A mirror for use in high power optical applications, the mirror comprising: a support plate comprising a synthetic diamond material; and a reflective coating disposed over the support plate, wherein the reflective coating comprises a bonding layer of carbide forming material which bonds the reflective coating to the synthetic diamond material in the support plate, a reflective metal layer disposed over the bonding layer, and one or more layers of dielectric material disposed over the reflective metal layer, wherein the bonding layer and the reflective metal layer together have a total thickness in a range 50 nm to 10 μm with the reflective metal layer having a thickness of no more than 5 μm, and wherein the support plate and the reflective coating are configured such that the mirror has a reflectivity of at least 99% at an operational wavelength of the mirror.