Two-state lasing in GaAs-based InAs/InGaAs quantum dot mode-locked laser

GaAs-based InAs/InGaAs quantum dot (QD) mode-locked lasers (MLLs) have been fabricated and characterized. The fundamental mode locking at 17.08 GHz was obtained, with the second harmonic mode locking at 34.165 GHz. When keeping the QD MLL gain section bias current (I
gain
) constant at 200 mA, two-state lasing was observed by changing the reverse bias at saturable absorber (SA) section (V
SA
) of QD MLL. The two lasing states at 1277 nm region and 1170 nm region are attributed to ground state (GS) and excited state (ES) transitions, respectively. For the first time, spontaneous emission (SE) measurement at the sidewall of the QD MLL gain section has been performed to investigate the two state transitions. The significant carrier recombination competition has been observed at the GS and ES transitions from SE measurement. Our study suggests the significance of the carrier competition in QD mode-locked laser performance and characteristics.