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

Transcription

1 ASE Suppression in a Diode-Pumped Nd:YLF Regenerative Amplifier Using a Volume Bragg Grating Spectral density (db) Mirror VBG Wavelength (nm) Frontiers in Optics 2007/Laser Science XXIII A. V. Okishev OSA 91st Annual Meeting University of Rochester San Jose, CA Laboratory for Laser Energetics September 2007

2 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

3 Summary Regenerative amplifier with volume Bragg grating (VBG) decreases out-of-band ASE to the more than 40-dB level VBG introduced in the diode-pumped regenerative amplifier (DPRA) as a spectrally selective element does not alter the performance of DPRA. Unseeded DPRA produces 150-pm FWHM with a mirror and a 43-pm-FWHM output spectrum with VBG. Out-of-band ASE filtering has been demonstrated even at a very low (comparable to ASE) level of DPRA seeding. DPRA with VBG as a subsystem for the optical parametric chirpedpulse amplifier (OPCPA) pump provides significant temporal contrast improvement for recompressed pulse. E16051

4 OMEGA EP is designed to achieve high radiation intensity on a target OMEGA target chamber OMEGA EP target chamber Performance capabilities Pulse width OMEGA Laser Bay Short-pulse Beam 1 1 to 100 ps Compression chamber Energy on 2.6 kj, 10- to 100-ps target (kj) grating limited <10 ps Intensity (W/cm 2 ) ~ Focusing (diam) >80% in 20 nm Beam Main amplifiers Booster amplifiers OMEGA EP Laser Bay Achieving high intensity on target requires high temporal contrast of the pulse to avoid target modifications before interaction with the main pulse. E16052

5 OPCPA is the heart of a laser system for generating high-intensity ultrashort pulses OPCPA pump Master oscillator Regenerative amplifier Power amplifier SHG Mode-locked laser Stretcher OPCPA amplifier Nd:glass main amplifier Compressor Target chamber 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. 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. E16053

6 Filtering the OPCPA pump to reduce the ASE bandwidth greatly improves the temporal contrast of the OPCPA signal Normalized intensity (db) Simulated signal amplified in conventional OPCPA system Simulated signal amplified with filtered pump pulse Time (ps) Time (ps) ASE FWHM = 140 pm ASE FWHM = 30 pm OPCPA parameters High-gain preamplifier ( mm of LBO) operated at saturation ASE with Gaussian spectral density and FWHM equal to 140 pm or 30 pm Energy of ASE in the duration of the pump is 0.1% of the total energy of the pump 1053-nm signal with 8-nm bandwidth E16054

7 Angularly tuned VBG is used in DPRA* as a spectrally selective element for ASE suppression VBG Nd:YLF Pump DPRA Optical spectrum analyzer (OSA) Seed Beam diagnostics (CCD) Energy meter Using an angularly tuned VBG doubles the number of passes for better spectral selectivity. E16055 * 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, (2004).

8 VBG with a 230-pm-FWHM filter function produces a 23-pm-FWHM filter function after 50 two-pass round-trips without distorting the temporal pulse shapes Simulations Intensity (arbitrary units) Wavelength offset (pm) One pass Two passes, 50 round-trips Experimental data Time (ns) Input pulse Filtered pulse E15623 Intensity (arbitrary units) Time (ns) Mirror VBG

9 The DPRA output-beam profile is not affected by using VBG as a resonator element DPRA with mirror (R = 99.8%) DPRA with VBG (R = 99.4%) DPRA output energy is reduced by ~25% due to lower VBG reflectivity. This decrease has been compensated for by the pump-diode-current increase. E15624

10 VBG narrows the unseeded DPRA output spectrum from 150 pm to 43 pm FWHM Spectral density (arbitrary units) Linear scale Wavelength (nm) Spectral density (db) Mirror VBG 0 10 log scale Wavelength (nm) 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. E15625

11 VBG in seeded DPRA provides out-of-band ASE suppression to the 40-dB OSA dynamic-range limit Spectral density (arbitrary units) Spectral density (db) Simulation Wavelength (nm) Mirror VBG Experimental Data Wavelength (nm) Linear scale log scale E15626 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 E ASE = 0.7 E in ; seed-pulse energy is pj; DPRA gross gain is >10 12.

12 Using a DPRA with VBG provides OPCPA output-contrast improvement even when the DPRA seed is 100 reduced Third-order scanning cross correlation of the OPCPA output pulses Prepulse contrast enhancement Intensity (db) Mirror VBG Time (ps) Time (ps) E16056 Nominal seed-pulse energy is 100 pj Seed-pulse energy is reduced to 1 pj

13 Summary/Conclusions Regenerative amplifier with volume Bragg grating (VBG) decreases out-of-band ASE to the more than 40-dB level VBG introduced in the diode-pumped regenerative amplifier (DPRA) as a spectrally selective element does not alter the performance of DPRA. Unseeded DPRA produces 150-pm FWHM with a mirror and a 43-pm-FWHM output spectrum with VBG. Out-of-band ASE filtering has been demonstrated even at a very low (comparable to ASE) level of DPRA seeding. DPRA with VBG as a subsystem for the optical parametric chirpedpulse amplifier (OPCPA) pump provides significant temporal contrast improvement for recompressed pulse. E16051