Unit 3A - Optical Fiber PDF

10/11/2024 Unit 3 Fiber Optics and Holography A Introduction, structure of optical fiber, Light guidance through optical fiber, Acceptance angle and Acceptance cone, Numerical aperture, B Types of optical fibers, Attenuation and Dispersion in optical...

10/11/2024 Unit 3 Fiber Optics and Holography A Introduction, structure of optical fiber, Light guidance through optical fiber, Acceptance angle and Acceptance cone, Numerical aperture, B Types of optical fibers, Attenuation and Dispersion in optical fiber, Applications of optical fibers. C Basic principle of holography, Recording of holograms, Reconstruction process, Applications of holography. OPTICAL FIBRES and OPTICAL COMMUNICATION 1  10/11/2024 OPTICAL FIBER A bundle of optical fibers A fiber optic audio cable being illuminated on one end Dr. Munendra 3 Introduction to Optical Fibers An optical fiber is a glass or plastic fiber that carries light along its length just like a pipe carries the water through it. Usually 120 micrometers in diameter Used to carry signals in the form of light over distances up to 50 km. Dr. Munendra 4 2  10/11/2024 Introduction (Cont…) Core – thin glass center of the fiber where light travels. Cladding – outer optical material surrounding the core Buffer Coating – plastic coating that protects the fiber. Dr. Munendra 5 CORE CHARACTERISTICS The diameter of light carrying region of the fiber is the core diameter. The larger the core the more rays of light that travel in the core. The larger the core the more optical power that can be transmitted. The core has a higher refractive index than the cladding. Dr. Munendra 6 3  10/11/2024 CLADDING It protects the core from absorbing surface contaminants. It adds mechanical strength to the fiber. It reduces the scattering losses. Dr. Munendra 7 BUFFER /COATING Adds further strength to the fiber. Mechanically isolates the fiber from small geometrical irregularities. Coating provides an outer envelope and protection to other layers in optical fiber. Dr. Munendra 8 4  10/11/2024 Advantages of Optical Fibre Thinner Less Expensive Higher Carrying Capacity Less Signal Degradation& Digital Signals Light Signals Non-Flammable Light Weight Dr. Munendra 9 Optical fibers for internet & telephone communication Information is “digitized” (converted to 0s and 1s); 0 = no light pulse, 1 = light pulse Advantages: – Clarity of signal: copper wires & electricity can lead to “cross- talk”, as electric current in one wire results in magnetic field which causes a small current in a neighboring wire; “cross-talk” does not occur in optical fibers – High information density: Two optical fibers can transmit the equivalent of 30,000 telephone calls simultaneously (in 1956, the 1st transatlantic cable could handle only 52 simultaneous conversations) – Low weight & volume: It requires 30,000 kg of Cu wire to transmit the same amount of information as 0.1 kg of optical fibers – Transmission at light speed (instead of at drift velocity in the case of Cu wires) Dr. – Long transmission distance: very low intensity attenuation in10 Munendra fibers 5  10/11/2024 Properties of optical fibers Fiber has to have two important properties: – Total internal reflection, so that light is contained within fiber – Low attenuation, so that light can be carried over long distances with minimal loss Structure – Inner core glass: high refractive index (contains light) – Cladding glass: lower refractive index – Outer polymer coating: adds strength & protects fiber Dr. Munendra 11 Refraction and Total Internal Reflection Dr. Munendra 12 6  10/11/2024 REVISION OF REFLECTION Dr. Munendra 13 Light Visible light is a form of electromagnetic wave Dr. Munendra 14 wavelength 7  10/11/2024 Light in materials When light enters a transparent medium, it loses some energy by moving electrons As a result, light slows down! And so, light bends! Why? Incident light Reflected light AIR GLASS Refracted light AIR Transmitted light Dr. Munendra 15 Bending of light The difference in the speed of light in different materials causes it to bend speed of light in vacuum Refractive index of material = Dr. Munendra speed of light in material 16 8  10/11/2024 Total internal reflection Consider light that goes from glass to air Critical angle is the angle at which the refracted light goes along the surface A light rays with greater angles will get totally reflected back into the glass This is the principle used in optical fibers AIR GLASS Dr. Munendra 17 Critical angles Critical angle for water/air is 48 degrees, for diamond/air is 24.5 degrees, and in optical fibers is 75 degrees Optical fiber Dr. Munendra Protective cladding 18 9  10/11/2024 Critical angles & Mirages Dr. Munendra 19 Rainbows Dr. Munendra 20 10  