Rydberg atom is a kind of alkali atoms whose outermost valence electrons are in the highly excited state with a large electric dipole moment and a very high polarization rate, which exhibits extreme sensitivity to external electric field and be utilized in microwave quantum sensing. Rydberg atom-based sensors have developed rapidly and have shown extensive applications in the fields of precise navigation, quantum metrology, communication and high-resolution remote sensing owing to its distinct advantages including high sensitivity, broad frequency bandwidth (DC-THz), SI traceability and self-calibration capability that are not available in conventional antenna-based receivers. As an emerging quantum sensor, the Rydberg atomic receiver employs laser-excited atoms to convert radio frequency (RF) fields into an optical signal that can be read out on a photodetector at baseband to detect the relevant information of the RF field. In Rydberg atom-based electrometry, under the effect of an intense coupling laser, the dispersion property of the medium changes, resulting in the absorption of the weak probe laser being reduced, which is the definition of electromagnetically induced transparency (EIT). Furthermore, when a RF field is applied near resonance using two Rydberg energy levels, the corresponding Rydberg energy levels split, resulting in a two-peak spectrum, known as Autler–Townes splitting (EIT-AT splitting). The EIT-AT splitting interval is proportional to the amplitude of the RF field, which can obtain a direct measurement of the external field by measuring the splitting interval. However, this practical deployment of this promising technology, particularly for advanced functionalities like direction positioning, faces numerous significant challenges. On the one hand, the existing single-beam detection architecture is incapable of acquiring the spatial phase differences critical for Angle of Arrival (AOA) estimation, which the multi-beam architecture could conceptually essential to overcome this limitation. On the other hand, the traditional optical beam splitters suffer from structural complexity and poor stability, which limit the application of this technology in reality positioning.

In this work, in order to achieve the probe laser array and meet the exact alignment requirements of the Rydberg atoms for both the probe laser and coupling laser, we report and present a multi-beam Rydberg atomic receiver for AOA estimation utilizing a cascade liquid crystal polarization grating (LCPG), achieving a 2×2 two-dimensional (2D) collimated laser beam with an interval of 2.42 mm. First, we utilized the polarization-selective properties of the LCPG to design a beam-splitting grating with a period of 21.32 μm, which can split the incident laser into a 2×2 array of beams with deflection angles. Second, to achieve a collimated array output, we further proposed a four-zone collimating grating with a period of 14.95 μm which can collimate the four beams separately. This cascaded gratings optical system has a compact structure with beam power uniformity better than 95% and the effective collimation is over a distance greater than 15 cm, overcoming the inherent shortcomings of discrete optical components effectively. Moreover, we employ the EIT in Rydberg atomic vapor cell in conjunction with a atomic superheterodyne technology to measure the phase difference at different locations of the incident RF signal simultaneously, realizing the estimation of the angle of arrival.

To demonstrate this approach, we simulated and measured phase differences of an incident X-band wave at 8.57 GHz by fixing the local oscillator signal (E_LO) and rotating the signal source (E_SIG) within the incident range of -40° to +40°. To simplify the complexity of the adjustment, we expanded the coupling laser to 5 mm to ensure full coverage of the 2×2 probe laser array and apply different photodetectors (PDs) to receive signals from each probe laser simultaneously and connect them to a multi-channel oscilloscope to obtain the phase difference $ \Delta \phi $ of the beat-note signal, thereby enabling precise determination of the AOA of the incident signal. The experimental results show that the functional relationship between the measured phase difference $ \Delta \phi $ and the AOA is highly consistent with the theoretical model and electromagnetic simulation results with the error less than 1.5°, indicating that the proposed atomic-based subwavelength multi-beam receiver can be used to determine the AOA of the RF signal.

This work not only validates the fundamental principle of quantum-based RF direction positioning using Rydberg atom-based sensors but also establishes a practical and robust foundational unit for future advanced systems. The proposed integrated beam array scheme is highly scalable and versatile, and the 2D configuration can be directly extended to full three-dimensional (3D) arrays for omnidirectional sensing. Furthermore, the inherent design of the 2×2 laser array allows for the arbitrary combination of any three beams within a larger array to perform AOA estimation, providing redundancy and flexibility. This capability paves the way for developing sophisticated quantum sensor networks capable of multi-parameter estimation and multi-target recognition in complex spectral environments. By integrating miniaturized photonic components with atomic sensing, this research opens up new, viable avenues for the development of next-generation navigation systems, adaptive radar, and non-cooperative remote sensing technologies, marking a transition from laboratory demonstration towards practical quantum-enabled instrumentation.

Although conventional electronic sensing technologies are mature, current marine environmental monitoring that primarily relies on them still faces limitations, such as poor interference immunity and high system complexity. While fiber optic sensing technology offers advantages like immunity to electromagnetic interference and structural flexibility, existing research has largely focused on single-parameter detection or wavelength-interrogation methods, leaving challenges such as cross-sensitivity in the simultaneous decoupling measurement of multiple parameters unresolved. This study aims to construct an all-fiber multi-parameter sensing probe that achieves high-sensitivity synchronous detection and effective decoupling of temperature, salinity, and pressure, thereby providing a reliable and easily integrable fiber optic sensing solution for in-situ marine environmental monitoring.

