Editor-in-Chief Adam Wax sorts out the pros and cons of Zoom meetings vs. in-person interactions.

Solid-state lighting has been moving into new directions since its emergence in the 1990s. Light-emitting diodes (LEDs) and LED-based luminaires have seen impressive developments over time and now much research interest is being devoted to the development of a closely aligned lighting technology, based on using laser diodes. In many ways, this is similar to LED-based lighting, but in many ways it is also very different because of the peculiarities of semiconductor lasers and of laser-emitted light. This broad overview looks at recent and on-going developments in the field of diode laser-based lighting technologies. In doing this, it examines the role of laser diodes, phosphors, and system design in enabling the creation of illumination systems that are fit for various application requirements. Phosphors suitable for pumping with laser diodes have been given emphasis. Sufficient tutorial context is also provided so that readers can easily appreciate the import of various developments. Finally, the review also briefly looks at anticipated future developments in this field.

All-optical logic gates (AO-LGs) are the key elements that play a pivotal role in the development of future all-optical networks and all-optical computing. A complete overview of the seven all-optical logic gates (i.e., AND, OR, NOT, XOR, XNOR, NAND, and NOR) based on their design techniques and applications are covered, including the latest technologies, such as topological photonics and artificial intelligence-based designs techniques. In addition, we have further categorized the AO-LGs as reconfigurable gates, simultaneous gates, reversible gates, modulation-based gates, and data rate-based gates. The techniques to implement these different classes of gates are reviewed and their limitations are discussed. We also discuss in brief the various simulation tools used to design and analyze the AO-LGs. Finally, the most feasible techniques for constructing optical integrated circuits based on the existing fabrication technologies and available resources are explored, and future prospects are outlined.

Guest editors Haojie Xia, Liandong Yu, and Lianxiang Yang introduce the Special Section on Advances in Optical Measurement for Geometrical and Mechanical Quantities.

In the practical application of speckle pattern interferometry, it is often necessary to measure the deformation or defect of some specimens placed in boxes. Transparent glass observation windows are often used on one side of these boxes. In laser speckle pattern interferometry, the glass windows often cause local overexposure in the field of view, which has a great influence on the measurement. A method for identification and elimination of overexposed areas based on polarized optics is proposed. A polarization camera is used for image acquisition; polaroid is used to filter out stray and chaotic interference light, greatly reducing the overexposed area in the imaging field of view, and then feature extraction technology is used to identify and extract the overexposed area caused by the glass window. Finally, according to the measurement information around the overexposed area, the phase of the overexposed area is fitted and corrected, so as to obtain the speckle pattern interferometry measurement results without overexposed points. Experimental results show that the proposed method can effectively extract and eliminate the overexposed area, and good measurement results can be obtained.

We investigate an approach to measuring harmful gases by enhanced Fano resonance generated in a universal multimode waveguide-microcavity model. Dynamic Fano resonance is theoretically described and experimentally proved to be associated with the phase shift between two waveguide modes as well as their amplitude ratio and coupling coefficient. The spectra can be engineered to form a Lorentz dip, various Fano lineshape, and Lorentz peak by controlling the coupling point in both microbottle and surface nanoscale axial photonics platforms. In principle, the model can be applied to any class of whispering gallery mode microcavity device. With sharp asymmetric lineshape, our model can improve the sensitivity by 51.5 times in theory when measuring the harmful gas refractive index change, which may open up opportunities for advancements in the harmful gas leakage detection applications.

The phenomenon of Talbot image formation on a stepped surface is described and demonstrated both theoretically and experimentally. The Talbot self-image shift on the surface step is observed and implemented for the measurement of the three-dimensional step height of a large discontinuous object with an optically plane surface. Due to the measurement of the phase not being required, the system is free from phase ambiguity problem. This means that the measurement range is larger as compared with the interferometric techniques. Based on the simulation analysis and experimental measurements, minimal and maximal measurable steps are calculated.

With the continuous improvement of the theory and method of gear measurement, the effectiveness of gear contact measurement is reduced, making it difficult to meet the need for 100% gear detection. To improve the measurement efficiency and make effective use of the full tooth surface data, a gear pitch rapid measurement method by point laser sensor is proposed. This method studies the principle of measuring the gear based on a laser point sensor, and the efficient model of collection and analysis of tooth surface data is established. Then the evaluation of the pitch deviation is completed, including the individual single pitch deviation, the single pitch deviation, the individual cumulative pitch deviation, and the total cumulative pitch deviation. Additionally, the pitch deviation of the multiple cross-sections is investigated, which may characterize the overall pitch deviation of the tooth surface. Compared to the gear measuring center P26, the experimental results show that the optical measurement of pitch deviation is feasible and efficient. Depending on the different measuring conditions, the offset status of the sensor can be selected to meet the different efficiency and accuracy requirements. The research and application potential of a point laser sensor in gear measurement is wide-ranging.

To locate the impact source on a composite material structure, an energy eigenvector and correlation interpolation (EECI) method is proposed based on a multi-fiber Bragg grating (FBG) sensor array. The fundamental frequency interference is eliminated by the wavelet transformation. The signal energy feature vector is extracted by the wavelet packet transformation. A two-dimensional correlation matrix is obtained by analyzing the correlation between impact response signals and signals in the reference database. The linear interpolation is then applied to accurately predict the impact location. The EECI method is verified on a carbon fiber composite plate with an effective test size of 400  ×  400 (mm). The experimental result shows that the mean absolute error is 16.18 mm and the mean relative error (MRE) is 4.04%. The influence of the FBG sensor arrays and the quantity on the localization accuracy is also discussed to optimize the layout of the localization system. It shows that the MRE is [25.32%, 44.50%] for a single FBG sensor array, [17.19%, 42.63%] for two FBG sensor arrays, [15.63%, 22.47%] for three FBG sensor arrays, which shows the advantages of multi-FBG sensor arrays.

