Investigate On The Performance Of BOTDR System Based On Encoding Technique In Radiation Environment

The safe operation of the spacecraft and the nuclear power plant is of great significance,which is necessary to use the sensor to measure the health of the equipment.Optical fiber sensor has a fairly promising applicative prospect in safety detection of radiation environment for its anti-electromagnetic interference,low mass,easily integrated,corrosion resistance.Brillouin Optical Time Domain Reflectometer(BOTDR)which is one of the most important optical fiber sensors,is a hot research point at present.The principle of its distributed sensing is on the basis that the frequency shift variation of the spontaneous Brillouin scattering is of a linear relation to the variation of temperature and strain.Optical fiber is the sensing part of the BOTDR,which plays a key role in the whole system.However,the radiation induced attenuation and fluorescence effect,will cause the fiber loss increases along with the increase of radiation dose.Thus,the intensity of the Brillouin back scattering light becomes weaker,resulting in the signal to noise ratio(SNR)of the distributed optical fiber sensor system decreased.Furthermore,the temperature and strain sensing becomes difficult.In order to obtain a better signal to noise ratio of the system and a longer sensing distance,the method of improving the power of the fiber is generally adopted.In the single pulse system,we usually increase width of the detection light to increase the power,but it cause the system to increase the length of the actual event point,the temperature and strain of the sensor sensitivity decreased.The contradiction between the spatial resolution and the dynamic range of the system restricts the application of BOTDR system in the radiation environment.Use the coded pulse as the detection signal can be ensure the spatial resolution is not reduced and increase the energy of the incident light at the same time,which is an effective and practical method for solving the cause induced radiation losses.Golay complementary sequence have good autocorrelation properties encoded with good peak to average power ratio,which is suitable as a probe light signal of BOTDR coding system.Based on the characteristics of the radiation type of BOTDR system in the radiation environment,the ground simulation experiment is carried out.The feasibility of temperature and strain sensing of BOTDR system in the radiation environment is verified by experiment.Under the radiation environment,the performance degradation caused by the radiation effect are analyzed in the experiment.Experimental results show that by improve the input optical power can improve the performance of BOTDR system.By analyzing the advantages and disadvantages of common coding BOTDR and the requirement of engineering application,a BOTDR system based on Gray complementary sequence is designed.The performance of BOTDR based on Gray complementary sequence under different radiation doses has been carefully analyzed.Experimental results show that,compared to the single pulse system,at the same spatial resolution,signal to noise ratio and dynamic range of the system improved 12dB.Golay complementary sequence technique solved the single pulse system under high doses of radiation caused by a lack of SNR falling sensing accuracy and wide pulse signal sensing error problem.