Fibre Optic Communication

ABSTRACT Fiber optics is one of the most advanced technologies which uses a simple basic physics principle called Total Internal Reflection. The basic requirements for the fibre optic communication are very easy and simple handling apparatus. There are lots of advantages in communicating with this technology. The applications of fibre optics cannot be explained within a day or in a bunch of sheets as there are infinite applications for it. Though there are some disadvantages, it is cleared by using latest technologies.

Fibre optic communication Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. Lets look at the fibre optic communication in detail.

Technology Modern fiber-optic communication systems generally include an optical transmitter to convert an electrical signal into an optical signal to send into the optical fiber, a cable containing bundles of multiple optical fibers that is routed through underground conduits and buildings, multiple kinds of amplifiers, and an optical receiver to recover the signal as an electrical signal. The information transmitted is typically digital information generated by computers, telephone systems, and cable television companies. The main parts of the optical fiber communication are: 1) Transmitter, 2) Receiver, 3) Fibre, 4) Amplifier (Silicon waveguide)

1) Transmitter:A device that accepts an electrical signal as its input, processes this signal, and uses it to modulate an opto-electronic device, to produce an optical signal capable of being transmitted via an optical transmission medium There is a variety of different aspects to any fibre optic transmitter. For any application, the different specifications need to be examined to ensure that the particular fibre optic transmitter will meet the requirements. a) One of the major aspects to any fibre optic transmitter, is its power level. It is obvious that the fibre optic transmitter should have a sufficiently high level of light output for the light to be transmitted along the fibre optic cable to the far end. Some fibre optic cable lengths may only be a few metres or tens of metres long, whereas others may extend for many kilometres. In the case of the long lengths, the power of the fibre optic transmitter is of great importance. b) The type of light produced is also important. Light can be split into two categories, namely coherent and incoherent light. Essentially, coherent light has a single frequency, whereas incoherent light contains a wide variety of light packets all containing different frequencies, i.e. there is no single frequency present. While some emitters may appear to emit a single colour, they can still be incoherent because the light output is centered on a given frequency or wavelength. c) The frequency or wavelength of the light can also be important. Often fibre optic systems will operate around a given wavelength. Typically the wavelength of operation is given.

d) It is also necessary to consider the rate at which the transmitter can be modulated as this affects the data rate for the overall transmission. In some instances low rate systems may only need to carry data at a rate of a few Mbps, whereas main telecommunications links need to transmit data at many Gbps.

Efficiency:The efficiency of an electro-optical transmitter is determined by many factors, but the most important are the following: spectral linewidth, which is the width of the carrier spectrum and is zero for an ideal monochromatic light source; insertion loss, which is the amount of transmitted energy that does not couple into the fibre; transmitter lifetime; and maximum operating bit rate

2) Reciever:The main component of an optical receiver is a photo-detector, which converts light into electricity using the photoelectric effect. The photodetector is typically a semiconductorbased photodiode. Several types of photodiodes include p-n photodiodes, a p-i-n photodiodes, and avalanche photodiodes. Metalsemiconductor-metal (MSM) photodetectors are also used due to their suitability for circuit integration in regenerators and wavelength-division multiplexers.The optical-electrical converters are typically coupled with a transimpedance amplifier and a limiting amplifier to produce a digital signal in the electrical domain from the incoming optical signal, which may be attenuated and distorted while passing through the channel. Further signal processing such as clock recovery from data (CDR) performed by a phase-locked loop may also be applied before the data is passed on.

Types of fibre optic transmitter There are two main types of fibre optic transmitter that are in use today. Both of them are based around semiconductor technology: l Light emitting diodes (LEDs) 2 Laser diodes

3) Fiber:An optical fiber consists of a core, cladding, and a buffer (a protective outer coating), in which the cladding guides the light along the core by using the method of total internal reflection. The core and the cladding (which has a lower-refractive-index) are usually made of high-quality silica glass, although they can both be made of plastic as well. Light is kept in the core of the optical fiber by total internal reflection. This causes the fiber to act as a waveguide.

