Michael T. Eismann shares his thoughts on the value of immediacy and accuracy in modern scholarly publishing.

This guest editorial introduces the Special Section on Self-Mixing Interferometry for Industrial Applications, which is published in Optical Engineering, Vol. 57, Issue 5.

This paper presents an experimental technique for two-dimensional imaging of dynamic acoustic pressure changes that is applied to visualize a stationary acoustic wave. This technique uses the optical feedback interferometry sensing scheme with a near-infrared laser diode and a two-axis scanning system. The stationary acoustic wave is generated by using a 40 kHz piezoelectric transducer pointing toward a concave acoustic reflector. The acoustic pressure dynamic changes are measured due to its impact on the propagating medium refractive index, which variation is integrated along the laser optical path from the laser diode to a distant mirror and back. The imaging system records a 100×50  pixels image of the acoustic pressure in 66 min.

This work proposes to measure the topography of microstructure surfaces using a self-mixing interference (SMI) configuration. The theoretical measurement model is built using beam-expanded plane wave method and considering SMI effect. The interference patterns for different objects are obtained based on the presented model. In addition, an algorithm for reconstructing the three-dimensional surface is implemented and applied onto an object with spherical surface. The presented work shows the potential application for topography measurement using a compact SMI configuration.

An enhanced laser self-mixing Doppler velocimeter by fiber Bragg grating (FBG) is proposed. In fiber sensing, FBG is often used as an optical filter to select the wavelength of the light using the narrow reflection spectrum. The Doppler frequency shift from the rotating target can be parsed in the self-mixing interferometry (SMI) signal. The frequency-fluctuant self-mixing signal is transformed into intensity variations by a FBG when the laser wavelength is set to the edge of the FBG steep transmission profile. An experimental comparison between the enhanced and the traditional SMI approaches is made. The magnitude of obtained FBG-enhanced SMI signal is about 40 times stronger than that from traditional SMI. The result of the experiments indicates that enhanced self-mixing Doppler velocimetry by FBG can be applied to velocity sensing. This method has potential application in the engineering industry because the FBG is easy to fabricate and connect to a SMI system and the wavelength of FBG can be changed to match the different lasers for an enhanced SMI signal.

We proposed an all-fiber tunable distributed Bragg reflector (DBR) laser rangefinder, which realized high-accuracy measurement of the absolute distance in the range of 1.88 to 3.33 m based on the self-mixing effect. The theoretical analysis of the all-fiber tunable DBR laser shows that wavelength tuning range and frequency of the laser can influence the measurement range and resolution. In addition, the experimental results indicate that the influence factors of measurement accuracy and sensitivity in absolute distance measurement are modulation response linearity, experimental equipment, and circuit bandwidth, which is in good agreement with theory analysis. Moreover, increasing the linear wavelength tuning range of the tunable DBR laser is an essential method to improve the measurement accuracy and sensitivity of the absolute distance measurement system.

We present an overview of the applications of self-mixing interferometer (SMI) to tasks of interest for mechanical engineering, namely high-resolution measurement of linear displacements, measurements of angles (tilt, yaw, and roll), measurements of subnanometer vibrations, and absolute distance, all on a remote target—representative of the tool-carrying turret of a tool-machine. Along with the advantages of SMI—compactness, low cost, minimum invasiveness, ease of use, and good accuracy, we illustrate the typical performance achieved by the basic SMI sensors, that is, the versions requiring a minimum of signal processing and discuss special features and problems of each approach.

A proximity detector based on self-mixing technique, well suited for different industrial applications, is demonstrated. Instead of using a light-source plus a detector, the proposed sensor is realized by a single laser source. Two different physical effects in the laser diode allow for a continuous detecting range, from 10 mm up to 80 mm. The main advantages of the sensor are target detection from just one point of view; multiple sensors configuration does not need optical filters; separation of source and detector is eliminated; and background rejection is intrinsically given by the self-mixing effect, which shows a sharp cut-off after the focus.

We present a self-mixing interferometer (SMI) vibrometer developed around a laser diode with a simple and compact analog processing of the fringe signal, and show that this instrument can readily detect ambient vibrations—which adversely affect mechanical material-working lathes. In addition, we find out that the vibrometer is an excellent intrusion detector, capable of discovering a man stepping several meters away from the sensor, thus hinting at an SMI application as an antithief sensor.

Operationally significant infrared search and track (IRST) systems have been primarily second-generation thermal imager technology with scanned time-delay-integration (TDI) detector operation. The benefit of the scanned technology provides for large aperture, gimbal-scanned sensors with extremely wide field of regard, but with low revisit rates. Dramatic progress in large format staring arrays has provided the possibility of higher performance systems with lower complexity. These large format infrared staring arrays may be able to provide systems with higher performance (due to detector count) with less complexity (fewer gimbal scan limitations). In fact, lower performance IRST systems may satisfy operational requirements without scanning or stare-step operation in a “strap-down” architecture. The first step in a full capability staring system IRST design requires a thorough knowledge of staring array IRST performance. This knowledge includes a basic understanding of signal to noise (SNR) in both undersampled and well-sampled systems, with and without a matched filter. For undersampled systems, unresolved targets result in low SNR in both the average case and worst-case scenarios. We assess (using SNR as our primary metric) how the staring IRST system benefits from typical staring operations, such as dither and stare step. We provide a comparison of staring IRST system performance in the midwave infrared (MWIR) and longwave infrared (LWIR) with three modes of operation: basic staring (no sensor movement), dither, and stare step. In addition, we introduce a metric that allows comparison of different types of IRST systems. We use this metric to compare the performance of MWIR and LWIR as well as staring, dither, and stare-step systems. In the future, we will compare scanned systems to staring IRST systems.

This study presents a Bayesian approach based on a color image demosaicking algorithm. The proposed method is composed of pointwise and patchwise measurements. The estimation of the missing pixel is formulated as a maximum a posteriori and a minimum energy function. By utilizing Bayesian theory and some prior knowledge, the missing color information is estimated with a statistics-based approach. Under the maximum a posteriori and Bayesian framework, the desired target image corresponds to the optimal reconstruction given the mosaicked image. Compared with existing demosaicking methods, the proposed algorithm improves the CPSNR, S-CIELAB, FSIM, and zipper effect measurements while maintaining high efficiency. Moreover, it handles Gaussian and Poisson noisy images better than other conventional images.

Until now, coding of range gates suffers from a nonlinear error in erroneous coding sequences. It is shown that the use of codes based on Hamiltonian paths eliminates ambiguities within the sequences. In particular, the application of gray codes as multigate sequences was successfully investigated for error-free coding of range gates. Two different coding sequences based on 3-bit and 4-bit gray codes were applied to sample a three-dimensional imaging range of 1.8 and 4.2 m with superresolution with three and four sampling points.

