Optimizing avalanche photodiodes (APDs) in standard complementary metal–oxide–semiconductor (CMOS) processes is challenging due to fixed doping concentrations of the available wells. A speed-improved APD in pin photodiode CMOS technology for high-sensitivity and high-speed applications using a lateral well modulation-doping technique is presented. The increased operating voltage of the presented device leads to a −3-dB bandwidth of 2.30 GHz with a multiplication factor of 20 for 1-μW optical power. This corresponds to a responsivity of 7.40  A/W. A multiplication factor of 44,500 was measured at 10-nW optical power. The thick absorption zone leads to an unamplified quantum efficiency of 72.2% at 635-nm wavelength.

We present a time-polarization multiplexing setup that overcomes the maximum amplification limit of semiconductor optical amplifiers (SOAs) in the pulsed regime. A coherent optical pulse at the input is equally divided into two, which are independently amplified, and coherently recombined to compose an oversaturation amplified optical pulse at the output. This simple topology, containing an SOA and few extra devices, offers 1.96-dB oversaturation gain in comparison with a single SOA. Furthermore, little experimental complexity and, thus, cost, are required by the proposed topology.

In optical interconnection technology, high-speed and large data transitions with low error rate and cost reduction are key issues for the upcoming 8K media era. The researchers present notable types of optical manufacturing structures of a four-channel parallel optical module by fully passive alignment, which are able to reduce manufacturing time and cost. Each of the components, such as vertical-cavity surface laser/positive-intrinsic negative-photodiode array, microlens array, fiber array, and receiver (RX)/transmitter (TX) integrated circuit, is integrated successfully using flip-chip bonding, die bonding, and passive alignment with a microscope. Clear eye diagrams are obtained by 25.78-Gb/s (for TX) and 25.7-Gb/s (for RX) nonreturn-to-zero signals of pseudorandom binary sequence with a pattern length of 231 to 1. The measured responsivity and minimum sensitivity of the RX are about 0.5  A/W and ≤−6.5  dBm at a bit error rate (BER) of 10−12, respectively. The optical power margin at a BER of 10−12 is 7.5 dB, and cross talk by the adjacent channel is ≤1  dB.

Modern advanced manufacturing and testing technologies allow the application of freeform optical elements. Compared with traditional spherical surfaces, an optical freeform surface has more degrees of freedom in optical design and provides substantially improved imaging performance. In freeform optics, the representation technique of a freeform surface has been a fundamental and key research topic in recent years. Moreover, it has a close relationship with other aspects of the design, manufacturing, testing, and application of optical freeform surfaces. Improvements in freeform surface representation techniques will make a significant contribution to the further development of freeform optics. We present a detailed review of the different types of optical freeform surface representation techniques and their applications and discuss their properties and differences. Additionally, we analyze the future trends of optical freeform surface representation techniques.

This guest editorial introduces the Special Section on Interferometry.

When ultrashort laser pulses are focused inside a glass at a high repetition rate, structural changes occur because of the heat accumulation from the train of laser pulses. This report describes phase measurement of the structural changes induced inside glass by phase-shifting digital holographic microscopy. Two-dimensional phase distribution across the structural change for static exposure is retrieved. By focusing femtosecond laser pulses at the interface between two glass substrates, melted materials can directly weld glass plates. Quantitative phase measurement of welded glass substrates revealed that the refractive index decreased in the laser-irradiated zone.

Depth-resolved wavenumber-scanning interferometry (DRWSI) is used to measure the contours or displacement fields inside a structure. One of the most promising phase retrieving algorithms of DRWSI is the eigenvalue decomposition and least-squares algorithm, because it can blindly evaluate the number of interferometric sources with a fine depth resolution. However, it is not robust to noise, in particular, salt noise or impulse noise. In order to significantly improve the immunity of DRWSI to noise, an updated eigenvalue decomposition algorithm is developed in this manuscript by employing the Spearman’s rank (SR) correlation function as a kernel function. Extreme experimental conditions under an environment with salt noise are designed to verify the performance of the new algorithm in DRWSI, called eigenvalue decomposition using SR correlation and Fourier transform. The results show that it is very effective for the phase reconstruction of DRWSI.

In optical interferometry, noise and distortions in the recovered wrapped phase are very common, and their nature is inherent to the quality and visibility of the fringe patterns that modulate the phase. Filtering these phase imperfections from the wrapped phase is not straightforward since we cannot directly apply filters without damaging their information, that is, its modulus 2π phase jumps. However, having a way to filter noise and distortions from the wrapped phase is desirable and very important because, at the end, the filtered phase is closer to the expected, errors are reduced, and the unwrapping task can be less complex. We propose a modulus 2π filtering method to remove noise and distortions directly from the wrapped phase without damaging its information. The presented method is a global filtering process, but we use the local frequencies from the wrapped phase in such a way that each pixel is tuned to its instant frequency.

In order to achieve the two-dimensional (2-D) velocity measurement of a flow field at extreme condition, a 2-D interferometric Rayleigh scattering (IRS) velocimetry using a multibeam probe laser was developed. The method using a multibeam probe laser can record the reference interference signal and the flow interference signal simultaneously. What is more, this method can solve the problem of signal overlap using the laser sheet detection method. The 2-D IRS measurement system was set up with a multibeam probe laser, aspherical lens collection optics, and a solid Fabry–Perot etalon. A multibeam probe laser with 0.5-mm intervals was formed by collimating a laser sheet passing through a cylindrical microlens arrays. The aspherical lens was used to enhance the intensity of the Rayleigh scattering signal. The 2-D velocity field results of a Mach 1.5 air flow were obtained. The velocity in the flow center is about 450  m/s. The reconstructed results fit well with the characteristic of flow, which indicate the validity of this technique.

We propose a technique to estimate the phase derivative in both x and y directions based on Riesz transform from a single speckle correlation fringes. The originality of this technique is to exploit Riesz transform for phase derivatives estimation, spatial modulation, speckle denoising, and measure of features similarity. Phase modulation process is realized by combining a digital spatial carrier and Riesz quadrature; speckle denoising is computed using Riesz wavelets transform, and the performance is evaluated by Riesz features SIMilarity. Before applying our method on real speckle correlation fringes, its performance is tested by numerical simulation.

The availability of more affordable, high resolution, infrared (IR) detector arrays has opened up the need for a larger set of IR materials. This has created a renewed interest in chalcogenide glasses as their transmission spans from 1 to 14  μm. The Naval Research Laboratory has shown that the chalcogenide glasses can be diffused to make gradient index (GRIN) lenses. GRIN materials are interesting because they can have unique dispersion properties that do not exist in the currently discovered homogeneous materials, a key parameter for optical designers. When fabricating a GRIN material, the ability to test and characterize its properties is essential. This has prompted the need and development of an IR Mach–Zehnder interferometer for relative index of refraction measurements of GRIN materials. These same, more affordable IR detector arrays have also allowed for what was commonly done in the visible to be developed in the IR. Now, two-dimensional change in refractive index can be measured without scanning, and the fringe density becomes less of an issue in the IR, allowing for larger changes in refractive index over a small area to be measured. An axial chalcogenide GRIN with Δn of 0.13 was measured at a wavelength of 3.39  μm.

