Guest Editors Claas Falldorf, Pascal Picart, Cosme Furlong, and Percival Almoro introduce the Special Section on Lensless Imaging in Optical Metrology.

To enhance the complexity of the output sequence of the optical injection structure and improve the security of the bit plane encryption, this work proposes an image encryption algorithm based on optical chaos and quadrature amplitude modulation (QAM)-like. The external disturbance generated by the mutual injection of the dual-drive laser improves the complexity of the optical chaos output from the laser in the optical injection structure. In the image encryption algorithm, this work adopts a QAM-like coding mode, which encrypts the pixel value of the plaintext image by transforming the coordinates of the QAM constellation diagram and the coding. In the coding stage, the different coding modes of QAM-like are selected using the encryption sequence composed of optical chaotic sequence and key to transform the color plaintext image into the coded image, and the pixel value of the plaintext image is changed. In the scrambling stage, the encrypted sequence is used to scramble the multi-dimensional rows and columns of the three-dimensional coded image, and the position information of the pixel points is encrypted. In the diffusion stage, the scrambled images of each color component are diffused by GF(17) multiplication and finally merged to generate ciphertext images. Simulation results show that the proposed image encryption algorithm has sufficient key space and information entropy is close to the theoretical maximum, which can effectively resist known plaintext attacks, selective plaintext attacks, and statistical feature attacks.

Volume holographic optical elements (vHOEs) are currently used in various applications, such as augmented reality displays or wearables. Yet, the use of vHOEs as illumination optics has not found its way into products. In automotive exterior lighting, vHOEs could enable unique styling, reduction of installation space, weight, and material. However, in headlamps, rear lamps, or signal lamps, several technical and conceptional challenges must be addressed. One of these challenges is to replace lasers, which are expensive and have high safety demands, by light emitting diodes (LEDs), which are widely used in the field of automotive lighting. The reconstruction of volume holograms with LEDs is straightforward for graphical holograms showing a three-dimensional scene. For automotive exterior lighting, however, the hologram is not just a design element, but in addition must feature a light distribution fulfilling all the legal requirements. The vHOE thus becomes the most important functional element of the lighting system projecting the light distribution without an additional lens. For instance, the low beam distribution must provide a sharp, asymmetric cutoff line and white color, which turns out to be difficult. We have developed an improved manufacturing technique of such vHOEs, using two spatial light modulators. We present the design, the holographic printer setup, and first experimental results of vHOE samples.

Scatterometry has been put into practical use for microstructure measurement of ultra-large-scale integration due to its high process compatibility. On the other hand, its application has been limited to periodic structures. By applying this method to isolated systems and using hard X-rays, it may be possible to significantly exceed a resolution of 10 nm, which is the limit of conventional optical measurement. We demonstrate the feasibility of this measurement by rigorous calculations. For this purpose, we measured the intensity of specular reflection and noise at the beamline of hard X-ray radiation. The virtual target is a 15-nm-wide lattice. The signal-to-noise ratio is low enough for a lattice with a period of 25 nm but 10 times higher for an isolated lattice.

Digital holography allows for the recording and reconstruction of three-dimensional images using interference and diffraction principles. The propagation of light from the hologram plane to the reconstruction plane is a crucial step, often achieved through Fresnel propagation, a method that inherently transforms the reconstructed pixel pitch to provide diffraction-limited imaging. However, the accuracy of this method is limited by the Fresnel approximation, especially in applications such as digital holographic microscopy. We present a simple method that significantly improves the accuracy of the Fresnel approximation by incorporating higher orders of the binomial approximation. We validate the effectiveness of our approach through high numerical aperture simulations and experimental results, demonstrating superior sub-micron resolution and reduced distortions compared with standard Fresnel propagation.

A multi-view occlusion removal method for digital lensless holographic microscopy (DLHM) is presented. Multiple DLHM holograms, whose individual reconstructions show occluded or partially occluded sample details, are recorded for different sample placements at its plane. A coordinated addition of the multiple DLHM recordings produces a composite hologram whose reconstruction allows the removal of the occlusions for a given imaging plane while increasing the reconstructed field of view. A theoretical model supports the method and its feasibility is tested with phase bio- and non-bio samples.

Digital lensless holographic microscopy (DLHM) allows the design of cost-effective systems using off-the-shelf materials, making this type of lensless microscope accessible to many users worldwide. However, these materials may have a limited optomechanical performance that is aggravated due to the sought compactness and the intended cost-effective manufacturing process. This problem particularly affects the illumination source, which is of critical importance for DLHM, as it defines the optical performance of the microscope. While recent reports show that the required point source can be built from a low-cost laser diode coupled to an also low-cost aspheric lens, the resulting illumination has a distorted wavefront that limits the performance of the microscope. A simple homemade setup to correct the distortion of such illumination source and its integration into a compact, cost-effective, DIY, and open-source-certifiable digital lensless holographic microscope, is presented. The distortion-corrected DLHM is validated by imaging calibrated test targets and biological samples, achieving a 12-fold extension on the distortion-free magnification range of previous designs and a doubling of the effective spatial resolution without significant increments in its overall cost.

