Analysis of CO₂ laser beam suitable for thick metal cutting

In dealing with CO₂ laser beams, it is often observed that the beam does not obey optical formula that are valid in lower power situations. This is particularly true with the beams of low order mode and power greater than 2 kW. Traditional approaches aimed at improving focussing characteristics of such beams may actually produce the opposite effect. An analysis was made concerning optical distortion caused by laser beam heating of transmission optics. The distortion in the output coupling mirror was observed to give rise to a spike that may be observed in the near-field. The distortion that takes place in the focussing lens has been found to cause serious deterioration of its focussing property. A beam mode that offers good focusing while avoiding the occurence of distortion was pursued theoretically. Good agreement between the theoretical and experimental observations of focussing properties was achieved. Additionally the resultant cutting performance correlates well to the measured focussing characteristics. As a result of this analysis it can be demonstrated that a 2 kW CO₂ beam can cut up to 20 mm thick mild steel with good surface quality.