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Optical Fibre Communications - Birth and Destiny of a SuperHighway

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  1. Optical Fibre Communications- Birth and Destiny of a SuperHighway November 2000

  2. Optical Fibre Now Connects the World • An incredibly successful innovation • 60 million kilometres of optical fibre are being installed around the Globe this year What the Wheel did for Transport, Optical Fibre has done forCommunications So where did the story begin

  3. In the Beginning • Existing copper technology offered insufficient bandwidth • 40 years ago work on guided optical communication started in the Harlow laboratories in the UK • Several potential guiding mechanisms were investigated • By late 1965, Charles Kao and his team believed optical fibre held most promise, and so they pioneered it’s use for communication • In 1966 they published the paper that started it all: • “Dielectric-fibre surface waveguides for optical frequencies”; K.C.Kao & G.A.Hockham, Proceedings of the IEE, Volume 113, No.7, July 1966 So how did we communicate before Optical Fibre?

  4. Fire Heliograph Aldis Lamp Early Optical Communication

  5. Copper Conductors - The Telegraph • Low loss and • No longer constrained by line of sight

  6. Copper Conductors - The Telephone • End customers had direct access to a network • Convenient human interface

  7. Copper ConductorsCoaxial Cable • Lower loss at higher electrical carrier frequencies • Carries many more speech channels than twisted-pair cables Multiple coax cable in 1970s: Rate = 120 Megabit/second Distance = 2 km • But: • Bulky • Performance Limited by Quantity & Conductivity of Copper • Insufficient Capacity to meet expected demand

  8. Long Haul Microwave Waveguide • Circular microwave waveguide • Propagates single HE01 Mode • Operated in the 30-110 GHz band • 10 cm diameter • Low loss: 2dB/km • Anticipated to carry 100 x 500 Mbit/sec channels • (a total of 50 Gigabit/second) • But: • Rigid • Expensive to manufacture • Difficult to install • Especially when compared to optical fibre

  9. Why Optical Communication? Light is a very high frequency electromagnetic wave • 600,000 Giga-Hertz “looks” Green Why use Light instead of Electrical signals? • Loss of Copper increases with frequency • Dielectric guiding can be Lossless • Laser just been invented

  10. The invention of the Laser stimulated research into Optical Communication: High Brightness High Spectral purity Early Semiconductor LaserPlus Human Hair Optical Laser invention Equivalent to a radio-wave oscillator but at Optical frequencies • 1958 Schawlow and Townes first propose Concept • 1960 First working Laser (Ruby) • 1961 First Gas Laser (Helium Neon) • 1962 First Semiconductor Laser (Gallium Arsenide)

  11. Roof to try to stop solar heating optical pipe beside Harlow labs Diversions Which Led Nowhere1 - Optical Pipes • 2.5 cm Steel pipes with silvered bore • Theory predicted low loss at shallow angles • But: • High Loss in Practice, 133 dB/km • Rigid • Very Temperature sensitive

  12. Idea was to steer the beam by deflecting the previous lens 2 - Confocal Lens SystemMultiple Lenses Spaced by Focal Length • But: • Alignment problems • Large Bend Radius • Very Temperature sensitive

  13. The Confocal lens system steered the beam by Refraction at the lens surface Refraction and Total Internal Reflection Total Internal Reflection can be Lossless

  14. Fibres made of Solid Dielectric (Glass) • Logie Baird is reported to have made bundles of bare glass fibres in 1927 • Well known that a solid glass fibre would guide light, but only if the surface is not touched

  15. Large Core Glass Fibre with Cladding • 1951 Idea first proposed for Light Guides, not for Communications • by Brian O’Brien, Van Heel, Moller Hansen, Lawrence Curtiss, Poor glass caused high loss in 1960s

  16. This is an incoherent fibre bundle (of many fibres), used for illuminating inaccessible locations Coherent fibre bundles are used in fibre Endoscopes to transfer images over a metre or so Clad Optical FibreFirst Invented for Optical Fibre Bundles These fibre bundles use fibres with Large Cores so they can collect more light

  17. “Single Mode” Optical Guide • If a guide is much narrower than wavelength of light l, then light only propagates along a Single Mode or path • The core is so small that the light spot is diffraction limited • This enables much shorter pulses to be transmitted • Single Mode operation enables Higher Information Rates

  18. Air-Clad Guide (Single Mode) Guide thickness must be ~0.1 microns • But • Fragile • High Optical Loss at Supports

  19. Supports Thin dielectric film ~0.1 microns thick 1 cm wide Lab experiments (1965), Prism coupling into, and out of bent Thin-Film Waveguide Thin Film Waveguides • But: • Loss • No Guidance in plane of the film • Only bends in one axis

  20. Their 1966 paper that started it all Charles Kao, measuring light propagation over short fibre sample George Hockham verified the concept using microwave theory and experiments George Hockham The Visionaries Alec Reeves (in 1937 invented the concept of digitising signals). He initially directed the team at Harlow, UK.

