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An advanced numerical model for simulating ultralong semiconductor optical amplifiers (UL-SOAs) is presented. The model specifically accounts for the very strong gain saturation and the ultrafast gain dynamics in UL-SOAs. Due to the UL-SOA's length, the weak but fast intraband effects can strongly interact with the input signals. For this reason, UL-SOAs have a tremendous four-wave mixing (FWM) efficiency even for large wavelength detunings. Therefore, it is necessary that the UL-SOA model is a time-domain model so that all FWM products and their interaction with other FWM products are automatically calculated. These FWM products need to be considered in simulations, because they contribute to the UL-SOA's saturation in addition to amplified spontaneous emission. Moreover, due to the time-domain modeling, pseudorandom bit sequence signals can be applied to the model for simulating telecommunication applications. Other aspects that should be considered when modeling UL-SOAs are the gain parametrization and the ultrafast gain dynamics. The wavelength-dependent material gain needs to be properly represented close to the transparency, since most of the device is deeply saturated. The gain dynamics have to be dynamically modeled with rate equations automatically incorporating the limited bandwidths of the different nonlinear gain effects. Although usually not considered for short SOAs, the implementation of weak nonlinear effects like free carrier absorption and two-photon absorption is important due to the long interaction length in UL-SOAs.