Abstract: A slab waveguide laser which provides efficient operation in predominantly a single mode operation includes a periodic modulation structure positioned between a slab waveguide and one of two cavity mirror reflectors. Alternatively, the periodic modulation structure may be included along the slab face. The periodicity of the periodic modulation structure is determined by the width of the waveguide to ensure that the periodic modulation is coherently self-imaged in one (or more) round trip(s) of the laser cavity. Because of the low loss propagation within the slab waveguide, overall cavity losses are minimized in both the lateral and transverse directions of the slab.