Theory of two coupled lasers

The phase-locking of two coupled lasers is studied using the model of Spencer and Lamb. Tuning curves for the odd and even normal modes of two coupled empty resonators are presented to show the origin of the strong frequency-pulling of the coupled lasers. It is shown that the even mode is preferable for combining the two laser beams in the far field. Scaling relations, obtained for the locking range, reduce the number of effective parameters of the coupled system. In the limits of very large gain and very weak coupling the locking range is found to be narrower than predicted by the Spencer-Lamb coupled laser theory.