Research On The Code Based Botdr System

Brillouin optical time-domain reflectometry(BOTDR)is a spontaneous Brillouin backscattering based distributed optical fiber sensing technology.By analyzing the detected Brillouin gain spectrum of spontaneous Brillouin backscattering,we can obtain the distributed temperature and strain along the fiber.Because of its good flexibility by accessing only one end of fiber to perform the measurement and the ability of breakpoint measurement,BOTDR has good potential in the health monitoring of large-scale structures such as dams,roads,bridges,oil and gas pipeline and submarine cables.By using an appropriate coded pulse as probe pulse in BOTDR,one can not only improve the SNR but also maintain the spatial resolution,which is a very practical way to improve dynamic range.However,the BOTDR sensor based on the code technique still has some shortcomings.Firstly,since the spontaneous Brillouin scattering signal in BOTDR is very weak,the coded probe pulse(CPP)needs to be amplified by an EDFA to achieve high SNR for the signal,before being input into the sensing fiber.However,the power of a long CPP is not flat because of the transient effect of EDFA,leading to the distortion of the decoded BGS distribution.In addition,it takes long time to obtain the BGS when using of traditional frequency sweeping method.In this work,two improved schemes have been made for the code-based BOTDR sensor to overcome the above two shortcomings.Firstly,the operation principle and the transient effect of EDFA are analyzed fundamentally,and a pre-depletion two-wavelength probe pulse is proposed to suppress the distortion of the coding probe pulse induced by EDFA.The effect of the proposed pre-depletion two-wavelength probe pulse is verified from the simulation and experiment.A comparing experiment research is made between the effect of coded probe pulse with and without the pre-depletion pulse.The experiment results indicate that the proposed pre-depletion two-wavelength probe pulse is beneficial to the improvement of the frequency measurement accuracy.Secondly,a BOTDR sensor that based on the combination of fast Fourier transform(FFT)technique and code technique is proposed by combining the FFT technique to the code-based BOTDR,to reduce the measuring time.The scheme of acquisition of spontaneous Brillouin scattering signal and the data processing are also proposed.Experiments are carried out beyond 10 km single-mode fiber,and the results show the capabilities of the proposed scheme to achieve 2m spatial resolution with 0.37MHz frequency uncertainty.The measurement time can be more than tens of times faster than traditional frequency sweeping method in theory.