Senior Editor Daniel LeMaster explains the nuances of OE’s signal- and image-processing paper policy.

Editor-in-Chief Adam Wax thanks a longtime senior editor for his service to the journal and welcomes a new one.

With the advancement of laser technology in recent decades, this device has been used in a wide range of applications, such as medical sciences, military, industry, holography, spectroscopy, and astronomy. In past years, military operations for better communication depended on radiofrequency. Vulnerability to security threats and exposure to electromagnetic interference were the main problems of this electromagnetic spectrum region. Therefore, attention was drawn to the visible and infrared (IR) regions. This spectrum provided a secure transfer of the data. The probability of intercepting a laser signal is very low due to the divergence of the narrow beam and the coherence of its optical beam. Hence, it has made this device a good candidate for secure military operations. As a result, the laser based-devices and laser-guided weapons (LGW), such as laser target designator and beam riders, have become an undeniable tool on battlefields. Reduction of the vulnerability to the LGW threats by providing laser detection, angle of arrival, wavelength discrimination, and temporal characterization is the main intention of the laser warning system (LWS). This system consists of three major subsystems. The optical subsystem will consist of spectral filters, focusing lenses, and the detection one, which is made of a few detectors in a unique array’s configuration [especially IR photodetector (IRPD)], and the processing subsystem. In the last decades, with the progress made in nanofabrication and nanotechnology, consequential research has been done to enhance the performance of IRPDs, such as increasing the production yield, making it simple in the fabrication processes, lowering the fabrication cost, and increasing the operating temperature. This review paper gives a particular concentration to the photodetectors used in the detection subsystem of LWSs. In addition, some nanostructures have been surveyed to enhance the optical coupling and light–matter interaction of IRPD. Also, all studied structures are compared in a table. Finally, the detection subsystem we simulated and built at the Nanoptronics Research Center will be analyzed.

Common line-structured light center extraction algorithms have limitations in accuracy and robustness when extracting the center of line-structured light obtained from objects with complex surface structures. In this study, we propose a high-precision center extraction algorithm for multi-curvature line-structured light. This algorithm improves the enhanced parallel thinning algorithm to eliminate redundant points and burrs and fills for missing center points at the end of the light stripe via the improved internal propulsion center extraction algorithm. Finally, sub-pixel centers are recalculated in the direction of the normal vector. A comparative experiment using three classical algorithms was conducted based on a tooth model with a complex curved surface. The experimental results show that the mean absolute error of this algorithm is <0.1, which is only half that of the best-performing dual-threshold grayscale center-of-gravity method in the classical method. Extraction of the center can be performed for multi-curvature line-structured light over 20 curvatures in the range of ( 0.0001 , 4.1439 ) / pixel ^{− 1}; moreover, the end of the light stripe and the identification of the truncation can be improved. Therefore, the proposed algorithm is suitable for contour detection of multi-curvature targets and effectively improves the accuracy of high-precision object detection.

We propose a solution to the issue of hyperspectral unmixing methods that only consider local spectral–spatial information of the pixel level or pixel block. The proposed method is a two-stage convolutional autoencoder network that takes into account global spatial context information. In stage-I, the parallel dual-branch module is dedicated to extracting multi-scale spatial and spectral features. The extracted spatial–spectral prior information is then propagated from stage-I to stage-II to assist in the extraction of joint spectral–spatial features. The proposed method also uses a spectral–spatial attention residual module to refine spectral–spatial features into distinct deep spectral–spatial features and suppress irrelevant redundant features. We validate the proposed method using synthetic and real datasets. It is found that the proposed method outperforms existing unmixing methods in terms of endmembers extraction and abundance estimation. The source code for the proposed model will be made public in the Github repository available at https://github.com/xzw001212/two-stage-DBMRANet.

