Scaling limits for optical interconnects: Directly modulated versus externally modulated links

We establish a lower bound on the energy-per-bit required for transmitting information using a photonic channel. We include in our analysis the energy required to convert information from the electronic to the photonic domain and back. We investigate links that employ a directly modulated laser as well as an external modulator. We show that the power dissipation of the channel also imposes a bound on the bandwidth density.