The overhead power transmission line, as an important component of equipment for long-distance power transmission, is threatened by icing and galloping, which will lead to equipment troubles and cause huge economic loss. Thus, there is a great need for on-line monitoring for transmission lines. Because the photoelectric composite cable, such as optical fiber composite overhead ground wire (OPGW) is widely used, it is possible to introduce distributed optical fiber sensors (DOFS) into transmission line status monitoring. Common schemes based on DOFS usually offer low sensing density and are hard to realize global awareness, while phase sensitive optical time domain reflectometry (Φ-OTDR), as a DOFS that has the characteristic of distributed sensing, is hoping to address the limitation. In this paper, by establishing mathematical models, proposing analytical method, and building demonstration devices to analyze the dynamic strain characteristics of overhead power transmission line, an Φ-OTDR based on-line monitoring scheme of transmission line status is presented and experimentally proved for the first time. The estimation error of sag is less than 5.8% on centimeter scale, and the estimation error of ice thickness is no more than 10.84% on sub-millimeter scale, which gives an accurate description of transmission line status and provides strong support for early warning of transmission line failures.

The demand for distributed acoustic sensors, which are capable of reconstructing the amplitude, frequency, and phase of an acoustic field, is increasing. These sensors are typically realized by phase-sensitive optical time domain reflectometry ( $\Phi$ -OTDR). However, common $\Phi$ -OTDR systems suffer from a fading phenomenon, which causes amplitude fluctuation on Rayleigh backscattering traces, and the scattered light may not have enough intensity in some regions. These areas have low amplitude backscattering that may be even lower than the system noise floor during some periods of time. Therefore, we cannot reconstruct the phase signal properly in such low intensity areas. Systems with multiple frequencies have been proposed to achieve fading suppression. However, online data processing of such schemes remains a challenge, especially for continuous in-field applications. Prediction of fading must be performed in real time. Any delay caused by massive calculations is not acceptable. In this paper, a continuous fading suppression method based on a $\Phi$ -OTDR system with three different probe frequencies is presented as well as a tracking algorithm for selecting the optimum probe signal for any time, which predicts the occurrence of fading before it actually occurs. The performance of the proposed method has been experimentally evaluated and statistically analyzed. The distortion induced by the fading effect could be suppressed to 1.15% under continuous real-time running. The method is highly effective and repeatable, which makes it suitable for practical online purposes.

We demonstrate an enhanced distributed acoustic sensing (DAS) system based on ultra-weak fiber Bragg grating (UWFBG) array and self-heterodyne detection. By using the interaction process between nearby UWFBGs, the distribution of acoustic field along the sensing fiber could be fully captured. Self-heterodyne detection was applied, leading to much lower noise density level for the proposed sensing system. The experimental results have shown that the acoustic field could be reconstructed with high fidelity even at the rear end of a 42 km long sensing fiber with the spatial resolution of 11.7 m. The proposed sensing system has a linear intensity response of R2=0.9988, while the peak-to-peak value (VPP) of dynamic strain was tuned from 9.28 n $\epislon$ to 176.96 n $\epsilon$ . The root-mean-square (RMS) measurement error of our sensing system was less than 1.32 n $\epsilon$ . The frequency response range of the proposed system was from 8 Hz to 1 kHz. The proposed sensing system offered a promising solution for the performance enhancement of DAS system that could achieve high spatial resolution, high sensing sensitivity, long sensing distance and dead zone free measurement at the same time.

Location error and false alarm are noticeable problems in fiber distributed acoustic sensing systems based on phase-sensitive optical time-domain reflectometry (Φ-OTDR). A novel method based on signal kurtosis is proposed to locate and discriminate perturbations in Φ-OTDR systems. The spatial kurtosis (SK) along the fiber is firstly obtained by calculating the kurtosis of acoustic signals at each position of the fiber in a short time period. After the moving average on the spatial dimension, the spatial average kurtosis (SAK) is then obtained, whose peak can accurately locate the center of the vibration segment. By comparing the SAK value with a certain threshold, we may to some degree discriminate the instantaneous destructive perturbations from the system noise and certain ambient environmental interferences. The experimental results show that, comparing with the average of the previous localization methods, the SAK method improves the pencil-break and digging locating signal-to-noise ratio (SNR) by 16.6 dB and 17.3 dB, respectively; and decreases the location standard deviation by 7.3 m and 9.1 m, respectively. For the instantaneous destructive perturbation (pencil-break and digging) detection, the false alarm rate can be as low as 1.02%, while the detection probability is maintained as high as 95.57%. In addition, the time consumption of the SAK method is adequate for a real-time Φ-OTDR system.

Phase-sensitive time-domain reflectometry (Φ-OTDR) can be used for fully distributed long-distance vibration monitoring. There is a fading phenolmenon in the Φ-OTDR, which will cause the signal intensity somewhere to be too low to extract the phase of the signal without distortion. In this paper, the Φ-OTDR based on space-division multiplexing (SDM) is proposed to suppress fading and we used multi-core optical fiber (MCF) to realize SDM. While inheriting the previous optimization strategy, we proposed a strategy based on frequency spectral similarity to process multiple independent signals obtained by SDM. And we compared the two methods. Through the experiments, the distortion rate can be reduced from an average level of 9.34% to less than 2% under continuous running of 270 s, which proves that SDM is a reliable technical route to achieve fading suppression. This method can effectively improve the fading suppression capability of the existed commercial systems.

Distributed optical fiber sensors (DOFSs) are a promising technology for their unique advantage of long-distance distributed measurements in industrial applications. In recent years, modern industrial monitoring has called for comprehensive multi-parameter measurements to accurately identify fault events. The hybrid DOFS technology, which combines the Rayleigh, Brillouin, and Raman scattering mechanisms and integrates multiple DOFS systems in a single configuration, has attracted growing attention and has been developed rapidly. Compared to a single DOFS system, the multi-parameter measurements based on hybrid DOFS offer multidimensional valuable information to prevent misjudgments and false alarms. The highly integrated sensing structure enables more efficient and cost-effective monitoring in engineering. This review highlights the latest progress of the hybrid DOFS technology for multi-parameter measurements. The basic principles of the light-scattering-based DOFSs are initially introduced, and then the methods and sensing performances of various techniques are successively described. The challenges and prospects of the hybrid DOFS technology are discussed in the end, aiming to pave the way for a vaster range of applications.