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E1-E2 CFA Overview of Optical Communication 15 OVERVIEW OF OPTICAL COMMUNICATION 15.1 LEARNING OBJECTIVES  Fiber-Optic Applications  Basic optical fiber communication system:  The Structure of an Optical Fiber  Principle o...

E1-E2 CFA Overview of Optical Communication 15 OVERVIEW OF OPTICAL COMMUNICATION 15.1 LEARNING OBJECTIVES  Fiber-Optic Applications  Basic optical fiber communication system:  The Structure of an Optical Fiber  Principle of Operation 15.2 INTRODUCTION The use of light for transmitting information from one place to another place is a very old technique. In 800 BC., the Greeks used fire and smoke signals for sending information like victory in a war, alerting against enemy, call for help, etc. Mostly only one type of signal was conveyed. During the second century B.C. optical signals were encoded using signaling lamps so that any message could be sent. There was no development in optical communication till the end of the 18th century. The speed of the optical communication link was limited due to the requirement of line of sight transmission paths, the human eye as the receiver and unreliable nature of transmission paths affected by atmospheric effects such as fog and rain. In the late 19th and early 20th centuries, light was guided through bent glass rods to illuminate body cavities. Alexander Graham Bell invented a 'Photophone' to transmit voice signals over an optical beam. By 1964, a critical and theoretical specification was identified by Dr. Charles K. Kao for long-range communication devices, the 10 or 20 dB of light loss per kilometer standard. Dr. Kao also illustrated the need for a purer form of glass to help reduce light loss. By 1970 Corning Glass invented fiber-optic wire or "optical waveguide fibers" which was capable of carrying 65,000 times more information than copper wire, through which information carried by a pattern of light waves could be decoded at a destination even a thousand miles away. Corning Glass developed fiber with loss of 17 dB/ km at 633 nm by doping titanium into the fiber core. By June of 1972, multimode germanium-doped fiber had developed with a loss of 4 dB per kilometer and much greater strength than titanium-doped fiber. In April 1977, General Telephone and Electronics tested and deployed the world's first live telephone traffic through a fiber-optic system running at 6 Mbps, in Long Beach, California. They were soon followed by Bell in May 1977, with an optical telephone communication system installed in the downtown Chicago area, covering a distance of 1.5 miles (2.4 kilometers). Each optical-fiber pair carried the equivalent of 672 voice E1-E2 CFA Version 3.0 April 2021 Page 182 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication channels. Today more than 80 percent of the world's long-distance voice and data traffic is carried over optical-fiber cables. An optical fiber is a thin, flexible, transparent fiber that acts as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communication. Fibers are used instead of metal wires because signals travel along them with less loss and are also immune to electromagnetic interference. With increase in population struggle for survival increased Its impacts on appearing in human life in many ways. There have been shortage of utilizes resources. The resources consist of materials, technology, money, human recourse, information, interconnectivity etc. Due to consistent pressure there has been different ways of innovations in almost every stream of life. In the field of telecommunication also development are happening in the fields of client terminals access technique, aggregation technique, multiplexing technique, transport technique. There has been different access technique and different type of client terminals as per respective access technique. The basic contents were limitations of transmission media and low order multiplexing and switching. The initial transmission started with attaching information leaflet with visions. The same concept was utilized on building semaphore. That came the evolution telegraphs lines after the invention of more score in which use of guided media has got important. In this era use of open wire communications having overhead line with minimal multiplexing was the latest things. However has the requirement of reliable telecommunication has increased need was well to have proper voice communications and switching like manual, electro mechanical, fully digital involving automatic increasing order of multiplexing were implemented. In this era the main access network comprised of cable network made up of copper and transmission network was predominately of over head lines. Later on seeing