Directed Energy Applications Research Articles

Modeling the atmospheric refractive index structure parameter using macrometeorological observations

The dynamic fluctuations in the atmospheric refractive index, commonly referred to as optical turbulence, cause phase distortions of the electromagnetic waves propagating through the atmosphere. The consequent scintillations have large implications for free-space optical communication, laser remote sensing, and directed energy applications. The refractive index structure parameter (C n 2), quantifying the strength of these fluctuations, is usually estimated using high-frequency micrometeorological measurements, employing sonic anemometer-thermometers or scintillometers. Despite providing highly accurate information, these systems are immensely complex and costly, especially for frequent field applications and remote locations. In this study, we have developed an empirical multinomial model for estimating C n 2 using three-year macrometeorological data and validated it against collocated and concurrent micrometeorological measurements, from a tropical semi-arid location. This simpler model would be handy for applications in remote locations having weather station measurements alone.

Lasers in Defence

Abstract: The development and usage of laser technology in fields such as defence, law enforcement, communication, and defence have had a major impact on the law enforcement sector. The idea of Light Amplification by Stimulated Emission of Radiation (LASER) was conceived in 1957, but it wasn't until 1960 that Theodore H. Maiman built the first practical application of the technology. In addition to its high coherence and intensity, laser illumination is also distinguished by its spatial coherence, enabling light pulses on the femtosecond timescale. It finds use in various disciplines, including holography, the space sciences, spectroscopy, medicine, microelectronics, power electronics, industrial engineering, and even directed energy weaponry. Laser target markers and beam riders are the most common methods for accurately controlling LGW towards ground objects. Recent advances in laser technology are intensively investigated for their potential use in target indicators and range finders. There has been an increase in the usage of lasers in recent years, with several new services in fields as diverse as commerce, industry, medicine, science, technology, and even the military. Applications like range finders, target designators, power beamers, LIDARs, and long-range, high-speed, secure communication systems have the potential to completely alter the face of combat. India's laser R&D centres are working on various laser systems and their supporting components and materials for use in directed energy applications. Laser crosslinks are a technology that can maintain data speeds in the tens of gigabits, and they give higher bit rates with decreased bit error rates owing to their small wavelengths.

An alternative pathway from hot dry rock to green hydrogen by organic Rankine cycle applications

Some researchers have been focused on electricity generation using thermal energy extraction from HDRs via Organic Rankine cycles. This new research topic creates a new pathway to reach green hydrogen for countries that have geothermal power such as Turkey. In this study, a well in the Nevşehir region that does not has the potential to be used in direct geothermal energy applications due to the absence of water was examined. By using the heat obtained from HDR by injected water, the estimated amount of electricity that can be produced with a single shaft binary organic Rankine cycle was revealed by calculating the thermal energy that can be obtained from the wells which have a surface temperature of 183 °C at 2900 m 2900 m depth. In the next step, the hydrolysis of water with electricity obtained from renewable energy and the hydrogen potential that can be produced were revealed. This hydrogen, which is estimated to be produced, can be fed directly to the natural gas lines or subjected to processes such as storage or industrial usage.

High-performance laser power converts for direct-energy applications.

Six-junction GaAs laser power converts (LPCs) were designed and fabricated. Each subcell is vertically connected by p++-AlGaAs: C/n++-AlGaAs: Si: Te (1:2) tunnel junction with good thermal stability and a record peak tunneling current density of 1867 A/cm2. The I-V characteristics of LPCs with an aperture of 10×10 mm2 were investigated as a function of laser power and temperature. Maximum conversion efficiency and output power of 57.7% and 15.4 W, respectively, and a continuous stable operation at 22.9 W for over 550 hours were demonstrated. The temperature coefficient of conversion efficiency and open-circuit voltage were -0.197%abs/°C and -8.15 mV/°C, respectively, under 808 nm laser illumination of 21.0 W. Furthermore, an array of 100 large-scale (41×46 mm2) LPCs with an output power of 179 W under 1 kW laser irradiation at 20 m wireless transmission was developed.

Adaptive Optics for Directed Energy: Fundamentals and Methodology

Adaptive optics and related wavefront correction technologies have played a significant role in directed-energy applications. The utility of adaptive optics for wavefront correction of the laser, the optics, and the optical path to a target is discussed. Various error sources are identified and quantified for wavefront correction of the optical path to a target, including turbulence, thermal blooming, and boundary-layer effects at the transmitter. The challenges of each error source in different scenarios result in different design choices for adaptive optics. The choice of laser source also results in different design approaches for adaptive optics and related technologies. The overall beam control architecture must also be considered for best performance. A methodology for integrated assessment of all effects is presented, involving wavefront performance budgets.