The light source in digital lensless holographic microscopy (DLHM) plays a key role in the optical performance of this microscopy methodology. The underlying physics that support DLHM rely on having an outward propagating spherical wavefront illuminating the sample to record on the surface of a digital camera the intensity of the diffracted wavefield that is magnified by free-space propagation; this first stage constitutes the recording. The information of the sample is numerically retrieved by computing the diffraction that an inward propagating spherical wavefront undergoes in the recorded intensity in a stage known as reconstruction. In both stages of this lensless microscopy method, it is assumed that perfect spherical wavefronts are utilized; hence, the closer the illumination light sources to exp[±ikr]/|r| are, the better the performance of the microscope. For the recovery of the information of the sample, the accuracy of the inward propagating spherical wavefront exp[−ikr]/|r| is guaranteed by its correct sampling in the numerical computation of the diffraction process; thus, the main challenge is the physical production of light sources to illuminate the sample whose amplitude profile is accurately described by exp[ikr]/|r|. In this work, the different approaches that our research group has explored to produce illuminating point sources for DLHM are evaluated. Point sources based on pinholes, coned-shaped optical fibers, optical pick-up units, and aspheric lenses are evaluated by rating their robustness, compactness, reliability, cost and easiness of production, and their performance on imaging test targets and biosamples. Some additional DLHM light sources reported in the literature are also briefly reviewed based on their available data.

We introduce a lensless long wave infrared (LWIR) sensing system, utilizing double-random phase encoding. The employment of thin random phase encoding elements eliminates the need for traditional optical lenses. For object classification, convolutional neural network is used to process the speckle patterns produced by the random phase encoding, thus avoiding the reconstruction problem associated with lensless imaging. This approach is attractive for applications demanding compactness and cost-efficiency for LWIR systems. Experiments are provided to illustrate the proposed system. Our results demonstrate that this system competes well with conventional lensed LWIR imaging methods in a binary classification task under noisy conditions, where noise is not known a priori. To the best of our knowledge, this is the first report on such approaches in the LWIR domain.

Speckle metrology techniques utilize the phenomenon of speckle patterns for various measurement applications. Speckle pattern interferometry and speckle shearography are the widely used speckle metrological techniques in diverse fields. In speckle interferometry, the phase map embedded in the speckle pattern fringes is directly proportional to the displacement; however, in speckle shearography, it is related directly to displacement derivative. We aim to explore the relationship between the extracted phase derivative from speckle fringe pattern and the phase from their corresponding shearing fringes along the x and y directions. A speckle fringe pattern and the sheared fringes along the x and y directions are numerically generated. From speckle fringe pattern, the phase derivatives along the x and y directions are extracted by using the Riesz transform algorithm, whereas from the shearing fringes, the phase distribution is extracted by using monogenic signal. The similarity between the phase derivate distribution from speckle fringe pattern and phase distribution from sheared fringe is quantitatively evaluated by using image quality index. Furthermore, application experimental data are also presented.

Referenceless phase holography (RELPH) is a lensless holographic method that generates the full (amplitude and phase) optical field if intensity and phase distributions of this field in one plane are given as numerical data. It is based on the interference of two pure phase fields that are produced by reflection of two mutually coherent plane waves at two phase modulating spatial light modulators (SLM). The optical field of any real or artificial three-dimensional (3D) scene can be displayed. This means that referenceless phase holography is a promising method for future 3D television or 3D cinema as well as for interferometric metrology. We introduce the theory of RELPH, possible technical realizations, and discuss the numerics. The possibilities and problems in calculating the diffraction fields of given 3D scenes are worked out, modifications of the algorithms are presented. Experiments are shown, not only proving the practicability of RELPH, but also confirming the various 3D cues, such as depth of field, occlusion, and parallax. Two approaches to multicolor display are presented and experimentally verified. The benefits and advantages of RELPH are outlined, open problems and necessary technological developments as well as possibilities and future prospects are discussed.

