A laser device, in particular a semiconductor laser, emitting optical radiation with a defined mode pattern can be produced from a standard Fabry-Perot (FP) laser by post-processing at the wafer level, i.e. before the wafer is separated into individual dies by cleaving/dicing. A sub-cavity is formed within the FP laser cavity. The sub-cavity has a predetermined length and is located between the FP facets. An aperiodic grating composed of a small number of contrast elements, typically less than 10, with predetermined inter-element separations and predetermined spacings relative to the sub-cavity is formed on or in the optical waveguide. The inter-element separations and the spacings relative to the sub-cavity produce a filtering function of the aperiodic grating for optical radiation propagating in the waveguide. The laser device is suitable for telecommunications applications due to its high side-mode-suppression ratio and narrow-linewidth.