06 Optical Amplifiers - Ece.ucy.ac.cy
12Optical AmplifiersStavros IezekielDepartment of Electrical andComputer EngineeringUniversity of CyprusSYSTEM CONSIDERATIONS HMY 645 Lecture 06 Spring Semester 201543Global telecommunications relies on optical fibre:pIN (t)pOUT (t) pIN(t - τ)Fibrep (t)L cτ/ngttoτAttenuation & dispersion Reduction in pulse energy Pulse spreadingAlthough optical fibre is an excellent transmission medium (lower loss and largerbandwidth compared to coaxial cable, for example), it is not perfect.t
5At low bit rates, the maximum transmission distance is limited by attenuation, whileat high bit rates the distance is limited by dispersion:Chromatic DispersionConsider a system transmitting ispersion-limitedPower20nm“Wavelength”FP Fabry-Perot laser diodeDFB distributed feedback laser diodeData distortion from dispersionChromatic Dispersion30020250200150100-100-200.80.6Frequency componentsof modulated signaltravel at differentvelocities in fibre0110010160 km1005010101Frequency (GHz)0.40.2Pow erPropagation DistanceTime/DistanceThe source is not spectrally pure, it hasa finite spectral width, which meansthat chromatic dispersion from the fibrewill lead to pulse spreading.80 km1’s0 km00’sTimeNRZ distortion very pattern dependent!
910Impact of attenuationFraction of P owerR emainingConsider a fibre with an attenuation factor of 0.25 dB/km:10.80.6After 80 km, only 1% ofinitial launch e (km)12Optical Signal RegenerationOptical regenerators are classified into three categories by the 3-R's scheme. It is necessary to re-amplify and reshape the pulses at regularintervals using iver3RPhotoreceiverRegeneratorElectronics:Clock recovery,pulse reshapingLaser transmitter3R retiming reshaping re-amplificationFibreoutput1R : re-amplification of the data pulse alone is carried out.2R : in addition to re-amplification, pulse reshaping is carried out.3R : in addition to re-amplification and reshaping, retiming of data pulse isdone.
PhotoreceiverFibreinputExample of a fibre-optic regenerator (622 Mb/s)RegeneratorElectronics:Clock recovery,pulse reshapingDecisionthresholdLaser transmitterFibreoutputDecision times Advantages:– Clock recovery– Pulse reshaping Disadvantages:O/E & E/O conversion neededBit rate is “locked in” – noupgradesSingle wavelength onlyPINphotodiodeTransimpedanceamplifierLaser driverLimiting amplifier1516Electronic regenerators make use of mature technology, but the ideal goal isto go towards all-optical regeneration:OPTICAL AMPLIFICATIONAll-optical 3R regeneration is an active research topic.Laserdiode
18Optical AmplifiersPhysical Principle of Optical Amplifiers All-optical components (i.e. optical input/output). Fibre-based amplifiersalso contain lasers, but this is to create a population inversion in the gainmedium. Have replaced electronics-based regenerators, in which optical signals hadto be photodetected, amplified electronically and then applied to opticalsource. Have revolutionised optical communications– used in wavelength division multiplexed (WDM) systems– allow the use of soliton transmission at ultra high bit rates (1000s ofGb/s) over thousands of km– Have removed the speed and wavelength bottleneck associated with allelectronic regeneration. An optical amplifier provides gain over a useful spectral range, as shownhere for an erbium-doped fibre amplifier:Note: This model does not apply to Raman amplifiers This broad spectral range enables a number of wavelengths to bemultiplexed onto a fibre, thus increasing the bit rate that can betransmitted.FibreAttenuation(dB/km)λ1550 nmOpticalamplifiergain(dB)40 nmλSpectrum of 16 amplified WDM channels (using EDFA)
