Editor-in-Chief Adam Wax writes about a new paper type in Optical Engineering.

We survey the design of the Polaroid SX-70 camera system, to exhibit what it means to design a tightly-coupled system (rather than a loosely coupled or modular one). The optics, the mechanics of the camera, and the chemistry of the film-picture unit are described as being adapted to each other so that we have a system, and the system as a whole is optimized with superfluous refinements avoided. We then discuss the picture-making experience inherent in such a system.

The nonlinear effect of fiber limits the further increase in pulse energy, and Mamyshev oscillator shows outstanding advantages in managing nonlinearity in waveguide medium, which is now associated with high peak power and high pulse energy. The potential applications of these laser sources based on Mamyshev mechanism have facilitated aggressive research and innovative ideas by researchers around the world. Here, we focus on the mode-locked principle and starting dynamics of Mamyshev oscillator. The review of Mamyshev technology is summarized from two starting modes of seed source injection and self-starting. Initial research and significant progress in this field, plus new insights and challenges of Mamyshev oscillator for ultrafast fiber laser technology are analyzed.

Guest editors Daniel Malacara-Hernández, Alfredo Dubra, Jim Schwiegerling, Pablo Artal, Yobani Mejía, and Eva Acosta Plaza introduce the Special Section on Advances in Optical Measurements and Instrumentation for Ophthalmology and Optometry.

A method is given to reconstruct a surface from measured surface gradient values located on a two-dimensional, square grid using a zonal method. Reconstruction is done square by square, using a surface elevation value calculated when processing the previous neighboring square as a reference elevation value and then calculating the difference from that value for the other three locations of the square. The curved surface area element defined by a grid square is, in general, a tilted, toric surface that is decomposed into four component surfaces; a tilted plane, a surface of revolution, and two cross-cylinder surfaces. Only the tilted plane and the oblique cross-cylinder surface contribute to elevation differences at the square corners so only they need to be included in the calculations. This unique use of available information allows the surface to be reconstructed without the iterative steps that are generally needed in zonal reconstruction methods. Methods to deal with boundary conditions and with measurement noise are also included in the implementation of the method.

We describe the construction and characterization of three Stokes lenses that can be used in optometric practice to measure different ranges of refractive astigmatism using subjective routines. The proposed set of Stokes lenses is assembled from Risley prism mounts coming from a Nidek phoropter in which the optical prisms are replaced by three different sets of cylindrical lenses ( ± 0.75, ± 1.25, and ± 1.75 D) coming from a trial lens case to cover a different astigmatism range (1.5, 2.5, and 3.5 D in modulus, respectively). We present the experimental characterization of the three Stokes lenses with the power vector notation and use an automatic lensmeter (Topcon CL-300) for dioptric power measurement. The measured dioptric power profiles are in concordance with the theoretical predictions, and statistical testing reveals no differences between measured and theoretical values for the three lenses, neither in spherical nor in cylindrical components (p ≥ 0.05 for all cases). Bland–Altman plots show very good agreement between measured and theoretical values with limits of agreement below 0.12 D for both M and J components for the three lenses. These Stokes lenses can be easily constructed from regular Risley prism mounts, so they can be used in almost any phoropter model for refractive error compensation.

Presence of a range of insects with different shape, size, and structural morphology of compound eye reveals the diverse pathways adopted by the nature striving to develop a flawless vision under given set of constraints. Exploration of various compound eyes allows researchers to understand better the various stages of visual system development in insects. By far, the most frequently used techniques for the investigation of structural morphology of a compound eye are scanning electron microscopy, transmission electron microscopy, microcomputed tomography, and histology. The nature of implementation of these techniques restricts in vivo studies, and also the requirements for a complicated and costly setup put limitations on its availability at small research facilities. Despite the widespread acceptance of the optical coherence tomography (OCT) as a standard tool for the various ophthalmological investigations, reports on compound eye study using OCT are uncommon. We demonstrate the application of a full-field OCT (FF-OCT) system for depth-resolved structural imaging and morphological analysis of the compound eye dioptric system. We report the application of a time-domain FF-OCT system for depth-resolved en face and three dimensional in vivo imaging of dragonfly’s prominent compound eye with resolutions of 2.8 and 7.2 μm in lateral and axial directions, respectively. The study shows the effectiveness of the FF-OCT approach for label-free and rapid structural morphology analysis of compound eyes. The results have been compared with the histology results reported in literature.

An investigation of the accuracy and reproducibility of refraction measurements of an at-home refraction measurement device is carried in a standardized experimental setup. The refraction measurement device is a handheld, low-cost, and simple to use refraction measurement device based on the inverse Shack–Hartmann technology. The device is aimed for consumers, telehealth, and at-home measurements. Users attach the device to their smartphone for control and analysis of the measurement. Looking through the device, users align patterns from the screen through two separated optical channels. The patterns combine on the user’s retina depending on their refraction values. A derivation of the formulation of the method is presented. The results of the accuracy assessment are analyzed through linear regression showing very good match between the input refraction values and the measured ones. Reproducibility result also show low variation between devices.

The potential of vertical-aligned spatial light modulators for use in adaptive-optics visual simulators is demonstrated. We performed visual acuity and contrast sensitivity tests in different subjects, with their eye’s optics corrected by the custom adaptive-optics system and with induced aberrations similar to those in severe keratoconus. We tested the system in a see-through configuration, achieving a total corrected field of view of ∼13 deg. The applicability of these modulation devices for a wearable visual adaptive optics system is discussed.

An optical calibration system for refractometers was designed and built using a singlet lens and a translatable detector that induces spherical error, making it a simple and easy-to-use system. The system was calibrated using spectacles and then validated by correcting blurred images created by a series of ophthalmic test lenses. A linear regression model of the correction of the ophthalmic test lenses showed nearly direct correlation with an offset of 0.12 D and an R² value of 0.9998. An updated version of the system was designed in Zemax OpticStudio with a singlet lens, which could induce spherical error accurate to that of the human eye within ≤ 0.05 D. In future work, this system will be manufactured and tested in the lab.

The human eye is a complex optical system with multiple elements. It is aspheric, nonsymmetric, and time dependent; yet, overall it has incredible performance. There have been various instruments developed over the years to measure and then to guide treatment to correct for ocular aberrations. As the development of these instruments (and treatments) has progressed, we have sought to correct more difficult cases, which may be more aberrated, time-dependent, or difficult in some other way. To this end, we have developed a new dynamic aberrometer that expands the boundaries of measurement capability with the aim of measuring and treating more difficult cases. This aberrometer has been designed that incorporates high-resolution Shack–Hartmann wavefront sensing, full gradient (spot) corneal topography, dynamic acquisition, and a subjective digital refractometer. This instrument is designed to measure extremely high aberrations and to provide information for treatment in multiple modalities. A small clinical study was conducted with subjects ranging from 23 to 64 years old to evaluate the effectiveness of the dynamic analysis at selecting a refraction. Examples are presented for measurements with keratoconus, irregular corneas, and tear-film irregularity. In the clinical study, young subjects showed an overall +0.27 D reduction in instrument induced myopia using dynamic measurement compared to a snapshot. The instrument has a large dynamic range for measuring subjects with keratoconus and other aberrated corneal conditions. The new instrument is effective at providing information needed for treatment in multiple modalities. The subjective digital refractometer corrects the fixation target for the objectively measured low-order aberrations (defocus and astigmatism). This provides immediate subjective feedback on the objective refraction and, with the ability to manually adjust the refraction parameters, the ability to compare objective and subjective refractions in the same setting.

