Editor-in-Chief Michael Eismann reflects on the responsibilities of mentorship.

This guest editorial introduces the Special Section on Atmospheric Propagation.

The performance of free-space optical applications can be improved using beams of different wavelengths for the auxiliary actions of pointing/tracking or turbulence correction. Chromatic dispersion owing to the atmosphere is an issue for multiwavelength systems, and the dispersion of electromagnetic signals is typically predicted based on refractive conditions from standard atmospheric models. However, for long near-horizontal paths near the Earth’s surface, substantial refractive index gradients that are associated with features such as inverse temperature layers and ducts can be encountered. These features can significantly alter the ray trajectory, the chromatic divergence, and the angle of arrival of directional beams relative to standard atmosphere predictions. A ray tracing approach was implemented to examine the chromatic divergence and angle of arrival of the rays through various practical and extreme atmospheric conditions involving a temperature inversion layer. Over a distance of 150 km along the ground, a brief encounter with the layer can cause pairs of rays with wavelengths 532 and 1550 nm to diverge up to 4.5 times greater than their standard atmosphere predictions. For a single wavelength, a linear increase of angle of arrival with initial launch angle was found for the standard atmosphere, but this trend was significantly altered in the presence of an inversion layer. Extreme refractive conditions with a large inversion layer were simulated to produce optical ducting over long distances. Chromatic separation of rays as large as 280 m was observed when only one of the two wavelengths remained in the duct.

We develop and study two approaches for the prediction of optical refraction effects in the lower atmosphere. Refraction can cause apparent displacement or distortion of targets when viewed by imaging systems or produce steering when propagating laser beams. Low-cost, time-lapse camera systems were deployed at two locations in New Mexico to measure image displacements of mountain ridge targets due to atmospheric refraction as a function of time. Measurements for selected days were compared with image displacement predictions provided by (1) a ray-tracing evaluation of numerical weather prediction data and (2) a machine learning algorithm with measured meteorological values as inputs. The model approaches are described and the target displacement prediction results for both were found to be consistent with the field imagery in overall amplitude and phase. However, short time variations in the experimental results were not captured by the predictions where sampling limitations and uncaptured localized events were factors.

Part I of this two-part paper uses wave-optics simulations to look at the Monte Carlo averages associated with turbulence and steady-state thermal blooming (SSTB). The goal is to investigate turbulence thermal blooming interaction (TTBI). At wavelengths near 1  μm, TTBI increases the amount of constructive and destructive interference (i.e., scintillation) that results from high-power laser beam propagation through distributed-volume atmospheric aberrations. As a result, we use the spherical-wave Rytov number and the distortion number to gauge the strength of the simulated turbulence and SSTB. These parameters simplify greatly given propagation paths with constant atmospheric conditions. In addition, we use the log-amplitude variance and the branch-point density to quantify the effects of TTBI. These metrics result from a point-source beacon being backpropagated from the target plane to the source plane through the simulated turbulence and SSTB. Overall, the results show that the log-amplitude variance and branch-point density increase significantly due to TTBI. This outcome poses a major problem for beam-control systems that perform phase compensation.

Part II of this two-part paper uses wave-optics simulations to look at the Monte Carlo averages associated with turbulence and time-dependent thermal blooming (TDTB). The goal is to investigate turbulence thermal blooming interaction (TTBI). At wavelengths near 1  μm, TTBI increases the amount of constructive and destructive interference (i.e., scintillation) that results from high-power laser beam propagation through distributed-volume atmospheric aberrations. As a result, we use the spherical-wave Rytov number, the number of wind-clearing periods, and the distortion number to gauge the strength of the simulated turbulence and TDTB. These parameters simply greatly given propagation paths with constant atmospheric conditions. In addition, we use the log-amplitude variance and the branch-point density to quantify the effects of TTBI. These metrics result from a point-source beacon being backpropagated from the target plane to the source plane through the simulated turbulence and TDTB. Overall, the results show that the log-amplitude variance and branch-point density increase significantly due to TTBI. This outcome poses a major problem for beam-control systems that perform phase compensation.

