Comb Operation in Terahertz Quantum Cascade Ring Lasers

In recent years, optical frequency comb sources have been realized in the mid-infrared and terahertz region by using quantum cascade lasers (QCLs) [1] , [2] . These electrically driven, on-chip sources provide high output power and a compact design. Commonly used waveguide designs include ridge type resonators supporting Fabry-Perot modes. The appearing effect of spatial hole burning in this resonator type prevents the exploitation of the whole gain bandwidth of the active region [3] . This limitation can be overcome using ring-shaped resonators, which exhibit either a standing-wave pattern formed by two counterpropagating waves, or support a traveling wave, which does not cause spatial hole burning.