Ideal amplifier:Opticalgain putInputPumpGAIN Disadvantages: Advantages:– Optical input & output– Photons in – more photons out– Transparent to both bit rate &modulation format– Supports many wavelengths No pulse reshaping Needs dispersioncompensationAdds noise to outputsignalPINGainGainPhasef Flat gain response Linear phase response WDM: Wavelength divisionmultiplexingPIN24Real amplifier:OutputPOUTInputPINGAIN NOISEGainGainTYPES OF OPTICAL AMPLIFIERPhasefPIN Gain saturation Nonlinearity
26Semiconductor Optical AmplifiersTypes of optical amplifierGain medium – semiconductorPump – injection current Semiconductor laser amplifiers (SLAs)Basic structure is similar to a laser diodeTravelling WaveSemiconductor OpticalAmplifier (SOA)1. Fabry-Perot amplifiers: essentially laser diodes that are biased below lasing(oscillation) threshold.2. Travelling-wave amplifiers: here, the facet reflectivities are virtually eliminatedby using anti-reflection coatings or angled facets.Angled-facet or tiltedstripe – the reflectedbeam at the facet isphysically separatedfrom the forwardbeam Fibre amplifiersMirror1. Making use of nonlinear effects, such as stimulated Raman or Brillouinscattering (these are also known as distributed fibre amplifiers).2. Rare earth doped fibres: most common type is erbium-doped (1.55μm centralwavelength), but praseodymium-doped also available (1.3μm).Buried-facetorwindow facet – theoptical beam spreadsin the transparentwindowFabry-PerotSemiconductor OpticalAmplifier (SOA)Important parameters for optical amplifiers include:i.ii.iii.GainNoise figureSaturation output power262728Travelling-waveSOA with angledfacetsToo much facet reflectivity in a Fabry-Perot SOA is not good.Packaged SOAAdvantage of SOAs is that theyare small and can be integratedwith other devices
29Doped Fibre AmplifiersPackaged erbium-doped fibre amplifier (EDFA)Gain medium – fibrePump – laserPump laser diodeCouplerRare-earth dopants (e.g. erbium)Note: Pump wavelength is differentfrom signal wavelengthIts wavelength isdependent on thedopantRaman and Brillouin optical amplifiers have a similar structure, but instead ofdoped fibre, they use highly nonlinear fibre.Erbium-doped fibreInput and output fibres32The most commonly used amplifiers are EDFAsOther doped fibre amplifiers31EDFAs have replaced theapproach taken with earlygeneration links that used allelectronic 3R regenerators.EDFAs are used in all modernlong-distance optical links,but usually the regenerationis 1R.Band nameMeaningWavelength 75
34Bandwidth of various fibre amplifiers (doped and Raman)Optical Amplifier Gain Characteristics- Travelling wave semiconductor optical amplifier (TWSOA), erbiumdoped fibre and Raman fibre amplifiers provide wide spectralbandwidth suitable for WDM applications.- Brillouin fiber amplifier has a very narrow spectral bandwidth 50MHz and it can be used for channel selection within a WDMsystem1440 1460 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 nmEDFA 47 nmEDFA 52 nmFluoride EDFA 62 nmTellurite EDFA 76 nm]TDFA 37 nmTDFA 35 nmRaman Fluoride EDFA 80 nmDist. Raman Fluoride EDFA 83 nmRaman TDFA 53 nmRaman 18 nmRaman 40 nmRaman 100 nmRaman 132 nm1440 1460 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 nmE-BandNote: TWSLA travelling wave semiconductor laser amplifier – just another name for TWSOAS-BandC-BandL-BandU-BandTDFA thullium-doped fibre amplifier, EDFA erbium-doped fibre amplifier35 Application 1: As in-line amplifiers in long-haul links to compensate forattenuation in the 1550 nm window. Mostly EDFAs and Raman.OpticalSourceOpticalfibreAPPLICATIONS OF OPTICALAMPLIFIERSOptical amplifiers boost thesignal at regular intervals (e.g.between 30 km to 80 km) tomake sure power level in linkdoes not drop below therequired receiver sensitivityOpticalReceiverOptical amplifier compensatesfor fibre loss at periodic intervals
