# Optical Link Design for Quantum Key Distribution-Integrated Optical Access Networks
## 1 Introduction  
The rapid expansion of artificial intelligence (AI)-driven services such as ChatGPT-4o, cloud computing, and wireless communications has significantly increased data traffic within optical access networks (OANs) [1, 2, 3, 4, 5, 6, 7, 8]. OANs deliver Internet services to multiple end-users through fiber-to-the-x (FTTx) technologies, primarily implemented as passive optical networks (PONs) that operate without optical amplifiers [1, 2, 3, 4, 5, 6]. These networks typically support transmission distances of 20-40 km using a time-division multiplexed/time-division multiple access (TDM/TDMA) scheme [1, 3, 4, 5]. For simplicity, this paper refers to such networks as TDM-PONs instead of TDM/TDMA-PONs [1]. In a TDM-PON, an optical line terminal (OLT) communicates with optical network units (ONUs) by assigning different time slots to each user [1]. A key advantage of this approach is that the branching point, or distribution cabinet, can be implemented using a simple power splitter, significantly reducing costs [6, 9]. However, a major drawback of TDM-PONs is their high power budget requirement, as splitting loss increases with the number of ONUs [2]. This issue becomes particularly critical at a line rate exceeding 10 Gb/s [3, 4, 5, 6, 7, 8]. For example, the 10 Gb/s symmetric XGS-PON, standardized in the International Telecommunication Union Telecommunication Standardized Sector (ITU-T) G.9807.1, requires a power budget of at least 28 dB to support a 20 km distance with 32 ONUs [10]. In this standard, bidirectionalcommunication is achieved using a 1577 nm wavelength for downstream (DS) transmission and a 1270 nm wavelength for upstream (US) transmission [10]. Looking ahead, ITU-T Q2/15 and G.Sup64 standardization indicate that future high-speed PONs (HSPs) will likely be based on 25 or 50 Gb/s TDM-PONs or \(N\times 50\) Gb/s time-and-wavelength-division multiplexing (TWDM)-PONs [1; 4; 11; 12], following the NG-PON2 (ITU-T G.989) framework [2].  
Beyond increasing transmission speeds, security has emerged as a critical concern for OANs which directly connect end users [9; 13; 14; 15; 16]. With the advent of autonomous vehicles, this security challenge may become a matter of life and death [17; 18; 16]. As OANs increasingly encompass mobile fronthaul applications, ensuring their secure operation would become imperative [19; 20]. Although TDM- and TWDM-PONs are vulnerable to security threats due to the broadcast nature of DS transmission [13], where signals are shared among multiple ONUs (including unintended recipients), quantum key distribution (QKD) technology can offer unconditional security [14; 21; 22]. However, the high cost of QKD remains a major barrier to its deployment in OANs, which are highly cost-sensitive [1; 2; 3; 4; 5; 6; 7; 8; 9]. In addition, the QKD can be operated in the presence of the Raman scattering induced by PON signals and the distribution loss associated with the point-to-multipoint (P2 MP) topology [14; 15; 23].  
In this paper, we provide a comprehensive review of the integration of QKD into OANs. The present review aims to clarify the implementation challenges that arise when integrating QKDs into OANs to enhance security. Previous research suggests that DS continuous-variable (CV)-QKD is a viable option for integration into PONs, particularly in terms of the number of subscribers that can be supported by QKD services. Section 2 reviews the existing PON standards, while Section 3 examines the research on QKD in P2 MP topologies. Section 4 suggests a framework for integrating DS CV-QKD with OANs and discusses key implementation challenges in CV-QKD. Section 5 presents the conclusions.