Time-Frequency Analysis For Distributed Optical Fiber Sensing Based On MZ Interferometry And Φ-OTDR

Optical fiber is used as the sensing medium in distributed fiber sensors, which can simultaneously and remotely detect the environment parameters along the fiber link with high accuracy. Currently, the distributed fiber sensing technology can be classified as three main categories, (a) interferometry based solutions, (b) backscattering based solutions, and (c) fiber grating based solutions, of which each has advantages and disadvantages. The distributed fiber sensing system based on MZ interferometer and the phase sensitive optical domain reflectometry (O-OTDR) combines the advantages of both sensing techniques, providing high spatial resolution and wide frequency range.In this thesis, theoretical analysis on signal detection technique, Wigner-Ville distribution and short-time Fourier transform is performed. Simulations and experiments are then demonstrated. Meanwhile, the distributed fiber sensing system based on MZ interferometry and the Φ-OTDR are extensively investigated.Firstly, the principle of the MZ interferometry and the Φ-OTDR technology is briefly introduced. According to the principle of the backscattering in an optical fiber, the model of the Φ-OTDR system based on Rayleigh backscattering is established. The operating principle and the characteristics of the Φ-OTDR system are briefly illustrated. Then the sensing principle of MZ interference technology is shown as well as its characteristic. Next, according to the features of the optical paths of the two systems, a joint system by combing them is discussed.Secondly, according to the sensitive characteristic of the Φ-OTDR to the external vibrations, the power of the position of external vibration on the temporal traces of the Rayleigh backscattering will be dramatically changed. But the Rayleigh backscattering traces corresponding to the power change of other positions is smooth and stable. Therefore the position of vibration can be obtained by averaging and differentiating the Rayleigh scattering traces. In the experiments, the signal-to-noise ratio (SNR) for location is～7dB in a 3km fiber link and the frequency resolution for vibration is～27Hz for a sweep signal from 4 kHz to 6 kHz.To analyze a non-stationary signal with high resolution of Wigner-Ville Distribution (WVD) but without weakness of fast Fourier transform (FFT), a joint time-frequency method, Wigner-Ville Distribution and Smoothed Pseudo Wigner-Ville Distribution (WVD-SPWVD), is applied. In the experiments, a non-stationary signal is generated by the piezoelectric tube (PZT) driven by a sweep signal with the frequency range of 4 kHz-6 kHz. The results obtained using WVD-SPWVD agree well with the frequency information of the sweep signal, which can be considered as an effective method to evaluate low frequency non-stationary vibration events in distributed fiber sensing system based on MZ interferometer and Φ-OTDR. For a high-frequency variation of 17 kHz～19 kHz, WVD-SPWVD outperforms other solutions. While for the SNR, SPWVD and STFT have better performance.In summary, WVD-SPWVD enables to greatly improve the frequency resolution at the cost of a slightly increased time consumption, compared with other methods.