Fiber-optic Fabry-Perot Research Articles

A Hybrid GAN-Inception Deep Learning Approach for Enhanced Coordinate-Based Acoustic Emission Source Localization

In this paper, we propose a novel approach to coordinate-based acoustic emission (AE) source localization to address the challenges of limited and imbalanced datasets from fiber-optic AE sensors used for structural health monitoring (SHM). We have developed a hybrid deep learning model combining four generative adversarial network (GAN) variants for data augmentation with an adapted inception neural network for regression-based prediction. The experimental setup features a single fiber-optic AE sensor based on a tightly coiled fiber-optic Fabry-Perot interferometer formed by two identical fiber Bragg gratings. AE signals were generated using the Hsu-Nielsen pencil lead break test on a grid-marked thin aluminum plate with 35 distinct locations, simulating real-world structural monitoring conditions in bounded isotropic plate-like structures. It is demonstrated that the single-sensor configuration can achieve precise localization, avoiding the need for a multiple sensor array. The GAN-based signal augmentation expanded the dataset from 900 to 4500 samples, with the Wasserstein distance between the original and synthetic datasets decreasing by 83% after 2000 training epochs, demonstrating the high fidelity of the synthetic data. Among the GAN variants, the standard GAN architecture proved the most effective, outperforming other variants in this specific application. The hybrid model exhibits superior performance compared to non-augmented deep learning approaches, with the median error distribution comparisons revealing a significant 50% reduction in prediction errors, accompanied by substantially improved consistency across various AE source locations. Overall, this developed hybrid approach offers a promising solution for enhancing AE-based SHM in complex infrastructures, improving damage detection accuracy and reliability for more efficient predictive maintenance strategies.

DC current sensor based upon a fused fiber optic Fabry-Perot interferometer (FPI) and copper wire

In order to meet the demand for direct current (DC) current measurements in industrial production, a new type of fiber optic DC current sensor, combining thin copper wire and fiber optic Fabry-Perot interferometer (FPI), is reported. FPI is a sandwich structure formed by splicing a single-mode fiber (SMF) and a quartz capillary, with a cavity length of approximately 200 μm. The copper wire, with a length of 3.2 cm and a diameter of 1 mm, is attached to the FPI to form the sensor. Due to the excellent thermal conductivity of copper, the results showed that the sensor increased the sensitivity from 4.1 pm/°C to 11 pm/°C. After applying direct current to the sensor, a large quantity of heat generated by the copper wire was absorbed by the FPI, resulting in a significant shift in the resonant wavelength of FPI, which can indirectly measure the current. Three current experiments were performed, and parabolic curves have been used in fitting of the Dip wavelength and the square of the current. The results provide high correlation coefficients with values exceeding 0.99. Therefore, this curve may be used to measure the square of the current, further obtaining the current. In summary, the sensor is compact and durable, easy to manufacture, and provides high correlation coefficients, providing an alternative for measuring DC current.

Highly sensitive push-pull differential pressure sensor with multi-parameter measurement capabilities

A multi-parameter sensor comprising of a Fabry–Perot (FP) pair and a fiber-optic FP microcavity was proposed for simultaneous measurement of differential pressure (DP), static pressure (SP) and temperature. The push-pull FP pair is used to measure the differential pressure, and wide measurement range can be achieved by adjusting the dimensions of the diaphragm. The self-enclosed fiber-optic microcavity is utilized to measure the static pressure. For multi-parameter measurement, a matrix equation is proposed to solve the differential pressure, static pressure and temperature by fully considering the relationship of the FP cavity lengths and measurands. The performance of multi-parameter sensing is verified experimentally. The sensitivities of differential pressure, static pressure and temperature are 294.4822 nm/KPa, 353.08 nm/MPa and 42.4056 nm/°C, respectively, in variation of cavity lengths. The sensing characteristics show good linearity. Such a sensor could find important applications in the fields of process industry and oil industry, etc.

Sensitivity enhancement of an all-solid FPE sensor via a programmable Vernier effect.