10/11/2024 Optical fibers Dr. Munendra 21 LIGHT PROPOGATION THROUGH FIBER Optical fibers work on the principle of Total Internal Reflection n1 and n2 be the refractive index of the core and cladding of the fiber respectively If angle of incidence and refraction are Φ1 & Φ2 then by Snell’s law of refraction sin Φ1 n2 = sin Φ2 n1 The critical angle is given by sin Φc = (n2 / n1) At the angles of incidence greater than critical angle, the light is reflected back into the originating dielectric medium.This phenomenon is known as “ Total Internal Reflection” Dr. Munendra 22 11  10/11/2024 LIGHT PROPOGATION THROUGH FIBER Because refractive index of the core is greater then that of the cladding , light traveling in the core will remain in it due to total internal reflection as long as the light strikes the core-cladding interface at an angle greater than the critical angle. The transmission of a light ray in an optical fiber is shown in fig.It is guided by the series of total internal reflection at the core cladding interface.this ray has an angle of incidence Φ at the interface,which is greater than critical angle and reflected at the same angle to the normal. Low index cladding Core of high index ……………………………………….…………………………………….. Φ Φ Φ Φ Low index cladding Dr. Munendra 23 Parameters of a Fiber Note that, for total internal reflection, θi should be such that the ø > øc (critical angle). Maximum value of θi for which ø = øc is Known as Acceptance Angle θ0. Dr. Munendra 24 12  10/11/2024 Acceptance angle may be defined as the maximum angle that a light ray can have relative to the axis of the fiber and propagate down the fibre. The light rays contained within the cone having a full angle 2θ0 are accepted and transmitted along the fiber. Therefore the cone is called the Acceptance cone. Light incident at an angle beyond θ0 refracts through the cladding and the corre- sponding optical energy is lost. Dr. Munendra 25 Consider an incident beam at just acceptance angle. In that case  = c Then 2 Sinc = n2 n1  Cosc = 1 −  2  n  n1  And Sin 0 Sin 0 Sin 0 n1 = = = Sin r Sin(90 − c ) Cosc n0 Solving these two equations, we get the value of acceptance angle  0 = Sin −1  n12 − n2 2    Dr. Munendra 26 13  10/11/2024 Parameters of a Fiber The fractional difference between the refractive indices of the core and the cladding is known as Fractional refractive index change and is represented by Δ. n −n = 1 2 n1 Sine of the Acceptance angle is known as the Numerical Aperture. This is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. NA = Sin 0 = n1 − n2. 2 2 NA = n1 2  n + n  n − n   2n  n − n  n1 − n2 = (n1 + n2 )( n1 − n2 ) =  1 2  1 2 2n1   1  1 2 2n1 = 2n1  2 2 2  2  n1   2  n1  Dr. Munendra 27 Types of fiber Optical fibers come in two types: Single-mode fibers – used to transmit one signal per fiber (used in telephone and cable TV). They have small cores (9 microns in diameter) and transmit infra-red light from laser. Multi-mode fibers – used to transmit many signals per fiber (used in computer networks). They have larger cores (62.5 microns in diameter) and transmit infra-red light from LED. Dr. Munendra 28 14  10/11/2024 Single-mode step-index Fiber Advantages: Minimum dispersion: all rays take same path, same time to travel down the cable. A pulse can be reproduced at the receiver very accurately. Less attenuation, can run over longer distance without repeaters. Larger bandwidth and higher information rate Disadvantages: Difficult to couple light in and out of the tiny core Highly directive light source (laser) is required. Interfacing modules are more expensive Dr. Munendra 29 Single Mode vs Multi Mode Single Mode Multi Mode Dr. Munendra 30 30 15  10/11/2024 Multimode Fiber further classified: Step Index Core vs Graded Index Core Graded-index Fiber: Optical fiber in Step-index Fiber: Fiber that has a which the refractive index of the uniform index of refraction throughout core is in the form of a parabolic the core that is a step below the curve, decreasing toward the index of refraction in the cladding cladding. Minimizes modal dispersion. Dr. Munendra 31 31 Dr. Munendra 32 16  10/11/2024 Classes of Fiber Optics Cladding Cores diameter Wavelength Light source diameter Single-mode 1,300 to 1,550 Laser,VCSEL 5~10 microns 125 microns fibers nm infrared Multi-mode Step 50, 62.5 or above 125~140 LED, ,VCSEL 850 to 1,300 nm Index fibers microns microns infrared Multi-mode Step 230~630 750~2000 LED, ,VCSEL 400~600 microns Index fibers microns microns infrared 750~2000 Multi-mode plastic 750~2000 microns 650 nm LED, visible red fibers microns Dr. Munendra 33 Parameters of a Fiber Normalized frequency d V= (n1 − n2 ) 2 2 (V- number)  d d = NA = n 2   1 Nm: Maximum number of 1 Nm = V 2 modes supported by a fiber 2 V < 2.405 : Single mode fiber V > 2.405 : Multi mode fiber Cut off Wavelength: λ at V = 2.405 Dr. Munendra 34 17  10/11/2024 Losses in fiber Mainly due to Attenuation and Dispersion Attenuation: reduction of light amplitude Dispersion: deterioration of waveform Dr. Munendra 35 Losses in fiber: ATTENUATION An optical signal propagating through a fiber will get progressively attenuated. The signal attenuation is defined as the ratio of the optical power (Po) from a fiber of the length L to the input power (Pi). Signal attenuation is a log relationship. Length is expressed in kilometers. Therefore, the unit of attenuation is decibels/ kilometer (dB/km). Attenuation is caused by absorption, and scattering. Each mechanism of loss is influenced by fiber- material properties, wavelength used and fiber structure. 10  Pi  = log  L  Po  Dr. Munendra 36 18  10/11/2024 ATTENUATION in optical fibers Attenuation in optical fiber is caused primarily by both scattering and absorption (intrinsic as well as extrinsic). Light scattering: The propagation of light through the core of an optical fiber is based on total internal reflection of the lightwave. Rough and irregular surfaces, even at the molecular level, can cause light rays to be reflected in random directions. This is called diffuse reflection or scattering, and it is typically characterized by wide variety of reflection angles.Light scattering depends on the wavelength of the light being scattered. Dr. Munendra 37 ATTENUATION in optical fibers Absorption: Material absorption can be divided into two categories. Intrinsic absorption losses correspond to absorption by fused silica (material used to make fibers) whereas extrinsic absorption is related to losses caused by impurities within silica. A) Intrinsic Absorption: Any material absorbs at certain wavelengths corresponding to the electronic and vibrational resonances associated with specific molecules. For silica molecules, electronic resonances occur in the ultraviolet region ( wavelength < 0.4um), whereas vibrational resonances occur in the infrared region (wavelength > 7um). Dr. Munendra 38 19  10/11/2024 B) Extrinsic Absorption: Extrinsic absorption results from the presence of impurities. Transition-metal impurities such as Fe, Cu, Co, Ni, Mn, and Cr absorb strongly in the wavelength range 0.6~1.6um. Their amount should be reduced to below 1 part per billion to obtain a loss level below 1dB/km. Such high-purity silica can be obtained by using modern techniques. The main source of extrinsic absorption in state-of-the-art silica fibers is the presence of water vapors. A vibrational resonance of the OH ion occurs near 2.73um. Its harmonic and combination tones with silica produce absorption at the 1.39um, 1.24um and 0.95um wavelengths. The three spectral peaks seen in next figure occur near these wavelengths and are due to the presence of residual water vapor in silica. Dr. Munendra 39 Absorption Losses In Optic Fiber 6 Rayleigh scattering 5 & ultraviolet Loss (dB/km) 4 absorption 3 Peaks caused Infrared by OH- ions 2 absorption 1 0 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Wavelength (m) Dr. Munendra 40 20  10/11/2024 Losses in fiber: DISPERSION Dispersion is the spreading out of a light pulse in time as it propagates down the fiber. Dispersion in optical fiber includes model dispersion, material dispersion and waveguide dispersion. 1. Model Dispersion in Multimode Fibers Multimode fibers can guide many different light modes since they have much larger core size. Each mode enters the fiber at a different angle and thus travels at different paths in the fiber. Since each mode ray travels a different distance as it propagates, the ray arrive at different times at the fiber output. So the light pulse spreads out in time which can cause signal overlapping so seriously that you cannot distinguish them any more. Model dispersion is not a problem in single mode fibers since there is only one mode that can travel in the fiber. Dr. Munendra 41 Dr. Munendra 42 21  10/11/2024 2. Material Dispersion Material dispersion is the result of the finite linewidth of the light source and the dependence of refractive index of the material on wavelength. Material dispersion is a type of chromatic dispersion. Chromatic dispersion is the pulse spreading that arises because the velocity of light through a fiber depends on its wavelength. 