UNIT II: light wave systems (6 L) - PowerPoint Presentation

1. UNIT II: light wave systems (6 L)System architectures, Point-to-Point links: system considerations, Design guide lines: optical power budget, rise time budget, Long haul systems ObjectiveTo apply subject understanding in Link Design.Course Outcome Perform Link power budget and Rise Time Budget by proper selection of components and check its viability.

2. (Remember) Arrange, Define, Duplicate, Label, List, Memorize, Name, Order, Recognize, Relate, Recall, Repeat, Reproduce and State.(Explain) Classify, Describe, Discuss, Explain, Express, Identify, Indicate, Locate, Recognize, Report, Restate, Review, Select, Translate.(Apply) Apply, Choose, Demonstrate, Dramatize, Employ, Illustrate, Interpret, Operate, Practice, Schedule, Sketch, Solve, Use, Write.(Analysis) Analyze, Appraise, Calculate, Categorize, Compare, Contrast, Criticize, Differentiate, Discriminate, Distinguish, Examine, Experiment, Question, Test.(Synthesis) Arrange, Assemble, Collect, Compose, Construct, Create, Design, Develop, Formulate, Manage, Organize, Plan, Prepare, Propose, Set up, Write.(Evaluation) Appraise, Argue, Assess, Attach, Choose Compare, Defend Estimate, Judge, Predict, Rate, Core, Select, Support, Value, Evaluate.

3. The Objective of this unit is to learn following criteria

4.

5.

6. The system should be able to adopt new technology as we should be able toaccommodate higher data rates with least possible changes

7. Major elements of an optical fiber linkTransmitter Regenerator Receiver

8. System Block DiagramSourceOptical detectorReceiverSinkConnectorDrivecircuitOptical sourceOpticalspliceOpticalamplifierFibreOpticalcouplerOptical RxOptical-to-electronicsOpticalTxRegeneratorTransmitterTheir main role is to transport information,  in the form of digital bit stream, from one place to another with high accuracy. The length of the link can vary from less than a kilometer to thousands of kilometers, depending on the application required.

9. SourceSourcecodingModulationMultiplexingModulationExternalInternal Analogue Digital Frequency Time Pulse shaping Channel coding Encryption etc.

10. Regenerator is a device to overcome attenuation problems. Electronic regenerator regenerates signals by first converting optical signals to electrical signals.The electrical signal is regenerated, converted back to optical, and further injected into the fiber.On WDM systems, each wavelength requires its own opto-electric amplifier, an expensive proposition if there are many wavelengths.Regenerator

11. An optic amplifier/repeater merely increases the power of the signal ( makes the light brighter).A regeneration station ("regen") reshape the digital signal into sharp, well-defined 1's and 0's.In general, with metro fiber routes, there are about 4 / 5 amps for every ‘regen’.As light travels down a fiber, it loses power, and the sharp transitions (representing binary data - or 1's and 0's) of the digital signal become smoothed out and loses power.This is rectified by placing amplifiers and regenerators into series with the fiber cable. 

12. Amplifier, Regenerator and example of a metro fiber routesAfter a certain distance(25-100 km) it becomes necessary to compensate for fiber loss. This can be done using regenerators that restore the SNR and pulse shape but not the BER.

13. Amplification and Regeneration of Optical pulses The periodic regeneration is usually required to regenerate the original waveform and synchronization of signals.Complete regeneration includes three regenerating operations with a signal viz.Regeneration of amplitude (amplification),Regeneration of signal waveform, and,Regeneration of synchronization.

14. Three R’s of complete Regeneration of Signals (of amplitude, signal waveform, and synchronization)

15. Receiver1st-stageamplifier2nd-stageamplifierPre-detectionfilteringSampler&detectorDemultiplexer EqualizerDemodulatorOutput signalDecoderDecryption

16. Digital LinksDigital Transmission SystemsThe simplest transmission link shown below is “point-to-point” link.Infor--mation sourceOptical trans--mitteruserOptical ReceiverOptical fiberSimple Block Diagram of Point-To-Point Link

