PASSIVE-MODE LOCKING IN STANDING-WAVE LASER RESONATORS

In standing-wave laser oscillators the field energy density periodically varies along the resonator axis and creates a complex refractive-index grating in the gain medium. This grating couples the originally independent counterpropagating waves of the individual axial cavity modes. The coupling induces a mode frequency shift that is a nonlinear function of the unperturbed mode frequency. The uneven shifts can give rise to a substantial broadening of the beat-note linewidth of the multi-axial-mode free-running laser and to a corresponding in the threshold intracavity power for self-starting passive mode locking. The theoretical results are in good qualitative and quantitative agreement with previously reported experimental observations and measurements.