A lens-free on-chip digital holographic microscope (LFOCDHM) is essential for a variety of biomedical applications such as cell cycle assays, drug development, digital pathology, and high-throughput biological screening. However, due to the unit magnification configuration of the lens-free system, the field-of-view (FOV) contains over a hundred times more cells than a conventional 10× microscope objective. Consequently, the segmentation process becomes labor-intensive and time-consuming due to the complex and variable morphology of cells within the large FOV. To address this issue, numerous deep learning-based cell segmentation methods have been proposed. Nevertheless, convolutional neural networks, limited by their localized receptive field, are unsuitable for segmenting and processing large FOV imaging results from LFOCDHM. Therefore, we propose a high-throughput live cell analysis processing method called Swin Transformer U-Net (STU-Net). Based on the reconstructed phase results, a shift window is utilized to compute the self-attention to extract its features at five scales, which can compute the normalized inner distance and pixel-level classification and achieve high-throughput accurate cell segmentation (accuracy >0.9743). We validated the robustness and generalizability of our STU-Net by the accurate segmentation of data from HeLa cell slides across the full FOV and live C166 cells in vitro. Given its capability for quantifying cell growth and proliferation based on the multi-cell parameters generated from segmentation results, the proposed approach is expected to provide a strong foundation for subsequent drug development and biological screening.

We demonstrate low-light quantitative phase imaging using the transport of intensity equation. The incident numbers of photons are experimentally determined using an optical power meter. Although the low-light phase imaging condition is vulnerable to noise, its application in astronomy and imaging of biological samples draws the attention of researchers. Low light is treated as a Poisson noise. We explore the method as an opportunity to study low-light phase imaging conditions and the possible potential to go toward ultra-low-light conditions. We have illustrated the same with the help of numerical simulation and experimental results.

We present recent advances in lensless random phase encoded imaging for cell identification as related to biomedical applications, accentuating the robustness of methodologies through an examination of previously published works. Following an explanation of the foundational principles of lensless random phase encoded imaging for cell identification, discussions proceed on the evaluation of lateral resolution capabilities, current computational approaches, and an exploration of the practical impacts of these advancements in cell identification for biomedical applications. In particular, we seek to foster a deeper understanding of the robustness of key biosensor parameters in non-reconstructive lensless imaging for cell identification and its role in computational imaging and biomedical diagnostics. To the best of our knowledge, this is the first review of lensless random phase encoding for automated cell classification and its robustness to noise, and key bio sensor parameters. These systems can be made into low-cost, compact, and field-portable biosensors that are attractive for constrained healthcare systems.

We construct a solid-flow coupled model of a blunt-nose front-window flight vehicle. Through this model, we analyze the propagation characteristics of the target light ray and the resulting imaging deviation under the combined effect of the flow field and the optical dome. The results show that the target light is deflected near the normal direction after entering the aero-optical flow field and away from the normal direction after entering the optical dome. The imaging deviations produced by the aero-optical flow field and the optical dome, respectively, are superimposed effects. Under the same flight conditions, the imaging deviation caused by the aero-optical flow field is smaller than that caused by the optical dome.

The hybrid iterative angular spectrum method is an effective approach for computing holograms with reduced interlayer crosstalk noise. However, the use of a random phase as the initial phase results in speckle noise across all reconstructed planes. We propose a hybrid iterative angular spectrum method based on the double-phase amplitude encoding method. This method eliminates the constraints of the initial phase by employing a quadratic phase instead of a random phase as the initial phase, successfully reducing speckle noise in the reconstructed images. Through simulation and experimental validation, the algorithm demonstrates superior reconstruction performance, making it applicable to various fields such as biology, medicine, and engineering design. It provides a robust solution to the challenges of multi-plane image reconstruction.

Optical readout of digital volume reflection holograms (DVRHs) using a liquid crystal spatial light modulator (LC-SLM) as a holographic optical element is demonstrated. This is achieved by first computing the optical field at the exit of the composite DVRH, which is generated numerically from multiple digital transmission holograms recorded at different wavelengths. This complex output field is then appropriately introduced as an electrical input into the LC-SLM operated in phase-only and amplitude-only modes. The effectiveness of the LC-SLM for optical readout with high wavelength selectivity is investigated.

Interferometric imaging technology requires accurate acquisition of the modulus, phase, and spatial frequency coordinate information of the complex coherence when reconstructing the target image. Due to the inherent jitter of optical fibers, it is relatively difficult to accurately obtain the phase information. Using the continuous hybrid input–output algorithm, the target image can be reconstructed when only the modulus and spatial frequency coordinate information of the complex coherence are known. However, in the actual imaging process, there are always certain errors in the measurement of the modulus and spatial frequency coordinate information, which affects the quality of the reconstructed image. Based on the theoretical calculation formula of the complex coherence modulus, we analyzed the factors affecting the measurement results of the complex coherence modulus and built an indoor testbed for experimental verification. Finally, through theoretical simulation, the impact of the error in the measurement of the complex coherence modulus on the quality of the reconstructed image was analyzed, proving the necessity of accurate measurement of the complex coherence modulus. The experimental and simulation results have important guiding significance for the design of subsequent interferometric imaging systems and experimental schemes.

