  Lowering the noise level of short pulse lasers has been a long-standing
effort for decades. Modeling the noise performance plays a crucial role in
isolating the noise sources and reducing them. Modeling to date has either used
analytical or semi-analytical implementation of dynamical methods or Monte
Carlo simulations. The former approach is too simplified to accurately assess
the noise performance in real laser systems, while the latter approach is too
computationally slow to optimize the performance as parameters vary over a wide
range. Here, we describe a computational implementation of dynamical methods
that allows us to determine the noise performance of a passively modelocked
laser within minutes on a desktop computer and is faster than Monte Carlo
methods by a factor on the order of 1000. We apply this method to characterize
a laser that is locked using a fast saturable absorber---for example, a
fiber-based nonlinear polarization rotation---and a laser that is locked using
a slow saturable absorber---for example, a semiconductor saturable absorbing
mirror.
