Editor-in-Chief Adam Wax bids farewell to longtime Senior Editor Khan Iftekharuddin and welcomes new Senior Editor Daniel LeMaster.

Guest editors Ettore Stella, Pietro Ferraro, Peter Lehmann, Armando Albertazzi Gonçalves, and Wolfgang Osten introduce the Special Section on Multimodal Optical Sensing and Processing.

We tackle the problem of robot localization by means of a deep learning (DL) approach. Convolutional neural networks, mainly devised for image analysis, are at the core of the proposed solution. An optical encoder neural network (OE-net) is devised to give back the relative pose of a robot by processing consecutive images. A monocular camera, fastened on the robot and oriented toward the floor, collects the vision data. The OE-net takes a pair of the acquired consecutive images as input and provides the relative pose information. The neural network is trained using a supervised learning approach. This preliminary study, made on synthetic images, suggests that a convolutional network and hence a DL approach can be a viable complement to the traditional visual odometry for robot ego-motion estimation. The obtained outcomes look very promising.

Optical projection tomography (OPT) is an advanced three-dimensional (3D) imaging technology, which uses the filtered backprojection (FBP) algorithm to recover the 3D volume with sufficient number of projections. As to in vivo imaging, it is urgent to reduce the number of projections because the acquisition time could be minimized. However, reconstructing from undersampled OPT data can lead to artifacts and the decline in image quality. The simultaneous iterative reconstruction technique (SIRT) is introduced to remove artifacts and improve the image quality. The image qualities reconstructed from FBP and SIRT separately are compared, and the structural similarity and peak signal-to-noise ratio are calculated. Through simulated phantoms and OPT data of in vivo zebrafish embryo, SIRT consistently outperforms FBP in terms of reduced artifacts and enhanced image contrast especially when the projection numbers are reduced. SIRT method can provide high-quality reconstruction with 50 or fewer projections, thereby significantly reducing the minimum acquisition time and light dose while maintaining reconstruction quality. Through optimization and GPU acceleration, the SIRT algorithm can converge faster so as to reduce the image processing time. To our knowledge, this is the first time one SIRT algorithm is used for reconstruction of sparse OPT data. The experimental results show that SIRT algorithm outperform FBP especially when the number of projections is reduced. In addition, SIRT performs better in the preservation of vascular signal, which is significant for the monitoring of angiogenesis.

The non-destructive behavior of terahertz (THz) radiation is used in a wide variety of applications in industry and research and has inspired researchers to explore different configurations of THz-based measurement systems to bring such systems from the laboratory to the field. In this work, an optical system is presented for terahertz reflectivity measurement based on refractive optical elements. The optical arrangement is housed in a handheld THz scanner head unit that is realized using additive manufacturing. The THz scanner head is modular, compact, lightweight, and easy to handle, making it field deployable and suitable for outdoor measurements. Further, the applicability of this THz scanner head is demonstrated in applications such as hydration sensing, surface roughness characterization, and interface detection in multilayered structures, which play key roles in industrial environment for non-contact quality control and assessment of consumer products.

Polymeric thin films represent an emerging industrial area driven by their enormous technological and commercial potential in interdisciplinary sectors such as chemistry, material science, engineering, and physics. The large selection in terms of materials/composites and the wide range of technological solutions that could be used for their fabrication could create confusion for the final user requiring a quantitative characterization of their properties. This analysis could be even more complex in the case of functionalized polymeric films such as the samples reported in this work. Here we present how thin polymer films can be wholly characterized by applying a multiplicity of optical methods. Films were realized by a special liquid one-step process. Moreover, such polymer films were functionalized here for the first time by mesoporous silica nanoparticles. The nanoparticles were added to a polymeric matrix. We show that a full characterization was achieved by employing three different microscope techniques, i.e., scanning electron microscope, digital holography (DH), and space-time DH. Exploiting such a multimodal methodology can be of great benefit for characterizing the functionalized polymeric thin films. In fact, multiple characterization in different conditions was possible. The results reported in terms of morphological information, thickness distribution, three-dimensional (3D) mapping, large field of view, high magnification, and super resolution of the zoomed area offer a good solution for testing materials and obtaining a quantitative characterization and whole inspection in the case of complex polymeric samples.

