Optics

This paper proposes a high-sensitivity and sensitivity-tunable vibration sensing system based on a switchable loop length optoelectronic oscillator (OEO). Carrier-sideband separation is realized by using an acousto-optic modulator (AOM), and the resonant cavity length is designed to be independent of the sensing fiber arm. Compared with a conventional 10 GHz OEO under the same total loop delay condition, the proposed architecture provides a theoretical sensitivity enhancement of approximately $1.93\times {10}^4$, without requiring a high RF oscillation frequency. Meanwhile, the system oscillates at only 80 MHz, which greatly reduces the implementation difficulty of the frequency detection circuit. The proposed scheme further introduces a mechanical optical switch (MOS) to select intra-loop fibers of different lengths, thereby reconfiguring the equivalent loop delay and the free spectral range of the OEO. Experimental results show that stable single-mode oscillation is achieved at 80.42 MHz with a side-mode suppression ratio of 51 dB. By selecting loop fiber lengths of 1200 m, 500 m and 0 m, frequency-to-displacement sensitivities of 0.892 GHz/cm, 1.93 GHz/cm and 9.27 GHz/cm are obtained respectively, with excellent linearity. A 600 Hz vibration signal is successfully demodulated with a signal-to-noise ratio of 72.1 dB. The proposed method provides a simple and reconfigurable solution for high-precision vibration measurement under different operating conditions. Full article

In the present work, we propose a simple analytical algorithm for unobscured two-mirror telescope designs. It is based on fitting of the closest aberration-free surface for each field position and allows for computation of the wave aberration distribution over a large aperture. All the equations necessary for the algorithm implementation are expressed in a general form allowing its application for complex freeform mirrors. It is shown using the example of a small telescope with $f^{\prime }=245$ mm, $F/\#=7$ and $2°\times 0.96°$ field of view, that the algorithm allows us to find a good starting point, and reach a high spatial resolution in comparison with off-axis Ritchey–Chrétien and Schiefspiegler telescopes. The algorithm can be of a specific interest for the development of highly specialized imaging systems of several types. Full article

This study investigates the feasibility of using an etched addressed fiber Bragg structure (EAFBS) as the sensing element in a microwave-photonic system for local pH monitoring. A theoretical model is developed that incorporates transmission-spectrum analysis of the addressed Bragg structure and evaluation of the effective refractive index of the guided mode as a function of the surrounding medium parameters. The model establishes a relationship between the refractive index of the surrounding medium and the spectral response of the EAFBS. Using the proposed mathematical model, numerical simulations are performed for three pH ranges corresponding to various liquid media. The results of numerical studies show that in all in the pH ranges, the address frequency varies nearly linearly, with a sensitivity of approximately 1.66 GHz/pH (pH range of 4–7), 2.64 GHz/pH (pH range of 7–10), and 1.92 GHz/pH (pH range of 10–13), whereas its temperature dependence in the range of 35–45 °C is approximately 1.06 GHz/°C. These results indicate that the proposed EAFBS-based approach can provide a foundation for high-speed pH sensing systems with a simplified interrogation scheme and potential capability for frequency-division multiplexing. Full article

We investigate the localization transition in a one-dimensional extended Aubry–André–Harper (AAH) model, emphasizing its implementation as a quantum simulator in ultracold atomic optical lattices. In the single-particle case, quasi-periodic hopping modulation allows the localization transition point to be clearly observed even for small system sizes. By driving the system into strongly Anderson localized states immediately after the transition, we observe a sharp many-body localization (MBL) transition upon introducing interactions, with the MBL transition point closely approaching that of Anderson localization. To demonstrate the effects of interactions, we map out a global phase diagram and find that critical states in this model are easily thermalized. Contrary to previous studies where enhanced interactions significantly promoted thermalization, we find that increasing interactions does not notably shift the MBL transition point. Importantly, our setup offers a practical and experimentally accessible platform for studying sharp MBL transitions using ultracold atoms in optical lattices, bridging MBL physics with advances in quantum optics and cold-atom technologies. Full article