Motion error compensation is a crucial aspect of processing inverse synthetic aperture light detection and ranging data. Motion phase error occurs mainly due to the relative motion between the target and the system, as well as vibration of either the system or the target, which significantly affects the image quality of optical synthetic aperture radar. Since spatial targets usually have a non-cooperative motion state with a high degree of motion parameter uncertainty, accurate estimation of cross-range phase error becomes challenging due to the presence of envelope tilt effect. We propose an adaptive compensation method that handles motion errors of maneuvering targets by estimating and compensating various types of errors introduced by the target motion process. Using the geometric and signal models to analyze error components, a compensation model is established that uses envelope contrast and image entropy as fitness functions. The bat algorithm is employed to solve this error model. Simulation and outdoor experimental results demonstrate that the proposed algorithm offers higher accuracy and better stability compared to traditional optimization algorithms.

Nowadays, more and more people are concerned about air quality. The particle size of aerosol is an important parameter of air quality. The classical laser particle size analyzer has some disadvantages, such as high price, large volume, and difficult processing. We introduce the scattering receiving screen into the forward light intensity distribution detection system, which only needs a commonly available CCD to obtain an image. Our method has the advantages of low cost and large detection angle. A genetic algorithm was then used to retrieve aerosol particle size distribution from scattered light intensity images. The experimental results show that the aerosol particle size distribution measured by the imaging method is close to that provided by a commercial particle size spectrometer. Moreover, the experimental results also show that this method can measure a polydisperse system. The method presented in this paper provides a simple and feasible scheme for the online detection of particle size distribution in atmospheric, medical, and industrial environments.

Industrial production often faces a variety of complex working conditions that lead to various defects, including Mura, on the surfaces of various industrial products. We propose a reconstruction network called RecTransformer, which is developed with a transformer for anomaly inpainting. RecTransformer is designed to effectively detect various types of surface defects despite using only a small number of defect samples. RecTransformer simplifies the defect detection problem to a patch-level image completion problem. Without using convolution, the given block image is processed by the transformer model to generate a defect-free reconstructed image. Herein, global semantic information is established, and an attention mechanism is built in the patch sequence, and the spatial information of the patches is determined by position encoding to complete the global image reconstruction process. With a limited number of defect samples as training data, the RecTransformer algorithm accurately reconstructs defects. It achieves an area under the receiver operating characteristic curve score of 97.6% for pixel-level segmentation on the testing dataset. Experiments conducted on a universal surface defect dataset demonstrate the effectiveness of the RecTransformer algorithm. RecTransformer can be adapted to detect various types of surface defects, including Mura in display devices, with only a small number of defect samples.

Fringe projection profilometry (FPP) has been widely used in various fields because of its high precision and efficiency. However, fringe images containing overexposed areas are obtained by traditional FPP for high-reflective object surfaces, which leads to errors in absolute phase and three-dimensional (3D) reconstruction results. A robust 3D reconstruction technique based on a polarization state-based ternary polarization coding strategy is proposed to address these errors. Instead of the light intensity coding patterns, the polarization coding patterns are used in this method. Next, the S₁ image is calculated using the polarization image captured by the camera. The Stokes parameter is utilized to segment the fringe codewords without using a specially designed threshold. The proposed method enhanced the stability of the fringes, improved the quality of the fringes, and reduced the number of fringe patterns. Experimental results verified that the proposed method has higher robustness than the conventional method in the measurement of objects with high-reflective surfaces. The results reveal that the proposed method can suppress the effect of highlight areas in the reconstruction of high-reflective surfaces to obtain accurate absolute phases and reconstruct the 3D point cloud data reliably.

A new sandwich sample holder is employed to examine the terahertz (THz) spectrum characteristics of an aqueous solution of bovine serum albumin (BSA) with the aim of exploring the interaction between water and BSA molecules and the effect of the BSA’s molecular vibration frequency on the THz absorption coefficient. In the frequency range of 0.1 to 2.5 THz, the spectral signal strength of the BSA aqueous solution is lower than that of the BSA solid sample; furthermore, the absorption coefficient reduces with the growth of its concentration and is always higher than the absorption coefficient of the solid BSA. The achieved results also reveal that the partial resonance absorption peaks of solution samples and solid samples are the same, signifying that a sandwich sample holder could be effectively employed to detect the BSA aqueous solution. We also utilize GaussianView6.0 software to conduct theoretical simulations on glycine small molecules and find out that their absorption coefficients in the presence of solution and solid conditions also exhibit similarities over a wide frequency range. The research results can be regarded as a solid reference for further exploring the THz absorption characteristics of protein aqueous solution.

