Fiber Optic Strain Sensors Research Articles

High-Resolution Optical Fiber Strain Sensor Array Based on Pound-Drever-Hall Technique With Fast Phase Regulation Method

Sensors that use Pound-Drever-Hall (PDH) technology benefit from the good performance and high resolution but are in shortage of large-scale multiplexing. This paper proposed a fast phase regulation method for the hybrid PDH multiplexing scheme based on space division multiplexing and time division multiplexing. Using both the information provided by an I/Q demodulator, the points from the error signal that the close-loop control requires can be rebuilt in the software with a phase rotation algorithm within a time period of less than 20 us. The relationship between the multiplexing scale and parameters selection is also given in this paper, showing that the system can have a larger multiplexing scale up to several tens sensors with appropriate parameters selection. A multi-channel strain array of up to 10 channels with a resolution better than 10 pϵ/√Hz@1 Hz over 50 Hz band per channel has been realized in this paper, and the system works with good channel crosstalk isolation. The experiment results show that the developed instrument has potential usage in the field of large-scale quasi-static strain measurement and earthquake observation.

Fiber optic temperature and strain sensor using dual Mach-Zehnder interferometers.

A fiber optic temperature and strain sensor using dual Mach-Zehnder interferometers (MZIs) is proposed. The dual MZIs were fabricated by fusion splicing of two different fibers between two single-mode fibers. The two fibers of thin-core fiber and small-cladding polarization maintaining fiber were fusion spliced with a core offset. As the responses of the two MZIs are different in terms of temperature and strain, simultaneous temperature and strain measurement were experimentally validated by selecting two resonant dips in the transmission spectrum to construct a matrix. Experimental results show that the proposed sensors had the maximum temperature sensitivity of 66.67pm/°C and the maximum strain sensitivity of -2.0p m/µε. The minimum discriminated temperature and strain of the two proposed sensors were 0.20°C and 0.71µε, and 0.33°C and 0.69µε, respectively. The proposed sensor has promising application prospects due to the merits of ease of fabrication, low costs, and good resolution.

Assembly-free ultra-sensitive miniaturized strain sensor based on an asymmetric optical fiber taper

This research reports an ultra-sensitive miniaturized optical fiber strain sensor based on a one-step assembly-free structure. A 10 µm thin asymmetrical taper waist of a single mode fiber (SMF) was designed and fabricated to form a Michelson Interferometer (MI). The sensor has been experimentally demonstrated for strain measurements with long-term stability. The sensor provides a ∼14 dB fringe visibility with a high extinction ratio in the reflection spectrum. Experimental results indicate that the sensor exhibits maximum strain sensitivity of −39.77 pm/µε in the measurement range of 0 to 1600µε. The sensor shows promising results in terms of stability and repeatability. The proposed sensor requires only one-step fabrication with great reproducibility and, therefore, can be easily mass-produced with a less stringent requirement in fabrication equipment. A highly sensitive response for measuring axial strain makes the sensor a viable contender for a number of specialized niche applications.

Optical fiber strain sensor with high precision and extended dynamic range based on a coupled optoelectronic oscillator.

We have proposed and experimentally demonstrated an optical fiber strain sensor with high precision and extended dynamic range based on a coupled optoelectronic oscillator (COEO). The COEO is a combination of an OEO and a mode-locked laser, sharing one optoelectronic modulator. The feedback between the two active loops makes the oscillation frequency equal to the mode spacing of the laser. It is equivalent to a multiple of the natural mode spacing of the laser, which is affected by the applied axial strain to the cavity. Therefore, we can evaluate the strain by measuring the oscillation frequency shift. Higher sensitivity can be obtained by adopting higher frequency order harmonics owing to the accumulative effect. We carry out a proof-to-concept experiment. The dynamic range can reach 10000 μ ε. Sensitivities of 6.5 Hz/μ ε for 960 MHz and 13.8 Hz/μ ε for 2700 MHz are obtained. The maximum frequency drifts of the COEO in 90 mins are within ±148.03 Hz for 960 MHz and ±303.907 Hz for 2700 MHz, which correspond to measurement errors of ±22 μ ε and ±20 μ ε. The proposed scheme has the advantages of high precision and high speed. The COEO can generate an optical pulse whose pulse period is influenced by the strain. Therefore, the proposed scheme has potential applications in dynamic strain measurement.