3. Waveguide Dispersion in single mode fiber Waveguide dispersion is only important in single mode fibers. It is caused by the fact that some light travels in the fiber cladding compared to most light travels in the fiber core. Since fiber cladding has lower refractive index than fiber core, light ray that travels in the cladding travels faster than that in the core. Waveguide dispersion is also a type of chromatic dispersion. It is a function of fiber core size, V-number, wavelength and light source linewidth. Dr. Munendra 43 Coupling Losses Dr. Munendra 44 22  10/11/2024 Fiber-Optic Applications The use and demand for optical fiber has grown tremendously and optical-fiber applications are numerous. Telecommunication applications are widespread, ranging from global networks to desktop computers. These involve the transmission of voice, data, or video over distances of less than a meter to hundreds of kilometers, using one of a few standard fiber designs in one of several cable designs. Carriers use optical fiber to carry plain old telephone service (POTS) across their nationwide networks. Local exchange carriers (LECs) use fiber to carry this same service between central office switches at local levels, and sometimes as far as the neighborhood or individual home (fiber to the home [FTTH]).. Dr. Munendra 45 Fiber-Optic Applications Optical fiber is also used extensively for transmission of data. Multinational firms need secure, reliable systems to transfer data and financial information between buildings to the desktop terminals or computers and to transfer data around the world. Cable television companies also use fiber for delivery of digital video and data services. The high bandwidth provided by fiber makes it the perfect choice for transmitting broadband signals, such as high- definition television (HDTV) telecasts. Intelligent transportation systems, such as smart highways with intelligent traffic lights, automated tollbooths, and changeable message signs, also use fiber-optic-based telemetry systems. Another important application for optical fiber is the biomedical industry. Fiber-optic systems are used in most modern telemedicine devices for transmission of digital diagnostic images. Other applications for optical fiber include space, military, automotive, and the industrial sector Dr. Munendra 46 23  10/11/2024 Optical communication has many well known advantages: Weight and Size Fibre cable is significantly smaller and lighter than electrical cables to do the same job. In the wide area environment a large coaxial cable system can easily involve a cable of several inches in diameter and weighing many pounds per foot. A fibre cable to do the same job could be less than one half an inch in diameter and weigh a few ounces per foot. This means that the cost of laying the cable is dramatically reduced. Material Cost Fibre cable costs significantly less than copper cable for the same transmission capacity. Dr. Munendra 47 Information Capacity The usual rate for new systems is 2.4 Gbps or even 10 Gbps. Thirty million calls is about the maximum number of calls in progress in the world at any particular moment in time. That is to say, we could carry the world's peak telephone traffic over one pair of fibres. Most practical fibre systems don't attempt to do this because it costs less to put multiple fibres in a cable than to use sophisticated multiplexing technology. Dr. Munendra 48 24  10/11/2024 No Electrical Connection In electrical systems there is always the possibility of “ground loops” causing a serious problem, especially in the LAN or computer channel environment. There is often a voltage potential difference between “ground” at different locations. It is normal to observe 1 or 2 volt differences over distances of a kilometer or so. Optical connection is very safe. Electrical connections always have to be protected from high voltages because of the danger to people touching the wire. In some tropical regions of the world, lightning poses a severe hazard even to buried telephone cables! Of course, optical fibre isn't subject to lightning problems but it must be remembered that sometimes optical cables carry wires within them for strengthening or to power repeaters. These wires can be a target for lightning. Dr. Munendra 49 No Electromagnetic Interference Because the connection is not electrical, you can neither pick up nor Create electrical interference (the major source of noise). This is one reason that optical communication has so few errors. There are very few sources of things that can distort or interfere with the signal. In a building this means that fibre cables can be placed almost Anywhere electrical cables would have problems, (for example near a lift motor or in a cable duct with heavy power cables). In an industrial plant such as a steel mill, this gives much greater flexibility in cabling than previously available. In the wide area networking environment there is much greater flexibility in route selection. Cables may be located near water or power lines without risk to people or equipment. Dr. Munendra 50 25  10/11/2024 Distances between