Recent Advancements in Ultrashort Pulse Lasers Shed New Light on Directed Energy Applications
Researchers from across the U.S. DoD seek to use the unique properties of ultrashort pulse lasers (USPLs) for directed energy applications. While the laser technology for a fielded system remains on the horizon, the fundamental physics and material science can be researched today with current USPLs. Given limited resources, the DoD research offices have complementary internal programs with the support of academic and industrial research, including numerous multidisciplinary university research initiatives (MURIs).
The result has been a rapid acceleration in the knowledge base of USPL effects and remarkable improvements in the modeling capabilities. Simultaneously, improvements have occurred in USPL technology to meet DoD requirements for force application and force protection. This article highlights some of the efforts, primarily in the basic research realm, where the DoD’s collaborative effort is making a significant contribution to this field.
OVERVIEW OF USPLs
Both basic and applied research with USPLs are active areas with numerous different applications in fields such as chemistry, material science, and directed energy. For the directed energy field, the ability to achieve extreme intensities coupled with lower loss transmission across long distances (compared to longer pulses and continuous-wave lasers) is fundamental to how USPLs are viewed as unique tools.
USPLs are systems that deliver extremely short pulses, generally less than 1 ps (1 ps = 10-12 s). With sufficient energy in a pulse, peak intensities of gigawatts (109 W), terawatts (1012 W), or higher can be achieved. Such high intensities easily access the regime of nonlinear optics, where higher-order terms for the refractive index of transparent media become relevant. In addition, because these pulses are essentially instantaneous on and then off, atoms and molecules react differently than longer-pulse excitations.
One particularly interesting effect of USPLs interacting with a transparent propagation medium (air, water, glass, etc.) is filamentation [1–3]. Filaments arise when the intensity of USPLs cause a small increase in the second-order nonlinear index of refraction, which acts like a slow lens (see Figure 1). As the pulse focuses itself, the intensity increases, thereby increasing the self-focusing to the point where the medium is ionized. In air, the free electrons generated by ionization have their own nonlinear effect: defocusing. If the self-focusing can be balanced by the defocusing and losses from ionization, a stable filament can be created in which the cross section of the laser (typically a diameter of 100 μm) is maintained over long distances even up to hundreds of meters .
A common hallmark of filamentation is the generation of a narrow cone of supercontinuum radiation that can span, for a driving near-infrared (NIR) laser, the spectrum from ultraviolet (UV) to mid-infrared (IR) and beyond . A clear advantage of a filament is the ability to affect matter in a consistent way without needing to know the precise location of a target.
Another advantage from the consistency of filaments is the ability to create a channel that assists in the creation of harmonics of the laser frequency, leading to ultrashort generation of extreme-UV (XUV) and soft x-rays as well as attosecond (10-18 s) pulses. These pulses are seen as unique tools to interrogate matter with immense precision from a relatively compact source.
While efforts to determine how filaments propagate and interact with targets have made significant gains, the applicability of this technology to military applications has been limited by the laser technology. Most systems today with adequate energy use titanium:sapphire (Ti:S) gain media and chirped pulse amplification to achieve the required intensities. This technology provides a trade-off between pulse energy and pulse repetition rate. Additionally, the complexity and sensitivity of this technology require highly skilled maintenance and strict control of the environment.
Nonetheless, as demonstrated by the European Teramobile project , this maintenance/control can be accomplished in a transportable system. Recent advances in ultrashort pulse fiber lasers  are seen as a significant game changer that could lead to both high pulse energies and higher repetition rates, with the possible added advantage of operating at "eye-safe" wavelengths. This is commensurate with increased research moving beyond Ti:S technology in the NIR to USPLs that operate across a wide range of IR wavelengths where transmission in air is actually better than at visible wavelengths (see Figure 2).
The following sections survey projects and findings from different research organizations united by the goal to advance the basic science of USPLs.