Tissue elasticity is one of the significant diagnostic parameters to study disease progression in biological tissues. Elastography measurements hold strong potential for determining the mechanical properties of malignant tissues in clinical practice. Optical methods for quantifying the elastic properties have received the attention of both research and medical community. In the current work, we calculate the elastic modulus of tissue-mimicking phantoms and ex-vivo biological tissue using holographic imaging by tracing Rayleigh wave propagation on the tissue surface. Rayleigh waves produced by the electromechanical actuator over a set of excitation frequencies are imaged by an ultrafast camera. The phase map reconstruction of the Rayleigh wave is carried out using a phase-shifting algorithm and the distance-time slope method is analyzed to compute Rayleigh wave velocities. The elastic modulus calculated by holographic imaging is validated against the gold standard mechanical compressional testing data. The quantitative results obtained from this current optical system combining holographic imaging and Rayleigh wave tracing are highly effective in ex-vivo and in-vivo tissue characterization and carry high potential as a promising tool in clinical translations and commercial adaptations.

Automatic optical inspection technology (AOI) is a visual inspection technology that has developed rapidly in recent years. The high speed and accuracy of AOI can greatly enhance the efficiency of modern industrial production. However, when this technology is applied to optical components inspection, it encounters a challenge that the specular highlight induces over or under exposure during the imaging process, and further results in a low imaging contrast and unclear defect details of the targets. To solve this problem, a dark-field polarization imaging setup based on a division-of-focal-plane polarization camera was adopted to achieve high contrast defect images. Meanwhile, algorithms based on the improved LeNet-5 convolutional neural network were developed to recognize the defects. An accuracy above 99.5% was obtained for the distinction of defective and non-defective samples, and an accuracy of 94.4% was reached for the various defect classification. Our work demonstrated an effective application of polarization imaging and machine learning in AOI of optical components manufacturing.

Based on angular spectrum diffraction theory, the scattering field of vector vortex beams (VVBs) passing through a rotating random rough surface is derived. The speckle produced by VVBs passing through the rotating ground glass is collected experimentally. The effects of ground glass roughness and rotational speed on the speckle and orbital angular momentum (OAM) of different order VVBs are discussed in detail. Furthermore, the speckle characteristics of the VVB are compared with the scalar vortex beam. The change in the OAM states of VVBs before and after passing through the rotating ground glass was detected by double-slit interference. The results show that the speckle of VVBs is generated after passing through the static ground glass, and the speckle contrast increases with the increment of roughness. Compared with the scalar vortex beam, the speckle contrast of the VVB changes faster with the roughness of ground glass. When the ground glass rotates, the speckle reduction effect appears and the contrast is reduced. When the rotational speed is >15 r / s, the speckle reduction effect is the strongest. The intensity distributions and OAM states of VVBs are not affected by the rotating ground glass. The research results provide a theoretical basis for the application of VVBs in optical communication, optical detection, and other fields.

Laser cladding is an advanced manufacturing technology for preparing high-performance metallurgical coatings on metal substrates that is widely used in industrial production. In order to improve the powder utilization rate and reduce the waste of powder in the coaxial powder feeding laser cladding process, the Euler–Lagrange theory was used to establish the simulation model of gas-powder two-phase flow in laser cladding. Fluent was used to simulate the powder particle size, powder particle shape factor, carrier gas flow rate, and powder feed rate under different process conditions, and the results were verified by experiments. Through analysis, under the process parameters of a carrier gas flow rate of 4 L / min and a powder feed rate of 15 g / min, the powder utilization rate was high and the surface of the cladding layer was smooth. When the particle size was 100 mesh and the particle shape factor approached 1.0, the concentration of powder focus was higher and the convergence effect was the best. The influence of powder process parameters on powder convergence was comprehensively analyzed, which is of great significance for the subsequent optimization design of nozzle and the improvement of powder utilization in the laser cladding process.

