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Over view of Transmission Technologies & Optical Fiber Communication


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Topics covered in this presentation:
GENERAL: History of Transmission Systems
Optical fiber communication,
History of OFC
ITU-T Recommendations
Fiber optic principle
Windows of operation
Trends in OF Communication
Fiber classification
OF Cable Types
Optical Fiber transmission impairments
Optical Sources and Detectors
Optical Link Characterization and Design

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Over view of Transmission Technologies & Optical Fiber Communication

  1. 1. Over view of Transmission Technologies & Optical Fiber Communication by Naveen Jakhar, ITS
  2. 2. 2 In this Presentation…  GENERAL: History of Transmission Systems  OFC • History of OFC • Advantages • Applications • ITU-T Recommendations • Fiber optic principle • Windows of operation • Trends in OF Communication • Fiber classification • OF Cable Types • Optical Fiber transmission impairments • Optical Sources and Detectors • Optical Link Characterization and Design September 12, 2016
  3. 3. 3 History of Transmission Systems The developments… • Open Wire Systems • Coaxial Cables • UHF Systems • Microwave Systems • Digital Transmission Systems • Satellite Communication Systems • Optical Fiber Cable September 12, 2016
  4. 4. OPEN WIRE SYSTEMS • Till 1950s, the long distance voice communication was almost entirely transported over Open Wire Carrier system. • The voice signals for these systems were multiplexed using FDM to a higher frequency carrier and carried through open wire systems. • These open wire systems were capable of carrying traffic of three to twelve subscribers at a time. 4September 12, 2016
  5. 5. COAXIAL CABLES • With the introduction of U/G symmetrical pair cable carrier system which was followed by the Coaxial Cable system, greatly enhanced, by the decade end, the simultaneous voice channel carrying capacity to 960 voice channels. • The first Coaxial Cable System was commissioned between Agra and Delhi in the year 1959. • Over the years, this system was improved and developed to carry 2700 simultaneous voice channels. 5September 12, 2016
  6. 6. UHF SYSTEMS • Close on the heel of coaxial systems, in the mid-60s wireless microwave systems were developed and inducted in the network. • The first Microwave system was installed in 1965 between Calcutta and Asansole. Microwave systems with 60, 300 and 1800 voice channels capacity were inducted into the telecom network subsequently. • These systems were mostly indigenous (developed and manufactured within the country). 6September 12, 2016
  7. 7. DIGITAL TRANSMISSION SYSTEMS • By mid-1980s Digital TAX exchanges were introduced in the network with the aim to improve STD services. • Till 1989, Coaxial cable and UHF transmission medias were used to provide connectivity. • Induction of Digital Transmission Systems which were mainly Digital UHF, Digital Microwave, Digital Coaxial and Optical Fiber Systems, started during 1989-90. U/G coaxial cable was initially used for the connectivity of large and medium cities and however, later on, it was also used for connecting small towns. • Media diversity was provided through Radio Relay (UHF and Microwave) Systems. These Radio relay systems were very reliable and beneficial particularly for connecting hilly and backward areas where laying and maintenance of underground cable is extremely difficult. 7September 12, 2016
  8. 8. SATELLITE SYSTEMS • Work for connecting far flung, inaccessible area, and island community started in late 70s by DoT. • The first Domestic Satellite Network was established by connecting Port- Blair and Car-Nicobar in Andaman & Nicobar islands, Kavaratti in Lakshadweep islands, Leh in Ladakh region and Aizwal in North Eastern region. These station were simultaneously linked to the gateway at Delhi and Chennai. This satellite network was commissioned in November 1980 through International Telecommunication Satellite. • Satellite Communication capacity increased with launch of INSAT-1 and INSAT-2 series satellites. MCPC - VSAT (Multi Channel per Carrier - Very Small Aperture Terminals) systems were developed and deployed in remote and inaccessible areas of Garhwal region of (then) Uttar Pradesh, Himachal Pradesh, Arunachal Pradesh, J&K, Orissa, Sikkim etc. for providing STD facilities. These terminals were linked to an Earth Station generally co- located with the TAX. 8September 12, 2016