The fringe projection 3D measurement method is a widely used 3D measurement technology, which utilizes a digital projector as the structural light source. However, the nonlinear intensity response of projectors results in non-sinusoidal fringe images captured by cameras, introducing phase errors that undermine measurement accuracy. We propose a nonlinear correction method based on precise pre-coding for digital fringe projection 3D measurement systems. By encoding only two sets of four-step phase-shifting fringe images into the projector and modeling the relationship among gamma value, pre-coding value, and ideal phase, the algorithm estimates accurate pre-coding values. Experimental validation demonstrates significant reductions in phase error standard deviation post-gamma correction on standard whiteboards, leading to an ∼84% increase in 3D point cloud height accuracy. In addition, the proposed method effectively mitigates the periodic water ripple phenomenon induced by nonlinear gamma effects. Experimental investigations on plaster sculpture and printed circuit board further validate the method’s superiority, achieving higher phase accuracy and point cloud precision, with a measurement accuracy of 0.1 mm.

Bidirectional reflection distribution function (BRDF) models can accurately capture the reflective properties of target surfaces, thus playing a crucial role in achieving realistic image rendering in the field of computer graphics. The reflection model based on microfacet theory, which relies on a single roughness parameter, inadequately simulates the reflection properties of rough metal surfaces, thereby limiting rendering realism. To address this issue, we propose an improved method that introduces a glossiness parameter into the microfacet theory, thereby constructing the roughness-glossiness-Cook-Torrance (RGCT) reflection model. The glossiness parameter accurately captures the ratio and distribution of specular and diffuse reflections, which is directly related to the glossy properties of metal surfaces, enhancing the accuracy of the model in simulating the interaction between light and rough metal surfaces. We conducted comparative experiments on rough metal surfaces in the near-infrared wavelength bands with incidence angles from 0 to 90 deg. Experimental results demonstrate that compared to the traditional Cook-Torrance (CT) model based on microfacet theory, the RGCT model reduces the root mean square error (RMSE) by 0.0041 and 0.0048 on average, improves the coefficient of determination R2 by 0.0649 and 0.1082 on average, and reduces the relative error by 8.23% and 15.84% in the 1064 and 1550 nm wavelength bands, respectively. This improvement enhances the accuracy of the model in simulating the interaction between light and rough metal surfaces.

In response to the issues of visual occlusion in single-camera fringe projection, which makes it difficult to achieve three-dimensional panoramic object shape reconstruction and complicates system calibration in multi-camera fringe projection, a new method is proposed that uses a dual-plane mirror to assist single-camera fringe projection for three-dimensional panoramic object shape reconstruction. This method is based on a coordinate transformation approach using camera-projector calibration, which utilizes the dual-plane mirror to transform from the real camera coordinate system to the virtual camera coordinate system, thereby achieving the transformation from a virtual point cloud to a real point cloud. The method is simple and easy to implement and does not require initial estimation of the initial parameters of the plane mirror, which will greatly reduce the 3D reconstruction error and improve the computational speed. Finally, experiments on stepped standard workpieces and complex-shaped objects are conducted for three-dimensional reconstruction. The experimental results show that compared with the standard gauge block, the measured average absolute error and relative error are 0.0815 mm and 0.6738685%, respectively. This method not only effectively solves the problem of visual occlusion in fringe projection but also partially addresses the issue of information loss in object reconstruction due to high surface reflectivity, providing new insights for solving the problem of high surface reflectivity on objects.

We address the problem of eliminating measurement errors by filtering in such a way as to minimize the smoothing of real data while attempting to remove effects introduced by hardware functions. A critical analysis was performed of a range of existing filters for X-ray mirrors with subnanometer precision, including not only classical smoothing filters but also specialized filters designed for filtering X-ray mirror problems. Because the standard filters under consideration did not allow us to remove diffraction rings and peaks for our X-ray mirrors, we developed a novel technique for such image filtering. The proposed Fourier filtering of the interferometer data allows us to eliminate some of the measurement errors, thereby increasing the accuracy of the measured X-ray mirrors in the subnanometer range.

Machine tools play an important role in the manufacturing industry. The straightness error of the guide rail during machining seriously affects the accuracy of the machine tool, which is usually measured based on the laser collimation principle by the quadrant detector (QD). Laser beam radius varies significantly with long propagation distances. However, the traditional measurement model hardly considers the impact of radius variation. Therefore, an improved straightness measurement model is proposed. The spot radius variation is taken into account in the improved model. Meanwhile, an accurate trajectory equation for the variation of beam radius with distance was quickly established using less measured data. It facilitates faster and more accurate straightness measurements in industrial sites. The feasibility of the improved model is verified at four distances during 5 m. At 3.5 m, the maximum calibration error of the improved model was −1.65 μm, which is 86.8% lower than the traditional model. Meanwhile, the root mean square error of the improved model was 0.7 μm, which was 83.1% lower than the traditional model. The maximum repeatability error of the straightness was 0.28 μm. The straightness measurement accuracy of the improved model is obviously improved at long distances. The improved model would have great potential for long-distance measurement using lasers based on QD.

