High-power lasers for directed-energy applications

Bessel-Gauss coherently combined beams

The application of a conical wavefront in the near field of the tiled aperture coherent beam combination (CBC) produces a segmented, similar to Bessel-Gauss irradiance pattern (BG-CBC), in the far field. The properties of such a structured optical field were numerically investigated. In contrast to that for the classical CBC, the power diffracted beyond the 'zero' BG-CBC diffraction order is more smoothly and homogenously distributed, not evidencing sharp maxima and dark regions typical for CBC. The novelty of the BG-CBC with segmented conical wavefront is a simple way of caustics elongation and redistribution of power density for the same CBC architecture.

High-performance laser power converts for wireless information transmission applications

Laser Power Converters (LPCs) are components of the laser wireless power transmission (LWPT) system receiving laser power. This paper proposes a comprehensive test method that employs continuous, pulse-pause, and short-time techniques to evaluate the performance of six-junction GaAs LPCs operating with an optical input at 808 nm. Additionally, we investigate the performance of LPCs with different areas and achieve a conversion efficiency over 60%. Furthermore, we apply LPCs with varying areas to wireless information transmission and successfully achieve a response time of 1.7 µs.

Analytical study on the atmospheric absorption properties of fiber lasers in a 1 µm band

Atmospheric absorption is one of the significant factors influencing the atmospheric propagation efficiency of high-power fiber lasers. Based on typical atmospheric environment parameters, the atmospheric absorption (aerosol and atmospheric molecular absorption) of fiber lasers with different linewidths and center wavelengths in a near 1 µm band is numerically calculated. The results show that the atmospheric absorption of common (several nanometer scales) and narrow linewidth (<1n m scale) lasers have distinctly different external characteristics, but the intrinsic mechanisms are interconnected. Due to the high wavelength selectivity of atmospheric molecular absorption, this work focuses on the factors influencing water vapor (main absorbing gas) absorption of different linewidth lasers and the corresponding low absorption region. Based on the fine atmospheric absorption spectra of different types of fiber lasers, the output spectra of fiber lasers can be artificially designed to avoid strong absorption during atmospheric propagation and achieve improved high-energy laser propagation efficiency. The above method provides a partial reference for designing and optimizing the light source parameters of high-power fiber lasers for atmospheric propagation.

High-precision calculation and experiments on the thermal blooming of high-energy lasers

Thermal blooming (TB) is one of the important factors affecting the quality of high-energy laser beams. Reasonable simulation of thermal blooming is important to the application of a high-energy laser. However, reported investigations on TB simulation are mainly based on one method, such as the perturbation method or the phase screen method, which often leads to obvious errors in some conditions. In the paper, the reasonable ranges of optical generalized distortion parameters for both methods are determined based on the reported experimental data, which solves the problem of accurate TB simulations for the first time. In addition, the dynamic effect of thermal blooming is also calculated. Finally, the formula method is presented to extract the phase of thermal blooming distortion. We then use LC-SLM (Liquid crystal spatial light modulator) to emulate thermal blooming effect in the lab. The experimental results are more consistent with the numerical simulation results than conventional phase extraction methods. Our work provides a quantitatively and programmable way to accurately simulate TB with LC-SLM in the lab.

Effect of reciprocity-breaking on fine-track tip/tilt systems

We analyze here a candidate system for correcting the wander of a self-channeled laser pulse using a fast-steering mirror along with a cooperative beacon imaged with a telescope. For our model system, the imaging telescope is coaxial with the propagation of the outgoing pulse. In the ideal case, any incoming light gathered from the beacon would be collimated, such that taking a centroid beacon image would yield the precise tip and tilt required for the self-channeled pulse to propagate back to the beacon on the reciprocal path. The degree to which reality differs from this ideal case determines the effectiveness of the wander correction. We simulate our system for a range of propagation and imaging conditions. We also show that in the absence of image noise (i.e., when the beacon power is arbitrarily high, and the signal-to-noise ratio is not an important consideration), the system exhibits its best performance when the receiving aperture diameter of the imaging system is close to the transverse size of the outgoing pulse, maximizing reciprocity. When realistic noise and finite beacon power are included in the simulation, however, we find that this reciprocity advantage may not be sufficient to compensate for the reduced photon count and resolving power of a small receiving aperture. In this case, the optimal aperture diameter will be the smallest possible, which allows for an acceptable signal-to-noise ratio.