Image sets and videos can be modeled as subspaces, which are actually points on Grassmann manifolds. Clustering of such visual data lying on Grassmann manifolds is a hard issue based on the fact that the state-of-the-art methods are only applied to vector space instead of non-Euclidean geometry. Although there exist some clustering methods for manifolds, the desirable method for clustering on Grassmann manifolds is lacking. We propose an algorithm termed as kernel sparse subspace clustering on the Grassmann manifold, which embeds the Grassmann manifold into a reproducing kernel Hilbert space by an appropriate Gaussian projection kernel. This kernel is applied to obtain kernel sparse representations of data on Grassmann manifolds utilizing the self-expressive property and exploiting the intrinsic Riemannian geometry within data. Although the Grassmann manifold is compact, the geodesic distances between Grassmann points are well measured by kernel sparse representations based on linear reconstruction. With the kernel sparse representations, clustering results of experiments on three prevalent public datasets outperform a number of existing algorithms and the robustness of our algorithm is demonstrated as well.

A multiple kernel learning (MKL) method is proposed for synthetic aperture radar (SAR) target recognition. The goal of the proposed MKL is to learn an optimal combined kernel to reduce the dimensionality of SAR images and maximize the separability of SAR targets. Orthogonal neighborhood-preserving projection (ONPP) can effectively reduce the sample dimensionality and maximally preserve the structure information but without the discrimination. On the contrary, maximum margin criterion (MMC) has the ability of classification but without the ability of preserving structure information. To realize the proposed goal, ONPP and MMC are combined within the graph embedding framework, where an optimal projective direction and basic kernel weights are automatically learned. Based on the obtained projection direction and kernel weights, all basic kernels are projected to generate the composite kernel. Moreover, the projection and transformation operations are based on three-dimensional (3-D) data generated by a series of basic kernel matrices, which can completely preserve the structure information in reproducing kernel Hilbert space. Numerical experiments on MSTAR dataset indicate that the proposed MKL method can effectively reduce the dimensionality of SAR images and achieve the outstanding recognition performance when compared with several state-of-the-art algorithms.

We introduce a phase mask with a left-handed azimuthal polarization to generate a special shape of point spread function. The proposed point spread function has low-intensity and small size in the central lobe. This central lobe of the point spread function is used to obtain an image on the detector, and therefore the lateral resolution is improved. The proposed method can achieve a remarkable improvement in the lateral resolution and the depth of field compared with conventional confocal scanning microscopy. The image simulation demonstrated that the images obtained by this proposed method have better imaging quality.

The performance of the “weighted Fourier phase slope” centroiding algorithm at the subpupil image of a Shack–Hartmann wavefront sensor for point-like astronomical guiding sources is explored. This algorithm estimates the image’s displacement in the Fourier domain by directly computing the phase slope at several spatial frequencies, without the intermediate step of computing the phase; it then applies optimized weights to the phase slopes at each spatial frequency obtained by a Bayesian estimation method. The idea was inspired by cepstrum deconvolution techniques, and this relationship is illustrated. The algorithm’s tilt estimation performance is characterized and contrasted with other known centroiding algorithms, such as thresholded centre of gravity (TCoG) and cross correlation (CC), first through numerical simulations at the subpupil level, then at the pupil level, and finally at the laboratory test bench. Results show a similar sensitivity to that of the CC algorithm, which is superior to that of the TCoG algorithm when large fields of view are necessary, i.e., in an open-loop configured adaptive optics system, thereby increasing the guide star limiting magnitude by 0.6 to 0.7 mag. On the other side, its advantage over the CC algorithm is its lower computational cost by approximately an order of magnitude.

Phase retrieval with unidirectional differential phase-contrast image requires integration with noisy data, which is an illposed inverse problem. Conventional direct integration method would result in severe streak artifacts. Total variation (TV) regularization-based method would reduce the streak artifacts, but the edges parallel with phase-contrast sensitivity direction are likely to be over smoothed. We propose an improved weighted TV regularization phase retrieval method by introducing a weighting factor to the conventional TV term. When applied to simulation and experimental data, this method shows an advantage of preserving the sharpness of the edges while preserving the ability of reducing streak artifacts compared with conventional TV-regularization method.

The solar-blind ultraviolet (UV) and visible (VIS) imaging system provides a valuable tool for search and rescue missions. However, due to atmospheric scattering and absorption effects, the UV images are significantly degraded, even missing the target in some frames. A framework based on a weighted mask, with three schemes suitable for various imaging conditions is proposed. Compared with traditional methods, this framework not only preserves low-intensity target regions but also highlights and tracks any suspicious target. Scheme 1 enhances the signal-to-noise ratio (SNR) by computing the accumulating weights of sequential frames, supporting temporal and average weighting means. The temporal weighting serves as a traditional recursive temporal filtering method, which has an effect similar to that of average weighting. Scheme 2 mitigates small platform drifts by introducing a Kalman filter. Scheme 3 mitigates large platform perturbations by eliminating interference from a moving background, which is achieved by determining the warping relationship from adjacent VIS frames. The experiments are designed to cover as many situations as possible, including low-SNR imaging on a static platform, high-SNR imaging on a flat-flying small drone, and strong/weak complex target imaging on a hovering platform. The experiments assess the proposed methods and validate their predicted performance.

A concept for targetless, computer-aided design (CAD)-based, close-range photogrammetry for online shape inspection is introduced. The shape of an object, which is arbitrarily located on a conveyor belt, is to be measured and compared with its nominal shape as defined by a CAD model. For most manufactured objects, deviations are only measured at a few given comparison points. These deviations can be estimated using local photogrammetry based on a priori geometrical information given by the CAD model and the comparison points. Our method results in faster output with higher precision, because we do not generate a shape representation of the entire measured object using typical photogrammetric methods. Images depicting the object from convergent angles are captured by an array of cameras in a precalibrated network, and the CAD model is matched and aligned, within the projective geometry of the camera network, to the depicted object in the images without the use of targets. An algorithm for solving this virtual projective targetless shape matching problem is presented.

Sampling moiré is introduced as white light-color moiré. The experimental results show that the red and blue color images of the sampling moiré fringes have phase difference π relative to each other. The principle of color sampling moiré is derived assuming that the Bayer pattern-CCD array can be considered a combination of a bicolor red–blue grating and a green smooth plate. The moiré fringes created by the superimposition of the red–blue and a binary grating are simulated under the circumstances, where the binary grating is illuminated by red, blue, and white sources. The results of the simulation confirm that the red and blue sources moiré fringes have a phase shift relative to each other and that the gray-scale moiré fringes of the white source have smaller contrast than those of color sources. Simulation of the color sampling moiré shows results similar to the experiments except that the phase difference of π differs close to the image edges in the latter, due to the color aberration of the lens. It is indicated that sampling moiré can be used as color moiré without a color source or a color grating, with low cost setup.

An automatic classification method based on machine learning is proposed to distinguish between true and false laser-induced damage in large aperture optics. First, far-field light intensity distributions are calculated via numerical calculations based on both the finite-difference time-domain and the Fourier optical angle spectrum theory for Maxwell’s equations. The feature vectors are presented to describe the possible damage sites, which include true and false damage sites. Finally, a kernel-based extreme learning machine is used for automatic recognition of the true sites and false sites. The method studied in this paper achieves good recognition of false damage, which includes a variety of types, especially attachment-type false damage, which has rarely been studied before.

