ltra-short (femtosecond), high-power laser pulses can exceed the threshold for nonlinear self-focusing in air. This results in an extended propagation from the dynamical balance between the plasma formation and the nonlinear focusing. Experiments were performed using the chirped-pulse-amplification (CPA) lasers in the Plasma Physics Division to study the physics of extended propagation in air and its effects on atmospheric breakdown, laser-induced electrical discharge, and chemical/biological (chem/bio) agent detection. Self-guiding of the laser beams for extended distances and formation of multiple laser and plasma filaments were observed. Time-resolved images of laser-induced electrical discharges showed the initiation and sustention of the discharges by the plasma filaments. Measured optical spectra of the white light generated in the laser propagation revealed the presence of molecular plasmas that are useful for identifying chem/bio agents. Potential applications include directed energy weapons, remote sensing for both chem/bio defense, and environmental air pollutant monitoring.

INTRODUCTION

Ultra-high-power lasers that can deliver intense radiation have raditionally resided in a few, very large national laboratories. This is because more energy is usually required as the power of the laser increases, and thus the size of the laser correspondingly increases. Therefore, research into the physics associated with intense radiation from these ultra-high-power lasers could only be carried out at these large institutions. In addition, the size and cost of the lasers severely limited the range of potential applications. This has all changed during the last decade when a new way of generating high-power lasers was discovered. This simple but effective "trick" to increase laser power starts with the recognition that power is, by definition, energy per unit time. Instead of increasing the energy carried in a laser pulse for a fixed time duration to obtain higher power, one can produce the same laser power if one decreases the pulse duration while maintaining the same amount of energy in it. By utilizing ultra-short laser pulses with durations as short as a few tens of femtoseconds (thousand-trillionth of a second), laser pulses with power as high as tens of terawatts (trillion watt) can now be obtained by using table-top sized laser systems. Research on these lasers can now be performed in reasonably sized laboratories, and many potential applications are envisioned.

Many interesting phenomena are associated with the interactions of these very intense and short laser pulses with various media. In particular, the propagation of a short intense laser pulse in a gas such as air is very different from that of a long or continuous wave (CW) laser pulse. For example, the high intensity of these pulses can produce nonlinear contributions to the index of refraction of the medium. The intensity of the laser pulse could also become so high that the air molecules would ionize and form a plasma. The inter-play between the laser pulse and the plasma that it creates can be very complicated and can profoundly affect the evolution of the laser pulse as it propagates through the atmosphere. Experiments using ultra-short (~100 fs), high intensity (>10¹³W/cm²) laser pulses have demonstrated long-distance self-guided atmospheric propagation,¹ air breakdown, filamentation, and white light generation. Intense, directed white light pulses have been generated and backscattered from atmospheric aerosols. The generation of pulsed THz radiation in plasma channels formed by femtosecond pulses has also been observed and analyzed. Although many of the observations cannot be completely explained, the experimental, theoretical, and numerical results obtained to date indicate potential applications for both passive and active remote sensing and induced electric discharges, among others. In addition, the individual micropulses in a shipboard free electron laser (FEL) system may exhibit short-pulse propagation characteristics. To achieve these potential applications, it is necessary to have a comprehensive and quantitative understanding of the physical mechanisms that govern the propagation of intense, short laser pulses in air.

The following sections begin with a description of the table-top ultra-high-power lasers in the laboratory. Next, the physics of propagating femtosecond terawatt laser pulses in air is discussed, with experimental demonstration of the novel phenomena of self-guided laser filaments and numerical verification of the experimental results. These filaments and the associated broadband radiation that they generate can be used in the remote sensing of cal/biological agents in defense or anti-terrorism applications or detecting hazardous air pollutants in environmental monitoring and enforcement. The plasma filaments associated with a self-guided femtosecond intense laser pulse can also be used for triggering high-voltage electrical discharges. This phenomenon is next discussed, with emphasis on the discharge initiation mechanism, by studying the time evolution of the discharge. There is considerable interest worldwide in studying this phenomenon so that it can be applied to areas such as lightning arrest around power plants. The concluding section summarizes research efforts at NRL in studying ultra-short intense laser pulse propagation in air.

NRL T³ AND TFL LASERS

The NRL High Field Physics Laboratory was one of the first laboratories to have a table-top terawatt (TW) laser system soon after the invention of the chirped pulse amplification (CPA) method. The T³ laser was installed in 1992 as the first commercial CPA laser ever built. It has been upgraded several times over the years and is still a state-of-the-art CPA laser. It is a solid state laser involving two lasing media, titanium-doped sapphire (Ti:sapphire) and neodymium-doped glass (Nd:glass). The lasing wavelength is in the infrared at 1054 nanometers (nm). Like most lasers, it consists primarily of a laser oscillator that generates the seed laser pulse and then a series of laser amplifiers to boost the energy in the pulse. The difference is that the seed laser pulse has a pulse length of only 100 femtoseconds (fs). As the laser pulse is amplified, the power and intensity of the pulse continuously increases and eventually will reach the breakdown threshold of the laser glass medium. To avoid such disastrous consequences, the CPA technique stretches the laser pulse after the oscillator with a diffraction grating to ~10,000 times and thus reduces the laser intensity and power by the same factor. The stretched pulse can now be safely amplified to the desired high energy per pulse. The stretching process is then reversed by re-compressing the amplified pulse with diffraction gratings in air or vacuum to produce a high-power, ultra-short pulse. One interesting observation is that in the stretched pulse, the frequency content of the pulse is arranged such that the high-frequency (pitch) components are moved to the back of the stretched pulse, reminiscent of the chirped tune of a singing robin, and hence the "chirped" pulse amplification technique.

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