Performance Analysis of the Injection of a Transparent Assist Light in a SOA Amplifying a CO-OFDM Transmission System

Using large optical bandwidth Semiconductor Optical Ampliflers (SOA) is exten- sively studied as a solution for amplifying the future broadband high data rate optical transmis- sion networks. Speciflc research is focused on the non-linear impacts of SOAs on the data pattern of amplifled signals. A solution to improve performances of a SOA used as an inline amplifler is to increase its linear operating regime by increasing its output saturation power. A method to enhance the output saturation power, without changing the amplifler operating point in terms of gain level and NF, is to inject into the SOA an optical continuous wave (CW) pump signal at the gain transparency wavelength (transparent assist light) using a counter-propagation conflguration, as shown in Figure 1(a). The improvement of its performances on conventional On-Ofi Keying (OOK) signals has been previously proved (1). In this paper, it is numerically investigated for a 10.94Gb/s QPSK (Quadrature Phase-Shift Keying) CO-OFDM (Coherent Optical-Orthogonal Frequency Division Multiplexing) signal including 128 subcarriers by using an experimentally validated simulation platform (2). An enhancement of about 4.2dB is obtained for the output saturation power. In addition, the coupling factor amplitude-phase (Alpha Factor) remains constant over a wide range of powers. By evaluating the Error Vector Magnitude (EVM) of a received QPSK CO-OFDM signal with this conflguration and by comparing with results where no assist light is used, an improvement in the EVM is observed for high input powers (Figure 1(b)) leading to a larger input power range (about 2.5dB more) for a given EVM. It can be shown that this improvement is due to a reduction of non-linear efiects, especially the Four-Wave Mixing (FWM) efiect which is the principal cause of the performance degradation for an OFDM signal, while the other parameters of the SOA remain unchanged.