Groundwater discharge into streams helps maintain flows during droughts and provides refugia for thermally sensitive species, as the relatively constant temperature of spring water buffers against instream diurnal and seasonal fluctuations. Unmanned aerial vehicle (UAV)-based thermal imagery represents an attractive option to monitor sites of groundwater-surface water mixing. However, the use of economical thermal sensors presents several challenges to obtaining a stable dataset for mosaicking. Here, we present a method for acquiring and postprocessing thermal infrared imagery at spring discharge sites using an inexpensive, uncooled microbolometer mounted on a small UAV. The procedure involves initial estimation and removal of pixel bias from the sensor output, and then compensation for temporal sensor drift by optimizing the stability of the signal at ground features detected within multiple frames. We illustrate this approach by presenting a case study at the Devils River, a groundwater dependent stream in Texas. Comparison with imagery acquired using a more expensive thermal camera system, designed to compensate for sensor drift at the time of data acquisition using knowledge of internal camera temperature, reveals that our method produces a more consistent final mosaic image. A good linear fit (r²  =  0.97) between the signal in the stabilized dataset and ground-based measurements of water temperature underscores the potential for this method to inexpensively produce high quality maps of surface temperature in ecologically important stream reaches.

This paper proposes a hierarchical feature-matching model for the typical faults detection, which is a big challenge in the trouble of a moving freight car detection system (TFDS) due to the constant color and complex background of images. The proposed model divides fault detection into two stages: image segmentation and parallel shape matching. In the process of segmentation, a fast adaptive Markov random field (FAMRF) algorithm is presented based on the image pyramid model and affinity propagation theory. In the process of shape matching, a shape descriptor named exact height function (EHF) is introduced on the basis of parallel dynamic programming. The experimental results indicate that the proposed hierarchical model combined with FAMRF and EHF can achieve automatic detection of an air brake system, bogie block key, and fastening bolt. The proposed model achieves high detection accuracy and great robustness, and it can be effectively applied to the fault detection in TFDS.

Visual resolvability and intersection space of view of a binocular microscopic vision system (MVS) in microassembly systems have influences on each other via analyzing theoretically. According to requirements of various microassembly tasks, it is an explicit problem how to find the best compromise between the visual resolvability of binocular MVS and its intersection space of view in three-dimensional space. A Pareto optimization-based method to make the trade-off between the visual resolvability and the intersection space of view of binocular MVS with relevant fixed parameters in microassembly systems is proposed. Through modeling and analyzing on the visual resolvability and intersection space of view of a binocular MVS, the topology and optical magnifications of two MVSs are considered as the key parameters to optimize its physical properties. A multiobjection optimization model based on a Pareto optimization method is formulated and used to find the Pareto optimal cure, namely the Pareto front, which represents the best trade-off between the visual resolvability and the intersection space of view of a binocular MVS. On these bases, the physical properties of a binocular MVS with structural parameters corresponding to the optimization solutions are numerically simulated. Accordingly, a binocular MVS with different parameters is established on an automatic microassembly system with a single manipulator with five degree-of-freedom and experimentally tested. The simulation and experimental results indicate that all of the optimal solutions, which locate on the Pareto front, are the best trade-off between the visual resolvability and the intersection space of view of a binocular MVS. According to the Pareto front, physical parameters of a binocular MVS can be chosen to get the optimal visual resolvability with a certain intersection space of view and vice versa.

There is an urgent demand for high-accuracy, real-time three-dimensional (3-D) shape measurements in industrial production as an ideal tool for quality control. Based on 3-D digital image correlation (3D-DIC), in this study, we measured micro-stair-steps of solder paste and printed circuits on circuit boards. The 3-D shapes of the circuit boards were successfully reconstructed, and the shapes of both solder paste and printed circuits were identified with high accuracy. To evaluate the performance of the 3D-DIC method, a stylus profiler was used to conduct shape measurements for printed circuit boards (PCBs), and comparisons were made between the results of 3D-DIC and the stylus profiler. The experimental results demonstrate that: (1) printed circuits with a length of several tens of microns can be measured with an accuracy of 3  μm using 3D-DIC in a field of view of 14  ×  12  mm²; (2) the time required for 3-D reconstruction was less than 1 s for a 500  ×  500 image with a grid step of 3 and a subset size of 19 on a consumer-grade computer (i7 4790k CPU, 16 GB of memory), thus meeting the requirements of online and real-time shape measurements of PCBs; and (3) for printed circuit measurements, the subset size chosen should be close to or slightly larger than the width of the printed circuit.

Modulation measurement profilometry can achieve the reconstruction of a tested object with complex or step-like surface. The technology applying coaxial and coimage plane optical systems in which the projection axis coincides with the observation axis can effectively avoid the defects of shadow, shade, and phase discontinuity. All of these weaknesses are largely present in optical three-dimensional sensing based on trigonometric measurement principle. However, in the practical application, the captured fringe patterns are always inevitably influenced by noise, which will directly affect the measurement accuracy. As a well-known and remarkable time-frequency analysis tool, the wavelet transform (WT) method has a strong noise immunity and is suitable to analyze nonstationary signals. Comparisons between the WT method and the Fourier transform method are made not only by computer simulations appearing robust within various types of noise but also the practical experiment. It shows that the WT method is helpful in the enhancement of measurement accuracy. Additionally, experiment further verifies its strong antinoise ability.

Optical lens distortions will seriously affect the measurement accuracy of the vision system. An independent lens distortion correction method based on cross-ratio invariability and compressed sensing is, thus, proposed to improve the measurement accuracy of the system. A virtual distortion correction template is designed according to the characteristics of the lens distortion. The ideal projection coordinate in the imaging plane of the camera and the distortion correction value of the characteristic points in the virtual correction template can be calculated based on the four-point cross-ratio invariance principle. The ideal projection coordinate of the other pixels in the image can then be reconstructed based on compressed sensing theory and, thus, correcting the distortion of all pixels. The experimental results show that for the same vision measurement system, when the independent distortion correction algorithm is used, its measurement accuracy is significantly improved, and the maximum error is only 0.069 mm. This method can correct the distortion effectively, thus improving the precision of the system.

For gas analysis systems based on terahertz time-domain spectroscopy (THz-TDS), relatively complex systems have been proposed to enhance the sensitivity to measure small amounts of gas. This paper proposes a simple system comprising a hollow optical fiber with an inner dielectric layer as a low-volume and long-path gas cell THz-TDS-based gas analysis system. The inner dielectric layer of the fiber not only reduces the transmission loss for THz waves, but also protects the metal layer of the fiber from reactive gases. The proposed dielectric-loaded fiber is fabricated by depositing a metal thin film on a polymer film tube. In a gas analysis experiment conducted using the fiber with a THz-TDS system, separating the absorption peaks of NH₃ gas and water vapor initially proved to be difficult, although NH₃ gas was successfully detected. However, by improving the frequency resolution via Fourier transform and introducing a longer hollow optical fiber, the two peaks were successfully separated and the system sensitivity improved.

