The train-induced vibration response provides a flexible solution for the real-time monitoring deformation of high-speed railway track slab in actual operation. This paper proposes a long-term real-time monitoring method for track slab deformation based on wavelet packet energy (WPE) using fiber optic accelerometers to record train-induced vibration. We found that the vibration response law of track slab deformation could be established by using the WPE of the frequency band covering the first- and second-order frequencies induced by the adjacent carriages. A field test was carried out for more than one year on the Beijing–Shanghai high-speed railway to investigate the train-induced vibration response law of track slab that was continuously deformed under a long-term temperature load. The maximum values of the WPE characteristic index appeared in winter and summer, and they were positively correlated with the temperature difference between the air environment and the track slab under the daily temperature load. These results were demonstrated to be consistent with the track slab deformation law for long-term and daily temperature loads. The novel method based on fiber optic accelerometers and WPE provides a new method for the long-term and real-time monitoring of track slab deformation.

In a multi-channel interferometric fiber-optic sensor system using space-division multiplexing (SDM) and phase-generated-carrier (PGC) demodulation, the phase delay and phase modulation depth fluctuation of each channel will affect the amplitude consistency and harmonic distortion of the demodulation results. In this paper, an improved demodulation scheme based on the Goertzel algorithm is proposed to calculate the multi-channel phase delay and phase modulation depth and to compensate for their fluctuations simultaneously. First, the carrier's 1^st to 6^th harmonic amplitudes in the interference fringe are extracted using the Goertzel algorithm. Then, the phase delay is calculated using the real and imaginary components of the 1^st harmonic amplitude. The phase modulation depth is calculated with a combinatorial operation of the 1^st to 6^th harmonic amplitudes. In addition, a reference channel is introduced to implement phase delay and modulation depth feedback control. The experimental results demonstrate that the improved scheme can effectively suppress the harmonic distortion and improve the amplitude consistency of multi-channel interferometric fiber-optic sensors with low resource consumption.

Strain rate is an important basic physical parameter in the fields of deformation observation, geodetic measurement, and geophysical monitoring. This paper proposes a novel fiber optic strain rate sensor (FOSRS) that can directly measure the strain rate through a differentiating interferometer that converts the strain rate to the optical phase. The sensing principle, sensitivity, resolution, and dynamic range of the proposed FOSRS are theoretically analyzed and verified by experiment. The experimental results show that the developed FOSRS with a 12.1 m sensing fiber has a flat sensitivity of 69.50 dB, a nanostrain rate ( n ε / s ) resolution, and a dynamic range of better than 95 dB. An ultrahigh static resolution of 17.07 pε / s can be achieved by using a 25.277 km sensing fiber for long baseline measurements. The proposed method significantly outperforms existing indirect measurement methods and has potential applications in geophysical monitoring and crustal deformation observation.

Jerk is directly related to a physical mutation process of structural damage and human comfort. A fiber optic jerk sensor (FOJS) based on a fiber optic differentiating Mach–Zehnder interferometer is proposed. It can directly measure jerk by demodulating the phase of interference light, which avoids the high-frequency noise interference caused by differentiating the acceleration. The sensing theory and sensor design are given in detail. The experimental and theoretical results agree, demonstrating that the FOJS has a high sensitivity, an ultralow phase noise floor, a wide measuring range, and good linearity. The impact test shows that the FOJS can directly measure jerk and has good consistency with a standard piezoelectric accelerometer. The FOJS has potential applications in earthquake engineering, comfort evaluations, and railway design. This is the first time that directly measuring jerk with an optical sensor is reported.

This paper proposes an interrogation method to improve the resolution of DFB (distributed feedback) fiber laser acoustic emission (AE) sensing system. In this method, the coefficients of three ellipses formed by every two-channel interference fringe are estimated using bias-corrected ellipse-specific fitting (BCESF). The system parameters can be further corrected using the coefficients of the three ellipses. A resolution of 2×10 ^-7 pm/ √Hz at 100 kHz, which is approximately 10 dB higher than the traditional method, is achieved using the system parameters correction. The carbon fiber reinforced plastic (CFRP) laminates tensile test shows that the improved DFB fiber laser AE sensing system exhibits better performance than the PZT sensor. The damage initiation, damage spread, and instability stages of CFRP laminates' failure process can be accurately recognized.

The fiber-optic sensor array based on path-matched differential interferometry (PMDI)-time division multiplexing (TDM) offers the advantages of minimal channel crosstalk and high sensitivity. The 3 × 3 optical fiber coupler demodulation is the commonly employed scheme for PMDI-TDM. However, due to the non-ideal characteristics of the 3 × 3 optical fiber coupler and inconsistencies in multi-channel photodetectors, harmonic distortion occurs during signal demodulation. This paper proposes a low harmonic distortion signal demodulation scheme for PMDI-TDM structure fiber-optic sensor arrays based on multi-channel parameter fitting and tracking. This scheme utilizes the random sample consensus (RANSAC) algorithm to accurately obtain the parameters of the ellipse formed by the interference fringes of each element sensor in the system. Furthermore, the multi-channel parallel extended Kalman filter (MPEKF) is used to realize the real-time tracking of multi-channel demodulation parameters. The RANSAC-MPEKF scheme can suppress the influence of outliers, and effectively eliminate the non-ideal characteristics of 3 × 3 coupler and the non-consistency of photodetector, thereby suppressing harmonic distortion of the demodulation results of each channel. The experimental results demonstrate that the proposed scheme can suppress the influence of outliers, .and achieves a demodulation rate ≥500 kHz with minimal resource consumption, making it particularly suitable for large-scale multiplexing systems with a THD value better than 0.12%.