Outgoing Editor-in-Chief Michael Eismann reflects on his tenure.

The differential image motion monitor (DIMM) is a widely used instrument for measuring atmospheric coherence length. We studied the feasibility to miniaturize the classic DIMM. By adopting more accurate formulas, the geometry of the classic DIMM can be altered mildly. A mini-DIMM can be developed under this new geometry condition. For example, a mini-DIMM may consist of two 6-cm apertures, located close to each other. We present the validation experiment where astronomical seeing was measured simultaneously by a mini-DIMM and a standard DIMM. Comparisons with a standard DIMM show good agreement between the two instruments.

Matrices provide a practical and elegant tool for describing the transformation properties of beam splitters and waveguide couplers acting on single-mode optical fields. Using a systematic approach, we show how the application of various physical constraints determines the form of the matrix for both classical fields and quantum number states. The goal is to provide a clear explanation of the conditions under which various matrix forms are appropriate to represent four-port couplers and beam splitters. Examples of calculations using the matrices are provided.

Ghost imaging via sparsity constraints (GISC) is an advanced imaging technique. The reconstruction quality of GISC is affected by the sparse ratio of the object, the regularization parameter, and the iteration number. Influences of these parameters on the peak signal-to-noise ratio (PSNR) of the reconstructed image are discussed and evaluated. The optimal regularization parameter and iteration number at different sparse ratios are given. Then the reconstructed images of GISC using the optimal parameters at different sparse ratios are shown. The improvement of the reconstruction quality of GISC utilizing the optimal parameters is confirmed through comparison with normalized ghost imaging. Finally, the reconstruction quality of GISC with random noise is analyzed, and a method to obtain the sparse ratio of the object by analyzing the signal of the bucket detector is discussed.

Aspheric lenses help meet the most demanding optical requirements while the precision injection molding technique hits the target for precision and cost. We developed a method of analyzing aberration terms in the transmitted “wavefront measurement,” determined by Shack–Hartmann wavefront sensing to estimate the fabrication errors of injection-molded aspheric elements. Considering aspheric element fabrication using a small training data set and F-measure fuzzy cluster analysis, an unsupervised learning method was applied to extract typical aberration terms from the wavefront polynomial. The experimental results suggest that these aberration terms, which are related to spherical (third-, fifth-, and seventh-order) and coma (third-order) aberration terms in the transmitted wavefront polynomial expansion, can be employed to estimate the surface error and decenter, respectively, of a lens from a specific mold cavity. The sampling lenses evaluated in the proposed measuring process were collected from different mold cavities according to their total working performance in the modulated transfer function measurement for the whole camera module. The performances of the typical aberration terms were discussed by comparing to the ones obtained from an interferometer and a profilometer. The proposed method could provide high detection efficiency and can thus be applied for the quality control of aspheric elements for mobile phones, where the existing errors are mainly spherical, coma, and astigmatism aberrations.

The restoration of nonuniform distorted infrared (IR) images is crucial for human visual perception and subsequent application tasks. However, existing methods sometimes fail to yield visually natural decompositions and perform insufficiently in the preservation of meaningful structures while suppressing disturbing noise. A spatially adaptive hybrid ℓ₁  −  ℓ₂ variational framework for the nonuniform intensity correction of IR images is proposed. Considering the piecewise constant characteristics of latent images, a weighted ℓ₁-norm regularization method is developed to constrain the local affinity of neighborhood pixels according to their intensity and structural priors, thereby significantly preserving structures while smoothly flattening areas. Additionally, an ℓ₂-norm guided local smoothness constraint is incorporated with an absolute scale term provided by coarse estimation to characterize the bias field component to restrict potential solutions and enforce the bias component to be textureless. Moreover, the proposed ℓ₁  −  ℓ₂ model is efficiently solved by an alternating direction method of multipliers scheme. Extensive experiments on both synthesized images and two real-world IR datasets indicate that the performance of the proposed method is superior to that of five existing algorithms both visually and numerically.

Risley prisms can serve as a beam steering device to move field of view and appear to be a promising solution to boresight adjustment in imaging tracking applications. It is desirable to deduce the inherent relation among target offsets, target’s line of sight (LOS) deviations from sensor’s boresight and the required prism rotations. The boresight adjustment model based on Risley prisms is described. Reverse ray tracing is performed and case examples are given to characterize the nonlinear relationship between target offset and its LOS deviation. A boresight adjustment method based on the inverse ray tracing and the two-step method is then illustrated. The simulation results show that the target’s LOS deviations from the boresight are stretched along the boresight’s radial direction relative to the target offsets. By the proposed boresight adjustment method, the required rotations of the prisms can be derived from the target offset. The singularity problem is also pointed out using this method, and the corresponding conclusions are consistent with those deduced in many works. The research demonstrates the inner mechanisms of the imaging tracking based on Risley prisms in principle. The proposed boresight adjustment method can provide guidance for controlling the circular motion of the prisms.

