Core Network Business_Tx PDF

TX Module Concept of Optical Fiber Communication Fundamental of Transmission INDEX NO. TOPIC PAGES 1. Concept of Optical Fiber Communication 2 2. Optical Fiber Splicing 21 3. 38 5. Fault Localization in OF Network using OTDR, Power Meter OFC laying and Installation practices (Sec-I) OFC laying and I...

TX Module Concept of Optical Fiber Communication Fundamental of Transmission INDEX NO. TOPIC PAGES 1. Concept of Optical Fiber Communication 2 2. Optical Fiber Splicing 21 3. 38 5. Fault Localization in OF Network using OTDR, Power Meter OFC laying and Installation practices (Sec-I) OFC laying and Installation practices (Sec-II) FTTH Technology 6. SDH and NGSDH 103 7. DWDM Technology-an overview 121 8. CPAN and OTN Technology overview 130 9. Terrestrial Radio Links 147 10. MLLN 161 11. Transmission media equipment planning and Installation 170 12. FSO & Li-Fi Communication 193 4. SDE to AGM (LICE) 56 92 Page 1 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication 1 CONCEPT OF OPTICAL FIBER COMMUNICATION 1.1 Objective After reading this unit, you should be able to understand:  Fiber-Optic Applications  Basic optical fiber communication system:  The Structure of an Optical Fiber  Principle of Operation – Theory 1.2 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 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 SDE to AGM (LICE) Page 2 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication 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 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. 1.3 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 SDE to AGM (LICE) Page 3 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication 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. 1.4  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…) 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 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 SDE to AGM (LICE) Page 4 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication 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. 1.5 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. Fig : 1 Fiber Optic System 1.5.1 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 SDE to AGM (LICE) Page 5 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication 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. 3a). 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, n1sin1= n2sin2 In addition to the refracted ray there is a small amount of reflected light in the medium with refractive index n1. Because n1 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 Figure 2. This angle is referred to as the critical angle, crit, and from Snell‘s law is given by Sincrit = n2/n1 Fig : 2 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. 1.5.2 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. 4. 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, SDE to AGM (LICE) Page 6 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication 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. Jacket Jacket Cladding Core Cladding Cladding (n2) Jacket Core (n2) Light at less than Angle of Angle of critical angle is incidence reflection absorbed in jacket Light is propagated by total internal reflection Fig. Total Internal Reflection in an optical Fibre Fig : 3 Propagation of light through fiber 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 1.5.3 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 Fig. 2. 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 SDE to AGM (LICE) Page 7 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication pulses with a laser or a light-emitting diode moves along the core without penetrating the cladding. Fig : 4 (a) Cross section and (b) longitudinal cross section of a typical optical fiber 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 Core 125 62.5 125 100 Cladding Core and Cladding Diameters FigTypical : 5 Typical Core and Cladding Diameter 1.6 Characteristics of Optical Fiber Like any communication system there are some important factors affecting SDE to AGM (LICE) Page 8 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication performance of optical fibers as a transmission medium. 1.6.1 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. BANDS IN OPTICAL FIBER 1.6.2 Windows in Fiber Optic A narrow window is defined as the range of wavelengths at which a fibre best operates. Typical windows are given below: Window Operational Wavelength 800nm - 900nm 850nm 1250nm - 1350nm 1300nm 1500nm - 1600nm 1550nm SDE to AGM (LICE) Page 9 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication Fig : 6 Operating Bands 1.6.3 Loss characteristics 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. 1. INTRINSIC ATTENUATION It is loss due to inherent or within the fiber. Intrinsic attenuation may occur as (I) Absorption - Natural Impurities in the glass absorb light energy. (II) Scattering - Light rays travelling in the core reflect from small imperfections into a new pathway that may be lost through the cladding. The most common form of scattering, Rayleigh scattering, is caused by small variations in the density of glass as it cools. These variations are smaller than the wavelengths used and therefore act as scattering objects (see Figure 2). Fig : 7 Rayleigh scattering Scattering affects short wavelengths more than long wavelengths and limits the use of wavelengths below 800 nm. Attenuation due to absorption is caused by the intrinsic properties of the material itself, the impurities in the glass, and any atomic defects in the glass. These impurities absorb the optical energy, causing the light to become dimmer. While Rayleigh scattering SDE to AGM (LICE) Page 10 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication is important at shorter wavelengths, intrinsic absorption is an issue at longer wavelengths and increases dramatically above 1700 nm. However, absorption due to water peaks introduced in the fiber manufacturing process are being eliminated in some new fiber types. Fig : 8 Absorption 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. Fig : 9 Attenuation Vs. Wavelength characteristic 2. EXTRINSIC ATTENUATION It is loss due to external sources. Extrinsic attenuation may occur as – (I) Macro bending - The fibre is sharply bent so that the light travelling down the fibre cannot make the turn & is lost in the cladding. (II) Micro bending - Microbending or small bends in the fibre caused by crushing contraction etc. These bends may not be visible with the naked eye. Attenuation is measured in decibels (dB). A dB represents the comparison between the transmitted and received power in a system. SDE to AGM (LICE) Page 11 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication Micro bend Micro bend Fig : 10 Micro bends Micro bend Fig. Loss and Bends Fig : 11 Macro bend 1.6.4 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 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. Dispersion of optical energy within an optical fiber falls into following categories:  Intermodal Delay or Modal Delay  Intramodal Dispersion or Chromatic Dispersion  Material Dispersion  Waveguide Dispersion SDE to AGM (LICE) Page 12 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication  Polarization –Mode Dispersion Fig : 12 Dispersion 1.6.5 BANDWIDTH-LENGTH PRODUCT Bandwidth is a length dependent. Longer fibre results in more pulse spreading and leads to lower BW. As a result, the fibre BW is often given in terms of the BW times kilometer product. A 1000 MHz x km fibre can usually operate with 100 MHz BW if a 10 km fibre is used or with a 1000 MHz BW if a 1 km fibre is used. ELECTRICAL AND OPTICAL BANDWIDTH A distinction must be made between electrical and optical BW. Electrical bandwidth (BWel) is defined drops to 0.707. The optical bandwidth (BWopt) is defined as the frequency at which the ratio, PLo/PLi dropped to 1/2. (The ratio Iout/Iin and PLo/PLi have maximum values of 1). Because PLi and PLo are directly proportional to Iin and Iout respectively (and not to Iin2 and Iout2 as in an all electrical system), the half power point is equivalent to the half current point. That is the point where Iout/Iin drops to 0.50, not to 0.707. This results in a BWopt that is larger than the BWel. BWel = 0.707 x BWopt It is important to realize that these two parameters represent two ways of describing the same system. For example, a system can be said to have an optical BW of 10 MHz, which implies that its electrical BW is 7.07 MHz. SDE to AGM (LICE) Page 13 of 194 For Restricted Circulation TX Module Fig : 13 1.7 Concept of Optical Fiber Communication ELECTRICAL AND OPTICAL BANDWIDTH Fiber Types 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. 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. 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) 1.7.1 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 SDE to AGM (LICE) Page 14 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication instance. Fig : 14 STEP-INDEX MULTIMODE FIBER 1.7.2 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. Fig : 15 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. 1.7.3 SINGLE-MODE FIBER 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. Fig : 16 1.8 SINGLE-MODE FIBER 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 tightSDE to AGM (LICE) Page 15 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication buffered cable, primarily used inside buildings. 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. Singlefiber 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. Fig : 17 Tight Buffer Tube Cable The structure of a 250um coated fiber (bare fiber) 2.  