High-Fidelity Frequency Division Multiplexed Distributed Acoustic Sensor Based on Desynchronized Dual-Frequency-Pulse Sequence

Distributed acoustic sensing (DAS) has emerged as a transformative technology for dense-arrayed measurements of acoustic wave or vibration. A fundamental trade-off exists between two key metrics of DAS: the maximum fiber length and the acoustic sampling rate. Frequency division multiplexing (FDM) technique is an effective approach to multiply the DAS sampling rate by the FDM number N. However, due to the phase offset and phase sensitivity fading effect of Rayleigh backscattering (RBS) in the fiber, the fidelity of FDM DAS outputs is degraded. Several schemes to mitigate such effects have been proposed, such as introducing an extra frequency component for compensation. In this work, we present an FDM DAS scheme to further enhance the fidelity based on desynchronized dual-frequency-pulse sequence. The pulse sequence contains 2N+1 frequency components, and has a global period of N(N+1) times of the pulse interval. Measurement results from all the interrogations and all the frequency components contribute to the fading compensation via matrix equations. In the demonstration experiment, the frequency response range achieves a fourfold enhancement from 1.0 kHz to 4.0 kHz over a 49.2 km sensing fiber and the spurious-free dynamic range (SFDR) reaches 36.4 dB when a 3.5 kHz sinusoidal strain is applied near the fiber end. In addition, our method is adaptable to the degenerated case where sinusoidal strain with a period of N+1 times of pulse interval is applied, and the plain case that no strain is applied to the fiber.