Objective:　Planar　waveguide　amplifiers　have　the　advantages　of　slab　and　fiber　ones　and　become　an　essential　branch　of　the　high　average　power　solid-state　lasers.　Yd:YAG　planar　waveguide　amplifiers　have　the　potential　for　higher　output　than　Nd:YAG　due　to　the　less　generated　heat　and　higher　extraction　efficiency　under　the　same　absorption　pump　power.　The　absorption　cross-section　of　Yd:YAG　is　smaller　than　Nd:YAG;　a　higher　doping　concentration　of　the　core　and　end-pumping　must　fully　absorb　the　pump　power.　To　maximize　extraction　efficiency,　the　doping　concentration　is　significantly　reduced　due　to　self-absorption.　To　ensure　that　the　pump　power　can　be　fully　absorbed,　extending　the　length　of　the　core　is　necessary.　However,　the　size　of　the　core　is　limited　by　the　process　conditions.　If　the　doping　concentration　is　sufficiently　small,　the　end-pumping　cannot　meet　the　absorption　of　pump　power　requirements.　In　this　study,　we　design　a　multipass　pumped　planar　waveguide　with　the　low-doped　Yd:YAG　core　to　prolong　the　pump　absorption　length.　The　amplifier　using　a　multipass　pumped　planar　waveguide　shows　higher　pump　absorption,　better　absorption　uniformity,　and　higher　optical-optical　efficiency　than　the　double-clad　planar　waveguide　amplifier.　We　hope　that　the　new　structure　can　provide　methods　and　ideas　for　designing　and　optimizing　planar　waveguide　lasers.　Methods:　Based　on　the　theory　of　single-mode　transmission　and　laser　mode　competition　in　the　planar　waveguide,　the　core　and　inner　cladding　of　the　planar　waveguide　are　Yb:YAG　with　a　thickness　of　0.2　mm　and　a　doping　concentration　of　1%　and　Er:YAG　with　a　thickness　of　0.5　mm　and　a　doping　concentration　of　0.5%,　respectively.　The　core　and　inner-claddings　form　the　core　area,　which　is　80.0　mm×16.0　mm×1.6　mm.　YAG　is　bonded　around　the　core　area　with　a　thickness　of　1.2　mm.　The　outer-claddings　of　1　mm　cover　the　core　and　expansion　areas,　forming　a　double-cladding　waveguide　with　the　core　area　and　a　single-cladding　waveguide　with　the　expansion　area.　A　multipass　pumping　ray　path　in　the　waveguide　is　formed　through　internal　reflections　on　the　waveguide　surfaces.　Symmetrical　double-end　pumped　and　single-pass　power　extraction　configurations　are　adopted　for　laser　amplification.　Based　on　the　Yb3+　laser　kinetics　model,　we　develop　a　three-dimensional　(3D)　laser　amplification　model　using　ray　tracing　and　finite　element　methods.　We　use　the　model　to　simulate　double-clad　planar　waveguide　and　multipass　pumped　planar　waveguide　amplifiers.　Besides,　we　compare　their　pump　absorption　and　amplification　extraction　characteristics　under　10　kW　pump　power　and　200　W-injected　seed　power.　The　temperature　distribution　is　simulated　based　on　the　results　from　the　laser　magnification　model.　Results　and　Discussions:　Compared　with　a　double-clad　planar　waveguide　amplifier,　a　multipass　pumped　planar　waveguide　amplifier　exhibits　higher　output　power　and　optical-optical　efficiency　(Fig.9).　The　simulation　results　using　the　laser　amplification　model　show　that　the　planar　waveguide　core　absorption　coefficient　is　0.24　cm-1,　which　is　77.3%　lower　than　the　passive　absorption　coefficient,　strongly　affected　by　nonlinear　absorption　(Fig.11).　The　pump　absorption　decreases　with　a　decrease　in　the　absorption　coefficient.　The　multipass　pump　absorption　efficiency　is　still　above　90%　due　to　an　increase　in　the　absorption　length　(Table　2).　The　absorption　power　density　distribution　of　the　core　simulated　by　the　laser　amplification　model　is　different　from　that　of　the　passive　absorption　coefficient　model,　which　is　caused　by　considering　the　effects　of　the　laser　and　pump　intensities　on　pump　absorption　(Fig.12).　Because　of　better　pump　absorption　uniformity,　although　the　absorbed　power　increases,　the　maximum　temperature　does　not　increase　significantly.　However,　the　temperature　distribution　in　the　width　direction　is　asymmetric,　which　may　cause　low　actual　beam　quality　(Fig.13).　The　maximum　thermal　stress　of　core　is　only　18.5%　of　the　safety　limit,　which　is　lower　than　that　in　the　double-clad　planar　waveguide　amplifier　(Table.3).　The　extraction　efficiency　of　the　multi-channel　pump　is　higher　(Fig.16)　since　the　high　pump　intensity　is　preferred　to　extract　the　absorbed　power　(Fig.15).　Conclusions:　A　noble　planar　waveguide　is　designed　for　a　high-power　planar　waveguide　laser　amplifier　with　the　low-doped　Yd:YAG　core.　multipass　pumping　of　the　core　is　achieved　through　bonding　YAG　around　the　core　area　and　high　reflection　coating　on　reflective　surfaces.　The　geometric　of　the　waveguide　is　reshaped　and　optimized　to　improve　pump　absorption　and　uniformity.　The　double-clad　planar　waveguide　amplifier　and　multipass　pumped　planar　waveguide　amplifier　are　simulated　using　a　3D　laser　amplification　simulation　model　combining　the　ray　tracing　and　finite　element　methods.　When　the　pump　power　is　10　kW,　the　optical-optical　efficiency　of　the　multipass　pumped　planar　waveguide　and　double-clad　planar　waveguide　amplifier　becomes　saturated　with　200　W　of　injected　seed.　When　the　injected　seed　is　200　W,　the　multipass　pumped　planar　waveguide　threshold　pump　power　becomes　500　W,　which　is　almost　the　same　as　the　double-clad　planar　waveguide　amplifier.　However,　the　slope　and　optical-optical　efficiencies　are　higher　than　those　of　the　double-clad　planar　waveguide　amplifier.　When　the　injected　seed　is　200　W,　and　the　pump　power　is　10　kW,　the　absorption　power　density　distribution　simulated　using　the　laser　amplification　model　significantly　differs　from　that　of　the　passive　absorption　coefficient　model.　The　simulation　results　using　the　laser　amplification　model　showed　that　in　the　multipass　pumped　planar　waveguide　amplifier,　the　pump　absorption　efficiency　is　93.3%,　the　output　power　is　7311　W,　and　the　optical-optical　efficiency　is　71.1%.　It　is　significantly　higher　than　that　of　the　double-clad　planar　waveguide　amplifier.　The　multipass　pumped　planar　waveguide　amplifier　shows　better　pump　absorption　uniformity　and　a　smaller　risk　of　thermal　damage.　©　2021,　Chinese　Lasers　Press.　All　right　reserved.