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) 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 SDE to AGM (LICE) Page 16 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication (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. Recommended for use between buildings that are unprotected from outside elements. Loose tube cable is restricted from inside building use. Fig : 18 Loose Tube Cable 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.) 1.9 ITU-T COMPLIANT FIBERS SDE to AGM (LICE) Page 17 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication 1.9.1 ITU-T G.651 compliant Multimode fibers and OM1 Multimode fibers can be classified further into two as Multimode 50/125 and Multimode 62.5/125. The classification is based on the core diameter of multimode fibers. 50/125 have a core diameter of 50 micrometers, whereas for 62.5/125 have a core diameter of 62.5 micrometers. Recent classification of multimode fibers divides them as OM1, OM2, OM3 etc. OM1 multimode fibers are 62.5/125 multimode fibers. OM2 and OM3 fibers are compliant with ITU-T G.651 recommendations. G.651 multimode fibers are used mainly in Local Area Networks (LAN). Multimode fibers are not suitable for Long haul applications. Cheaper transmission devices like lasers etc. make Multimode fibers attractive for short distance transmission within the 300 to 500 meters reach. For a 10GBASE-SR system demanding 2000 MHz*km, OM2 multimode fiber can be used for a distance of up to 82 meters and OM3 fibers can be used for 300 meters. An OM2 fiber having a bandwidth of 500 MHz*km can be used for 550 meters on a 10BASE-SX/LX networks. ITU-T does not have any specification for 62.5/125 multimode fibers. OM1 Fibers also known as 62.5/125 Multimode fibers are popular in United States. OM2 and OM3 multimode fibers are also known as ITU-T G.651 fibers. The core of MMF 50/125 has a graded index refractive index profile, which is gradually changing from the center of the core to the cladding that enables multiple modes with near equal velocity to travel inside the fiber. 1.9.2 ITU-T G.652 Compliant Single Mode fibers This is the most common single mode fiber in the world. It is designed to have minimum dispersion at around 1310nm, which is supposed to be transmission window for single mode fibers. Conventional single mode fibers can be used at 1550nm with the use of dispersion compensation modules. G.652A is the first single mode fibers ITU-T classified. G.652B fibers are also known as conventional type single mode fibers and many installers intend to use 652B fiber by mentioning simply G.652. The major difference is in attenuation at both 1310nm and 1550nm and polarization mode dispersion. 652B fibers have a PMD as low as 0.2 ps/sqrt.km where as for 652A fibers have a PMD of 0.5 ps/sqrt.km. Attenuation is low for G.652B fibers. Similarly G.652C and G.652D fibers differ in PMD value. PMD for G.652C fiber is 0.5 ps/sqrt.km, where as for G.652D fibers have a PMD of less than or equal to 0.2 ps/sqrt.km. Both these optical fibers are known as low water peak fiber having low attenuation at 1360nm through 1480nm, the wavelength range which is not yet used commonly for transmission. 1.9.3 ITU-T G.653 Compliant Dispersion shifted fiber SDE to AGM (LICE) Page 18 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication These fibers are designed to utilize the low attenuation window of 1550nm by minimizing the dispersion value at around 1550nm. The purpose was good, but it generated Non-linear effects in the transmission which caused more troubles. 1.9.4 ITU-T G.654 Compliant Cut-off Shifted fiber This fiber is also known as low attenuation fiber. Some manufacturers have extremely low attenuation at 1550nm for this fiber. The application area demands low attenuation like those in Submarine optical fiber cables and terrestrial ultra long haul optical networks. Low attenuation at 1550nm range makes this fiber suitable for 400km span without repeaters. The low attenuation ranges from 0.15 – 0.16 dB/km. 1.9.5 ITU-T G.655 Compliant Non-zero dispersion shifted fiber NZDSF was introduced in the mid 1990s for WDM applications. NZDSF is the short of Non-zero dispersion shifted fiber. These are wide band transmission The nonlinear effects are successfully solved in G.655 fibers. The non-linear effects due to zero dispersion at 1550nm in G.653 fibers are solved by G.655 fibers which are having a non-zero value for dispersion at this wavelength range. ITU-T specifies up to G.655E fibers (latest) from G.655A fibers which are not currently in use. G.655 fibers are most suitable for DWDM applications. It has a positive nonzero dispersion value over the entire C-band, which is the spectral operating region for eribium doped optical fiber amplifiers. Version G.655b was introduced to extend WDM application into the S-band. Version G.655c specifies a lower PMD value of 0.2 ps√km than the 0.5 ps/√km value of G.655a/b 1.9.6 ITU-T G.656 Compliant Low Slope Dispersion Non-zero Dispersion shifted fiber This is another type non-zero dispersion shifted fiber which has more stricter and low dispersion slope which enables to guarantee the DWDM performance in wide wavelength range. It has a positive chromatic dispersion value ranging from 2 to 14 ps/(nm-km) in the 1460 to 1625 nm wavelength band. Here dispersion slop is significantly lower than in G.655 fibers It means that the chromatic dispersion changes slower with the wavelength so that dispersion compensation is simpler or not needed. This allows the use of CWDM without chromatic dispersion compensation. 1.9.7 G. ITU-T G.657 Compliant Bend Insensitive fiber G.657 fibers are the new comers in the market, but became a super hit in the FTTH market. More and more installers are looking for G.657 fibers. As the name indicates, the bend insensitive fibers are suitable for applications where multiple bends will be present. Insensitivity to bends makes them suitable for installation at home and office SDE to AGM (LICE) Page 19 of 194 For Restricted Circulation TX Module Concept of Optical Fiber Communication environment. G.657A is intended to compatible with G.652 D fibers. Interconnectivity with the existing G.652 fibers are guaranteed for the G.657 A fibers. ITU-T G.657B fibers are free from all backward compatibility requirements and do not require complying with conventional single mode fibers. The difference between 657A and B fibers is in the bending radius. G.657B can be bend at 7.5mm radius and less for some manufacturers. Single mode optical fibers complying with ITU-T G.657A was developed with the purpose of using at FTTH sites. G.657A category fibers are therefore compliant with G.652 category fibers also. This back compatibility makes the G.657A category fibers suitable for access networks used for FTTH. The other category, G.657.B does not need to be compliant with G.652 fibers. Therefore G.657.B category fibers are mostly used in indoor fiber optic cables that are installed with field installable optical connectors. 1.10 ADSS (All-Dielectric Self Supporting cables) All Dielectric Self-Supporting cable or more commonly referred to as ADSS cable is a type of fiber optic cable that is used in aerial applications. This type of cable does not need a messenger to support it, so it can be installed in a single pass. This cable construction does not contain any metallic elements so it is also non-conductive. The cable inside multi-loose tube filled with a water-resistant filling compound or design for wate rblocked with water blocking material in side cable. The cable high tensile by aramid yarns and FRP(fibre-reinforced polymer) strength member rod inside. Fig : 19 Cross-sectional view of ADSS All-dielectric, self-supporting aerial cables(ADSS) are designed to support themselves when strung between structures such as utility poles. All dielectric cables are suitable for applications which are adjacent to aerial power transmission standards lines. 1.11 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. Optical fiber classification depends on more than the number of modes that a fiber can propagate. The optical fiber's refractive index profile and core size further distinguish different types of single mode and multimode fibers. SDE to AGM (LICE) Page 20 of 194 For Restricted Circulation TX Module SPLICING OF OPTICAL FIBER 2 SPLICING OF OPTICAL FIBER 2.1 OBJECTIVE After reading this unit, you should be able to: 1. Understand the Different types of splicing methods 2. Preparing cable for splicing 3. Carrying out splicing operations 2.2 INTRODUCTION Splicing often is required to create a continuous optical path for transmission of optical pulses from one fiber length to another. The three basic fiber interconnection methods are: de-matable fiber-optic connectors, mechanical splices and fusion splices. De-matable connectors are used in applications where periodic mating and de-mating is required for maintenance, testing, repairs or reconfiguration of a system. The penalty for this flexibility is the larger physical size and higher cost, as well as higher losses of optical power (typically 0.2 to 1 dB) at the connector interface. Mechanical splices are available for both multimode and single-mode fiber types and can be either temporary or permanent. Typical mechanical splices for multimode fiber are easy to install and require few specialized installation tools. Insertion loss, defined as the loss in optical power at a joint between identical fibers, typically is 0.2 dB for mechanical multimode splices. Since single-mode fibers have small optical cores and hence small mode-field diameters (MFD), they are less tolerant of misalignment at a joint. Consequently, mechanical splices capable of achieving acceptable performance within a single-mode system loss budget are somewhat more expensive to purchase, more time consuming to install, and may require capital equipment outlays on par