To achieve accurate detection of different space x-ray sources, three types of filters are designed for the follow-up x-ray telescope onboard the Einstein probe (EP). Two of them are supported by PI mesh and Ni stiffener, which are studied in this paper. Since visible light blocking performance of these filters is important for x-ray detection, the transmittance and visible light irradiation damage performance of them are measured and presented. With a self-built double-beam laser system, the detection sensitivity of transmittance at 532 nm is increased to 10 ^{− 9} compared with 10 ^{− 5} for the spectrophotometer. Moreover, the tests of visible light irradiation damage performance under vacuum and air conditions have been carried out using this system, which proves that the filters have good reliability. Finally, the topography of these filters is studied and described with a microscope.

A distributed optical current transducer (DOCT) and a current measurement method are proposed to solve the problem that the straight-through optical current transducer is vulnerable to crosstalk magnetic field interference. We established a mathematical model of the magnetic interference degree of DOCT. Analyzing and summarizing the characteristics of the magnetic interference degree distribution of DOCT revealed that there are six magnetic interference zeros outside of DOCT. Then based on the background of three-phase common box current measurement in a gas-insulated substation, a current measurement method of anticrosstalk magnetic field interference is proposed, that is, adjusting the direction of DOCT to make the interference current at zero magnetic interference point, to achieve the effect of anticrosstalk magnetic field interference. The experiment shows that the measurement error of DOCT is <0.15 % , and the interference of the crosstalk magnetic field can be ignored.

In our study, a communication experiment with a 13-km horizontal urban link was conducted based on space diversity technology to examine the communication performance and scintillation effect of this link. Four different distribution models, including the Gamma–Gamma distribution model, were selected. Although the scope of application of the other distribution models was smaller than that of the Gamma–Gamma distribution model, they had better fitting effects than the Gamma–Gamma distribution model did regarding the use of space diversity technology in extreme urban environments with strong or weak turbulence; the degree of fit could reach more than 0.9. We concluded that the space diversity technology had a certain influence on the probability density function of the light intensity fluctuation and a significant inhibitory effect on the scintillation light intensity, resulting in a gain of more than an order of magnitude for the bit error rate.

We propose a tunable reflective Fourier ptychographic microscopy approach in which the resolution and field of view are variable with different objective lenses and a digital micromirror device. Several objective lenses are applied to tunable reflective Fourier ptychographic microscopy without additional optical alignment while also enhancing the resolution. To demonstrate the feasibility of the new technique, tunable reflective Fourier ptychographic microscopy with 10 × and 50 × objective lenses is demonstrated. Moreover, we compare the resolution of the new technique and conventional Fourier ptychographic microscopy to verify the improved resolution of the new technique.

Based on the refraction characteristic of uniform and transparent liquid, the measurement error of the liquid refractive index using the internal standard method is analyzed, and an improved laser speckle method is proposed. This method designs a double-cell structure composed of two identical rectangular parallelepiped glass cells with one cell containing the liquid to be measured and the other one being empty. This double-cell structure is placed in the built optical system based on laser speckle, and then the refractive indices of sodium chloride solution and sucrose solution with various concentrations are measured by the improved method. Compared with the measurement values of the Abbe refractometer and the internal standard method, the results show that the improved method can significantly reduce the measurement error and improve the measurement accuracy.

