Coherent Detection Brillouin Optical Time Domain Analysis Sensing Technology

Due to the unqiue characteristics of distributed optical fiber sensing(e.g.,low loss,low weight,and immune to electro-magnetic interference),it can realize distributed physical quantities measurements such as temperature,strain,pressure,magnetic field,and electricfield,and has been widely used in engineering construction,energy,chemical,aerospace,transportation,military and other fields.Realization of distributed fiber sensing is normally based on the natural scattering processes presented in an optical fiber,such as Rayleigh scattering,spontaneous Raman scattering,and spontaneous or stimulated Brillouin scattering.The backscattered signals can be interrogated in the time domain,frequency domain,or correlation domain,resulting in different types of distributed fiber sensors with their unique sensing properties.Among them,distributed Brillouin fiber sensors can offer remarkable sensing performance in terms of sensing distance,measurement certainty,and spatial resolution.For instance,a distributed fiber sensor with hundred kilometers of sensing distance,several centimeter or sub-centimeter spatial resolution,and sub-MHz measurement certainty has been experimentally demonstrated,by using interrogating methods such as Brillouin optical time domain analysis or Brillouin correlation domain analysis.Performance of the distributed Brillouin fiber sensor is directly decided by the signal-tonoise ratio(SNR)of the measured Brillouin gain(phase),such as measurement range,measurement certainty,and spatial resolution.In order to improve SNR,distributed Raman amplification,optical pulse coding,and coherent detection are the most-commonly used approaches.Besides,coherent detection can effectively eliminate the effect of pump power jitter,non-local effect,on the other hand,increase the dynamic measurement range,exhibiting some unique advantages as compared with the most-commonly used direct detection scheme.In this dissertation,we mainly focuse on coherent detection Brillouin optical time domain analysis sensor,theoretically analyzed and experimentally demonstrated the effects of chromatic dispersion(CD)and phase fluctuation induced by optial fiber link transmission on the extraction of Brillouin gain and phase,some approaches have been further proposed to eliminate their detrimental effects.Besides,we design an advanced Brillouin gain and phase extraction scheme,the sensing performance is dramatically improved by using Brillouin phase/gain ratio to retrieve BFS compared with current BFS retrival schemes.Detailed description of our work is listed as follows:(1)The detrimental effect of chromatic dispersion(CD)induced by optial fiber link transmission on the extraction of Brillouin gain is theoretically analyzed and experimentally demonstrated,when utilizing the commonly-used dual-sidebands phase modulation to generate Brillouin probe light.Two Brillouin probe light generation schemes,based on dual-sidebands intensity modulation and single-sideband intensity modulation,respectively,have been proposed to effectively eliminate the effect of CD.The averaged Brillouin frequency shift(BFS)estimation error along the whole sensing fiber is reduced by 6MHz,by using the proposed dual-sidebands intensity modulated probe compared with dual-sidebands phase modulated probe.Furthermore,using single-sideband intensity modulated probe to eliminate the effect of CD is more robust than using dual-sidebands intensity modulated probe,as it is independent of the sensing fiber length and the modulation frequency.(2)The effect of phase fluctuation induced by optial fiber link transmission on the extraction of Brillouin gain and phase has been theoretically analyzed and experimentally demonstrated.A phase fluctuation cancellation(PFC)scheme based on optical wavelength multiplexing has been proposed and experimentally demonstrated.Compared with the case without PFC,the BFS estimation errors are reduced from 4.9MHz to 0.4MHz(when fitting Brillouin gain spectrum)and from 2.6MHz to 0.2MHz(when fitting Brillouin phase spectrum)after PFC is performed.(3)The effects of phase fluctuation caused by optical fiber link transmission and Brillouin gain distortion induced by experimental components when using current Brillouin gain and phase extraction schemes have been theoretically analyzed and experimentally demonstrated.A coherent detection scheme with microwave interference carrier suppression is proposed to eliminate their effects for precise Brillouin gain and phase extraction.By further using Brillouin phase/gain ratio for BFS retrival instead of the combination of frequency scanning and curve fitting,sensing speed improvement by >30 times and measurement uncertainty reduction by ~4 times(assisted by digital signal denoising)have been achieved.Our researches mentioned above show the strategies to design robust,high speed,and high accuracy coherent detection Brillouin optical time domain analysis sensors,meanwhile light the potential ways for making further progress.