Dual-wavelength or multi-wavelength digital holography extends the axial measurement range to the order of several microns or more, but the noise enlargement effect greatly affects the measurement accuracy. To address this problem, an unwrapping method is proposed based on synthetic-wavelength phase and searching process. This methodology implements noise suppression using phase unwrapping at the single-wavelength level and averaging. We introduce the analytical approach and provide a simulation analysis to demonstrate the effectiveness of this noise suppression method. The proposed method is used to measure a standard step and the results demonstrated the feasibility of the proposed method in practical usage.

Polynomial-based calibration methods for a telecentric structured light system, composed of a camera with a telecentric lens and a pin-hole projector, can prevent measurement error introduced by projection calibration and thus have high measurement accuracy. However, using multiple calibration plate positions to cover the measuring volume increases the time to carry out the projection and capture the fringe images. To decrease the execution time and maintain high accuracy, we propose an improved calibration method for a telecentric structured light system based on the principle of projective geometry of a projector. Placing a calibration plate with ring markers in only three positions within the measuring volume, we can establish the conversion relationships between the phase and depth and between the pixel position and transverse coordinates. For validation, a telecentric structured light system was set up and calibrated with the proposed calibration method. The calibrated system was applied for three-dimensional shape measurements of a calibration plate and a cylinder surface. Experimental results demonstrate that the proposed telecentric structured light system calibration method has a measurement accuracy similar to the polynomial-based system calibration method.

Active imaging systems can provide increased contrast-to-noise ratio (CNR) and targeting performance over passive systems in low-light and long-range applications. We use both a radiometric model and the Night Vision Integrated Performance Model to compare the performance of active continuous-wave (CW) and laser range-gated (LRG) imaging systems with laser illumination at 1.6 and 2.1 μm, corresponding to the shortwave infrared (SWIR) and extended SWIR (eSWIR) bands, respectively. The imager performance is characterized by CNR as a function of range, as well as pixels-on-target. The modeling results demonstrate increased performance in the eSWIR band over the SWIR band in the majority of cases and increased performance for LRG systems over CW systems in all cases. The in-progress design of an active imaging testbed to confirm these modeling results with field imagery is discussed.

Formation of laser-induced surface micro/nanostructures and chemical modification on the surface of polyethersulfone (PES) film and composite membrane were studied. Laser irradiation was carried out by ultraviolet (248 nm) laser pulses at a fluence below the ablation threshold. The motivation was to investigate the effect of the structure (film and asymmetric/symmetric membrane), as well as addition of polyvinylpyrrolidone, on morphological and chemical changes after laser irradiation. Nano grains were formed on the surface of the membrane, where the difference in density was observed due to the different membrane structures and composition; while nano ripples were formed on the surface of the PES film. To monitor the modification of chemical properties of the surface after irradiation, attenuated total reflectance Fourier transform spectroscopy as well as contact angle measurement were performed. Finally, the physicochemical modifications caused by irradiation were discussed.

A polarization-insensitive four-port beam splitter based on the plus-shaped ridge structure is introduced. The ridge of this grating is a plus-shaped resin material. Based on the rigorous coupled-wave analysis and the simulated annealing algorithm, the transmission efficiency of the zeroth order under TE polarization is 1.79%, and the efficiencies of 22.21%/22.19% for the ± first/± second orders are diffracted. Meanwhile, the transmission efficiency of the zeroth order under TM polarization is 0.13%, and the efficiencies of 23.25%/23.23% for the ± first/± second orders are diffracted. In addition, the diffraction efficiency of each order is computed while the wavelength, grating period, and duty cycle are varied around the optimized parameters. The study reveals that the grating has excellent tolerance and stability. Hence, it is inferred that plus-shaped ridge four-port grating exhibits promising potential for advancement in the field of lasers.

