## 3 Experimental Setups  
In our experiments, we investigated the three colorless US transmitters in WDM-PON scenarios, see Figure 4. For all experiments, the transmitted DMT signal is generated offline with MATLAB and converted to an analog waveform by an AWG (Keysight M8195A--60 GSa/s, Santa Rosa, CA, USA). The analog signal is amplified and a DC bias current is added before being fed to an RSOA. In all cases, a 1 mm long InGaAsP quantum well C-band RSOA (provided by III-V labs, 10G-RSOA-11-S, Palaiseau, France) is placed at the ONU. The distribution fiber (DF) links the ONU to the RN. At the RN, a flat top optical band-pass filter (OBPF) emulates the WDM filter. RN and CO are connected by the feeder fiber (FF). The three implementations of the RSOA based transmission links are shown in Figure 4 and described in more detailed now. Figure 4a shows the setup for the ASE spectrum-sliced scheme. In this scheme, the current of an RSOA in the ONU is modulated with the data. This encodes the information onto the ASE output power of the RSOA. At the remote note, an OBPF (tuned to the emission center of the RSOA, which is at 1537.92 nm) band limits the modulated ASE-signal to the assigned wavelength channel. The filtered signal is then sent to the CO. The filter bandwidth has been chosen to be identical to the filter bandwidth of the other schemes. Figure 4b shows the setup for the self-seeded RSOA-FCL. Here, a Faraday rotator (FR) is placed at ONU after the RSOA. In addition, a Faraday rotator mirror (FRM) is placed at the RN after a coupler behind the WDM filter. The FRM and the RSOA reflective facet form the Fabry-Perot resonator cavity embedded in the DF. In case of RSOA with high polarization dependent gain (HPDG), the FR and FRM cancel out the birefringence effect in the cavity and maintain a constant polarization at the RSOA input [42]. The coupler at the RN has a coupling ratio of 90/10, where 90% of the power is kept in the cavity and 10% is transmitted to the CO. The large amount of the power kept in the cavity is required to overcome cavity losses and thereby to achieve a net gain and with this lasing. In addition, the high power in the cavity maintains the RSOA in the nonlinear operation region for better noise and residual modulation suppression [17; 43]. Figure 3: Constellation diagrams of modulation formats used for the bit- and power-loaded DMT signal generation. Binary phase shift keying (BPSK) encodes 1 bit/symbol. Quadrature phase shift keying (QPSK) encodes 2 bit/symbol. The quadrature amplitude modulation (QAM) formats: 8 QAM, 16 QAM, 32 QAM, and 64 QAM encode 3, 4, 5 and 6 bit/symbol, respectively. Figure 4c shows the experimental setup for the externally seeded RSOA scheme. Here, a laser source (emitting at 1537.92 nm, similar to the other setups) at the CO seeds the RSOA at the ONU through the fiber link. The seeding power entering the RSOA has been optimized for the highest SNR. The RSOA now only serves as a modulator and amplifier for the respective laser wavelength. In the three schemes, we implement only a single optical channel measurement due to lack of equipment, however more than 16 channels have been demonstrated [2; 21]. In all cases, the link budget between the ONU and the CO is \(\sim\)12 dB (i.e., combination of all insertion losses of components in the link between ONU and CO). At the CO, a direct detection receiver detects the signals from the three transmitters. Here, a 90/10 coupler sends 10% of the power to an optical spectrum analyzer (OSA) to monitor the signal. A variable optical attenuator (VOA) is used to control the power entering the preamplifier Erbium doped fiber amplifier (EDFA). A pre-amplified direct detection receiver is used here to guarantee that the signal quality measured here is as much as possible independent of the receiver scheme. A 5 nm flat top OBPF suppresses the out of band ASE noise from the EDFA. The optical signal is finally detected by a photodiode (PIN, responsivity 0.6 A/W), which is directly connected to a RTO (DSOX96204Q--20 GHz bandwidth and 40 GSa/s).