A grating based line-narrowing apparatus having a prism beam expander with at least four prisms. Each prism is positioned at an incidence angle between 67 and 71 degrees. A single layer coating of high refractive index, robust material, such as Al.sub.2 O.sub.3, is applied to the hypotenuse face of each prism, providing an efficient, anti-reflection coating. In a preferred embodiment the incident angle of the four prisms are each at about 68.2 degrees. The laser of the preferred embodiment has similar line narrowing efficiency and bandwidth as that for a laser with a 3 prism, ideal multi-layer anti-reflection coated beam expander. The primary benefit of the new four prism arrangement is that the prism coatings should be much less expensive and have greatly increased durability. Applicants have tested the new prisms for 30 million, 10 mJ pulses in air with no noticeable degradation. No present day dielectric multi-layer stack coating could survive such exposure.