Generation of coupled modes in an unmatched three-mirror laser cavity

An extraordinary phenomenon of generation of coupled types of oscillations with a transverse field distribution corresponding to the TEM$_{pq}$ eigenmodes with different pairs of indices $p$ and $q$ in the main laser cavity and in an external cavity has been experimentally detected. The excitation of coupled modes depends on the configuration and tuning of partial cavities. To solve the system of integral equations for a three-mirror cavity with mismatched spherical mirrors in the quasioptical approximation, it has been proposed to use modified boundary conditions including the coupling coefficients for eigenmodes in partial cavities.