Analytical study on the steady-state thermal blooming effect of high-power ytterbium-doped fiber lasers propagating through the atmosphere

Thermal blooming effect is one of the significant factors affecting the propagation performance of high-power ytterbium-doped fiber lasers (YDFLs) in the atmosphere. In this paper, two 20 kW YDFL systems with typical wavelengths (1070 nm and 1080 nm) are fabricated for propagation comparison experiments, which are used to investigate the thermal blooming effect induced by high-power YDFL propagation through the atmosphere. Under approximately the same laser system parameters (except wavelength) and atmospheric environment, the 1070 nm laser has better propagation characteristics than the 1080 nm laser. Due to the combined effect between the different central wavelengths of the two fiber lasers and the spectral broadening caused by output power scaling, the thermal blooming caused by the different absorptivity of water vapor molecules to the two fiber lasers is the main factor for the variation of the propagation properties. Through theoretical analysis and numerical calculation of factors affecting the thermal blooming effect, and considering the industrial manufacturing difficulty of YDFLs, a reasonable selection of fiber laser parameters can effectively improve atmospheric propagation performance and reduce manufacturing costs.

Confined-doped fiber enabled kilowatt-level all-fiber laser with 1.28 GHz linewidth

In this manuscript, a narrow linewidth fiber amplifier based on confined-doped fiber is established, and the power scaling and beam quality maintaining capabilities of this amplifier are investigated. Benefitted from the large mode area of the confined-doped fiber and precisely controlling the Yb-doped region in the fiber core, the stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) effects are effectively balanced. As a result, a 1007 W signal laser with just 1.28 GHz linewidth is obtained by combining the advantages of confined-doped fiber, near-rectangular spectral injection, and 915 nm pump manner. As far as we know, this result is the first beyond kilowatt-level demonstration of all-fiber lasers with GHz-level linewidth, which could provide a well reference for simultaneously controlling spectral linewidth, suppressing the SBS and TMI effects in high-power, narrow-linewidth fiber lasers.

1064 nm InGaAs metamorphic laser power converts with over 44% efficiency

InGaAs metamorphic laser power converters (LPCs) have the potential to deliver electrical energy over distances of several kilometers. In this study, metalorganic chemical vapor deposition (MOCVD) was used to grow InGaAs-based LPCs with an absorption wavelength of 1064 nm. At step thicknesses of 2800 nm, overshoot thicknesses of 6000 nm, reverse component and thicknesses of 2.4% and 700 nm, respectively, a surface roughness of 6.0 nm and InGaAs (24%) lattice relaxation of 93.7% of the InGaAs metamorphic buffer were obtained. The I-V characteristics of LPCs with 10 × 10 mm² apertures were investigated as a function of laser power and temperature. The maximum conversion efficiency of 44.1% and 550 hours of continuous stable operation at 4 W were demonstrated. Under 1064 nm laser illumination of 4 W, the temperature coefficients for the conversion efficiency and open-circuit voltage were -0.1%abs/°C and -1.6 mV/°C, respectively, and the LPC output power fluctuation was less than 0.5% during 216 hours of continuous temperature change from 20 to 100°C.

Compact integrated aberration-compensating module for a 9 kW Nd:YAG slab laser based on an off-axis stable-unstable resonator

An integrated aberration-compensating module (IACM), consisting mainly of an adjustable slab-aberration compensator, a one-dimensional Shack-Hartmann wavefront sensor, and a data processor, which meet the urgent requirements of correcting the specific wavefront aberrations of a slab laser based on an off-axis stable-unstable resonator, is designed and experimentally demonstrated. Benefits include compactness, robustness, simplicity, automation, and cost-effectiveness. The particular wavefront aberrations of the 9 kW level quasi-continuous-wave Nd:YAG slab laser, which have characteristics of asymmetry, large amplitude and gradient, high spatial frequency, and low temporal frequency, were measured and theoretically analyzed. In the experiment, the wavefront aberrations of the slab laser were corrected by the IACM. At the average output power of 9 kW, the diffraction-limited factor β was improved from 20.3 times diffraction limit (DL) to 3.6 times DL. The peak-to-valley and root-mean-square values of aberrations were reduced from 9.6 to 0.85 µm and from 2.86 to 0.18 µm within five iterations of the IACM, respectively. Moreover, The IACM is capable of maintaining the compensating surface figure after power-off.