the limitations of over head lines like deterioration weather due to electro magnetite interference less carrying capacity ete. were found. Use of optical fibre as a transmission media got thrust due to less cost, improve technology in multiplexing, virtually infants capacity and immunity to electro-magnetic interference. Requirement of bandwidth which was around 20Kbps have reached to around 1Gbps. The accesses network is also converging with the development of IP & MPLS technologies of dada communication. Multiplexing is also migrating in TDM, FDM to packet base statistical multiplexing. Client terminals are also converging having all capabilities of voice, video, text, web and multimedia. The network is converging to one by using architecture of Next Generation network. Applications which were accesses network depended are also becoming universally accessible and a accesses network agnostic. The human interface is also improve presentably because of manufacturing line terminal incorporating signals of E1-E2 CFA Version 3.0 April 2021 Page 183 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication sensory organs like touch, vision, mind etc.. Today client terminals have improve GUI based web interface having faster processing multimedia capacity and capability to communicate to multiple secessions over multiple windows having full mobility as well as portability. Due to competitions and rapid growth of innovation, the world are become faster and expectations of prominent service delivery are also been increased. Delay in providing services has also been reduced and overall connectivity in becoming P-P i.e. pair to pair. 15.3 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.  Long distance communication backbones  Inter-exchange junctions  Video transmission  Broadband services  Computer data communication (LAN, WAN etc.)  High EMI areas  Non-communication applications (sensors etc…) 15.4 ADVANTAGES OF OPTICAL FIBER COMMUNICATION Fiber Optics has the following advantages: Wider bandwidth: The information carrying capacity of a transmission system is directly proportional to the carrier frequency of the transmitted signals. The optical carrier frequency is in the range 1013 to 1015 Hz while the radio wave frequency is about 106 Hz and the microwave frequency is about 1010 Hz. Thus the optical fiber yields greater transmission bandwidth than the conventional communication systems and the data rate or number of bits per second is increased to a greater extent in the optical fiber communication system. Further the wavelength division multiplexing operation by the data rate or information carrying capacity of optical fibers is enhanced to many orders of magnitude. Low transmission loss: Due to the usage of the ultra low loss fibers and the erbium doped silica fibers as optical amplifiers, one can achieve almost lossless transmission. In the modern optical fiber telecommunication systems, the fibers having a transmission loss of 0.2dB/km are used. Further, using erbium doped silica fibers over a short length in the E1-E2 CFA Version 3.0 April 2021 Page 184 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication transmission path at selective points; appropriate optical amplification can be achieved. Thus the repeater spacing is more than 100 km. Since the amplification is done in the optical domain itself, the distortion produced during the strengthening of the signal is almost negligible. Dielectric waveguide: Optical fibers are made from silica which is an electrical insulator. Therefore they do not pickup any electromagnetic wave or any high current lightning. It is also suitable in explosive environments. Further the optical fibers are not affected by any interference originating from power cables, railway power lines and radio waves. There is no cross talk between the fibers even though there are so many fibers in a cable because of the absence of optical interference between the fibers. Signal security: The transmitted signal through the fibers does not radiate. Further the signal cannot be tapped from a fiber in an easy manner. Therefore optical fiber communication provides hundred per cent signal security. Small size and weight: Fiber optic cables are developed with small radii, and they are flexible, compact and lightweight. The fiber cables can be bent or twisted without damage. Further, the optical fiber cables are superior to the copper cables in terms of storage, handling, installation and transportation, maintaining comparable strength and durability. 