with fusion splicing. Typical insertion losses for single-mode mechanical splices range from 0.05 to 0.2 dB. 2.3 SPLICING Splices are permanent connection between two fibers. The splicing involves cutting of the edges of the two fibers to be spliced. This cut has to be carefully made to have a smooth surface and is generally achieved by a special cutting tool. The two ends, thus, prepared are then brought together and made to butt against each other. The fibers are then fixed permanently and reinforced. The fixing process can be achieved in a number of ways. It could be mechanically fixed permanently through uses of epoxies or through fusion. Mechanical splicing doesn‘t physically fuse two optical fibers together, rather two fibers are held butt-to-butt inside a sleeve with some mechanical mechanism. You will get worse insertion loss and back reflection in mechanical splices than in fusion splices (the second type we are introducing below).Mechanical splicing is mostly used for emergency SDE to AGM (LICE) Page 21 of 194 For Restricted Circulation TX Module SPLICING OF OPTICAL FIBER repairs and fiber testing. You can check out some mechanical splice products here. Fig : 20 Mechanical splice The second type splicing is called fusion splicing. In fusion splicing, two fibers are literally welded (fused) together by an electric arc. Fusion splicing is the most widely used method of splicing as it provides for the lowest insertion loss and virtually no back reflection. Fusion splicing provides the most reliable joint between two fibers. Fusion splicing is done by an automatic machine called fusion splicer (fusion splicing machines). Fig : 21 Fusion Splice SPLICE LOSSES Splice losses can be divided into two categories as shown in Table.  Extrinsic and  intrinsic splice loss factors SDE to AGM (LICE) Page 22 of 194 For Restricted Circulation TX Module SPLICING OF OPTICAL FIBER Table: Extrinsic and intrinsic splice loss factors Extrinsic, or splice process-related, factors are those induced by the splicing methods and procedures. Splice process factors include lateral and angular misalignment (separation and transverse offset between the fiber cores, axial tilt), fiber end quality, contamination and core deformation. They can be controlled or minimized by the skill of the individual doing the splicing, and by the automated fiber alignment and fusing cycles on newer equipment. Additional splice process factors exist for mechanical (butt-spliced) joints, including fiber-end separation, fiber-end angle and fresnel reflection. Fig : 22 Fiber misalignment The second category of losses is related to the properties of the fibers spliced and is referred to as intrinsic splice loss. Intrinsic parameters include variations in fiber diameter (both core and cladding), index profile, Numerical aperture, Mode Field Diameter (MFD) and non-circularity of the fiber cores. SDE to AGM (LICE) Page 23 of 194 For Restricted Circulation TX Module Fig : 23 SPLICING OF OPTICAL FIBER Intrinsic splice loss due to core diameter and NA mismatch For single-mode dispersion non-shifted fibers, the dominant fiber-related factor is MFD mismatch. The intrinsic loss contribution due to MFD mismatch may be estimated from Figure. Fig : 24 Single-mode intrinsic splice loss due to MFD mismatch As shown in Figure, the actual splice loss (bi-directional average) is practically non-directional, (e.g., similar fiber-related loss will be seen across the joint regardless of the direction of optical propagation). Also, the intrinsic loss is relatively low for MFD mismatches expected within typical manufacturer‘s tolerances. For example, the worstcase, fiber-related bi-directional loss for fibers having a 9.3 ± 0.5 micron MFD specification would be approximately 0.04 dB. SDE to AGM (LICE) Page 24 of 194 For Restricted Circulation TX Module 2.4 SPLICING OF OPTICAL FIBER SPLICING METHODS The following three types are widely used: 1. 1. Adhesive bonding or Glue splicing. 2. Mechanical splicing. 3. Fusion splicing. Adhesive Bonding or Glue Splicing This is the oldest splicing technique used in fiber splicing. After fiber end preparation, it is axially aligned in a precision V–groove. Cylindrical rods or other kind of reference surfaces are used for alignment. During the alignment of fiber end, a small amount of adhesive or glue of same refractive index as the core material is set between and around the fiber ends. A two component epoxy or an UV curable adhesive is used as the bonding agent. The splice loss of this type of joint is same or less than fusion splices. But fusion splicing technique is more reliable, so at present this technique is very rarely used. 2. Mechanical Splicing This splicing is mainly used for temporary splicing in case of emergency repairing. This method is also convenient to connect measuring instruments to bare fibers for taking various measurements. The mechanical splices consists of 4 basic components : (i) An alignment surface for mating fiber ends. (ii) A retainer. (iii) An index matching material. (i) A protective housing. A very good mechanical splice for M.M. fibers can have an optical performance good as fusion spliced fiber or glue spliced. But in case of single mode fiber, this type of splice cannot have stability of loss. 3. Fusion Splicing The fusion splicing technique is the most popular technique used for achieving low splice losses. The fusion can be achieved either through electrical arc or through gas flame. The process involves cutting of the fibers and fixing them in micro–positioners the fusion splicing machine. The fibers are then aligned either manually or automatically core aligning (in case of S.M. fiber) process. Afterwards, the operation that takes place involve withdrawal of the fibers to a specified distance, preheating the fiber ends through electric arc and bringing together of the fiber ends in a position and splicing through high temperature fusion. If proper care is taken and splicing is done strictly as per schedule, SDE to AGM (LICE) Page 25 of 194 For Restricted Circulation TX Module SPLICING OF OPTICAL FIBER then the splicing loss can be minimized as low as 0.01 dB/joint. After fusion splicing, the splicing joint should be provided with a proper protector to have following protections: (a) Mechanical protection (b) Protection from moisture. Sometimes the two types of protection are combined. Coating with Epoxy resins protects against moisture and also provides mechanical strength at the joint. Now–a–days the heat shrinkable tubes are most widely used, which are fixed on the joints by the fusion tools. 2.5 PRINCIPLE OF FUSION SPLICING TECHNIQUE: It is most widely used method for splicing optical fiber. There are a number of fusion welding machines manufactured by different companies, some of them are fully automatic and controlled by a microprocessor and some are partly automatic and manually controlled. In some cases, the fiber ends & the fusion process can be seen on a TV-monitor screen. The process can be sub-divided into the following three steps : (a) Axial alignment. (b) Perfusion & (c) Actual fusion welding. In case of the old machines the axial alignment is done manually by manipulating a number of knobs and is observed with the help of a high power microscope. This is normally followed in case of multimode fiber. In case of modern machines, prealigned, V-grooves are provided a finer adjustment is done, if necessary. For single mode fiber, other techniques are followed. The best one is fully automatic core alignment method which is now days used. After alignment is done, the ends of the fibers are fire polished by an electric arc and this method is called pre-fusion. During this process, the fiber ends are kept separated at a distance, after this they are brought closer and the process is called as fiber end feedings. This feeding process is continued during actual fusion by electric arc to prevent a reduced section at the point of welding. The process of perfusion, fiber ends feed and actual fusion is critical to a good weld and is frequently automatically controlled by the fusion machine. The fusion time of single mode fiber is less than that the multimode fiber. The Introduction of single mode fiber for use in long haul network, brought with it different fiber construction and cable design, from that of multimode cables. The design of the cable, the brittleness of the fibers and the requirement of accurately aligning the single mode fiber cores, required splicing techniques different to those used for multimode fibers, where aligning of the cladding is done. Due to this sophisticated splicing machines were developed. SDE to AGM (LICE) Page 26 of 194 For Restricted Circulation TX Module SPLICING OF OPTICAL FIBER The main functions of the above are: (1) Auto active alignment of the core. (2) Auto arc fusion. (3) Video display of the entire process. (4) Indication of the estimated loss at the slice. In this core profile alignment system (CPA), the two fibers ends to be spliced are cleaved and then clamped in accurately machined V- grooves. A video image proceeding technique is used to detect the boundary between the core and cladding glasses in the fibers on each side of the splice point. The core boundaries in the fibers and aligned in the horizontal and vertical plane by microprocessor controlled micropositioners. When the optimum alignment is achieved, the fibers are automatically fused under the microprocessor control. The machine then measures the radial and angular offsets of the fibers and uses these figures to calculate a splice loss estimated, which is used only as a guidance. The operator of the machine observes the alignment and fusion processes on a video screen showing horizontal and vertical projection of the fibers and then decides the