Types of fibre:Fibers which support many propagation paths or transverse modes are called multi-mode fibers (MMF), while those which can only support a single mode are called single-mode fibers (SMF). Multi-mode fibers generally have a larger core diameter, and are used for short-distance communication links and for applications where high power must be transmitted. Single-mode fibers are used for most communication links longer than 550 metres (1,800 ft).

4) Amplifier:The transmission distance of a fiber-optic communication system has traditionally been limited by fiber attenuation and by fiber distortion. By using opto-electronic repeaters, these problems have been eliminated. These repeaters convert the signal into an electrical signal, and then use a transmitter to send the signal again at a higher intensity than it was before. Because of the high complexity with modern wave length-division multiplexed signals (including the fact that they had to be installed about once every 20 km), the cost of these repeaters is very high. An alternative approach is to use Silicon waveguide

Silicon waveguide:For every 50 miles or so the signal must be re-amplified, cleaned up and re-launched. Light is dimmed by miles of fibre, and the crisp on-and-off pulses that represent the ones and zeros of a digital signal become misshapen and fuzzy. Now Cornell University researchers have demonstrated that all this can be done on a single photonic microchip, according to a Cornell University press release. Previously the researchers had demonstrated a light amplifier on a silicon chip using a process called four-wave mixing, which could amplify an optical signal by ‘pumping’ with another beam of light. Four-wave mixing has been used to amplify light in devices made of optical fibre, but the process requires tens of metres of fibre. The Cornell researchers used silicon waveguides only a few hundred nanometres across and 1.8 centimetres long embedded in a single silicon chip. The tight dimensions of the waveguide, smaller than the wavelength of the light travelling through it, forces two entering beams of light — the signal and the ‘pump’ — to exchange energy. Some photons from the pump are converted to the same wavelength as the signal, amplifying it, while others come out at a wavelength equal to twice the pump wavelength minus the signal wavelength. That last effect can be used to convert a signal from one wavelength to another. The researchers found that pumping a pulsed signal with a continuous wave light beam at another frequency amplifies the signal but doesn’t clean up the pulses. However, if the arrangement is changed so that the light carrying the signal acts as the pump, the output is both amplified and sharpened.

Disturbances 1) Attenuation.:Attenuation is a reduction in the transmitted power, has long been a problem for the fiber optics community. The increase in data loss over the length of a fiber has somewhat hindered widespread use of fiber as a means of communication. However, researchers have established three main sources of this loss: a) absorption and b) scattering

a) Absorption Absorption occurs when the light beam is partially absorbed by lingering materials, namely water and metal ions, within the core of the fiber as well as in the cladding. Though absorption in standard glass fibers tends to increase between the critical lengths of 700 and 1550 nanometers (nm) (Hecht, August 2000), almost any type of fiber at any length will have light absorbed by some of the traces of impurities that inevitably appear in all fibers. As the light signal travels through the fiber, each impurity absorbs some of the light, weakening the signal; therefore, longer fibers are more prone to attenuation due to absorption than shorter ones. b) Scattering Scattering, another significant aspect of attenuation, occurs when atoms or other particles within the fiber spread the light. This process differs with absorption in that, for the most part, foreign particles on the fiber are not absorbing the light, but the light signal bounces off the particle rather than the fiber’s wall and spreads the signal in another direction (Single-Mode, 2000). For glass fibers, the foremost type of scattering is Rayleigh scattering, which somewhat contrasts with the accepted definition of scattering. With this process, atoms or other particles within the fiber fleetingly absorb the light signal and instantly re-emit the light in another direction. In this way, Rayleigh scattering appears very much like absorption, but it absorbs and re-directs the light so quickly that it is considered scattering (Hecht, August 2000). Both scattering and absorption are cumulative, in that they keep building up. Light is absorbed and scattered continuously, so the signal at the end of the fiber is almost never exactly the same signal as it was at the beginning. However, for the most part, the signal loss is minimal and does not greatly hinder the communication. 2) Dispersion Three Types of Dispersion i) Material Dispersion Dispersion in optical fibers can be categorized into three main types. The first is material dispersion, also known as chromatic dispersion. This type of intramodal dispersion results from the fact that the refractive index of the fiber medium varies as a function of wavelength (Keiser, 1983). Since neither the light source nor the fiber optic cable is 100 percent pure, the pulse being transmitted becomes less and less precise as the light’s

