Excimer laser with line narrowing

A line-narrowing unit for use in an excimer or molecular fluorine laser system for generating a line-narrowed DUV/VUV laser beam, comprising:

• a beam expanding prism made of a bulk material which is substantially transparent at an emission wavelength of said excimer or molecular fluorine laser system, said prism being arranged at a particular orientation within said resonator for expanding the beam such that substantially the entire spectral bandwidth of said beam remains within an acceptance angle of said resonator while a beam divergence is decreased and a geometric cross-sectional axis extent of said beam is increased by passing through said prism;
• and
• a line selection element arranged within said resonator after said beam expanding prism at a particular orientation such that only a selected portion of the spectral bandwidth of the incident beam remains within an acceptance angle of said resonator after said line selection element,
• wherein said prism includes an entrance surface at which a laser beam is incident and refracted into the bulk material, and an exit surface at which said beam exits substantially normally to said exit surface, and
• wherein the apex angle between said entrance surface and said exit surface of said prism is within a range of apex angles between 37.5° and 42.5°, and a beam incidence angle at said entrance surface is more than 65°.