There are many challenges in characterizing open-air laser propagation. Varying measurement methodologies with diverse instrumentation can provide contradictory results for both optical turbulence and extinction. This paper assesses instrumentation accuracy w.r.t. a propagating laser and explores the optimal experimental setup. Accordingly, an open-air experiment was conducted to characterize the atmosphere and high-energy laser (HEL) propagation with commonly deployed instrumentation. Presented here is a comparative evaluation of the instrumentation results to far-field HEL measurements. The differential image motion monitor (DIMM) and the wide angle tele-radiometric transmissometer (WATT) provided measurements with the lowest mean percent error for optical turbulence and extinction, respectively. This suggests that path-integrated dual-sided instrumentation outperforms nodal measurements. However, it is found that nodal measurements perform best near and at the laser’s aperture height if dual-sided instrumentation is not available. Additionally, a quality-control routine is outlined for all deployed instruments considered in the evaluation. The experimental results in this paper yield instrumentation performance for characterizing static laser propagation over land. Further research is recommended to assess instrumentation over longer optical paths both static and dynamic.
This paper explores the fundamental phenomenology of weather-driven diurnal and nocturnal optical turbulence trends. Examining long duration persistent atmospheric measurements at Townes Institute Science and Technology Experimentation Facility (TISTEF), an outdoor laser range operated by the University of Central Florida (UCF), reveals key correlations between observed meteorological quantities and optical turbulence strength. A distributed set of meteorological instruments provide information on local conditions via temperature, pressure, relative humidity, net radiation, wind anemometers, cloud ceilometer, and a sky imager. The strength of optical turbulence is captured via a boundary layer scintillometer (BLS) and the delayed tilt anisoplanatism (DELTA) sensors. The paper compares the turbulence measurements against the performance of a physical weather-driven turbulence model and a deductive machine learning (ML) based turbulence model. These models attempt to accurately capture the relationship and phenomenology between meteorological conditions and optical turbulence. Additionally, the paper discusses an instrument concept that could augment current turbulence forecasting techniques to have improved short term forecasts.
This research summarizes the events of a laser propagation test at the TISTEF laser range during January 2024. A 1064 nanometer (nm) continuous-wave (cw) fiber laser was focused at 1 kilometer and propagated in a variety of conditions over a week-long period. Meteorological instruments including a Scintec BLS900, MZA DELTA, and sonic anemometers were deployed along the optical path. The propagated beam spot was recorded at 100 Hz from both transmit and receive site locations. The processed imagery from both cameras generated beam profile data such as short-term spot size, long-term spot size, and beam wander. These statistics were explored as a function of measured atmospheric parameters such as visibility, refractive index structure parameter, wind speed, and more.
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.
Recently, the technique of infrared/terahertz Double Resonance Spectroscopy (DRS) was proposed for remote sensing of trace gases in atmospheric conditions. The atmospheric window of transmission is in the 9-11 µm range, which makes CO2 lasers highly suited for this application. DRS is a valuable detection method because of increased measurement specificity due to terahertz ro-vibrational signature detection. Preliminary DRS measurements utilizing a pulsed CO2 laser source with a known trace gas in a vacuum chamber are discussed. A plan is then presented for future DRS experiments at ambient pressure using ultrafast laser techniques.
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.
Studies were performed using self-channeled femtosecond laser pulses (filaments) interacting with various materials at a
distance of 30 meters. Using time resolved optical shadowgraphy, the filament interaction with the target is observed.
Shockwaves in both the target and the surrounding atmosphere are observed and their velocities measured. Estimations
of the shockwave energy are made from these observations. In transparent targets, optical coupling into the target
material is observed. This coupling results in optical damage lines in the material. Results of the filament interactions
will be discussed along with supporting modeling.
In this work we present the status of our high repetition-rate/high power EUV source facility. The masslimited
target concept has demonstrated high conversion efficiencies (CE) previously, with precision solid
state lasers. Currently, experiments are in progress with high power high repetition-rate (3-4 kHz) Qswitched
laser modules. We present a new dedicated facility for the high power EUV source. Also, we
present a precision EUV energy-meter, which is developed for absolute EUV energy measurements.
Spectral measurements of the tin-doped droplet laser plasma are performed with a flat-field spectrometer
(FFS) with a back-illuminated CCD camera. We address the issue of maintaining the calibration of the
EUV optics during source operation at non-optimum intensity at high repetition-rate, which is required for
CE improvement studies. Here we present the unique metrology for measuring EUV energies under a
variety of irradiation conditions without degrading EUV optics, even at high repetition rates (multi-kHz).
Tin-doped droplet target has been integrated with several lasers including high power high repetition rate lasers
and demonstrated high conversion efficiencies for all the lasers. This implies the EUV source power is linearly
increasing as the laser frequency goes higher. The target exhibit very low out-of-band radiation and debris emission.
The drawback of increasing the repetition rate of the target and the laser will be limited. The total amount
of tin consumed for a EUVL source system is also small enough to be operated for a long term without large effort
for recycling of the target materials. We address and demonstrate in this paper the primary issues associated
with long-term high power EUV sources for high volume manufacturing (HVM) using tin-doped droplet target.