quality of the splice. The manual part of the splicing is cleaning and cleaving the fibers. For cleaning the fibers the following material are used. (i) Hexane - jelly cleaning. (ii) Di-chlorine methyl of Acetone or Alcohol - to remove primary coating. (iii) Freon gas - to clean the bits of scrapper or stripper. With the special fiber cleaver or cutter, the cleaned fiber is cut. The cut has to be so precise that it produces an end angle of less than 0.5 deg on a prepared fiber. If the cut is bad, the splicing losses will increase. The shape of the cut can be monitored on the video screen. Some of the defects noted while cleaving are as below: (1) Broken ends. (2) Ripped ends. (3) Slanting cuts. It is desirable to limit the average splice loss to less than 0.1 dB. The completed splice should be inspected & if not satisfactory, redone. The Splice loss indicated by the splicing machine should not be taken as the final value, as it is only an estimated loss and so after splicing is over the splice loss measurement is to be taken by an OTDR. This makes use of the relative level of back scattered light at 2 points one before and one after the splice point to determine the apparent splice loss. SDE to AGM (LICE) Page 27 of 194 For Restricted Circulation TX Module SPLICING OF OPTICAL FIBER -fusing the ends of two clean, cleaved fibers with an electric arc. Requirement of a good splice Following are the requirements of a good splice. (i) Low loss. (ii) Stability of loss (iii) Reliability. (iv) Ease of reopening. (v) Low cost. Considering all the above facts, suitable methods of splicing should be selected. The Following are the common steps in every splicing method. (A) Fiber and preparation: (I) Fiber stripping. (II) Fiber cleaning. (III) Fiber cutting. (B) Axial alignment. (C) Actual splicing of two fiber ends. Fig : 25 Fusion Splice SDE to AGM (LICE) Page 28 of 194 For Restricted Circulation TX Module 2.6 SPLICING OF OPTICAL FIBER MATERIALS REQUIRED CABLE END PREPARATION FOR SPLICING Cable end preparation and splicing must be performed by personnel trained and familiar with handling of optical fiber cable, components, and splicing accessories. Mishandling of fiber cable can cause damage to the fiber and result in cable length cuts or system degradation. 2.7  Tape measure  Marker (or equivalent) for marking cable jacket  Splicing Machine  Outer Jacket remover  Sheath remover  Diagonal cutters  Buffer remover  Fiber stripper  Gel cleaner  Sleeves etc. FIBER OPTIC CABLE SPLICING PROCEDURE Ensure all required materials are on hand. It is recommended that the processes of cable end preparation, fiber splicing, and splice closure assembly be performed from beginning to end with minimal interruption. If for any reason actions are interrupted, ensure fiber cable end and fibers are adequately protected. Determine end location of cable where the splice point is to be located. 1. Strip fiber cable jacket. Strip back about 1 meters of fiber cable jacket to expose the fiber loose tubes or tight buffered fibers. Use cable rip cord to cut through the fiber jacket. Then carefully peel back the jacket and expose the insides. Cut off the excess jacket. Clean off all cable gel with cable gel remover. Separate the fiber loose tubes and buffers by carefully cutting away any yarn or sheath. Leave enough of the strength member to properly secure the cable in the splice enclose. SDE to AGM (LICE) Page 29 of 194 For Restricted Circulation TX Module SPLICING OF OPTICAL FIBER Fig : 26 Strip fiber cable jacket 2. Strip fiber tubes. For a loose tube fiber cable, strip away about 0.9 meters of fiber tube using a buffer tube stripper and expose the individual fibers. 3. Clean cable gel. Carefully clean all fibers in the loose tube of any filling gel with cable gel remover. Fig : 27 Cleaning of gel 4. Preparation of Cable Joint Closure for Splicing The type of preparation work performed on the cable prior to splicing differs on the type of joint closure and fiber organizer used. However, the following steps shall be usually common for different types of joint closure. (a) The strength member of each cable shall be joined to each other and/or the central frame of the joint closure. (b) The joint closure shall be assembled around the cable. (c) The sealing compound or heat shrink sleeve shall be applied to the cables and closure, or prepared for application after splicing is complete. (d) Tags which identify the fibers number shall be attached at suitable location on the SDE to AGM (LICE) Page 30 of 194 For Restricted Circulation TX Module SPLICING OF OPTICAL FIBER fibers. (e) Splice protectors shall be slipped over each fiber in readiness for placing over the bare fiber after splicing. Follow the splice closure assembly instructions to build the closure unit, attach the cable ends, and fabricate the end seal around the cables to be spliced. Repeat the above steps for all cables that are planned to enter the closure so that closure end plate seal and fabrication is complete. Fig : 28 Splice closure preparation Strip first splicing fiber. Hold the first splicing fiber and remove the 250um fiber coating to expose 5cm of 125um bare fiber cladding with fiber coating stripper tool. For tight buffered fibers, remove 5cm of 900um tight buffer first with a buffer stripping tool, and then remove the 5cm of 250um coating. 5. Place the fusion splice protection sleeve. Put a fusion splice protection sleeve onto the fiber being spliced. 6. Clean the bare fiber. Carefully clean the stripped bare fiber with lint-free wipes soaked in isopropyl alcohol. After cleaning, prevent the fiber from touching anything. 7. Fiber cleaving. With a high precision fiber cleaver, cleave the fiber to a specified length according to your fusion splicer‘s manual. SDE to AGM (LICE) Page 31 of 194 For Restricted Circulation TX Module SPLICING OF OPTICAL FIBER Fig : 29 Fiber cleaving 8. Prepare second fiber being spliced. Strip, clean and cleave the other fiber to be spliced. 9. Fusion splicing. Place both fibers in the fusion splicer and do the fusion splice according to its manual. Fig : 30 Fusion splicing 10. Heat shrinks the fusion splice protection sleeve. Slide the fusion splice protection sleeve on the joint and put it into the heat shrink oven, and press the heat button. SDE to AGM (LICE) Page 32 of 194 For Restricted Circulation TX Module Fig : 31 SPLICING OF OPTICAL FIBER Heat shrinks the fusion splice protection sleeve. 11. Place splice into splice tray. Carefully place the finished splice into the splice tray and loop excess fiber around its guides. Ensure that the fiber‘s minimum bending radius is not compromised. Fig : 32 Splice trey SDE to AGM (LICE) Page 33 of 194 For Restricted Circulation TX Module SPLICING OF OPTICAL FIBER Fig : 33 Splice trey 12. Close the splice tray. After all fibers have been spliced, carefully close the splice tray and place it into the splice enclosure. 13. Mount the splice enclosure. Close and mount the splice enclosure if all splices meet the specifications. Fig : 34 Splice closure 14. Placing of completed joint in pitJoint shall be taken out from the vehicle and placed on the tarpaulin provided near the pit. The joint closure shall be fixed to the bracket on the pit wall and pit closed. Fig : 35 2.8  Placing of completed joint in pit WARNING Do not use a voltage other than the allowable power voltage indicated. Doing so may cause a fire or electric shock. SDE to AGM (LICE) Page 34 of 194 For Restricted Circulation TX Module SPLICING OF OPTICAL FIBER  To reduce the risk of fire, electric shock or malfunction, do not get liquid or metallic objects inside the splicer. Check for condensation before operating. If necessary, allow the condensation to evaporate before using the splicer.  Do not make mechanical or electrical modifications to the splicer, this may expose you to dangerous voltages or other hazards.  If liquid, a metallic object or other foreign substance gets inside the splicer, immediately turn off the power and disconnect the power source. Contact qualified service personnel.  This fusion splicer performs an arc discharge. Avoid the use of the splicer in a hazardous location in which flammable gas can generate or only electrical  Do not touch the electrodes. Doing so may cause personal injury or electric shock. If an abnormal condition such as unusual noise, smoke or unusual odor occurs, immediately turn off the power and disconnect the power source. Next, contact the maintenance service center.  Do not let water come in contact with the battery. Safety and protective devices to prevent danger are built in the battery, but if these devices are damaged, excessive current flow may cause abnormal chemical reaction in the battery fluid, heat generation, bursting and fire may result.  Do not use or leave the battery exposed to high temperature conditions, such as a fire.  Only use the specified battery charger. Not doing so can cause the battery to be overcharged or excessive current flow may cause abnormal chemical reaction in battery fluid, heat generation, bursting and fire could result.  Make sure the polarities are correctly connected. Reversed connections may cause abnormal chemical reaction in battery fluid, heat generation, bursting and fire could result.  Do not attach the battery to a power supply plug or directly to a car's cigarette lighter. Excessive current flow may cause heat generation.  Use the battery only for the application for which it was designed. Not doing so will result in a loss of performance and a shortened life expectancy. Also excessive current flow may cause loss of control during charging or discharging of the battery, heat generation, bursting and fire.  