Effective performance of laser systems intended for power delivery on a distant object requires an Adaptive Optics System (AOS) to correct distortions of the laser beam caused by atmospheric perturbations along the propagation path. The turbulence-induced effects are responsible for beam wandering and intensity scintillation, resulting in degradation of the beam quality and waning of the power density on the target. Adaptive optics methods are used to compensate these negative effects, though an effective AOS performance requires a reference wave formed, for example, by the beacon on the target. One way of forming such a beacon is by using an ultra-short laser pulse (USLP) delivered a tight light spot. Multiple physical phenomena play a key role at USLP propagation in turbulent atmosphere, including: (a) atmospheric perturbations and random amplitude-phase modulation of the propagating beam; (b) spatial modulation of the wavefront and beam shape expressed as its self-focusing caused by the non-linear effects in the atmosphere; (c) a laser noise at USLP propagating in dispersive and nonlinear media.
This presentation discusses the requirements to the USLP-based beacon that can support optimal operation of the AOS for effective correction of the wavefront of the outgoing power beam.
A concept for all-optical remote detection of radioactive materials is presented and analyzed. The presence of excess radioactivity increases the level of negative ions in the surrounding air region. We model irradiated air to estimate the density of negative ions and use a set of coupled rate equations to simulate a subsequent laser-induced avalanche ionization. This can act as a source of seed electrons for a laser-induced avalanche ionization breakdown process. We examine avalanche ionization behavior in several laser parameter regimes, and determine the time required for saturation of the breakdown for both a single seed ion as well as for a population of ions present in the focused volume of a highintensity laser pulse, corresponding to two methods of remotely measuring the ion density, which is a signature of radioactive materials.
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
Effective performance of laser systems intended for power delivery on a distant object requires an adaptive optics system to correct the laser beam distortions caused by atmospheric perturbations along the propagation path. The turbulence-induced effects are responsible for beam wobbling, wandering, and intensity scintillation, resulting in degradation of the beam quality and decline of the power density on the target. Adaptive optics methods are used to compensate these negative effects. In its turn, operation of the Adaptive Optics System (AOS) requires a reference wave that can be generated by the beacon on the target. This report discusses the requirements to the beacon that can support optimal correction of the wavefront. Post-processing of the beacon-generated light field enables retrieval and detailed characterization of the turbulence-perturbed wavefront — data essential to control the adaptive optics module of a high-power laser system.
The effect of laser noise on the atmospheric propagation of high-power CW lasers and high-intensity short pulse lasers in dispersive and nonlinear media is studied. We consider the coupling of laser intensity noise and phase noise to the spatial and temporal evolution of laser radiation. High-power CW laser systems have relatively large fractional levels of intensity noise and frequency noise. We show that laser noise can have important effects on the propagation of high-power as well as high-intensity lasers in a dispersive and nonlinear medium such as air. A paraxial wave equation, containing dispersion and nonlinear effects, is expanded in terms of fluctuations in the intensity and phase. Longitudinal and transverse intensity noise and frequency noise are considered. The laser propagation model includes group velocity dispersion, Kerr, delayed Raman response, and optical self-steepening effects. A set of coupled linearized equations are derived for the evolution of the laser intensity and frequency fluctuations. In certain limits these equations can be solved analytically. We find, for example, that in a dispersive medium, frequency noise can couple to and induce intensity noise, and vice versa. At high intensities the Kerr effect can reduce this intensity noise. In addition, significant spectral modification can occur if the initial intensity noise level is sufficiently high. Finally, our model is used to study the transverse and longitudinal modulational instabilities. We also present atmospheric propagation examples of the spatial and temporal evolution of intensity and frequency fluctuations due to noise for laser wavelengths of 0.85 μm , 1 μm , and 10.6 μm .
The effect of laser noise on the atmospheric propagation of high-power CW lasers and high-intensity short pulse lasers in dispersive and nonlinear media is studied. We consider the coupling of laser intensity noise and phase noise to the spatial and temporal evolution of laser radiation. High-power CW laser systems have relatively large fractional levels of intensity noise and frequency noise. We show that laser noise can have important effects on the propagation of high-power as well as high-intensity lasers in a dispersive and nonlinear medium such as air. A paraxial wave equation, containing dispersion and nonlinear effects, is expanded in terms of fluctuations in the intensity and phase. Longitudinal and transverse intensity noise and frequency noise are considered. The laser propagation model includes group velocity dispersion, Kerr, delayed Raman response, and optical self-steepening effects. A set of coupled linearized equations are derived for the evolution of the laser intensity and frequency fluctuations. In certain limits these equations can be solved analytically. We find, for example, that in a dispersive medium, frequency noise can couple to and induce intensity noise, and vice versa. At high intensities the Kerr effect can reduce this intensity noise. In addition, significant spectral modification can occur if the initial intensity noise level is sufficiently high. Finally, our model is used to study the transverse and longitudinal modulational instabilities. We also present atmospheric propagation examples of the spatial and temporal evolution of intensity and frequency fluctuations due to noise for laser wavelengths of 0.85 μm, 1 μm, and 10.6 μm.
