Distributed Brillouin Optical Fiber Sensing Based On Channel Estimation

Distributed optical fiber sensors have the unique advantages of small size,light weight,electrical insulation,anti-electromagnetic interference and long-distance transmission and sensing,being widely used in various fields such as transportation,industrial mining,defense military,biomedical,aerospace.Among them,the Brillouin Optical Time Domain Analysis(BOTDA)technology based on stimulated Brillouin scattering effect(SBS)has become one of the hot spots in the academic and application fields,due to its ability to realize distributed sensing with distance of up to hundreds of kilometers,spatial resolution as high as the scale of millimeter,as well as simultaneous measurement of temperature and strain.However,in the up-to-date dynamic measurement occasion,the traditional BOTDA measurement method of point-by-point frequency sweep fundamentally limits the measurement efficiency of the system,making it difficult to adapt to new application requirements.This dissertation focuses on the bottleneck of the dynamic response capability of BOTDA system.Inspired by the intrinsic connection between optical fiber sensing and optical fiber communication technology,combining with the advanced signal processing methods of optical orthogonal frequency division multiplexing(OFDM)technology,this dissertation proposes a BOTDA based on OFDM channel estimation(OFDM-BOTDA for short),in order to break through the limitations of traditional measurement schemes and improve the measurement efficiency of BOTDA systems.The main research work and innovations are summarized as follows:(1)The OFDM-BOTDA sensing technology is proposed for eliminating frequency sweeping that fundamentally limits the measurement efficiency in traditional BOTDA.Through in-depth analysis of the mathematical model of OFDM-BOTDA,the OFDM detection optical signal and demodulation algorithm are designed according to the characteristics of BOTDA system.By utilizing advantages of OFDM such as multi-carrier multiplexing,adaptive control of sub-carrier spacing to MHz level,flexible coding and decoding and advanced signal processing algorithm,a BOTDA without sweeping is implemented.The experimental results demonstrate the feasibility and reliability of the scheme.The 2km fiber measurement time is reduced to 30 ms,which greatly improves the measurement efficiency of BOTDA.In addition,the adverse effects of OFDM peak-to-average power ratio(PAPR)on the performance of OFDM-BOTDA system are studied.By analyzing the mathematical connotation of PAPR,it is proposed to directly map OFDM symbols using constant amplitude zero autocorrelation sequence(CAZAC),which greatly reduced signal PAPR to 4dB.(2)In order to improve the signal-to-noise ratio(SNR)for OFDM-BOTDA and avoid superposition average measurement,Golay pulse coding is proposed to further improve the system measurement efficiency.Based on the characteristics of OFDM-BOTDA,it is pointed out that the general time domain decoding technology is not applicable and a frequency domain decoding technique suitable for OFDM-BOTDA multi-carrier system is proposed.Using 512-bit Golay pulse coding,the measurement time of 10 km fiber is successfully reduced to 2ms.The measurement errors of Brillouin phase spectrum(BPS)and Brillouin gain spectrum(BGS)in OFDM-BOTDA system and the errors of Brillouin frequency shift(BFS)calculated by BPS and BGS curve fitting are analyzed.The results show that BFS calculated by BPS fitting presents smaller measurement uncertainty when using narrower fitting intervals,especially for pulse coding systems.(3)Aiming at improving the spatial resolution of the OFDM-BOTDA system,a multi-K value detection technique is proposed,combining the BPS / BGS ratio detection(named K-value detection)and OFDM multi-carrier characteristics.The limitation of pulse width and system noise on the effective linear region of K value in K-value detection technology is thoroughly analyzed.It is pointed out that multi-K value detection technology has larger measurement range and higher measurement accuracy.The influence of optical carrier crosstalk and subcarrier crosstalk on multi-K value detection technique in OFDM-BOTDA are detailly studied.The solutions of increasing the optical carrier and subcarrier frequency spacing and subcarrier interleaving are proposed respectively.When using OFDM signal with total bandwidth of 500 MHz,subcarrier number of 32 and pump pulse width of 50 ns,80MHz single K-value detection linear range and multi-K value detection based on 4 subcarriers are realized,and finally the system spatial resolution is improved to 6.8m.