The initial phase of structured illumination is an important parameter in structured illumination microscopy (SIM). Its estimation accuracy directly affects the reconstruction quality of SIM super-resolution images. However, when the modulation of the system is less than 0.02, the current phase estimation algorithm will cause an estimation error above 0.2 rad. An algorithm based on multi-image correlation processing in frequency domain (MCF) is proposed to solve this problem. Simulation and experimental results show that the MCF algorithm greatly improves both the initial phase estimation accuracy and the reconstruction quality of super-resolution images for low modulation SIM systems with a random phase shift. This means that the MCF algorithm can extend the scope of the SIM technology, especially for low modulation systems or systems lacking precise phase-shifting components.

Optical camera communication (OCC) traditionally uses the bright and dark stripes of a rolling shutter pattern captured by the complementary metal–oxide–semiconductor image sensor to realize communication. We assign a unique identification [LED’s unique identification (LED-ID)] to each LED and introduce an LED-ID detection and recognition method of OCC using discrete Fourier transform (DFT). It is different from the traditional LED modulation and demodulation method. In this approach, LED-IDs are modulated to possess different frequency characteristics instead of transmitting data. Taking the DFT of the rolling shutter pattern to obtain a spectrum image, we can recognize the LED-ID according to the characteristics of the spectrum image. The experiment results show that the proposed method has good performance, with at least 426 recognizable LED-IDs and a recognition distance reaching 3 m. Therefore, the proposed method can be considered as one of the effective methods for OCC.

This work aims to provide a quick solution for impressing a well-defined and repeatable speckle pattern on the surface of a material sample. The proposed technique is based on a water-soluble stabilizer on which the speckle pattern generated and optimized via computer is printed. To verify the application, a two-dimensional (2D) digital image correlation (DIC) system is employed to measure the full-range strain distribution at the macroscopic level during a tensile test on open-hole aluminum specimen. The experimental setup consists mainly of an action camera, a macro lens, and an open-source 2D DIC software. The measured data obtained from the DIC are compared to the other ones provided both from a traditional measurement method based on strain gauge and by a numerical simulation. The results indicate that the approach is both accurate and reliable to obtain stress-strain curves especially in the presence of plastic deformations.

Dioptric power measurement is an essential part of the characterization and quality control of lens manufacturing. In this framework, a fairly simple and potentially cost-effective method for lens characterization by interferometric fringe evaluation is reported. This technique consists of acquiring images of straight and parallel fringes projected onto a planar screen through the lens to be tested and in the regions surrounding the lens. Upon phase evaluation procedures, two apparent three-dimensional shapes of the screen are obtained, namely, the background contour and the one visualized through the test lens. The lens power is obtained as a function of the ratio between the two shapes and the parameters of the optical setup. The fringe pattern evaluation was performed by four-stepping and phase unwrapping methods. The results obtained by this technique were compared with the ones obtained by a commercial focimeter in the measurement of unifocal and progressive power lenses.

Shearography is a very effective non-destructive testing method for detecting the internal defects of composite materials. How to improve the detection efficiency of this technique to help achieve the rapid and automatic recognition of multiple defects has always been a hot research topic for researchers. Based on the results of the shearography, we propose a method to identify multiple defects in the phase map using the spatial characteristics of Freeman chain code. First of all, we use Canny operator to binarize the phase map, then refine the fringe by morphological means, and finally identify the defects by the spatial characteristics chain code. In addition, Otsu is used to assist in judging the fracture or incomplete defects. For the fringes under vacuum loading or thermal loading, this method can detect the defects quickly and successfully. The theoretical description is given and the effectiveness and practicability of the method are verified by experiments.

We analyze the Crown of Light (COL) cut using an angular spectrum evaluation tool. Several light performing features of the COL are discussed. In particular, it is found that the angular spectrum of the COL tends to be concentrated and that this maximizes the brilliance and sparkle probability when the COL is aimed at localized light sources. It is contended that the COL represents a novel paradigm in diamond cuts. A distinctive feature of the COL cut is its dome shaped crown.

Annular subaperture stitching interferometry (ASSI) is a common approach for the measurement of aspherical surfaces. A common obstacle of ASSI is the occurrence of lateral displacement errors when the sensor or specimen is repositioned between the subaperture measurements. Our contribution focuses on modeling of the statistical displacement errors. A virtual experiment is presented simulating the propagation of the displacement errors through a cumulative and a global stitching algorithm to the retrieved surface form. For the considered experimental setup, the uncertainty in lateral position depends on the positioning uncertainties of the employed motion system and the uncertainty in the absolute distance measurement between the sensor and specimen. The lateral displacement uncertainty is determined experimentally employing a calibratable lateral grating. Thus, it is traceable to the SI unit of the length (meter). The experimental results show that the lateral displacement errors may be modeled by a normal distribution, and the results of the virtual experiment indicate that the statistical lateral displacement errors transfer linear through the stitching procedure and also cause a normal distributed topography error. This enables the assignment of an expanded uncertainty to each individual sample point employing the Zernike polynomial expression of the topography measurement.