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.
Proc. SPIE. 9979, Laser Communication and Propagation through the Atmosphere and Oceans V
KEYWORDS: Oscillators, Mode locking, High power lasers, Laser applications, Laser beam propagation, Fiber lasers, Directed energy weapons, High power fiber amplifiers, Atmospheric turbulence, Atmospheric propagation
To achieve the power levels necessary for directed energy applications with fiber or slab lasers, it is necessary to combine multiple lasers into a single beam director. Here we compare the performance of incoherent and coherent beam combining strategies and address three important issues that should be considered before a beam combining architecture is implemented. First, we consider the difficulty in phase locking high-power fiber and slab lasers. The large linewidths of high-power fiber and slab lasers induce random phase fluctuations occurring on sub-nanosecond time scales. To coherently combine these high-power lasers can involve rapid and precise phase control to compensate for these fluctuations. Even with a master oscillator - multiple power amplifier system, the coherence length of the beams to be combined is very short necessitating continuous precise control of optical path lengths. Second, we consider the dephasing effects of atmospheric turbulence. We find that in moderate to strong turbulence conditions and kilometer propagation distances, coherent combining at the transmitter plane has negligible impact on the energy delivered to a target. Finally, we consider the multifaceted task of coherent combining at the target plane. This is effectively an adaptive optics situation in which the distortions caused by atmospheric turbulence are partially compensated for.
This paper discusses an advanced target in the loop (ATIL) system with its performance based on a nonlinear phase conjugation scheme that performs rapid adjustment of the laser beam wavefront to mitigate effects associated with atmospheric turbulence along the propagation path. The ATIL method allows positional control of the laser spot (the beacon) on a remote imaged-resolved target. The size of this beacon is governed by the reciprocity of two counterpropagating beams (one towards the target and another scattered by the target) and the fidelity of the phase conjugation scheme. In this presentation we will present the results of the thorough analysis of ATIL operation, factors that affect its performance, its focusing efficiency and the comparison of laboratory experimental validation and computer simulation results.
A viable beam control technique is critical for effective laser beam transmission through turbulent atmosphere. Most
of the established approaches require information on the impact of perturbations on wavefront propagated waves.
Such information can be acquired by measuring the characteristics of the target-scattered light arriving from a small,
preferably diffraction-limited, beacon. This paper discusses an innovative beam control approach that can support
formation of a tight laser beacon in deep turbulence conditions. The technique employs Brillouin enhanced fourwave
mixing (BEFWM) to generate a localized beacon spot on a remote image-resolved target. Formation of the
tight beacon doesn’t require a wavefront sensor, AO system, or predictive feedback algorithm. Unlike conventional
adaptive optics methods which allow wavefront conjugation, the proposed total field conjugation technique is
critical for beam control in the presence of strong turbulence and can be achieved by using this non-linear BEFWM
technique. The phase information retrieved from the established beacon beam can then be used in conjunction with
an AO system to propagate laser beams in deep turbulence.
Compact size sources of high energy protons (50-200MeV) are expected to be key technology in a wide range of scientific applications 1-8. One promising approach is the Target Normal Sheath Acceleration (TNSA) scheme 9,10, holding record level of 67MeV protons generated by a peta-Watt laser 11. In general, laser intensity exceeding 1018 W/cm2 is required to produce MeV level protons. Another approach is the Break-Out Afterburner (BOA) scheme which is a more efficient acceleration scheme but requires an extremely clean pulse with contrast ratio of above 10-10. Increasing the energy of the accelerated protons using modest energy laser sources is a very attractive task nowadays. Recently, nano-scale targets were used to accelerate ions 12,13 but no significant enhancement of the accelerated proton energy was measured. Here we report on the generation of up to 20MeV by a modest (5TW) laser system interacting with a microstructured snow target deposited on a Sapphire substrate. This scheme relax also the requirement of high contrast ratio between the pulse and the pre-pulse, where the latter produces the highly structured plasma essential for the interaction process. The plasma near the tip of the snow target is subject to locally enhanced laser intensity with high spatial gradients, and enhanced charge separation is obtained. Electrostatic fields of extremely high intensities are produced, and protons are accelerated to MeV-level energies. PIC simulations of this targets reproduce the experimentally measured energy scaling and predict the generation of 150 MeV protons from laser power of 100TW laser system18.
