Tradeoff between the Brillouin and transverse mode instabilities in Yb-doped fiber amplifiers.

The Brillouin instability (BI) due to stimulated Brillouin scattering (SBS) and the transverse (thermal) mode instability (TMI) due to stimulated thermal Rayleigh scattering (STRS) limit the achievable power in high-power lasers and amplifiers. The pump power threshold for BI increases as the core diameter increases, but the threshold for TMI may decrease as the core diameter increases. In this paper, we use a multi-time-scale approach to simultaneously model BI and TMI, which gives us the ability to find the fiber diameter with the highest power threshold. We formulate the equations to compare the thresholds of the combined and individual TMI and BI models. At the pump power threshold and below, there is a negligible difference between the full and individual models, as BI and TMI are not strong enough to interact with each other. The highest pump threshold occurs at the optimal core size of 43 µm for the simple double-clad geometry that we considered. We found that both effects contribute equally to the threshold, and the full BI and TMI model yields a similar threshold as the BI or TMI model alone. However, once the reflectivity is sufficiently large, we find in the full BI and TMI model that BI may trigger TMI and reduce the TMI threshold to a value lower than is predicted in simulations with TMI alone. This result cannot be predicted by models that consider BI and TMI separately. Our approach can be extended to more complex geometries and used for their optimization.

Accurate and efficient modeling of the transverse mode instability in high energy laser amplifiers.

We study the transverse mode instability (TMI) in the limit where a single higher-order mode (HOM) is present. We demonstrate that when the beat length between the fundamental mode and the HOM is small compared to the length scales on which the pump amplitude and the optical mode amplitudes vary, TMI is a three-wave mixing process in which the two optical modes beat with the phase-matched component of the index of refraction that is induced by the thermal grating. This limit is the usual limit in applications, and in this limit TMI is identified as a stimulated thermal Rayleigh scattering (STRS) process. We demonstrate that a phase-matched model that is based on the three-wave mixing equations can have a large computational advantage over current coupled mode methods that must use longitudinal step sizes that are small compared to the beat length.

MeV electron acceleration at 1 kHz with <10 mJ laser pulses: erratum.

In this erratum the funding section of Opt. Lett.42, 215 (2017)OPLEDP0146-959210.1364/OL.42.000215 has been updated.

MeV electron acceleration at 1 kHz with <10 mJ laser pulses.

We demonstrate laser-driven acceleration of electrons to MeV-scale energies at 1 kHz repetition rate using <10 mJ pulses focused on near-critical density He and H2 gas jets. Using the H2 gas jet, electron acceleration to ∼0.5 MeV in ∼10 fC bunches was observed with laser pulse energy as low as 1.3 mJ. Increasing the pulse energy to 10 mJ, we measure ∼1 pC charge bunches with >1 MeV energy for both He and H2 gas jets.

Generation of axially modulated plasma waveguides using a spatial light modulator.

We demonstrate the generation of axially modulated plasma waveguides using spatially patterned high-energy laser pulses. A spatial light modulator (SLM) imposes transverse phase front modulations on a low-energy (10 mJ) laser pulse which is interferometrically combined with a high-energy (130-450 mJ) pulse, sculpting its intensity profile. This enables dynamic and programmable shaping of the laser profile limited only by the resolution of the SLM and the intensity ratio of the two pulses. The plasma density profile formed by focusing the patterned pulse with an axicon lens is likewise dynamic and programmable. Centimeter-scale, axially modulated plasmas of varying shape and periodicity are demonstrated.

Multi-MeV Electron Acceleration by Subterawatt Laser Pulses.

We demonstrate laser-plasma acceleration of high charge electron beams to the ∼10 MeV scale using ultrashort laser pulses with as little energy as 10 mJ. This result is made possible by an extremely dense and thin hydrogen gas jet. Total charge up to ∼0.5 nC is measured for energies >1 MeV. Acceleration is correlated to the presence of a relativistically self-focused laser filament accompanied by an intense coherent broadband light flash, associated with wave breaking, which can radiate more than ∼3% of the laser energy in a ∼1 fs bandwidth consistent with half-cycle optical emission. Our results enable truly portable applications of laser-driven acceleration, such as low dose radiography, ultrafast probing of matter, and isotope production.

Shock formation in supersonic cluster jets and its effect on axially modulated laser-produced plasma waveguides.

We examine the generation of axially modulated plasmas produced from cluster jets whose supersonic flow is intersected by thin wires. Such plasmas have application to modulated plasma waveguides. By appropriately limiting shock waves from the wires, plasma axial modulation periods can be as small as 70 µm, with plasma structures as narrow as 45 µm. The effect of shocks is eliminated with increased cluster size accompanied by a reduced monomer component of the flow.