We investigate the transmission performance of 28-Gb/s nonreturn-to-zero on-off keying (NRZ-OOK), duobinary, and four-level pulse amplitude modulation signals over different fiber lengths without any inline amplification or pre/ post-processing digital signal processing technique by utilizing an O-band 18G-class directly modulated laser (DML). By experiment, it is verified that negligible sensitivity penalty is introduced by DML chirp effect after fiber transmission, which is due to the extremely low dispersion of O-band wavelengths. Meanwhile, the results indicate that NRZ-OOK outperforms the other two formats in receiver sensitivity, and for 20-km transmission, a power budget of 25.5 dB is achieved without pre-amplification. According to the experimental results, O-band DML-based NRZ-OOK transmission is a cost-effective solution for future 100G-EPON.
