Highly unidirectional InAs∕InGaAs∕GaAs quantum-dot ring lasers

We report fabrication and characterization of semiconductor ring lasers with quantum-dot active region. The InAs∕InGaAs∕GaAs “dots-in-a-well” ridge-waveguide ring lasers are monolithically integrated with coupling waveguides and monitoring quantum-dot photodetectors. The lowest threshold current density for semiconductor ring lasers is demonstrated. When enhanced by a forward biased S-section waveguide, stable unidirectional operation with record suppression ratio of counterpropagating waves exceeding 30dB is achieved.