Three classes of array modes of closely spaced antiguides are analyzed: modes consisting of coupled fundamental (element) modes, modes consisting of coupled first-order (element) modes, and modes adjacent to coupled fundamental modes. The behavior of coupled fundamental modes as a function of lateral index step is analyzed, and explained from a ray-optics point of view. It is found that at resonance, for both coupled fundamental and first-order modes, the array-mode propagation constant is virtually identical to the propagation constant of the mode of a single, unperturbed antiguide. It is also found that the array-mode radiation loss at resonance is equal to the radiation loss of a single element, taken separately, divided by the number of array elements. Several types of mode discrimination mechanisms are discussed: the ones inherent to arrays of antiguides (i.e., mode-dependent lateral radiation loss and field overlap with the active region), interelement loss, and losses in monolithic diffractive spatial filters based on the Talbot effect. Near and at resonance interelement loss suppresses both adjacent and out-of-phase modes, thus allowing for sole in-phase-mode operation. Monolithic diffractive spatial filters strongly suppress out-of-phase modes both near and far from the in-phase-mode resonance; and discriminate against adjacent modes near the in-phase-mode resonance. For devices with 3 μn-wide antiguide cores and 1-μm wide interelement spacing, intermodal discrimination values of 15-20 cm-1 can be achieved. Excellent agreement is found between experimental data (i.e., diffraction-limited in-phase-mode operation to 10 × threshold and 450 mW) and theoretical predictions based on the effective-index method. © 1990 IEEE