COMP3310/6331 Media: Fiber Dr Markus Buchhorn: [email protected] Optical Fibre Characteristics • When copper just won’t do… • Light (weight), very robust to oxidation, water, electrical interference, … • Can go a long way • But – not very flexible (it breaks, cracks), – not easy to join (need to melt it) • Easy to make thin cables – Copper down to 0.03mm diameter – Standard Fibre down to 8micron diameter (0.008mm) • And can go well below 1micron in the lab • Not as cheap(?) 2 Quick change in language • In copper we talk about frequency (f or ) [in Hz or s-1] • In Fibre we talk about wavelength (λ) [in meters] • High frequency ≡ Short wavelength • c (speed) = f * λ (speed in that media) 3 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 λ λ time cvacuum = 300,000km/s ccopper,glass ≈ 2/3 c Huge change in performance • Copper we use kHz (103) to MHz (106), Wireless to GHz (109) • Optical we start at THz (1012) and up • VDSL 12 MHz = 25 m • Wifi 2.4GHz = 12.5 cm • Yellow light = 600nm = 500 THz 4 How it works – Physics! • Index of refraction = c/(speed of light in that material) = “n” • Cross from one material to the other = change of speed = change of direction • Make a fibre cable: – Take a glass fibre core, – Wrap it in different glass – Protect it with a plastic jacket 5 Core Cladding ncladding < ncore Jacket How it works • Get the angle and change of material right: – Total internal reflection • Within a ‘critical angle’ • Step-index fibre • Graded-index fibre 6 nclad ncore Animations from http://www.thefoa.org/tech/ref/basic/fiber.html nclad ncore Each ray = a “mode” Modal distortion Straight line is slowest Reduces modal distortion Multimode vs Singlemode • These are “multi-mode” fibres – With significant modal distortion • Make core much narrower – a few wavelengths? – “single mode” fibre • Performance goes way up • So does Cost 7 Fibre standards • For the cable – Multimode fibres, OM1 (62/125μm), OM2-OM5 (50/125 μm) – Singlemode fibres, ITU G.652-G.657 (9/125 μm) – Performance expectations more than actual manufacturing • For the connector – So, so many standards • Vary by sector • 30+ on Wikipedia • Do not mix up your cables – MMF into SMF does mechanically work, but “performance suffers”. 8 Fibre connectors • Need to be “perfect”! • Glass face to Glass face – Dust is a serious enemy – Most use curved (or sometimes angled) faces, reduce reflections at the end • Terminating copper cables involves scissors and pliers • Terminating fibre involves melting and polishing glass/plastic – Thinner than a human hair • Splicing fibre directly via melting (good), or glueing (less good) – But glue is cheaper… 9 Losses in fibre • Attenuation 0.4-3 dB/km, due to – Scattering (structures+materials in the fibre) – Absorption (materials in the fibre) – Distances of many km are trivial • 8km still yields 75% of the original light. • Attenuation depends on wavelength • Fibres have multiple passbands – Due to various materials and manufacturing techniques – Always improving, both absorption and range 10 First Window Second Window Third Window Other losses in fibre • Chromatic dispersion – Index of Refraction varies with wavelength – A pure single wavelength is hard to do (even for a laser) – Soliton pulses fix it • Polarisation mode dispersion – Core shape helps fix it 11 “Loss budgets” • You have an energy budget – How much you can send – How much you need to receive (for a reasonably clear signal) • Loss due to the fibre [0.4-1.0 db/km for SMF] • Loss due to (mechanical) connectors [0.3dB each] • Loss due to (physical) splicing [0.3dB each] 12 Going round corners • Copper has tight bending radius – N times diameter • Fibre does not – Fractures, causing attenuation and/or interference – MMF better than SMF • Fibre testing – Send a pulse, look what gets through and what comes back! 13 THz channels • A single wavelength of light at 500 THz can carry 1+Pb/s? – In theory… a very faint signal! • Electronics can’t (yet) keep up or see – better to be clearer and slower! – Use ASK, PSK • Can also do Polarisation Division Multiplexing – Frequency-DM becomes Wavelength-DM – multiple wavelengths of light • WDM can be coarse (CWDM) and