Optical fiber consists of thin glass fibers or plastic that can carry information at frequencies in the visible light spectrum and beyond. The typical optical fiber consists of a very narrow strand of glass called the core. Around the core is a concentric layer of glass called the cladding?
An optical transmission system has three basic components
(a) Light source
(b) Transmission medium (fiber optics)
Light source: In such a system a pulse of light indicates bit 1 and the absence of light indicates bit 0. Light source can be an LED or a laser beam.
Transmission medium: Transmission medium is the ultra-thin fiber of glass.
Detector: A detector generates an electrical pulse when the light falls on it,
A typical core diameter is 62.5 microns .Typically cladding has a diameter of 125 microns. 100 microwatts power (roughly) a light emitting diode can couple into an optical fiber. Coating the cladding is a protective coating consisting of plastic, it is called the Jacket.
The loss in signal power as light travels down the fiber is called attenuation. An important characteristic of fiber optics is refraction. Refraction is the characteristic of a material to either pass or reflect light. When light passes through a medium, it “bends” as it passes from one medium to the other. An example of this is when we look into a pond of water If the angle of incidence is small, the light rays are reflected and do not pass into the water.
If the angle of incident is great, light passes through the media but is bent or refracted. Optical fibers work on the principle that the core refracts the light and the cladding reflects the light. The core refracts the light and guides the light along its path. The cladding reflects any light back into the core and stops light from escaping through it - it bounds the medium! Fast data transmission rate is an advantage to using fiber optics data transmission.
Types of Optical Fibers
Single Mode Fiber
The various characteristics of Single mode fiber are:-
1. The diameter of glass core in single mode fiber is very small ranging from 8 to 10 microns.
2. In this mode, light can propagate only in a straight line, without bouncing.
3. Fiber glass has lower density (index of refraction) that creates a critical angle close enough to 90° such that the beam propagates in a straight line.
4. In this case, propagation of different beams is almost identical and delays are negligible. The beams arrive at destination together and can be recombined with little distortion to the signal.
5. Single mode fibers are more expensive and are widely used for long distance communication.
6. These types of fibers can transmit data at 50 Gbps for 100 kilometers without amplification.
1. In multimode fiber, multiple beams travel in the core in different paths.
2. In multimode fiber, the diameter of core is about 50 microns.
3. Multimode fibers are further categorized into Step index fibers and Graded index fibers.
a) Step Index Fiber
1. Density of core remains constant from the centre to the edges.
2. A beam of light moves in a straight line in this medium until it reaches the interface of core and the cladding.
3. At this interface, the angle of ray is changed due to the change in density.
4. In this mode, some beams travel in a straight line through the core and reaches destination without reflection or refraction.
5. The beams that strike the interface of core and cladding at and angle smaller than critical angle penetrate the cladding and are lost.
6. The beams striking at angle greater than critical angle are reflected back in core and form total internal reflection.
7. In this fiber, a ray with smaller angle of incidence requires more bounces thus will take more time to reach the destination whereas the ray with high angle of incidence will require less number of bounces and will reach the destination in lesser time.
b) Graded Index Fiber
1. A graded index fiber has different densities at the core and at the edges. Density is highest at the centre of the core and decreases gradually to its lowest at the edge.
2. Because of this difference in densities, different beams refract at different angles into a curve
3. Only the horizontal beams move in a straight line due to constant density at the centre.
Advantages of Optical Fiber
1. They are not affected by electrical and magnetic interference as the data travel in form of light.
2. Optical fiber offers higher bandwidth than twisted pair or coaxial cable.
3. Optical fibers are thin, lighter in weight and small in size as compared to other wired Medias. It is easier to group several optical fibers in one bundle.
4. Glass is more resistant to corrosive materials as compared to copper. Hence can be laid in different environments.
5. In optical fibers, attenuation (loss of signal) is very low. Therefore these fibers can run several kilometers without amplification.
6. Fibers do not leak light and are quite difficult to tap. So they provide security against potential wiretappers.
7. There is no cross-talk problem in optical fibers.
8. They are highly suitable for environments where speed is needed with full accuracy.
9: Photons in fiber do not affect one another (as they have no charge) and are not affected by stray photons outside the fiber. But when electrons move in a wire they affect each other and are themselves affected by electrons outside the wire.
10. The size (diameter) of the optical fibers is very small (it is comparable to the diameter of human hair). Therefore a large number of optical fibers can fit into a cable of small diameter.
11. The material used for the manufacturing of optical fibers is "silica glass". This material is easily available. So the optical fibers cost lower than the cables with metallic conductors.
12. As the light rays have a very high frequency in the GHz range, the bandwidth of the optical tiber is extremely large. This allows transmission of more number of channels. Therefore the information carrying capacity of an optical fiber is much higher than that of a co-axial cable.
Disadvantages of Optical Fiber
1. Fiber optics cables are fragile i.e. more easily broken than wires.
2. Being fragile, optical fibers need to be put deep into the land. This causes a lot of installation cost. Also the interface used for these fibers are expensive.
3. Optical fibers are unidirectional for two-way communication, two fibers are required.
4. It is a newer technology and requires skilled people to administer and maintain them.
Characteristics of Optical Fiber Cables:
Fiber optic cables have the following characteristics:
1. Fiber optic cabling can provide extremely high bandwidths in the range from 100 mbps to 2 gigabits because light has a much higher frequency than electricity.
2. The number of nodes which a fiber optic can support does not depend on its length but on the hub or hubs that connect cables together.
3. Fiber optic cable has much lower attenuation and can carry signal to longer distances without using amplifiers and repeaters in between.
4. Fiber optic cable is not atlected by EMI effects and can be used in areas where high voltages are passing by.
5. The cost of tiber optic cable is more compared to twisted pair and co-axial.
6. The installation of fiber optic cables is difficult and tedious.
1. Optical fiber transmission systems are widely used in th~ backbone of networks. Current optical fiber systems provide transmission rates from 45 Mb/s to 9.6 Gb/s using the single wavelength transmission.
2. The installation cost of optical fibers is higher than that for the co-axial or twisted wire cables.
3. Optical fibers are now used in the telephone systems.
4. In the Local Area Networks (LANs).