Standoff spectroscopy via remote generation of a backward-propagating laser beam

Texas A&M University, College Station, TX 77843, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.81). 02/2011; 108(8):3130-4. DOI: 10.1073/pnas.1014401107
Source: PubMed

ABSTRACT In an earlier publication we demonstrated that by using pairs of pulses of different colors (e.g., red and blue) it is possible to excite a dilute ensemble of molecules such that lasing and/or gain-swept superradiance is realized in a direction toward the observer. This approach is a conceptual step toward spectroscopic probing at a distance, also known as standoff spectroscopy. In the present paper, we propose a related but simpler approach on the basis of the backward-directed lasing in optically excited dominant constituents of plain air, N(2) and O(2). This technique relies on the remote generation of a weakly ionized plasma channel through filamentation of an ultraintense femtosecond laser pulse. Subsequent application of an energetic nanosecond pulse or series of pulses boosts the plasma density in the seed channel via avalanche ionization. Depending on the spectral and temporal content of the driving pulses, a transient population inversion is established in either nitrogen- or oxygen-ionized molecules, thus enabling a transient gain for an optical field propagating toward the observer. This technique results in the generation of a strong, coherent, counterpropagating optical probe pulse. Such a probe, combined with a wavelength-tunable laser signal(s) propagating in the forward direction, provides a tool for various remote-sensing applications. The proposed technique can be enhanced by combining it with the gain-swept excitation approach as well as with beam shaping and adaptive optics techniques.

1 Follower
  • Source
    • "[6] "
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on the lasing action of atmospheric air pumped by an 800 nm femtosecond laser pulse with peak power up to 4 TW. Lasing emission at 428 nm increases rapidly over a small range of pump laser power, followed by saturation above ~ 1.5 TW. The maximum lasing pulse energy is measured to be 2.6 uJ corresponding to an emission power in the MW range, while a maximum conversion efficiency of is measured at moderate pump pulse energy. The optical gain inside the filament plasma is estimated to be excess of 0.7/cm. The lasing emission shows a doughnut profile, reflecting the spatial distribution of the pump-generated white-light continuum that acts as a seed for the lasing. We attribute the pronounced saturation to the defocusing of the seed in the plasma amplifying region and to the saturation of the seed intensity.
    Optics Letters 04/2014; 39(7):1725-8. DOI:10.1364/OL.39.001725 · 3.18 Impact Factor
  • Source
    • "This scheme has the ability to produce a backward propagating infrared pulse at 845 nm and has been experimentally observed on a small scale in the lab in ambient air [5] [6]. Furthermore, a different method involving lasing in atmospheric nitrogen has also been proposed [3] [7]. This scheme uses a femtosecond pulse to create a filament at a specified distance by using a temporal prechirp. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Remote chemically specific detection of trace impurities in the atmosphere from distances on the order of kilometers is an important problem from both an environmental and a national defense viewpoint. A new scheme is discussed consisting of the remote generation of a backward propagating stimulated Raman pulse. This pulse is then used to drive a coherent anti-Stokes Raman scattering scheme, resulting in a strong chemically specific signal propagating back to the detector.
    Journal of Modern Optics 08/2013; 61(1). DOI:10.1080/09500340.2013.846428 · 1.17 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The ability to reliably detect hazardous airborne biological materials is an inherently difficult and important problem for Defence (i.e., force protection), national security (i.e., detection of anthro-pogenic and natural hazards, evaluation of risks for first responders) and for ecological monitoring (i.e., air pollution). There are a multitude of approaches to the problem of biological detection, all of which differ in sophistication, sensitivity, efficiency, response time, deployment feasibility and cost. A key chal-lenge of providing sound scientific advice on a prototype detection system is the ability to evaluate against these criteria in order to select the optimal solution for the operational requirements. An example of such a study is presented in the current paper. 1 release point (a) Vignette four release impact (probability of infection) 10 3 10 4 10 5 0.1 1 10 Q(x) distance (km) simulation theory (p = 1 8) Q(x) ∝ x − 2+p 1+2p (b) Comparison between theory and simulation Figure 1. (a) Simulation results of the release impact (vignette four, atmospheric stability condition -neutral). Shown in (b) is a comparison of the theoretical scaling of active sample collection (Eq. 11) to simulation results for the release in (a) along the indicated trajectory. Aerosol sample collection is a necessary step for many airborne biological detection systems. In these detection systems, detection is only realised when collection is combined with identification. Aerosol collection can be conditionally subdivided into two major groups: passive and active. Passive methods usually include a collection area onto which hazardous material can deposit via turbulent diffusion. Active methods rely on an active (or forced) air collection, whereby a significantly larger volume of air is sampled by a driven infiltration process. As such, active sampling systems are relatively faster and more efficient but much more costly, difficult to deploy and less scalable than passive systems. In this paper we compare the performance of active and passive methods of aerosol collection. We demon-strate how a release of hazardous material can be modelled as a tracer puff that evolves downstream in accordance with the analytical framework of turbulent dispersion, and incorporates the basic meteorologi-cal conditions of the atmospheric boundary layer. We propose a simple physics based model for active and passive aerosol collection where the sampling time is constrained by the travelling time of the hazardous plume over the sampling point. We employ these models to estimate and compare relevant parameters of active and passive systems of aerosol collection analytically. Drawing on the simulation results from other DSTO work which examined the effects of feasible biological releases into the environment, we can validate some of these trends numerically.
Show more


Available from