Editor-in-Chief Michael Eismann discusses changes occurring because of the COVID19 pandemic.

An algorithm is presented that can be used to obtain accurate optic axis orientation of birefringent tissue samples. A single-mode fiber (SMF)-based polarization-sensitive optical coherence tomography with a single linearly polarized incident light was used in the measurements in which the light reflected from the sample and detected by the spectrometer is linearly polarized light at 45 deg with respect to the experimental horizontal axis. By employing polarization controllers to completely specify the parameters of the fiber system, the absolute optic axis orientation could then be estimated accurately by analyzing the Mueller matrices of the fiber system and sample. The proposed algorithm has been validated in the quantification of the optic axis orientation of a quarter-wave plate. Finally, several birefringent tissue samples were imaged with this SMF-based system.

This guest editorial summarizes the Special Section on Machine Learning in Optics.

A multimodal image fusion method based on the joint sparse model (JSM), multiscale dictionary learning, and a structural similarity index (SSIM) is presented. As an effective signal representation technique, JSM is derived from distributed compressed sensing and has been successfully employed in many image-processing applications such as image classification and fusion. The highly redundant single dictionary always has difficulty satisfying the correlations between images in traditional JSM-based image fusion. Therefore, the proposed fusion model learns a more compact multiscale dictionary to effectively combine the multiscale analysis used in nonsubsampled contourlet transformation with the single-scale joint sparse representation used in image domains to solve the issues of single-scale sparse fusion and to improve fusion quality. The experimental results demonstrate that the proposed fusion method obtains the state-of-the-art performances in terms of both subjective visual quality and objective metrics, especially when fusing multimodal images.

Astronomical images collected by ground-based telescopes suffer from degradation and perturbations attributed to atmospheric turbulence. We investigate the application of convolutional neural networks (CNNs) to ground-based satellite imaging to address two existing problems. First, the multiframe blind deconvolution (MFBD) algorithms that can extract well-resolved images from these degraded data frames are computationally expensive, requiring supercomputing infrastructure for relatively fast performance and currently cannot be done in real time. Because of this, it is difficult to optimize collection parameters to maximize the likelihood of producing a resolved image with MFBD. Second, the space-object National Imagery Interpretability Rating Scale (SNIIRS) allows human analysts to provide a quantitative score of image quality based on identification of target features. It is naturally difficult to automate this scoring process, not only because the scale is based on identifiable features but also because the images may be in an almost-resolved image quality regime that is difficult to handle for traditional computer vision techniques. For both applications, we present our results using CNNs on data collected at the Maui Space Surveillance Site as well as a new synthetic dataset we introduce containing over a million SNIIRS rated pairings of perturbed and pristine ground-based satellite images.

Many optical systems are used for specific tasks such as classification. Of these systems, the majority are designed to maximize image quality for human observers. However, machine learning classification algorithms do not require the same data representation used by humans. We investigate the compressive optical systems optimized for a specific machine sensing task. Two compressive optical architectures are examined: an array of prisms and neutral density filters where each prism and neutral density filter pair realizes one datum from an optimized compressive sensing matrix, and another architecture using conventional optics to image the aperture onto the detector, a prism array to divide the aperture, and a pixelated attenuation mask in the intermediate image plane. We discuss the design, simulation, and trade-offs of these systems built for compressed classification of the Modified National Institute of Standards and Technology dataset. Both architectures achieve classification accuracies within 3% of the optimized sensing matrix for compression ranging from 98.85% to 99.87%. The performance of the systems with 98.85% compression were between an F  /  2 and F  /  4 imaging system in the presence of noise.

Arabic script is inherently cursive in nature; therefore, optical character recognition (OCR) techniques developed for other scripts are generally inappropriate for Arabic. We present an OCR system for the recognition of handwritten Arabic words. Several original contributions have been made. First, we have proposed a novel feature vector to represent handwritten Arabic words based on the ellipsoid approximation of maximally stable extremal regions (MSERs). This feature vector is compact, robust, and well suited to handle writer-induced variation in handwritten script. Second, we present a new database, Saudi Arabian city name (SACN) database consisting of handwritten Saudi Arabian city names, which is the first such publicly available database. Finally, we present promising experimental results on two databases. One of the main experiments compares the proposed MSER feature vector with several well-established feature extraction methods using the proposed SACN database. The proposed feature vector performed better than all the standardized methods, with 100% correct identification rate. Another experiment is designed to compare our system to other recent studies on handwritten Arabic text recognition using the extensively employed IFN/ENIT database. Using the standard experimental protocol, our system outperformed all others with 92.64% correct identification rate.

Carbon fiber reinforced plastics (CFRPs) have been used to replace metallic alloys in many industries because of their high strength-to-weight ratio. Due to their anisotropic behavior, low-velocity impacts can produce defects whose effects on material performance are hard to foresee. Nondestructive testing (NDT) methods are a convenient alternative to evaluate their integrity. Shearography, an image-based optical interferometric technique for measuring deformation, is one of these NDT possibilities. The segmentation of defects in the resulting images provided by such a method is essential to correctly locate and indicate the severity of impact damage. This task is especially intricate for shearography images with barely visible impact damage because of their usual low signal-to-noise ratio. We compare a combination of wavelet decomposition with multithresholds introduced in a previous publication with a U-net convolutional neural network for analyzing impact damage in CFRP plates. Both tools are detailed and then evaluated using the Matthews correlation coefficient and the equivalent diameter criterion. The results showed that U-net provided a better impact damage characterization in both evaluation metrics, allowing a safer defect detection that is less dependent on the inspector’s ability to interpret them.

