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Data traffic in optical networks keeps increasing with the emergence of higher throughput wireless technologies and the growing importance of multimedia services for the functioning of the society. Network operators are challenged to enhance the capacity of their networks while keeping expenses under control. Filterless Optical Network (FON) is considered a cost-effective solution to scale capacity of optical networks. In FON, Wavelength Selective Switches, traditionally used to build optical switching devices, are replaced by simpler and cost-effective passive splitters and combiners. Optical amplifiers (OAs) are crucial to compensate for the propagation loss of the fibers, filtering and splitting losses of the nodes and to ensure that power of the optical signal is above the sensitivity level of the receiver. Lower node losses due to the absence of filtering and the fact that FONs are anticipated to be used in short-range metro-area networks allow operators to achieve further savings by reducing the number of OAs in the network. However, passive components operate on the entire optical spectrum and OA-related noise gets propagated through the network, reducing the quality of the optical signals. Thus, while minimizing the cost, OA placement algorithm in FON guarantee the feasibility of the connections. Free propagation of the signals requires FONs to be divided into fiber trees to prevent undesired laser-loop effects. Different partitions may lead to different costs of OAs, so the one, minimizing the cost, must be found. In this thesis we investigate the problem of OA placement in metro-area and core networks, aiming at minimizing OA's cost, in two …