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

简介：Backconversionisanintrinsicphenomenoninnonlinearfrequencydown-conversionprocesses.However,thephysicalreasonforitsoccurrenceisnotwellunderstood.Here,wetheoreticallyrevealthatbackconversionistheresultofaπ-phasejumpassociatedwiththedepletionofoneinteractingwave.Bysuppressingtheidlerphasejumpthroughadeliberatecrystalabsorption,thebackconversioncanbeinhabited,thusenhancingtheconversionefficiencyfromthepumptothesignal.TheresultspresentedinthisLetterwillfurthertheunderstandingofnonlinearparametricprocessesandpavethewaytowardthedesignofhighlyefficientdown-conversionsystems.