We explore the blending of model-based and deep learning approaches for target recognition in inverse synthetic aperture radar (ISAR) imagery. It evaluates five different approaches, namely, a model-based geometric hashing approach, a supervised deep learning approach, and three different blending models that fuse the model-based and deep learning approaches. The model-based approach extracts scattering centers as features and requires domain experts to identify and characterize important features of a target, which makes the training process very costly and hard to generalize when the image quality degrades in low signal-to-interference-plus-noise-ratio conditions. Next, a deep learning algorithm using a convolutional neural network is considered to extract the spatial features when raw ISAR data are used as input. This approach does not need an expert and only requires the labels of images for training. Finally, three model-based and deep learning approaches are blended together at the feature level and decision level to benefit from the advantages of both approaches, achieving a higher performance. The results show that the blending of the two approaches achieves a high performance while providing explainable inferences. The performance of the five different approaches is evaluated under varying conditions of occlusion, clutter, masking of the target, and adversarial attacks. It is empirically shown that the model-based and deep learning approaches are able to complement each other and can achieve better classification accuracy upon fusing the integrated approach.

Reliable phase-only spatial light modulators (SLMs) are in demand for accurate phase modulation in a wide range of fields. Due to the nonlinear optical response of liquid crystals and the limited manufacturing process available, the spatial nonuniformity of the phase modulation by the pixels should be measured and/or calibrated. We propose an in situ calibration method based on digital holography to calibrate the spatial nonuniformity of phase modulation of the SLM. The SLM panel is divided into blocks composed of pixels. The differential phase on hundreds of blocks can be reconstructed through the holograms. The distribution of modulated phase can then be derived after eliminating statics phase anomalies. The spatial nonuniformity of the panel can be measured for calibration with high efficiency. A modulated phase step on the SLM was calibrated to increase linearly. The spatial nonuniformity was calibrated to decrease by more than 75% using only a beam splitter and an imaging sensor. The in situ strategy for low cost and efficient calibration was demonstrated with optical experiments using a 4K (3840  ×  2160  pixels) phase-only SLM.

In optical metrology, the phase-shifting technique is used to retrieve the phase information from interferograms. Such displacement can be performed by mirrors attached to electromechanical devices (such as piezoelectric or moving mounts), gratings, or polarizing components, which need to be calibrated to associate the displacement of the device with respect to the induced phase shift. For this purpose, we present a closed-form formula to calculate of the original phase step between two randomly shifted fringe patterns by extending the Gram–Schmidt orthonormalization algorithm. To demonstrate its feasibility, we perform an evaluation that consists of three cases that represent different fringe pattern conditions. First, we evaluate the accuracy of the method in the orthonormalization process by estimating the test step using synthetic normalized fringe patterns with no background, a constant amplitude, and different noise levels. Second, we evaluate the formula with a variable amplitude function on the fringe patterns and a constant background. Third, we evaluate non-normalized noisy fringe patterns in which we include the comparison of prefiltering processes such as the Gabor filters bank, Hilbert–Huang transform, and isotropic normalization process and a high-pass filter to emphasize how they affect the calculation of the phase step.

Automated situation awareness (ASA) in a complex and dynamic setting is a challenging task. The accurate perception of environmental elements and events is critical for the successful completion of a mission. The key technology to implement ASA is target detection. However, in most situations, targets of interest that are at a distance are hard to identify due to the small size, complex background, and poor illumination conditions. Thus, multimodal (e.g., visible and thermal) imaging and fusion techniques are adopted to enhance the capability for situation awareness. A deep multimodal image fusion (DIF) framework is proposed to detect the target by fusing the complementary information from multimodal images with a deep convolutional neural network. The DIF is built and validated with the Military Sensing Information Analysis Center dataset. Extensive experiments were carried out to demonstrate the effectiveness and superiority of the proposed method in terms of both detection accuracy and computational efficiency.

We provide an optimized closed-loop algorithm for color control of a multichannel light-emitting diode (LED) lighting system using a multispectral sensor. The proposed control scheme makes it possible to maintain a desired lighting ambience in a room by taking into account external lighting sources. It rejects disturbances specific to LEDs (spectral drift) or independent from them (variations in external lighting, changes in the room access conditions, or changes in the lighting inside the room such as a light addition). This control scheme consists of using a multispectral sensor, a regulator, an anti-windup system, an efficient optimization algorithm, and a set of colored LEDs. Experimental results show the high performance of a preindustrial system with a color difference Δu^′v^′  =  0.0004. This value is below the threshold of the color difference that is considered noticeable by the human eye.

The existence of blemishes deteriorates the imaging quality of fiber-optic imaging elements and reduces the yield of finished products in factories. The effective detection of blemishes is a prerequisite for analyzing the causes of blemishes and preventing their generation. An independently developed detection device for fiber-optic imaging elements based on machine vision is implemented to detect blemishes automatically and accurately. The blemish distributions of three typical fiber-optic imaging elements, including a fiber-optic plate (FOP), a fiber-optic inverter (FOI), and a fiber-optic taper (FOT), are compared with each other, and the causes of blemishes in fiber-optic imaging elements are analyzed. The distribution of blemishes in the FOP is random, and the average number (AN) of blemishes in the first zone (central zone) and the second zone (outer zone) with the same area is similar. The AN of blemishes for the FOI prepared by twisting the FOP at high-temperature increases from 272.8 (FOP) to 2125.0, and the AN in the outer zone is 1542.5, higher than that in the central zone. More blemishes are generated by the twisting process, and the distribution of these blemishes is close to the outer zone. A significant amount of chicken wires is found in the outer zone, but the majority of blemishes are still spots. The AN of blemishes for the FOT prepared by stretching the FOP increases from 383.0 (FOP) to 515.0, which is attributed to the increase of the number of blemishes in the first zone from 190.8 to 427.0, but the AN of blemishes in the second zone decreases from 192.3 to 88.0. The stretching process stimulates the formation of blemishes with a distribution that is close to the central ring. These blemishes are randomly distributed inside or at the boundary of the multifiber. Most of the blemishes are still spots.

