Publisher's Note: This paper, originally published on 12 March 2024, was replaced with a corrected/revised version on 17 April 2024. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
Modern USPL (Ultra Short Pulse Laser) development is trending towards higher repetition rates and higher average power systems. High peak power, low repetition rate USPLs have long been used to generate laser filaments, which consist of a plasma channel and region of focused high intensity propagation. Filamentation leads to heat deposition in the air from linear and nonlinear effects, producing a gas density depression that persists over hydrodynamic timescales (milliseconds). This is long after the femtosecond pulse has passed. In the “single shot” (approximately 10 Hz) regime of filamentation, the time between pulses allows the air density to return to equilibrium before the next pulse arrives. Prior work has experimentally measured the single shot gas density depression via interferometry and demonstrated that high repetition rate filamentation leads to deflection of subsequent pulses due to residual heating from the prior pulses. This work experimentally examines USPL thermal blooming as a function of laser repetition rate. Residual heating effects between pulses are demonstrated through measurements of the energy deposition by the laser filament. The temporally and spatially resolved energy deposition is extracted from interferometric measurements of the phase shift due to the gas density depression. Comparison is made between experimentation and modeling, as well as verification of past results. This work demonstrates how atmospheric propagation of modern high average power, high repetition rate USPL pulses differ from traditional single shot USPL systems.
The majority of filamentation studies have focused on near infrared (NIR) filaments, which have been demonstrated to propagate over many times the Rayleigh range with clamped intensity, electron plasma density, and beam diameter. Long wavelength infrared (LWIR) laser light sources, however, have not been extensively studied for filamentation. Here, we discuss filamentation in both wavelength regimes and introduce a new ultrafast CO2 system capable of producing high-power 10 µm picosecond pulses. Future work is outlined which will significantly increase the output power of the CO2 laser for upcoming studies and allow direct comparison of NIR and LWIR filaments.
Filaments, formed by ultrashort pulsed laser (USPLs) with high peak powers, deliver high intensities and a plasma channel to km-scale distance, without the need for focusing elements. These properties make them viable for long-range outdoor applications, including propagation to or at high altitudes where air pressure is a fraction of that at sea level. Since filament formation and characteristics are known to vary with air pressure, here, we analyze how critical filamentation thresholds and properties change as pressure decreases, through experiment and simulation. This study indicates that filament applications are indeed feasible over long distances to or at high altitudes.
Laser filaments generated by ultrashort pulse (USP) lasers achieve diffractionless propagation for distances surpassing the Rayleigh distance, making them highly beneficial to long-range outdoor applications. However, filaments generated by a single USP are limited to a clamped electron density, intensity, and lifetime. Here, we demonstrate how spatial and temporal engineering can overcome these limitations and enhance a variety of filament applications. We also prove the robustness of structured filaments in propagation studies on a turbulent, kilometer scale range. A strong understanding of beam engineering and generating structured filaments has the potential to improve many applications.
Laser filaments generate intensities at remote distances that exceed the plasma and ablation thresholds of solid materials, but intensity clamping limits the impact of a single pulse. To overcome this fundamental restraint, we have engineered a high-energy solid-state Titanium:Sapphire laser to generate nanosecond-duration bursts of ultrashort pulses. This temporal structuring of the laser energy enhances nonlinear propagation and several interaction mechanisms with solid targets including ablation, acoustic shockwave production, and remote RF generation. This presentation will discuss the impact of the pulse parameters and burst format on these effects in both low and high-altitude environments through experiments and simulations.
This work presents the initial activation of the Mobile Ultrafast High-Energy Laser Facility (MU-HELF) located on a 1 km test range at the Townes Institute Science and Technology Experimentation Facility (TISTEF). The MU-HELF was designed to study nonlinear laser propagation effects including filamentation and produces pulses at 800 nm with current peak powers as high as 5 TW. The pulse width, energy, size, and focusing conditions of the launched beams are all readily adjustable. Several data collection techniques have been implemented that enable high-resolution, single-shot beam profiles, spectra, and energy measurements at any point along the range. Atmospheric conditions are also continuously measured during laser propagation using the array of monitoring equipment available at TISTEF. The newly active test facilities and data collection procedures demonstrated here will drive future in-depth high-intensity laser propagation studies and development of field-deployable applications.
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