  9. 9. OPTICAL FIBER CABLE • Introduction of Optical Fiber Cable Systems in the country started in 1989-90. • These systems are capable of carrying large no. of voice channels compared to the existing technologies that were available at that time and offer the circuit at low cost per kilometer of circuit. The DoT deployed these OFC systems in a big way for connectivity right upto the level of Tehsils. • By the year 2000, a huge network of optical fiber cable was in place and a large number of PDH & SDH technology OFC systems were deployed for providing backbone connectivity to switching network. • From 2002-03, DWDM technology systems were inducted over the OFC backbone. 9September 12, 2016
  10. 10. 10September 12, 2016
  11. 11. 11September 12, 2016
  12. 12. September 12, 2016 12 Optical Fiber Communication
  13. 13. 13 • 1790: Optical Semaphore invented by Claude Chappe of France. • 1880: Photophone invented by A.G. Bell at Washington. • 1940: Optical guides reflective coated to carry visible light. • 1960: LASER invented by Theodore Maiman. • 1963: Unguided communications with LASER. • 1966: OPTICAL FIBER invented by Corning Glass researchers: ROBERT MAURER DONALD KECK & PETER SCHULTZ Optical Communications - Historical Perspective September 12, 2016
  14. 14. • Material for fiber was fused silica with special properties like: – Extreme purity – A high melting point – Low refractive index. Initially very high loss fiber was developed. Typical loss of ~17db/km [at λ =820 nm] • 1970: low loss fiber developed. OFC systems became practical. • Currently : fiber losses=<0.2-0.35 db/km 14 HISTORICAL PERSPECTIVE (contd…) September 12, 2016
  15. 15. 15 The Developments September 12, 2016
  16. 16. 16 • High information carrying capacity • Low attenuation • Plentiful Resource • Greater safety • Immunity to RFI • Immunity to EMI • No cross-talk • Higher Security • Small size and light weight • Less Corrosion • Less temperature sensitive ADVANTAGES OF FIBER COMMUNICATIONS September 12, 2016
  17. 17. 17 • High information carrying capacity: A valid comparison would be on the basis of cost per meter per telephone channel, rather than just cost per meter. • Resource plentiful: The basic materials are either silicon dioxide for glass fibers or transparent plastic which are plentiful. • Less attenuation: A typical fiber attenuation is 0.3 dB/km. Whereas a coaxial cable (RG-19/U) will attenuate a 100MHz signal by 22.6 dB/km. • Greater safety: Optic fibers glass/plastic, are insulators. No electric current flows through them. ADVANTAGES OF FIBER COMMUNICATIONS September 12, 2016
  18. 18. 18 • Immunity to RFI: Fibers have excellent rejection of radio-frequency interference (RFI) caused by radio and television stations, radar, and other electronics equipment. • Immunity to EMI: Fibers have excellent rejection of electromagnetic interference (EMI caused by natural phenomena such as lightning, sparking, etc). • No cross-talk: The optic wave within the fiber is trapped; none leaks out during transmission to interfere with signals in other fibers. • Higher Security: fibers offer higher degree of security and privacy. Advantages of Fiber communications (2) September 12, 2016
  19. 19. 19 • Small size and light weight: typical optical cable fiber dia 125µm, cable dia 2.5 mm and weight 6 kg/km. A coaxial cable (RG-19/U), outer dia 28.4 mm, and weight 1110 kg/km. • Less Corrosion: Corrosion caused by water/chemicals is less severe for glass than for copper. • Less temperature sensitive: Glass fibers can withstand extreme temperatures glass fiber unaffected. Advantages of Fiber communications (3) September 12, 2016
  20. 20. Applications of OF Communications 20 • Telecommunications Long-Distance Transmission Inter-exchange junction Fiber in the loop (FITL) -- FTTC, FTTB, FTTH • Video Transmission Television broadcast, cable television (CATV), remote monitoring, etc. • Broadband Services provisioning of broadband services, such as video request service, home study courses, medical facilities, etc. • High EMI areas Along railway track, through power substations can be suspended directly from power line towers, or poles. • Military applications • Non-communication fiber optics: e.g. fiber sensors. September 12, 2016
  21. 21. 21 • Dark fiber is optical fiber infrastructure that’s in place, but not being used • Its like an unplugged electrical extension cord • It can be ‘lit’ with active telecom equipment, when required by TSPs or other end-users • Provides significant cost savings and substantial time- efficiencies to end users • In India, companies registered as IP-I can provide assets such as Dark Fiber. The ‘Dark Fiber’ Concept September 12, 2016