Video Compressive Imaging (VCI) enables low-dimensional detectors to capture high-dimensional data through incoherent encoding. However, traditional pseudo-random coding often exhibits structural sampling that leads to detail loss. While adjusting the sampling rate can balance structured sampling and incoherence, the reconstruction quality remains unsatisfactory. To overcome this limitation, we propose a high-frequency coding method that mitigates the structural problems of pseudo-random coding by reducing low-frequency components. Simulation results show that this method significantly improves image detail reconstruction, with an average peak signal-to-noise ratio (PSNR) increase of 1.6% across various sampling rates. At a 20% sampling rate, the PSNR improvement reaches around 6%. Furthermore, the method integrates easily into existing VCI systems, offering substantial improvements in image reconstruction quality and reliability compared to pseudo-random coding. Full article

Laser-induced acoustic communication is a highly adaptable cross-medium technique that combines the advantages of optical transmission through air and acoustic transmission underwater. However, poor signal stability at high repetition frequencies currently hinders its widespread application. To address this, this paper proposes an innovative scanning focal-point method to enhance stability. Traditional methods such as beam scanning, focus control, and distributed interaction are primarily aimed at enhancing sound pressure in a specific direction, achieving near-field/far-field focusing, or improving the signal-to-noise ratio through coherent synthesis of ultrasonic intensity. In contrast, the method proposed in this paper is intended to avoid the interference of droplets and vapor generated by single-point breakdown under high repetition frequencies, which would otherwise degrade the laser-acoustic conversion efficiency. It is therefore an active defense strategy specifically targeting the stability of laser-induced acoustic communication. First, optical simulation software was used to analyze the effects of surface ripples and bubbles on focal spot displacement and size. Next, a single-pulse experimental system was developed to measure the range and duration of surface depressions caused by optical breakdown. Finally, a scanning focal-point system was constructed for comparative experiments, with results recorded via hydrophones and high-speed cameras. The maximum laser-induced acoustic signal generated by the scanning focal-point method is 7.4 times that produced by single-point breakdown. The experimental results demonstrate that the scanning focal-point method can effectively avoid the influence of water surface disturbance and steam on the optoacoustic conversion efficiency and significantly improve the amplitude and stability of the laser-induced acoustic signal. Full article

All-cellulose composite (ACC) films, with their excellent tunable optical and mechanical properties, combined with biodegradability, represent a highly promising material for applications in the packaging and flexible electronics sectors. The optical properties of ACC films are critically governed by their microstructure, which is determined by drying conditions. In this study, the effects of drying conditions on the structure–property relationships of ACC films were systematically investigated. First, ACC films were fabricated via the partial dissolution of microcrystalline cellulose powder in ionic liquids, followed by a film-casting process. Subsequently, various drying conditions under different temperatures and pressures were applied to finalize the films. XRD characterization demonstrated the coexistence of cellulose I and cellulose II structures. Optical and morphological tests revealed that (1) drying without pressure resulted in obvious shrinkage and deformation, with the diameter reduced by 70%; (2) the high-temperature/high-pressure drying method promoted a dense structure, resulting in ACC films with high transmittance (>90%) and low haze (<10%); and (3) ACC films dried under different hot-press temperature conditions showed similar transmittance and a large difference in haze, which could be related to the micro-pores formed within films. The systematic correlation between structure and optical properties established in this work provides a clear pathway for the tailoring of the optical performance of cellulose films through controlled drying conditions. Full article

We present a method for quantifying ultrashort pulse-shape instability in a train of pulses using multi-shot second-harmonic-generation frequency-resolved optical gating (SHG FROG). All versions of multi-shot FROG have previously shown the ability to qualitatively distinguish stable from unstable pulse trains, as systematic differences appear between measured and retrieved FROG traces when instability is present. This has proved possible because the recently introduced retrieved-amplitude N-grid algorithmic (RANA) approach provides highly reliable pulse retrieval, even for unstable pulse trains and in the presence of noise, thereby eliminating the possibility that algorithm stagnation, which also yields such systematic differences, could be confused for such instability. In other words, RANA’s excellent performance ensures that any non-random discrepancies between measured and retrieved FROG traces reflect physical pulse-shape instability rather than algorithmic stagnation. To quantify such instability, we now introduce an instability parameter, $R$. It involves an extension of the well-known statistical “Runs” test, which has been used for decades to test for systematic error in fits to one-dimensional (1D) data. A runs test counts the “runs”—consecutive points in the plot of the difference between the data and fit with the same sign (+ or −), yielding an evaluation of the goodness of the fit, largely independent of random error. Specifically, the more runs, the better the fit. However, because FROG traces are functions of two variables, we must extend the usual 1D runs test to two dimensions, that is, to enumerate the 2D runs—“hills” and “valleys” in the difference between measured and retrieved 2D FROG traces. Many small 2D runs indicate only random noise-like differences, that is, a good fit, and, hence, a stable pulse train, whereas few large runs reflect systematic error, that is, a poor fit, and, hence, pulse-shape instability. Finally, because random noise could contribute numerous meaningless runs in the wings of a FROG trace, where the intensity is near zero, we must also weight each hill and valley by its average measured trace intensity in order to minimize its effects. We show that R is intuitive and reasonable and, in addition, is independent of pulse complexity and trace size. As a result, it provides a clear metric of pulse-shape stability vs. instability. Full article