This work presents a highly sensitive hybrid-integrated optical receiver with a custom designed linear transimpedance amplifier (TIA). The choice of the circuit and diode technology, considering the impact of process parameters on the optical sensitivity and the input referred noise current, are elaborated. The TIA has been designed in a 55-nm complementary metal oxide semiconductor process and was combined with a commercial InGaAs/InP PIN-diode. Measurements resulted in a transimpedance of 9.2 kΩ and a bandwidth of 1.4 GHz. The optical sensitivity of the hybrid receiver is −27.45 dBm for 2 Gbps, with a wavelength of 1550 nm and a bit error rate of ^{10 − 9}. The input referred root mean square (RMS) noise current results in 110 nA. The fabricated chip has an area of 410 μm × 410 μm and a power consumption of 31 mW at a 1 V supply voltage. The performance of the designed TIA is able to match other designs used in high sensitivity applications such as continuous variable quantum key distribution or optical wireless communication, while consuming significantly less power and having a smaller chip area.

Segmented reflectors are frequently used in automotive lighting applications. Extended light sources are one of the limiting factors on the compactness of segmented reflectors. We propose the technique of folding the light source for achieving highly compact reflector designs while improving their efficiency. We then use the macrofocal approach to design legal low-beam reflectors based on the proposed folded light source technique. We show that our method can deliver 51.2% more optical flux, which results in a significant improvement in the efficiency equal to 26.2%. This technique would allow optical engineers to design more compact and energy-efficient products for automotive headlamps and for general lighting applications.

We propose a technique for additive manufacturing of polymer step-index (STIN) optics. The process is composed of aerosol jet multi-material deposition followed by laser-assisted stereolithography in an alternating fashion to fabricate multi-material three-dimensional objects. The layer-wise dynamic tuning of the refractive index provides the possibility to fabricate optics with tailored index profiles. The index range achieved by the presented approach is considerably higher than the common index tuning techniques. Using this technique, a planar multilayer STIN lens is fabricated. The index profile of the fabricated lens has similar progression to the designed profile. In addition, through simulation and also optical characterization, the lensing functionality of the structure is observed.

The behavior of erbium doped fiber amplifiers (EDFA) in a master oscillator power amplifier configuration was characterized, for the first time, in-situ using a Co60 γ-ray source. The results indicate that power degradation and photo-anneal recovery processes simultaneously exist. A fiber optimized for efficiency also provides a radiation harder EDFA with reduced power degradation after a 10 kRad (100 Gy) exposure.

Magnesium fluoride (MgF₂) is a widely used optical window material in lasers. Laser-induced damage in MgF₂ materials involves complex thermal-mechanical coupling issues. With the rapid development of high-power fiber laser technologies and application of optoelectronic countermeasures, it is necessary to investigate the damage mechanism of 1.06 μm high-power continuous-wave laser on MgF₂ optical windows to clarify the laser damage threshold and factors influencing laser-irradiated MgF₂ window mirrors. Therefore, based on the theory of heat conduction and elastic mechanics, a simulation study was conducted using the finite element method. First, based on the thermo-mechanical theory, established a thermo-mechanical damage model for a laser-irradiated MgF₂ crystal. Second, we calculated the temperature, stress, and strain fields of single-crystal MgF₂ material under the action of a 100 W / cm² laser. When the laser was irradiated for 4.921 s, thermal stress-induced burst damage was observed, but no melting damage occurred. Finally, the impact of parameters such as the laser power density, spot size, and laser action time on the damage effect was discussed using the parametric scanning method. The calculation results showed that the aforementioned factors significantly impact the damaging effect. Moreover, under the same laser parameters, material burst due to thermal stress is expected to precede the melt damage.

The eigenmode expansion method is applied to discretize the stepped shine grating into a binary shine grating. Various strategies that increase the directionality of the coupling efficiency have been optimized and replicated using this binary shining grating. We discussed and analyzed the impact of distributed Bragg reflectors, slits, and metal reflectors on the coupling efficiency of grating couplers. A metal reflector and slit were included in the design of a binary shine grating coupler. The coupling efficiency from the fiber to the waveguide of vertically incident electrically polarized light is 75.07% at 1600 nm. A 3 dB bandwidth of 77 nm from 1554 to 1631 nm is obtained.