High accuracy and low-cost indoor localization has received much attention for the past few years. Visible light (VL) localization, which reuses indoor LED luminaire infrastructure as artificial visual landmarks, has emerged as a promising technology. Using only lights as localization beacons, it can provide highly accurate indoor localization without reconstructing the whole environmental scenario. We propose a localization system that utilizes luminaire distribution patterns on the ceiling as visual landmarks without the costly luminaire circuits as in previous methods. For pattern detection, a unique descriptor for each luminaire cluster is derived through image processing followed by perspective rectification. Then, each pattern’s ID in a lookup table is retrieved with earth mover’s distance under rotation transformation. Using all individual luminaires in each pattern, our system achieves accurate and robust localization under noise and interference. Furthermore, our system can automatically generate pattern dictionaries of hundreds to thousands of unique IDs resulting in great scalability. Evaluations are conducted through real-world experiments. Compared with previous systems, the proposed method achieves superior performance in terms of detection range, localization, and orientation accuracy. Specifically, our system achieves high average three-dimensional localization accuracy of 2.47 cm and orientation accuracy of 0.87 deg under different heights for the best dictionary setting. Finally, an implementation on a mobile phone demonstrates the potential to run on resource-constrained platforms in real time.

This research proposes a terahertz (THz) metalens for focusing THz waves with an electrically controllable focal length. The dynamic of tuning of THz metalenses with graphene provides high controllability, fast responses, and compact structures for beam focusing. An array of graphene microrings has been presented with significant phase variations possibility. The three-dimensional finite-difference time-domain method has been employed for the analysis of the proposed structure. Applying a voltage to the graphene layer changes the graphene Fermi level, and the focal length can be controlled. As the chemical potential of graphene varies from 0.1 to 0.3 eV, the focal length changes 20.5λ (from 4 to 12.2 mm at the frequency of 0.75 THz). In addition, by biasing every array of graphene microrings with different gate voltages, we can electrically rotate the focal beam up to 32.02 deg. An equivalent circuit model (CM) is proposed for fast analysis and the verification of the FDTD results as well. The achieved results of the CM comply very well with those derived by the FDTD method. The proposed structure has the potential to be used in THz imaging.

Our work is devoted to the optimization of 4-component RGBW smart lighting systems that are able to obtain the resulting white light with a combination of high color rendering and luminous efficacy for a certain fixed correlated color temperature. Three approaches for the determination of the optimal brightness contributions of the light emitting diodes (LEDs) are studied. They are accordingly based on: (1) maximization of the color rendering, (2) prioritizing to reduce the color rendering maximum by 1 to 6 units to increase luminous efficacy, and (3) getting a constant value of RGB contribution and luminous efficacy while maintaining high color rendering. A comparison of the three systems with different base white LEDs is demonstrated. The obtained theoretical results are verified experimentally.

Optimization of the illumination system of an ultraviolet light-emitting diode (UVLED)-based lithography system by reducing the system complexity and spatial volume has been a research focus. This work proposes a compact lithography illumination system that applies a specially designed embedded compound parabolic concentrator (ECPC) to replace the traditional collimation module and the zoom lenses module. The ECPC, comprised of two detachable parabolic surfaces, precisely collimates all light beams from the UVLED to a preset divergence angle of 6 deg and provides an adjustable lithography coherent factor. The final illumination system offers a square illumination field with a uniformity of up to 1.3% in the large coherent factor and a linearly adjustable numerical aperture in the range of 0.02 ∼ 0.04. The final exposure result of 0.7 μm based on the experiment setup further verifies the performance of the designed illumination system.

The feasibility of the proposed new process chain was demonstrated by generating an anti-reflective (AR) structure array with continuous profile on a chalcogenide glass substrate. First, a soft mold made of polydimethylsiloxane (PDMS) was created by casting and lift-off process off a Si master mold with original AR features. The master mold can be reused to create multiple PDMS molds. Afterwards, a nonisothermal compression molding process was conducted using PDMS soft mold to replicate a moderate aspect ratio AR structure array from PDMS mold onto chalcogenide glass surface. PDMS mold was first heated to a low temperature around 190°C such that its rigidity can be maintained, whereas the chalcogenide glass was heated to a relatively higher temperature around 240°C for molding. Surface profiles for both PDMS mold and molded glass were evaluated using both Wyko optical surface profilometer and scanning electron microscope tool. For optical performance, an optical light transmission test was conducted. We demonstrated that PDMS can be a good candidate for mold material in chalcogenide glass compression molding process in replicating large surface arrays with moderate aspect ratio structures.

