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.

The last years have witnessed the wide application of Distributed Acoustic Sensor (DAS) systems in several fields, such as submarine cable monitoring, seismic wave detection, structural health monitoring, etc. Due to their distributed measurement ability and high sensitivity, DAS systems can be employed as a promising tool for the phase sensitive optical time domain reflectometry (Φ-OTDR). However, it is also well-known that the traditional Φ-OTDR system suffers from Rayleigh backscattering (RBS) fading effects, which induce dead zones in the measurement results. Worse still, in practice it is difficult to achieve the optimum matching between spatial resolution (SR) and signal to noise ratio (SNR). Further, the overall frequency response range (FRR) of the traditional Φ-OTDR is commonly limited by the length of the fiber in order to prevent RBS signals from overlapping with each other. Additionally, it is usually difficult to reconstruct high frequency vibration signals accurately for long range monitoring. Aiming at solving these problems, we introduce frequency division multiplexing (FDM) that makes it easier to improve the system performance with less system structure changes. We propose several novel Φ-OTDR schemes based on Frequency Division Multiplexing (FDM) technology to solve the above problems. Experimental results showed that the distortion induced by fading effects could be suppressed to 1.26%; when the SR of Φ-OTDR is consistent with the length of the vibration region, the SNR of the sensing system is improved by 3 dB compared to the average SNR with different SRs; vibration frequencies up to 440 kHz have been detected along 330 m artificial microstructures. Thus, the proposed sensing system offers a promising solution for the performance enhancement of DAS systems that could achieve high SNR, broadband FRR and dead zone-free measurements at the same time.

Fully distributed fiber sensors, such as phase sensitive optical time domain reflectometry (Φ-OTDR) systems, have drawn significant attention from researchers, especially for use in geophysical applications. Distributed sensing, cost efficiency, wide dynamic range, good spatial resolution, and high accuracy make these sensors ideal for industrial use and for replacing traditional geophones. However, inevitable drifts in the central frequency of laser sources always cause low frequency noise in the output, which could easily be mistaken with real sub-Hertz environmental vibrations. This deteriorates the data accuracy, especially when dealing with low frequency seismic waves. In this study, we propose a method in which adding an extra probe frequency to a Φ-OTDR setup provides a reference frequency. This reference frequency provides information regarding changes in the laser source and other environmental noises, such as humidity and temperature, helping to refine extracted results from low frequency noise. This feature is also very useful for frequency domain analysis, where we may lose the near DC band information during mathematical measurements. Regarding the adjustable properties of this reference frequency, it can be implemented in various Φ-OTDR applications and commercial devices.

In conventional heterodyne phase-sensitive optical time domain reflectometry (Φ-OTDR), the optical path difference (OPD) between sensing and reference arms deteriorates the signal-to-noise ratio (SNR). We counteract this effect by introducing a loop path in reference arm. This facilitates the use of laser source with low coherence length. Compared with an ordinary Φ-OTDR system, SNR increased from ~1 dB to 5 dB. Regardless of OPD compensation features, the probability to getting a better output increases by generation of multiple reference beams (and consequently multiple beat signals), as compared to conventional systems. The method is cost efficient and suitable for practical purposes.