In microbiology research, there is a strong need for next-generation imaging and sensing instrumentation that will enable minimally invasive and label-free investigation of soft, hydrated structures, such as in bacterial biofilms. White-light interferometry (WLI) can provide high-resolution images of surface topology without the use of fluorescent labels but is not typically used to image biofilms because there is insufficient refractive index contrast to induce reflection from the biofilm’s interface. The soft structure and water-like bulk properties of hydrated biofilms make them difficult to characterize in situ, especially in a nondestructive manner. We build on our prior description of static biofilm imaging and describe the design of a dynamic growth flow cell that enables monitoring of the thickness and topology of live biofilms over time using a WLI microscope. The microfluidic system is designed to grow biofilms in dynamic conditions and to create a reflective interface on the surface while minimizing disruption of fragile structures. The imaging cell was also designed to accommodate limitations imposed by the depth of focus of the microscope’s objective lens. Example images of live biofilm samples are shown to illustrate the ability of the flow cell and WLI instrument to (1) support bacterial growth and biofilm development, (2) image biofilm structure that reflects growth in flow conditions, and (3) monitor biofilm development over time nondestructively. In future work, the apparatus described here will enable surface metrology measurements (roughness, surface area, etc.) of biofilms and may be used to observe changes in biofilm structure in response to changes in environmental conditions (e.g., flow velocity, availability of nutrients, and presence of biocides). This development will open opportunities for the use of WLI in bioimaging.

Surface metrology must increasingly contend with submicron films, whose prevalence now extends to products well beyond semiconductor devices. For optical technologies such as coherence-scanning interferometry (CSI), transparent submicron films pose a dual challenge: film effects can distort the measured top surface topography map and metrology requirements may now include three-dimensional maps of film thickness. Yet CSI’s sensitivity also presents an opportunity: modeling film effects can extract surface and thickness information encoded in the distorted signal. Early model-based approaches entailed practical trade-offs between throughput and field of view and restricted the choice of objective magnification. However, more recent advances allow full-field surface films analysis using any objective, with sample-agnostic calibration and throughput comparable to film-free measurements. Beyond transparent films, model-based CSI provides correct topography for any combination of dissimilar materials with known visible-spectrum refractive indices. Results demonstrate single-nm self-consistency between topography and thickness maps.

With the goal of designing interferometers and interferometer sensors, e.g., LADARs with enhanced sensitivity, resolution, and phase estimation, states using quantum entanglement are discussed. These states include N00N states, plain M and M states (PMMSs), and linear combinations of M and M states (LCMMS). Closed form expressions for the optimal detection operators; visibility, a measure of the state’s robustness to loss and noise; a resolution measure; and phase estimate error, are provided in closed form. The optimal resolution for the maximum visibility and minimum phase error are found. For the visibility, comparisons between PMMSs, LCMMS, and N00N states are provided. For the minimum phase error, comparisons between LCMMS, PMMSs, N00N states, separate photon states (SPSs), the shot noise limit (SNL), and the Heisenberg limit (HL) are provided. A representative collection of computational results illustrating the superiority of LCMMS when compared to PMMSs and N00N states is given. It is found that for a resolution 12 times the classical result LCMMS has visibility 11 times that of N00N states and 4 times that of PMMSs. For the same case, the minimum phase error for LCMMS is 10.7 times smaller than that of PMMS and 29.7 times smaller than that of N00N states.

This paper concerns a numerical technique and associated modeling results for evaluating coherent phase transfer function (CPTF) degradation that is induced by wave aberrations of the imaging train of a laser Fizeau interferometer. The CPTF is defined as the ratio of the Fourier transform of the phase portion of the complex image of a phase object formed by the interferometer to the Fourier transform of the phase object itself. A unit step function is used as the object. The aberrated image is calculated using an optical design program. The traditional instrument transfer function is the amplitude portion of the CPTF. It has been found that the phase portion of the CPTF is needed for properly characterizing the frequency response of the system when a significant amount of coma is involved. Based on the results of the study, an aberration tolerance criterion is proposed for the optical design of a laser Fizeau interferometer.

Biological cells are usually transparent with a small refractive index gradient. Digital holographic interferometry can be used in the measurement of biological cells. We propose a dual-wavelength common-path digital holographic microscopy for the quantitative phase imaging of biological cells. In the proposed configuration, a parallel glass plate is inserted in the light path to create the lateral shearing, and two lasers with different wavelengths are used as the light source to form the dual-wavelength composite digital hologram. The information of biological cells for different wavelengths is separated and extracted in the Fourier domain of the hologram, and then combined to a shorter wavelength in the measurement process. This method could improve the system’s temporal stability and reduce speckle noises simultaneously. Mouse osteoblastic cells and peony pollens are measured to show the feasibility of this method.

Functional surfaces with a rising degree of complexity are becoming increasingly important for modern industrial products. It is common knowledge that one cannot produce surfaces better than it is possible to measure them. Consequently, the demand for their effective and precise measurement has increased to the same extent as their production capabilities have grown. Important classes of optical functional surfaces are aspheres and freeforms. Both types of surfaces have become essential parts of modern optical systems such as laser focusing heads, sensors, telescopes, glasses, head-mounted displays, cameras, lithography steppers, and pickup heads. For all of them, the systematic quality control in the process of their fabrication is essential. We review the challenges of asphere and freeform testing and how available metrology systems cope with it. A special focus is on tilted wave interferometry and how it compares to other methods.

Surface topography measurement for metal additive manufacturing (AM) is a challenging task for contact and noncontact methods. We present an experimental investigation of the use of coherence scanning interferometry (CSI) for measuring AM surfaces. Our approach takes advantage of recent technical enhancements in CSI, including high dynamic range for light level and adjustable data acquisition rates for noise reduction. The investigation covers several typical metal AM surfaces made from different materials and AM processes. Recommendations for measurement optimization balance three aspects: data coverage, measurement area, and measurement time. This study also presents insight into areas of interest for future rigorous examination, such as measuring noise and further development of guidelines for measuring metal AM surfaces.

A method is proposed to automatically evaluate the focal planes of spherical particles. This method compares the correlation coefficients of multiple reconstructed planes relative to a reference plane. The particles are located where a minimum correlation is found, and reconstructions are made using an angular spectrum propagator. The Hough transform is employed to segment the hologram, thereby enabling the detection of circular shapes, such as Airy patterns, and the edges of the particles themselves. The autofocus is improved by creating a correlation matrix using an iterative process, which reduces the computational cost of the particle display processes in their respective focal planes. A theoretical model was studied to estimate the longitudinal and transverse magnifications of the focused particles caused by the influence of aberrations in the reconstruction of digital holograms due to the spherical reference wave used. Experimentally, laser light was used to illuminate 5-_μm latex particles, which was recorded by a CCD camera with a 9.9-_μm pixel size. The reconstructions measured an average particle radius of 71.3±16.3  _μm in the average focal plane, which was estimated to be 60.65±0.22  mm from the hologram where the magnifications were considered.