  22. 22. 22 Series of Recommendations by the ITU-T, A to Z G series: Transmission systems and media, digital systems and networks Some of the G series: G.600-G.699: Transmission media and optical systems characteristics G.700-G.799: Digital terminal equipments G.800-G.899: Digital networks G.900-G.999: Digital sections and digital line system ITU-T Recommendations September 12, 2016
  23. 23. 23 G.600-G.699: Transmission media and optical systems characteristics G.600-G.609: General G.610-G.619: Symmetric cable pairs G.620-G.629: Land coaxial cable pairs G.630-G.639: Submarine cables G.640-G.649: Free space optical systems G.650-G.659: Optical fibre cables G.660-G.679: Characteristics of optical components and subsystems G.680-G.699: Characteristics of optical systems ITU-T Recommendations September 12, 2016
  24. 24. 24 Fiber optic principle September 12, 2016
  25. 25. 25 Ray Theory: • A number of optic phenomena are adequately explained by considering light as narrow rays. • The theory based on this approach is called geometrical optics. • These rays obey following rules: 1. In a vacuum, rays travel at a velocity of c = 3x108 m/s. In any other medium, rays travel at a slower speed, given by v = c/n n = refractive index of the medium. 2. Rays travel straight paths, unless deflected by some change in medium. 3. If any power crosses a medium-boundary, the ray direction is given by Snell’s law: n1 sin θi = n2 sin θr THEORY OF FIBER OPTICS September 12, 2016
  26. 26. 26 INCIDENT RAYS REFLECTED RAYS REFRACTED RAYS 1 1 3 2 2 3 n2 θr θi Principle of Total Internal Reflection n1 = 1.465 n2 = 1.461 n1 September 12, 2016
  27. 27. 27 The Optical Fiber Cladding (n2) 125 µmCore (n1) 6-10 µm Refractive index n1 > n2 September 12, 2016
  28. 28. 3 2 1 3 2 1 Light Propagation in fiber Core (n1) Cladding (n2) 28September 12, 2016
  29. 29. 29 Air 1.0 Carbon dioxide 1.0 Water 1.33 Ethyl alcohol 1.36 Magnesium fluoride 1.38 Fused silica 1.46 Polymethyl methacrylate polymer 1.5 Glass 1.54 Sodium chloride 1.59 Zinc sulfide 2.3 Gallium arsenide 3.35 Silicon 3.5 Indium gallium arsenide phosphoide 3.51 Aluminium gallium arsenide 3.6 Germanium 4.0 Index of Refraction in different materials September 12, 2016
  30. 30. 30 DUAL NATURE OF LIGHT Wave Nature of Light : • Many light phenomena can be explained by realizing that light is an electromagnetic wave having very high oscillation frequencies. • The wavelength of light beam: λ = v/f {where, v = velocity of light , f = frequency} Particle Nature of light : • Sometimes light behaves as though it were made up of very small particles called photons. The energy of a single photon in Joules is: Wp = hf {where, h = 6.626 x 10-34 js [Planck’s constant], f = frequency} September 12, 2016
  31. 31. Relation between λ and refractive index When light waves enter a medium, their wavelength is reduced by a factor equal to the refractive index n of the medium but the frequency of the wave is unchanged if λ0 is the vacuum wavelength of the wave the wavelength of the wave in the medium, λ' is given by
  32. 32. 32 1015 1014 1013 1012 1011 1010 109 108 107 106 105 104 103 102 101 RADIO POWER MICROWAVE ULTRAVIOLET INFRARED Electromagnetic Spectrum V I S I B L E L I G H T UHF VHF HF MF LF VLF Hz September 12, 2016
  33. 33. • Light – Ultraviolet (UV) – Visible – Infrared (IR) • Communication wavelengths – 850, 1310, 1550 nm – Low-loss wavelengths • Specialty wavelengths – 980, 1480, 1625 nm UV IR Visible 850 nm 980 nm 1310 nm 1480 nm 1550 nm 1625 nm λ 125 GHz/nm Optical Spectrum
  34. 34. • Visible light has a wavelength range of 0.4-0.7 µm • Silica glass fiber attenuates light heavily in visible & UV regions. • Glass fiber is relatively efficient in wavelength ranges upto and in the infrared region. • Three windows of operation are at 850, 1310 and 1550 nm. 34 Window Concept in OFC Spectrum September 12, 2016
  35. 35. Window Concept in OFC Spectrum Window Concept in OFC Spectrum
  36. 36. 36 Window Concept in OFC Spectrum September 12, 2016
  37. 37. 37 Window Concept in OFC Spectrum First Window This is the band around 800-900 nm. This was the first band used for optical fiber communication in the 1970s and early 1980s. The fiber losses are relatively high in this region, Therefore, the first telecom window is suitable only for short-distance transmission. This window was relevant only for the initial silica fiber, which had different attenuation characteristics compared to low loss fiber developed later on. September 12, 2016