wavelengths are separated over long distances (Thoughts, 2000). The exact same effect occurs when a glass prism disperses light into a spectrum. ii) Wave-guide Dispersion Wave-guide dispersion, another type, is very similar to material dispersion in that they both cause signals of different wavelengths and frequencies to separate from the light pulse (Keiser, 1983). However, wave-guide dispersion depends on the shape, design, and chemical composition of the fiber core. Only 80 percent of the power from a light source is confined to the core in a standard single-mode fiber, while the other 20 percent actually propagates through the inner layer of the cladding. This 20 percent travels at a faster velocity because the refractive index of the cladding is lower than that of the core (Keiser, 1983). Consequently, signals of differing frequencies and wavelengths are dispersed and the pulse becomes indistinguishable. An increase in the wave-guide dispersion in an optical fiber can be used in order to counterbalance material dispersion and shift the wavelength of zero chromatic dispersion to 1550 nanometers. Engineers used this concept to develop zero-dispersion-shifted fibers designed to have larger wave-guide dispersion (see Figure 2). Developers doped the core with erbium in order to increase the difference between the refractive indices of the cladding and the core, thus enlarging wave-guide dispersion (Lerner, 1997). iii) Modal Dispersion The third and final significant type of dispersion is related to the fact that a pulse of light transmitted through a fiber optic cable is composed of several modes, or rays, of light instead of only one single beam; therefore, it is called modal dispersion (Thoughts, 2000). Since the rays of the light pulse are not perfectly focused together into one beam, each mode of light travels a different path, some short and some long. As a result, the modes will not be received at the same time, and the signal will be distorted or even lost over long distances (Thoughts, 2000). Solitons, or stable waves, could quite possibly be the ultimate solution to dispersion, but this phenomenon is still being carefully studied and tested (Lerner, 1997). However, other dispersion compensation devices are in commercial use today. For example, erbium-doped fibers, as discussed earlier, and wavelength-division multiplexing (WDM) effectively shift most systems to 1550 nanometer at zero dispersion (Hecht, July 2000). Although high-performance WDM systems have been developed from novel fiber designs, the dispersion slope and the mode-field diameter of the fiber are still producing complications. New fiber designs, such as reduced-slope fibers, have low dispersion over a very wide range because the slope of dispersion is less than 0.05 ps/nm2-km, which is significantly lower than standard fibers (Hecht, July 2000). New concepts and designs are continually being developed to reduce dispersion in optical fibers.

Advantages:1) Extremely high bandwidth In an optical fibre system, a greater volume of information can be carried out. The rate of transmission of information is directly proportional to the frequencies. The light has higher frequencies than the radio or microwave frequencies. And also we can send much number of lights of different frequencies in a single fiber. Hence it is better than radio and microwave. 2) Ease of handling:The cross section of the optical fibre is about 100 micrometers, whereas wires are bigger in size and weigh more. Fibres are quite flexible and strong. 3) Cheap: Optical fibers are cheaper. As it is made from silicon which is available abundantly, it is cheaper. 4) Non hazardous:A wire communication could, accidentally short circuit high voltage and produce spark and can ignite the inflammable gase present in that environment. It is not possible in optical fibre as it is an insulator. 5) Optical fibres are immune to EMI and RFI In optical fibres, information is carried out through photons which are immune to external noise, Electro Magnetic Interference (EMI) and Radio Frequency Interference (RFI). 6) Optical reduce the reduce cross talk possibility:The light wave propagating in the fibre is entirely trapped and cannot leak out. Similarly the light outside cannot couple into the fibre from outide. Because of this feature , cross talk susceptibility is greatly reduced. 7) Optical fibres have low loss per unit length:The transmission per unit length of an optical fibre is very less i.e., 4 dB/km whereas it is 2 dB/km for cables. 8) Resistance:Optical fibre are resistant to heat, light,corrosion, radiationetc., 9) Grounding:It does not require any grounding whereas the copper wire needs. 10) Security It has higher security and there is no way to tap the signal in the optical fibre.