Defect detection for specular surfaces plays a vital role in precision manufacturing. However, traditional defect detection methods are unsuitable for specular surfaces because of their specular reflection property. The defect detection on specular surfaces is usually performed by inspectors, which makes the defect detection a time-consuming and unstable task. Deflectometry has been widely used in defect detection for specular surfaces combined with machine learning. Nevertheless, conventional deflectometry methods use the local curvature deviation map based on the unwrapped phase, which can only detect geometrical defects. Moreover, hand-crafted features need to be defined for each specific task. We present a method based on deflectometry and deep learning. Deflectometry provides the input images for the network, and the deep learning network completes the identification and location of defects. In deflectometry, the proposed method uses the light intensity contrast map to replace the local curvature map, which can detect both geometrical and textural defects. Based on conventional networks, depthwise separable convolution kernel is applied to reduce parameters, and residual convolution block is utilized to alleviate vanishing or exploding gradients. A subnet for feature aggregation is used to obtain multiscale information of defect features. Performance evaluation based on experiment results proved the effectiveness of the proposed method.

We report a highly sensitive electrolytic copper (Cu) detection method using nanoparticle enhanced laser-induced breakdown spectroscopy (NELIBS) for metal smelting processes. The sulfate solution Cu with content ranging from 10 to 50 g/L was prepared and measured on the nanoparticle chip. The experimental results showed that the Cu spectral line intensity of NELIBS increased 5.5 to 9.4 times compared with typical LIBS without nanoparticles. The calibration curve was established based on the spectral line intensity at Cu I 327.40 nm according to the different Cu content in the solution. The determination coefficient R² of the calibration curve was 0.99. The method of laser-induced breakdown spectroscopy based on drying liquid droplets on the nanoparticle chip can be applied to the rapid determination of Cu content in the copper electrolyte.

Photonic chips have great potential for neural network computing due to their fast speed, low power consumption, and parallelism. We propose a quantized neural network modeling method based on microring resonators (MRR). We analyze the optical properties of the MRRs and utilize lasers with different wavelengths as inputs of the neural network. The quantization aware method is adopted to train the neural network, and the stochastic search method is utilized to determine hyperparameters of the network. We transform the network parameters and hyperparameters into MRR parameters to simulate neural network matrix multiplication operations. Finally, we used the Mixed National Institute of Standards and Technology database for testing the proposed model. For 4-, 5-, and 6-bit quantization of weight parameters, we obtain classification accuracies of 94.23%, 94.73%, and 96.11%, respectively. Thus our study demonstrates the feasibility of building a neural network inference system using a microring structure and provides a theoretical support for applying MRRs in neural networks.

The optimization and uncertainty analysis of laser-based optical sensors in the design phase is a challenging task due to the presence of stochastic laser speckle effects. We present an accurate, efficient, and versatile simulation framework for the design of optical sensor assemblies, capable of handling objective as well as subjective speckle effects. The framework integrates the stochastic nature of laser speckle with the deterministic properties of ray-tracing simulations, enabling the simulation of sensitivities to translational as well as rotational target motion and reliable performance estimation, even for more complex optical assemblies. To validate the simulation results for translation and rotation, they are compared against the experimental data of four speckle-based optical sensor assemblies as well as against analytical relations for speckle pattern motion. The accuracy of the developed framework is demonstrated by simulation errors for correlation peak shift of the speckle pattern of less than 2  μm rms and 2.4  μm rms for translation and rotation, respectively. For the center of gravity shift as additional simulation output for an integrated laser sensor for sensing translations in all three degrees of freedom, a simulation error of 2.6  μm rms was obtained, which also lies well below the resolution of the designed optical sensor assemblies.

Aiming at the problem of a spur gear fault of the rotation vector (RV) reducer in industrial robots, an instantaneous phase detection technology based on eccentric modulation and laser displacement sensor is proposed. Taking the RV-20E reducer as the object, through the analysis of the RV reducer gear fault mechanism, an RV reducer fault diagnosis test bed is developed to simulate the actual operating conditions of the RV reducer of industrial robot. The instantaneous phase information of the RV reducer is collected, and the adaptive permutation entropy is used to process the instantaneous phase data, which can judge the gear fault type quickly and effectively. The test results show that the fault diagnosis test bench based on this detection technology can effectively and quantitatively judge the fault type of the spur gear of the RV reducer.

We report a precise and stable laser beam positioning method with a high scanning speed by a pixelated binary grating (PBG) realized by a compact on-off state digital micromirror device (DMD). The overall procedure for PBG pattern generation is proposed, analyzed, and experimentally verified. The resulting performances correspond to an angular resolution of 0.01 deg and angular repeatability of   ±  2.54  ×  10^−  4  deg over a beam steering range of   ±  7.5  deg. The scanning speed of 40 deg/s is achieved in continuous scanning with a resolution of 0.01 deg. Arbitrary positions can be targeted in the scanning range with DMD’s frame rate of 4000 Hz. To verify its applicability in precision metrology, absolute distances to multiple off-axial targets were determined in parallel with a high speed. This method will open a wider application of DMD in precision LIDARs, three-dimensional calibration of precision machines, and high-resolution laser patterning.

Due to the influence of many factors in monocular vision pose measurement, the reliability of pose measurement results needs to be evaluated by uncertainty. The difficulties of the traditional guide to the expression of uncertainty in measurement (GUM) method in actual pose evaluation are analyzed, and the adaptive Monte Carlo method (AMCM) is used to evaluate the uncertainty of pose results. According to the mathematical model of pose measurement, the factors affecting pose results are analyzed in detail. The uncertainty evaluation model is established by the error traceability method and the distribution of uncertainty components is analyzed reasonably. Both the Monte Carlo method (MCM) and AMCM are used to evaluate the uncertainty of pose results. The feasibility and effectiveness of the AMCM evaluation method are verified. The experimental results show that MCM and AMCM, as the supplement of the GUM method, can well solve the problem of pose evaluation in the actual measurement system. However, AMCM is more convenient and efficient than MCM to solve the problem of too large or too small sampling times.

