Analysis and suppression of the leakage light-induced noises in a large-capacity FBG sensing system

The leakage light caused by the finite extinction ratio (ER) of probe pulses leads to excess reflected noises in a large-capacity fiber Bragg grating (FBG) array, thereby limiting the performance of the optical time domain reflectometry-based sensing system, and currently, there are no studies specifically addressing this issue. In this study, we analyze the impact of the leakage light-induced noises on the reflected signals from the FBGs, and propose a spectral compensation method using valley-based noise referencing to suppress the leakage light-induced spectral distortion. Unlike conventional approaches that rely on complex hardware configurations to enhance the ER, the proposed method operates purely in the signal processing domain and requires no hardware modification. The theoretical analysis demonstrates that the leakage light noise can be extracted from the time domain signals and adopted as a reference. Thus, the corrected spectrum of each FBG can be obtained by subtracting the reference from the original spectrum. A 20-km FBG sensing array with 10000 FBGs is demodulated with the proposed method, and four FBGs from the far-end of the array are heated to verify the sensing performance. Experimental results show that the temperature-wavelength coefficient is corrected from 5.357 to 11.204 pm/°C with a demodulation precision of 3.297 pm and an average wavelength drift of 7.169 pm, verifying the effectiveness of the proposed method in suppressing the leakage light-induced noises.