High-power, high-gain, wideband amplifiers, operating in the GHz regime, have important applications, ranging from satellite communication, radar sources, and electronic countermeasures. These applications require high-power and broadband operation to achieve the necessary waveform diversity and agility. We propose a Cherenkov maser amplifier (CMA), which is capable of providing high power, high efficiency, and extremely broadband operation in the GHz regime. In the CMA, a subluminal hybrid TEM mode in the corrugated waveguide interacts with an electron beam. This interaction leads to high-gain amplification over an extremely wide range of input frequencies. The interaction is analyzed and simulated in the linear and nonlinear regimes. We show that intrinsic conversion efficiencies can be as high as 30%. Conversion efficiencies greater than 40% have been simulated by spatially tapering the corrugated waveguide, and saturation distances can be shortened by prebunching the electron beam. Simulation results indicate that the wideband CMA configuration operating in the GHz regime can generate power levels in the GW range, employing electron beams in the multi-kA and low MeV range.

Long-range propagation of laser pulses in the atmosphere is of interest in free-space communication, remote detection, and directed energy, among others. We analyze and numerically simulate the propagation and stability of laser pulses propagating in the vertical direction. Stability of the dynamic equilibrium of the laser pulse is analyzed and numerically simulated, taking into account the variation of the air density, linear and nonlinear dispersion, and turbulence level, as a function of altitude. In the dynamic equilibrium, the spot size, pulse duration, wave-front curvature, phase, and chirp evolve self-consistently along the propagation path. Significant intensity enhancement at remote distances is achieved by tailoring the laser parameters in such a way that the transverse and longitudinal focal points nearly coincide. In addition, the hybrid filamentation-modulation instability is analyzed and simulated, indicating that the beam breaks up transversely and longitudinally. Our model enables laser pulse propagation studies over distances that ordinarily would be computationally challenging.

Long-range propagation of laser pulses in the atmosphere has numerous applications. In this presentation, we analyze and numerically simulate the propagation of laser pulses over distances >50 km in the vertical direction. A dynamic equilibrium is derived where the spot size, pulse duration, wavefront curvature, phase, and chirp evolve self consistently along the path of propagation. The model takes into account the variation of air density, linear and nonlinear dispersion, and turbulence with altitude. By tailoring the initial chirp, wavefront curvature, and ratio of laser power to nonlinear focusing power, the pulse can be focused at remote locations with significant intensity enhancements. The hybrid filamentation-modulation instability, which is responsible for transverse and longitudinal beam breakup, is analyzed and numerically simulated. Our model allows for simulation of propagation over distances that would be computationally challenging using full scale wave-optic codes.

This paper proposes a method for generating tunable, low-frequency signals in the ionosphere and propagating them back to the ground. The concept is based on a ground-based, mobile, modulated rf source, which generates signals in the ionosphere at frequencies in the megahertz range. These low-frequency signals propagate to the observation point on the ground. The low-frequency signal is parametrically generated by a ground-based, modulated high-frequency rf source. By tuning the modulation frequency to the local plasma frequency in the ionosphere, a resonant condition generates a low-frequency signal that propagates to the observation point on the ground. The ionospheric electrons are subjected to the modulated rf source and experience a nonlinear force-a ponderomotive force-which resonantly drives a current. The frequency of the driving current is at the modulation frequency, that is, the local plasma frequency. The driving current, driven by a resonance, increases in time, and generates a low-frequency signal. In an example, we find that a 1-MW source at 94 GHz and modulated at 9 MHz can generate a 9-MHz signal 500 km from the source at an intensity greater than the minimum detectable level. The low-frequency signal from the ionosphere can propagate back to the ground-based detector by means of surface waves. This concept may have applications in compact, mobile, and low-cost over-the-horizon radar and as a method to monitor ionospheric parameters.

We study the generation of spin-orbit (SO) modes via four-wave mixing (FWM)-based parametric amplification. SO modes carry quantized total angular momentum (TAM), and we show that FWM processes that generate new signals conserve TAM. This is a generalization of prior research which operated in a regime where FWM processes conserved spin and orbital angular momenta independently. We calculate the growth rates of new modes for both degenerate and nondegenerate pump configurations. Our theory is validated against numerical simulations for the cases where the generated signals are in the same SO mode(s) as the pump(s). We also calculate the growth rates of signals in SO modes other than the pumps.

The propagation of laser pulse trains (LPT) through the atmosphere is analyzed and simulated. The effects of group velocity dispersion, focusing, chirping, and atmospheric turbulence are included in the analysis. The model is general and is solved for pulses longer than several laser wavelengths. We present examples of the propagation of LPTs with parameters chosen to replicate recent experiments which use LPTs to generate rf radiation in the atmosphere. Propagation of LPTs over extended distances is of particular interest. The pulse length of the individual micro-pulses used in the simulation ranges from similar to 100 fs -100 ps.

