Envelope tracking for linearization of semiconductor optical amplifier in coherent optical OFDM systems

Since the last decade, the society has become more connected increasing the expectations about high data rates, low cost of implementation, and flexibility for the next generation of communication networks. Thus, to meet these requirements the coherent optical orthogonal frequency division multiplexing technique (CO-OFDM) has pointed out as a suitable technology. CO-OFDM can achieve high spectral efficiency, robustness to chromatic dispersion, and convenient implementation flexibility and capabilities of compensating nonlinear impairments with digital signal processing methods. Hence it is convenient to research novel techniques that improve performance, mitigating nonlinear distortions by increasing the dynamic range of nonlinear devices such as semiconductor optical amplifiers (SOA). In this work, some nonlinear companding methods are studied and optimized to get the lower error vector magnitude (EVM) as a function of SOA input power. Also, a deep analysis of how the SOA behavior is affected by the change of its power supply (bias current) is presented, giving an important comprehension of the SOA nonlinear behavior in terms of EVM, output power, and gain. These inherent nonlinear effects associated with SOA may translate into a transmission performance loss for non-constant envelope modulation formats. However, a variety of linearization schemes may be adopted for coping with these impairments and offering an effective system design. In this doctoral dissertation, an envelope tracking (ET) technique is proposed for linearizing an SOA-based CO-OFDM transmitter. Optimized design of the ET subsystem is performed under various scenarios, with the eventual joint use of peak to average power ratio (PAPR) reduction either via hard-clipping or nonlinear companding. A thorough SOA carrier density analysis is performed to assess the effectiveness of the proposed scheme. Moreover, the robustness of the proposed approach against some parameters variation both inside the ET path (DAC characteristics and bandwidth limited envelope generation) is exposed