Ultra-low frequency dynamic strain detection with laser frequency drifting compensation based on a random fiber grating array

Dynamic strain sensing over a frequency range from 0.01 to 20 Hz can be used for monitoring earthquakes and volcanoes, charting rock and petroleum formations beneath the earth. However, significant laser frequency drifting (LFD) has limited the detection in this frequency range, especially for distributed frequency detection with phase optical time domain reflectometry (OTDR), where the LFD will introduce a time dependent noise destroying the dynamic strain reconstruction. In this study, a simple and effective method that utilizes the referenced random fiber grating to monitor the variation of laser frequency has been both theoretically analyzed and experimentally demonstrated. During the maximum up to 200 s data acquisition time, the frequency variation of a distributed feedback (DFB) laser with MHz linewidth is obtained from the referenced portion of sensing signal, and then the 1 Hz and 0.01 Hz dynamic strain variations with amplitude of 30 µ ε are reconstructed with strain measurement standard deviation of 66 n ε . Due to signal-to-noise ratio (SNR) enhanced Rayleigh traces from random fiber gratings, a minimum detectable frequency drifting of 7.28 MHz could be achieved over the optical frequency of 2 × 10 14 H z .