By harnessing the heterogeneous combination of waveplates, we demonstrated a wavelength-tunable passband-squeezed polarization-diversified fiber loop (PDFL) comb filter. The PDFL comb filter is composed of a polarization beam splitter (PBS), two polarization-maintaining fiber (PMF) segments of the same length, one waveplate combination (WC) of dual quarter-wave plates (QWPs), and another WC of a QWP and a half-wave plate. The former and latter WCs are located before the first and second PMF segments, respectively, and the second PMF is butt-coupled to the PBS so that its slow-axis orientation angle (OA) should be 22.5 deg for the horizontal axis of the PBS. This is the first report on the implementation of the wavelength tunability in the passband-squeezed PDFL comb filter using this heterogeneous WC. Through the Jones calculus, we derived the sinusoidal passband-squeezed transmittance function of the filter. From the derived transmittance, we found the slow-axis OA sets of the four waveplates where the extra phase ϕ of the transmittance function could be continuously changed from 0 deg to 360 deg and graphically represented these OA sets as functions of ϕ along with the relationship between the OA sets. For eight OA sets selected from these OA sets, allowing ϕ to regularly increase from 0 deg to 315 deg by 45 deg, we calculated the passband-squeezed transmission spectra of the filter and confirmed a redshift of the passband-squeezed spectrum by 0.1 nm per set. Then, this theoretical prediction was validated by experimental verification, which corroborates that our filter can offer the wavelength tunability of the passband-squeezed comb spectrum.

We report the design, fabrication, and characterization of 1310-nm-emission buried-heterostructure (BH) spot-size-converter (SSC)-integrated distributed-feedback (DFB) laser diode (LD) with record power efficiency of 20% at optical power of more than 100 mW at 75°C and current of 400 mA. The SSC also functions as semiconductor optical amplifier so that with laser oscillation takes place in the DFB section, the SSC provides optical gain while shaping the optical mode to give a lowest possible far-field (FF) of 9 ° ( h ) × 14.5 ° ( v ) . The measured coupling loss of the SSC-integrated DFB LD output to 10 μm-radius-of-curvature lensed single-mode-fiber is ∼0.9 dB. For Co-Package Optics application, the SSC-integrated DFB LD is a potentially viable candidate for External-Laser-Source module, or for hybrid-integrated laser-source on silicon photonics (SiPh) platform via flip-chip and butt-coupling of light to SiPh waveguide.

We design a multispectral transmission waveguide based on phoxonic crystals at midwave infrared (MWIR) frequencies. The phoxonic crystal slot-waveguide architecture is realized using a germanium (Ge)-slot waveguide, surrounded by a supercell array of oxide holes in silicon–germanium (SiGe) membrane tailored photonic and phononic crystal bandgap. The plane wave simulations for both photonic and phononic crystal unit cells were performed to confirm the geometry of the phoxonic supercell. The bandgap analysis shows the capability of the proposed architecture to confine photons of the terahertz frequency range within the slot waveguide by isolating them from the phonons of gigahertz frequency range. The phononic and photonic bandgaps were simultaneously engineered by varying the periodic variation of the density function and dielectric permittivity, respectively. The computational approach shows the suppression in photon-phonon scattering as validated by a uniform transmission of ∼99.8 % over a broad range of 3 to 5 μm wavelengths. The designed phoxonic crystal waveguide can be fabricated with planar processing technology and used in many applications where multispectral control of mid-IR signals is required.