A phase error compensation method is proposed for phase measurement profilometry. The gamma nonlinearity of a projector–camera pair can induce higher-order harmonics in a deformed pattern, resulting in periodic phase errors in the unwrapped phase map. To reduce the phase error caused by gamma nonlinearity, a model relating the phase error and practical wrapped phase is established, and a phase matrix for compensation is obtained based on the model instead of solving the gamma value of the projector–camera pair (γ). In addition, the compensation accuracy theoretically decreases with the ratio of the third-harmonic coefficient to the fundamental frequency components of the deformed pattern (β). The simulated measurement shows that the root-mean-square (RMS) before and after compensation is 0.0052 and 2.6840  ×  10^−  4  rad when β is 0.0063, respectively, and it is 0.0755 and 0.0175 rad when β is 0.0880, respectively. The experimental results indicate that the RMS of the unwrapped phase in a uniform plane before and after compensation is 0.0639 and 0.0156 rad, respectively, when β is ∼0.09 using the proposed method. The periodic phase error caused by gamma nonlinearity decreases dramatically using the proposed method.

The imaging and digital processing of anisotropic optical properties of in vivo skin may be helpful in the assessment of its heterogeneous anatomy and physiological functioning. The interaction between polarized light and this organ provides information about the structural order beneath the epidermal layer. However, penetration depth is limited by optical absorption and scattering that are related to local skin characteristics such as pigment density, connective tissue organization, and cutaneous layer thickness. In this work, in vivo healthy skin of a murine model was analyzed using Mueller imaging polarimetry. A gray-level co-occurrence matrix analysis of diattenuation, birefringence, and depolarization measures in four different skin locations showed texture properties that can be used to distinguish local physiology.

Phase-shifting shadow moiré is an efficient technique for three-dimensional (3-D) shape measurement, but the technique is always suffered from the error of phase shifter. A three-frame random phase-shifting algorithm is proposed to relax the requirements of repeatability and accuracy of the phase shifter in a shadow moiré system. The proposed method can act as a bridge between the three phase-shifting analysis methods and two-shot phase-shifting methods. The value of introduced arbitrary and unknown phase shift can be calculated by a determinate formula. The algorithm has the advantages of being free from the disturbance of background elimination and less computation. In addition, the proposed method is immune to nonsinusoidal fringe patterns and can be implemented easily in many applications. Both simulation and optical experimental results verify the feasibility of this method.

This paper proposes a grating eccentricity error correction technique for high-precision angular measurement via image processing. We first built a mathematical model of grating eccentricity error, then we established an error-correct arithmetic based on the least square method. We tested the proposed method to find that it yields a standard error deviation of only 6.33″, compared to the uncorrected standard error deviation of 22.33″. This method is based on a single detector, which is inexpensive and performs well. The results presented here may provide a theoretical and technological foundation for further research on small-size, high-resolution photographic rotary encoders.

In the large sky area multiobject optical fiber spectroscopy telescope project, to capture the spectrum of a particular object, the optical fiber positioner must position the optical fiber end face to a specified location on the focal plane. The accuracy of the optical fiber positioner is guaranteed by feedback from photogrammetry. Photogrammetry accuracy is based on accurate calibration. However, given the complexities in the optical fiber focal plane and the optical fiber positioner, the accurate standard point is considerably difficult to obtain, which results in insufficient calibration accuracy. To solve this problem, a convenient calibration method based on the combination of small, planar targets is proposed. In this method, each optical fiber positioner positions the optical fiber to several designed locations, which are relatively accurate. These points form a high-precision, two-dimensional point array that can be used as the planar target. In this manner, each optical fiber positioner can be regarded as a small, high-precision planar target. All small, high-precision planar targets are assembled to form the flexible planar target, which is used for calibration. The experimental result indicates that this method is highly accurate and can be applied in focal plane calibration.

Polarization measurements can be used to differentiate objects of different geometry and material. Polarimetry has been used as a tool to differentiate between two composite geometries, each comprised of a model satellite bus, a solar panel, and a dish antenna. Each component demonstrates unique polarization signatures that can be obtained by analyzing the reflection of passive illumination. An approach for characterizing geometries using Stokes vector–vector space has been demonstrated to be very effective in differentiating between composite geometries representative of orbiting satellites. Our approach lends itself well to a statistical analysis that is applicable for a computational method to provide target identification of space objects.

For digital holographic applications, we have studied the refocus criteria operating on the high-pass filtered and the nonfiltered reconstructed complex amplitudes (RCAs), respectively. A set of 10 criteria have been investigated for applications on phase, amplitude, and mixed objects in off-axis digital holography. Without filtering, the criteria always exhibit opposite behaviors for amplitude and phase objects and cannot work normally for mixed ones. With high-pass filtering on the RCAs, most of the criteria can be used regardless of the phase or amplitude nature of the objects without noises. But, in real applications with noises, they show different robustness. The investigations indicate that the criterion based on the integrated amplitude shows good exactitude and strong robustness, exhibiting a great potential in real applications, especially for multiobject systems.

An application of circular grating Talbot interferometer for measurement of low-frequency vibrations in a plate is demonstrated. Experiments were conducted to measure/monitor the in-plane low-frequency vibration from 1 to 25 Hz in a plate object. The measurement system is advantageous over accelerometer, because it is easy to construct, less expensive, and robust. This method can be implemented conveniently to quantitatively measure temporal and continual displacements of the vibration object with variable sensitivity and also applicable to shallow curved objects.

Weak-light phase locking is a key technology for Taiji space gravitational wave detection and its pathfinder mission. Previously, the phase locking was achieved by a complicated technique, which controls the frequency of the laser via a piezo-electric actuator (kHz range or more) and a temperature actuator (sub-Hz range). We propose an easy phase-locking strategy, which is based on the electro-optic modulator (EOM). Compared with the traditional way, this strategy only needs to modulate the driven voltage of the EOM, and the frequency bandwidth can cover all ranges. An experiment is also established to prove the feasibility of the method. The results show that the noises are <80  μrad  /  Hz^1/2 in frequencies from 0.2 to 1 Hz, and the thermal drift is the main noise source in our recent system.

Strain measurements on pipelines provide a nondestructive means to evaluate their in-service conditions. They have been proposed to detect abnormal operating pressures, pipe wall problems, or intrusive pipeline events. For such applications, optical fiber sensors are especially appealing because of their hazard and electromagnetic interference-free nature. In this work, two different types of optical fiber sensors are compared for use to monitor the hoop strain on a pressurized pipe, the well-developed fiber Bragg grating (FBG) sensor and a proposed simple fiber sensor based on multimode interference (MMI). The FBG sensor shows a better linearity of strain measurement, while the MMI sensor provides a larger wavelength shift with strain at relatively low pressures. An intensity-based detection of vibrations, simulating an intrusive pipe drilling event, was achieved using the proposed MMI sensor.

Aiming at such blade features as thin wall, mutability, and vulnerability, this paper proposes a quick and reliable measuring method, which has significantly improved the accuracy and efficiency of blade measurement. Using conoscopic polarization holography, the proposed method is able to eliminate the influences of significant change in the curvature of blade profile, thus realizing the efficient collection of data. Then, through the accurate registration based on a K-dimensional tree (KD-tree), the error distribution and the deviations of relevant geometric parameters of blade profile are obtained, providing reliable data for the accurate adjustment of precision machining parameters. The experimental results show that the efficiency and accuracy of the proposed method are improved significantly with a measuring repeatability of <8  μm and a measuring time of about 5 min.

