High Spatial Resolution Sensing Based On Forward Stimulated Brillouin Scattering

Guided acoustic wave Brillouin scattering has gained considerable interests in recent years owing to its capacity to detect mechanical features of materials surrounding the optical fiber.More recently,related research has successfully realized distributed liquid sensing based on forward stimulated Brillouin scattering?FSBS?,which has great potential in oil and gas exploration,ocean exploration,pollution monitoring,leak detection,etc.However,due to the long acoustic wave lifetime and poor signal-to-noise ratio?SNR?,current spatial resolution is restricted to several decameter to one hectomete,indicating that distributed FSBS sensor is far from practical expectations.This paper aims to propose a new sensing scheme which will improve the spatial resolution and thus greatly improve the practicability of distributed FSBS sensor.Here we propose a novel optomechanical time-domain analysis?OM-TDA?with compact setup to greatly improve the performance of spatial resolution.The separation of activation and probing processes for acoustic wave permits short probe pulse to be employed,eliminating the restriction of acoustic wave lifetime.Coherent stimulated forward Brillouin scattering is employed,Brillouin optical time-domain analysis is used to detect the intensity evolution of probe pulse,combined with more robust demodulation algorithm and auxiliary technique to strengthen the acoustic wave,considerably enhancing the SNR.In the theoretical research,mathematical model of OM-TDA is established,based on which we anticipated the experimental results and analyzed the influence of FSBS intensity and Kerr effect.The simulation manifested that the four-wave mixing terms in the Kerr effect cause distortion of the FSBS spectrum.When the injected optical power is high,the demodulation algorithm deviates greatly from the actual situation,and the demodulation error of the system increases.These results guide us on how to choose the power of light.In the experimental research,the FSBS gain of different transverse acoustic modes is measured,among which theR_0,7 mode has the largest gain.The phase sensitive characteristic in the OM-TDA is studied.The phase difference between the beatnote of activation pulse and that of probe pulse greatly affects the strength and direction of FSBS energy transfer in the read pulse.The effects of excitation pulse width and read pulse width on the FSBS spectrum are studied.Longer excitation pulse generates narrower the FSBS spectrum,while the read pulse width has no effect on FSBS spectrum.The pseudo-random phase-encoded four-tone excitation pulse is employed to generate stronger acoustic field,enhancing the intensity of FSBS interaction.Based on theoretical and experimental research,high spatial resolution sensing experiment is carried out by selecting reasonable experimental parameters.A distributed measurement with 2 m spatial resolution is demonstrated in air and alcohol identification,which is almost one order magnitude better than other works and greatly fosters its practical applications.