Objective　A　vertical-　external-　cavity　surface-　emitting　laser　(VECSEL)　offers　unique　advantages　such　as　high　power,　good　beam　quality,　and　designable　emitting　wavelength.　Additionally,　the　external-　cavity　structure　allows　for　the　convenient　insertion　of　other　optical　components,　thus　enabling　the　VECSEL　to　operate　in　mode-　locking,　single-　frequency　running,　or　wavelength-　tuning　mode.　These　characteristics　render　the　VESCEL　a　desirable　candidate　for　various　applications　not　realizable　by　conventional　lasers　(e.　g.,　solid-　state　lasers,　gas　lasers,　fiber　lasers,　or　laser　diodes).　In　particular,　the　peak　power　pulses　generated　by　a　mode-　locked　VECSEL　can　be　used　in　diverse　fields　such　as　multiphoton　imaging,　high-　resolution　time-　domain　terahertz　spectroscopy,　and　supercontinuum　generation.　However,　obtaining　high　peak-　power　pulses　from　a　mode-　locked　VECSEL　is　not　trivial.　To　increase　the　peak　power　of　mode-　locked　pulses,　one　can　increase　the　average　output　power　of　the　laser,　reduce　the　pulse　time　width,　or　reduce　the　pulse　repetition　rate.　However,　these　three　methods　are　mutually　restrictive;　thus,　the　pulse　peak　power　in　a　mode-　locked　VECSEL　can　only　be　improved　to　a　certain　extent.　In　particular,　the　carrier　lifetime　of　the　semiconductor　gain　media　used　in　a　VECSEL　is　short,　i.　e.,　in　the　nanosecond　level,　which　significantly　limits　the　further　reduction　of　the　repetition　rate　of　mode-　locked　laser　pulses.　Consequently,　the　increase　in　the　peak　power　of　pulses　generated　from　the　mode-　locked　VECSEL　is　restricted　considerably.　Methods　In　this　study,　a　custom-　designed　saturable　Bragg　reflector　(SBR)　was　used　as　a　saturable　absorber,　where　a　moderate　saturation　fluence　can　effectively　balance　between　the　cycling　power　and　multi-　pulse　generation　in　the　resonant　cavity,　thereby　maintaining　a　sufficiently　high　average　output　power　at　low　repetition　rates　and　significantly　improving　the　peak　power　of　the　mode-locked　laser　pulses.　The　epitaxial　structure　of　the　gain　chip　used　in　the　experiment　(Fig.　1)　is　a　reverse-　order　structure　of　the　active　region,　followed　by　a　distributed　Bragg　reflector　(DBR).　First,　an　Al0.86GaAs　etching　stop　layer　was　deposited　on　the　GaAs　substrate.　Subsequently,　a　GaAs　protective　layer,　a　high-barrier　Al0.6GaAl　window　layer,　an　active　region,　and　a　DBR　were　deposited.　Finally,　the　entire　structure　was　terminated　with　an　oxygen-resistant　GaAs　layer.　Unlike　gain　chips　achieved　via　reverse　growth,　the　SBR　exhibits　an　epitaxial　structure　(Fig.　1)　that　is　consistent　with　the　normal　order　of　a　DBR　followed　by　an　absorption　region.　The　saturable　absorber　is　a　single　InGaAs　quantum　well　located　in　the　final　quarter-wavelength　layer　of　the　DBR,　with　a　thickness　of　10　nm.　The　quantum　well　is　intentionally　set　at　the　peak　position　away　from　the　laser　standing　wave　to　obtain　a　large　saturation　fluence.　Additionally,　because　the　saturation　fluence　of　the　SBR　should　not　be　excessively　high,　we　implemented　a　single　quantum　well　with　a　commonly　used　thickness　of　10　nm.　The　laser　resonant　cavity　used　in　the　experiment　(Fig.　2)　comprises　six　mirror　cavities,　including　a　DBR　each　at　the　bottom　of　both　the　gain　chip　and　SBR.　