Frequency combs are powerful tools for precision metrology, spectroscopy, and communications. In the terahertz (THz) domain, the quantum cascade laser (QCL) offers an on-chip platform for self-started comb generation via four-wave mixing. However, the development of highly stable THz QCL frequency combs is limited by complex external locking hardware, e.g., femtosecond laser optics, phase-locked loops, microwave injection circuits, etc. Specifically, optical injection locking of THz QCL combs has never been reported due to the lack of stable THz local oscillators. Here, we demonstrate a compact mutual optical injection locking scheme between a single-mode QCL and a THz QCL comb emitting around 2.56 THz. Experimental results show that when the frequency detuning between the two QCLs is tuned, mutual optical injection locking can be clearly observed. Furthermore, it is found that the optical injection locking dynamics show strong master-slave configuration asymmetry. When the single-mode QCL is used as a master, the comb behavior is strongly perturbed, which is reflected by the repetition frequency shift and linewidth broadening. The locking bandwidth is measured to be 30 MHz. Conversely, when the comb laser is used as a master, unstable regions are suppressed and the locking bandwidth expands to 94 MHz. Within the locking regime, the phase noise and Allan deviation of radio frequency signals are remarkably improved. The proposed scheme enables compact THz comb stabilization and improved repetition frequency signal generation, paving the way to chip-scale THz comb stabilizations, and optical frequency divisions.
Wu et al. (2026) studied this question.