Influence of Pump Light on LP01 and LP11 Modes in Few-Mode Fiber Brillouin Optical Time Domain Reflectometry

The quality of pump pulse in few-mode-fiber Brillouin optical time domain reflectometry (FMF-BOTDR) is vital for the spontaneous Brillouin scattering of modes LP01 and LP11 because it is the comprehensive effect of the main laser linewidth and pulse width, which is firstly discussed as we know. Numerical and experimental analysis are made for the amplitude and linewidth distribution, corresponding to the signal-noise ratio (SNR) and frequency resolution in BOTDR, respectively. Simulation shows the linewidths and peak values of Brillouin scattering have the same tendency for the LP01 mode and LP11 mode when the laser linewidth is less than 1 MHz but decreases slowly until they are the same when the laser linewidth is wider than 1 MHz. With the pulse width widening, the Brillouin linewidths for LP01 and LP11 modes both decrease sharply, almost to the natural linewidth of fiber 41 MHz and 35 MHz. Experimental results show that the amplitude distribution for the LP01 mode is always larger than for the LP11 mode if the main laser has the same linewidth and the frequency fluctuation is at least 2 MHz with the fiber laser and LP11 mode. The above results could provide improved sensing resolution for FMF-BOTDR sensing system.