Do not disassemble or modify the battery. Safety and protective devices to prevent danger are built in the battery. If these devices are damaged, excessive current flow may cause loss of control during charging or discharging of the battery, heat generation, bursting and fire.  Do not place the battery close to heat sources or leave exposed directly to the sun for long periods of time. Safety and protective devices to prevent danger are built SDE to AGM (LICE) Page 35 of 194 For Restricted Circulation TX Module SPLICING OF OPTICAL FIBER in the battery. If these devices are damaged, excessive current flow may cause loss of control during charging or discharging of the battery, heat generation, bursting and fire. 2.9 CAUTION  Avoid places with too much dust or dirt. Dirt or dust that can accumulate in the fusion splicer causing short circuits or insufficient cooling, which may lead to splicer malfunction or deterioration, resulting in fire or electric shock.  Always use and store the splicer in the locations defined in this manual. Not doing so may cause splicer malfunction or deterioration, resulting in fire or electric shock.  To reduce the risk of electric shock, do not plug/unplug the power cord or remove the battery with wet hands.  Disconnect the power cord by grasping the plug, not the cord.  The battery's optimum charging temperature range is 0 to 45°C. Whenever possible, place the charger in a location that is within this temperature range. Do not charge the battery at extremely low temperature (below 0°C). Doing so may lead to deterioration in performance and battery leakage.  If you are not going to use the splicer for a while, remove the battery before storing it. Not doing so will shorten a battery life.  Only use 99% pure alcohol to clean the splicer. To prevent malfunction and damage, do not use any other kind of chemicals.  The heating plate of the heat shrink oven may be hot during and after heating. Do not touch it directly.  Do not operate the splicer in rain. Doing so may cause the battery or AC power supply to be short-circuited. 2.10 CONCLUSION The most important task in the design of fiber optic link is to determine the maximum range of the optical transmission path, being in fact the balance of optical power in the link. Balance of power is a comparison of the power at the input of the optical link with the losses in fiber optic cables and other path components. This help to find the optimal parameters of transmitting and receiving devices to ensure proper signal transmission. Fiber optic cable fusion splicing provides the lowest loss connection. High precision fusion splicers are generally bulky and expensive. With proper training, a fiber splicing technician can routinely achieve less than 0.1 dB insertion loss for single mode fiber. Splices are critical points in the optical fiber network, as they strongly affect not only the quality of the links, but also their lifetime. In fact, the splice shall ensure high SDE to AGM (LICE) Page 36 of 194 For Restricted Circulation TX Module SPLICING OF OPTICAL FIBER quality and stability of performance with time. High quality in splicing is usually defined as low splice loss and tensile strength near that of the fiber proof test level. Splices shall be stable over the design life of the system under its expected environmental conditions. At present, two technologies, fusion and mechanical, can be used for splicing glass optical fibers and the choice between them depends upon the expected functional performance and considerations of installation and maintenance. These splices are designed to provide permanent connections. The most basic fiber optic measurement is optical power from the end of a fiber. This measurement is the basis for loss measurements as well as the power from a source or presented at a receiver. Fiber optic power meter is a test instrument used for absolute optical fiber power measurement as well as fiber optic loss related measurement. SDE to AGM (LICE) Page 37 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER 3 FAULT LOCALIZATION IN OF NETWPRK USING OTDR, POWER METER 3.1 OBJECTIVE After reading this unit, you should be able to understand:  Working principle of OTDR  Operation of OTDR  Use of OTDR in fault localization  Power Meter 3.2 INTRODUCTION An optical time domain reflectometer (OTDR) is a fiber optic tester for the characterization of fiber and optical networks. The purpose of an OTDR is to detect, locate, and measure events at any location on the fiber link. One of the main benefits of an OTDR is that it operates as a one-dimensional radar system, allowing for complete fiber characterization from only one end of the fiber. An OTDR generates geographic information regarding localized loss and reflective events, providing technicians with a pictorial and permanent record of the fiber‘s characteristics, which may be used as the fiber‘s performance baseline. 3.3 OPTICAL TIME DOMAIN REFLECTOMETER An OTDR sends short pulses of light into a fiber. Light scattering occurs in the fiber due to discontinuities such as connectors, splices, bends, and faults. An OTDR then detects and analyzes the backscattered signals. The signal strength is measured for specific intervals of time and is used to characterize events. An OTDR uses the effects of Rayleigh scattering and Fresnel reflection to measure the fiber's condition, but the Fresnel reflection is tens of thousands of times greater in power level than the backscatter. Rayleigh scattering occurs when a pulse travels down the fiber and small variations in the material, such as variations and discontinuities in the index of refraction, cause light to be scattered in all directions. However, the phenomenon of small amounts of light being reflected directly back toward the transmitter is called backscattering. Fresnel reflections occur when the light traveling down the fiber encounters abrupt changes in material density that may occur at connections or breaks where an air gap exists. A very large quantity of light is reflected, as compared with the Rayleigh scattering. The strength of the reflection depends on the degree of change in the index of refraction. SDE to AGM (LICE) Page 38 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER The OTDR technique consists of sending impulses to the fiber and measuring the time delay and intensity of the backscattered signal. The backscatter effect occurs because of the same reasons that we have attenuation on optical fiber, scattering. What happens is that some of the light gets reflected back due to changes in the molecular density of the glass. Measuring this light is equivalent to measuring fiber attenuation. The structure of an OTDR is basically a light source to emit signal pulses and an optical receiver connected to a data processing unit. The emitted signal is sent directly into the fiber and the incoming reflection directed to the receiver by a beamsplitter. The light source is synchronized with the receiver so that time delay between outgoing and incoming signals can be measured. 3.3.1 A Use for Rayleigh Scatter As light travels along the fiber, a small proportion of it is lost by Rayleigh scattering. As the light is scattered in all directions, some of it just happens to return back along the fiber towards the light source. This returned light is called backscatter as shown below. Fig : 36 Rayleigh Scattering Principle The backscatter power is a fixed proportion of the incoming power and as the losses take their toll on the incoming power, the returned power also diminishes as shown in the following figure. Fig : 37 Backscatter Power SDE to AGM (LICE) Page 39 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER The OTDR can continuously measure the returned power level and hence deduce the losses encountered on the fiber. Any additional losses such as connectors and fusion splices have the effect of suddenly reducing the transmitted power on the fiber and hence causing a corresponding change in backscatter power. The position and degree of the losses can be ascertained. 3.3.2 Measuring Distances The OTDR uses a system rather like a radar set. It sends out a pulse of light and ‗listens‘ for echoes from the fiber. If it knows the speed of light and can measure the time taken for the light to travel along the fiber, it is an easy job to calculate the length of the fiber. Fig : 38 Measuring Distances 3.3.3 To Find the Speed of the Light Assuming the refractive index of the core is 1.5, the infrared light travels at a speed of This means that it will take This is a useful figure to remember, 5 nanoseconds per meter (5 nsm-1). If the OTDR measures a time delay of 1.4us, then the distance travelled by the light is The 280 meters is the total distance traveled by the light and is the ‗there and back‘ distance. The length of the fiber is therefore only 140m. This adjustment is performed automatically by the OTDR – it just displays the final result of 140m. SDE to AGM (LICE) Page 40 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER 3.4 WORKING PRINCIPLE OF OTDR Fig : 39 OTDR BLOCK DIAGRAM A. Timer The timer produces a voltage pulse which is used to start the timing process in the display at the same moment as the laser is activated. B. Pulsed Laser The laser is switched on for a brief moment. The ‗on‘ time being between 1ns and 10us. We will look at the significance of the choice of ‗on‘ time or pulsewidth a little bit later. The wavelength of the laser can be switched to suit the system to be investigated. C. Directive Coupler The directive coupler allows the laser light to pass straight through into the fiber under test. The backscatter from the whole length of the fiber approaches the directive coupler from the opposite direction. In this case the mirror surface reflects the light into the avalanche photodiode (APD). The light has now been converted into an electrical