  The exponential growth of demand for high-speed internet and high bandwidth
applications such as interactive entertainment in access networks mandates the
requirement for higher optical signal speeds. On the other hand, since cost and
energy efficiency should be concurrently preserved in access networks, PONs
have emerged as a breakthrough solution. This survey shows that future-proof
PONs should be supported by an adaptively modulated WDM-OFDM architecture to
maximize signal capacity and transmission-reach, while the employment of
reflective semiconductor optical amplification and re-modulation can
potentially prevent the employment of an additional light source.
