Interferometric Strain Sensor Research Articles

Low-temperature cross-sensitivity strain sensor based on a microbubble Fabry-Pérot interferometer with a thin wall

A Fabry-Pérot interferometer (FPI) strain sensor with low-temperature cross-sensitivity is prepared by a simple, cost-effective, and safe process. The sensor is fabricated by splicing two segments of the single-mode fiber and several arc discharges to form a unique circular bubble and a thin wall. The thin air-bubble wall deforms more easily under stress due to the good stress concentration ability of the tapered structure, resulting in a high strain sensitivity of 10.78 pm/µε. The temperature sensitivity of the sensor is 1.24 pm/°C and the temperature cross-sensitivity is 0.115 με/°C in the temperature range of 25 to 200 °C, which reduces the temperature-induced measurement error. The experimental results show that the sensor with good stability and repeatability has large commercial potential in measuring strain in complex environments.

High-sensitivity interferometric high-temperature strain sensor based on optical harmonic vernier effect

We present a fiber optic vernier harmonic sensor for simultaneous detection of temperature and strain in high temperature conditions based on a parallel dual fiber Fabry-Perot interferometers (FPI). Two air-cavity FPIs were assembled from SMF, MMF and a hollow silica tube (HST). A fiber Bragg grating (FBG) is inscribed in one of the arms of the Fabry-Perot interferometers (FPIs) for temperature compensation. Based on the fiber optic vernier harmonic effect, the strain sensitivity of the sensor is further increased to 178.75 pm/µε, with a good linear response to strain and temperature in the range of 24 ℃-900 ℃. And the fabrication is straightforward and economy. It is a potential tool for micro strain measurements under extreme high temperature conditions.

Ultra-High Sensitivity and Temperature-Insensitive Optical Fiber Strain Sensor Based on Dual Air Cavities.

This study proposed an all-fiber Fabry-Perot interferometer (FPI) strain sensor with two miniature bubble cavities. The device was fabricated by writing two axial, mutually close short-line structures via femtosecond laser pulse illumination to induce a refractive index modified area in the core of a single-mode fiber (SMF). Subsequently, the gap between the two short lines was discharged with a fusion splicer, resulting in the formation of two adjacent bubbles simultaneously in a standard SMF. When measured directly, the strain sensitivity of dual air cavities is 2.4 pm/με, the same as that of a single bubble. The measurement range for a single bubble is 802.14 µε, while the measurement range for a double bubble is 1734.15 µε. Analysis of the envelope shows that the device possesses a strain sensitivity of up to 32.3 pm/με, which is 13.5 times higher than that of a single air cavity. Moreover, with a maximum temperature sensitivity of only 0.91 pm/°C, the temperature cross sensitivity could be neglected. As the device is based on the internal structure inside the optical fiber, its robustness could be guarantee. The device is simple to prepare, highly sensitive, and has wide application prospects in the field of strain measurement.

A Fabry–Pérot interferometer strain sensor composed of a rounded rectangular air cavity with a thin wall for high sensitivity and interference contrast

A Fabry–Pérot interferometer (FPI) strain sensor with high sensitivity and interference contrast is designed and fabricated by a simple, cost effective, and safe process which can be scaled up for commercial production. The excellent strain sensitivity of 8 pm/μɛ and interference contrast of 25 dB stem from the special structure with the unique air cavity, small wall thickness, and excellent reflection surface of the air cavity. The strain-temperature cross-sensitivity of the sensor is only 0.59 μɛ/°C in the temperature range of 20–80°C. The sensor has large potential in applications such as strain sensing.

High Sensitivity Fiber Interferometric Strain Sensors Based on Elongated Fiber Abrupt Tapers.

We demonstrate high-sensitivity fiber strain sensors based on an elongated abrupt taper. The fiber abrupt taper, with a tapered diameter ranging from 40–60 μm, was made by using a hydrogen microflame to break the waveguide adiabaticity so as to convert the fundamental mode into cladding modes. The abrupt taper was further uniformly tapered by using a normal moving flame with a torch diameter of 7 mm to elongate the tapered region until the tapered diameter was down to 2.5–5 μm. The excited high-order modes were confined to propagate along the cladding and then recombined at the rear edge of the fiber taper to produce interferences with extinction ratios of up to 16 dB. The tapered region was pulled outwardly to change the optical path difference (OPD) between modes to measure the tensile strain with all the interfering wavelengths blue-shifted. The measured best strain sensitivity was 116.21 pm/με and the coefficient of determination R2 of linear fitting exhibits high linearity. This strain sensor based on elongated abrupt taper is several times higher than that of most of the fiber strain sensors ever reported.

