Study On Long-distance BOTDA Fiber Sensing Based Ondistributed Brillouin Amplification

Stimulated Brillouin scattering(SBS)is widely used in the fields of distributed fiber sensing,fiber laser,and microwave photonics,et al..This thesis presents the major issues and research progress of the Brillouin optical time-domain analysis(BOTDA)sensor system,then gives the introduction of key devices in BOTDA.Furthermore,we proposed three types of novel structures for long-distance BOTDA using distributed Brillouin amplification(DBA),and a novel structure of coherent BOTDA using frequency-comb pulse for high-spatial-resolution sensing.The detailed work is summarized as follows:(1)In order to solve the frequency drift because of the independent pump source in DBA-based long-distance BOTDA,we proposed a novel BOTDA structure using firstorder DBA,which adopts a single laser.The frequency drift is completely avoided in this scheme.The feasibility of optical pulse coding(OPC)in this system is confirmed.The frequency comb technique is introduced in this system to expand the pulse gain spectrum;thereby the measurement error due to pulse distortion can be avoided.The influence of transient gain(TG)fluctuation is explored in detail.Results show that sufficient number of frequency combs can make the output pulse approach to initial shape in DBA-BOTDA using narrow pulse.In order to control the TG fluctuation in an acceptable range to suppress the decoding error,the number of frequency combs,coded pulse space and coding length must be optimized.The relevant research has been published on IEEE Photonics Technology Letters with first author.(2)A novel BOTDA configuration based on the second-order DBA was proposed,which can realize the more flat gain distribution.The signal-to-noise ratio(SNR)can be enhanced by ~3dB compared with the first-order DBA-based BOTDA.In addition,the power distributions of first-order DBA pump,second-order DBA pump and pulse were numerically simulated.Simulation result shows that the measured pulse agrees well with the theoretical prediction.Using the second-order pumping,~99km sensing distance and ~5m spatial-resolution was realized,in which only ~6dBm second-order DBA pump and ~1.5dBm first-order DBA pump were employed(two orders of magnitude lower than the distributed Raman amplification(DRA)based BOTDA).The relevant research has been published on Optics Express with second author.(3)The long-distance sensing characteristic of coherent BOTDA using frequencycomb-based DBA pumping was explored.Due to the use of coherent detection,faster data acquisition speed can be achieved by the simultaneous demodulation for multiple frequency-comb probe components.Because the DBA pump itself using frequencycomb acts as probe light,a wide-band gain from DBA pump is experienced for pulse light.Thus the frequency-shift dependence of waveform and peak power for pulse can be avoided,making the non-local effect to be suppressed significantly.Long-distance sensing along ~74.2 km fiber with a spatial resolution of ~6 m was demonstrated.(4)On the basis of coherent BOTDA using frequency-comb DBA pump,we proposed a novel frequency-comb-based high-spatial-resolution coherent BOTDA configuration which uses one of the double-sideband(DSB)pulses.Results show that,for the BOTDA system using DSB pulse,Brillouin gain spectrum(BGS)splitting is generated from the non-negligible impact of nonlinearity caused by the higher pulse peak power.The impact of nonlinear interaction on BGS reconstruction can be suppressed by using one of the side-bands.High-spatial-resolution sensing can be achieved by combining this technique with phase-shift pulse(PSP).Experimentally,sensing with a spatial resolution of ~60cm along ~592m fiber was realized.The relevant researches of(3)and(4)have been published on Optics Express with second author.