  Increased traffic demands globally and in particular in short-reach links in
data centers will require optical communication systems to continue scaling at
an accelerated pace. Nevertheless, energy constraints start to limit the bit
rate that can be practically transmitted over optical systems both at the
shortest distances in data centers and at the longest distances in ultra-long
submarine links. Short-reach links in data centers face strict constraints on
power consumption, size, and cost, which will demand low-power solutions that
scale to bit rates beyond 100 Gbit/s per wavelength, while accommodating
increased losses due to longer fiber plant, multiplexing of more wavelengths,
and possibly optical switching. At the longest distances, submarine optical
cables longer than about 5,000 km face energy constraints due to power feed
limits at the shores, which restricts the electrical power available to the
undersea optical amplifiers, ultimately limiting the optical power and
throughput per fiber. This dissertation presents strategies to mitigate these
limitations. For short-reach links in data centers, we propose low-power
coherent receiver architectures that completely avoid high-speed
analog-to-digital converters and digital signal processors. For long-haul
submarine links, we demonstrate how optimizing the channel power allocation
under a constraint on the amplifier power maximizes the information-theoretic
capacity per fiber.
