Editor-in-Chief Adam Wax discusses several issues related to authorship of scientific papers.

The super-resolution (SR) reconstruction of remote sensing images is a low-cost and efficient method to improve their resolution, and it is often used for further image analysis. To understand the development of SR reconstruction of remote sensing images and research hotspots and trends, we examined its history and reviewed existing methods categorized into traditional, learning-based, and deep-learning-based methods. To evaluate the reconstruction performance, we conducted experiments comparing various algorithms for the single- and multi-frame SR reconstruction of remote sensing images considering three datasets. The experimental results indicate the advantages and limitations of single- and multi-frame reconstruction, with the latter showing a higher performance. Finally, we provide directions for future development of this SR reconstruction.

The Kaczmarz algorithm is an iterative method for solving linear equations in the form of Ax  =  b. It is widely used in computed tomography and digital signal processing (DSP) but has yet to be adopted in computer-generated holography. Phase retrieval algorithms, such as Gerchberg–Saxton or Fienup, are significantly more popular in this field. However, we propose a unique and alternative approach to projecting a replay field through discrete Fourier transform matrices and have shown that there are legitimate benefits to implementing this approach. The gradient descent iteration mechanism adopted by Kaczmarz, for instance provides finer granularity control over the individual pixels in the replay field. We consequently demonstrate the quality of the image is significantly improved when compared with Gerchberg–Saxton.

Video super-resolution (VSR) aims to generate high-resolution (HR) video by exploiting temporal consistency and contextual similarity of low-resolution (LR) video sequences. The key to improving the quality of VSR lies in accurate frame alignment and the feature fusion of adjacent frames. We propose a dual channel attention deep and shallow super-resolution network, which combines with HR optical flow compensation to construct an end-to-end VSR framework HOFADS-VSR (attention deep and shallow VSR network union HR optical flow compensation). HR optical flow calculated by spatiotemporal dependency of consecutive LR frames is used to compensate adjacent frames to implement accurate frame alignment. Deep and shallow channels with attention residual block restore small-scale detail features and large-scale contour features, respectively, and strengthen the rich features of global and local regions through weight adjustment. Extensive experiments have been performed to demonstrate the effectiveness and robustness of HOFADS-VSR. Comparative results on the Vid4, SPMC-12, and Harmonic-8 datasets show that our network not only achieves good performance on peak signal-to-noise ratio and structural similarity index but also the restored structure and texture have excellent fidelity.

We describe the flight testing and the integration process of the Microsoft HoloLens 2 as head-mounted display (HMD) with DLR’s research helicopter. In the previous work, the HoloLens was integrated into a helicopter simulator. Now, while migrating the HoloLens into a real helicopter, the main challenge was the head tracking of the HoloLens, because it is not designed to operate on moving vehicles. Therefore, the internal head tracking is operated in a limited rotation-only mode, and resulting drift errors are compensated for with an external tracker, several of which have been tested in advance. The fusion is done with a Kalman filter, which contains a non-linear weighting. Internal tracking errors of the HoloLens caused by vehicle accelerations are mitigated with a system identification approach. For calibration, the virtual world is manually aligned using the helicopter’s noseboom. The external head tracker (EHT) is largely automatically calibrated using an optimization approach and therefore, works for all trackers and regardless of its mounting positions on vehicle and head. Most of the pretests were carried out in a car, which indicates the flexibility in terms of vehicle type. The flight tests have shown that the overall quality of this HMD solution is very good. The conformal holograms are almost jitter-free, there is no latency, and errors of lower frequencies are identical with the performance that the EHT can provide, which in combination greatly improves immersion. Profiting from almost all features of the HoloLens 2 is a major advantage, especially for rapid research and development.

Surface inspections are important in steelmaking processes to characterize the final product’s quality. We present a method to measure surface depth profile using laser scattering geometry. This technique is used to analyze the focus effect on microscopic analyses of steel inclusions using the EN-10247 standard. The results presented herein offer promising new perspectives for the metal manufacturing industry through cost-effective solutions that attain quasi in-line process inspection capabilities.

