This Editorial by the Editor-in-Chief, Michael T. Eismann, gives an overview of Review Clubs.

The windowed Fourier ridges (WFR) algorithm is shown to be a phase-shifting algorithm for phase extraction from a carrier fringe pattern. However, the former only provides a phase estimation with a controllable phase error, whereas the latter pursues exact phase extraction. This link not only is interesting but also enhances the understanding of different phase extraction techniques. Advantages and disadvantages of the WFRs algorithm are discussed.

This letter proposes a method of configuring a testing target to evaluate the performance of adaptive optics microscopes. In this method, a testing slide with fluorescent beads is used to simultaneously determine the point spread function and the field of view. The point spread function is reproduced to simulate actual biological samples by etching a microstructure on the cover glass. The fabrication process is simplified to facilitate an onsite preparation. The artificial tissue consists of solid materials and silicone oil and is stable for use in repetitive experiments.

The configuration of a gas analysis system using the combination of a single-wavelength quantum cascade laser and a hollow-optical-fiber gas cell was optimized to improve measurement sensitivity. On the basis of theoretical calculation, a 6-m looped fiber gas cell was introduced into the system to more sensitively measure the concentration of nitric monoxide (NO) gas. High-precision measurement is achieved by precisely scanning the wavelength of the laser around the sharp absorption peak of NO gas. Concentrations of NO gas as low as 100 ppb were successfully measured.

This guest editorial introduces the Special Section on Imaging Spectrometry.

A fast and precise registration method for multi-image snapshot Fourier transform imaging spectroscopy is proposed. This method accomplishes registration of an image array using the positional relationship between homologous points in the subimages, which are obtained offline by preregistration. Through the preregistration process, the registration problem is converted to the problem of using a registration matrix to interpolate subimages. Therefore, the hardware interpolation of graphics processing unit (GPU) texture memory, which has speed advantages for its parallel computing, can be used to significantly enhance computational efficiency. Compared to a central processing unit, GPU performance showed ∼27 times acceleration in registration efficiency.

A prototype fiber-based imaging spectrometer was developed to provide snapshot hyperspectral imaging tuned for biomedical applications. The system is designed for imaging in the visible spectral range from 400 to 700 nm for compatibility with molecular imaging applications as well as satellite and remote sensing. An 81×96  pixel spatial sampling density is achieved by using a custom-made fiber-optic bundle. The design considerations and fabrication aspects of the fiber bundle and imaging spectrometer are described in detail. Through the custom fiber bundle, the image of a scene of interest is collected and divided into discrete spatial groups, with spaces generated in between groups for spectral dispersion. This reorganized image is scaled down by an image taper for compatibility with following optical elements, dispersed by a prism, and is finally acquired by a CCD camera. To obtain an (x,y,λ) datacube from the snapshot measurement, a spectral calibration algorithm is executed for reconstruction of the spatial–spectral signatures of the observed scene. System characterization of throughput, resolution, and crosstalk was performed. Preliminary results illustrating changes in oxygen-saturation in an occluded human finger are presented to demonstrate the system’s capabilities.

Accurate estimation or retrieval of surface emissivity from long-wave infrared or thermal infrared (TIR) hyperspectral imaging data acquired by airborne or spaceborne sensors is necessary for many scientific and defense applications. This process consists of two interwoven steps: atmospheric compensation and temperature–emissivity separation (TES). The most widely used TES algorithms for hyperspectral imaging data assume that the emissivity spectra for solids are smooth compared to the atmospheric transmission function. We develop a model to explain and evaluate the performance of TES algorithms using a smoothing approach. Based on this model, we identify three sources of error: the smoothing error of the emissivity spectrum, the emissivity error from using the incorrect temperature, and the errors caused by sensor noise. For each TES smoothing technique, we analyze the bias and variability of the temperature errors, which translate to emissivity errors. The performance model explains how the errors interact to generate temperature errors. Since we assume exact knowledge of the atmosphere, the presented results provide an upper bound on the performance of TES algorithms based on the smoothness assumption.

The Gough Map, one of the earliest surviving maps of Britain, was created and extensively revised over the 15th century. In 2015, the map was imaged using a hyperspectral imaging system while in the collection at the Bodleian Library, Oxford University. The goal of the collection of the hyperspectral image (HSI) of the Gough Map was to address questions such as enhancement of faded text for reading and analysis of the pigments used during its creation and revision. In particular, pigment analysis of the Gough Map will help historians understand the material diversity of its composition and potentially the timeline of, and methods used in, the creation and revision of the map. Multiple analysis methods are presented to analyze a particular pigment in the Gough Map with an emphasis on understanding the within-material diversity, i.e., the number and spatial layout of distinct red pigments. One approach for understanding the number of distinct materials in a scene (i.e., endmember selection and dimensionality estimation) is the Gram matrix approach. Here, this method is used to study the within-material differences of pigments in the map with common visual color. The application is a pigment analysis tool that extracts visually common pixels (here, the red pigments) from the Gough Map and estimates the material diversity of the pixels. Results show that the Gough Map is composed of at least five kinds of dominant red pigments with a particular spatial pattern. This research provides a useful tool for historical geographers and cartographic historians to analyze the material diversity of HSI of cultural heritage artifacts.

Imaging spectrometers are frequently used in remote sensing for their increased target discrimination capabilities over conventional imaging. Increasing the spectral resolution of these sensors further enables the system’s ability to discriminate certain targets and adds the potential for monitoring narrow-line spectral features. We describe a high spectral resolution (Δλ=1.1  nm full-width at half maximum) snapshot imaging spectrometer capable of distinguishing two narrowly separated bands in the red-visible spectrum. A theoretical model is provided to detail the first polarization grating-based spatial heterodyning of a Savart plate interferometer. Following this discussion, the experimental conditions of the narrow-line imaging spectrometer (NLIS) are provided. Finally, calibration and target identification methods are applied and quantified. Ultimately it is demonstrated that in a full spectral acquisition the NLIS sensor is capable of less than 3.5% error in reconstruction. Additionally, it is demonstrated that neural networks provide greater than 99% reduction in crosstalk when compared to pseudoinversion and expectation maximization in single target identification.

