Influence of finite extinction ratio on performance of phase-sensitive optical time-domain reflectometry

The leakage light of optical pulses due to finite extinction ratio (ER) of an electro-optic modulator (EOM) leads to Rayleigh backscattered noises over the entire fiber length, and limits spatial resolution and sensing range in phase-sensitive optical time-domain reflectometry (Φ-OTDR). Two configurations are proposed to improve the ER of optical pulses for better spatial resolution over long sensing length. With ER of 55 dB using a nonlinear optical loop mirror, we achieved 2 m spatial resolution over 8.4 km sensing length; while with ER of 60 dB obtained by two cascaded EOMs, we can achieve a 1 m spatial resolution over the same range. Experimental results and analysis show that leakage of the optical pulses acts as a noise floor, which limits the highest spatial resolution over the same sensing range.