Super-resolution (SR) aims to increase the resolution of imagery. Applications include security, medical imaging, and object recognition. We propose a deep learning-based SR system that takes a hexagonally sampled low-resolution image as an input and generates a rectangularly sampled SR image as an output. For training and testing, we use a realistic observation model that includes optical degradation from diffraction and sensor degradation from detector integration. Our SR approach first uses nonuniform interpolation to partially upsample the observed hexagonal imagery and convert it to a rectangular grid. We then leverage a state-of-the-art convolutional neural network architecture designed for SR known as residual channel attention network (RCAN). In particular, we use RCAN to further upsample and restore the imagery to produce the final SR image estimate. We demonstrate that this system is superior to applying RCAN directly to rectangularly sampled LR imagery with equivalent sample density. The theoretical advantages of hexagonal sampling are well known. However, to the best of our knowledge, the practical benefit of hexagonal sampling in light of modern processing techniques such as RCAN SR is heretofore untested. Our SR system demonstrates a notable advantage of hexagonally sampled imagery when employing a modified RCAN for hexagonal SR.

Phase shifting profilometry (PSP) has been commonly used in three-dimensional shape measurement. However, image saturation is a challenging problem leading to phase and measurement errors when measuring the shiny surface. We propose a new method using pixel-wise composed fringe pattern based on multi-intensity matrix projection of neighborhood pixels. First, a multi-intensity projection matrix is established and projected onto the object. The optimal intensity (the maximum projection intensity without saturation) for each pixel and the mask of each intensity can be obtained by analyzing the proportion of saturated pixels in the captured matrix region. Then, a set of new fringe pattern images without saturation can be obtained with phase shifted sinusoidal fringe patterns of different intensity levels and the corresponding mask. With PSP, the object can be reconstructed. Compared with existing methods, the proposed method can significantly reduce the number of projection operations and time consumption. The performance of the proposed method is verified by the experiments.

Although various camera calibration methods have been proposed, most of these methods cannot deliver accurate determination of the intrinsic camera parameters, because of the coupling errors existing between intrinsic and extrinsic camera parameters and the use of explicit distortion models. Here, we propose a model-free method for accurate camera calibration, which utilizes phase-shifted fringe patterns shown on a liquid crystal display (LCD) screen for estimating the intrinsic parameters of a camera and correcting lens distortion. Horizontal and vertical fringe patterns are consecutively displayed on the LCD screen, which is placed at two positions parallel to the camera sensor plane. These fringe patterns are captured by the camera from the front viewpoint for calculating the absolute phase maps. Then, the images with lens distortion can be transformed to the distortion-free images using an inverse mapping operation. Subsequently, the principal point coordinates are calculated according to the geometric imaging relationship between the positions of the two LCD screens. Finally, the focal length can be estimated using the similarity of triangles formed by the obtained principal point coordinates and the obtained phase maps at the two positions. Both simulated and real experiments are performed to verify the validity of the proposed method. The results demonstrate that this method not only successfully eliminates coupling errors but also perfectly corrects lens distortion.

The development of temperature sensors with ultra-fast response, high sensitivity, and wide range of temperature is of important academic value for the study of flame structure and combustion characteristics under extreme conditions. Recently, tunable diode laser absorption spectroscopy (TDLAS) is developed rapidly as an important means of combustion diagnostics due to its outstanding characteristics, such as non-invasiveness, rapid response, and high realizability. The absorption lines of TDLAS are extended to the mid-infrared region. Two spectral lines of H₂O at center wavelength of 2136.14 and 1874.31  cm^−  1 are used as candidate pair, which provides a wide measuring range (800 to 4000 K) and high-temperature sensitivity (4.55 at 1000 K) sensor for temperature measurement. We used two quantum cascade lasers with wavelength of 4.68 and 5.34  μm to scan the selected line pair of H₂O rapidly in the combustion environment and obtained the temperature information of the combustion field. Our work provides a good solution for the temperature measurement of extreme combustion environment with ultra-fast response, high sensitivity, and wide temperature range.