17. Point-to-Point LinksKey system requirements needed to analyze optical fiber links:1. The desired (or possible) transmission distance2. The data rate or channel bandwidth3. The desired bit-error rate (BER)(a) Core size(b) Core index profile(c) BW or dispersion(d) Attenuation(e) NA or MFDMMF or SMFLED or laserpin or APD(a) Emission wavelength(b) Spectral line width(c) Output power(d) Effective radiating area(e) Emission pattern(a) Responsivity(b) Operating λ(c) Speed(d) Sensitivity

18. Selecting the FiberOther factors to consider: attenuation (depends on?) and distance-bandwidth product (depends on?) cost of the connectors, splicing etc.Then decideMultimode or single modeStep or graded index fiberBit rate and distance are the major factors

19. Based on above information, selection of optical fiber (single/multi mode) is done and following details about the fiber is required.Core size core refractive index profile Band width or dispersion Attenuation NA or Mode field diameter

20. Selecting the Optical SourceEmission wavelength depends on acceptable attenuation and dispersionSpectral line width depends on acceptable ………… dispersion (LED  wide, LASER  narrow) Output power in to the fiber (LED  low, LASER  high)Stability, reliability and costDriving circuit considerations

21. Then, appropriate source is selected (LASER/LED) and following information about the source is required. 1.Emission wavelength 2. Spectral line width 3. Output power 4. Effective radiating area 5. Emission pattern 6. Number of emitting modes.

22. Typical bit rates at different wavelengthsWavelengthLED SystemsLASER Systems.800-900 nm (Typically Multimode Fiber)150 Mb/s.km2500 Mb/s.km1300 nm (Lowest dispersion)1500 Mb/s.km25 Gb/s.km(InGaAsP Laser)1550 nm (Lowest Attenuation)1200 Mb/s.kmUp to 500 Gb/s.km(Best demo)

23. Selecting the detectorType of detectorAPD: High sensitivity but complex, high bias voltage (40V or more) and expensive PIN: Simpler, thermally stable, low bias voltage (5V or less) and less expensiveResponsivity (that depends on the avalanche gain & quantum efficiency)Operating wavelength and spectral selectivitySpeed (capacitance) and photosensitive area Sensitivity (depends on noise and gain)

24. Design ConsiderationsLink Power BudgetThere is enough power margin in the system to meet the given BERRise Time BudgetEach element of the link is fast enough to meet the given bit rate These two budgets give necessary conditions for satisfactory operation

25. Link power budget and Rise time budget are the methods to analyze working of desired system & it’s performance.

26. System ConsiderationsSelection of “Wave Length” to transmit the data decides the components which operates in this wavelength region. For example, if transmission distance is small, operating wavelength may be 800-900 nm and if it is large; the wavelength would be 1300/ 1550 nm (where low attenuation & dispersion occurs).

27. While selecting a photo detector, we determine, how much minimum optical power must to fall on the photo-detector to satisfy BER requirement at the specified data rate.Selection of source requires the parameters to be seen as signal dispersion, data rate, transmission distance and cost.Selection of optical fiber depends on type of source and amount of dispersion that can be tolerated.

28. In the Link power budget; for a specific BER, power margin between ‘optical transmitter output’ and ‘minimum receiver sensitivity’ is determined and then margin is allocated to connectors, splices, fiber losses etc.In Rise time budget, it is checked that the desired overall system performance is achieved or not? It determines the dispersion limitations of an optical fiber link.

29. So while talking about RTB, The system speed is observed limited by the factors as Transmitter rise time ttxGroup velocity dispersion rise time tGVD Modal dispersion rise time tmodReceiver rise time trx Here, σλ is the half power spectral width of the source, D is dispersion, L is length, q is a constant (ranging from 0.5-1.0), B0 is band width of 1km length of cable.

30. Rise-Time Budget (1)A rise-time budget analysis determines the dispersion limitation of an optical fiber link. The total rise time tsys is the root sum square of the rise times from each contributor ti to the pulse rise-time degradation:The transmitter rise time ttxThe group-velocity dispersion (GVD) rise time tGVD of the fiberThe modal dispersion rise time tmod of the fiberThe receiver rise time trxHere Be and B0 are given in MHz, so all times are in ns.

