A Study on Noise Reduction of Φ-OTDR System Based on VSS-NLMS Algorithm

To improve the signal-to-noise ratio (SNR) of vibration location information in Φ-OTDR system, a variable step size normalized least mean square (VSS-NLMS) denoising method is proposed in the article. First, the mathematical principle of VSS-NLMS method is established. The method can adaptively filter the input signal and adjust the parameters. It can also update the step factor adaptively. The simulation results show that the SNR of this method is improved from -1.16 dB to 12.01 dB. Second, a heterodyne coherent optical system is built. After the detected beat signal is demodulated by I/Q quadrature demodulation, the amplitude signal and phase signal of the Rayleigh backscattered light are acquired. The amplitude signal determines the location information and the phase signal determines the vibration signal. Finally, the wavelet denoising method, the empirical mode decomposition soft threshold (EMD-soft) denoising method and the variational mode decomposition nonlinear wavelet threshold (VMD-NWT) denoising method are compared with the VSS-NLMS denoising method in the experiment. The experimental results show that the location information SNR of the VSS-NLMS denoising method is improved to 59.31 dB, 46.81 dB, 50.14 dB, 34.00 dB and 67.09 dB for 5 groups of 100 Hz - 500 Hz vibration events at the location of the 10.14 km sensing fiber. At the same time, the vibration signals of 5 groups are demodulated. This method has laid a foundation for the application of oil and gas pipelines.