Ever since the invention of single mode fiber optic cable decades ago, the industry has continued to develop new ways of increasing the amount of data that can be transmitted over an optical fiber link. Two significant developments have improved fiber utilization: (1) the simultaneous transmission of multiple lasers of different wavelengths over a single fiber — a technique called wavelength division multiplexing (WDM), and (2) coherent transmission using digital signal processors (DSPs) to modulate and detect multi-levels in both phase and amplitude of laser light on two polarizations, resulting in increased spectral efficiency. This white paper reviews the technological advancements that have increased the capacity of information that can be transmitted over a single mode fiber link and discusses how parameters in coherent transmission such as modulation order, baud rate, and transmission shaping determine overall fiber capacity.

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