Objective　High-power　ultrashort-pulse　lasers　operating　in　the　picosecond　and　femtosecond　time　domains　have　important　applications　in　strong-field　physics,　biomedical　imaging,　optical　frequency　conversion,　and　nuclear　laser　fusion.　The　direct　output　of　mode-locked　lasers　has　a　more　promising　signal-to-noise　ratio,　beam　quality,　and　stability　than　the　power　amplification　method　for　high-power　ultrashort　pulse　generation.　Commonly　used　mode-locking　methods　for　high-power　mode-locked　lasers　include　semiconductor　saturable　absorber　mirror　mode-locking　(SESAM),　Kerr　lens　mode-locking　(KLM),　and　non-linear　mirror　modelocking　(NLM).　In　particular,　the　NLM　mode-locking　method,　which　utilizes　a　nonlinear　crystal　and　an　output　coupling　mirror,　has　demonstrated　significant　potential　owing　to　its　high　stability,　wide　range　of　applicable　wavelengths,　and　large　modulation　depth.　However,　achieving　saturation　in　the　NLM　method　requires　a　high　power　density,　resulting　in　a　peak　power　density　within　the　cavity　that　is　typically　lower　than　that　required　for　reflectivity　saturation.　The　quality　factor　(Q)　of　a　cavity　in　an　NLM　mode-locked　laser　increases　with　the　peak　power　density　within　the　cavity.　This　phenomenon　results　in　Q-switching　instability,　making　it　challenging　to　attain　stable　continuous-wave　mode-locking　(CWML).　This　challenge　can　be　addressed　by　introducing　an　early　saturation　tendency　for　the　nonlinear　reflectivity　of　NLM　devices.　This　ensures　a　balance　between　gain　and　loss　at　the　saturation　point,　ultimately　achieving　stable　continuous-wave　mode-locked　pulses.　Hence,　the　development　of　a　straightforward,　stable,　and　dependable　method　to　achieve　early　saturation　is　important　for　achieving　high-power　mode-locked　outputs.Methods　In　this　paper,　we　present　a　novel　NLM　structure　involving　two　crystals　that　undergo　frequency　doubling　twice.　This　design　introduces　loss　by　incorporating　a　crystal　that　doubles　the　second　harmonic,　achieving　early　saturation　of　reflectivity　to　effectively　suppress　Q-switching　instability　and　facilitate　CWML.　Based　on　the　single-crystal　NLM　nonlinear　reflectance　expression,　we　derive　an　NLM　reflectance　expression　for　the　two　crystals　undergoing　frequency　doubling　twice.　The　materials　and　lengths　of　the　two　crystals　are　determined　using　a　reflectance　formula　combined　with　their　crystal　properties.　The　calculation　results　show　that　the　addition　of　a　second　frequency-doubling　crystal　introduces　a　rollover　in　the　nonlinear　reflectivity　curve,　thereby　forming　a　new　saturation　point.　This　reduces　the　saturated　peak　power　density　and　modulation　depth　of　the　NLM,　theoretically　proving　the　potential　of　suppressing　Q-switching　and　mode-locking.　The　experimental　validation　involved　a　cavity　comprising　a　semiconductor　laser　pump　source,　a　large-core　crystal　waveguide,　a　4f　system,　a　dispersion　compensation　device,　a　polarizer,　and　an　NLM　device.　In　this　experiment,　the　first　frequency-doubling　crystal　was　deployed　and　adjusted　to　the　optimal　state,　which　resulted　in　unstable　Q-switched　and　mode-locked　pulses.　Subsequently,　with　the　addition　of　a　second　frequency-doubling　crystal　and　adjustments　under　the　same　conditions,　the　output　signal　transitioned　from　unstable　Q-switching　and　mode-locking　to　stable　CWML.　This　observation　proves　the　effectiveness　of　our　early　saturation　method　employing　a　second　frequency-doubling　crystal　to　achieve　CWML.Results　and　Discussions　The　output　power　of　the　laser　varies　with　the　pump　power　[Fig.　4(a)],　with　a　maximum　value　of　26.5　W.　In　the　mode-locking　experiment,　the　laser　spectral　curve　at　the　maximum　output　power　[Fig.　4(b)]　exhibits　a　center　wavelength　of　1030.1　nm　and　a　spectral　width　of　1.3　nm.　A　signal　diagram　of　the　mode-locked　pulses　detected　by　the　oscilloscope　is　shown　in　Fig.　4(c).　The　radio　frequency　spectrum,　presented　in　Fig.　4(d),　reveals　a　repetition　frequency　of　31.2　MHz　and　a　signal-to-noise　ratio　of　46　dB,　demonstrating　excellent　inter-pulse　stability　without　side　peaks.　The　autocorrelation　curve　in　Fig.　4(e)　exhibits　a　modelocked　width　of　0.95　ps.　Fig.　4(f)　depicts　the　measurements　of　the　beam　radius　with　a　calculated　beam　quality　factor　of　M2=1.05.Conclusions　In　this　paper,　we　present　a　novel　NLM　crystal　waveguide　laser　that　effectively　suppresses　the　Q-switching　instability.　The　position　of　the　saturation　point　can　be　tuned　by　adjusting　the　crystal　thickness,　thereby　achieving　a　simple,　stable,　and　reliable　structure.　We　derive　a　nonlinear　reflectivity　expression　and　verify　that　the　reflectivity　produces　a　rollover　to　achieve　early　saturation.　Stable　continuous　mode-locking　can　be　experimentally　obtained　to　verify　the　feasibility　of　this　structure.　A　CWML　output　with　an　average　power　of　26.5　W,　a　pulse　width　of　0.95　ps,　and　a　repetition　frequency　of　32.2　MHz　is　obtained.　Future　enhancements,　such　as　utilizing　a　double-clad　crystal　waveguide　and　optimizing　the　equivalent　transmittance,　are　expected　to　achieve　higher-power　CWML　pulse　outputs.　This　novel　NLM　device　exhibits　significant　potential　for　the　development　of　high-power　mode-locked　lasers.　©　2024　Science　Press.　All　rights　reserved.