A grating based line narrowing device for line narrowing lasers producing high energy laser beams. Techniques are provided for minimizing adverse effects of hot gas layers present on the face of the grating. In preferred embodiments a stream of gas is directed across the face of the grating. In other embodiments the effect of the hot gas layer is reduced with the use of helium as a purge gas and in other embodiments the purge gas pressure is reduced to reduce the optical effects of the hot gas layer.