Mode splitting and the coherent instability in high-gain lasers

Abstract

A high-gain continuous-wave laser may exhibit an output instability in the form of a periodic pulse train or chaotic signal. In the time domain, the instability is interpreted as resulting from the inability of the medium’s polarization to respond quickly enough to perturbations in the optical field of the cavity. In the frequency domain, it can be interpreted as arising from the splitting of a single longitudinal mode into several oscillating lines, each of which satisfies the same cavity-resonance condition as the original mode. We show that the mode-splitting interpretation of the instability yields a new understanding of the complex behavior reported in xenon, helium–xenon, and helium–neon lasers. The fundamental frequencies of the instability and the possibility of chaotic output are shown to be consistent with unequally spaced resonant modes that arise close to the laser threshold. With saturation, nonresonant harmonics of the fundamental frequencies appear in the laser output. These harmonics have fixed phases relative to the resonant modes and lead to the complex asymmetries of the pulse shapes. Interaction between the resonant and nonresonant modes can trigger period doubling in the pulse train.