Eldridge Briscoe

Remote atmospheric breakdown for standoff detection by using an intense short laser pulse

A remote atmospheric breakdown is a very rich source of UV and broadband visible light that could provide an early warning of the presence of chemical-biological warfare agents at extended standoff distances. A negatively chirped laser pulse propagating in air compresses in time and focuses transversely, which results in a rapid laser intensity increase and ionization near the focal region that can be located kilometers away from the laser system. Proof-of-principle laboratory experiments are performed on the generation of remote atmospheric breakdown and the spectroscopic detection of mock biological warfare agents. We have generated third harmonics at 267 nm and UV broadband radiation in air from the compression and focusing of femtosecond laser pulses. Fluorescence emission from albumin aerosols as they were illuminated by the femtosecond laser pulse has been observed.

Streamerless guided electric discharges triggered by femtosecond laser filaments

The time evolution of electrical discharges induced and guided between the cathode of a Van de Graf generator and a ground sphere was studied using a 100 fs Ti:Sapphire laser. Nonlinear focusing and ionization effects produce optical and plasma filaments in the discharge region. Streak camera images often exhibit streamers that propagate towards the cathode, but sudden discharge triggering is frequently observed with no streamer precursors. The typical discharge triggering delay time was measured to be 150 ns. Similar time delays were obtained from an air chemistry code used to model the long time behavior of the plasma induced by the short laser pulse. The model shows that ohmic heating of the filament plasma persists over long time scales and inhibits the decay of electron density due to recombination and attachment processes. Eventually the rise in electron temperature causes the avalanche rate to increase to the point where breakdown occurs. The hydrodynamic density reduction process reported by Tzortzakis et al. [Phys. Rev. E 64, 057401 (2001)] is also taken into consideration. Its main effect is found to be a hastening of the breakdown process.

Measurements of energetic electrons from the high-intensity laser ionization of gases

Electrons ionized from tightly bound atomic states by a high-intensity laser pulse can gain energies from one to millions of electron volts dependent on the intensity of the pulse. We have currently been investigating hundreds of kilovolt to megavolt electrons produced by ionization of krypton and argon with terawatt laser pulses. Angular and energy distributions have been measured to determine the usability of this electron source as an injector for higher energy accelerators. Studies have included pressure dependence, angular ejection angle energy dependence, and polarization dependence. In particular, the energy-dependent ejection angle of electrons has been used to produce electron beams with energies peaked at 600 keV. Numerical simulations of these electrons show that 4 MV electron beams with excellent beam quality and femtosecond pulse widths can be produced from this electron source using higher power laser pulses.

Longitudinal compression of short laser pulses in air

We have performed laboratory experiments to study long distance propagation of large bandwidth ultrashort laser pulses in air. Initial pulse length, frequency chirping, and laser pulse energy were varied where the maximum propagation distance was up to 105 m. We have demonstrated the compression of initially negatively chirped low intensity laser pulses due to the linear group velocity dispersion of air. The characteristics of the compressed pulse such as pulse duration and spectral chirping were found to be significantly affected by the laser pulse intensity, with higher intensities corresponding to longer minimum compressed pulse duration.

Direct characterization of self-guided femtosecond laser filaments in air

High-power femtosecond laser pulses propagating in air form self-guided filaments that can persist for many meters. Characterizing these filaments has always been challenging owing to their high intensity. An apparently novel diagnostic is used to directly measure the fluence distribution of femtosecond laser pulses after they have formed self-guided optical filaments in air. The diagnostic is unique in that the information contained in the filaments is not lost owing to the interaction with the apparatus. This allows filament characteristics such as energy and size to be unambiguously determined for the first time.

