Bandwidth control technique for a laser

A process for producing effective bandwidths of a pulse laser beam of a narrow band electric discharge laser having a line narrowing unit comprising a grating and a fast tuning mechanism, said process comprising the steps of:

• A) monitoring said laser beam to determine bandwidth of individual laser pulses,
• B) periodically adjusting the tuning mechanism during a series of pulses so that the nominal wavelengths of some pulses in said series of pulses are slightly longer than a target wavelength and the nominal wavelengths of some pulses in said series of pulses are slightly shorter than the target wavelength in order to produce for the series of pulses an average spectrum centered approximately at the target wavelength with average spectral deviation from the target wavelength approximately equal to a desired deviation.