Extrinsic Absorption


At 1380 nm we find an extremely interesting  phenomenon . The attenuation rises sharply to well over 3dB/km rendering the section to about 1420nm practically useless for long distance communications. This is due to water in the form of hydroxyl ions in the glass absorbing  energy with a wavelength of 1380 nm and nearabouts. During the manufacture the water content should be kept as low as  1 part to 10,000000000.

This is an example of extrinsic absorption , whereby impurities are allowed to permeate the material. Extrinsic absorption  allows energy from the light to be transferred to the atoms of the impurity.

There is little that one can do about  hydroxyl ions  but the answer to any other impurities is to manufacture in a super-clean environment.




This is again a manufacturing defect and is caused by higher and lower molecular density fluctuations relative to the average in a fiber. Light interacts with these areas and becomes partially scattered in unpredictable directions. This is pictured below in a diagram from the US Navy (NEETS):



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As you can see, light now "bounces" everywhere rather than being a ray following a coherent path. This will produce a severe attenuation in any rays entering the cladding and alter the phase of others, which may lead to some signals arriving  at opposite phases simultaneously and cancelling each other.



There are two types of scattering. The most serious for us is Rayleigh scattering which occurs when the defect is less than 1/10th of the operating wavelength of the light. The loss caused is proportional to the fourth power of the wavelength, and as the wavelength increases the loss caused by Rayleigh scattering decreases.

The second type of scattering is known as Mie scattering which occurs when the defective area is greater than 1/10th of the wavelength. Modern techniques in manufacturing mean that Mie scattering is not common in good cables as very few large defects of this nature are present.