IAN LANG ELECTRONICS

Optoelectronics

Distortion of Signals.

Our second type of dispersion is intramodal, which is also known for reasons that will become clear as chromatic dispersion. This dispersion is of primary concern in single mode systems as the use of single mode eliminates intermodal dispersion- you cannot have one ray travelling faster than itself.
Intramodal dispersion is something more of an esoteric concept than is intermodal. It is in fact a combined result of material and waveguide dispersions.
Both material and waveguide dispersions are to do with the refractive indices of the fiber materials and the wavelengths of the light source, which may produce a high degree of coherence but still have some width over the spectrum; that it is to say they produce light at more than one frequency and hence more than one wavelength simultaneously. Even a LASER, contrary to popular belief, produces light at more than one wavelength, although it is very narrow in spectral width. The diagram below serves to illustrate the difference:

As can be seen the LED produces light over a bandwidth of 20nm and the LASER over only 2nm. However the power of the output is greater in a narrow band, and this is particularly so for the LASER. The differing wavelengths will have a differing speed and thus we get intramodal dispersion.

Now, the value of the resulting dispersion is not constant. and at a point it will pass through an area of zero dispersion. This is caused when the material and waveguide dispersions cancel each other out. In pure silica this occurs at about 1300 nm but by altering the size of the core and/or doping the material it is possible to shift the material dispersion wavelength, and thus the zero dispersion wavelength towards any window. usually this will be the minimum loss window at 1550 nm.
Cables treated in such a way are known as dispersion-shifted fiber, and the tradeoff is that a very slightly increased amount of attenuation is introduced.
The technique when used in multi-mode fibres is more commonly called minimum dispersion shifting.