Laser triangulation and photometric stereo are commonly used three-dimensional (3-D) reconstruction methods, but they bear limitations in an underwater environment. One important reason is due to the refraction occurring at the interface (usually glass) of the underwater housing. The image formation process does not follow the commonly used pinhole camera model, and the image captured by the camera is a refracted projection of the object. We introduce a flat refraction model to describe the geometric relation between the refracted image and the real object. The model parameters were estimated in a calibration step with a standard chessboard. The proposed geometric relation is used for rebuilding underwater 3-D shapes in laser triangulation and photometric stereo. The experimental results indicate that our method can effectively correct the distortion in underwater 3-D reconstruction.

A key requirement to put terahertz (THz) imaging systems into applications is high resolution. Based on a self-developed THz quantum cascade laser (QCL), we demonstrate a THz inline digital holography imaging system with high lateral resolution. In our case, the lateral resolution of this holography imaging system is pushed to about 70  μm, which is close to the intrinsic resolution limit of this system. To the best of our knowledge, this is much smaller than what has been reported up to now. This is attributed to a series of improvements, such as shortening the QCL wavelength, increasing Nx and Ny by the synthetic aperture method, smoothing the source beam profile, and diminishing vibration due to the cryorefrigeration device. This kind of holography system with a resolution smaller than 100  μm opens the door for many imaging experiments. It will turn the THz imaging systems into applications.

Color constancy is the feature of the human vision system (HVS) that ensures the relative constancy of the perceived color of objects under varying illumination conditions. The Retinex theory of machine vision systems is based on the HVS. Among Retinex algorithms, the physics-based algorithms are efficient; however, they generally do not satisfy the local characteristics of the original Retinex theory because they eliminate global illumination from their optimization. We apply the sparse source separation technique to the Retinex theory to present a physics-based algorithm that satisfies the locality characteristic of the original Retinex theory. Previous Retinex algorithms have limited use in image enhancement because the total variation Retinex results in an overly enhanced image and the sparse source separation Retinex cannot completely restore the original image. In contrast, our proposed method preserves the image edge and can very nearly replicate the original image without any special operation.

We present an experimental multichannel polarization imaging system based on a polarization diffraction grating (PDG). The PDG simultaneously creates six diffraction orders, each having a different polarization state. For this proof-of-concept experiment, we encoded the PDG onto a parallel-aligned liquid-crystal spatial light modulator (LCSLM), using a double-pass optical architecture. We use a birefringent object where the top is covered with a retardation film and the bottom is isotropic. We show experimental results, where we simultaneously produce six images of this birefringent object, each with a different polarization state. The proposed technique can produce a single-shot imaging polarimeter.

Transmission map estimation is one of the most important parts of single image dehazing, which is known as an under-constraint problem. Various assumptions, some have been summarized as priors or models, are proposed to solve this problem. However, many previous methods cannot honestly reflect their theory in the results, due to not considering all of the employed assumptions simultaneously. Meanwhile, most other methods avoiding this defect are with inappropriate assumptions. We try to solve this problem by proposing a method that simultaneously considers the dark channel prior and the piecewise smoothness assumption. It is achieved by minimizing an energy function based on all of the employed assumptions. To maintain a reasonable run time, the minimization problem is mapped into a graph cut problem with a specific graph build strategy. The method is compared with state-of-the-art methods on both synthetic and natural images. Experimental results show that the proposal is promising for haze removal quality.

The integration of an edge-preserving filtering technique in the classification of a hyperspectral image (HSI) has been proven effective in enhancing classification performance. This paper proposes an ensemble strategy for HSI classification using an edge-preserving filter along with a deep learning model and edge detection. First, an adaptive guided filter is applied to the original HSI to reduce the noise in degraded images and to extract powerful spectral–spatial features. Second, the extracted features are fed as input to a stacked sparse autoencoder to adaptively exploit more invariant and deep feature representations; then, a random forest classifier is applied to fine-tune the entire pretrained network and determine the classification output. Third, a Prewitt compass operator is further performed on the HSI to extract the edges of the first principal component after dimension reduction. Moreover, the regional growth rule is applied to the resulting edge logical image to determine the local region for each unlabeled pixel. Finally, the categories of the corresponding neighborhood samples are determined in the original classification map; then, the major voting mechanism is implemented to generate the final output. Extensive experiments proved that the proposed method achieves competitive performance compared with several traditional approaches.

We present the design concept and initial simulations for a polychromatic full-field fan-beam x-ray fluorescence computed tomography (XFCT) device with pinhole collimators and linear-array photon counting detectors. The phantom is irradiated by a fan-beam polychromatic x-ray source filtered by copper. Fluorescent photons are stimulated and then collected by two linear-array photon counting detectors with pinhole collimators. The Compton scatter correction and the attenuation correction are applied in the data processing, and the maximum-likelihood expectation maximization algorithm is applied for the image reconstruction of XFCT. The physical modeling of the XFCT imaging system was described, and a set of rapid Monte Carlo simulations was carried out to examine the feasibility and sensitivity of the XFCT system. Different concentrations of gadolinium (Gd) and gold (Au) solutions were used as contrast agents in simulations. Results show that 0.04% of Gd and 0.065% of Au can be well reconstructed with the full scan time set at 6 min. Compared with using the XFCT system with a pencil-beam source or a single-pixel detector, using a full-field fan-beam XFCT device with linear-array detectors results in significant scanning time reduction and may satisfy requirements of rapid imaging, such as in vivo imaging experiments.

This paper proposes an approach for the full-field surface reconstruction of three-dimensional (3-D) objects. The method uses a fringe projection profilometry system to obtain a 3-D point cloud with a multiview and proposes an objective function that involves all of the parameters of rotation and translation. The feature points of the surface of the object are extracted to determine the initial value of the objective function, which is then optimized to achieve accurate registration of the point clouds. The experimental results demonstrate that the proposed method is accurate and effective.

An asymmetric technique for optical image encryption is proposed using Kolmogorov phase screens (KPSs) and equal modulus decomposition (EMD). The KPSs are generated using the power spectral density of Kolmogorov turbulence. The input image is first randomized and then Fresnel propagated with distance d. Further, the output in the Fresnel domain is modulated with a random phase mask, and the gyrator transform (GT) of the modulated image is obtained with an angle α. The EMD is operated on the GT spectrum to get the complex images, Z1 and Z2. Among these, Z2 is reserved as a private key for decryption and Z1 is propagated through a medium consisting of four KPSs, located at specified distances, to get the final encrypted image. The proposed technique provides a large set of security keys and is robust against various potential attacks. Numerical simulation results validate the effectiveness and security of the proposed technique.