Lower accuracy of spectral information estimation from RGB values in liquid crystal displays affects high-precision color reproduction and display evaluation based on spectra. To improve the precision of spectra estimation, we propose an emission spectra estimation method based on a dimensionality transform strategy. Some metric learning techniques and kernel tricks are used for spectra dimensionality transform. The redundancy of spectral signals and nonlinearity of the display system have been removed; namely, the color feature is optimized. Then, a support vector regression model is applied in this dimensionality-transformed space to predict the emission spectra. The proposed method is validated by comparing the International Commission on Illumination (CIE) 1976 (L*, u*, v*) uniform color space and DE 2000 color differences between the estimated and true spectra for three different displays. The results manifest that the accuracy of spectra estimation obviously improves after applying the dimensionality transform strategy. The proposed method is significant for display color management, high-fidelity color reproduction, and display evaluation.

We introduce a method for designing a flattened optical frequency comb (OFC) utilizing cross-polarization modulation (XPolM) within a semiconductor optical amplifier-Mach Zehnder interferometer (SOA-MZI) framework. The proposed OFC generation technique is highly efficient in producing 64 optical comb lines with a minimal power deviation of 1.8 dB and 10 GHz adjacent frequency spacing which is operating within a bandwidth of 642 GHz. The performance of the proposed OFC generation method is comprehensively examined through theoretical and analytical investigations, considering key parameters that include carrier density, confinement factor, differential gain, and injection current of the SOA. By optimizing these parameters, the proposed OFC holds promise for diverse applications including RF photonics, spectroscopy, metrology, quantum optics, and high-speed optical communication systems.

We present a combination of split ring resonators for THz absorption applications. This combination creates a fractal organization with an ultra-wideband characteristic between 45.5 and 56.7 THz. During the design process, a very thin ZnSe substrate with a thickness of 0.6 μm was used, and copper resonators were placed on the substrate layer. The back layer was also covered with a very thin layer of copper to prevent the transmission of electromagnetic waves. The proposed design exhibits a wideband absorption characteristic with peak resonance points at 46.36, 47.57, 48.98, 50.39, 52.97, 54.62, and 56.1 THz. Furthermore, the electric field distributions at these peak resonances were obtained to further explore the absorption characteristics. The absorption response was also analyzed for both transverse electric and transverse magnetic modes to demonstrate mode stabilization in the absorption performance. Finally, polarization and angle of incidence variations were studied, and polarization independence was achieved. This design is a promising candidate for THz imaging and stealth applications due to its wideband performance.

As most cameras are currently built to be used alongside machine learning algorithms, image quality requirements still emanate from human perception. To redefine key performance indicators (KPI) for machine vision, optical designs are tested and optimized before their conception using differentiable simulation methods and gradient backpropagation to jointly train an optical design and a neural network. Although this helps to design optical systems for improved machine learning performance, it remains unstable and computationally expensive to model complex compound optics such as wide-angle cameras. We focus on optimizing the distortion profile of ultra wide-angle designs as it constitutes the main KPI during the optical design. Along the way, we highlight the benefits of controlling the distortion profile of such systems, as well as the challenges related to using learning-based methods for optical design.

We propose an efficient multiobjective design approach for a plasmonic nanoslit array device based on extraordinary optical transmission using the Q factor analysis. This approach leads to a substantial reduction in computation time required for the analysis of resonance in slit array by computing the Q factor at a singular frequency, corresponding to the eigenfrequency of the dispersive wave equation, rather than by performing calculations for the overall transmission for evaluating transmission bandwidths. For rigorous numerical analysis, we investigate the optical properties of plasmonic nanoslit array devices such as transmission, return loss (RL), and Q factor using the finite element method. Furthermore, to investigate the signal distortion that the light may undergo, we calculate the group delay time and dispersion based on various structures of nanoslit array filters. Among them, the three objective functions are defined as transmission, RL, and Q factor, with slit height, slit width, and array period as design variables, respectively. Multiobjective optimization is carried out by using Pareto optimality and particle swarm optimizations. The proposed method allows us to evaluate the Q factor much faster than estimating the bandwidth from the full transmission spectrum of the periodic slit array, which significantly reduces the computational time. The Q factor analysis can be used in the multiobjective optimization process that would otherwise be impossible due to time constraints.