The spectral region of midinfrared has proven its great potential in various fields, such as materials characterization, medical examination, security, and product inspection. However, the development of room-temperature sensitive midinfrared photodetectors is inevitably in strong demand. Graphene-based detectors are expected to potentially overcome many of the shortcomings of earlier detectors. Unfortunately, low light absorption originating from a single atomic layer is still an obstacle for sensitive sensing using graphene-based photodetectors. To optimize the performance of plasmonic antenna, we have proposed wideband bull’s-eye-shaped antennas using cavity-mode resonance for the midinfrared region. The proposed multi-resonant frequency plasmonic antennas are integrated with graphene-based detectors for midinfrared detection. We demonstrated that the graphene device integrated with the multifrequency plasmonic antenna enabled the room-temperature sensitive midinfrared detection. In addition, such a device can achieve frequency filtering measurement through the bull’s eye shaped plasmonic antenna. The coupling of antenna onto the device largely spares the integration space. We believe that such a device is practical in various applications that require room-temperature, frequency selective sensitive photo detection.

A high extinction ratio (ER) wideband mid-infrared polarization beam splitter based on a GaAs hybrid plasma tridirectional coupler is proposed; it consists of a Si₃N₄–GaAs waveguide (SGW), GaAs waveguide, and GaAs hybrid plasma waveguide. TE0 and TM0 are coupled to the SGW and GaAs waveguide, respectively, by adjusting the width of the waveguide to meet the phase matching condition. When the wavelength is 3.5 μm, the ER and insertion loss (IL) of TE (TM) mode are 48.9 (51.85) and 1.36 (0.3) dB, respectively. In the 400 nm operating bandwidth range of 3300 to 3700 nm, the ER of TE and TM modes is >22 dB. The designed polarizing beam splitter has a high ER and low IL. Because of its large signal transmission capacity, it has potential applications in air molecule detection and analysis and optical communication.

To explore the nonlinear absorption (NLA) properties of the SnO₂ composite system, SnO₂ / Ag / SnO₂ (SAS) composited films were prepared by magnetron sputtering at room temperature. The NLA properties were investigated using femtosecond Z-scan technology at a wavelength of 800 nm. The results show that as the sputtering power of the Ag layer increases, the NLA type of the composite films changes from reverse saturated absorption to saturated absorption. According to the research results, adding the Ag layer can regulate the NLA type of composite films. In addition, by increasing the sputtering power of the Ag layer and the incident laser energy intensity, the NLA behavior of the composite films is improved. Compared with the monolayer SnO₂ film, the NLA coefficient (β) of the composite film has increased by 17 times. We provide a new perspective for the application of SAS composite films in optoelectronic devices.

Due to the natural dispersion effect of the material itself, chromatic aberration is produced when light is incident, so it is important to eliminate chromatic aberration, especially for metalens. We use the phase change material (PCM) Ge₂Sb₂Te₅ (GST-225) in the wavelength range of 4 to 5 μm and design two-layer vortex metalenses, the first one at a single wavelength of 5 μm, to verify the feasibility of a two-layer metasurface for vortex metalens. The second is a two-layer vortex achromatic metalens, where the two layers are modulated by the propagation phase and geometric phase, respectively, and the design of the vortex achromatic metalens is achieved by a combination of compensation phase and adjustment of the crystallization rate of the PCM. The results show that the designed metasurface achieves achromatic effect in the designed wavelength band, which provides an idea for the design of the achromatic vortex metalens.