We investigate the three-beam conjugate enhanced micro-vibration detection system. We introduce a conjugate light to the classical dual-beam heterodyne laser vibrometer to generate the three-beam interference. Detection of the micro-vibration signal is then enhanced by matching the power and phase of conjugate light and measured light. We also present the basic principle of three-beam conjugate enhanced micro-vibration detection and the conjugate enhancement conditions. Moreover, the expressions of micro-vibration signal amplitude amplification and resolution enhancement are derived and compared with the demodulation amplitude and limited resolution of classical dual-beam laser vibrometer. Experiments and numerical results further verify the amplitude amplification and resolution enhancement of the three-beam conjugate enhanced micro-vibration system. The archived results indicate that this system is especially suitable for the high-resolution detection of weak vibration targets, e.g., micro-nano structures.

We present a simple method to measure the two principal planes and the effective focal length of an imaging optical system. It is based on measurements of transverse magnification of the image and both the object and image positions from two arbitrary reference points. A simple linear equation is derived to find the distances of the principal planes from the reference points, which enables the computation of the effective focal length. Experimentally, the method only needs a standard optical bench, thus avoiding the use of a nodal slide. The accuracy obtained for a practical example with an optical system composed of two achromatic doublets is smaller than 1 mm, or better than 2%, for all three measured values (two principal planes and the one effective focal length) as compared with the results of optical ray tracing performed with OSLO software.

We propose a model for coherence scanning interferometry using familiar Fourier optics methods and the spectrum of plane waves for the case where light source spectral bandwidth limits the fringe contrast as a function of optical path length. The model is straightforward to implement, is computationally efficient, and reveals many of the common error sources related to the optical filtering properties of the imaging system. We quantify the limits of applicability of the model related to the geometrical approximations for Fourier optics, particularly for high numerical apertures, and when using the fringe contrast for determining surface heights. These limitations can be overcome using a three-dimensional imaging model.

Multi-camera stereo-digital image correlation (stereo-DIC) methods have gained popularity as non-contact methods for measuring high-spatial-resolution deformation of large engineering structures. Common multi-camera stereo-DIC methods can be classified into two types: target-based and calibration-based methods. Considering the wide application prospects of multi-camera stereo-DIC methods, it is important to study and compare the performances of these two types of multi-camera stereo-DIC methods. A four-point bending test of reinforced seawater sea-sand coral concrete (Coral-SWSSC) beams was conducted in this study to investigate the performance of multi-camera stereo-DIC in deformation measurement of slender components, and the measurement accuracy, robustness, and applicability of the two types of multi-camera stereo-DIC methods were compared and discussed. Furthermore, deformation analysis of reinforced Coral-SWSSC beams based on high-resolution multi-camera stereo-DIC methods was performed. The analysis showed that beams strengthened with basalt fiber-reinforced polymer wrapped steel tube and ultra high performance concrete (UHPC) can fully exploit the properties of Coral-SWSSC; the two strengthening materials demonstrated a higher compatibility, which compensates for the low strength defect of the Coral-SWSSC, making it a viable strengthening method. This study may promote the application of multi-camera stereo-DIC methods to measuring high-resolution deformation of large engineering structures.

A low-cost near-infrared spectrometer based on fiber beam scanning and linear variable filter (LVF) was proposed and demonstrated. The key elements of the spectrometer include a piezoelectric driven fiber scanner, an LVF monochromator, and a single point detector. The single fiber scanner provides a new way to continuously sweep the LVF. A PIN photodiode behind the LVF monochromator can acquire more spectral information than the traditional linear array detection. The performance of the spectrometer was verified with a narrow band tunable laser and a broadband light source. With the help of a deconvolution algorithm, the spectral resolution of the proposed spectrometer is up to 0.32% of the center wavelength (≈5  nm@1550  nm).

