Fiber-optic Fabry-Perot Interferometer 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.

Part-per-Billion (Ppb)-Level Acetylene Sensor Employing Optical Quartz Enhanced Photoacoustic Spectroscopy (QEPAS) with an Erbium-Doped Fiber Amplifier (EDFA) and a Fiber-Optic Fabry–Perot Interferometer

An all-optical double-pass quartz enhanced photoacoustic spectroscopy (QEPAS) sensor for the detection of acetylene (C2H2) at part-per-billion (ppb) levels was developed and experimentally validated. By employing an erbium-doped fiber amplifier (EDFA) capable of emitting up to 1 W of optical power, a commercial quartz tuning fork (QTF) resonating at 30.7 kHz, and double-pass acoustic microresonators, the amplitude of the QEPAS signal was significantly enhanced, thereby improving the sensitivity of the C2H2-QEPAS sensor. Instead of using piezoelectric detection in conventional QEPAS, a highly sensitive fiber-optic Fabry–Perot interferometer (FPI) was employed to measure the vibration of the QTF prong. A working point self-stabilizing technique was exploited to improve the demodulation sensitivity and stability. The linear response of the sensor to laser power and C2H2 concentration confirmed that no saturation occurred. With an excitation laser power of 1 W and a C2H2 absorption line of 1532.83 nm, a C2H2 minimum detection limit (MDL) of 19.1 ppb was achieved, corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 1.58 × 10−8 cm−1∙W∙Hz−1/2. Owing to its high sensitivity, long-term stability, and immunity to electromagnetic interferences, the QEPAS sensor provides considerable potential for remote gas sensing in environmental monitoring and other applications.

Diamond-Based Fiber-Optic Fabry–Perot Interferometer with Ultrawide Refractive-Index Measurement Range

Diamond-Based Fiber-Optic Fabry–Perot Interferometer with Ultrawide Refractive-Index Measurement Range

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.

Temperature-compensated high strain measurement based on a Fabry–Perot interferometer with the virtual-assisted Vernier effect

A highly sensitive optical fiber Fabry–Perot interferometer (FPI) for strain measurement with temperature compensation is proposed. Instead of using another actual reference interferometer, a virtual FPI is constructed to superpose with the sensing FPI to form the Vernier effect. The fundamental and the first-order harmonic Vernier effect are generated to increase the sensitivity by adjusting the parameter of the virtual FPI. In order to separate the strain from the environment temperature, an FBG is cascaded to distinguish the applied temperature. Experimental results demonstrate that, with the help of the fundamental Vernier effect, the sensitivity and temperature of the FPI increases from 1.05 pm/°C to 10.63 pm/°C in the temperature range of 40-120°C, and the sensitivity of strain increases from 2.635 pm/µε to 33.11 pm/µε in the strain range of 0-400 µε. In order to access the tracking points more easily and further enhance the sensitivities, the first-order harmonic Vernier effect is generated by modifying the virtual FPI. Results show that the temperature and strain sensitivities are 21.25 pm/°C and 62.25 pm/µε, respectively. In addition, with the help of the FBG, the strain can be separated from the temperature by solving the cross-sensitivity matrix.

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.

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.

Fast Demodulated White-light Interferometry-based Measurement and Theoretical Verification of Mechanical Sensitivity of MEMS Biomimetic Microphone with Fiber-optic Fabry-Perot Interferometer

Fast Demodulated White-light Interferometry-based Measurement and Theoretical Verification of Mechanical Sensitivity of MEMS Biomimetic Microphone with Fiber-optic Fabry-Perot Interferometer

Ultrasensitive Hydrogen Sensor with Vernier Effect by Dual Parallel Structured Fiber-Optic Fabry-Perot Interferometers

Ultrasensitive Hydrogen Sensor with Vernier Effect by Dual Parallel Structured Fiber-Optic Fabry-Perot Interferometers

Fast-Response Fiber-Optic FPI Temperature Sensing System Based on Modulated Grating Y-Branch Tunable Laser

In this paper, a cost-effective and miniaturized instrument is proposed, which is based on a tunable modulated grating Y-branch (MG-Y) laser for rapid temperature measurement using a Fabry-Perot interferometer (FPI) sensor. The FPI sensor with a 1 463-µm cavity length is a short segment of a capillary tube sandwiched by two sections of single-mode fibers (SMFs). This system has a broad tunable range (1 527 nm–1 567 nm) with a wavelength interval of 8 pm and a tuning rate of 100 Hz. Temperature sensing experiments are carried out to investigate the performance of the system by demodulating the absolute cavity length of the FPI sensor using a cross-correlation algorithm. Experimental results show that the sensor can reach the response time as short as 94 ms with the sensitivity of 802 pm/°C. Benefiting from the homemade and integrated essential electrical circuits, the entire system has the small size, low cost, and practical application potential to be used in the harsh environment for rapid temperature measurement.