In this work, it is demonstrated that the tilt angle of an etalon can be determined by analyzing some features of an intensity distribution pattern (interferogram) with machine learning algorithms. These features present a strong nonlinear relationship with the etalon tilt angle, showing several discontinuities and ambiguities within a 20° range. Here, a regression based on an ensemble of artificial neural networks was implemented to correctly estimate the tilt angle. By using this ensemble, the tilt angle was estimated with a mean absolute error of 0.028° and root squared error of 0.047°, for a measurement range between —10.013° and 10.013°. Finally, it is shown that in this way both magnitude and direction of the tilt angle can be determined from just an image and additionally that to perform this task a simple optical setup was required, reducing its overall cost. Full article

Bent-core liquid crystals are renowned for their remarkable optical and ferro-electrical properties, making them highly sought after for various applications. However, to harness their full potential, a thorough understanding of their structural mechanisms and fluctuations during phase transitions is imperative. In this study, we conducted an in-depth analysis of the structural conformation of a V-shaped liquid crystal, specifically (E) 1,2-phenylene bis[4-((E)-(4-pentyloxy chloro phenyl) diazenyl) benzoate], referred to as V1, utilizing density functional theory (DFT) calculations at the B3LYP/6-311G(d,p) level. Geometry optimization and frequency calculations of the most stable conformers were performed at the same theoretical level. Our investigation into the mesomorphic behavior of V1 unveiled two enantiotropic phase transitions: Isotropic (Iso) → Nematic (N) → Smectic A (SmA) → Crystalline (Cry), with decreasing temperature. To elucidate the molecular alterations of V1 at the microscopic level, Fourier Transform Infrared (FT-IR) and Fourier Transform Raman (FT-Raman) spectra were recorded across various temperature ranges. Remarkably, the simulated vibrational spectra exhibited a striking resemblance to the experimentally observed vibrational spectra at room temperature, validating the accuracy of our computational approach. These findings hold immense promise for advancing further research and facilitating the development of novel applications leveraging the unique properties of bent-core liquid crystals. Full article

To circumvent the prohibitive cost of large-format infrared focal plane arrays and the significant spatial–spectral mismatch caused by spectral smile in conventional long-slit configurations, this work develops a low-cost short-wave infrared (SWIR, 1000–2500 nm) hyperspectral imaging system utilizing digital micromirror device (DMD) scanning paired with a single-element detector. A comprehensive analytical model for a prism–reflection grating (P-RG) compound dispersive element is established, enabling the joint optimization of the prism apex angle and grating period to achieve quantitative compensation of spectral distortion across the entire waveband. Based on this model, the optical system is integrated and optimized, while a centroid localization algorithm is implemented to facilitate online calibration of model parameters and real-time reconstruction of the hyperspectral data cube at the DMD plane. Experimental results demonstrate that both smile and keystone distortions are suppressed below $5\mathsf{\mu }\mathrm{m}$ throughout the 1000–2500 nm range, which is superior to the single DMD pixel pitch of $7.6\mathsf{\mu }\mathrm{m}$. The full-field modulation transfer function (MTF) at the Nyquist frequency (32.9 lp/mm) exceeds 0.7, approaching the diffraction limit. Characterization confirms that the system provides 510 spectral channels with an average resolution of 3.57 nm and a spatial resolution of 2.5 $\mathsf{\mu }\mathrm{m}$. By effectively eliminating spectral overlap and cross-column crosstalk on the DMD encoding surface, this system provides a high-fidelity optical front-end for single-pixel imaging, offering a viable technical pathway for the development of affordable SWIR hyperspectral instrumentation. Full article