The infrared (IR) detection methods are more threatening than other detection methods, such as radar or sonar detections, because the object detected by the IR sensor cannot recognize the situation being detected. Recently, many studies for actively reducing IR signal contrast by controlling the surface temperature of the object are being conducted. We propose an active IR stealth technique using repulsive particle swarm optimization algorithm for IR low-observability. We present a pixel-wise application method of an active IR stealth algorithm to synchronize those signals from the object and the background in the midwavelength IR region. The proposed pixel-wise application method of a statistical active IR stealth technique, which determines the IR stealth temperature for the object surface, is proved to be an effective method for various known background images by reducing the average contrast radiant intensity between the object and background up to 96%.

This paper describes a way to synthesize a larger coherent aperture from smaller apertures combined with motion, when only intensities are measured. It relies on collecting intensity patterns in two planes for each aperture, for example, the aperture plane and an image plane, and using a phase-retrieval algorithm to reconstruct the optical field in the aperture plane. As the sensor moves forward, a larger two-dimensional aperture is synthesized, allowing a much finer resolution image to be reconstructed. An algorithm to correct for the relative pointing (tip and tilt phases) and piston errors between different apertures and at different times is needed to phase up the synthetic aperture. Results of simulations, including the effects of speckle, are shown, and practical considerations are evaluated.

Object detections are critical technologies for the safety of pedestrians and drivers in autonomous vehicles. Above all, occluded pedestrian detection is still a challenging topic. We propose a new detection scheme for occluded pedestrian detection by means of lidar–radar sensor fusion. In the proposed method, the lidar and radar regions of interest (RoIs) have been selected based on the respective sensor measurement. Occluded depth is a new means to determine whether an occluded target exists or not. The occluded depth is a region projected out by expanding the longitudinal distance with maintaining the angle formed by the outermost two end points of the lidar RoI. The occlusion RoI is the overlapped region made by superimposing the radar RoI and the occluded depth. The object within the occlusion RoI is detected by the radar measurement information and the occluded object is estimated as a pedestrian based on human Doppler distribution. Additionally, various experiments are performed in detecting a partially occluded pedestrian in outdoor as well as indoor environments. According to experimental results, the proposed sensor fusion scheme has much better detection performance compared to the case without our proposed method.

Recent advances in photolithography allowed the fabrication of high-quality wire grid polarizers for the visible and near-infrared regimes. In turn, micropolarizer arrays (MPAs) based on wire grid polarizers have been developed and used to construct compact, versatile imaging polarimeters. However, the contrast and throughput of these polarimeters are significantly worse than one might expect based on the performance of large area wire grid polarizers or MPAs, alone. We investigate the parameters that affect the performance of wire grid polarizers and MPAs, using high-resolution two-dimensional and three-dimensional (3-D) finite-difference time-domain simulations. We pay special attention to numerical errors and other challenges that arise in models of these and other subwavelength optical devices. Our tests show that simulations of these structures in the visible and near-IR begin to converge numerically when the mesh size is smaller than ∼4  nm. The performance of wire grid polarizers is very sensitive to the shape, spacing, and conductivity of the metal wires. Using 3-D simulations of micropolarizer “superpixels,” we directly study the cross talk due to diffraction at the edges of each micropolarizer, which decreases the contrast of MPAs to ∼200∶1.

We propose an optical security technique for image encryption using triple random-phase encoding (TRPE). In the encryption process, the original image is first double-random-phase encrypted. The obtained function is then multiplied by a third random-phase key in the output plane, to enhance the security level of the encryption process. This method reduces the vulnerability to certain attacks observed when using the conventional double random-phase encoding (DRPE). To provide the security enhancement of the proposed TRPE method, three attack cases are discussed: chosen-plaintext attacks, known-plaintext attacks, and chosen-ciphertext attacks. Numerical results are presented to demonstrate feasibility and effectiveness of the proposed method. Compared with conventional DRPE, the proposed encryption method can provide an effective alternative and has enhanced security features against the aforementioned attacks.

We present a Wiener Teager–Kaiser approach for phase derivative estimation from a single speckle correlation fringe. In principle, the Teager–Kaiser operator estimates the energy of the fringe pattern and extracts its phase derivatives using an energy separation algorithm. However, in the estimation of the energy, this operator presents a computation error mainly due to a high frequency component. In this work, we addressed this error in mean square error sense by applying the Wiener filter on the operator prior to phase derivative computation. The performance of our proposed method on simulated and real fringe improves significantly the accuracy of the Teager–Kaiser operator.

A combination method using numerical orthogonal polynomials and overlapping averaging is presented for freeform surface estimation. The whole effective aperture of freeform surface is decomposed into multiple overlapped subapertures. The corresponding local surface over each subaperture is reconstructed by numerical orthogonal polynomials suited for general shaped aperture. Then, the whole freeform surface is obtained by overlapping averaging approach from multiple local surfaces. The performance of the presented combination method is discussed and demonstrated by examples and further verified by an experiment. The results show that the combination method could reach nanometer accuracy. Meanwhile, the local deformations of freeform surface can be characterized finely.

This paper aims to establish confidence intervals of the quantification of brightness enhancement effects resulting from the use of pulsing bright light. It is found that the methods used so far may yield significant bias in the published results, overestimating or underestimating the enhancement effect. The authors propose to use a linear algebra method called the total least squares. Upon an example dataset, it is shown that this method does not yield biased results. The statistical significance of the results is also computed. It is concluded over an observation set that the currently used linear algebra methods present many patterns of noise sensitivity. Changing algorithm details leads to inconsistent results. It is thus recommended to use the method with the lowest noise sensitivity. Moreover, it is shown that this method also permits one to obtain an estimate of the confidence interval. This paper neither aims to publish results about a particular experiment nor to draw any particular conclusion about existence or nonexistence of the brightness enhancement effect.

A low-cost method of detecting lasers based on detecting coherence properties of received light is presented. The method uses an unbalanced Mach–Zehnder interferometer with a modulating piezo-mounted mirror in one arm to discriminate against incoherent background light and identify the presence of laser radiation at the nW level against much brighter backgrounds. The wavelength of the coherent input can be determined by comparing the intensities of the modulation frequency harmonics.

We proposed a plasmonic lens (PL) to achieve far-field focusing performance under a 532 nm wavelength. This lens is produced by perforating multiple concentric rings on the gold layer, and the radius and width of the ring can be designed flexibly. Its machining process has adopted the electron beam lithography and lift-off technology. The lenses are designed within a small range of phase delay and the small-sized slits of the PL are replaced with a larger-sized one, which has almost no impact on either the full width at half maximum (FWHM) or the focal length of the PL. With an increase of the slits width, a further enhancement of the spot intensity has been observed. Meanwhile, both the numerical relationships of simulation and experiment are consistent. This method can reduce the difficulty of PL processing, by which the PL with large-sized slits has almost the same performance as that of small-sized ones. The method is of great significance to the low-cost production of modern optoelectronic devices, and it would promote potential applications in optical interconnection devices, controllable focusing, and superresolution imaging.