We numerically calculated and analyzed the influence of a 7  ×  1 fiber power combiner on supercontinuum (SC) laser beam quality in the range of 0.5 to 2.3  μm. The calculation results show the M² oscillates within a range for a specific wavelength as the length of the output fiber changes. When the length of the output fiber is fixed, whether single input fiber or all fibers are injected with SC lasers, the M² of the SC shows a nonlinear change and oscillates continuously throughout the entire spectrum, while it first decreases and then increases as the wavelength increases as a whole. Analysis of the numerical results shows that by changing the type of input fiber and the taper waist structure, the beam quality of both long-band and short-band parts of the SC laser can be improved. To verify theoretical calculations, the divergence angle of SC was measured as the beam quality after being transmitted by one 7  ×  1 fiber power combiner in the spectral range from 0.5 to 2.3  μm. The divergence angle first decreases but then increases with a prolonging wavelength, and the curve of the experimental result renders a basic match with the calculation. To the best of our knowledge, currently no overall evaluation is reported on the beam quality of combined SC sources.

Process parameter optimization is important for improving the lapping effect of optical elements. Based on a temperature-controlled lapping disc, we present a method using fuzzy theory and orthogonal experiment to optimize lapping process parameters. Unlike the theory of optimization target set with a single evaluation criterion, a comprehensive optimization target with multiple evaluation criteria based on a fuzzy algorithm is proposed in this method. The lapping disc temperature, lapping disc rotation speed, and lapping pressure were selected as the optimization process parameters, and the material removal rate (MRR), surface change uniformity (SCU), and fuzzy synthetic index of lapping (FSILAP) were used as the optimization targets to conduct comparative experiments and analysis. The experimental results showed that when using the optimal process parameter combination obtained by the comprehensive index FSILAP as the optimization target, balanced results for MRR and SCU could be obtained. Notably, the result obtained when using a single evaluation criterion as the optimization target was suboptimal. The proposed method is effective and practical for the optimization of lapping process parameters.

All-dielectric waveguide modulators nowadays are critical for on-chip photonic circuit’s integration. We propose and theoretically demonstrate a graphene-based all-dielectric waveguide modulator with offset nanowires. The displacement between the nanowires causes the electric field parallel to the graphene to increase, thereby realizing an enhanced field modulation effect. We obtain the best modulation performance by optimizing the structural parameters and changing the offset distance of the nanowires. Analysis of the simulation results indicate that the structure offers a high modulation depth (>0.13  dB  /  μm) covering the S  ,  C, and L bands. Benefitting from the strong subwavelength confinement and excellent broadband modulation performance, the proposed optical waveguide modulators offer a significant potential to realize various long-wave integrated modulators, interconnects, and optoelectronic devices.

A dual-core photonic crystal fiber polarization beam splitter filled with salt water in the central ellipse is designed and optimized by the full-vector finite element method. For a polarization beam splitter length of 31.1245  μm, the orthogonally polarized light with a bandwidth of 180 nm around 1.55  μm can be separated effectively and the extinction ratio (ER) is 139.88 dB. The photonic crystal fiber polarization beam splitter offers advantages such as the short length, high ER, and broadband characteristics and have large potential in optical communication.

An analytical method to investigate the characteristics of the visible-light positioning using received signal strength (RSS) is proposed, and the formation mechanism of the positioning error is explored. The positioning error is studied in terms of two contributing factors being the original measurement noise of RSS and the dilution of precision (DOP), which transforms the measurement error to the positioning error. It is found that the DOP is related to light-emitting diode (LED) parameters, receiver parameters, and the geometric deployment of the LED beacons. In particular, the LED beacon geometric deployment in the form of triangle, square, and hexagon cell geometries are characterized and optimized. Numerical results show that the mean positioning error of the triangle deployment is smaller than the other two geometries under the same condition, and the side length of the cell should reduce as the half power angle of the LED decreases. The results of this paper provide a guideline to design the RSS-based visible-light positioning (VLP) systems.

