We demonstrate an ultra-dense wavelength-division- multiplexed (UD-WDM) passive optical network (PON) where 12.5-GHz spaced 1 GbE ${ imes}$ 256 optical channels are distributed using 12.5- and 200-GHz arrayed waveguide gratings in series. For the generation of upstream signals, we use reflective semiconductor optical amplifiers. We use two optical fiber amplifiers at the optical line terminal to amplify downstream and upstream channels.