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TCOM 503 Fiber Optic Networks

TCOM 503 Fiber Optic Networks

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TCOM 503 Fiber Optic Networks

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  1. TCOM 503 Fiber Optic Networks Spring, 2007 Thomas B. Fowler, Sc.D. Senior Principal Engineer Mitretek Systems

  2. Topics for TCOM 503 • Week 1: Overview of fiber optic communications • Week 2: Brief discussion of physics behind fiber optics • Week 3: Light sources for fiber optic networks • Week 4: Fiber optic components fabrication and use • Week 5: Fiber optic components, modulation of light • Week 6: Optical fiber fabrication and testing of components • Week 7: Noise and detection

  3. Topics for final exam (revised) • Principles of fiber optic cable and devices (reflection, refraction, interference, diffraction) • Types of fiber optic cable • Types of distortion and other problems involved with optical fiber • Operation of LEDs and lasers • Operation of detectors • Operation of EDFAs • Resonant couplers/wavelength selective couplers & splitters • Other optical devices • Isolators - Fabry-Perot filters • GRINs - Dielectric filters • FBGs - Modulators & Modulation types • Optical fiber construction, fabrication • Optical test instruments and how to interpret their displays

  4. Optical fiber standard dimensions • Core, cladding, jacketing standardized • Jacket: 245 mm Source: Corning

  5. Requirements for fabricating useful optical fiber • Materials must be extremely pure • Impurity < 1 part per billion for metals • Impurity < 1 part per 10 million for water • About 1000 times more pure than traditional chemical purification techniques allow • Dimensions must be controlled to extremely high degree • Core size, position, cladding size tolerances ~ 1 micron or less • Roughly 1 wavelength of light • Refractive indices must also be very precisely controlled • Must be made in long lengths • Must have tensile strength

  6. Purification of silica a two step process • First: use distillation • Heat silica to boiling point (2230o C), condense gas • Metals are heavier and do not boil at this temperature • Yields impurity levels of ~ 10-6 • Second stage takes place when fiber fabricated

  7. Fiber fabrication process • Called “Outside Vapor Deposition Process” or OVD process • Stages • Laydown • Consolidation • Drawing

  8. First stage: Laydown • Vapor deposition from ultrapure vapors • Soot preform made when vapors exposed to burner and form fine soot particles of silica and germanium (From particles of silica and germania) Source: Corning

  9. Laydown (continued) • Particles deposited on surface of rotating bait rod • Core first • Then silica cladding • Vapor deposition process purifies fiber material as impurities do not deposit as rapidly • Preform is somewhat porous at this stage

  10. Second stage: consolidation • Bait rod removed • Placed in high-temperature consolidation furnace • Water vapor removed • Preform sintered into solid, dense, transparent glass • Has same cross-section profile as final fiber, but is much larger (1-2.5 cm, final: 125 mm = .0125 cm)

  11. Done in “draw tower” Glass blank from consolidation stage lowered into draw furnace Tip heated until “gob” of glass falls Pulls behind it a thin strand of glass Gob cut off Strand threaded into computer-controlled tractor assembly Sensors control speed of drawing to make precise diameter Third stage: drawing 1850-2000o C Source: Corning

  12. Drawing (continued) • Diameter measured hundreds of times per second • Ensures precise outside dimension • Primary and secondary coatings (jackets) applied • At end, fiber wound onto spools for further processing Gob forming, Source: Corning

  13. Draw tower Source: Axsys

  14. Other methods used to make fiber • Vapor phase axial deposition (VAD) • Batch process • Preforms can be drawn up to 250 km • Flame hydrolysis • Soot formed and deposited by torches

  15. VAD process (continued) Source: Dutton

  16. Other methods used to make fiber (continued) • Modified chemical vapor deposition (MCVD) • Silica formed inside silica tube in gaseous phase reaction • Soot deposited on inside of tube • Burners traverse tube • Sinters soot • Produces highly controllable RI profile • At end, tube evacuated, sides collapse

  17. MCVD process Source: Fotec

  18. Types of optical fiber • Single mode glass—long distance communications • Multimode glass—short distance communications • Plastic—consumer short distance, electronics & cars • Hybrid or polymer clad (glass core, plastic cladding)—lighting, consumer applications

  19. Basic structure of all optical fiber • Core—carries most of light • Cladding—confines light to core • In some fibers, substrate glass layer to add strength • Inner jacket or primary buffer coating—mechanical protection • Outer jacket or secondary buffer coating—mechanical protection Source: Optical Cable Corporation

  20. Plastic optical fiber (POF) • 1000 mm diameter, 980 mm core • Strong • Uses LEDs in visible range, 650 nm • Not suitable for long-distance uses • Does not transmit infrared Source: Pofeska/Mitsubishi Rayon Co.