A system has been developed to measure spherocylindrical spectacle lenses. The global pandemic limited access to regular eye exams. The system bypasses this limitation by providing at-home prescription measurements. The power and orientation of the spectacle lenses are obtained with a cell phone camera, a displayed or printed target, and a magnetic stripe card. The spatially varying magnification of the lenses is calculated by examining the image captured through the lens of the target at a fixed distance. This information is then used to calculate a clinical prescription, i.e., Sph/Cyl × Axis. The system is tested with 48 single-power spherocylindrical lenses with a range of −11.25 to +4.25 D for Sph, −5.25 to −0.25 D for Cyl, and a whole range of Axis. The results are plotted in comparison with these reference prescriptions provided by a commercially available lensmeter and show good agreement. The image processing and clinical prescription calculation are discussed here.

A digital-holography (DH) system is created in a heterodyne-pulsed configuration, meaning that the reference and signal pulses are nondeterministically correlated in time. Using the off-axis image plane recording geometry, two performance metrics are measured: (1) the total-system efficiency and (2) the ambiguity efficiency. These metrics are compared against the same measured efficiencies for a DH system in a homodyne-pulsed configuration, which uses deterministically correlated reference and signal pulses. The total-system efficiency of both systems is found to be consistent with one another, showing that no new component efficiencies are required when switching from a homodyne- to a heterodyne-pulsed configuration. Additionally, an instantaneous phase modulation model is used to characterize system performance in terms of nonideal pulse overlap. Such a model validates the use of the ambiguity efficiency for future efforts.

We propose a practical near real-time compression-based class-associative multiple target detection technique for color images. The size of the color images is reduced to its utmost ratio to speed up the processing time and to increase the storage capacity of the recognition system. A fringe-adjusted joint transform correlation technique is employed to successfully detect the compression-based multiple targets in colored images. In addition, to eliminate the false alarms and zero-order terms due to multiple desired and undesired objects in an input scene, we have used the shifted phase-encoded and the reference phase-encoded techniques. The performance of detecting the class-associative multiple targets for large compression ratios (up to 94%) and under strong noisy conditions (Gaussian and salt and pepper noises) is assessed through many computer simulations experiments. Furthermore, detecting small targets or even a small part of a target is presented for images occluded up to 90%.

Plenoptic imaging offers not only two-dimensional projections but also adds light array directions, thus supporting single-shot all-in-focus imaging. Its poor spatial resolution becomes an obstacle to high-quality all-in-focus imaging performance. Although various super-resolution (SR) methods have been designed and combined with multifocus image fusion (MFIF), high-quality multifocus fused SR images can be reconstructed for various applications, almost all of them deal with MFIF and SR separately. To our best knowledge, we first unify MFIF and SR problems as the multifocus image SR fusion (MFISRF) in the optical perspective and thus propose a dataset-free unsupervised framework named deep fusion prior (DFP) to address such MFISRF, particularly for plenoptic SR all-in-focus imaging. Both numerical and practical experiments have proved that our proposed DFP approaches or even outperforms those state-of-the-art MFIF and SR method combinations. Therefore, we believe DFP can be potentially used in various computational photography applications. The DFP codes are open source and available at http://github.com/GuYuanjie/DeepFusionPrior.

Polarized light-matter interactions are mathematically described by the Mueller matrix (MM)-valued polarized bidirectional reflectance distribution function (pBRDF). A pBRDF is parameterized by 16 degrees of freedom that depend upon scattering geometry. A triple degenerate (TD) MM assumption reduces the degrees of freedom to eight: one for reflectance, six for non-depolarizing properties, and one for depolarization. When the non-depolarizing dominant process is known or assumed (e.g., Fresnel reflection), the degrees of freedom are further reduced to two. For a given material, if the TD model is appropriate and the dominant non-depolarizing process is known, then these two degrees of freedom can be estimated from as few as two polarimetric measurements. Thus, the MM can be extrapolated from a reduced number of measurements. The primary contribution of this work is the development and demonstration of a linear estimator for an MM’s dominant eigenvalue (i.e., single depolarization parameter) that requires fewer measurements than a full MM reconstruction. MM extrapolations from single snapshot acquisitions with a Sony Triton 5.0MP polarization camera are performed at 30 acquisition geometries and two wavelengths on an ensemble of LEGO bricks treated to have varying surface roughness. These extrapolated MMs are compared with MMs reconstructed from a complete dual rotating retarder Mueller imaging polarimeter. The flux error mean and mode are 11.06% and 1.03%, respectively, despite a 10 × reduction in the number of polarimetric measurements.

The diffractive optical elements (DOEs) have the advantages of lightweight, low-cost, loose surface-figure tolerances and convenient folding, which provides a direction for next-generation large (>20 m) space telescopes. However, the cophasing of the DOEs limits its application in telescopes and decreases the optical system performance. The phase diversity method is studied and introduced to solve the cophasing detection of the two segmented DOEs. An improved experimental system is set up using the phase diversity method, and the experiment results show that the piston error and tilt error of the two mirrors are 0.03λ and 0.05 μrad, respectively, which meet the actual needs of the cophasing errors.

Within telecommunications, it is well established that optical preamplified receivers offer better sensitivity than linear-mode (LM) avalanche photodiodes (APDs) for direct-detection measurements. Long-range light detection and ranging (LiDAR), however, has predominantly made use of LM APDs for detection. This discrepancy arose due to the high coupling loss between the multimode return signals in LiDAR and single-mode optical preamplifiers. Recently, few-mode (FM) optical preamplifiers have been experimentally demonstrated, which solve this modal mismatch. We evaluate the applicability of FM preamplified receivers to long-range pulsed LiDAR in comparison with currently deployed LM APDs by considering the two receivers’ noise, gain, and bandwidth characteristics. We show that FM preamplified receivers can offer an improvement in both receiver signal-to-noise ratio and bandwidth.

We analyze the erythrocyte aggregation by the biospeckle laser phenomenon and calculate characteristic parameters to develop non-invasive hemorheological techniques. Biospeckle is more complex than other light-based techniques, such as light transmission, providing information about the dynamics of the scatterers, their size, and morphology. Dextran 500 kDa combined with plasma and phosphate-buffered saline was used to induce controlled in vitro erythrocyte aggregation. Red blood cell aggregation was studied using an objective speckle setup in a microchip sample chamber. The speckle grain sizes were determined, and parameters to characterize the morphology of the erythrocyte aggregates were proposed. The inertia moment and the correlation coefficient were determined to assess the cells aggregation dynamics.