Many works around the digital speckle correlation (DSC) are to improve the computational efficiency and measurement accuracy in recent years. This work aims to improve the efficiency and accuracy of DSC for both single-point and full-field points used in mechanical properties test of materials. For this purpose, first, a subpixel initial guess for the inverse compositional Gauss–Newton algorithm (IC-GN) with the first-order shape function is introduced for single-point image registration. Then, with the aid of strategy of subset image edge extend interpolation (SIEEI), the efficiency and accuracy of full-field displacement and deformation measurement are improved simultaneously. The SIEEI is employed to reduce mean squared difference errors caused by traditional bicubic interpolation algorithm. Comparative studies between the traditional IC-GN algorithm and the proposed algorithms are presented using simulated speckle images and CCD images. The proposed method achieves more executive efficiency and more accuracy for single-point and full-field points image registration. The computational efficiency of the proposed algorithms increases 7.5% for full-field registration using CCD images. The mean of squared difference errors of the SIEEI method is less than the traditional bicubic interpolation algorithm. The presented approach shows great potentials for challenging application, such as mechanical properties test of materials.

Compared to the dual-energy scintillator detectors widely used today, energy-resolved photon-counting x-ray detectors show the potential to improve material identification in various radiography and tomography applications used for industrial and security purposes. However, detector effects, such as charge sharing and photon pileup, distort the measured spectra in pixelated, photon-counting detectors operating under high flux. These effects result in a significant performance degradation of the detectors when used for material identification where accurate spectral measurements are required. We have developed a semianalytical, postdata acquisition, computational algorithm that corrects the measured attenuation curve for severe spectral distortions caused by the detector. The calibration of the algorithm is based on simple attenuation measurements of commercially available materials using standard laboratory sources, enabling the algorithm to be used in any x-ray setup. The algorithm is developed for correcting spectral data acquired with the MultiX ME100 CdTe x-ray detector but could be adapted with small adjustments to other photon-counting, energy-resolved detectors with CdTe sensors. The validation of the algorithm has been done using experimental data acquired with both a standard laboratory source and synchrotron radiation. The experiments show that the algorithm is fast, reliable at x-ray flux up to 5  Mph  /  s  /  mm² and greatly improves the accuracy of the measured spectrally resolved linear attenuation, making the algorithm useful for both security and industrial applications where photon-counting detectors are used.

The phase-shifting algorithm is widely used for noncontact three-dimensional (3-D) reconstruction, traditionally leveraging gray values to recover phase angles. It is difficult to reconstruct the shape of an object with a specular or dark surface. We propose an algorithm for accurate phase recovery based on high dynamic range imaging. Unlike most existing methods that use gray values to calculate the phase angle, the proposed method uses E values, which are the recovered irradiance from low dynamic range images, to calculate the phase angle. Experiments show that the proposed method improves phase recovery accuracy and achieves good results for the 3-D reconstruction of specular or dark objects.

The inclusion of phosphor into a high brightness light-emitting diode (LED) package is a complicated task since LEDs are encapsulated with a phosphor and epoxy mixture to convert blue photons to white light. Moreover, this common practice may cause high temperatures and fractures in the gold wire bonds of the chip or solder balls due to local heating and thermal stresses leading to device failures. Furthermore, at elevated junction temperatures, the light conversion efficiency of the phosphor reduces and decreases the overall optical efficiency of an LED. Although, remote phosphor technique has been already applied to LED systems, the high power requirements have needed better performing methods. Thus, an immersion liquid cooled remote phosphor-coated system has been proposed and experimentally and computationally investigated. First, a set of experiments was performed, which includes the combined effects coming from both optical and thermal improvements with the proposed liquid cooled remote phosphor-coated technique, where the total light extraction enhancement was obtained in excess of 25%. Then, the same problem has been computationally studied for investigation of solely optical enhancements, which has shown that remote phosphor-coated LED package with a liquid coolant of suitable refractive index at the optical path has enhanced the overall lumen performance about 13%, whereas the rest of the improvements of 12% were due to thermal enhancements.

A bandwidth reconfigurable and passband switchable filter is proposed and experimentally demonstrated. Based on the same structure, the filter can realize a single broadened response, two passbands with tunable bandwidth or multiband filter. A single broadened response adopts binary phase-shift keying modulation, in which the bandwidth is related to the modulation data pattern. By using the programmable single-sideband suppression carrier signals, the position of the optical lines can be settled down properly and the corresponding arbitrary passbands are constructed. The proposed filter is flexible, which offers possibilities to process multitype microwave signals simultaneously.

This work describes a design process that greatly increases the depth of field of a simple three-element lens system intended for biometric iris recognition. The system is optimized to produce a point spread function that is insensitive to defocus so that recorded images may be deconvolved without the knowledge of the exact object distance. This is essentially a variation on the technique of wavefront encoding; however, the desired encoding effect is achieved by aberrations intrinsic to the lens system itself, without the need for a pupil phase mask.

A reactive Eu(III)-complex Eu(TTA)₃  (  UA  )   was synthesized with 2-thenoyltrifluoroacetone (TTA) as primary ligand and undecylenic acid (UA) as active ligand. The structure of the complex was characterized by elemental analysis, FT-IR spectra, and UV–vis absorption spectra. The results indicate that the Eu(III) ion is successfully coordinated with primary ligand TTA and active ligand UA. Fluorescence spectra suggest that the complex exhibits excellent red emission of Eu(III) ion with a peak at 611 nm being monitored at 365 nm of near ultraviolet. Meanwhile, the complex has also shown a long fluorescence lifetime of 1.4259 ms and a high quantum yield of 86.7%. The singlet and triplet energy levels for the ligands of rare earth complexes were calculated using Gaussian 03 program package. Intramolecular energy transfer diagram was established and an energy transfer process following Dexter electron exchange mechanism was demonstrated, which provides the experimental data and theoretical foundation for the development of high-performance luminescent materials. The result of TGA shows that the complex keeps stable at 156°C or below. All the results demonstrate that the reactive Eu(III)-complex can be used as the red-light material in white light-emitting diodes phosphor.

The author solves the differential equations of small displacement linear elastic theory for the surface stresses in ring-mounted glass lenses. He tests his formulation on two cases for which the solutions are known. The results are favorable. He then analyzes an actual lens mount design from his personal experience. He finds his results to be in disagreement with the most commonly used formulation, the results of which are much higher for the tensile stress. He attributes the discrepancy to inconsistent geometric considerations in Delgado and Hallinan’s formulation. The author tests his closed-form solution against a Nastran finite element model and finds the results in good agreement outside the immediate contact zone. In the contact zone, Nastran demonstrates its instability in calculating the surface stresses as it approaches the point of contact.

We propose a unique method for producing nondiverging optical array and elliptical hollow beam in a controlled manner using aberration patterns generated from oblique illumination of axicon. The optical arrays with propagation invariance property are persisted for short ranges in the focal depth, whereas diverging array with a constant number of bright spots is produced beyond bottle beam. The measured variation in the geometrical parameters of obliquely illuminated axicon setup has facilitated precise control on the dimension of optical array and shape of the elliptical hollow beam, respectively. The theoretical analysis confirms the experimental results for the generation of short-range nondiverging optical array and elliptical dark hollow beam with fine control. To the best of our knowledge, this is the first experimental approach to extend the potential of axicon beyond generation of Bessel and circular hollow beams.

