In a laser-processing machine including a CO2 laser, laser radiation for material processing is generated by molecular oscillations. The generated laser radiation is usually guided through a gas atmosphere, which is kept free from substances that absorb the generated laser radiation with particular measures.
The measurement of the photo-acoustical effect was published, e.g. by L. B. Kreutzer: Laser opto-acoustical spectroscopy, A new technique of gas analysis, Anal. Chem. 46 239A, (1974).
German patent, DE 195 35 720 A1, discloses a method and an arrangement for leak-proofing of housings, in which gas escaping from the housing is irradiated by a light bundle from a light source, which is designed such that when the housing is not tight, the photo-acoustical effect can be measured. To improve the measurement, a feedback circuit is suggested.
To control the gas atmosphere, conventionally a molecular sieve is used (see e.g., European patent, EP 0 749 800) or nitrogen is used as a filling gas for the beam path (see e.g. WO 95/33594).
Applications against or shielding from gas entering from the outside are also known in the art.
CO2 laser radiation is absorbed by many molecules to a higher or smaller degree. A precondition for absorption is that one of the molecular compounds has the matching binding energy. Examples of such gaseous substances that must be kept away from the beam path are SF6, C2H4, halogenized hydrocarbons, ammonia, alcohols, acetone, and CO2.
The detrimental effect of these gases is not absorption per se and therefore weakening of the power of the laser radiation required for processing, but the optical effect on the laser radiation produced through absorption, which widens the beam and distorts the phase front. The absorption that is relevant for laser processing machines has hardly any negative effect on the power. The actual detrimental effect is the negative influence of laser radiation resulting from the temperature increase and the resulting change of the refractive index.
Investigations have shown that impurities of <100 ppb (0.1 ppm) SF6, which has the highest known absorption at 10 μm wavelength, is sufficient to decisively impair cutting of steel sheets with 3 kW laser power.