We report the development of a portable coherent Doppler lidar (CDL) system incorporating onboard field programmable gate array for real-time data processing. An ordinal statistic-based mean Doppler frequency estimation algorithm with high accuracy and low computational cost is proposed. The basic characteristics of the proposed method were specified with the use of mathematical modeling through the algorithm, realized as MATLAB computer code that includes parts simulating lidar signals and data processing. A comparison of the proposed method with established and well-used determination methods of the maximum Doppler frequency is accomplished. The ordinal statistics method is easy to implement and facilitates obtaining computational results swiftly. To demonstrate the performance of this lidar system, an intercomparison field campaign between lidar and ultrasonic anemometer is carried out. The excellent degree of correlation experimentally confirmed the feasibility of developing affordable, low-power consumption, compact, and robust CDL systems for various industrial and scientific applications.

Electrical impedance tomography (EIT) is a nondestructive imaging technique for estimating the electrical impedance distribution within a conductive process. EIT is employed in many fields, including medical, geology, and industrial. The pressure sensor design is one of the most recent applications of this method. In this application, a thin and flexible conductor uses as the pressure sensor to apply physical pressure on a specific region and cause local variations in the electrical impedance of that region. We present the principles of designing and implementing a multifrequency EIT system for pressure sensing and imaging. Considering this, the theoretical topics and the hardware components of an EIT system are described. Subsequently, with the aim of examining the designed system, the conductive fabric was employed as the pressure sensor. In numerous experiments, the reconstructed electrical impedance images of the applied pressure demonstrated that there is a relatively linear relation between the pressure and the impedance variations at each frequency. The experimental results reveal that the designed system successfully detects the pressure area and determines its magnitude in the applied pressure region.

High-precision, stable tracking technology on a mobile platform is a key technology for realizing the integration of airborne search and tracking. In this study, coarse-fine compound technology based on gyrostabilization and miss tracking is applied. First, the working principle of the coarse and fine composite system is analyzed, and then the coarse and fine composite algorithm is modeled. Thereafter, the error of the coarse and fine composite axis is analyzed, and the precision tracking servo control algorithm is designed. Finally, using the model and algorithm designed to control the single reflector and galvanometer, the coarse tracking of the space target and the fine tracking of the source are completed under the condition that the equivalent sine amplitude is 5 deg and the frequency is 0.2 Hz. At the same time, a test system was built for performance verification. The algorithm achieved a coarse-tracking azimuth-axis tracking accuracy [root mean square (RMS)] of 26.3 μrad and a pitch-axis tracking accuracy (RMS) of 28.9 μrad. After composite tracking was switched on, precision tracking azimuth-axis tracking accuracy (RMS) improved to 7.9 μrad and pitch-axis tracking accuracy (RMS) improved to 6.3 μrad, both better than 10 μrad. This study provides new insights into the stability and precision tracking of single cameras on airborne platforms. At the end of this work, the outdoor experiment to verify the influence of random interference on the system performance under given conditions and the subsequent optimization direction of the system are given.

Existing roughness machine vision inspection methods based on designed indices require the artificial construction of features associated with roughness based on image information. Such methods are less universal but highly demanding for the imaging environment and cannot be directly applied to general industrial environments. The roughness machine vision inspection methods based on traditional deep learning, on the other hand, rely heavily on a large number of training samples, and the model needs to be retrained when the recognition of new categories is required by the inspection task. Therefore, a machine vision inspection method for milling surface roughness grade based on the improved model-agnostic meta-learning (MAML++) network was proposed to address the above problems. This method could not only automatically extract the image features associated with roughness but also recognize the surface roughness grade of milling workpieces by a small number of labeled samples. It reduced the need for sample size and can recognize new categories without retraining the model. The experimental results showed that the method exhibited high accuracy in roughness inspection tasks with different sample distributions and had promising robustness to complex light environments. In addition, in terms of the requirement for recognizing new roughness categories, a low-cost and fast roughness inspection task could be accomplished by acquiring fewer labeled samples. In summary, the proposed model has the potential to be a fast inspection system that can be applied to surface roughness inspection for most industrial applications. It provides the technology for inline inspection of machine vision roughness in general industrial lighting environments.

Optical elements are used in various fields, in which it is required to measure spherical, aspherical, and free-form surface shapes. We use a nanoprofiler previously developed by our research group to measure a cylindrical surface shape. Unlike spherical or aspherical shapes, the cylindrical surface shape of free-form surface is rotationally asymmetrical and thus difficult to determine the measured position. Therefore, we processed fiducial marks on the measured sample and specified the measurement position by fiducial marks. Then we performed comparative measurements with a Fizeau interferometer using a computer generated hologram (CGH) as a reference plane. The nanoprofiler and interferometer measured to accuracies of 43.7 nm peak-to-valley (PV) and 60.9 nm PV, respectively; the difference between the two was 17.2 nm PV. Furthermore, a spherical shape and an aspherical shape were similarly measured by both devices. The spherical shape had a difference of 8.4 nm PV and that of the aspherical shape was 9.7 nm PV; thus the difference between the measurements of the cylindrical surface shape was larger. Considering that the guaranteed accuracy of CGH for measuring a cylindrical surface shape is ∼30 nm PV, we concluded that this is a reasonable result and fiducial marks are effective to determine the position.

An improved composite Fourier transform profilometry is proposed in this study to quickly measure the three-dimensional (3D) contour of an object. Three groups of fringe patterns with different frequencies are combined in this method. Compared with the other composite profilometry, it solves the problem of phase error propagation in phase unwrapping. The absolute phase of the object is accurately calculated and the measuring accuracy is improved greatly by filtering out the background direct current noise. Both simulation and experiment results show that the 3D contour of the measured object can be reconstructed quickly and accurately by this method. It has great application potential in the field of real-time 3D measurement.

In the field of microstructuring and nanostructuring ultrashort pulsed (USP) laser processing attracts increasing attention due to its ability to generate high-precision structures. A simulation of the USP process can lead to a reduced process development time and help to achieve a better geometrical quality of the manufactured microstructures. To predict the ablation shape, temperature distribution, and distortion in a USP ablation process, a detailed simulation of the physical processes during and after the laser ablation is required. Different simulation tools such as multiscale simulations are already established but still need different and accurate input parameters regarding the material properties of the workpiece to be machined. A material characterization procedure that can be used in a standardized way for different materials and processing stations needs to be developed. The procedure determines the absorption coefficient, penetration depth, and ablation threshold precisely matched to a USP machine and the material used. Based on the material characterization procedure a calibration of the used simulation has to be carried out as precisely as possible. The simulation can then be applied for a digital process development and subsequently validated with specific experiments.