THE NAVAL RESEARCH LABORATORY (NRL) AND AIR FORCE RESEARCH LABORATORY (AFRL)
NRL and AFRL are performing theoretical and experimental research on the propagation of USPLs through the atmosphere. The utility of USPLs for defense applications depends on their ability to deliver high-intensity pulses to targets at tactically relevant ranges, often through challenging atmospheric conditions. Because of the relatively low energy per pulse of present-day USPL systems (less than 1 J), most applications rely on self-focusing to deliver the required intensity. When the peak laser power is many times larger than the critical self-focusing power (about 5 GW in atmospheric air), the pulse can break up into several filaments (see Figure 3) . The distance at which filamentation occurs can be difficult to control, especially when turbulence, aerosols, dispersion, and other processes affect the propagation [9, 10].
NRL is modeling the propagation of USPLs through the atmosphere using its internally developed HELCAP code . HELCAP can model high-energy laser effects such as thermal blooming, as well as USPL effects such as self-focusing, dispersion, plasma generation, and spectral broadening. In addition, the code can model propagation through turbulence and aerosols. Recent theoretical studies include modeling nonlinear self-focusing in turbulence to control the focal range , long-range self-channeling of ultrashort pulses in turbulence , and possible challenges for adaptive optics for USPLs .
NRL and AFRL have also designed and constructed unique experimental facilities to characterize USPL propagation through turbulence. These facilities include NRL’s propagation laboratory (30–90-m range), AFRL’s PHEENIX laser propagation range (180– 540-m range) , and the propagation range at the Naval Surface Warfare Center Carderock Division (NSWCD) (~1-km range). These facilities use a turbulence generator that consists of several heated wires that extend the length of the propagation path and generate Kolmogorov turbulence conditions characteristic of the atmosphere  (see Figure 4). At the longer-range facilities, the turbulence generator can controllably create optically weak (Rytov variance <<1) to optically strong (Rytov variance >5) turbulence.
At the NSWCCD facility, turbulence conditions characteristic of slant paths and boundary layers can be simulated. The Ti:S USPLs being used in these experiments include the kilohertz Ti:S femtosecond laser (kTFL) with 20-mJ/pulse and optical parametric amplifier to access 1.1- to 2.6-μm wavelengths (NRL range); the PHEENIX 40-TW, 10-Hz laser (AFRL range); and the Astrella laser (currently installed at the NSWC range), which is a ruggedized, portable laser capable of a 7 mJ/pulse at a 1-kHz rep-rate. Experiments (e.g., Figure 5) have demonstrated long-range, nonlinear self-channeling of ultrashort pulses (up to 10 Rayleigh lengths) through deep turbulence (Rytov variances >1).
Future research may include the use of adaptive optics to improve ultrashort pulse propagation. Additionally, AFRL investigates how the plasma generated by USPL filamentation interacts with large external electric fields, including guiding electric discharges by placing the filament between high-voltage electrodes. Research has been conducted to understand the complex dynamics of filamentation-driven discharges. Initially, the hypothesis was that the filament acted as a conducting wire placed between the electrodes. Experiments reveal that the process is more complex, driven by space charge and shock wave mechanisms [16–18]. As a spin-off from this research, AFRL is currently analyzing the broadband electromagnetic radiation emission from the filament as a means to characterize the properties of USPL filament-generated plasma.
THE AIR FORCE OFFICE OF SCIENTIFIC RESEARCH (AFOSR)
AFOSR has a basic research portfolio dedicated to the interaction of USPLs with matter. The objective of the program is to explore and understand the broad range of physical phenomena accessible via the interaction of USPL sources with matter to further capabilities of interest to the Air Force, including but not limited to, directed energy. The high peak powers accessible with USPL sources give rise to a rich assortment of nonlinear laser-matter interaction physics.