The calculation, interpretation, and implications of radiometric sensitivity metrics for Earth-observing multispectral and hyperspectral imaging sensors are discussed. The most commonly used sensor performance metric is signal-to-noise ratio, from which additional noise equivalent quantities can be computed, including noise equivalent spectral radiance (NESR), noise equivalent delta reflectance (NEΔρ), noise equivalent delta emittance (NEΔϵ), and noise equivalent delta temperature (NEΔT). For hyperspectral sensors, these metrics are typically calculated from an at-aperture radiance (typically generated by MODTRAN) that includes both target radiance and nontarget (atmosphere and background) radiance. Unfortunately, these calculations treat the entire at-aperture radiance as the desired signal, even when the target radiance is only a fraction of the total (such as when sensing through a long or optically dense atmospheric path). To overcome this limitation, an augmented set of metrics based on a contrast signal-to-noise ratio, including their noise equivalent counterparts (CNESR, CNEΔρ, CNEΔϵ, and CNEΔT), is developed. These contrast metrics better quantify sensor performance in an operational environment that includes remote sensing through the atmosphere.

In hyperspectral target detection, one must contend with variability in both target materials and background clutter. While most algorithms focus on the background clutter, there are some materials for which there is substantial variability in the signatures of the target. When multiple signatures can be used to describe a target material, subspace detectors are often the detection algorithm of choice. However, as the number of variable target spectra increases, so does the size of the target subspace spanned by these spectra, which in turn increases the number of false alarms. Here, we propose a modification to this approach, wherein the target subspace is instead a constrained subspace, or a simplex without the sum-to-one constraint. We derive the simplex adaptive matched filter (simplex AMF) and the simplex adaptive cosine estimator (simplex ACE), which are constrained basis adaptations of the traditional subspace AMF and subspace ACE detectors. We present results using simplex AMF and simplex ACE for variable targets, and compare their performances against their subspace counterparts. Our primary interest is in the simplex ACE detector, and as such, the experiments herein seek to evaluate the robustness of simplex ACE, with simplex AMF included for comparison. Results are shown on hyperspectral images using both implanted and ground-truthed targets, and demonstrate the robustness of simplex ACE to target variability.

We study the generalization and scalability behavior of a deep belief network (DBN) applied to a challenging long-wave infrared hyperspectral dataset, consisting of radiance from several manmade and natural materials within a fixed site located 500 m from an observation tower. The collections cover multiple full diurnal cycles and include different atmospheric conditions. Using complementary priors, a DBN uses a greedy algorithm that can learn deep, directed belief networks one layer at a time and has two layers form to provide undirected associative memory. The greedy algorithm initializes a slower learning procedure, which fine-tunes the weights, using a contrastive version of the wake-sleep algorithm. After fine-tuning, a network with three hidden layers forms a very good generative model of the joint distribution of spectral data and their labels, despite significant data variability between and within classes due to environmental and temperature variation occurring within and between full diurnal cycles. We argue, however, that more questions than answers are raised regarding the generalization capacity of these deep nets through experiments aimed at investigating their training and augmented learning behavior.

Drill cores provide direct evidence for geological prospecting, and compiling drill core record information has been a vital tool in geological exploration. We design and fabricate a drill core spectral imaging system that covers a wavelength range of 400 to 2500 nm with a spectral resolution of 3 to 12 nm. A visible and near infrared and a shortwave infrared spectrometer are mounted together to cover the wavelength range. This paper describes the mechanical structure, light path structure, parameters, and software processing chain of the system. With the spectral imaging system and custom-design software, a mineral core sample is analyzed to validate system performance.

This paper discusses computational modeling and experimental demonstration of a Fresnel zone light field spectral imaging system. This type of system couples an axial dispersion binary diffractive optic with a light field detector design providing a snapshot spectral imaging capability. The computational model was validated experimentally and provides excellent predictions of potential system capabilities. Additionally, the experimentally demonstrated prototype was able to digitally refocus monochromatic images by wavelength across greater than a 100 nm bandwidth. Through simulation, the demonstrated system was approximated to have a full range from ∼400 to 800 nm at close to a 15-nm spectral sampling interval. We also demonstrated experimentally the capability of resolving between and processing two different spectral signatures in a single snapshot. The type of system demonstrated here offers substantial new capability as an optically simple, snapshot spectral imager. The experimental proof of concept and computational model set the stage for further development of these types of systems.

Results of a method of estimating index of refraction from passive, polarimetric hyperspectral imaging radiance measurements are presented. As off-nadir viewing hyperspectral imaging platforms gain prominence, estimating index of refraction, which is invariant to viewing angle, may prove advantageous to estimating the emissivity, which is not. Results show that index of refraction can be retrieved to within 8% rms error for fused silica and sapphire glass targets, while simultaneously estimating object temperature. The accuracy and self-consistency of this technique for estimating index of refraction are shown to compare favorably to the maximum smoothness temperature–emissivity separation algorithm. Additionally, the results show that atmospheric downwelling radiance can also be accurately estimated, to within the noise of the instrument, concurrently with index of refraction.

This paper presents best practices for modeling the resolution of dispersive imaging spectrometers. The differences between sampling, width, and resolution are discussed. It is proposed that the spectral imaging community adopt a standard definition for resolution as the full-width at half maximum of the total line spread function. Resolution should be computed for each of the spectral, cross-scan spatial, and along-scan spatial/temporal dimensions separately. A physical optics resolution model is presented that incorporates the effects of slit diffraction and partial coherence, the result of which is a narrower slit image width and reduced radiometric throughput.

