The　exploration　and　utilization　of　marine　resources　has　promoted　the　rapid　development　of　marine　science　and　technology,　and　has　put　forward　higher　requirements　for　underwater　communication　technology.　Long　distance　underwater　wireless　optical　communication　(UWOC)　requires　the　selection　of　light　source　on　the　transmitter　side.　Laser　diodes　(LDs)　have　excellent　portability　and　maneuverability,　and　have　been　widely　used　in　the　UWOC　systems.　However,　their　beam　quality　is　not　so　good　and　it　is　difficult　to　modulate　under　high　power.　In　recent　years,　vertical-external-cavity　surface-emitting　laser　(VECSEL)　has　received　much　attention　due　to　its　high　output　power　and　good　beam　quality.　This　work　is　to　explore　the　advantages　of　using　a　490-nm　blue　VECSEL　as　a　light　source　in　UWOC,　and　to　improve　the　performance　of　the　UWOC　system　by　the　soft-decision　pulse-position　modulation　(PPM).　First,　the　optical　power　attenuation　coefficient　of　the　channel　is　obtained,　and　the　measured　c　is　about　0.0591　m–1　in　a　96-m-long　tap　channel.　Subsequently,　soft-decision　and　hard-decision　are　simulated　and　experimentally　verified.　Both　simulations　and　measurements　show　that　the　bit　error　rate　(BER)　can　be　significantly　reduced　with　soft-decision.　Afterwards,　we　improve　the　system　by　using　the　soft-decision　algorithm　and　investigate　the　communication　performance　of　64　PPMs　at　different　bandwidths　by　adjusting　the　PPM　signal　rate.　Finally,　50　MHz　is　chosen　as　a　signal　rate　in　the　experiment.　Then　a　UWOC　system　is　demonstrated　in　this　work.　The　transmitter　side　consists　of　a　490-nm　VECSEL　light　source　with　an　acousto-optic　modulator　(AOM).　The　pseudo-random　binary　sequence　(PRBS)　is　loaded　into　the　arbitrary　waveform　generator　(AWG)　for　digital-to-analog　conversion　after　PPM　modulation,　and　the　analog　signal　is　sent　to　the　driver　of　the　AOM　for　acousto-optic　modulation　of　the　incident　beam.　The　laser　is　focused　before　entering　the　AOM　and　then　collimated　after　having　exited　to　reduce　its　divergence.　The　modulated　laser　beam　passes　through　a　distance　of　96　m　in　the　tank　by　using　multiple　mirrors　on　both　sides　of　the　tank.　Then,　the　beam　is　focused　by　a　lens　to　the　avalanche　photodiode　(APD)　for　photoelectric　conversion　in　the　end,　and　the　signal　is　processed　by　a　mixed　signal　oscilloscope　(MSO)　after　data　acquisition.　A　soft-decision　algorithm　is　introduced　to　further　optimize　the　performance　of　the　PPM　modulation.　When　the　optical　signal　passes　through　a　relatively　long　distance　of　96　m,　the　measured　BER　is　as　low　as　1.9　×　10–5.　This　indicates　that　the　soft-decision　PPM-based　490　nm　blue　VECSEL　UWOC　system　performs　very　well.　©　2024　Authors.