15.5 FIBER OPTICS BASICS: PRINCIPLES OF OPTICAL COMMUNICATION Optical Fiber is new medium, in which information (voice, Data or Video) is transmitted through a glass or plastic fiber, in the form of light, following the transmission sequence give below: (1) Information is encoded into Electrical Signals. (2) Electrical Signals are converted into light Signals. (3) Light Travels down the Fiber. (4) A Detector Changes the Light Signals into Electrical Signals. (5) Electrical Signals are decoded into Information. - Inexpensive light sources available. - Repeater spacing increases along with operating speeds because low loss fibres are used at high data rates. E1-E2 CFA Version 3.0 April 2021 Page 185 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication Figure 76: Fiber Optic System 15.6 PRINCIPLE OF OPERATION - THEORY Speed of light is actually the velocity of electromagnetic energy in vacuum such as space. Light travels at slower velocities in other materials such as glass. Light travelling from one material to another changes speed, which results in changing its direction of travel. This deflection of light is called Refraction. The amount that a ray of light passing from a lower refractive index to a higher one, is bent towards the normal, but light going from a higher index to a lower one, refracting away from the normal, as shown in the figures. The basics of light propagation can be discussed with the use of geometric optics. The basic law of light guidance is Snell‘s law (Fig. 77). Consider two dielectric media with different refractive indices and with n1 >n2 and that are in perfect contact, as shown in Figure. At the interface between the two dielectrics, the incident and refracted rays satisfy Snell‘s law of refraction—that is, n1sinφ1= n2sinφ2 In addition to the refracted ray there is a small amount of reflected light in the medium with refractive index n1. Because n1 is greater than n2 then always 2 > 1. As the angle of the incident ray increases there is an angle at which the refracted ray emerges parallel to the interface between the two dielectrics. This angle is referred to as the critical angle, crit, and from Snell‘s law is given by Sinφcrit = n2/n1 E1-E2 CFA Version 3.0 April 2021 Page 186 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication Figure 77: Snell’s law If the angle of incidence increases amore than the critical angle, the light is totally reflected back into the first material so that it does not enter the second material. The angle of incidence and reflection are equal and it is called Total Internal Reflection. 15.6.1 Propagation Of Light Through Fibre The optical fiber has two concentric layers called the core and the cladding. The inner core is the light carrying part. The surrounding cladding provides the difference refractive index that allows total internal reflection of light through the core. The index of the cladding is approximately 1% lower than that of the core. Typical values for example are a core refractive index of 1.47 and a cladding index of 1.46. Fiber manufacturers control this difference to obtain desired optical fiber characteristics. Most fibers have an additional coating around the cladding. This buffer coating is a shock absorber and has no optical properties affecting the propagation of light within the fiber. Figure shows the idea of light travelling through a fiber. Light injected into the fiber and striking core to cladding interface at greater than the critical angle, reflects back into core, since the angle of incidence and reflection are equal, the reflected light will again be reflected. The light will continue zigzagging down the length of the fiber. Light striking the interface at less than the critical angle passes into the cladding, where it is lost over distance. The cladding is usually inefficient as a light carrier, and light in the cladding becomes attenuated fairly. Propagation of light through fiber is governed by the indices of the core and cladding by Snell's law. Such total internal reflection forms the basis of light propagation through a optical fiber. This analysis consider only meridional rays- those that pass through the fiber axis each time, they are reflected. Other rays called Skew rays travel down the fiber without passing through the axis. The path of a skew ray is typically helical wrapping around and around the central axis. Fortunately skew rays are ignored in most fiber optics analysis. E1-E2 CFA Version 3.0 April 2021 Page 187 of 271 For Restricted Circulation  E1-E2 CFA Overview of Optical Communication Jacket Jacket Cladding Core Cladding (n2) Cladding Core (n2) Jacket Light at less than Angle of Angle of critical angle is incidence reflection absorbed in jacket Light is propagated by total internal reflection Figure 78: Propagation of light Fig. Total Internal through Reflection fiber in an optical Fibre The specific characteristics of light propagation through a fiber depends on many factors, including - The size of the fiber. - The composition of the fiber. The light injected into the fiber 15.6.2 Geometry Of Fiber The optical fibers used in communications have a very simple structure. A hair-thin fiber consist of two concentric layers of high-purity silica glass the core and the cladding, which are enclosed by a protective sheath as shown in Figure. Core and cladding have different refractive indices, with the core having a refractive index, n1, which is slightly higher than that of the cladding, n2. It is this difference in refractive indices that enables the fiber to guide the light. Because of this guiding property, the fiber is also referred to as an ―optical waveguide.