Synchronous intensification of spatial resolution and sensing range has attracted widespread concern for distributed acoustic sensor. The pulse compression approach as a representative technology is adopted for addressing this issue, while the performance is suffered from the Rayleigh side-lobe noise due to the insufficient side-lobe suppression ratio (SLSR). In this work, the poly-phase coding phase-sensitive optical time domain reflectometer (Φ-OTDR), possessing an ultra-high SLSR, is proposed to immensely alleviate the Rayleigh side-lobe noise influence. The spatial resolution is relevant to the single code width, as well as the sensing range and SLSR are relevant to the coding length. An instructive model is deduced to evaluate the influence of SLSR to Φ-OTDR system's SNR. According to the model, a theoretical SLSR value is derived, making the Rayleigh side-lobe noise could be neglected. In the proof-of-concept experiments, the modulated poly-phase coding with ultra-high 127.08 dB SLSR guarantees a 204.3 km sensing range and 2 m spatial resolution. Two sinusoidal perturbances with 20 Hz and 60 Hz are exerted at 173.3 km and 203.8 km on the fiber. As a result, the strain resolutions are achieved at 287.3 pϵ/√Hz and 142.1 pϵ/√Hz, respectively. This work demonstrated that ultra-high SLSR is the foundation for achieving excellent performance for compression pulse distributed acoustic sensors.

A crosstalk-free frequency division multiplexing distributed acoustic sensing (CFFDM-DAS) technology for partial discharge (PD) measurement in power cable is proposed in this paper. The multi-carrier crosstalk-free probe pulse, along with raised cosine roll-off processing, is constructed and implemented to avoid inter-channel crosstalk between the different frequency, significantly increasing multiplexing density. In this way, the CFFDM-DAS is modeled and established with an increased crosstalk suppression ratio of 61.39 dB compared to traditional multiplexing techniques. As a result, the frequency bandwidth of the proposed method is extended to 100 kHz using 10-channel multiplexing with a 5 km sensing range. Validation experiments confirm that the sensing system exhibits an excellent wideband response and reliable acoustic signal reconstruction capabilities. Discharge experiments on a 10 kV XLPE power cable with void insulation defect are conducted. The measurement of PD with the proposed system demonstrates its capacity for detecting discharges with a minimal apparent charge of approximately 31 pC and effectively capturing most PD events. The proposed CFFDM-DAS system presents a promising approach for the reliable and continuous monitoring of PD activity.

The acoustic emission property and transient process of cylindrical cavity discharge is comprehensively modeled and validated for power cable in this paper. An electrostatic field model is established to analyze the discharge duration and power density vary with discharge magnitude and cylindrical cavity height. The relationship between the acoustic source and PD activity is constructed through discharge power density and duration. Furthermore, a transient model for acoustic wave propagation in multilayer power cable is established. Numerical analysis with finite element analysis is conducted to analyze the effects of different discharge magnitudes under the same cavity height and different cavity height under the same discharge magnitude on the acoustic waves at the surface of the power cable. The simulation reveals that with higher discharge magnitude resulting greater intensity of the acoustic wave and with longer cavities resulting in lower acoustic frequencies. Ultimately, a series of validation experiments are implemented to acquire the statistical acoustic characteristics. The intensity of acoustic wave is positively correlated with the discharge magnitude in R-square of 0.9895, while the frequency is negatively correlated with cavity height in R-square of 0. 9589.

Due to the inherent $\pi $ -phase constraint in phase unwrapping algorithms, the dynamic strain range of phase-sensitive optical time domain reflectometry ( $\Phi $ -OTDR) is significantly limited. $\Phi $ -OTDR with modified architectures have proven effective in expanding the dynamic strain range. However, such methods still subject to the $\pi $ -phase constraint, resulting in inconsistent enhancement of the dynamic strain range across the frequency response bandwidth. This work innovatively proposes a least squares linear programming phase unwrapping (ls-LPPU) algorithm, which determines the wrapping orders under large dynamic strains by minimizing the squared optical path difference between two wavelengths, thereby achieving temporally independent phase unwrapping. Experimental results demonstrate that the proposed method achieves precise restoration of large dynamic strains ranging from 13.8 to $44.6~\mu \varepsilon $ , representing an enhancement of at least 90-fold. Furthermore, under a pulse repetition frequency of 10 kHz, error-free recovery of large dynamic strains with frequencies spanning 0.9–4.5 kHz is realized. The ls-LPPU algorithm facilitates the restoration of large dynamic strains by directly obtaining the phase wrapped orders. Meanwhile, its temporally independent characteristic eliminates the $\pi $ -phase constraint, achieving the decoupling of dynamic strain extension and frequency response bandwidth.