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.　One　approach　for　obtaining　such　high-power　ultrashort　pulses　is　the　use　of　the　amplification　systems,　such　as　chirped　pulse　amplification　(CPA)　or　a　master　oscillator　power　amplifier　(MOPA).　However,　amplifier　chains　produce　amplified　spontaneous　emission　(ASE),　and　the　complex　structure　of　the　amplifier　reduces　the　stability　of　the　system.　Another　method　is　to　use　the　direct　output　from　the　modelocked　oscillator,　represented　by　a　semiconductor　saturable-absorber　mirror　(SESAM)　mode-locked　disk　laser,　which　was　demonstrated　in　2000.　The　disk　gain　medium　has　a　high　area-to-volume　ratio,　which　leads　to　excellent　heat　dissipation　and　therefore　stabilizes　the　operation　at　high　power.　However,　its　low　single-pass　gain　requires　a　complex　multi-pass　pump　for　improving　the　absorption　efficiency　of　the　pumped　light,　which　can　reduce　the　system　stability.　In　2022,　our　group　reported　a　large　core-size　crystal　waveguide　mode-locked　laser,　providing　a　new　solution　for　realizing　high-power　mode-locked　lasers.　Crystal　waveguides　used　as　gain　media　have　good　mode　limiting　capability　and　high　heat　dissipation　capability,　and　they　can　achieve　a　stable　high-power　output.　We　believe　that　this　new　mode-locked　oscillator　has　potential　for　power　scaling.　As　studies　related　to　dispersion　compensation　for　crystal　waveguide　mode-locked　lasers　have　been　scarce,　investigation　of　dispersion　compensation　to　find　ways　to　increase　power　is　essential.　Methods　The　semiconductor　saturable-absorber　mirror　used　for　mode-locking　introduces　the　tendency　of　the　laser　towards　Qswitching　instabilities;　thus,　the　Q-switching　mode-locking　(QML)　operation　of　the　laser　can　be　realized　with　a　repetition　frequency　of　the　order　of　kHz.　This　leads　to　the　generation　of　high-energy　pulses　that　will　cause　irreversible　damage　to　the　SESAM.　The　cavity　design　follows　the　conditions　proposed　by　Hönninger　et　al.　with　regard　to　achieving　continuous-wave　mode　locking　(CWML).　We　calculate　the　threshold　powers　for　different　spot　radii　on　the　SESAM　and　different　cavity　lengths　(Fig.　1)　and　then　select　suitable　cavity　parameters.　The　spot　radius　on　the　SESAM　is　set　as　200　μm,　the　cavity　length　is　4.5　m,　and　the　theoretical　value　of　power　for　CWML　is　3.4　W.　To　achieve　a　stable　CWML,　a　commercial　SESAM　with　a　small　modulation　depth　is　selected,　an　output　coupler　with　a　lower　transmittance　is　used,　multiple　4f　systems　are　employed　to　increase　the　cavity　length,　and　the　focal　length　ratio　of　the　last　set　of　mirrors　is　changed　to　scale　the　spot　radius　on　the　SESAM.　Furthermore,　we　analyze　the　main　sources　of　dispersion　in　the　cavity　and　experimentally　implement　dispersion　compensation　by　inserting　five　Gires-Tournois-Interferometer　(GTI)　mirrors　in　the　cavity　(Fig.　3)　and　the　-23500　fs2　group　velocity　dispersion　(GVD)　per　round　trip　is　achieved.　The　experimental　results　show　a　significant　improvement　compared　to　the　configuration　without　dispersion　compensation,　and　no　obvious　saturation　is　observed　in　the　output　power　curve　[Fig.　5(a)],　indicating　further　power　expansion.　Results　and　Discussions　At　a　pump　power　of　160　W,　the　waveguide　core　absorbs　a　pump　power　of　84　W,　the　pump　absorption　efficiency　is　52.5%,　the　output　power　is　21　W,　and　the　optical-to-optical　efficiency　is　25%.　The　beam　quality　values　in　the　x　and　y　directions　of　the　output　beam　are　1.17　and　1.05,　respectively,　which　indicates　impressive　beam　quality.　The　wide-span　radio　frequency　(RF)　spectrum　and　autocorrelation　curve　indicate　single-pulse　operation.　Figure　6(a)　shows　a　single-pulse　shape,　illustrating　an　autocorrelation　trace　with　an　full　width　at　half-maximum　(FWHM)　of　∼　2　ps,　corresponding　to　a　time-bandwidth　product　of　0.4.　The　time-bandwidth　product　is　closer　to　the　Fourier　limit　compared　with　that　of　the　configuration　without　dispersion　compensation.　We　analyze　the　reasons　for　the　low　optical-to-optical　efficiency　and　suggest　ways　for　further　power　expansion:　1)　No　significant　saturation　is　observed　in　the　experiment　and　a　higher　output　can　be　obtained　when　a　high-power　pump　laser　is　used.　2)　Using　a　crystal　waveguide　with　a　large　core　size　as　a　double-cladding　structure,　optimizing　the　length　of　the　crystal　waveguide,　or　connecting　multiple　crystal　waveguides　in　series　can　help　to　improve　the　pumping　efficiency　and　increase　the　output　power.　3)　The　focal　length　ratio　of　the　last　set　of　mirrors　can　be　adjusted　so　that　the　spot　area　on　the　SESAM　will　also　increase　by　a　corresponding　multiple;　when　the　power　in　the　cavity　increases　by　a　certain　multiple,　then　stable　mode　locking　can　be　achieved.　4)　With　an　increase　in　the　output　power,　the　use　of　an　output　coupler　with　a　higher　transmission　ratio　can　improve　the　slope　efficiency　of　the　laser.　Conclusions　An　all-solid-state　passively　mode-locked　laser　based　on　a　Yb∶YAG　large-core-diameter　crystal　rectangular　waveguide　is　reported.　Using　a　GTI　mirror　to　compensate　the　dispersion　in　the　cavity　as　well　as　using　a　laser　with　average　power　of　16　W,　a　pulse　width　of　2　ps,　time-bandwidth　product　of　0.4,　and　repetition　rate　of　31.7　MHz　can　be　obtained.　The　mode-locked　output　characteristics　of　the　large-core-diameter　crystal　rectangular　waveguide　mode-locked　laser　are　experimentally　studied.　The　main　sources　of　intracavity　dispersion　are　analyzed,　and　methods　to　expand　the　output　power　are　proposed.　©　2023　Science　Press.　All　rights　reserved.