  21. What the Paper Says • Charles Kao and George Hockham became confident of the potential of optical fibre as a future trunk transmission medium after detailed study. They published their findings in the definitive paper in 1966* • The waveguide structure: • "Theoretical and experimental studies indicate that a fibre of glassy material ... cladded structure with a core ....and an overall diameter of about 100  represents a practical waveguide with important potential as a new form of communication medium.” • The Loss: • "The realisation of a successful fibre waveguide depends, ..on the availability of suitable low-loss dielectric material. The crucial material problem appears ...difficult but not impossible. Certainly, the required loss figure of around 20 dB/km is much higher than the lower limit of loss imposed by fundamental mechanisms.” * "Dielectric-fibre surface waveguides for optical frequencies" Proc. IEE July 1966

  22. Clad Single-Mode Fibre • Very Small Glass Core surrounded by Glass Cladding of lower refractive index • Cladding Diameter = 125 microns • Refractive index n1 = ~ 1.54, (n1 - n2)/n1 = ~ 1% • Core Diameter = 6 microns • Difficult to work with in the early days due to Small Core

  23. Red light scattering out of the side of the fibre due to imperfections Others believed it impossible in 1966 • Incredibly high Absorption Loss > 1000 dB/km • High Scattering Loss • Difficult to join fibre ends

  24. Major Breakthrough in Fibre Loss • Early fibres made by Rod-in-Tube process • Core rod assembled inside Cladding tube (like seaside rock) • Could not push loss below ~200 dB/km • Molten glass contaminated by crucible etc. • In 1970 Corning Glass company invented a New Process • They created ultra-pure glass in situ, by Oxidation of Silicon TetraChloride and Germanium TetraChloride vapour • These chemicals were already being produced in high purity for the semiconductor industry Low loss made fibre communications possible

  25. Deposition Fibre Preform ManufactureDeposition and Collapse into Preform Deposition • Reactant gases are passed through a rotating Silica tube • The tube is heated by an Oxy-Hydrogen burner that traverses its length Collapse • Further heating at a higher temperature during rotation of the tube causes it to collapse uniformly Preform • The short thick preform rod has a refractive index profile identical to that of the desired fibre, but many times larger diameter

  26. Fibre Drawing • The Short Thick Preform is Drawn into a Long Thin fibre • The bare fibre is immediately coated with Acrylate resin to protect the surface

  27. Fibre Loss Tumbled over the Years • Absorption loss was reduced by Purifying the materials • Eliminated Fe, Cu, Cr, Co, Water • Eventually, Two Fundamental Limits were reached: 1 Infra-Red absorption of Silica at much longer wavelengths 2 Rayleigh scattering, ~ 4th power of optical frequency • Pushed operation to longer wavelengths (1.5 microns) • Same mechanism gives us Blue Sky and Red Sunsets • Lowest loss fibre ever, was ~ 0.16 decibel per km in 1979 0.2 decibel per kilometre is typical today

  28. Evolution of Fibre Loss v. Wavelength In one decade the optimum operating wavelength moved from 0.85 to 1.55 microns

  29. Fibre Cables • One or several fibres are then incorporated into a cable • Provides further protection appropriate to the application

  30. core cladding fusion splice Fibre Connector Splicing and Connectors • Fibres are joined by butting the cleaved ends together • Then fusing them together using an electric arc

  31. First Applications Immune to Electrical Interference No Radiated Signals Fibre cable compared alongside coax and multi-copper pair cables Why use Optical Fibre? • Next: • Occupied Less Duct Space • Next • Longer Distances • Now: • Very Low Cost • Very High Information capacity

  32. Simple Fibre Transmission System • Transmitter • Converts electrical signal into modulated optical signal • Fibre to Guide the Light • Receiver • Converts modulated optical signal back into electrical signal

  33. Photodiode and Low Noise electrical pre-amplifier Laser mounted in package Packaged Components

  34. STL/ Nortel Networks Fibre Transmission System 1967 Range 20 metres Fibre Bundle of 27 fibres Loss = 1200 dB/km at wavelength of 0.85 microns Gallium Arsenide Light Emitting Diode, Silicon PIN Photo-Detector Purpose: Transmission of Video in high electrical interference environments