Signal interference between light detection and ranging (lidar) sensors has the potential to degrade range data integrity, which may challenge multilidar applications such as autonomous vehicles if not adequately considered. Our work proposes a methodology for evaluating lidar interference, analyzes distinct lidar interference phenomena, and validates a previously proposed model of interference between two circularly scanning, pulsed lidar sensors. The comparison between Monte Carlo simulated and experimentally observed interference events suggests that lidar interference may be inferred through geometrical approximations of lidar scanner arrangements. The experimental data quantify the occurrences of at least two modes of lidar interference—direct and scattered interference. When present, direct interference was found to occur one to two orders of magnitude greater in total occurrence than scattered interference. However, direct interference’s effects represented an average range error of <1  m. Alternatively, scattered interference was observed with an average range error two to four orders of magnitude greater than direct interference and proportional to the maximum range of the victim lidar. Further characterizations of interference phenomenon are presented that include the angular distribution of erroneous ranging data, range distributions of errors, loss of in-tolerance points, and potential radiometric influences.

The temperature compensation is essential to the strain measurement in distributed optical fiber sensor (DOFS) due to the temperature-strain cross talk. We demonstrated that the temperature sensitivities and characteristics were different among fibers with different coatings. Compared with the uncoated fiber, polyimide coating could increase the temperature-sensitivity coefficient of fibers, while acrylate coating decreases it. The temperature-sensitivity coefficient changed significantly when the fibers were coated with materials due to different thermal expansion coefficient. To study the temperature compensation for stain measurement with DOFS, the experiments were conducted to measure the spectral shifts of scattered light due to temperature and strain changes for acrylate-coated fiber and polyimide-coated fiber bonded to the same steel plate. The temperature-induced strain of steel plate surface was transferred to the optical fibers together with stress-induced strain. Because of different temperature-sensitivity coefficients and fiber coatings, the measured surface strain distributions on the steel plate are different. The expansion coefficient of steel plate measured by polyimide fiber in the experiment was 11.88  ×  10^−  6  K^−  1, which coincided with theory value. A hypothesis for analyzing the strain transfer of fiber was found in the experiment. When bonded on the substrate for measuring strain, the total elongation of the fiber is consistent with the total elongation of the substrate along the direction of the fiber within the bonded-length range.

Synchronous measurement of temperature and the deformation field can be achieved through reasonable assignment of R, G, and B channels of a CCD camera in a high-temperature environment. The R and G channels are used for colorimetric temperature measurement, and the B channel is adopted for deformation measurement. However, the external blue compensation light source may cause crosstalk of the R and G channels, leading to inaccurate measurement. To eliminate such an interference phenomenon, we first use the function relationship between the illumination of the object surface and the exposure time to calibrate the camera parameters at room temperature, thus removing the initial gray level of the R and G channels in the high-temperature image. This contributes to improving the low-temperature measurement accuracy for the temperature field, with a calculation error of <5  %  . In contrast, an experiment shows that the temperature deviation of the low-temperature region calculated using the uncorrected image is up to hundreds of degrees. Then, the radiation intensity of channel B is removed using the temperature balance relationship, which increases the calculation accuracy by 15% in comparison to the uncorrected result. The algorithm is validated by the heating experiment of C/SiC material with an oxy-acetylene flame. The results suggest that the proposed improved algorithm is of great significance for the evaluation of materials at high temperatures.

Accurate temporal characterization both in intensity and phase distribution is important in the diagnosis of the petawatt (PW) class. We present a single-shot picosecond frequency-resolved optical gating (ps-FROG) setup based on an autocorrelator with ps measurement range that is spectrally resolved through a fine grating. The modified ptychographic-based algorithm with a changing update coefficient was used for the reconstruction of the pulse distribution; it can better adapt to the reconstruction of pulse with a large time–bandwidth product. We calibrated and verified the homemade ps-FROG in a 100-μJ ps laser system and used it to characterize the pulse distribution generated by the PW laser system of the Shen Guang II facility. The system shows good performance and high accuracy in reconstructing the intensity and phase distributions of a ps pulse, which provides reference for accurately adjusting the grating pair to acquire the pulse width as a preset.

A robust calibration approach for a telecentric stereo camera system for three-dimensional (3-D) surface measurements is presented, considering the effect of affine mirror ambiguity. By optimizing the parameters of a rigid body transformation between two marker planes and transforming the two-dimensional (2-D) data into one coordinate frame, a 3-D calibration object is obtained, avoiding high manufacturing costs. Based on the recent contributions in the literature, the calibration routine consists of an initial parameter estimation by affine reconstruction to provide good start values for a subsequent nonlinear stereo refinement based on a Levenberg–Marquardt optimization. To this end, the coordinates of the calibration target are reconstructed in 3-D using the Tomasi–Kanade factorization algorithm for affine cameras with Euclidean upgrade. The reconstructed result is not properly scaled and not unique due to affine ambiguity. In order to correct the erroneous scaling, the similarity transformation between one of the 2-D calibration plane points and the corresponding 3-D points is estimated. The resulting scaling factor is used to rescale the 3-D point data, which then allows in combination with the 2-D calibration plane data for a determination of the start values for the subsequent nonlinear stereo refinement. As the rigid body transformation between the 2-D calibration planes is also obtained, a possible affine mirror ambiguity in the affine reconstruction result can be robustly corrected. The calibration routine is validated by an experimental calibration and various plausibility tests. Due to the usage of a calibration object with metric information, the determined camera projection matrices allow for a triangulation of correctly scaled metric 3-D points without the need for an individual camera magnification determination.

Digital holography is attracting attention because it can record instantaneous three-dimensional (3D) information and record dynamic phenomena. However, when recording high-speed phenomena, the frame rate ranges from tens of thousands to hundreds of millions, and the calculation time of the reconstructed images is a problem. We have developed a special-purpose computer for high-speed 3D imaging using digital holography. The developed special-purpose computer has four calculation modules and has achieved a calculation time 68 times faster than that of a personal computer with 48 cores. With the developed computer, a total of 32 reconstructed images can be calculated in 0.69 ms from four holograms of 128  ×  128  pixels with eight varying depths.