The correlation-based image subset matching methods such as digital image correlation (DIC) are widely used for measuring the deformation fields. The specimen surface must be covered with speckle patterns, or more exactly, random intensity distributions with sufficient contrast, which act as a carrier of deformation information, to acquire reliable and accurate matching in the correlation calculation. The requirement of the artificial speckles on the specimen surface inevitably contaminates the specimen. To maintain the cleanliness of the specimen surface, an in situ deformation estimation method without speckle fabrication is developed and its result is demonstrated. Topological similarity measurement (TSM) is a feature vector-based displacement estimating algorithm for full-field deformations that uses feature vectors that encode the relative positions of neighboring mark spots. Compared with the DIC method, this approach makes up for its deficiency in displacement estimation of speckle-free flat surface by incorporating a topological arrangement-based feature vector, subpixel center location method, and biharmonic spline interpolation method. Experimental results indicate that the proposed TSM method is feasible to become the preferred displacement estimation method for uniform deformation. As a complement to the DIC method, the TSM method is more suitable for uniform deformation distribution, which requires high measurement accuracy but cannot use DIC.

By constructing two phase-shifted fringe images and a flat image, the 2 + 1 phase-shifting algorithm can reduce the error caused by motion and realize real-time and high-resolution three-dimensional (3-D) measurement. However, the nonlinear gamma of the digital light processing and the ambient light will affect the light-field distribution of the flat image, and extensive experiments have shown that the distribution of the captured flat image is inconsistent with the DC component of the deformed patterns. Therefore, based on the linear calibration model, a more realistic DC component of the deformed patterns is deduced from the captured flat image. The AC components of the deformed patterns can be extracted more genuinely by subtracting the deduced DC component from deformed patterns. So, the influence of ambient light can be suppressed efficiently to improve 3-D measuring accuracy. The experimental results show the feasibility and validity of the proposed method.

Light detection and ranging (LiDAR) return signal generation technology applied in the LiDAR indoor test and simulation is significant to design, develop, test, and validate a LiDAR’s capability and performance. To generate a target’s information carried by the return signal, the dimensional decomposition and equivalent generation method of the LiDAR return signal are proposed. The target four-dimensional (4D) information is decomposed into one-dimensional (1D) intensity information, 1D range information, and two-dimensional (2D) angle–angle spatial information. The 1D intensity information is simulated by the absorption of prism pairs, while the 1D range information is simulated by the combination of electrical and optical time delay. The 2D angle–angle spatial information is implemented by the stack of segmented digital mirror array device slice images in sequence. Moreover, a LiDAR return scene projector (LRSP) prototype is developed and its performance is measured. The results show that its energy dynamic range is 51.25 dB. The distance simulation range is 240.15 m to 22.5 km (1.601 to 150  μs). The simulation accuracy of the target’s depth is <9  cm (0.6 ns). The spatial resolution of 64  ×  64  pixels is verified by vertical and horizontal line pairs test. Because the LRSP has 12 image slices, its resolution is 64  ×  64  ×  12  pixels in three-dimensional (3D) space. Finally, the prototype is demonstrated by reconstructing a staircase. The energy dynamic and 2D angle-angle spatial resolution are improved significantly compared with the existing LRSPs.

The trend toward the use of light-emitting diodes (LED) for photometric and colorimetric measurements has put pressure on studies that aim to improve spectral uncertainties with calibration methodologies. Recent studies have shown good results using artificial neural networks (ANN) to calibrate a colorimeter; however, this methodology presents the difficulty of finding the segmentation of the input variables for ANN training. We propose the use of a new calibration methodology for a photometer, an ANN that transforms the output of the sensor into luminous transmittance without the need of a filter in a prototype that uses an RGB sensor and a white LED as an illuminant to verify the possibility of calculating the luminous transmittance of a sample. A 14,031 spectra dataset was built that covered the entire input range for training the final ANN, a four-input multilayer perceptron, and 20 neurons in the hidden layer. The ANN was validated with errors smaller than a class L photometer. The prototype was tested for measuring luminous transmittance, under the D₆₅ illuminant of colored filters samples with all results within the ±3  %   points error range. The prototype overcame commercial meters with better results. The ANN can effectively calculate luminous transmittance and can be tested to calculate other photometric values that use different weighting functions with the same hardware. The methodology presents an option for a meter with a correction method that depends only on software. This allows the development of a compact and low-cost photometer.