An experiment was conducted to study turbulence along a 149-km path between the Mauna Loa and Haleakala mountain tops using digital cameras and light-emitting diode (LED) beacons. Much of the path is over the ocean, and a large portion of the path is 3 km above sea level. On the Mauna Loa side, six LED beacons were placed in a roughly linear array with pair spacings from 7 to 62 m. From the Haleakala side, a pair of cameras separated by 83.8 cm observed these beacons. Turbulence along the path induces tilts on the wavefronts, which results in displacements of the LED spots in the images. The image motion is caused by unwanted noise sources such as camera platform motion. Differential motion between spots cancels much of this noise, and this differential motion is weighted by the turbulence along the path in different ways depending on the geometry between the sources and the cameras. A camera motion insensitive weighting function is developed to deal with this observational issue. A linear combination of these weighting functions is then used to generate a composite weighting function, which better rejects turbulence near the sources and receivers and is most sensitive to turbulence in the portion of the path out over the ocean. This technique is used to estimate turbulence in this region. The long range involved caused very strong scintillation in the image, which added new challenges to the data processing. A resulting estimate for Cn2 of 4  ×  10^−  17  m^{−  2  /  3} is in good agreement with the Hufnagel–Valley HV5/7 model and the results of numerical weather modeling.

Long-range optical imaging applications are typically hindered by atmospheric turbulence. The effect of turbulence on an imaging system can manifest itself as an image blur effect usually quantified by the phase distortions present in the system. The blurring effect can be understood on the basis of the measured strength of atmospheric optical turbulence along the propagation path and its impacts on phase perturbation statistics within the imaging system. One method for obtaining these measurements is by the use of a dynamically ranged Rayleigh beacon system that exploits strategically varied beacon ranges along the propagation path, effectively obtaining estimates of the aberrations affecting an optical imaging system. We developed a method for extracting tomographic turbulence strength estimations from a dynamically ranged Rayleigh beacon system that uses a Shack–Hartmann sensor as the phase measurement device. The foundation for extracting tomographic information from strategically range-varied beacon measurements obtained in rapid sequence is presented along with modeled example cases for typical turbulence scenarios. Additionally, the processing algorithm was used to simulate identification of isolated strong turbulence layers. We present the chosen processing algorithm’s foundation and provide discussion of the utility of this algorithm as an atmospheric turbulence profiling methodology.

We advance the benefits of previously reported four-dimensional (4-D) weather cubes toward the creation of high-fidelity cloud-free line-of-sight (CFLOS) beam propagation for realistic assessment of autotracked/dynamically routed free-space optical (FSO) communication datalink concepts. The weather cubes accrue parameterization of optical effects and custom atmospheric resolution through implementation of numerical weather prediction data in the Laser Environmental Effects Definition and Reference atmospheric characterization and radiative transfer code. 4-D weather cube analyses have recently been expanded to accurately assess system performance (probabilistic climatologies and performance forecasts) at any wavelength/frequency or spectral band in the absence of field tests and employment data. The 4-D weather cubes initialize an engineering propagation code, which provides the basis for comparative percentile performance binning of FSO communication bit error rates (BERs) as a function of wide-ranging azimuth/elevation, earth-to-space uplinks. The aggregated, comparative BER binning analyzes for different regions, times of day, and seasons applying a full year of 4-D weather cubes data provided numerous occasions of clouds, fogs, and precipitation events. The analysis demonstrated the utility of 4-D weather cubes for adroit management of CFLOS opportunities to enhance performance analyses of point-to-point as well as evolving multilayer wireless network concepts.

Performance metrics such as mean-bit-error rate and probability of fade for free-space optical communication (FSOC) applications using intensity-modulation direct detection are theoretically calculated based on probability density functions (PDFs) describing irradiance fluctuations. Theoretical calculations using common PDF models can result in significant errors in prediction of performance metrics. In particular, these models do not consider the change in skewness of the distribution as the aperture size increases, and often positively skewed distributions (right tailed) are used to model scenarios where the true statistics are negatively skewed (left tailed). We evaluate the magnitude of errors in the prediction of bit-error rate and probability of fade based on simulation data for a collimated Gaussian beam in a realistic FSOC scenario for two strengths of turbulence and varying aperture sizes. The PDF models considered are lognormal, gamma–gamma, inverse-Gaussian-gamma, fractional exponential, exponentiated-Weibull, 3-parameter-Weibull, and normal distributions.

