Semiconductor laser amplifiers (SLAs) are multi-functional devices, as they can perform the following functions: amplification, modulation, detection and wavelength conversion. For the optimization of SLAs, a good numerical simulator is required. This gives insight into both the internal behavior of SLAs and the variation of certain system parameters due to changes in the SLA structure or external conditions. This paper describes an extensive numerical model for SLAs. It includes large-signal time dependent analysis, residual mirror reflection, amplified spontaneous emission (ASE) and multiple electrodes. SLAs with active layers made of bulk or MQW material can both be simulated. Important parameters such as the bit-rate, amplification, noise figure, chirp, ASE-spectrum and polarization-dependency can be obtained. It is also possible to use the simulator to investigate the influence of the accumulation of forward and backward propagating ASE in cascaded SLAs. The simulator is used for the design of a fast external modulator and for a wavelength convertor at 1.55 micrometers . Further, a multi-functional two electrode polarization-intensive amplifier-modulator- detector structure is proposed and optimized. This SLA can be used as an insert-drop transparent node for application in LANs and access/distribution networks.