All-solid, open-cavity fiber optic Fabry-Perot etalon (FPE) sensors possess a wide static pressure detection range, yet their low sensitivity significantly restricts their application. This study proposes a programmable Vernier effect to improve the gas pressure sensitivity of FPE sensors substantially. By effectively modulating the emission spectrum of a widely tunable laser using a variable optical attenuator (VOA), the emission spectrum at different modulation lengths is expected to produce an optical beating in conjunction with the transmission spectrum of the FPE sensor, thereby realizing the Vernier effect. Experimental results indicate that by utilizing the proposed programmable Vernier effect, the pressure sensitivity of the FPE sensor has increased to -612.21 pm/kPa, demonstrating an amplification in sensitivity of approximately -153 times, consistent with the theoretical results. Owing to the programmable Vernier effect, which flexibly enhances the sensitivity of the FPE sensor, this sensor demonstrates considerable potential for gas pressure monitoring under various extreme conditions.

Enhancing the sensitivity of a humidity sensor by using PVA and GQDs hybrid diaphragms and the harmonic Vernier effect

What we believe to be a novel fiber optic Fabry-Perot interferometer (FPI) humidity sensor based on polyvinyl alcohol (PVA) doped graphene quantum dots (GQDs) was proposed and experimentally studied. This sensor consisted of a sensing FPI and a reference FPI in parallel. The two sensing cavities FPI were composed of humidity sensitive materials PVA and PVA-GQDs, respectively. Experimental comparative studies had found that doping GQDs in PVA increases humidity sensitivity by 2.1 times. Four reference cavity FPIs were prepared by splicing single-mode fiber and quartz capillary, and then they were combined with two sensing cavity FPIs to form two Vernier effect (VE) sensors and two harmonic Vernier effect (HVE) sensors. Experimental research had found that the sensitivities of PVA as a sensing material for the VE sensor and HVE sensor were -1.0804 nm/%RH and-1.6566 nm/%RH, respectively. The sensitivities of PVA-GQDs as sensing materials for the VE sensor and HVE sensor were-3.1527 nm/%RH and 7.3343 nm/%RH, respectively. Moreover, both HVE sensors had minimal temperature crosstalk. PVA-GQDs were excellent humidity sensitive materials that significantly improve the sensitivity of humidity sensors, making it promising candidate for humidity sensing in various applications.

High sensitivity optical fiber temperature sensor based upon a polydimethylsiloxane filled Fabry-Perot interferometer

A high-sensitivity temperature sensor based upon an optical fiber Fabry-Perot interferometer (FPI) filled with polydimethylsiloxane (PDMS) is reported that employed a single mode fiber (SMF) and a hollow core fiber (HCF) to enhance the sensitivity. The sensing characteristics were investigated from 20 °C to 40 °C. The experimental results demonstrate that the sensor is sensitive to temperature due to the high thermal optical coefficient (TOC) and high thermal expansion coefficient (TEC) of PDMS, achieving a high sensitivity of 2.155 nm/°C. The designed sensor shows great potential for high-precision temperature sensing applications due to its high sensitivity, compact structure, and simple preparation.

In situ monitoring of cycling characteristics in lithium-ion battery based on a two-cavity cascade fiber-optic Fabry-Perot interferometer

The state characterization inside the lithium-ion battery during charge/discharge cycling is extremely crucial for understanding the electrochemical reaction mechanism. However, current methods exhibit a challenge to overcome the specific battery environment obstacles, including strong redox properties, strong electromagnetic interference, and fast reaction processes. Hence, more efforts are still needed to monitor the actual state inside the battery accurately and reliably. To address this issue, we designed and developed a compact two-cavity cascade fiber-optic Fabry-Perot interferometer (FPI) sensor that can be safely implanted in batteries to measure internal temperature and pressure simultaneously. With its high pressure and temperature sensitivity of 26.6 ​nm/kPa and 107 ​nm/°C, this sensor exhibits an ultra-low cross-sensitivity of −40 ​Pa/°C. During charge/discharge cycling tests, regular cyclic pressure and temperature signals are obtained at various rates cycling in real-time and in situ, revealing details about the actual state characterization inside the battery. From the experiment results, the pressure inside the battery is divided into reversible changes caused by respiration effects and irreversible changes caused by trace gas production. Furthermore, the FPI sensor provides a more precise temperature than thermocouples that measure the surface temperature of the battery, reflecting the internal/external temperature difference to a maximum of 3.5 ​°C at 1 ​C rate cycling. This operando FPI sensor provides a valuable technological tool for battery performance testing and safety monitoring.