An innovative constellation-shaped three-dimensional 14-quadrature amplitude modulation QAM (3D-14QAM) mapping scheme based on regular tetrahedron is proposed. The non-uniform 14QAM signals are designed by Huffman code and mapped onto the designed 3D constellation based on regular tetrahedron. The simulation demonstration is carried out successfully by VPI software in a 4  ×  100  Gb  /  s wavelength division multiplexing (WDM) polarization division multiplexed coherent optical communication system with a transmission distance of 300 km. For WDM system, the simulation results show that the proposed scheme is capable of achieving about 1.8 dB OSNR gain over the traditional 3D-16QAM scheme at the hard-decision forward error correction (HD-FEC) limit of 3.8  ×  ^{10  −  3}. In addition, the proposed 3D-14QAM can obtain 1.1 dB OSNR gain compared with the traditional 3D-16QAM for single-channel scenario at the HD-FEC. The outstanding performance indicates that the proposed scheme can provide a performance improvement for both single carrier system and WDM system. And benefited from the ability of suppress nonlinearity, the proposed scheme can obtain a larger gain in the WDM system than a single carrier system.

We propose an improved Gaussian curve fitting method based on the Hilbert transformation (HTG) to tackle the ineffectiveness of the traditional peak-seeking algorithm in detecting the multi-peak Fiber Bragg grating (FBG) reflection spectra. A five-point sliding filter is used to process the FBG reflection spectral signal, de-noise and smooth the noise, and select the optimal threshold point by the Hilbert transformation (HT). The sub-spectra of the multiple FBG reflection spectral signals were derived and the initial positioning of the spectral peaks were achieved. The Levenberg–Marquardt (LM) algorithm is used to extract the Bragg wavelength from the segmented sub-spectral signals as well as optimize the Gaussian curve fitting coefficients. The HTG-LM algorithm is then proposed, and is optimized and utilized to achieve precise positioning of the spectral peaks. The theoretical analysis and experimental results showed that the proposed HTG-LM algorithm could dynamically detect the multiple reflection spectra of the FBG sensing system with good stability, and at the same time, reduce the amount of peak-seeking data, which is highly beneficial to improve the signal demodulation rate. The peak detection accuracy of the proposed algorithm is better than 1 pm and the precision is better than 4  ×  10^−  7  pm, which indicates that this HTG-LM algorithm provides an accurate demodulation algorithm for the FBG sensor networks. As a result, it is a promising multi-peak detection algorithm proposed by this paper to be applied to the FBG sensing systems.

A spectral economized scheme for optical quantization is proposed by using cascaded unbalanced Mach–Zehnder modulators (UMZMs) to introduce phase differences between two arms. Less spectra are needed in this scheme since the period of sampling pulse can be modulated to be larger. Simulation results show that the effective number of bits (ENOB) reaches 4 when only 2 additional wavelengths and 4 periods are employed, in contrast to the 15 additional wavelengths required by traditional frequency quantization schemes. Larger NOB is predicated when more wavelengths are used in the quantization system. The influence of noise on ENOB is also studied. It is revealed that the timing and intensity jitters do not influence ENOB. Instead, the ultimate ENOB is limited by the largest phase difference introduced by UMZMs. The proposed scheme can be used in both low- and high-resolution optical quantizers, indicating promising spectrum economized applications for future optical circuits.

