This paper proposes an optical fiber evanescent wave sensor for phase transition detection of organic compounds, which was validated using n-octadecane. The sensor is constructed by arc-discharge splicing single-mode fiber (SMF) into a waist-enlarged fusion taper (WEFT) structure using a fiber fusion splicer. When two WEFTs are connected in series, they form a Mach-Zehnder interferometer (MZI). Since n-octadecane has different refractive indices in its solid and liquid states during the phase change, the change in refractive index causes variation in the interference dips in the spectrum, enabling the distinction between the solid and liquid states. However, traditional wavelength and intensity tracking methods require precise numerical analysis, limiting their practical applications. Therefore, we propose using machine learning to assist the WEFT structure in phase change detection. During the heating and cooling processes, the K-means algorithm is first applied to classify the solid and liquid states, corresponding to the two phases of the transition. Subsequently, a Gaussian mixture model (GMM) is used for optimization, allowing for accurate differentiation between the liquid and solid states of n-octadecane. The results show that during the heating and cooling processes, after training on the spectral data, the average silhouette coefficients were 0.8619 and 0.8813, respectively, and the log-likelihood values were -21.8062 and -1.175. The sensor we propose has a simple structure and is easy to manufacture. Combined with machine learning algorithms, it holds great potential for application in the field of phase change energy storage. (c) 2026 Chinese Laser Press

To meet the growing demand for intelligent sensing in complex underwater acoustic environments, this work presents a high-sensitivity, broadband fiber-optic extrinsic Fabry-P & eacute;rot interferometric (EFPI) acoustic sensor integrated with machine learning (ML) algorithm. Leveraging the high Young's modulus and corrosion resistance of titanium (Ti) diaphragm, an extrinsic Fabry-P & eacute;rot interferometric (Ti-EFPI) acoustic sensor is developed. For intelligent signal processing leading to acoustic classification, a pipeline encompassing signal enhancement, multidimensional feature extraction, and classifiers was employed for accurate identification of underwater sound sources. Experimental results show that the Ti-EFPI sensor maintains an average sensitivity of approximately -120 dB re 1V/ mu Pa over the 100 Hz-20 kHz range, and exhibits 51-73 dB higher sensitivity than the conventional piezoelectric hydrophones. The system achieved identification accuracies exceeding 91% for four different types of underwater acoustic sources. This work effectively combines high-performance fiber-optic sensing with advanced machine learning algorithms for intelligent signal processing, offering a compact and efficient solution for intelligent ocean acoustic monitoring.

We propose and demonstrate an intensity-demodulated Fiber Bragg Grating (FBG) vibration sensing system based on optical carrier microwave interferometry (OCMI). The system contains two FBGs with different wavelengths, one as the sensing element and the other as the reference element. Vibration-induced wavelength shifts in the sensing FBG cause the shift of the OCMI pattern, resulting in the change of the power of the modulated radio frequency (RF) signal, which enables real-time vibration detection. The core contributions of this work are its tunable sensitivity and concise demodulation method. System sensitivity can be flexibly tailored by adjusting the modulated RF signal or by selecting FBGs with different wavelength spacings. Experimental results validate the system's operation in the 1-3000 Hz frequency range. We demonstrate this tunability at a 100 Hz vibration frequency: for the OCMI with 1 GHz Free Spectral Range (FSR), the sensitivity reaches 171 mV/ V and 182 mV/V at RF frequencies of 2.46 GHz and 3.54 GHz, respectively, whereas it drops to 68 mV/V when the FSR is reduced to 0.72 GHz. The system's capability for dynamic response was further verified by successfully capturing human voice signals. This work presents a novel application of a specific OCMI architecture tailored for real-time dynamic sensing that combines tunable sensitivity, measurement flexibility, and a concise demodulation architecture.

In this paper, we propose a novel flexibly switchable frequency optoelectronic oscillator (OEO) based on a multi-passband microwave photonic filter (MPF). The multi-passband MPF based on a Solc-Sagnac interferometer with two polarization-maintaining fibers (PMFs) in the loop is employed to perform microwave frequency selection. We can readily achieve a switchable multi-passband MPF by simply varying the polarization state through rotating polarization controllers (PCs) within the Solc-Sagnac interferometer. Two fiber rings cascaded with a dual-path structure are used for fine frequency selection, stabilizing OEO oscillation and increasing the side mode suppression ratio (SMSR). In our experiment, two segments of PMF (2 m and 3 m) have been applied in the Solc-Sagnac interferometer and signals at four switchable frequencies of 210.55 MHz, 479.35 MHz, 662.75 MHz, and 1115.08 MHz with an SMSR of about 50 dB have been generated. The proposed frequency-switchable OEO has the advantages of easy manufacture, great flexibility, effectiveness, and good stability.