In this presentation we outline the results of the analysis and numerical simulations of the physical phenomena associated with the propagation of laser pulse trains (LPT) in air. In the performed studies the intensity of the micro-pulses in the LPT is far below that of tunneling ionization. The ionization process relies on the background level of radioactivity, which plays an important role in initiating a collisional ionization process. The focused LPT ionizes the air forming a plasma filament. The ponderomotive forces associated with the LPT drive the plasma oscillations predominantly in the radial direction. As the plasma density builds up on axis, the latter portion of the LPT is defocused, resulting in scattering of the incoming laser radiation and shortening of the laser’s interaction length. In our model, a low intensity LPT photo-ionizes background negative ions (produced by ambient ionizing radiation) and provides the seed electrons necessary to initiate collisional ionization. The driven radial electron currents in turn generate directed rf radiation. The frequency of the rf radiation is given by 1/Tp where Tp is the separation time of micro-pulses in LPT.

In this paper, we analyze and numerically simulate mechanisms for generating directed rf radiation by a low-intensity laser pulse train (LPT) propagating in air. The LPT ionizes the air, forming a plasma filament. The ionization process relies on the background level of radioactivity which plays an important role in initiating a collisional ionization process. In our model a low-intensity LPT photoionizes background negative ions (produced by ambient ionizing radiation) and provides the seed electrons necessary to initiate collisional ionization. The intensity of the LPT is far below tunneling ionization levels. The ponderomotive forces associated with the LPT and self-fields drive plasma oscillations predominately in the radial direction. The driven radial electron currents in turn generate directed rf radiation. As the plasma density builds up on axis, the later portion of the LPT can defocus and limit the interaction length. The spectrum of the rf radiation consists of the fundamental frequency associated with the pulse separation time as well as harmonics. The rf generation mechanism is analyzed using fluid equations which incorporate, among other things, the effects of background radioactivity, photoionization, collisional ionization, ponderomotive and space-charge effects, and electron attachment-recombination processes. As an example, for a specific set of parameters, the rf spectrum and intensity are compared to experimental data.

This paper discusses the results of experimental studies of low intensity laser pulse train (LI-LPT) propagation in air. The train of ultra-short laser pulses of adjustable repetition rate became possible with the intra-cavity longitudinal mode selector that improves the efficiency of the mode locking mechanism. This technique enabled the generation of a LPT with an envelope duration TPL ≈ 40 ns (FWHM). The envelope is filled with a train of micro-pulses that form a temporal comb with an individual micro-pulse duration τL ≥ 150 ps. The micro-pulse separation time, TP, can be tuned from ~10 ns to 0.45 ns. Depending on the pump level, the total energy ELPT of the LPT is in the range of 150 mJ and 1.2 J. When focused in air, the LPT with peak micro-pulse intensity ranging from 5×1014 W/cm2 to 1016 W/cm2 generates a plasma. The laser induced plasma leads to laser light scattering, broadband luminescence, and generation of rf radiation. We report the first results of the experimental studies of the interaction of the LI-LPT with air. Theoretical analysis and simulations of the filamentation and rf radiation have been carried out. The results of the experiment are in agreement with the theoretical and simulation results.

We present an erratum to our Letter [Opt. Lett. 47, 3447 (2022)10.1364/OL.457709]. In the Letter we provided an example calculation for how to use our results to predict the signal-to-noise ratio for an OAM-multiplexed communication system. This erratum corrects the parameter name for which numerical values are provided. The calculations in the original Letter were performed using the correct values for all parameters; therefore, this correction does not affect the results and conclusions of the original Letter.

Compact, high-power, and wide band amplifiers, operating in the GHz frequency regime, have a number of important applications, ranging from rf communications with ground and airborne systems to electronic disruption. Amplifiers in this frequency range are key to achieving the necessary waveform diversity and agility for these applications. In this work the fundamental limits of the Cherenkov Maser Amplifier (CMA) in generating high-power levels over a wide frequency range is analyzed. In the CMA, a high-current, relativistic electron beam is injected and propagates in a dielectrically lined waveguide. The dielectric layer reduces the phase velocity of the hybrid TEM-TM mode of the wave guide to equal the electron beam velocity. This coupling allows for GW output powers throughout the 1–4 GHz range. Conversion efficiency can be enhanced by pre-modulating the electron beam or tapering the phase velocity along the length of the wave guide.

A Cherenkov maser amplifier (CMA) for generating high-power levels over a wide frequency range is proposed, analyzed, and numerically simulated. The CMA is a wideband amplifier consisting of an annular relativistic electron beam in a cylindrical waveguide, having an inner conductor and outer layer of dielectric material all enclosed by an outer conductor. The interaction between the hybrid TEM/TM subluminal mode of the waveguide and the relativistic electron beam leads to amplification over a wide range of input frequencies in the gigahertz regime. The interaction is analyzed and simulated in the linear and nonlinear regimes. We show that conversion efficiencies can be enhanced by spatially tapering the dielectric waveguide. In addition, by premodulating the electron beam, efficiencies can be further enhanced and saturation distances reduced. Conversion efficiencies greater than 25% have been simulated by premodulating the electron beam and/or spatially tapering the dielectric waveguide over distances of a few meters. Simulation examples indicate that the ultrawideband CMA configuration operating in the gigahertz regime can generate power levels in the gigawatt range, employing electron beams in the multi-kiloampere and low megaelectronvolt range.

