Testing Long Haul, High Speed Fibre Optic Networks:
Chromatic Dispersion, Polarization Mode Dispersion and Spectral Attenuation
one of the big advantages of fibre optics is its capability for long distance high speed communications. Attenuation at long wavelengths is low. Fibres can be fusion spliced with virtually no loss. High-powered lasers and fibre amplifier regenerators mean long distances are easily obtained.
However over very long distances, new factors in fibre performance become important.
Chromatic Dispersion
Chromatic dispersion (CD) is caused by the fact that singlemode glass fibres transmit light of different wavelengths at different speeds. The ratio of the speed of light in a medium to the speed in a vacuum defines the index of refraction or refractive index of the material. For optical fibre, the effective index of refraction is about 1.45, so the speed of light in glass is about 2/3 the speed of light in a vacuum. But the index of refraction, and thereby the speed of light in the fibre, is a function of the wavelength of light, the principle we all know from seeing a prism break light into a spectrum.
Most sources used in long distance fibre optic links are lasers which have very little spectral width. And fibres are optimized for the wavelength of use. Both these factors minimize the effects of chromatic dispersion but cannot totally stop it. As the pulse proceeds down the fibre, the light of longer wavelength travels slightly faster and spreads the pulse out as shown here.
What Causes Chromatic Dispersion
There are two factors that cause chromatic dispersion: material dispersion and waveguide dispersion.
Material dispersion is caused by the variation of the index of refraction in a given material, glass in this case, over wavelength. Looking at the graph below, the variation of the index of refraction over the entire spectrum covered by fibre optics may seem small, only a few percent, but when you are dealing with very high speed pulses over very long distances it can add up.
Waveguide Dispersion
Waveguide dispersion is a bit more complex. In singlemode fibre, the wavelength of the light is not that much bigger than the core of the fibre and (we’ll leave out the complex physics) as a result the light travelling down the fibre actually travels in an area that exceeds the diameter of the core, which we call the “mode field diameter” of the fibre. The mode field diameter is a function of the wavelength of the light, with longer wavelengths travelling in a larger mode field diameter. Thus part of the light is travelling in the geometric core of the fibre and part is travelling in the cladding. Since the core is made of a higher index of refraction glass than the cladding, the light in the cladding travels faster than the light in the core. Longer wavelengths have larger mode field diameters so they suffer more material dispersion.