Accurate measurement of small forces is essential in fields such as microrobots, nanotechnology, and biological cells. This article introduces a high-sensitive optical fiber sensor for micro-newton forces measurement utilizing neural network-assisted demodulation. This sensor employs a single-mode-multimode-single-mode (SMS) multimodal interference structure, in which a vulcanized silicone rubber film is embedded to achieve the force measurements. This design enables mechanical sensing at the micro-newton level. In traditional interference dip wavelength tracking methods, measurement results often exhibit nonlinear fitting, which increases the system demodulation complexity. Therefore, to enhance measurement accuracy, we introduced a long short-term memory (LSTM) algorithm combined with a Bayesian optimized LSTM algorithm for data processing. This approach effectively captures the sensor's response characteristics under different external forces and significantly improves fitting accuracy. After optimization, our sensor achieved a measurement accuracy of 0.99982, with errors reduced to 23.87 $\mu $ N. The manufacturing process of this sensor is simple and compact, providing essential technical support for the advancement of biomedicine and nanotechnology.

This study presents a high-precision liquid-level sensing system that integrates a titanium diaphragm extrinsic Fabry-Perot interferometer (Ti-EFPI) with a microwave photonic filter (MPF). The titanium diaphragm, characterized by its inherently high Young's modulus and outstanding corrosion resistance, is particularly suitable for prolonged operation in underwater environments. The MPF technology enables the conversion of wavelength shifts of interference patterns in optical-domain, caused by cavity length variations, into the corresponding passband's frequency shifts in the electronic domain. By monitoring changes of the central frequency of the MPF's passband, liquid level can be measured with high accuracy. In our experiment, we achieved the liquid-level sensitivities of 0.373 MHz/cm with the theoretical detection resolution of 0.0027 cm during the drop of the liquid level. The long-term stability (< +/- 4 kHz drift) has also been obtained in the experiment. The proposed liquid level sensing system method enjoys high sensitivity, good stability, low manufacturing cost and the ease of fabrication, making it promising for industrial applications.

We propose and experimentally demonstrate a high-resolution fiber optic liquid level sensing system based on optical carrier microwave interferometry (OCMI). This system employs a custom-designed, rubber-diaphragm-based Fiber Bragg Grating (FBG) sensor head to convert small hydrostatic pressure variations into axial strain on the sensing FBG, and translates its wavelength response into the corresponding notch frequency shift in microwave domain via an OCMI. The experimental results demonstrate a liquid level average sensitivity as high as -23.80 MHz/cm over a 400 mm measurement range, achieving a theoretical resolution of 0.42 μm. Furthermore, the system exhibited high stability, small hysteresis, and a capability for tunable sensitivity. An investigation into temperature cross-sensitivity reveals a low residual thermal effect of -0.305 cm/°C. This work leverages microwave photonic interrogation for liquid level sensing. By integrating a simple, custom-designed FBG sensor head with an OCMI system, we surpass the resolution limits of conventional optical methods, and also expand the measurement range, presenting a promising solution for high-precision liquid level monitoring.

In this paper, we propose and demonstrate a sensitivity-enhanced sensing interrogation scheme based on an Optoelectronic oscillator (OEO), in which a switchable dual-passband microwave photonic filter (MPF) is introduced into the loop. The switchable dual-passband MPF is a combination of a modified fiber Mach-Zehnder interferometer (FMZI), an electro-optical modulator (EOM), a roll of dispersion compensating fiber (DCF), and a photodetector (PD). The dual-passband switching of the MPF can be achieved by simply adjusting the polarization state via rotating a polarization controller (PC) in the FMZI. The sensitivity can be improved by a factor of two by tracking the frequency corresponding to the central frequency of the high-frequency passband relative to the low-frequency passband. Temperature-sensing experiments were conducted to verify the concept of enhanced sensitivity. Experimental results on temperature sensing show that tracking low- and high-frequency OEO signals yields sensitivities of 5.23 MHz/degrees C and 10.84 MHz/degrees C, respectively, and temperature resolutions of 0.009 degrees C and 0.004 degrees C, thereby increasing sensitivity and resolution.