We analyze the diffraction field when changes in the curvature function of the boundary condition are implemented. The study is performed using differential geometry models with a curvature function displaying local behavior. Depending on the sign of curvature, we classify the diffraction field as elliptic, hyperbolic, or parabolic. In particular, it is shown that the optical field is organized around the parabolic regions, which correspond to focusing regions. The model is experimentally corroborated by applying a coordinate transformation to the transmittance of a zone plate. The reason to use this transmittance comes from the fact that its diffraction field displays multiple foci allowing identification, description, and control of bifurcations and morphogenesis effects, which are studied using the curvature function.

A simultaneous phase-shifting Twyman interferometer with a point source array is proposed. We use a point source in combination with a grating and select the four (±1,±1) diffraction orders to generate a point source array. With this configuration, we can acquire four independent Twyman interferometers. Adjust the offset amounts of each point source to introduce different phase shifts in the interferograms, and realize dynamic measurement. We introduce 0, π/2, π, and 3π/2 in the interferograms. The initial phase will be exactly retrieved by employing the four-bucket algorithm. The experimental results show the feasibility and precision of the interferometer.

A beam pen lithography system is proposed for ultraviolet (UV) patterning of complicated patterns in a maskless manner. The system consists of a single UV light-emitting diode (LED), a double-sided microlens/spatial-filter array (MLSFA), and a motorized x-y stage. The double-sided MLSFA contains a pinhole array sandwiched by two microlens arrays. The microlens arrays are fabricated by an excimer laser micromachining system to achieve accurate and optimized lens profiles obtained from optical simulation. Techniques have been developed in the fabrication processes to guarantee good alignment between the pinhole array and the two microlens arrays. Finally, arrayed UV spots are formed with feature size around 4 to 5  μm. Through mechanical movement of stage and intensity control on the UV LED, a number of complicated patterns are experimentally demonstrated using this type of beam pen lithography. It offers a simple, inexpensive, and portable choice on maskless UV patterning with great potential for future applications.

One of the key issues in conventional wide-angle lenses is the well-known cosine-fourth power law problem causing the illumination falloff at its image space. This paper explores methods of improving illumination in the image space in panomorph lenses. By tracing skew rays within the defined field of view and pupil diameter, we obtained the actual position of the three-dimensional pupil model of the entrance pupil (EP) and exit pupil (XP). Based on the law of irradiance transport conservation, the relation between the area of the EP projection and illumination in the image space is derived to investigate the factors affecting the illumination on the peripheral field. A panomorph lens has been optimized as an example by providing a self-defined operation in the optimization process. The characteristic of the EP and XP in panomorph lenses is qualitatively analyzed. Compared with the conventional design method, the proposed design strategy can enhance the illumination with and without polarized light based on qualitatively evaluating the area of projected EP. It is demonstrated that this method enables the enhancement of the illumination without additional film coating.

We theoretically simulate the performance of ultranarrow emitters for the first time to achieve record high coverage for the International Telecommunication Union Radiocommunication Sector BT.2100 (Rec.2100) and National Television System Committee (NTSC) color gamut. Our results, employing more than 130-m parameter sets, include the investigation into peak emission wavelength and full width at half maximum (FWHM) values for three primaries that show ultranarrow emitters, i.e., nanoplatelets are potentially promising materials to fully cover the Rec.2100 color gamut. Using ultranarrow emitters having FWHM as low as 6 nm can provide the ability to attain 99.7% coverage area of the Rec.2100 color gamut as well as increasing the NTSC triangle to 133.7% with full coverage. The parameter set that provides possibility to fully reach Rec.2100 also has been shown to match with D65 white light by making use of the correct combination of those three primaries. Furthermore, we investigate the effect of the fourth color component on the CIE 1931 color space without sacrificing the achieved coverage percentages. The investigation into the fourth color component, cyan, is shown for the first time to enhance the Rec.2100 gamut area to 127.7% with 99.9% coverage. The fourth color component also provides an NTSC coverage ratio of 171.5%. The investigation into the potential of emitters with ultranarrow emission bandwidth holds great promise for future display applications.

Surface control and phase matching of large laser conversion optics are urgent requirements and huge challenges in high-power solid-state laser facilities. A self-adaptive, nanocompensating mounting configuration of a large aperture potassium dihydrogen phosphate (KDP) frequency doubler is proposed based on a lever-type surface correction mechanism. A mechanical, numerical, and optical model is developed and employed to evaluate comprehensive performance of this mounting method. The results validate the method’s advantages of surface adjustment and phase matching improvement. In addition, the optimal value of the modulation force is figured out through a series of simulations and calculations.

High-power laser facilities, such as Laser MegaJoule, are currently being operated for inertial confinement fusion experiments. Emission of volatile organic compounds (VOC) and moreover semivolatile organic compounds (SVOCs) from seals in laser environment is of tremendous importance for the optics lifetime and laser performance. That is why all the seals were screening in the same conditions: 48 h at 30°C and three successive cycle of 1.5 h at 50°C. This paper focuses on the qualification test performed on three seals: two ethylene propylene diene monomer (EPDM) and one fluoropolymer (FPM). It is shown that the molded and the extruded EPDM do not outgas the same amount neither the same molecules whereas EPDM and FPM outgas nearly the same level of phthalates.

We propose a 3-dB mode insensitive power splitter (MIPS) capable of broadcasting and combining optical signals. It is fabricated with two identical few-mode fibers (FMFs) by a heating and pulling technique. The mode-dependent power transfer characteristic as a function of pulling length is investigated. For exploiting its application, we experimentally demonstrate both FMF-based transmissive and reflective star couplers consisting of multiple 3-dB mode insensitive power splitters, which perform broadcasting and routing signals in few-mode optical fiber networks such as mode-division multiplexing (MDM) local area networks using star topology. For experimental demonstration, optical on-off keying signals at 10  Gb/s carried on three spatial modes are successfully processed with open and clear eye diagrams. Measured bit error ratio results show reasonable power penalties. It is found that a reflective star coupler in MDM networks can reduce half of the total amount of required fibers comparing to that of a transmissive star coupler. This MIPS is more efficient, more reliable, more flexible, and more cost-effective for future expansion and application in few-mode optical fiber networks.