An automatic fishing net detection and recognition method for underwater obstacle avoidance is proposed. In the method, optical gated viewing technology is utilized to obtain high-resolution fishing net images and extend detection distance by suppressing water backscattering and background noise. The fishing net recognition is based on the proposed histograms of slope lines (HSLs) descriptors plus a support vector machine classifier. The extraction of HSL descriptors includes five steps of contrast-limited adaptive histogram equalization, the Gaussian low-pass filtering, the Canny detection, the Hough transform, and weighted vote. In the proof experiments, the detection distance of the fishing net reaches 5.7 attenuation length and the recognition accuracy reaches 93.79%.

Image stacking is a well-known method that is used to improve the quality of images in video data. A set of consecutive images is aligned by applying image registration and warping. In the resulting image stack, each pixel has redundant information about its intensity value. This redundant information can be used to suppress image noise, resharpen blurry images, or even enhance the spatial image resolution as done in super-resolution. Small moving objects in the videos usually get blurred or distorted by image stacking and thus need to be handled explicitly. We use image stacking in an innovative way: image registration is applied to small moving objects only, and image warping blurs the stationary background that surrounds the moving objects. Our video data are coming from a small fixed-wing unmanned aerial vehicle (UAV) that acquires top-view gray-value images of urban scenes. Moving objects are mainly cars but also other vehicles such as motorcycles. The resulting images, after applying our proposed image stacking approach, are used to improve baseline algorithms for vehicle detection and segmentation. We improve precision and recall by up to 0.011, which corresponds to a reduction of the number of false positive and false negative detections by more than 3 per second. Furthermore, we show how our proposed image stacking approach can be implemented efficiently.

This work presents a comprehensive experimental study on the effect of glass frit thickness on the residual stress after laser bonding. The shear force and thermal residual stress of the bonding area due to laser bonding at different thicknesses of glass frit was investigated via experiment, respectively. Two method of ANSYS simulation and stress measurement were carried out. The result shows that the thinner the glass frit, the less residual stress of the after bonding sample; the shear force would increase with the thinning of the glass frit under the sufficient adhesion between the glass frit and the glass substrate. Thereby, an appropriate and sufficiently thin glass frit layer is recommended during the glass/glass laser bonding.

Head pose is useful information for many face-related tasks, such as face recognition, behavior analysis, human–computer interfaces, etc. Existing head pose estimation methods usually assume that the face images have been well aligned or that sufficient and precise training data are available. In practical applications, however, these assumptions are very likely to be invalid. This paper first investigates the impact of the failure of these assumptions, i.e., misalignment of face images, uncertainty and undersampling of training data, on head pose estimation accuracy of state-of-the-art methods. A learning-based approach is then designed to enhance the robustness of head pose estimation to these factors. To cope with misalignment, instead of using hand-crafted features, it seeks suitable features by learning from a set of training data with a deep convolutional neural network (DCNN), such that the training data can be best classified into the correct head pose categories. To handle uncertainty and undersampling, it employs multivariate labeling distributions (MLDs) with dense sampling intervals to represent the head pose attributes of face images. The correlation between the features and the dense MLD representations of face images is approximated by a maximum entropy model, whose parameters are optimized on the given training data. To estimate the head pose of a face image, its MLD representation is first computed according to the model based on the features extracted from the image by the trained DCNN, and its head pose is then assumed to be the one corresponding to the peak in its MLD. Evaluation experiments on the Pointing’04, FacePix, Multi-PIE, and CASIA-PEAL databases prove the effectiveness and efficiency of the proposed method.

The DAISY descriptor has been widely used in dense stereo matching and scene reconstruction. However, DAISY is vulnerable to similar feature regions because the construction method of DAISY sequentially arranges the description of center and neighbor sample points and does not consider their relationships. To enhance the discriminative power of the local descriptor and accelerate the speed of dense matching and scene reconstruction, we propose a low-dimensional local descriptor. The proposed descriptor is inspired from the local binary pattern (LBP). In image space, LBP describes local detail texture by computing the difference between center and neighbor sample points. We introduce this advantage in scale space to extend the DAISY descriptor and make it more efficient for dense matching similar features in the different regions. On this basis, a two-dimensional discrete cosine transform (2D-DCT) is utilized to reduce the dimensions of the descriptor as well as reduce the computation cost of dense matching and scene reconstruction. Through a variety of experiments on the benchmark laser-scanned ground truth scenes as well as indoor and outdoor scenes, we show the proposed descriptor can get more accurate depth maps and more complete reconstruction results than that of using other common descriptors, and the computational speed is much faster than that of using DAISY.

Infrared depth recognition technology with an efficient custom-made signal smoothing algorithm is used as a base for a precise inspection system. The main purpose of this paper is to introduce the basics of an algorithm that will improve precision and stabilize distance measurements in projects with camera image and depth sensors. Such results are within the reach of currently available hardware but not with the available software, for which there is a lack of suppliers support. The second goal is to prove that golden sample data matrix compiled from multiple sensors data can be taken into consideration as a simple and general automated optical inspection system.

High-spatial-frequency optical fields or sources are often encountered when simulating directed energy, active imaging, or remote sensing systems and scenarios. These spatially broadband fields are a challenge in wave optics simulations because the sampling required to represent and then propagate these fields without aliasing is often impractical. To address this, two spatial filtering techniques are presented. The first, called Fresnel spatial filtering, finds a spatially band-limited source that, after propagation, produces the exact observation plane field as the broadband source over a user-specified region of interest. The second, called statistical or quasihomogeneous spatial filtering, finds a spatially band-limited source that, after propagation and over a specified region of interest, yields an observation plane field that is statistically representative of that produced by the original broadband source. The pros and cons of both approaches are discussed in detail. A wave optics simulation of light transiting a ground glass diffuser and then propagating to an observation plane in the near-zone is performed to validate the two filtering approaches.

