Distributed fiber acoustic sensing (DAS) technology is an advanced sensing technology that utilizes backward Rayleigh scattering in optical fibers to locate and recover changes in environmental physical quantities at any position on an optical fiber link. It is widely applied in health monitoring of large infrastructure such as oil and gas pipelines, bridges and tunnels, and transportation tracks. However, traditional DAS systems possess defects like interference fading and conflicts between sensing distance and spatial resolution. To address these issues, frequency scanning schemes and linear frequency modulation pulse schemes have been successively proposed. For the frequency scanning scheme, to achieve high-resolution strain measurement, the frequency scanning step must be reduced, which leads to a longer measurement cycle and a narrowed frequency response range of dynamic strain. Increasing the frequency scanning step, on the other hand, further restricts the strain resolution. For the linear frequency modulation pulse scheme, the maximum value of a single strain measurement of the system is limited by the modulation bandwidth of the linear frequency modulation pulse. For example, measuring 1000 με requires a modulation bandwidth of over 150 GHz, imposing excessively high demands on device indicators. We aim to develop a solution that can combine the high strain resolution of linear frequency modulation with a wide range of strain measurement.

We propose and experimentally validate a dynamic strain sensing method based on linear frequency modulation pulse sequence. This method combines linear frequency modulation pulse technology and frequency scanning technology. The detection light pulse is modulated into a linear frequency modulation pulse sequence with center frequency scanning through phase modulation. The dynamic strain causes frequency shift and time shift phenomena on the pulse sequence frequency scanning spectrum and the linear frequency modulation pulse RBS space-time distribution diagram respectively. The frequency shift result enables a larger strain measurement range, and the time shift result leads to a high strain resolution. The pulse reorganization demodulation algorithm is employed for demodulation, and the linear frequency modulation pulse measurement distortion value can be identified and the large strain measurement result of the frequency scanning pulse sequence can be used for numerical prediction. Meanwhile, the measurement range of the system is extended by the cyclic calibration correlation method, and the actual strain value is finally calculated.

The pulse sequence frequency scanning gauge factor and linear frequency modulation pulse time shift gauge factor of the system are calibrated by applying different strain values to the PZT. The measured pulse sequence frequency scanning gauge factor is -150.8 MHz/με, the maximum value of strain that can be measured by the pulse sequence is approximately 1260 nε, and the linear frequency modulation pulse time shift gauge factor is -251.6 ns/με (Fig. 5). When an amplitude modulated signal is applied to PZT, the strain is demodulated by a pulse recombination demodulation algorithm. Both small and large amplitude strains are well restored, and the strain sensitivity reaches 30 pε/Hz^1/2 (Fig. 7). Since the pulse reorganization demodulation algorithm uses a cyclic calibration operation, the curve cross-correlation calculation results can be expanded through continuous iteration of the calibration value, and finally a disturbance with an amplitude of 600 με is demodulated (Fig. 9).

This study proposes and experimentally verifies a dynamic strain sensing method based on a linear frequency modulation pulse sequence. The detection light pulse is modulated into a linear frequency modulation pulse sequence with a center frequency scan through phase modulation. According to the injection order of the pulse sequence, the original data is reorganized into a pulse sequence frequency scan result with a frequency shift characteristic and a linear frequency modulation pulse result with a time shift characteristic. The demodulation is carried out using a pulse reorganization demodulation algorithm. The experimental results demonstrate that when this method measures low-frequency dynamic strain (<10 Hz) at a distance of 10.45 km, the maximum value of the single strain measurement of the traditional single pulse detection scheme is expanded by over 6 times to 1260 nε. Simultaneously, the measurement range of the system is increased to 600 με through the cyclic calibration correlation method, the strain resolution is 4 nε, and the strain sensitivity reaches 30 pε/Hz^1/2. The proposed method has significant advantages and application potential in the field of large strain measurement range and high strain sensitivity measurement.