Millisecond-long suppression of spectroscopic optical signals using laser filamentation.
TL;DR: It is demonstrated that the transient change in refractive index associated with relaxation of the gas can be used to reject both continuous and time-varying spectroscopic signals, including emission from laser-produced plasmas, which can augment temporal gating of simple optical detectors.
Abstract: Ultrashort laser pulse filamentation in air can extend the delivery of focused laser energy to distances greatly exceeding the Rayleigh length. In this way, remote measurements can be conducted using many standard methods of analytical spectroscopy. The performance of spectroscopic techniques can be enhanced by temporal gating, which rejects the unwanted noise and background. In the present work, we investigate the thermal relaxation of air in the wake of single-filament plasmas using shadowgraphy. We demonstrate that the transient change in refractive index associated with relaxation of the gas can be used to reject both continuous and time-varying spectroscopic signals, including emission from laser-produced plasmas. This method can augment temporal gating of simple optical detectors.
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TL;DR: In this paper , the authors directly measured and simulated the dynamics of water droplets with a radius of about 5 μ m, typical of fog, under the influence of optical and acoustic interactions that are characteristic of femtosecond filaments.
Abstract: Atmospheric aerosols, such as water droplets in fog, interfere with laser propagation through scattering and absorption. Femtosecond optical filaments are shown to clear foggy regions, improving the transmission of subsequent pulses. However, the detailed fog-clearing mechanism had yet to be determined. Here, we directly measure and simulate the dynamics of water droplets with a radius of about 5 μ m, typical of fog, under the influence of optical and acoustic interactions that are characteristic of femtosecond filaments. We find that, for filaments generated by the collapse of collimated near-infrared femtosecond pulses, the main droplet-clearing mechanism is optical shattering by laser light. For such filaments, the single-cycle acoustic wave launched by filament-energy deposition in air leaves droplets intact and drives negligible transverse displacement, and therefore, negligible fog clearing. Only for tightly focused nonfilamentary pulses, where local energy deposition greatly exceeds that of a filament, do acoustic waves significantly displace aerosols.
2 citations
TL;DR: In this paper , the cumulative effects of high-repetition-rate pulse trains have been investigated for short plasma filaments produced by pulses of moderate energy (0.4mJ) and repetition rates up to 100 kHz.
Abstract: Cumulative effects are crucial for applications of laser filaments, such as for the remote transfer of energy and the control of electric discharges. Up to now, studies of cumulative effects in the air of high‐repetition‐rate pulse trains have been performed at lower rates than 10 kHz. Herein, the nonlinear effects associated with short plasma filaments produced by pulses of moderate energy (0.4 mJ per pulse) and repetition rates up to 100 kHz are experimentally characterized. With increasing repetition rate, a decrease in absorption, fluorescence emission, and breakdown voltage and concurrently an increase in peak intensity and third‐harmonic‐generation efficiency are observed. Hydrodynamic simulations of the heated gas show that the observed decreases are directly related to a quasi‐stationary state of reduced gas density in the filament. However, further investigations are required to fully understand the physics underpinning the observed sharp reduction of the breakdown voltage at 100 kHz repetition rates. The results may prove relevant for energy and information delivery applications by laser‐induced air waveguide or electric discharge and lightning control.
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TL;DR: The self-channeling of ultrashort laser pulses through 20 m of air was demonstrated and a preliminary model is shown to explain these results.
Abstract: The self-channeling of ultrashort laser pulses through 20 m of air was demonstrated. The channeled pulse was measured to have 0.75 mJ of energy, a diameter of 80 microm FWHM, and a modulated spectrum. All these values were measured to be fairly constant during the propagation of the pulse. A preliminary model is shown to explain these results.
1,205 citations
TL;DR: In supplement to Edlén's recently revised formula for relative refractivity, the density dependence of refractive index is re analyzed and new formulas are presented which are more useful over a wide range of pressure, temperature, and composition than any presently available.
Abstract: The theoretical background and present status of formulas for the refractive index of air are reviewed. In supplement to Edlen's recently revised formula for relative refractivity, the density dependence of refractive index is reanalyzed. New formulas are presented for both phase and group refractive index which are more useful over a wide range of pressure, temperature, and composition than any presently available. The application of the new formulas to optical distance measuring is briefly discussed.
543 citations
TL;DR: The experiment shows that external focusing strongly influences the plasma density and the diameter of femtosecond Ti-sapphire laser filaments generated in air and is in good qualitative agreement with the results of numerical simulations.
Abstract: Our experiment shows that external focusing strongly influences the plasma density and the diameter of femtosecond Ti-sapphire laser filaments generated in air. The control of plasma filament parameters is suitable for many applications such as remote spectroscopy, laser induced electrical discharge, and femtosecond laser material interactions. The measurements of the filament showed the plasma density increases from 10(15)cm(-3) to 2 x 10(18)cm(-3) when the focal length decreases from 380 cm to 10 cm while the diameter of the plasma column varies from 30 microm to 90 microm. The experimental results are in good qualitative agreement with the results of numerical simulations.
303 citations
TL;DR: In this article, the authors demonstrate remote elemental analysis at distances up to 90m, using a laser-induced breakdown spectroscopy scheme based on filamentation induced by the nonlinear propagation of unfocused ultrashort laser pulses.
Abstract: We demonstrate remote elemental analysis at distances up to 90m, using a laser-induced breakdown spectroscopy scheme based on filamentation induced by the nonlinear propagation of unfocused ultrashort laser pulses. A detailed signal analysis suggests that this technique, remote filament-induced breakdown spectroscopy, can be extended up to the kilometer range.
248 citations
06 Jun 2003
TL;DR: In this article, the backscattered fluorescence of nitrogen from long filaments generated by intense ultrafast Ti-sapphire laser pulses propagating in air is studied and it shows an exponential increase with increasing filament length, indicating amplified spontaneous emission (ASE).
Abstract: The backscattered fluorescence of nitrogen from long filaments generated by intense ultrafast Ti-sapphire laser pulses propagating in air is studied It shows an exponential increase with increasing filament length, indicating amplified spontaneous emission (ASE)
230 citations