Optical Fibre System

ByMohd Nasir bin SaidTelecommunications DepartmentAdvance Technology Training Centre Kulim Kedah Darul Aman

What is "Fiber Optics"?

It's the communications technology that works by sending signals down hair thin strands of glass fiber

History

It began about 30 years ago in the R&D labs (Corning, Bell Labs, ITT UK, etc.) and was first installed in Chicago, IL, USA in 1976. By the early 1980s, fiber networks connected the major cities on each coast.

By the mid-80s, fiber was replacing all the telco copper, microwave and satellite links.

In the 90s, CATV discovered fiber and used it first to enhance the reliability of their networks, a big problem. Along the way, the discovered they could offer phone and Internet service on that same fiber and greatly enlarged their markets.

Computers and LANs started using fiber about the same time as the telcos

Other applications developed too: aircraft, ship and automobile data busses, CCTV for security, even links for consumer digital stereo!

LightWhat is light? Energy of from electromagnetic wave and

particle.

What is photon? Particle from of light Photonics to lights is like Electronics to

current

The electromagnetic Spectrum

The light used in optical fiber network is one type of electromagnetic energy.

When an electric change moves back and forth, or accelerates, a type of energy called electromagnetic energy is produced.

This energy is the form of waves can travel through a vacuum, the air, and through some materials like glass.

An important property of energy wave is the wavelength Radio, microwave,radar, visible light,x-rays are all types

of electromagnetic energy.

The electromagnetic spectrum

Wavelenghts are not visible to the human eye are used to transmit data over optical fiber.

These wavelenghts are slightly longer than red light and are called infrared light.

These wavelenghts were selected because they travel through optical fiber better than other wavelenghts.

Ray model of light

When electromagnetic waves travel out from a source, they travel in straight lines

These straight lines pointing out from the source are called rays. In the vacuum of empty space, light travels continuously in a straight

line at 300,000km per second. However,light travel at different, slower speed other through other

materials like air,water and glass. When a light ray called the incident ray,crosses the boundry from

one material to another , some of the light energy in the ray will be reflected back.

This is why you can see yourself in window glass. The light that is reflected back is called reflected ray.

Ray model of light The light energy in the incident ray that is not reflected will enter the

glass Refracted ray-The entering ray will be bent at an angle from its

original path. How much the incident light ray is bent depends on the angle at

which the incident ray strikes the surface of the glass and the different rates of speed at which light travels through the two substance.

The optical density of the glass determines how much the rays of light in the glass.

Optical density refers to how much a light ray slows down when it passes through a substance.

The greater the optical density of a material, the more it slows light down from its speed in a vacuum.

The ratio of the speed of light in a material to the speed of light in a vacuum is called the Index of Refraction

When light traveling in a transparent material meets the surface of another transparent material two things happen:-

a) some of the light is reflected –reflection

b) some of the light is transmitted into the

second transparent material -refraction

The bending of light is called refraction and it depends upon the fact that light travels at one speed in one material and at a different speed in a different material.

As a result each material has its own Refractive Index which we use to help us calculate the amount of bending which takes place. Refractive index is defined as:

n = C

where ; n is the refractive index C is the speed of light in a vacuum is the speed of light in the material

The indexes of refraction of several common materials are given above

*Vacuum -1.0 *Air 1.0003 *Water-1.33 *Ethyl Alcohol -1.36 *Silicon -3.4**Index of refraction is based on a

wavelength of light emitted from a sodium flame (5890 Å)

Snell Law

How a light ray reacts when it meets the interface of two transmissive materials that have different indexes of refraction can be explained with Snell’s law.

Snell’s law simply states n1 sin 1 = n2 sin 2

wheren1 = refractive index of material 1 (unit less)n2 = refractive index of material 2 (unit less)1 = angle of incidence (degrees)2 = angle of refraction (degrees)

Critical Angle The critical angle is defined as the minimum angle of

incidence at which a light ray may strike the interface of two media and result in an angle of refraction of 90 or greater

This definition pertains only when the light ray is traveling from a more dense medium into a less dense medium. The critical angle can be derived from Snell’s law as follows:

n1 sin 1 = n2 sin 2

sin 1 = n2 sin 2 n1

TIR

The transmitted ray now tries to travel in both materials simultaneously for various reasons this is physically impossible so there is no transmitted ray and all the light energy is reflected. This is true for any value of 1, the angle of incidence is equal to or greater than

We can define the two conditions necessary for TIR to occur:

1. The refractive index of the first medium is greater than the refractive index of the second one.

2. The angle of incidence, 1, is greater than or equal to the critical angle, c

The phenomenon of TIR causes 100% reflection. In no other situation in nature, where light is reflected, does 100% reflection occur. So TIR is unique and very useful.

Numerical Aperture

The numerical aperture of a core is the range of angles of incident light rays entering the fiber that will be completely reflected.

Modes- The paths which a light ray can follow when travelling down a fiber.

By controlling both conditions, the fiber run will have total internal reflection. This give a light wave that can be used for data communications.

Fiber Construction And Geometry

Te core is the light transmission element at the center of the optical fiber.

Cladding is also made of silica but with a lower index of refraction than the core.Light rays travelling through the fiber core reflect off this core to cladding interface as they move through the fiber by TIR

Surrounding the cladding is a buffer material that is usually plastic. The buffer material helps sheid the core and cladding from damage.

The strenght material surrounds the buffer,preventing the fiber cable from being strecthed when installer pull it.The material used is often Kevlar, the same material used to produce bulletproof vest.

The outer jacket surrounds the cable to protect the fiber against abrasion,solvents and other contaminations.

Type of Fiber andMode of Propogation

• Single Mode

• Multimode – Step index - Graded Index

If the diameter of the core of the fiber is large enough so that there are many paths light can take through the fiber, the fiber is called “ multimode” fiber.

Single mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber.

Transmission mode

For short distance

Easy to work with.LANsFor very high pulse rates

For short distanceEasy to work with.LANsProvides more bandwidth than (c)Most common and widely used type

For long distanceDifficult to work with.Phone companies and CATV companies

Figure 1.9 Core index profiles: (a) single-mode step index; (b) multi-mode step index; (c) multi-mode graded index

Single Mode

High Bw applications- 4 Ghz Low losses 0.3-0.5 db/km Small Core area 8-10 micron Tx at 1300nm-1550nm wavelength Higher cost.

Multimode Step Index

BW of 10Mhz/km Loss of 5-20db/km Large core 200-1000micron Cladding 1035 micron Limited transmission distance Tx at 660-1060

Multimode Graded index

BW up to 600Mhz/km Losses of 2 to 10 db/km Cores of 50/62.5/85/100 micron Cladding of 125 and 140 micron Effective with laser or LED sources

TQ

Q & A ?