This study adopts a systematic experimental approach, which primarily consists of two parts: sensor fabrication and performance testing. The sensor fabrication procedure is outlined as follows. First, a photonic crystal fiber is spliced with a single-mode fiber using a fusion splicer, and a tapered region is formed through a tapering process to construct a Mach–Zehnder interferometer. Subsequently, a hollow-core fiber is spliced to an SMF, cleaved approximately 80 μm from the splice point, and a polymer film is formed using UV-curable adhesive to fabricate a Fabry–Pérot interferometric cavity. Finally, a fiber Bragg grating is connected in series with the above structures via single-mode fiber to form the complete composite sensor. Performance testing was conducted on a dedicated optical platform. A broadband light source with a wavelength range of 1250–1650 nm and an optical spectrum analyzer with a resolution of 0.04 nm were employed. For salinity testing, a series of NaCl solutions with concentrations ranging from 0.5 to 3.0 mol/L were prepared, and the solutions were injected via a syringe into a glass capillary tube containing the packaged sensor to complete the measurements. Pressure testing was performed by adjusting the immersion depth of the sensor in a 1.6 m water column to achieve continuous pressure variation within the range of 2.94–8.82 kPa. Temperature testing involved placing the sensor in a water environment on a temperature-controlled stage, with heating and cooling tests carried out in the range of 29–39 ℃ with a step size of 1 ℃. Based on the test results, a sensitivity matrix was constructed to enable synchronous decoupling measurement of temperature, salinity, and pressure.

The influence of salinity, pressure, and temperature on the spectral characteristics of the composite fiber sensor is summarized as follows. For salinity sensing, the intensities of the interference dips Dip1 and Dip2 in the transmission spectrum increase linearly with rising salinity, with Dip1 exhibiting a sensitivity of −25.1557 dB/(mol/L), while the FBG reflection peak remains stable, confirming its temperature-sensing independence. Under pressure variation, the depths of the interference dips decrease regularly with increasing pressure. The pressure sensitivities of Dip1 and Dip2 are 2.2878 dB/kPa and 1.456 dB/KPa, respectively, and the FBG reflection peak shows negligible response to pressure change, indicating good pressure-temperature decoupling capability. Temperature experiments reveal a stable response of the FBG reflection peak with a sensitivity of −0.0465 dB/℃, whereas the interference dips exhibit piecewise-linear temperature dependence. Based on the above experimental results, a sensitivity matrix was established, which successfully enables simultaneous decoupling measurement of salinity and pressure. The average measurement errors for salinity and pressure are no more than 0.029 mol/L and 0.28 kPa, respectively, demonstrating reliable multi-parameter sensing performance of the sensor for marine environmental monitoring.

This paper proposed and experimentally validated an MZI sensor for simultaneous measurement of temperature, liquid salinity, and pressure. By tapering a PCF used for salinity sensing and splicing it with an FBG for temperature compensation on one side and a polymer-film FPI for pressure sensing on the other, a complete MZI sensor capable of multi-parameter measurement was constructed. The transmission of input light through the fiber sensor was analyzed theoretically in detail, and its performance was demonstrated through comparative experimental validations. The results show that the fabricated sensor exhibits linear spectral responses to variations in liquid salinity and pressure, with salinity and pressure sensitivities reaching −25.1557 dB/(mol/L) and 2.2878 dB/kPa, respectively, while temperature compensation via the FBG achieves a temperature sensitivity of −0.0465 dB/℃. By constructing a sensing matrix based on the salinity–pressure sensitivities of two selected interference dips and the FBG reflection peak, simultaneous measurement of the three parameters is enabled.

Although this sensor demonstrates excellent sensing performance in laboratory conditions, systematic research on environmental stability and long-term reliability is still required for practical marine applications. Subsequent work will be conducted at three levels: 1) material level: conduct long-term seawater immersion and accelerated aging tests on polymer films to evaluate their service life; simultaneously explore inorganic film alternatives (e.g., Al₂O₃, SiO₂) to fundamentally enhance the chemical stability of the sensing unit. 2) Encapsulation level: design a multi-layer encapsulation structure comprising “flexible buffer-rigid protection-anti-fouling coating” with a focus on micro-area coating protection for vulnerable areas like tapered PCFs. Conduct high hydrostatic pressure cycling tests and seal integrity verification. 3) System validation layer: construct a multi-parameter coupled environmental simulation chamber to conduct long-term drift and cross-sensitivity re-evaluation tests. Plan real-sea deployment trials to validate the sensor's long-term reliability under typical oceanic conditions. In summary, this sensor demonstrates strong potential in structural design, sensing performance, and decoupling capability. Future work will follow a three-pronged research approach integrating materials, packaging, and systems to progressively advance it from laboratory prototyping to practical in-situ ocean monitoring applications. This will provide a novel all-fiber optic sensing solution for high-precision, interference-resistant, long-term continuous observation of ocean temperature, salinity, and depth.

Chirality, a fundamental geometric property ubiquitous in nature, confers upon matter the ability to interact asymmetrically with circularly polarized light, manifesting as unique optical responses such as circular dichroism (CD) and optical rotation. These properties render chiral structures highly promising for applications in biochemical sensing, optical field manipulation, quantum information, and enantiomer separation. However, conventional fabrication methods relying on mechanical engraving or chemical synthesis face inherent limitations including weak chiroptical responses, low design freedom for geometric configurations, and challenges in scalable production. Recent advancements in nanofabrication technologies, particularly electron beam lithography (EBL), focused ion beam (FIB), and direct laser writing (LDW), have provided revolutionary means for the precise and customized construction of artificial chiral micro/nanostructures. These techniques, characterized by high resolution, excellent programmability, and three-dimensional manufacturing capabilities, are powerfully driving the transition of chiral photonics from fundamental research to functional device applications.

