## 1 Introduction
However, in both cases, controlled offset filtering by a WDM filter or an additional delay line interferometer was required, which makes the system sensitive to any wavelength drift. In addition, the extra lasers at the CO increases cost and energy consumption. In these three schemes, the modulation bandwidth limitation of RSOAs is a challenge that has to be addressed to fulfill the high data rate requirements of WDM-PONs. There have been different approaches to alleviate this issue. One solution is to improve the speed of the RSOAs by optimizing the physical design of the device [26]. Initial implementations of RSOA, mainly in the externally seeded scheme, employed non-return-to-zero (NRZ) amplitude-shift keying (ASK) modulation with either electronic [27; 28] or optical signal processing [28; 29] or a combination of electronic and photonic signal processing [30] to demonstrate 10 Gbit/s. Return-to-zero (RZ) and NRZ are compared in [31]. Advanced modulation formats have also been demonstrated for higher spectral efficiency with electronic and optical equalization in [12]. Polar RZ PAM4 with delay interferometer optical signal processing has achieved 20 Gbit/s [32]. Recently, multicarrier schemes such as adaptive orthogonal frequency division multiplexing (Adaptive-OFDM) [33] have been implemented for improved performance of the externally seeded scheme. Adaptive OFDM [25] and bit- and power-loaded DMT [14], both with optical equalization have already demonstrated beyond 25 Gbit/s transmission. In general, DMT has been used to maximize capacities of bandwidth limited intensity modulated direct detected (IM-DD) systems [25; 34; 35; 36]. DMT is a type of OFDM with real-valued output signals that can be easily detected with inexpensive direct detection receivers. As a multicarrier format, DMT also brings the flexibility of adapting the modulation format and power of any individual subcarrier to cope with the frequency dependence of the channel signal-to-noise ratio (SNR). In this paper, we employ bit- and power-loaded DMT to show that the ASE spectrum-sliced scheme, the self-seeded RSOA-FCL, and the externally seeded RSOA configurations are viable US transmitters. The results are an expansion of our recent work [37], where only the maximized bit rates are presented. In this work, the SNR offered by each sources has been measured and compared. It is found that the SNR improves by more than 5 dB when switching from an ASE spectrum-sliced scheme to a self-seeded RSOA-FCL, and it improves by another \(\sim\)5 dB when switching from a self-seeded RSOA-FCL to an externally seeded RSOA. Finally, transmission experiments have been performed. We demonstrate 6.25, 20.1, and 30.7 Gbit/s in back-to-back (BtB) and 4.17, 10.1 and 24.5 Gbit/s over 25 km NZDSF (non-zero dispersion shifted fiber) for the ASE spectrum-sliced, RSOA-FCL and externally seeded RSOA sources, respectively. No offset filtering was needed in any of these experiments [14; 25]. The organization of the paper is as follows. First, we discuss the steps for generating and analyzing a DMT signal. In Section 3, we present the experimental setups. In Section 4, we measure the SNR of the RSOA-FCL by varying WDM filter bandwidth, cavity length and bias current. We compare  
Figure 1: Wavelength division multiplexed passive optical network (WDM-PON) topologies with reflective semiconductor optical amplifier (RSOA) based colorless upstream (US) transmitters: (**a**) Amplified spontaneous emitter (ASE) spectrum-sliced transmitter: the US signal is directly modulated on to the ASE of the RSOA. The WDM filter at the remote node (RN) passes only the assigned sub band from each RSOA at the optical network units (ONUs); (**b**) Self-seeded RSOA-FCL (fiber cavity laser): a Fabry-Perot cavity indicated by the red dashed lines is formed between the RSOA and a mirror at the RN. The cavity is embedded in the distribution fiber (DF) of the PON; (**c**) Externally seeded RSOA: a laser at the central office (CO) seeds the RSOA at the ONU.