Simulating atmospheric turbulence is an essential task for evaluating turbulence mitigation algorithms and training learning-based methods. Advanced numerical simulators for atmospheric turbulence are available, but they require evaluating wave propagation, which is computationally expensive. We present a propagation-free method for simulating imaging through turbulence with applications in ground-to-ground imaging. The key idea behind our work is a method to draw intermodal and spatially correlated Zernike coefficients. By establishing the equivalence of the angle-of-arrival correlation by Basu, McCrae, and Fiorino (2015) with the multiaperture correlation by Chanan (1992), we show that the Zernike coefficients can be drawn according to a covariance matrix defining the correlations. We propose fast and scalable sampling strategies to draw these samples. The method allows us to compress the wave propagation problem into a sampling problem, hence making the simulator significantly faster than existing ones. Experimental results show that the simulator has an excellent match with the theory and real turbulence data. We anticipate the simulator, which balances speed and accuracy, will be a useful tool for various computational imaging applications.

Underwater imaging has been increasingly employed in vision-based marine research. However, the inappropriate installation of a light source and the complex underwater environment will result in the uneven illumination and overexposure on the captured images. To address these issues, an underwater image enhancement framework for autonomous underwater vehicles platform is proposed, which consists of underwater light source optimization and illumination nonuniformity correction. The light source optimization method improves the imaging quality by computing an appropriate angle of the light casting. In this way, the center of the field of view is always well lit. In addition, an adaptive filter-based illumination correction algorithm is proposed to solve the uneven illumination caused by the artificial light source. During this process, image block segmentation and the measure of image enhancement index are applied to improve the adaptability and reduce the calculation errors of the filter parameters. A dataset with real underwater images collected under different natural conditions has been built and tested. The experimental results indicate that the proposed method is more adaptive and effective than the typical methods.

We present a solution to the problem of light scattering by spherical particles, adapted for interpreting the signals of portable lidars for autonomous vehicles. The solution was obtained for typical wavelengths used in laser sensing tasks: 0.355, 0.532, 0.905, 0.940, 1.064, 1.55, 2.15, and 10.6  μm. The solution was obtained within the framework of the Mie scattering theory for water and ice. The inherent high-frequency oscillations in the backscattering direction are smoothed out by means of a moving average, which allows one to construct fast and efficient algorithms for particle size distributions observed in the atmosphere. The resulting solution is presented in the form of a data bank, which is available in the public domain.

Lensless inline digital holographic microscopy (LI-DHM) and Fourier ptychographic microscopy (FPM) are two widespread quantitative phase imaging (QPI) techniques. They have been employed in various fields, especially for biological slice imaging because of their simplicity in use, stability in structure, and also large field of view. Spherical phase response (for example from HeLa cells) is commonly observed in biological imagery. As a consequence, for calibration and validation purposes, small (several to tenth of microns in diameter) transparent microbeads have been used as standards. Phase imaging of their large counterparts (hundreds of microns in diameter) using either LI-DHM or FPM has not been reported so far. We are aiming to analyze the phase response of a 146-μm soda-lime microsphere. It has been immersed in Canada balsam to reduce phase difference and to avoid overexposed diffraction rings. The phase estimation issue has been tackled using approaches that involve either Gerchberg–Saxton type algorithms or an inverse problem-based procedure. Confronting the results confirms the QPI capability for both imaging techniques to assess phase responses from such a large transparent object.

Four different methods for calculating the radial profile of the Fourier transform of a circularly symmetric function are discussed and evaluated. Applications to optics, in which the circularly symmetric function represents the distribution of field across a circular aperture, are many. The four approaches include: (1) numerical integration of the zero-order Hankel transform equation (using the Gauss–Kronrod method); (2) extraction of the radial profile of a two-dimensional Fourier transform; (3) the projection-transform method that makes use of the projection-slice theorem of Fourier analysis; and (4) the quasifast Hankel transform introduced by Siegman. The results indicate that the projection-transform method is the fastest method and that it has accuracy that is second only to numerical integration.

Based on chaotic biological cryptography and computer-generated holography, a symmetric–asymmetric hybrid encryption and decryption system was proposed. This method has multiple keys, in which two decryption keys are different from the encryption keys. The encryption and decryption system possesses both symmetric and asymmetric key cryptography features. The biometric code is unique and unchangeable, not easy to be stolen or lost, which further enhances the security of the system. Chaos technology can reduce the amount of information, when combined with the advantages of the digitization of computer-generated holography, which can be conducive to the preservation and the transmission of encrypted information. The numerical simulation results show that this scheme is feasible and has better security and higher robustness.

