A beam delivery system of a projection exposure system comprises a laser generating a beam of laser light from a plurality of longitudinal laser modes in a cavity, wherein light generated by a single longitudinal laser mode has an average line width λlat, wherein the laser light of the beam has, at each of respective lateral positions of the beam, a second line width λlat corresponding to lateral laser modes, and wherein the laser light of the beam has, when averaged over a whole cross section thereof, a line width λb corresponding to plural lateral laser modes, and wherein λm