Resolution is an important index for evaluating the performance of a low-light-level image intensifier. The traditional method of image tube resolution measurement is to observe the resolution target on the image intensifier through a microscope and give a subjective judgment. The disadvantage is that the difference in the visual acuity between testers will lead to variability in measurement results. In addition, the process of focusing the resolution image is very time-consuming and eye-consuming. To solve these problems, an objective evaluation system of image tube resolution based on fast Fourier transform (FFT) that provides an efficient and feasible objective evaluation scheme for the estimation of image tube resolution is proposed. In this system, the resolution target is first focused on the cathode surface of the image tube through an optical system, and then the image is taken by a high-resolution camera. The region of interest of the collected image shows that the gray-scale sequence along the direction of the stripe change reflects the frequency of the stripe. When the gray-scale sequence is transformed to the frequency domain using FFT, the clarity and resolution of the stripe will be correlated with some quantities in the frequency domain. We extract the clarity-resolution values from the frequency domain of all stripe elements and use the generated clarity threshold combined with the linear fitting strategy to achieve the resolution value. The experimental results show that, in terms of accuracy, the test results of the system are consistent with the subjective evaluation results. For the tubes with resolutions between 61.37 and 66.49  lp  /  mm, the accuracy of our system is higher than that of human judgment. For repeatability, the measurement results of the system are in good agreement with the subjective evaluation results of the tubes with resolutions between 50 and 62  lp  /  mm. Therefore, our system can be regarded as a reasonable alternative to the subjective evaluation method, which will greatly reduce the variability caused by different testers.

Ultraviolet communication (UVC) with its ability to operate in non-line-of-sight (NLOS) offers significant advantages as compared to conventional optical wireless communication systems. NLOS UVC is a short-distance communication system due to high path loss and turbulence-induced fading. We propose an effective cooperative multirelay system to combat the effect of attenuation and fading in NLOS UVC, thereby making it suitable for long-distance communication. We investigate the performance of the proposed system in terms of outage probability by considering both variable-gain and fixed-gain amplified and forward relaying. Further, we consider higher-order modulation schemes to improve the data rate of the system and derive innovative closed-form expressions of the average symbol error rate and diversity order for the variable-gain relaying. We demonstrate performance gains of the considered multirelay system over nonrelayed links for different system parameters. We present numerical results to confirm the accuracy of the derived analytical expressions.

Daylight is an inherent attribute of a sustainable building. Artificial reproduction of natural illumination under varying needs and environmental conditions has been made possible after the appearance of the new wave of solid-state light sources. Our work is devoted to the development of white light-emitting diode (LED) clusters consisting of red, green, blue, and white (RGBW) LEDs for implementation in a smart lighting system that is able to reproduce light with correlated color temperature (CCT) similar to daylight, high values of color rendering index, and luminous efficacy. A method for determination of the optimal contribution of each of the four LEDs is demonstrated and discussed. We show that only three LEDs—green, blue, and white (no red) can be used in many cases for reproducing daylight. Both luminous efficacy of radiation and actual luminous efficacy of the considered RGBW clusters as functions of the CCT are analyzed.

The excitation of surface-plasmon-polariton (SPP) waves guided by a columnar thin film (CTF) deposited on a one-dimensional metallic surface-relief grating was studied employing the rigorous coupled-wave approach, when the grating plane, the plane of incidence, and the morphologically significant plane of the CTF are all different. The incident plane wave in this grating-coupled configuration could be either p- or s-polarized. The absorptance was plotted against the polar angle of incidence at a fixed value of the free-space wavelength, and absorptance peaks were correlated with the solution of the dispersion equation of the underlying canonical boundary-value problem for SPP waves. Both p-polarized and s-polarized plane waves can excite SPP waves, provided that either the plane of incidence and/or the morphologically significant plane of the CTF do not coincide with the grating plane. None, one, or multiple SPP-wave excitations are possible, depending on the orientations of the grating plane and the morphologically significant plane with respect to the plane of incidence. The direction of propagation of an SPP wave thus excited may not wholly lie in the plane of incidence.

We present the setting and design of an aplanatic optical system for three-point objects and images. The system is composed of a singlet aspheric lens and its entrance pupil. The aspheric lens and the entrance pupil are such that the three-point images and objects lay on the same plane. The image at the center is free of spherical aberration, and the other off-axis images are free of coma.

In ultrasmooth surface machining technologies, such as fluid jet polishing (FJP), a better understanding of the material removal mechanism is urgently required for the improvement of surface quality. The shape of the material removal on a polished surface after impact of a liquid jet containing nanoparticles was first measured and then compared with the calculated spatial distribution of the particle-wall collision along the solid surface. The consistency implies that the impact of particles on the workpiece surface is the main reason for material removal in FJP. Thus, to further understand the mechanism of the material removal of FJP, the characteristics of the particle-surface collision of a liquid jet are essential. For this, the effects of jet velocity, particle diameter, and particle density on the particle-surface collisions of a liquid jet were numerically investigated. The results show that the probability of the nanoparticle-surface collision of a liquid jet has a linear relationship with both the jet speed and particle density. Meanwhile, for the particle diameter, the relationship is nonlinear when the particle diameter is <500  nm and has little influence on the collision probability (P_C); whereas, the P_C increases almost cubically when the particle diameter is >500  nm. The present results provide more knowledge about the particle-surface collisions in a liquid jet and may be helpful for further understanding of the material removal mechanism in ultrasmooth surface processing technologies.

Infrared (IR) film systems can accumulate considerable stress due to their large thickness. In addition, the thermal stress of the films will further increase due to a significant difference in thermal expansion coefficient between the film materials and the substrate. Stress accumulation limits the passband transmittance of the 2.76 to 3.04  μm traditional IR interference filter to about 80%. To increase the passband transmittance and guarantee the mechanical properties, four kinds of samples were designed, prepared, and measured. Through matching the film systems and proposing an alternative deposition method, high-performance IR bandpass filters with an average transmittance of 92.2% have been prepared. The improved transmittance provides a necessary study of developing IR systems and improving imaging quality.