In this article, a respiration monitoring sensor based on an agarose-coated fiber Bragg grating (AC-FBG) and an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated, which achieves high sensitivity and high-speed respiration signal detection by combining fiber-optic sensing and OEO-based demodulating technology. The proposed sensor captures subtle respiratory variations with exceptional precision. The experimental investigation reveals that the absolute sensitivity of the proposed sensor reaches 0.07391%relative humidity (RH), which is 19.9 times higher than that measured with the optical method. The response frequency of the proposed OEO-based demodulation system is more than 6.69 Hz. Owing to its excellent sensing performance, the sensor not only monitors the human body's regular breathing mode and special breathing modes (e.g., rapid respiration, deep respiration, and apnea) but also realizes the monitoring of the weak respiratory signals at a distance. The measured data from the respiratory monitoring system proposed in this article show a high level of agreement with the ground-truth breathing data, which provides new technological means for the evaluation of respiratory function in a variety of key medical scenarios.

To address the increasing demand for versatile applications, there is a pressing need for high-resolution sensing devices that can operate in intricate environmental conditions. This work demonstrates a highly sensitive Ushaped surface plasmon resonance (SPR) sensor with two independent sensing channels, based on tantalum pentoxide (Ta2O5) film sensitization technology. We systematically optimized the sensor's key structural parameters using the transmission matrix method. The experimental findings demonstrate that the refractive index (RI) sensitivity of channel B reaches up to 4161 nm/RIU, which is twice that of channel A (2005 nm/RIU). Temperature variations have a significant impact on channel B (-0.25 nm/degrees C) due to the Ag/Ta2O5 film coating, while channel A (just silver film) stays practically unaffected within the measurement range of 15 degrees C-35 degrees C. Furthermore, the U-shaped sensor features high sensitivity, compact structure, and good stability, which establishes a novel platform for dual-channel measurement applications.

Amortized neural inverse design typically remains closed-world: component choices are fixed vocabulary tokens, coordinate grids are frozen at training time, and continuous variables are discretized into sequence tokens. Multilayer optical coatings are an industrially important instance, coupling material sequence, layer thickness and wavelength-dependent response. We present IrisFlow, a query-based, open-vocabulary flow-matching framework instantiated in coatings: the target reflectance/transmittance spectrum, wavelength grid, candidate-material optical constants and layer count are supplied at query time. Candidate materials enter as wavelength-aware optical tokens rather than learned identities; material sequences are sampled by discrete flow matching over the query's candidate bank, thicknesses by continuous flow matching without discretization. A single 136M-parameter model designs 2-100-layer stacks. Across a 224-task benchmark it reconstructs in-distribution targets faithfully and retains same-order accuracy on a 15-material held-out bank without retraining; it reconstructs bands up to 1100 nm beyond its training envelope, designs against analytic application specifications and outperforms an autoregressive baseline on that baseline's material library. With optical constants calibrated to our deposition process, IrisFlow designs four color-displaying coolers, fabricated by ion-assisted evaporation: the three chromatic devices reach a CIEDE2000 color error of 3.1-5.2 while retaining 93-95

In this paper, an agarose-coated fiber Bragg grating (AC-FBG)-based humidity sensing system with high sensitivity, leveraging the Vernier effect in an optoelectronic oscillator (OEO), is proposed and experimentally demonstrated. Changes in ambient humidity induce effective expansion and contraction of film morphology on the AC-FBG, leading to shifts of the Bragg wavelength. The Bragg wavelength variations of the AC-FBG can be converted to the changes of the microwave signal through the OEO system, effectively improving the resolution of sensing demodulation. The sensitivity of the proposed humidity sensing system is further enhanced by employing the Vernier effect in the OEO through the cascading of two FBGs, where tunable sensitivity can be achieved by adjusting the spacing between two FBGs. The proof-of-principle experimental results show that the sensitivity of the proposed AC-FBG-based humidity sensing system based on OEO without the Vernier effect is about 7.31 kHz/%RH, while the sensitivity based on the OEO with the Vernier effect is improved to 1.032 MHz/%RH, representing approximately 141.2 times higher than the system without the Vernier effect. The proposed scheme offers the advantages of simplicity, compact configuration, high resolution, and tunable sensitivity.

