Fabrication of Fabry-Perot laser diodes is cheap, but unfortunately the laser emission tends to be multimode, and hence of less interst for optical telecommunications applications. A way to overcome this drawback is to introduce a perturbation along the Fabry-Perot cavity acting as an interferometric filter and hence a single longitunidal mode emission with a narrow linewidth is achieve at a low cost. This technique can be transferred to many type of wavewguides.