Distributed fiber-optic acoustic sensing is capable of continuously monitoring the spatially distributed evolution of environmental variables along a sensing fiber, thus has been extensively used in various fields, such as geophysical exploration, seismic surveillance, and structure health monitoring. Despite its outstanding performance, the technology is still restricted by the trade-off between frequency response and dynamic strain measurement range. Firstly, the conventional spectral analysis method is fundamentally limited by the time-consuming frequency scanning, which hinders its capability in monitoring dynamic events. Secondly, the traditional phase- demodulation approach offers high frequency response, but it suffers from the measurement slew-rate when monitoring vibration signal with large amplitude. To address this challenge, a novel frequency-comb spectrum-correlation reflectometry (OFC-SCR) based distributed fiber-optic acoustic sensing technique has been proposed in Light: Science & Applications .
Known as a groundbreaking tool for precision metrology, the optical frequency comb (OFC) enables parallel multi-frequency interrogation thanks to its evenly spaced and highly stable spectral lines across a broad spectral range. In this study, the research team employs a digitally synthesized OFC probe light to interrogate the broadband Rayleigh backscattering optical spectra along the sensing fiber. An interleaved OFC configuration is developed for parallel interrogation of the Rayleigh backscattering spectrum, with a dual-sideband scheme to further suppress cross-correlation errors, as shown in Fig. 1. As a result, the need for time-consuming frequency scanning is eliminated, and the measurement speed is improved by more than an order of magnitude compared to the state-of-the-art fast frequency-scanning approaches.
The experimental setup of the proposed OFC-SCR is shown in Fig. 2. Each comb-tooth samples a specific optical frequency within the spectrum, thus allowing broadband Rayleigh backscattering optical spectrum pattern to be captured by a single-shot measurement. The time evolution of the detected Rayleigh backscattering patterns and the cross-correlation spectra along the sensing fiber is shown in Fig. 3. The dynamic strain measurement range of the OFC-SCR is determined by the modulation bandwidth of the interrogation OFC, and no longer restricted by the measurement slew-rate, which is more than tenfold larger than that of the traditional phase demodulated methods with the same parameter settings.
The system also shows excellent high-frequency vibration detection capability. In the experiment, up to 24 kHz vibration signal can be successfully detected with a sensitivity as low as 11.4 pε/√Hz, as presented in Fig. 4, noting that the vibration frequency is very close to the 25 kHz Nyquist sampling frequency upper limit.
The proposed OFC-SCR simultaneously achieves high frequency response, wide dynamic measurement range, high sensing sensitivity, and excellent robustness, significantly pushing the performance boundaries of distributed fiber-optic acoustic sensing. The OFC probe light based parallel of interrogation methodology represents a fundamentally new framework for dynamic spectral analysis, and may revolutionize the implementation of fiber-optic sensing.
Light Science & Applications
Frequency-comb enabled spectrum-correlation reflectometry for distributed fiber-optic sensing