Mode switching through exceptional points induced by lasing–inversion coupling

The gain–loss coupling in optical cavities induces exceptional points (EPs), where two optical modes coalesce. The large modal overlap near an EP intensifies gain competition, favoring single-mode lasing. Recent studies further revealed self-modulation closer to the EP that transforms the lasing mode into a frequency comb. Such EP-enabled comb formation suggests a previously unaccounted-for mechanism that overcomes the strong gain competition and drives a second mode to threshold. Here, using a Bloch coupled-mode theory based on the Maxwell–Bloch equations, we show that the second threshold arises from dynamical couplings among the population inversion, the lasing field, and an inactive optical mode. The lasing–inversion coupling produces extra linear EPs, whose spectral structure governs switching among single-mode lasing and frequency combs with different repetition rates. This above-threshold mode-switching mechanism suggests a possible route toward reconfigurable photonic systems, with potential relevance to adaptive optical communication links and dual-comb spectroscopy.