A theoretical nonlinear treatment of coupled quantum cascade lasers (QCLs) by a monolithic Talbot cavity all grown on the same chip is presented, analyzed and the results are compared to recent experiments. The model is capable of computing numerically the stability or instability of the supermodes of the coupled system and can capture possible bifurcations into pulsating intensities. The model is derived by using an equivalent ring laser model that contains several separated gain section all coupled by an integrated Talbot cavity. In the small signal gain limit it captures the threshold gain of the various supermodes and matches the results of previous calculations in the literature in the same limit.