The Research Of High Extinction Ratio Optical Pulse Modulation And Its Application On Performance Enhancement Of OTDR Sensing System

Optical fiber sensing system based on optical time domain reflection (OTDR) can obtain time and space distribution of the measuring information by injecting an optical probe pulse into the sensing fiber and monitoring its back scattering signal. Compared with the traditional sensors, in recent years it is particularly concerned by researchers for its advantages of continuous distributed, high sensitivity, no electromagnetic radiation, strong anti-interference ability, good chemical stability and so on.The extinction ratio of optical probe pulse is closely related to the performance (sensitivity, dynamic range, etc) of sensing system based on OTDR. This paper analyzes the relationship between the extinction ratio of optical probe pulse and the performance of several typical OTDR sensing systems. It provides a theoretical basis for the selection of the extinction ratio when the sensing system is designed. The optical probe pulse with high extinction ratio is very important to improve the performance of this kind of sensing system.In order to obtain high quality optical pulses, generally an electro-optic modulator (EOM) is used to convert continuous-wave (CW) light produced by laser into optical probe pulse. However, the working point of the EOM may drift for long term operation, resulting in the variation of probe pulse extinction ratio and degradation of signal-to-noise ratio for the sensing system. Therefore stabilizator has to be introduced to lock the working point of EOM automatically. Traditionally, a disturbance is added on the optical signal and a small part of the signal is coupled out for feedback, the drifting of EOM work point is compensated according to the feedback. However this optical coupler based feedback structure is difficult to ensure stability and extinction ratio of optical pulses at the same time. In this paper, a novel method for the modulation of optical probe pulse with high extinction ratio is proposed to solve these limitations of traditional method. Optical coupler is replaced by an optical switch synchronized with the probe pulse.Due to the limitations of traditional measurement methods, a dual pulses interferometric method is proposed for the dynamic extinction ratio (DER) comparison between the proposed and traditional modulator. Contrast measurement results have shown that the proposed method can achieve higher dynamic extinction ratio and stability comparing to traditional method. The contrast tests were made to evaluate the performance of the Brillouin Optical Time Domain Reflectometer (BOTDR) and Phase Optical Time Domain Reflectometer ((p-OTDR) sensing system based on the proposed and traditional modulator. Experimental results have shown that the performance of the BOTDR and (p-OTDR sensing system can be obviously improved with the proposed high extinction ratio optical pulse modulator. When DER was improved from 35 to 65 dB, the maximum uncertainty of temperature measurement at the end of a 48.5 km sensing fiber has been reduced from 5.2 to 0.8℃ with a 25 m spatial resolution in BOTDR system and the noise power at the sensing fiber with 7 km has been reduced from -5.3 dBm to -39 dBm in cp-OTDR system. The high extinction ratio optical probe pulse generation method is quite suitable for the performance enhancement of an OTDR sensing system, which has wide application potential.