Polycrystalline synthetic diamond material

A method of fabricating a polycrystalline CVD synthetic diamond wafer comprises growing a first thickness of polycrystalline CVD synthetic diamond wafer 1 on a substrate in a first reactor using process gases comprising a carbon-containing gas and hydrogen, removing the substrate from the first reactor, removing the first wafer from the substrate, cutting at least one second smaller wafer 2 from the first wafer, locating the second wafer on a carrier 3, and growing further polycrystalline CVD synthetic diamond material 4 on the second wafer to a second thickness in a second reactor using process gases comprising nitrogen, a carbon-containing gas and hydrogen. The first and second reactors may be microwave plasma CVD reactors and are preferably the same reactor. A polycrystalline CVD synthetic diamond material has a specified average thermal conductivity through its thickness and comprises first and second thicknesses of diamond material grown under first and second growth conditions and separated by a growth event interface. The total thickness of the wafer may be at least 2.75 mm. The wafer may be used as a heat spreading substrate for mounting the active mirror of a disk laser.