Investigation of an Intensified Thermo-Chemical Experimental Set-Up for Hydrogen Production from Biomass: Gasification Process Integrated to a Portable Purification System—Part II

Biomass gasification is a versatile thermochemical process that can be used for direct energy applications and the production of advanced liquid and gaseous energy carriers. In the present work, the results are presented concerning the H2 production at a high purity grade from biomass feedstocks via steam/oxygen gasification. The data demonstrating such a process chain were collected at an innovative gasification prototype plant coupled to a portable purification system (PPS). The overall integration was designed for gas conditioning and purification to hydrogen. By using almond shells as the biomass feedstock, from a product gas with an average and stable composition of 40%-v H2, 21%-v CO, 35%-v CO2, 2.5%-v CH4, the PPS unit provided a hydrogen stream, with a final concentration of 99.99%-v and a gas yield of 66.4%.

Energy storage enhancement of paraffin with a solar-absorptive rGO@Ni film in a controllable magnetic field

Magnetically driven photothermal conversion and energy storage techniques can enhance the energy storage performance of phase change materials (PCMs) and thus have immense potential in energy applications. Paraffin is a common PCM that melts slowly and has poor rate of heat storage capacity. In this study, the effect of a controllable magnetic field on the photothermal and heat storage properties of paraffin was investigated. First, a reduced-graphene-oxide/nickel foam (rGO@Ni foam) composite film with good photothermal ability, corrosion resistance, and oxidation resistance was prepared, and photothermal conversion and energy storage processes under different magnetic fields were experimentally studied. The movement of the composite film could be adjusted by the action of magnetic force, especially to enhance the phase interface movement, accelerate paraffin melting, and improve the photothermal capacity and heat storage characteristics of the material. The composite film and phase interface could be dynamically tuned by regulating the magnetic field, thus enhancing the photothermal conversion effect. In particular, with increasing magnetic field strength, the thermal energy storage efficiency and speed of phase interface movement increased by 29% and 50%, respectively. The proposed magnetic force-driven method will enhance solar energy conversion and promote direct solar energy applications.

Step bulk arrangement of 19 fiber laser collimators for directed energy applications

We designed an arrangement of 19 high-power fiber laser collimators on an aluminum step bulk in coherent combination for use in directed energy applications. Strong environmental turbulence degrades coherent beam combining advantages; therefore, we only consider low-turbulence levels. The combining elements are single mode 3 kW high-power fiber lasers, which are collected by 19 right-angle prisms in two concentric hexagonal rings for combination. With this configuration, a high combining efficiency of 88%, a high filling factor up to f = 0.9 and lower cost production of the beam combining process are possible.

Weak turbulence effects on different beams carrying orbital angular momentum.

The study of beams carrying orbital angular momentum (OAM) has been of interest for its use in free-space optical communications (FSOC), directed energy applications, and remote sensing (RS). For FSOC and RS, it is necessary to measure the wavefront of the beam to recover transmitted or environmental information, respectively. In this computational study, common OAM beams such as the Laguerre-Gaussian (LG), Bessel-Gaussian (BG), and Bessel beams are propagated through atmospheric turbulence and compared to their Gaussian beam counterpart. The turbulence is simulated using multiple phase screens within the framework of a split-step method. Beam metrics used to quantify beam propagation will include the spatial coherence radius, OAM spectrum, on-axis intensity, spot size, divergence, and on-axis scintillation. Atmospheric turbulence along the path is limited to the weak scintillation limit, where beam parameters can be predicted analytically using the Rytov approximation. The results show that BG beams and multiplexed BG beams retain more OAM information than their LG and Bessel beam counterparts. The LG beam on-axis intensity and on-axis scintillation are seen to be independent of OAM mode. The scintillation of the LG beam is less than a BG, Bessel, and Gaussian beam across low- and high-order OAM modes. Insight into these results is discussed through studying the beam divergence, while the initial spot sizes of the Gaussian, LG, and BG beams are limited to the same spatial extent.

GEOTHERMAL ENERGY RESOURCE POTENTIAL IN NIGERIA FROM AEROMAGNETIC REVIEWS: ANOTHER PANACEA FOR ENERGY CRISES AND ENVIRONMENTAL GLOBAL WARMING

An effort has been made in this paper to advocate a panacea to the prevailing energy crisis in Nigeria by reviewing the country’s geothermal energy resource potential from regional aeromagnetic calculations. There is widespread occurrence of warm/hot springs in Nigeria, which gives credence to the availability and abundance of geothermal reserves. Results of aeromagnetic investigations also confirm that areas with anomalous high heat flow exist in Nigeria, showing high prospects of employing geothermal energy for direct and indirect energy applications. It is, therefore, imperative for Nigeria to take a quantum leap in holistic renewable energy research and development through appropriate government science advice, policy and environmental management.

