# 10 Gb/s Bidirectional Transmission with an Optimized SOA and a SOA-EAM Based ONU
## 1 Introduction  
The explosive growth of network data requires higher transmission capacity and flexibility from optical communication networks, which is accelerating the development of next-generation passive optical networks (PONs) [1; 2]. Thus, research on the application of passive devices is gradually advancing, including monolithically integrated reflective semiconductor amplifiers (RSOAs) [3], dual electro-absorption modulated lasers (DEMLs) [4], distributed feedback semiconductor optical amplifiers (DFB-SOAs) [5], dual-output-DEMLs [6], and electro-absorption modulated laser semiconductor optical amplifiers (EML-SOAs) [7], which have been applied to 1.25 Gb/s, 2.5 Gb/s, or beyond for next-generation PON2s (NGPON2s) [8; 9]. This means that in facing the needs of future users, it is imperative to deploy a small-footprint, large-capacity, low-budget, and dense network. In the actual design of optical network units' (ONUs) structure, the performance of the light source, modulator, and power amplifier determine the transmission capacity and signal quality of the system [1]. It is desired that the whole system applies amplifiers and modulators with a limited volume and affordable costs, such as semiconductor optical amplifiers (SOAs) and semiconductor optical amplifier electro-absorption modulators (SOA-EAMs).  
Unstable received output is an issue at the ONU. Facing the existing issue, gain-control would be considered [10]. Compared with an erbium-doped fiber amplifier (EDFA) [11], an SOA is an integrated device with little volume, a long service life, low cost, and a mature manufacturing process. Thus, an SOA (as the amplifier of an ONU) can better meet the requirements of ONU design for an NGPON2 [12]. Additionally, an SOA can be used for all-optical wavelength conversions (AOWC) [13]and all-optical switches [14], which means that it is necessary to make an SOA choose the required proper condition to fit different functions. Hence, we show the performance of an SOA under different bias currents and input powers for selecting the operating environment in distinct applications.  
Reflective SOAs (RSOAs) [3], electro-absorption modulators (EAMs) [15], RSOA-EAMs [16; 17], and SOA-EAMs have been used in actual optical links as the modulators [18]. However, ONUs composed of reflective modulators are affected by inherent interference noise from backward reflected light [19], and the high insertion loss of EAMs will lead to the serious degradation of an optical signal-to-noise ratio (OSNR). Therefore, this paper mainly discusses the modulation performance of SOA-EAMs, which can compensate the loss from EAMs by SOAs. Additionally, the overshoot mode caused by an SOA saturation effect can compensate for the undershoot mode of an EAM to output an undistorted waveform. Additionally, the chirp of an SOA can also be compensated for by the chirp of an EAM [17; 18]. The integrated SOA-EAM combination currently employed in NGPON2-ONUs has a wider modulation bandwidth and a lower chirp than common modulators. This can increase system tolerance, improve optical signal quality in an optical access network, and enable a PON's configuration to reach a longer range.  
In this paper, we first characterize principal SOA performance via a small-signal model analysis method, obtaining the appropriate operating point of the input optical power and the bias current-carrying signal in Section 2. We then construct a carrier-distributed dense wavelength division multiplexed PON (DWDM-PON), discussing the modulation characteristics of the SOA-EAM and obtaining the sensitivities in single-ONU and multi-ONU transmissions for 10 Gb/s at a standard forward error correction (FEC) level [20] in Section 3. The paper ends with the conclusion in Section 4.