Frequency Demodulation of Dynamic Stress Based on Distributed Polarization Coupling System

In this paper, a white light interferometer-based distributed polarization coupling system is developed for measuring the frequency of dynamic stress in two ways. First, the movable mirror of a Michelson interferometer (MI) scans the dynamic coupling point to obtain its interferogram. The Hilbert transform and fast Fourier transform (FFT) are then used to extract the envelope of the interferogram and obtain the frequency spectrum of the envelope, respectively. This first method achieved single and double dynamic stress measurements with an absolute error less than 0.47 Hz. Second, the movable mirror of MI is adjusted to compensate for the optical path difference generated in the polarization-maintaining fiber. FFT and wavelet transform are applied to frequency-invariant and frequency-varying signals, respectively. The second method achieved a frequency measurement up to 1 kHz and real-time monitoring of dynamic stress. The frequency measurement errors for this method are less than 2 Hz. Finally, we compared the advantages and disadvantages of the two methods. While the first method is capable of distributed dynamic stress sensing, the second method can measure the instantaneous frequency of dynamic stress at a certain position. The two methods proposed in this paper may have practical applications in dynamic stress sensing in the future.