Temperature-immune Fabry-Perot cavity sensor based on an opened hollow-core anti-resonant fiber.

A new approach to conquer the thermal phase drift of an optical fiber Fabry-Perot interferometer (FPI) sensor is proposed and experimentally demonstrated. By employing a hollow-core anti-resonant fiber (HC-ARF) and optimizing the fusion splicing (includes mode field adaptation) between the lead-in single-mode fiber (SMF) and the HC-ARF, a high spectral resolution (λ/Δλ ≈ 3.8 × 104) optical fiber air-cavity FPI sensor with a fringe visibility higher than 7 dB is constructed. To eliminate the thermal phase drift (i.e. temperature crosstalk) of the sensor that originates from the intrinsic thermal expansion effect of the silica material of the HC-ARF, the FPI air cavity is connected to the external environments, by which the effect of air expelling from the cavity with temperature increasing can well compensate the temperature-induced cavity elongation. As a result, the thermal phase drift of the FPI is reduced to zero at a temperature range of ∼ 80-110 °C and within the temperature range of 40-80 °C, the thermal phase drift is still halved compared with the sealed FPI cavity. The nearly zero thermal phase drift of a FPI at such a temperature range has never been achieved before, to our best knowledge. As a proof of concept, a temperature-immune fiber-optic strain sensor is demonstrated. This work offers a new and efficient approach to eliminate the thermal phase drift (i.e. temperature crosstalk) of a fiber-optic device, which may significantly improve the measurement accuracy and detection limit of fiber-optic FPI sensors. Furthermore, the principle and schema can be generalized to a wide variety of fiber-optic devices.

Influence of thermal coupling on the temperature compensation of fiber optic strain sensors

Abstract Fiber Bragg Grating (FBG) thermometers are commonly used for temperature compensation of FBG strain sensors. It is shown theoretically and experimentally that the thermal coupling between the underlying substrate and the respective sensors causes a delayed response to temperature changes. This can be explained by the heat capacity of the underlying substrate causing a temperature gradient between the surrounding and the substrate. In this contribution, we apply our theoretical result to achieve an improved accuracy in the strain measurement by an approximate factor of up to 3.5. In addition, experiments are carried out in which the thermal coupling in the system is adjusted by using thermal conduction paste. The results also show significantly improved temperature compensation.

Experimental investigation of surface-mounted optical fibre strain sensor using neural network analysis

Experimental investigation of surface-mounted optical fibre strain sensor using neural network analysis

Distributed Optical Fiber Strain Sensors Packaged with Polyurethane and GFRP

To apply optical fiber sensors in an extreme environment and fulfill the rapid curing requirement of the thermosetting packaging process on glass fiber reinforced polymers (GFRP) prepreg, a novel fabricating method is proposed in this paper. It packages optical fiber by pouring and brushing the waterborne polyurethane into dry weave GFRP clothes that have bare optical fibers on one side. A special mold with a slot for laying and fixing package materials is designed. The sample with the size of 20 mm × 80 mm × 1 mm is made by adopting the developed mold. The GFRP side of the sample was pasted on the carbon fiber beam with a constant section, and a static loading test was conducted. The strain of the cantilever beam was collected by the packaged distributed optical fiber sensor and demodulated by the OFDR system. According to the Ko theory, the measured strain was converted into the displacement of the cantilever beam. The displacement of the free end of the cantilever beam was compared with the position measured by the machine vision, and the error between the two was small. The experimental results show that the ultra-thin distributed optical fiber strain sensor obtained by rapid packaging can be used for the deformation monitoring of large structures such as wind turbine blades.

