Coupled mode theory is used to describe the behavior of an external laser cavity consisting of a diode laser array and a diffractive mode-selecting mirror. The mirror is designed to establish a uniform-amplitude, uniform-phase fundamental mode. Coupled mode theory is then used to study the behavior of higher-order modes. We show that the maximum discrimination against higher-order modes occurs when the round-trip cavity length satisfies certain Talbot relations. In addition, this high modal discrimination can be maintained for arrays with large numbers of lasers without incurring significant loss in the fundamental mode.