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.

Development and Prospect of Wireless Power Transfer Technology Used to Power Unmanned Aerial Vehicle

Recently, unmanned aerial vehicles (UAV) have been widely used in the military and civil fields. However, the battery power is a key factor that restricts the operation range of the UAV. Using wireless power transfer (WPT) technology to power UAVs can improve the endurance of UAVs and enhance their maneuverability and flexibility. In this paper, the WPT technology is divided into three types: near-field WPT technology, far-field WPT technology and solar-powered UAV. The developments, challenges and prospects of these three types of WPT technologies used to power UAVs are summarized. For each type of WPT technology, the basic working principles are first introduced. The development of each type of WPT technology, as well as the challenges and application prospects in UAV charging, is introduced. The related works consist of academic and industry research, ranging from prototypes to commercial systems. Finally, three types of WPT technology used in UAV charging are compared and discussed, and the advantages and disadvantages of each type of WPT technology are shown. The related research showed that using WPT technology to power the UAV is a promising way to enhance the endurance of the UAV.

694 W sub-GHz polarization-maintained tapered fiber amplifier based on spectral and pump wavelength optimization

The comprehensive suppression of the stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) is a critical issue for the power scaling of fiber laser with sub-GHz spectral linewidth. In this manuscript, a narrow linewidth and polarization-maintained (PM) fiber amplifier based on tapered Yb-doped fiber (T-YDF) is established, and the effects of spectral linewidth, spectral shape and pump wavelength on the SBS and/or TMI thresholds are investigated. Up to 694 W polarization-maintained fiber laser with just ∼790 MHz linewidth is obtained by combining the advantages of tapered Yb-doped fiber, near-rectangular spectral injection and 915 nm pump manner. This work could provide a well reference solution for the realization of high-power ultra-narrow linewidth fiber lasers.

An Application of Artificial Neural Networks to Estimate the Performance of High-Energy Laser Weapons in Maritime Environments

Efforts to develop high-energy laser (HEL) weapons that are capable of being integrated and operated aboard naval platforms have gained an increased interest, partially due to the proliferation of various kinds of unmanned systems that pose a critical asymmetric threat to them, both operationally and financially. HEL weapons allow for an unconstrained depth of magazine and cost exchange ratio, both of which are essential characteristics to effectively oppose small unmanned systems, compared to their kinetic weapons counterparts. However, HEL performance is heavily affected by atmospheric conditions between the weapon and the target; therefore, the more precise and accurate the atmospheric characterization, the more accurate the performance estimation of the HEL weapon. To that end, the Directed Energy Group of the Naval Postgraduate School (NPS) is conducting experimental, theoretical and computational research on the effects of atmospheric conditions on HEL weapon efficacy. This paper proposes a new approach to the NPS laser performance code scheme, which leverages artificial neural networks (ANNs) for the prediction of optical turbulence strength. This improvement could allow for near real-time and location-independent HEL weapon performance estimation. Two experimental datasets, which were obtained from the NPS facilities, were utilized to perform regression modeling using an ANN, which achieved a decent fit (R2 = 0.75 for the first dataset and R2 = 0.78 for the second dataset).

Towards Ultimate High-Power Scaling: Coherent Beam Combining of Fiber Lasers

Fiber laser technology has been demonstrated as a versatile and reliable approach to laser source manufacturing with a wide range of applicability in various fields ranging from science to industry. The power/energy scaling of single-fiber laser systems has faced several fundamental limitations. To overcome them and to boost the power/energy level even further, combining the output powers of multiple lasers has become the primary approach. Among various combining techniques, the coherent beam combining of fiber amplification channels is the most promising approach, instrumenting ultra-high-power/energy lasers with near-diffraction-limited beam quality. This paper provides a comprehensive review of the progress of coherent beam combining for both continuous-wave and ultrafast fiber lasers. The concept of coherent beam combining from basic notions to specific details of methods, requirements, and challenges is discussed, along with reporting some practical architectures for both continuous and ultrafast fiber lasers.

