## 1 Introduction
Therefore, in future PON standardization, it may happen that the O-band window is full, so that it may become of interest to move again to the C-band. This is the rationale behind our paper. In fact, it is necessary to study DML based IM-DD systems in C-band 1550 nm. In the IM-DD system in [17], the eye skewing effect caused by adiabatic chirp and higher CD in the C-band mentioned before is mitigated by the intensity directed equalizer. Apart from IM-DD PON systems, several studies have been carried out in C-band. To overcome the higher CD in C-band, single sideband (SSB) modulation has been proposed in fiber-wireless systems [25] and radio-over-fiber links [26], and the tunable dispersion compensation module (TDCM) has been proposed in 5G networks [27]. In the high bit rate PON channel, severe optoelectronic (O/E) bandwidth (BW) limitations and power fading due to CD are two of the main issues [28]. The main limitation of the DML based IM-DD system (compared to EML and MZM based systems) is the interaction induced by CD and the adiabatic chirp of DML, which is a nonlinear impairment, in general complex to be analyzed. Frequency fluctuations in the transmitted signal (i.e., chirp) due to the higher dispersion in C-band over the link can generate intersymbol interference (ISI) at receiver [29]. The chirp of DML therefore limits the maximum transmission distance. As discussed in [17], directly modulated PAM symbols have different optical frequencies due to different intensity levels. The resulting eye skewing effect and other nonlinear distortions are caused when PAM modulated signals are transmitted over high dispersion links in C-band. Therefore, the ISI of adjacent symbols is different for the different intensity levels of PAM symbols. A fiber with negative dispersion was designed in [30] to gain from the positive chirp of DML. In [31], a tunable optical filter (TOF) was designed to reduce the impairments. RX offline digital signal processing (DSP), e.g., feed forward equalization (FFE) and decision feedback equalization (DFE), can be used to partially overcome the impairments. In this paper, we experimentally analyzed three problems: (1) the E/O frequency response of the 10G-class DML, (2) the frequency response of the DML based IM-DD system over a 25-118 km SMF, and (3) the maximum ODN loss (max. ODN loss) that can be achieved using this system employing electrical duobinary (EDB), NRZ-OOK (also termed as pulse amplitude modulation (PAM-2)), and PAM-4 signals as a function of extinction ratio (in a range from about 1 to 9 dB) at 25 Gbps over an SMF of up to 25 km in C-band using direct detection. We considered feed-forward equalization (FFE) and decision feedback equalization (DFE) approaches at the receiver, which were performed by using the offline processing approach in our laboratory experiments. We experimentally obtained that PAM-2 should be the the most appropriate modulation format of 25G-PON. The paper is organized in the following way: details of the experimental setup are described in Section 2. The experimental results are reported in Section 3. In Section 3.1, the E/O frequency response of our 10 G-class DML is characterized. In Section 3.2, we experimentally and theoretically analyze the channel frequency response of SMF in a DML based IM-DD system. In Section 3.3, we compare the performance of EDB, PAM-2, and PAM-4 signals as a function of ER. We conclude in Section 4.