A generalized inverse matrix-recurrent neural network (GIM-RNN) data processing algorithm for unknown emissivity was proposed to measure high-temperature by multiwavelength pyrometer (MWP). First, emissivity classification was realized quickly according to a solution from generalized inverse algorithm to underdetermined multiwavelength equation group. According to the relationship between the emissivity of 2 adjacent channels of the 6 channel thermometer used in this paper, 243 emissivity models (1 * 3 * 3 * 3 * 3 * 3) were designed for classification. Twelve of them were shown on the schematic diagram in the text. To make the figure clear and easy to observe, the prediction results of six common emissivity models were listed as the experimental results. Then, it was input into the corresponding RNN subnetwork to inverse temperature precisely. Simulation results showed that in the range of 1500 to 3000 K temperature, the relative error of the test set was within 1.0% for the network trained after classification by GIM, whereas the relative error of the test set was within 1.2% for the network trained without GIM classification. After 5.0% random noise was added to the inputting data, the relative error still was controlled within 1.5%, which reflected the good antinoise performance of the algorithm. Multispectral measurement data of rocket engine plumes is processed by the proposed algorithm in this manuscript. The inversion results are consistent with the theoretical results. It is indicated that the proposed algorithm has good adaptability to different materials. It is expected to become a general data processing algorithm for MWP.

The signal-to-noise ratio (SNR) of heterodyne detection is seriously reduced by the spatial phase distortion, so the compensation method of the phase distortion is of great significance for improving the performance of the heterodyne system. By replacing the single detector with an array detector in the system, the previous compensation method based on sequence shift and optimization algorithm has a certain effect. However, the method also has various shortcomings, such as long processing time, poor search stability, and possible false alarm, which severely limits its practical application. To solve the above-mentioned problems, the autocorrelation operation is used to achieve the equiphase superposition of the signals output by the elements of the array detector, thereby realizing the phase compensation. In comparison to the previous method, our method does not need an optimization algorithm, which avoids the long iterative operations and considerably improves the processing efficiency. Besides, this method avoids the false alarm caused by the optimization algorithm. The numerical calculation indicates that in case of severe phase distortion, the proposed method can increase the SNR by dozens of dB compared with the single-point detector system, thus proving the effectiveness of this method. The study results may provide a feasible and effective phase compensation method for improving and promoting the laser heterodyne detection performance.

A low-cost NO₂ trace concentration measuring device based on a cavity ring-down spectroscopy (CRDS) technique is presented. We theoretically and experimentally demonstrate that, if the nonresonant direct reflection is effectively isolated, an inexpensive broadband diode laser (LD) can excite multiple cavity modes and stabilize the cavity output signal with an optical feedback (OF) method. To improve the measuring accuracy for low concentrations of NO₂ gases, we also design and machine a cavity with several mixing rooms. Several standard concentrations NO₂ gases are used to test the apparatus. The results prove a great agreement between measured and theoretical values over the full range.

When laser interferometry is applied to measure the form deviation of a helical tooth flank, dense interference fringes are almost inevitable because of the distorted shape of the helical tooth flank. The high-density fringe leads to poor processing accuracy of interferograms and is an important indicator of the interferogram quality. An optical path optimization method based on position errors analysis is proposed to modulate the fringe density. First, some factors that may affect fringe density in the optical system are modeled. Consequently, the incident angle of the measured light is proved to be the main factor affecting the fringe density and is the primary parameter that modulates the interference fringe density. Subsequently, an analytical model is established, and a fringe density modulation method based on the optimized incident angle is proposed for designing the optical path. Finally, simulated and actual experimental results are provided to verify the correctness of the analysis of the main factor affecting the fringe density and feasibility of the modulation method.

Approximate and rigorous methods are widely used to model light scattering from a surface. The boundary element method (BEM) is a rigorous model that accounts for polarization and multiple scattering effects. BEM is suitable to model the scattered light from surfaces with complex geometries containing overhangs and re-entrant features. The Beckmann–Kirchhoff (BK) scattering model, which is an approximate model, can be used to predict the scattering behavior of slowly varying surfaces. Although the approximate BK model cannot be applied to complex surface geometries that give rise to multiple scattering effects, it has been used to model the scattered field due to its fast and simple implementation. While many of the approximate models are restricted to surface features with relatively small height variations (typically less than half the wavelength of the incident light), the BK model can predict light scattering from surfaces with large height variations, as long as the surfaces are locally flat with small curvatures. Thus far, attempts have been made to determine the validity conditions for the BK model. The primary validity condition is that the radius of curvature of any surface irregularity should be significantly greater than the wavelength of the light. However, to have the most accurate results for the BK model, quantifying the validity conditions is critical. This work aims to quantify the validity conditions of the BK model according to different surface specifications, e.g., slope angles (SA) and curvatures. For this purpose, the scattered fields from various sinusoidal and combinations of sinusoidal profiles are simulated using the BEM and the BK models and their differences are compared. The result shows that the BK model fails when there are high SA ( ⪆ 38 deg) and small radii of curvature ( ⪅ 10 λ) within a sinusoidal profile. Moreover, it is shown that for a combination of sinusoidal profiles the BK model is valid for profiles with a high maximum slope angle value ( ⪆ 38 deg) if the average of positive SA is low ( ⪅ 5 deg).

Wideband metasurfaces usually have increased structure complexity as compared to their narrowband counterparts, by integrating multiple in-plane or cascaded resonant structures. We propose a dielectric grating structure as a quarter-wave plate with alleviated structure complexity and wide bandwidth at terahertz frequencies, by simply enlarging the period from subwavelength to superwavelength scale. From the modal theory, the superwavelength grating supports more than one waveguide modes. The superposition of the fundamental mode and higher-order mode with diverse dispersion characteristics is used to engineer the spectral response and to achieve a wideband linear-to-circular polarization conversion, which is validated by experimental study.

A polarization beam splitter (PBS) is proposed based on an asymmetric directional coupler consisting of a silicon nitride assisted waveguide and two ridge waveguides on a lithium niobate on insulator. Due to the silicon nitride buffer layer, the phase-matching condition is satisfied for TE polarization, whereas TM polarization has a significant phase mismatch by choosing reasonable widths of three waveguides. A 112.2-μm-long PBS is designed, and the gap width is chosen to be 300 nm to make it easy to fabricate. Numerical simulations show that the designed PBS has a broad bandwidth (>80 nm) for an extinction ratio of >15 dB and an ultralow insertion loss (<0.35 dB).

Diffractive optical elements (DOEs) have been widely used to realize beam shaping. However, the speckle noise always appears in the reshaped beam, which induces beam quality degradation. We proposed a regional padding algorithm based on the Gerchberg–Saxton algorithm to implement speckle-reduced beam shaping. A variable region is set in the DOE to iterate and optimize the phase of each part DOE. Zero padding is used around the DOE to decrease the sampling interval on the output plane. When the region expands and padded value reduction is controlled slowly, more sampling points on the output plane can be optimized effectively and suppress speckle noise. Numerical simulations and optical experiments have been performed to verify the feasibility of the proposed method.