Except　for　the　output　coupler,　which　presents　a　certain　transmittance　at　the　laser　wavelength,　all　other　mirrors　demonstrate　high　reflectivity　at　the　laser　wavelength.　Results　and　Discussions　Based　on　the　experimental　result,　the　temporal　waveform　of　the　pulses　exhibits　double　or　triple　pulses　connected　to　each　other　(Fig.　3).　We　believe　that　this　may　be　due　to　the　low　intensity　of　the　intra-cavity　pulses　(corresponding　to　a　relatively　high　saturation　fluence　of　the　SBR)　and　the　inadequateness　of　one　pulse　in　fully　saturating　the　SBR.　Therefore,　before　the　SBR　is　fully　recovered,　absorption　continues　to　occur　on　one　or　two　following　pulses,　thus　resulting　in　double　or　triple　pulses.　Stable　continuous-wave　(CW)　fundamental　mode-locked　pulse　trains,　steady　second-harmonic　mode-locked　pulses,　and　higher-order　unstable　harmonic　mode-locked　pulses　are　obtained　(Fig.　5).　In　our　opinion,　the　high-order　harmonics　occurred　because　as　the　pump　power　increases,　the　pulse　intensity　inside　the　cavity　increases,　thus　resulting　in　a　low　saturation　fluence　of　the　SBR.　After　a　pulse　saturates　the　SBR,　owing　to　the　long　resonant　cavity　length,　the　SBR　has　sufficient　time　to　recover.　Therefore,　one　or　more　subsequent　pulses　can　saturate　the　SBR　again,　thus　resulting　in　multiple　pulses　in　the　cavity,　which　eventually　evolve　into　higher-order　harmonic　mode　locking.　The　interconnected　double　or　triple　pulses　mentioned　above,　as　well　as　the　occurrence　of　high-order　harmonic　mode　locking　can　be　described　by　numerically　solving　the　delay　differential　equations　for　passive　mode　locking,　and　the　evolution　of　intra-cavity　pulses　over　time　can　be　obtained　(Fig.　4).　The　measured　fundamental　CW　mode-locked　pulse　has　a　period　of　approximately　14.92　ns,　a　repetition　rate　of　67　MHz,　and　a　pulse　width　of　2.08　ps　(Fig.　6).　When　the　absorbed　pump　power　increases　to　18.9　W,　an　output　power　of　0.325　W　can　be　obtained.　When　the　absorbed　pump　power　exceeds　21.7　W,　the　maximum　output　power　of　the　second　harmonic　mode　locking　is　0.836　W.　The　maximum　output　power　of　the　fourth-harmonic　mode　locking　is　0.683　W　(Fig.　7).　Under　fundamental,　second-　and　fourth-harmonic　mode　locking,　the　maximum　peak　powers　of　the　pulses　are　2.33,　3.00,　and　1.23　kW,　respectively.　Conclusions　The　short　carrier　lifetime　(of　nanosecond　level)　of　a　VECSEL　significantly　limits　the　further　reduction　of　the　pulse　repetition　rate　under　passive　mode　locking,　thereby　limiting　the　improvement　to　the　pulse　peak　power.　For　saturable　absorbers　used　to　commence　mode　locking,　a　relatively　high　saturation　fluence　may　generate　interconnected　double　or　triple　pulses,　whereas　a　relatively　low　saturation　fluence　may　result　in　multiple　pulses　in　the　resonant　cavity.　This　study　utilizes　a　custom-designed　SBR　with　a　moderate　saturation　fluence　to　achieve　both　low　repetition　rates　and　high　average　output　power　levels　in　a　passive　mode-locked　VECSEL.　We　experimentally　achieved　a　repetition　rate　of　67　MHz.　In　the　case　of　fundamental,　second-　and　fourth-harmonic　mode　locking,　the　corresponding　peak　powers　of　the　pulses　are　2.33,　3.00,　and　1.23　kW,　respectively,　which　demonstrate　a　passive　mode-locked　VECSEL　with　a　repetition　rate　below　100　MHz　and　a　peak　power　above　1　kW　simultaneously.　©　2025　Science　Press.　All　rights　reserved.