A PANDA polarization-maintaining few-mode ring-core fiber (PM-FM-RCF) structure with two air holes around the ring core is proposed. The relative mode multiplicity factor (RMMF) is defined to evaluate the spatial efficiency of the designed PM-FM-RCF. The performance analysis and comparison of the proposed PANDA PM-FM-RCFs considering three different types of step-index profiles are detailed. Through modal characteristic analysis and numerical simulation, the PM-FM-RCF with a lower refractive index difference ( Δnoi=1.5%) between the ring core and the inner central circle can support up to 16 polarization modes with large RMMF at C-band, which shows the optimum modal properties compared with the PM-FM-RCF with higher Δnoi. All the supported polarization modes are effectively separated from their adjacent polarization modes with effective refractive index differences ( Δneff) larger than 10−4, which also show relatively small chromatic dispersion ( −20 to 25  ps/nm/km), low attenuation ( <1.4  dB/km), and small bending radius ( ∼8  mm) over the C-band. The designed PM-FM-RCF can be compatible with standard single-mode fibers and applied in multiple-input multiple-output-free spatial division multiplexing optical networks for short-reach optical interconnection.

In optical communication, the behavior of the ultrashort pulses of optical solitons can be described through nonlinear Schrodinger equation. This partial differential equation is widely used to contemplate a number of physically important phenomena, including optical shock waves, laser and plasma physics, quantum mechanics, elastic media, etc. The exact analytical solution of ( 1+n)-dimensional higher order nonlinear Schrodinger equation by He’s variational iteration method has been presented. Our proposed solutions are very helpful in studying the solitary wave phenomena and ensure rapid convergent series and avoid round off errors. Different examples with graphical representations have been given to justify the capability of the method.

We had proved that a kind of graphene nanomaterial “carboxyl-functionalized graphene oxide (GO-COOH)” possessed nonlinear saturable absorber (SA) property. The modulation depth of a GO-COOH water solution was measured as ∼8%. Moreover, a GO-COOH-based SA device was made and applied in an erbium-doped fiber laser. In this fiber laser, Q-switching pulses and mode-locked pulses were both obtained. With an increase in the pump power, the pulse width of Q-switching pulses decreased from 9.05 to 2.49  μs. The mode-locked pulse width was 844 fs, and the corresponding spectral bandwidth was 3.64 nm. Moreover, polarization adjusting or control was not needed during the whole process of mode locking. It illustrated that the proposed fiber laser incorporating GO-COOH could endure bigger intracavity birefringence. Our results indicated that the GO-COOH nanomaterial was a promising SA for generating high-performance pulse lasers.

Transverse mode instability (TMI) limits power scaling of fiber lasers. A semianalytical model is established to calculate the TMI threshold in high-power fiber laser systems of the multi-kW-class. A linear inner-cladding fiber can mitigate the TMI effect by smoothing the heat profile and decreasing the nonlinear coupling coefficient along the fiber. We investigate strong pump absorption of a linear inner-cladding fiber, which leads to shorter fiber length. Utilizing a 915-nm copumping scheme, the linear inner-cladding fiber can realize 10-kW output power in single-mode regime theoretically.

The simultaneous wavelength exchange and 2R regeneration between two degraded optical time division multiplexing (OTDM) signals is experimentally demonstrated. By exploiting the self-phase modulation effect and offset filtering in a single highly nonlinear fiber (HNLF), it achieves 2R optical regeneration. Meanwhile, it realizes wavelength exchange using a bidirectional configuration. Optical wavelength exchange and 2R of 20- and 50-Gbit/s OOK signals at 1557.3 and 1552.3 nm is implemented with a power penalty of ∼0.5  dB at a bit-error rate of 10−9. The results show that the crosstalk is negligible for both channels in opposite directions. This bidirectional configuration exhibits excellent performance for simultaneous wavelength exchange and 2R regeneration between two degraded OTDM signals with different wavelengths.

Double-layer optical fiber coating is performed using a melt polymer satisfying a Sisko fluid model in a pressure-type die. For this purpose, wet-on-wet coating process is applied. The assumption of fully developed flow of a Sisko fluid model, two-layer liquid flows of an immiscible fluid is modeled in an annular die of length L, where the bare glass fiber is dragged at a higher speed. The nonlinear governing equations are modeled and then solved by utilizing optimal homotopy asymptotic method (OHAM). The convergence of series solution is established. The convergence of OHAM is also verified by the Adomian decomposition method. In addition, the shear-thinning and shear-thickening characteristics of the non-Newtonian Sisko fluid are examined, and a comparison is made with the Newtonian fluid. At the end, the present work is also compared with the experimental results already available in the literature by taking the non-Newtonian parameter that tends to zero.

The modeling of the simultaneous propagation of high-power and bidirectional data along the same 10-km-long single-mode fiber is discussed. The intense signal carries the energy needed to supply an instrument in the context of cabled submarine observatories. The considered mathematical description takes into account the fiber’s nonlinear behavior in terms of Raman and Brillouin scattering to describe spectral propagation in the static regime. By testing our model against measurements, its validity is evaluated. Preliminary results are promising and reveal the path to follow for its improvement.

We investigate a (2+1)×1 side-pumping combiner numerically and experimentally for high-power fiber laser based on tandem pumping for the first time. The influence of taper ratio and launch mode on the 1018-nm pump coupling efficiency and the leakage power into the coating of the signal fiber (LPC) is analyzed numerically. A side-pumping combiner is developed successfully by tapered-fused splicing technique based on the numerical analysis, consisting of two pump fibers ( 220/242  μm, NA=0.22) and a signal fiber ( 40/400  μm, NA=0.06/0.46). The total 1018-nm pump efficiency of the combiner is 98.1%, and the signal light insertion loss is <3%. The results show that, compared with laser diodes pumping, the combiner appears to have a better LPC performance and power handling capability when using 1018-nm fiber as the pump light. Meanwhile, an all-fiber MOPA laser based on tandem pumping with 1080-nm output of 2533 W and the slope efficiency of 82.8% is achieved based on the home-made combiner.

A physical encryption scheme for orthogonal frequency-division multiplexing (OFDM) visible light communication (VLC) systems using chaotic discrete cosine transform (DCT) is proposed. In the scheme, the row of the DCT matrix is permutated by a scrambling sequence generated by a three-dimensional (3-D) Arnold chaos map. Furthermore, two scrambling sequences, which are also generated from a 3-D Arnold map, are employed to encrypt the real and imaginary parts of the transmitted OFDM signal before the chaotic DCT operation. The proposed scheme enhances the physical layer security and improves the bit error rate (BER) performance for OFDM-based VLC. The simulation results prove the efficiency of the proposed encryption method. The experimental results show that the proposed security scheme not only protects image data from eavesdroppers but also keeps the good BER and peak-to-average power ratio performances for image-based OFDM-VLC systems.