This review systematically summarized fabrication strategies and recent research progress for chiral structures based on these mainstream nanofabrication technologies. For electron beam lithography, the research focus has shifted from mature two-dimensional planar chiral arrays (e.g., swastika-shaped structures) to more challenging quasi-three-dimensional architectures. Innovative processes such as multi-layer alignment, utilization of proximity effects, and exposure on non-planar substrates successfully realized complex chiral metasurfaces featuring multilayer stacking and suspended bridging, significantly enhancing optical chiral responses (e.g., CD values exceeding 20°). Focused ion beam technology, leveraging its integrated capabilities for milling, deposition, and imaging, demonstrated unique advantages in direct forming of 3D chiral structures. Fabrication of asymmetric aperture arrays via tilted scanning achieved tunable CD transmittance up to 78%. Combined with the "nano-kirigami" technique, FIB induced controllable buckling and twisting of 2D thin films, constructing dynamic 3D biomimetic cilium structures with strong chiroptical responses (g-factor reaching 0.8). Direct laser writing, especially based on two-photon polymerization, excelled in the freeform fabrication of complex 3D chiral architectures. This technique efficiently produced various 3D chiral models including helices, vortices, and petal-like structures. Integration with functional materials like semiconductor quantum dots and liquid crystal elastomers enabled the fusion of chirality with properties such as photoluminescence and stimulus-responsiveness, expanding functional dimensions. Recent breakthroughs, such as the "3D Pin" technique, further broadened material choices by enabling direct printing of chiral structures from high-purity inorganic semiconductors.

These precisely tailored chiral structures demonstrated remarkable performance across several cutting-edge application domains. In sensing, chiral metasurfaces based on materials like magnesium hydride and silicon achieved high-sensitivity, real-time detection of parameters such as hydrogen concentration and pressure through changes in their chiroptical signals. For optical modulation, mechanisms like photoinduced carrier excitation enabled dynamic and reversible switching of chiroptical responses (e.g., the sign of CD), laying groundwork for reconfigurable photonic devices. In light absorption and particle separation, designed chiral metamaterials achieved efficient selective absorption of specific circularly polarized light and generated chirality-dependent optical gradient forces at the nanoscale, offering novel principle-based solutions for high-efficiency separation of chiral particles below 10 nm. It indicate that nanofabrication technologies have become the core driver for the design and manufacturing of artificial chiral structures. Future research in this field will concentrate on several directions: At the process level, continuous pursuit of higher resolution, efficiency, lower cost, and development of multi-material and heterogeneous integration strategies is essential. Regarding structural design, exploration of more complex multi-level chiral, dynamically reconfigurable, and intelligently responsive architectures is needed. For applications, promoting chiral micro/nanodevices towards broader interdisciplinary fields such as on-chip integrated systems, biocompatible sensing platforms, and quantum light source manipulation is crucial. With the deepening integration of nanofabrication technologies with materials science, photonics, and life sciences, high-performance, multifunctional, and integrated chiral micro/nanodevices will undoubtedly inject new vitality into the future development of information, energy, and health technologies.

In the rapidly developing field of modern photonics, the precise and reliable control of the light polarization state stands as a crucial and indispensable foundation that strongly supports a wide range of core technological applications, including high-speed optical communication, advanced quantum information processing, and high-resolution optical imaging systems. Conventional Hermitian optical systems typically manipulate the polarization of light through passive geometric structural design, relying on fixed physical configurations rather than active tuning mechanisms. Their inherent confinement by the fundamental law of energy conservation gives rise to notable and unavoidable drawbacks in terms of dynamic response performance, operational flexibility, and adaptability to complex working environments. Non-Hermitian physics, as an innovative and revolutionary theoretical framework, breaks through the inherent shackles of this traditional Hermitian system by introducing gain and loss as active and controllable tuning degrees of freedom into optical systems, thereby opening up new possibilities for optical manipulation.

As a promising interdisciplinary product integrating non-Hermitian physics and nanophotonics, non-Hermitian metasurfaces have rapidly emerged as a prominent and highly focused research direction in the photonics community in recent years. The theoretical foundation of non-Hermitian polarization metasurfaces is mainly established on the principles of non-Hermitian Hamiltonians and the time-domain coupled-mode theory (TCMT), with their most prominent and distinctive characteristic being the existence of exceptional points (EPs), which are unique to non-Hermitian systems. When the key parameters of the optical system evolve to an exceptional point, two or more eigenvalues and their corresponding eigenstates undergo simultaneous degeneracy, a phenomenon that does not occur in traditional Hermitian systems. This degeneracy further triggers drastic and remarkable changes in the overall system responses, such as efficient chiral mode conversion, unidirectional polarization rotation, and enhanced light-matter interaction.

By fully taking advantage of the intrinsic and unique properties of exceptional points (EPs) and parity-time (PT) symmetry, such non-Hermitian metasurfaces can realize a series of advanced polarization manipulation effects that are extremely hard or even impossible to achieve with traditional optical methods. These effects include asymmetric transmission of polarized light, topological phase transitions in polarization states, and polarization-dependent mode switching, all of which have important implications for the development of next-generation optical devices.

