Investigation of 4-PAM modulation format for use in WDM-PON optical access systems

In this paper, we investigate the performance of four level pulse amplitude modulation (PAM-4) format in fiber optical access networks. PAM-4 modulation format has higher spectral efficiency if compared to widespread used non-return-to zero (NRZ) line code and therefore potentially can provide higher data transmission speeds in fiber optical networks with limited bandwidth. When transmission speed is increased, chromatic dispersion (CD) can be the main distance-limiting factor for PAM-4 modulation format. For this reason in this paper we investigate maximal transmission distance with dispersion compensation and without dispersion compensation in 4-PAM modulated wavelength division multiplexed passive optical network (WDM-PON), as well as minimal available channel spacing. It is shown that dispersion compensation with fiber Bragg grating dispersion compensation module (FBG DCM) can improve the performance and maximal available reach of PAM-4 modulated optical signal by 25.4% or extra 15 km in length.