Measurements of intense femtosecond laser pulse propagation in air

The nonlinear self-focusing of an intense femtosecond pulse propagating in air can be balanced by the plasma defocusing as the laser intensity is increased above the threshold for multiphoton ionization. The resultant laser∕plasma filament can extend many meters, suitable for many applications such as remote atmospheric breakdown, laser induced electrical discharge, and femtosecond laser material interactions. Direct (bore-sight) measurements of filament size and fluence over 4 m showed a preservation of the total energy in the filament during propagation. This indicates the energy lost in creating the central plasma column through multiphoton ionization was continuously being replenished from the surrounding radiation. Electrical measurement of the filament conductivity estimated the plasma density to be 1×1016cm−3 and electrical discharges triggered by a femtosecond laser filament were found to occur at substantially reduced breakdown fields.

Ultraviolet light generation by intense laser filaments propagating in air

Sub-millimeter diameter filaments can develop from laser self-focusing in air. These filaments produce ultraviolet radiations in form of third harmonics and frequency upshifting. Using high-power laser we observed generation of third harmonic radiation in air.

Characterization of the third-harmonic radiation generated by intense laser self-formed filaments propagating in air

We perform laboratory experiments to study ultraviolet radiation generated by intense self-formed laser filaments produced by propagating high-power femtosecond laser pulses in air. The laser used in the experiment is a 0.5 TW Ti:sapphire system with the center wavelength at 800 nm. The observed ultraviolet emission occurs in the form of the third harmonic and frequency-upshifted radiation from the fundamental. We present direct characterization of the generated harmonic and frequency-upshifted radiation, including transverse imaging and spatially resolved spectral measurements.

Remote Underwater Laser Acoustic Source

Summary form only given. Through the mechanisms of nonlinear whole-beam self focusing (NSF) and linear group velocity dispersion (GVD), an ultrashort laser pulse can propagate relatively long distances underwater at moderate intensity (up to distances on the order of the attenuation length, approximately 10 meters in sea water), then quickly converge to an intense focus within a few centimeters at a predetermined remote location. Optical breakdown would then generate a plasma and an acoustic shock at this location. Such an acoustic source could be useful for sonar imaging and other Navy applications. Previous experiments at NRL indicate that ~1 mJ of coupled laser energy will produce a 200 dB, microsecond-timescale acoustic pulse, a source level more than adequate for high resolution acoustic imaging applications. This technique has the capability to improve on previous laser acoustic generation schemes in two important ways: 1) the present scheme allows for laser propagation through many meters of water, and 2) the photoacoustic energy conversion efficiency can be on the order of tens of percent for optical breakdown, versus 10-4 or less for other schemes relying on thermal expansion of water. The NRL research program aims to study the physics of intense underwater laser propagation and acoustic generation, including: GVD; nonlinear refractive index effects such as NSF, filamentation, and self-phase modulation; scattering; absorption; and laser-induced breakdown. NRL FY05 experiments include efforts to generate and tailor an appropriate frequency-chirped pulse at 400 nm, to measure the GVD of water, and to demonstrate GVD/NSF-induced pulse compression. Broadband 2nd harmonic generation at 400 nm with conversion efficiency up to 12%, and pulse energies up to 2 mJ has been demonstrated. NRL intense underwater laser propagation simulations predict an upper limit on initial pulse power due to beam filamentation instability of 15 Pnsf, where Pnsf=lambda2 /2pin0n2. A lens-aided compression scheme is under investigation which may increase this limit. Initial experimental and simulation results will be presented

Direct measurements of the dynamics of self-guided femtosecond laser filaments in air

Imaging of femtosecond laser filaments is accomplished by utilizing a recently developed diagnostic which is not damaged by the filaments and which greatly reduces the nonlinearities in the detection system. This calibrated detection system allows quantities such as the filament energy and fluence distribution to be determined with greater detail, accuracy, and confidence than was previously possible. The diagnostic is placed on a rail so that filament images can be obtained at a large number of positions along the propagation path. It is found that filaments formed from 450-fs pulses carry higher fluence and propagate farther than filaments formed from 50-fs pulses.