The focus of this paper is to analyze in detail the nonlinear polarization rotation phenomenon in the Semiconductor Optical Amplifier (SOA) according to the injection conditions. To this end, we have developed a numerical model based on the coupled mode theory and the formalism of Stokes. The obtained results are in agreement with the experimental measurements that have been carried out in free space, which allows optimum control and preservation of the polarization state of the injected and collected signals. 1. Introduction Semiconductor optical amplifiers (SOAs) are key enablers for optical networks. They are considered among the most promising technologies for the next generation of optical networks. They can have a significant impact on the architecture and shape the economics of future optical networks due to their attractive features, such as the compact size, the high potential of monolithically integration, and the multifunctionality combined with strong nonlinearities. They have huge potential for use in wavelength division multiplexing (WDM) and access, metropolitan, and core networks. We can achieve various optoelectronic functions for all-optical communication systems, by exploiting nonlinear effects taking place within the SOA, such as cross-gain modulation (XGM), cross-phase modulation (XPM), four-wave mixing (FWM), and cross-polarization modulation (XPolM). Therefore, SOAs can provide integrated functionality of internal switching and routing functions that are required for a feature-rich network. Space switches, wavelength converters, and wavelength selectors, which are made from SOAs, can lead to large cost reductions and improved performance in future optical network equipment. SOAs are naturally polarization sensitive in terms of gain and effective refractive indices. The difference between the confinement factor of the transverse electric (TE) mode and that of the transverse magnetic (TM) mode results in an optical anisotropy and birefringence [1]. When two signals are injected in the SOA, an additional birefringence and gain compression affects the SOA. The two signals affect one another by producing different phase and gain compression on the TE and TM components. Consequently, this results in a rotation of the polarization state for each signal. The SOA bias current and the input signal power are the parameters that determine the magnitude of the polarization rotation. In order to analyze the polarization rotation phenomenon in detail and to predict the SOA operational characteristics, mathematical models are required. Many
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