signal. D. Amplifying and Averaging The electrical signal from the APD is very weak and requires amplification before it can be displayed. The averaging feature is quite interesting and we will look at it separately towards the end of this tutorial. E. Display The amplified signals are passed on to the display. The display is either a CRT like an oscilloscope, or a LCD as in laptop computers. They display the returned signals on a simple XY plot with the range across the bottom and the power level in dB up the side. SDE to AGM (LICE) Page 41 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER The following figure shows a typical display. The current parameter settings are shown over the grid. They can be changed to suit the measurements being undertaken. The range scale displayed shows a 50km length of fiber. In this case it is from 0 to 50km but it could be any other 50km slice, for example, from 20km to 70km. It can also be expanded to give a detailed view of a shorter length of fiber such as 0-5m, or 25-30m. Fig : 40 An OTDR display-no signal The range can be read from the horizontal scale but for more precision, a variable range marker is used. This is a movable line which can be switched on and positioned anywhere on the trace. Its range is shown on the screen together with the power level of the received signal at that point. To find the length of the fiber, the marker is simply positioned at the end of the fiber and the distance is read off the screen. It is usual to provide up to five markers so that several points can be measured simultaneously. F. Data Handling An internal memory or floppy disk can store the data for later analysis. The output is also available via RS232 link for downloading to a computer. In addition, many OTDRs have an onboard printer to provide hard copies of the information on the screen. This provides useful ‗before and after‘ images for fault repair as well as a record of the initial installation. 3.4.1 A Simple Measurement If we were to connect a length of fiber, say 300m, to the OTDR the result would SDE to AGM (LICE) Page 42 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER look as shown in the following figure. Fig : 41 Measurement through OTDR Whenever the light passes through a cleaved end of a piece of fiber, a Fresnel reflection occurs. This is seen at the far end of the fiber and also at the launch connector. Indeed, it is quite usual to obtain a Fresnel reflection from the end of the fiber without actually cleaving it. Just breaking the fiber is usually enough. The Fresnel at the launch connector occurs at the front panel of the OTDR and, since the laser power is high at this point, the reflection is also high. The result of this is a relatively high pulse of energy passing through the receiver amplifier. The amplifier output voltage swings above and below the real level, in an effect called ringing. This is a normal amplifier response to a sudden change of input level. The receiver takes a few nanoseconds to recover from this sudden change of signal level. 3.4.2 Dead Zones The Fresnel reflection and the subsequent amplifier recovery time results in a short period during which the amplifier cannot respond to any further input signals. This period of time is called a dead zone. It occurs to some extent whenever a sudden change of signal amplitude occurs. The one at the start of the fiber where the signal is being launched is called the launch dead zone and others are called event dead zones or just dead zones. SDE to AGM (LICE) Page 43 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER Fig : 42 Dead Zone 3.4.2.1 Why is there a Dead Zone? An OTDR is designed to detect the backscattered level all along the fiber link by measuring backscattered signals, which are much smaller than the signal that was injected into the fiber. The photodiode, the component receiving the signal, is designed to receive a given level range. When a strong reflection occurs, the power received by the photodiode can be more than 4,000 times higher than the backscattered power, saturating the photodiode. The photodiode requires time to recover from its saturated condition. During this time, it will not detect the backscattered signal accurately. The length of fiber that is not fully characterized during this period (pulse width + recovery time) is termed the dead zone. Fig : 43 The OTDR dead zone 3.4.2.2 Overcoming the Launch Dead Zone As the launch dead zone occupies a distance of up to 20 meters or so, this means that, given the job of checking a 300m fiber, we may only be able to check 280m of it. The customer would not be delighted. To overcome this problem, we add our own patch cord at the beginning of the system. If we make this patch cord about 100m in length, we can guarantee that all launch dead zone problems have finished before the customers‘ fiber is reached. SDE to AGM (LICE) Page 44 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER Fig : 44 A patchcord overcomes dead Zone problems The patch cord is joined to the main system by a connector which will show up on the OTDR readout as a small Fresnel reflection and a power loss. The power loss is indicated by the sudden drop in the power level on the OTDR trace. 3.4.3 Length and Attenuation The end of the fiber appears to be at 400m on the horizontal scale but we must deduct 100m to account for our patch cord. This gives an actual length of 300m for the fiber being tested. Immediately after the patch cord Fresnel reflection the power level shown on the vertical scale is about –10.8dB and at the end of the 300m run, the power has fallen to about –11.3 dB. A reduction in power level of 0.5 dB in 300 meters indicates a fiber attenuation of: Most OTDRs provide a loss measuring system using two markers. The two makers are switched on and positioned on a length of fiber which does not include any other events like connectors or whatever as shown in the following figure. SDE to AGM (LICE) Page 45 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER Fig : 45 Using two markers for loss measurement The OTDR then reads the difference in power level at the two positions and the distance between them, performs the above calculation for us and displays the loss per kilometer for the fiber. This provides a more accurate result than trying to read off the decibel and range values from the scales on the display and having to do our own calculations. 3.5 AN OTDR DISPLAY OF A TYPICAL SYSTEM The OTDR can ‗see‘ Fresnel reflections and losses. With this information, we can deduce the appearance of various events on an OTDR trace as seen on next page. A. Connectors A pair of connectors will give rise to a power loss and also a Fresnel reflection due to the polished end of the fiber. B. Fusion Splice Fusion splices do not cause any Fresnel reflections as the cleaved ends of the fiber are now fused into a single piece of fiber. They do, however, show a loss of power. A good quality fusion splice will actually be difficult to spot owing to the low losses. Any signs of a Fresnel reflection is a sure sign of a very poor fusion splice. C. Mechanical Splice Mechanical splices appear similar to a poor quality fusion splice. The fibers do have cleaved ends of course but the Fresnel reflection is avoided by the use of index marching SDE to AGM (LICE) Page 46 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER gel within the splice. The losses to be expected are similar to the least acceptable fusion splices. Fig : 46 A typical OTDR trace D. Bend Loss This is simply a loss of power in the area of the bend. If the loss is very localized, the result is indistinguishable from a fusion or mechanic splice. 3.6 EVENT INTERPRETATION IN OTDR In general, two types of events occur: reflective and non-reflective. 3.6.1 Reflective Events Reflective events occur where discontinuity arises in the fiber, causing an abrupt change in the refractive index. Reflective events can occur at breaks, connector junctions, mechanical splices, or the indeterminate end of fiber. For reflective events, connector loss is typically around 0.5 dB. For mechanical splices, though, the loss typically ranges from 0.1 to 0.2 dB. SDE to AGM (LICE) Page 47 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER Fig : 47 A reflective event at a connector junction If two reflective events occur very close together, the OTDR may have problems measuring the loss of each event. In this case, it displays the loss of the combined events, which typically occurs when measuring a short fiber length, such as a fiber jumper. Fig : 48 A reflective event at two connector junctions located close together In the case of a fiber end, the reflective event will fall into the noise and prevent taking the attenuation measurement. SDE to AGM (LICE) Page 48 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER Fig : 49 A reflective event at a fiber end Fiber ends can also cause a non-reflective event. In this case, no reflectance is detected. 3.6.2 Non-reflective Events Non-reflective events occur where discontinuities are absent in the fiber and are generally produced by fusion splices or bending losses, such as macro bends. Typical loss values range from 0.02 to 0.1 dB, depending on the splicing equipment and operator. Fig : 50 A non-reflective event For non-reflective events, the event loss can appear as an event gain, displaying a step-up on the OTDR trace. 3.7 OTDR MEASUREMENTS SDE to AGM (LICE) Page 49 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER An OTDR can perform the following measurements:  For each event: Distance location, loss, and reflectance  For each section of fiber: Section length, section loss (in dB), section loss rate (in dB/km), and optical return loss (ORL) of the section  For the complete terminated system: Link length, total link loss (in dB), and ORL of the link. 3.7.1 Measurement Methods The OTDR lets technicians perform measurements on the fiber span in several ways: full-automatic, semi-automatic, and manual measurement functions. Technicians can also use a combination of these methods. 