Strategies toward the realization of two-dimensional broadband and coherent quantum cascade ring laser arrays

We present 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 devices operate at temperatures of 380 K and above. A reduction in threshold current density and enhanced radiation efficiency are observed as compared to Fabry-Pérot lasers. The devices exhibit single-mode emission at a wavelength around 8 m with a side-mode suppression ratio of 30 dB at room temperature. Tuning of the resonance is achieved by variation of the grating period or change in temperature. The emitters exhibit a low-divergence ring-shaped beam pattern with a lobe separation of 1.5 deg. Based on a direct coupling scheme, phase locking of two ring-CSELs is demonstrated with a fringe visibility of 60. Coherent operation of ring-type lasers results in a narrow spectral line and thus in an enhancement of the spectral brightness.