| In recent years,distributed fiber sensors based on forward stimulated Brillouin scattering(FSBS)have become a frontier hotspot in the field of fiber sensing because of their capability to detect the mechanical properties of material outside of the fiber.The unique transverse acoustic resonance produced by FSBS can not only measure the temperature and strain,but also detect the acoustic impedance of the external dielectric material outside of the fiber,which has great application potential in liquid detection,nano precision cladding diameter measurement,etc.Based on the basic theory of acoustic guided Brillouin scattering(GAWBS)and FSBS,this thesis described the principle of distributed liquid acoustic impedance measurement based on FSBS.A novel post-processing was proposed for distributed acoustic impedance sensing by extracting the 2nd-order local spectrum.The opto-mechanical time-domain analysis(OMTDA)sensing experiment was optimized,and the influences of nonlinear effects on FSBS sensors were discussed detailly.The main researched contents and novelty of this thesis are summarized as follows:(1)A new distributed FSBS acoustic impedance sensing scheme based on local spectrum of 2nd-order sideband demodulation was proposed.The previous reports were based on the 1st-order sideband to extract the local gain spectrum of FSBS.Our experiments showed that: The 1st-order sideband gain spectrum was greatly saturated after a certain distance,which limited the sensing distance due to the high-order sideband generation resulting from cross phase modulation(XPM)induced by FSBS.Therefore,by demodulating the 2nd-order sideband,the pulse energy can penetrate into the fiber deeply,thus the signal-to-noise ratio and sensing distance can be effectively improved while keeping the higher spatial resolution.Finally,combining the 1st-order and 2nd-order sideband demodulation,the liquid acoustic impedance measurement with 20 m spatial resolution was realized along 1630 m standard single-mode fiber.This work was of significance for the distance extension of distributed forward Brillouin sensors.(2)The impacts of nonlinear effects on distributed FSBS sensing was experimentally studied and the optimization approach was given,using optomechanical time-domain analysis(OMTDA)sensor as an example.The excitation of FSBS often requires high pulse power(Watt level)because of the lower gain coefficient.Due to the co-propagation of reading pulse and scattered light,high-power pulse will induce various nonlinear effects in FSBS sensing system.Using the previously reported method based on activation-reading time-domain separation,the influence of nonlinear effect due to activation pulse can be effectively avoided.However,the nonlinear effect of reading pulse had a direct impact on sensing performance.Based on this consideration,we studied the influence of nonlinear effects on FSBS sensing and its physical mechanism under different power of reading pulse.It was found that due to the automatic wave vector matching characteristics caused by copropagating,the dual frequency pulse components of the activation pulse modulation instability(MI)sideband and the stimulated Raman scattering(SRS)Stokes sideband still had a coherent effect on the induced acoustic field of FSBS,so the MI and SRS of the activation pulse had no significant effect on the excitation of FSBS.Under the condition of high-power reading pulse,the high-frequency and low-frequency power distribution of reading pulse and the extracted FSBS local spectrum showed serious frequency shift asymmetry.Kerr four-wave mixing(Kerr-FWM)and cascaded FSBS effects made complex energy transfer between different sidebands,which aggravated the above asymmetry.The variation process of the 1st and 2nd-order scattering sidebands with the reading pulse power along 4.7 km SMF was shown,and the optimization region was found,inside which a perfect FSBS local spectrum was obtained,and the sensing distance extension can be achieved. |
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