Transient hydraulic pressure is crucial for hydraulic pressure and pipeline leak detection in the fields of petroleum pipelines, gas pipelines, and so on. Real-time, long-distance, online, high-pressure, and high-precision measurement requirements can be precisely achieved through fiber optic sensing technology. In response to the need for pipeline leak detection, we propose a fiber-optic transient hydraulic pressure sensor based on single-hole-dual-core fiber Bragg grating (SHDC-FBG). By inscribing FBG on SHDCF using a deep ultraviolet laser, we conduct modal field analysis on the SHDCF and theoretical simulation analysis of the resonance wavelength of the SHDC-FBG. The simulation results closely match experimental data, demonstrating that air holes induce significant pressure-driven refractive index changes in the center core. Furthermore, the static and dynamic pressure sensing characteristics have been discussed. Experimental results indicate that the SHDC-FBG sensor exhibits a sensitivity of -5.4 pm/MPa within a wide dynamic hydraulic range of 0-20 MPa. To measure the hydraulic leak process, we compare the SHDC-FBG sensor with a conventional resistive pressure sensor. The experimental results show that the performance of the two sensors is essentially the same. Nevertheless, the compact size, low cost, and electromagnetic immunity make our sensor more competitive in harsh, long-distance environments.

In this paper, a stable femtosecond electro-optic frequency comb (EOFC) based on a dual-loop optoelectronic oscillator (DL-OEO) and a parametric frequency mixer is proposed and experimentally demonstrated. The DL-OEO is composed of an intensity modulator, a phase modulator, and two optoelectronic feedback loops, where optical injection into a semiconductor laser with a Period-one (P1) state is used in the DL-OEO loop for selecting the corresponding microwave signal. Hence, a seed EOFC with tunable and high repetition frequency can be generated and simultaneously be pulse-compressed through the long optical fiber used in the DL-OEO. Then, the seed pulse is sent into a parametric frequency mixer to further compress the pulse width to the order of femtoseconds. In the proof-of-principle experiment, EOFCs with tunable repetition frequency from 10.20 GHz to 18.21 GHz were generated, where the pulse width can be compressed to similar to 300 fs, a 10 dB spectral width of 20.75 nm, and 216 comb lines. The proposed femtosecond EOFC has broad application prospects in precision measurement, optical communication, quantum computing, and other fields.

In this paper, a two-tap microwave photonic filter (MPF) based temperature compensated optical fiber current sensing system has been proposed and experimentally demonstrated. Two fiber Bragg gratings (FBGs) are employed in the experiment, and the light signals modulated by radio-frequency (RF) signals reflected by two FBGs can be treated as two tap signals, which enter the dispersion-compensated fiber for the delay and beat through the photodetector to recover the RF signals, then a two-tap MPF is formed. Two FBGs are considered as the sensing and temperature compensation elements, respectively. The sensing FBG is affixed on the Terfenol-D. The variation of the current applied to the solenoid changes the magnetic field, and axial stretching strain is generated on the sensing element and sensing FBG's wavelength will change, which results in the shifts of the dip frequency of the MPF, by measuring which the current can be demodulated. Meanwhile, applying current will cause internal heating of the solenoid, and to further reduce the effect of the variation of temperature on current measurement, the temperature compensation FBG is affixed on the temperature compensated material Monel400, and both two FBGs are placed inside the solenoid and good temperature compensation can be obtained. In the three-time repetitive continuous experiment, the current sensitivity as current increases of the system with temperature compensation is 4.23 MHz/A. The current measurement errors of the systems without and with temperature compensation are 0.83 A and 0.12 A, respectively. Our proposed system offers the advantages of easy fabrication, high current resolution with temperature compensation, good repeatability and high system stability, high current measurement precision, etc.

In this work, a novel highly sensitive optical fiber temperature sensor utilizing a Mach-Zehnder modal interferometer (MZMI) is presented, which uses a down taper between liquid-sealed waist-enlarged fused tapers (WEFTs) structure. Through the optical fiber tapering process, the single-mode fiber (SMF) is fused into the WEFT, which can excite cladding modes and form an MZMI that is sensitive to the external environment. To enhance the evanescent waves coupling with the ambient condition, a down taper is embedded in the WEFT structure and a mixture of glycerol and ethanol with a high thermos-optic coefficient is used to wrap the WEFT structure. The experimental results indicate that the maximum temperature sensitivity is up to 1.95 nm/degrees C, which is 21.4 times higher than that of the WEFT structure. The proposed scheme is simple, compact, and costeffective, which has good application potential for cold chain transportation, smart buildings, and greenhouse cultivation.