We designed an 8×8 double-layer photonic network (DLPN) using ring-assisted directional coupler (RADC) switches to minimize crosstalk and accommodate fabrication errors. The RADC photonic switch was realized in a silicon-on-insulator (SOI) platform with enhanced optical confinement using the finite difference time domain method. Electro-optic tuning was achieved by injecting carriers across the P-i-N junction. Our proposed RADC switches have a 5% fabrication tolerance on waveguide dimensions, resulting in optimized crosstalk performance. A comparative study of ring-assisted Mach Zehnder interferometer switches and the proposed RADC switches was conducted in the DLPN architecture, considering various performance metrics such as extinction ratio, free spectral range, quality factor, finesse, crosstalk, and insertion loss. The proposed RADC switches achieved a 35% footprint reduction and crosstalk reduction (−20.71 to −35.07 dB) in waveguide crossings within the double-layer architecture.

We designed and fabricated a 2×2 channel multi-mode interference-Mach-Zehnder interferometer polymer thermo-optic switch. The thermo-optic switch comprises a borosilicate glass substrate, a core layer of JA-206, and an upper cladding layer of RZJ-390. The performance of this thermo-optic switch was simulated using the finite-difference beam propagation method and finite element method, and the results show that the switch is characterized by low power consumption and higher extinction ratio (ER). Furthermore, this study utilized a femtosecond laser to investigate the optimal preparation parameters. The results indicated that setting the laser power at 1 mW and the scanning speed at 4 mm/s ensures an efficient manufacturing process. With the operating wavelength of 1550 nm, the tests show that the driving power of the thermo-optic switch is about 5.5 mW, and the ER reaches 23.37 dB. In addition, after a continuous test lasting 12 h, the rise and fall times of the switch were measured to be <1.7 ms. Therefore, it meets the optical interconnection requirements of electro-optical printed circuit boards.

A novel ultra-wideband polarization beam splitter (PBS) with dual-core conjoined tube negative curvature fiber (DC-CTNCF) is studied and proposed. By introducing a double conjoined tube in the x-direction, the extra loss caused by inter-tube coupling and vibration can be reduced, and the crosstalk effect between adjacent tubes can be suppressed, which in turn improves the performance and stability of the DC-CTNCF PBS. By optimizing the fiber structural parameters, the DC-CTNCF PBS with a length of only 5.03 mm achieves an ultra-wide bandwidth of 590 nm in the wavelength range of 1.44 to 1.99 μm, and the ratio of the loss of the higher-order modes (HOM) to the fundamental mode is larger than 100 in the corresponding wavelength range, indicating that the PBS has excellent single-mode properties. The DC-CTNCF PBS represents a significant improvement in the fields of fiber optic communications and fiber optic sensing, as it surpasses the length limitation of current PBS designs and increases the coupling length by an order of magnitude.

A mathematical model for simulating the thermal radiation process of a laser-heated surface-absorbing approximate-transparent infrared window is demonstrated. The model is based on the heat conduction equation and Kirchhoff’s law. The simulation was conducted for a germanium window to calculate the radiometric quantities including radiance, exitance, and flux. The calculated results of the radiance reveal that the infrared radiation of the germanium window has approximately the Lambertian property. Numerical calculations demonstrate that following continuous irradiation of a 160-W/cm2 near-infrared laser for 500 ms, the maximum radiant exitance at the surface of the window reaches the order of magnitude of 1 W/m2 in the medium-wave infrared band and 10 W/m2 in the long-wave infrared band. A comparative study was conducted to investigate the radiant flux differences of intrinsic germanium, n-type doped germanium, and p-type doped germanium under the same conditions.

The solar-blind ultraviolet photodetector can find extensive applications in fire prevention, ozone hole monitoring, navigation, and more, for the advantage of no solar background radiation in the ultraviolet-c band (UVC, 200 to 280 nm) at the Earth’s surface. However, most of the UVC photodetectors reported in recent years, including gallium oxide photodetectors, zinc oxide–based photodetectors, and aluminum nitride photodetectors, suffer from the complex and costive fabrication process, which requires high-temperature material fabrication and high fabrication costs. Here, we report an ultraviolet (UV) photodetector composed of magnesium-doped zinc oxide (MgZnO) photosensitive functional material via a simple low-temperature sol–gel fabrication method. In the study, first, the synthesizing method of the MgZnO photosensitive functional layer is systematically investigated. Then, the optical bandgap change of MgZnO with the doped Mg concentrations is explored. The physical model of the relationship between the Mg-doped concentrations and the optical bandgap of the MgZnO photosensitive functional layer is established by spectroscopic methods. Based on the doping study, a highly responsive MgZnO UV photodetector has been designed and fabricated. The MgZnO UV photodetector shows a response bandwidth of 200 to 280 nm, a high responsivity of 108 mA/W, and rise and fall times as low as 0.4 and 2.4 s, respectively. The work reported here may open an easy, low-cost route for developing UV photodetectors.