  38. 38. 38 Window Concept in OFC Spectrum Second Window This is the window around 1310 nm which came into use in the mid 1980s. This band had the property of zero dispersion of light waves(on single-mode fiber). The fiber attenuation in this window is about 0.35-0.4 dB/km. This is the band in which the majority of long distance communications systems were designed. September 12, 2016
  39. 39. 39 Window Concept in OFC Spectrum Third Window The window from around 1510 nm to 1625 nm has the lowest attenuation available on current optical fiber (about 0.26 dB/km). In addition optical amplifiers are available which operate in this band. Almost all new communications systems, from the late 1990s operate in this window. The loss peaks at 1250 and 1400 nm are due to traces of water in the glass. September 12, 2016
  40. 40. Wavelength bands used in OFC BAND DESCRIPTION WAVELENGTH RANGE nm O Band Original 1260-1360 E Band Extended 1360-1460 S Band Short wavelength 1460-1530 C Band Conventional 1530-1565 L Band Long wavelength 1565-1625 U Band Ultra long wavelength 1625-1675 40September 12, 2016
  41. 41. 41 Window Concept in OFC Spectrum • The potential transmission capacity of optical fiber is enormous. • The second window is about 100 nm wide and ranges from 1260 nm to 1360 nm (loss of about 0.4dB/ km). The third & fourth window is around 100 nm wide and ranges from 1530 nm to 1625 nm (loss of about 0.26 dB/km). • The useful range is therefore around 200 nm. • A λ-range of 100nm will correspond to a frequency bandwidth of 30 THz (on a centre wavelength of 1000nm). • Assuming that a modulation technique resulting in 1 bit/Hz of analog bandwidth is available, then we can expect a digital bandwidth of 3 ×1013 bits per second (30Tbps)! September 12, 2016
  42. 42. 42 G.655 standard OF cable •Single mode •1550 nm •Carries up to 200 λ DWDM •10 Gbps to 40 Gbps per λ- commercially deployed •100G and beyond 100G products are under development. Example: Bharti-Singtel Chennai-Singapore Submarine OFC link is 104λ x STM-64 ! (as in 2004-05) working on G.655 NZDSF Current trends September 12, 2016
  43. 43. 43 Bell Labs in Sep’2009 announced ultra-high speed transmission of more than 100 Petabits per second.kilometer, shown over a distance of 7000kms. 155 λ x 100G DWDM was used for the experiment. Employs Advanced DSP with Coherent Detection. Corning Inc. has developed a new multi-core fiber design. In Jan’2013, NEC Labs and Corning announced transmission speeds upto 1.05 Pb/s over 52.4km of a single 12-core fiber. Latest trends September 12, 2016
  44. 44. 46 Classification of Optical Fibers September 12, 2016
  45. 45. • Basic fiber has a core with refractive index n1 surrounded by cladding layer with refractive index n2, n1 > n2 • Change in RI is achieved by selectively doping the core (like with GeO2). • The difference between n1 and n2 is less than 0.5%. • The cladding layer is surrounded by one or more protective coating. Construction of Optical Fiber CORE CLADDING n2 n2 n1 > n2n1 > n2 47September 12, 2016
  46. 46. 48 Material Classification: • Liquid core fiber. • Fused-silica-glass fiber: have silica-core and silica-cladding. • Plastic-clad-silica (PCS) fiber: have silica core and plastic cladding. • All-plastic fiber : have both core and cladding made up of plastic. • Compound glass fiber such as fluoride glass fiber. Modal classification : • Fibers can be classified based on number of modes available for propagation - Single-mode (SM) fiber - Multi-mode (MM) fiber Classification based on refractive index profile : • Step index (SI) fiber • Graded index (GRIN) fiber CLASSIFICATION OF OPTICAL FIBER September 12, 2016
  47. 47. 49  Single-mode (SM) fiber • one mode of light at a time through the core • modal dispersion is greatly reduced • a higher bandwidth capacity  Multi-mode (MM) fiber • has larger core, than the SM fiber • numerous modes or light paths, can be carried simultaneously through the waveguide. o Step Index (SI)- there is a step in the refractive index at the core and cladding interface o Graded-index (GRIN) refers to the fact that the refractive index of the core is graded- it gradually decreases from the center to outward of the core; reducing modal dispersion. Classification of Optical Fiber September 12, 2016
  48. 48. 50 8-12 µm 125µm 50 - 100µm 125µm 50 µm 125µm c) Multi mode GRIN fiber (Graded-Index) b) Multi mode step-index fiber a) Single mode step-index fiber Classification of Optical fiber Index profile Index profile Index profile September 12, 2016
