We propose a multilevel transmission technique that employs optical amplitude domain multiplexing. With this technique we can easily increase the capacity of TDM-PON systems based on on-off keying modulation without any loss budget reduction or the need to employ higher frequency drive circuits. We report an experimental result for the proposed multilevel transmission system whose receiver comprises only commercially available electric devices. The experimental result reveals good performance in a quaternary transmission indicating that our technique is suitable for upgrading TDM-PON systems.