One of the primary challenges in laser-matter interactions lies in the efficiency of coupling laser light to targets in the presence of generated plasmas. For this project, we investigated the feasibility of novel strategies for upgrading capabilities of laser systems to improve this efficiency. The study employed analysis and modeling to evaluate strategic upgrade paths for developing laser performance, control, and diagnostic capabilities.
Increasing the coupling of laser energy to targets on fusion-class laser systems improves the chances for both ignition and laser machine safety by reducing the operating fluences/intensities. Hence, the objective of this project was to evaluate subsets of potential laser capability upgrades that offer potential for increasing the coupling of high-energy laser light to targets. The subsets were classified in the following manner:
Subset 1 dealt with the safe increase of energy and/or peak power, which are often constrained by the fluence or intensity limitations of the final optics. Candidate solutions included more precise beam-shaping, beam-shaping on each beamline, and softer apodization of beam edges that are most influenced by accumulated B-integral and/or elimination of diffraction gratings that induce amplitude modulation when away from relay planes. Subsets 2 and 3 involved evaluating prospects for a more intelligent, faster modification of the laser pulse format (either amplitude, wavelength, or both) versus time. This could enable further mitigation of the imperfectly controlled exchange of energy between beams crossing at or near the target. Subset 4 augmented the suite of diagnostics available to diagnose the mechanisms that interfere with efficient coupling of laser energy to targets.
All four thrust areas were explored, and the following results emerged. (1) Discussions with plasma and laser physicists helped identify strategies for increased laser energy and peak power. (2) The feasibility of pulse shaping with 1-ps resolution was explored in simulations, and a patent was filed on a novel pulse shaping concept known as Spectrally Transcribed And Chirp Corrected Arbitrary Temporal Optimizer (STACCATO). (3) The feasibility of implementing dynamic wavelength modifications within the laser pulse was analyzed and the conclusion was that it is feasible at the master oscillator but limited by downstream optical components unless diffraction gratings and other diffractive optical elements can be bypassed or instead implemented as features for beam-slewing and zooming. (4) The novel diagnostic architecture can simultaneously resolve time and spectrum for measuring the stimulated Raman scattering signatures of laser pulses modified by the plasma included from the target. The following subsections provide detail on the conclusions of investigations into the four areas.
The Laser Energy Optimization by Precision Adjustments to the Radiant Distribution (LEOPARD) programmable spatial shaper was originally developed to introduce dynamic shadow blockers and tailor laser beam profiles for optimized energy extraction efficiency (Heebner et al. 2010). The peak energy of a high-energy laser system is often limited by surface flaws that are initiated and grown proportional to local fluence. Deployment of shadow blockers addresses this for up to a limited number of residual flaws (Heebner et al. 2011). The peak power of a high-energy laser system is often limited by local intensification due to beam filamentation (B-integral). This can be initiated by nearfield beam contrast or sharply defined beam edges. Tailoring the injected intensity profile addresses this by precompensating the contrast to target a flat output fluence profile. Intensification at beam edges can still occur if the edges are too sharp. Programmable spatial-shaping methods can address this by softening the apodization of the beam edges, typically on a static serrated edge mask, or by locally reducing the beam intensity just inside of the beam edges using programmable spatial shaping.
There is a growing demand from high-energy-density (HED)/inertial confinement fusion (ICF) laser users for enhanced laser pulse shaping capability at picosecond resolution. However, there are currently no demonstrated options for the generation of optical waveforms or measurement of experimental optical signatures with picosecond features over long record lengths. The STACCATO concept resulted from a need to meet this demand. Many optical arbitrary waveform generators that have been published in literature can achieve ultrafine, femtosecond resolution. However, due to the very limited record lengths of less than 10 ps for these solutions, these waveform generators have not displaced electro-optic modulators that achieve nanosecond record lengths with extremely coarse resolution at 250 ps. The STACCATO concept was developed and patented under this project (Heebner 2016). The mechanism is shown in Figure 1 and builds upon the traditional spectral pulse shaper, but performs wavelength to time conversion or an all-optical Fourier transform.
STACCATO can also dynamically alter the spectrum (i.e., wavelength) of a waveform in time. This is accomplished by writing phase ramps in the spectral domain that become phase ramps or instantaneous frequency shifts in the time domain.
One application of dynamic wavelength modification is beam-slewing at target. Laser–target interactions might benefit from the capability to slew the focal spot by approximately hundreds of microns in a few nanoseconds for the duration of the drive pulse. This might be accomplished by introducing a small chirp (linear wavelength sweep) to the pulse and implementing a diffraction grating to convert this to a sweep in beam pointing. This would be very similar to the smoothing by spectral dispersion (SSD) concept only with a linear instead of sinusoidal sweep. The linear wavelength sweep is constructed of a sawtooth pattern of ramps that effectively achieve an all-optical serrodyne or time prism. A related application of dynamic wavelength modification is beam zooming at target. This may be an important capability for direct drive or shock ignition experiments. In these schemes, a target capsule implodes while being driven by a symmetric arrangement of focused laser spots. The ability to shrink the focus of the spots as the target collapses would be beneficial. It was determined that this could be accomplished again by introducing a small chirp to the pulse and implementing a quadratic phase mask to convert this to a zoom in beam focus.
