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Final ofc ppt


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this is helpfull for optical fiber communication total theory

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Final ofc ppt

  2. 2. DEPT OF ELECTRONICS & COMMUNICATION ENGINEERING,JBIET Optical Fiber Communication By:- Mr. Rajkumar D Bhure Assoc.Prof. ECE Dept.
  3. 3. Spectrum for communication
  4. 4. Attenuation Vs Wavelength
  5. 5. Fiber vs. Copper Optical fiber transmits light pulses  Can be used for analog or digital transmission  Voice, computer data, video, etc. Copper wires (or other metals) can carry the same types of signals with electrical pulses
  6. 6. Advantages of OFC over conventional Copper Wire Communication Small Size and light Weight Abundant Raw Material Availability Higher Band Width Low Noise Low transmission loss and high signal security Highly Reliable and easy of maintainance Fibers Are Electrically Isolation Highly transparent at particularo Wave Length No possiblityof ISI and echoes cross talketc. Immunity to natural hazardous.
  7. 7. Index of Refraction(n) When light enters a dense medium like glass or water, it slows down The index of refraction (n) is the ratio of the speed of light in vacuum to the speed of light in the medium n=c/v Water has n = 1.3  Light takes 30% longer to travel through it Fiber optic glass has n = 1.5  Light takes 50% longer to travel through it
  8. 8. Material Used Basic materials are plastic and Sand(Sio2) The Dopents used increase or decrease RI are:- GeO2 and P2O5--- To increase RI B2O3------ To Decrease RI
  9. 9. Elements ofOptical communication System
  10. 10. Optical Fiber Core  Glass or plastic with a higher index of refraction than the cladding  Carries the signal Cladding  Glass or plastic with a lower index of refraction than the core Buffer  Protects the fiber from damage and moisture Jacket  Holds one or more fibers in a cable
  11. 11. Plastic Optical Fiber Large core (1 mm) step-index multimode fiber Easy to cut and work with, but high attenuation (1 dB / meter) makes it useless for long distances
  12. 12. Singlemode Fiber Singlemode fiber has a core diameter of 8 to 9 microns, which only allows one light path or mode  Images from (Link Ch 2a) Index of refraction
  13. 13. Multimode Step-Index Fiber Multimode fiber has a core diameter of 50 or 62.5 microns (sometimes even larger)  Allows several light paths or modes  This causes modal dispersion – some modes take longer to pass through the fiber than others because they travel a longer distance  See animation at link Ch 2f Index of refraction
  14. 14. Step-index Multimode Large core size, so source power can be efficiently coupled to the fiber High attenuation (4-6 dB / km) Low bandwidth (50 MHz-km) Used in short, low-speed datalinks Also useful in high-radiation environments, because it can be made with pure silica core
  15. 15. Singlemode FIber Best for high speeds and long distances Used by telephone companies and CATV
  16. 16. Multimode Graded-Index Fiber The index of refraction gradually changes across the core  Modes that travel further also move faster  This reduces modal dispersion so the bandwidth is greatly increased Index of refraction
  17. 17. Step-index and Graded-index Step index multimode was developed first, but rare today because it has a low bandwidth (50 MHz-km) It has been replaced by graded-index multimode with a bandwidth up to 2 GHz-km
  18. 18. Types of Optical Fibers
  19. 19. Total Internal ReflectionSnells Law :n1 sin1  n2 sin2Re flection Condition1  3When n1  n2 and as 1 increases eventually 2goes to 90 deg rees and nn1 sinc  n2 or sinc  2 n1c is called the Critical angleFor 1  c there is no propagating refracted ray
  20. 20. Light Ray confinement nSinθ0=n1Sinθ; SinΦ=n2/n1; NA= n1(2Δ)1/2 ; ΔT=L Δ n12/cn2
  21. 21. Acceptance AngleThe acceptance angle (i) is thelargest incident angle ray thatcan be coupled into a guided raywithin the fiberThe Numerical Aperture (NA) isthe sin(i) this is definedanalogously to that for a lens 1 1 1NA = (n12 - n2 2 2 ) = 2 2 (2D n ) = n(2D ) 2 n1 - n2 n + n2Where D º and n º 1 f# º f = f n 2 D FullAccep tan ceAngle 1 = 2 ×NA
  22. 22. Sources and Wavelengths Multimode fiber is used with  LED sources at wavelengths of 850 and 1300 nm for slower local area networks  Lasers at 850 and 1310 nm for networks running at gigabits per second or more
  23. 23. Sources and Wavelengths Singlemode fiber is used with  Laser sources at 1300 and 1550 nm  Bandwidth is extremely high, around 100 THz-km
  24. 24. Fiber Optic Specifications Attenuation  Loss of signal, measured in dB Dispersion  Blurring of a signal, affects bandwidth Bandwidth  The number of bits per second that can be sent through a data link Numerical Aperture  Measures the largest angle of light that can be accepted into the core
  25. 25. Attenuation and Dispersion
  26. 26. Measuring Bandwidth The bandwidth-distance product in units of MHz×km shows how fast data can be sent through a cable A common multimode fiber with bandwidth- distance product of 500 MHz×km could carry  A 500 MHz signal for 1 km, or  A 1000 MHz signal for 0.5 km  From Wikipedia
