Mitigation of Spectral Shadowing Effect in Direct Detection Double-Pulse FBG-Assisted Phase-OTDR

FBG-assisted phase-OTDR consists of a phase-OTDR interrogating a cascade of FBGs, offering a better SNR than conventional Rayleigh-based phase-OTDR. However, using a cascade of FBGs induces spectral shadowing. In this letter, a method for the mitigation of spectral shadowing effect is proposed and explained for direct detection double-pulse FBG-assisted phase-OTDR. The approach is based on the addition of a third optical pulse in the interrogating signal. The letter provides the theoretical background to understand the proposed method and details the experimental results performed for the validation of the approach. The spectral shadowing reduction efficiency depends on the parasitic frequency and the length of the FBG cascade. As an example, a 30 dB compensation efficiency is achieved for a 1kHz parasitic frequency and a 50m long sensor.