High-Resolution and Large-Dynamic Range Fiber-Optic Sensors Based on Dual-Mode Direct Spectrum Interrogation Method

Conventional optical fiber temperature/strain sensors often have to make compromises between the resolution and the dynamic range. Here we present a new method that meets the measurement requirements for both high resolution and large dynamic range. A high-quality optical fiber Fabry-Perot Interferometer (FPI) constructed using a pair of chirped fiber Bragg gratings is employed as the sensor and a dual-mode direct spectrum interrogation method is proposed to identify the small drift of external temperature or strain. As a proof-of-concept illustration, a temperature resolution of 0.2 °C within 30–130 °C is demonstrated. For strain sensing, the resolution can be 10 µε within 0–1000 µε. The measurement resolution can be improved further by routinely increasing the reflectivity of the CFBG and the cavity length and the sensor can also be mass-produced. This new sensing schema not only resolves the conflict between the resolution and the dynamic range of fiber-optic temperature/strain sensors but can also be extended to other sensors and measurands.

Multiplexing and passive demodulation of intrinsic low-finesse Fabry-Perot interferometers using free-running DFB diode lasers

This paper presents a novel and simple technique for passive quadrature demodulation of multiplexed low-finesse fiber-optic Fabry-Perot interferometers. The approach proposed here uses two quadrature channels with very close wavelengths generated by two thermostabilized DFB diode lasers operating in continuous-wave mode. A coherent light correlation technique was used to demultiplex the wavelength channels and signals from interferometers at different positions along the fiber. The proposed technique does not require optical filters for spectral channel demultiplexing, making it suitable for demodulating relatively long interferometers with any free spectral range. An in-line array of four identical 13.1 cm long interferometers was used to demonstrate the method's performance. Experimental results demonstrate a phase resolution of 1.38 phase degrees, corresponding to a temperature resolution of 3 × 10−3 °C. Numerical simulations indicate that the main factor limiting the length of the sensing interferometer, and thus the resolution of the system, is the wavelength variation of the temperature-stabilized, independently operating lasers. The number of spectral channels can be easily extended to realize three or four-wavelength demodulation schemes.

A Review of Wearable Optical Fiber Sensors for Rehabilitation Monitoring.

As the global aging population increases, the demand for rehabilitation of elderly hand conditions has attracted increased attention in the field of wearable sensors. Owing to their distinctive anti-electromagnetic interference properties, high sensitivity, and excellent biocompatibility, optical fiber sensors exhibit substantial potential for applications in monitoring finger movements, physiological parameters, and tactile responses during rehabilitation. This review provides a brief introduction to the principles and technologies of various fiber sensors, including the Fiber Bragg Grating sensor, self-luminescent stretchable optical fiber sensor, and optic fiber Fabry-Perot sensor. In addition, specific applications are discussed within the rehabilitation field. Furthermore, challenges inherent to current optical fiber sensing technology, such as enhancing the sensitivity and flexibility of the sensors, reducing their cost, and refining system integration, are also addressed. Due to technological developments and greater efforts by researchers, it is likely that wearable optical fiber sensors will become commercially available and extensively utilized for rehabilitation.

