Chengyong Feng

Optimizing sub-ns pulse compression for high energy application.

We demonstrate ∼ 40X pulse compression (down to ∼ 300 ps) with ∼ 1 joule, nanosecond pulses for high energy applications requiring ≥ 1 gigawatt of peak power. Our method is based on the established principle of stimulated Brillouin scattering (SBS). To push the SBS technique to its highest peak-power limit, a combination of theoretical modeling and experiments is used to identify and optimize all critical parameters, including optical configuration, interaction length, intensity matching, choice of gain medium and thermal stability. Pulse compression results are presented both at 1064 nm and 532 nm, with performances close to the theoretical limit and excellent shot-to-shot reproducibility.

Ring-shaped backward stimulated Raman scattering driven by stimulated Brillouin scattering

Backward stimulated Raman scattering is generated in water, pumped by pre-compressed pulses from a single-cell stimulated Brillouin scattering pulse compressor. The maximum energy efficiency of 9% is achieved by employing a circularly-polarized pump pulse at its energy of 50 mJ, around which point the backward stimulated Raman scattering also exhibits a ring-shaped profile. The correlations between spatial and temporal profiles as well as the intensities of the backward stimulated Raman and the stimulated Brillouin scattering generated from Raman cell indicate that the ring-shaped backward stimulated Raman is driven by intense stimulated Brillouin scattering. We demonstrate the latter process to be much more efficient for the backward Raman generation than the conventional process in which the laser itself pumps a backward stimulated Raman beam. It is shown that a further increase in pump energy leads to a drop in efficiency, combined with a break-up of the ring pattern of backward stimulated Raman. These effects are associated with filament generation above a certain threshold.

Spatio–temporal characterization of pulses obtained from a high-energy sub-nanosecond laser system

Spatio-temporal profiles of laser pulses, obtained from each stage of a high-energy sub-nanosecond laser system, are investigated. The laser system is composed of a Q-switched Nd:YAG unstable oscillator, a chain of Nd:YAG amplifiers, a second-harmonic generator, and a high-energy pulse compressor based on stimulated Brillouin scattering (SBS). A curved energy front, i.e., the pulses emerging away from the beam center being gradually delayed from the center pulse, is shown to originate from the unstable oscillator. Our comparative study shows that injection seeding will enlarge the energy front curvature, via reduction of the effective gain. After the laser amplifiers, the energy front curvature is more than doubled due to the gain saturation effect. The latter also modifies the spatial pulse width distribution. While there is a negligible pulse duration spread across the oscillator beam, the amplified pulses are found to have gradually reduced pulse duration away from the beam center. More interestingly, after the SBS pulse compression, not only the pulse width but also the delay is compressed down. This is, to the best of our knowledge, the first study of the spatio-temporal profile of the SBS compressed pulse. To compare with the experiments, two numerical models are developed to simulate the evolution of spatio-temporal profiles within the Nd:YAG laser system and during the SBS pulse compression, respectively. The first model is demonstrated to reproduce the experimental results very well, while the second model predicts part of the features of the SBS compressed pulse. The limitation on the latter is discussed.

Videos of light filamentation in air

Light filaments are of interest for applications in remote sensing, communications, and the control of electronic discharges. Different plasma dynamics and emitted radiation have been observed according to the initial pulse characteristics and beam collimation. To help observing and understanding these observations, a new technique of creating a movie of the moving light pulse and the plasma emission in its wake is presented. Over 1,000 synchronized frames of a streak camera are combined to produce the 4D (2 D space, time in ps and wavelength) movie.

High-energy sub-phonon lifetime pulse compression by stimulated Brillouin scattering in liquids

Multi-Joule level stimulated Brillouin scattering (SBS) pulse compression below the acoustic phonon lifetime is demonstrated with a energy-scalable generator-amplifier setup. Single-pass compression of pulses longer than 20τB (τB as phonon lifetime) to as short as 0.5τB with ~100 mJ pulse energy is realized from the generator, by choosing the focusing length to match approximately with the full length at half maximum of the input Gaussian pulses. The interaction length is identified, both experimentally and numerically, as the key parameter in achieving sub-phonon lifetime pulse compression, with its main mechanism being the steepening of the Stokes pulse trailing edge via energy exchange process. After combining the generator with an amplifier that involves only collimated beams and serves as energy booster, the compression of 9 ns, 2 J pulses at 532 nm into 170 ps, 1.3 J per pulse is achieved in water, with very good stability in both pulse energy and duration. This work demonstrates for the first time the efficient high-energy SBS sub-phonon lifetime pulse compression, and paves a way to the reliable generation of sub-200 ps laser pulses with Joule-level energy.

Exploiting shock wave and self-absorption for high resolution laser induced breakdown spectroscopy

The shock wave created by a high energy UV filament is sufficient create a low pressure absorber enabling higher resolution laser induced breakdown spectroscopy, through reduction or pressure broadening. Isotopic selectivity is demonstrated.

HV discharge acceleration by sequences of UV laser filaments with visible and near-infrared pulses

We investigate the triggering and guiding of DC high-voltage discharges over a distance of 37 cm by filaments produced by ultraviolet (266 nm) laser pulses of 200 ps duration. The latter reduce the breakdown electric field by half and allow up to 80% discharge probability in an electric field of 920 kV m–1. This high efficiency is not further increased by adding nanosecond pulses in the Joule range at 532 and at 1064 nm. However, the latter statistically increases the guiding length, thereby accelerating the discharge by a factor of 2. This effect is due both to photodetachment and to the heating of the plasma channel, that increases the efficiency of avalanche ionization and reduces electron attachment and recombination.

Beam control through nonlinear propagation in air and laser induced discharges

Various models of filamentation in air are presented and the remaining theoretical challenges are pointed out. Means to extend the range of filaments are reviewed and proposed. Filaments offer promise of guiding electrical discharges over large gaps. Experiments of UV filament induced discharge are presented.

Resolving isotopic emission lines using a spatial heterodyne spectrometer

A modified Spatial Heterodyne Spectrometer (SHS) is used for measuring atomic emission spectra for instance in LIBS experiments. This device is basically a Fourier Transform Spectrometer, but the Fourier transform is taken in the directions perpendicular to the optical propagation and heterodyned around one specific wavelength. Therefore, this device uses no movable parts. The resolution of the device is sufficient to distinguish common isotopic lines.

Generation of 300 ps laser pulse with 1.2 J energy at 532 nm by stimulated Brillouin scattering in water

We experimentally demonstrate SBS pulse compression in water from 10 ns, 2.3 J to 300 ps, 1.2 J. To our best knowledge, this is the highest compressible energy that has been achieved at 532 nm.