A magnetorheological finishing (MRF) method, which is named lap-MRF, is proposed in this paper. A lap, instead of a large polishing wheel, is used to expand the polishing area, which significantly improves the material removal rate. Lap-MRF uses the flexible magnetorheological fluid (MR fluid) as a polishing pad to match the surface well. Moreover, the MR fluid in the polishing zone can be renewed continuously to ensure the stability of the material removal rate. The principle of Lap-MRF and the theoretical model of the material removal rate are presented. The effects of the spindle speed and the working gap distance on the polishing normal pressure and the material removal rate are analyzed. Finally, a series of polishing experiments and simulations are carried out. The volume removal rate for K9 is up to 0.76  mm³  /  min and the comprehensive material removal stability is 6.7%. Using MATLAB simulation process method, the convergence efficiency for a ϕ1000  mm mirror is 98.5%. The results verify the validity of the proposed method.

The goal of the paper is to simplify a simulation of cascaded Fresnel diffraction (CFD) by reducing the number of integrals and giving a solution for arbitrary plane. The simplification becomes possible with a use of CFD theorem. CFD theorem is formulated and proved. CFD theorem for simplification of one-lens model and two-lens model is demonstrated, and their mathematical models are obtained. As a result, the number of integrals describing diffraction and light propagation in lens models is decreased multiple times. We consider the optical field in any plane, making our approach more universal by assuming ideal point spread function. This approach may be applicable to numerical simulations as well as analytical investigations, optimizations, and designs. Our work benefits Fresnel diffraction method for modeling complex optical systems.

Fiber nonlinear (NLI) noise, as well as amplified spontaneous emission (ASE) noise, degrades the performance of coherent optical fiber communications. We find that the temporal distribution of NLI noise, which is unlike that of ASE noise and depends on the waveform of signal pulse, has an impact on the estimation of error vector magnitude (EVM) and bit error ratio (BER). The influences of NLI noise, as well as ASE noise, on EVM and BER are analyzed for quadrature phase-shift keying and M-ary quadrature amplitude modulation systems. Modified theoretical formulas are proposed to estimate EVM and BER for the system under the effect of NLI noise, ASE noise, or mixing of these two noises. For system with mixing of ASE and NLI noises, according to our model, the optimal channel power for the best EVM is not corresponding to the best optical-signal-to-noise ratio. Our model is validated by numerical simulations.

We propose a high-birefringence twin-core hollow optical fiber, which is composed of two rectangular Ge-doped cores, annular silica cladding, and a large central air hole. The rectangular cores are weakly coupled with the air hole and arranged in the same silica cladding. The mode birefringence in single-mode operation of 1.55  μm reaches 2.48  ×  10^−  4, which is comparable with traditional panda polarization maintaining fiber. By introducing an extra stress zone, the mode birefringence can be improved further by two times. The high birefringence of the twin-core hollow fiber is induced by strong asymmetry of the waveguide structure. The characteristics of the guiding mode, birefringence, and evanescent field are analyzed numerically by full-vector finite-element method. The effect of geometric and material parameters is discussed. The proposed scheme of twin-core hollow fiber can overcome the polarization fading effect and signal cross talk occurring in traditional multicore fibers and is much desirable for improving the performance of in-fiber integrated interferometers and biochemical sensors.

The characteristics of one-pump fiber optical parametric amplifiers (1-P FOPAs) are theoretically investigated as the temporal width of the input pump increases from the pulsed to the continuous wave (CW) regime. Coupled nonlinear Schrödinger equations are numerically solved, and the pulse width of the pump is gradually increased to simulate the evolution of the gain and output spectra of the 1-P FOPA from the pulsed to the CW operation. The results show that the quasi-CW operation is achieved when the pulse width is about 500 fs. Also, when the temporal width of the pump increases and lies in the picosecond regime, the characteristics of FOPA recover those of the FOPA operated in the CW regime.

A miniature in-fiber Fabry–Perot interferometric (FPI) sensor is demonstrated. Through manufacturing microhole in a single-mode fiber with excimer laser, a single microhole FPI sensor and a dual microhole FPI sensor are fabricated, and the temperature sensing characteristics of them are investigated experimentally. The results show that the spectra of two sensors shift to short wavelength with an increase in the temperature, and the relationships between wavelength shift and temperature change have better linearity in temperature range of 30°C to 85°C. The temperature sensitivity of single microhole FPI sensor is 0.160  nm  /    °  C, and the dual microholes FPI sensor has two temperature sensitivities corresponding to different FP cavities. Furthermore, the temperature sensitivity can be adjusted by changing the distance between two microholes. When increasing the distance from 1 to 2 mm, the higher temperature sensitivity can be changed from 0.176 to 0.192  nm  /    °  C, and the temperature-dependent wavelength shift still keeps a good linearity. Compared with conventional fiber Bragg grating temperature sensor, this kind of FPI sensor has not only different dual temperature sensitivities but also higher sensitivity and stability, so that this kind of microhole FPI sensor can be used in many fields.

The stability of gain-clamped erbium-doped fiber amplifiers (EDFAs) may be affected by the parameter drift of active and passive devices. We propose a hybrid gain-control scheme to enhance the working stability, which consists of an optical dual-fiber-Bragg-grating (FBG)-based linear cavity and an electrical fuzzy-based controller. The principle of optical gain clamping is then depicted and a detailed design about dual-channel fuzzy controller is given. The experimental results show that, in the dual-FBG configuration, with 20.2-dB mean gain, the designed L-band EDFA simultaneously guarantees the ±0.1-dB stability (with 30-dB dynamic range) and the ∼0.4-dB flatness in the range of 1570 to 1610 nm. In addition, under the electrical feedback control, the flatness stability of gain spectrum reaches ±0.18  dB within 60 min and the mean gain stability is about ±0.34  dB within 10-h continuous operation.

A silicon microresonator consisting of an asymmetric embedded dual microring (EDMR) with a U-bend feedback coupled waveguide (FCW) is proposed and investigated for possible applications in sensing. A Fano resonance originates from the optical interference between the EDMR and racetrack-like resonator (RTR) in the proposed device. The interference between a high-Q EDMR cavity and a low-Q RTR cavity can increase the extinction ratio (ER) of Fano spectrum. The slope of Fano resonance can be well tuned by changing loss factor β of FCW. We can see the periodic spectrum transits from Lorentz profile to Fano profile by changing optical path length L of FCW. The maximum ER of Fano resonance is as high as −45  dB. Our sensor exhibits sensitivity of 1.2  ×  10⁴  dB  /  nm, and a minimum detection limit of 2  ×  10^−  7  RIU. Moreover, for such a structure, the fabrication is simple and CMOS compatible. The sensor’s performance is simulated for ethylene glycol (C₂H₆O₂) solution. The microcavity with optimized geometric structures presented provides the potential for ultracompact sensing applications.