Continuous wave high-power laser propagation in water is affected by strong thermal lensing and thermal blooming already at short distances

When laser beams propagate through media with non-vanishing absorption, the media is heated resulting in a change of the refractive index, which can lead to thermal lensing and thermal blooming. However, experimental details about both phenomena for propagations in water are lacking, especially for high-power lasers in the kilowatt range. We show that significant thermal lensing occurs only for high input powers before the onset of convective flow, while for low input powers, no strong thermal lens arises. After the onset of water flow, thermal blooming occurs at low input powers comparable to that known for propagations over kilometres in the air. However, for high input powers a thermal blooming on a qualitatively higher level is shown. By wavefront sensing, the change of refractive index distribution in water is investigated. This clearly shows the fast development of a strong thermal lens for high input powers and the onset of convection. Furthermore, a qualitatively good agreement of the accompanying simulations is observed. It is found that the absorption coefficient is linear with a value of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mu ={13.7}\,{\mathrm{m}^{-1}}$$\end{document}μ=13.7m-1 at least up to 7.5 kW, i.e. 8 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm{kW/cm}^2$$\end{document}kW/cm2. However, the directed transmission into an aperture is only constant before any thermal lensing of blooming occurs.

Coherent beam combination of seven 1.5 µm fiber amplifiers through up to 1 km atmospheric turbulence: near- and far-field experimental analysis

A laser testbed based on active coherent beam combination (CBC) of seven 1.5 µm, 3 W fiber amplifiers was developed for applications requiring high power such as power density deposition on targets or free space laser communication. For the first time to our knowledge, the frequency-tagging locking of optical coherence by single-detector electronic-frequency tagging technique was implemented in the field in real atmospheric turbulence conditions in a target-in-the-loop configuration. Successful combination was achieved after horizontal propagation of 311 m and 1 km, at 1.5 m above the ground, while the estimated average turbulence strength was Cn2∼4.10^-14m^-2/3. We present the CBC laser bench and an embedded near-field interferometer called PISTIL (PISton and TILt) able to measure the relative phase shift of each emitter. We show that this measurement can provide information on relative turbulence-induced phase variation of the combined laser beams. In particular, the far-field beam envelope wandering can be estimated through this diagnosis. Results are supported by an analytical model and confirmed by numerical post-analysis of measured far-field interference. This additional interferometer may improve CBC beam pointing through turbulence.

740-watt level optical tap coupler using side-polished large-mode-area double clad fibers for a high power fiber laser

We fabricated a fiber-optic directional coupler based on evanescent field coupling between side-polished large mode area (LMA) double clad fibers (DCFs) for a high power fiber laser. The tapping ratio of the fabricated coupler was measured to be - 32 dB. The fundamental mode coupled in a core of the lower side-polished fiber (SPF) was transferred to the upper SPF without clad-mode coupling. Two SPFs were directly faced to increase an optical handling power up to 740 W. The tapping ratio of the coupler was constantly maintained at the applied laser output. The beam quality of the laser including the fabricated coupler was maintained to be 1.22, without mode distortion by the coupler.

Optimal Path Configuration with Coded Laser Pilots for Charging Electric Vehicles Using High Intensity Laser Power Beams

Wireless power transmission (WPT) for wireless charging has been gaining wide attention as a promising approach to miniaturizing the battery size and increasing the maximal total range of an electric vehicle (EV). With an appropriate charging infrastructure, WPT holds great potential to accelerate the acceptance of EVs through users’ higher satisfaction, reducing EV cost, and increasing the driving range and capability. A WPT system based on high-intensity laser power beaming (HILPB) provides an optimal solution for wirelessly charging electric vehicles from a distance of several meters. Despite a large number of WPT approaches, the problem of optimal path configuration for charging EV remains an unexplored area. This paper proposes a method to determine the optimal power transmission path in environments where multiple power transmitters (PTXs) and power receivers (PRXs) are operated simultaneously. To this end, we modeled the HILPB power that reaches a PRX equipped with a photovoltaic (PV) array and validated the model by simulating the WPT process in an environment with multiple PTXs and PRXs using a direct-sequence optical code division multiple access (DS-OCDMA) system. In the simulation environment, upon receiving a request from a PRX, a PTX sent its power channel information through optically encoded laser pulses using each available wireless power channel (WPC). The PRX calculated the maximum deliverable power of a PTX and WPC based on the received channel power indicator of the incident laser beam. Based on the calculation results, it selected the optimal PTX and WPC for its maximum power requirement (MPQ). The MPQ of each PRX was satisfied by applying the algorithm for selecting the PTX according to the alignment and characteristics of the PTXs and PRXs. We modeled a power reception model of the PRX based on a PV array using coded laser pilots and validated it through experimentation. We discussed some algorithms that select the most suitable PTX among several PTXs for which several EVs receive the power it needs.

