Chromatic Dispersion in Optical Fibers

Posted by Kevin Miller on Wed, Aug 24, 2011 @ 12:08 PM

Chromatic dispersion is a phenomenon that is an important factor in fiber optic communications.  It is the result of the different colors, or wavelengths, in a light beam arriving at their destination at slightly different times.  The result is a spreading, or dispersion, of the on-off light pulses that convey digital information.  Special care must be taken to compensate for this dispersion so that the optical fiber delivers its maximum capacity.

Chromatic dispersion is commonplace, as it is actually what causes rainbows - sunlight is dispersed by droplets of water in the air.  Sir Isaac Newton observed this phenomenon when he passed sunlight through a prism and saw it diverge into a spectrum of different colors.  This dispersion occurs because different colors, or light frequencies, act slightly differently as they pass through a medium such as glass.  In fiber-based systems, an optical fiber, comprised of a core and cladding with differing refractive index materials, inevitably causes some wavelengths of light to travel slower or faster than others.

Chromatic dispersion is a serious consideration in long-haul optical fibers.  Its effect is essentially to stretch or flatten the initially sharply-defined binary pulses of information.  This degradation makes the signals (1s and 0s) more difficult to distinguish from each other at the far end of the fiber.  The result is that at any given length, the effective information capacity, or bandwidth, of the fiber optic cable can be significantly reduced.  Dispersion is added as the modulated beam of light, consisting of a number of closely spaced wavelengths, travels down this nearly transparent waveguide.

The bottom line is that chromatic dispersion becomes a major consideration and must be accounted for when developing or deploying fiber optic equipment for use in telecommunications, cable TV, or other high-speed optical networks.

Fortunately, techniques have been developed that help compensate for the negative effects of chromatic dispersion.  One method involves pre-compensating the signal for the anticipated dispersion before it's sent down the optical fiber.  Another method calls for using dispersion compensating fiber at the end of a length to correct or reverse the dispersion that was realized as the signal traversed the optical fiber.  As a result, these techniques are widely used to help solve the problem of chromatic dispersion.

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Topics: optical fiber, dispersion compensating fiber