dense (DWDM) – Usually use each wavelength as a separate channel 14 Add-Drop Multiplexing - optically • Given N wavelengths (channels) on a fibre – How do I add one more, or take one out? • Active electronic separation vs Passive optical separation of channels 15 Transmitting over light • Convert digital data to optical signalling (modulation…) – OOK->QPSK++ • You can pulse an led • Fast-ish, and very cheap • Don’t get very bright or nicely shaped pulses • Too broad a colour range • Used in MMF • You can chop a laser • Semiconductor lasers are now 100micron in size • And wavelength tunable on the fly • And use an “Optical Mach-Zehnder” modulator • Used in SMF 16 Multi-core cable design • Individual fibres are fragile • Fibres with sub-fibres or hollow channels • Cable bundles up to 1024 fibres • Costs as much to bury/hang as a single fibre • Connect buildings, cities, countries • People often still want their own fibre path – Security – Guaranteed performance – Avoid interference – New technologies – Concept of ‘dark’ and ‘grey’ fibre • An empty glass you can fill how you want, or • A slot alongside others to add your wavelength 17 How fast can you go? • Depends on “fast” • Highest absolute speed (2012): 1Pb/s over 50km (that’s 1,000,000Gb/s…) – And (2017)10.2Pb/s over 11km • High speed over long distances (2009): 100Gb/s over 7000km times 155λ – And (2016) 65Tb/s over 6600km (?) • Various techniques, with multiple wavelengths, multiple cores, pre-distortion, … • Better measure: bandwidth*distance – 2016: 4*105 (Tb/s).km – 2012: 5*104 (Tb/s).km – 2012 used very specialised fibre, 2009 was commodity fibre – 2020 Internet traffic estimate: 375,000PBytes/month = 1.4Pb/s… – And these measures don’t include energy costs! 18 How far can you go? • Without effort, 1-2 km over MMF, 50-100 km over SMF • Want more? Brighter lasers – gets difficult • Regenerate/Repeat every 50-100 km – Expensive optics and electronics, reconstruct signal and retransmit – perfectly – Optical-electronic-optical “OEO” interfaces • Amplify every 50-100 km – Cheap electronics, and can even be done optically (erbium doping) – Amplifies signal and noise. – Record: 2015 – Melbourne to Melbourne, via Sydney&Perth = 10,358km @100Gb/s? 19 Undersea cabling 20 Underwater cables are safe! 21 Doing more with less • Unlike copper, fibre is not (easily) a shared medium – Point-to-point • Can use a single fibre for RX and TX at the same time – Optical splitters at both ends – Can get crosstalk, in connectors, and within fibre • Still more common to have a fibre pair though 22 The Last mile and fibre • Lots of discussion about ‘costs’ – Compare capital costs of installing fibre vs copper – Compare running costs of fibre vs copper – Compare performance of fibre vs copper • General approach – mix it up: – Push fibre as close as you “can” (afford, achieve) • To the home? To the driveway? To some nearby node? – then use copper for the last bit (with DSL) – While keeping an eye on the electronics 23 Exchange Range 24 Copper links = 4km circle Fibre links = 40+km circle Radius2 # of houses Exchange Range 25 Some FTTx models • Leverage what is already in the ground/on the poles – Most homes have landline phones = capital investment • Reduce the average distance of copper – Push fibre as deeply as “affordable” (by who?) 26Exchange Copper (POTS – DSL) FTT Home FTT Kerb 20m < 300m < 4km FTT Node What needs power? Who is sharing? How many houses per ‘box’ What needs managing? Home Compare FTTx with HFC • Fibre To T he … (FTTx): – FTTPremises/FTTHome, – FTTBasement, – FTTCurb/Kerb/dp – FTTNode • Hybrid Fibre Coax (HFC) – “FTTN” to get you close, – Then shared (coax) copper • To a lot of houses, 50-100’s in NBN • And again try to make it cheap to run… – What is active, smart, powered, etc. – And what isn’t. 27 (G)PON • (Gigabit) Passive Optical Networking • Using the flexibility of optics – And the allure of cheap broadband – With minimal fibre usage… • TDM out to the active Optical Network Units or Terminals (ONU/ONT) • TDM out to the (passive) splitters – From the Optical Line Terminals (OLT) • WDM: RX and TX on a single fibre, – SDM: Make more money out of the cable 28
