A waveguide including a folded, coated reflector highly reflective of radiation having two different wavelengths over a wide range of incidence angles, including grazing incidence angles. The waveguide can be employed for guiding laser radiation useful for medical or other applications, such as a combined HeNe and CO.sub.2 laser beam. In some embodiments, the invention is a hollow waveguide formed by folding a flat, coated substrate into a polygonal shape. Because the coated substrate is folded, rather than rolled into a cylindrical tube, the bending stress on the coating is concentrated at the corners of the polygonal waveguide rather than distributed over the waveguide's entire surface. Although the coating may crack at the corners of the waveguide's polygonal cross-section, the flat surfaces between the waveguide edges will be intact, and the cracks in the corners will have an insignificant effect on waveguide performance.