TCOM 503 Fiber Optic Networks

1. TCOM 503Fiber Optic Networks Spring, 2006 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 (continued); Modulation of light • Week 6: Optical fiber fabrication and testing of components • Week 7: Noise and detection

3. Useful web sites • A tutorial on testing optical systems: ftp://ftp.flukenetworks.com/public/cabling/DSP4000Series/DSP4000_CBT_2_1.EXE • A demo version of an optical network design program from RSOFT: http://www.rsoftdesign.com/products/evalform.cfm • Physics demos relevant to the course: http://www.colorado.edu/physics/2000/index.pl

4. Useful web sites (continued) • HP/Agilent material on bit error rate (BER) testing: http://we.home.agilent.com/cgi-bin/bvpub/agilent/reuse/cp_ReferenceRedirector.jsp?CONTENT_NAME=AGILENT_EDITORIAL&CONTENT_KEY=1000000386:epsg:apn&STRNID=03&LANGUAGE_CODE=eng&COUNTRY_CODE=ZZ

5. Noise and detection • Steps in signal reception and recovery • Contributors to signal degradation • Measure of performance: bit error rate • Ways of dealing with transmission problems

6. Basic problem • Maximize speed while keeping bit error rate (BER) low • BER is ultimate figure-of-merit for a given link of given speed • All other variables must be juggled to keep BER very low • Distance • Power • Coding • Fiber • Detector • Amplifiers

7. Typical system Source: Dutton

8. Steps in signal detection and recovery Bias control Automatic gain control PLL Bandpass filter l clock Detector (PIN or APD) Decoder Received bit stream Amplifier Pre-amplifier

9. Tasks of receiver • Decide where bits begin and end • Long strings of 0s or 1s may mean no light transitions for many bits • Decide what light amplitude represents a 0 and what represents a 1 • Involves a “decision point” which may be dynamic

10. The problem • Signal transmitted • Signal received

11. Steps in signal detection and recovery (continued) • Signal split into wavelengths • Each wavelength fed into receiver • Optical signal converted to electronic form using PIN or APD • Electronic signal amplified and filtered through bandpass filter to remove low and high frequency components • Further amplification of signal • Feedback loop stabilizes signal strength • Phase locked loop (PLL) used to recover timing • Timing used to determine when to make 0/1 decision • Bit stream fed into decoder (higher layer processing)

12. Limits to receiver performance • Reliable detection of a bit requires theoretical minimum of 21 photons • Real receivers require about 10x this amount • May be still higher if large amounts of noise present • Quantum efficiency of PIN diodes and other detectors goes down as speed increases • 10 Gbps ~ 0.8 • 20 Gbps ~ 0.65 • 40 Gbps ~ 0.33

13. Contributors to signal degradation • Noise • Jitter • Dispersion • Reflections • Scattering

14. Jitter • Difference between correct timing of a pulse and timing detected by receiver • Noise and distortion introduce slight timing errors • Vary at random • Causes • Nature of detection equipment • Smearing and distortion of pulses due to dispersion, action of filters and other components Source: www.ultranet.com

15. Noise • Random glitches in signal • Less of a problem in optical than in electrical circuits • Causes • Stray light • Imperfect components such as filters, switches • Light sources all have some noise • Amplifiers • Receiver circuitry

16. Dispersion • Smearing of pulses due to primarily to chromatic effects Source: Dutton

17. Reflections • Light transmitted backward due to imperfect components • Many devices use mirrors or rely on interference • Can reflect light if not perfect • Splices and connectors also are a source • Can occur at any junction between materials of different RI

18. Scattering • Light sent in random directions • Causes • Imperfections in fiber • SBS: diffraction caused by acoustic vibrations in fiber • Originates with electric field of light beam • SRS: diffraction caused by molecular vibrations in fiber • Originates with electric field of light beam

19. Signal (bit stream) recovery: PLL • Objective is to output a sine wave of same frequency and phase as input • This allows use of the output to “clock” the input pulses and determine when to read them in order to decide if 1 or 0 was transmitted

20. PLL (continued) • VCO produces clock frequency close to frequency being received • VCO output fed to comparison device which compares phase of input, VCO • Output of phase detector proportional to difference between input, VCO (error signal) • VCO adjusted to minimize error signal • Output taken from VCO

21. Decision problem • Given a certain observed photocurrent (or proportional voltage) received, which bit (0 or 1) is most likely to have been transmitted? • Probability question • Answer depends on average receiver response to transmission of 0 or 1 • Also depends on likelihood that 0 or 1 transmitted (so-called a priori knowledge) • Usually 50% in this case, but in multi-symbol environments can vary considerably • With this information a maximum likelihood algorithm can be generated

22. Decision problem (continued) • To get average response curves, create histogram • Send large number of 1s, record signal levels received • Divide into bins based on current received • Plot number received vs. current • Smooth out • Repeat for large number of 0s

23. Determining decision curve 25 20 Number in this range 15 10 5 4.0 5.0 3.0 4.5 5.5 2.0 2.5 .5 1.5 3.5 1.0 Photodetector current (ma)

24. Decision problem (continued) Area=fraction of time 0 will be decided when 1is correct Area=fraction of time 1 will be decided when 0is correct