‖ As a minimum there is also a further layer known as the secondary cladding that does not participate in the propagation but gives the fiber a minimum level of protection, this second layer is referred to as a coating. Light rays modulated into digital pulses with a laser or a light-emitting diode moves along the core without penetrating the cladding. Figure 79: (a) Cross section and (b) longitudinal cross section of a typical optical fiber E1-E2 CFA Version 3.0 April 2021 Page 188 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication The light stays confined to the core because the cladding has a lower refractive index—a measure of its ability to bend light. Refinements in optical fibers, along with the development of new lasers and diodes, may one day allow commercial fiber-optic networks to carry trillions of bits of data per second. The light stays confined to the core because the cladding has a lower refractive index—a measure of its ability to bend light. Refinements in optical fibers, along with the development of new lasers and diodes, may one day allow commercial fiber-optic networks to carry trillions of bits of data per second. The diameters of the core and cladding are as follows. Core (m) Cladding (m) 8 125 50 125 62.5 125 100 140 Fibre sizes are usually expressed by first giving the core size followed by the cladding size. Thus 50/125 means a core diameter of 50m and a cladding diameter of 125m. 125 8 125 50 125 62.5 125 100 Core Cladding Figure 80: Typical Core and Cladding Diameter Typical Core and Cladding Diameters 15.7 FIBRE TYPES – SINGLE MODE AND MULTI-MODE The refractive Index profile describes the relation between the indices of the core and cladding. Two main relationships exist: (I) Step Index (II) Graded Index The step index fibre has a core with uniform index throughout. The profile shows a sharp step at the junction of the core and cladding. In contrast, the graded index has a non- uniform core. The Index is highest at the center and gradually decreases until it matches with that of the cladding. There is no sharp break in indices between the core and the cladding. E1-E2 CFA Version 3.0 April 2021 Page 189 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication By this classification there are three types of fibres : (I) Multimode Step Index fibre (Step Index fibre) (II) Multimode graded Index fibre (Graded Index fibre) (III) Single- Mode Step Index fibre (Single Mode Fibre) 15.7.1 Step-Index Multimode Fiber Step Index multimode Fiber has a large core, up to 100 microns in diameter. As a result, some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternative pathways cause the different groupings of light rays, referred to as modes, to arrive separately at a receiving point. The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. The need to leave spacing between pulses to prevent overlapping limits bandwidth that is, the amount of information that can be sent. Consequently, this type of fiber is best suited for transmission over short distances, in an endoscope, for instance. Figure 81: STEP-INDEX MULTIMODE FIBER 15.7.2 Graded-Index Multimode Fiber It contains a core in which the refractive index diminishes gradually from the center axis out toward the cladding. The higher refractive index at the center makes the light rays moving down the axis advance more slowly than those near the cladding. Figure 82: GRADED-INDEX MULTIMODE FIBER Also, rather than zigzagging off the cladding, light in the core curves helically because of the graded index, reducing its travel distance. The shortened path and the higher speed allow light at the periphery to arrive at a receiver at about the same time as the slow but straight rays in the core axis. The result: a digital pulse suffers less dispersion. 