A high-precision error measurement method of angular displacements based on the biaxial of the reciprocal roll angles (RRA) is presented. The resolution and precision of the angular displacement measurement systems are guaranteed using the principle of small-angle measurements, along with a photoelectric autocollimator and reflector. Based on the method of the biaxial RRA and the principle of full-circle closure, a displacement measurement system covering the full circumferential range is built. The displacement measurement system can recognize the reference angle error elimination, the calibration angle error, and mutual compensation. The total error model of the angular displacement error measurement and the biaxial turntable is established, and the main error terms are analyzed using the total error model of the turntable and the components. The traditional “Fourier harmonic analysis” and sparse decomposition methods are used to correct the system error components. The simulation accuracy of the corrected turntable is similar to that of the error-free term, and the analysis results are used in the system-optimized configuration, including precision distribution. The experiment verifies the feasibility and effectiveness of the proposed high precision, angular displacement, and error measurement method.

To obtain satisfactory performance in characterizing optical freeform surfaces with local features, this paper proposes a model of a radial basis function with slope-based shape factor and distribution (RBF-SSD). Compared to previous RBF-slope models with only slope-based shape factors, the RBF-SSD model relates both shape factors and distribution with the surface slope, ensuring greater fitting ability can be achieved when fitting a surface with local features. Fitting experiments for two different surfaces demonstrated the fitting ability of the RBF-SSD model. An off-axis three-mirror system with 3  °    ×  3.6  °   field of view was designed as an example to show the optical design efficacy of our model.

Some widely used optical measurement systems require a scan in wavelength or in one spatial dimension to measure the topography in all three dimensions. Novel hyperspectral sensors based on an extended Bayer pattern have a high potential to solve this issue as they can measure three dimensions in a single shot. This paper presents a detailed examination of a hyperspectral sensor including a description of the measurement setup. The evaluated sensor (Ximea MQ022HG-IM-SM5X5-NIR) offers 25 channels based on Fabry–Pérot filters. The setup illuminates the sensor with discrete wavelengths under a specified angle of incidence. This allows characterization of the spatial and angular response of every channel of each macropixel of the tested sensor on the illumination. The results of the characterization form the basis for a spectral reconstruction of the signal, which is essential to obtain an accurate spectral image. It turned out that irregularities of the signal response for the individual filters are present across the whole sensor.

It is difficult to cut through a transparent material such as polyethylene (PE) film with a continuous-wave (CW) laser of the near-infrared wavelength, because of the low absorption of laser energy. The plasma induced by a pulsed laser, however, can damage the surface of a film, which can change the transparency of the material. In this study, a transparent PE film with a thickness of 0.6 mm was irradiated by various combinations of focused laser pulses and a CW laser. The test conditions and the processes were recorded visually by a Schlieren optics system on the side surface of the film and by a high-speed camera on the front surface. From the results, it can be concluded that, though it is difficult to cut through a transparent material such as PE film with only a pulsed or CW laser alone, once the transparency of the surface has been modified by multiple focused laser pulses, the PE film can be cut through easily by a CW laser with a near-infrared wavelength.

A silicon Mach–Zehnder modulator (MZM) with quasi-TM mode propagation is investigated for non-return-to-zero on–off keying modulation. The quasi-TM PN phase shifter phase-loss characteristics have been determined and shown to exhibit better performance compared to quasi-TE phase shifter with the same waveguide cross-sectional area and device parameters. The phase shifter modulation efficiency is 1.02 V.cm. The MZM transfer characteristics are determined for the dual-arm push–pull driving scheme, and a traveling-wave electrode structure is employed to enhance the device bandwidth. A 3-dB bandwidth of 74 GHz is obtained at 2.5-V reverse bias. The modulator high-speed characteristics are studied for different data rates over single-mode fiber transmission. A 30-km fiber transmission with an open eye at 160 Gbps with 2-V peak-to-peak drive is obtained with an extinction ratio of 2.3 dB and bit-error-rate (BER) of ∼2.77  ×  10^−  06. The effect of fiber dispersion on the BER shows dispersion tolerance from 0 to −1.73  ps  .  nm^−  1  .  km^−  1 for 30-km fiber transmission below the hard-decision forward error correction threshold at 100-Gbps operation.

A wave-front shaping optimization algorithm is proposed to image through scattering media with higher accuracy. Pearson correlation coefficient (PCC), structural similarity (SSIM), and gradient structural similarity (GSSIM) are introduced as the loss functions to recover images. We have concluded that GSSIM has the optimal detail recovery performance among complex multivalued targets, followed by SSIM and PCC. To find the optimal phase mask, we propose the squirrel search algorithm and compared it with two classical global optimization algorithms including the genetic algorithm, the simulated annealing algorithm, and the commonly used continuous sequence algorithm. The simulation and experimental results show that the proposed squirrel search algorithm has a faster convergence speed and higher robustness and stability, which indicates an improved reconstruction quality. This algorithm will help realize imaging through scattering media better and quicker, which is meaningful in dynamic biological tissue imaging and related fields such as optical detection.