Fiber-optic sensors (FOSs) have garnered significant attention as ideal tools for remote health monitoring and disease diagnosis, owing to their compact dimensions, resilience to electromagnetic interference, and seamless integration into communication networks. Here, we present a high-precision and wearable fiber Bragg grating (FBG) sensing system based on an optoelectronic oscillator (OEO) for monitoring vital signs. The sensor head is a commercial FBG embedded within a polydimethylsiloxane (PDMS) substrate, which can achieve an elastic modulus akin to human skin. In contrast to traditional demodulation methods that employ costly and bulky FBG demodulation equipment, we utilize OEO for demodulation, which has garnered adjustable sensitivity, rapid demodulation speed, and ultrahigh instrument-limited resolution (approximately several orders of magnitude higher than optical-domain demodulation methods). By employing a mixer to down-convert the detection frequency to below 100 kHz, we achieve a sensitivity similar to high-frequency detection, thus significantly reducing the overall detection cost. Compared to intensity-based demodulation methods, our scheme, which employs frequency-based demodulation, offers greater resistance to interference. Furthermore, we have demonstrated the capability of the system for monitoring vital signs such as respiration rate, wrist pulse, limb movements, and facial muscle activity. This study establishes a new technical foundation for high-resolution and wearable vital signs monitoring technology, aiming to meet the future demands of remote disease diagnosis and health monitoring.

The mode-locking characteristic and microwave pulses regulation based on a self-mode-locking optoelectronic oscillator (SML-OEO) are analyzed and experimentally demonstrated. The oscillating modes in the SML-OEO cavity can be phase-coherent by means of the inherent oscillatory characterization via the opto-electronic loop. A self-excited microwave frequency comb (MFC) with the characteristics of a periodic rectangular pulse is generated. The phenomenon of nonlinear dynamical bifurcation in the SML-OEO, which leads to an amplification of the time scale in the system, where a single-cycle oscillatory pulse with a period of τ evolves into an oscillatory pulse with a period of 2τ. On this basis, by injecting a microwave signal with a period of 2τ/N (N is an integer) into the OEO loop, the gain in the OEO cavity is regulated so that the SML-OEO forms a new mode-locked state. The temporal domain characteristic forms a periodic rectangular pulse with a period of 2τ/N. The experiment results show that the microwave pulse of SML-OEO can be effectively regulated from 30 ns to 3.75 ns as N is set from 3 to 8.

A sensing system that simultaneously measures refractive index (RI) and temperature using a hybrid fiber interferometers-based microwave photonic filter (MPF) has been proposed and experimentally demonstrated. An extrinsic Fabry-Perot interferometer (EFPI) based on single-mode fibers (SMFs) is employed for RI sensing and the temperature can be measured by using a Sagnac interferometer (SI) fabricated with a section of polarization maintaining fiber (PMF). By tracking the MPF's dual-passband at the different central frequencies, the proposed sensor can effectively achieve dual-parameter sensing. Compared with the wavelength interrogation in optical domain, the proposed sensing system using the frequency interrogation in electrical domain, has an unlimited measurement range in principle. The experimental results indicate that the proposed sensing system realizes an RI sensitivity of 1197.604 MHz/RIU in the range from 1.332-1.412 RIU and a temperature sensitivity of 0.934 MHz/°C in the range from 20-50 °C. Moreover, the proposed system, possessing advantages such as high resolution, the ease of fabrication, and good stability, has great potential for applications in the field of multi-parameter sensing, such as biochemical detection, environmental monitoring, and industrial production.

This article proposes a silicone-encapsulated wearable fiber-optic sensor based on a single-modemultimode- few-mode-multimode-single-mode fiber-optic Mach-Zehnder interferometer (MZI) configuration, achieving simultaneous extraction of respiratory and heartbeat signals through the variational mode decomposition (VMD) algorithm. The sensing region is fully encapsulated within a soft and elastic silicone layer, improving the durability and maintaining high biocompatibility. The proposed sensor exhibits high curvature sensitivity, achieving an average sensitivity of -17.45 dB/m(-1), with a maximum sensitivity of -69.42 dB/mm(-1) within the curvature range of 2.77- 3.92 m(-1), and an average of 22.76 dB/m(-1) with a maximum of 65.25 dB/mm(-1) in the extended range of 3.92-4.96 m(-1). Experimental results show that the sensor can be freely worn at different positions on the chest, and it successfully acquires both respiratory and heartbeat signals under various breathing patterns. The respiratory and heart rates (HRs) measured by the system show good agreement with manual breath counting and a commercial HR monitor, respectively. With advantages, including cost-effective fabrication, simplified manufacturing process, and low-temperature crosssensitivity, this design provides a practical approach for implementing fiber-optic sensors in clinical respiration and HR monitoring.