Reliability　and　characterization　of　850　nm　50　Gbit/s　PAM-4　vertical-cavity　surface-emitting　lasers　(VCSELs)　are　presented.　These　VCSELs　have　demonstrated　a　threshold　current　of　0.8　mA　and　a　slope　efficiency　of　0.95　W/A.　The　maximum　optical　output　power　of　9　mW　is　achieved　at　a　thermal　rollover　current　of　13.5　mA.　The　optical　power　is　up　to　5　mW　and　the　-3dB　bandwidth　is　in　excess　of　17　GHz　at　25　degrees　C　and　6　mA　bias.　The　current　density　and　power　dissipation　density　are　low　to　15　kA/cm(2)　and　25.5　kJ/cm(2),　respectively.　The　standard　deviations　of　photoluminescence　peak　wavelength　and　Fabry-Perot　cavity　wavelength　of　epitaxial　wafer　are　0.75　nm　and　2.2　nm,　respectively.　After　1500　h　of　the　reliability　study　no　degradation　or　failures　of　the　22　VCSELs　are　observed　at　80　degrees　C　in　a　heating　chamber　at　a　bias　of　6　mA.　Considering　high　optical　absorption　of　DX　center,　the　impurity　doping　concentration　of　3　pairs　of　N-DBRs　that　were　adjacent　to　active　region　are　optimized.　The　additional　SiO2　passivation　layer　not　only　can　provide　moisture　resistance　but　also　provide　a　photon　lifetime　tuning.　The　output　power　increases　by　optimizing　thickness　of　SiO2　layer　reducing　power　dissipation　density.　Single　thin　oxide　aperture　is　employed　by　slowing　down　the　oxidizing　rate　and　improving　temperature　during　a　VCSEL　oxidation　process　to　thereby　reduce　stress　concentration　of　an　oxidation.　Single　thin　oxide　aperture　may　limit　the　-3dB　bandwidth,　but　the　modulation　characteristics　can　be　improved　by　adopting　advanced　modulation　techniques　such　as　4-level　pulse　amplitude　modulation　(PAM-4).