An analytical model of hybrid accumulation architecture based on charge-domain and digital-domain time-delay-integration complementary metal-oxide-semiconductor image sensor (TDI-CIS) in the scanning direction is proposed. Optical performance of signal-noise-ratio, dynamic range, and modulation transfer function of the charge-domain, digital-domain, and hybrid accumulation scheme is simulated and analyzed. The synthetical evaluation target (SET) is defined to obtain the best performance under different distribution methods of the charge-domain and digital-domain at a fixed TDI stage for a hybrid accumulation scheme. According to the simulation results, the hybrid accumulation scheme whose charge-domain accumulation stage is 8 and digital-domain accumulation stage is 16 has the optimal SET, which is 12.99% higher than a 128-stage digital-domain accumulation scheme and 25% higher than the 128-stage charge-domain accumulation scheme.

A feasible way to improve the manufacturing efficiency of large reaction-bonded silicon carbide optics is to increase the processing accuracy in the ground stage before polishing, which requires high accuracy metrology. A swing arm profilometer (SAP) has been used to measure large optics during the ground stage. A method has been developed for improving the measurement accuracy of SAP using a capacitive probe and implementing calibrations. The experimental result compared with the interferometer test shows the accuracy of 0.068  μm in root-mean-square (RMS) and maps in 37 low-order Zernike terms show accuracy of 0.048  μm RMS, which shows a powerful capability to provide a major input in high-precision grinding.

This work presents the use of a recently developed interferometric system based on the swept source optical coherence tomography (SS-OCT) technique, which allows the characterization of transparent and semitransparent multilayer systems employing a tunable fiber-optic laser with a coherence length suitable for achieving long-deep range imaging (<10  cm). The inclusion of fiber Bragg gratings in the system allows it to perform a self-calibration in each sweep of the light source. Measurements carried out on cuvettes, ampoules, small bottles, and glass containers used in the pharmaceutical industry are presented. The thicknesses of the walls and the distance between them were determined. Transparent and semitransparent objects of a multilayer type of different thicknesses were also measured. The configuration presented allows extension of the measurement range obtainable with the usual OCT systems, demonstrating the potentiality of the proposed scheme to carry out quality control in industrial applications.

Virtual binocular sensors, composed of a camera and catoptric mirrors, have become popular among machine vision researchers, owing to their high flexibility and compactness. Usually, the tested target is projected onto a camera at different reflection times, and feature matching is performed using one image. To establish the geometric principles of the feature-matching process of a mirror binocular stereo vision system, we proposed a single-camera model with the epipolar constraint for matching the mirrored features. The constraint between the image coordinates of the real target and its mirror reflection is determined, which can be used to eliminate nonmatching points in the feature-matching process of a mirror binocular system. To validate the epipolar constraint model and to evaluate its performance in practical applications, we performed realistic matching experiments and analysis using a mirror binocular stereo vision system. Our results demonstrate the feasibility of the proposed model, suggesting a method for considerable improvement of efficacy of the process for matching mirrored features.

A distributed vibration sensing technique using double-optical-pulse based on phase-sensitive optical time-domain reflectometry (ϕ-OTDR) and an ultraweak fiber Bragg grating (UWFBG) array is proposed for the first time. The single-mode sensing fiber is integrated with the UWFBG array that has uniform spatial interval and ultraweak reflectivity. The relatively high reflectivity of the UWFBG, compared with the Rayleigh scattering, gains a high signal-to-noise ratio for the signal, which can make the system achieve the maximum detectable frequency limited by the round-trip time of the probe pulse in fiber. A corresponding experimental ϕ-OTDR system with a 4.5 km sensing fiber integrated with the UWFBG array was setup for the evaluation of the system performance. Distributed vibration sensing is successfully realized with spatial resolution of 50 m. The sensing range of the vibration frequency can cover from 3 Hz to 9 kHz.

A plug-in module acceleration feedback control (Plug-In AFC) strategy based on the disturbance observer (DOB) principle is proposed for charge-coupled device (CCD)-based fast steering mirror (FSM) stabilization systems. In classical FSM tracking systems, dual-loop control (DLC), including velocity feedback and position feedback, is usually utilized to enhance the closed-loop performance. Due to the mechanical resonance of the system and CCD time delay, the closed-loop bandwidth is severely restricted. To solve this problem, cascade acceleration feedback control (AFC), which is a kind of high-precision robust control method, is introduced to strengthen the disturbance rejection property. However, in practical applications, it is difficult to realize an integral algorithm in an acceleration controller to compensate for the quadratic differential contained in the FSM acceleration model, resulting in a challenging controller design and a limited improvement. To optimize the acceleration feedback framework in the FSM system, different from the cascade AFC, the accelerometers are used to construct DOB to compensate for the platform vibrations directly. The acceleration nested loop can be plugged into the velocity loop without changing the system stability, and the controller design is quite simple. A series of comparative experimental results demonstrate that the disturbance rejection property of the CCD-based FSM can be effectively improved by the proposed approach.