To improve the phase-measuring profilometry measurement speed, phase-shifted fringes can be encoded into color channels. However, for colored objects, the intensity of the fringes in each channel is affected by surface texture. Therefore, a measurement method based on color coding and sinusoidal correction is proposed. First, two color images are encoded. The red (R) and blue (B) channels for the first image and the green (G) channel for the second image are used in a three-step phase-shifting process. Meanwhile, the G channel of the first coded image and the R and B channels of the second coded image are corresponding solid colors. Two color-coded images are projected onto the object’s surface, and the CCD captures the modulated deformed fringe pattern. After decoupling, the channels of the images are separated, the pure color channels are extracted, and the reflectivity of each point on the object surface in the RGB channels is calculated. Then, the intensity of the phase-shifted fringes in the two images is corrected by the calculated reflectivity. A 24-color standard color card and color lion mask were used to verify the method. Compared with the traditional color coding method, we were able to reduce the influence of color texture and obtain better results.

The determination of residual water content is vital for the quality control of high explosives, such as octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). However, conventional analytical methods are time-consuming and inaccurate. We have proposed a simple, sensitive, and accurate method for moisture determination based on terahertz (THz) time-domain spectroscopy. A model based on effective medium theory was implemented by a third-order extension of the Landau, Lifshitz, and Looyenga model to predict the moisture content of HMX based on THz spectroscopy. The predicted values of moisture content based on the model were found to be in excellent agreement with the results obtained using gravimetric measurements. Overall, the results proved the effectiveness of the proposed method for rapid and accurate moisture determination of HMX.

Hyperspectral cameras capture images where every pixel contains spectral information of the corresponding small area of the depicted scene. Spatial misregistration—differences in spatial sampling between different spectral channels—is one of the key quality parameters of these cameras, because it may have a large impact on the accuracy of the captured spectra. Spatial misregistration unifies various factors, such as differences in the position of the optical point spread function (PSF) in different spectral channels, differences in PSF size, and differences in PSF shape. Ideally, there should be no difference in spatial sampling across the spectral channels, but in any real camera, all these factors are present to some degree. Our work shows the magnitude of the spectral errors caused by these spatial misregistration factors of different magnitudes and in various combinations when acquiring hyperspectral images of real scenes. The spectral errors are calculated in Virtual Camera software where high-resolution airborne images of real-world scenes and several PSFs of different hyperspectral cameras are used as the input. The misregistration factors are simulated. Two different methods for quantifying spatial misregistration in the lab are also tested and compared using the correlation with the errors in the real-world scenes as the criterion. The results are used to suggest the best camera characterization approach that would adequately predict spatial misregistration errors and allow reliable comparison of different hyperspectral cameras to each other.

Traceable, highly accurate calibration of wavefront sensors such as Shack–Hartmann sensors is a challenging task and an active field of research. We have developed a measurement system for the traceable calibration of wavefront sensors employing spherical wavefronts and a point light source in combination with a three-axis linear stage. We obtain an absolute calibration of sensor errors, including the reference spot position and wavefront gradient-dependent errors for each microlens individually. We discuss the main error influences and present an initial measurement uncertainty budget for the calibration. The calibration can be performed with an expanded measurement uncertainty (95% coverage interval) of better than 5  μrad for the wavefront gradient deviation.

The vehicle LIDAR has been widely used in the fields of three-dimensional city modeling, terrain mapping, the agroforestry ecosystem, military detection, and unmanned driving for its advantages of high resolution, strong anti-interference ability, small volume, and light weight. The measurement accuracy of vehicle LIDARs is influenced by the internal heat source of the system and the external ambient temperature to a great extent. A compact vehicle LIDAR system is designed and analyzed. Combined with the distribution characteristics of the internal heat source of the LIDAR system, the temperature distribution and thermal deformation in the LIDAR system under different external temperatures are analyzed. On this basis, the laser path deviation of the LIDAR system caused by thermal deformation is also analyzed and compared. Finally, when the ambient temperature changes from −10  °  C to 50°C, the deviation of the laser output angle is within ±0.1 mrad by optimizing the design, which meets the requirements of the measurement accuracy of the vehicle’s LIDAR.

