Suppression of internal parasitic oscillation in large volume CO2 laser amplifier

Internal parasitic oscillation in a large booster amplifier is caused by the coupling of high gain media with the reflections of the amplifier's inner surface. The energy and onset time of internal parasitic oscillation, observed in the final amplifier of LEKKO II, were quantitatively measured as a function of the small signal gain coefficient. It was found that the oscillation was caused by the reflection on the FRP inner surface of the amplifier. Because of its low total reflectance and diffused reflective characteristics, MgO was coated on the inner surface. Specular and total reflectances of this mixture were 32% str. and 45%, and were lower by about 1/12 and 1/4 than those of FRP, respectively. Coating the FRP chamber with MgO also increased the gain threshold for the internal parasitic oscillation by 0.7/m and the utilizable maximum gain by 0.4/m. Finally, the mechanism of internal parasitic oscillation was analyzed quantitatively by a multireflection model; the theoretical gain thresholds were 2.6-2.7/m for the FRP wall and 3.6-3.7/m for the MgO coating.