Low divergence single-mode surface emitting quantum cascade ring lasers

We describe the fabrication and operation of surface emitting second-order distributed feedback quantum cascade ring lasers. The devices exhibit single-mode emission at a wavelength of 3.95 mu m with a side mode suppression ratio of 25 dB. A linear tuning coefficient of 0.13 cm(-1)/K is observed. A single longitudinal mode in the ring shaped resonator results in a highly symmetric far-field pattern and a low beam divergence, represented by a full width at half maximum of similar to 3 degrees. Based on these characteristics the presented compact coherent light source may find its way into today's midinfrared spectroscopy applications. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3000630]