Laser damage of fused silica is a critical problem that limits the fluence increase of high-power laser systems. The reaction of plasma processing of fused silica was analyzed. The redeposited layer was removed and scratches of the fused silica were passivated by plasma to increase the laser-induced damage threshold (LIDT). It was found that the deposition generated during the etching process adheres to the surface of fused silica to limit the further increase in the LIDT. The LIDT of fused silica tends to increase and then decrease as fused silica is processed by atmospheric pressure plasma. Accordingly, a competitive evolution for etching and deposition affecting the LIDT of fused silica was established. The etching and deposition relationship analyzed by the model can effectively increase the LIDT of fused silica without deteriorating the original surface of fused silica by choosing an appropriate dwell time.

To improve the estimation accuracy of spectral reflectance from the given trichromatic value, a new two-step k-nearest neighbors locally weighted linear regression method is proposed. The algorithm has good local learning ability and can take into account the similarity of colorimetric and spectral reflectance space. The simulated and practical imaging experiments were carried out with Munsell matte and glossy dataset, respectively. Experimental results show that the mean root mean square error values of the spectral reflectance estimated by our model in simulated RGB, practical imaging Adobe RGB. and raw RGB data experiments are 0.00731, 0.01519, and 0.01453, respectively, and the mean color difference values under CIE standard illuminant D65 are 0.380, 1.311, and 1.180, respectively. In addition, we showed the calculation time cost of various models in the practical experiment. The calculation time of one sample for the proposed method is 0.094 s. The proposed method is better than several state-of-the-art methods in terms of comprehensive estimation performance and running efficiency.

In a traditional lensless in-line holographic microscope (LIHM), the field of view (FOV) is generally limited by the size of the imaging sensor, which is usually several millimeters wide. We propose a scanning LIHM system using a line-scan camera to achieve wide field-of-view holographic imaging. In addition, the iterative holographic reconstruction method is applied to obtain the sample image with enhanced contrast and resolution using two hologram measurements, which are acquired by slightly shifting the sample-to-sensor distance. The capability of the proposed scanning LIHM system was demonstrated by experiments with a U.S. Air Force target and a biological sample of leaf vein. The reconstructed sample image had a FOV of 2.87  cm  ×  4.70  cm (2048  ×  3357  pixels), which was much larger than the typical FOV of several square millimeters in a traditional LIHM system.

The measurement of the minimum temperature difference (MTD) at which a four-bar target can be resolved is an important task for assessing the performance of infrared imaging equipment. Recently, there have been efforts to use an automatic objective approach based on machine learning to replace the classical manual subjective MTD measurement approach, which is heavily influenced by the psychological subjective factors of the operator and is limited in accuracy and processing speed. However, the scale variability of the four-bar targets and the high intrinsic target-background similarity in infrared images have been observed to present a challenging problem for automatic MTD measurement. We present a concealed multiscale feature extraction network (CMF-Net), a backbone for automatic multiscale four-bar target detections, in an attempt to address the aforementioned challenges. We first propose a multilevel feature pyramid module to construct a robust representation of multiscale four-bar targets. Next, to distinguish between the four-bar targets from the background, a target saliency enhancement module (TSE) is introduced to capture discriminative features about the target by encoding local and global contextual information. Experiments show that the proposed method is effective and achieves state-of-the-art four-bar target detection results on infrared-image dataset compared with existing methods.

We argue for the integration of the statistical models already widely used in radar technology into lidar technology. The aim is to assess the validity or degree of confidence of an alert to be issued in view of not overloading the pilot with nuisance alerts. We present the basics of the detection theory. We give three examples of simulations illustrating the use of these statistical models either for designing lidars or for preparing lidar missions. We describe the simulator having been developed and used. We also present the idea of developing mixtures of statistical models as an approach to thresholding and object classification at mission time. Some experimental data are presented to validate both the simulator output and the use of mixtures of models for object segmentation or classification.

Deterministic illumination diffractive-diffusers have nonperiodic short and medium-scale topography. Because of the deterministic locations of vertical sidewalls at the phase transition boundaries, over-coating diffractive diffusers with thin-film antireflection layers perturbs their function, resulting in performance deviations and nonuniformities. To mitigate these effects, we added antireflection random nanostructures on the surface of three different classes of fused-silica multiphase diffractive diffusers, using reactive-ion plasma etching. The diffusers were measured before and after the random nanostructures addition, using a polarized-laser scatterometer with a dynamic range of nine orders of magnitude. The bidirectional scatter distribution function was measured over the entire equatorial plane of incidence, to analyze the directionality of scattered light and the impact of the antireflective nanostructure presence on the optical performance of the diffusers. The overall reflectivity suppression was measured across the illumination patterns directions, as well as, across the entire 180-deg angle-sweep. The designed deterministic illumination patterns and their contrast were unaffected by the presence of the random antireflective structures, whereas Fresnel reflectivity was reduced by an order of magnitude on average.

Various techniques for image color reproducibility under low-light conditions have been proposed, such as high sensitivity, a combination of visible (VIS) and infrared (IR) light, and coloring monochromatic images. However, when the illuminance falls below a certain level, color images cannot be obtained without prior color information. Previously, the exclusive use of IR without VIS illumination was proposed to achieve a pseudocolor image (basic IR color). It improved visibility compared with conventional monochrome images. However, there are cases, depending on the objects, when basic IR light cannot reproduce the correct color. An image processing method for enhancing color reproducibility is proposed, particularly for objects that are not suitable for the basic IR color (enhanced IR color). Moreover, we developed an algorithm to combine the advantages of both VIS and IR colors by utilizing signals of six wavelengths: three wavelengths each of VIS and IR. The proposed method includes an automatic transition between the optimal combination of VIS and IR colors when the illumination level changes, thus providing images with superior color reproducibility under various illumination levels compared with basic IR color.

To observe targets in expanded depths of view in light field imaging, we design an open-source light field parallel refocusing code-free GUI tool named OpenRefocus, which is based on classical spatial projection integration. OpenRefocus has been successfully used for refocusing light field images selected from open-source datasets as well as captured using commercial light field cameras faster than other released light field refocusing programs. Considering its fast processing speed and user-friendly operations, OpenRefocus can be a useful tool for light field refocusing in many light field imaging applications.

