NonlinearopticalpropertiesofstimulatedBrillouinscattering(SBS)tosignaldetectioninwaterareanalyzed.Withthethresholdcharacteristics,SBSonlyoccurswhenthehighpowerlaserisfocusedintheSBScell.Whenthereisanobjectpresentinfrontofthefocus,itleadstolowerincidentintensityandthenSBSdoesnotoccur.ThebackwardSBSsignaldependsonthefocusinglocation.ThenonlinearopticalpropertiesofSBSprocessinthefocusingregimeareanalyzedtheoretically.Withtheobjectcomingneartothefocusingcenter,thebackwardStokessignalrisesupfromzerotoamaximum,andthengrowstosaturation.Thedelaytimeoftheechosignaltopumpsignalcangivetheobjectlocation.Inexperiment,thepeakpositionofvaryingrateofenergycangiveobjectlocation.