It is well known that a translating mask can optically encode low-resolution measurements from which higher resolution images can be computationally reconstructed. We experimentally demonstrate that this principle can be used to achieve substantial increase in image resolution compared to the size of the focal plane array (FPA). Specifically, we describe a scalable architecture with a translating mask (also referred to as a coded aperture) that achieves eightfold resolution improvement (or 64∶1 increase in the number of pixels compared to the number of focal plane detector elements). The imaging architecture is described in terms of general design parameters (such as field of view and angular resolution, dimensions of the mask, and the detector and FPA sizes), and some of the underlying design trades are discussed. Experiments conducted with different mask patterns and reconstruction algorithms illustrate how these parameters affect the resolution of the reconstructed image. Initial experimental results also demonstrate that the architecture can directly support task-specific information sensing for detection and tracking, and that moving objects can be reconstructed separately from the stationary background using motion priors.

The metrology of freeform wavefront can be performed by the use of a noninterferometric method, such as a Shack–Hartmann sensor (SHS). Detailed experimental investigations employing an SHS as metrology head are presented. The scheme is of nonnull nature where small subapertures are measured using an SHS and stitched to give the full wavefront. For the assessment of complex misalignment errors during the spiral scanning, a library of residual slope errors has been created, which makes the alignment process fast converging for minimizing the scanning errors. A detailed analysis of the effects of slope and positioning error on reproducibility is presented. It is validated by null test where a null diffractive optical element has been used in a Mach–Zehnder configuration for compensating the freeform shape. A freeform optics is measured by both measurement schemes, and the results are in good agreement. Further, the nonnull-based scanning subaperture stitching scheme is also validated by performing measurements on an aspheric surface and compared with the measurements from the interferometric method (Zygo Verifire).

The experiment is designed and taken to measure the link gain in a single-input multiple-output ultraviolet (UV) communication system with diversity reception, and the correlation of multichannel is also taken into account. Theoretical and experimental research on the multireceiver UV communication system suggests that diversity reception is an effective way to gain high BER performance even if the link gain correlation is non-negligible (with normal level correlation coefficient). The link gain of diversity reception is compared particularly with the gain from expanding the detecting area to find its boundary for performance improvement and the distance limit between receivers. The experimental results provide more reliable guidelines for receiver design in UVC systems and other scattering wireless optical communication channels with diversity reception applied such as multiple-input multiple-output.

The corrective calibration of the removal function plays an important role in the magnetorheological finishing (MRF) high-accuracy process. This paper mainly investigates the asymmetrical characteristic of the MRF removal function shape and further analyzes its influence on the surface residual error by means of an iteration algorithm and simulations. By comparing the ripple errors and convergence ratios based on the ideal MRF tool function and the deflected tool function, the mathematical models for calibrating the deviation of horizontal and flowing directions are presented. Meanwhile, revised mathematical models for the coordinate transformation of an MRF machine is also established. Furthermore, a Ø140-mm fused silica plane and a Ø196  mm, f/1∶1, fused silica concave sphere samples are taken as the experiments. After two runs, the plane mirror final surface error reaches PV 17.7 nm, RMS 1.75 nm, and the polishing time is 16 min in total; after three runs, the sphere mirror final surfer error reaches RMS 2.7 nm and the polishing time is 70 min in total. The convergence ratios are 96.2% and 93.5%, respectively. The spherical simulation error and the polishing result are almost consistent, which fully validate the efficiency and feasibility of the calibration method of MRF removal function error using for the high-accuracy subaperture optical manufacturing.

We present an analysis of the shape, surface quality, and imaging capabilities of custom three-dimensional (3-D) printed lenses. 3-D printing technology enables lens prototypes to be fabricated without restrictions on surface geometry. Thus, spherical, aspherical, and rotationally nonsymmetric lenses can be manufactured in an integrated production process. This technique serves as a noteworthy alternative to multistage, labor-intensive, abrasive processes, such as grinding, polishing, and diamond turning. Here, we evaluate the quality of lenses fabricated by Luxexcel using patented Printoptical^© technology that is based on an inkjet printing technique by comparing them to lenses made with traditional glass processing technologies (grinding, polishing, etc.). The surface geometry and roughness of the lenses were evaluated using white-light and Fizeau interferometers. We have compared peak-to-valley wavefront deviation, root mean square (RMS) wavefront error, radii of curvature, and the arithmetic roughness average (Ra) profile of plastic and glass lenses. In addition, the imaging performance of selected pairs of lenses was tested using 1951 USAF resolution target. The results indicate performance of 3-D printed optics that could be manufactured with surface roughness comparable to that of injection molded lenses (<inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:mi<Ra</mml:mi< <mml:mo<<</mml:mo< <mml:mn<20</mml:mn< <mml:mtext<  </mml:mtext< <mml:mi<nm</mml:mi< </mml:mrow< </mml:math< </inline-formula<). The RMS wavefront error of 3-D printed prototypes was at a minimum 18.8 times larger than equivalent glass prototypes for a lens with a 12.7 mm clear aperture, but, when measured within 63% of its clear aperture, the 3-D printed components’ RMS wavefront error was comparable to glass lenses.

In the optical stabilization control system (OSCS) control system based on a charge-coupled device (CCD), stabilization performance of the line-of-sight is severely limited by the mechanical resonance and the low sampling rate of the CCD. An approach to improve the stabilization performance of the OSCS control system with load restriction based on three loops, including an acceleration loop, a virtual velocity loop, and a position loop, by using MEMS accelerometers and a CCD is proposed. The velocity signal is obtained by accelerators instead of gyro sensors. Its advantages are low power, low cost, small size, and wide measuring range. A detailed analysis is provided to show how to design the virtual velocity loop and correct virtual velocity loop drift. Experimental results show that the proposed multiloop feedback control method with virtual velocity loop in which the disturbance suppression performance is better than that of the dual loop control with only an acceleration loop and a position loop at low frequency.