  27. 27. Numerical Aperture If the core and cladding have almost the same index of refraction, the numerical aperture will be small This means that light must be shooting right down the center of the fiber to stay in the core
  28. 28. Fiber Manufacture
  29. 29. Three Methods Modified Chemical Vapor Deposition (MCVD) Outside Vapor Deposition (OVD) Vapor Axial Deposition (VAD)
  30. 30. Modified Chemical Vapor Deposition (MCVD) A hollow, rotating glass tube is heated with a torch Chemicals inside the tube precipitate to form soot Rod is collapsed to crate a preform Preform is stretched in a drawing tower to form a single fiber up to 10 km long
  31. 31. Modified Chemical Vapor Deposition(MCVD)
  32. 32. Outside Vapor Deposition (OVD) A mandrel is coated with a porous preform in a furnace Then the mandrel is removed and the preform is collapsed in a process called sintering  Image from
  33. 33. Vapor Axial Deposition (VAD) Preform is fabricated continuously When the preform is long enough, it goes directly to the drawing tower  Image from
  34. 34. Drawing Apparatus The fiber is drawn from the preform and then coated with a protective coating
  35. 35. Fiber Performance
  36. 36. Attenuation Modern fiber material is very pure, but there is still some attenuation The wavelengths used are chosen to avoid absorption bands  850 nm, 1300 nm, and 1550 nm  Plastic fiber uses 660 nm LEDs  Image from (Link Ch 2n)
  37. 37. Optical Loss in dB (decibels) Power In Power Out Data Link If the data link is perfect, and loses no power  The loss is 0 dB If the data link loses 50% of the power  The loss is 3 dB, or a change of – 3 dB If the data link loses 90% of the power  The loss is 10 dB, or a change of – 10 dB If the data link loses 99% of the power  The loss is 20 dB, or a change of – 20 dB dB = 10 log (Power Out / Power In)
  38. 38. Absolute Power in dBm The power of a light is measured in milliwatts For convenience, we use the dBm units, where -20 dBm = 0.01 milliwatt -10 dBm = 0.1 milliwatt 0 dBm = 1 milliwatt 10 dBm = 10 milliwatts 20 dBm = 100 milliwatts
  39. 39. Three Types of Dispersion Dispersion is the spreading out of a light pulse as it travels through the fiber Three types:  Modal Dispersion  Chromatic Dispersion  Polarization Mode Dispersion (PMD)
  40. 40. Modal Dispersion Modal Dispersion  Spreading of a pulse because different modes (paths) through the fiber take different times  Only happens in multimode fiber  Reduced, but not eliminated, with graded-index fiber
  41. 41. Chromatic Dispersion Different wavelengths travel at different speeds through the fiber This spreads a pulse in an effect named chromatic dispersion(group Delay) T=1/vg =1/L *dВ/dώ Chromatic dispersion occurs in both singlemode and multimode fiber  Larger effect with LEDs than with lasers  A far smaller effect than modal dispersion
  42. 42. Modal Distribution In graded-index fiber, the off-axis modes go a longer distance than the axial mode, but they travel faster, compensating for dispersion  But because the off-axis modes travel further, they suffer more attenuation
  43. 43. Equilibrium Modal Distribution A long fiber that has lost the high-order modes is said to have an equilibrium modal distribution For testing fibers, devices that can be used to condition the modal distribution so that measurements will be accurate
  44. 44. Mode Stripper An index-matching substance is put on the outside of the fiber to remove light travelling through the cladding
  45. 45. Mode Scrambler Mode scramblers mix light to excite every possible mode of transmission within the fiber  Used for accurate measurements of attenuation  Figure from (Link Ch 2o)
  46. 46. Semiconductor Optical Sources
  47. 47. Source Characteristics Important Parameters  Electrical-optical conversion efficiency  Optical power  Wavelength  Wavelength distribution (called linewidth)  Cost Semiconductor lasers  Compact  Good electrical-optical conversion efficiency  Low voltages  Los cost
  48. 48. Semiconductor Optoelectronics Two energy bands  Conduction band (CB)  Valence band (VB) Fundamental processes  Absorbed photon creates an electron-hole pair  Recombination of an electron and hole can emit a photon Types of photon emission  Spontaneous emission  Random recombination of an electron-hole pair  Dominant emission for light emitting diodes (LED)  Stimulated emission  A photon excites another electron and hole to recombine  Emitted photon has similar wavelength, direction, and phase  Dominant emission for laser diodes
  49. 49. Basic Light Emission Processes Pumping (creating more electron-hole pairs)  Electrically create electron-hole pairs  Optically create electron-hole pairs Emission (recombination of electron-hole pairs)  Spontaneous emission  Simulated emission
  50. 50. Semiconductor Material Semiconductor crystal is required Type IV elements on Periodic Table  Silicon  Germanium Combination of III-V materials  GaAs  InP  AlAs  GaP  InAs …