Accurate axial localization of microparticles is a key requirement in in-line digital holography (ILDH), particularly under noisy conditions and for weakly scattered objects. This work presents experimental and simulated benchmarking of three widely used focus metrics: maximum intensity, complex amplitude, and Kurtosis. Experimental holograms of microparticles with different diameters were recorded using a compact ILDH system, while simulated holograms of a 10 µm particle were generated. Numerical reconstruction was performed using a Fresnel convolution approach with FFT-based propagation over a range of axial distances. The performance of each focus metric was evaluated based on peak definition, robustness to coherent noise, and consistency across particle sizes and configurations. The results show that both maximum intensity and Kurtosis provide consistent and reliable axial localization, with very similar behavior across all cases. In contrast, the complex amplitude metric is more sensitive to noise and exhibits larger fluctuations in the axial response. These results indicate that simple intensity-based metrics can achieve accurate localization under moderate signal-to-noise conditions, while higher-order statistical metrics improve robustness in more challenging scenarios. This work provides practical guidelines for selecting autofocus criteria in ILDH systems for particle imaging and holographic metrology. Full article

In this work, we report a theoretical study of the energy, momentum, and angular momentum of non-diffracting Tricomi beams. By utilizing the vector potential in the Lorenz gauge, we derive the explicit analytical expressions for the electric and magnetic field components of non-diffracting Tricomi beams. A canonical theory is introduced to describe the energy, momentum, spin angular momentum (SAM), and orbital angular momentum (OAM) of the non-diffracting Tricomi beams. The effects of the asymmetry constants, topological charge, and half-cone angle on the energy, momentum, SAM, and OAM of the non-diffracting Tricomi beams are simulated and analyzed. This study provides fundamental physical insights into the dynamical characteristics of non-diffracting Tricomi beams relevant to potential optical manipulation applications. Full article

We propose and analyze a new class of photonic neurons based on spatial solitons generated in photorefractive media. They are designed to operate entirely within the optical domain. By engineering single-node and multi-node multichannel architectures, we demonstrate the feasibility of constructing balanced, scalable, and reconfigurable structures capable of emulating neural behaviors such as symmetric signal splitting, plasticity, and dynamic adaptation. The optimization of geometric parameters—including soliton waveguides features, input distances, and incidence angles—proves crucial for ensuring the stability of solitonic propagation and the proper functioning of interaction nodes. The results lay the groundwork for the development of high-performance optical neural circuits, with potential applications in distributed signal processing, neuromorphic artificial intelligence, and reconfigurable optical memories. Full article

This paper addresses the problem of counting stacked components in industrial scenarios and proposes a method that combines close-range scanning for complete contour acquisition with deep learning for quantity recognition: The contour acquisition system consists of a line array camera and a linear laser. Both are arranged horizontally at a certain angle, and the laser line is perpendicular and in the same direction as the stacking of the components. The system scans and connects single-row pixels along the stacking direction to obtain the contour. This method effectively avoids the occlusion problem caused by uneven stacking of components. The quantity recognition algorithm adopts a network structure similar to Encoding–Decoding using the component gap (cls: 0 indicates not, 1 indicates yes) and the endpoint coordinates of the separation line segment [cls, x1, y1, x2, y2] to form a label. Multi-scale anchors are introduced to predict the translation distance of the line segment (positive or negative, indicating direction). The prediction head is fully convolutional, and the loss for regression is computed using the predicted endpoints of the ground-truth line segments. A line segment redundancy removal method is proposed to output the predicted confidence (conf) and coordinates [conf, px1, py1, px2, py2] for each component gap. The self-built dataset is used for training and validation. Experiments show that the recognition accuracy of each image reaches 95.79%, and the gap recognition accuracy reaches 99.62%, which can meet the requirements of automation. Full article