  49. 49. 51 Output of Single mode fiber Output of Multi-mode fiber September 12, 2016
  50. 50. 52 Optical Fiber Cable Types September 12, 2016
  51. 51. 53 • Cabling is needed to protect the fiber from mechanical damage and environmental degradation. • OF Cables have following common parts- Cabling of Optical Fiber September 12, 2016
  52. 52. OF CABLE cross section 1.Optical fibre 2.Central strength member 3.Filling compound 4.Loose tube 5.Filler 6.Wrapping tape 7.Optional aramid or glass strength members 8.Sheath September 12, 2016 54
  53. 53. Cable Components September 12, 2016 55 Component Function Material Buffer/ loose tube buffer Protect fibre From Outside Nylon, Mylar, Plastic Central Member Facilitate Stranding, Temperature Stability, Anti- Buckling Steel, Fibre glass Primary Strength Member Tensile Strength (pulling, shearing, and bending) Aramid Yarn, Steel Cable Jacket Contain and Protect Cable Core Abrasion Resistance polyethylene, polyurethane, polyvinyl chloride or teflon. Cable Filling Compound Prevent Moisture intrusion and Migration Water Blocking Compound Armoring Rodent Protection, Crush Resistance Steel Tape
  54. 54. • Centre Strengthening Member – GRP(glass reinforced plastic), FRP(fiber reinforced plastic) • Loose Tube Buffers – 2.4 mm dia, Fibres are placed inside along with jelly. • Primary Strength Member – Aramid Yarn • Inner Sheath – Black • Outer Nylon Sheath - Orange OF Cable Construction 56September 12, 2016
  55. 55. Loose Tube Buffers •The Fibers are loosely drawn inside the Buffer Tubes to take care of Temp. variations •The OF Cable which is used outside is known as Loose Tube Buffers •The Correction Factor is 0.98/0.985  980 meters of OFC will contain 1000 meters Fiber inside (Cable length is less by 1.5 to 2%) 57September 12, 2016
  56. 56. 58 • Conventional Loose-tube OFC • Armoured OFC (Underground Installation - Directly Buried) • Aerial Optical Fibre Cables • Ribbon OFC – high/very high fiber-count, for OAN • Micro-duct OFC- high fiber count in same duct • ADSS(All-Dielectric Self-Supporting) – aerial installations • OPGW (Optical Ground Wire)- power line installations Optical Fiber Cable Types September 12, 2016
  57. 57. Construction Of Cable 59September 12, 2016
  58. 58. Fiber Count in Cable 60 •6 fiber •12 fiber •24 fiber •48 fiber •96 fiber Standard OFC length on drum is 2000M (2Km). Other drum lengths like 4km are also available. September 12, 2016
  59. 59. Ribbon-fiber Cable 61September 12, 2016
  60. 60. Microduct OFC 62September 12, 2016
  61. 61. Armored Cable September 12, 2016 63 Aerial Cable/Self- Supporting
  62. 62. ADSS Cable (33 KV) 64September 12, 2016
  63. 63. OPGW (400 KV) 65September 12, 2016
  64. 64. These are tight Buffered cable •Has only one fibre per cable •Connector ended •Used in the indoor applications •Connecting equipment to outside OFC cable •Connecting meters to the equipment micro meter Pig Tail Cable September 12, 2016 66
  65. 65. Specification Of OFC Fibre - Core - 8-10 Microns (Single Mode) 50 - 100Microns (Multimode) Cladding - 125 Microns (overall Dia) Attenuation - better than 0.5 db /KM Primary Coating 250 Microns UV cured Acrylate Secondary Coating –2.4 mm nylon PE Jelly filled tube Central Strength Member – Fibre Reinforced Plastic (FRP) Moisture Barrier- non metallic polythylene sheet free from pinholes and other defects Polythene sheath Polythene free from pin holes September 12, 2016 67
  66. 66. Nylon Outer Sheath (0.7mm thickness)- Protective sheath against termite & partially against rodent Strength to withstand a load - 3X9.8 W Newtons, where W is weight of O/F cable per KM in Kg MAX Strain allowed in fibre - 0.25% MAX attenuation variation - Permissible + 0.02 dB from normal 20 degree centigrade to 60 degree centigrade Flexibility – Maximum bending radius allowed 24d, d is the diameter of OF cable Cable drum lengths - 2 KM +10% Cable ends - one end fitted with grip Other end sealed with cap Specification Of OFC September 12, 2016 68
  67. 67. September 12, 2016 69 OF Cable jointing
  68. 68. OF Cable jointing Jointing of optical fiber is imperative in fiber communication. For this the following are used- CONNECTORS COUPLERS SPLICES
  69. 69. OPTICAL FIBER CONNECTORS Connectors used for arranging transfer of optical energy from one fiber optic component to another in an optical fiber system Components include fiber, filter, coupler, opto electronic devices etc.