Terahertz (THz) technology has become popular worldwide as a new approach to detecting biomolecules because the vibrational and rotational energy levels of many biomolecules fall in the THz band and because the THz wave has the characteristics of low electronic energy, which will not damage the samples to be measured. Many biomolecules need to maintain their biological activity in liquid environment. However, as a polar molecule, water has a strong absorption of THz wave, which is mainly because the vibration frequency of hydrogen bond in aqueous solution is within the THz frequency range. Therefore, the best solution is to reduce the action distance between the aqueous solution and THz wave and control it within 100  μm. Microfluidic chips can meet such requirements. Therefore, the combination of THz technology and microfluidic technology can study the dynamic characteristics of biomolecules in an aqueous solution. The microfluidic chip was fabricated using ZEONOR 1420Rs. The THz transmittance of the material can exceed 95%. The depth of the microchannel in the microfluidic chip is 50  μm. In addition, the chip has the characteristics of good airtightness, portability, convenient disassembling, and reusability. Seventeen kinds of electrolytes were tested with the chip. The results show that the THz spectral intensity of electrolyte composed of different anions and cations, so the spectral characteristics of other electrolyte solutions can be obtained according to the spectral information of these detected ions.

A spectral purity Al/Yb/Al filter for the wavelength range of 50 to 100 nm has been designed, fabricated, and characterized. The spectral analysis and transmittance measurements were performed in the wavelength range of 50 to 80 nm and 50 to 100 nm. The spectral analysis measurement results show that high-order harmonics greater than 10 nm can be efficiently suppressed. Transmittance measurement results show that the filter has the ability of harmonic suppression in the wavelength range of 50 to 95 nm. The fitting result of the transmittance measurement result indicated that lower measured transmittance was mainly caused by the interdiffusion and oxidation of the filter.

Spatial light modulators (SLMs) provide a powerful technology to perform high-resolution and high-speed phase modulation of laser radiation for the generation of structured laser beams. The possibility of using SLMs for dynamic laser manipulation of airborne light-absorbing particles due to the action of the photophoretic forces, demonstrating different types of manipulation, including confinement of the trapped particles, two- and three-dimensional guiding of the trapped particles, their controllable revolution, and the transfer of trapped particles between different optical traps, is shown. For this purpose, different phase masks that can be used for the generation of the desired optical traps with the help of SLMs are provided, opening new possibilities for using SLMs for laser manipulation in air and allowing for the realization of all-optical conveyors for the fabrication of microdevices in the future.

Computed tomographic imaging spectrometers capture hyperspectral images in realtime. However, postprocessing the imagery can require enormous computational resources; thus, limiting its application to nonrealtime scenarios. To overcome these challenges, we developed a highly parallelizable algorithm that exploits spatial shift-invariance. To demonstrate the versatility of our algorithm, we developed implementations on a desktop and an embedded graphics processing unit. To our knowledge, our results show the fastest image reconstruction times reported.

We developed an open source optical design program in C++ with a genetic and bisection optimization method. The genetic algorithm brings the system close to the global optimum using the survival of the fittest principle. After an abort criterion has been reached, a local optimization via the bisection method optimizes the system further until a new optimum is reached. We test our algorithm with different population numbers and search area sizes for the genetic selection and investigate how the calculation time or the accuracy of our algorithm behaves when we change those parameters. It is shown that our algorithm is able to optimize complex optical systems. The user can add different wavelengths, field positions, or a minimum surface gap. It is also possible to change the weight factor of specific parameters in the merit function. We explain the working principle of our algorithm and compare the results with commercial tools.

We have utilized optical-pump terahertz-probe spectroscopy to analyze the relaxation process of photoinduced nonequilibrium carriers in bulk ZnTe with a crystal orientation of   ⟨  110  ⟩  . The experimental results showed that the concentration of nonequilibrium carriers increases with the increase in the energy of pump laser. When the pump energy calculated by a single exponential fitting function reached 63  μJ, the concentration of nonequilibrium carriers attained saturation. In addition, for low pump energy, the relaxation time did not change with increasing pump energy. On the contrary, when the pump energy was high, the relaxation time increased in proportion to the pump laser energy.

We numerically present a method to generate a high-energy square pulse in dual-amplifier figure-eight microstructured optical fiber laser (DAF8MOFL) using a genetic algorithm (GA). We explore a large region of the parameter space to find adequate parameters that promote the generation of high-performance square pulse. We demonstrate that, by utilizing the GA-based optimization technique, the parameters of the DAF8MOFL can be conveniently optimized to achieve targeted output pulse shape according to real-parameter constraints. We show that by adequately adjusting the cavity parameters, high-energy pulse with a square profile and picosecond pulse width can be obtained without the pulse breaking.

When a link budget for Earth-to-space connectivity needs to be prepared for a scenario in which optical technology is used, in terms of atmospheric turbulence random degradation, the most important losses to account for are those due to scintillation and beam wander fading processes. Currently, the standard approach is to include the margins for scintillation and beam wander separately in the link budget, treating the statistical distribution of both processes independently. In essence, this approach is incorrect as the actual statistics for the received irradiance at the spacecraft are governed by the composite channel arising from the joint interaction between these two phenomena. A method to calculate a fading loss for the composite channel is developed. The calculation is performed by inverting the cumulative distribution function of the governing statistical model for a given outage probability p that the received irradiance is below a certain threshold value. A tractable approximation solution is found, and an accuracy analysis is conducted to give the limits of applicability of such an expression. Finally, the derived expression is used in a practical example framed in an Earth-to-space optical geosynchronous orbit feeder link scenario in which the fading loss approximation derived for the composite channel is found to be below 1 dB error from the exact solution for most of the range of interest, and well below 0.1 dB error near the optimum transmitter size.

