By Acacia Communications | Posted on January 8, 2019
The simplest form of fiber optic communication is coupling a laser to one end of a fiber, turning the laser on and off to transmit ones and zeros, and connecting a photodetector to the other end to record the photocurrent. This simple modulation format is called on-off keying (OOK), and the reception is called direct detection.
Besides optical power in single-mode optical fiber, phase and polarization are two additional orthogonal dimensions that can be used to transmit information. Coherent communication uses these three dimensions, increasing the fiber’s data-carrying capacity by at least a factor of four in gigabits per second (Gb/s). Today, a capacity increase of a factor of 12 is deployed, which is achieved as a result of the receiver’s ability to read the amplitude and phase of the signal. Equalization permits practical complex multilevel modulation, such as 16-QAM (quadrature amplitude modulation) (Figure 1).
Figure 1. Data constellations showing the signal locations in the complex plane (real and imaginary axes). Single-polarization on-off keying (OOK) and dual-polarization 16-QAM (quadrature amplitude modulation). Courtesy of Acacia Communications Inc.