Fabry-Perot Interferometric Strain Sensors Research Articles

A Fabry-Perot interferometer based on probe-embedded bubble for ultrasensitive strain measurement

A high-sensitivity Fabry-Perot interferometer (FPI) strain sensor based on tapered single-mode fiber (SMF) probe embedded in microbubble is proposed and verified. The two reflective surfaces of this FPI are composed of the two tapered SMF ends and assembled within a bubble. The distance between the reflection face can be flexibly adjusted to enhance the strain sensitivity of the sensor. Experimental result shows that the strain sensitivity of proposed FPI sensor reaches to 142.2 pm/με. Meanwhile, the sensor demonstrates low temperature sensitivity of 2.19 pm/°C, which achieves an extremely low temperature-strain crosstalk of less than 16.6 nε/°C. Moreover, the proposed FPI sensor possesses advantages of high sensitivity, low-cost and compact length, which shows potential in areas of micro strain measurement.

Ultra-high sensitivity strain sensor based on biconical fiber with a bulge air-bubble.

An ultra-high sensitivity Fabry-Perot interferometer (FPI) strain sensor is proposed and experimentally demonstrated. It is composed of a biconical fiber with a bulge air-bubble at its waist. Because of the good stress concentration capability of the tapered fiber, the bulge air-bubble is easily deformed when it is stressed, resulting in an ultra-high strain sensitivity of 101.7 pm/µɛ, which is the highest strain sensitivity among the direct wavelength interrogation-based strain sensors reported so far, to the best of our knowledge. Moreover, the proposed sensor has a low temperature sensitivity of ∼1pm/∘C; thus an extreme low temperature cross sensitivity of less than 10 nɛ/°C can be achieved.

Tapered Hollow‐Core Fiber Air‐Microbubble Fabry–Perot Interferometer for High Sensitivity Strain Measurement

AbstractA highly sensitive and ultracompact (12.5 µm) optic‐fiber strain sensor based on air‐microbubble Fabry–Perot interferometer (FPI) formed in a tapered hollow core fiber (HCF) is proposed. The sensor is fabricated by fusion splicing the lead‐in and lead‐out single‐mode fibers (SMFs) with a HCF, and then the HCF is tapered at the center to construct an air‐microbubble FPI. The application of the tapered operation not only provides an ultrasmall FPI preparation method, but also increases the strain sensitivity of the proposed FPI sensor. The simulation model is established based on the COMSOL to study the response to axial strain, and the results show that the existence of tapered area can effectively improve the strain sensitivity. Accordingly, the high strain sensitivity of 8.62 pm µε−1 is achieved with a good linear response (R‐square: 0.996) in the wide strain range of 0–3200 µε, which is almost four times larger than that of the all‐fiber FPI strain sensors without the tapered area (about 2 pm µε−1). In addition, the sensor has a negligible temperature sensitivity of 0.32 pm °C−1. Therefore, the highly sensitive, lower temperature crosstalk, low‐cost, and ultracompact optic‐fiber strain sensor would have a broad prospect of application in practice.

“Bellows spring-shaped” ultrasensitive fiber-optic Fabry-Perot interferometric strain sensor

All-silica fiber-optic Fabry-Perot strain sensor is rapidly gaining widespread adoption in many fields due to its compact structure, low cost and immunity to electromagnetic interference. However, the conventional configuration suffers from low sensitivity due to the limitation of its inherent structure feature. In this paper, we demonstrate an ultrasensitive fiber-optic Fabry-Perot interferometric strain sensor based on silica bellows spring structure. Utilizing the cascaded microbubbles, the sensitivity of the single-microbubble sensor is enhanced up to 203.8 pm/με and that of the cascaded two-microbubble sensor reaches 518.8 pm/με. The sensitivity is reinforced by 5-to-12 folds compared with the highest level ever reported, which could also be multiplied by increasing the microbubble number. The temperature-induced strain error is much low, less than 0.01 με/°C, showing great thermal stability and high-temperature application potential. Moreover, the strain sensor also provides high-quality spectrum, controllable free-spectral range and low cost.

A Fabry–Perot Interferometer Strain Sensor Based on Concave-Core Photonic Crystal Fiber

A high-sensitivity Fabry–Perot interferometer (FPI) strain sensor based on a concave-core photonic crystal fiber (CPCF) was proposed and demonstrated. The FPI was formed by directly splicing a CPCF to a standard single-mode fiber. The CPCF was fabricated by cleaving a high-numerical-aperture solid-core photonic crystal fiber under axial tension using a traditional fiber cleaver. An FPI strain sensor with a cavity length of 4.85 μ m and a strain sensitivity of 31.58 pm/ μ ϵ was successfully produced. The sensor also demonstrated a low temperature sensitivity of 0.653 pm/°C, reducing the cross-sensitivity between tensile strain and temperature. The benefits of the proposed sensor include simple fabrication, high strain sensitivity, and low temperature cross-sensitivity, making it attractive for practical applications.