Localization of Dual Partial Discharge in Transformer Windings Using Fabry–Pérot Optical Fiber Sensor Array

The power transformer is one of the most critical core devices for energy exchange in power systems, and its safe and stable operation is directly related to the reliability of the power grid. Partial discharge is the main cause of insulation degradation and failure of high-voltage electrical equipment. Online monitoring and accurate localization of partial discharge can provide information on the aging of power equipment, which is of great value for improving the safe operation and maintenance of the power grid. Internal dual partial discharge in transformer windings is a more complex type of fault. Since it is located inside the windings, the signal is attenuated and distorted, making it difficult for traditional monitoring methods to capture such partial discharge signal The Fabry–Perot optical fiber sensor is an ultrasonic detection method that can be built into the transformer interior. This sensor has high sensitivity and a small size, enabling flexible placement at different locations inside the transformer for precise partial discharge detection. Especially for the narrow space inside the high and low voltage windings, F–P sensors can form an array, utilizing the array’s directivity to locate the fault points. In this study, an ultrasonic detection system based on the F–P optical fiber sensor array was developed. The system utilizes a directional cross-localization algorithm based on the multiple signal classification (MUSIC) algorithm to accurately locate dual partial discharge sources. This partial discharge detection system was applied to a 35 kV single-phase transformer, enabling the localization of dual partial discharge sources within the high and low voltage windings. Combined with experimental results, this method exhibits high localization accuracy and is particularly suitable for detecting partial discharge phenomena that occur within or between transformer windings.

Miniaturized and highly sensitive fiber-optic Fabry–Perot sensor for mHz infrasound detection

Infrasound detection is important in natural disasters monitoring, military defense, underwater acoustic detection, and other domains. Fiber-optic Fabry–Perot (FP) acoustic sensors have the advantages of small structure size, long-distance detection, immunity to electromagnetic interference, and so on. The size of an FP sensor depends on the transducer diaphragm size and the back cavity volume. However, a small transducer diaphragm size means a low sensitivity. Moreover, a small back cavity volume will increase the low cut-off frequency of the sensor. Hence, it is difficult for fiber-optic FP infrasound sensors to simultaneously achieve miniaturization, high sensitivity, and extremely low detectable frequency. In this work, we proposed and demonstrated a miniaturized and highly sensitive fiber-optic FP sensor for mHz infrasound detection by exploiting a Cr-Ag-Au composite acoustic-optic transducer diaphragm and a MEMS technique-based spiral micro-flow hole. The use of the spiral micro-flow hole as the connecting hole greatly reduced the volume of the sensor and decreased the low-frequency limit, while the back cavity volume was not increased. Combined with the Cr-Ag-Au composite diaphragm, a detection sensitivity of −123.19 dB re 1 rad/μPa at 5 Hz and a minimum detectable pressure (MDP) of 1.2 mPa/Hz1/2 at 5 Hz were achieved. The low detectable frequency can reach 0.01 Hz and the flat response range was 0.01–2500 Hz with a sensitivity fluctuation of ±1.5 dB. Moreover, the size of the designed sensor was only 12 mm×Φ12.7 mm. These excellent characteristics make the sensor have great practical application prospects.

Temperature-Decoupled Single-Crystal MgO Fiber-Optic Fabry-Perot Vibration Sensor Based on MEMS Technology for Harsh Environments.

A high-temperature-resistance single-crystal magnesium oxide (MgO) extrinsic Fabry-Perot (FP) interferometer (EFPI) fiber-optic vibration sensor is proposed and experimentally demonstrated at 1000 °C. Due to the excellent thermal properties (melting point > 2800 °C) and optical properties (transmittance ≥ 90%), MgO is chosen as the ideal material to be placed in the high-temperature testing area. The combination of wet chemical etching and direct bonding is used to construct an all-MgO sensor head, which is favorable to reduce the temperature gradient inside the sensor structure and avoid sensor failure. A temperature decoupling method is proposed to eliminate the cross-sensitivity between temperature and vibration, improving the accuracy of vibration detection. The experimental results show that the sensor is stable at 20-1000 °C and 2-20 g, with a sensitivity of 0.0073 rad (20 °C). The maximum nonlinearity error of the vibration sensor measurement after temperature decoupling is 1.17%. The sensor with a high temperature resistance and outstanding dynamic performance has the potential for applications in testing aero-engines and gas turbine engines.

