## 1 Introduction  
The time division multiplexed (TDM) passive optical network (PON) has been one of the most important architectures to support the increasing bandwidth and traffic requirements in access networks. The standardization of 25 Gb/s per \(\lambda\) (25G-PON) has been recently released by IEEE (25G-EPON [1]). Standardization of 50 Gbps PONs is under development by IEEE and ITU-T. 50 Gbps PONs exploit two wavelengths per direction, each at 25 Gbps [2, 3]. The 25G-PON physical layer will preserve the intensity modulation and direct detection (IM-DD) scheme to allow low-cost transmitter (TX) and receiver (RX) optics. For the first time in a PON standardization release, both the uplink and downlink will operate in O-band (around 1300 nm) to cope with chromatic dispersion over the PON standard reach from 0 to (at least) 20 km over single-mode fiber (SMF) [1, 2, 3, 4, 5, 6, 7, 8]. Our group has analyzed in previous papers the performance of directly modulated laser (DML)-based IM-DD PON systems in O-band [9, 10, 11]. In [12, 13], the channel frequency response of a DML-based IM-DD system was investigated in O-band through simulations and theoretical calculations. In general, DML represents a good choice when it operates in a system with low chromatic dispersion (CD), for example, in O-band, because of the low cost and small footprint advantages [14, 15, 16], while, because of laser chirp effects, it is usually less suitable for C-band operation. However, it was shown in [17] that DML, when operated in some specific situations, can even achieve a better performance than electro-absorption modulated laser (EML) and Mach-Zehnder modulator (MZM) in C-band, since under proper combination of adiabatic and nonadiabatic chirp, DML can give a higher nonfaded bandwidth than EML and MZM. The modulation bandwidth of DML is generally smaller and the chirp is generally higher than those of external modulators. External modulators, therefore, are more suited to higher data rate and longer reaches transmission (i.e., \(>\)10 km) [18]. However, DML is better suited to optical access networks, such as PON, due to low cost, low power consumption, low insertion loss, compactness, and high optical output power [19, 20]. The bandwidth of commercial DMLs can currently reach 21 GHz [21]. A 65 GHz bandwidth low chirp (enhancement linewidth factor \(\alpha<1.0\)) DMLmodule was demonstrated in [22], in which a high data rate transmission of 294.7 Gbps over 15 km single mode fiber (SMF) was achieved without using complex high speed electronic compensations, a strong optical confinement effect, or optical fiber amplifiers. Moreover, 100 Gb/s commercial DML has yet to be released [23]. Regarding modulation formats for 25 and 50 Gbps PON, besides traditional binary NRZ (return-to-zero, and possibly in a duobinary version), two other modulation formats, quaternary pulse amplitude modulation (PAM-4) and duobinary PAM-4 (DB-PAM-4) have been studied in the literature. When using DML lasers, the key physical layer impairments are well known to be the mix of laser chirp, fiber chromatic dispersion, and the limitations in the available modulation bandwidth. For instance, in [24], the measured channel frequency response of a DML based IM-DD system over a 0-20 km SMF and the measured receiver sensitivity of an on-off keying (OOK) signal obtained at low extinction ratio (ER) (below 2.7 dB) were shown. Typically, at bit rates above 10 Gbps for PON applications, chromatic dispersion (CD) may have an important impact, and this is the reason why, as mentioned before, both 25G- and 50G-PON currently under proposal in the ITU-T and IEEE standardization bodies operate in the O-band (around 1300 nm), where CD is at its minimum. Compared to O-band, there are two main advantages of C-band: lower optical loss and lower optical nonlinearity (because CD is not zero at 1550 nm; on the contrary, CD equals to zero at 1300 nm in O-band). Moreover, both downstream and upstream transmission in both 25G- and 50G-PON standardizations currently operate in O-band.