The measurement range of sensors based on optical resonators is often limited by the cavity linewidth. The usual Pound–Drever–Hall current feedback can extend the sensor’s measurement range to the tuning range of the laser. However, this method is still insufficient for sensors with high dynamic range requirements. We present an optical frequency shift detection technique that combines temperature and current tuning, which expands the dynamic range of sensors based on optical resonators. When the optical frequency shift in the optical resonators is beyond the current feedback measurement range, temperature feedback is used to alter the wavelength of the laser. As a result, the resonance curve of the resonators is adjusted to fit the current tuning measurement range. The method has high accuracy for current feedback and a large measurement range for temperature feedback. Experimental results show that the method has a range of measurement of 8.14 GHz, 14 times higher than the traditional current feedback method, with a measurement accuracy of 38 kHz and a dynamic range of 106 dB.

We fabricated buried circular cladding waveguides with different cross-sectional diameters in neodymium-doped yttrium aluminate crystals via femtosecond laser direct writing at a repetition rate of 500 kHz. Continuous-wave waveguide lasers operating at 1079 nm were successfully achieved at room temperature using an optical pump with 803-nm light. In addition, the guiding properties of the cladding waveguides fabricated using various laser-writing conditions were investigated. The fabricated waveguides exhibited excellent performance, with an insertion loss of 1.2 dB at a wavelength of 1.06 μm. Confocal micro-Raman and micro-photoluminescence analyses were performed to investigate the crystalline lattice modifications induced by the FsLDW. The lasing slope efficiency of the cladding waveguide with a diameter of 40 μm reached 29.2% with a maximum output power of 183 mW, and the cladding waveguide with a diameter of 60 μm exhibited a lasing slope efficiency of 29.9% and a maximum output power of 155 mW.

A wide angle of incidence (AOI) mirror at 266 nm was fabricated through electron beam evaporation for fourth harmonic laser systems. The optical and microscopic morphology of the mirror was characterized by a spectrophotometer, scanning electron microscope, X-ray diffractometer, and atomic force microscope. The reflectivity of the films reaches 93.2% at 266 nm in the range of 0- to 45-deg AOI. The mirror was annealed at temperatures of 200°C, 300°C, and 400°C, respectively. The characterization results indicate that the annealing of mirror at 400°C led to the crystallization of HfO2 in a monoclinic crystalline phase. The highly reflective band of the annealed films at 400°C is obviously shifted to the shorter wavelength by 8 nm at 0- to 45-deg AOI. The annealing temperature has no significant effect on the surface morphology and roughness of the film.

Stress is a natural response of the body to threatening and challenging situations. Although stress can sometimes have positive impacts, such as enhanced alertness and improved performance, it can also cause harmful effects, such as sustained high blood pressure, anxiety, and depression, especially when prolonged. Continuous monitoring of stress-related molecules, known as stress biomarkers, could enable early diagnosis of stress conditions and therefore improve recovery and reduce healthcare costs and long absences from work. We present a highly sensitive grating-coupled surface plasmon resonance (SPR) sensor for detecting stress biomarkers. The gold-coated sensor chip operates with a tunable laser within the wavelength range of 1527 to 1565 nm. This sensing method is based on detecting a shift of the SPR wavelength, which occurs due to a change in the refractive index of the medium that is caused by the presence of analytes near the plasmonic grating. The sensor chip was tested with four stress-related biomarkers: glucose, creatinine, lactate, and cortisol. With the current version of the sensor, containing no recognition element, the achieved detection limits for these analytes were 5.9, 7.1, 36.9, and 10.7 mM, respectively, which are close to the physiological values of these analytes in body fluids, such as sweat. This proof-of-concept work demonstrates the sensitivity and physiologically relevant detection limits of the presented compact sensor chip and its potential for future healthcare applications, such as continuous stress monitoring, when developed further.

Tunable thermal absorber has become a research hotspot for new devices with high efficiency and low loss, which has a broad application prospect in the fields of infrared communication and material detection. A non-reciprocal thermal absorber composed of InP magneto-optical nanowire arrays is proposed, which can realize strong non-reciprocal absorption/radiation at ∼8 to 10 μm by utilizing the surface plasmon resonance and multiple reflections among the nanowires. The optical properties of nanowire arrays are closely related to their structural parameters. We analyze the effects of variables such as the diameter, length, shape, and period spacing of the nanowires and find that only the cylindrical nanowire arrays expand the band of non-reciprocal effects, creating a completely new non-reciprocal peak at 9.44 μm. The electric field energy distributions of different cross sections intuitively explain the enhancement mechanism of cylindrical nanowire arrays and the physical mechanism of the nonreciprocal effect. In addition, we compare the computational results of effective medium theory (EMT) with finite difference time domain and analyze the applicable conditions for fast prediction using EMT. Last, we explore the photothermal effect when the nonreciprocity of the array is maximized, and the results show that for 9.44-μm light, a luminous flux of 1 mW/μm2 can bring about a nonreciprocal temperature difference of 15 K, which endows the array with significant tunable absorption/emission capability, which is helpful for the development and application of future nonreciprocal thermal absorbing devices for the mid-infrared wavelength band.