Ultrashort-throw projectors allow large screens in very limited space and without concerns about shadows that obstruct the image, which is the development trend of the projector industry. However, the technical problems of image distortion and uneven illumination restrict its industrialization development. The freeform mirror is the core component of the ultrashort-throw projection system, and the high-performance design method and corresponding manufacturing technology are the key factors of ultrashort-throw projectors to achieve rapid development. In this paper, the initial shape of the freeform mirror is designed by geometric construction with energy mapping. The ultrashort-throw projection imaging system model is established. After optimization, the throw ratio is 0.2, the projection screen is 80 inches, the total length of the projection lens is 155 mm, the field of view is 152 deg, and the edge illumination of the system is 97% of the center illumination. The assembly and adjustment method of the lens and the processing and coating method of the mirror are studied, and the experimental prototype is assembled and tested. This paper aims to establish the theory of freeform surface design and provide theoretical and technical support for the construction and system design of off-axis complex freeform surface.

An analog of an electromagnetically induced transparency C4 symmetric device with a silicon array-dielectric layer-Au film F–P cavity is designed. Through the coupling between the waveguide and F–P cavity modes, the polarization-independent terahertz electromagnetic induced transparency (EIT)-like effect is obtained. The coupling analysis between the two types of modes in different orders shows the EIT-like effect, but the orders could affect the EIT-like properties. Moreover, the C4 symmetry structure makes it have polarization-independent characteristics. This research is conducive to obtaining polarization-independent EIT-like devices and to further research on the coupling between different kinds of modes for obtaining EIT-like devices with different performance requirements.

This work presents a theoretical approach to a highly multimode-guided wave-based fourth harmonic (FH) generation method by exploiting a zero-order Bessel–Gauss beam through the total internal reflection quasi phase-matching technique. An anisotropic crystal featuring a rectangular structure of beta barium borate has been used to carry out this analysis. In contrast to the conventional frequency conversion scheme using a Gaussian beam, the projected scheme yields a maximum six times superior efficiency of 6.9547% in a crystal length of nearly 6.3 mm, corresponding to a 266-nm FH wavelength. The analysis has also taken into account optical losses, such as absorption, Goos–Hänchen-shift, surface roughness, and the effect of the nonlinear law of reflection. Finally, a systematic study has been performed to describe the influence of parametric modification, such as variations in slab length and thickness, to provide a more comprehensive picture of the overall analysis.

When using Newton’s ring to test lens surface radius of curvature, the air gap in between the master plate and the lens can have either a positive or a negative meniscus shape. The optics for the negative meniscus shape air gap has been widely analyzed and the results are assumed to be valid for the positive meniscus shape air gap. Accurate and approximate analysis are performed in this paper for the positive meniscus shape air gap. Analysis results of this paper show that for the same parameters involved, the Newton’s ring number for the positive meniscus shape air gap is larger than the Newton’s ring number for the negative meniscus shape air gap. The difference between these two Newton’s ring numbers increases as the value of h / r increases, where h is the lens half diameter and r is the master plate surface radius of curvature. For h / r = 0.87, the ratio of these two numbers can be as large as 2.

Unidirectional absorbers have potential applications in infrared detection, energy conversion, and solar energy harvesting. In this work, the absorption properties of a typical photonic crystal (PC) unidirectional absorber in the visible- and near-infrared modulated by graphene are systematically investigated. This is achieved by placing a typical non-periodic PC in an air medium. When the electromagnetic wave is incident vertically, double peak absorption is formed at wavelengths of 560 nm as well as 755 nm. The achieved non-reciprocal absorption is attributed by the surface plasmon resonance between the semiconductor and metal defect layers. In this work, the effect of period numbers is discussed to improve absorption performance. Meanwhile, the role of the metal defect layer is explored by the variation of the electric field inside the device. And the unidirectional absorption performance was verified. In addition, the absorption characteristics of the two types of semi-conductor material devices are analyzed. Compared to existing unidirectional devices, this structure has the advantages of small size, high efficiency, wide bandwidth, and easy optoelectronic integration. This structure, based on its own absorption and emission characteristics, can facilitate the development of LIDAR stealth devices, and thus, the advancement of infrared radar steganography.

A series of on-chip optical links of 50-μm length, utilizing 650 to 850 nm propagation wavelength, with Si avalanche-mode optical sources, silicon nitride-based waveguides, and Si Ge detectors, have been designed and realized, with a 0.35-μm SiGe radio frequency bipolar integrated circuit process. The optical coupling between the optical source and the detectors was realized by a set of dedicated designed optical waveguides, which were all fabricated with components of the SiGe radio frequency process. All components were fully integrated on the same silicon chip. The Si avalanche-mode light-emitting diodes (Si AMLEDs) emitted in the 650- to 850-nm wavelength regime. Correspondingly, small microdimensioned detectors utilize SiGe detector technology with detection efficiencies of up to 0.85 in the same wavelength regime and with a transition frequency of up to 20 GHz. Best performances for the optical links as realized show optical coupling of up to 5 GHz with a total optical link budget loss of −40 dB. A set of link results are presented and several interpretations are given on current realizations. The technology is particularly suitable for realization of low-cost on-chip optical signal processing, optical interconnects, and various types of on-chip microsensors.

High peak-to-average power ratio (PAPR) remains the main challenge of coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. Selective mapping (SLM) is a well-known effective PAPR reduction technique, but it suffers from high computational complexity. To reduce the PAPR of OFDM signals, a low-complexity, partition-recombined SLM (PRSLM) scheme is proposed. The PRSLM scheme can generate more candidate signals than that of the conventional-original SLM (COSLM) scheme. The real and imaginary components of time-domain symbols are first partitioned, and then recombined to generate new candidate signals, which reduces computational complexity significantly. A 40-GBaud single-polarization CO-OFDM transmission system is set up to evaluate the performance of the PRSLM scheme. Simulation results show that the proposed scheme achieves 61.9% reduction in complex multiplication complexity, and 60.3% reduction in addition complexity compared with the COSLM scheme when the FFT size is 128. Furthermore, the proposed scheme achieves a similar PAPR reduction performance to the COSLM scheme without bit error rate degradation. In a nutshell, the PRSLM scheme improves noise tolerance and implementation efficiency of high-speed optical communication as well as alleviates nonlinear impairments.

Bit-error detection and correction is a very important part of a data communication network. In this article, we propose an optical approach to designing an error detection and correction circuit using Hamming code-based frequency encoded binary data and Toffoli gates. Here, the Toffoli gates are made of semiconductor optical amplifiers (SOAs) that work based on the nonlinear polarization rotation property. The characteristics of SOA, including very fast switching action (about 0.01 ns), low power consumption, frequency independent cross-polarization gain in the C-band, cascading property, etc., make the scheme attractive. The frequency encoded data-based proposed scheme will be effective for WDM communication networks. MATLAB simulation results testify to the feasibility of the proposed scheme.