The mathematical model of an optical ring resonator (RR) configuration using a single directional optical coupler (DOC) with a uniform fiber Bragg grating (FBG) incorporated and an RR configuration using two DOCs with an FBG incorporated is developed in the Z-domain. The developed mathematical model is used for the evaluation of transmission response. Spectral characteristics in terms of peak transmittance, resonant frequency, and free spectral range are determined for a single fiber ring configuration using two DOCs with an FBG incorporated. The results are found to be in very close agreement with measured values. The transmission characteristic of the single waveguide structure comprising a ring of length 2 mm enclosing a Bragg grating is analyzed. By properly optimizing the grating reflectivity and power coupling ratios of the coupler, a narrow full width at half maximum of 4.72 GHz that is around 78 times sharper than the normal FBG resonance is obtained. Further narrowing of the spectral width is achieved by increasing the RR length. The grating lengths are chosen judiciously to minimize the side peaks. The group delay and dispersion characteristics are addressed.

Nonlinear interaction induced by Kerr effects is a significant problem in the efforts made to ensure the successful development of transmission systems, such as mode-division multiplexing (MDM)-based optical multimode communication systems. A technique for reducing the interaction of the modes in MDM systems based on shaping envelopes of propagated signals is proposed. The envelopes of modes that carry m-array quadrature amplitude modulation (mQAM) are optically formed with a sinusoidal envelope to lower fiber nonlinearities by reducing the effective intensity and interference time between modes. The transmission performance of the proposed system is analytically characterized by developing an analytical model to estimate induced nonlinear phase noise and numerically investigated by examining the signal-to-noise ratio versus mode power. The effect of sinusoidal enveloped (SE)-4QAM format on the transmission distance is also explored for different mode combinations. Single-, two-, and five-mode transmissions are carried out to investigate the proposed method’s efficiency on the reduction of nonlinear interaction. In our system, the modes carry SE-4QAM format at rate 20  Gsymbol  /  s. The results show a significant enhancement in the performance of the MDM system when modes are modulated by SE-4QAM format. For example, the transmission distances of LP01 and LP11 are lengthened by 56.5% and 150% at the symbol error rate of 10^−  5, respectively.

Frequency-modulated continuous-wave (FMCW) lidars combine the advantages of lasers and FMCW radars and have a wide range of applications in three-dimensional imaging and meteorological observation. However, its spatial resolution is directly limited by the nonlinearity of laser frequency sweep. A nonlinear correction method is proposed using a singular value decomposition (SVD)-least squares algorithm, which can calculate the nonlinear coefficient of each order accurately by minimizing the error between the actual phase and the fitted phase. In the simulation, the results demonstrate that the proposed method can achieve significantly better performance on nonlinearity and root-mean-square error (RMSE) than that of the conventional high-order ambiguity function (HAF) algorithm. Calculation results show that, for the SVD algorithm, the nonlinearity and RMSE are reduced by 50.37% and 52.55%, respectively, compared with those of the HAF algorithm under the signal-to-noise ratio of 25 dB, proving the performance improvement of the proposed algorithm.

We demonstrate the output of femtosecond (fs) green and ultraviolet (UV) lasers through second-harmonic generation using type-I phase-matched K₃B₆O₁₀Br (KBOB) nonlinear optical (NLO) crystals. The wavelengths of the fs green and UV lasers are 515 and 400 nm, respectively. The 1030-nm fs laser is used to pump the KBOB crystal to generate the green output. When the laser pump power reached 880 mW with a repetition rate of 1 kHz, an output power of 235 mW was obtained from the green laser with an optical conversion efficiency of 26.7%. When a pump power of 5.87 W was attained with repetition rate of 10 kHz, a green output power of 1.03 W was generated. The fs UV laser is pumped by an 800-nm fs laser, with 132-mW output power achieved when the pump power was 470 mW with an optical conversion efficiency of 28.1%. The experimental results indicate that the KBOB crystal is a promising NLO crystal for generating fs green and UV lasers because of its excellent NLO properties.

Ultrahigh peak power laser systems require highly absorptive and high-damage threshold beam dumps, which allow safe and reliable termination of the beam path in vacuum. We present promising optical materials for high-power laser beam dump designs. In order to evaluate the optimal beam dump performance of the various materials, we characterized the optical properties of these materials and measured their damage thresholds. The tested beam dump materials include highly absorptive (>99.8  %  ) Vantablack coatings and absorptive glasses with different optical densities. The laser-induced damage threshold (LIDT) tests were performed using 800-nm ultrafast laser beams with a pulse duration of 40 fs at a 1-kHz repetition rate. The LIDT test methods, such as S-on-1 and R-on-1, were used for determining the damage threshold. The Vantablack coatings showed a moderate ultrafast damage threshold while being highly absorptive and applicable on metallic substrates. The absorptive glasses with high optical density have higher damage resistance and can be used for low-repetition rate applications with the highest intensities. By comparing the optical properties and LIDT results, we discuss the suitability of these beam dump materials for ultrafast high-power laser systems.

Microelectromechanical systems (MEMS) have produced high-quality, high-bandwidth, small form factor, and inexpensive fast steering mirror (FSM) devices potentially suitable for a large variety of applications, such as image stabilization and beam pointing in satellite-based and ground-based, free-space optical communication systems. However, one outstanding question for this application is power handling. The absorption of the mirror substrate is low, but non-negligible, so the question remains of whether thermal loading from laser radiation on a MEMS mirror will deform its surface and, if so, to what extent. We show experimental results of optical performance changes due to thermal loading for MEMS two-axis FSM devices from Mirrorcle Technologies, Inc. Results and reproducible behavior are reported and compared in ambient versus vacuum conditions, where the benefits of convective cooling are absent. Finite element analyses corroborate the experimental results and show that the mirror substrate can deform due to thermal expansion imbalances. The deformation changes the focusing characteristics of the mirror, with a peak to valley defocus (second-order Zernike mode) of up to 50 nm when the mirrors are tested in ambient and up to approximately 450 nm when under vacuum. Such defocusing negatively impacts the link budget for laser-based satellite communications.