Estimation of blade forces in wind turbines using blade root strain measurements with OpenFAST verification

This paper introduces an inference method for computing the forces and bending moments on operating wind turbine blades using strain measurements and supervisory control and data acquisition (SCADA) data. Operational data from four months of a Clipper Liberty C96 2.5 MW turbine instrumented with interferometric strain sensors at the blade roots as well as SCADA data such as wind speed, rotor hub speed, and blade pitch angle allow for accurate calculation of blade forces and moments. To perform such calculations, certain structural properties of the turbine blades must be inferred in the absence of detailed, proprietary information. This is done by inferring missing information from the National Renewable Energy Laboratory (NREL) 3 MW WindPACT reference wind turbine specifications. The derived forces and moments computed on the blades of the Clipper turbine are compared with the behavior of the NREL 3 MW reference turbine according to OpenFAST simulation outputs. Comparison of blade root reaction forces to OpenFAST outputs match closely, demonstrating that this inference method can be used to successfully estimate the internal forces and bending moments acting on the blades. These methods are useful on turbines for which all the structural information is not available.

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.

High sensitivity strain sensor based enhanced polarized mode coupling and FBG sensitization

A highly sensitive strain sensor based on enhanced polarized mode coupling is proposed and demonstrated experimentally. The proposed Sagnac interferometric strain sensor comprises a segment of Panda-type polarization maintaining fiber (PMF) serving as a sensing component and a commercial fiber Bragg gratings (FBG) serving as a sensitizing component. The sensitization effect of FBG has been analyzed and demonstrated experimentally. The core of single mode fiber is intentionally collimated and fusion spliced to one stress applied zone built-in Panda type PMF to excite effectively the polarized mode. The spectra shift towards long wavelength with increasing strain. The measured strain sensitivity reaches up to 176 pm/με. The proposed fiber strain sensor characterizing simple fabrication and high sensitivity can be a good candidate in the field of strain measurements.

High sensitivity Sagnac interferometric strain sensor based polarization maintaining fibre enhanced coupling

A high sensitivity, simple fabrication all-fibre strain sensor is proposed and demonstrated experimentally. The sensor consists of a section of Panda-type polarization maintaining fibre (PMF) built into a Sagnac interferometer. The Panda-type PMF is intentionally spliced to the single-mode fibre through enhanced fusion splicing to excite high-order polarized modes. The enhanced fusion splicing characterizes a large magnitude of overlap. The measured strain sensitivity reaches up to 32 pm/με and the strain resolution of 0.63 με has been achieved. The proposed fibre strain sensor characterizing easy fabrication and high sensitivity can be a good candidate in the field of structural health monitoring.

Estimation of Blade Forces in Wind Turbines Using Blade Root Strain Measurements with Openfast Verification

This paper introduces an inference method for computing the forces and bending moments on operating wind turbine blades using strain measurements and supervisory control and data acquisition (SCADA) data and in the absence of comprehensive design specifications. Operational data from four months of a Clipper Liberty C96 2.5 MW turbine instrumented with interferometric strain sensors at the blade roots as well as SCADA data such as wind speed, rotor hub speed, and blade pitch angle allow for accurate calculation of blade forces and moments. To perform such calculations, certain structural properties of the turbine blades must be inferred in the absence of detailed, proprietary information. This is done by inferring missing information from the National Renewable Energy Laboratory (NREL) 3 MW WindPACT reference wind turbine specifications. The derived forces and moments computed on the blades of the Clipper turbine are compared to the behavior of the NREL 3 MW reference turbine according to OpenFAST simulation outputs. Comparison of blade root reaction forces to OpenFAST outputs match closely, demonstrating that this inference method can be used to successfully estimate the internal forces and bending moments acting on the blades.

Temperature self-compensation strain sensor based on cascaded concave-lens-like long-period fiber gratings.

A robust, novel, to the best of our knowledge, fiber Mach-Zehnder interferometer strain sensor is designed and experimentally implemented. The sensor consists of two concave-lens-like long-period fiber gratings and is engraved by the high frequency ${{\rm CO}_2}$CO2 laser. The concave-lens-like grids can excite higher-order cladding modes to interfere with the fundamental mode, which increases the light-material contact. The excellent interference spectra are obtained and analyzed theoretically. The experimental results show that the strain sensitivity of the sensor can reach 0.011 dB/µ$\unicode{x03B5}$ε in the range of 0-2160 µ$\unicode{x03B5}$ε. The resolution of the sensor is up to 0.91 µ$\unicode{x03B5}$ε. Moreover, the temperature crosstalk can be self-compensated by monitoring a pair of split interfering dips. These outstanding characteristics make it very suitable as a candidate for strain measurement.

Sensitivity-Enhanced Extrinsic Fabry-Perot Interferometric Fiber-Optic Microcavity Strain Sensor.

This study presents an extrinsic Fabry–Perot interferometric (EFPI) fiber-optic strain sensor with a very short cavity. The sensor consists of two vertically cut standard single-mode fibers (SMFs) and a glass capillary with a length of several centimeters. The two SMFs penetrate into the glass capillary and are fixed at its two ends with the use of ultraviolet (UV) curable adhesives. Based on the use of the lengthy glass capillary sensitive element, the strain sensitivity can be greatly enhanced. Experiments showed that the microcavity EPFI strain sensor with initial cavity lengths of 20 μm, 30 μm, and 40 μm, and a capillary length of 40 mm, can yield respective cavity length–strain sensitivities of 15.928 nm/με, 25.281 nm/με, and 40.178 nm/με, while its linearity was very close to unity for strain measurements spanning a range in excess of 3500 με. Furthermore, the strain–temperature cross-sensitivity was extremely low.

