Extrinsic Absorption

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Absorption is a major cause of signal loss in optical fiber , it is defined as the portion of attenuation resulting from the conversion of optical power into another energy form , such as heat , Absorbtion is explained by 3 factors :-

- Imprerfections in atomic structure of the fiber material

- The intrinsic (basic fiber materials properites).

-Extrinsic (presence impurities fiber materials properites )

 Extrinsic Absorption

Extrinsic absorption results from the presence of impurities. Transition-metal impurities such as Fe, Cu, Co, Ni, Mn, and Cr absorb strongly in the wavelength range 0.6~1.6um. Their amount should be reduced to below 1 part per billion to obtain a loss level below 1dB/km. Such high-purity silica can be obtained by using modern techniques.

The main source of extrinsic absorption in state-of-the-art silica fibers is the presence of water vapors. A vibrational resonance of the OH ion occurs near 2.73um. Its harmonic and combination tones with silica produce absorption at the 1.39um, 1.24um and 0.95um wavelengths. The three spectral peaks occur near these wavelengths and are due to the presence of residual water vapor in silica.

In new kind of glass fiber, known as dry fiber, the OH ion concentration is reduced to such low levels that the 1.39um peak almost disappears. Such fibers are used to transmit WDM (Wavelength Division Multiplexer) signals over the entire 1.30um to 1.65um wavelength range.

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source :/https://www.fiberoptics4sale.com/blogs/archive-posts/95052294-optical-fiber-attenuation