Inorganic perovskite materials (IPMs) seem to overcome the limitation of the stability of organic perovskites to a large extent. Herein, we discuss the design and the development of periodic nanophotonic structure (PNS)-based two-terminal IPM/Si tandem solar cells through the optical optimization process, computed using rigorous coupled-wave analysis. The IPM taken as the top active layer is cesium lead iodide (CsPbI₃), as it perfectly matches with the top cell requirements to design with a bottom Si-based tandem cell. The PNS is designed on to the tandem cell’s top layer. The top cell IPM layer thickness is kept fixed at 100 nm to limit the problems associated with thicker perovskite layers’ depositions and all the required tandem cell parameters are then optimized accordingly. To highlight the effectiveness of the proposed design, it is comparatively analyzed with Lambertian limiting values and bare and planar tandem structures. The results predict a notable performance enhancement for the planned design that accounts for around 40% comparative short-circuit current density increase. The complete spectral analysis presented provides insight for quantitative performance enhancement of the CsPbI₃ cell due to PNS that leads to overall tandem cell performance improvement. The absorption enhancement is credited to the PNS at the top as it leads to better index matching, reduction in reflections, and better trapping of full-spectrum photons.

We propose reconfigurable photonic switches with very low input power, ultracompact size, fast response, and low optical loss for high-speed optical network and on-chip interconnect applications. The photonic crystal waveguide-based nanostructure is used to realize the reconfigurable 3  ×  3 and 4  ×  4 optical switches. The two different operating wavelengths of 1550 and 1630 nm play a significant role in the nanophotonic switching platform. The reconfiguration of optical signal path is effectively attained using these wavelengths with low delay time and high data rate. The performance of nanoswitch parameters is numerically investigated by the finite-difference time-domain method. The proposed optical switch is designed with a low input power of 0.5 mW, fast response time of 607.33 fs, and high data rate of 1.646 Tbps. This high-performance device can be suitable for lightwave communication network, quantum computing, and nanoscale photonic integrated systems.

We have investigated the supermode properties, namely, mode effective refractive index (RI) (n_eff), effective area (A_eff), and chromatic dispersion (CD), in three-core homogeneous strongly coupled multicore fibers (SC-MCFs) for different core RI profiles (step index and RI dip) and in different possible core arrangements (triangular and linear). Further, the impacts of fiber parameters, namely, core radius (a), relative RI difference between core and cladding (Δ), and core pitch (Λ), on the differential group delay (DGD) between different supermodes have also been analyzed for all the considered configurations of SC-MCFs. The analysis presented has been done numerically using the FemSIM simulation platform. It has been observed that core arrangement has significant impact on the levels of n_eff, A_eff, CD, and DGD, and it also affects the degeneracy of the supermodes in a three-core homogeneous SC-MCF. Further, there exist certain core pitches in which the A_eff values of fundamental supermodes in different core layouts are equal. Furthermore, there exists a certain relative RI difference between the core and cladding (Δ) values in which DGD values in all the considered SC-MCF configurations are equal. On the other hand, over a range of Δ values, DGD in a linear layout is flattened compared to a triangular layout. Incorporating the RI dip structure in the cores of SC-MCFs affects the CD levels significantly. Therefore, by careful control of the fiber parameters and core arrangement, large mode effective area and low and flattened dispersion SC-MCF can be designed that may be suitable in dense wavelength division multiplexing application.

We propose to investigate the performance of a satellite-to-ground quantum key distribution (QKD) system that uses key retransmission as a method for improving the system reliability. We develop analytical frameworks based on two three-dimensional Markov chain models allowing us to comprehensively analyze the proposed system’s performance in terms of key loss rate, link utilization, and delay outage rate. Our performance analysis takes into account the physical layer impairments induced by free-space optical channel and receiver noise. Numerical results quantitatively demonstrate that the performance of satellite-to-ground QKD system is significantly improved due to key retransmission. In addition, the appropriate selection of system parameters corresponding to different turbulence conditions to achieve the best performance improvement is also provided.

The effect of the sign of the dispersion parameter on nonlinear pulse propagation in a birefringent optical fiber is analyzed via numerical analysis. First, the concept of effective cross-dispersion is introduced to demonstrate the simultaneous existence of positive dispersions of opposite polarizations within the same waveguide, leading to the generation of a dispersive wave and supercontinuum. Then, a general set of equations is deduced to calculate the dispersive waves corresponding to x and y polarizations considering a birefringence-dependent wavelength. Using this, it is established that solitons and dispersive waves can be induced corresponding to each polarization even if the dispersions are normal. Finally, we demonstrate that spectral broadening can also be induced corresponding to one polarization despite the existence of positive dispersion, i.e., in the complete absence of solitons for the entire duration of pulse propagation.