All-SiC fiber-optic sensor for pressure and temperature dual-mode sensing in harsh environments

This paper presents a fiber-optic Fabry–Perot (FP) sensor with an all-silicon carbide (SiC) structure for pressure and temperature dual-mode sensing in harsh environments. The all-SiC sensing diaphragm and vacuum FP cavity were formed by direct bonding of a 6 H-SiC (N-doped) diaphragm with a 6 H-SiC (V-doped) substrate. The variation in length of the FP cavity was used to measure the external pressure, while the principle of temperature sensing was due to the temperature dependence of the refractive index and thermal expansion of the SiC substrate. The pressure and temperature characteristics of the sensor were calibrated from 25 °C up to 700 °C over a pressure range of 0–1.0 MPa. The pressure sensor achieved high sensitivity (174.3 nm/MPa) and linearity (R2 = 0.999 at 700 °C), the spectral sensitivity of the temperature sensing being approximately 32.12 pm/°C. Moreover, the temperature-pressure cross-sensitivity was as little as 4.98 °C/MPa. The dual-mode sensing capability of the all-SiC fiber-optic sensor demonstrated great promise for applications in extremely high-temperature environments.

A Five-Hole Pressure Probe Based on Integrated MEMS Fiber-Optic Fabry-Perot Sensors.

The five-hole pressure probe based on Micro-Electro-Mechanical Systems (MEMS) technology is designed to meet the needs of engine inlet pressure measurement. The probe, including a pressure-sensitive detection unit and a five-hole probe encapsulation structure, combines the advantages of a five-hole probe with fiber optic sensing. The pressure-sensitive detection unit utilizes silicon-glass anodic bonding to achieve the integrated and batch-producible manufacturing of five pressure-sensitive Fabry-Perot (FP) cavities. The probe structure and parameters of the sensitive unit were optimized based on fluid and mechanical simulations. The non-scanning correlation demodulation technology was applied to extract specific cavity lengths from multiple interference surfaces. The sealing platform was established to analyze the sealing performance of the five-hole probe and the pressure-sensitive detection unit. The testing platform was established to test the pressure response characteristics of the probe. Experimental results indicate that the probe has good sealing performance between different air passages, making it suitable for detecting pressure from multiple directions. The pressure responses are linear within the range of 0-250 kPa, with the average pressure sensitivity of the five sensors ranging from 11.061 to 11.546 nm/kPa. The maximum non-linear error is ≤1.083%.

Label-free and selective heparin detection by surface functionalized fiber Fabry-Perot interferometer biosensor

Heparin (Hep) is the most commonly used anticoagulant drug in clinical settings, and precise dosage control is crucial. To enable rapid and accurate quantification of Hep concentration in clinical practice, a label-free and selective fiber-optic Fabry-Perot interferometric (FPI) biosensor detection method based on graphene oxide (GO) was proposed. This method utilizes protamine (PRTM) sulfate as the modified layer and achieves Hep detection through layer-by-layer (LBL) assembly. In this study, we developed a label-free and selective fiber optic FPI biosensor for the detection of Hep using GO. The GO on the surface layer of the fiber optic FPI biosensor was treated with specific functionalization, it enables specific interaction with Hep. The binding of Hep to the functionalized surface induces a change in the luminal light signal, which can be monitored within the cavity for rapid and accurate detection of Hep. The results demonstrate that this biosensor exhibits good sensitivity, with a value of 10.82 nm/mM in the low-concentration control group, and an average limit of detection (LOD) calculated as 31μg/mL in the low-concentration group. This study presents a label-free, selective and efficient method for biomedical science testing that holds great potential for widespread application in clinical medicine.