25. Limitations on achievable bit rates

26. Math background • Normal or Gaussian distribution: • Commonly referred to as N(m,s2) • Cumulative normal distribution, P(X), is integral of this from negative infinity to X

27. Math background (continued) • Complementary cumulative distribution function, Q(X), given by • It immediately follows that P(X) + Q(X) = 1

28. Math background (continued) • Pdf and cdf fornormal distri-bution Q(a)

29. Math background (continued) • Probability of value falling between a and b

30. Math background (continued)

31. Decision problem (continued) • If • s0 = variance of received photocurrent for 0 transmitted • s1 = variance of received photocurrent for 1 transmitted • I0 = mean value of received photocurrent for 0 transmitted • I1 = mean value of received photocurrent for 1 transmitted Then threshold photocurrent (decision point) Ith given by Ith = (s0 I1 + s1 I0)/(s0 + s1)

32. Decision problem (continued) • Can also be shown that bit error rate (BER) given by Q ((s0 I1 + s1 I0)/(s0 + s1)) where Q is the upper tail of Gaussian distribution • Smaller if argument larger • This can be shown to be approximately equal to Q((Imin2/(4NoB))1/2) where Imin = minimum signal amplitude B = bandwidth N0 = noise power

33. Decision problem (continued) • BER goes up if noise power increases Log of BER

34. Bit error rates (BER) • Early days (1970s) networks had error rates of 10-6 to 10-5 on slow copper links • Protocols designed to handle these high error rates • Nowadays layered protocols would choke on such rates • Single bit error could cause retransmission of up to 3000 cells • Acceptable rate today is 10-12 to 10-14

35. State of the art in Optical Fiber

36. Some current system components

37. Tradeoffs in designing faster systems • Receiver sensitivity • Double speed requires doubling power at receiver (because it is received only half as long for each bit) • Higher sensitivity • Double power of transmitter • Shorten link by 15 km • Use multilevel coding

38. Tradeoffs in designing faster systems: Signal Bandwidth • Signal increases bandwidth of laser source by double the signal bandwidth • Modulating signal at 10 Gbps means increasing effective output bandwidth by 20 Gbps • At 1550 nm, 1.5 nm corresponds to about 125 GHz • Relevant formulae • For Gaussian pulse of time duration t0, spectral width sn = 1/pt0 • Bandwidth consumed by frequency range Df given by

39. Tradeoffs in designing faster systems: Signal Bandwidth where Df0 is center frequency, Dl0 is center wavelength, n is index of refraction (~1.5) • Dispersion problem • Doubling speed doubles dispersion • Doubling speed also halves pulse length, so same amount of dispersion has 2x effect • Net result is that doubling speed multiplies dispersion effects by 4 • 2.4 Gbps link 1000 km long can only be 65 km at 10 Gbps • Note that broad spectral range of LEDs precludes their use in long distance applications, as well as in applications where closely spaced wavelengths are needed

40. Tradeoffs in designing faster systems: nonlinear effects • Stimulated Brillouin Scattering (SBS) and Stimulated Raman Scattering (SRS) kick in at high powers • Large electric fields trigger vibrations in lattice which cause it to look like a diffraction grating • Incoming pulses are therefore scattered • Power threshold for SBS can be as low as 10 mW • Effectively imposes limit on maximum power that can be used to overcome noise and attenuation

41. System design • Electronics cost and complexity go up rapidly as speed increases • Theoretical limitations indicate that 10 Gbps may be practical limit for PCM systems • Scattering • Dispersion • 40 Gbps systems may appear • Note that 40 Gpbs corresponds to bandwidth of 80 GHz • 80 GHz requires about 1 nm, without guard bands • This is wider than ITU grid spacing

42. System design (continued) • More practical to increase speed in other ways • WDM • Multilevel codes • Solitons • Require optical time domain multiplexing

43. System parameters dependent on power • Signal-to-noise ratio (SNR) • Signal power/noise power • Can be improved up to a point by increasing laser power • Inter-symbol interference (ISI) • Merging of bits due to dispersion • Partially compensated with more power • Extinction ratio • If zero bit represented by power level > 0, then extinction ratio is power level of 1/power level of 0 • Low value can be compensated with more power

44. Determining how good a signal is as carrier of information • Classic measure is “eye diagram” • Made by superimposing large number of signal traces overtop of one another Source: Dutton

45. Eye diagram (continued) • Vertical opening ~ difference in signal level from 0 to 1 • Noise, other factors will reduce • Horizontal opening ~ amount of jitter • Large amount of jitter will reduce horizontal width • Thickness of bands at zero crossing also measure of jitter • Overall size of opening measure of how easy it is to correctly detect 1s and 0s • If eye is closed, nearly impossible to detect Source: Dutton

46. Eye diagram (continued) Source: Dutton

47. Eye diagrams (continued) 900 Mbps Cypress PSI system 2.5 Gbps Cypress PSI SONET system Source: Cypress Semiconductor

48. Eye diagrams (continued) Source: LeCroy

49. Eye diagram of VCSEL at 10 Gbps Source: IBM Micro News, Vol. 6, No. 2