A narrow band F2 laser system useful for integrated circuit lithography. An output beam from a first F2 laser gain medium (master oscillator) is filtered with a pre-gain filter to produce a seed beam having a bandwidth of about 0.1 pm or less. The seed beam is amplified in a power gain stage which includes a second F2 laser gain medium. The output beam of the system is a pulsed laser beam with a full width half maximum band width of about 0.1 pm or less with pulse energy in excess of about 5 mJ. In a preferred embodiment the pre-gain filter includes a wavelength monitor that permits feedback control over the centreline wavelength so that the pre-gain filter optics can be adjusted to ensure that the desired bandwidth range is injected into the power gain stage.