The concentration and composition of bioaerosols in the ambient air are closely related to human health. Obtaining real-time information on the types of bioaerosols is a challenge for bioaerosol monitoring today. Aiming to realize automatic detection with low cost and low false alarm rate, we describe a bioaerosol detection system with dual fluorescence receiving channels excited by ultraviolet light. The system uses 280-nm UV-LED to excite the intrinsic fluorescence of biological particles, and according to the emission bands of NADH, riboflavin, and tryptophan, it is divided into two fluorescence bands: 330 to 420 nm and 420 to 650 nm. A series of experiments were conducted to evaluate the detection capability of the system. The results showed that the system has good consistency with the culture method in concentration detection and has good reproducibility. We detected multiple types of aerosol particles. By comparing the fluorescence intensity of the two bands, the system can not only distinguish between fluorescent and nonfluorescent particles but also preclassify the biological particles into four categories: proteinaceous substances and microorganisms with tryptophan as the main component, dust particles in the air, plant cells and debris, and cigarette burning smoke.

Integrated structural defects are one of the most common defects in the microchannel plate fabrication and physicochemical treatment stage of low-light-level (LLL) image intensifiers. Its appearance seriously affects the imaging quality. The traditional evaluation method of such defects is solely based on visual observation, and its main disadvantages are strong subjectivity and large uncertainty. To address the above problems, two objective evaluation methods of integrated structural defects of an LLL image intensifier are introduced. The first is based on local image patch extreme, and the second is based on frequency fast Fourier transform. The basic idea of the first method is to scan the effective area (EA) of the target image globally and then extract the target defect by adopting a series of local image patch extremes adjacent in the Y-axis. Experimental results reveal that this method shows excellent performance in detecting and positioning accuracy while the time cost is satisfactory. The general steps of the second method are to scan the EA globally, and then perform a fast Fourier transform on the obtained image patch; through bandpass filtering in the frequency domain and inverse Fourier transforming the convoluted image, structures or objects projecting the similar characteristics of integrated structural defects are detected. This method not only maintains good detection integrity but also ensures high detection accuracy. Through experiments on several defective image intensifiers and compared with traditional relevant technologies, we conclude that the proposed methods can realize the high-precision objective detection of integrated structural defects of LLL image intensifiers and can be used as effective alternatives to the traditional evaluation method of such defects.

Accurate laser positioning system (ALPS) is widely used in large space indoor dynamic positioning. To improve the positioning accuracy and refresh frequency, the inertial measurement unit (IMU) with a high frame rate is used for fusion, so it is necessary to calibrate the external parameters of ALPS and IMU. Based on the analysis of the calculation process and error description of traditional calibration, we propose a calibration method of spatial multipose transformation (SMT). In this calibration method, the target plate performs multiple attitude transformations to obtain the attitude data and process its error. Then, the Lodrigues matrix is introduced in the solution process to reduce the dimension of unknown variables in the equation, and it is optimized combined with SVD decomposition. At the same time, to further adapt to the application of fusion, a more intuitive error description method is proposed to evaluate the influence of calibration results on fusion accuracy. The simulation experiments indicate that the proposed method enables the calibration error to have accuracy within 0.02 mm. Furthermore, the results of the validation experiments show that the proposed calibration solution method improves in accuracy and robustness with respect to the conventional method. It provides a technical basis for realizing higher precision indoor dynamic positioning applications in large space.

The infrared (IR) imaging community has a need for direct IR detector evaluation due to the continued demand for small pixel pitch detectors, the emergence of strained-layer-superlattice devices, and the associated lateral carrier diffusion issues. Conventional laser speckle-based modulation transfer function estimation is dependent on Fresnel propagation and limited to paraxial geometries, preventing the utilization of this approach on small pixel pitch IR devices. This paper presents a generalized approach for determining a focal plane array (FPA) system input power spectral density, utilizing numerical evaluation of Rayleigh–Sommerfeld speckle irradiance autocorrelation functions, speckle irradiance spectral analysis using the Wigner distribution function, and experimental error quantification incurred from making wide-sense-stationary assumptions regarding the associated laser speckle random process. The effort’s final result is an experimental demonstration of an FPA resolution estimation technique valid in nonparaxial test scenarios.

This study presents a real-time hardware implementation of a novel physical layer security algorithm developed for visible-light communications (VLCs) based on precoded spatial modulation (SM). The demonstration was carried out on a low-cost 70  cm  ×  40  cm  ×  40  cm miniature room model with four light-emitting diodes (LEDs) as a test-bed for conducting experiments in the field of VLC. The test-bed is a 10:1 shrunk replica of a conventional room and can be built with simple office supplies totaling <$10, excluding drive and collection optoelectronic components. While being cost-friendly, the test-bed also allows for (i) integrating optical components and (ii) carving desired window and door patterns with different cardboard color tones. Hence, the effects of reflections from different colored walls and the effect of external light sources can be observed on the performance of the secure VLC system. We successfully demonstrate the operation of the zero-forcing precoder and the SM on the built set up to provide robust and secure communication among the transmitting LEDs and the receivers, representing the legitimate user and the eavesdropper, for the first time in the literature. The secrecy capacity improvement is also noted, validating the proposed approach in realistic environments.

We propose and demonstrate a scheme for atmospheric transfer of microwave signals at X-band with sub-hundred-femtosecond level precision using active optical phase correction. With the proposed scheme, we have transferred a microwave signal at 9.192 GHz over a 60-m atmospheric link. The timing fluctuations and Allan deviations are both measured and analyzed to assess the performance of our scheme. The root-mean-square timing drift of the transferred microwave signal with timing fluctuation suppression is 44 fs, and the Allan deviation is 9  ×  ^{10  −  14} at 1 s and 1  ×  ^{10  −  17} at 1000 s with optical phase correction. The transmission link achieved a lower instability than currently reported cesium fountain clocks, which means this proposed transfer scheme could be employed to transfer cesium atomic clock over the atmospheric transmission link.

