  Quantum cascade lasers (QCLs) have emerged as promising candidates for
generating chip-scale frequency combs in mid-infrared and terahertz
wavelengths. In this work, we demonstrate frequency comb formation in ring
terahertz QCLs using the injection of light from a distributed feedback (DFB)
laser. The DFB design frequency is chosen to match the modes of the ring cavity
(near 3.3 THz), and light from the DFB is injected into the ring QCL via a bus
waveguide. By controlling the power and frequency of the optical injection, we
show experimentally and theoretically that combs can be selectively formed and
controlled in the ring cavity. The potential for soliton generation and
efficient power extraction through the bus waveguide presents exciting
opportunities for integrating this technology in compact comb designs.