Scattering of light from a surface depends on its microroughness. In general, total integrated scattering of a surface is expressed in terms of root-mean-square microroughness (σ_rms) using empirical relations. These empirical relations assume that the σ_rms is measured over the spatial scales ranging from 0 to surface length/diameter (L). In reality, σ_rms will be measured over finite spatial lengths/scales that result in band-limited microroughness (σ_bl) and hence band-limited integrated scatter (BLIS). BLIS depends on the microroughness of surface over the finite spatial bandwidths rather than infinite spatial scales. Scatter from super polished optical mirrors peaks in specular direction and falls off exponentially. Hence, BLIS measurements in near-specular direction provide crucial information in understanding the relation between surface microroughness and scatter. Our work gives a detailed description of BLIS measurements carried out on super polished optical surfaces with different surface microroughness and a comparison between the scatter and surface microroughness measurements.

With the increase in digital image correlation (DIC) applications, it is an important task to realize the fully automatic measurement and provide an automatic deformation measurement sensor. Keeping this in mind, we propose an automatic speckle region selection method for DIC. This method can be divided into three main steps: initial contour selection, active contour computation, and morphological operation. The sum of modulus of the local intensity gradient vector of the local image is used to determine the initial contour, the active contour computation is driven by improved automatic local Gaussian distribution fitting energy, and the morphological closing operation is finally conducted to achieve better robustness. Experimental results show that the root mean squared error of the localization is less than 0.8260 pixels. We further integrate the proposed method into an open-source DIC software, and the fully automatic deformation analysis is realized. The fully automatic deformation measurement will further widen the application scope of DIC, especially in the field of smart robotics and smart city.

Aiming at small size, compact structure, strong light-collecting ability, and low loss requirements of optical coherence tomography (OCT) probes used for human body detection, we design an all-fiber OCT probe, which is composed of a single-mode fiber and a multimode fiber (MMF) with spherical lens. By measuring the optical performance of these probes, we found that, when the length of the MMF of the composite probe is 372  μm and the diameter of the ball lens is 300  μm, it has the strongest focusing effect and a large reflected light signal receiving angle. Furthermore, the OCT probe studied was applied to a self-made spectral domain OCT system for validation. The results show that our probe can obtain a clear two-dimensional grayscale image of three layers of glass and fish scales, i.e., the probe manufacturing scheme can be used. Furthermore, it meets the needs of miniature OCT probes, which provides more effective options for measuring human blood vessels or other biological tissues.

A laser signal simulation system based on a backward transmission screen is proposed. The system is used to test the performance of laser-guided weapons in a laboratory environment. The system enables the simulation of the laser-guided weapons by controlling the movement of a two-axis motor and changing the laser intensity. In addition, using the ScramNet ring topology, real-time hardware-in-the-loop simulation (HILS) can be implemented through the system. In terms of the scale and the complexity of the system, this laser simulator is relatively more compact and simpler than other previously reported cases. Moreover, the diffusivity of the laser beam can be highly improved through the transmission screen. This implies that diffused laser signals reflected from a target can be accurately simulated through the system. A closed-loop HILS for a target tracking scenario is done using the laser signal simulation system. The system satisfies the designed requirements and proves itself to be an innovative simulation method for laser-guided weapons with semiactive seekers.

We present five-splitting equal-intensity laser output based on a T-shaped reflection grating under normal incidence, which is aimed to reduce polarization loss. Although five-splitting has been reported by a single-layer grating, a set of parameters can only be effective for either TE or TM polarization. Both TE and TM polarizations can be operated simultaneously by T-shaped arrays. For only one set of groove profiles, splitting efficiencies of 18.26%/19.32%/18.53% and 18.34%/18.25%/19.56% can be reached in ±2nd  /    ±  1st  /  0th orders for TE and TM polarizations, respectively. Within the incident wavelength of 1544 to 1552 nm and the incident angle range of 0 deg to 0.19 deg, each reflective efficiency for a five-splitting laser output is >17  %  . The diffraction mechanism inside the grating can be explained by the modal method. The polarization-independent T-shaped grating with five-splitting laser output presented here should be useful in various laser fields due to good manufacturing tolerance and incident performance.

