Coherent optical transmission is a technique that uses modulation of the amplitude and phase of the light, as well as transmission across two polarizations, to enable the transport of considerably more information through a fiber optic cable. Using digital signal processing at both the transmitter and receiver, coherent optics also offers higher bit-rates, greater degrees of flexibility, simpler photonic line systems, and better optical performance.
Coherent Optics: Taking Fiber Optics to the Next Level | Network World
