**5. Conclusions**

Nowadays the WDM-PON systems rely on fixed wavelength transmitters and are expected to become more cost-efficient at high per user data rates. It was examined that different types of optical modulation formats are available for passive WDM fiber optical access networks. Implementation and research of multilevel modulation formats like PAM-4 and duobinary can dramatically improve the spectral efficiency and available bitrate by using the bandwidth of already existing optical, electro-optical or electrical devices. Theoretical simulations and experimental research methods showed possibility to double the available transmission speed in optical access networks by using the same bandwidth, e.g., instead of 10 Gbit/s transmit 20 Gbit/s signal by using 10 GHz electrical and electro-optical equipment, if PAM-4 modulation method is used. In our research we investigated existing optical modulation formats—widely used NRZ, DB and PAM-4 for optical access networks, by experimentally demonstrating and modeling system transmission in RSOFT OptSim simulation environment and Matlab software. As it shown by simulation results, narrowest channel spacing provides higher spectral efficiency. However, better signal quality and system performance are achieved with larger channel spacing interval, e.g., 100 GHz, mainly due to crosstalk between channels. From experimental data we can clearly see that the chromatic dispersion limits transmission capacity when bit rates increase. Implementation of the efficient compensation solution may sufficiently extend the reach of optical link and improve the transmission quality in our investigated WDM-PON systems.