The Slanted Light Interrogation for Cross-Correlated Encoded Recording (SLICER) optical recorder concept was developed and demonstrated under a separate Lawrence Livermore National Laboratory project and patented by the Laboratory (Muir and Heebner 2017a). The experimental demonstration resulted in a single shot 50 ps record with 0.4 ps resolution at 1053 nm (Muir and Heebner 2017b). The SLICER mechanism operates by implementing a thick piece of birefringent material to create two collinear, cross-polarized and delayed replicas of a signal under test. The phases acquired by the time-staggered replicas are then simultaneously modified with a slanted incidence auxiliary short pulse—sub-picosecond—pump as they traverse a semiconductor. After overlaying the two phase-shifted copies in time with a second crossed birefringent plate and destructively interfering them with a crossed polarizer, a rolling light shutter is achieved. Only the time pixel coincident with the arrival of the pump-induced rolling shutter is transmitted as a time sample and recorded on a camera's spatial pixel. This mapping of time to space occurs along one axis of the camera while the second axis could be exploited to record spectral content dynamically as found in a spectrogram. The performance envelope of SLICER can be extended to a broad range of spectral wavelengths by selection of the semiconductor material's bandgap energy. For this project, we found that introducing a diffraction grating behind the SLICER device could enable the camera to record dynamic wavelength shifts versus time resulting from stimulated Raman scattering in a plasma. This in turn could be used as a probe diagnostic to infer the evolution of the properties of the plasma while undergoing interactions with intense laser beam(s).
This work focuses on new and ongoing strategies for optimizing the coupling of laser energy to targets, and thus serves to forward progress for inertial fusion energy and stockpile stewardship science. It supports the NNSA goal of strengthening the Lawrence Livermore science, technology, and engineering knowledge base for inertial fusion, as well as meeting the DOE strategic objective of delivering scientific discoveries and major scientific tools that transform our understanding of nature.
Four potential thrust areas consisting of laser capability upgrades were explored and evaluated for their promise of increasing the coupling of high energy laser light to targets. Thrust area 1 focused on increasing the available laser energy, and is expected to be challenging while some improvements can still be made with regard to beam shaping. Thrust areas 2 and 3 may hold significant leverage in adding high temporal resolution intelligent pulse shaping and dynamic wavelength modification schemes for low cost in the laser system front end with the novel STACCATO concept. Thrust area 4 focused on novel diagnostics for measuring signatures of laser pulses modified by the plasma, and it was determined that stimulated Raman scattering may be accomplished with a hybrid approach of the recently demonstrated SLICER single-shot cross-correlator and a spectrometer.
Heebner, J. E., et al. 2010. "A Programmable Beam Shaping System for Tailoring the Profile of High Fluence Laser Beams." Paper presented at Laser Damage Symposium XLII: Annual Symposium on Optical Materials for High Power Lasers, Boulder, CO, Sept 26–29, 2010. doi. 10.1117/12.867728. LLNL-PROC-462911.
——— 2011. "Programmable Beam Spatial Shaping System for the National Ignition Facility." Paper presented at SPIE LASE, San Francisco, CA, Feb 19, 2011. doi. 10.1117/12.875794. LLNL-PROC-469093.
——— 2017. “Arbitrary Pulse Shaping with Picosecond Resolution over Multiple Nanosecond Records,” U.S. Patent Application, PCT/US17/21887.
Muir, R. and J. E. Heebner. Forthcoming. "All Optical Sampling by Slanted Light Interrogation for Cross-Correlated Encoded Recording (SLICER)." U.S. Patent filed May 11, 2017.
Muir, R. and J. E. Heebner. 2017. "Single-Shot Optical Recorder with Sub-Picosecond Resolution and Scalable Record Length on a Semiconductor Wafer." Optics Letters 42 (21): 4414–4417. doi:10.1364/ol.42.004414. LLNL-JRNL-729266.
Heebner, E. 2017. “Arbitrary Pulse Shaping with Picosecond Resolution over Multiple Nanosecond Records,” U.S. Patent Application, PCT/US17/21887.
Muir, R. and J. Heebner. 2017b. "Single-Shot Optical Recorder with Sub-Picosecond Resolution and Scalable Record Length on a Semiconductor Wafer." Optics Letters 42, no. 21 (2017): 4414-417. doi:10.1364/ol.42.004414. LLNL-JRNL-729266.