With the increase in varieties of services in network, time-sensitive services (TSSs) appear and bring forward an impending need for delay performance. Ultralow-latency communication has become one of the important development goals for many scenarios in the coming 5G era (e.g., robotics and driverless cars). However, the conventional methods, which decrease delay by promoting the available resources and the network transmission speed, have limited effect; a new breakthrough for ultralow-latency communication is necessary. We propose a de-optical-line-terminal (De-OLT) hybrid access–aggregation optical network (DAON) for TSS based on software-defined networking (SDN) orchestration. In this network, low-latency all-optical communication based on optical burst switching can be achieved by removing OLT. For supporting this network and guaranteeing the quality of service for TSSs, we design SDN-driven control method and service provision method. Numerical results demonstrate the proposed DAON promotes network service efficiency and avoids traffic congestion.

In an attempt to meet the goal of distributing millimeter-wave (mm-wave) signals, recent years have witnessed significant relevance being given to combining radio frequency with optical fiber technologies. The future of radio-over-free-space-optics technology aims to build a universal platform for distributing millimeter waves for wireless local area networks without using expensive optical fibers. This work is focused on simultaneous transmission of four independent OFDM-based channels, each carrying 20 Gbps to 40 GHz data, by mode-division multiplexing of Laguerre–Gaussian mode with vortex lens and Hermite–Gaussian mode to realize a total transmission of 80 Gbps to 160 GHz data over 50-km free-space optical link. Moreover, the performance of the proposed system is also evaluated under the influence of various atmospheric turbulences, such as light fog, thin fog, and thick fog.

We demonstrated strain sensing of a microfiber with a microarched transition region, which was fabricated by flame heated tapering. Due to multimode interference of different propagation modes of microfiber, two main transmission dips were observed at 1215.0 and 1469.8 nm. Enhanced by the microarched transition region, the depth of the dip was up to 19 dB at 1215.0 nm. The position of the dip red-shifted while the axial strain changed from 0 to 1166.2  μϵ. The axial strain sensitivity was up to 56.6  pm/μϵ, which was one order of magnitude higher than that of the traditional optical strain sensor based on microfiber or fiber Bragg grating. The linear correlation coefficient was 98.21%. This kind of microfiber with a microarched transition region can be widely used in various physical, chemical, and biological sensing and detection fields.

An investigation of laser cleaning of the oxide layers on the surface of the hot-rolled steel sheets was presented both experimentally and theoretically. By using a ∼100-ns pulsed laser, the optimized experimental parameters, such as the scanning speed, spatial overlap between two adjacent scans, and laser power density, were studied to obtain efficient and high-quality cleaning and avoid the formation of new oxide layer due to the heating by the laser. The temperature elevation and ablation depth were simulated and the data agree well with the experimental results. The change of composition of the surface layer, the surface roughness, and hardness due to laser ablation were also measured.

We report on a compact conductively cooled high-repetition-rate nanosecond Nd:YAG laser. The oscillator was an laser diode side-pumped electro-optical (EO) Q-switched Nd:YAG rod laser adopting unstable cavity with a variable reflectivity mirror. A pulse train of 142 mJ with duration of 10 ns, repetition rate of 80 Hz at 1064 nm has been achieved. Maximum pulse energy was obtained at the pump energy of 1380 mJ, corresponding to the optical–optical conversion efficiency of 10.3%. The peak power was deduced to be 14.2 MW. The near-field pattern demonstrated a nearly super Gaussian flat top profile. To our knowledge, this is the highest repetition rate operation for a conductively cooled EO Q-switched Nd:YAG rod laser.

We analyze the channel properties of a nonline-of-sight (NLOS) ceiling-to-device and device-to-device visible light communication systems by considering various receivers' orientation and variable fields of view (FOVs). Analyses based on the recursive indoor channel model show that for a particular transmitter configuration, the pure NLOS path can offer higher 3-dB channel bandwidth (up to 14 MHz) compared with the link with LOS and NLOS components. We also show how the receiver rotation (orientation) influences the probability of receiving signals via the NLOS path compared with the LOS and NLOS paths. Moreover, based on the experimental campaign, we demonstrate that shadowing observed at the receiver due to people’s movement results in decreased received power level (up to 1.8 dB), thus resulting in reshaping of the probability density function of received power.

An image encryption technique has been proposed using deoxyribonucleic acid (DNA) operations and chaotic map in this scheme. First, initial conditions of row encryption and column encryption are calculated. Then, a two-dimensional sine iterative chaotic map with infinite collapse (ICMIC) modulation map (2D-SIMM) is adopted to produce chaotic sequences. Extended exclusive OR (XOR) is executed to enhance security. A mask matrix is produced by 2D-SIMM. It performs XOR operation with the DNA-encoded matrix. Finally, the revised DNA-encoded matrix is performed two-by-two DNA complementary rules and executed DNA decoding to obtain the cipher image. Experiment results prove that the proposed scheme is secure enough and can resist various attacks.

An admirable and efficient Nd:YVO4 laser at 1085 nm is demonstrated with a compact 35 mm plano-plano cavity. A chosen narrow bandpass filter with high-transmittance (HT) coating at 1064 nm ( T=96%) and optimized part-reflection (PR) coating at 1085 nm ( T=15%) is used as the output coupler. In the continuous-wave (CW) regime, the maximum output power reaches 3110 mW at the pump power of 11.41 W. Based on a Cr:YAG crystal with initial-transmittance of 91%, the first passively Q-switched Nd:YVO4 laser at 1085 nm is achieved. When the pump power is changed from the threshold of 4.50 to 6.08 W, the dual-wavelength lines at 1064 and 1085 nm are generated simultaneously. However, at the pump power of above 6.08 W, the single-wavelength line at 1085 nm is achieved. The largest output power, the highest peak power, and the narrowest pulse width are 1615 mW, 878 W and 26.2 ns, respectively.

We study Cr/Sc-based multilayer mirrors designed to work in the water window range using hard and soft x-ray reflectivity as well as x-ray fluorescence enhanced by standing waves. Samples differ by the elemental composition of the stack, the thickness of each layer, and the order of deposition. This paper mainly consists of two parts. In the first part, the optical performances of different Cr/Sc-based multilayers are reported, and in the second part, we extend further the characterization of the structural parameters of the multilayers, which can be extracted by comparing the experimental data with simulations. The methodology is detailed in the case of Cr/B4C/Sc sample for which a three-layer model is used. Structural parameters determined by fitting reflectivity curve are then introduced as fixed parameters to plot the x-ray standing wave curve, to compare with the experiment, and confirm the determined structure of the stack.

The nonlinear propagation characteristics of multiwavelength optical signals in silicon waveguides are investigated for all-optical regeneration. Our experiment and simulation show that the multiwavelength regenerators based on silicon waveguides can be developed with a clock-pump scheme by properly setting the signal and pump power levels, ensuring that the Q-factor degradation induced by the Kerr nonlinear cross talk of the input signals is <1.0  dB and the clock-pump power is no more than the saturated input level related to the nonlinear loss. A three-wavelength regeneration experiment based on the clock-pump four-wave-mixing scheme was demonstrated in the silicon nanowire waveguide, and both the extinction ratio and Q-factor are improved by >3.0  dB for 12.5  Gbit/s on–off keying signals. The feasibility of an eight-wavelength regeneration with the clock pump is also verified by simulation.

