Spatial Nonlocality from Finite Extinction Ratio in BOTDR over Heterogeneous Fiber Links

Abstract

Finite extinction ratio in pulse-modulated Brillouin optical time-domain reflectometry (BOTDR) produces residual inter-pulse Brillouin backscatter outside the nominal pulse gate. We model and experimentally quantify this effect as spatial nonlocality in a heterogeneous fiber link and define the spectral contamination ratio (SCR) as the contaminating Brillouin-band power normalized to the local Brillouin-band power. Measurements were performed using a 1.2 km standard single-mode fiber (SMF) followed by 0.1 km of dispersion-compensating fiber (DCF), whose separated Brillouin frequency shifts allow section-to-section leakage to be resolved spectrally. At 20 m spatial resolution, increasing the extinction ratio from 18.5 to 33.0 dB reduced the SMF-band contamination in the DCF gates from -5.3 to -14.8 dB. Over the 16-33 dB range, the SCR showed fitted slopes of -1.064 and -1.246 dB/dB for the two contamination directions. These results quantify finite-ER-induced spatial nonlocality and provide a link-specific metric for extinction-ratio selection in BOTDR.

4 pages