A high-efficiency, multiwavelength beam-expander optical system that employs dielectric-enhanced mirrors is disclosed. Each mirror includes a reflective multilayer coating formed from alternating layers of HfO2 and SiO2 that define, in order from the substrate surface, at least first and second sections, wherein the HfO2/SiO2 layer thicknesses are generally constant within a given section and get smaller section by section moving outward from the substrate surface. The first and second sections are respectively configured to optimally reflect different operating wavelengths so that the beam-expander optical system has an optical transmission of greater than 95% at the different operating wavelengths.