## 1 Introduction
### Experimental Setups and Parameters
The wavelengths of the two ONUs are 1565.495 nm and 1564.678 nm, respectively, which give rise to a wavelength spacing of \(-0.8\) nm for effectively mitigating the optical beating interference effect [16]. Such a wavelength spacing is large enough to allow the involved two ONUs to use free-running lasers. For ONU2 occupying the high-frequency spectral region, a tunable optical filter (TOF, Yenista XTM-50 filters) is used to produce an optical single sideband signal (OSSB) for mitigating the channel fading effect [29]. The optical filter bandwidth and its passband central frequency are adjusted to ensure that the channel interference effects between different ONUs are negligible. Two Erbium-doped fiber amplifiers (EDFAs) each followed by a 0.8 nm bandwidth TOF fix the ONU output optical powers at \(-6\) dBm. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Inter-ONU**} & \multicolumn{2}{c}{**Upstream**} \\ \hline ONU1 & CH1 & CH2 & CH3 & CH4 \\  & 3.76 & 3.88 & 6.82 & 16.82 \\ ONU2 & CH1 & CH2 & CH3 & CH4 \\  & 3.88 & 3.88 & 7.41 & 16 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Channel bitrate (GBPS). Figure 4: Experimental setup of a 27 km, 62.47 Gbit/s upstream IMDD PON with inter-ONU communications. (**a-d**): Electrical signal spectra. (**e.f**): Optical signal spectra. (**g-i**): The Brillouin backscattering effects with all the channels being switched off. Blue/Orange: electrical/optical signal spectra. At the remote node, for the proof-of-concept demonstrations, a 50:50 passive optical coupler and an optical circulator are used for realizing the simultaneous upstream and direct inter-ONU communications. The optical power launched into the 25 km feeder fiber is \(\sim\)6 dBm. For the upstream communications, in the OLT, the received optical signals' spectra are plotted in Figure 4e. After direct detection, the corresponding electrical signal spectra containing eight channels are presented in Figure 4d. In addition, for the direct inter-ONU communications, the ONU-received optical and electrical signals' spectra are also illustrated in Figures 4f and 4c, respectively. The optical detection and signal demodulation processes of the upstream signals and the direct inter-ONU communication signals are similar. A 40 GHz bandwidth PIN photodetector is used to convert the two optical signals to an electrical signal consisting of eight independent channels. Following the PIN, a 40 GHz@64 GSample/s digital sampling oscilloscope (Keysight UXR0402A) digitizes the received electrical signal. The signal demodulation processes are conducted offline, which include S/P conversion, CP removal, 256-point FFT operation, conventional single-tap subcarrier equalization, multi-channel de-aggregation, and BPSK/QAM decoding. For the multi-channel de-aggregation process, two cascaded FFT operations with the sizes of 32 and 16 are, respectively, implemented for each ONU. For the considered PON architectures, both the direct inter-ONU and upstream communications suffer from the Rayleigh and Brillouin backscattering effects. Because the fibers between each ONU and the optical coupler are only \(\sim\)2.2 km as seen in Figure 4, the upstream communication-experienced backscattering effects arising from the broadcasted direct inter-ONU signals are negligible in comparison with the backscattering effects suffered by the direct inter-ONU communications [16]. This is further verified by Figure 4g,i, where the Stokes and anti-Stokes components of the upstream communication-suffered Brillouin backscattering effects are not obvious in comparison with the backscattered components suffered by the direct inter-ONU communications. For the inter-ONU communications, the Brillouin backscattering effects lead to a power peak at a radio frequency of \(\sim\)10.8 GHz in the directly detected direct inter-ONU communication signals, as seen in Figure 4h.