Mumbai University > Electronics Engineering > Sem7 > Optical Fiber Communication

Marks: 10M

Year: May 2013

• Important Factors responsible for power loss in optical fibre are as follows:

  i. Intrinsic absorption by basic constituent atoms of fibre material

  ii. Extrinsic absorption by impurity atoms in silica material

  iii. Absorption by atomic defects in the composition of glass

  Intrinsic absorption:

  1. Intrinsic absorption is caused by the basic fiber material properties. If a fiber was absolutely pure with no impurities or imperfections then all absorption would be intrinsic. Intrinsic absorption thus sets the minimum level of absorption.

  2. The Intrinsic absorption in the ultra violet region is caused by the electronic absorption bands.

  3. Basically, absorption occurs when a light particle (photon of light) interacts with an electron in the valence band and excites it to a higher energy level.

  4. Also in the infrared and far infrared normally at wavelengths above 7µm, fundamentals of absorption band from the interaction of photon with molecular vibration within the glass occur.

  5. Strong absorption bands occur due to oscillations of structural units such as Si-O (9.2µm), P-O (8.1µm), B-O (7.2µm) &Ge-O (11.0µm) within the glass.

  6. An interaction between a vibrating band and electromagnetic field of optical signal results in transfer of energy from the field to the band, resulting in absorption.

  Extrinsic absorption:

  i) Extrinsic absorption is caused by the impurities introduced into the fiber material. Trace metal impurities such as iron, nickel, and chromium are introduced into the fiber during fabrication.

  ii) Extrinsic absorption is caused by electronic transition of these metal ions from one energy level to another.

  iii) Extrinsic absorption also occurs when hydroxyl ions $(OH^-)$ are introduced into the fiber, water in silica glass forms a silicon-hydroxyl (Si-OH) band. This band has a fundamental absorption at 2700nm.

  iv) These harmonics increases extrinsic absorption at 1383nm, 1250nm and 950nm.

  v) These absorption peaks define 3 regions or windows of operation. The first window is centered at 850nm. The second window is centered at 1300nm. The third window is centered at 1550nm.

Absorption by atomic defects:

1. Atomic defects are imperfections in the atomic structure of fiber material.

   e.g. Missing molecules high density clusters of atom group’s oxygen defects in the glass structure.

2. These imperfections in atomic structure include absorption by the presence of above defects.

3. The absorption loss due to atomic defects becomes significant if the fiber is exposed to ionizing radiation, as might occur in nuclear reactor environment, in medical radiation therapies, in space missions that pass through the earth’s Van Allen Belts or in accelerator instruments.