Optical Fibers

  • Introduction to Optical Fibresclip_image001

Optical Fibres are thin rods of glass wrapped in a low density plastic and cabling. In modern days, it is used as an instrument in micro surgery to project images from inside the body and help surgeons see in hard to reach places. It is also used widely in communications, both in computer networks as a fast Internet connection source and in telecommunications both transcontinentally and transoceanically.

We’ll now discuss some concept that are relevant to Optical Fibres, starting with Reflection and Refraction.

  • Reflection and Refraction

The concepts of reflection and refraction are very important in the design of Optical Fibres and has a key part in how Optical fibres work. The law of reflection states that a beam of light striking a flat surface (the incidence ray) would be reflected at the same angle at the normal, the medium dividing the incidence ray and the reflective ray and is perpendicular to the surface of reflection. That would also mean that the angle of incidence (from the incidence ray to the normal) is also equivalent to the angle of reflection (from the reflective ray to the normal). Refer to Diagram 1.

Diagram 1

Snell’s Law, named after the Dutch mathematician Willebrord van Roijen Snell, states that the product of the refractive index and the sine of the angle of incidence of a ray in one medium is equal to the product of the refractive index and the sine of the angle of refraction in a successive medium. This can be represented algebraically by n1 sinclip_image0031 = n2 sinclip_image003[1]2, where n1, n2 are the two values of refractive index and clip_image003[2]1,clip_image003[3]2 are the angles of the incidence and refraction. Refer to Diagram 2.

Diagram 2

We shall now move on to Critical Angles.

  • Critical Angles

Generally, the refractive index of a denser transparent substance is higher than that of a less dense material, which would mean that light would travel slower in the substance. A general rule is that when a ray travels from a medium to another with a higher refractive index, it’ll bend towards the normal. When a ray travels from a medium to another with a lower refractive index, it’ll bend away from the normal.

Due to this fact, as the angle of incidence of a ray increases in a medium with a higher refractive index, the ray passing through to a medium with lower refractive index will bend away from the normal until it meets 90 deg. with the normal, and on the boundary between the two mediums, which is then referred to as the critical angle. Refer to Diagram 3.

Diagram 3

Next we’ll be looking at the physics behind Optical Fibres.

Physics in Optical Fibres

Optical Fibres relies heavily on two concepts of physics, the concepts of refraction, refractive indexes, critical angles and Total Internal Reflection. The concepts of refraction states that a ray of light travelling from a medium with a higher refractive index to a medium with a lower refractive index would bend away from the normal. With this in mind, it also states that a critical angle would be reached when the ray of light increases to an angle that will bend it 90 deg. away from the normal. The concept of Total Internal Reflection is apparent when the ray of light travelling from a higher refractive index medium to a lower refractive index medium has an angle so great it is able to refract the light greater than the critical angle, resulting in the ray reflecting back into the first high refractive index medium.

The main function for Optical Fibres is to send information through it by transmitting a beam of light from one end to another, trying to have as little quality loss as possible. The design of modern Optical Fibres reflect this and relies much on the physics involved.

Optical Fibres are more commonly used in surgery as a means of a tiny camera for surgeons in a patient’s body and used together in their thousands in Fibre Optic communication cables for telephones and high-speed Internet connections. Each Optical Fibres has a core of thin glass, which is wrapped then by a cladding (usually a plastic with a lower refractive index than glass) and is then finally all wrapped in a harder plastic buffer to prevent damage. Refer to Diagram 4.

Diagram 4

When transmitting a signal, an L.E.D. or laser is used to send a beam of light in on/off pulses (digital) down the Optical Fibre to be received on the other end. It is either transmitted straight down the Optical Fibre, resulting in longer distance travel, or it is transmitted at an angle which is calculated to be greater than critical angle of the glass to the plastic, resulting in Total Internal Reflection and the ability for the beam of light to turn corners. Refer to Diagram 5.

Diagram 5

The angle of transmission of the beam into the glass is usually greater than 82 deg, which is required to achieve Total Internal Reflection when the light hits the plastic cladding. If the angle is less than 82 deg, the beam of light would be refracted out of the Optical Fibre. Refer to Diagram 6.

Diagram 6

That concludes the project on Optical Fibres, and other relevant information. To find out more, go to the next section ‘Relevant Links’.

  • Relative Links

Here are some relevant links to provide you with more information on Optical Fibres and related material.

HowStuffWorks – comprehensive site on all you need to know about Optical Fibres, along with other interesting things.
Corning Optical Fibres – discovery centre with animated explanations.
What are Optical Fibres made of? – detailed site on what optical fibres are made of.
Light and Optics: Arizona State University – brief explanation of how optical fibres work.

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