# Optical Link Design for Quantum Key Distribution-Integrated Optical Access Networks
## 2 High-Speed PON
### Towards a Beyond-10G-Class TDM-PON  
Figure 1a illustrates the DS transmission in a TDM-PON. A TDM signal is transmitted from an OLT to all connected ONUs via a power splitter. Each ONU extracts its designated data while discarding the rest. For the US transmission, as depicted in Figure 1b, ONUs transmit data within their assigned time slots, necessitating a scheduling mechanism. The widespread adoption of a TDM-PON in OANs is primarily driven by its cost-effectiveness, which stems from the use of standard fixed-wavelength optical transceivers for each direction (i.e., DS or US) and a power splitter for signal distribution [3]. Widely deployed symmetric 10G-class TDM-PONs have been standardized by the Institute of Electrical and Electronics Engineers (IEEE) and the Full-Service Access Network (FSAN) and ITU-T, and are referred to as XGS-PON [10] and 10-Gigabit Ethernet PON (EPON) [24], respectively.  
For all PONs operating at 10 Gb/s or lower, DS signals are transmitted over the C-band and S-band, while US signals are transmitted over the O-band (see Table 1) [5].  
The IEEE 802.3 ca has standardized the 25G EPON to address the growing bandwidth demands [5]. With the implementation of 100 Gb/s Ethernet, the commercialization of 25 GHz transmitters, such as electro-absorption modulated laser (EML) or directly modulated laser, along with 25 GHz avalanche photodiodes (APDs), has become feasible. However, higher line rates have posed new challenges in maintaining the required 29 dB power budget, which is essential for delivering Internet services to 32-128 ONUs in a TDM-PON. This challenge arises from an additional 5 dB receiver sensitivity penalty, induced by the increased line rate from 10G to 25G [5]. To mitigate this issue, forward error correction (FEC) has been enhanced, APD noise levels have been reduced, and the launched power has been increased to as high as 10.5 dB [25], compensating for the penalty by 1 dB, 1 dB, and 3 dB, respectively [5]. The stringent power budget constraints necessitate the use of on-off-keying (OOK) modulation [3; 4; 5]. Additionally, due to the significant chromatic dispersion (CD) penalties at 25G after 20 km of standard single-mode fiber (SSMF), the US wavelength has been shifted to the 1358 nm region (see Table 1).  
The ITU-T has initiated efforts to standardize the 50G-class PONs [4]. It is anticipated that 50G PONs will require OOK modulation, semiconductor optical amplifier-integrated 50 GHz EML operating in the O-band, and a digital signal processor to compensate for CD-induced distortion [4; 31]. This suggests that beyond-10G-class TDM-PONs may enable the allocation of the C-band for QKD, where propagation loss is minimal, provided that legacy 10G-class PONs are no longer in use. Adequate spectral separation between QKD and PON signals can help mitigate noise from Raman scattering [9]. However, spontaneous  
\begin{table}
\begin{tabular}{c c c} \hline
**PON Standard** & **DS Wavelength (nm)** & **US Wavelength (nm)** \\ \hline GPON (ITU-T G.984) [26] & 1480–1500 & 1290–1330 \\ EPON (IEEE 802.3 ah) [27] & 1480–1500 & 1260–1360 \\ XG-PON1 (ITU-T G.987) [28] & 1575–1580 & 1260–1280 \\ XGS-PON (ITU-T G.9807.1) [10] & 1575–1580 & 1260–1280 \\
25G EPON (IEEE 802.3 ca) [29] & 1340–1342, 1358–1360 & 1260–1280, 1290–1310, 1319–1321 \\
50G PON (ITU-T G.9804) [11] & 1340–1344 & 1260–1280, 1290–1310 \\ NG-PON2 (ITU-T G.989) [30] & 1596–1603 & 1524–1544 \\ \hline \end{tabular}
\end{table}
Table 1: Wavelength plan for PON.  
Figure 1: (a) DS and (b) US transmission in TDM-PON. (CO: central office, PS: power splitter).  
Raman scattering generated by O-band PON signals remains non-negligible, given that high-power transmitters are required for beyond-10G-class TDM-PONs.