A multi-user visible-light communication (VLC) system based on 4  ×  4 multiple-input multiple-output (MIMO) was investigated. The system consists of four light-emitting diodes (as a transmitter) and four photodetectors (as a receiver). The proposed system was simulated for two, three, and four users. The objective of this study is to optimize the system power consumption and to utilize the sum rate on the system coverage area. By expanding the system from a 2  ×  2 MIMO-VLC to a 4  ×  4 MIMO-VLC, the results demonstrate that the total sum rate of the system can be improved. The findings reveal that the 4  ×  4 MIMO-VLC achieves a maximum sum-rate enhancement of 143% compared with the 2  ×  2 MIMO-VLC. The results also show that the efficiency of 4  ×  4 MIMO-VLC system with non-orthogonal multiple access (NOMA) and gain ratio power allocation (GRPA) was increased by around 181% compared to 2  ×  2 MIMO-NOMA-based VLC with GRPA in combination. For the 4  ×  4 MIMO-NOMA-VLC device with GRPA, the sum bit rate for two, three, and four users was increased by 40%, 33%, and 24%, respectively. Through this study, it can be concluded that GRPA scheme plays an important role in utilizing MIMO-VLC system sum rate of the MIMO-VLC system.

We propose a 16-ary quadrature amplitude modulation (16-QAM) signal decision algorithm termed as the joint support vector machine (SVM) and K-nearest neighbor (KNN) module (JSKM). The M-ary SVM makes the first decision for all the received symbols. Subsequently, the KNN algorithm makes the second decision for the received symbols inside the decision boundary of the SVM. In this manner, the advantages of the SVM can be exploited and the KNN algorithm can help overcome the limitations of the SVM. The proposed algorithm was implemented to perform the simulation of a 112-Gbps 16-QAM coherent optical transmission system. The results demonstrated the superiority of the JSKM algorithm as it could increase the transmission distance by 440 and 227 km compared to those obtained using the KNN and M-ary SVM algorithms, respectively.

We presented an optical system that could measure the viscosity coefficient of liquid in a micro-area. The orbital rotation of a polystyrene microsphere was realized by a dual-beam fiber-optic trap with a transverse offset. The rotation rate increased with the viscosity coefficient of the environmental medium. On this basis, the viscosity coefficients of ethanol solutions with different concentrations were measured successfully. The volume of solution samples was less than 1  μL. This provides a basis for the viscosity measurement of rare liquid or enchylema, which is of great significance for biological applications such as cell characteristics and reaction dynamics.

We propose the two-stage cascaded-tapered silica photonic crystal fiber (PCF) for the supercontinuum (SC) generation. The cascaded-tapered silica PCF is designed to have a spatial periodic structure. The physical scenarios of the spectral broadening due to the interaction between the structure-induced periodic dispersion and Kerr nonlinearity under different pulse widths and peak powers are investigated. It is found that when the pump pulses with width of <100  fs and peak power of not more than 10 kW are propagated in the cascaded-tapered silica PCF, the SC with good coherence can be generated. It is believed that the research results have potential applications in the nonlinear photonics and spectroscopy.

Low lamp input-to-TEM₀₀ mode laser output conversion efficiency and strong thermal lensing effects are bottleneck issues in commercially available single-rod/single-beam lamp-pumped lasers. A six-rod/six-beam concept is proposed to substantially enhance lamp input-to-TEM₀₀ mode laser output conversion efficiency. A 4-kW-single-thick arc lamp was used to efficiently pump six thin Nd:YAG rods simultaneously within a modified two-ellipse pump cavity. All of the design parameters were numerically optimized by Zemax^® and LASCAD^® software. Since each rod absorbed only a small part of the lamp power, significantly reduced thermal lensing and thermal stress problems were predicted. Asymmetrical laser resonators were adopted to enable a large mode-matching between the lamp pump mode and the fundamental laser mode within each rod. 34.82-W total TEM₀₀ mode laser power was numerically calculated, corresponding to 0.87% lamp input power-to-TEM₀₀ mode laser power conversion efficiency and a 2.18 times improvement over that of the state-of-the-art lamp-pumped laser. A TEM₀₀ mode power-to-multimode power ratio of 59.0% was also obtained, being 2.40 times more than that of the most efficient single-rod TEM₀₀ mode lamp-pumped laser. Six TEM₀₀ mode laser beams emitting from the single laser head may provide a cost-effective laser solution to several processes in industry and scientific research. For the extraction of a single TEM₀₀ mode laser beam, a laser resonant cavity with folding mirrors was also presented. A 15.34-W laser beam brightness figure of merit was numerically calculated, corresponding to a 13-fold increase over that from each of the six TEM₀₀ mode laser beams.