Early research work in this emerging and promising field mainly focused on the design, fabrication, and basic performance testing of static geometric structures. Researchers successfully achieved various basic polarization control functions, such as the conversion from linear polarization to circular polarization and the precise selection of specific circular polarization states, through the rational, precise, and elaborate design of subwavelength meta-atoms with specific shapes, sizes, and orientations. By further integrating gain and loss functional materials (such as semiconductor materials and plasmonic materials) into these carefully designed geometric structures, PT-symmetric metasurfaces were successfully fabricated and characterized. A variety of novel and interesting physical effects, including polarization-dependent laser emission with high directionality and unidirectional invisibility of polarized light, were experimentally observed, measured, and verified on these specially designed PT-symmetric metasurfaces. In recent years, research efforts in this dynamic field have gradually advanced towards deeper theoretical exploration, more refined experimental manipulation, and more practical application-oriented directions. Researchers have successfully achieved precise full coverage control of arbitrary polarization states on the Poincaré sphere by introducing anisotropic meta-atoms with tailored optical properties and gradient phase distributions across the metasurface. The organic and effective combination of geometric phase effects and non-Hermitian optical phenomena has also provided strong and effective support for the implementation of practical optical functions, such as broadband polarization conversion with high efficiency, efficient vortex beam generation with controllable topological charge, and polarization-insensitive light absorption.

In the ongoing study of non-Hermitian polarization metasurfaces, a key and significant breakthrough lies in achieving independent, precise, and synchronous control of multiple polarization degrees of freedom. Specifically, based on structure-optimized coupled resonator metasurfaces with carefully designed resonant modes, researchers have successfully achieved synchronous and precise control of two key polarization parameters: polarization ellipticity and polarization orientation. After artificially introducing well-controlled non-Hermitian coupling effects between orthogonal polarization modes, polarization-dependent nonreciprocal transmission phenomena were also clearly observed and systematically studied in experiments. These phenomena are the core working mechanisms of essential photonic devices such as optical isolators and circulators in modern optical communication and quantum information systems.

To effectively overcome the inherent limitations of traditional static metasurface devices, such as their fixed performance and lack of dynamic adjustability, researchers have in recent years begun to actively explore and develop dynamically tunable non-Hermitian polarization metasurfaces. By integrating functional materials and components such as electro-optic crystals, thermo-optic polymers, and optomechanical structures into the overall metasurface design, real-time, flexible, and reversible control over gain/loss distribution and polarization response has been successfully achieved, greatly expanding the practical application scenarios of these metasurfaces.

This paper focuses on the core theme of polarization control, comprehensively reviewing the relevant theoretical foundations, key experimental methods, and representative research achievements of non-Hermitian polarization metasurfaces from three important aspects: system geometric parameters optimization, polarization degrees of freedom control, and dynamic regulation by external fields. Looking ahead to the future development prospects of this exciting field, the deep and seamless integration of non-Hermitian physics with emerging advanced technologies such as machine learning, artificial intelligence, and topological photonics is expected to bring entirely new innovative breakthroughs to the study of non-Hermitian polarization metasurfaces.

To investigate the formation, evolution, and regulation mechanisms of topological states induced by the combined effects of disclination topological defects and Floquet periodic modulation in honeycomb photonic lattices, and to explore how disclination structures with different rotational symmetries regulate the generation, localization, and propagation characteristics of vortex beams. In contrast to conventional topological photonic systems, where topological states are mainly associated with edges or boundaries, this study focuses on higher-order localized topological states bound to structural defects inside the lattice. By constructing $ {\rm{C}}_{5} $ and $ {\rm{C}}_{7} $ disclination honeycomb photonic lattices, this work aims to reveal the relationship between disclination-induced degenerate states, fractional charge localization, and vortex modes carrying orbital angular momentum. Furthermore, the role of Floquet periodic modulation in reconstructing the band structure, opening band gaps, and controlling localized topological states is analyzed. The study provides a theoretical basis for realizing stable topological vortex optical fields and offers guidance for the design of tunable topological photonic devices based on defect engineering and nonequilibrium periodic driving.

Honeycomb photonic lattice models containing disclination structures with $ {\rm{C}}_{5} $ and $ {\rm{C}}_{7} $ rotational symmetries are constructed. These two types of disclination topological defects are introduced by removing or inserting a 60° wedge angle in an ideal honeycomb lattice, corresponding to fivefold and sevenfold coordination around the defect core, respectively. Such geometrical operations break the original translational and rotational symmetries of the lattice and induce effective curvature or gauge-field-like responses near the defect center. Based on Wannier center theory and fractional charge theory, the formation mechanisms of disclination states, corner states, and edge states are analyzed, and the connection between localized fractional charge distributions and higher-order topological states is discussed. For the optical propagation model, the paraxial wave equation is employed to describe the evolution of light fields in the photonic lattice. Under the linear approximation, the nonlinear term is neglected, and the problem is reduced to an eigenvalue equation similar to a two-dimensional Schrödinger equation with an effective periodic potential. The finite-difference method is then used to discretize the system and solve the eigenvalue problem numerically, from which the band structure, eigenvalue spectrum, and eigenmode distributions of the $ {\rm{C}}_{5} $ and $ {\rm{C}}_{7} $ disclination structures are obtained. To further introduce nonequilibrium topological control, a helical periodic potential is applied to implement Floquet periodic modulation. By transforming the system into a co-rotating reference frame, an equivalent gauge-field theoretical model is established, in which the helical modulation acts as a periodically driven effective vector potential. Finally, vortex modes carrying orbital angular momentum are constructed by linearly combining degenerate disclination states with an additional π/2 phase shift, and their propagation behavior in the periodic potential is studied through numerical simulations over long propagation distances.

