2 Fiber-Optic Communications Systems Chapter 9Fiber-Optic Communications Systems
3 9.1 System Design Considerations Design is based onApplicationType of signalDistance from transmitter to detectorPerformance standardsResource constraints (time, money, etc.)ImplementationComponentsFormat, power, bandwidth, dynamic rangeAmplification
4 9.1 System Design Considerations Design is based onImplementationComponentsFormat, power, bandwidth, dynamic rangeAmplification, amplitude, and spacingMultiplexingSecurity requirementsAcceptable noise levels
5 9.1 System Design Considerations System Power BudgetMost important parameter is throughput or transfer function.Output power must be greater than the input sensitivity of the receiver.System budgetAmount of power lost or gained in each componentSystem power marginAllows for component tolerances, system degradation, repairs and splices
6 9.1 System Design Considerations Power at the SourceTransmitter must be appropriate for the applicationNumber of signalsWavelength of signalType of transmitter device (LED, Laser)ModulationMode structureTunabilityWDM and amplification capabilityCoupling efficiency
7 9.1 System Design Considerations Power in the FiberMatchingSource output pattern, core-size, and NA of fiberCoupling is criticalPower at the DetectorSensitivity is the primary purpose of the detectorMinimum sensitivity yet still meets standardsMust support the dynamic range of the power levels
8 9.1 System Design Considerations Fiber AmplificationFor those fibers that require amplificationTwo types:Repeaters are rarely used.Optical amplifiers are the preferred amplification.Use manufacturers specifications to ensure optimization of the input signal.
9 9.1 System Design Considerations Amplifier PlacementDepends onType of amplifierTransmitterReceiverRise timeNoise and error analysisCan be insertedBefore regenerationBetween regenerators
10 9.1 System Design Considerations System Rise Time BudgetDetermines the bandwidth carrying capabilityTotal rises time is the sum of the individual component rise times.Bandwidth is limited by the component with the slowest rise time.
11 9.1 System Design Considerations Rise Time and Bit TimeRise time is defined as the time it takes for the response to rise from the 10% to 90% of maximum amplitude.Fall time is the time the response needs to fall from 90% to 10% of the maximum.Pulse width is the time between the 50% marks on the rising and falling edges.
12 9.1 System Design Considerations Transmitters, Receivers, and Rise TimeRise time of transmitter is based on the response time of the LED or laser diode.Rise time of the receiver is primarily based on the semiconductor device used as the detector.
13 9.1 System Design Considerations Fiber Rise TimeComes directly from the total dispersion of the fiber as a result of modal, material, wave guide, and polarization mode dispersionTotal Rise TimeSum of all the rise times in the system
14 9.1 System Design Considerations Round Trip DelayTime needed for the signal to reach the furthest point of the network and returnDispersion CompensationAllows for lowering the fiber dispersion characteristicsadd fiber with dispersion of the opposite magnitudeOnly available type: chromatic dispersion
15 9.1 System Design Considerations Single Channel System CompensationImplementationLong length of small amplitude dispersion fiberShort length of large amplitude dispersion fiber (distributed compensation)Multi-Channel System CompensationLarge effective area fibersReduced dispersion fibers
16 9.1 System Design Considerations Single Channel System CompensationNoise and Error AnalysisDetermines the type of amplification requiredMinimizing System NoiseAdditional Noise SourcesExtended pulse widthModal properties of fibersChirpFresnel reflectionFeedback noise
17 9.1 System Design Considerations Multiple Channel SystemChannel Density and SpacingStandards have been defined by ITU-TWDM, TDM, and NoiseInterchannel crosstalk: Data from adjacent channels gets mixedDispersion in adjacent channelsNon-linearities at high powers causes interferenceNarrow bandpass filtering at the receiver
18 9.1 System Design Considerations WDM Power ManagementMethods must ensure that all power levels fall with acceptable range.Gain flattening is the process of adjusting the amplitudes of wavelengths to be the same.
19 9.2 From the Global Network to the Business and Home Long-Haul CommunicationsTerrestrial cablesTelegraph cable across the English Channel in 1850First transatlantic cable in 1866Transatlantic telephone cable in 1957Transatlantic fiber-optic cable in 1988Optical amplifiers replaced repeaters in 1990s
20 9.2 From the Global Network to the Business and Home Undersea CablesMust be capable of low loss and dispersionMust limit optical noiseMust have a pressure resistant coveringAmplifier gain below 10 dBPrecise dispersionRepeatered systems has pump laser and amplifierUnrepeatered system has optical amplifiers spaced out over the length of the fiber
21 9.2 From the Global Network to the Business and Home Terrestrial CablesLong-haul lengthsEasy repairAmplification needed less oftenWhen is terrestrial, satellite or undersea cabling used?Depends on politics and economy rather than technology or geography
22 9.2 From the Global Network to the Business and Home Metro and Regional NetworksPSTN: Public switched telephone networks for regions (little population)MANs: Metropolitan area networks (more densely populated areas such as towns and universities)LANs: Local area networksWANs: Wide area networks
23 9.3 Special Fiber-Optic Communications Systems Soliton CommunicationsForm of dispersion compensationCombination of chromatic and self-phase modulationCoherent Communications SystemsUses WDM bandwidth more efficientlyPossible improvement in receiver sensitivity
24 9.3 Special Fiber-Optic Communications Systems Optical CDMAMaximizes the bandwidth in LANs without special filtering devicesSpreads the signal energy over a wider frequency band than necessary
25 9.3 Special Fiber-Optic Communications Systems Free Space OpticsSignal travels through space rather than a fiberRelies on line of sightFree of FCC regulationsBandwidth is not held to that of the fiber usedFiber Optics and the Future“Where you go, then so shall I.”