We present a technique for measurement of the intensity profile of high-power laser beams. It is based on the application of a matrix of the copper-coated optical fibers. Laser radiation propagating through a copper-coated optical fiber is partially scattered and subsequently absorbed by the copper layer. The change of an electrical resistance of the copper coating caused by its heating is measured by an ohmmeter. It is estimated that this method allows measurements of the intensity profile of laser beams with an average power exceeding 10 kW level. The optical intensity profile and the angle of divergence of the single-mode laser beam were measured using the proposed technique.

We treat wideband subwavelength guided-mode resonant gratings with grating-depth dependent duty cycles. By rigorous numerical computations, we visualize Bloch modal evolution while transforming the grating profile from trapezoid to triangle. Parametric optimization is achieved using the coordinate transformation method of Chandezon. With the increase of the profile base angle, the higher mixed resonant modes TM_{1  ,  1  &  2} attract and combine and interact with the modes at both sides, forming wide reflection bands. This modal combination and interaction are the key determinant of broadband reflectivity spectral location and width. The optimized structure exhibits 99% reflectivity across a 613-nm spectral range, spanning a 1438- to 2051-nm wavelength range with a fractional bandwidth of ∼35  %  . It is shown that gratings with trapezoidal profiles possess a good tolerance to groove depth variation, thus being easier to fabricate with a diamond-tip than triangular profile-based gratings.

Nanostructures supporting surface plasmon resonance are crucial for efficiency enhancement in thin film solar cells. A plasmonic nanostructure over GaAs layer consisting of Al nanospheres on the front surface and Ag nanocylinders on the rear surface of the GaAs layer with SiO₂ as the back contact is studied for the first time. Using the finite-difference time-domain method, it is found that the optimal nanostructure enhances the short circuit current density by 45.38% and 14.76% in comparison to bare and Si₃N₄-coated thin film GaAs solar cells, while the enhancement in efficiency (η) is 46.83% and 15.11%, respectively. The plasmonic scattering by nanostructure and propagation of waveguide modes at the metal/dielectric interface are mainly responsible for these enhancements.

A tunable Fano resonance based on a microring resonator is proposed. The double microring is nested in parallel in the U-shaped feedback waveguide with the graphene embedded in part of the waveguide as the active modulation region of Fano resonance. When different voltages are applied to the graphene in the feedback waveguide or microrings, the Fano resonance exhibits different and asymmetrical shapes and has a larger extinction ratio than the traditional Lorentz shape. The maximum extinction ratio of the proposed device is up to 71.50 dB. The proposed structure will have a wide application in sensors, switches, and integrated optoelectronic devices.

Experimental demonstration of a quantum random number generator based on one single-photon avalanche diode (SPAD) detector, a T  /    (  T  −  t  )   pulse-shaped laser, and an field-programmable gate array (FPGA) acquisition module is presented. An integrated laser driver drives an external laser diode at 670 nm wavelength, whereas the SPAD with a photon detection probability of 18.5% is integrated together with an active quenching-resetting circuit. The SPAD detector generates counts for the interarrival time (IAT) measurement system implemented in an FPGA, where the change of the IATs between consecutive pulses is used to derive a random bit stream. It is shown how the application of a pulse-shaped laser driver can increase the performance of the system as compared to the continuous-wave operation of the laser diode to achieve the maximum generation rate of 5 Mbps while using a single SPAD. The generated numbers pass all randomness tests of the National Institute of Standards and Technology (NIST), Dieharder, and ENT test (pseudorandom number sequence test) suits.

Graphene has attracted widespread attention in dynamic optoelectronic devices due to its tunable electrical and optical properties. But different modulation capabilities of the graphene-based designs at different frequencies are less studied. We study the electrical tunability of transmissive metalenses based on graphene when working at three frequencies 0.3, 1.25, and 2 THz, respectively. The constitutive meta-atoms are composed of graphene patches and metallic gratings for efficient phase shift in the orthogonal polarization. Although the conductivity of graphene is tunable at all the frequencies, responses of meta-atoms show weak and strong dependence on the Fermi level at the low and high frequencies, respectively. Therefore, the focal length of the metalens is not electrically tunable at 0.3 THz. In contrast, the metalenses designed at 1.25 and 2 THz show electrically adjustable focal lengths, and the tuning range of the focal length increases with frequency. The research here provides clear guidance for the design of graphene metalenses with different electrical tunabilities for a variety of application scenarios.

