Baseband Linearity and Equalization in Fiber Optic Digital Communication Systems

If a single pulse of optical energy propagates along a dielectric waveguide, pulse broadening can occur for one or more of the following reasons: material dispersion, individual mode waveguide dispersion, or differences in the group delays of different guide modes. In addition, the pulse shape may become only statistically defined because of random mode coupling and/or statistical fluctuations of the optical source. If a sequence of digitally on-off modulated pulses is injected into a dielectric waveguide, those pulses may begin to overlap after a sufficient distance of propagation. In general, the optical powers in the pulses will not add in a linear manner.1" On the other hand, as will be shown below, the guide may be pseudo-linear in power. T h a t is, for the purpose of processing the power received at the output end of the t The fiber is a medium which is linear in E field propagation (from Maxwell's equations). It is usually excited by a power-modulated source, and its output field is detected bv a square-law (power sensitive) device. Even if the input pulses are separate, the response of the square-law detector will in general contain cross terms resulting from the overlap in the output pulses. 1175