As a precision position sensor, the photoelectric encoder has difficulty avoiding eccentricity or tilt error in the industrial manufacturing and assembly process. At present, manual error correction is often used to adjust. The adjustment efficiency is low, and it has difficulty cooperating with other types of error compensation methods. In view of these problems, we proposed a photoelectric encoder eccentricity error detection and compensation method based on digital signal bandpass filtering that effectively corrects the signal deviation caused by the installation eccentricity in the output signal and judges the direction and magnitude of the eccentricity error to realize the encoding correction of eccentricity and inclination error of the device. The mathematical model of the eccentric error generation principle of the photoelectric encoder combined with the Lissajous figure was derived. We then demonstrated the design of a photoelectric encoder error detection platform to detect photoelectric encoders of different diameters in real time. Finally, using the advantages of field programmable gate array high-speed parallel processing, the error compensation algorithm was written into the development chip to improve the efficiency of processing the encoder output error signal. This method can adapt to the eccentricity error correction and compensation of different models of photoelectric encoders and can greatly improve the efficiency and accuracy compared with previous detection methods.

The development of precision manufacturing puts forward higher requirements for the measurement accuracy of grating heterodyne interference displacement measurement, while its nonlinear error is one of the main factors that influence measurement accuracy. A synthetic model of nonlinear error including six error sources is established, and the influence of various nonlinear error sources is analyzed according to the phase expression of nonlinear error derived from the model. When the installation error γ  =  θ  =  1  deg and the performance of optical elements α  =  β  =  0.5  deg, dε₁  =  0.1  deg, dε₂  =    −  0.1  deg, T_p  =  0.95, R_p  =  0.05, R_s  =  0.98, T_s  =  0.02, η_p  =  0.4, η_s  =  0.35, nonlinear error will reach 2.32 nm. We provide a theoretical basis for compensating the nonlinear error of grating interferometers.

Light fields from localized light sources create a variety of interesting glory phenomena when reflected by small glass spheres (microbeads). Starting with single glass spheres, the interplay of the underlying optical phenomena, i.e., refraction, reflection, diffraction, and dispersion, have been studied both theoretically and experimentally. In a second step, we considered retroreflecting screens formed by flat arrays of tiny glass spheres when exposed to divergent illumination. The arising glories show a specific ring pattern and appear freely floating around the light source as their blurred real image. The investigated glory structures will have numerous applications in sophisticated lamp and illumination designs, as demonstration tools in optical education or advertising, and artistic installations, in particular when appearing as three-dimensional objects by stereoscopic observation.

Optics imperfection is one of the fundamental errors that produce periodic nonlinearities which limits displacement measurement accuracy with heterodyne interferometers. A corner cube-based retroreflector is commonly used to increase measuring resolution, but it causes changes to incident polarization that introducing the two-frequency mixing. A mathematical model of periodic nonlinearities is established, taking into account of those facts for a double-pass interferometer, for analyzing and design. To minimize the periodic nonlinearity caused by the corner cube, we propose a polarization-maintaining retroreflector (PMR) instead of a common one. The theoretical analysis shows that the PMR can reduce the periodic nonlinearity caused by the corner cube in heterodyne interferometers significantly. Furthermore, polarization leakage coefficients and total periodic nonlinearities measurements are carried out to verify the performances of the PMR. Experimental results indicate that the periodic nonlinearity decreases from 8 to 2 nm after the normal corner cube is replaced by the PMR, and their periods of the phase shifts are consistent with the theoretical analysis.

A diffractive waveguide based on surface relief grating is one of the most significant components for near-eye displays (NEDs). Here, we propose a colorful NED system using meta-grating as the coupler to achieve high exit-pupil brightness uniformity and color uniformity. The whole system has a lightweight and compact configuration and allows for flexible integration with ordinary smart glass. Our work provides a feasible way to design a colorful and miniaturized NED.

Photon Doppler velocimetry (PDV) is a ubiquitous diagnostic method in dynamic compression experiments. It typically requires the roughening of the test surface to ensure scattering of the probe light back into the PDV probe at non-normal angles of incidence. Although surface scattering plays an essential role in the quality of PDV data, surface scattering properties for common materials and preparations have not previously been studied with specific relevance to PDV. Here, we present detailed measurements of the bidirectional reflectance distribution function for four different surface preparations (Scotch-Brite hand and drill roughened, ball-rolled, and grit blasted) and four different materials (copper, aluminum, stainless steel, and tantalum). These measurements employ a conventional PDV probe and obtain a diffraction limited angular resolution with 10 pW accuracy and 2.6% repeatability. In addition to scattering data, we employ scattering theory and simulations to accurately emulate the measured data. We also present a straightforward method to derive the average scattering distribution from surface profilometry and observe several qualitative aspects of the scattering data that may help to optimize PDV signals.

It is well known that tunable lenses, with refractive power that can be varied, e.g., by changing the curvature of a membrane, can replace the motion of lens groups in zoom systems. Similar to classical zoom systems, the performance of these systems is heavily influenced by the fundamental first-order layout. Moreover, the first-order layout sets the most important requirements for the employed tunable lenses. In this contribution, we present a method for the analysis of a large number of possible first-order solutions for typical requirements and for the selection of the most promising layouts. The first-order solution space is mapped, allowing the layouts to be automatically filtered and plotted depending on pre-defined characteristics. Ray tracing of the marginal and chief rays combined with the traditional thin lens aberration theory provide efficient estimations of the expected installation space requirements and performance for each first-order layout. Using an example, we demonstrate good agreement between these estimations and the corresponding real lens layout, optimized by commercial raytracing software. The presented design method for zoom systems based on tunable lenses is compared with similar approaches for classical zoom lenses.

A multiband tunable metamaterial terahertz absorber based on VO₂ material is introduced and studied. The absorber consists of gold planes and periodic metallic materials. The first layer is gold plane, the second layer is polyimide, the third layer is silicon film, and the fourth layer is super surface. From the simulation results, we can see that the phase transition occurs when the temperature of VO₂ is greater than or equal to 68°C. After the phase transition, there are three absorption peaks, and the absorption rates are more than 99% at 2.53, 5.7, and 8.67 THz, realizing perfect absorption. When the temperature of VO₂ is less than 68°C, the maximum absorption rate is 30.4%, which fails to meet the absorption requirements. By increasing the thickness of polyimide, the absorption spectrum moves slightly to the low frequency band to achieve redshift. Compared with most of the proposed multiband absorbers, the metamaterial absorber with adjustable absorption spectrum has better application prospects. VO₂ is a phase change material with insulator metal phase transition characteristics and reversible process. By using the phase transition characteristics of VO₂, the absorber has a switching function and is conducive to the flexible regulation of the absorption spectrum.