More explicitly, the USPL laser program is interested in mechanisms to control dynamics of femtosecond laser propagation in transparent media (e.g., filamentation) as well as concepts for monochromatic and tunable laser-based sources of secondary photons (e.g., x-rays and gamma rays) and particle beams (e.g., protons and neutrons). If successful, the research portfolio will develop the capability to (1) propagate ultraintense laser pulses kilometers downrange through the atmosphere to produce unique nonthermal effects on materials, components, and systems; and (2) produce a compact, transportable single source of both photon and particle radiation for nondestructive evaluation (NDE) of critical DoD components.
The delivery of high-energy mid-IR USPLs to a remote target is limited by high atmospheric transmission windows and turbulence conditions and is unavoidably influenced by intrinsic nonlinear optical properties of air. Under continuing AFOSR funding, new paradigms have emerged, predicting novel nonlinear effects that modify and can potentially enhance the effective USPL delivery range.
Currently, high-power USPLs are the exclusive domain of sources with wavelengths around 1 μm—the Ti:S multi-TW laser with a wavelength of 0.8 μm being the dominant source. Sources in the NIR exhibit poor atmospheric transmission and are strongly distorted through turbulent pathways. Moving to longer wavelengths and into mid-wave IR (MWIR) 3.5–4.2-μm or long-wave IR (LWIR) 8–12-μm high-transmission windows offers many potential advantages over 1-μm sources, such as improved atmospheric transmission and longer propagation paths through turbulence.
Specifically, these wavelengths are already exploited for a host of applications, including remote sensing and thermal imaging (seeing through fog, etc.). However, these advantages are offset by the need for large beam launch apertures due to rapid diffractive spreading, which becomes more severe at longer wavelengths—Rayleigh range. If this diffractive spreading could be offset by somehow confining the beam waist, the obvious advantage of MWIR and LWIR sources could revolutionize long-range, high-energy USPL delivery. This could result in a significantly reduced launch aperture and propagation over tens of Rayleigh ranges.
Research has identified two paradigms at longer wavelength that profoundly modify the physics and consequently the manner in which such high-energy MWIR and LWIR pulses propagate in the atmosphere. One is electromagnetic optical carrier shock waves, and the second is many-body weak Coulomb correlations between remote ionized electrons (plasma). These enable low-loss long-range transmission well beyond the classical Rayleigh range for LWIR multi-Joule ultrashort pulses [19, 20].
The USPL portfolio is also exploring the fundamental role that the laser wavelength, fixed by the choice of gain medium, plays in dictating the laser matter interaction physics. Coherent sources of MWIR radiation are of great interest for a wide range of scientific and technological applications, from spectroscopy and frequency metrology to information technology, industrial process control, photochemistry, photobiology, and photomedicine. The MWIR spectrum, which may be defined as wavelengths beyond 2 μm, covers important atmospheric windows; and numerous molecular gases, toxic agents, air, water, soil pollutants, components of human breath, and explosive agents have strong absorption fingerprints in this region.
Coherent MWIR sources also offer important technologies for atmospheric chemistry, free-space communication, imaging, rapid detection of explosives, chemical and biological agents, nuclear material, and narcotics, as well as applications in air- and sea-borne safety and security. The timely advancement of capable coherent MWIR sources is, therefore, vital to future progress in many application areas across a broad range of scientific, technological, and industrial disciplines.
AFOSR has a number of basic research efforts exploring the wavelength dependence of strong field processes, with particular emphasis on the MWIR spectral region. The 3–5-μm atmospheric propagation window is an area of particularly high interest to the DoD, with numerous useful applications, including chemical detection via light detection and ranging (LIDAR), remote aerosol detection via MWIR molecular spectroscopy, and directed energy.
"The utility of USPLs for defense applications depends on their ability to deliver high-intensity pulses to targets at tactically relevant ranges, often through challenging atmospheric conditions."
The AFOSR research portfolio also has a strong focus on the interaction of USPL pulses with solid materials. Contrary to irradiation with conventional laser sources, the laser energy deposition occurs on timescales shorter than the electron-phonon coupling time, leading to high-quality, reproducible material processing with minimal thermal collateral damage. Much of the research to date has been phenomenological; the physical processes are not understood in detail, and many open questions remain unanswered.
