How does Fibre Optics Work?

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Fibre optics make use of a glass fibre that is thin, transparent, flexible, and compact

Fibre optics cable starts as a large solid glass rod with a dimension of several centimetres’ and approx. 1 metre long. It is then reconstructed into thin strands of fibre with a diameter of 125 microns and several kilometres long.

 

Glass rod ready to be made into a strand of fibre

 

Optical fibre uses light in the infrared region. The wavelengths used are longer than visible light and are used for data transfer. It transmits light and digital information over very long distances by using total internal reflection.

Light travels down a fibre optic cable by bouncing repeatedly off the walls. Each tiny photon or particle of light bounces down the fibre like a person sliding down a large enclosed water slide at a fun park.

 

Total Internal Reflection

Refraction

 

Now you might expect a beam of light, traveling in a clear glass fibre, simply to leak out of the edges. But if light hits glass at a really shallow angle of less than 42 degrees, it reflects back in again—as though the glass were really a mirror.

This is called total internal reflection. It’s one of the things that keep light inside the fibre.

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Light waves have three primarily properties that reflect total internal reflection:

  • Reflection
  • Refractive index
  • Refraction

 

Fibre Optics and Reflection

When a beam of light hits a refractive material the direction of the refractive light will depend on the angle of incidence of the light beam. As this angle changes so does the direction of the refractive light. At a certain critical angle, the light no longer passes through the interface between the two media but is reflected as if the interface were a mirror.

The critical angle is dependant on the refractive index so will be different for most materials. The critical angle is what makes data transmission through optic cable possible.

 

 

 

Total internal reflection is the reflection of light that occurs when it strikes an interface at an angle of incidence that is greater than the critical angle for the pair of media.

Total internal reflection is achievable if we increase the incident angle rather than increase the refractive index. The light will come back to the first medium. This is used to transmit the light. When a laser strikes the interface at an angle greater than the critical angle total internal reflection will happen.

 

Fibre Optics Core

Because of this the light can continue to travel along to core without being affected by the outer coating. The other thing that keeps light in the fibre is the structure of the glass, which is made up of two separate parts.

The central part of the glass called the Core is what the light travels through. One thing to note is that the core size is different for a multimode fiber and a single mode fiber. Wrapped around the outside of the Core is another layer of glass called cladding.  The core is optically denser than the cladding.

 

What is Numerical Aperture - Video 9 Thumbnail

 

The core is made from pure glass such as silicon dioxide. The cladding is also made from glass but is doped with small amounts or germanium and boron to reduce its index of refraction.

 

Fibre Optic Cladding

The cladding’s job is to keep the light signals inside the core. It can do this because it is made of a different type of glass to the core which has a lower refractive index compared to the core which has a higher refractive index. Although the cladding does not carry light it performs an essential part of the fibres composition.

The cladding is not just a simple coating it keeps the value of the critical angle constant throughout the entire fibre length. The cladding does not absorb any of the light from the core. As a result the light can travel long distances. However light signals will degrade due the presence of impurities that are present in the glass at time of manufacture.

The two materials chosen for core and cladding are carefully selected to ensure that total internal refraction occurs—that means that as light enters the boundary between core and cladding, it is bent in such a way that it remains in the core.

The core and the cladding are quite delicate and need the protection of a buffer coating to prevent any physical damage or moisture from damaging the fibre.

 

What is Refractive Index?

Speed of light changes when it passes through different mediums.  This change in speed is expressed by the refractive index (n).

Refractive index is the speed of light in a vacuum divided by the speed of light in a material. Refractive index measures how much a material refracts light. This refractive index can be tailored to specific applications.

Various wavelengths respond differently to the refractive index of the material.

The index of refraction, n, is a known value which can be obtained from the fiber cable manufacturer. The index of refraction changed based on the wavelength being utilised.

 

 

Manufacturers Index of refraction chart

 

Snell’s Law

In 1621, a Dutch physicist named Willebrord Snell derived the relationship between the different angles of light as it passes from one transparent medium to another. When light passes from one transparent material to another, it bends according to Snell’s law.

 

Willebrord Snell

 

What is the Refraction of Light?

As light transmits through different materials it’s velocity changes. Light travelling through a vacuum travels faster that light travelling through denser material such as glass. The abrupt slowing of the light at the vacuum glass interface causes it to bend.

As the light ray passes from one transparent medium to another, it seems to change direction; this phenomenon is called refraction of light. How much that light ray changes its direction depends on the refractive index of the mediums.

 

What is Fibre Optics

 

This refraction technique is effectively used in optical fibres.

 

Conclusion

Fiber optic cables, glass or plastic can be seen as a transmission medium – a “pipe” to transmit data over short distances or very long distances at extremely high speeds. Both the core and the cladding are glass but have different “refractive indexes” which essentially means that light travels at different speeds through the materials.

The result is that light pulses produced from lasers or LEDs at one end of a fibre optic cable are sent through the fibre optic core and are reflected back to the core when the light hits the fibre optic cladding, thus keeping the light within the centre core.