Numerical results show that both $ {\rm{C}}_{5} $ and $ {\rm{C}}_{7} $ disclination honeycomb photonic lattices can generate stable localized disclination states under appropriate structural parameters. The number of disclination states is closely related to the rotational symmetry of the system. Specifically, five disclination states are formed in the $ {\rm{C}}_{5} $ structure, while seven disclination states appear in the $ {\rm{C}}_{7} $ structure. Several of these states exhibit clear degeneracy in the eigenvalue spectrum, such as the degenerate pairs observed in the corresponding disclination-state modes. These results are consistent with the theoretical analysis based on Wannier center displacement and fractional charge localization, indicating that disclination defects can induce higher-order localized topological states by modifying the local topological response of the lattice. After the introduction of Floquet periodic modulation, the band structure of the system is significantly reconstructed. The helical periodic potential introduces a periodic driving along the propagation direction, causing the bands to fold in quasienergy space and leading to the opening of new band gaps near the original band-crossing or band-approaching regions. Meanwhile, some originally degenerate disclination states undergo rearrangement in their energy ordering and mode distributions. For example, in the $ {\rm{C}}_{5} $ structure, the degenerate disclination states change their relative spectral positions after helical modulation, showing that Floquet periodic modulation can not only regulate the band width and band-gap size, but also affect the formation, separation, and stability of localized topological states. This demonstrates that disclination defects and Floquet modulation have a synergistic effect in controlling topological states in nonequilibrium photonic systems. The vortex modes constructed from degenerate disclination states show robust propagation behavior. For the $ {\rm{C}}_{5} $ structure, degenerate disclination states are linearly combined to form a complex vortex mode with a well-defined phase-winding feature. A similar construction is applied to the $ {\rm{C}}_{7} $ structure using its corresponding degenerate disclination states. During propagation, the amplitude distributions of the vortex beams remain relatively stable in both structures, with only small overall fluctuations even after 100 propagation periods. The phase distributions exhibit a continuous variation from -π to π around the vortex core, confirming that the constructed modes possess typical vortex phase structures and well-defined orbital angular momentum. These results indicate that disclination structures can effectively induce and maintain stable topological vortex beams, while Floquet periodic modulation further enhances the controllability and robustness of their propagation.

The study demonstrated that disclination topological defects can effectively induce higher-order localized topological states in honeycomb photonic lattices, and that the number and distribution of these states are strongly determined by the rotational symmetry of the disclination structure. Floquet periodic modulation provides an additional dynamic control mechanism by reconstructing the band structure, opening band gaps, and regulating the degeneracy and spatial distribution of disclination states. Vortex modes formed by linear combinations of degenerate disclination states exhibit stable amplitude distributions, clear phase-winding characteristics, and strong propagation robustness over long propagation distances. These findings reveal the synergistic mechanism of disclination defects and nonequilibrium periodic driving in controlling topological optical fields. They provide theoretical insights into topological defect states, Floquet topological photonics, and vortex beam manipulation, and may contribute to the future design of robust topological photonic devices for optical information processing, vortex beam control, and topological light transport.

High-precision temperature measurement is critical for aerospace, industrial process control, biomedical microenvironment monitoring and other applications. Conventional fiber Bragg grating (FBG) temperature sensors suffer from intrinsically low sensitivity (~10 pm/°C) restricted by the thermo-optic effect and thermal expansion coefficient of silica fiber. Existing sensitivity enhancement strategies, such as coating high-thermal-expansion polymers or packaging with metal substrates, usually introduce extra stress cross-sensitivity and poor long-term reliability, and cannot meet the demand for miniaturized all-fiber sensing devices. Meanwhile, traditional optical Vernier effect relies on envelope fitting of cascaded interferograms, which inevitably induces severe envelope broadening, deteriorated detection limit and narrowed dynamic range as the magnification factor increases. This inherent performance trade-off has become a critical technical limitation restricting the application of Vernier-enhanced fiber sensors in high-precision temperature measurement fields. To break through these inherent bottlenecks, this paper proposes and fabricates a parallel FBG high-sensitivity temperature sensor combining femtosecond laser micromachining and an envelope-free Vernier demodulation algorithm, aiming to achieve simultaneous optimization of sensitivity, detection limit and measurement range for high-precision temperature detection.

In this work, parallel FBG sensing arrays are fabricated via femtosecond laser point-by-point inscription within the same cross-section of standard single-mode fiber, leveraging the cold-processing property, nanoscale spatial resolution and three-dimensional fabrication flexibility of femtosecond laser. The spatially parallel configuration effectively eliminates axial stress crosstalk and internal temperature gradient errors commonly existing in cascaded or series grating structures, ensuring synchronous and proportional thermal drift of each FBG resonance peak under uniform temperature excitation. This one-step in-fiber fabrication avoids the alignment error and extra insertion loss caused by cascading discrete fiber devices, and fully maintains the structural integrity of the all-fiber sensing system. For signal demodulation, an envelope-free Vernier method based on a virtual theoretical reference arm and intercept tracking strategy is developed. Instead of building a physical reference interferometer, a set of reference resonance peaks is numerically generated with a designed free spectral range. The discrete high-Q resonance peaks of parallel FBGs are used to equivalently replace the continuous cosine spectrum of conventional interferometers. By tracking the wavelength drift of the intercept between the fitting curve and a fixed horizontal reference line, envelope extraction and fitting procedures are completely avoided. The virtual reference arm can flexibly adjust its free spectral range via algorithm parameters to match different magnification demands, without reprocessing or modifying the physical sensing structure. Both four-FBG and simplified two-FBG parallel structures are fabricated and tested, and the detection limit performance is quantitatively evaluated via Monte Carlo simulations with 200 independent noise-injection experiments.