A Q-switched ytterbium-doped fiber laser incorporating a molybdenum ditelluride saturable absorber for operation at 1046 nm is demonstrated. The laser cavity has a low Q-switching threshold of ∼48  mW. At the maximum pump power, the laser is able to generate pulses with a duration, repetition rate, and output power of 4.88  μs, 39.6 kHz, and 0.76 mW, respectively, as well as a pulse energy of 18.9 nJ. The output of the laser can be tuned from 1064 to 1099 nm, giving a tuning range of 35 nm. The generated pulses have a signal-to-noise ratio of around 60 dB. No interference frequency components or spectral modulation is observed, indicating that the proposed system is highly stable and suitable for use as a fiber laser source in the 1.0-μm region.

Algorithmic and architectural options and trade-offs between performance and complexity/power dissipation are necessary to consider in the transfer of coherent technology from long-haul transmission to short-reach application. An adaptive equalization algorithm cascading polarization demultiplexing and frequency domain equalization is proposed to target passive optical network applications that feature extremely low computational complexity. Three coordinate rotation digital computer-based vector rotators are introduced to realize polarization demultiplexing without performing multiplications. A simplified gradient descent algorithm is proposed to search and trace the state of polarization. A nonbutterfly adaptive equalizer in the frequency domain for each polarization is designed to handle link chromatic dispersion and the imperfection of the transceiver frequency response. The performance of the proposed equalization scheme is experimentally verified by a 32-GBaud dual-polarization Nyquist quadrature phase-shift keying system over 20-km standard single-mode fiber transmission. With the proposed method, the number of hardened multipliers for field-programmable gate array realization is reduced by ∼85  %   compared with a conventional 2  ×  2 multiple-input multiple-output finite impulse response filter with the same receiver sensitivity.

We report on a long-pulse, quasicontinuous wave partially end-pumped slab (INNOSLAB) amplifier at 1319 nm. The low-pulse-energy seed laser was amplified to 8.24 mJ at repetition of 500 Hz by five-pass amplification. The beam quality factors are 1.70 and 1.28 in the horizontal and vertical directions, respectively. The optical–optical efficiency was 1.16% under the pump power of 187.5 W. The experimental results match well with the numerical simulation. The optical–optical efficiency can be improved to about 10% with higher absorption efficiency of the pump source and higher overlapping efficiency between the pump light and 1319-nm seed laser. And further improvements can be realized by optimizing the input pulse energy density and the pass of amplification.

Free-space optical (FSO) communication has gained popularity for wireless applications over legacy radio frequency for advantages such as unlicensed operation, spatial reuse, and security. Even though FSO communication can achieve high bit rates, range limitation due to strong attenuation and weather dependency has always restricted its practical applications to controlled settings such as building-to-building communication. Futuristic mobile and secure ad hoc FSO network applications such as smart cars and air-subsea links need more efficient and autonomous link acquisition capabilities, which can be enabled by in-band full-duplex (IBFD) operation. We proposed an IBFD-FSO transceiver prototype consisting of off-the-shelf components to demonstrate the concept of IBFD operation by the isolation technique. We also developed a laser-based IBFD-FSO link model by incorporating an atmospheric attenuation model and self-interference cancellation model. We found that, for clear weather, the maximum achievable link range using commercially available components can be up to 120 m. We also determined weather-dependent performance of the FSO link in terms of visibility and transmit power.

We investigate the performance of 25-Gbps dual-polarized orthogonal frequency division multiplexing (OFDM)-based modulation in a directly modulated distributed feedback (DFB)-laser over 25 km of single-mode fiber. A Volterra equalizer is used to compensate for the nonlinear effects of the optical fiber. The results show that FBMC-OQAM modulation outperforms OFDM, universal filtered multicarrier (UFMC), and generalized frequency division multiplexing (GFDM) waveforms. Indeed, a target bit error rate of ∼3.8  ×  10^−  3 [forward error correction (FEC) limit] for FBMC, UFMC, OFDM, and GFDM can be achieved at −30.5, −26, −16, and −14.9  dBm, respectively. The effect of the DFB laser is also investigated for UFMC, OFDM, and GFDM, and they undergo a Q penalty of 2.44, 2.77, and 4.14 dB, respectively, at their FEC limit points. For FBMC-OQAM, the signal is perfectly recovered when excluding the DFB laser at −30.5  dBm.

All-optical photonic integrated devices have gained great attention in the field of optical computing and large-scale integration. All-optical logic gates are useful for optical signal processing and optical communication network. The flexible devices presented here satisfy the functionality of NAND (NOT-AND), NOR (NOT-OR), and XNOR (exclusive NOR) logic gates using only one structure with proper changes in the phase of an applied light signal. The design of all-optical logic gates is implemented with photonic crystal waveguides using square lattice silicon rods. The performance of the structure is simulated, verified, and analyzed by the finite-difference time-domain method, with the principle of interference effect at a wavelength of 1550 nm. The contrast ratio (CR) of NAND, NOR, and XNOR logic gates is 17.59, 14.3, and 10.52 dB, respectively, with an optimized size of 7.2  μm  ×  5.4  μm.

Laser ablation (LA) has been acknowledged as a universal laser processing tool to develop new devices. Typically, LA involves removal of material with a high energy femtosecond (fs) laser, which has a relatively low-pulse repetition rate (kHz). This constricts the processing speed to avoid separation between successive pulses during the device patterning. Hence, it is reasonable to use a high-repetition-rate laser as an alternative approach to enable high-speed laser processing. We experimented on the surface ablation of poly allyl diglycol carbonate (PADC) using a high-repetition-rate (MHz) fs laser. The objective of this study is to investigate the laser-ablated pattern on PADC and the effect of the number of laser pulses on the ablation rate. For the purpose of this experiment, straight trenches were formed directly on the PADC surface with 800-nm fs-pulsed laser writing. It was found that the ablation mechanism in the high-repetition-rate regime was primarily controlled by the number of irradiated pulses. At a fixed pulse energy, the ablation rate dropped from 9.6 to 3.4  pm  /  pulse for 1  ×  10⁵ and 8  ×  10⁵  pulses, respectively. This outcome stemmed from the disruption of laser focusing via thermally induced surface morphology changes due to excessive energy accumulation.

