Impact of output coupler configuration on operating characteristics of semiconductor ring lasers

We have investigated semiconductor (GaAs/AlGaAs) ring lasers with three different types of output coupler in order to identify the influence of the coupling geometry on the performance of such lasers. Ring lasers with multimode interference (MMI) couplers show significantly better characteristics than devices with Y-junctions and directional couplers. This improvement is explained by the high tolerance of MMI couplers to changes in the position of the input field, the refractive index and the operating wavelength that occur through changes in device operation. The work was also motivated by the need to improve the currently poor efficiency of ring laser devices and several ways of achieving improvement have arisen from the present investigation. All of the devices investigated had a bend radius of 400 μm, the resonator length was between 2.5 mm and 3.2 mm and a strip-loaded waveguide geometry was used. The lowest threshold current obtained was 80 mA in a device with a directional coupler, and the highest differential quantum efficiency achieved was 8%, using a 280-μm-long MMI coupler