A tightly interlaced matched ambidextrous bilayer (TIMAB) is a sculptured thin film consisting of a cascade of one-dimensional cells made up of two layers of chiral constituents that are identical except for their structural handedness and are as thick as their structural period. Periodic TIMABs, where the structural period of the chiral constituents is the same in all bilayer cells, exhibit a narrow polarization-universal bandgap on axial excitation. We propose linearly chirped TIMABs, in which the structural period of the chiral constituents increases with the position of the cell in the structure and show that they exhibit a significantly wider bandgap (more than four times) than the periodic TIMABs.

Optical encoders play a crucial role in optical communication applications, and there is a substantial demand for innovative designs that are both ultra-compact and high speed. The proposed structure consists of silicon rods in the air background using a square lattice. Nano-coupling rods are used in combination with a waveguide-shaped tuning fork. The photonic bandgap is being calculated using the plane wave expansion method and finite-difference time-domain method to analyze the performance characteristics of optical encoders, such as transmission spectra, electric field view, contrast ratio, and response time. The proposed encoder operates at 1550 nm, allowing it to carry out encoder operations in which only one input port is active, whereas the others are deactivated. The proposed encoder is designed with a fast response time of 137.2 fs, a high data rate of 7.28 Tbps, and an ultra-compact size of 80.06 μm2.

Optical fiber temperature sensors based on fluorescence lifetime are widely utilized in a multitude of fields. The integral ratio method (IRM) and fast Fourier transform (FFT) method are the most commonly employed techniques for obtaining fluorescence lifetime. However, both methods have been found to have insufficient precision to varying degrees. We employed the extended Kalman filter (EKF) algorithm to fit the fluorescence attenuation curve, and the weighted linear least squares method is utilized to obtain a more accurate fluorescence lifetime. A comprehensive hardware system is fabricated for verification purposes. The software simulation demonstrates that the EKF algorithm enhances the precision of the fluorescence attenuation curve by up to 96.469%. The results of the test demonstrate that the temperature sensor we proposed has a resolution of 0.07°C. In comparison to the IRM and FFT methods, we enhanced the average accuracy of the fluorescence lifetime by 17.819% and 4.626%, respectively, whereas the average deviation of the temperature is reduced by 80.224% and 54.701%, respectively. We provide an effective way for the algorithm design of high-accuracy optical fiber temperature sensors based on fluorescent lifetime.

An optoelectronic oscillator (OEO) with wide tuning range and stabilized output is proposed and experimentally demonstrated. In the proposed OEO, a narrow-passband microwave photonic filter (MPF) is implemented by a joint operation of a dispersion-induced MPF, an infinite impulse response (IIR) filter based on a double-parallel recirculating delay line, and a two-tap finite impulse response (FIR) filter based on a polarization multiplexed dual-loop. By changing the time delay difference between the two polarization states of the polarization division multiplexing emulator within the dispersion-induced MPF, the central frequency of the narrow-passband MPF can be adjusted, and the tunability of the OEO is realized. A theoretical derivation is achieved, which is verified by an experiment. Tunable oscillation signals from 8.09 to 12.1 GHz at an average tuning step of ∼500 MHz are generated. With the help of the IIR and two-tap FIR filters, the strong stability and high side-mode suppression performance of the OEO is realized, where the maximum frequency drift of the generated 8.09 GHz signal is observed to be 2.1 kHz over 1 h and its side-mode suppression ratio reaches 61 dB with a frequency span of 5 MHz. The phase noise performance of the scheme is also studied.

The application of complex measurement matrices in image-compressed sensing (CS) has attracted significant attention; however, effectively combining their structural, random, and orthogonal characteristics remains a substantial challenge. We propose a complex measurement matrix that integrates randomness and orthogonality into a structured discrete Fourier transform (DFT) matrix, termed ROS-DFT. We first introduce randomness to the DFT matrix through random phase encoding (RPE). Subsequently, sequences generated by the Logistic map are employed to shuffle index arrays of the matrix, which further augments matrix randomness. Finally, the Gram–Schmidt orthogonalization is applied to the columns of the measurement matrix, recovering column orthogonality, which is lost during the randomization operation. In addition, sparsification of the measurement matrix is achieved by randomly preserving a fixed number of non-zero elements in each column to reduce the storage space and minimize the computational complexity. Extensive qualitative and quantitative experiments demonstrate the effectiveness of the proposed method in significantly enhancing the quality of reconstructed images across various CS ratios compared with state-of-the-art methods.