This paper discusses a novel type of beam director for effective laser beacon formation in deep turbulence conditions. The concept of the proposed beam director is based on an innovative approach employing a Brillouin enhanced four-wave mixing (BEFWM) mechanism for generating a tight (small spot size) laser beacon on a remote image-resolved target. The BEFWM technique enables both amplification and total (phase and amplitude) conjugation of the beacon-forming beam without the need for wavefront sensors, deformable mirrors or predictive feedback algorithms. Total conjugation is critical for beam control in the presence of strong turbulence, whereas conventional adaptive optics methods do not have this capability. The phase information retrieved from the beacon beam can be used in conjunction with an AO system to propagate laser beams in deep turbulence.
Recently, a new method of remote detection of concealed radioactive materials was proposed. This method is based on
focusing high-power short wavelength electromagnetic radiation in a small volume where the wave electric field exceeds
the breakdown threshold. In the presence of free electrons caused by ionizing radiation, in this volume an avalanche
discharge can then be initiated. When the wavelength is short enough, the probability of having even one free electron in
this small volume in the absence of additional sources of ionization is low. Hence, a high breakdown rate will indicate
that in the vicinity of this volume there are some materials causing ionization of air. To prove this concept a 0.67 THz
gyrotron delivering 200-300 kW power in 10 microsecond pulses is under development. This method of standoff
detection of concealed sources of ionizing radiation requires a wide range of studies, viz., evaluation of possible range,
THz power and pulse duration, production of free electrons in air by gamma rays penetrating through container walls,
statistical delay time in initiation of the breakdown in the case of low electron density, temporal evolution of plasma
structure in the breakdown and scattering of THz radiation from small plasma objects. Most of these issues are discussed
in the paper.
A remote atmospheric breakdown (RAB) is a very rich source of ultraviolet (UV) and broadband visible light that could provide the early warning to the presence of CW/BW agents through spectroscopic detection, identification and quantification at extended standoff distances. A low-intensity negatively chirped laser pulse propagating in air compresses in time due to linear group velocity dispersion and focuses transversely due to non-linear effects resulting in rapid laser intensity increase and ionization near the focal region that can be located kilometers away from the laser system. Proof of principle laboratory experiments are being performed at the Naval Research Laboratory on the generation of RAB and the spectroscopic detection of mock BW agents. We have demonstrated pulse compression and focusing up to 105 meters in the laboratory using femtosecond pulses generated by a high power Ti:Sapphire laser. We observed nonlinear modifications to the temporal frequency chirp of the laser pulse and their effects on the laser compression and the positions of the final focus. We have generated third harmonics at 267 nm and white light in air from the compressed pulse. We have observed fluorescence emission from albumin aerosols as they were illuminated by the compressed femtosecond laser pulse.
The experimental results on generation of high gain-length product for 13.5 nm radiation from 2 - 1 transition in hydrogen-like Li III ions are presented for 1 micrometers subpicosecond pumping laser. The comparison with earlier results, obtained with 0.25 micrometers subpicosecond pumping laser, is discussed in terms of gain generation efficiency. The results for discharge created pre-plasma in L equals 4 mm and L equals 14 mm microcapillaries are also presented.
