Subject :- Applied Physics 2.

Topic :- Laser.

Difficulty :- Medium.

As a light signal propagates through a fiber it suffers loss of amplitude and change in shape which is called as attenuation and dispersion respectively.

Attenuation basically attenuation is gradual loss in intensity of light as it travels through the fiber. It is express in decibel per Km dB/km. the attenuation coefficient α is given 

  α=  (log10 pi/po )/L      pi= input power 
                               p0= output power
                                L = length of fiber

The main contributors to attenuation are

Absorption: - Even highly purified glass absorbs light in specific wavelength region. These absorptions are inherent property of glass itself. This is called as intrinsic absorption. However, intrinsic losses are insignificant. But the impurities are the major source of the spectra. The absorption of light either through intrinsic or extrinsic. Process contributes a transmission loss because that much energy is subtracted from the light propagating through the fiber.

Rayleigh Scattering: - Glass is a discarded structure having local microscopic variations in the density which is in turn cause local variation in Refractive index. light propagating through such a structure suffers scattering losses. The amount of scattering is proportional to 1/λ^4 .

Geometric effect or bending loss: - These are fiber losses introduced during manufacturing process. Irregularities in fiber dimension mat arise in the drawing process in coating and cabling process or in the installation process.

However, there is a minimum attenuation at around a particular band of wavelength. It is called an “optical window, such windows are selected for data transmission through the fiber. Two windows are used in optical communication

First window between 800 μm to 900 μm
Second window between from 1300 μm to 1600 μm

Dispersion: It refers to the broadening of pulse as light travel through a fiber. It is measured in ns / km

1.Intermodal dispersion: - A ray of light launched into fiber fallows a zigzag path. when many modes are propagating they travel with different net velocities and comes out at different times which cause broadening of pulse. Low N.A fiber exhibit smaller dispersion also narrow spectral width of source limit the dispersion.

2.Waveguide dispersion: - the effective refractive index for any mode varies with wavelength which cause pulse spread in. This is a waveguide dispersion.

3.Material dispersion: - it is based on the wavelength of optical signal. Light waves of different wavelengths travel at different speeds in a medium. The short wavelength waves travel slower than long wavelength wave. Thus, material dispersion can be reduced. By using more and more monochromatic source.