Miniaturized fiber Fabry-Pérot interferometer for strain sensing

A temperature-insensitive miniaturized all-fiber Fabry–Perot interferometer strain sensor was proposed and experimentally studied. This sensor with an air cavity was made by splicing a short section of hollow-core fiber with two standard single mode fibers. The experimental results showed that these sensors have an fringe contrast exceeding 22 and 13 dB for the air cavity with a length of 167.08 and 557.11 μm, and its reflection spectrum shifts linearly to long wavelength by increasing the strain from 0 to 3000 μe with a strain sensitivity of 1.8 and 1.7 pm/μe, respectively. This strain sensor also showed good repeatability and long time stability, while its temperature sensitivity was only about 0.95 pm/°C in the range of 0°C–180°C for the 557.11 μm air cavity, so there is less strain–temperature cross-sensitivity. Moreover, this sensor was easy to be fabricated and there was no requirement for chemical etching, careful cleaving, and special fusion. It may be potentially used for strain sensing applications with high resolution and miniaturized size. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1510–1514, 2016

Hybrid Fibre Optic Sensor Network for Real-time High Temperature Performance Monitoring of Steel Structures

In and post a fire event, an accurate and real-time evaluation and monitoring of a structure’s performance can assist firefighters for efficient survivor rescuing, which significantly improve the fire rescuing safety both for fire fighters and the trapped survivors. However, due to the lack of durable sensors, the structural performance of steel structures in fire conditions is challenging to be evaluated in real time, especially when the associated civil structures are in a large scale. In this paper, a fiber optic sensor network is developed and used to monitor the structural performance of the steel structures in high temperature environments. The fiber optic sensor network has the capacity of real-time large strain measurement up to 12% and temperature of up to 700oC simultaneously. The capability of large strain measurement up to 12% enables the sensor system to monitor the strain distribution of steel structures during fire events in real time. An one-story one-bay steel frame (A36 steel) is used as an example in this paper to perform the structural performance assessment of steel structures in high temperature using the developed sensor network. The simulated fire tests were performed using high temperature furnace through gradual temperature increase from room temperature to 800 °C at a rate of 10 °C/min. The evaluated fiber optic sensor network consists of two long period fiber grating (LPFG) temperature sensors, five movable extrinsic Fabry-Perot interferometric (EFPI) large strain sensors, and two hybrid EFPI/LPFG sensors, which were distributed along the steel frame inside and around the heating zone of the frame. Experimental results demonstrated that the developed sensor network effectively monitored the plastic hinge formation and failure mode of the steel frame, approving the feasibility of the sensor network for steel structure evaluation in high temperature environments.

Self-repairing, interferometric waveguide sensor with a large strain range

We demonstrate a polymer waveguide, Fabry-Perot interferometer strain sensor fabricated through a self-writing process in a photopolymerizable resin bath between two silica optical fibers. The measurable strain range is extended through sensor self-repair and strain measurements are demonstrated up to 150% applied tensile strain. The sensor fabrication and repair is performed in the ultraviolet wavelength range, while the sensor interrogation is performed in the near-infrared wavelength range. A hybrid sensor is fabricated by splicing a short segment of multimode optical fiber to the input single-mode optical fiber. The hybrid sensor provides the high quality of waveguide fabrication previously demonstrated through self-writing between multimode optical fibers with the high fringe visibility of single-mode propagation. The peak frequency shift of the reflected spectrum Fabry-Perot sensor is extremely linear with applied strain for the hybrid sensor, with a sensitivity of 2.3×10(-3) per nanometer per percent strain. The calibrated peak frequency shift with applied strain is the same for both the original sensor and the repaired sensor; therefore, the fact that the sensor has self-repaired does not need to be known. Additionally, this calibration is the same between multiple sensor fabrications. In contrast to a conventional air gap Fabry-Perot cavity sensor, no decrease in the fringe visibility is observed over the measurable strain range.

Microbubble based fiber-optic Fabry–Perot interferometer formed by fusion splicing single-mode fibers for strain measurement

We demonstrate an all-fiber optical Fabry-Perot interferometer (FPI) strain sensor whose cavity is a microscopic air bubble. The bubble is formed by fusion splicing together two sections of single-mode fibers (SMFs) with cleaved flat tip and arc fusion induced hemispherical tip, respectively. The fabricated interferometers are with bubble diameters of typically ~100 μm. Strain and temperature sensitivities of fabricated interferometers are studied experimentally; a strain sensitivity of over 4 Pm/με and a thermal sensitivity of less than 0.9 Pm/°C is obtained.