  70. 70. Fiber connectors 72September 12, 2016
  71. 71. COUPLERS Fiber optic couplers either split optical signals into multiple paths or combine multiple signals on one path. . The number of input and output ports, expressed as an N x M configuration, characterizes a coupler . Fused couplers can be made in any configuration, but they commonly use multiples of two (2 x 2, 4 x 4, 8 x 8, etc.).
  72. 72. SPLITTERS The simplest couplers are fiber optic splitters . These devices possess at least three ports . A TYPICAL ‘T’COUPLER
  73. 73. SPLICES Splice is a permanent interconnection between two fibers Two types of splices – •Mechanical splice •Fusion splice
  74. 74. Mechanical splices
  75. 75. FIVE GENERAL STEPS TO COMPLETE FUSION SPLICE 1. Strip, clean & Cleave 2. Load Splicer 3. Splice Fibers 4. Diagnose and Correct If Errors Occur 5. Remove and Protect Splice
  76. 76. CLEAVER September 12, 2016 79
  77. 77. LOAD SPLICER September 12, 2016 80
  78. 78. Ribbon fusion splicer September 12, 2016 81
  79. 79. LAYING OF CABLE • Optic fiber cables are laid underground as well as overhead. • Underground laying is much frequent practice. • Over ground laying is used in special cases • A large collection of accessories are required to make a strong and reliable overhead OFC alignment. • Sometimes ordinary overhead alignments are erected for emergent situations 82September 12, 2016
  80. 80. 83 Properties of Optical Fiber and Transmission Impairments September 12, 2016
  81. 81. 84 Losses in Optical Fibers • There are several points in an optic system where losses occur. • These are: –couplers –splices –Connectors –Fiber itself September 12, 2016
  82. 82. Classification of fiber losses • Losses due to absorption. – Even the purest glass will absorb heavily within specific wavelength regions. Other major source of loss is impurities like, metal ions and OH ions. • Losses due to scattering: – caused due to localized variations in density, called Rayleigh scattering and the loss is: L = 1.7(0.85/λ)4 dB/km λ is in micrometers • Losses due to geometric effects: – micro-bending – macro-bending • Losses are also termed as Attenuation in a fiber 85September 12, 2016
  83. 83. Losses due to micro bending September 12, 2016 86
  84. 84. Losses due to macro bending September 12, 2016 87
  85. 85. 88 • Dispersion is spreading of the optical pulse as it travels down the length. • Dispersion limits the information carrying capacity of fiber • Dispersion is classified as : Chromatic Dispersion , Modal Dispersion, and PMD • Chromatic dispersion consists of: – Material Dispersion – Waveguide Dispersion • Modal Dispersion: – pulse spreading caused by various modes (only for MM fiber). – For visible light, refraction indices n of most transparent materials (e.g., air, glasses) decrease with increasing wavelength λ DISPERSION IN FIBER September 12, 2016
  86. 86. 89 Consequences of Dispersion September 12, 2016
  87. 87. MATERIAL DISPERSION • Pulse spreading caused due to variation of velocity with wavelength • Every laser source has a range of optical wavelengths; figure shows examples for LD and LED laser sources September 12, 2016 90
  88. 88. MATERIAL DISPERSION September 12, 2016 91 FIBER LOGIC 1 LOGIC 1 λ1 λ λ λ λ λ 0 2 1 0 2 λλ λ λ1 0 2 1.0 0.5 Light source spectrum
  89. 89. How to reduce material dispersion? • By using sources with smaller band width or spectral width September 12, 2016 92 LED 20-100 nm LD(semiconductor) 1-5 nm YAG laser 0.1 nm He Ne laser 0.002nm
  90. 90. waveguide dispersion • The figure below shows the light distribution inside the fiber (in the core and cladding) for different wavelengths • Dispersion directly proportional to wavelength September 12, 2016 93 .