A comparative study and experimental observations of optical fiber sagnac interferometric based strain sensor by using different fibers

Abstract Designing of strain monitoring system is a vibrant topic for the researchers. Strain monitoring systems are required in various fields such as mechanics, aeronautics, metallurgy and health monitoring of complex structures etc. Researchers designed different strain monitoring systems based on requirements. In this paper, we have reported a comparative study of Optical Fiber Sagnac Interferometer (OFSI) based strain sensors using three different types of fiber, i.e Polarization Maintaining Fiber (PMF), Standard Photonic Crystal Fiber (PCF) and High Birefringence Photonic Crystal Fiber (Hi-Bi PCF). The transmission spectrums of the sensors were recorded using Optical Spectrum Analyzer (OSA). We have experimentally analyzed the response and sensitivity of the sensors at different applied strains. The experimental results show that the optical interferometric strain sensor using PMF is having highest sensitivity in comparison to other sensors.

Experimental study on an interferometric strain sensor based on hollow-core photonic bandgap fiber for intrusion detection

In this paper, the design and preliminary performance results of a Mach–Zehnder interferometric (MZI) strain sensor based on hollow-core photonic bandgap fiber (HC-PBG) for intrusion detection are proposed. An experiment is conducted to test the system before deployment in the field. Firstly, to judge the necessity of using the HC-PBG, an experiment is performed to compare the normalized strain responsivity (NR) of the HC-1550 and the conventional SMF-28. By interchanging the investigated optical fibers as a sensing arm of the MZI, it is possible to compare the obtained results with a conventional standard silica fiber and the HC-PBG. The experimental results show that the NR of the HC-PBG is about 3.6 – 5.3 dB better than that of the SMF depending on the frequency in the frequency range 500 Hz to 2 kHz. Secondly, we concentrate our study and measurements on the HC-1550. A simulation of intrusion detection is introduced by using the proposed interferometric fiber-optic strain sensor to detect the dynamic strain induced phase shift as a result of varying the amplitude and frequency of the input signals to a PZT tube. The investigated input signal frequencies are from 500 Hz to 2 kHz. An experiment is performed to measure the induced strain from the PZT for different input voltages and frequencies, and using the phase generated carrier (PGC) demodulation technique for extracting the measured signal information. This technique is an effective way to test the performance and the functionality of the system in lab before the field trials. Experimental results show the importance of using HC-PBG for sensing applications and the ability of the system to effectively be used for borders monitoring and securing important facilities.

“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.

An Embeddable Strain Sensor with 30 Nano-Strain Resolution Based on Optical Interferometry

A cost-effective, robust and embeddable optical interferometric strain sensor with nanoscale strain resolution is presented in this paper. The sensor consists of an optical fiber, a quartz rod with one end coated with a thin gold layer, and two metal shells employed to transfer the strain and orient and protect the optical fiber and the quartz rod. The optical fiber endface, combining with the gold-coated surface, forms an extrinsic Fabry–Perot interferometer. The sensor was firstly calibrated, and the result showed that our prototype sensor could provide a measurement resolution of 30 nano-strain (nε) and a sensitivity of 10.01 µε/µm over a range of 1000 µε. After calibration of the sensor, the shrinkage strain of a cubic brick of mortar in real time during the drying process was monitored. The strain sensor was compared with a commercial linear variable displacement transducer, and the comparison results in four weeks demonstrated that our sensor had much higher measurement resolution and gained more detailed and useful information. Due to the advantages of the extremely simple, robust and cost-effective configuration, it is believed that the sensor is significantly beneficial to practical applications, especially for structural health monitoring.

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

Strain Sensing Comparison Between Single and Multimode Fibers using Optical Low Coherence Interferometry

Optical low coherence interferometry is one of the accurate optical sensing technologies and widely utilized in various physical sensing properties. The system principle is to characterize the relative interferogram movement distance caused by the various strain on a sensing arm. An interferometric strain sensor from two-stage Mach-Zehnder interferometer was demonstrated for double sensitivity improvement. The strain performance comparison between single and multimode fibers will be analyzed for fiber sensing applications. The stepper motor was set up with a movement distance of 20 nm in every step and the velocity could achieve 10000 step/s. The fiber strain was characterized as 22.22 μe on a 9-cm length. The experimental results demonstrated the multimode fiber sensitivity is higher than single mode fiber. Repeatability of both single and multimode are uncertain. The interferogram movement distance from the multimode fiber was higher than a single mode and demonstrates higher sensitivity

Temperature and index insensitive strain sensor based on a photonic crystal fiber in line Mach–Zehnder interferometer

Mach–Zehnder interferometer strain sensors made from pieces of photonic crystal fiber spliced to standard single mode fibers are fabricated with different lengths. High strain sensitivity of 2.1pm/με at 1550nm is achieved with the length of 45mm. The interference is induced by the core mode and the high order core mode due to the special air hole structure of PCF, which is independent of the surrounding refractive index. The temperature sensitivity (∼13.24pm/°C) is relatively low. This structure is good in avoiding cross sensitivity in strain measurement and is fabricated with a simple process and low cost.