Multimode Instability in a CO2 Laser with Saturable Absorber

Instabilities in the two-mode oscillation of a CO 2 laser with a saturable absorber are studied both experimentally and theoretically. Depending on the passive Q-switching behavior in one of the modes, exclusive mode exchange and simultaneous two-mode oscillation are observed. These instabilities are reproduced with the rate-equation model in which detailed vibrational relaxation mechanism and spatial mode coupling are considered. The rate-equation analysis predicts chaotic two-mode oscillation. Structure of the strange attractor of the two-mode chaos is compared with that in the single-mode case through the return map method and the fractal dimension measurement.