Absorption is a major cause of signal loss in an optical fiber. It is defined as the portion of attenuation resulting from the conversion of optical power into another energy form, such as heat. In optical fibers it is explained by three factors:
1. Imperfections in the atomic structure of the fiber material
2. The intrinsic or basic fiber-material properties
3. The extrinsic (presence of impurities) fiber-material properties
Imperfections in the atomic structure induce this assimilation process by the presence of missing molecules or oxygen defects. It is also induced by the diffusion of hydrogen molecules into the glass fiber. Since intrinsic and extrinsic material properties are the main cause, they are discussed further.
An intrinsic condition is caused by basic fiber-material properties. If an optical fiber were absolutely pure, with no imperfections or impurities, then all assimilations would be intrinsic. Intrinsic sets the minimal level of assimilation.
In fiber optics, silica (pure glass) fibers are used predominately. Silica fibers are used because of their low intrinsic material assimilation at the wavelengths of operation.
In silica glass, the wavelengths of operation range from 700 nanometers (nm) to 1600 nm. The picture below shows the level of attenuation at the wavelengths of operation. This wavelength of operation is between two intrinsic regions. The first region is the ultraviolet region (below 400-nm wavelength). The second region is the infrared region (above 2000-nm wavelength).
Intrinsic conditions in the ultraviolet region is caused by electronic assimilation bands. Basically, this occurs when a light particle (photon) interacts with an electron and excites it to a higher energy level. The tail of the ultraviolet band is shown in the picture above.
The main cause of the intrinsic condition in the infrared region is the characteristic vibration frequency of atomic bonds. In silica glass, this is caused by the vibration of silicon-oxygen (Si-O) bonds. The interaction between the vibrating bond and the electromagnetic field of the optical signal causes the intrinsic condition. Light energy is transferred from the electromagnetic field to the bond. The tail of the infrared band is shown in the picture above.
Extrinsic is caused by impurities introduced into the fiber material. Trace metal impurities, such as iron, nickel, and chromium, are introduced into the fiber during fabrication. Extrinsic is caused by the electronic transition of these metal ions from one energy level to another.
Extrinsic also occurs when hydroxyl ions (OH-) are introduced into the fiber. Water in silica glass forms a silicon-hydroxyl (Si-OH) bond. This bond has a fundamental absorption at 2700 nm. However, the harmonics or overtones of the fundamental assimilation occur in the region of operation. These harmonics increase the extrinsic condition at 1383 nm, 1250 nm, and 950 nm. The picture above shows the presence of the three OH- harmonics. The level of the OH- harmonic absorption is also indicated.
These assimilation peaks define three regions or windows of preferred operation. The first window is centered at 850 nm. The second window is centered at 1300 nm. The third window is centered at 1550 nm. Fiber optic systems operate at wavelengths defined by one of these windows.
The amount of water
(OH-) impurities present in a fiber should be less than a few parts per
billion. Fiber attenuation caused by extrinsic-absorption is affected by
the level of impurities (OH-) present in the fiber. If the amount of
impurities in a fiber is reduced, then fiber attenuation is reduced.