  51. 51. Direct and Indirect Materials Relationship between energy and momentum for electrons and holes  Depends on the material Electrons in the CB combine with holes in the VB Photons have no momentum  Photon emission requires no momentum change  CB minimum needs to be directly over the VB maximum  Direct band gap transition required Only specific materials have a direct bandgap
  52. 52. Light Emission The emission wavelength depends on the energy band gap Eg  E2  E1 ch 1.24   Eg E Semiconductor compounds ghave different  Energy band gaps  Atomic spacing (called lattice constants) Combine semiconductor compounds  Adjust the bandgap  Lattice constants (atomic spacing) must be matched  Compound must be matched to a substrate  Usually GaAs or InP
  53. 53. Common SemiconductorCompounds GaAs and AlAs have the same lattice constants  These compounds are used to grow a ternary compound that is lattice matched to a GaAs substrate (Al1-xGaxAs)  0.87 <  < 0.63 (mm) Quaternary compound GaxIn1-xAsyP1-y is lattice matched to InP if y=2.2x  1.0 <  < 1.65 (mm) Optical telecommunication laser compounds  In0.72Ga0.28As0.62P0.38 (=1300nm)  In0.58Ga0.42As0.9P0.1 (=1550nm)
  54. 54. Optical Sources Two main types of optical sources  Light emitting diode (LED)  Large wavelength content  Incoherent  Limited directionality  Laser diode (LD)  Small wavelength content  Highly coherent  Directional
  55. 55. Avoiding losses in LED Carrier Photonconfinement Confinement Band-gap and refractive index engineering. Heterostructured LED
  56. 56. Double Heterojunction LED (important) Fiber Optics Epoxy  Double Metal contact heterostructure n AlGaAsp GaAs (active region)  Burrus type p Al GaAs LED n+ GaAs Metal contact  Shown bonded to a fiber with index- matching epoxy.
  57. 57. Double Heterostructure The double heterostructure is invariably used for optical sources for communication as seen in the figure in the pervious slide. Heterostucture can be used to increase:  Efficiency by carrier confinement (band gap engineering)  Efficiency by photon confinement (refractive index) The double heterostructure enables the source radiation to be much better defined, but further, the optical power generated per unit volume is much greater as well. If the central layer of a double heterostructure, the narrow band-gap region is made no more than 1mm wide.
  58. 58. Photon confinement - Reabsorption problem Source of electrons Active region (micron in thickness) Source of holesActive region (thin layer of GaAs) has smaller band gap, energy of photonsemitted is smaller then the band gap of the P and N-GaAlAs hence could notbe reabsorbed.
  59. 59. Carrier confinement electrons holes n+-AlGaAs p-GaAs p+-AlGaAsSimplified band diagram of the ‘sandwich’ top show carrier confinement
  60. 60. Light Emitting Diodes (LED) Spontaneous emission dominates  Random photon emission Spatial implications of random emission  Broad far field emission pattern  Dome used to extract more of the light  Critical angle is between semiconductor and plastic  Angle between plastic and air is near normal Spectral implications of random emission  Broad spectrum   1.452 kT p
  61. 61. Laser Diode Stimulated emission dominates  Narrower spectrum  More directional Requires high optical power density in the gain region  High photon flux attained by creating an optical cavity  Optical Feedback: Part of the optical power is reflected back into the cavity  End mirrors Lasing requires net positive gain  Gain > Loss  Cavity gain  Depends on external pumping  Applying current to a semiconductor pn junction  Cavity loss  Material absorption  Scatter  End face reflectivity
  62. 62. Laser Diode Stimulated emission dominates  Narrower spectrum  More directional Requires high optical power density in the gain region  High photon flux attained by creating an optical cavity  Optical Feedback: Part of the optical power is reflected back into the cavity  End mirrors Lasing requires net positive gain  Gain > Loss  Cavity gain  Depends on external pumping  Applying current to a semiconductor pn junction  Cavity loss  Material absorption  Scatter  End face reflectivity
  63. 63. Optical Feedback Easiest method: cleaved end faces  End faces must be parallel  Uses Fresnel reflection  n 1  2 R   n 1  For GaAs (n=3.6) R=0.32 Lasing condition requires the net cavity gain to be one R1 R2 expg  a  L  1  g: distributed medium gain  a: distributed loss  R1 and R2 are the end facet reflectivitys
  64. 64. Phase Condition The waves must add in phase as given by 2 L  z  2 m Resulting in modes given by 2Ln  m Where m is an integer and n is the refractive index of the cavity
  65. 65. Longitudinal Modes
  66. 66. Longitudinal Modes The optical cavity excites various longitudinal modes Modes with gain above the cavity loss have the potential to lase Gain distribution depends on the spontaneous emission band Wavelength width of the individual longitudinal
  67. 67. Mode Separation Wavelength of the various modes 2Ln  m 1 1    m  m1  2 L n    m m  1 The wavelength separationof the modes is 2 Ln 2 A longer cavity   2   m 2 Ln  Increases the number of modes  Decrease the threshold gain There is a trade-off with the length of the laser cavity