A Method for Predicting the Maximum Detectable Distance of φ-OTDR

This letter proposes a method for predicting the maximum detectable distance of phase-sensitive optical time-domain reflectometry (
$\varphi $
-OTDR). Based on analyzing the noise composition of the output photocurrent signal of
$\varphi $
-OTDR, the relationship between the ratio of the standard deviation of the photocurrent signal to the standard deviation of thermal noise and the complete distortion rate of the demodulated signal is obtained from actual measurements. The critical value of the ratio is determined according to the critical value of the complete distortion rate. Finally, based on the known optical fiber attenuation coefficient, the standard deviation of the photocurrent signal at starting position of optical fiber, the balanced photodetector (BPD) output voltage without light and the ratio determined by the complete distortion rate, a formula for calculating the maximum detectable distance is proposed. The results indicate that when the optical fiber length is not greater than 35.7 km, the maximum deviation is only 1.0 km. It can effectively predict the maximum detectable distance of
$\varphi $
-OTDR.