Replicated lightweight composite mirrors are gaining increasing attention for space applications due to potential weight savings, cost reductions, and faster manufacturing times over traditional glass mirrors. However, dimensional stability remains one of the most critical issues for these resin-based high precision optics. Composite mirrors with a surface figure error (SFE) of 0.03λ were manufactured using a UV-cured replicated layer (RL). The effects of cure state and environmental exposure on the stability of the mirror were evaluated by measuring changes in SFE using interferometric imaging. Higher stability was observed for the replicated epoxy mirror that underwent a secondary cure, after the initial UV cure. SFE for these mirrors increased by only 5-nm (RMS) when exposed to 100% RH for 12 days. The mirrors that were UV cured alone under identical exposure conditions increased in SFE by over 500% of that value. The increased hygroscopic stability is consistent with a reduced amount of unreacted polar epoxide groups that can hydrogen bond with absorbed moisture diffusing into the network. In addition, the mirror with the secondary cure remained stable up to 125°C while the mirror with a lower degree of cure exhibited warpage under identical thermal condition due to additional cure shrinkage.

An interrogation system for large-capacity fiber-distributed sensing was proposed and implemented. The system is based on the time- and wavelength-division multiplexed method and ultraweak fiber Bragg grating (FBG) sensor arrays. A narrow-linewidth tunable light source consisting of a distributed feedback laser array is used, which can be directly modulated into nanosecond pulses with a wide tuning range. Theoretically, the system can achieve a demodulation of >10,000 gratings and the FBG can be placed closely. An experimental system was implemented and the performance of the established system was tested, including its wavelength demodulation, time-division multiplexing, and temperature-sensing characteristics. The prototype achieved 27-nm wavelength demodulation and successfully demodulated 10 ultraweak grating strings of the same wavelength. By rationally planning the grating network, we can evaluate from the experimental results that the prototype can achieve at least 220 grating demodulation.

The polarization holographic gratings recorded in Au nanoparticles-doped methyl orange/polyvinylpyrrolidone (MO/PVP) composite films were investigated. The polarization state of diffraction beams was controlled by a probe light and holographic gratings together. The first-order diffraction efficiency of the MO/PVP film doped with Au nanoparticles was measured to be 3.2 times larger than that of pure composite film. Meanwhile, an obvious enhancement of photoinduced birefringence of MO molecules was also demonstrated. This enhancement of diffraction efficiency and photoinduced birefringence was discussed in terms of the localized field effect via the surface plasmon resonance of Au nanoparticles, which was confirmed by finite-difference time-domain simulation.

It is shown, experimentally, that in silver- and copper-containing zinc–phosphate glasses, metal molecular clusters are formed during the glass synthesis. X-ray irradiation of these glasses led to the considerable increase of its luminescence in visible spectral range. This effect is caused by the transformation of the charged metal molecular clusters into the neutral state. Luminescence and excitation spectra of the glass, doped with silver and copper simultaneously, change significantly in comparison with the spectra of glasses doped with one metal. The reason for this can be the formation of hybrid AgnCum molecular clusters. The computer simulation of the structure and optical properties of such clusters by the time-dependent density functional theory method is presented. It is shown that the optimal luminescent material for photonics application, in comparison with other studied materials, is glass, containing hybrid molecular clusters.

Bifacial transparent perovskite solar cells (BTPSCs) were designed to harvest more solar energy and ensure higher efficiency than conventional PSCs. A series of BTPSCs was successfully prepared using transparent ultrathin Au electrodes with different thicknesses. The transmittance and resistance of Au electrodes played a major role in achieving photo-to-electricity conversion efficiency (PCE). Engineering the light-harvesting ability of the fabricated BTPSCs led to the highest PCE of 14.74%. Reflecting-light intensity and illumination angle were further observed to be the key factors affecting PCE. These BTPSCs could be applied on building integration of photovoltaics (PVs), such as semitransparent PV windows or venetian blinds. Another alternative application is to use these BTPSCs as the wings of unmanned aerial vehicles.

An accurate and efficient demodulation algorithm for a fiber Bragg grating (FBG) strain sensor has been proposed and demonstrated. An accurate demodulation can be achieved by scanning only the 0.15-nm reflection spectrum of the FBG sensor instead of scanning the complete reflection spectrum. The proposed algorithm was first used to calculate the integration of 15 regions (10 pm per region) to calculate the wavelength drift. This algorithm is good in that the demodulation time can be greatly reduced without degrading the demodulation accuracy. Hence, the efficient algorithm, which can support a short demodulation time of a low-cost demodulation system, is very suitable for high demodulation rate requirements of the high-speed sensing systems.

The dependence of refractive indices of liquid crystal (LC) on temperature is represented by the Haller approximation model, and its dependence on the wavelength is expressed by the extended Cauchy model. We derived the refractive indices expressions of nematic LC E7 depending on temperature and wavelength simultaneously by combining these two models. Based on the obtained expressions, one can acquire the refractive indices of E7 at arbitrary temperature and wavelengths. The birefringence, variation rate of refractive indices, macroscopic order parameter Q, and orientational order parameter ⟨P2⟩ of E7 were then discussed based on the expressions.

The 3×3 coupler is used to close the Sagnac loop to provide passive biasing of the interferometer, which allows a balanced-detection configuration in interferometric fiber-optic sensing. Yet, the system asynchrony caused by photodetectors and converters leads to the deviation of eventual retrieval result, which may affect the judgment of disturbance prediction. To improve the performance of fiber interferometers, a compensation demodulation method is proposed. Compared with previous compensation methods, which focus on a signal processing process, this work offers a solution through an approach in the signal source acquiring process, the origin of the problem. By inserting a controllable optic delay device, a modified interferometric fiber-optic structure is presented. Operation of the compensation demodulation method is verified in an experiment along a 30-km-long sensing fiber. The performance of the structure is also compared with the conventional one. Experimental results show that the improved structure is advantageous for considerate compensation effect, accurate calibration, and low location error for the application of disturbance localization.

Photoplethysmography (PPG) is a widely used technique for measuring blood oxygen saturation, commonly using an external pulse oximeter applied to a finger, toe, or earlobe. Previous research has demonstrated the utility of direct monitoring of the oxygen saturation of internal organs, using optical fibers to transmit light between the photodiode/light emitting diode and internal site. However, little research into the optimization and standardization of such a probe has yet been carried out. This research establishes the relationship between fiber separation distance and PPG signal, and between fiber core width and PPG signal. An ideal setup is suggested: 1000-μm fibers at a separation distance of 3 to 3.5 mm, which was found to produce signals around 0.35 V in amplitude with a low variation coefficient.

This erratum corrects a misspelling in the author list.