Stewart M. Cameron

High-power optical parametric chirped-pulse amplifier system with a 1.55??m signal and a 1.064??m pump

We have designed and built a chirped-pulse parametric-amplifier system that utilizes a 10 Hz, 300 ps, Nd:YAG pump laser system; a 1.575??m fiber oscillator and amplifier as the signal source; and rubidium titanyl phosphate and potassium titanyl arsenate nonlinear crystals. We obtained 260 fs, 30 mJ pulses centered at 1.550??m, representing a gain of >109 and a peak power of 100 GW. To our knowledge, these are the highest energy and peak power pulses ever produced in the 1.5–2.0??m region

Time-resolved measurements of self-focusing pulses in air

The spatial, spectral, and temporal properties of self-focusing 798-nm 100-fs pulses in air are experimentally measured with high-resolution, single-shot techniques at a set propagation distance of 10.91 m. The data, obtained with an initially collimated Gaussian beam, show significant evolution of spatial narrowing as well as temporal and spectral changes at intensities lower than those required for significant ionization of air.

Observation of nonlinear optical phenomena in air and fused silica using a 100 GW, 1.54 µm source

A 100-GW optical parametric chirped-pulse amplifier system is used to study nonlinear effects in the 1.54 mum regime. When focusing this beam in air, strong third-harmonic generation (THG) is observed, and both the spectra and efficiency are measured. Broadening is observed on only the blue side of the third-harmonic signal and an energy conversion efficiency of 0.2% is achieved. When propagated through a 10-cm block of fused silica, a collimated beam is seen to collapse and form multiple filaments. The measured spectral features span 400-2100 nm. The spectrum is dominated by previously unobserved Stokes emissions and broad emissions in the visible.

Optically activated switches for low jitter pulsed power applications

Optically activated high voltage switches are commonly used in pulsed power systems for reliable low jitter, multichannel and multiswitch (low inductance) applications. In addition to low jitter switching, optical activation provides a high degree of electrical isolation between the triggering and switching power systems simplifying pulsed power design. The disadvantages of optical triggering for large-gap gas switches are the optical energy, line-of-sight optics, and system maintenance required to obtain reliable operation. This paper describes two technologies which can reduce or eliminate these disadvantages and provide more flexible optically activated switches for pulsed power systems. One approach is to reduce the optical trigger energy requirement for gas gap switches. Shorter optical pulses require less energy to initiate a plasma discharge. An experiment is being assembled to trigger a 50-100 kV gas gap switch with 120 fs wide optical pulses. Lower trigger energy has also been demonstrated by the introduction of metallic aerosols into a gas gap W. Frey (1997). The apparatus will be added to this experiment to reproduce these results and determine the optical energy and power density requirements over a range of wavelengths and pulse widths. The status of this experiment will be discussed. A second approach uses solid state switching in two configurations: (1) as the main switch and (2) as a trigger. High-gain photoconductive semiconductor switches (PCSS) are practical for some direct pulsed power switching applications. We have demonstrated switching up to 220 kV and 8 kA. Higher power optically activated switching can be obtained by combining solid state and gas gap switching technologies. Multimegavolt (MMV) switches can be triggered with fiber-optically triggered, remotely located PCSS. By placing the compact PCSS trigger extremely close to the trigger point, reliable, low-jitter, high power switching is achieved with low energy fiber-optic trigger systems that can easily be controlled and adjusted from a remote control center. Power for the trigger system can be derived from the electrical fields near the trigger, so all electrical cables to the trigger system are eliminated and replaced with 100 micron diameter fibers that trigger and monitor the operation of the system. Results from experiments with PCSS triggered gas gap switches and the design for a PCSS triggered multimegavolt switch will be reported. PCSS switching properties including new picosecond pulse results and fabrication procedures for improved longevity will also be described. A 120 fs wide 780 nm laser pulse was used to radiate THz bandwidth pulses with a GaAs PCSS operating in the linear mode. New approaches for PCSS contact fabrication are being developed and tested to simplify the growth procedure, increase the current per filament capability, and improve device longevity. Progress continues to make PCSS a more useful component for pulsed power applications.

Bifurcation mode of relativistic and charge-displacement self-channelling

Stable self-channeling of ultra-powerful (P{sub 0} - 1 TW -1 PW) laser pulses in dense plasmas is a key process for many applications requiring the controlled compression of power at high levels. Theoretical computations predict that the transition zone between the stable and highly unstable regimes of relativistic/charge-displacement self-channeling is well characterized by a form of weakly unstable behavior that involves bifurcation of the propagating energy into two powerful channels. Recent observations of channel instability with femtosecond 248 nm pulses reveal a mode of bifurcation that corresponds well to these theoretical predictions. It is further experimentally shown that the use of a suitable longitudinal gradient in the plasma density can eliminate this unstable behavior and restore the efficient formation of stable channels.

