This　paper　presents　improved　underwater　wireless　optical　communication　(UWOC)　based　on　a　490　nm　passively　mode-locked　vertical-external-cavity　surface-emitting　laser　(VECSEL)　with　the　repetition　rate　of　1.46　GHz　and　the　pulse　width　of　2.25　ps.　After　conducting　a　comprehensive　evaluation　of　the　performance　characteristics　of　quasi-cyclic　low-density　parity-check　(QC-LDPC)　coding　and　pulse-position　modulation　(PPM)　in　relation　to　bit-error-rate　(BER)　and　communication　capacity　across　different　signal-to-noise　ratios　(SNRs),　a　passively　mode-locked　VECSEL　based　UWOC　system　is　designed　and　experimentally　performed.　The　UWOC　system　employs　the　QC-LDPC　encoding　technique　and　PPM　modulation　scheme　that　are　most　suitable　for　the　system.　Compared　with　the　case　without　QC-LDPC,　the　minimum　received　optical　power　with　coding　is　improved　by　3.5　dBm,　and　the　SNR　with　coding　is　improved　by　0.4　dB　under　condition　of　256-PPM　and　forward　error　correction　(FEC)　threshold　of　3.8×10−3.　Meanwhile,　compared　with　the　UWOC　system　using　a　continuous-wave　(CW)　VECSEL　as　the　light　source,　the　attenuation　coefficient　of　the　UWOC　system　based　on　the　mode-locked　VECSEL　is　decreased　by　0.04　m-1,　and　the　BER　of　the　mode-locked　VECSEL　based　UWOC　system　is　decreased　by　0.3　dB.　©　2023　Elsevier　B.V.