15.7.3 Single-Mode Fiber E1-E2 CFA Version 3.0 April 2021 Page 190 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication It has a narrow core (nine microns or less), and the index of refraction between the core and the cladding changes less than it does for multimode fibers. Light thus travels parallel to the axis, creating little pulse dispersion. Telephone and cable television networks install millions of kilometers of this fiber every year. Figure 83: SINGLE-MODE FIBER 15.8 CABLE CONSTRUCTION There are two basic cable designs are: 1. Tight Buffer Tube Cable 2. Loose Buffer Tube Cable Loose-tube cable is used in the majority of outside-plant installations and tight- buffered cable, primarily used inside buildings. 15.8.1 Tight Buffer Tube Cable With tight-buffered cable designs, the buffering material is in direct contact with the fiber. This design is suited for "jumper cables" which connect outside plant cables to terminal equipment, and also for linking various devices in a premises network. Single- fiber tight-buffered cables are used as pigtails, patch cords and jumpers to terminate loose-tube cables directly into opto-electronic transmitters, receivers and other active and passive components. Multi-fiber tight-buffered cables also are available and are used primarily for alternative routing and handling flexibility and ease within buildings. The tight-buffered design provides a rugged cable structure to protect individual fibers during handling, routing and connectorization. Yarn strength members keep the tensile load away from the fiber. E1-E2 CFA Version 3.0 April 2021 Page 191 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication Figure 84: Tight Buffer Tube Cable The structure of a 250um coated fiber (bare fiber)  Core (9um for standard single mode fibers, 50um or 62.5um for multimode fibers)  Cladding (125um)  Coating (soft plastic, 250um is the most popular, sometimes 400um is also used) 15.8.2 Loose-Tube Cable The modular design of loose-tube cables typically holds 6, 12, 24, 48, 96 or even more than 400 fibers per cable. Loose-tube cables can be all-dielectric or optionally armored. The loose-tube design also helps in the identification and administration of fibers in the system. In a loose-tube cable design, color-coded plastic buffer tubes house and protect optical fibers. A gel filling compound impedes water penetration. Excess fiber length (relative to buffer tube length) insulates fibers from stresses of installation and environmental loading. Buffer tubes are stranded around a dielectric or steel central member, which serves as an anti-buckling element. The cable core, typically uses aramid yarn, as the primary tensile strength member. The outer polyethylene jacket is extruded over the core. If armoring is required, a corrugated steel tape is formed around a single jacketed cable with an additional jacket extruded over the armor. Loose-tube cables typically are used for outside-plant installation in aerial, duct and direct-buried applications. Loose tube cable is designed to endure outside temperatures and high moisture conditions. The fibers are loosely packaged in gel filled buffer tubes to repel water. E1-E2 CFA Version 3.0 April 2021 Page 192 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication Recommended for use between buildings that are unprotected from outside elements. Loose tube cable is restricted from inside building use. Figure 85: Loose Tube Cable 15.8.3 Elements In A Loose Tube Fiber Optic Cable: 1. Multiple 250um coated bare fibers (in loose tube) 2. One or more loose tubes holding 250um bare fibers. Loose tubes strand around the central strength member. 3. Moisture blocking gel in each loose tube for water blocking and protection of 250um fibers 4. Central strength member (in the center of the cable and is stranded around by loose tubes) 5. Aramid Yarn as strength member 6. Ripcord (for easy removal of inner jacket) 7. Outer jacket (Polyethylene is most common for outdoor cables because of its moisture resistant, abrasion resistant and stable over wide temperature range characteristics.) 15.9 TYPES OF FIBER OPTIC CABLE (MOST POPULAR FIBER OPTIC CABLE TYPES) 15.9.1 Indoor Cables Simplex Fiber Cables A single cable structure with a single fiber. Simplex cable varieties include 1.6mm & 3mm jacket sizes. E1-E2 CFA Version 3.0 April 2021 Page 193 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication Figure 86: Simplex Fiber Cables Duplex Fiber Optic Cable This cable contains two optical fibers in a single cable structure. Light is not coupled between the two fibers; typically one fiber is used to transmit signals in one direction and the other receives. Figure 87: Duplex Fiber Optic Cable 15.9.2 Outdoor Loose Tube Fiber Optic Cables Tube encloses multiple coated fibers that are surrounded by a gel compound that protects the cable from moisture in outside environments. Cable is restricted from indoor use, typically allowing entry not to exceed 50 feet. Figure 88: Outdoor Loose