Telecommunications Networking IFiber Optics: OverviewBasic Fiber Optic SystemThe Radio SpectrumOptical Transmitter: exampleOptical Transmitter ExampleSlide 7Optical FiberCauses of AttenuationCauses of Pulse Spreading: Modal Delay SpreadModal Delay SpreadSlide 12Causes of Pulse SpreadingSlide 14Pulse Spreading: ExamplesOptical ReceiverSlide 17Causes of Errors in Optical Fiber SystemsSlide 19Optical Fiber System: exampleFiber Optic System: exampleCopyright 1998, S.D. Personick. All Rights Reserved.Telecommunications Networking ILectures 12&13 Fiber OpticsCopyright 1998, S.D. Personick. All Rights Reserved.Fiber Optics: Overview•1966 C. Kao et. al, propose that strands of glass can be produced, which can carry light over long distances (>2 km)•1970 First demonstration of a fiber with less than 20dB/km of loss (Maurer, et.al., at Corning)•1975-77 Experiments and field trials•1979 Real systems are placed in serviceCopyright 1998, S.D. Personick. All Rights Reserved.Basic Fiber Optic SystemGlass fiberDetector/ReceiverDigital pulses (On/Off)Digital pulsesOptical TransmitterLight SourceCopyright 1998, S.D. Personick. All Rights Reserved.The Radio Spectrum•AM Radio ~ 1 MHz (300 meter wavelength)•Television ~ 50-500 MHz•Digital cordless phone ~ 900 MHz•Wireless LAN ~ 2.5 - 5 GHz•DBS ~ 10 GHz (0.3 meter wavelength)•Visible light ~ 4-7.5 x 10**14 Hz (0.8-0.4 um)•Fiber optics ~ 0.9 - 1.55 um (not visible)Copyright 1998, S.D. Personick. All Rights Reserved.Optical Transmitter: example50 ohm resistor2VPeak Light Emitting DiodeLight output~20 mA peakCopyright 1998, S.D. Personick. All Rights Reserved.Optical Transmitter ExamplebiasData in1V peakdrivercurrentLight outputcurrentLight outputLaserCopyright 1998, S.D. Personick. All Rights Reserved.Optical Transmitter: example•Current = 20mA = .020A•# electrons per second = .020 A/[1.6 x 10**-19] Coulombs per electron = n•# photons produced/second =n x [Quantum Efficiency]•optical power out = n x QE x [~1.5 x 10**-19 Joules per photon] ~ .020 QE x [1.5/1.6] (W) ~20 x [1.5/1.6] x QE (mW)•If QE~ 20%, then power out ~3.75 milliwattsCopyright 1998, S.D. Personick. All Rights Reserved.Optical FiberFiberOptical PulsesOptical PulsesOptical output pulses are attenuated and spread in timecompared to optical input pulsesCopyright 1998, S.D. Personick. All Rights Reserved.Causes of Attenuation•Light is absorbed by fiber impurities and the principal fiber material itself•Light is “scattered” out of the fiber because of the inherently random density fluctuations of any “glass” (Rayleigh scattering) as well as by more macroscopic density fluctuations•Typical long distance fiber: <0.5 dB per kmTypical plastic fiber: >100 dB per kmCopyright 1998, S.D. Personick. All Rights Reserved.Causes of Pulse Spreading:Modal Delay Spread“Multimode” FiberT (min) = nL/c, where c/n = speed of light in fiberT (max) = T(min) x [1/cos(max angle that is captured)]c =300,000,000 m/s, n~1.5... n/c ~ 5ns/m corecladdingCopyright 1998, S.D. Personick. All Rights Reserved.Modal Delay Spread•The rays in the previous slide represent thesolutions of Maxwell’s equations…each of which is called a “mode”•If one actually solves Maxwell’s equations,one finds a discrete set of modes, each correspond-ing to a ray at a different angle relative to the axis•The spacing between these allowed rays is •If is large enough (e.g., 0.2 radians corresponding to ~11.4 degrees, then only the axialray is below the critical angle. DDCopyright 1998, S.D. Personick. All Rights Reserved.Modal Delay Spread•If only the axial ray is below the critical angle, then there is only one solution to Maxwell’s equations (one mode) which is guided by the fiber.•Such a fiber is called a single mode fiber•With only one ray (mode) there is no modal pulse spreading!Copyright 1998, S.D. Personick. All Rights Reserved.Causes of Pulse SpreadingWavelengthDispersion: ps/km-nmZero dispersion at ~1.3 umDispersion: a change in the delay down the fiber as the wavelength changes- ps/[nm-km]Copyright 1998, S.D. Personick. All Rights Reserved.Causes of Pulse Spreading•Pulse spreading can be caused by the variation of delay vs angle in multimode fibers (delay spreading). Typical plastic multimode fiber: >100 ns/km•Pulse spreading can also be caused by the variation of delay with wavelength (“dispersion”). Typical glass fiber with 900 nm LED source ~5 ns/km; with 1550 nm laser source < 0.1 ns/kmCopyright 1998, S.D. Personick. All Rights Reserved.Pulse Spreading: Examples•Multimode fiber: Maximum angle captured in fiber is 0.2 radians (for example)~11.5 degrees. 1/cos(0.2 rad) = 1.020. Delay spreading = 5 ns/m x 0.02 = 0.1 ns/m = 100 ns/km•Single mode fiber +900 nm LED source: Dispersion at 900 nm wavelength ~100 ps/nm-km. LED sprectral width ~50 nm. Dispersion ~ 100 x 50 = 5000ps/km =5ns/kmCopyright 1998, S.D. Personick. All Rights Reserved.Optical ReceiverAmplifier +RegeneratorPhotodiodeOutput pulsesCopyright 1998, S.D. Personick. All Rights Reserved.Optical Receiver•The detector converts photons to electrons~ 0.5 mA/mW (output current/input power)•The amplifier amplifies the weak current that is produced by the detector in a typical optical fiber application•The regenerator produces a new electrical pulse stream (clock recovery, comparitor, D-flip flop)Copyright 1998, S.D. Personick. All Rights Reserved.Causes of Errors in Optical Fiber Systems•Noise produced by the amplifier in the receiver•“Quantum” noise associated with the detection process•Intersymbol interference due to pulse spreading•Bottom line: In a typical fiber optic system, we require ~20,000 received photons per pulse to produce an error rate of 10**-9; assuming that we don’t have a significant amount of intersymbol interference (pulse spreading <0.5 pulse spacing)Copyright 1998, S.D. Personick. All Rights Reserved. Fiber Optic System: exampleLight sourceGlass fiberDetector/ReceiverDigital pulses (On/Off)Digital pulsesLight SourceOptical TransmitterAssume: Bit rate = 100Mbps; Optical transmitteroutput = 1 mW; Coupling loss into fiber = 3dB;Pulse spreading<0.1 ns/km; Fiber loss = 0.5 db/km;Required optical energy per received pulse: 20,000 photons x 1.5 x 10**-19 J/photonCopyright 1998, S.D. Personick. All Rights Reserved.Optical Fiber System: example•Receiver requires 20,000 photons per received optical pulse = 20,000 x 1.5