We demonstrate a tunable laser consisting of two gain sections with different bandgap energies using selective intermixing of InGaAsP multiple quantum wells. The selectivity in the intermixing process is achieved using different capping films during a high-temperature rapid annealing process. The region that was capped by a SiO_yN_x film had its bandgap energy blueshifted to 1530 nm while the region that was capped by a SiN_x film had its bandgap energy unchanged at 1560 nm. Wavelength tuning is achieved by injecting current separately into the two gain sections. When the currents injected into the two sections are varied, the combination of the gain spectra leads to a laser wavelength in the overall effective gain whose peak position depends on the magnitudes of the two injected currents. The fabricated device was capable of producing laser emission that was tunable from 1538 to 1578 nm with almost constant output power from 1544 to 1566 nm.

A hybrid optical amplifier is proposed using a combination of Er-Yb codoped waveguide amplifier (EYDWA) and fiber optical parametric amplifier (FOPA). The scheme is investigated for 100  ×  40  Gb  /  s dense wavelength division multiplexed (DWDM) system at 0.2-nm channel spacing. The parameters are optimized to achieve a flat gain of >30.6  dB, with a gain ripple of 2.67 dB. This is obtained over a wavelength range of 1542 to 1562 nm at −10-dBm optimum input power per channel, without using any costly gain flattening scheme. For higher input powers up to 10 dBm, an increase in gain ripple from 2.67 to 6.13 dB is observed. The gain ripple of 2.67 dB of the proposed EYDWA–dual pump FOPA configuration is also much better than 10.32 dB for the hybrid EYDWA–single pump FOPA. The obtained high and flat gain, along with the low noise figure, confirms the usefulness of the proposed amplifier for applications in long-haul DWDM systems.

A multifunction all-optical routing device for a dynamic network is presented. The proposed routing device consists of two arrayed waveguide gratings (AWGs), 12 optical circulators, and six tunable fiber Bragg gratings (TFBGs) to realize the functions of all-optical multichannel reconfigurable optical add/drop multiplexer (ROADM) and optical wavelength interleaver. The AWGs have the characteristics of wavelength cycling of light. So even if there is no additional optical architecture in the proposed optical device, it can have the function of an optical interleaver. Both ROADM and optical interleaver are the keys to enable the modern optical communication services and support the remote configuration of optical paths and the reconfiguration of network topologies. The experimental data show that the functions of add/drop and wavelength interleaver can be implemented. Three dense wavelength division multiplexing (DWDM) channels are used to evaluate the performance of ROADM of the proposed routing device. The results of this study demonstrated the feasibility of this routing device for the DWDM networks.

A highly stable mode-lock fiber laser with a thin film coating of graphene oxide (GO) nanoparticles on a D-shaped fiber is proposed and demonstrated. The GO-coated D-shaped fiber, which serves as the cavity’s saturable absorber (SA), is fabricated by drop-casting GO nanoparticles on the smooth and consistently polished area of the D-shaped fiber made using a disk-polishing machine. Mode-locking is induced through the interaction of the evanescent field from the D-shaped fiber and GO nanoparticle layer. The cavity has a length of 12.5 m with a fundamental frequency of 16.5 MHz and operates in the anomalous dispersion regime. Stable soliton mode-locking is obtained above a pump power of 76.6 mW, with a central lasing wavelength of 1555.7 nm and well-defined Kelly’s sidebands. The generated pulses have a repetition rate of 16.5 MHz and pulse duration of 1.18 ps over a pump power range of 76.6 to 280.5 mW, with only minor fluctuations observed. A signal-to-noise ratio of 58.3 dB is computed, indicating a highly stable output. The proposed SA fabrication technique provides a simple, cost-effective, and consistent method of generating mode-locked pulses in fiber lasers.

A low-complexity optical phase noise mitigation scheme, based on a radio frequency pilot (RFP) and partition phase correction (PPC), is proposed. For diverse channel configurations, an optimized RFP with fixed parameters is first employed in coarse phase tracking for generality purposes. Based on the reliable predecision results, we propose a noniterative PPC algorithm to further suppress the residual noise. Numerical simulations demonstrate that compared with other phase noise compensation algorithms, the proposed scheme has better robustness and lower computational complexity. In addition, the performance degradation caused by decision errors is also discussed, and a moderate partition length with small floating range can guarantee satisfactory correction quality.

This study proposes the use of heterodyne detection and wavelength scanning techniques to solve the problems of small signal and large noise in a Rayleigh Brillouin optical time-domain analysis (BOTDA) system and thereby improve the system performance. Based on an analysis of the Rayleigh BOTDA system, heterodyne detection is used to enhance the signal intensity and wavelength scanning is used to reduce the fading noise. Experimental results obtained using the proposed techniques show that the amplitude fluctuation and signal-to-noise ratio in a 50-m-long fiber near the fiber end with 17 wavelength scans-based averaging are, respectively, reduced by 0.91 dB and increased by 4.19 dB compared to those with a single wavelength. Furthermore, the Brillouin frequency shift in a 70-m-long fiber heated to 50°C and inserted at the center of the sensing fiber can be measured accurately with a maximum fluctuation of 0.19 MHz; this is equivalent to a temperature measurement accuracy of 0.19°C. These results indicate that the proposed techniques can realize high-accuracy measurement, and therefore, they show great potential in the field of long-distance and high-accuracy sensing.

The average bit-error-rate,   ⟨  BER  ⟩  , of optical wireless system using higher order mode laser beam is investigated when atmospheric turbulence shows anisotropic and non-Kolmogorov characteristics. Results reveal that increase in anisotropy in both x- and y-directions positively affects the optical wireless systems performance. Increase in the beam order results in an increase in   ⟨  BER  ⟩   for any anisotropy level, and thus, higher order beams adversely affect the optical wireless systems performance. Larger structure constant, beam source size, and propagation distance result in larger   ⟨  BER  ⟩  , but larger wavelength, inner scale length, and signal-to-noise ratio tend to reduce   ⟨  BER  ⟩  . Increase in the power-law exponent of non-Kolmogorov turbulent spectrum first increases the   ⟨  BER  ⟩   until a certain value, and then   ⟨  BER  ⟩   starts to decrease when the power-law exponent is further increased. Adverse effect of higher order laser beam holds to be valid for any power-law exponent of non-Kolmogorov turbulence.

For microseparation devices, the quality of the microholes, e.g., smooth surfaces and edges, is of crucial importance for high-performance separating ability. Here, we used water-assisted femtosecond laser perforating technology to fabricate high-quality size-controllable (from several to tens of micrometers) micropore arrays on ultrathin aluminum foil surface, which have smooth edges without fragments and debris. The micropore arrays can effectively filtrate particles with diverse diameters. Compared to the micropores prepared in air under the same laser processing parameters, the water-assisted micropores have greatly improved the surface quality, and the particle separation ratio can be increased by ∼40  %  . This method for preparing high-quality micropore arrays can also be applied to other sheet materials, such as titanium, silicon, and even plastic, and so on, which can be widely used in the fields of microfluidic devices for microseparation.

