ASE Suppression in a Diode-Pumped Nd:YLF Regenerative Amplifier Using a Volume Bragg Grating

A. V. OkishevUniversity of RochesterLaboratory for Laser Energetics

Frontiers in Optics 2007/Laser Science XXIIIOSA 91st Annual Meeting

San Jose, CA16–20 September 2007

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Collaborators

A. V. Okishev, C. Dorrer, and J. D. Zuegel

Laboratory for Laser Energetics, University of Rochester

V. I. Smirnov

OptiGrate, Orlando

L. B. Glebov

CREOL, University of Central Florida, Orlando

Regenerative amplifier with volume Bragg grating (VBG) decreases out-of-band ASE to the more than –40-dB level

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• VBGintroducedinthediode-pumpedregenerativeamplifier(DPRA) as a spectrally selective element does not alter the performance of DPRA.

• UnseededDPRAproduces150-pmFWHMwithamirrorand a43-pm-FWHMoutputspectrumwithVBG.

• Out-of-bandASEfilteringhasbeendemonstratedevenataverylow(comparable to ASE) level of DPRA seeding.

• DPRAwithVBGasasubsystemfortheopticalparametricchirped-pulse amplifier (OPCPA) pump provides significant temporal contrast improvement for recompressed pulse.

Summary

OMEGA EP is designed to achieve high radiation intensity on a target

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Achieving high intensity on target requires high temporal contrast of the pulse to avoid target modifications before interaction with the main pulse.

OMEGA

Laser Bay Mainamplifiers

OMEGA EP Laser Bay OMEGA EP Laser Bay

Compressionchamber

OMEGA EPtargetchamber

Boosteramplifiers

Beam 1 2 3 4

OMEGA targetchamber

Performance capabilities

Short-pulseBeam 1

Pulse width 1 to 100 psEnergy on target (kJ)

2.6 kJ, 10- to 100-psgrating limited <10 ps

Intensity (W/cm2) ~2 × 1020

Focusing (diam) >80% in 20 nm

OPCPA is the heart of a laser system for generating high-intensity ultrashort pulses

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The regenerative amplifier is an optimal subsystem of the OPCPA pump for ASE spectral filtering because of its high gain and multiple number of roundtrips, which allows using spectral filters in a multipass configuration.

ASE-induced temporal variations of the OPCPA pump lead to spectral-density variations of an amplified pulse that, in turn, deteriorate the temporal contrast of the recompressed pulse.

OPCPA pump

Masteroscillator

Regenerativeamplifier

Poweramplifier SHG

Mode-lockedlaser

Stretcher OPCPAamplifier

Nd:glass mainamplifier

CompressorTarget

chamber

Filtering the OPCPA pump to reduce the ASE bandwidth greatly improves the temporal contrast of the OPCPA signal

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OPCPA parameters• High-gainpreamplifier(2 × 29.75 mm of LBO) operated at saturation• ASEwithGaussianspectraldensityandFWHMequalto140pmor30pm• EnergyofASEinthedurationofthepumpis0.1%ofthetotalenergyofthepump• 1053-nmsignalwith8-nmbandwidth

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Simulated signal amplified inconventional OPCPA system

Simulated signal amplified with filtered pump pulse

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ASE FWHM = 140 pm ASE FWHM = 30 pm

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Angularly tuned VBG is used in DPRA* as a spectrally selective element for ASE suppression

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Using an angularly tuned VBG doubles the number of passes for better spectral selectivity.

* The DPRA design is identical to the one described in A. V. Okishev and J. D. Zuegel, “Highly stable, all-solid-state Nd:YLF regenerative amplifier,” Appl. Opt. 43, 6180–6186 (2004).

VBGNd:YLF

Pump

Seed

Opticalspectrumanalyzer

(OSA)

Beamdiagnostics

(CCD)

Energymeter

DPRA

VBGwitha230-pm-FWHMfilterfunctionproducesa 23-pm-FWHMfilterfunctionafter50two-passround-tripswithout distorting the temporal pulse shapes

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Experimental data

Simulations

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One passTwo passes, 50 round-trips

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Input pulseFiltered pulse

The DPRA output-beam profile is not affected by using VBG as a resonator element

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DPRA output energy is reduced by ~25% due to lower VBG reflectivity. This decrease has been compensated for by the pump-diode-current increase.

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DPRA with mirror(R = 99.8%)

DPRA with VBG(R = 99.4%)

VBG narrows the unseeded DPRA output spectrum from150pmto43pmFWHM

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In the DPRA with VBG, the spectral width and shape are defined by the common action of the VBG reflection curve and Nd:YLF gain profile.

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Linear scale

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log scale

VBG in seeded DPRA provides out-of-band ASE suppression to the –40-dB OSA dynamic-range limit

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Simulations show that for a reliable OSA recording of an ASE filtering effect the injected energy must be about equal to the DPRA ASE. In this case EASE = 0.7 Ein; seed-pulse energy is 0.0025 pJ; DPRA gross gain is >1012.

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1053.0 1053.3 1053.6Wavelength (nm) Wavelength (nm)

Simulation Experimental Data

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Using a DPRA with VBG provides OPCPA output-contrast improvement even when the DPRA seed is 100× reduced

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Third-order scanning cross correlation of the OPCPA output pulses

Nominal seed-pulse energy is 100 pJ

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Prepulse contrast enhancement

Seed-pulse energy is reduced to 1 pJ

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Regenerative amplifier with volume Bragg grating (VBG) decreases out-of-band ASE to the more than –40-dB level

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Summary/Conclusions

• VBGintroducedinthediode-pumpedregenerativeamplifier(DPRA) as a spectrally selective element does not alter the performance of DPRA.

• UnseededDPRAproduces150-pmFWHMwithamirrorand a43-pm-FWHMoutputspectrumwithVBG.

• Out-of-bandASEfilteringhasbeendemonstratedevenataverylow(comparable to ASE) level of DPRA seeding.

• DPRAwithVBGasasubsystemfortheopticalparametricchirped-pulse amplifier (OPCPA) pump provides significant temporal contrast improvement for recompressed pulse.