The　facet　coating　of　GaAs-based　laser　diodes　(LDs)　stressed　by　constant　current　was　studied　in　detail　using　focused　ion　beam,　high-resolution　transmission　electron　microscopy,　and　energy-dispersive　X-ray　spectroscopy　techniques.　Our　analysis　found　that,　for　Si/Al2O3　facet　coating,　silicon　near　the　active　area　exposed　to　high　laser　intensity　becomes　diffused,　making　it　thicker　than　the　Si　layer　outside　the　active　area.　Oxygen　diffused　into　the　Si　layer　and　the　Si　layer　got　oxidized.　Such　change　of　facet-coating　thickness　and　composition　causes　the　facet　reflectivity　to　fluctuate　and　carriers　to　recombine　nonradioactively　and　eventually　lead　to　catastrophic　optical　damage.　We　conclude　that　the　performance　of　LDs　could　be　improved　by　optimizing　their　facet　coating.