Apparatus for depositing germanium and carbon onto one or more substrates comprises a vacuum chamber, at least first and second magnetron sputtering devices and at least one movable mount for supporting the one or more substrates within the vacuum chamber. The first magnetron sputtering device is configured to sputter germanium towards the at least one mount from a first sputtering target comprising germanium, thereby defining a germanium sputtering zone within the vacuum chamber. The second magnetron sputtering device is configured to sputter carbon towards the at least one mount from a second sputtering target comprising carbon, thereby defining a carbon sputtering zone within the vacuum chamber. The at least one mount and the at least first and second magnetron sputtering devices are arranged such that, when each substrate is moved through the germanium sputtering zone on the at least one movable mount, germanium is deposited on the said substrate, and when each substrate is moved through the carbon sputtering zone on the at least one movable mount, carbon is deposited on the said substrate.