Underwater wireless optical communication (UWOC) is useful for transmitting data short distances in the ocean. In most cases, such devices are placed on underwater vehicles to make it easier to establish a communication link. However, the vehicle often works in dark water. Thus it needs a light emitting diode (LED) as an illuminant for its camera to observe the surroundings, but this is noise to the UWOC system. Although such noise could be eliminated by inserting an optical filter in the UWOC receiver, the effectiveness depends on the incident angle of light into the filter. Because the illumination distribution of LED is quite complicated, there are some special requirements for UWOC systems to eliminate such noise. First, we simulate the incident angle distribution of LED illumination noise into the receivers of a bidirectional UWOC system with the Monte Carlo method. Then we analyze its influence to design an UWOC system. The results show that the wavelength of UWOC should be shorter than that of the LED illuminant to make them work simultaneously. Under the condition that the noise can be eliminated effectively, the spectrum width of the communication signal and the optical filter should be wider to enlarge the receiving field. According to the theoretical conclusion, we experimentally demonstrate a half-duplex UWOC system with an LED illuminant in dark water, where the attenuation coefficient is about 0.20 m ^{− 1}. In the system, each terminal has a white LED illuminant with 6.01 W optical power, a blue LED illuminant with 7.72 W optical power, and a UWOC transmitter based on a purple LED with about 1.25 W optical power. The UWOC realized a 27.78 Mbps rate and 19 m distance when the illuminants were open. The proposed UWOC system and its design criteria are useful for UWOC applications.

Visible light communications (VLCs) using light-emitting-diodes (LEDs) as communication devices have developed rapidly with the widespread deployment of solid-state illumination. However, the modulation bandwidth of LEDs is one of the dominant issues that limit the VLC system efficiency. Although plenty of techniques have been investigated to deal with this restriction, few people pay attention to the theoretical analysis of LED illumination process. The light-conversion concept shows significant influence on the white LED transmission modeling and receiving technologies. We propose an LED response modeling approach based on the white light generation process and its spectral characteristics, as well as the corresponding signal receiving methods. A dual-pole zero-forcing equalizer (ZFE) is designed, and the analytical signal-to-noise ratios (SNRs) of the output signal with different receiving processes are derived. Simulation results show that SNR is effectively improved by adopting the proposed ZFE, and the dual-pole equalizer can effectively improve system bit error rate performance with different transmission rate. Under the practical lighting scenario, transmission rate can reach up to 70 Mbps over a 1.5 m line-of-sight link. Meanwhile, a minimum data rate of 30 Mbps can be supported within a coverage area of 3 m in diameter.

A multiple-image encryption method based on a computer-generated phase-only hologram (POH) algorithm and chaotic systems is proposed. In the proposed method, first, a modified Gerchberg–Saxton (GS) algorithm is applied to transform the multiple-image into corresponding sub-sampled POHs. Then, the multiple POHs are combined using spatial division multiplexing (SDM). The combined hologram is then mapped into a digital image, and each pixel of the digital image is transformed by a chaotic system to improve security and form the final ciphertext. The modified GS algorithm is employed to generate a sub-sampled POH, which is a prerequisite for SDM. The adoption of SDM eliminates issues such as information leakage due to inter-image crosstalk and ensures the quality of decrypted images. The transformation based on a chaotic system leads to nonlinearity and unpredictability in the encryption process, which further increases the complexity of the encryption system. Numerical simulation demonstrates the security and feasibility of the proposed multiple-image encryption method.

Vibration greatly impairs the imaging quality of airborne synthetic aperture ladar (SAL), so the vibration phase error must be estimated and compensated before imaging. Limited by the laser’s average power, the high pulse repetition frequency of the SAL will result in lower peak-power of the laser, thereby reducing the echo signal’s single-pulse signal-to-noise ratio (SNR). In the case of a low SNR, the coherence between channels worsens, and the vibration phase-error compensation method based on interferometry is difficult to apply. We analyze the imaging SNR and resolution of the echo signal when the airborne SAL observes noncooperative targets by intentionally broadening the transmitting and receiving beams, discussing the imaging processing flow of the low SNR signal under conditions of beam-broaden and platform vibration. The data processing flow includes the phase correction of the echo signal using the transmitting reference channel signal. Based on the flight parameters of the airborne SAL, the echo was dechirped and narrow-band filtered to reduce noise. After subaperture division, the spatial correlation algorithm was used to estimate and compensate the vibration phase error, and the range-Doppler algorithm was used to achieve strip imaging. At the same time, the phase gradient autofocus algorithm was introduced to improve the focusing effect. The processing results of flight experiment data of the airborne SAL corresponding to noncooperative targets at 1 km demonstrated the effectiveness of the proposed method.