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CO2 Laser Output Windows

A common problem for windows used in high-power lasers is that of thermal distortion of the beam caused by thermally induced refractive index gradients as the window is heated by the transmitted beam. This issue is minimised by the use of optical CVD diamond as the window material has a high thermal conductivity, a low absorption coefficient and a low value of the temperature coefficient of the refractive index. This enables diamond products like exit windows, output couplers, beamsplitters and lenses for high power Co2


lasers. The extreme high laser damage treshold allows more compact systems with diamond optics and enables maintenance free optics because of the very long lifetime of the optic. The following table compares the values of these quantities for diamond and ZnSe at 10.6μm wave

  CVD Diamond (Optical) ZnSe
  Thermal conductivity (W/mK) 2,000 17
  Absorption @ 10.6μm (cm-1) 0.1-0.03 (typical: 0.05) 0.0005
  Absorption of AR coating per surface (%) 0.1 0.1
  Dn/dT (10-6K-1) 10 57


The potential advantages of diamond for this application are further illutrated in the pictures, which show the result of 2-D computational modelling of the thermal and refractive index gradients of a diamond and a ZnSe window both AR coated on both surfaces and cooled at the edge when traversed by a 10.6μm wavelength, 5kW Co2 laser beam.