Evaluating damage in smart composite laminates using embedded EFPI strain sensors

Extrinsic Fabry-Perot interferometric (EFPI) strain sensors (SSs) were assessed for their sensing capacity and survivability via surface mounting and interior embedment in the carbon/epoxy beams. Artificial delaminations of two different sizes were used to simulate delamination. The embedded EFPI-SSs along with surface-mounted SGs were used to evaluate the effect of the delaminations on bending stiffness induced by quasi-static loading on a comparative basis. The linear responses of EFPI-SSs up to a bending strain of 0.5% were in good agreement with the interpolated strains of surface strain measurements. The embedded EFPI-SSs were able to differentiate the effect of the tensile delaminations on bending stiffness. The interior strain data from embedded EFPI-SSs were shown to be very useful to extrapolate the strain difference on both sides of the delaminations as well as deducing local stresses.

Dual fiber-optic Fabry-Perot interferometer strain sensor with low-cost light-emitting diode light source

A prototype strain sensor based on a dual fiber-optic Fabry- Perot interferometer FFPI was successfully implemented experimen- tally. The light source was a low-cost communication light-emitting diode, replacing the expensive laser diode. No costly Faraday isolator was re- quired in this sensing system. The dynamic strain modulation in the sensing part of this setup was performed using a piezoelectric trans- ducer. The interesting dynamic strain signal could be extracted at the optical receiver by optical signal demodulation in the reference FFPI. The temperature variation of the sensing part was simultaneously de-

Composite-cavity-based Fabry-Perot interferometric strain sensors

A composite-cavity-based Fabry-Perot interferometric strain sensor system is proposed to gain the minimum cross sensitivity to temperature and a high multiplexing capability at the same time. The interrogation of the sensor system is based on a white-light interferometric technology, and the demodulation is achieved by analyzing the coherence spectra. A demonstration system with two sensors is presented and tested.

Demodulation of Fiber-Optic Sensors for Frequency Response Measurement

The neural-network-based processing of extrinsic Fabry-Perot interferometric (EFPI) strain sensors was investigated for the special case of sinusoidal strain. The application area is modal or cyclic testing of structures in which the frequency response to periodic actuation must be demodulated. The nonlinear modulation characteristic of EFPI sensors produces well-defined harmonics of the actuation frequency. Relationships between peak strain and harmonic content were analyzed theoretically. A two-stage demodulator was implemented with a Fourier series neural network to separate the harmonic components of an EFPI signal and a backpropagation neural network to predict the peak-to-peak strain from the harmonics. The system performance was tested using theoretical and experimental data. The error for high-strain cases was less than about 10% if at least 12 harmonics were used. The frequency response of an instrumented cantilever beam provided the experimental data. The demodulator processing closely matched the actual strain levels

Demodulation of extrinsic Fabry-Pe´rot interferometric sensors for vibration testing using neural networks

Strain level measurement on a periodically actuated and instrumented structure can provide information about the health of that structure. A simple demodulation system employing artificial neural networks (ANNs) is analyzed for an extrinsic Fabry-Perot interferometric (EFPI) strain sensor. The harmonic content of the nonlinear sensor output for the sinusoidal strain case is used to predict the maximum strain level. Implementations of the demodulation system are demonstrated for both simulated and experimental data using back-propagation neural networks. The simulation uses the theoretical response of the EFPI sensor and the experiment uses an EFPI-instrumented smart composite beam to obtain training and testing data. Excitation is provided by a piezoelectric actuator operating from 50 Hz to 1 kHz. System performance is compared for two-stage and single-stage networks and for differing types of data preprocessing. The ANN systems successfully extract the signal harmonics and predict the peak strain levels. Data preprocessing using principal component analysis produces the best accuracy. The architecture of an EFPI-based health monitoring system is proposed.

Digital and analog readout systems for fiber-optic strain sensors as applied to the monitoring of roller element bearing systems.

A rotating machinery test rig was instrumented with fiber Fabry-Perot interferometer strain sensors for condition monitoring of rolling element bearings. Strain variations produced by ball passes were observed and analyzed in the time and frequency domain. Wavelength division multiplexing was utilized to simultaneously monitor the sensors with analog and digital readout systems--analog for high bandwidth and digital for high dynamic range and the monitoring of multiple sensors. The effects of imbalance on the shaft, changes in rotational speed, effects on the rotor system, and detection of bearing defects were investigated. Frequency peaks observed in the bearing sensor spectra closely matched predicted values. Imbalance and rotational speed tests showed good agreement with expected trends, and bearing defects were successfully detected.

