Wednesday, 29 October 2014

Losses in Optical Fiber

Signal Degradation in Optical Fiber :

  • Attenuation and dispersion determine the maximum distance an optical signal can be transmitted before the receiver is unable to detect it.
  • The attenuation and dispersion of a fiber are wavelength dependant.

Transmission Losses in Fibers:

Transmission loss or attenuation of the signal in an optical fiber is measured in dB/km.

Types of Losses

–Material Absorption
–Rayleigh scattering
–Waveguide imperfections
–Radiative loss

Material Absorption

•Intrinsic Absorption:

–Intrinsic absorption losses correspond to absorption by fused silica.
–intrinsic material absorption for silica in the wavelength range 0.8~1.6um is below 0.1dB/km.

•Extrinsic Absorption:

–Extrinsic absorption is related to losses caused by impurities within silica.
–The main source of extrinsic absorption silica fibers is the presence of water vapors.

Rayleigh Scattering

•Silica molecules move randomly in the molten state and freeze in place during fiber fabrication.
•Density fluctuation lead to random fluctuations of the refractive index.
•Light scattering in such a medium is known as Rayleigh scattering.

Waveguide Imperfections

•Imperfections at the core-cladding interface, such as random core-radius variations, can lead to losses.
•This has been taken good care of in optical fiber manufacturing and the core radius is made sure not to vary significantly along the fiber length.

Radiative Losses

•Radiative losses occur whenever an optical fiber undergoes a bend of finite radius of curvature.
•Fibers can be subjected to two types of bends.

–Macroscopic bending: 

Bends having diameter smaller than the specified by manufacturer e.g. when a fiber cable turns a corner.

–Microscopic bending: 

Random microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables, e.g. deformation of axis.

Gap- Loss :

•Gap-loss happens when there is a space, breakage, between fiber connection.
•Light can cross this gap, but spreads out and is weakened and diffused