All-normal-dispersion dissipative soliton fiber laser using an offset-splicing graded-index-multimode-fiber-based saturable absorber

All-normal-dispersion (ANDi) dissipative soliton mode-locking is realized based on nonlinear multimode interference (NMI), which is implemented by offset-splicing three pieces of graded-index multimode fibers (GIMFs) and acts as a saturable absorber. The higher-order modes can be excited by offset-splicing GIMFs (OS-GIMFs), which eliminates adding the step multimode fiber (SIMF) into the resonant cavity and the precise length requirement of the SIMF. In the experiment, the stable dissipative soliton mode-locking at 1030 nm can be obtained with the pulse width of 7.3 ps and the repetition rate of 20.52 MHz, and the bandwidth is 6.98 nm. The maximum output is 3.2 mW with the pump power of 257 mW. The OS-GIMFs can significantly improve the saturated absorption and can easily realize dissipative soliton mode-locking in ANDi regions, which makes it attractive in ultrafast photonics.

Influence of thermal blooming on the beam quality of an array of Hermite-Gaussian beams propagating in the atmosphere

The influence of thermal blooming on the quality of an array of Hermite-Gaussian (H-G) beams propagating in the atmosphere is studied, where the incoherent combination is considered. An analytical expression of the equivalent distortion parameter of such an array is derived and validated. As the mode order or the inverse radial fill factor of an array of H-G beams increases, the thermal blooming effect weakens, requiring more time to reach steady-state thermal blooming. The focal shift of an array of H-G beams in the atmosphere is also investigated. Owing to the thermal blooming effect in the atmosphere, the actual focus moves away from the geometric focus as the mode order decreases, which is different from the behavior in free space. Additionally, for an array of multimode beams, the actual focus moves away from the target as the weighting factor of TEM₀₀ increases.

Photonic integrated circuits based hybrid integration for wavelength beam combining

In this Letter, we have demonstrated wavelength beam combining (WBC) through hybrid integration of photonic integrated circuits (PICs) to significantly reduce the size, weight, and operation power of the laser combining system. The hybrid integration WBC includes III/V semiconductor optical amplifiers (SOAs), which provide gain, and the silicon nitride PICs, which perform as the external cavity. We first show that the arrayed waveguide grating (AWG) -based hybrid laser defines the lasing wavelength through the AWG passband. We then demonstrate that the AWG successfully forms multiple channel lasers by combining SOAs in the hybrid platform.

Characterization of Absorption Losses and Transient Thermo-Optic Effects in a High-Power Laser System

(1) Background: The modeling, characterization, and mitigation of transient lasers, thermal stress, and thermo-optic effects (TOEs) occurring inside high energy lasers have become hot research topics in laser physics over the past few decades. The physical sources of TOEs are the un-avoidable residual absorption and scattering in the volume and on the surface of passive and active laser elements. Therefore, it is necessary to characterize and mitigate these effects in real laser systems under high-power operations. (2) Methods: The laboratory setup comprised a 10-kW continuous wave laser source with a changeable beam diameter, and dynamic registration of the transient temperature profiles was applied using an infrared camera. Modeling using COMSOL Multiphysics enabled matching of the surface and volume absorption coefficients to the experimental data of the temperature profiles. The beam quality was estimated from the known optical path differences (OPDs) occurring within the examined sample. (3) Results: The absorption loss coefficients of dielectric coatings were determined for the evaluation of several coating technologies. Additionally, OPDs for typical transmissive and reflective elements were determined. (4) Conclusions: The idea of dynamic self-compensation of transient TOEs using a tailored design of the considered transmissive and reflecting elements is proposed.