Sagnac interferometer-based optical fiber strain sensor with exceeding free spectral measurement range and high sensitivity

Sagnac interferometer-based sensors have shown high sensitivity, but limited measurement range due to the free spectral range (FSR) limitation especially the higher the sensitivity. In this paper, we designed an optical fiber Sagnac interferometer with a polarization maintaining fiber (PMF) and a fiber Bragg grating (FBG) in the loop. The strain sensing performance was experimentally conducted. The PMF in the Sagnac interferometer showed a high strain measurement sensitivity with a limited measurement range due to the FSR. On the contrary, FBG in the Sagnac interferometer showed a low strain sensitivity but a large measurement range. By combining the PMF and FBG in a Sagnac interferometer, a high strain sensitivity of about 17.08 pm/µε with a large measurement range of 0–5187 µε was experimentally achieved. The method used in this paper can also be utilized to expand the sensing range in other interferometers. The proposed PMF and FBG constructed Sagnac interferometer shows the merits of simple structure, high sensitivity, and large measurement range, which make it a competitive candidate in strain monitoring.

Study of a Fiber Optic Fabry-Perot Strain Sensor for Fuel Assembly Strain Detection.

This paper proposes a fiber optic Fabry-Perot (F-P) strain sensing system using non-scan correlation demodulation applied to the health monitoring of the pressurized water reactor's fuel assembly structures. The structural design and sensing mechanism analysis of the sensor were carried out, and the strain transfer model from the fuel sheet to the strain gauge was established. After the sensor fabrication and installation, the static tests have been conducted, and the results indicate that the sensing system can accurately measure the microstrain with a sensitivity of up to 12.6 nm/με at a high temperature (300 °C). The dynamic testing shows that the sensing system has a good frequency adaptation at 10-500 Hz. Thermal-hydraulic experiments show that the sensing system can run stably in a nuclear reactor, with high temperature, high pressure, and high-velocity flow flushing; additionally, the consistency deviation of the measured data is less than 1.5%.

Strain optical fiber sensor with modified sensitivity based on the vernier effect

An optical fiber strain sensor with adjustable sensitivity is proposed and demonstrated. The strain sensing setup employs a fiber optic Michelson interferometer (sensing element) and an extrinsic Fabry–Perot interferometer, in which both structures excited the Vernier effect. By adjusting the cavity length of the extrinsic Fabry–Perot interferometer, it is possible to control the M-factor. The set-up offers both sensitivity enhancement and magnification mode, where the maximal sensitivity is 4.38 pm/με (M-factor = 2.3). This design improves the sensor performance and its implementation is not intricate.

Time-dependent structural response estimation method for concrete structures using time information and convolutional neural networks

In this study, a structural response estimation method for evaluating the long-term strain, which can serve as the basis of assessing structural safety of concrete structures, is presented. Characteristics of short-term deformation caused by environmental influences and long-term inelastic deformation caused by combination of rheological effects and environmental influences are identified. To reflect these time-dependent short- and long-term deformation characteristics in the proposed method, the strain and corresponding time information collected from a structural health monitoring (SHM) are used. In the proposed method, the relationship between a strain response measured from a concrete structure and the corresponding time information including the year, month, day, and hour is defined by a convolutional neural network (CNN), which is a deep learning technique. The method assumes that the main sources of the strain in the structure are environmental influences, such as temperature and humidity, which have daily and seasonal periodicity, and rheological effects in concrete, such as creep and shrinkage, but it also assumes that the quantitative information on these sources is not available (e.g., environmental temperature and humidity, and rheological strain were not reliably measured). Thus, the CNN method developed in this study is trained only with strain and time data collected over several years, and used to estimate strain values within a specific timeframe, e.g., when the safety evaluation of the concrete structure is required or in case of SHM system failure or data loss. The presented method was developed and validated using SHM data from a real structure instrumented with fiber optic strain sensors. In addition, exploration of the constitution of the input of the CNN identified the type of time information that is the most effective in the long-term strain estimation of the developed method.