We report here the characteristic of a 1.55-μm fiber gas Raman amplifier worked in an ethane-filled hollow-core fiber (HCF). This amplifier is composed of a CW C-band tunable, narrow linewidth seed laser, and a pulsed 1064.6-nm microchip pump laser. Through a dichroic mirror, the seed laser, together with a 1064.6-nm pump source, is coupled into the ethane-filled HCF and then efficient 1553-nm Raman laser generates by the stimulated Raman scattering of ethane molecules. A maximum Raman laser average power of 43.8 mW (pulse energy 87.6  μJ) with a Raman conversion efficiency of 47.5% (quantum efficiency of ∼70  %  ) is obtained using 8.7-m HCF filled with 1.4-bar ethane. A method to obtain efficient 1.55-μm fiber laser sources is demonstrated.

The rigorous coupled-wave approach (RCWA) and the differential evolution algorithm (DEA) were coupled in a practicable approach to maximize absorption in optical structures with three-dimensional morphology. As a model problem, optimal values of four geometric parameters and the bandgaps of three i-layers were found for an amorphous-silicon, multiterminal, thin-film tandem solar cell comprising three p  –  i  –  n junctions with a metallic hexagonally corrugated backreflector. When the optical short-circuit current density was chosen as the figure of merit to be maximized, only the bandgap of the topmost i-layer was significant and the remaining six parameters played minor roles. While this configuration would absorb light very well, it would have poor electrical performance. This issue arises because the optimization problem allows for the thicknesses and bandgaps of the semiconductor layers to change. We therefore devised another figure of merit that takes into account bandgap changes by estimating the open-circuit voltage. The resulting configuration was found to be optimal with respect to all seven variable parameters. The RCWA + DEA optimization approach is applicable to other types of photovoltaic solar cells as well as optical absorbers, with the choice of the figure of merit being vital to a successful outcome.

Acesulfame potassium (AK) is widely used in the food industry. However, it does harm to the liver and nervous system. Here, a rapid method based on surface-enhanced Raman spectroscopy (SERS) was developed for the detection of AK in mouthwash. As the results show that the characteristic peaks of AK were at 549, 641, and 1650  cm^−  1. The vibrational modes of each peak were calculated by density functional theory. This method can detect AK in mouthwash samples within 3 min without any complicated pretreatment. These results suggested that this SERS-based method for rapid determination of AK in mouthwash was sensitive and reliable. It is expected that this method possesses great potential in monitoring the quality of mouthwash.

Nonlinear optical properties in La-modified   (  1-x  )  Pb  (  Mg_1/3Nb_2/3  )  O₃-xPbTiO₃ (PMN-PT) (3/75/25) transparent ceramic are studied. The optical nonlinearity is investigated employing Z-scan technique at the off-resonant wavelengths of 515 and 1030 nm. The two-photon absorption coefficient is measured up to be 9.6  cm  /  GW at 515 nm, and three-photon absorption coefficient is found to be 0.091  cm³  /  GW² at 1030 nm. When an external electric field was applied to the PMN-PT, we found that the electric field increased from 0 to 3  kV  /  cm in the closed-aperture Z-scan experiment with the wavelength of 515 nm, whereas the nonlinear refractive index n₂ increased from 1.85  ×  10^−  8  esu to 3.87  ×  10^−  8  esu, an increase of 110%. The large nonlinear effect could be applied in areas such as sensor protection and telecommunications and to write permanent refractive index structures into the interior of optical materials, which may make a difference in integrate optoelectronic devices progress.

We demonstrate a plasmonic analog of electromagnetically induced transparency in a simple planar metamaterial structure, which consists of two bright three-dimensional (3-D) Dirac semimetal films (DSFs). Through the coupling in the asymmetric elements, a pronounced transparency peak within a broad transmission resonance spectrum is activated. In addition, the transmission spectrum can be tuned by varying the Fermi energy of 3-D Dirac semimetal. Moreover, higher group delays of more than 6 ps are obtained under different energies, and we can achieve the capability of controlling the group delay by changing Fermi energy because of the spectral tunability. The proposed DSF-based plasmonically induced transparency system may pave the way for potential applications in tunable terahertz switching, sensors, and slow-light devices.

A dispersion tailored properties of fiber Bragg grating (FBG)-based vibration sensor has been explored, assuming a highly perturbed apodization profile. Most of the FBG designed so far for vibration sensing applications are nonuniform. This paper deals with the modeling of perturbation as a more complicated chirp-type nonlinear apodization profile. It can significantly reduce the dispersion-induced signal distortion and side lobe intensity followed by the improvement of sensor performance. The study reveals that an accurate modeling of vibration sensor is quite feasible while opting the proposed apodization profile. FBG sensor parameters are optimized in such a way that the sensor may operate only under lowest order modes. To the author’s knowledge, sensing capability of FBG-based microaccelerometer having arbitrary chirp type of apodization profile has not been discussed before. It is assumed that small perturbation theory is valid while keeping the influence of microbend geometry in analysis. The standard matrix method is used to study dispersion characteristics due to the complicated apodized profile. The simulation results are validated with the experimental result for the known cases.

We demonstrate the broadband stimulated emission generation in the Er^3  +  /  Tm^3  +  /  Ho^3  + codoped optical fiber rod (ETHOFR). The absorption spectra, broadband emission, energy level, electron transition process, and optical stimulated emission of this codoped material are analyzed. Broadband spectrum amplification from 1.7 to 2.1  μm is observed when ETHOFR is excited by 793-nm laser diode (LD). The optical gain shows similar wavelength in dependence on luminescence properties. This material holds potential in the application of weak optical signal amplification.

A side-hole fiber surface plasmon resonance (SPR) sensor is proposed and numerically analyzed to solve the cross-sensitivity problem of temperature and magnetic field. In the side-hole fiber SPR sensor, the side-holes I and II are filled with magnetic fluid and a mixture of ethanol–glycerin, respectively, resulting in a loss spectrum with two channels. Each channel has different sensing characteristics. When the external magnetic field intensity increases, the magnetic field sensitivities of channels I and II are 1.098 and −0.018  nm  /  Oe, respectively. Moreover, with the increasing temperature, the temperature sensitivities of channels I and II are −5.909 and −4.211  nm  /    °  C, respectively. Therefore, the variations of magnetic field intensity and temperature can be simultaneously measured by detecting resonant wavelengths of channels I and II of side-hole fiber SPR effects, resulting in the influence of temperature in a side-hole fiber SPR magnetic field sensor being eliminated. The side-hole fiber SPR sensor has obvious advantages of compact structure and high sensitivity, and it also has solved the problem of liquid storage glassware to compact the structure of the fiber SPR sensor further.

An optical fiber ring resonator (OFRR), a wavelength sensor for testing a single-mode laser system in a wide range of temperature, is presented. We will show that it is possible to calibrate, in relative form, a scale of wavelength, to determine, accurately enough, a thermally induced laser detuning, using the free spectral range of an OFRR. The optical circuit was constructed using 2  ×  2 (50/50) optical fiber coupler obtaining an OFRR of 10-cm ring radius. A single-mode diode laser system has been launched into the OFRR, and different experiments have been performed. We tested the OFRR performance, considering fluctuations in the laser wavelength caused by small temperature instabilities, measuring the output intensity from the ring resonator. Theoretical simulations and experimental results were in agreement with the expected behavior. Furthermore, OFRR systems can be used as an excellent control tool to test the wavelength stability for a narrowband laser diode and act as a part of a control system.

This erratum corrects a missing citation in the paper “Robust and efficient method for matching features in omnidirectional images.”