In recent years, rhenium disulfide (ReS₂) has attracted much attention in polarization dependent photonic applications due to its anisotropic optoelectronic response. To reveal the anisotropic response mechanisms, single crystal layered ReS₂ flake with lateral size of about 10 μm is obtained using mechanical exfoliation method, and the ultrafast carrier dynamics of an individual flake is studied using transient absorption microscopy. The excited carrier concentration of layered ReS₂ flake exhibits obvious polarization dependence of the pump light, which is caused by the anisotropic linear absorption and electron mobility rates in different orientations. These results could provide more references for the application of ReS₂ in polarization sensitive detectors and optoelectronic devices.

Silicon waveguides are particularly appealing for the implementation of all-optical (AO) signal processing devices and switches due to the improved fabrication technology of silicon. Therefore, a silicon-on-silica waveguide is employed as the building block for simulating fundamental AO logic operations, including XOR, AND, OR, NOT, NOR, NAND, and XNOR, at 1.33-μm telecommunications wavelength. The proposed waveguide consists of two microring resonators and three strip waveguides. The operation concept of these logic gates relies on the constructive and destructive interference that results from the phase difference induced by incident optical beams. The performance of the target logic gates is assessed against the contrast ratio (CR) metric. The simulation results suggest that, by exploiting the proposed waveguides, these gates can operate with higher CR and faster speed compared to other designs.

Image degradation caused by bad weather, such as haze, rain, and snow, significantly degrades a vision system’s performance, imposing challenges to the autonomous motion of mobile robots. Therefore, it is crucial to restore the degraded images for mobile robots operating outdoors. Spurred by this concern, a framework that restores degraded images and affords simultaneous localization and mapping (SLAM) under multiple bad weather conditions is presented. Specifically, the developed architecture combines the advantages of convolutional neural network features and weather features. It improves the accuracy of identifying the degradation type by developing a weather inference module based on ensemble learning. According to the internal mechanism of image degradation, a degraded image restoration method is proposed utilizing physical models, with a subsequent operation refining the preliminary restored results. To improve our method’s adaptability to real scenes, unpaired real-world weather images are introduced into the degradation removal algorithm through generative adversarial networks. The proposed weather persistent assumption combines weather inference and degradation restoration modules in the SLAM system to capture multiple weather conditions, which improves the system’s accuracy and running speed. Comprehensive experiments evaluating the developed framework and its components highlight that the proposed weather inference and degraded image restoration methods achieve a highly appealing effect. The final experimental results demonstrate that our framework autonomously identifies weather types, triggers the corresponding restoration, and realizes accurate localization in multiple bad weather conditions. It affords SLAM accuracy under multiple bad weather conditions that is close to raw SLAM in clear weather.

Within the last decades, a large number of techniques for contrast enhancement has been proposed. There are some comparisons of such algorithms for few images and figures of merit. However, many of these figures of merit cannot assess usability of altered image content for specific tasks, such as object recognition. In this work, the effect of contrast enhancement algorithms is evaluated by means of the triangle orientation discrimination (TOD), which is a current method for imager performance assessment. The conventional TOD approach requires observers to recognize equilateral triangles pointing in four different directions, whereas here convolutional neural network models are used for the classification task. These models are trained by artificial images with single triangles. Many methods for contrast enhancement highly depend on the content of the entire image. Therefore, the images are superimposed over natural backgrounds with varying standard deviations to provide different signal-to-background ratios. Then, these images are degraded by Gaussian blur and noise representing degradational camera effects and sensor noise. Different algorithms, such as the contrast-limited adaptive histogram equalization or local range modification, are applied. Then accuracies of the trained models on these images are compared for different contrast enhancement algorithms. Accuracy gains for low signal-to-background ratios and sufficiently large triangles are found, whereas impairments are found for high signal-to-background ratios and small triangles. A high generalization ability of our TOD model is found from the similar accuracies for several image databases used for backgrounds. Finally, implications of replacing triangles with real target signatures when using such advanced digital signal processing algorithms are discussed. The results are a step toward the assessment of those algorithms for generic target recognition.

This erratum corrects the article’s issue of publication.