Experimental results demonstrate that the parallel FBG sensors fabricated by femtosecond laser exhibit excellent spectral quality and stable thermal response. All FBG resonance peaks show good linear red-shift characteristics with temperature rise, without obvious crosstalk or peak distortion. Multiple repeated temperature cycling tests also verify that the sensor exhibits favorable repeatability and low hysteresis, with negligible wavelength deviation after multiple heating-cooling cycles. After envelope-free Vernier demodulation, the four-FBG sensor achieves temperature sensitivities of 110.18 pm/°C (M=10) and 208.51 pm/°C (M=20), corresponding to 11.2-fold and 21.3-fold enhancement compared with a single bare FBG, respectively. The optimized parallel two-FBG structure delivers a temperature sensitivity of 244.48 pm/°C at M=20, which is 19.5 times higher than that of a single FBG, with a linear correlation coefficient R² above 0.999 under all test conditions. The experimental sensitivity values under different magnification factors are in good agreement with the theoretical calculation results, which confirms the correctness of the proposed envelope-free Vernier amplification model. Benefiting from the noise averaging effect of multi-peak fitting, the temperature detection limit is reduced by 43.34% to 0.0931 °C at M=10, breaking the inherent trade-off between sensitivity and detection limit in traditional Vernier sensors. Unlike the conventional envelope-based Vernier effect, the proposed method preserves the narrow linewidth of FBG resonance peaks without spectral broadening, thus effectively expanding the effective dynamic measurement range while enhancing sensitivity. Compared with traditional envelope-based Vernier sensors with equal magnification, the proposed scheme avoids the sharp shrinkage of measurement range, and achieves a better balance between sensitivity and measurable range. The simplified two-FBG structure achieves comparable magnification performance with significantly reduced device footprint and fabrication cost.

This work proposes, fabricates and experimentally characterizes a parallel FBG high-sensitivity temperature sensor integrated with femtosecond laser inscription and envelope-free Vernier demodulation. By inscribing parallel FBG arrays in the same cross-section of single-mode fiber via femtosecond laser, axial crosstalk and temperature gradient errors are effectively suppressed. The envelope-free Vernier scheme with virtual reference arm and intercept tracking replaces traditional envelope fitting, fundamentally eliminating measurement errors induced by spectral broadening and envelope distortion. The sensor achieves over 19.5-fold sensitivity enhancement relative to a single FBG, and realizes simultaneous improvement in sensitivity, detection limit and measurement range. Furthermore, the proposed sensing structure and demodulation framework can be extended to multi-parameter monitoring such as micro-strain, refractive index and micro-pressure, showing broad prospects in industrial precision manufacturing and biomedical scenarios. With the advantages of compact structure, no physical reference arm required and good compatibility with existing femtosecond micromachining processes, this work provides a promising technical approach for high-precision temperature sensing scenarios and the development of miniaturized high-performance optical fiber sensors.

Diffractive waveguide serves as a core optical scheme and mainstream solution for consumer augmented reality (AR) near-eye displays, featuring thin structure, expandable exit pupil and high ambient transmittance. Nowadays, lightweight, high-imaging-quality AR glasses have become a mainstream demand for wearable smart devices, and diffractive waveguide technology is widely adopted in commercial products due to its excellent optical performance and structural advantages. Traditional surface relief gratings have defects such as chromatic dispersion, narrow field of view (FOV), low coupling efficiency and uneven brightness. These defects cause severe color separation, luminance fluctuation and obvious rainbow artifacts, which hinder the development of compact single-layer full-color AR devices with high image quality. Subwavelength-structured metasurfaces achieve precise and flexible manipulation of light phase, polarization and amplitude, and provide effective approaches to address the inherent limitations of diffractive waveguides.

Based on the grating equation, k-space analysis and geometry-determined theoretical efficiency limit, researchers summarized three typical achromatic strategies. These three technical routes have different working principles and applicable scenarios for full-color waveguide design. Layer-stacked metasurfaces realized independent red-green-blue (RGB) phase modulation by vertically arranging multiple functional layers, and achieved an average diffraction efficiency higher than 89.2% under mixed RGB illumination. Nevertheless, this scheme imposes ultra-high requirements on component alignment and fabrication. The high-order diffraction matching method assigned distinct diffraction orders to three primary wavelengths to unify the in-waveguide diffraction angles. This approach fundamentally eliminates chromatic dispersion and realizes single-layer achromatic coupling with a theoretical FOV over 45°. The geometric compensation strategy does not suppress intrinsic dispersion. By optimizing waveguide thickness, multicolor beams converge accurately at the out-coupler, offering a feasible technical route for compact full-color 3D display devices.

Multi-zone in-couplers and imaginary refractive index engineering effectively narrowed the practical efficiency gap between actual performance and theoretical limits. The minimum field efficiency was increased from 20% to 25.3%. Combined with inverse topological optimization, this method suppresses angle-dependent efficiency fluctuations and balances the output uniformity of RGB light. Silicon carbide (SiC) has excellent thermal conductivity and mechanical strength, which greatly raises the upper limit of full-color FOV and fundamentally suppresses rainbow artifacts. Polarization multiplexing divides the whole field of view into two orthogonal polarization channels, achieving a horizontal FOV exceeding 80° on a monolithic waveguide. Geometric-phase-based polarization multiplexing enables various advanced functions: spin-split metagratings realize low-crosstalk stereoscopic imaging without extra waveguide layers; hybrid polarization-volume holographic gratings adopt dual-focus design to alleviate vergence-accommodation conflict; polarization steering components combine segmented fields of view to double the total FOV without increasing waveguide thickness.

