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.