Free-electron lasers (FELs) capable of operating in vibrational infrared (IR) (3 - 30 micrometer) regime have been developed at a small number of fixed sites around the world. However, there is a need for portable systems capable of operating in the 3 - 5 and 8 - 13 micrometer atmospheric windows for remote sensing applications. Wider use of FELs is inhibited by system size, cost, and the shielding requirements associated with the production of high current 15 - 45 MeV electron beams which are currently needed to lase in the infrared. The concept of an electromagnetic wiggler, in which the magnetostatic wiggler system of fixed transverse magnets is replaced by an intense counter-streaming microwave or optical radiation beam, has been of interest from the early work on FELs as a means of reducing the required electron beam energy. Although there has been little experimental progress to date due to the lack of high power wiggle radiation sources, the recent development of high power millimeter-wave gyrotrons has led to a re-evaluation of the feasibility of electromagnetic wigglers. We have published earlier studies of the possibility of IR FELs using low energy beams (3 - 5 MeV) using a gyrotron-powered millimeter-wave wiggler. Both waveguide cavity and open-mirror resonator (quasioptical) gyrotron (QOG) powered wigglers were considered. In this talk we present a new wiggler design, in which the millimeter-wave power generated by the QOG is coupled into a corrugated waveguide and compressed. This has the possibility of substantially increasing the single-pass FEL gain over previously published design concepts. Designs for proof-of- principle low voltage infrared FEL experiments based on both radio-frequency (rf) linear accelerator and electrostatic accelerator technology are presented together with point designs for portable systems covering the infrared windows in the atmosphere.
The nonlinear, self-consistent propagation of ultra-intense, subpicosecond laser pulses in plasmas is analyzed. Large amplitude wakefields are generated when the laser pulse length is approximately equal to the plasma wavelength. Relativistic optical guiding is found to be ineffective in preventing diffraction of pulses with lengths less than the plasma wavelength. Short, intense laser pulses can be effectively guided with preformed plasma density channels. Simulations based on a 2D-axisymmetric fluid model are discussed.
Stimulated backscattered harmonic radiation generated by the interaction of an intense pump laser field with an electron beam or plasma is analyzed using a nonlinear, relativistic, fluid theory valid to all orders in the pump laser amplitude. The backscattered radiation occurs at odd harmonics of the doppler shifted incident laser frequency. The growth rate and saturation level of the backscattered harmonics are calculated and thermal limitations are discussed. This mechanism may provide a practical method for producing coherent radiation in the XUV regime.
1407_46The generation of nonlinear plasma wakefields by an intense, short laser pulse and the relativistic optical guiding of intense laser pulses in plasmas are studied with a nonlinear, self-consistent model of laser-plasma interactions. Nonlinear steepening and period lengthening of the plasma waves are observed, and expressions are obtained for various nonlinear wakefield quantities. Relativistic focusing with the self-consistent plasma response shows that laser pulse fronts and laser pulses shorter than a plasma wavelength, 2(pi) c/(omega) (rho ), are not relativistically guided and will continuously erode due to diffraction.
1467_37A number of proposed applications of electromagnetic waves require that the radiation beam maintain a high intensity over an appreciable propagation distance. An example of this is the possibility of a power satellite providing electrical energy to a host of other satellites by means of directed radiation beams. This is referred to as power beaming. Another example is the possibility of accelerating particles to ultra-high energies by utilizing high-power laser beams. Other applications include advanced radar and directed-energy sources. The quest to achieve these objectives has led to a resurgence of research on propagation of radiation beams and diffraction theory. Diffraction causes a beam of radiation to spread out in the lateral direction and, from energy conservation, the intensity drops off correspondingly. Briefly, the objective of much of the research being carried out is: 'Can diffraction be overcome?' The authors present a survey and critique of the analyses and experimental tests of solutions of the wave equation in connection with so-called diffractionless and other directed radiation beams. The examples discussed include electromagnetic missiles, Bessel beams, electromagnetic directed energy pulse trains, and electromagnetic bullets.
A free-electron laser facility (FEL) is being constructed at the National
Institute of Standards and Technology (NIST) in collaboration with the Naval
Research Laboratory (NRL) . The FEL will be driven by the electron beam from the
NIST racetrack microtron (RTM). The anticipated performance of the FEL is: (1)
wavelength variability from 200 run to 10 tim; (2) continuous train of 3-ps pulses at
66 MHz; and (3) average power of 10 W to 200 W. This excellent performance will be
achieved primarily because of the unique characteristics of the RTM. This
accelerator will provide a continuously pulsed electron beam with high brightness
and low energy spread at energies from 17 MeV to 185 MeV. For FEL operation high
peak current is required and a new injector for this purpose has been designed. The
undulator for the project is 3.64-m long with 130 periods and a peak field of 0.54
T. The construction of the undulator is nearly complete and delivery is expected
shortly. The 9-m optical cavity has been designed and is under construction. An
experimental area is being prepared for FEL users which will have up to six
stations. Initial operation of the FEL is scheduled for 1991. The NIST-NRL FEL
will provide a powerful, tunable light source for research in biomedicine, materials
science , physics , and chemistry.