31. Rise-Time Budget (2)31

32. Optical power-loss modelTry Ex: 8.1

33. In a fiber optic system, optical fiber loss occurs due to :Source to fiber coupling lossesConnector lossSplices lossFiber attenuationSystem margin loss due to component ageing & temp. fluctuations

34. The link power budget provides calculation details for probable losses and additional power margin for component ageing & temp. fluctuations. The power budget equation Pd = Ps – Pr Where Ps is power of the source and Pr is receiver sensitivity.

35.

36. Since, power launched to fiber Pf = Pr + losses and also, Pf = η.Ps ( η is quantum efficiency)thus,Pf – Pr = lossesOr, η.Ps - Pr = losses Or, Pr = η.Ps – lossesHere Losses = m.Lc + n.Ls + .D + s

37. Losses = m.Lc + n.Ls + .D + swhere, Lc- connector loss in dB.Ls- splice loss in dBm-No. of connectorsn-No. of splices-Fiber attenuation in dB/ KmD-transmission distance in Km.s-system margin(6-8 dB)

38. The final power budget equation is Pd = Ps – Pr = Ps – (η.Ps – losses) = Ps – η.Ps + losses = Ps (1 - η) + losses = Ps (1 - η) + m.Lc + n.Ls + .D + sSo the power budget equation isPd = Ps (1 - η) + m. Lc + n. Ls + .D + s

39.

40. Summary of power budget

41. Summary of fiber optic loss budget

42. Power Budget ExampleSpecify a 20-Mb/s data rate and a BER = 10–9. With a Si pin photodiode at 850 nm, the required receiver input signal is –42 dBm. Select a GaAlAs LED that couples 50 mW into a 50-μm core diameter fiber flylead. Assume a 1-dB loss occurs at each cable interface and a 6-dB system margin. The possible transmission distance L = 6 km can be found from PT = PS – PR = 29 dB = 2lc + αL + system margin = 2(1 dB) + αL + 6 dBThe link power budget can be represented graphically (see the right-hand figure).

43. Example: Spreadsheet Power Budget

44. In telecommunication, the term long-haul communications has the following meanings:1. In public switched networks, pertaining to circuits that span large distances, such as the circuits in inter-LATA, interstate, and international communications.2. In the military community, communications among users on a national or worldwide basis.Long-haul systems

45. Basically; Long-haul optics refers to the transmission of visible light signals over optical fiber cable for great distances, especially without or with minimal use of repeaters. Normally, repeaters are necessary at intervals in a length of fiber optic cable to keep the signal quality from deteriorating to the point of non-usability. In long-haul optical systems, the goal is to minimize the number of repeaters per unit distance, and ideally, to render repeaters unnecessary.Long-haul systems

46. The long-haul communications are characterized byHigher levels of users, such as the National Command Authority,More stringent performance requirements, such as higher quality circuits,Longer distances between users, including world wide distances,Higher traffic volumes and densities,Larger switches and trunk cross sections, andFixed and recoverable assets.The "Long-haul communications" usually pertains to the defense services e.g U.S. Defense Communications System.Long-haul systems

47. Actually, Advances in fiber optic technology have made long-haul communications systems reach distances that were once unheard of. Today's fiber optic transmission links transmit multiple channels of video and audio signals over a worldwide distances, and can reach high traffic volumes. This distance is made possible by a number of devices that amplify optical signals and combine larger & larger numbers of signals for transmission over a single optical fiber.

48. A basic long-haul CATV transmission system designed to carry 77 channels of CATV VSB/AM signals for 100km in a basic point-to-point configuration.

49. Long-Haul L-Band Satellite Transport Using CWDM

50. A bidirectional application that multiplexes both L-Band and CATV VSB/AM signals. This configuration also incorporates a WDM channel at 1310nm as well as six channels in the C-Band region.

51. Long-Haul System Using DWDMA unidirectional application where the DWDM transmits eight 1550nm signals over one single-mode fiber. These transmitters could represent a variety of video, audio, and/or data signal inputs.

52. Long-Haul Systems Using EDFA and DCMThe use of EDFAs has gained momentum, especially in long-haul communications systems where one EDFA can replace as many as five conventional amplifiers in the transmission path.Figure below illustrates a DWDM configuration similar to the one shown in last slide; however, in this setup, an EDFA has been added to boost the transmission distance to greater than 200km.

53. Thank You