Enhanced stimulated Raman scattering during intense laser propagation.

Stimulated Raman scattering is ubiquitous in many high-intensity laser environments. Parametric four-wave mixing between the pump and Raman sidebands can affect the Raman gain, but stringent phase matching requirements and strongly nonlinear dynamics obscure clear understanding of its effects at high laser powers. Here we investigate four-wave mixing in the presence of strong self-focusing and weak ionization at laser powers above the Kerr critical power.

Nonlinear underwater propagation of picosecond ultraviolet laser beams.

Meter-scale nonlinear propagation of a picosecond ultraviolet laser beam in water, sufficiently intense to cause stimulated Raman scattering (SRS), nonlinear focusing, pump-Stokes nonlinear coupling, and photoexcitation, was characterized in experiments and simulations. Pump and SRS Stokes pulse energies were measured, and pump beam profiles were imaged at propagation distances up to 100 cm for a range of laser power below and above self-focusing critical power. Simulations with conduction band excitation energy =9.

A laser-plasma accelerator driven by two-color relativistic femtosecond laser pulses.

A typical laser-plasma accelerator (LPA) is driven by a single, ultrarelativistic laser pulse from terawatt- or petawatt-class lasers. Recently, there has been some theoretical work on the use of copropagating two-color laser pulses (CTLP) for LPA research. Here, we demonstrate the first LPA driven by CTLP where we observed substantial electron energy enhancements.

Implementation of a long range, distributed-volume, continuously variable turbulence generator.

We have constructed a 180-m-long distributed, continuously variable atmospheric turbulence generator to study high-power laser beam propagation. This turbulence generator operates on the principle of free convection from a heated surface placed below the entire propagation path of the beam, similar to the situation in long-distance horizontal propagation for laser communications, power beaming, or directed energy applications. The turbulence produced by this generator has been characterized through constant-temperature anemometry, as well as by the scintillation of a low-power laser beam.

High-power lasers for directed-energy applications.

In this article, we review and discuss the research programs at the Naval Research Laboratory (NRL) on high-power lasers for directed-energy (DE) applications in the atmosphere. Physical processes affecting propagation include absorption/scattering, turbulence, and thermal blooming. The power levels needed for DE applications require combining a number of lasers.

Femtosecond laser-induced breakdown spectroscopy of surface nitrate chemicals.

Ultrashort laser-induced breakdown spectroscopy was used to detect the emission radiation from the breakdown of surface contaminants by a femtosecond laser pulse. This study focused on the detection of visible to near-infrared radiation signatures from molecular fragments of the nitro (NO(x)) group present in the breakdown plasma, where target chemicals of potassium nitrate (KNO(3)) and sodium nitrate (NaNO(3)) were used. Spectral signatures at a wavelength region around 410 nm were observed for both KNO(3) and NaNO(3), and were identified as the fluorescence transitions of the NO(x)-molecular structures.

Application of a scattered-light radiometric power meter.

The power measurement of high-power continuous-wave laser beams typically calls for the use of water-cooled thermopile power meters. Large thermopile meters have slow response times that can prove insufficient to conduct certain tests, such as determining the influence of atmospheric turbulence on transmitted beam power. To achieve faster response times, we calibrated a digital camera to measure the power level as the optical beam is projected onto a white surface.

Absorption and scattering of 1.06 microm laser radiation from oceanic aerosols.

The absorption and scattering of oceanic aerosols are characterized using low- and high-power lasers in the near IR (1.064 microm). The imaginary part of the refractive index of sea salt inferred from low-power absorption measurements is 200x less than the commonly accepted value from the literature.

Tunable, high peak power terahertz radiation from optical rectification of a short modulated laser pulse.

A new way of generating high peak power terahertz radiation using ultra-short pulse lasers is demonstrated. The optical pulse from a titanium:sapphire laser system is stretched and modulated using a spatial filtering technique to produce a several picosecond long pulse modulated at the terahertz frequency. A collinear type II phase matched interaction is realized via angle tuning in a gallium selenide crystal.

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.

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.