Estimation of the Performance Improvement of Pre-Amplified PAM4 Systems When Using Multi-Section Semiconductor Optical Amplifiers  
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SOAs also suffer from low gain saturation power (the order of 10 mW) which distorts the amplified signal even when the average input signal power is of the order of hundreds of microwatts [8; 9].  
The NF and saturation power of SOAs can be improved by subdividing the amplifier into separate sections and applying a different bias current to each subsection [10]. The concept of a multi-section SOA [10] is shown in Figure 2. In Figure 2a, for a single-section SOA, a large DC bias current is applied to the SOA resulting in equal carrier injection being applied along the entire length of the SOA. For the multi-contact SOA, the SOA is subdivided into smaller sections with each section biased independently. The SOA is operated with favorable current biasing arrangements, whereby a stronger bias current is applied towards the output facet allowing for an improvement of the effective NF and increase in the effective saturation power relative to single-section SOAs [10; 11; 12]. This favorable biasing current arrangement can be done without increasing the overall injected current and hence keeping the power consumption constant. In separate works, regarding reflective SOAs (RSOA), with the RSOA subdivided into two subsections was found to have improved the direct modulation electro-optic response of the RSOA [13]. To date, no study (neither experimental nor numerical) has been performed on investigating the benefits of amplifying information-carrying modulated optical signals using multi-section SOAs, only the NF and saturation powers have been measured and/or calculated [10; 11; 12]. In addition, a simplified numerical model for multi-section SOAs has not yet been developed that would allow estimating the amplification performance of modulated signals in multi-section SOAs.  
In this paper, we show how to create a simplified multi-section SOA model and use that model to evaluate the amplification performance of multi-section SOAs, and as comparison to conventional single-section SOAs for 28 Gbaud PAM4 signals. We are modifying our simplified SOA model in [14] to consider multi-section SOAs. We find that in order to replicate the experimental results for amplification of PAM4 signals using SOAs in [15], we need only consider amplified spontaneous emission (ASE) noise generated in the SOA and thermal noise in an optical receiver, and we achieve similar bit-error rate (BER) performance to experimental 28 Gbaud PAM4 systems [15]. The input power dynamic range (IPDR) is an important metric for amplified systems using SOAs [8], because the IPDR indicates the useful input power range over which signals can be amplified and the data retrieved without error [8]. We show that a multi-section SOA could improve the IPDR by more than 3 dB relative to the IPDR of a similar gain single-section SOA. The results, from this simple system simulator for multi-section SOAs, motivate the further experimental investigation of multi-section SOAs for datacom applications.  
Figure 1: (**a**) Simplified schematic showing communication between two clusters of racks in a datacenter. The transponders use externally modulated lasers (EML) to communicate at up to 400 Gbit/s between the top of the rack aggregators (ToR Agg.); (**b**) Proposed use of semiconductor optical amplifiers (SOA) as a pre-amplifier to overcome losses within an optical switch. Eyediagrams of the simulated PAM4 signals are shown, in (**b**) a slightly distorted eyediagram due to the SOA nonlinearities are shown.