For the first time, we propose a dual-hop multiple input multiple output (MIMO)-based convergent underwater wireless optical communication (UWOC)–free-space optical (FSO) system. The UWOC and FSO links are Gamma–Gamma (GG) distributed. Closed-form expression for the average bit error rate (ABER) is derived for the proposed MIMO-based dual-hop UWOC-FSO convergent system using the GG cumulative distribution function. The end-to-end system performance analysis is carried out by considering the turbulence, attenuation, and pointing error effects for UWOC and FSO links. For the UWOC link, different oceanic conditions, such as the clear ocean, coastal ocean, and turbid harbor, are considered. Various atmospheric effects, such as clear air, fog, rain, drizzle, and haze, are considered for the FSO link. The analytical results of the proposed MIMO-based convergent system are compared with single-input single-output (SISO) system. As a result, it is observed that the proposed MIMO 2  ×  3 scheme offers an improvement of 35 dB in the average signal-to-noise ratio compared with the SISO system at ABER of 10^−  5 in the case of weak pointing error.

To improve the deficiencies of the current pressure-sensing structure, such as temperature crosstalk and large size, a cascaded polymer microcavity structure is prepared in the optical fiber end surface, which combined with the fiber forms an ultracompact three-beam Fabry–Pérot interferometer. Two different size interferometers are prepared to monitor the details of their interference peak changes with temperature and pressure, and the sensitivity matrix method is used to measure the ambient temperature and pressure simultaneously. The proposed sensing probe effectively avoids the crosstalk problem caused by the temperature during the pressure measurement process. In addition, the two fiber polymer probes are very compact, and the axial lengths along the fiber are only 25 and 30  μm, respectively. Therefore, the sensing probe can be applied to the sensing detection of pressure in narrow areas.

Breast cancer is one of the leading causes of death throughout the world. Early detection of breast cancer can prevent the death of many women throughout the world. There are many methods to detect tumor in breast tissue samples. Plasmonic-based sensors are of great importance in recent optical technology. A perfect semiconductor metamaterial absorber based on surface plasmon resonance is designed, and its sensing capability to detect breast cancer is investigated using numerical simulations. The indium antimonide (InSb) is used as an optimized semiconductor in the structure and the absorption value of 99.22% is obtained. The thermal, material, optical, and geometrical tunability of the structure is investigated. The maximum refractive index sensitivity as high as 24,680 nm/RIU is obtained. Moreover, it is shown that the structure can effectively work as a breast cancer detector. Furthermore, the temperature sensitivity of the structure is also analyzed, and the ultra-high value of 10,830 nm/K is obtained for thermo-optical sensitivity of this design/device.

Periodic surface grating structures made of machinable infrared-transparent dielectrics can offer broadband reflection and transmission in THz waves. We present geometrically varying periodic metasurface made by micromachining and integrated above a gradient-index dielectric multilayer to achieve a near-perfect longpass filter. We utilized techniques in two-dimensional and three-dimensional rigorous, coupled-wave analysis for understanding quasioptical electromagnetic wave coupling within photonic gratings. The low-index pyramidal metasurface shape factors show at-period interference to allow near-perfect transmission. A purely infrared dielectric and polymeric low-index material is compared to seek trade-offs in infrared absorption. We also establish relationships between minimum deposited layer thickness and longpass filter performance. This micromachined metamaterial focal plane filter-on-chip can perform combined antireflection and Bragg band-pass filtering at the near- to far-infrared wavelengths. Its capability can be used for infrared/THz focal plane thermal imaging for space-based applications, understanding the Earth’s surface and atmosphere, and observing cryogenic features of far bodies and exoplanets.

A 1  ×  4 optical demultiplexer (OD) based on the self-collimation (SC) effect in hole-type silicon photonic crystal (PC) was proposed, and its performance was numerically demonstrated. The 1  ×  4 OD consists of two cascaded Mach–Zehnder interferometers and an independent mirror. It has a total of four splitters and five mirrors. The simulation results indicate that by selecting the correct optical path length, the input SC beam with four evenly spaced frequencies can be demultiplexed into four output ports in the 1  ×  4 OD. If the center working wavelength is 1550 nm, the free spectral range of the 1  ×  4 OD is about 23 nm, which almost covers the whole optical communication C-band window. The size of the structure is on the order of 50  μm. Because of its simple structure, small size, and single material, it has potential application value in future photonic integrated circuits.

This erratum corrects misnumbered videos in the paper.