For an optical tracking system, due to the limited sampling rate and time delays in the image-based tracking loop, control bandwidth is usually restricted. An optimized controller based on Youla–Kucera parameterization is proposed to improve both vibration rejection and target tracking performance. This optimized controller combined the position error signal and the control output to compensate for the original feedback loop. The performance in low frequency was enhanced using an optimized Q₃₁-filter, although the close-loop bandwidth of the tip-tilt mirror was not improved. The stability and robustness of this control configuration were analyzed by gain margin and phase margin of the open-loop transfer function. Because of relying on a simple and low-frequency model of the tip-tilt mirror control system, this new controller did not lead to a compromise between vibration rejection and noise propagation in the loop. Simulations and experiments were used to testify to the effectiveness of the new controller.

We present an approach for reduction of off-axis aberrations of imaging optical systems. The idea is to reduce aperture-dependent off-axis aberrations of optical systems by reduction of the effective aperture of off-axis beams depending on the angle of incidence. The effective aperture of beams is reduced to a greater extent for beams with greater angle of incidence. This is achieved by the use of specific three-dimensional vignetting stop structures, which can be implemented in different ways. These structures work as angle-dependent vignetting stop for the subsequent optical system. This approach can be applied to various optical systems to improve their off-axis optical resolution and extend the usable field of view, without the use of additional optical elements like lenses or mirrors.

We present phase masks (PMs) made from birefringent CaCO₃ crystal plates to create laser beams at an imaging plane with given shapes and intensity distributions close to flat-top distributions. The principle of PMs is based on creating the phase shift of π or 2π (depending on the initial plate thickness) in linearly polarized radiation passing through etched areas with given shapes. The phase shift in these areas is converted into the intensity distribution at an analyzer output, which can be projected with a demagnification by a high-quality lens into its imaging plane aligned with a microprocessing plane or target. In this research, we provide theoretical discussion of choosing PM configurations. PMs with the square and quadrating square shapes (such configurations are in demand in laser microprocessing and molecular optics) were fabricated by laser-induced microplasma and successfully tested in the experimental setup with a pulsed laser (with a wavelength of 1.06  μm and pulse duration of 120 ns). PMs were also used in this experimental setup for laser ablation of polished steel samples. Contours of footprints on samples were well matched with formed beam shapes.

Dynamics of a third-order optical phase locked loop (OPLL) with a resonant type loop filter (LF) has been studied analytically and numerically. The range of stable synchronous operating zone of such OPLL has been estimated analytically in system and signal parameters and obtained predictions are in good agreement with numerical results. Beyond the stable region, it is found that chaotic oscillation of phase error occurs through a sequence of period doubling bifurcation. In addition to this, tracking of such periodic as well as chaotic dynamics of third-order OPLL by another OPLL in master-slave configuration has also been explored here. The tracking is possible by slave OPLL within a very short range of effective frequency offset by keeping other design parameters within the stable mode of operation. The synchronization of the master and slave OPLLs has been studied in generalized sense by considering auxiliary system. The study is very important and helpful for optical signal processing and also in chaos-based secure communication.

Atmospheric intensity scintillation effect deteriorates the communication quality of space chaos laser communication. To analyze the effect of internal mismatch and internal mismatch induced by intensity scintillation on bit error rate (BER) for space downlink chaos laser communication system, numerical simulation results based on BER formula are conducted and analyzed. Numerical results demonstrate that BER worsens with the effect of intensity scintillation. Although the probability that received optical power causes larger mismatch noises is small, its corresponding larger instantaneous BER deteriorates the ensemble average BER significantly. Results also indicate that internal mismatch deteriorates BER more than external mismatch. These results are beneficial for designing practical space chaos laser communication system with optimal performance.

The performance of dual-hop mixed radio frequency (RF)/free space optical (FSO) communication system with partially coherent beams (PCBs) is investigated on the basis of decode-and-forward (DF) protocol. Specifically, both the RF and FSO channels are assumed to follow F-distribution with proper selection of parameters. Taking the path loss, atmospheric turbulence, and pointing errors into consideration, the closed-form expressions for the probability density function (PDF) and cumulative distribution function of the received signal-to-noise ratio are derived in terms of hypergeometric function in RF and FSO links, respectively. Subsequently, the closed-form expressions for outage probability, average bit error rate, and ergodic channel capacity of this system with M-ary phase-shift keying and M-ary quadrature amplitude modulation schemes are obtained on the basis of DF protocol. The results show that shadowing in RF link as well as the pointing error and atmospheric turbulence in FSO link would result in significant performance deterioration in dual-hop mixed RF/FSO system, while PCB can bring significant performance improvement of mixed RF/FSO systems with pointing errors over weak to moderate turbulence. Furthermore, the correctness of the analytical expressions is verified by Monte Carlo simulations. This work would benefit the design and development of the mixed RF/FSO systems.

We demonstrate a fifth generation (5G) new radio (NR) signal transmitted by an analog optical and seamless antenna wireless connection at the frequency band of 60 GHz to exploit a high-frequency unlicensed frequency range. An optical frequency doubling technique, using two Mach–Zehnder modulators operating in a carrier suppressed and linear regime, respectively, was adopted to obtain the desired millimeter wave frequency at the photodetector’s output. The proposed system was tested with the 5G NR signals with a maximum bandwidth of 200 MHz and 64 quadrature amplitude modulation format. It was shown that the signal transmitted through the optical fiber and free space optical link with 1 m long seamless antenna transmission at 62 GHz was capable of meeting the signal quality requirements in terms of error vector magnitude. Moreover, the system phase noise performance showed an almost negligible difference between the various system configurations.