An innovative scheme, named orthogonal frequency division multiplexing (OFDM)-based two-dimensional (2D) generalized optical quadrature spatial modulation (GOQSM), is proposed for bandlimited multiple-input multiple-output optical wireless communication (MIMO-OWC) systems. In this scheme, by way of combining two consecutive time slots into one time block, 2D GOQSM mapping can be executed in spatial domain and time domain. Therefore, more spatial bits can be transmitted, resulting in the increase of system spectral efficiency. Furthermore, discrete Fourier transform (DFT) spreading is further introduced in OFDM-based 2D GOQSM systems, so as to substantially reduce the peak-to-average power ratio (PAPR) of the transmitted signals. To evaluate and compare the performance of OFDM-based 2D GOQSM systems with other traditional systems, numerical simulations are conducted under various spectral efficiencies. The obtained simulation results prove that the proposed OFDM-based 2D GOQSM scheme with DFT spreading can achieve better bit error rate performance and lower PAPR than other benchmark schemes.

Small size, high-brightness fiber-coupled laser modules have always been the ultimate goal that all researchers are pursuing. A high-brightness 525 nm wavelength fiber-coupled system is designed and evaluated. Based on a multichip 2D green laser diode array, fast-axis collimators are set inside the light source, and a beam shaping system that can rearrange the beam and improve the beam quality in both axes is designed. The simulation results indicate that 24 single emitters are coupled into a 50μm / 0.15 NA optical fiber successfully and the output power is 22.55 W. The brightness of 16.25 MW / ( cm² · sr ) is calculated with fiber coupling efficiency is 97%.

The power scintillation of Ince Gaussian (IG) beam is investigated numerically based on the split step method. The effect of the IG beam modes on mitigating the effect of atmospheric turbulence is studied. Furthermore, the power scintillation indices of IG beams with different modes are compared with the fundamental Gaussian beam power scintillation indices. Moreover, the ability of different parameters, namely the beam waist, operating wavelength, and the receiver aperture, to reduce the scintillation is examined. The obtained results show that IG beams with even modes have the advantage over Gaussian beams for links longer than 2 km. Moreover, IG beams with even modes are able to reduce the scintillation more than the ones with odd modes. Yet, increasing the beam waist and the receiver radius reduces the scintillation of the beams with odd mode more than the beams with even mode. Our results contribute to the improvement of free-space optical communication systems operating in turbulent atmosphere.

A better understanding of stress effects that affect the bending losses in active and passive optical fibers allows us to improve fiber system designs and helps to optimize refractive index profiles in high power, large mode area laser fibers. Bending an optical fiber affects the light in a fiber core by two different phenomena. First, the curved shape of the waveguide changes the light propagation. The second phenomenon is the refractive index change caused by the mechanical stress in a bent optical fiber. The refractive index changes due to bending stresses are estimated by the elasto-optic and stress-optic models. The light propagation in a curved waveguide can be modeled by applying the electromagnetic wave theory together with the conformally transformed refractive index profiles that include the stress effects. The modeled refractive index profiles that include the bending stress-induced index changes are compared with the refractive index profiles that were measured from actual bent optical fibers. We tested if this comparison would allow us to estimate stress-optic coefficient C₂ values in stress-optic model. Measured bend loss values are compared to the bend loss values simulated with the modeled refractive index profiles.

The existence of atmospheric turbulence seriously restricts the application of laser technology. However, the acoustic wave will affect the air density during the propagation process, change the original structure of the air turbulence, and then affect the propagation of the laser in the air turbulence. To study the effect of disturbance of atmospheric turbulence by acoustic waves on laser propagation, the propagation characteristics of lasers in local atmospheric turbulence under coherent acoustic wave perturbation are investigated experimentally. First, a coherent acoustic wave generator and a closed local air turbulence simulation device are designed. The propagation experiments of laser in a closed local air turbulence environment disturbed by coherent acoustic waves are carried out. It is found that the coherent acoustic wave will interfere with the wave reflected from the inner wall of the closed local air turbulence simulation device, and the sound pressure forms a regular standing wave intensity distribution pattern. Moreover, the variations of laser beam drift fluctuation variance, scintillation index, and beam diameter with coherent acoustic wave frequency, power, and sound source array position are studied. The results show that the original structure of air turbulence will be changed in the process of coherent acoustic wave propagation, which will affect the beam drift fluctuation variance, scintillation index, and beam diameter changes of laser propagation in air turbulence. The beam drift fluctuation variance, scintillation index, and beam diameter change regularly with the change of the frequency and power of the sound source. Changes of the fluctuations with frequency are relatively gentle, whereas changes with power are severe. This paper explores the influence of coherent acoustic waves disturbing air turbulence on laser propagation characteristics, and the research results provide a basis for improving or disturbing laser propagation performance in atmospheric turbulent environments.

In recent years, plastic optical fiber (POF) has been considered as a promising cost-effective scheme for short-distance data communications, multimedia communication in cars, and in-house networks. However, due to the intrinsic nature of the relatively large numerical aperture of POF and the high attenuation rate, implementing high data rates over 100-m POF transmission length will be a significant challenge. We propose a scheme of high-speed 100-m POF transmission system based on a visible red laser and a cascaded neural network (NN) post-equalizer. To mitigate the nonlinear distortion induced by the POF, three different NNs, i.e., convolutional NN (CNN), long and short-term memory NN (LSTM), and cascaded NN structure consisting of convolutional layers and LSTM (CNN-LSTM), are employed as the post-equalizer. Experimental results show that using three different post-equalizers can significantly improve the system performance compared with the Volterra equalizer baseline. Among them, CNN-LSTM can outperform the others in terms of the bit error rate (BER) and the system Q-factor in the low nonlinear region. When the system operating in strong nonlinear region, CNN can achieve optimal performance at a lower system overhead of complexity. Finally, we successfully demonstrated a 100-m POF transmission system using 16 quadrature amplitude modulation discrete Fourier transform-spread orthogonal frequency division multiplexing modulation format at 1.8 Gbps with BER below 3.8 × ^{10 − 3} by utilizing CNN-LSTM.

Modulation format identification (MFI) and optical performance monitoring are important for elastic optical networks. We propose a cascaded module-based MFI and optical signal-to-noise ratio (OSNR) estimation method. Polar coordinates were used as features to distinguish the modulation formats (MFs) and estimate the OSNR. Dilated convolution was adopted to increase the receptive field and reduce computational complexity. Four commonly used MFs were investigated: dual-polarization (DP) quadrature phase shift keying, 8QAM, 16QAM, and 32QAM. A 32G baud DP transmission system is established, and the results are discussed. The results show that MFI accuracy can reach 100% when the OSNR is less than the 7% forward error correction limit, and the accuracy of OSNR monitoring is close to 100%. Two factors, sample length and nonlinearity, were further investigated. The results show that the MFI and OSNR estimators can achieve perfect performance when the sample length is 3000. Simultaneously, the accuracy of the MFI remained at 100%, and the accuracy of the OSNR estimator decreased by 1% to 2% when the launch power changed from 0 to 5 dBm. Furthermore, two modules with the same structure, DC-CNN and CNN, were compared. The results show that the two models can achieve similar accuracies and that the DC-CNN has the least number of parameters. Finally, experimental verification was conducted to ensure the practical applicability of the proposed method.

