A hybrid distributed optical fiber vibration and temperature sensor based on optical Rayleigh and Raman scattering

A hybrid distributed optical fiber vibration and temperature sensing system is proposed and experimentally demonstrated. By the system structure of optical time domain reflectometer (OTDR) and wavelength division method, the Rayleigh and Raman scattering light in multi-mode fiber (MMF) are extracted respectively for fiber vibration event detection and hot spot monitoring. The vibration event causes the polarization state and phase of the optical signal propagating in MMF to change, which makes the OTDR trace become smooth from the vibration position to the fiber end, so that it provides a reliable way to locate the vibration events. And then, a data processing method that averages the data within the vibration position and at least 50 continuous positions behind is proposed to improve the signal to noise ratio (SNR) of the vibration signal and finally by fast Fourier transform (FFT), the vibration spectrum is accurately obtained. In addition, the backscattered Raman Stokes and anti-Stokes signals are separated by the wavelength division multiplexer (WDM) and then they are converted into voltage signals. By conventional temperature demodulation algorithm, the temperature information along the MMF is obtained with measurement accuracy of about 1.5 °C. The experimental results show that the proposed hybrid distributed vibration and temperature sensing system possess good performance and as it is compatible with MMF that has been laid on the power cable, it is of great significance for engineering application. • Rayleigh and Raman scattering signals in multi-mode fiber are extracted for hybrid sensing. • Optical polarization state testing function is added to Raman temperature system. • Trace averaging method is adopted to locate the vibration event. • Averaging the data within a position region is necessary for spectrum analysis.