Research On Distributd Optical Fiber Sensor Based On Stimulated Brillouin Scattering

Distributed fiber optic sensing technologies have been widely used in energy security,structure monitoring,smart city and other fields due to the advantages of small size,low cost and immune to electromagnetic interference.Among different existing technologies,Brillouin optical time-domain analysis(BOTDA)has attracted substantial attention due to its capability to measure environmental quantities over very long optical fibers with a high spatial resolution.However,the allowable probe power is limited by the non-local effect in BOTDA system,thus the sensing performance is difficult to further improve.When the pulse used in BOTDA is shorter than 12 ns,the incomplete activation of the acoustic filed results in a large broadening of the Brillouin gain/loss spectrum(BGS/BLS)and a reduction of its peak amplitude,as a result,the spatial resolution is limited to 1 m.It is proved that the noise in long-distance BOTDA systems is mainly determined by the receiver,but the impact of detection schemes on the sensing performance still remains unclear.Besides,the measurement accuracy is related to the BGS linewidth,but the fixed spatial resolution in the current BGS linewidth reduction scheme limits the flexibility of BOTDA sensor.In this thesis,several innovative schemes are proposed to overcome the limitations and to further improve the sensing performances.The main research efforts are summarized as follow.1)The robustness of BOTDA sensor based on dual-tone probe wave with fixed frequency separation is deeply studied.It is discovered that when the distribution of Brillouin frequency shift(BFS)along the sensing fiber is not entirely uniform,non-local effect might be imposed at the front part of sensing fiber,which gives rise to systematic frequency error on the local estimated BFS.Aiming at solving this problem,we propose an approach that acquires both the upper and lower probe sidebands simultaneously using two photodiodes,and the average between the Brillouin gain and loss spectral is calculated to eliminate the detrimental impact of the non-local effect.Moreover,the impact of probe power on the distortions of both pump pulse and Brillouin gain/loss spectrum is experimentally investigated.Applying the proposal,the maximum allowable power of the injection probe wave is improved from-8 dBm/sideband to 0 dBm/sideband.Besides,a new BOTDA technique for enhancing the probe power up to+5 dBm is proposed and demonstrated,which is based on a conventional dual sideband probe setup but the probe waves are modulated by a FSK signal.In this technique,the pump distortion is compensated in the time domain by the FSK probe signal.By employing a 105 km sensing fiber,this technique is experimentally validated with 2 m spatial resolution.Furthermore,this technique shows a good tolerance for the sensing fibers with large Brillouin frequency shift difference.2)The signal-to-noise ratio(SNR)of direct-detection BOTDA is modelled and experimentally validated,with and without the use of optical pre-amplification.Theoretical and experimental results verify that increasing the probe power reaching the photodetection is always beneficial to enhance the SNR.It turns out that,the pre-amplification together with a good-quality photodetector and a proper post-processing filtering,can achieve the highest SNR for direct-detection BOTDA.Such an optimal SNR presents only a 2.3 dB penalty compared to the ideal shot noise limited technique,which however must be accomplished by rather sophisticated configurations.In addition,the model here established enables the theoretical evaluation and prediction of the SNR at any fiber position for any given experimental condition.Moreover,the impact of fiber parameters on BOTDA system is investigated theoretically and experimentally.The results show that the attenuation coefficient of optical fiber plays an important role in long-distance BOTDA,while the impact of effective core area can be ignored.A 100 km-long sensing range with 2 m resolution is achieved using low loss fiber in the dual-sideband BOTDA system.3)In order to both adjust the spatial resolution and narrow the BGS linewidth in BOTDA system,a novel scheme based on differential pulse pair technique is proposed.By designing the phase and amplitude of the pulse properly,the higher peak gain and narrower linewidth of BGS are achieved compared to the conventional single pulse scheme with same spatial resolution.The pulse parameters for different spatial resolution are analytically solved and the performances of the sensor are theoretically and experimentally studied.Experimental results show that the linewidth of BGS is narrowed to 19 MHz and 22 MHz in the case of 2 m spatial resolution and 1 m spatial resolution,which indicates 2.3 times and 4.3 times frequency accuracy improvements are achieved,respectively.Furthermore,the advantages of our proposal for small Brillouin frequency shift(BFS)change and the capability for sub-meter spatial resolution are also investigated.4)A novel post-processing technique is proposed to retrieve.a flexible and variable spatial resolution from conventional Brillouin optical time-domain analyzers,using a pulse longer than the acoustic settling time.The negative impact of acoustic transient effect is eliminated,enabling a Brillouin response proportional to the spatial resolution and a Brillouin gain spectrum close to natural linewidth.This leads to a better overall sensing performance,in particular for sub-metric spatial resolutions,with no compromises on sensing range and measurement time.The proposed technique realizes 10 cm spatial resolution in a 10-km-long sensing fiber.In addition,the scheme has the advantages of simple experimental structure,fast data processing time and stable measurement over long time.