A heat-affected zone significantly reduces the mechanical properties of carbon fiber-reinforced plastics (CFRP). However, the beam quality, especially the stability of the beam pointing, limits the heat-affected zone (HAZ) minimization studies. In this study, the HAZ is reduced by improving the pointing stability of the laser beam. A beam pointing stabilization system (BPSS) based on position-sensitive devices and fast steering mirrors (FSMs) was developed. The research focuses on building a control model from the position-sensitive devices’ position deviation signals to the FSMs’ corrected control angles. The control model can detect and predict the beam pointing deviations and correct the attitudes of the FSMs to correct the beam pointing deviations, improving the beam pointing stability. The performance of the proposed BPSS and the corresponding control model were experimentally verified. The results indicate that the pointing deviations of the corrected beam in the X and Y directions are reduced by 78.08% and 70.28%, respectively. Additionally, the HAZ of CFRP processed with the corrected laser is reduced by 30.8%. The quality of the laser cutting of CFRP is significantly improved.

We present the design of uncoupled-coupled group multi core fiber (U-CG-MCF) with 125-μm cladding diameter for high spatial density optical space division multiplexing links. Design approaches used for uncoupled MCF and coupled MCF are combined in the proposed design that has two groups of cores. All the three cores that belong to same group have strong coupling among them and form supermodes. As the number of cores involved in strong coupling is less than the total number of six cores and coupling is suppressed between cores in adjacent groups, the receiver complexity will be minimal for the proposed fiber. Trench-assisted heterogeneous core group profiles are used in this work to suppress the crosstalk between similar supermodes in adjacent core groups and to reduce the core group separation. Coupling coefficient in the order of 10 ^{− 3} per meter is achieved at 1.55-μm wavelength for all the supermodes with a small core group separation of 34 μm. Simulation results promise the applicability of the proposed 125-μm U-CG-MCF with minimum receiver complexity for future high spatial density optical links in C and L wavelength bands.

A weight recognition scheme is proposed based on the analysis of plastic optical fiber (POF) specklegrams. A simple experiment is performed for data acquisition comprising specklegram images corresponding to different external applied weights ranging from 0 to 3 kg in steps of 0.5 kg on the POF. These specklegram images are further split into training, validation, and test datasets for employment in the convolutional neural network (CNN). The model is trained and validated with a sufficient number of epochs (iterations) to obtain optimal training and validation accuracies (>90 % ); it is then used for recognizing (classifying) the unseen test dataset images. A user-defined CNN model is optimized using four different optimizers: Adam, AdaMax, Nadam, and RMSProp. The recognition (or test) accuracy of these optimizers is compared. The Nadam optimizer has the highest recognition accuracy of 93.1% for increasing weights and 91.9% for decreasing weights. Furthermore, we investigate the impact of two parameters, namely temperature effect and information loss of specklegrams, on the model’s weight recognition accuracy. The temperature effect is studied for finite temperatures ranging from 29°C to 35°C with 3°C step fluctuations around the ambient temperature of 25°C. We find that the maximum deviation in recognition accuracy is about 1.2%. To quantify the information content (speckles) in each blocked specklegram, Shannon entropy (SE) is estimated. These images are then used in our existing CNN model with the Nadam optimizer to evaluate recognition performance with blocking. With increasing blocking, we observe a decrease in SE and recognition accuracy. This analysis suggests that, even with information loss in the specklegrams at ≤20 % blocking, a good representation of weights with high recognition accuracy (>80 % ) can be obtained. The presence of a small physical obstacle in front of the imaging system can cause information loss in specklegrams to manifest in practical scenarios.

One of the ways to diagnose cancer in its early stages, which can lead to timely treatment, is to use Raman spectroscopy. The Raman spectrum of a material is created by the interaction of light with the material; it determines the exact characteristics and properties of the material, which can be used to distinguish cancerous cells from healthy ones. However, the intensity of this spectrum is weak, and the optical fibers used in these devices must have very little loss and thus have low sensitivity to bending. This paper proposes a circular photonic crystal fiber with a structure in which the losses in different bending radii are significantly reduced. The modified state of the fiber at the wavelength of 1.55 μm supports the transmission of 10 modes propagated in the core. The loss for the fundamental mode at this wavelength and bending radius of 2 mm equals 3.82 × 10 ^{− 16} dB / m, which is considered a low value. As the bending radius decreases further, low-order modes diminish, which are more sensitive to bending than high-order modes. However, their losses are lower than in high-order modes. The critical bending radius for this fiber, with the loss reaching 0.1 dB / m, is equal to 0.096 mm. This low critical bending radius shows the low sensitivity of the proposed fiber to bending. The fiber was examined in different bending radii and wavelengths, and the parameters of this structure were presented for the lowest bending losses, which can be used in Raman spectrometers as spectrum collecting fibers and improve cancer diagnosis devices with Raman spectrum.

Pyroelectric detectors are used for gas analysis and flame detection because of their fast response and excellent performance. Most pyroelectric devices are based on monocrystalline lithium tantalate or pyroelectric lead zirconate titanate thin films deposited on a silicon (Si) substrate. In comparison, recently discovered pyroelectric-doped hafnium oxide (HfO₂) offers the possibility of manufacturing completely complementary metal-oxide-semiconductor (CMOS)-compatible devices on large Si wafers. This is a promising approach to simplifying mass production of the sensor element and realizing new sensor structures with a high performance. Si substrates were structured with trenches and filled with thin-doped HfO₂ layers by atomic layer deposition to multiply the pyroelectric current responsivity. An effective pyroelectric coefficient of up to 1300 μC / m² / K was measured. Micromechanical structuring of the 6-in Si wafers was used to improve the thermal conversion of the sensor element. The applied plasmonic absorbers increase the infrared light absorption to >80 % for the spectral range of 3 to 5 μm, which was determined using Fourier transform infrared reflection measurements. In the first step, the performance of the sensor element was evaluated with an analog transimpedance amplifier with a feedback resistance of 5 GΩ. A specific detectivity D ^* > 1 · 10⁷ cm√Hz / W was measured for the frequency range of 1 to 10 Hz. In addition, an application-specific integrated circuit was designed for the electrical signal conditioning to build a fully CMOS-compatible pyroelectric detector. It offers a simple to manufacture, flexible configuration, and digital communication interface with a signal-to-noise performance close to analog detectors. We present the measurement results of different sensor elements and detector types.

