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Highlights • FBG-based inclinometer for slope displacement monitoring is presented. • Relationship of deflections of FBG-based inclinometer versus strains was derived. • Effectiveness of FBG-based inclinometers for slope monitoring was verified. Abstract Landslide monitoring and forecasting have always been important research topics in geotechnical engineering. This study presents a self-designed inclinometer using fiber Bragg grating (FBG) sensing technology, which was used to obtain the slope internal displacement. It is well known that FBG sensors can be effectively utilized for strain measurement; however, it is difficult to directly use FBG strain sensors to measure the displacement. Therefore, based on the conjugate beam method, a mathematical equation to convert strains to lateral displacements is established, which has been verified by the finite element numerical model considering different beam lengths and two bending types. Furthermore, field monitoring suitability tests of an FBG-based inclinometer were carried out, which shows that the proposed deflection solution based on FBG strain sensors can accurately calculate the shear displacement of a slope. [ABSTRACT FROM AUTHOR]

Landslides and other geological disasters occur frequently and often cause high financial and humanitarian cost. The real-time, early-warning monitoring of landslides has important significance in reducing casualties and property losses. In this paper, by taking the high initial precision and high sensitivity advantage of FBG, an FBG strain sensor is designed combining FBGs with inclinometer. The sensor was regarded as a cantilever beam with one end fixed. According to the anisotropic material properties of the inclinometer, a theoretical formula between the FBG wavelength and the deflection of the sensor was established using the elastic mechanics principle. Accuracy of the formula established had been verified through laboratory calibration testing and model slope monitoring experiments. The displacement of landslide could be calculated by the established theoretical formula using the changing values of FBG central wavelength obtained by the demodulation instrument remotely. Results showed that the maximum error at different heights was 9.09%; the average of the maximum error was 6.35%, and its corresponding variance was 2.12; the minimum error was 4.18%; the average of the minimum error was 5.99%, and its corresponding variance was 0.50. The maximum error of the theoretical and the measured displacement decrease gradually, and the variance of the error also decreases gradually. This indicates that the theoretical results are more and more reliable. It also shows that the sensor and the theoretical formula established in this paper can be used for remote, real-time, high precision and early warning monitoring of the slope. [ABSTRACT FROM AUTHOR]

• A quasi-distributed fiber-optic displacement sensor (QDFODS) was presented. • The decrease of the light intensity with displacement variation was reported. • Laboratory experiments and field monitoring application of the QDFODS were conducted. In this paper a quasi-distributed fiber-optic displacement sensor (QDFODS) for landslide monitoring using an optical time domain reflectometer (OTDR) was demonstrated. The new sensor was connected in series by several fiber-optic displacement sensors (FODSs). Their configurations introduce power loss through the decrease of their fiber bowknot radius when displacement was applied. The decrease of the light intensity with displacement variation was reported. The proposed FODS had a maximum measurement distance and an effective initial measurement displacement of 36 mm and 0.98 mm. The authors performed the laboratory experiments, and the sliding damage of the rock mass with two-stage sliding was effectively predicted by the QDFODS. Further, a field monitoring application of the QDFODS was conducted to confirm the feasibility for determining the progressive deformation characteristics of the slope, such as the magnitude of movement and positions of the potential sliding surfaces. This capability demonstrates the promising applications of the QDFODS in the monitoring of civil engineering, especially slopes, foundation pits and tunnels. [ABSTRACT FROM AUTHOR]

• The development of fiber optic sensors in geotechnical engineering is fully reviewed. • Three algorithms for displacement calculation were discussed by numerical simulation. • A comparison analysis of optical fiber sensors for geotechnical monitoring is given. Prediction of displacement or strain is an important means and factor for evaluating the safety of geotechnical structures, such as slopes, dams, tunnels and excavation engineering. In recent years, fiber optic displacement sensors have been extensively used in civil engineering due to their obvious advantages of light weight, high precision, strong durability, wide measurement range and long-distance transmission. This paper reviews the development of two common types of fiber optic sensors (fiber Bragg grating sensors and bend loss based fiber optic sensors) for geotechnical health monitoring, and the characteristics and the state of the art research were analyzed and discussed in details. Based on the measured strains, three algorithms for transforming monitored data to required displacement were investigated. Comparison analysis regarding typical advantages and disadvantages of these fiber optic sensing technologies for geotechnical health monitoring was also presented and discussed in this paper. [ABSTRACT FROM AUTHOR]