Analysis of the caustics of partially coherently combined truncated Gaussian beams

A theoretical model of the caustics of partially coherently combined beams (CBCs) was derived to model CBCs in the case of truncated Gaussian beams. The model enables the analysis of different lattice structures and partial coherence effects, including incoherent beam combining. The impacts of the coherence state, geometry of the array and truncation losses were analyzed. An optimal truncation level of about 6% was found and a maximum CBC efficiency of 62% was determined for this CBC system. With an increase in the beam numbers in the lattice, the relative caustics length decreases, which is consistent with the general rules of diffraction theory.

2.05 kW all-fiber high-beam-quality fiber amplifier with stimulated Brillouin scattering suppression incorporating a narrow-linewidth fiber-Bragg-grating-stabilized laser diode seed source

We experimentally demonstrate an all-fiber high-power fiber amplifier with high beam quality and a slope efficiency of 81.8%, using a fiber-Bragg-grating-stabilized laser diode as a narrow spectral linewidth (0.08 nm) seed source. During amplification, the spectral linewidth of the laser output is broadened from 0.08 to 0.24 nm due to nonlinear phenomena. To the best of our knowledge, we report the first experimental observation of the suppression of stimulated Brillouin scattering (SBS), with increased output power. In addition, we investigated the SBS suppression by simultaneously measuring the optical backscattered power, backscattered spectrum, and output spectrum at different values of output power. The beam quality, M², was measured to be ∼1.28 at the maximum output power of 2.05 kW, and modal instability was not observed.

Narrow-linewidth all-fiber amplifier with up to 3.01 kW output power based on commercial 20/400 μm active fiber and counterpumped configuration

A multikilowatt all-fiber amplifier with narrow spectral linewidth is investigated by using commercial 20/400 μm active fiber due to its more extensive application prospects. Stimulated Brillouin scattering (SBS) is suppressed effectively by combining multiple approaches including optimizing linewidth-broadened seed, shortening fiber length, and a counterpumped configuration. Consequently, the output power is pushed up to 3.01 kW with M²=1.17, and the linewidth is maintained at 48 GHz. The detailed characteristics of SBS are presented and analyzed in the frequency and time domains as output power is gradually raised. A high optical-to-optical efficiency is obtained with 85.6% and amplified spontaneous emission is reduced to 40 dB with respect to a laser signal. The proposed architecture reaches a relatively high output power with excellent beam quality under the condition of the equivalent linewidth for the commercial 20/400 μm active fibers in robustly all-fiberized structures.

Distinctive roles of elevated absorbing aerosol layers on free-space optical communication systems

The impact of enhanced local heating due to absorption of solar radiation by elevated layers of aerosol black carbon (BC) in the lower troposphere in the performance of free-space optical (FSO) communication links is investigated. It is seen that a strong elevated BC layer at an altitude around 4.5 km enhances the atmospheric stability locally and leads to a large reduction in the atmospheric refractive index structure parameter (Cn2), leading to improved performance of the FSO communication links. For layers in the tropical atmosphere with sufficiently high BC concentration, the signal attenuation due to BC absorption is alleviated by the large reduction in Cn2 due to BC-induced warming and brings down the link outage probability. Synergy between reduction in Cn2 and long wavelength transmission improves the link budget significantly by reducing the beam wander and number of adaptive optics units required.

Behavior of tiled-aperture arrays fed by vector partially coherent sources

This paper explores the far-zone behavior of partially coherent arrays. We derive an expression for the far-zone spectral density valid for any array composed of circular elements and fed by fields with Schell-model cross-spectral density functions. This expression is written as the sum of convolution integrals, making it easy to physically interpret. We discuss this expression at length and present examples. Lastly, we validate our analysis by comparing Monte Carlo averages from wave-optics simulations with theory. We conclude this paper with a brief summary of the results and potential uses of our work.

Array tilt in the atmosphere and its effect on optical phased array performance

This paper investigates atmospheric array tilt and its effect on target-in-the-loop optical phased array (OPA) performance. Assuming a direct-solve, piston-only-phase-compensation OPA, two expressions for the atmospheric array tilt variance are derived using Mellin transform techniques. The first-the "full" array tilt variance-is germane when the OPA is sensitive to atmospheric tilt and is shown to significantly impact OPA target-plane intensity. The second-the Zernike-tilt-removed array tilt variance-is relevant when a separate system compensates for atmospheric tilt (the more likely scenario) and is shown to negligibly affect OPA performance. To show how atmospheric array tilt errors affect target-plane intensity, moments of the far-zone (or focused) array intensity, as functions of the array tilt variance, are derived and discussed. Lastly, Monte Carlo simulation results are presented to validate the theoretical array tilt variance expressions.