3.7.1.1 Full-Automatic Function Using the full-automatic function, the OTDR detects and measures all of the events, sections, and fiber ends automatically, using an internal detection algorithm. 3.7.1.2 Semi-Automatic Function Selecting the semi-automatic function, the OTDR measures and reports an event at each location (distance) with a marker. Markers can be placed either automatically or manually. The semi-automatic function is of high interest during span acceptance (after splicing), when technicians completely characterize all events along the span in order to establish baseline data. Because automatic detection will not detect and report a nonreflective event with a zero loss, it places a marker at that location so that the semiautomatic analysis will report the zero loss. 3.7.1.3 Manual Measurement Function For even more detailed analysis or for special conditions, technicians completely control the measurement function manually. In this case, technicians place two or more cursors on the fiber in order to control the way the OTDR measures the event. Depending on the parameter being measured, technicians may need to position up to five cursors to perform a manual measurement. While this is the slowest and most cumbersome method of measurement, it is important to have this capability available for fiber spans with unusual designs and construction that are difficult to analyse accurately using automated algorithms. SLOPE Measure the slope (in dB/km), or fiber linear attenuation, using either the 2-point method or the least squares approximation (LSA) method. The LSA method attempts to determine the measurement line that has the closest fit to the set of acquisition points. The LSA method is the most precise way to measure fiber linear attenuation, but it requires a continuous section of fiber, a minimum number of OTDR acquisition points, and a relatively clean backscatter signal, which is free of noise. SDE to AGM (LICE) Page 50 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER 3.8 MEASUREMENT ARTIFACTS AND ANOMALIES IN OTDR Occasionally, the backscattered trace displays unexpected results and events. GHOSTS False Fresnel reflections, termed ghosts, on the trace waveform may be occasionally observed. Ghosts can result from a strong reflective event on the fiber, causing a large amount of reflected light to be sent back to the OTDR, or an incorrect range setting during acquisition. Fig : 51 Ghosts on an OTDR trace In both cases, the ghost can be identified because no loss is incurred as the signal passes through this event. In the first case, the distance at which the ghost occurs along the trace is a multiple of the distance of the strong reflective event from the OTDR. The use of index-matching gel at the reflection point can reduce the reflection. In addition, selecting a shorter pulse width, selecting a reduced power setting on the OTDR (some OTDRs provide this option), or adding attenuation in the fiber before the reflection can reduce the injected power. If the event causing the ghost is situated at the end of the fiber, a few short turns around a suitable tool, such as a pen, pencil, or mandrel, will sufficiently attenuate the amount of light being reflected back to the source and eliminate the ghost. This technique is known as a mandrel wrap. Be sure to select a mandrel of the appropriate diameter for the type of cable, jacketed fiber, or coated fiber used, eliminating permanent damage to the fiber span. Never bend a fiber or cable to introduce attenuation without using a suitable mandrel, which will prevent excess bending. SPLICE GAIN It is important to note that an OTDR measures splice loss indirectly, depending on information obtained from the backscattered signal. It is assumed that the backscattering coefficients of the fiber spans are identical all along the link under test. If this is not the case, then measurements can be inaccurate. One common example is the observance of SDE to AGM (LICE) Page 51 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER apparent splice gains or gainers. The inaccuracy is quite small, but with today‘s fusion splicing equipment and experienced technicians making very low loss splices, it is possible for the effect to make the splice appear to be a gain instead of a loss. If fibers of different mode field diameters, such as core size, are joined, the resulting OTDR trace waveform can show higher backscattering levels. This result is due to the increased level of backscattered signal reflected back to the OTDR in the downstream fiber. This phenomenon can occur when joining different types of fiber in a multimode span or joining two fibers with different backscatter coefficients. Fig : 52 3.9 A positive splice from A to B OPTICAL POWER METER The power meter is the standard tester in a typical fiber optic technician‘s tool kit. It is an invaluable tool during installation and restoration. The power meter‘s main function is to display the incident power on the photodiode. Transmitted and received optical power is only measured with an optical power meter. For transmitted power, the power meter is connected directly to the optical transmitter‘s output. For received power, the optical transmitter is connected to the fiber system. Then, the power level is read using the power meter at the point on the fiber cable where the optical receiver would be. The power meter, as it is commonly called, measures the optical power of light present on a fiber optic cable. An optical power meter (OPM) is a device used to measure the power in an optical signal. This light can be generated directly from the output of a fiber optic transmitter device or from another common fiber optic testing device: a laser light source. The optical power is measured in dBm or in mW. A typical optical power meter consists of a calibrated sensor, measuring amplifier and display. The sensor primarily consists of a photodiode selected for the appropriate range of wavelengths and power levels. On the display unit, the measured optical power and set wavelength is displayed. A traditional optical power meter responds to a broad spectrum of light; however SDE to AGM (LICE) Page 52 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER the calibration is wavelength dependent. This is not normally an issue, since the test wavelength is usually known, however it has a couple of drawbacks. Firstly, the user must set the meter to the correct test wavelength, and secondly if there are other spurious wavelengths present, then wrong readings will result. 3.9.1 DETECTOR SPECIFICATIONS Currently, power meter photodiodes use Silicon (for multimode applications), Germanium (for single-mode and multimode applications), and Indium Gallium Arsenide (InGaAs) (for single-mode and multimode applications) technologies. As shown in the following figure, InGaAs photodiodes are more adapted to the 1625 nm wavelength than Germanium (Ge) photodiodes, because Ge photodiodes are quite sensitive and drop off rapidly at the 1600 nm window. Fig : 53 Responsivity of the three typical detector types Features found on more sophisticated power meters may include temperature stabilization, the ability to calibrate to different wavelengths, the ability to display the power relative to ―reference‖ input, the ability to introduce attenuation, and a high power option. 3.9.2 DYNAMIC RANGE The requirements for a power meter vary depending on the application. Power meters must have enough power to measure the output of the transmitter (to verify operation). They must also be sensitive enough, though, to measure the received power at the far (receive) end of the link. Long-haul telephony systems and cable TV systems use transmitters with outputs as high as +16 dBm and amplifiers with outputs as high as +30 dBm. Receiver power levels can be as low as –36 dBm in systems that use an optical preamplifier. In local area networks (LANs), though, both receiver and transmitter power levels are much lower. The difference between the maximum input and the minimum sensitivity of the power meter is termed the dynamic range. While the dynamic range for a given meter has limits, the useful power range can be extended beyond the dynamic range by placing an attenuator in front of the power meter input. However, this limits the low-end SDE to AGM (LICE) Page 53 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER sensitivity of the power meter. For high power mode, use an internal or external attenuator. If using an internal attenuator, it can be either fixed or switched. Typical dynamic range requirements for power meters are as follows:  +20 to –70 dBm for standard power applications  +26 to –55 dBm for high power applications such as Analog RF transmission in cable TV (CATV) or video overlay in passive optical network (PON) systems.  –20 to –60 dBm for LAN applications Sometimes optical power meters are combined with a different test function such as an Optical Light Source (OLS) or Visual Fault Locator (VFL), or may be a sub-system is a much larger instrument. When combined with a light source, the instrument is usually called an Optical Loss Test Set. Optical Loss Test Sets (OLTS) are available in dedicated hand held instruments and platform-based modules to suit various network architectures and test requirements. They are used to measure optical power and power loss, and reflectance and reflected power loss. The products may also be used as optical sources or optical power meters, or to measure optical return loss or event reflectance. 