Stabilized Terahertz Quantum Cascade Laser Dual-Comb Sources with a Hybrid Locking

Frequency combs show various application potentials in high precision spectroscopy, imaging, communications, and so on. In the terahertz (THz) region, semiconductor-based quantum cascade lasers (QCLs) are good candidates for frequency comb and dual-comb operations. THz dual-comb sources can be obtained by beating two THz QCL combs with a slight difference in the repetition frequencies. A free-running THz dual-comb source normally shows relatively large phase noise, which strongly hinders its high precision applications. Different approaches, e.g., digital algorithms, active phase locking of one dual-comb line, self-reference techniques, etc., have already been employed to stabilize THz dual-comb sources. Up to now, a complete locking of a THz dual-comb source using hardware locking elements has never been demonstrated. Here, we propose a hybrid locking method to simultaneously stabilize both dual-comb offset and repetition frequencies of a THz QCL dual-comb source. Experimental results demonstrate that the stability of all dual-comb lines is improved significantly when the proposed hybrid locking is applied to the dual-comb source. Under the hybrid locking condition, the measured "maxhold" dual-comb line width can reach a record of 5.7 kHz. Furthermore, the recorded time trace of the dual-comb signal shows pulse-like behavior during a large time scale of 100 mu s, which verifies that the proposed method functions well for an active locking of a THz QCL dual-comb source.