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L5 Optical Fiber Link and LAN Design

L5 Optical Fiber Link and LAN Design. Table of content. Transmission Type Elements in Network Design Factors for Evaluating Fiber Optic System Design Link Budget Considerations Power Budget Power Budget Requirement Example : Long-haul Transmission System Example : LAN.

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L5 Optical Fiber Link and LAN Design

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  1. L5Optical Fiber Link and LAN Design

  2. Table of content • Transmission Type • Elements in Network Design • Factors for Evaluating Fiber Optic System Design • Link Budget Considerations • Power Budget • Power Budget Requirement • Example : Long-haul Transmission System • Example : LAN

  3. Table of content (cont.) • Bandwidth Budget • System Rise Time • Example on STM-4, STM-16 and STM-64 • Budget Summary • Sensitivity Analysis • Eye Diagrams • Signal to Noise Ratio (SNR) • Cost/ Performance Considerations • Summary

  4. Transmission Types • Two types of transmissions: 1. Link (point to point) 2. Network a. point to multipoint b. Mesh c. Ring

  5. Elements of Link/ Network Design • Transmitter : Operating wavelength (), Linewidth (), Rise time, Bit-rate, Line format, Power level • Fiber : SMF/MMF, Fiber type – SMF28, DSF, etc, Cable loss, Spool length • Rx : PSEN, PSAT, Rise time

  6. Elements of Link/ Network Design (cont.) • Connection: No. of splice, Splice loss No. of connectors, Connector Loss • In Line Devices: Splitter, Filter, Attenuator, Amplifier Insertion loss, Gain

  7. The Main Problems Attenuation and Loss Dispersion The Main Question • In Digital System - Data Rate - Bit Error Rate • In Analog System - Bandwidth - Signal to Noise Ratios

  8. Factors for Evaluating Fiber Optic System Design

  9. Factors for Evaluating Fiber Optic System Design (cont.)

  10. LEDs Output Power Modulation Bandwidth Center Wavelength Spectral Width Source Size Far-Field Pattern Laser Diodes Output Power Modulation Bandwidth Center Wavelength, Number of Modes Chirp, Linewidth Mode Field of the Gaussian beam Optical Transmitter/ Sources

  11. Multimode Fiber Attenuation Multimode Dispersion Chromatic Dispertion Numerical Aperture Core Diameter Single-Mode Fiber Attenuation Chromatic Dispersion Cutoff Wavelength Spot Size Optical Fiber

  12. Optical Receiver/ Photodiode • Risetime/Bandwidth • Response Wavelength Range • Saturation Level • Minimum Detection Level

  13. RX TX OA OA Medium and Devices Simple Link

  14. Link Budget Considerations Three types of budgets: (1) Power Budget • Bandwidth or Rise Time Budget • ?

  15. POWER BUDGET

  16. Power Budget Requirements: • PB : PRX> PMIN • PRX = Received Power • PMIN = Minimum Power at a certain BER • PRX = PTX – Total Losses + Total Gain - PMARGIN • PTX = Transmitted Power • PMARGIN≈ 6 dB

  17. Requirements Cont’d: • Loss,L = LIL + Lfiber + Lconn. + Lnon-linear LIL = Insertion Loss Lfiber = Fiber Loss Lconn.= Connector Loss Lnon-linear= Non-linear Loss • Gain,G = Gainamp + Gnon-linear Gainamp = Amplifier Gain Gnon-linear = Non-linear Gain

  18. dB, dBm, mW dB = 10 log (P1/P2)

  19. Decibel to Power Conversion

  20. Decibel to Power Conversion

  21. Connector Splice Example: Power Budget Measurement for Long Haul Transmission 185 km PSEN = -28 dBm PTx = 0 dBm IS THIS SYSTEM GOOD? Attenuation Coefficient,  = 0.25 dB/km Dispersion Coefficient, D = 18 ps/nm-km Number of Splice = 46 Splice Loss = 0.1 dB Connector Loss = 0.2 dB PMargin = 6 dB

  22. Total Losses = 46.3 + 4.6 + 0.4 = 51.3 dB Simple Calculation…. Fiber Loss = 0.25 dB/km X 185 km = 46.3 dB Splice Loss = 0.1 dB X 46 = 4.6 dB CONCLUSION: BAD SYSTEM!! Connector Loss = 0.2 dB X 2 = 0.4 dB PMargin = 6 dB PRX = PTX – Total Losses – PMargin = 0 – 51.3 – 6 PRX = -57.3 dB Power Budget, PRX< PSEN !!

