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What are the reasons for signal attenuation in optical fiber? (Refer part I)

Marks: 10M

Year: Dec2013, Dec2012, Dec2014, May2014

An optical signal becomes increasingly distorted as it travels along the fiber.

I. Dispersion:

• Dispersion is the broadening of actual time-width of the pulse due to material properties and imperfections.

• As pulse travels down the fiber, dispersion causes pulse spreading. This limits the distance travelled by the pulse and the bit rate of data on optical fiber.

• In a fiber three distinct types of distortion are observed:

1. Intramodal dispersion:

• Pulse broadening within a single mode is called as intramodal dispersion or chromatic dispersion.

• The two main causes of intramodal dispersion are as follows:

  a. Material dispersion:

  • It is the pulse spreading due to the dispersive properties of material.

  • It arises from variation of refractive index of the core material as a function of wavelength.

  b. Waveguide dispersion:

  • It occurs because a single mode fiber confines only about 80% of the optical power to the core.

  • Dispersion thus arises since the 20% light propagating in the cladding travels faster than light confined to the core.

2. Intermodal dispersion:

• Dispersion caused by multipath propagation of light energy is referred to as intermodal dispersion.

• In digital transmission, we use light pulse to transmit bit 1 and no pulse for bit 0. When the light pulse enters fiber it is breakdown into small pulses carried by individual modes. At the output individual pulses are recombined and since they are overlapped receiver sees a long pulse causing pulse broadening.

3. Polarization mode dispersion:

• Birefringence is the optical property of a material having refractive index that depends on the polarization and propagation direction of light.

• Varying birefringence along the length of the fiber causes each polarization mode to travel at slightly different velocity and polarization orientation changes with distance.

• The resulting difference in the propagation time between the orthogonally polarized modes causes pulse spreading and this leads to polarization mode dispersion.

II. Attenuation

When the light travels in the optical fiber then due to transmission characteristics it gives signal loss. Different factors are responsible for signal attenuation inside the fiber.

Bending loss:

• Once fiber is laid as cables for communication purpose bending losses occur.

• Optical fibers suffer radiation losses at bends or curves on their paths.

The two types of bending losses are as follows:

a. Macrobending loss:

• Fiber is subjected to gentle bend over a large arc, this bend causes failure of total internal reflection condition of a optical signal at the core-cladding interface which leads to leakage of optical signal from the fiber. This causes macrobending loss.

b. Microbending losses:

• Microbends are caused either by non-uniformities in the manufacturing of the fiber or by non-uniform lateral pressures created during the cabling or packaging losses.

• When a light beam strikes these imperfections in the fiber, the condition of total internal reflection is not attained and the beam leaks out of the core causing microbending losses.

Absorption loss

Absorption is caused by three different mechanism:

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

Scattering loss

Scattering losses in glass arise from microscopic variations in the material density, from compositional fluctuations and from structural inhomogeneties or defects occurring during fiber manufacture.