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This paper numerically investigates a passive mode-locking system based on nonlinear polarization rotation in a semiconductor optical amplifier (SOA). The principle of polarization-dependent gain saturation induced by the high-intensity part of the optical pulse in a SOA makes it possible to reshape the pulse in such a way that the low-intensity part is cut away. This will result in pulse narrowing and, when realized in a loop with sufficiently high gain, will lead to mode locking. The reshaping is realized by simple polarization controlling elements. Extensive numerical simulations have been done for commercially available SOA with a consistent pulse propagation model.