Phase and amplitude beam shaping with two deformable mirrors implementing input plane and Fourier plane phase modifications

We find that ideas in optical image encryption can be very useful for adaptive optics in achieving simultaneous phase and amplitude shaping of a laser beam. An adaptive optics system with simultaneous phase and amplitude shaping ability is very desirable for atmospheric turbulence compensation. Atmospheric turbulence-induced beam distortions can jeopardize the effectiveness of optical power delivery for directed-energy systems and optical information delivery for free-space optical communication systems. In this paper, a prototype adaptive optics system is proposed based on a famous image encryption structure. The major change is to replace the two random phase plates at the input plane and Fourier plane of the encryption system, respectively, with two deformable mirrors that perform on-demand phase modulations. A Gaussian beam is used as an input to replace the conventional image input. We show through theory, simulation, and experiments that the slightly modified image encryption system can be used to achieve arbitrary phase and amplitude beam shaping within the limits of stroke range and influence function of the deformable mirrors. In application, the proposed technique can be used to perform mode conversion between optical beams, generate structured light signals for imaging and scanning, and compensate atmospheric turbulence-induced phase and amplitude beam distortions.

Multi-aperture laser transmissometer system for long-path aerosol extinction rate measurement

We present the theory, design, simulation, and experimental evaluations of a new laser transmissometer system for aerosol extinction rate measurement over long paths. The transmitter emits an ON/OFF modulated Gaussian beam that does not require strict collimation. The receiver uses multiple point detectors to sample the sub-aperture irradiance of the arriving beam. The sparse detector arrangement makes our transmissometer system immune to turbulence-induced beam distortion and beam wander caused by the atmospheric channel. Turbulence effects often cause spatial discrepancies in beam propagation and lead to miscalculation of true power loss when using the conventional approach of measuring the total beam power directly with a large-aperture optical concentrator. Our transmissometer system, on the other hand, combines the readouts from distributed detectors to rule out turbulence-induced temporal power fluctuations. As a result, we show through both simulation and field experiments that our transmissometer system works accurately with turbulence strength Cn2 up to 10^-12  m^-2/3 over a typical 1-km atmospheric channel. In application, our turbulence- and weather-resistant laser transmissometer system has significant advantages for the measurement and study of aerosol concentration, absorption, and scattering properties, which are crucial for directed energy systems, ground-level free-space optical communication systems, environmental monitoring, and weather forecasting.

Experimental observations of a laser suppression imaging system using pupil-plane phase elements

To help diminish the undesirable effects of laser irradiation on an imaging sensor, a pupil-plane phase element was introduced to broaden the point spread function, thereby reducing the focused laser irradiance. A sharpened image was subsequently restored via Wiener deconvolution. Successful experimental demonstrations employing a spatial light modulator in the pupil plane are reported for the vortex, axicon, and cubic phase. Furthermore, to circumvent information loss owing to zero values in the modulation transfer function, we demonstrate how images with different phase elements, combined with a joint deconvolution operation, provide an improved image.

Mode instability dynamics in high-power low-numerical-aperture step-index fiber amplifier

The study on mode instability (MI) in the large-mode-area fiber is generating great interest regarding the high-power applications of fiber lasers. To the best of our knowledge, we have investigated for the first time the dynamics of the output beam from a kilowatt-level all-fiber amplifier based on the low-numerical-aperture (<0.04) step-index (SI) fiber before and after the onset of the MI, including the temporal dynamics and mode evolution. The temporal power fluctuations indicate three evolution stages apart from the onset threshold of the MI, defined as stable, transition, and chaotic regions. In addition, the mode decomposition technique is utilized to accurately observe and investigate the mode evolution and relevant modal content corresponding to the transition and chaotic regions in the SI fiber laser for the first time. According to the mode decomposition results, the reduction of the extracted power can be explained by the high bending loss of the high-order mode excited in the MI process. Finally, the difference of MI dynamics between the fiber lasers based on the SI fiber and rod-type photonic crystal fiber is discussed.

