## 1 Introduction  
In the 5G and beyond era, ultra-reliable low-latency communication (URLLC) is increasingly critical for supporting use cases associated with factory automation, transportation, and electrical power distribution [1, 2]. The International Telecommunication Union (ITU-T) has specified that the maximum end-to-end user-plane latency for 5G URLLC services is 1 ms [3]. For delivering future 6G services, even more challenging latency targets of <1 ms are envisaged, which has already been accepted by both academia and industry [4]. As a direct result, exploring cost-effective solutions capable of satisfying such an extremely low latency requirement imposes unprecedented pressure on the telecommunications R&D community worldwide. On the other hand, from the practical network implementation point of view, intensity modulation and direct detection (IMDD) passive optical networks (PONs) offer a cost-effective solution to construct the next generation of x-haul mobile access networks [5], where standard single-mode fibers (SSMFs) are utilized to connect an optical line terminal (OLT) (interconnected to the central unit or distributed unit) and optical network units (ONUs) (interconnected to the distributed unit or remote unit). For a typical 20 km PON, a one-way fiber propagation delay is ~0.1 ms (~5 \(\upmu\)s/km [6]), which is nonnegligible in comparison with other latency-generating factors such as forward error correction (FEC) time delay (<0.5 \(\upmu\)s [7]). In addition, numerous URLLC applications including autonomous vehicles, industrial automation, and telemedicine not only have stringent low latency requirements, but also impose a strong need for dynamically establishing communication connections between different users within a single mobile access network [7; 8]. However, the communications between different end-users in a single PON-based mobile access network require passing the source ONU data to the OLT and then to the destination ONUs, which leads to a round-trip fiber propagation delay of ~0.2 ms (for 20 km PONs). Therefore, to effectively meet the ultra-low latency requirements of these applications, direct inter-ONU communications (also termed ONU-to-ONU communications in this paper), without passing data to the OLT and compromising upstream transmission performances, are highly desirable [9; 10; 11; 12; 13; 14]. For practically achieving such direct inter-ONU communications in a PON, it is necessary to equip the remote node with the capability of redirecting and broadcasting each ONU signal to all other ONUs. For achieving this functionality, several techniques have been reported, where corresponding PON remote nodes are modified using cyclic arrayed waveguide gratings [10; 11], fiber Bragg gratings [12], optical switches [13; 14], passive optical couplers [9; 15; 16], cascaded optical interleavers [17], and their combinations [18]. The techniques based on dynamic optical switches [13; 14; 19] cause their remote nodes to be active, thus leading to relatively high power consumption. A ring network topology would also deliver the desirable direct inter-ONU communications [20; 21], which, however, may require modifying the existing network architecture. In comparison, the optical coupler-based remote nodes are simple, cost-effective, and transparent to wavelengths [9; 15; 16; 22]. However, for PONs with a large number of ONUs, the optical coupler-based remote nodes introduce extremely high power losses for inter-ONU communications. For example, for a PON using a 64-by-3 optical coupler for supporting the inter-ONU communications between 64 ONUs, the inter-ONU signal power losses can be as large as ~43 dB. Existing optical coupler-based techniques use extra optical amplifiers to compensate for such power loss [23], which, however, result in active remote nodes, and are thus not desirable for PON application scenarios. Considering the fact that future PONs are expected to support more ONUs, it is thus vital to explore advanced passive remote nodes with low inter-ONU signal power losses.