Reflection-Based Optical Fiber Strain Sensor Using Polarization Maintaining and Thin Core Fibers

A strain fiber optic sensor is proposed and experimentally demonstrated in this letter. The sensor operates in reflection mode, and its assembly relies on a single splice point between a thin core fiber and polarization-maintaining fiber. The system offers competitive sensitivities of 14.68 and 6.9 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> , determined by the PM-fiber length. In addition, the one-way ANOVA analysis indicates a minimal error (MSE=0.34) and low probability of measurement overlap ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text {Prob}&gt;\text {F}= 5.59882\times 10^{-85}$ </tex-math></inline-formula> ). The fabrication, the sensor dimensions, and the statistical analysis make the proposed sensor an attractive alternative for monitoring strain.

Improving the measurement range of FFPI strain sensing using second-order control PDH technology.

A novel demodulation method is presented to expand the measurement dynamic range for fiber optic strain sensors using PDH technology. The new control algorithm uses two integrators to form a 2nd order control, and the FFPI strain sensor can have a dynamic range of 20 dB/octave larger than the PID control method when the input signal frequency decreases a magnitude. A strain resolution of 4.7 pɛ Hz-1/2@10Hz, a 118 dB@10Hz dynamic range without consecution, and 158 dB with consecution is obtained. The experiment results show that the new control method can improve the sensing system's dynamic range with the corner frequency unchanged and without the system noise level degradation.

Experimental study on uplift mechanism of pipeline buried in sand using high-resolution fiber optic strain sensing nerves

Reliable assessment of uplift capacity of buried pipelines against upheaval buckling requires a valid failure mechanism and a reliable real-time monitoring technique. This paper presents a sensing solution for evaluating uplift capacity of pipelines buried in sand using fiber optic strain sensing (FOSS) nerves. Upward pipe-soil interaction (PSI) was investigated through a series of scaled tests, in which the FOSS and image analysis techniques were used to capture the failure patterns. The published prediction models were evaluated and modified according to observations in the present study as well as a database of 41 pipe loading tests assembled from the literature. Axial strain measurements of FOSS nerves horizontally installed above the pipeline were correlated with the failure behavior of the overlying soil. The test results indicate that the previous analytical models could be further improved regarding their estimations in the failure geometry and mobilization distance at the peak uplift resistance. For typical slip plane failure forms, inclined shear bands star from the pipe shoulder, instead of the springline, and have not yet reached the ground surface at the peak resistance. The vertical inclination of curved shear bands decreases with increasing uplift displacements at the post-peak periods. At large displacements, the upward movement is confined to the deeper ground, and the slip plane failure progressively changes to the flow-around. The feasibility of FOSS in pipe uplift resistance prediction was validated through the comparison with image analyses. In addition, the shear band locations can be identified using fiber optic strain measurements. Finally, the advantages and limits of the FOSS system are discussed in terms of different levels in upward PSI assessment, including failure identification, location, and quantification.

Discussion on pile axial load test methods and their applicability in cold regions

The measurement of pile axial load is of great significance to determining pile foundation design parameters such as skin friction and end bearing capacity and analyzing load transfer mechanisms. Affected by the temperature and ice content of frozen ground, the interface contact relationship between pile foundation and frozen soil is complicated, making pile axial load measurements more uncertain than that in non-frozen ground. Therefore, it is necessary to gain an in-depth understanding of the current pile axial load test methods. Four methods are systematically reviewed: vibrating wire sensors, strain gauges, sliding micrometers, and optical fiber strain sensors. At the same time, the applicability of the four test methods in frozen soil regions is discussed in detail. The first two methods are mature and commonly used. The sliding micrometer is only suitable for short-term measurement. While the Fiber Bragg grating (FBG) strain gauge meets the monitoring requirements, the Brillouin optical time-domain reflectometer (BOTDR) needs further verification. This paper aims to provide a technical reference for selecting and applying different methods in the pile axial load test for the stability study and bearing capacity assessment of pile foundations in cold regions.

