The output coupler usually has reflectance in the range 40% to 70%. In order to optimize power and spatial profile of the laser beam, the output coupler and rear mirror have optical surfaces with well-defined radii of curvature. As the output coupler transmits a high-power laser beam, it is made of ZnSe in order to minimize absorption and thermal lensing. The rear mirror usually has reflectance 99.5% which means that the transmitted laser beam has low power. However, as absorption in the coating is higher than in the output coupler, it is more important to use a substrate material with high thermal conductivity. Therefore, Germanium or Gallium Arsenide are used in most lasers.