The results of studies of 7.5–8.0 μm range surface-emitting ring quantum-cascade lasers are presented. A second-order diffraction grating with a calculated coupling coefficient of ~ 9 cm-1 is formed on the entire surface of the ring cavity by focused ion beam milling. Surface-emitting lasing at room temperature near 7.75 μm with a threshold current density of ~ 8 kA/cm2 and an outer radius of the ring cavity of 202 μm is demonstrated. The results of studying the intensity distribution in the far-field near the normal to the surface showed the presence of two maxima. It is shown that the implemented coupling coefficient is not sufficient to ensure single-mode lasing in the studied ring quantum-cascade lasers.
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