Design of a multi-physics coupling MEMS pressure sensor

The pressure measurement of explosion shock wave puts forward high requirements on the temperature resistance and response speed of the sensor. In this paper, a multi-physics coupling pressure sensor is designed to meet requirements. The sensor mainly consists of a Fabry-Perot cavity optical fiber sensing unit, a piezoelectric pressure sensing unit and a thermometric resistance temperature measurement module. Firstly, the structure of the sensor is designed. The top and bottom surfaces of the Fabry-Perot cavity are composed of a silicon diaphragm and a quartz glass. The piezoelectric pressure sensing unit is an AlN film including its electrodes. Pt thermometric resistance is developed to realize real-time temperature monitoring. Secondly, the fabrication process of the sensor is discussed, especially three key technologies. An Al film is sputtered to increase the reflectivity of the quartz glass. The SU-8 photoresist is applied to accurately control the thickness of Fabry-Perot cavity. Etching is adopted to work out the graphics of the AlN piezoelectric film. At last, a complete fabrication process of the sensor is described. The fabrication of multilayer films begins with a double-sided polishing silicon wafer, while Fabry-Perot cavity was from a double-sided polishing quartz glass. Then two parts of the sensor are combined to ensure the multi-physics coupling sensor to achieve complete function.

Highly sensitive optical fiber pressure sensor based on the FPI and Vernier effect via femtosecond laser plane-by-plane writing technology.

In this paper, a highly sensitive pressure sensor based on fiber-optic Fabry-Perot interferometers (FPIs) and the Vernier effect (VE) is proposed and experimentally demonstrated. We employ a closed capillary-based F P I s for the sensing cavity, and an F P I r created through femtosecond laser refractive index modulation for the reference cavity, which remains impervious to pressure changes. Connecting these two FPIs in series produces a VE-based cascaded sensor with a clear spectral envelope. The femtosecond laser micromachining technique provides precise control over the length of F P I r and facilitates adjustments to the VE's amplification degree. Experimental results reveal significant pressure sensitivities of -795.96p m/M P a and -3219.91p m/M P a, respectively, representing a 20-fold and 80-fold improvement compared to F P I s (-39.80p m/M P a). This type of sensor has good sensitivity amplification and, due to its all-fiber structure, can be a promising candidate for high-temperature and high-pressure sensing, especially in harsh environments.

Miniature Optical Fiber Fabry-Perot Interferometer Based on a Single-Crystal Metal-Organic Framework for the Detection and Quantification of Benzene and Ethanol at Low Concentrations in Nitrogen Gas.

This study reports for the first time, to the best of our knowledge, a real-time detection of ultralow-concentration chemical gases using fiber-optic technology, combining a miniaturized Fabry-Perot interferometer (FPI) with metal-organic frameworks (MOFs). The sensor consists of a short and thick-walled silica capillary segment spliced to a lead-in single-mode fiber (SMF), housing a tiny single crystal of HKUST-1 MOF, imparting chemoselectivity features. Ethanol and benzene gases were tested, resulting in a shift in the FPI interference signal. The sensor demonstrated high sensitivity, detecting ethanol gas concentrations (EGCs) with a sensitivity of 0.428 nm/ppm between 24.9 and 40.11 ppm and benzene gas concentrations (BGCs) with a sensitivity of 0.15 nm/ppm between 99 and 124 ppm. The selectivity study involved a combination of three ultralow concentrations of ethanol, benzene, and toluene gases, revealing an enhancement factor of 436% for benzene and 140% for toluene, attributed to the improved miscibility of these conjugated ring molecules with the alkane chains of the ethanol-modified HKUST-1. Experimental tests confirmed the sensor's viability, demonstrating significantly improved response time and spectral characteristics through crystal polishing, indicating its potential for quantifying and detecting chemical gases at ultralow concentrations. This technology may prevent energy resource losses, and the sensor's small size and robust construction make it applicable in confined and hazardous locations.

Recent Progress in MEMS Fiber-Optic Fabry-Perot Pressure Sensors.

Pressure sensing plays an important role in many industrial fields; conventional electronic pressure sensors struggle to survive in the harsh environment. Recently microelectromechanical systems (MEMS) fiber-optic Fabry-Perot (FP) pressure sensors have attracted great interest. Here we review the basic principles of MEMS fiber-optic FP pressure sensors and then discuss the sensors based on different materials and their industrial applications. We also introduce recent progress, such as two-photon polymerization-based 3D printing technology, and the state-of-the-art in this field, e.g., sapphire-based sensors that work up to 1200 °C. Finally, we discuss the limitations and opportunities for future development.