A precise approach to determining the optimum location of observation plane or optimum size of array aperture to achieve maximum power-in-the bucket in the coherent beam combining

The power-in-the bucket and the intensity distribution generated by the coherent combination of a definite number of similar laser beams with Gaussian profiles are numerically studied at different observation planes in the far-field. A precise method is introduced for optimal engineering of the array aperture size and the observation plane location in the far-field. A relation between the array aperture size and the observation plane distance is proposed using the Fresnel zone concept for coherent beam combining to achieve the maximum power-in-the bucket (PIB). The results reveal that the maximum PIB in a small bucket radius of 5 or 10 cm in the observation plane (radii in directed-energy applications) can be obtained either when the observation plane is located at the primary region of the far-field for a specified array aperture size or when the array aperture size is equal to or less than the size of the first Fresnel zone . Also, the proposed approach is independent of array geometry so that it will be practical for different array geometries, namely square, circular, and hexagonal. Furthermore, it is also independent of other parameters such as numbers and divergence of beams. The maximum amount of PIB in a specified observation plane location can be obtained for array aperture sizes lower than the first Fresnel zone, regardless of the number of beams or their waist radius.

Measuring the turbulence profile in the lower atmospheric boundary layer.

Optical turbulence can have a severe effect on the propagation of laser beams through the atmosphere. In free space optics and directed energy applications, these laser beams quite often propagate along a slant or vertical path. In these cases, the refractive index structure function parameter cannot be assumed constant, since it varies with height. How it varies with height, especially in the first few meters above the ground, is not well behaved. Turbulence height profiles have been measured since the 1970s, mainly for astronomical observations. These profiles are usually measured for the atmospheric boundary layer (the layer of air from the ground up to approx. 1km during day and 100m during night) and some kilometers above it. We have measured the temperature fluctuations in the first few meters above ground level using a system containing eight resistance thermometer devices, mounted in a row at different spacings. Measurements were made flying this system under a tethered balloon or mounted on a telescoping mast. The temperature structure function parameter, CT2, can be estimated from the temperature fluctuations measured by the 28 different probe pairs and the unique distances between the two probes. Finally, Cn2 is estimated from this temperature structure function parameter and compared to values predicted by a turbulence profile model.

Diffractive anisoplanatism and tracker bandwidth limitations.

This work develops the concept of "diffractive anisoplanatism," a phenomenon that limits tracker performance for directed-energy applications by introducing differences between point-source-beacon tilt measurements and scoring-beam centroid motion. Our theoretical analysis of this phenomenon, checked against wave-optics simulations, highlights two relevant effects: diffractive conversion of phase to amplitude in the beacon light, and diffractive spreading of the scoring beam into regions outside of the geometric cone sampled by the beacon. In this work, we derive expressions for the variance and power spectral density of the differential jitter between beacon tilt and scoring-beam centroid motion. Additionally, we find a scenario-dependent frequency fS above which corrections of atmospheric tilt will increase, rather than decrease, scoring-beam centroid jitter on target. Diffractive anisoplanatism provides a fundamental-physics limit on tracker performance that should be considered alongside other practical limitations.

Controlling the Spatial Coherence of an Optical Source Using a Spatial Filter

This paper presents the theory for controlling the spectral degree of coherence via spatial filtering. Starting with a quasi-homogeneous partially coherent source, the cross-spectral density function of the field at the output of the spatial filter is found by applying Fourier and statistical optics theory. The key relation obtained from this analysis is a closed-form expression for the filter function in terms of the desired output spectral degree of coherence. This theory is verified with Monte Carlo wave-optics simulations of spatial coherence control and beam shaping for potential use in free-space optical communications and directed energy applications. The simulated results are found to be in good agreement with the developed theory. The technique presented in this paper will be useful in applications where coherence control is advantageous, e.g., directed energy, free-space optical communications, remote sensing, medicine, and manufacturing.

A Fast Two-Stream-Like Multiple-Scattering Method for Atmospheric Characterization and Radiative Transfer

AbstractMultiple-scattering effects can significantly impact radiative transfer calculations for remote sensing and directed-energy applications. This study describes the development and implementation of a fast-calculating two-stream-like multiple-scattering algorithm that captures azimuthal and elevation variations into the Air Force Institute of Technology Center for Directed Energy’s Laser Environmental Effects Definition and Reference (LEEDR) atmospheric characterization and radiative transfer code. LEEDR is a fast-calculating, first-principles, worldwide surface-to-100-km, atmospheric characterization package for the creation of vertical profiles of temperature, pressure, water vapor content, optical turbulence, atmospheric particulates, and hydrometeors as they relate to line-by-line layer transmission and radiance from the ultraviolet to radio frequencies. The newly implemented multiple-scattering algorithm fully solves for molecular, aerosol, cloud, and precipitation single-scatter layer effects with a Mie algorithm at every atmospheric layer. A unique set of asymmetry and backscattering phase-function parameter calculations accounts for radiance loss due to the molecular and aerosol constituent reflectivity within a layer and accurately characterize diffuse layers that contribute to multiple-scattered radiances in inhomogeneous atmospheres. LEEDR is valid for spectral bands between 200-nm and radio wavelengths. Accuracy is demonstrated by comparing LEEDR results with published sky radiance observations and experimental data. Determining accurate aerosol loading via an iterative visibility/particle-count calculation method is ultimately essential to achieve agreement between observations and model results for realistic atmospheres.

