We estimate internal parameters of a Q-switched ${\mathrm{CO}}_2$ laser by fitting trajectories of the four-level model to measured scalar time series. The four-level model is a five-dimensional nonlinear system of ordinary differential equations. A multiple shooting technique is used to construct the unobserved time courses of the population densities and to reveal the dependence of the parameters on the excitation current. For excitations barely above the laser threshold large pulse variations are identified as an effect of small variations of the pump parameter.

• Received 13 November 2000

DOI:https://doi.org/10.1103/PhysRevE.64.016222