Dielectric multilayers consisting of alternating layers of two different materials with thicknesses irregularly decreasing with depth in the structure are included in the cuticle of some beetles whose shell exhibits broadband reflection in the optical wavelength range. Emulating these structures, we propose and numerically analyze irregularly chirped dielectric multilayers. Analysis was performed using a dedicated genetic algorithm (GA) that searches for the multilayer configurations maximizing the reflection for normal incidence over a large wavelength range. We found that the GA leads to the irregularly chirped reflectors that significantly outperform the regularly chirped ones proposed and analyzed in the literature.

In modern optical element manufacturing, center artifacts are a common problem. A center artifact is a shape error that is rotationally symmetrical, steep, and localized at the center. These properties cause characteristic image defects different from those caused by ordinary irregularities. However, tolerancing center artifacts has not been fully discussed or properly carried out. We propose a simple mathematical model for center artifacts using normal distribution function as a figure model and showing that this function can be represented by a polynomial including odd-order terms. Our method enables appropriate optical simulation and tolerancing for center artifacts using general optical design software.

The evaluation of optical system performance in fog conditions typically requires field testing. This can be challenging due to the unpredictable nature of fog generation and the temporal and spatial nonuniformity of the phenomenon itself. We describe the Sandia National Laboratories fog chamber, a new test facility that enables the repeatable generation of fog within a 55  m×3  m×3  m (L×W×H) environment, and demonstrate the fog chamber through a series of optical tests. These tests are performed to evaluate system image quality, determine meteorological optical range (MOR), and measure the number of particles in the atmosphere. Relationships between typical optical quality metrics, MOR values, and total number of fog particles are described using the data obtained from the fog chamber and repeated over a series of three tests.

In order to improve the thermal characteristics of single-chip semiconductor lasers and increase the output power of the device, a new type of vertical packaging structure of heat sink is proposed and analyzed. The heat sink retains the advantages of simplicity and being easy to apply, and the performance of heat dissipation has been improved obviously. The new heat sink structure is believed to be more suitable for packaging of the high-power semiconductor laser chips by heat conduction. Finite-element thermal analysis was used to simulate the thermal field distribution and thermal vector distribution in the conventional structure and the new structure. The simulation results show that the thermal resistance of the conventional structure is 2.0  K/W and the thermal resistance of the new heat sink is less than 1.6  K/W. The theoretical calculation results show that the output power of the packaged laser by new heat sinks can be significantly improved.

A simple high-power thin-disk pumping configuration using a radiation combination of four commercially available laser–diode stacks is introduced. Two setup modifications are presented to compensate the nonsuitable shape of the pumping spot arising from low beam quality in our combination method. The effects of setup modifications on pumping spot shapes are confirmed by ray tracing simulations using Trace-pro^™ software. All setups are arranged in the laboratory, and the experimental measurements show pumping spots improvements on the disk due to modifications in agreement with simulation results. Output power measurements show that by adapting the pumping spot size to the disk cooling capacity the modified setups can deliver higher output powers and efficiencies. Furthermore, the modifications reduce the laser threshold and improve output laser beam quality. Hence, the modifications make the simple four laser–diodes beam combination applicable for thin-disk laser pumping.

We researched and demonstrated two-dimensional (2-D) deflectors constructed by on-axis type acousto-optic deflectors (AOD) in order to attain a multibeam recording for an internal drum scanning exposure system. First, we researched an on-axis AOD using anisotropic Bragg diffraction to obtain high diffraction efficiency. A significant improvement of diffraction efficiency was observed when acoustic waves are travelling in the [110] axis of paratellurite (TeO2) crystal, an incident light of linear polarization is traveling into a TeO2 along the [001] optic axis, the crystal is rotated about the [110] axis, and furthermore the direction of polarization of incident light is adjusted to the eigenmode of a TeO2 crystal. Second, we configured 2-D deflectors by cascading two AODs and achieved the multibeam laser recording for the internal drum scanning exposure system.

We propose an optical thresholder having a flat power transfer function both in low- and high-level outputs. The optical thresholder consists of two cascaded Mach–Zehnder interferometers (MZIs) with a nonlinear microring resonator in each MZI. We theoretically analyze the power transfer function and the response speed of the thresholder made of silicon waveguides. A thresholder having the threshold of 7 mW with a response speed of 13 GHz was obtained with a ring radius of 5  _μm. The response speed can be increased to 23.7 GHz by adjusting device parameters with the same ring radius at the cost of increased threshold of 22 mW. Further increased response frequency can be realized by reducing the ring radius. A thresholder with a ring radius of 2  _μm shows the response frequency of 40.5 GHz.

A photonic approach for frequency-sextupled microwave signal generation without filter or precise phase control is demonstrated by computer simulations and experiments. Without any filter, a frequency-sextupled microwave signal is generated by adjusting bias voltages of the cascade modulators. This structure largely reduces the dependence of particular phase relation that is built between the different modulated signals. The approach is verified by simulations and experiments, and stable 18- and 24-GHz frequency-sextupled signals are generated by 3 and 4 GHz local signals without filter or precise phase control.

We describe a systematic approach to design, optimize, and characterize a Fourier-domain mode-locked (FDML) laser with an erbium-doped fiber amplifier (EDFA) as the optical gain medium. A highly stable temporal intensity profile is obtained by minimizing chromatic dispersion and polarization fluctuations. The obtained bandwidth of 21 nm, tuning speed of 50 kHz, and output power of 5 mW are the highest reported so far with an EDFA-based FDML laser.

As a promising candidate technology for the next generation communication systems, visible light communication (VLC), combined with high modulation and coding schemes, can be used to achieve throughput much higher than the traditional RF wireless ones. We propose adopting multiple light-emitting diodes (LEDs) on the transmit side to form a multiple-input signal-output (MISO) VLC system. Through the maximum ratio transmit beamforming, the signals from the multiple LEDs can be added coherently at the receiver side, and, therefore, the signal-to-noise ratio of the system can be improved slightly. Meanwhile, a method of channel estimation with superposed signal is employed for better channel estimation. Extensive lab experiments demonstrate that a two-LED MISO-VLC system can achieve a data rate of 1.0  Gbit/s over a free-space link of 1.2 m.