  91. 91. 94 Chromatic Dispersion For G.652 fiber, CD is nil at 1310nm September 12, 2016
  92. 92. 95 Polarization Mode Dispersion (PMD) Most single-mode fibers support two perpendicular polarization modes, a vertical one and a horizontal one. Because these polarization states are not maintained, there occurs an interaction between the pulses that results is a smearing of the signal. PMD has more impact on higher bit-rates, more than 10Gbps. September 12, 2016
  93. 93. 96 Dispersion Coefficients • CD Coefficient - CD Coefficient, indicated as D, is expressed in ps/( - It specifies the arrival time delay in picoseconds, that would be included per 1km of the transmission fiber if the wavelength deviates by 1nm. • PMD Coefficient - It is indicated by PMDQ and the unit is ps /(km)-1/2 September 12, 2016
  94. 94. 97 Optical Sources and Detectors September 12, 2016
  95. 95. 98 • The basic elements in transmitters: Light source, Electronic interfaces, Electronics processing circuitry, Drive circuitry, optical interfaces, output sensing and stabilization, Temperature sensing and control. • Most common light sources (the device which actually converts electrical signals to its optical equipment) : – LEDs – LASER diodes. • Laser power is very sensitive to temperature. Hence temperature sensing and control is required • Operating characteristics of a laser are notably, threshold current, output power, and wavelength change with temperature Optical Sources September 12, 2016
  96. 96. LED vs LASER Diode 99 LED - LIGHT EMITTING DIODE - Shorthaul and medium haul communication systems where - Power requirements are small - Low bit rate optical communication - broad spectral width is not a problem LD - LASER (Light Amplification by Stimulated Emission of Radiation) Diode - Used for long distance and high bit-rates -very narrow spectral width (0.1 to 2nm) Cooled DFB Lasers are available in precisely selected λs (for DWDM applications)September 12, 2016
  97. 97. Lasers • Active Transmit device—Converts electrical signal into light pulse. • Conversion, or modulation is normally done by externally modulating a continuous wave of light or by using a device that can generate modulated light directly. • Light source used in the design of a system is an important consideration because it can be one of the most costly elements. • Its characteristics are often a strong limiting factor in the final performance of the optical link • Light emitting devices used in optical transmission must be compact, monochromatic, stable, and long-lasting. September 12, 2016 100
  98. 98. Semiconductor Lasers • Two type • Febry Perot- Normally used in SONET/SDH systems • Distributed Feedback- well suited for DWDM applications, as it emits a nearly monochromatic light, is capable of high speeds, has a favorable signal-to-noise ratio. • The ITU draft standard G.692 defines a laser grid for point-to-point WDM systems based on 100-GHz wavelength spacing with a center wavelength of 1553.52 nm September 12, 2016 101
  99. 99. External Modulation in DFB Laser September 12, 2016 102
  100. 100. 103 • The basic elements in an optical receiver: Detector, Amplifier, Decision circuits • The detectors used in fiber optic communications are semiconductor photodiodes or photo detectors. • It converts the received optical signal into electrical form. – PiN photodiode: cheaper, less temperature sensitive, and requires lower reverse bias voltage. – Avalanche PhotoDiode (APD): used where high receive sensitivity and accuracy is required. – But APDs are expensive and more temp sensitive DETECTORS September 12, 2016
  101. 101. Optical Link Design
  102. 102. 105 An Optical Fiber System consists of :  a transmitter to convert electrical signals to optical  a receiver to convert optical signal to electrical  a medium - optical fiber cable. Basic Fiber Optic Communications system September 12, 2016
  103. 103. • Decibels (dB): unit of level (relative measure) – X dB is 10-X/10 in linear dimension e.g. 3 dB Attenuation = 10-.3 = 0.501 – Standard logarithmic unit for the ratio of two quantities. In optical fibers, the ratio is power and represents loss or gain. • Decibels-milliwatt (dBm) : Decibel referenced to a milliwatt X mW is 10×log10(X) in dBm, Y dBm is 10Y/10 in mW. 0dBm=1mW, 17dBm = 50mW • Wavelength (λ): length of a wave in a particular medium. Common unit: nanometers, 10-9 m (nm) – 390nm (violet) to 700nm (red) is visible. In fiber optics primarily use 850, 1310, & 1550nm • Frequency (ν): the number of times that a wave is produced within a particular time period. Common unit: TeraHertz, 1012 cycles per second (Thz) • Wavelength x frequency = Speed of light ⇒ λ x ν = C Some terminology 106September 12, 2016
