Polarization fading elimination for ultra-weak FBG array-based Φ-OTDR using a composite double probe pulse approach

We proposed and experimentally demonstrated a technique using a composite-double-probe-pulse (CDPP) to eliminate the influence of polarization fading for Ultra-weak FBG (UWFBG) array-based phase-sensitive optical time-domain reflectometry (Φ-OTDR). The CDPP is composed of two optical pulses whose spatial interval is equal to twice the spatial interval of adjacent UWFBGs in the UWFBG array. The first optical pulse has a long duration, and the second one is composed of two continuous short pulses with orthogonal polarization states. The width of the short pulses is half of the long pulse, and their modulation frequencies are different from the long pulse. Using the proposed approach, we experimentally demonstrated that distributed quantitative measurements can be realized with high sensitivity based on only direct detection scheme, while the influence of polarization fading is effectively mitigated.