A digital carrier synchronization module with high working frequency is indispensable for high-speed digital coherent optical receivers to recover the transmitted symbols. We proposed a method to increase the working frequency of the digital carrier synchronization (DCS) module based on the commonly used M’th power algorithms. Parallel architecture can increase the throughput of digital signal processing (DSP) modules for a given working frequency. pipelined architecture (PA) leads to a reduction in the critical path, and thus it can be exploited to increase the throughput of DSP modules by increasing the working frequency. It is demonstrated that in PA the working frequency is not limited by the computation time of the M’th power subfunction with the highest complexity because it is feedforward and thus pipelining registers can be introduced to reduce the critical path inside it. Instead, the phase unwrapping subfunction (PUS) becomes the bottleneck of the working frequency because it requires the immediately preceding result and cannot be implemented in PA, which results in the longest critical path among the DCS module. To solve this problem, we propose a feedforward look-up-table-based PUS design that can greatly reduce the critical path and increase the working frequency. Experimental DCS implementation in a Xilinx Virtex7 field programmable gate array shows that with this method the working frequency of the DCS module for quadrature phase-shift keying (QPSK) signals can be increased by 63.8%. Furthermore, using experimental and simulation data, it is demonstrated that the performance of the DCS module with increased working frequency is close to that of the off-line DCS algorithms for QPSK signals.

Design and modeling of wavelength tunable filters based on the linear electro-optic (Pockels) effect in strained silicon waveguides is presented. The structure is based on the silicon grating-assisted contradirectional couplers. The basic idea is to utilize a silicon nitride (Si3N4) layer on top of the silicon waveguides to induce a relatively large Pockels effect in the waveguides by breaking the centrosymmetry of the Si crystal due to the surface strain. Wavelength tuning is obtained by applying DC voltages to the device contacts on top of the Si3N4 layer. Based on the presented structure, a wavelength tunable filter at the C-band communication window is designed and its characteristics were investigated by full-vectorial finite-element and finite-difference time-domain methods for electrostatic and optical responses. A 3.5-nm wavelength shift for an applied voltage of 1 V and a 7.5-nm shift for an applied voltage of 5 V are obtained with negligible loss. The presented device is superior to the previously reported structures in terms of tunability and loss.

A regrowth-free single-mode laser that is made using standard UV photolithography is reported. The laser achieves a single-mode side-mode suppression ratio of 37 dB, linewidth of 450 kHz, and tunes across 2.9 nm and is suitable for monolithic integration as a distributed feedback replacement, due to a large free spectral range of 60 nm.

Under investigation is the (3+1)-dimensional coupled nonlinear Schrödinger equations, which describe the propagation of the soliton in the inhomogeneous parity-time (PT)-symmetric coupler with gain or loss. Employing the Hirota method and symbolic computation, we obtain the one- and two-soliton solutions under a variable-coefficient constraint. Bäcklund transformation and the corresponding one-soliton solutions are derived. Via graphic analysis, we observe the linear-, parabolic-, and periodic-shaped solitons with different values of the self-phase modulation and cross-phase modulation. Increase of the diffraction and dispersion leads to the increase of both the soliton amplitudes and the velocities. However, ϱ(z) and γ do not affect the soliton amplitude and velocity, with ϱ(z) being the coupling between the modes propagating in the two fibers and γ describing the PT-balanced gain or loss.

The classic Raman coupled equations, which describe the principles of the interaction between the incident and generated laser, have important applications in numerous domains. However, the classic Raman equations ignored the off-axis generated laser energy, which cannot be ignored in certain circumstances. We improved the classic equations by considering the off-axis laser energy of the stimulated Raman scattering. A single-pass multiwavelength potassium gadolinium tungstate crystal Raman generator pumped by a picosecond Nd:YAG laser was realized. On the basis of theoretical calculation and experimental verification, the improved equations produced numerical results for the output spectrum and output laser conversion efficiency that have a better agreement with the experimental results than the classic equations.

We studied the pulsed laser diode (LD)-pumped saturable output coupler (SOC) passively Q-switched Nd:YVO4 transmission microchip laser both numerically and experimentally. We demonstrated numerically that the timing jitter decreased with increasing pump power both in the continuous wave (CW) LD pump scheme and the pulsed LD pump scheme. Compared with the CW pump scheme, the pulsed LD pump scheme showed a better performance in timing jitter control due to the much higher peak power and shorter duration of the pulses. Experiments were also carried out to verify the theoretical analysis results. As the pump power of the CW LD increased, the timing jitter value of our laser decreased from 1.68% at 30 mW pump power to a constant around 0.86% at 90 mW. The output frequency can be controlled by changing the pump spot size in the rectangular shape pulsed pump scheme, and it was achieved from 333.3 to 71.4 kHz, while the relative timing jitter decreased from 0.10% to 0.03% accordingly. Additionally, the microchip laser had a good stability of output power, and the power fluctuation was below 2% in a 2-h measurement.

Diode-pumped passively Q-switched laser operation of a Nd:GdNbO4 crystal at 1066 nm using Cr4+:YAG as a saturable absorber was demonstrated for the first time. Using two different Cr4+:YAG crystals with initial transmission of 95% and 90% as the saturable absorber for Q-switching, a maximum average output power of 0.50 and 0.41 W was obtained at a pulse repetition frequency of 28.7 and 17.8 KHz, respectively. The shortest pulse width of 23 ns, the largest pulse energy of 22.7  _μJ, and the highest peak power of 974.1 W were obtained when the Cr4+:YAG crystal was used with an initial transmission of 90%. The shortest pulse width of 33.3 ns, the largest pulse energy of 17.3  _μJ, and the highest peak power of 519 W were obtained when the Cr4+:YAG crystal was used with an initial transmission of 95%. All the results indicate that the Nd:GdNbO4 crystal is a material suitable for diode-pumped passively Q-switched lasers.

