Experimental and theoretical study of optical frequency combs in gain-switched Discrete Mode Lasers under optical injection into suppressed longitudinal modes.

In this work, we investigate how low-power continuous-wave (CW) optical injection into suppressed longitudinal modes far from the free-running mode (q = 0) of a single-mode discrete-mode laser (DML) influences the optical frequency combs (OFCs) generated in the DML under gain-switching (GS). In the absence of optical injection, the gain-switched DML emits an OFC at q = 0. Optical injection on a distant suppressed mode (q ≠ 0) produces a second OFC at the injected mode and can also enhance the spectral quality of the q = 0 comb. The effects of optical injection depend on injection power and modulation frequency, and different injected modes lead to OFCs with distinct spectral characteristics. The system is frequency tunable over more than 4 THz. These findings demonstrate a novel, robust, and simple approach for generating single- or dual-comb outputs from a GS-DML, offering potential benefits in multiwavelength communications, broadband spectroscopy, and metrology.