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 et al. [Appl. Opt.54, F201 (2015)APOPAI0003-693510.1364/AO.54.00F201] and Nelson et al. [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.

High-power lasers for directed-energy applications: comment

Sprangle et al. [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 1 GHz over 7 km distance. In this comment we also challenge the major conclusion of Sprangle et al. [Appl. Opt.54, F201 (2015)APOPAI0003-693510.1364/AO.54.00F201] and the more recently published paper by Nelson et al. [Appl. Opt.55, 1757 (2016)APOPAI0003-693510.1364/AO.55.001757] regarding inefficiency of coherent beam combining under typical atmospheric conditions.

Atmospheric propagation and combining of high power lasers: comment

Nelson et al. [Appl. Opt.55, 1757 (2016)APOPAI0003-693510.1364/AO.55.001757] recently concluded that coherent beam combining and remote phase locking of high-power lasers are fundamentally limited by the laser source linewidth. These conclusions are incorrect and not relevant to practical high-power coherently combined laser architectures.

Temporal coherence effects on target-based phasing of laser arrays

This paper studies how temporal coherence (in particular, linewidth broadening introduced to suppress stimulated Brillouin scattering) affects target-based phasing of fiber laser arrays. A radio-frequency modulated array whose elements are fed by a broadband laser source phasing on a remote step target is theoretically analyzed. An expression for the detector plane irradiance, ultimately used to phase the array on the target, is derived and discussed in detail. Simulation results of a seven-element hexagonal array phasing on a distant step target with scattering surfaces separated by many coherence lengths are presented to validate the theoretical findings.

Implementation of a long range, distributed-volume, continuously variable turbulence generator

We have constructed a 180-m-long distributed, continuously variable atmospheric turbulence generator to study high-power laser beam propagation. This turbulence generator operates on the principle of free convection from a heated surface placed below the entire propagation path of the beam, similar to the situation in long-distance horizontal propagation for laser communications, power beaming, or directed energy applications. The turbulence produced by this generator has been characterized through constant-temperature anemometry, as well as by the scintillation of a low-power laser beam.

Monolithic fiber end cap collimator for high-power free-space fiber-fiber coupling

In this paper, we present the design, construction, and testing of a monolithic fiber end cap collimator for high-power free-space fiber-fiber coupling applications. The collimator is based on a large-sized fiber end cap and a spherical lens design on the output facet. Values of the spot size and working distance are theoretically analyzed based on Gaussian approximation and ABCD transmission matrix. The free-space fiber-fiber coupling process is also analyzed for different lens curvature radii and coupling distances. In the experiment, a collimated laser beam is obtained with Rayleigh length of about 400 mm. A high-power laser with 1.1 kW output is tested on the end cap collimator, which only heats up by 7°C at the output facet without active cooling. Free-space fiber-fiber coupling between two 20/400 μm fibers is achieved based on these collimators, with measured coupling loss lower than 0.3 dB.

Atmospheric propagation and combining of high-power lasers

In this paper, we analyze beam combining and atmospheric propagation of high-power lasers for directed-energy (DE) applications. The large linewidths inherent in high-power fiber and slab lasers cause random phase and intensity fluctuations that occur on subnanosecond time scales. Coherently combining these high-power lasers would involve instruments capable of precise phase control and operation at rates greater than ∼10  GHz. To the best of our knowledge, this technology does not currently exist. This presents a challenging problem when attempting to phase lock high-power lasers that is not encountered when phase locking low-power lasers, for example, at milliwatt power levels. Regardless, we demonstrate that even if instruments are developed that can precisely control the phase of high-power lasers, coherent combining is problematic for DE applications. The dephasing effects of atmospheric turbulence typically encountered in DE applications will degrade the coherent properties of the beam before it reaches the target. Through simulations, we find that coherent beam combining in moderate turbulence and over multikilometer propagation distances has little advantage over incoherent combining. Additionally, in cases of strong turbulence and multikilometer propagation ranges, we find nearly indistinguishable intensity profiles and virtually no difference in the energy on the target between coherently and incoherently combined laser beams. Consequently, we find that coherent beam combining at the transmitter plane is ineffective under typical atmospheric conditions.