  104. 104. • Attenuation = Loss of power in dB/km – The extent to which optical power from the source is diminished as it passes through a given length of fiber-optic (FO) cable, tubing or light pipe. This specification determines how well a product transmits light and how much cable can be properly illuminated by a given light source. • Optical Signal to Noise Ratio (OSNR) = Ratio of optical signal power to noise power for the receiver. (OSNR = 10log10(Ps/Pn)). Some more terminology 107September 12, 2016
  105. 105. dB versus dBm • dBm used for output power and receive sensitivity (Absolute Value) A dBm is a specific measurement referenced to 10-3 watts or 1 milliwatt (mW). The calculation, where X is the measured power in watts, for laser output measured in dBm: Examples 10dBm 10 mW 0 dBM 1 mW -3 dBm 500 uW -10 dBm 100 uW -30 dBm 1 uW 108September 12, 2016
  106. 106. dB versus dBm • dB used for power gain or loss (Relative Value) For example, output power in Watts (A) compared to input power in Watts (B) used to represent attenuation of a fiber related to the Common (base 10) logarithm value: 109September 12, 2016
  107. 107. Bit Error Rate (BER) • BER is a key objective of the Optical System Design • Goal is to get from Tx to Rx with a BER < BER threshold of the Rx • BER thresholds are on Data sheets • Typical minimum acceptable rate is 10 -12 110September 12, 2016
  108. 108. Optical Budget Optical Budget is affected by: – Fiber attenuation – Splices – Patch Panels/Connectors – Optical components (filters, amplifiers, etc) – Bends in fiber – Contamination (dirt/oil on connectors) Basic Optical Budget = Output Power – Input Sensitivity Pout = +6 dBm R = -30 dBm Budget = 36 dB 111September 12, 2016
  109. 109. Optical Link Budget 112 Pt - (Lcp+ Lct+ Lsp+ Lfb+ Msys) ≥ Srec where Pt = light source transmitting power, in dBm Lcp =coupling loss source to fiber, in dB Lct =connector’s losses (2nos, source to fiber & fiber to detector), in dB Lsp =splicing losses, in dB Lfb =fiber loss, in dB Msys =system loss margin requirement, in dB Srec =required PD receiver sensitivity, in dBm September 12, 2016
  110. 110. 113 Transmitter Receiver Fiber Fiber Splice Receiver Sensitivity Margin LINK POWER BUDGET P O W E R September 12, 2016
  111. 111. 114 An Optical Link is required to be commissioned between two Stations A & B. Do the Power Budgeting. Check its feasibility. What is the Total Link Loss? Data is given below :- • Distance between two stations = 69 km. • Splice Loss. = 0.1 dB / Splice. • Connector Loss. = 1 dB / Connector. • Coupling Loss (Source to fiber). = 3 dB. • Laser Output. = 0 dBm. • Receiver Sensitivity. = -37 dBm. • fiber Loss. = 0.4 dB/km. • System Margin. = 3 dB. • Extra Cable to be kept at Joint = 20 m / Joint. • fiber Length to be taken. = 102% of Cable Length. • Shrinkage. = 1 %. • Extra Cable at Terminals = 100m each • Cable Length on drum = 2km /cable drum EXERCISE September 12, 2016
  112. 112. 115 Distance Between Station A & B = 69 km. Cable Length after taking Shrinkage = 69x101% = 69.69 km. Number of Cable Drums required = 69.69/2 = 35. Total number of Splices in the Cable Route = 35- 1= 34. Extra Cable to be kept at Joints = 20x34 = 680 m. Leading-in Cable at Both Ends = 100+100=200m. As the Cable Length exceeds 70 km, there will be one more Joint in the Route and we need to provide additional 20 meter of cable at Joint Location, Hence : Total number of Splices in the Link = 34+1= 35 Cable Length after keeping provision for Joint = 69.69+0.70+0.20 =70.59 km Fiber Length. =70.59 x 102% = 72.00 km. SOLUTION Contd… September 12, 2016
  113. 113. 116 Link Loss: Source to Fiber Coupling Loss = 03.00 dB. Connectors Losses = 1 x 2 = 02.00 dB. Fiber Loss = 0.4 x 72.0 = 28.80 dB. Splicing Loss = 0.1 x35 = 03.50 dB. Total link loss = 37.30 dB. Laser Output – Link Loss = 0 – 37.30 = -37.30dBm. Projected loss by including 3dB Margin = -40.30 dBm Which is beyond Receiver Sensitivity level of -37 dBm. Hence Link is NOT Feasible! SOLUTION September 12, 2016
  114. 114. Thank You 117September 12, 2016