Dynamics of optimized stable channel formation of intense laser pulses with the relativistic/charge-displacement mechanism

Studies of the dynamics of stable relativistic/ponderomotive channel formation demonstrate that the use of an appropriate longitudinal gradient in the electron density can significantly enhance the efficiency of the power compression. A unidirectional stable zone locking rule, which allows the operating point of the system to enter the region of stable channelled propagation, but blocks departures from it, is established. These characteristics are extremely favourable for kilovolt x-ray amplification, charged-particle acceleration and the initiation of nuclear reactions.

Adaptive remote sensing techniques implementing swarms of mobile agents

Measurement and signal intelligence of the battlespace has created new requirements in information management, communication and interoperability as they effect surveillance and situational awareness. In many situations, stand-off remote-sensing and hazard-interdiction techniques over realistic operational areas are often impractical and difficult to characterize. An alternative approach is to implement adaptive remote-sensing techniques with swarms of mobile agents employing collective behavior for optimization of mapping signatures and positional orientation (registration). We have expanded intelligent control theory using physics-based collective behavior models and genetic algorithms to produce a uniquely powerful implementation of distributed ground-based measurement incorporating both local collective behavior, and niter-operative global optimization for sensor fusion and mission oversight. By using a layered hierarchical control architecture to orchestrate adaptive reconfiguration of semi-autonomous robotic agents, we can improve overall robustness and functionality in dynamic tactical environments without information bottlenecking.

Laser filamentation of a femtosecond pulse in air at 400 nm

Summary form only given. Nonlinear propagation of ultrashort laser pulses in air has been a subject of great interest in recent years, with a large amount of work being done at 800nm due to the availability of broadband IR laser materials. Recent studies have begun to examine the nonlinear propagation of laser pulses at wavelengths other than 800nm. In this paper we present what to the best of our knowledge is the first rigorous measurements of the characteristics of laser filaments at 400 nm.

Dynamical Behavior of Multi-Robot Systems Using Lattice Gas Automata

Recent attention has been given to the deployment of an adaptable sensor array realized by multi-robotic systems (or swarms). Our group has been studying the collective, autonomous behavior of these such systems and their applications in the area of remote-sensing and emerging threats. To accomplish such tasks, an interdisciplinary research effort at Sandia National Laboratories are conducting tests in the fields of sensor technology, robotics, and multi- agents architectures. Our goal is to coordinate a constellation of point sensors using unmanned robotic vehicles (e.g., RATLERs, Robotic All-Terrain Lunar Exploration Rover- class vehicles) that optimizes spatial coverage and multivariate signal analysis. An overall design methodology evolves complex collective behaviors realized through local interaction (kinetic) physics and artificial intelligence. Learning objectives incorporate real-time operational responses to environmental changes. This paper focuses on our recent work understanding the dynamics of many-body systems according to the physics-based hydrodynamic model of lattice gas automata. Three design features are investigated. One, for single-speed robots, a hexagonal nearest-neighbor interaction topology is necessary to preserve standard hydrodynamic flow. Two, adaptability, defined by the swarms rate of deformation, can be controlled through the hydrodynamic viscosity term, which, in turn, is defined by the local robotic interaction rules. Three, due to the inherent nonlinearity of the dynamical equations describing large ensembles, stability criteria ensuring convergence to equilibrium states is developed by scaling information flow rates relative to a swarms hydrodynamic flow rate. An initial test case simulates a swarm of twenty-five robots maneuvering past an obstacle while following a moving target. A genetic algorithm optimizes applied nearest-neighbor forces in each of five spatial regions distributed over the simulation domain. Armed with this knowledge, the swarm adapts by changing state in order to avoid the obstacle. Simulation results are qualitatively similar to a lattice gas.

Observation of multiple pulse-splitting of ultrashort pulses in air

Summary form only given. It has been shown in 3 + 1 dimensional Kerr-nonlinearity self-focusing models, that group velocity dispersion is responsible for the temporal pulse-splitting of ultrashort pulses during propagation. Previous experiments have demonstrated pulse splitting due to the Kerr nonlinearity for short pulse propagation in bulks or gaseous media. However, studies in gaseous media are often in a focused geometry, or use pressurized gaseous media. This experiment elucidates the relationship between pulse splitting and spot-size change and does not use any optic to initiate self-focusing. We find pulse splitting occurs at a distance merely 0.7/spl times/ the diffraction length and occurs before spatial collapse to a filament. In addition, multiple pulse splitting is also observed. Peak fluence information from the beam-profile is monitored, indicating nonlinear loss mechanisms. We believe this is the first data on multiple pulse-splitting events in air.