Spectral characteristics of half-ring quantum-cascade lasers

A cavity scheme based on a half-ring with different radii is proposed and fabricated for 7–8 $\mu$m quantum-cascade lasers. A quantum-cascade laser with a half-ring cavity radius of 191 $\mu$m demonstrated lasing with a spectral width of 474 nm (82 cm$^{-1}$) at low temperatures. The free spectral range in these lasers was determined by the whispering gallery modes typical for ring cavities. The laser bandwidth at room temperature was 190 nm (31 cm$^{-1}$), which may be related to an increase in the internal losses with increasing temperature. An increase in the cavity radius to 291 $\mu$m made it possible to achieve room-temperature lasing with whispering gallery modes and a bandwidth of 249 nm (40 cm$^{-1}$) by reducing losses on the mirrors.