1. Copyright 1998, S.D. Personick. All Rights Reserved. Telecommunications Networking I Lectures 12&13 Fiber Optics

2. Copyright 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 service

3. Copyright 1998, S.D. Personick. All Rights Reserved. Basic Fiber Optic System Glass fiber Detector/Receiver Digital pulses (On/Off) Digital pulses Optical Transmitter Light Source

4. Copyright 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)

5. Copyright 1998, S.D. Personick. All Rights Reserved. Optical Transmitter: example 50 ohm resistor 2V Peak Light Emitting Diode Light output ~20 mA peak

6. Copyright 1998, S.D. Personick. All Rights Reserved. Optical Transmitter Example bias Data in 1V peak driver current Light output current Light output Laser

7. Copyright 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 milliwatts

8. Copyright 1998, S.D. Personick. All Rights Reserved. Optical Fiber Fiber Optical Pulses Optical output pulses are attenuated and spread in time compared to optical input pulses

9. Copyright 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: 100 dB per km

10. Copyright 1998, S.D. Personick. All Rights Reserved. Causes of Pulse Spreading: Modal Delay Spread “Multimode” Fiber T (min) = nL/c, where c/n = speed of light in fiber T (max) = T(min) x [1/cos(max angle that is captured)] c =300,000,000 m/s, n~1.5... n/c ~ 5ns/m core cladding

11. Copyright 1998, S.D. Personick. All Rights Reserved. Modal Delay Spread The rays in the previous slide represent the solutions 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 axial ray is below the critical angle.

12. 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!

13. Copyright 1998, S.D. Personick. All Rights Reserved. Causes of Pulse Spreading Wavelength Dispersion: ps/km-nm Zero dispersion at ~1.3 um Dispersion: a change in the delay down the fiber as the wavelength changes- ps/[nm-km]

14. 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/km

15. Copyright 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/km

16. Copyright 1998, S.D. Personick. All Rights Reserved. Optical Receiver Amplifier + Regenerator Photodiode Output pulses

17. Copyright 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)

18. 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)

19. Copyright 1998, S.D. Personick. All Rights Reserved. Fiber Optic System: example Light source Glass fiber Detector/Receiver Digital pulses (On/Off) Digital pulses Light Source Optical Transmitter Assume: Bit rate = 100Mbps; Optical transmitter output = 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/photon

20. Copyright 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 x 10**-19 J per pulse = 3 x 10**-15 J/pulse Bit (pulse) rate is 100Mbps; therefore the average received power level must be greater than 0.5 x (3 x 10**-15) x (10**8)= 1.5 x 10**-7 watts = 1.5 x 10**-4 mW~ -38.2 dBm

21. Copyright 1998, S.D. Personick. All Rights Reserved. Fiber Optic System: example Transmitter average power into the fiber is: 1 mW x 0.5 (coupling loss) x 0.5 (duty cycle) = 0.25 mW Allowable loss = 0.25/0.00015 = 1.66 x 10**3 ~ 32.2 dB At 0.5 dB/km loss, we can allow ~64 km of fiber Checking the pulse spreading; we get: 64 km x 0.1 ns/km = 6.4 ns (0.64 x pulse spacing) Bottom line, if we allow up to 50 km of fiber, we will be within the pulse spreading limit, and we will have about 14 x 0.5 = 7 dB of margin w.r.t. noise limited operation