We report a high-energy (∼97  _μJ), high-peak power (∼20  kW), single-frequency, linearly polarized, near diffraction-limited (M²<1.2) ∼4.8-ns pulsed laser source at 775 nm with a 260-Hz repetition rate. This laser was achieved by frequency doubling of a high-energy linearly polarized all-fiber-based master oscillator–power amplifier, seeded by a single-frequency semiconductor distributed feedback laser diode at 1550 nm. The frequency doubling is implemented in a single-pass configuration using a periodically poled lithium niobate crystal, and a conversion efficiency of 51.3% was achieved.

We systematically study the effects of computer-generated hologram characteristics and spatial light modulator (SLM) performance on the quality of the optical vortex (OV). To improve the energy efficiency of the OV, the blazed fork gratings are introduced to transfer most of the energy to +1 diffraction order of SLM, and the relationship among three kinds of holograms is discussed. A mathematical model, based on the digital blazed grating theory, is proposed to analyze the diffraction angle and energy efficiency of +1 order. The theoretical analysis is further validated by experimental results. The research provides useful guidelines for OV-related applications.

Recently, vehicular visible light communication (V2LC) is proposed as a solution to increase road safety and realize autonomous driving. The system performance for the outdoor V2LC system is investigated. In the system, the main distortion is caused by ambient-induced shot noise and thermal noise. With path loss and atmosphere turbulence, a statistical channel model is established for outdoor V2LC. Based on the channel model, the probability density function of the composite channel gain is derived. Moreover, the theoretical expressions of the outage probability and the bit error rate are derived, respectively. Numerical results show that the derived theoretical expressions are very accurate and can evaluate the system performance without time-intensive simulations.

This paper presents a highly sensitive photonic crystal fiber (PCF) refractive index sensor based on the surface plasmon resonance (SPR) effect operating in the telecommunication wavelengths. Gold is used as the plasmonic material due to its chemical stability and titanium dioxide (TiO2) is used to shift the resonance wavelength in the telecommunication bands. Both materials are deposited sequentially on the PCF surface, which is comparatively easy to fabricate. Numerical investigations show that the proposed sensor exhibits very high wavelength sensitivity of 10,800  nm/RIU and amplitude sensitivity of 514  RIU−1 in the sensing range between 1.46 and 1.48. Moreover, it exhibits maximum sensor resolution of 9.25×10−6  RIU and high linearity over a wide sensing range. The proposed sensor can be practically realized due to its simple and straightforward structure.

Acousto-optic figures of merit of TlBr-TlI solid solution (KRS-5) crystals are calculated for different propagation directions and polarizations of interacting waves. The transverse, collinear, and semicollinear regimes of acousto-optic interaction are considered in this paper. Optimal diffraction configurations applicable in acousto-optic devices operating in the far-infrared region of spectrum are found in the cases of longitudinal and shear acoustic waves in the cubic crystalline material.

Theoretical and experimental investigations on the response time improvement of unbiased long-wave infrared (LWIR) HgCdTe detectors operating at temperatures T=230  K were presented. Metal–organic chemical vapor deposition technology is an excellent tool in fabrication of different HgCdTe detector structures with a wide range of composition and donor/acceptor doping and without postgrown ex-situ annealing. The time constant is lower in biased detectors due to Auger-suppression phenomena and reduction of diffusion capacitance related to a wider depletion region. The relatively high bias current requirements and excessive low-frequency noise, which reduces the detectivity of biased detectors, inspire research on the time constant improvement of unbiased detectors. The response time of high-operating temperature LWIR HgCdTe detectors revealed complex behavior being dependent on the applied reverse bias, the operating temperature, the absorber thickness and doping, the series resistance, and the electrical area of the devices. The response time of 2 ns was achieved for unbiased 30×30  μm HgCdTe structures with λ50%=10.6  μm operating at T=230  K.

A metamaterial (MTM) absorber-based multifunctional sensor is numerically and experimentally realized using meander-line resonators. The proposed sensor device can be used to measure pressure, density, and humidity with perfect signal absorption characteristics at the frequency range of X band. The structure consists of a sensor layer sandwiched between two dielectric slabs. The sensor layer is used to detect unknown environmental parameters with respect to the electromagnetic responses of the material under test. A meander-line type resonator is chosen to achieve highly efficient electrical response at the related frequency range. It is well known that an MTM can be efficiently used for sensing purposes in a microwave regime if the absorption characteristic is approximately linear over the certain frequency band. The proposed model is numerically analyzed for pressure, density, and humidity sensing applications. In addition, the experimental study is carried out to prove the sensing ability of the suggested structure in the case of the density sensing application. The meander-line-based MTM absorber can be used in many application areas, such as the agriculture, medical, and defense industries.

The absorption spectra of methyl red (MR) dissolved in methanol, ethanol, dimethyl sulfoxide, and hexane were measured. It was found that the peaks of the absorption spectra of MR solutions shifted toward shorter wavelengths with increasing solvent polarity. Using the Z-scan method, the nonlinear optical properties of the four kinds of MR solutions were determined at wavelength 441.6 nm. The effective nonlinear indexes of refraction (n2) of MR solutions increased with the increasing thermo-optical coefficients of the solvents. In addition, the optical limiting effects of the MR solutions were investigated at 441.6 and 535 nm, and the results indicate that they possessed strong optical limiting effects.