The Photon Marathon – Explaining Chromatic Dispersion To Engineering Technology Students

Dispersion or pulse spreading is a basic topic in an undergraduate engineering technology course in fiber optic communication systems. Students need to understand what causes dispersion and to be able to calculate pulse spread and determine how it limits the length and data rate of a communications system. Chromatic dispersion, which results from the wavelength dependency of the velocity of light in an optical fiber, is the primary source of pulse spread in modern communications systems. The standard formula for calculating pulse spread due to chromatic dispersion can be derived using basic calculus and the derivation is comprehensible by most engineering technology students. However, although this formula is quite useful, it does not quantify all of the effects of chromatic dispersion on optical pulses. To get the total picture, one can always solve the pulse propagation equation, but this kind of rigorous analysis is more suited for engineering graduate students. A reasonable compromise between these two extremes is to model an optical fiber as a linear system and many texts do so using a Gaussian impulse response and pointing to references to justify the choice of Gaussian.