Coupling coefficients of coupled laser cavities.

We examine the properties of the coupling coefficients of two laser cavities that are coupled by a narrow gap. In particular, we explore the validity of a heuristic formula for the coupling coefficients proposed in an earlier publication. It turns out that the heuristic formula is applicable only if the coupling gap is lossless and off-resonance. It becomes inapplicable if the gap approaches resonance or has significant losses. The results of this study are significant for the theory of coupled-cavity lasers since they show that it is necessary to compute the coupling coefficients with the help of the complete theory.