AFOSR has numerous basic research initiatives aimed at developing a rigorous understanding of the femtosecond laser-solid interaction near and beyond the material damage threshold. Such a rigorous understanding is expected to result in the ability to control and optimize laser properties to predictably perform tailored material modification as desired for important defense capabilities.
THE ARMY RESEARCH LABORATORY (ARL)
As the DoD’s premier laboratory for ground forces, ARL plays a key role in guiding basic research toward Army applications. Directed energy has been an area of research at ARL for many decades, and USPLs have factored into that for more than a decade. As an integral part of ARL, the Army Research Office (ARO) (which is discussed in following text) has been key in motivating the state of the art in USPL research and technology across the U.S. and narrowing the gap in capabilities and understanding compared to global peers.
The concept of using the long propagation properties of filaments to guide other forms of energy has proven difficult to implement, due to limitations of USPL technology. Plasma recombination happens within a few nanoseconds, requiring a 100-MHz or higher repetition rate for a quasi-steady-state effect, which currently beyond the state of the art for high-energy ultrashort pulses.
Professor Howard Milchberg’s group at the University of Maryland  has recently demonstrated that the heat dissipated from plasma recombination can alter air’s refractive index sufficiently to provide a quasi-steady-state channel provided a repetition rate of 1 kHz or higher. This coincided with USPL technology advances that meet this requirement in a more compact, transportable system. ARL is collaborating with the Milchberg group to further investigate the ability to channel a fiber laser with a filament thermal waveguide. This ability would provide a radical new method of inscribing an atmospheric channel that better resists turbulence, thereby lessening the demands on adaptive optics.
ARL also examined the effects of filament ablation on solid, opaque targets, including metals, ceramics, and polymers. Femtosecond laser machining (FLM) has attained widespread success in achieving material removal with minimal damage effects. However, FLM uses a short local-length lens to produce high intensities on a target whereas filaments focus more gradually and have a trailing plasma column. Kiselev et al.  demonstrated that filament-induced laser machining (FILM) is able to achieve accurate ablation at long range. Our initial study  sought to compare the effects on a target between FLM and FILM with and without a focusing lens. We demonstrated that while FILM is not as efficient in material removal, it is consistent in ablation geometry across a range far larger than FLM Rayleigh ranges.
Analysis of the ablation craters indicated generation of laser-induced periodic surface structures (LIPSS). LIPSS has been an enigmatic research area owing to the elusiveness of a comprehensive theoretical explanation. The two dominant forms of LIPSS are low-spatial frequency LIPSS (LSFL) where the peak-to-peak (P2P) spacing is ¾ the laser wavelength (λ), and high-spatial frequency LIPSS (HSFL), where P2P is ¼ λ. To date, the prevailing theory of photon-phonon interference to explain LSFL has not been successfully extended to HSFL. Our data add to the puzzle by generating extremely low-spatial frequency LIPSS (~30–50 λ) on polymers; additionally, we confirm that the polarization direction is maintained during filamentation . LIPSS is of interest for generating unique surface textures that increase surface area, enhance light absorption, and can change hydrophilicity.
"The USPL portfolio is also exploring the fundamental role that the laser wavelength, fixed by the choice of gain medium, plays in dictating the laser matter interaction physics."
THE ARMAMENT RESEARCH, DEVELOPMENT, AND ENGINEERING CENTER (ARDEC)
Applications of USPLs, particularly filamentation, will be driven by the capabilities of new laser architectures. Motivated by U.S. Army ARDEC interest in filamentation in air, MIT-Lincoln Labs (MIT-LL) designed and built a laser that meets all specifications. The project called for a repetition rate of 5 kHz, and the seed laser selected could be configured for pulses at multiples of 625 Hz up to 5 kHz. Because the laser was designed to study self-focusing and filamentation in air, steps needed to be taken to avoid self-focusing in the laser rods.