Increased capacity demands and radio frequency (RF) congestion impacts on current communication networks have brought greater attention to free-space optical (FSO) communication as a viable augmentation technology for terrestrial, aerial, and space-based communication infrastructure. As a complementary alternative to RF communication systems, FSO can support high link bandwidths and provide high data security without RF spectral constraints. The performance of FSO links, however, can be significantly impacted by receive power variation caused by propagation and scattering losses along with losses due to atmospheric turbulence. Depending on the FSO application, these loss mechanisms can dynamically change, impacting link performance at different time scales. We investigate subcarrier phase-shift keying (PSK) and quadrature amplitude modulation (QAM) intensity modulation that can be adapted to dynamically changing link conditions to optimize bandwidth utilization. Using custom subcarrier intensity modulation (SIM) modems, the performance of binary PSK (BPSK), QPSK, 8PSK, 16APSK, and 16-QAM waveforms is reported. The impact of adaptive equalization is also characterized, and the initial performance of a subcarrier multiplexed system is presented. This work represents the first experimental evaluation of SIM waveforms using a laboratory scintillation playback system based on scintillation recorded over real-world propagation paths.

The ultra-high link latency of the conventional central reservation switching method in satellite optical networks limits link utilization. In this study, a novel fully distributed competitive satellite optical burst switching (DC-SOBS) mechanism was proposed to overcome the drawbacks of the optical circuit switching method and achieve link multiplexing. Furthermore, the proposed method addressed the low link utilization problem caused by the high link latency of the central-control-related methods. The DC-SOBS mechanism in the ring network of six Geostationary Orbit (GEO) satellites improved link utilization by three orders of magnitude compared with the reservation method. A data conflict processing method was proposed to ensure that the same information was provided to various optical switching nodes, and the same algorithm was used to achieve a consistent scheduling result in each node.

Simultaneous generation of red (R), green (G), and blue (B) wavelengths in different grating structures of 5% MgO doped domain engineered LiNbO₃ is studied theoretically by employing the type-0 quasiphase-matched (QPM) technique. The spectral output from two cascaded, four segmented, and two-phase shifter (PS) QPM structures using second-harmonic generation, difference-frequency generation, sum-frequency generation, and third harmonic generation were analyzed and interpreted. We observed RGB in the range of 620 to 650 nm, 530 to 580 nm, and 430 to 468 nm for red, green, and blue, respectively, for various configurations of input wavelength, period, device length, segment length, location, and width of the PS domain. The influence of the input pump, operating temperature, pump detuning, PS width, and phase matching periods are analyzed for all the proposed schemes. Fluctuation in the output wavelengths of about 10 to 20 nm was observed due to the temperature variations from 25°C to 150°C in steps of 25°C. Further, the advantages of using a MgO-PS QPM structure over the other proposed schemes have also been addressed for their potential application in a laser-based projection display system.

The major purpose of this study is to investigate two strategies for additive manufacturing processes of SS316L using the direct laser metal deposition method (DLMD). In additive manufacturing (AM), the influence of scanning patterns was the first strategy, and the other strategy was the effects of laser power reduction in various layers. The experiments used a semiconductor diode laser with a coaxial nozzle head. The goal of this study was to determine how these two techniques influenced the properties of additive-produced components. Geometrical metrics such as width, height, and penetration, as well as particle size, and microhardness profiles, were assessed in the macrosection of the fabricated wall samples. The sample’s height stability was also investigated. The results revealed that the scanning pattern significantly affects all aspects of additive-manufactured components. The results indicated that higher stability is achieved when using a unidirectional scanning sequence. The average particle size of specimens obtained using the unidirectional and bidirectional scanning patterns was 8.6 and 9.69  μm, respectively. The pattern of particle size variation in the samples demonstrates that the LMD walls in the beginning and end are larger than in the samples at the center, the microhardness fluctuation pattern is in the opposite direction of particle size. The width of the samples was also found to decrease as the laser power was reduced from the substrate upward.

In the past, several frequency-domain channel-estimation methods were proposed for coherent optical orthogonal frequency division multiplexing systems, which use offset-quadrature-amplitude modulation (CO-OFDM-OQAM). A common challenge for these methods is that the training sequences they use, even though they increase the magnitude of the pseudo-pilots, can make the peak-to-average power ratio (PAPR) of the signal worse. To address this problem, an innovative modified frequency-domain channel estimation method is designed and investigated for a polarization-division-multiplexed CO-OFDM-OQAM system that uses dual-dependent pilots. The operating principle of this new method is described in detail. Then, its PAPR performance and channel-estimation capability are examined using numerical simulations. The proposed method not only ensures a sufficiently large magnitude of the pseudo-pilot but also does not seriously degrade the PAPR of the signal. It is also found that the magnitude of its pseudo-pilot (2.3136) exceeds that of the interference approximation method with real pilots (1.1086), and it is only slightly worse than that of the enhanced IAM with complex pilots (2.6074). The PAPR of the new method (max 11.4 dB) is much lower than that of other frequently used methods. Finally, the channel-estimation capability of the proposed method is validated for both back-to-back and long-distance transmission scenarios. Furthermore, the BER performance of several different channel-estimation methods is also compared. The obtained results confirm the above observations.

Owing to the transverse wind, a laser beam drifts when transmitting in an atmospheric channel. The moving speed of the light spot image at the receiving end was deduced using cross-correlation algorithm. By establishing experiments with communication distances of 1.3 km and 100 km, the relationship between the transverse wind and the moving speed of the spot particle image on the optical transmission path was obtained. The results show that the correlation coefficients between the transverse wind and spot particle image moving speed are 0.92 and 0.83, respectively, which proves the feasibility of using the cross-correlation algorithm to obtain the transverse wind speed in atmospheric turbulence. And from this approach, the transverse wind speed and the wind vector moving image on the transmission path can be analyzed easily.

