Tunable and switchable harmonic h-shaped pulse generation in a 3.03 km ultralong mode-locked thulium-doped fiber laser

Weexperimentallydemonstratedatypeoftunableandswitchableharmonich-shapedpulsegenerationinathulium-dopedfiber(TDF)laserpassivelymodelockedbyusinganultralongnonlinearopticalloopmirror.Thetotalcavitylengthwas~3.03km,thelongesteverbuiltforaTDFlasertoourbestknowledge,whichresultedinanultralargeanomalousdispersionover-200ps~2aroundtheemissionwavelength.Theproducedh-shapedpulsecanoperateeitherinafundamentalorinahigh-orderharmonicmode-locking(HML)statedependingonpumppowerandintra-cavitypolarizationstate(PS).Thepulseduration,nomatteroftheoperationstate,wastunablewithpumppower.However,pulsebreakingandself-organizingoccurred,resultinginhigh-orderHML,whenthepumppowerincreasedaboveathreshold.Atafixedpumppower,theorderofHMLwasswitchablefromonetoanotherbymanipulatingthePS.Switchingfromthe8thuptothe48thorderofHMLwasachievedwithafixedpumppowerof~4.15W.OurresultsrevealedthedetailedevolutionandswitchingcharacteristicsoftheHMLandindividualpulseenvelopewithrespecttoboththepumppowerandPS.Wehavealsodiscussedindetailthemechanismsofboththeh-shapedpulsegenerationandtheswitchingofitsHML.Thiscontributionwouldbehelpfulforfurtherin-depthstudyontheunderlyingdynamicsoflong-durationparticular-envelopepulseswithultralargeanomalousdispersionandultralongroundtriptime.