Regenerators As a signal travels along a communication line it loses strength (is attenuated) and picks up noise. The traditional way to regenerate the signal, restoring its power and removing the noise, is to use a either a repeater or an amplifier. In long-line optical transmission cables now in use by the telephone companies, the repeater spacing is typically 120 kilometres. This Compares with 12 km for the previous coaxial cable electrical technology. The number of required repeaters and their spacing is a major factor in system cost. Dr. Munendra 51 Open Ended Capacity The maximum theoretical capacity of installed fibre is very great (almost infinite). This means that additional capacity can be had on existing fibres as new technology becomes available. All that must be done is change the equipment at either end and change or upgrade the regenerators. Better Security It is possible to tap fibre optical cable. But it is very difficult to do and the additional loss caused by the tap is relatively easy to detect. There is an interruption to service while the tap is inserted and this can alert operational staff to the situation. In addition, there are fewer access points where an intruder can gain the kind of access to a fibre cable necessary to insert a tap. Dr. Munendra 52 26  10/11/2024 However, there are some limitations: Joining Cables The best way of joining cables is to use “fusion splicing”. This is Where fibre ends are fused to one another by melting the glass. Making such splices in a way that will ensure minimal loss of signal is a skilled task that requires precision equipment. One of the major system costs is the cost of coupling a fibre to an integrated light source (laser or LED) or detector on a chip. This is Done during manufacture and is called “pigtailing”. Dr. Munendra 53 Bending Cables As light travels along the fibre, it is reflected from the interface between the core and cladding whenever it strays from the path straight down the center. When the fibre is bent, the light only stays in the fibre because of this reflection. But the reflection only works if the angle of incidence is relatively low. If you bend the fibre too much the light escapes. The amount of allowable bending is specific to particular cables because it depends on the difference in refractive index, between core and cladding. The bigger the difference in refractive index, the tighter the allowable bend radius. Optics for Transmission Only Until very recently there was no available optical amplifier. The signal had to be converted to electrical form and put through a complex repeater in order to boost its strength. Recently, optical amplifiers have emerged and look set to solve this problem but are quite costly. Dr. Munendra 54 27  10/11/2024 Gamma Radiation Gamma radiation comes from space and is always present. It can be thought of as a high-energy X-ray. Gamma radiation can cause some types of glass to emit light (causing interference) and also gamma radiation can cause glass to discolor and hence attenuate the signal. In normal situations these effects are minimal. However, fibres are probably not the transmission medium of choice inside a nuclear reactor or on a long-distance space probe. (A glass beaker placed inside a nuclear reactor for even a few hours comes out black in color and quite opaque.) Electrical Fields Very high-voltage electrical fields also affect some glasses in the same way as gamma rays. One proposed route for fibre communication cables is wrapped around high-voltage electrical cables on transmission towers. This actually works quite well where the electrical cables are only of 30 000 volts or below. Above that (most major transmission systems are many times above that), the glass tends to emit light and discolour. Dr. Munendra 55 Sharks Eat the Cable(?) In the 1980s there was an incident where a new undersea fibre cable was broken on the ocean floor. Publicity surrounding the event suggested that the cable was attacked and eaten by sharks. It wasn't just the press; this was a serious claim. It was claimed that there was something in the chemical composition of the cable sheathing that was attractive to sharks! Another explanation was that the cable contained an unbalanced electrical supply conductor. It was theorised that the radiated electromagnetic field caused the sharks to be attracted. Gophers (and Termites) Really Do Eat the Cable Gophers are a real problem for fibre cables in the United States. There is actually a standardised test (conducted by a nature and wildlife organisation) which involves placing a cable in a gopher enclosure for a fixed, specified length of time. In other countries termites have been known to attack and eat the plastic sheathing. Dr. Munendra 56 28