Reduction of coupling coefficients for distributed Bragg reflection in corrugated narrow-rib waveguides

The coupling coefficient in corrugated rib waveguides with rectangular teeth is calculated as a function of the rib widths. For sufficiently wide ribs (a few wavelengths in the material) the coupling coefficient asymptotically approaches its slab waveguide limit. But for narrower ribs the coupling coefficient drops rapidly with the rib width, whereas the power filling factor is almost unaffected owing to the strong field confinement. The effect on the threshold conditions of DFB lasers is also considered.