The possibility to measure the coupling constant C between eigenstates in a laser is demonstrated. Spatial separation of orthogonally polarized modes is shown to provide a simple differential tool to measure their coupling. The difference in the behaviors of linear and helicoidal eigenstates enlightens the influence of the nature of the eigenstates on their interaction in the active medium. The obtained experimental values are C=0.16 for linear and C=0.4 for helicoidal eigenstates in a longitudinally monomode Nd:YAG laser. The method is applicable to most kinds of laser. © 1997 American Institute of Physics.