For use in a waveguide gas laser a capillary tube of glass or ceramic has an inner surface defining a longitudinal capillary opening through which the laser gas flows. At least a portion of the inner surface is corrugated with corrugations or channels with a periodicity .LAMBDA. where .LAMBDA. = 1/2.lambda., .lambda. being the laser gas wavelength. The tube includes a diffused region extending outwardly from the opening. The diffused region of a depth d on the order of 1.lambda. to 3.lambda. acts as a waveguide for the waves, with the corrugations producing distributed feedback. The evanescent component of the waves travelling in the diffused region interact with the laser gas in the opening, gaining energy therefrom, and thereby amplifying the waves travelling in the diffused region, which exit the diffused region, surrounding the opening, as a beam of wavelength .lambda..