Single-crystal sapphire fiber-based strain sensor for high-temperature applications

Single-crystal sapphire fibers have a very high melting point (up to 2050°C), which renders them a very good candidate for sensing applications at a very high temperature. We present in this paper the recent work of developing single-crystal sapphire fiber extrinsic Fabry-Perot interferometric strain sensors based on the white-light interferometric spectrum demodulation technique. Prototype sapphire strain sensors were fabricated and tested at high temperatures up to 1004°C. The preliminary experimental results indicate that the sensors are promising to be used under high-temperature environments for making strain measurements with strain measurement resolution of 0.2-μ strain.

Characterization of Fiber Optic Sensors for Structural Health Monitoring

This paper presents experimental results on thermomechanical behavior of Extrinsic Fabry-Perot Interferometric fiber optic strain sensors (EFPI-FOSS). The objective of this study was to determine the accuracy, strength characteristics, and durability properties of both bare (nonembedded) EFPI sensors, and embedded optical fiber sensors in either a neat resin or aerospace grade composite laminate. Experimental results suggest that the embedded EFPI sensors provide reliable strain measurements for values exceeding 10,000 μ∊ under static loading conditions. A major portion of this study focused on evaluating the long term tension–tension fatigue behavior of optical fiber sensors. Test data suggest the EFPI sensors provide reliable data up to 1 million cycles at fatigue strain levels below 3,000 μ∊. For fatigue strain levels above this value, failure of the fiber optic sensor was observed. While the sensor failed it did not influence the strength and fatigue life of the composite coupons. Considering the design strains used in aerospace components, these results provide evidence that the EFPI sensors will survive during the life of typical aerospace structures.

Fiber-optic extrinsic Fabry-Perot interferometer strain sensor with <50 pm displacement resolution using three-wavelength digital phase demodulation

A fiber-optic extrinsic Fabry-Perot interferometer strain sensor (EFPI-S) of ls = 2.5 cm sensor length using three-wavelength digital phase demodulation is demonstrated to exhibit <50 pm displacement resolution (<2nm/m strain resolution) when measuring the cross expansion of a PZT-ceramic plate. The sensing (single-mode downlead-) and reflecting fibers are fused into a 150/360 microm capillary fiber where the fusion points define the sensor length. Readout is performed using an improved version of the previously described three-wavelength digital phase demodulation method employing an arctan-phase stepping algorithm. In the resent experiments the strain sensitivity was varied via the mapping of the arctan - lookup table to the 16-Bit DA-converter range from 188.25 k /V (6 Volt range 1130 k ) to 11.7 k /Volt (range 70 k ).

In situ cure monitoring of composite materials in autoclave molding

A real-time measurement of internal and residual strains in GFRP laminates with an extrinsic Fabry-Perot interferometric (EFPI) optical fiber strain sensor is performed in an autoclave molding. Another piezoceramic material is presented as a sensor for monitoring of resin curing. It is made of crystalline lead zirconate titanates (PZT) thin films synthesized by hydro-thermal method on titanium substrate. It is called PZT-S/A. Each sensor is embedded into pre-preg sheets before the molding. The ion viscosity and the temperature of the epoxy resin matrix are also simultaneously measured with a dielectric sensor and a thermocouple, respectively. The results of the experiments show that the internal and residual strains of GFRP laminates during the molding can be monitored by the optical fiber sensor. It is also shown that PZT-S/A is available for monitoring the curing state of the resin till the gelation point.

Prediction of Impact Contact Forces of Composite Plates Using Fiber Optic Sensors and Neural Networks

Real-time determination of contact forces due to impact on composite plates is necessary for on-line impact damage detection and identification. We demonstrate the use of fiber optic strain sensor data as inputs to a neural network to obtain contact force history. An experimental and theoretical study is conducted to determine the in-plane strains of a clamped graphite/epoxy composite plate upon low-velocity impacts using surface-mounted extrinsic Fabry-Perot interferometric (EFPI) strain sensors. The plate is impacted with a semispherical impactor with various impact energies using the drop-weight technique. The significant features of the strain and contact force response are contact duration, peak strain, and strain rise time. We have designed and built an instrumented drop-weight impact tower to facilitate the measurement of contact force during an impact event. The impact head assembly incorporates a load cell to measure the contact forces experimentally. An in-house finite-element program is used to establish the validity of the EFPI fiber optic sensor contact force response. The finite-element model is based on a higher-order shear deformation theory and accounts for geometric nonlinearity. Experimental load cell data and finite-element impact-induced contact force responses are in close agreement. The load cell data is used to train a three-layer feed-forward neural network which utilizes the Delta Bar Delta back-propagation algorithm. The output of the neural network simulation is the contact force history and the inputs are fiber optic sensor data in two different locations and time in 10-ms intervals. The efficiency and accuracy of the neural network method is discussed. The neural network scheme recovers the impact contact forces without using any complex signal processing techniques.