Comprehensive comparison shows that combining high-order diffraction with high-index SiC serves as the optimal scheme for compact single-layer, large-FOV full-color waveguides, which can effectively eliminate rainbow artifacts. Polarization multiplexing promotes system function expansion and optimizes user experience. At present, laboratory research results of metasurface waveguides are relatively abundant, but there is still a large gap between laboratory prototypes and mass-produced commercial products. Currently, all existing technical routes face three major industrialization bottlenecks: insufficient precision of large-area nanoimprint and deep-UV lithography lowers mass production yield; the overall efficiency of high-order gratings fails to satisfy the demands of practical display applications; the integration of tunable metasurfaces and waveguides remains in the early stage of development. Future research will focus on three key directions: inverse design optimization based on physics-driven neural networks, integrated fabrication of wide-bandgap SiC materials, and the development of multi-dimensional tunable metasurfaces for dynamic FOV and focal plane regulation. Metasurface diffractive waveguides have evolved from discrete component optimization to full light-field manipulation, and will support the development of next-generation spatial computing optical hardware.

Vortex beams, characterized by a spiral phase wavefront and a central phase singularity, carry orbital angular momentum (OAM). The infinite orthogonal states provided by OAM offer a new degree of freedom for manipulating optical fields, showing tremendous potential in high-capacity spatial division multiplexing communications, optical micromanipulation, and quantum information processing. Compared to continuous-wave (CW) vortex beams, pulsed vortex lasers with high peak power hold significantly greater application value in nonlinear optical frequency conversion and laser micromachining. However, traditional pulsed vortex lasers generally rely on bulky solid-state crystal resonators or complex spatial optical paths, making it difficult to meet the urgent demands of modern photonics for device miniaturization and high stability. Furthermore, high-power pumping in bulk crystals induces severe thermal lensing effects, which drastically distort the fragile spatial phase of vortex beams. To achieve compact and highly stable pulsed vortex sources, dielectric optical waveguides act as an ideal platform. The micrometer-scale geometric structure of the waveguide endows the device with an extremely high surface-to-volume ratio, which significantly enhances the heat dissipation capability of the system. In terms of pulse generation, passive Q-switching using two-dimensional saturable absorbers (SAs) is highly favored due to its compact footprint. Topological insulators, particularly bismuth selenide (Bi₂Se₃), feature a Dirac-cone electronic band structure and exhibit remarkable broadband nonlinear optical responses with large modulation depths. This study aims to propose and demonstrate a highly stable, compact passively Q-switched waveguide vortex laser by combining a femtosecond laser direct written (FsLDW) Nd:YAG waveguide with a high-quality Bi₂Se₃ SA grown by chemical vapor deposition (CVD).

A circular depressed-cladding waveguide was fabricated in a 1 at.% Nd³⁺-doped YAG crystal using the FsLDW technique. The fabrication utilized a Ti:sapphire regenerative amplifier delivering 120 fs pulses at a central wavelength of 795 nm with a repetition rate of 1 kHz. Parallel low-refractive-index damage tracks were inscribed to form a circular cladding boundary with a radius of 15 μm, perfectly preserving the original luminescence properties and lattice quality of the crystalline core. The geometric morphology and guiding characteristics were verified via optical microscopy and micro-Raman spectroscopy. Concurrently, a high-quality Bi₂Se₃ thin film was synthesized on a CVD method. Atomic force microscopy (AFM) and Raman spectroscopy were employed to characterize its morphology and structural properties. The nonlinear optical absorption parameters of the Bi₂Se₃ film were quantitatively evaluated using an open-aperture Z-scan technique driven by a 1030 nm femtosecond fiber laser. A compact laser resonator was subsequently constructed. An 808 nm continuous-wave Ti:sapphire laser served as the pump source, coupled into the waveguide via a spherical plano-convex lens. The Bi₂Se₃ film was inserted into the cavity as the SA. Notably, a spiral phase plate (SPP) with a topological charge of 1 was utilized as the output coupler. Compared to spatial light modulators or liquid crystal q-plates, the pure dielectric SPP possesses an exceptionally high laser damage threshold, directly generating the vortex beam while ensuring the robustness of the cavity.

The AFM measurements indicate that the average thickness of the synthesized Bi₂Se₃ film is approximately 16 nm. The Z-scan experimental results and subsequent nonlinear transmission fitting reveal that the Bi₂Se₃ SA exhibits an excellent modulation depth of 8.6% and a saturation intensity of 26.25 GW/cm² at 1 μm, indicating its strong capability for pulse modulation. In the laser performance evaluation, stable CW and passive Q-switching operations around 1064 nm were systematically compared under both TE and TM polarized pumping. After inserting the Bi₂Se₃ SA, stable Q-switched pulse trains were generated. The pulse temporal characteristics evolved typically with the increase of absorbed pump power. Specifically, at an absorbed pump power of 850 mW, the laser yielded the optimal pulsed performance: the shortest pulse width compressed to 39 ns, and the highest repetition rate reached 9.1 MHz. The corresponding maximum single-pulse energy was calculated to be 4.42 nJ. The recorded pulse trains exhibited uniform amplitude and high temporal stability without distinct amplitude fluctuations. Furthermore, the near-field intensity distributions captured by a CCD camera demonstrated a typical annular (donut-shaped) hollow profile, confirming the high generation quality of the vortex beam. More importantly, comparing the beam profiles at low and high pump powers, the annular contour showed no obvious distortion or degradation. This phenomenon provides strong evidence that the unique geometric structure of the circular waveguide effectively mitigates the thermal lensing effect through rapid transverse heat dissipation. Theoretical estimations further confirm that the maximum intracavity peak pulse energy fluence (1.3 mJ/cm²) is well below the physical damage threshold of the Bi₂Se₃ film (3.18 mJ/cm²).

