## 1 Introduction  
Space division multiplexing (SDM)-based free-space optical (FSO) links have garnered significant attention in recent years due to their potential to address the growing demand for high-capacity communication networks [1, 2, 3, 4]. These links aim to effectively overcome the last-mile connectivity challenge by efficiently bridging the gap between end users and the installed fiber infrastructure [5]. To further enhance this approach, a hybrid mode division multiplexing (MDM)-FSO bidirectional link can be implemented. This hybrid system offers a reliable and scalable solution for fiber-to-the-home (FTTH) applications, ensuring high-speed, cost-effective, and flexible connectivity to meet the increasing data traffic demands of modern households and businesses [2, 4, 6]. Such systems leverage the unique advantages of MDM technology, including improved spectral efficiency and channel capacity, combined with the flexibility of FSO links in overcoming geographic and infrastructural constraints. By integrating these technologies, MDM-FSO links not only address bandwidth limitations but also provide a resilient alternative in areas where traditional fiber deployment is challenging or cost-prohibitive. Figure 1 illustrates the proposed hybrid MDM-FSO architecture and its potential application in FTTH networks. The deployment illustrated in Figure 1 highlights a section of the MDM network that uses three-mode fiber as independent channels. It takes into account key parameters such as the number of spatial modes (applied to both MMF and FMF), modal dispersion, mode coupling, and mode-dependent loss (MDL). Additionally, factors like crosstalk between modes, fiber geometry, operating wavelength, and signal modulation format are considered, as they all have an impact on the capacity and transmission quality of the MDM system. An FSO link is employed under certain atmospheric conditions to address disruptions in a wired segment. A power splitter/mode multiplexer is implemented, which features one input and multiple outputs corresponding to the number of modes that the FSO link is intended to replace, depending on the specific type of channel (LP\({}_{01}\), LP\({}_{11}\), LP\({}_{02}\)). Finally, the quality of the signal within the network is assessed at the receiver level using a demultiplexer and mode selector. Different MDM signals can be transmitted through spatially engineered fibers, enhancing channel capacity and spectral efficiency [7; 8]. For the final meters of a communication link, high-capacity free-space optical (FSO) technology serves as an effective solution, particularly in scenarios where deploying telecommunication fiber cables is impractical or cost-prohibitive [8]. FSO technology is a notable application in establishing connections between offices within the same building or across university campuses, where short-range, high-speed links are essential. Telecom carriers have been making substantial investments in boosting the capacity of fiber backbones. However, to maximize the potential of FSO's existing capabilities and enhance revenue generation, this backbone expansion must be complemented by a comparable increase in data demand and accessibility at the network edge, where numerous end users connect to the system [9]. MDM-based FSO systems represent a promising solution for addressing these demands and can be implemented in a variety of scenarios. Specific applications include long-range wireless metropolitan area networks (WMANs), cellular backbones [10; 11], and seamless connectivity between WLAN networks on campuses and in enterprise environments. Moreover, FSO technology is highly adaptable and versatile, making it an excellent choice for wireless video surveillance and monitoring systems and for delivering broadband services to underserved or remote areas. Due to its ease of installation, flexibility, and re-deployable nature, FSO is particularly well-suited for disaster recovery scenarios, providing rapid and reliable connectivity when traditional infrastructure is damaged.