  35. STL/ Nortel Networks Fibre Transmission System 1973 Range: 2 kilometres Single Fibre (Multimode) Loss = 25 dB/km at wavelength of 0.85 microns Gallium Arsenide Sawn-Cavity Laser, Silicon Avalanche Photodiode Purpose: Transmission of Video to show viability of Singlefibre transmission

  36. Fibre Transmission System 1977 Hitchin-Stevenage Field Experiment Range 3 to 6 kilometres between Regenerators Remotely powered Regenerators in roadside foot-way boxes 4 Fibre Cable (Multimode) Loss = 5 dB/km at wavelength of 0.85 microns 6 Gallium Arsenide Narrow Stripe Lasers 6 Silicon Avalanche Photodiodes Purpose: To demonstrate practical operation in UK Telephone network STL/STC/Nortel Networks

  37. 1988 First Transoceanic Optical fibre system, North America - UK 1980 First deep optical cable trial, Loch Fyne Scotland Optical Fibre Across the Ocean Floor Signal regularly boosted by: Regenerators ~ 100km OR Optical Amplifiers ~35 km

  38. Progress through the 1990s • Total Capacity per fibre has increased Nearly 1000 Times in the last decade: • 1989 560 Megabits per second • 1999 320 Gigabits per second (= 10 Gbit/sec x 32 Colours) Two Key Innovations made it possible: 1 Optical Amplifiers 2 Wavelength Division Multiplexing

  39. Optical Amplifier Module Optical Amplifiers Few metres of special optical fibre doped with Erbium ions Optical Fibre splices Weak Light Signal In Amplified Light Signal Out Pump Pump Laser at shorter wavelength

  40. Single output to be coupled into optical transmission fibre Modulated Lasers data data data data Wavelength Division MultiplexingMulti-Coloured transmission • Several different coloured signals are combined into a single light beam by means of a dispersive element such as a prism or a grating • We use as many as 160 different Infra-Red “Colours” • These are passed through a Single fibre Simultaneously • Signals separated the same way at the far end

  41. Wavelength Division Multiplexing • Frequency Division Multiplexing at Optical frequencies

  42. Time Division Multiplexing • We pass Many signals simultaneously through a Single optical fibre (or copper conductor), by sampling each sequentially in Time

  43. Jacksonville State-of-the-Art Commercial system is: 10 Gbit/sec per Colour Carries > 75% of US Internet Backbone Traffic

  44. UTA Paris 10 Gbit/sec Being Deployed in Europe

  45. Control room Irish Fibre Network New Esat Network - 10 Gigabit/s per Wavelength Route Distance Optical Amplifiers • Fenian St – Galway 205 km 3 • Galway – Limerick 118 km 1 • Limerick – Cork 130 km 1 • Cork – Waterford 180 km 2 • Waterford– Dundrum 250 km 3 • Dundrum – Fenian St. 20 km 0

  46. Optical Amplifier Optical Amplifier Optical Amplifier Optical Amplifier Optical Amplifier Optical Amplifier Optical Amplifier Optical Amplifier Optical Amplifier Optical Amplifier Optical WavelengthMultiplexer Multiwavelength Optical Amplifier Fibre Ring Network - Irish Republic • 4 Wavelengths used now • Expandable up to 32 wavelengths (320 Gbit/sec) Fenian St Dundrum 4l 4l 4l 4l Galway 4l 4l • Optical signals travel both ways round the ring • This gives protection against Cable Cuts 4l 4l 4l Waterford Limerick 4l Cork 4l 4l 4l 4l 4l

  47. Cost perGigabit - Kilometre 1993 1998 2002 Optical Economics 1.6Tb/s Capacity per Fibre Moore’s Law 320Gb/s 2.5b/s 50Mb/s 1984 1994 1998 2000 Growing Much faster than Moore’s law And Benefiting Customers

  48. And even 100 Gigabit/second High Speed Transport R&D • Continued Research to achieve still higher Bit rates/Colour • We have Laboratory systems running at 40, 80 Gigabit/sec Light travels less than a millimetre during the light pulse duration I used to be impressed by the Speed of Light!

  49. From One Colour to Many • We have gone from a Single Track: • To a Multi-Lane Super-Highway • Product today uses 32 Colours (x10Gbit/sec) • Next generation will carry 160 Colours • And there are still demands to reduce congestion further How will these demands be met?

  50. How to get More through a Fibre Cable • Higher Bit Rates per Colour • More Colours • More Optical Bandwidth & Higher Spectral Efficiency If growth in demand continues at the present rate, we will eventually run out of Optical bandwidth • More Fibres in Parallel • Multi-Fibre cables • Typical cables in North America today contain ~50 fibres • State-of-the-art multifibre cable contains 864 fibres within a 1inch diam. But: • Difficult to repair if someone accidentally cuts the cable!