When a signal passes through an optical fiber, then signal intensity losses may occur. This is called attenuation. There are many reasons for losses in optical fibers.

This loss is generally expressed in decibel (dB) and is defined as the ratio of injected optical power P_i into the fibre to the received optical power P₀ from the fibre, i.e., attenuation  = 10/L Log (P_i) / (P₀)  dB / km where L is the length of the fibre in km.

Let us discuss them one by one:

Material Absorption

This loss mechanism is basically related to the material composition and the fabrication process of the fiber, which results in the dissipation of some of the transmitted optical power as heat in the waveguide. The absorption of the light signal may be due to inherent property (crystal structure) of the glass or due to impurities present within the glass material.

Linear Scattering

This is due to the transfer of some or all of the optical power contained within one propagating mode to a different mode. In this type of loss, no change of  frequency occurs on scattering. The linear scattering caused by material property (i.e., density fluctuations), produces an attenuation proportional to 1 / ג⁴ known as Rayleigh scattering. On the other hand, linear scattering, caused by imperfections in the fiber geometry (e.g. irregularities in core-cladding interface, core-cladding refractive index difference along the length of the fibre, diameter fluctuations, bubbles etc.), is known as Mie scattering.

Fiber Bending

As the name suggests, this loss is due to the bending in the fiber. This bending can be microbend or macrobend. optical fibres suffer radiation losses at bends on their paths. In this loss, the part of the propagating mode which is outside of the bend is required to travel faster than that on the inside so that a wavefront perpendicular to the direction of propagation is maintained. Hence, part of the mode in the cladding needs to travel faster than the velocity of light which is impossible. As a result, the energy associated with the part of the mode is lost through radiation. The large bending losses tend to occur in the multimode fibres at a critical radius of curvature R_c which is given by

R_c = 3n₁²ג / 4π(n₁² – n²₂) ^3/2

These are the various reasons for losses in optical fibers.