Generalized Cross-Correlation Strain Demodulation Method Based on Local Similar Spectral Scanning.

Optical fiber measurement technology is widely used in the strength testing of buildings, the health testing of industrial equipment, and the minimally invasive surgery of modern medical treatment due to its characteristics of free calibration, high precision, and small size. This paper presents an algorithm that can improve the range and stability of strain measurements in order to solve the problems of the small range and measurement failure of optical fiber strain sensors based on optical frequency-domain reflectometry (OFDR). Firstly, a Rayleigh scattering model based on the refractive index perturbation of an optical fiber is proposed to study the characteristics of Rayleigh scattering and to guide the strain demodulation algorithm based on the spectral shift. Secondly, a local similar scanning method that can maintain a high similarity by monitoring local Rayleigh scattering signals (LSs) before and after strain is proposed. Thirdly, a generalized cross-correlation algorithm is proposed to detect spectral offset, solving the problem of demodulation failure in the case of a Rayleigh scattering signal with a low signal-to-noise ratio. Experiments show that the proposed method still has high stability when the spatial resolution is 3 mm. The measurement precision is 6.2 με, which proves that the multi-peaks or pseudo-peaks of the traditional algorithm in the case of a large strain, the high spatial resolution, and the poor signal-to-noise ratio are solved, and the stability of the strain measurement process is improved.

Femto-strain resolution fiber-optic sensing with an ultra-simple white-light round trip filtering method.

In the past decade, laser-driven resonant fiber-optic sensors (RFOSs) have been reported touching their ultimate resolution limit. The practicability of these high-performance sensors is, however, discounted because of high system complexity and dependence on narrow-linewidth lasers. In this paper, a novel, to the best of our knowledge, white-light-driven RFOS is established based on a round trip filtering (RTF) method. Via measuring the RTF loss of an add-drop fiber ring resonator (FRR) sensor, strain signal can be read out with an ultra-simple open-loop configuration. In the sensing experiment, even a resolution of several femto-strain around 1 kHz is demonstrated, representing the highest resolution level of RFOS to date. Thanks to the obvious superiority in both resolution, simplicity, and cost over traditional laser-driven RFOSs, the proposed white-light-driven RFOS is believed to be a milestone in the development of fiber-optic strain sensors.

Study on strain sensing property of fiber Bragg grating based on surface strain measurement of mechanical structure.

Strain monitoring is of great significance to identify the failure of key mechanical components and ensure the good operation of mechanical equipment. In terms of the common issue of the low sensitivity of fiber Bragg grating (FBG) strain sensor in strain measurement on the mechanical structure surface, this paper describes a flexible hinge strain sensor with the FBG as the sensitive element. A theoretical analysis of the sensitivity of the sensor is performed, a copper alloy material with a large elastic modulus is selected, the simulation analysis and optimization design for the structural parameters of the sensor are carried out by using ANSYS and Solidworks software, then the object of the sensor is developed according to the simulation results, and finally, the performance of the sensor is tested. The results show that the maximum deformation the sensor can measure is 0.5299 µε, the sensitivity of the sensor is ∼1.8870 pm/µε, which is approximately twice that of the bare FBG sensor, the linearity is greater than 99%, and the relative standard deviation of the repeatability is 0.9%. The results of the study provide a reference for developing the optical fiber strain sensor and further improving its sensitivity.

A strain reflection-based fiber optic sensor using thin core and standard single-mode fibers

We propose and demonstrate a fiber optic strain sensor based on a simple splice between a thin core fiber and a piece of conventional single-mode fiber. Mode dispersion generates an interference reflection spectrum sensitive to strain and temperature. The sensor offers a competitive strain sensitivity of 2.4 pm/μɛ, good linearity (R2=0.9969), and a cursory repeatability analysis yielded variation. The thermal studies indicate minimal cross-sensitivity (2.9 μɛ/°C). The sensor offers a simple implementation without any intricate manufacturing.