External optical feedback induced noise in DFB fiber laser sensor system

External optical feedback effects due to reflection, Rayleigh backscattering and coherent Rayleigh backscattering in DFB fiber laser sensor system has been investigated. If the feedback intensity exceeds critical amount, excess noise would be induced in the demodulator. The maximum tolerable intensity back-reflection coefficient R_c and backscattering coefficient Sc into a fiber distributed feedback (DFB) laser with lead fiber length from 1 m~37.5 km before the onset of instabilities were shown. R_c was found to decrease with increasing lead fiber length while Sc was relatively invariable with varying fiber length. The coherent Rayleigh backscattering (CRBS) would induce neglectable noise with a lead fiber exceeding 13.5 km. To eliminate these noises, one or two isolators should be incorporated in the system.