Ring-cavity surface-emitting lasers as a building block for tunable and coherent quantum cascade laser arrays

We describe ring-cavity surface-emitting lasers (ring-CSELs) based on quantum cascade structures as an elementary building block for two-dimensional quantum cascade laser arrays. The light emitters operate at high temperatures as high as 380 K and above. The devices facilitate a reduction in threshold current density as well as enhanced radiation efficiency in comparison to Fabry-Pérot lasers. Single-mode emission is observed at a wavelength of around 8 μm with a side-mode suppression ratio of 30 dB at room temperature. A tuning of the resonance is achieved by a variation in the grating period or a change in temperature. Phase locking of two ring-CSELs is demonstrated which is based on a direct coupling scheme. Coherent operation of ring-type lasers results in light emission at an identical wavelength and thus in an enhancement of the spectral brightness.