Influence of Laser Linewidth on Phase-OTDR System Based on Heterodyne Detection

A theoretical analysis of the influence of laser's linewidth on phase-OTDR sensing system is proposed and corresponding experiment is demonstrated. The linewidth factor of laser source is introduced into the signal expressions of phase-OTDR system under the nonquasi monochromatic light condition. The effect of linewidth can be transformed into an exponential decay item of the signal. Even if the sensing range is always smaller than the coherent length of the laser source, the detection SNR will decrease as the linewidth increases. A Phase-OTDR strain detection experiment with linewidth controllable laser is performed to verify the derived theoretical analysis result. The result shows that when linewidth increases from 2 kHz to 1 MHz, the detection SNR of the same vibration signal will decrease 4.3 dB. In order to select the appropriate laser source for phase-OTDR system, the coherent length and influence of linewidth ought to be taken into account to ensure the detection performance of the system.