The Fourth-Generation Laser for Ultra-broadband Experiments (FLUX)

Solid-state, high-energy lasers, based on neodymium-doped glasses, dependably support high-energy laser−matter interactions. Because they operate in the near-infrared (NIR) wavelength range, an unfavorable one for laser–matter interaction, high-efficiency nonlinear frequency conversion to the ultraviolet (UV) is required at the end of these systems. Third-harmonic generation (THG) on lasers such as the National Ignition Facility, Laser Mégajoule, and OMEGA is a complex process. It relies on second-harmonic generation of the fundamental pulse from 1ω (1053 nm) to 2ω (526.5 nm) followed by nonlinear mixing of this new 2ω pulse with the unconverted fraction of the initial 1ω pulse, which leads to an output pulse at 3ω (351 nm). Potassium dihydrogen phosphate (KDP) and partially deuterated KDP (DKDP) are the only nonlinear crystals that can be grown with sufficient optical quality at apertures large enough for frequency conversion of kilojoule pulses. Owing to their relatively low nonlinearity (~0.2 pm/V at the typical phase-matching angles for mixing 1ω and 2ω pulses), and the need to operate at intensities below the laser-damage threshold, requires that, for efficient frequency conversion of nanosecond pulses, these KDP nonlinear crystals be relatively thick, typically of the order of 1 cm, which restricts the achievable fractional bandwidth Δω/ω below 0.1%.

Simulations show that broadband spectrally incoherent pulses can mitigate laser–plasma instabilities that undermine laser−matter interaction. There is currently no laser facility that can produce high-energy pulses with sufficient fractional bandwidth (~1%, i.e., 10 THz at 351 nm) to significantly damp laser−plasma instabilities. Up until now, no practical scheme has been demonstrated for frequency converting spectrally incoherent pulses from IR to UV, the wavelength range where most of the high-energy physics supported by solid-state lasers has been performed.

The FLUX laser is one potential path to increased-bandwidth, next-generation inertial confinement fusion drivers that will support high-energy laser target experiments at unprecedented fractional bandwidth Δω/ω > 1 %. The FLUX laser will rely on optical parametric amplification (OPA) and sum-frequency-generation (SFG) in a novel noncollinear angularly dispersed scheme to generate high-energy, spectrally incoherent 3ω pulses (~200 J). These technologies have been thoroughly investigated via simulations and proof-of-concept experiments. Other technologies for producing high-energy broadband UV pulses, including stimulated rotational Raman scattering and excimer lasers, have not yet been demonstrated at sufficient bandwidth.

The results of the FLUX technology demonstration and experimental campaigns will support the design of an upgraded OMEGA with 60 broadband beams. The FLUX laser is currently under construction with an estimated completion of the laser by the end of 2023, and shots to target by 2024/2025.

This work, which supports the exploration of inertial confinement fusion as a future source of energy, and the development of new laser and materials technologies, is funded by the Department of Energy National Nuclear Security Administration, the Department of Energy Office of Science, the University of Rochester, and the New York State Energy Research and Development Authority.