Absorption measurements in continuous high-power CO2 laser processing of materials

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Abstract
The absorption coefficient of the C02 laser light has been measured when metallic target of steel (35 NCD 16) and pure iron are irradiated with laser intensities varying from i03 to lO7Watt/cm 2 with helium gas shielding. The diagnostic used consists of an integrating sphere which collects all the laser light reflected by the target. This set-up allows a temporal resolution of this measurement of about 0. 1 ms. As the laser intensity increases, a clear correlation between the time-dependance of the increase of absorption and of the appearance of the plasma luminosity is evidenced. For higher intensities, when a keyhole regime is reached, this correlation disappears and the absorption reaches 95 %. This very high level of absorption is interpreted as a result of the geometrical trapping of the laser light inside the keyhole and a low contribution of plasma plume absorption to this process.

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