Very low threshold current operation of semiconductor ring lasers

Semiconductor ring lasers with a threshold current as low as 12.5 mA have been fabricated in GaAs/AlGaAs. The low threshold current is due to a passivating layer of silicon nitride and an optimised fabrication process that results in very smooth sidewalls. The threshold current was analysed and it was found that a considerable part of the current is lost by spreading towards the inside of the pillbox structure. The remaining loss mechanisms were identified as internal loss (1 dB), coupling loss (7-8 dB) and scattering loss (2-4 dB). The scattering loss was found to be independent of the radius for radii between 30 μm and 145 μm. The L-I curve displays several kinks, which are explained by shifts of the mode position and thus the coupling ratio at the Y-junction due to temperature changes with pumping