Holographic search algorithms such as direct search (DS) and simulated annealing allow high-quality holograms to be generated at the expense of long execution times. This is due to single iteration computational costs of O  (  N_xN_y  )   and number of required iterations of order O  (  N_xN_y  )  , where N_x and N_y are the image dimensions. This gives a combined performance of order O(Nx2Ny2). We use a technique to reduce the iteration cost down to O  (  1  )   for phase-sensitive computer-generated holograms, giving a final algorithmic performance of O  (  N_xN_y  )  . We do this by reformulating the mean-squared error (MSE) metric to allow it to be calculated from the diffraction field rather than requiring a forward transform step. For a 1024  ×  1024-pixel test images, this gave us a ≈50,000  ×   speed-up when compared with traditional DS with little additional complexity. When applied to phase-modulating or amplitude-modulating devices, the proposed algorithm converges on a global minimum MSE in O  (  N_xN_y  )   time. By comparison, most extant algorithms do not guarantee that a global minimum is obtained. Those that do, have a computational complexity of at least O(Nx2Ny2) with the naive algorithm being O  [    (  N_xN_y  )    !    ]  .

Currently, the highest power quantum cascade lasers (QCLs) include (i) an efficient active region design, which minimizes the energy fraction transferred into heat, (ii) a relatively large number (40) of cascades, which increases the mode confinement factor, Γ, and (iii) narrow buried-heterostructure waveguides, which efficiently cool the active region through efficient lateral heat flow. This combination allows watt-level continuous wave (cw) emission powers at room temperature. Moreover, narrow waveguides favor the fundamental TM₀₀ lateral mode, resulting in a nearly Gaussian beam. To the extent that power efficiency is already largely optimized, power scaling in narrow-stripe QCLs is only possible through increasing the laser length, which is limited by the needs for robust construction and transparent antireflection coatings. Broad-area (BA) QCLs with a small number of cascades can also deliver watt-level cw powers at room temperature, profiting from the enhanced vertical heat flow. Power can be scaled for these lasers also via the laser width and is nearly unlimited. The usual drawback of the BA lasers is poor beam quality. We discuss the use of tapered QCLs and the first demonstration of room temperature cw operation of the BA QCL in fundamental TM₀₀ mode. A stable and reproducible transverse optical mode is achieved using a double-taper waveguide geometry. The laser emits at 4.6  μm and has a demonstrated room temperature cw power of 100 mW.

As optical distribution frames (ODFs) carry increasing amounts of data, issues relating to their port management are becoming crucial. Image backhaul inspection has been widely accepted as an effective technology to monitor the occupancy status of ODF ports. However, the massive amount of on-site images requires a large team of off-site technologists to manually identify the occupancy status of each ODF port, which is time-consuming. We employ the you only look once version 3 (YOLOv3) network to automatically recognize ODF port occupancy. The YOLOv3 is a state-of-the-art convolutional neural network that has been shown to be very efficient for object detection in terms of processing speed and accuracy for the common objects in context. To accommodate ODF images with densely arranged small objects, high resolutions, closely spaced adjacent ports, and occlusion, we modified the original YOLOv3 with four-scale feature fusion, anchor box dimension clustering, and soft nonmaximum suppression filtering. Experiments showed a 7.38% increase in the original YOLOv3 detection accuracy rate of 91.45%. The new method can update the image backhaul inspection to automatically realize port resource management. The number of required port management technologists is considerably reduced, and the accuracy of port resources is increased, resulting in significant network investment savings.

A multirod solar side-pumping concept is proposed to study the feasibility of multirod/multibeam laser systems in large-sized solar furnaces. This scheme was based on the one-megawatt solar furnace (MWSF) in Odeillo, France, which collected and concentrated solar rays into an input face of a solar homogenizer placed at the focal zone. At its output face, 12 laser heads were positioned, each of which was composed of a two-dimensional compound parabolic concentrator that focused the uniform solar radiation from the homogenizer’s output to an Nd:YAG rod mounted inside a fused silica flow tube. Each laser rod was pumped by one-twelfth of the total concentrated solar power transmitted through the homogenizer, ensuring a good thermal performance under highly concentrated solar power. A 22.84-kW total multimode solar laser power, corresponding to 12.48-W  /  m² collection efficiency and 2.28% solar-to-laser power conversion efficiency, was numerically determined. The latter represents an enhancement of 1.75 times over the previous numerically calculated value for a multirod/multibeam laser scheme based on another one-MWSF near Tashkent, Uzbekistan.

