Losses in optical fibers

      Attenuation or loss in optical fibers basically refers to the loss of power. During transit, light pulse loses some of their photons, thus reducing their amplitude. Attenuation for a fiber is usually specified in decibels per kilometer. The degree of attenuation depends on the wavelength of light transmitted.

Attenuation measures the reduction in signal strength by comparing the output power with input power. Measurements are made in decibels (dB). The basic measurement for loss is done by taking the logarithmic ratio of input power (P_i) to the output power (P_o).           

Material absorption losses - It is a loss mechanism related to the material composition and fabrication process of the fiber which results in the dissipation of some of the transmitted optical power as heat in waveguide. The absorption of light may be intrinsic (caused by one or more major components of glass) or extrinsic (caused by impurities within the glass).

Linear scattering losses - Linear scattering mechanisms cause the transfer of some or all of the optical power contained within one propagating mode to be transferred linearly(proportionally) into a different mode. This process tends to result in attenuation of the transmitted light as the transfer may be to a leaky or radiation mode which does not continue to propagate within the fiber core, but is radiated from the fiber. It is mainly of two types:

a)     Rayleigh scattering         (b) Mie scattering

Bending loss

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

a)     Macroscopic bends (having radii that is large as compared with the fiber diameter)

b)     Random microscopic bends of fiber axis

       Here we shall confine our study of losses by elaborating the the bending loss experiment and how it is performed remotely.