Yb^3  +  :  KY  (  WO₄₎₂ (Yb:KYW) possesses various excellent properties, such as its highly polarized emission cross section, that makes it an excellent option for developing highly efficient diode-pumped solid-state lasers. A high-energy, high-beam-quality passively Q-switched Yb:KYW laser was demonstrated. The small-signal gain characteristics of the Yb:KYW crystal under quasi-continuous-wave laser diode (LD) pumping were obtained by theoretical analysis. Under LD end-pumping at a repetition rate of 500 Hz and a pumping width of 200  μs, the Q-switched Yb  :  KYW  /  Cr^4  +  :  YAG laser operated at a wavelength of 1025 nm, and a single-pulse energy of 0.5 mJ was obtained with a pulse width of 5.8 ns. Near-diffraction-limited beam quality with an M² value of ∼1.02 was achieved. The compact setup provided efficient second-harmonic generation at 512.5 nm via type-I phase matching using a LiB₃O₅ (LBO) crystal, and the output single-pulse energy was 0.13 mJ. We believe that these results further demonstrate the utility of Yb:KYW as a passively Q-switched laser crystal for solid-state lasers, particularly for applications that require nonlinear frequency conversion.

An acousto-optical filter with tunable polarization based on photonic crystal fibers is used to propose a method of optical modulation, from the modulation by positions of ultrashort pulses in the frequency domain (PPFDM) and the modulation in dual optical sideband simultaneously. So apply this method for generating optical logic gates. It can be shown that AND and OR logic gates can be obtained when a soliton of 55.5-fs time width is encrypted initially generating two bits of information by the modulation method proposed here. We analyzed the optical double-sideband PPFDM modulation for the pulses (bits), polarized in the two input modes, allowing a variation in the ε_cod encoding parameter for each pulse. One input pulse in relation to another is allowed a phase difference Δϕ  =  1.28π, because analyzing the frequency curve as a function of a phase variation, it is observed that it is in this phase that we obtain the most reliable logic gates.

An all-optical half adder (AOHA) on a square lattice photonic crystal structure with silicon rods in air is proposed. The AOHA design consisted of constructive and destructive interference-based XOR gate and AND gate, which were respectively devised by Y- and T-shaped waveguides to produce Sum and Carry logical outputs. The band structure of the proposed device was assessed by the plane wave expansion (PWE) method, and the electric field distributions and functional parameters were estimated by the finite difference time domain (FDTD) method. The functional parameters obtained for the AOHA include a contrast ratio of 9.42 dB for the Sum and 12.87 dB for the Carry, response time of 0.888 ps, operating bit rate of 1.126 Tb/s, and footprint of 290  μm². The normalized intensity of the AOHA was >86  %   and <12  %   for logic “1” and logic “0,” respectively. Hence, the proposed AOHA design is highly suitable for photonic networks and photonic integrated circuits.

We present the design of an innovative dual-band terahertz sensor based on a metamaterial array and a microfluidic channel. The sensing performance of two ultra-high absorption peaks—with absorption rates of 99.96% and 99.85%—generated in the frequency band of 0.2 to 1.2 THz is analyzed. Refractive indexes of analytes are predicted to be in a specific refractive index range, demonstrating the very low prediction error rate of the proposed sensor. Further analyses show that the designed sensor has application potential in label-free and rapid detection.

Knowledge of slow crack growth rate in a window material is required to determine the design life of a window in which cracks grow under the influence of applied service stress. Parameters that characterize crack growth rate can be measured by dynamic fatigue in which coupons are taken to failure at several constant stress rates. The slower the stress rate, the more time is available for cracks to grow during the strength measurement, and the lower the stress at which the coupon fails. That is, coupons tested at a slower stress rate are weaker than coupons tested at a faster stress rate. The rate of slow crack growth is commonly fit to either a power law or an exponential law. The exponential law provides the more conservative estimate of design life and is prescribed for use by the U.S. National Aeronautics and Space Administration to design windows for manned vehicles. Well-established analytical equations are used to derive the power law crack growth parameters from dynamic fatigue measurements of as-polished (unindented) coupons. There are no exact analytical equations to derive power law parameters from indented coupons or to derive exponential law parameters from unindented or indented coupons. Approximate procedures have been used to derive crack growth parameters when there are no exact equations. We now describe a numerical method that gives the power and exponential laws for both unindented and indented coupons by least-squares fitting of dynamic fatigue measurements. A MATLAB code is provided to carry out the calculations.

A simple and fast method is proposed for designing and optimizing broadband and narrowband terahertz (THz) graphene absorbers in the desired frequency bands or any number of desired frequencies. Also, the maximum bandwidth for multilayered structures is proposed by this method. This method uses a genetic algorithm, transmission line theory, and impedance matching technique. The proposed THz absorbers are perfect frequency-tunable structures with absorption values more than 90%, which consist of multilayered graphene separated by a dielectric material. Continuous graphene is utilized in the structure of these absorbers but metamaterial graphene can also be used. The optimization parameters are the thickness of dielectrics, refractive indices of dielectrics and the chemical potential of the graphene layers. These graphene-based absorbers have high potential for different THz applications such as filters, detectors, switches, modulators, etc.

Laser wireless power transmission (LWPT), which can realize long-range power transmission, has been received widespread attention in recent years. The influence of high-power laser nonuniform irradiation on the output power and conversion efficiency of photocells has attracted much attention. This article conducted simulations and experiments on nonuniform irradiation with a constant power density and on nonuniform irradiation with constant incident power. The current I, voltage V, output power P_L, conversion efficiency η_L, internal resistance R₀, and maximum power point resistance R_Lmax of a 1  cm  ×  1  cm single-junction gallium arsenide photocell under nonuniform irradiation were studied. Under the condition of nonuniform irradiation with a constant laser power density, the conversion efficiency of the photocell remained constant, but R₀ increased nonlinearly from 0.5 to 5  Ω. The η_L decreased from 45% to 5% when the load resistance is constant. Under the condition of nonuniform irradiation with constant laser power, the photoelectric conversion efficiency and R₀ changed little, and the η_L remained about 45%, for it is not affected by nonuniform irradiation. The output characteristics of the photocells under uniform irradiation were also studied, and the results were consistent with the above conclusions. When the photocells with constant temperature were irradiated in-homogeneously, it was also revealed that the internal resistance increased and that the conversion efficiency decreased due to the irradiation power variation. The empirical formula was derived through circuit transformation and curve fitting.