Single-mode lasing on radial modes in ring cavity quantum-cascade lasers

The results of stable single-mode lasing in ring cavity quantum-cascade lasers are presented. The slits of the second-order grating with a variable etching depth during movement along the ring cavity are formed by direct ion-beam lithography. Single-mode lasing at a wavelength of 7.65 $\mu$m with a signal-to-noise suppression ratio of (22–25) dB just above the laser threshold is obtained. Increasing the current pumping by 40% leads to an increase in the side-mode suppression ratio up to 28 dB.