Durable window materials with minimal optical loss are important for future high-energy laser (HEL) systems that will operate at or near the megawatt level. Sapphire is recognized as a promising HEL window candidate due to its outstanding optical transparency and mechanical properties. However, its weak scattering characteristics and absorption levels near the 1-μm wavelength region need to be lowered further for future HEL applications. In our study, the weak absorption and scattering of sapphire samples provided by three vendors were measured. Ultraviolet–visible spectroscopy measurements were made in the wavelength range 190 to 600 nm and several absorption bands due to vacancies and impurities were detected. The bulk absorption of several samples at wavelengths of 355, 532, and 1064 nm were measured using photothermal common-path interferometry and the absorption coefficient values obtained were in the range of ^{10  −  5} to 10^−  2  cm^−  1 and increased as the wavelength decreased. An empirical weak absorption tail model was used to fit the measured data. In addition, scattering measurements on all samples were made at 405, 532, 633, 1064, and 1550 nm using an instrument developed to assess the bidirectional scatterance probability distribution function. The total integrated scatterance was in the range of ^{10  −  4} to ^{10  −  2} and increased for all samples as the wavelength decreased. Surface roughness was found to contribute insignificantly to the scattering loss, while bulk defects along with subsurface damage have major impact. A simple single-scatter model was developed and applied to the measured bulk scattering data. The model suggests that impurity particles, porosity, and other density variations exist with a range of sizes that contribute to scattering. Overall, the measurements indicate that both weak absorption and scattering losses are strongly related to defect structures such as lattice disorder and impurities that were introduced during crystal growth or postgrowth processing. Understanding these defects and their contributions to optical loss can lead to improved manufacturing and processing methods.

Recently, the use of carrier frequencies in the domain of wireless bearer has been enhanced to fulfill the data transmission need. The research activities are coordinated to wireless communication with high information rates and amended security. An idea of the slotted rectangular patch antenna that depends on the photonic crystal structure is offered for a wireless communication system. The proposed antenna is contrasted with the accustomed slotted antenna. In addition, the investigation of the designed antenna is performed with various radius values and lattice constant. The negligible return loss of −43.79  dB and voltage standing wave ratio of 1.1 are accomplished at 2-μm radius with the lattice constant of a  =  5  μm at f_r  =  2.23  THz.

A photonic crystal fiber (PCF) made up of a spider-web such as cladding and an octagonal core is proposed for sensing applications. Three widely used industrial liquids and/or by-products, namely, water, acetic acid, and glycerol, have been investigated using the full vector finite element method. Extremely high sensitivities of 99.5%, 99.7%, and 99.9% were achieved at 4.5 THz for water, acetic acid, and glycerol, respectively. In addition to the sensitivity, extremely low effective material losses of 0.0004138, 0.000306, and 0.000113  cm^−  1 were achieved for water, acetic acid, and glycerol, respectively. Other parameters such as confinement loss, nonlinearity, and dispersion equally showed well-enhanced results compared to previous related works. The excellent sensing performance and ultralow confinement loss of this proposed PCF chemical-sensor make it a worthy candidate for use in many industrial sectors.

Our paper presents the design, fabrication, and characterization of metamaterial absorber-based microwave imaging detector that is operating in industrial, scientific, and medical (ISM) band. The results reveal that the structure has almost perfect absorption at 2.39 GHz in the simulation and 2.51 GHz in the experimental measurement. For energy-harvesting applications, Schottky diodes have been used and 11.8-mV dc voltage across a Schottky diode has been observed with 84.2% dc conversion efficiency in harvesting application. To show the different incident angle imaging, MATS-1000 antenna training kit is used, and different imaging pictures are given, which are obtained by MATLAB with the help of a microcontroller card. Both experimental and simulation study results verify that the microwave detector generates accurate images with negligible distortions. The innovative side of this study when it is compared with similar studies can be sorted as having more dc obtained voltage, incident angle characterization, and operation frequency. Simulated and measurement results show that the proposed structure can effectively be used in the imaging at ISM frequency band, which is the most common frequency band in wireless appliances.

Fiber optic microstructure turbulence sensors (FOMTS) with fiber Bragg gratings can measure the rate of tangential velocity change caused by ocean turbulence. The theoretical principles of the FOMTS and demodulation were verified by a prototype. The demodulation system resolution reached 0.001  pm  /  Hz, the sensitivity was 4.37  ×  10^−  16  m² s²  /  kg, and its natural frequency in air was 595 Hz, similar to the theoretical values. Its resonance frequency in water (534 Hz), exceeded the highest frequency of ocean turbulence (200 Hz). Furthermore, comparative experiments between the FOMTS and a commercially available sensor were performed. Good signal correspondence was observed, proving the effectiveness of the FOMTS and verifying the design principle.

This erratum corrects an error in Eq. 1.