We propose a polarization-insensitive broadband tunable metamaterial-based perfect absorber with high absorption. This absorber is composed of three layers of structure, a gold substrate at the bottom, the first polyethylene cyclic olefin copolymer (Topas) dielectric and vanadium dioxide (VO₂) as the top layer. Our theoretical results led us to show that this absorber has more than 90% absorption at 3.42 to 7.22 THz with a continuous bandwidth of 3.8 THz, whereas more than 99% absorption in the range of 3.98 to 6.68 THz with a continuous bandwidth of up to 2.7 THz in the case of vertical incidence of electromagnetic waves. In addition, near-perfect amplitude modulation can be achieved by varying the conductivity of the VO₂ where the absorbance can be continuously tuned from 1.7% to 100%. It is concluded that the performance of our reported absorber is significantly improved compared to that of previously reported VO₂ absorbers. Moreover, perfect absorption and resonant frequency drift can be explained with impedance matching and propagation phase theories, respectively. In addition, our absorber has polarization insensitivity as well as wide-angle absorption due to the presence of rotational symmetry in the absorber structure. Finally, we believe that the absorber proposed is strongly competitive in the terahertz (THz) range for designing THz modulators, cloaking, optic-electro switches, and photodetector.

Fresnel coefficients (FCs) corresponding to a lossless magnetic achiral/chiral (ACC) interface under variable angles of incidence (of s-polarized light) and permittivity/permeability ratios are derived including the nonmagnetic-magnetic case, examining possible anomalous Brewster and critical angle conditions. Bimodal and single-mode propagation (including also cases where both characteristic modes (right- and left-circular) in the chiral region enter evanescence) across a magnetic chiral/achiral (CAC) interface are then examined for possible anomalies. Simulation results are presented along with interpretations, supported extensively by verified tabular data. Finally, the concepts behind examining magnetic chiral interfaces in the presence of realistic dielectric loss are introduced for the ACC interface, indicating how the Snell’s laws are modified for each mode, and the decay behavior of the fields has to be rederived to accommodate the effect of loss. To introduce possible application areas, the characteristics of lossy, magnetic, and chiral slab resonators are presented, specifically relative to transverse resonances. Finally, a magnetic, chiral Fresnel zone plate is introduced via a methodology for examining its imaging and tuning properties relative to the two circular modes.

A hollow-core negative curvature fiber (HC-NCF) with high birefringence for temperature sensing based on surface plasmon resonance effect is proposed. For the HC-NCF, the cladding layer is composed of six silica tubes, and the thickness and radius of the two silica tubes in the y-axis direction are different from those of other ones. Moreover, the two silica tubes in the y-axis direction are filled with the gold wires, and the ethanol to be tested is filled in other silica tubes and air area. As the temperature changes, the coupling state between the x-pol core mode and surface plasmon polarization mode will change. The temperature sensitivity can be up to 3.1 nm / ° C in the range of 20°C to 40°C. Moreover, the proposed HC-NCF temperature sensor simultaneously achieves the maximum R of 3.333 × 10 ^{− 2} ° C and maximum FOM of 269 ° C ^{− 1}, which indicates that it has high sensitivity, good linearity, high R, and large FOM. The proposed HC-NCF temperature sensor is expected to be useful in the field of the environmental monitoring.

An integrated device for graphene electro-optic (EO) modulation and wavelength-mode division hybrid multiplexing (WMDM) is proposed. The device is composed of three parts: EO modulator, wavelength division multiplexer (WDM), and mode division multiplexer (MDM). The EO modulator and WDM consist of a one-dimensional photonic crystal nanobeam cavity coupled with a nanowire waveguide. The modulation of a certain wavelength can be achieved by utilizing the characteristic that the chemical potential of graphene can be adjusted with voltage. The MDM adopts nanowire waveguides to realize mode conversion. The performance parameters of the integrated device are analyzed using the three-dimensional finite-difference time-domain method of numerical analysis. The results show that the integrated device can simultaneously complete the modulation and WMDM of the fundamental transverse electric (TE₀) modes and first-order TE (TE₁) modes at 1570.0 and 1573.2 nm, respectively. The extinction ratio is greater than 23.2 dB, the insertion loss is <0.36 dB, and the channel crosstalk is < − 22.7 dB. In addition, the modulation speed reaches 18 GHz.

The doping of rare earth elements revolutionized applications of optoelectronic devices. Europium (Eu) ions acting as activators can provide the fundamental effect on luminescence of host materials. We investigate photoluminescence, lattice damage formation of Eu-implanted potassium titanyl phosphate (KTiOPO₄) crystals using 400-keV Eu ions with a fluence of 5 × 10¹⁵ ions / cm² at room temperature. Effective doping is achieved in the depth range of about 50 to 200 nm from the surface of KTiOPO₄ crystal, and the luminescent activity of Eu ions is inhibited due to significant implantation damage. However, the samples showed intense luminescence emission at room temperature through the lattice recovery after annealing at 600°C and 700°C. The lattice damage of as-implanted and annealed samples was investigated by Rutherford backscattering spectrometry in channeling configuration. Furthermore, the relative strain depth curves of Eu implanted KTiOPO₄ samples at different annealing temperatures were studied using the high resolution x-ray diffraction and Raman spectra. The present research reveals that Eu implanted KTiOPO₄ samples have great prospects in the field of optical applications and waveguide quantum reservoirs.

In the past few decades, many efforts have been made to solve the intercarrier interference (ICI) because of the loss of the orthogonality for the spectral efficient frequency division multiplexing (SEFDM) optical communication systems. In this paper, a deep neural network (DNN) is introduced to deal with the ICI. The intrinsic relationship between the mechanism of ICI damage and the DNN is studied and analyzed. Based on this analysis, the performance of DNN-ICI decoder compared with the conventional algorithms is demonstrated by simulation for optical SEFDM intensity modulation/direct detection (IM/DD) communication systems. The results show that the DNN-ICI decoder is greatly superior to other schemes in terms of bit error rate (BER) with a simple designed DNN under different bandwidth compression factors. Besides, the proposed methods are also robust to the fiber lengths. All the results indicate that the proposed DNN-ICI decoder has great potential to be used in SEFDM IM/DD optical systems.

Three-dimensional (3D) spatial data, terrain type, and physical properties of field terrain are very important information for mobile robots in trajectory planning, force adjustment, motion control, dynamic parameter optimization, and so on. Obtaining the information in advance through vision can provide a prior perception to help make reasonable decisions and improve security and adaptability. We present the first monocular-vision-based multiinformation map construction framework for field terrain. The current state-of-the-art physical properties inference algorithm is promoted in illumination robustness, time consumption, and terrain richness. Our improvement makes it possible to apply physical properties inference to the real-time mapping of field terrain. An improved monocular vision and inertia fused localization scheme is used to introduce the gravity direction. Depending on the obtained terrain segmentation and camera pose, an unsupervised monocular scale-consistent terrain dense depth estimation method with comprehensive geometric constraints is designed. Finally, the components are systematically combined to form a complete framework, and the ambiguity of inference results between multiview frames is addressed. Comprehensive experiments are conducted to verify the effectiveness of each component and the performance of the overall framework. Our multiinformation map construction framework can output a gravity-aligned 3D RGB map, terrain map, friction property map, and stiffness property map at once with high accuracy.