Based on one-dimensional photonic crystals (1D PCs), we studied an innovative multispectral compatible stealth material for visible light, infrared, and 1.06-μm laser. The fabricated PCs had advantages of different colors, low emissivity in the atmospheric windows, and low reflectance at 1.06-μm waveband. According to some relevant experiments, the prepared films possessed colors of yellow, green, and blue, which could be used to simulate the color of the desert, woodland, and ocean, respectively. The infrared stealth performance of films showed that the thermal radiation in the atmospheric windows of 3 to 5  μm and 8 to 14  μm could be reduced effectively. In addition, the films’ reflectance spectra measured by spectrometer indicate that the reflectance at 1.06  μm is below 20%, which, in practice, could enormously reduce the echo power of incident lasers.

Most of the natural materials have very weak interaction with the terahertz (THz) electromagnetic waves. Designing an absorber in the THz range is of considerable interest. We present multiband THz metasurface absorbers formed by the metallic Hilbert fractals separated with a dielectric layer from the metallic ground-plane. Numerical simulations reveal that the absorption spectra of the second-order Hilbert fractal absorber have four absorption bands located at 1.61, 3.41, 4.61, and 5.61 THz with absorption efficiency of 1, 0.97, 0.99, and 0.93, respectively. We also compare three orders of Hilbert fractals as resonant elements and examine the role of different geometrical parameters on the absorption spectra. The proposed absorbers can be readily tuned to desired frequencies for applications, such as imaging, filtering, sensing, and selective thermal emission.

Visible light communication (VLC) is a data transmission technology that uses the LED lighting infrastructure to simultaneously illuminate and communicate. The ubiquitous existence of LED lamps opened a new opportunity for addressing VLC in many indoor communication scenarios. The motivation for the presented application is the efficient management of warehouses supported by autonomous navigation robots that grab goods and deliver them at the packaging station. This functionality demands bidirectional communication between infrastructures and vehicles. We propose links for infrastructure-to-vehicle (I2V), vehicle-to-infrastructure (V2I), and vehicle-to-vehicle (V2V) to perform indoors, bidirectional communication. A bidirectional communication system between a static infrastructure and a mobile robot (I2V) is proposed. The LED lamps of the warehouse illumination system are used to lighten the space and to transmit information on position and racks’ contents. The mobile robots communicate with the infrastructure (V2I) to transmit information on the items that are being removed and carried to the packaging station. The communication among robots (V2V) provides information on the number of items intended to be collected when the vehicles are in the same lane. The proposed coding schemes are used as modulation for the ON-OFF keying method. Trichromatic white LEDs and a photodetector based on a-SiC:H/a-Si:H with selective spectral sensitivity are used at the emitter and receiver. Position information is provided by each LED lamp to the vehicle by adequate modulation of the RGB emitters. The decoding strategy is based on accurate calibration of the output signal. Different scenarios were designed and tested. Requirements related to synchronous transmission and flickering were addressed to enhance the system performance. The decoding task is discussed using a bit parity error control methodology to ensure simultaneous detection and correction of bit errors. The consequent increase of bit error rate in the VLC transmission is discussed in the I2V link.

We report, in this study, the impact of gold nanoparticles on low-voltage operating GaN ultraviolet (UV) photodetector through parameters such as detectivity, responsivity, repeatability, stability, and photo-to-dark current ratios. These studies revealed prominent increases in detectivity and responsivity of the device due to Au nanoparticle application, thus allowing it to detect weaker signals and obtain higher on/off current ratios. After the application of different quantities of Au nanoparticles to the device surface, the investigated photoelectric properties were enhanced reaching a peak detectivity of 1.09  ×  10¹¹  Jones and a photo-to-dark current ratio of 486 at 2-V bias. Moreover, the photocurrent increased by two-fold (from 35.3 to 70.6 nA) at a low bias of 0.5 V following the application of the Au nanoparticles to the device surface. The modified GaN photodetector with a remarkable detectivity offers solutions toward improving detectivity of UV photodetectors. In consequence, having established such prosperous properties under relatively low-voltage levels with this practical and cost-effective fabrication route provides a good approach toward a wide range of UV optoelectronic devices.

We present thermal control of electromagnetically induced transparency (EIT) by actively modulating the dark mode in terahertz (THz) metamaterials, including a cut wire and a split-ring resonator (SRR). By integrating indium antimonide (InSb) into the SRR and increasing the temperature, the active modulation of EIT is realized. The coupling mechanism is numerically analyzed through the coupled oscillator model, and the result of fitting the EIT intensity agrees well with the simulation results when the temperature changes from 200 to 240 K. By analyzing the electric field distribution, the physical mechanism is the change in the damping rate of the dark mode resonator due to the increase in InSb temperature. Our work has practical significance in designing tunable THz functional devices.

Erratum corrects an error in the affiliation of two authors.

This erratum corrects affiliation information.