3.9.3 THE PROCEDURE TO MEASURE THE OPTICAL LOSS IN THE FIBER OPTIC CABLE Set the power meter to the wavelength of the light source you are using  Connect a short fiber jumper cable between the light source and the power meter. See Figure 2.  Make note of the power level, in dBm. We will call this ―Reading A‖.  Connect the fiber cable under test to the output of the light source  Connect the power meter, set at the same wavelength as the power source, to the far end of the fiber cable under test. See Figure 3.  Make note of the power level, in dBm. We will call this ―Reading B‖.  The optical loss in the fiber cable is equal to ―Reading A‖ minus ―Reading B Optical Loss = “Reading A” – “Reading B” SDE to AGM (LICE) Page 54 of 194 For Restricted Circulation TX Module FAULT LOCALIZATION IN OF N/W USING OTDR, POWER METER Fig : 54 Connection of fiber jumper cable between the light source and the power meter Fig : 55 Figure When multimode fiber is used, measurements should be made at 850nm or 1310nm. It is preferred that measurements be made at both wavelengths, if possible, as the optical loss can vary significantly as the wavelength varies when multimode fiber is used. When single mode fiber is used, measurements should be made at 1310nm as this is the most common wavelength used with single mode fiber. These measurement procedures should be repeated for every fiber cable in the system. 3.10 CONCLUSION The OTDR is a more sophisticated measurement instrument. It uses a technology that injects a series of optical pulses into the fiber under test and analyses the light scattering and the light reflection. This allows the instrument to measure the intensity of the return pulse in functions of time and fiber length. The OTDR is used to measure the optical power loss and the fiber length, as well as to locate all faults resulting from fiber breaks, splices or connectors. OTDRs are also used for maintaining fiber plant performance. An OTDR allows you to see more details on cable installation, termination quality and provides advanced diagnostics to isolate a point of failure that may hinder network performance. An OTDR allows discovery of features along the length of a fiber that may affect fiber reliability. OTDRs characterize features such as attenuation uniformity and attenuation rate, segment length, location and insertion loss of connectors and splices, and other events such as sharp bends that may have been incurred during cable installation. SDE to AGM (LICE) Page 55 of 194 For Restricted Circulation TX Module OFC LAYING AND INSTALLATION PRACTICES 4 OFC LAYING AND INSTALLATION PRACTICES (SECTION -I) TRENCHING AND LAYING OF PLB DUCT 4.2 OBJECTIVE After reading this unit, you should be able to:  Know the soil classification  develop installation work plan and identify dependencies if any  determine the statutory permissions required and the relevant authorities involved  liaise with authorities and obtain relevant clearances/ municipal approvals  Understand the trenching  Understand the laying/construction practices  Back Filling and Dressing of the Trench 5.1 INTRODUCTION On the basis of the survey reports routes for OF cable laying shall be finalized. Road Cutting Permission shall be obtained from road and rail authorities for laying the Optical Fiber Cable along the finalized roads and at rail / road crossing along the route. Generally O.F. Cable may preferably be laid straight as far as possible along the road near the boundaries, away from the burrow pits. When the O.F. Cable is laid along the National Highways, Cable should run along the road land boundary or at a minimum distance of 15 meters from the center line of the road where the road land is wider. As the OFC carries high capacity traffic and is planned for about 25 to 30 years of life. It is essential that the cable is laid after obtaining due permission from all the concerned authorities to avoid any damage (which may result in disruption of services / revenue loss) and shifting in near future due to their planned road widening works. Trenching is the traditional cable laying method in which an above ground trench is excavated to produce an open cable laying environment. In areas where the aesthetic and technical above ground considerations are not of priority, such as open country areas, trenching is still an economical and timely method to lay cable. Horizontal directional drilling is the in-word in trenchless technology for installations below the ground for installing cables, ducts, pipes etc. Due to its ease of maneuverability the technology is considered best for working in city conditions minimizing utility damages. SDE to AGM (LICE) Page 56 of 194 For Restricted Circulation TX Module 4.3 OFC LAYING AND INSTALLATION PRACTICES EXCAVATION AND BACKFILLING FOR OPEN CUT TRENCHING The Contractor shall carry out excavation and backfilling of trenches in all kinds of soil strata for laying PLB HDPE pipe, RCC pipe and GI pipe. Soil shall be classified under two broad categories Rocky and Non Rocky, The soil is categorized as rocky if the cable trench cannot be dug without blasting and / of chiseling. All other types of soils shall be categorized as Non Rocky including Murrum & soil mixed with stone or soft rock. ROCKY SOIL. The terrain which consists of hard rocks or boulders where blasting/ chiseling is required for trenching such as quartzite, granite, basalt in hilly areas and RCC (reinforcement to be cut through but not separated) and the like. NON ROCKY SOILS This will include all types of soil- soft soil/hard soil/murrumie. any strata, such as sand, gravel, loam, clay, mud, black cotton murrum, shingle, river or nullah bed boulders, soling of roads, paths etc. (All such soils shall be sub-classified as kachcha soil) and hard core, macadam surface of any description (water bound, grouted tarmac etc.), CC roads and pavements, bituminous roads, bridges, culverts (All such soils shall be classified as Pucca soils) 4.4 GENERAL GUIDE LINES FOR TRENCHING AND PLB DUCT LAYING BEFORE START OF INSTALLATION Before the start of trenching following are to be ensured:  obtain OFC route plan from the planning team or the supervisors as per which OFC has to be laid  verify the proposed route to ensure that bend ratios meet manufacturer's specifications and industry standards  ensure that site is made safe and secure for cable installation in coordination with labour workers  develop installation work plan and identify dependencies if any  determine the statutory permissions required and the relevant authorities involved  liaise with authorities and obtain relevant clearances/ municipal approvals  ensure availability of all required trenching, cable laying, pipe laying, OFC laying and splicing equipments and spares for timely completion of installation activity and the availability of test equipments like OTDR and Power meter for carrying SDE to AGM (LICE) Page 57 of 194 For Restricted Circulation TX Module OFC LAYING AND INSTALLATION PRACTICES out optical tests DURING THE TRENCHING AND LAYING OF PLB DUCT  ensure cable drum is placed near site location and test cable on drum for optical continuity  ensure trenching is carried out by labour workers as per the route plan requirements and site terrain  ensure minimum radius is maintained, where bends are necessary  ensure use of specially designed dispensers to place the ducts in the trench asstraight as possible  ensure pipe/ ducts are placed at lower appropriate depths as per the laying standards after approval from competent personnel  ensure that ducts are free from twists, collapsed portions and that all such portions are rectified by using appropriate couplers  ensure proper uncoiling of PLB ducts 4.5 TYPES OF PIPE TO BE USED FOR OPTICAL FIBRE CABLE Optical Fibre Cables should be pulled through Permanently Lubricated PLBHDPE Duct of 40 mm-OD and 33 mm ID Pipe in 200/500/1000 Meter Coil.  Wherever DWC pipe or GI pipes or R.C.C. pipes are used for protection, the two ends of the pipe should be properly sealed to protect HDPE pipe from sharp edge of GI pipe and to bar the entry of rodents. For providing additional protection Split RCC/GI pipes should be used from top instead of full RCC/GI pipes.  Use of normal duty DWC (Double walled corrugated) HDPE pipe – ISI marked and anti-rodent conforming, choosing suitable DWC from nominal OD/ID dia 50/38,63/50,77/63,90/76,120/103,145/126,160/136,175/148 mm).  It is recommended that where ever OFC is passing over the ground surface (exposed outside) and more prone to damage, GI pipe may be used preferably. Depending upon the site conditions and cost consideration one of the protection viz DWC / GI / RCC pipe may be used. 4.6 STEPS INVOLVED IN OF CABLE TRENCHING AND LAYING OF PLB DUCT The Optical Fibre Cable shall be laid through PLB HDPE Ducts buried at a nominal depth of 165cms. All Depths should be measured from the top of pipe. The steps involved in OF Cable trenching are as under: SDE to AGM (LICE) Page 58 of 194 For Restricted Circulation TX Module  OFC LAYING AND INSTALLATION PRACTICES Excavation of trench up to a nominal depth of 165 cms.in non-Rocky soil, according to construction specifications. Along National/State Highways/other roads and in built up /rural areas. Under exceptional conditions/ genuine circumstances due to site constraints/ soil conditions, relaxation can be granted by the competent authority for excavation of trench to a depth lesser than 165cm. Such relaxation shall be given as per the laid down norms/ procedures being followed by the concerned CPSUs for their own works and with the approval of the competent authority. The payment in such cases shall be made on pro-rata basis as per the existing norms adopted by the concerned CPSUs. Fig. 1 shows the dimensional view of excavation of