  21. Numerical aperture • Light must fall at an angle such that it can enter fiber core, before total internal reflection takes over • This angle is called “numerical aperture”

  22. Basic cable construction: types • Tight buffered • No room for fibers to move inside of cable • Loose tube • Multiple fibers loose inside of outer plastic tube • Advantage is that with extra length of fiber inside tube due to curling, less likelihood of damage in sharp bends • Loose tube with gel filler • Multiple fibers immersed in gel inside of plastic tube Source: Dutton

  23. Typical indoor cable • Single core or double core • Utilize substrate for additional strength (aramid or fiberglass) Source: Dutton

  24. Tight buffered indoor cable • Application: building risers • 6 or 12 fibers typically • Central strength member supports weight of cable • Tight buffering means that fibers are not put under tension due to their own weight Source: Dutton

  25. Outdoor cable • More rugged, larger number of fibers per cable • 6 fibers/tube, 6 tubes = 36 fibers • 8 fibers/tube, 12 tubes = 96 fibers • Steel or plastic used for strength member • Outer nylon layer in locations where termites are a problem Source: Dutton

  26. Outdoor cable (continued) Source: Dutton

  27. Submarine cable • Smaller number of fibers because mechanical requirements much greater • 4 to 20 typically • Must withstand high pressure, damage from anchors, trawlers, etc. • Cables for shallow water are in greatest danger • Typically heavily armored Source: Dutton

  28. MM 0.90/ftSM 0.53/ft Indoor/OutdoorArmoredDirect bury MM 6.65/ftSM 2.45/ft Source: Mohawk/CDT

  29. Splicing: fiber geometry parameters • Always necessary to splice fiber • 3 parameters are most important to making good splices • Cladding diameter tolerance • Must be tight so that cores meet • Typical spec is 125 mm + 1.0 mm, removes this as problem • Core/cladding concentricity • Must be tight so that cores meet • Fiber curl • Must be minimal so that cores meet

  30. Fiber geometry parameters (continued)

  31. Cable connecting and splicing • Problem of splicing and joining fibers • Core is very small • Any irregularity can lead to significant loss of power or complete failure • Light is not like electricity • Travels in waveguide and is a guided wave • Requirements for good connection • Precisely square cuts • Ends polished flat • Ends butted together • Nearly exact matchup

  32. Ways of joining fibers • Fusion splicing (welding) • Commonly done in field • Index matching epoxy glue • Common done in field • Mechanical connectors • Used in field, but connectors attached in factory • Not suitable for fiber breaks

  33. Dangers to fiber optic cable • Excessive tension • Bends of small radius • Not generally problem with outdoor or undersea cables because physical size keeps bend radius to ~3 feet • Physical damage from animals, earth moving equipment • Installation damage • Lifting • Pulling through conduits • Water inflo • Lightning

  34. Cabling environments • Long-haul outdoor • Usually direct bury • Campus area outdoor • Direct bury • Conduit • Outdoor overhead • Undersea • Most difficult environment • Indoor • Benign environment • Installation usually most difficult problem

  35. Fusion splicing • After cutting and polishing, ends are butted and then fused by heat • Requires high temperature • Can yield losses as low as 0.1 db (~loss on 1 km of fiber) Source: Dutton

  36. Steps in fusion splicing • Strip primary coating on each fiber • Cleave ends square • Position ends a few mm from each other and clamp • Align ends and bring closer together • Electric arc started and melts glass, joining fibers

  37. Problems with splicing • When fiber ends melted and touched together, surface tension effects tend to align outside of cladding • Result is OK for multimode fibers with larger cores • May not work for single mode fibers • Single mode fibers require more precise alignment • Use of laser shining into one fiber with detector in the other • One fiber moved with precision actuator to position it • Other problems can occur during join • Surface tension can change fiber position as join is made • Other systems use magnified image of fiber ends displayed on a screen

  38. Problems with splicing (continued) • Heating is difficult • Best results when glasses melt and fuse • Can change refractive index and hence cause losses • Idea is to melt only a very thin layer on each end

  39. Joining with epoxy glue • Ends cleaved and polished, space between them filled with epoxy resin • Same RI as fiber core • Fibers held in place mechanically • Low cost but subject to many problems • Concentricity • Differing outside diameters • Circularity of outside • Tolerances of alignment device • Long-term stability of epoxy

  40. Mechanical spicing and bonding • Fibers inserted into silica sleeves • Aligned as with welding method • Bonded with epoxy • High quality but costly

  41. Losses in fiber splices • Extrinsic—caused by joining method but unrelated to fiber properties • Intrinsic—Caused by some inherent property of the fiber

  42. Longitudinal misalignment Some light not within NA Endfaces form Fabry-Perot interferometer Lateral misalignment 2.5 microns ~ 1 db loss Ends not square Surfaces cannot be joined closely Angular misalignment Losses due to NA 2o ~ 1 db Fiber end rough or irregular Scattering No close contact Extrinsic losses Source: Dutton

  43. Concentricity Axes of core and fiber differ Greater for SM Core shape Not problem for MM For SM, causes fiber to be birefringent Different RIs for different polarizations Leads to PMD Core diameter Losses traveling from large to small diameter Cladding diameter If diameters differ, cores cannot be aligned NA and Refractive Index Some light reflected if these differ Intrinsic losses Source: Dutton

  44. Purely mechanical connectors • Today’s most common interconnection device • Not fitted in field, as equipment expensive ($100K) and process difficult • Early connectors were poor • Latest generation much better • Components • Ferrule: long thin cylinder for alignment • Connector body: holds ferrule • Cable attachment mechanism: holds cable in body • Coupling device: where cables mate • Fiber optic cables generally do not use male/female connection method common for electronic cables

  45. Purely mechanical connectors Source: Goff

  46. Connector evolution Source: Goff

  47. Commonly used fiber optic connectors Source: Goff; Fotec

  48. Source: Goff; Fotec

  49. Connectors (continued) L to R: SC-DC, LC, MT-RJ, SC, Volition, Opti-Jack Source: Fotec

  50. Fiber optic connector selection guide