Modeling coupled microchip lasers requires complex coupling coefficients

Two coupled microlasers display instabilities in the intermediate regime between locked and independent states of oscillations. The frequencies and amplitudes of these oscillations exhibit strongly asymmetric behaviors for positive and negative detunings. Our experimental findings cannot be accounted for by coupled-equation models with purely real or imaginary coupling coefficients but are well described if complex coupling coefficients are introduced. We describe a method to determine experimentally the coupling coefficient and study its dependence as a function of the separation between lasers.