Propagation theory and numerical simulation of high-power optical pulse in laser amplifying medium

Taking into account the nonlinear effect, dispersion, gain distribution and loss of amplifying medium of a laser amplifier, the physical model that depicts the propagation characteristics of an optical pulse in the medium is established. In the example of Ti:sapphire amplifying medium, split-step Fourier transform method is used to solve the model numerically. The frequency chirp produced by Kerr effect is analyzed and the influences of gain saturation, group velocity dispersion and Kerr effect on the optical pulse are discussed in detail. The results indicate that the gain saturation of amplifying medium introduces the optical pulse distortion. Group velocity dispersion does not affect the pulse propagation characteristics. Kerr effect produces a frequency chirp and the gain saturation brings a larger frequency chirp at the front edge than that at the lagging edge of the pulse. It is the combined action of Kerr effect and gain saturation that makes the peak of the pulse spectrum shift to low frequency region, thus losing the symmetry of its initial profile. Therefore, in high-power laser amplification system, gain saturation and nonlinear effect are main factors that cause deformation of optical pulse, so they have to be considered seriously.