This　paper　reports　on　the　mode-locked　operation　of　a　980-nm　external-cavity　passively　mode-locked　laser　with　extremely　narrow　RF　linewidth.　Optical　pulses　with　10-ps　pulse　duration　were　generated　at　a　repetition　rate　of　955　MHz,　with　an　average　output　power　of　39.3　mW　-　which　corresponds　to　a　peak　power　of　4.1　W,　generated　directly　from　the　oscillator.　The　RF　spectrum　displays　a　-3dB　RF　linewidth　of　only　∼40　Hz,　as　well　as　a　60-dB　dynamic　contrast,　revealing　the　exceptionally　low-noise　fundamental　mode-locked　operation　of　this　laser.　At　a　repetition　rate　of　∼1　GHz,　the　highest　peak　power　of　5.26　W　was　achieved,　albeit　with　an　increased　-3dB　RF　linewidth　of　∼100　Hz.　The　twosection　chip　incorporated　an　active　region　with　a　dual　InGaAs　quantum　well　sandwiched　by　an　asymmetrical　waveguide,　and　was　operated　at　room　temperature.　By　taking　advantage　of　the　broad　tunability　of　the　repetition　rate　which　externalcavity　lasers　can　afford,　we　also　investigated　the　limits　of　stable　fundamental　mode-locked　operation　at　the　lowest　repetition　rates　(or　maximum　external　cavity　lengths).　©　2013　SPIE.