In this work, a set of coupled equations is presented that describes the spot size and pulse length evolution of a Kerr-focused ultra-short laser pulse in a turbulent and group velocity dispersive atmosphere. Solutions to the equations are compared against Monte Carlo simulations for focused and collimated beams in weak, moderate, and strong turbulence. The results indicate good agreement except when the beam accumulates excessive wings in the transverse profile and/or undergoes pulse splitting, such that the self-similar evolution assumption in the coupled equation derivation breaks down.

The theoretical framework for Raman spectroscopy using a UV probe laser pulse train consisting of multi femtosecond pulses is developed. We show selective excitation of a single Raman mode by tuning the pulse parameters.

In inertial confinement (ICF) experiments at the NIKE laser facility, the high-power krypton fluoride (KrF) laser output beams propagate through long (similar to 75 m) air paths to achieve angular multiplexing, which is required because the KrF medium does not store energy for a sufficiently long time. Recent experiments and simulations have shown that, via stimulated rotational Raman scattering, this propagation can spectrally broaden the laser beam well beyond the similar to 1 THz laser linewidth normally achieved by the induced spatial incoherence (ISI) technique used in NIKE. These enhanced bandwidths may be enough to suppress the laser-plasma instabilities which limit the maximum intensity that can be incident on the ICF target. In this paper we investigate an alternative technique that achieves spectral broadening by self-phase modulation in Xe gas, which has a large, negative nonlinear refractive index similar to 248 nm, and thus completely avoids transverse filamentation issues. The collective, nonlinear atomic response to the chaotic, nonsteady state ISI light is modeled using a two-photon vector model, and the effect of near-resonant behavior on the spectral broadening is studied.

Remote detection of a distant, shielded sample of radioactive material is an important goal, but it is made difficult by the finite spatial range of the decay products. Here, we present a proof-of-principle demonstration of a remote detection scheme using mid-infrared (mid-IR) (λ = 3.9 μm) laser-induced avalanche breakdown of air. In the scheme's most basic version, we observe on-off breakdown sensitivity to the presence of an external radioactive source. In another realization of the technique, we correlate the shift of the temporal onset of avalanche to the degree of seed ionization from the source. We present scaling of the interaction with laser intensity, verify observed trends with numerical simulations, and discuss the use of mid-IR laser-driven electron avalanche breakdown to detect radioactive material at range.

Several recently proposed methods for detecting radioactivity at range involve driving laser induced avalanche breakdown seeded by electrons or negative ions whose density are elevated in the vicinity of a radioactive source. Using a chirped, mid-IR laser, we drive breakdowns at 1 meter standoff distances and monitor the breakdown timing using the backscattered spectrum. In addition to the on/off radiation detection based on the increased probability of finding a seed electron in the focal volume, we also can determine the spatial distribution of these seed electrons in the focal volume through temporal information encoded in this backscatter spectrum. We demonstrate that the backscatter spectrum is a superior detection method relative to visible plasma fluorescence, total pump backscatter, or absolute backscatter timing in its ability to determine the relative radiation level. We discuss scaling to longer focal geometries inherent in remote sensing and possible limitations to the technique, supported by modeling.

We demonstrate standoff detection of radioactive material based on avalanche breakdown of irradiated air with mid-IR laser pulses. With radiation present, breakdowns experience a temporal shift in their evolution, and can exhibit on-off sensitivity.

High-average power, ultra-broadband, mid-IR radiation can be generated in a nonlinear medium by illuminating it with a multi-line laser radiation. Propagation of a multi-line CO2 laser beam in a nonlinear medium, e.g. gallium arsenide or chalcogenide, will generate directed, broadband, IR radiation in the atmospheric window (2-13 μm). A 3-D laser code for propagation in a nonlinear medium has been developed to incorporate extreme spectral broadening resulting from the beating of several wavelengths. The code has the capability to treat coupled forward and backward propagating waves. In addition, we include transverse and full linear dispersion effects. Methods for enhancing the spectral broadening are proposed and analyzed; in particular, grading the refractive index radially will tend to guide the CO2 radiation and extend the interaction distance, allowing for enhanced spectral broadening. Finally, we show that the laser phase noise associated with the finite CO2 linewidths can significantly enhance the spectral broadening. In a dispersive medium laser phase noise results in laser intensity fluctuations. These intensity fluctuations result in spectral broadening due to the self-phase modulation mechanism.