The teaching of chaos and nonlinear dynamics remains a significant challenge in physics education, as these concepts are often introduced through abstract mathematical models that are difficult to visualize. In this work, we propose an experimental approach based on electro-optics in nematic liquid crystals as an effective and accessible platform for teaching these phenomena. In particular, the system exhibits a transition from ordered convective patterns to strongly disordered turbulent regimes, which can be directly observed in real time using simple optical techniques. This experimental framework enables students to explore key concepts of nonlinear physics, including instability thresholds, pattern formation, and the emergence of complex dynamical behavior. The transition occurs through the nucleation and growth of turbulent domains, facilitating the understanding of nonequilibrium dynamics. From a pedagogical perspective, the proposed experiment combines strong visual impact with experimental controllability and accessibility, making it suitable for undergraduate students in physics, mathematics, and engineering. Furthermore, the integration of AI-assisted analysis provides students with an accessible framework to process experimental data, identify dynamical regimes, and explore complex systems through novel data-driven methodologies. Full article

The present work investigates the linear regime of propagation of modulated vector optical fields in isotropic dispersive media by focusing on the formation of complex vector vortex structures with amplitude-type singularities. A mathematical algorithm designed to derive novel exact analytical solutions for the linear vector amplitude equation is presented, enabling the systematic development and classification of diffraction-free vector solutions. Various types of solutions for the two orthogonal components of the vector amplitude function are obtained, resulting in non-trivial spatial amplitude structures in their cross sections. The proposed approach allows for precise analytical governance of the spatial and polarization properties of the obtained vortices via the vortex parameter n. The presented model offers a comprehensive framework for generating different types of vector vortex structures by choosing the values of the parameters n and m, depending on the initial phase of the components. The derived solutions extend the capabilities of conventional phase modulation techniques. It is demonstrated that by changing the vortex parameter n, the structural complexity of both the amplitude distributions and polarization patterns increases. A number of numerical simulations, based on the obtained analytical solutions, are performed. They validate the model and clearly illustrate the characteristic vectorial features via detailed vector diagrams. Full article

The development of efficient, single-phase-excitable white-light phosphors remains a critical challenge for solid-state lighting applications. In this work, white-light-emitting CaWO₄:Eu³⁺/g-C₃N₄ composites were successfully developed by integrating red-emitting CaWO₄:7%Eu³⁺ with blue-emitting graphitic carbon nitride (g-C₃N₄). Under 365 nm near-UV excitation, the composite exhibits dual-band emission originating from the ⁵D₀ → ⁷F₂ transition of Eu³⁺ (~616 nm) and the intrinsic band-edge luminescence of g-C₃N₄ (~460 nm). The optimal white-light performance is achieved at a g-C₃N₄ content of 0.5 wt%, yielding CIE chromaticity coordinates of (0.294, 0.324) and a correlated color temperature (CCT) of 7673 K. This sample demonstrates a photoluminescence quantum yield (PLQY) of 3.25%. Moreover, the CaWO₄:Eu³⁺/g-C₃N₄ composite shows enhanced thermal stability, retaining 78% of its initial emission intensity at 175 °C, with an activation energy of 0.41 eV—significantly higher than that of the pristine CaWO₄:Eu³⁺ (0.22 eV). These results indicate that the CaWO₄:Eu³⁺/g-C₃N₄ heterostructured phosphor is a promising candidate for single-phase-excitable white-light applications. Full article

Reconfigurable intelligent surfaces (RISs) have recently attracted attention as a potential solution for improving the reliability of optical wireless communication links, especially when direct transmission (DT) becomes severely degraded due to dynamic channel conditions. In this study, an RIS-assisted architecture based on a modulating retroreflector is proposed for underwater optical wireless communications (MRR-UOWC). In the considered system, both the DT path and the RIS-assisted path transmit the same information simultaneously at the same data rate. The propagation channels are modeled by taking into account propagation loss, Gamma–Gamma turbulence, and pointing error effects. At the receiver, the signals arriving through the direct path and the RIS-reflected path are coherently combined. To evaluate the effectiveness of this configuration, two diversity combining techniques, namely selection combining (SC) and maximum ratio combining (MRC), are investigated. Closed-form analytical expressions for the outage probability (${\mathrm{P}}_{\mathrm{out}}$), average bit-error rate (BER), and ergodic capacity ($\overline{C}$) are derived using the probability density function (PDF), cumulative distribution function (CDF), and moment-generating function (MGF) of the end-to-end signal-to-noise ratio (SNR). The analysis indicates that jointly exploiting the DT and RIS-assisted links can provide noticeable performance gains by leveraging the complementary characteristics of the two propagation paths. Full article