A novel and simple fiber-optic sensor for measuring a large displacement range in civil engineering has been developed. The sensor incorporates an extremely simple bowknot bending modulation that increases its sensitivity in bending, light source and detector. In this paper, to better understand the working principle and improve the performance of the sensor, the transduction of displacement to light loss is described analytically by using the geometry of sensor and principle of optical fiber loss. Results of the calibration tests show a logarithmic function relationship between light loss and displacement with two calibrated parameters. The sensor has a response over a wide displacement range of 44.7 mm with an initial accuracy of 2.65 mm, while for a small displacement range of 34 mm it shows a more excellent accuracy of 0.98 mm. The direct shear tests for the six models with the same dimensions were conducted to investigate the application of the sensor for warning the shear and sliding failure in civil engineering materials or geo-materials. Results address that the sliding displacement of sliding body can be relatively accurately captured by the theory logarithmic relation between sliding distance and optical loss in a definite structure, having a large dynamic range of 22.32 mm with an accuracy of 0.99 mm, which suggests that the sensor has a promising prospect in monitoring civil engineering, especially for landslides. [ABSTRACT FROM AUTHOR]

• A simple macro-bending loss optical fiber crack sensor with a large displacement range was proposed. • The decrease of the light intensity with displacement variation was reported. • Verification experiments and application monitoring of the sensor were conducted. An optical fiber crack sensor based on macro-bending loss for detecting the crack opening displacement (COD) is developed in this paper. The sensor incorporates an extremely simple design, light source and detector. The decrease of the light intensity with displacement variation was reported. The verification experiments to the sensing principle were conducted. The sensor shows a response over a wide range of 212.1 and 87.9 mm in shrinkage and elongation, respectively. The response has two distinct nonlinear regions. A slab cracking model was established and random COD conditions were simulated. It is found that the predicted displacements of the sensor are nearly identical to the measured COD, and both relative errors are basically within 10%. Additionally, the sensor can be connected in series on one optical link to realized quasi-distributed monitoring; if crack shrinkage or elongation can be known in advance, the sensor by designed properly has a larger range of 300 mm. This optical fiber crack sensor is characterized by large measurement range, simple design, high flexibility and low economic cost, which shows a promising application in the field of displacement monitoring. [ABSTRACT FROM AUTHOR]

A self-referenced phase generation carrier (SR-PGC) algorithm is proposed in this paper. The influence related to modulation depth, light intensity and carrier phase delay can be theoretically eliminated. In classical PGC algorithm, the orthogonal components are obtained by mixing the detected signal with the proper multiples of carrier frequency and low-pass filtering, which are related to several parameters (modulation depth, light intensity, carrier phase delay, etc.). We notice that the differential signals of them also contain the parameters above. In the proposed SR-PGC algorithm, the orthogonal components are taken as the reference signals, and they are divided by their own differential signals, so that the parameters above can be eliminated in real-time. The algorithm was implemented in the field-programmable gate array-based electronic processing circuit, and a large dynamic range of 178.2 dB@200 Hz was achieved. Within 1 kHz bandwidth, the phase resolution reached 2 μ rad/Hz 1 / 2. A maximum light intensity rejection ratio of 37.0 dB was obtained. While the modulation depth is between 1 rad and 3.5 rad, the signal-to-noise-and-distortion (SINAD) and the total harmonic distortion (THD) can reach as good as 74.0 dB and −76.8 dB respectively, and the fluctuation of the SINAD and THD in 4 h were both within 1.2%. These experimental results prove the high stability of SR-PGC algorithm. • A self-referenced phase generation carrier (SR-PGC) algorithm is proposed. • The SR-PGC algorithm can recover phase-shift and suppress nonlinear distortion. • The demodulation system gets better phase resolution and larger dynamic range. • The experimental results prove the high stability of SR-PGC algorithm. [ABSTRACT FROM AUTHOR]

• ITO thin film electrode was modified on optical fiber F-P interferometer to form optoelectrode. • The optoelectrode can be used to real-time online detect the conductivity of electrolyte. • This optical fiber optoelectrode has the advantage of simple preparation, and convenient operation. In this paper, an F-P interferometer optoelectrode structure functionalized by conductive indium tin oxide (ITO) thin film is first proposed. When the F-P cavity is filled with electrolyte, the effective refractive index in the cavity can be modulated by applying an electrical potential to form an electric double layer on the interface of the electrolyte and electrode. Significantly, this work presents a novel structure of integrated F-P interferometer optoelectrode functionalized by conductive ITO to measure the conductivity of the electrolyte. And it is the first time to use interference spectrum to react the conductivity of solution, which provides a new method for conductivity detection. This simple preparation, convenient operation, and low-costed optical fiber F-P interference optoelectrode has great potential in the biochemical analysis, biosensing, electrochemical analysis and battery electrode surface monitoring. [ABSTRACT FROM AUTHOR]