVariable resistance – Two lamps in series

or

Each lamp is filled with 170 Torr of Xe w/ 18 mm boregives an arc length of 15.35 in for one lamp or 30.7 in. for twowith a ko = 22.82, or 45.64 when running two in series*

IkR o /= IkV o *=

*Information given by Fenix Technology (vendor of the flashlamps)

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Simulation of PFN

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Lighting and SimmeringProblem: 1 kV per inch of arc length required to light lampsSolution: Creation of an electric field between anode and the wall of the flashtube

Use of high turn-ration pulse transformers (1:36) to convert -520 V pulses to -18.7 kV20-30 Hz

The lamps are simmered with two 60 W simmer units providing 1A of current (max. voltage 1500 V)

1.6 A boost after current pulse for 8 ms

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Preliminary Circuit DesignThe “switch” for this RLC circuit consists of two thyristors in series from Dynex Semi-Conductor (DCR1050F) Rated for

4 kV1 kA of current RMS12 – 15 kA non-repetitive surge currents

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Measured PFN Pulse

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Problem with “Blow Out”

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Schematic of PFN w/ Added High-Voltage Transformer

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Resulting Pulse with HV Transformer

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Resulting Pulse with HV Transformer

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Power Delivered to the FlashlampsNot the energy stored in capacitor

Tolerance of components

Loss in components increases as charge voltage increases

Lamp resistance gets smallInductorWire

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Single Pass Gain ResultsExperimental Setup:

6 mm diameter, 20 ns, 1 mJ pulses from a single mode Nd:YLF oscillator are used

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Measured Single Pass Gain

Compared to similar work done by:Dane et al. – Journal of Quantum Electronics 31, 148-163 (1995)

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Spatial Gain Results Along 14 cm Dimension

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Future WorkObtain four passes of amplification

Preliminary results are advantageousPossibly phase-conjugate after four passesAchieve 20-25 J output after eight passesSuccessfully double to 527 nmOperate amplifier at high average power (>1Hz)Pump both sides of the Ti-Sapphire amplifier by operating two units simultaneously

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AcknowledgementsThanks to my committee members:

Dr. Jorge Rocca, Dr. Siu Au Lee and Dr. Carmen Menoni

I would also acknowledge everyone who has helped me during my time at the lab

David Alessi, Mike Grisham, Scott Heinbuch, Brad Luther, Paul Platte, Mike Purvis, Brendan Reagan and David Springer.

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Questions?

Documents

Design and Construction of a High Energy, High Average ......5 Slab Amplifier Specifications Must amplify infrared radiation at 1053 nm to be doubled to 527 nm Green pump should have