Physics of Optical Fibers

Physics of Optical Fibers Points : Physics of Optical Fibers, Electromagnetic Spectrum, Reflection of Light, Refraction of Light, Total Internal Reflection, Numerical Aperture, The working of optical fibers depends on the basic principles of optics and the interaction of light with matter. From physical point of view, light can be considered either electromagnetic waves or as photons, quanta of electromagnetic energy. Both points of views are valid and valuable. But the most useful point of view is that light is rays and travel in straight lines, between or within optical elements, which can reflect or refract (bend) light rays at their surfaces. Electromagnetic Spectrum The visible light lies in the range from 380mm (violet) to 780 nm (red) and is bordered by ultra-violet radiation at the- lower wavelength and infra-red radiation at the upper wavelength. The electromagnetic waves spectrum and their applications are shown.

Reflection of Light When light rays fall on a surface between two medium, a part of it is reflected back into the first medium. The amount of this reflected light depends on the angle of incidence to the normal. The angle of incidence is always equal to the angle of reflection, as shown.

Refraction of Light When light rays enter from a less dense medium to a denser medium, it is always bend toward the normal, as shown in the figure.

n₂ > n₁
The amount of refraction or bend depends on the refractive index of the two media. The refractive index is given by:
n = speed of light in free space/speed of light in a given material n for free space (vacuum) = 1
n for water 1.33
n for glass= 1.5

The refractive index of a medium depends on the wavelength of light. For wavelength of the infra-red light, it constantly decreases as the wavelength increases.

Total Internal Reflection When light rays enter from a denser medium into a less dense medium, it is bend away from the normal in the second medium, i.e.,

Angle of refraction (β) > Angle of incidence (α)

As the angle of incidence is increased, the angle of refraction will also increase. If it continues, a stage will reach when the angle of refraction becomes 90°, as shown. Thus the refracted rays move parallel to the interface between the two media. This angle of incidence at which the light rays become parallel to the interface between the two media is called “critical angle”. The critical angle depends upon the ratio of the refractive indices of the two media.

When the angle of incidence becomes greater than the critical angle, then there are no refracted rays in the less dense medium. The light rays are refracted back into the denser medium. This is called total internal reflection.

The total internal reflection takes place only when the light rays enter from a denser medium into a less dense medium and never in the reverse case.

Numerical Aperture (NA) The phenomenon of total internal reflection is used in optical fibers by making the “core glass” in the middle of the fibers with refractive index n₁ and “cladding glass” with refractive index n ₂, such that n₁ > n₂, as shown.

To launch the light into the core of the optical fibers, the acceptance angle - the angle over which light rays entering the fibers will he guided along its core, is normally measured as numerical aperture. Numerical aperture for a light that enters a fiber from air is given by;

NA = (n₀² — n₁²)^1/2
n_o = refractive index of the core
n₁ = refractive index of the cladding