The need for robust, versatile, and rapid analysis standoff detection systems has emerged in response to the increasing threat to homeland security. Laser Induced Breakdown Spectroscopy (LIBS) has emerged as a novel technique that not only resolves issues of versatility, and rapid analysis, but also allows detection in settings not currently possible with existing methods. Several studies have shown that femtosecond lasers may have advantages over nanosecond lasers for LIBS analysis in terms of SNR. Furthermore, since femtosecond pulses can travel through the atmosphere as a self-propagating transient waveguide, they may have advantages over conventional stand-off LIBS approaches1. Utilizing single and multiple femtosecond pulse laser regimes, we investigate the potential of femtosecond LIBS as a standoff detection technology. We examine the character of UV and visible LIBS from various targets of defense and homeland security interest created by channeled femtosecond laser beams over distances of 30m or more.
Plasmas produced by laser-matter interactions are a known source of electromagnetic radiation. However, little has been done to systematically study the electromagnetic radiation emitted from laser produced plasmas. It is our intent to provide detailed time and frequency domain measurements of such emitted radiation. An ultra-fast femtosecond high intensity laser and a superheterodyne receiver are employed to study laser-matter interactions for various materials in the frequency range 1-40GHz.
Laser interactions with bulk transparent media have long been investigated for material processing applications involving ablation and shock wave generation in both the nanosecond and femtosecond pulse width regimes1. Shock waves have been studied in fused silica and other optical glasses but previously have been characterized by the morphology of the concurrent ablation. We perform ablation at distances of 30 meters using the non-linear self-channeling effect. Using silicon wafers as targets because of their clearly defined ablation zones, we examine the effect that the filament has on the thin SiO2 layer coating the wafer's surface. It is observed that the surface layer experiences a shock wave resulting from the explosive forces produced by the plasma. The use of several laser pulses in burst mode operation leads to the observation of multiple shock fronts in the material, and the possibility of shock wave addition for higher damage. Optical interferometry will be used to characterize the shock wave dynamics, using both traditional means of focusing in the near field and at 30 meters using propagating self-channeled femtosecond pulses. The novelty of using self-channeling laser pulses for shock wave generation has many implications for military applications. These experiments are to be performed in our secure test range using intensities of 1014W/cm2 and higher incident on various transparent media. Interferometry is performed using a harmonic of the pump laser frequency. Experiments also include burst-mode operation, where a train of ultra-fast pulses, closely spaced in time, and novel new beam distributions, strike the sample.
Hydrogen-like line emission from lithium has long been considered a candidate for EUV light source for lithography. We have completed the evaluation of the potential of lithium as a laser-plasma source, both theoretically and experimentally. Theoretical calculations show optimum intensity region for lithium for attaining high conversion is close to 5.0 x 1011 W/cm2, with plasma temperature near 50 eV. Experimental studies compare directly, the conversion efficiency and optimum irradiation conditions for both planar tin and lithium solid targets. Best conversion efficiency found in this study is 2% for lithium, while CE measured is better than 4% for tin target at identical experimental conditions.
Light sources based on laser plasmas using tin as target material are known to provide high conversion efficiency of laser power to emission in the 13.5 nm spectral region. In addition, laser plasmas produced from microscopic droplet targets enable the utilization of the mass-limited concept which minimizes the effect of target debris produced from the laser plasma interaction. By combining the mass-limited target concept and tin as the choice of target material, we are developing an extreme-ultraviolet (EUV) light source that can supply high power while remaining essentially debris-free. This source uses tin-doped microscopic droplet liquid targets that are generated at high-repetition rates (>30 kHz), which allows convenient upward power scaling when coupled with a high averaged-power laser.
Detailed studies of the radiation from this source have been made using a precision Nd:YAG laser. Broad parametric studies of the conversion efficiency along with in-band spectroscopy of this EUV source have been performed. The parametric dependence of conversion efficiency is established based on measurements made by the Flying Circus diagnostic tool and a calibrated high-resolution flat-field spectrometer. These measurements have been independently confirmed by the Flying Circus 2 team.
The 13 nm emission that results from laser plasmas created from tin targets, results from a milliard of transitions occurring in many ions of tin (Sn6+-Sn13+). Understanding the energy manifolds within these multiple states will further our ability to manipulate energy into the narrow emission band demanded by EUV Lithography. A combined experimental theoretical program is underway to measure and interpret the detailed EUV emission spectra from laser plasmas suitable for EUVL, particularly mass-limited droplet laser plasmas. We employ high resolution spectroscopy in the 2 - 60 nm wavelength regions to characterize the emission from the plasma. This is interpreted with the aid of combined hydrodynamic/ radiation transport computer models. The results of this study will have impact on the in-band EUV conversion efficiency, estimation of the out-of-band short-wavelength emission, and in the development of electron temperature plasma diagnostics.
A high repetition-rate laser plasma source, possessing distinct radiation and particle emission characteristics, is now a principal candidate light source for the next generation of technology for the fabrication of computer chips. For these sources to satisfy this critical need they will need to meet unprecedented levels of performance, stability and lifetime. We review here some of the principal diagnostics of the EUV radiation that are now being utilized in the metrology, spectroscopy and imaging of these sources.
We are making a comprehensive study of the ablation of elemental materials by femtosecond lasers. Specifically, we are examining the ablation of a wide range of metals, under vacuum and in ambient air, using 850-nm wavelength, 100-fs laser pulses in an intensity range approaching and extending beyond the air ionization threshold. We compare ablation rates and examine in detail the morphology and structural integrity of the ablation region, towards gaining greater knowledge of the interaction science as well as constructing empirical models for fabrication guidelines across the periodic table.
The droplet laser plasma source has previously been shown to have many attractive features as a continuous, almost debris-free source for EUV and x-ray applications. In a combined experimental and theoretical study, we analyze the interaction physics between the laser light and the target over a range of conditions.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.