High reflective (HR) films with low stress and high damage threshold are deposited on a high-density deformable mirror (HDM) for a high-power laser system. The parameters of the laser system are about 5 J, 6 ns, and 200 Hz @ 1064 nm. The surface figure of the HDM is measured before and after coating, respectively. After coating, the surface error of HDM is only 39-nm rms, and the results show that the residual stress of HR films is very small. Then, a Φ50-mm sample and the HDM are tested successively in the high-power laser system. The temperature rise of the sample and HDM are measured in real-time. Finally, the temperature of HDM has risen by 10°C, while the sample is changed by only 1°C.

The design and simulation of two bio-optical sensors for the noninvasive measurement of glucose levels in urine of diabetic patients are presented. While the sensors are of the same shape and structure, their quality factors (QFs), sensitivities, and sizes are different. The structure developed is based on two-dimensional (2D)-hexagonal-latticed photonic crystals containing silicon rods immersed in urine. By creating line and point defects in the structure, two resonant modes are localized in the photonic bandgap (the photonic bandgap is a frequency range corresponding to mirror action for incident light of frequency within that range). Sensor 1 design was based on localized resonant mode 1, while sensor 2 uses localized resonant mode 2. The mechanism of these sensors relies on the change of the refractive index of urine as a result of the change in glucose concentration, which in turn causes shifts in resonant modes of the sensors. For tuning in the wavelength of 1.55  μm, the scaling method is utilized. The major difference between the proposed sensors and previously reported bio-optical sensors is the determination of glucose level based on the shift in resonant wavelengths. Previous works relied on the change of the amplitude of emerging waves for the same purpose. Consequently, the sensors presented here exhibit a higher QF and operate within the wavelength window of 1.55  μm. QF calculations and sensitivity simulations for sensor 1 yielded approximate values of 3800 and 500 nm/refractive index unit (RIU), respectively. For sensor 2, QF and sensitivity were 5540 and 500  nm  /  RIU, respectively. The sizes of sensor 1 and sensor 2 are 14.93  ×  8.8  μm² and 15.7  ×  9.47  μm², respectively. The 2D finite difference time domain and plane wave expansion methods were relied upon in the simulations.

A real-time hybrid optoelectronic analog-to-digital converter (HOE-ADC) based on optical sampling and electronic quantization is proposed and experimentally demonstrated. In the proposed HOE-ADC, a broadband semiconductor laser is intensity modulated by electronic pulses to generate an optical sampling pulse train. The optical sampling pulse is split into multiple channels with different optical fiber delay, and the analog signal is modulated on the multichannel optical sampling pulses using a Mach–Zehnder modulator array. By broadening the optical pulses, which can be done by employing optical-fiber chromatic dispersion, the bandwidth of the sampled pulse is reduced to match the sample rate of a low-speed electronic ADC, where the broadened optical sampling pulses are quantified. The nonlinearity of optical devices is calibrated through digital signal processing. A four-channel HOE-ADC is experimentally demonstrated. A sampling rate of 12  GS  /  s and a system bandwidth of 10 GHz are achieved. The effective number of bit (ENOB) for a 1.5-GHz target analog signal is measured to be 5.9 bit. Further, the primary influence factors, such as delay errors, for the performance of ENOB are also discussed.

Methyl parathion (MP) and fenthion are both organophosphorus pesticides that have been widely used in agriculture. However, they significantly pollute the environment. A silver nanorod array chip prepared under low temperature conditions was designed to detect the residues of fenthion on the surface of fruit peel and methyl parathion in water. The results show that the characteristic peaks of MP were at 811, 855, 1158, 1236, and 1324  cm^−  1 and the characteristic peaks of fenthion were at 853, 1156, 1224, and 1590  cm^−  1. The vibrational modes of each peak were calculated using the density function theory. This method can detect MP in different water samples and fenthion on apple peel within 3 min without any complicated pretreatment. The results suggest that the method for rapid determination of organophosphorus pesticides based on surface-enhanced Raman spectroscopy was sensitive and reliable. It is expected that this method has great application potential in environmental pollution monitoring.

A low-cost camera-based method of detecting continuous-wave (cw) lasers has been developed at Defence Science and Technology Laboratory. The detector uses a simple optical modification to a standard color camera combined with image processing techniques to distinguish lasers from other illumination sources and measure the wavelength, direction, and irradiance of the laser light. Such a detector has applications in collecting information on aircraft laser dazzle incidents, providing the evidence required to inform on aircrew laser exposure events and to assess if engagements are eye safe. A prototype has been developed using entirely commercially available off-the-shelf (COTS) components, costing ≈£600, and assessed in laboratory conditions with the capability of measuring laser wavelengths to ±5  nm and irradiances to ±10  %  . A realistic handheld laser engagement scenario, using a range of relevant wavelengths and irradiances, was simulated during the Moonraker trial where the prototype was capable of measuring laser wavelengths to an accuracy of ±10  nm and peak irradiances to ±25  %  . All laser engagements were detected over a total data collection period of 9 h with zero false alarms. Comparisons were made with a COTS laser detector, which showed an equivalent performance. This technology offers a low-cost approach to cw laser detection, which is capable of extracting a range of parameters while maintaining a relatively wide field of view and angular resolution.

This work systematically reports on the preparation of Nd^3  +-doped silicate glass (NDSG) and its waveguide structure, as well as the optimization effects of annealing procedures on the waveguide performances. The NDSG was synthesized by the melt-quenching technique. Its absorption spectrum and photoluminescence properties were recorded by a JASCO V-570 spectrophotometer and an Edinburgh FLS920P fluorescence spectrometer, respectively. A planar optical waveguide was fabricated in the NDSG using 0.4-MeV proton implantation at a fluence of 8  ×  10¹⁶  ions  cm^−  2. Its optical properties were optimized by 1-h stepwise annealing treatments with the temperature range of 260°C to 410°C. The distribution of the refractive index and the profile of near-field intensity were investigated by the prism-coupling equipment and the end-face coupling system, respectively. The obtained results show the prospects for using NDSG waveguides in planar photonic devices.