  23. ? How To Solve? Answer… Place an amplifier But… What is the gain value? And… Where is the location?

  24. First we calculate the amplifier’s gain.. Gain  PSEN - PRX Gain  -28 – (-57.3) Gain  29.3 dB Gain  30 dB To make it easy, Now…Where to put the amplifier?

  25. Three choices available for the location Power Amplifier – At the transmitter Preamplifier – At the receiver In Line – Any point along fiber

  26. Hence … Let us check one by one… Power Amplifier: PTX + Gain = POUT 0 + 30 = 30 dBm But is there any power amplifier with 30 dBm POUT? NO, THERE ISN’T

  27. Hence … What about Preamplifier? Remember… POUT received = -57 dBm Preamplifier with 30 dB available? Yes But, can it take –57 dBm? Typically, NO

  28. Let us check In Line Amplifiers 30 dB gain amplifier available here… But, What value can it take? Typically –30 dBm So… Now, we can find the location…

  29. Where is the –30 dBm point? PTX – Loss At That Point = 0 dBm – 30 dB Loss At That Point = -30 dBm Assume Other Loss = 0, Loss At That Point = Fiber Loss, 30 =  x Length of That Point Remember  = 0.25, Point Length = 30/0.25 = 120 km But 120 km from Tx, No. of splice = 120/4 = 30 Splice Loss = 0.1 dB x 30 = 3 dB

  30. + 1 connector at Tx 2 connectors Also remember connector loss at amplifier and Tx… Connector Loss = 0.2 dB x 3 = 0.6 dB Actually, at 120 km, Total Losses = Fiber Loss + Splice Loss + Connector Loss = 30 + 3 + 0.6 = 33.6 dB 33.6 dB > 30 dB!! NOT GOOD! Now, We have excess of 3.6 dB…Find the distance, Fiber Loss Length = 3.6/0.25 = 14.4 km Good Location = 120 km – 14.4 km = 105.6 km

  31. Connector Splice 185 km PSEN = -28 dBm PTx = 0 dBm 105.6 KM Let us confirm the answer… At 105.6 km from Tx, Fiber Loss = 0.25 x 105.6 = 26.4 dB No. of Splice at 105.6 km = 105.6/4 =26.4 = 27 Splice Loss = 0.1 x 27 = 2.7 dB Total Losses = 26.4 + 2.7 = 29.1 dB 29.1 dB < 30 dB !! CONFIRM…105.6 KM IS A GOOD LOCATION!!

  32. Example: Power Budget Measurement for LAN Server A Server B 500 m Using 850nm PSEN = -25 dBm PTx = -15 dBm IS THIS SYSTEM GOOD? Attenuation Coefficient,  = 4.5 dB/km Dispersion Coefficient, D = 18 ps/nm-km Number of Splice = 0 Splice Loss = 0 dB Connector Loss = 0.5 dB PMargin = 2 dB

  33. BANDWIDTH BUDGET

  34. System Rise Time • Calculate the total rise times Tx, Fiber, Rx • Calculate Fiber rise time, TFiber Tfiber = D x  x L D = Dispersion Coefficient  = Linewidth L = Fiber Length Tx Rise Time, TTX = normally given by manufacturer Rx Rise Time, TRX = normally given by manufacturer

  35. Total Rise time, Tsys: Tsys=1.1(TTX2+TRX2+Tfiber2)1/2

  36. T T’ Bandwidth Budget RX TX OA OA Δτ = T’ - T Medium and Devices

  37. What is a good Rise time? • For a good reception of signal Tsys < 0.7 x Pulse Width (PW) • PW = 1/BitRate for NRZ 1/2BitRate for RZ

