Frequency Combs in Injected Terahertz Ring Quantum Cascade Lasers

Optical frequency combs are laser sources characterized by a series of discrete, equally spaced lines in the frequency domain. Integrated, chip-scale frequency combs have a wide range of applications, including communications, frequency synthesis, optical ranging, and gas spectroscopy. In the terahertz (THz) and mid-infrared (MIR) regions, quantum cascade lasers (QCLs) have emerged as the leading compact sources for generating these frequency combs. Of particular interest are ring QCLs, which have shown promise in generating dissipative Kerr solitons (DKS) — ultra-short pulses that can travel through dispersive, nonlinear media while preserving their shape and amplitude. These pulsed sources in the THz range are valuable for applications such as nonlinear spectroscopy, which requires high optical intensities, and ranging applications, which benefit from short pulse durations. In this work, we demonstrate frequency comb formation in ring terahertz QCLs through the injection of light from a distributed feedback (DFB) laser. The DFB laser’s design frequency is selected to match the resonant modes of the ring cavity, centered near 3.3 THz, with light injected into the ring QCL via a bus waveguide. By tuning the power and frequency of the injected light, we show that comb formation within the ring cavity can be selectively initiated and controlled. Numerical simulations indicate that this comb is primarily frequency-modulated, with the optical injection acting as a trigger for comb generation. Our study also highlights the potential of the ring QCL to serve as a filter, effectively shaping the output of the DFB QCL. Additionally, we conducted a similar experiment using a racetrack structure instead of a ring and observed comparable phenomena. This approach shows great promise for applications in terahertz photonic integrated circuits, where waveguide couplers efficiently facilitate the injection and extraction of radiation from the ring cavity. These findings open new possibilities for generating novel comb states in the terahertz range, such as frequency-modulated waves and solitons, thereby expanding the functionality and application scope of THz QCLs in integrated photonics.