A compact, highly thermally stable, passively Q-switched Nd:YAG waveguide vortex laser was successfully demonstrated. The integration of the FsLDW-customized circular depressed-cladding waveguide and the CVD-grown Bi₂Se₃ topological insulator effectively overcomes the thermal and physical bulkiness bottlenecks of traditional solid-state vortex lasers. The robust thermal management capability of the waveguide physically locks the high-order transverse mode, ensuring distortion-free annular vortex beam output even under intense pumping conditions. The experimental results verify the excellent nonlinear optical modulation properties and impressive thermal damage resistance of the Bi₂Se₃ film. This research provides a novel experimental paradigm and solid foundation for the development of miniaturized, highly stable all-solid-state pulsed vortex sources, paving the way for advanced on-chip photonic integrated circuits and optical manipulation systems.

Thin-film lithium niobate (TFLN) inherits excellent intrinsic properties of bulk lithium niobate and features high refractive index contrast, strong optical field confinement and device miniaturization capability. It has become a core platform for next-generation photonic integrated circuits. However, the wide bandgap, weak light absorption and low electrical conductivity of TFLN prevent it from achieving efficient photodetection independently. Heterogeneous integration with high photosensitivity materials is the core technical solution to this problem. Transition metal dichalcogenides (TMDs), especially 2H-phase molybdenum ditelluride (2H-MoTe₂), exhibit narrow bandgap, strong near-infrared absorption and good compatibility with TFLN platforms, making them ideal photosensitive materials for TFLN waveguide-integrated photodetectors. Existing TFLN waveguide-integrated two-dimensional material photodetectors mostly rely on external bias voltage to separate and transport photogenerated carriers, which increases system power consumption and circuit design complexity. Existing self-powered detection schemes suffer from poor compatibility with TFLN planar microfabrication processes, and easily introduce interface defects and lattice damage. This work aims to develop a low-power, high-compatibility and high-performance self-powered photodetector integrated on TFLN waveguide platform.

The device was fabricated based on TFLN proton-exchange waveguide platform. Few-layer 2H-MoTe₂ was used as both light absorption layer and carrier transport channel. An asymmetric Schottky junction was constructed using gold (Au) and mechanically exfoliated few-layer graphite (Gr) electrodes with distinct work functions. All contacts between MoTe₂ and electrodes were realized by dry transfer process to form van der Waals heterointerfaces, which avoided Fermi level pinning effect and interface damage caused by traditional metal evaporation. The device structure consisted of a 500 μm thick silicon substrate, a 2 μm thick SiO₂ dielectric cladding layer, and a 0.5 μm thick x-cut lithium niobate single-crystal thin film with 5 μm wide proton-exchange waveguides. The source electrode was a evaporated Au (30 nm)/Cr (40 nm) stack, and the drain electrode was few-layer Gr prepared by dry transfer. Atomic force microscopy (AFM) was used to characterize the thickness of Gr and MoTe₂ layers. Raman spectroscopy was employed to verify the crystal phase and structural integrity of the materials after transfer. An Au/MoTe₂/Au field-effect transistor was fabricated to determine the carrier type of MoTe₂. A photoelectric test system was built to measure the I-V characteristics, photocurrent-power dependence, photoresponse time and dark current noise power spectral density of the device at 1310 nm communication wavelength. The actual optical power coupled into the waveguide was calibrated using a reference waveguide with identical dimensions.

The Au-MoTe₂-Gr photodetector was successfully fabricated and exhibited both self-powered and bias-driven operation modes. AFM characterization showed that the thickness of Gr layer was approximately 35 nm and that of MoTe₂ layer was about 25 nm. Raman spectra confirmed that both materials maintained excellent 2H crystal phase without phase transition or lattice damage after dry transfer. The transfer characteristic curve of the Au/MoTe₂/Au transistor indicated typical p-type semiconductor transport behavior of the few-layer MoTe₂. Energy band analysis revealed that Au formed a high-barrier Schottky contact with MoTe₂, while Gr formed a low-barrier contact. A net built-in electric field directed from Gr to Au was established in the channel, which drove the separation and transport of photogenerated electron-hole pairs at zero bias voltage. The I-V curves showed obvious unidirectional rectification characteristics in dark environment, verifying the successful construction of the asymmetric Schottky junction. Under 0.5 V forward bias, the device achieved a maximum responsivity of 42.65 mA/W at 1310 nm wavelength. In self-powered mode, the responsivity reaches 2.22 mA/W and the maximum optical on/off ratio is as high as 2.1 × 10⁶. This result is at a relatively high level among reported waveguide-integrated photodetectors, demonstrating its good application potential in on-chip integrated photonic systems. The device exhibited fast photoresponse speed, with rise time of 81 μs and fall time of 291 μs in self-powered mode, corresponding to a bandwidth of approximately 4.3 kHz. The specific detectivity was calculated to be 7.5 × 10⁷ Jones under self-powered operation. The photocurrent showed sublinear dependence on incident optical power, with power-law exponents of 0.47 under 0.5 V bias and 0.55 in self-powered mode. This phenomenon was attributed to the saturation of trap states and enhanced carrier recombination in the MoTe₂ channel with increasing incident optical power.

The Au-Gr asymmetric Schottky junction design provides a practical technical path for low-power and high-compatibility self-powered photodetection on TFLN waveguide platforms. The fabricated device achieves stable dual-mode operation at 1310 nm communication wavelength, combining high on/off ratio, fast photoresponse and low power consumption. It demonstrates great application potential in on-chip integrated photonic systems.