Causes of Light Losses in Fiber Optics

In the light loss in fiber, there are different factors involves, in which the three are most fundamental causes.

  1. Material loss
  2. Light scattering
  3. Waveguide & bend losses

Material Loss

The loss caused by the material and its impurities rather than by structural defects in fiber is called material loss also known as absorption loss some times.

The best example of the material loss is the absorption of light by material, due to light interacting with the molecular structure of the material.

The loss due to the atomic structure of the material itself is relatively small. For example in germanium Silicon glass this loss is less than 0.1 dB/km with λ between 0.8 -> 1.6 µm. at λ ≅ 1.3µm, the loss is nearly zero.

The loss due to impurities can be reduced by better manufacturing process. In improved fibers, the internal impurities are practically negligible.

The largest loss is caused by OH-ions and these cannot be sufficiently reduced. Because the OH impurity causes loss for particular wave length bands, the worst loss (about 4 dB/km) occurs near λ = 1.4µm for an impurity concentration of 1ppm.

Material Loss
Figure 1: Material Loss

The OH absorption peaks for concentration larger than 1ppm can be shown as:

The losses peaks occur at points of wavelengths. i.e,

0.93 µm     1st one

1.25µm      2nd one

1.40µm      3rd one

Light Scattering

The reflection of light in many directions is called light scattering. The scattered light does not propagate down the fiber but it is lost.

Light can be scattered by molecules of the material by structural imperfections and impurities.

Typically light scattering is caused by an obstruction. So when the light is scattered by an obstruction then the result is power loss. The term obstruction refers to the density variations in the material that result in changes in the refractive index.

There are two types of scattering. i.e. scattering by small obstruction & large obstruction.

Scattering by Small Obstructions

In this type when the index variations are molecular in size, then power loss is due to Rayleigh scattering. These small obstructions, which are inherent in manufacturing process and cannot be eliminated because it behaves like point sources, scattering light in all directions. The Rayleigh scattering loss greatly depends on wave lengths. It varies as 1 / λ4, means that due to longer wave lengths the power loss is lower. The affects of Rayleigh scattering losses are shown in figure 1 above.

Scattering by Large Obstructions

By large obstructions, macrobands or by fiber deformation the light can be scattered. But this type of loss is minimized by well improved manufacturing techniques.

By figure we can show this type of scattering as:

Scattering By Large Obstruction
Figure 2: Scattering by large obstruction

Waveguide & Microbend Losses

The waveguide or optical waveguide is the name of optical fiber. The structure variations in the fiber or fiber deformation couse the radiation of light away from fiber and so power is lost.

A change in diameter of fiber also causes the radiation loss shown as:

Waveguide Microband Losses
Figure 3: Waveguide Microband Losses

Here in figure the angle α1 is smaller than the critical angle so that is why that ray leaves the fiber. In the absence of the deformation the ray would be confined to the fiber as shown by the dashed line (blue color).

The micro bend is the very minute disturbances in core sized. It also causes radiation of light, shown by figure as:

Waveguide Microband Losses
Figure 4: Waveguide Microband Losses

Figure 4

So losses caused by imperfection and deformation of fiber structure are called bend losses.

Thus any variations in refractive index along the fiber will cause a similar energy loss. However in most of the cases the loss results from scattering of light.