  38. Example: Rise Time Budget Measurement for Long Haul Application Tx rise time, TTX = 0.1 ns Rx rise time, TRX= 0.5 ns Linewidth() = 0.15 nm Dispersion Coefficient, D = 18 ps/nm-km Fiber length = 150km Bit Rate = 622Mbps Format = RZ

  39. Total Rise time, TSYS = 1.1 TLS2 + TPD2 + TF2 = 1.1 0.01 + 0.25 + 0.16 TSYS = 0.77 ns Simple Calculation…. Fiber rise time, TF =Length x D x Linewidth() = 150 km x 18 x 0.15 nm = 0.4 ns

  40. Let say, Bit Rate = STM 4 = 622 Mbps Format = RZ Tsys < 0.7 x Pulse Width (PW) Pulse Width (PW) = 1/(622x106) = 1.6 ns 0.77 ns< 0.7 x 1.6 ns 0.77 ns < 1.1 ns !! Good Rise Time Budget!!

  41. Let say, Bit Rate = STM 16 = 2.5 Gbps Format = RZ Tsys < 0.7 x Pulse Width (PW) Pulse Width (PW) = 1/(2.5x109) = 0.4 ns 0.77 ns< 0.7 x 0.4 ns 0.77 ns ≥ 0.28 ns !! Bad Rise Time Budget!!

  42. Option Power Budget Bandwidth Budget Financial A Source (LED vs. LD) Δλ 850nm Mediocre Bad Cheap 1310nm Good Good Less expensive 1550nm Very good Very good Expensive Modulation Bandwidth LED NA Bad Cheap LD NA Good Expensive Output Power LED Mediocre NA Cheap LD Good NA Expensive Radiation pattern LED (far-field pattern) NA Bad Cheap LD (Gaussian beam) NA Good Expensive Budget Summary

  43. B Fiber Option Power Budget Bandwidth Budget Financial Attenuation MM Mediocre Mediocre Cheap SM Good Good Expensive Dispersion MM Mediocre Mediocre Cheap SM Good Good Expensive Numerical Aperture (NA) MM Mediocre Mediocre Cheap SM Good Good Expensive Core Diameter MM Mediocre Mediocre Cheap SM Good Good Expensive Budget Summary

  44. C Receiver (PIN vs. APD) Option Power Budget Bandwidth Budget Financial Rise time/ Bandwidth PIN Mediocre Mediocre Cheap APD Good Good Expensive Response wavelength range PIN Mediocre Mediocre Cheap APD Good Good Expensive Saturation Level PIN Mediocre Mediocre Cheap APD Good Good Expensive Minimum detection level PIN Mediocre Mediocre Cheap APD Good Good Expensive Budget Summary

  45. Sensitivity Analysis • Minimum optical power that must be present at the receiver in order to achieve the performance level required for a given system.

  46. Factors will affect this analysis 1. Source Intensity Noise - Refers to noise generated by the LED or Laser • Phase Noise - the difference in the phases of two optical wavetrains separated by time, cut out of the optical wave • Amplitude Noise - caused by the laser emission process. 2. Fiber Noise • Relates to modal partition noise 3. Receiver Noise • Photodiode, conversion resistor

  47. 4. Time Jitter and Intersymbol Interference • Time Jitter - short term variation or instability in the duration of a specified interval • Intersymbol Interference • result of other bits interfering with the bit of interest • inversely proportional to the bandwidth • Eye diagrams - to see the effects of time jitter and intersymbol interference

  48. 5. Bit error rate - main quality criterion for a digital transmission system BER = Q [IMIN2/ (4 . N0. B) ] where : N0 = Noise power spectral density (A2/Hz) IMIN = Minimum effective signal amplitude (Amps) B = Bandwidth Q(x) = Cumulative distribution function (Gaussian distribution)

  49. Eye Diagrams

  50. Signal to Noise Ratio SNR = S/N S - represents the information to be transmitted N - integration of all noise factors over the full system bandwidth SNR (dB) = 10 log10 (S/N)

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