Basic Principles of Fiber Optics Series: Refraction

Written by Ben Hamlitsch, trueCABLE Technical and Product Innovation Manager RCDD, FOI

Refraction, or the change in the direction of light as it changes speeds passing from one material into another, is a key component in fiber-optic transmission. The principles that cause an object in water to look like it is bent are the same principles that keep light contained within the core of an optical fiber even though it curves, bends, and transmits long distances.

If you shine a beam of light (a bundle of parallel rays) through the air, it will travel in a straight line. Rays of light usually travel in straight lines until they hit something.

If a ray of light hits the surface of a sheet of glass, some light will be reflected by the surface of the glass. However, much of the light will pass through the glass because it is transparent. Water affects light rays in a similar way. Some light will be reflected off the surface of the water, but much of the light will pass through the transparent water. When a ray passes from air into glass, the direction in which the light ray is traveling changes. The light ray appears to bend as it passes through the surface of the glass.

What Causes Refraction?

Refraction occurs when light waves change speed as they travel between two materials, each with a different refractive index, or index of refraction.

How quickly light travels through a medium is determined by its refractive index. Light travels more slowly in a material with a high refractive index and faster in a material with a low refractive index. It is also important to mention that even though light slows down when it passes through a lower refractive index, into one with a higher refractive index, it will speed up again once it passes through a medium with a lower refractive index.

As the light passes through these different mediums, it causes the light to bend. We can describe it this way. Glass and water are thicker and heavier than air. They are said to be 'denser' than air.

What happens is that light slows down when it passes from the less dense air into the denser glass or water. This slowing down of the ray of light also causes the ray of light to change direction. It is the change in the speed of the light that causes refraction.

This is a bit like when a car goes partly off the road and some of the wheels go into the softer edge of the road. The softer surface drags on the wheels and slows them down. This can cause the car to pull to the left, slightly changing its direction.

When light changes velocity as it travels from one medium into another, refraction occurs. The amount of refraction is determined by the relative difference in the light’s velocity in each of the media. The greater the difference, the greater the refraction.

To explain refraction in more detail, we must also look at the wave nature of light. Light consists of two perpendicular waves, and it travels in a path that forms right angles with both waves. As the light waves meet the boundary between media with different refractive indexes, that portion of the light wave changes velocity or experiences a phase velocity change, while the rest of the light wave maintains its original velocity. This change typically causes the light to change direction. This is described by the Snell’s Law.

Calculating the Index of Refraction

Up to this point, we have discussed the velocity of light. We have learned that it can change depending on the wavelength and the index of refraction of the medium it is passing through. We also know that the slower the light travels, the higher the index of refraction. The speed of light in a medium is inversely proportional to its index of refraction.

It is also important to understand that all materials have a different index of refraction. This is done by taking measurements at a wavelength of 598 nm. When these measurements are conducted, we can determine the index of refraction of different materials. For instance, in the table below, we show various materials and their respective indices of refraction.

The model used to calculate the index of refraction is reflected in the image below. This image illustrates light passing from one medium with a lower index of refraction to another medium with a higher index of refraction.

How can we take the complicated physics of refraction and relate it to fiber optic cabling networks and how these principles function in a fiber optic cable? When light passes from the light source (VCSEL or laser) into the fiber cable, there are two materials with different indices of refraction. These are the core and cladding. As the light passes into the core (high index of refraction) and bounces off of the cladding (low index of refraction), it stays within the fiber core and travels to its end destination. This is called total internal reflection. This means that the refractive index of the core, is always greater than the index of the cladding. Light will then be guided through the core, and the fiber optic cable can act as an optical waveguide, sending the digital data from point A to point B.

These principles allow light to travel down a fiber optic cable for long distances with minimal loss. If we dig into these principles, it can help us understand what is happening within a fiber optic cable.

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