A controllable humidity sensitivity measurement solution based on a pure FPI fiber tip and the harmonic Vernier effect

Hygroscopic films used to enhance the humidity sensitivity of fiber-optic sensors often experience material aging and nonlinear response issues. To address this issue, this study proposes a controllable humidity sensitivity measurement solution that offers a simple and cost-effective fabrication process without requiring humidity-sensitive material sensitization. The solution is based on a Fabry-Perot interferometer (FPI) fiber tip and the harmonic Vernier effect. By employing a cascade structure of single-mode fiber (SMF)-hollow core fiber (HCF)-no core fiber (NCF) with fiber grinding techniques, the FPI-based fiber sensing probe constructed a resonance cavity to detect small variations in air refractive index (RI) induced by humidity. A reference spectrum, which can flexibly control parameters, such as free spectral range (FSR), contrast, phase, and intensity, is superimposed onto the sensing spectrum acquired by the FPI fiber sensing tip. By extracting features from the internal envelope generated by the harmonic Vernier effect, the mapping relationship between the center wavelength of the inner envelope and the humidity is determined. The harmonic Vernier effect amplifies the humidity sensitivity of the fiber-optic FPI sensing tip, with the magnification factor determined by the optical path length (OPL) detuning of the created reference and sensing spectra. For the harmonic order j = 4, a humidity sensitivity of approximately 76.01 pm/% RH@ 24 ℃ was achieved without using hygroscopic material sensitization. The proposed solution offers numerous advantages and enables high-precision and high environmental reliability humidity monitoring.

Liquid crystal-embedded fiber optic fabry perot temperature sensor based on Vernier effect

An optical temperature sensing method with ultra-high sensitivity and a wide measurement range is proposed. The performance enhancements come from the high thermos-optical coefficient of liquid crystal (LC) material, and the Vernier effect produced by LC birefringence characteristics. This paper proposes and demonstrates a method to enhance the measurement range of the LC temperature sensor. A miniature LC temperature sensor was made by using an optical fiber Fabry-Perot interferometer (FPI). To compensate for and attenuation of LC and improve the contrast of the FPI, silver films are coated on one end of the optical fiber. Due to the high birefringence of the LC in the interferometer, a Vernier spectrum can be observed in the reflection. The temperature measurement range of 20–32 °C and the temperature sensitivity of the Vernier peaks of a sensor sample is about 42.18 nm/°C, which is much higher than the sensitivity of the cascaded FBG in structure. The limited measurement range is due to the free spectral range (FSR) limitation. In this context, the FBG peak can be used as a temperature reference while switching the adjacent Vernier peaks. Hence, this LC temperature sensor exhibits high sensitivity as well as a wide measuring range, which would broaden the application scope of the LC temperature sensors.

Versatile Optofluidic Fabry-Perot Sensor for Multiple Physical Parameters in Microfluidic Chips

An optofluidic sensor, which is composed of a fiber Fabry-Perot interferometer (FPI) and a microfluidic chip, is experimentally demonstrated to measure aerostatic pressure, local temperature, and fluidic flow rate in microfluidic chips. The aerostatic pressure sensitivity is as high as −2393.3 nm/Bar. By demodulating the aerostatic pressure and temperature with a sensitivity matrix, both the physical parameters can be predicted simultaneously. The noise-equivalent detection limit (NEDL) of aerostatic pressure is as low as 48.4 μBar. In addition, the experiment results show that the fluidic flow rate sensitivity is −1.8574 nm/(μl/min) and the NEDL of fluidic flow rate is 97.1 nl/min, respectively. The miniature optofluidic sensor based on the optical fiber Fabry-Perot interferometer provides a promising cost-effective sensing platform for monitoring multiple physical parameters in the microfluidic chips, which is of great significance for on-chip biochemical reactions.

Novel microspherical fiber Fabry–Pérot interferometer based on the Vernier effect for micro-displacement sensing

A novel optical fiber Fabry–Perot interferometer (FPI) based on the Vernier effect is proposed for micro-displacement sensing. The sensor is composed of an optical fiber microsphere and a section of multimode optical fiber with a flat end surface. The air cavity in the middle is used as the Fabry–Perot cavity. The extinction ratio of the reflection spectrum is significantly improved by the optical fiber microsphere, and the extinction ratio is ∼12 dB. First, a single FPI is tested for micro-displacement. A sensitivity of 2.13 nm/µm and linearity of 99.31% are obtained within the range of 0–1.872 µm via phase demodulation. The fabricated optical fiber FPI is used as the sensing arm and cascaded with the designed analog reference arm. The micro-displacement sensitivity of the sensing arm can be improved to 28.92 nm/µm through the Vernier effect, i.e., 13.58 times that of a single FPI. The fabricated sensor exhibits the characteristics of high sensitivity, contrast, and linearity; a compact structure; and low cost. It can be used for micro-displacement detection. In addition, the analog reference arm is unaffected by the environment, and it can freely adjust sensitivity magnification. Accordingly, it is applicable to micro-displacement detection scenarios under different sensitivity requirements.