To avoid intracavity damage, MIT-LL stretched the laser pulse in time and frequency in a standard technique known as chirped amplification. To achieve a short pulse, a large bandwidth is needed. To broaden the bandwidth, two host materials were used for the Yb3+ ions. Yb3+:YAG is a standard laser rod material and was used in the power stages of the system where the beam was larger. Yb3+:GSAG was developed for this laser and has gain at slightly longer wavelengths. Sections large enough for the first amplifier stage were successfully grown, cut, and polished by MIT-LL. The additional bandwidth allowed for compression down to several picoseconds. The laser system was built successfully and is an asset available for basic and applied research that requires high-repetition-rate picosecond laser pulses.
THE ARMY RESEARCH OFFICE (ARO)
The ARO is the ARL directorate with the primary mission of funding extramural basic research. As part of the tri-Service MURI program, ARO has managed a large MURI team of U.S. universities (including the University of Central Florida [UCF], the University of New Mexico [UNM], Rensselaer Polytechnic Institute, Northwestern University, the State University of New York [SUNY] Buffalo, the University of North Carolina [UNC] Charlotte, and Southern Methodist University [SMU]), collaborating with a number of European institutions and several government laboratories. Facilities include a 500-mJ, 30-fs, 10-Hz NIR and a 2-mJ, 5-fs (single-cycle) NIR laser at UCF; a 200-mJ, 40-fs, 10-Hz NIR and a 300-mJ, 200-fs UV laser at UNM; indoor laser range facilities at UCF and UNM; a 15-km and upper atmospheric overhead range in Florida; and a mobile USPL lab with high-speed tracking unit at UCF.
Areas of investigation include the nature and modeling of the filament physics, arrays of filaments, microwave guiding and focusing, the formation of virtual hyperbolic metamaterials from the beams, filament-aerosol (cloud) interactions, phase-controlled structured filaments, large filament arrays, backward emission and lasing, and millimeter-wave and terahertz generation.
The MURI program is in year 6. Research in an ARO single-investigator program at the University of Maryland has demonstrated that the shock wave caused by the filament formation  forms a low-pressure guide for high-voltage electrical discharges, that filaments can be formed that are topologically protected from dissipation or interaction, and that the air guide can be selectively heated by coherently exciting rotational modes of the air molecules. In addition, the research is exploring potential electrical phenomena in the filament generated air guide.
Furthermore, research funded by the Defense Research Advanced Projects Agency (DARPA), AFOSR, and the Office of the Secretary of Defense (OSD) at the University of Colorado Boulder has demonstrated bright, coherent extreme UV and soft X-rays from extreme High Harmonic Generation in NIR filaments in noble gases. Research in the ARO single investigator program has demonstrated the "UV surprise," unexpected strong generation of soft X-rays from femtosecond UV filaments, and the investigation of generating higher energy X-rays (approaching 10 keV) in femtosecond 10-μm laser filaments is ongoing.
USPLs provide a unique method to access a new class of interactions with matter that lends itself to DoD applications. We are now at the cusp of the technological development of USPL sources enabling new theoretical and experimental results to advance laser-based directed energy weapons. And basic research is stretching the bounds of wavelength, pulse duration, laser repetition rate, and material interactions toward new and exciting regimes.
Yet, the community currently remains relatively small and interdependent for experimental and modeling support. While each Service has its own viewpoint on the application of USPLs, the core physics remains the same.
It is conceivable that fieldable USPLs will be achievable in the mid-term future that can serve a wide variety of applications including stand-off detection, remote ablation, guiding electrical discharges, and cooperative effects with other forms of electromagnetic energy. By furthering the advancement of USPLs and nonlinear optics, the DoD has served to put the United States at the forefront of these quickly developing fields.
This work is supported by the Office of Naval Research, the High Energy Laser Joint Technology Office, the NRL Base Program, the AFRL Base Program, the OSD MURl Program, and ARO. The authors also thank Prof. Jerome Moloney from the University of Arizona for his assistance with the HITRAN model.
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