Product Code: JLA_8_6_275

Authors:
C. R. Hsu
C. E. Albright
A. Khakhalev
Welding Engineering, The Ohio State University

Contamination in the cavities of high power industrial CO₂ lasers is a cause of laser performance degradation. Such degradation can result in laser downtime and significant loss of productivity. The laser cavity gas supply can contribute to this contamination, but there is confusion about the level of gas impurity concentration which can cause a degradation process. The overall objective of this investigation was to study the effects of cavity gas contaminants on the performance of an industrial fast‐axial‐flow CO₂ laser. A 3 kW fast‐axial‐flow CO₂ laser operated for 2100 h prior to this investigation was employed in a series of gas contamination experiments. The operating power was selected as 1500 W. After cleaning the laser cavity, the absorptivities of output coupler and rear mirror were measured after laser operation using both ultra‐high‐purity laser cavity gases (99.999%) and premixed contaminated helium (100 ppm, 1000 ppm and 5000 ppm for C₃H₈, C₂H₄, CH₄ and O₂). The output powers were also measured. The results show that increased absorptivities in the output coupler were observed with both hydrocarbon and oxygen impurities compared to those with ultra‐high‐purity gases (99.999%). The increased absorptivities in the output coupler and rear mirror seem to correlate with the total carbon content in the laser cavity due to the gaseous hydrocarbon impurities. Deposition on the surfaces of corner mirrors were identified as carbon using energy dispersive electron spectroscopy analysis. Several flakes of deposited carbons were also identified on the surface of the NaCl window inside the resonator after operating the laser using 5000 ppm propane in helium. 1000 ppm oxygen in the laser gas mixture resulted in an increase of 1.4 % in the output power (4.8 % for 5000 ppm oxygen). 1000 ppm of propane resulted in a decrease of 17 % in the output power. A 44.8 % of power drop was observed when using 5000 ppm propane. The negative effect of contamination (especially hydrocarbon) on high power laser performance was demonstrated.