## 2 Experimental Setup and Principles  
The experimental setup of a 25+ Gbps IM/DD PON system is shown in Figure 1. It resembles a PON architecture based on point-to-multipoint passive optical power splitting in the outside distribution network. We consider a downstream transmission at net-rates above 25 Gbps over a single wavelength in the C-band. It is fully compatible with XG(S)-PON and 10G-EPON, and thus, it enables smooth migration of customers to higher net-rates without needing to change the ODN. Furthermore, the system's throughput can be increased by exploiting the WDM technique and adding new wavelengths as required. Of course, such a solution requires the wavelength management. In the OLT, the output of a continuous-wave laser source (CW) operating at 1550 nm wavelength is connected to the input of the dual-arm Mach-Zehnder modulator (MZM, Sumitomo, T.DEH1.5-40-ADC). An S\({}_{1}\)(t) input of the MZM, having a 3 dB bandwidth of 30 GHz, 9 dB insertion loss, and 15 dB extinction ratio, is driven using analog waveforms from an arbitrary waveform generator (AWG, Tektronix, AWG70001A) having 13 GHz analog bandwidth, up to 50 GSa/s sample rate and 8 bits vertical resolution. These waveforms are obtained using a 215-1 long pseudo-random bit sequence (PRBS15) and bit-to-symbol (B2S) mapping performed in MATLAB prior to the digital-to-analog conversion (DAC). The electrical signal after the output of the AWG was linearly amplified by an electrical broadband amplifier (EA) and fed into the electrical RF input of one arm of the dual-drive MZM. The second arm was loaded with a 50-Ohm load. To optimize the BER performance, we adjusted a bias voltage of the MZM. In such a way, we impact the MZM's chirp parameter. This adjustment ensures the best possible performance in terms of the dispersion-tolerance, which has a direct impact on signal quality.  
Note that the modulator's chirp is a key factor enabling dynamic tunability of the transmission reach in IM/DD-based optical fiber links [26]. After the MZM, the modulated optical signal is coupled into the feeder fiber and transmitted over a 21 km long standard single-mode fiber (SMF) link. The input optical power of +4 dBm ensures the ODN compliance to the E1 (18-33 dB) power budget class for several modulation format alternatives. After the downstream transmission, the optical signal is passively split by a 1-to-N (1:N) optical power splitter. In an ONU, the split optical signal is pre-amplified by a SOA(Samsung Electronics, OA40B3A, InP/InGaAsP, 1550 nm) prior to the detection. The variable optical attenuator (VOA), placed before the receiver, is used to adjust a received optical power required for the OPB assessment and to protect it from the overload. The receiver consists of a PIN photodiode (Discovery, DSC-R409, 30 GHz, 0.7 A/W) and a digital storage oscilloscope (DSO, Agilent Technologies, DSOX93304Q, 33 GHz, 80 GSa/s). The DSO is used to limit the receiver's minus 3 dB bandwidth to 8 GHz using a using a 4 th order Bessel-Thompson filter, to digitize the received waveforms and to store the digitized for further post-processing. The applied low-pass filtering (LPF) is imposed to emulate the bandwidth limitations at the receiver's (Rx) side aiming to investigate whether components intended for the 10G operation can be used for higher line rates. The offline digital signal processing (DSP) module consists of clock recovery, resampling, linear post-equalization (if considered), and bit-error-rate (BER) estimation. The signal is post-equalized by a linear symbol-spaced equalizer that uses feed-forward and feedback taps to compensate for analog imperfections. For BER estimation, we use a direct error counting. It relies on the bit-by-bit comparison of the transmitted and received bit sequences. More than 10\({}^{5}\) bits are available for the BER estimation at the receiver.