Physical realization of Schell‐model sources using a fast steering mirror

AbstractA simple optical system is built to quickly realize partially coherent sources for directed energy applications. The apparatus implements a recently developed technique in which any uniformly correlated or Schell‐model source can be generated by summing many statistically independent randomly tilted beams. This result means that only a tip‐tilt or fast steering mirror (FSM) is needed to realize Schell‐model sources, which in contrast to existing synthesis techniques, is both fast and flexible. A brief review of the theory necessary to understand and implement the tilt‐only synthesis technique is presented. The FSM apparatus is then described. Lastly, experimental results of a Schell‐model source are compared with theoretical predictions to validate both the synthesis technique and FSM apparatus.

Power-law Schell-model sources

A new type of Schell-model source is developed that has a spectral degree of coherence, or spatial power spectrum, which is described by a power-law function. These power-law sources generally produce cusped, or peaked far-zone spectral density patterns making them potentially useful in directed energy applications. The spectral degrees of coherence, spatial power spectra, and spatial coherence radii for power-law sources are derived and discussed. Two power-law sources are then synthesized in the laboratory using a liquid crystal spatial light modulator. The experimental spectral densities are compared to the corresponding theoretical predictions to serve as a proof of concept.

High-power lasers for directed-energy applications: reply.

The comment by Vorontsov and Weyrauch [Appl. Opt.55, 9950 (2016)APOPAI0003-693510.1364/AO.55.009950] is aimed at rebutting the critiques in Sprangle etal. [Appl. Opt.54, F201 (2015)APOPAI0003-693510.1364/AO.54.00F201] and Nelson etal. [Appl. Opt.55, 1757 (2016)APOPAI0003-693510.1364/AO.55.001757]. In the comment, Vorontsov and colleagues describe their experiments aimed at demonstrating the feasibility of coherent combining of lasers on a distant target, using relatively low-power lasers and a cooperative retro-reflective target. The Naval Research Laboratory has demonstrated the capability to project high power on a distant target by making use of an incoherent combining architecture. The proof-of-concept experiments were performed in a realistic environment without employing cooperative targets and without sophisticated adaptive optics instrumentation.

Interference effects in laser-induced plasma emission from surface-bound metal micro-particles.

The light-matter interaction of an optical beam and metal micro-particulates at the vicinity of an optical substrate surface is critical to the many fields of applied optics. Examples of impacted fields are laser-induced damage in high power laser systems, sub-wavelength laser machining of transmissive materials, and laser-target interaction in directed energy applications. We present a full-wave-based model that predicts the laser-induced plasma pressure exerted on a substrate surface as a result of light absorption in surface-bound micron-scale metal particles. The model predictions agree with experimental observation of laser-induced shallow pits, formed by plasma emission and etching from surface-bound metal micro-particulates. It provides an explanation for the prototypical side lobes observed along the pit profile, as well as for the dependence of the pit shape on the incident laser and particle parameters. Furthermore, the model highlights the significance of the interference of the incident light in the open cavity geometry formed between the micro-particle and the substrate in the resulting pit shape.

High-power lasers for directed-energy applications: comment.

Sprangle etal. [Appl. Opt.54, F201 (2015)APOPAI0003-693510.1364/AO.54.00F201] recently concluded that our experiments on coherent combining of laser beams over an atmospheric path [Opt. Lett.36, 4455 (2011)OPLEDP0146-959210.1364/OL.36.004455] were "effective only because at these low-power levels the linewidth of the lasers was very narrow… and the level of atmospheric turbulence was low…." These conclusions are inaccurate, not relevant to practical high-power coherently combined laser systems, and contradict our most recent experiments with coherent combining of 21 laser beams with a linewidth of about 1GHz over 7km distance. In this comment we also challenge the major conclusion of Sprangle etal. [Appl. Opt.54, F201 (2015)APOPAI0003-693510.1364/AO.54.00F201] and the more recently published paper by Nelson etal. [Appl. Opt.55, 1757 (2016)APOPAI0003-693510.1364/AO.55.001757] regarding inefficiency of coherent beam combining under typical atmospheric conditions.