Application 1) Medical application a) ophthalmology:A laser beam guided by an optical fibre is used to reattach detached retinas and to correct defects in vision. b) Cardiology:In cardiology, laser angioplasty is expected to do away with the ballon angioplasty and bypass surgery. A special catheter is developed for the

removal of blocks in the veins. The catheter consists three channels-one is for visualization, the second for delivering laser power and an open tube to flush or suck the debris.

2) Military application a) An aircraft, a ship or a tank needs tons of copper wires for wiring of the communication equipment, control mechanisms, instrument panel illumination etc. Use of fibres in place of copper wires reduces much weight and also maintains true communication silence to the entry. b) Fibre guided missiles are pressed into service during the resent wars. Sensors are mounted on the missile which transmit video information through the fibre to a ground control van and receive commands from the van again,. The control van continuosly monitors the course of the missile and if necessary corrects it to ensure that the missile precisely hits the target.

3) Entertainment application:A coherent optical fibre bundle is used to enlarge the image displayed on a TV screen. Conventional optical projection system ifs bulky and expensive. 4) Fibre optic sensors:A fiber optic sensor is a sensor that uses optical fiber either as the sensing element ("intrinsic sensors"), or as a means of relaying signals from a remote sensor to the electronics that process the signals ("extrinsic sensors"). Fibers have many uses in remote sensing. Depending on the application, fiber may be used because of its small size, or the fact that no electrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using different wavelengths of light for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using a device such as an optical time-domain reflectometer. a) Pressure sensor It is used to determine the pressure applied on the fiber using dispersion in the fiber. Here it acts as active sensor. b) Temperature sensor It is used to calculate the temperature applied on it. Here it acts as passive sensor. In this two well connected fibre with one end coated with silicon is used. When the light is passed it travels twice. Due to absorption of silicon depends on temperature the amount of light absorbed is calculated and the temperature is also calculated. c) Pollution Dector:A smoke and pollution detector can be built using the optical fibres. A beam of light radiating from one end can be connected by the optical fibre. If foreign particles are present in the region between the two fibres, they scatter light. The variation in intensity of the light is collected by the second optical fibre reveals the presence of foreign particles.

5) Communication application

If we use LAN system to connect all the systems in a bank or industry it may cost lot if it is large. Whereas it cost less if use optical fibre instead.It ios also easy to manage and maintain it.Moreover the speed is very high wjen compared to parallel connection of wires Telephone cables connecting various countries come under the category of long-haul system.A 6600km long submarine transatlantic telephone cable is commisioned under the international cooperation in 1980. Many such projects are completed since then. Disadvantages:1) It is difficult to join two optical fibres, rather than cable wires. 2) Care should be taken while introducing the light waves into fibre such that the angle of incidence should be greater than the critical angle. 3) The disadvantages of fiber optic systems include problems with the relative newness of the technology, the relatively expensive cost, and the lack of component and system standardization.

However, these disadvantages are already being eliminated because of increased use and acceptance of fiber optic technology.

Prepared by:

S.Balaji – 80808133007 – [email protected] T.Ananth – 80808133002 – [email protected] Second year E.C.E., Jayaram College of Engineering and Technology, Trichy. Refernce: Mr.E.Gopinath M.Sc,M.Phil. www.wkipedia.com www.tpub.com www.library.thinkquest.org www.radioeletronics.com www.howstufworks.com