Tube Fiber Optic Cables E1-E2 CFA Version 3.0 April 2021 Page 194 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication 15.9.3 Aerial/Self-Supporting Figure 97 (aerial/self-supporting) fiber cables are designed to be strung from poles outdoors and most can also be installed in underground ducts. They have internal stress members of steel of steel or aramid yarn that protect fibers from stress. Aerial cable provides ease of installation and reduces time and cost. Figure 8 cable can easily be separated between the fiber and the messenger. Temperature range -55 to +85°C. Figure 89: Aerial cable 15.9.4 Direct-Buried Armored Fiber Optic Cable Armored cables are similar to outdoor cables but include an outer armor layer for mechanical protection and to prevent damage. They can be installed in ducts or aerially, or directly buried underground. Armor is surrounded by a polyethylene jacket. Armored cable can be used for rodent protection in direct burial if required. This cable is non-gel filled and can also be used in aerial applications. The armor can be removed leaving the inner cable suitable for any indoor/outdoor use. Temperature rating -40 to +85°C. Figure 90: Armored cable 15.9.5 Submarine Fiber Optic Cable (Undersea Fiber Optic Cable) Submarine cables are used in fresh or salt water. To protect them from damage by fishing trawlers and boat anchors they have elaborately designed structures and armors. Long distance submarine cables are especially complex designed. E1-E2 CFA Version 3.0 April 2021 Page 195 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication Figure 91: Submarine cables 15.9.6 ITU-T Complaint Fibers  G.651 Multimode Fiber  G.652 Standard Fiber  G.653 Dispersion Shifted Fiber  G.654 Loss minimized Fiber  G.655 Non Zero Dispersion Shifted Fiber  G.656 Medium Dispersion Fiber (MDF), designed for local access  G.657 Bending Loss Insensitive Fiber 15.10 CHARACTERISTICS OF OPTICAL FIBER 15.10.1 Wavelength It is a characteristic of light that is emitted from the light source and is measures in nanometers (nm). In the visible spectrum, wavelength can be described as the colour of the light. For example, Red Light has longer wavelength than Blue Light, Typical wavelength for fibre use are 850nm, 1300nm and 1550nm all of which are invisible (Infrared). 15.10.2 Windows A narrow window is defined as the range of wavelengths at which a fibre best operates. Typical windows are given below: E1-E2 CFA Version 3.0 April 2021 Page 196 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication Windows Operational Wavelength 800nm - 900nm 850nm (Ist Window) 1250nm - 1350nm 1310nm (2nd Window) 1500nm - 1600nm 1550nm (3rd Window) 15.10.3 Attenuation Attenuation in optical fiber is caused by intrinsic factors, primarily scattering and absorption, and by extrinsic factors, including stress from the manufacturing process, the environment, and physical bending. The primary factors affecting attenuation in optical fibers are the length of the fiber and the wavelength of the light. Figure shows the loss in decibels per kilometer (dB/km) by wavelength from Rayleigh scattering, intrinsic absorption, and total attenuation. Figure 92: Attenuation Vs. Wavelength characteristic 15.10.4 Dispersion Dispersion is the spreading of light pulse as its travels down the length of an optical fibre as shown in figure The varying delay in arrival time between different components of a signal "smears out" the signal in time. This causes energy overlapping E1-E2 CFA Version 3.0 April 2021 Page 197 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication and limits information capacity of the fiber. Dispersion limits the bandwidth or information carrying capacity of a fibre. The bit-rates must be low enough to ensure that pulses are farther apart and therefore the greater dispersion can be tolerated. Figure 93: Dispersion 15.11 CONCLUSION Fiber optic technology is a revolutionary technological departure from the traditional copper wires twisted-pair cable or coaxial cable. The usage of optical fiber in the telecommunications industry has grown a few decades ago. Today, many industries particularly telecommunications industry chooses optical fiber over copper wire because of its ability to transmit large amount of information at a time. E1-E2 CFA Version 3.0 April 2021 Page 198 of 271 For Restricted Circulation E1-E2 CFA Overview of Optical Communication An optical fiber is a flexible filament of very clear glass capable of carrying information in the form of light. Optical fibers are hair-thin structures created by forming pre-forms, which are glass rods drawn into fine threads of glass protected by a plastic coating. E1-E2 CFA Version 3.0 April 2021 Page 199 of 271 For Restricted Circulation