30Optical amplifiers compensate for loss, but they also introduce noise:Dispersion (ps/nm)201300nmFibre Loss (dB/km)80-100kmThe other problem is that forstandard single-mode fibre, the1550 nm window offers lowloss, but minimal chromaticdispersion is at 1310 nm.Amplifier1550nmwindow100-10-20PowerInputAfter LossAfter Amplifier-3012501350145015501650Wavelength (nm)Added NoiseHence some kind of “dispersionmanagement” or dispersion compensation isrequired, e.g. by using dispersion-shifted fibre(DSF)WavelengthHence a low noise figure is important, as well as saturation power (being able to handlemedium power levels) Application 2: As power amplifiers to increase source power (post-amplifiers):PS (dBm)OpticalTransmitterG(dB) Application 3: As pre-amplifiers to improve receiver sensitivity:Optical inputOutput power (dBm) PS G Most laser diodes used in optical transmitters have powers of a few mW, butfibre can handle of the order of 100 mW before optical nonlinear effects occur.So a power amplifier can be used to boost signal immediately after the source. SOAs are useful because they can be integrated with lasers, but EDFA poweramplifiers are also available with output powers around 100 mW. Amplifier adds noise, but this is attenuated by the fibre Important that the amplifier is not saturated by the transmitterOpticalReceiver Optical amplifier is placed immediately before the optical receiver in orderto improve sensitivity. At this point the signal is weak, so good gain is required, but even moreimportant is the fact that the amplifier must not add a lot of noise, so a lownoise figure is required (typically less than 5 dB).
Application 4: As booster amplifiers in distribution networks (e.g. local access)to compensate for losses in a fibre splitter:Selecting Amplifiers for Applications 1,2,3TypeGainMaximum OutputpowerNoise figurePower AmplifierHigh gainHigh output powerNot very importantIn-lineMedium gainMedium outputpowerGood noise figureLow output powerLow value 5 dBessentialPreamplifierHigh gainStar coupler: splits into N fibres; has insertion and splitting loss4344 Other applications: It also possible to take advantage of nonlinearities insemiconductor optical amplifiers to perform operations such as erSOAλ2timetime Input wavelength 1 drives the SOA into compression, and so modifies the gainthat wavelength 2 sees. After filtering, the output appears on wavelength 2 asan inverted version of the input on wavelength 1.FIGURES OF MERIT FOR OPTICALAMPLIFIERS
Important figures of merit & considerations for an amplifierProperties of Ideal Optical Amplifiers Provide high gain– (30 dB or more) Include:– Gain– Bandwidth– Gain saturation– Noise Have a wide spectral bandwidth– to allow several wavelengths to be transmitted Provide uniform (i.e. flat) gain vs. λ– to maintain relative strength of spectral components Allow bi-directional operation– i.e. gain in both directions Have low insertion loss– to maximise benefits of amplifier gain Have no crosstalk– i.e. no interference between different spectral components Have wide dynamic range– gain should not saturate with high input powers Have a good conversion efficiency– pump power converted to amplifier gainGain profile of erbium-doped silica fibreHigh gain over a wide spectral bandwidth, but the gain profile is not flat.Spectrum of EDFA with1480 nm pumpASE: Amplified spontaneous emission noise
Typical gain versus power profile for optical amplifier:EDFA gain versus pump level51EDFA Basic tical filterWeak inputsignal at1.55μmERBIUM-DOPED FIBRE AMPLIFIERS– BASIC PHYSICSLaser diodepump at 980 nmor 1480 nm,Up to 50 mW powerAmplificationsection witherbium dopedsilica fibre,a few tens of metres(Er3 ions, 100 – 100 ppm) Amplifiedsignal at1.55µm Gain20 to 30 dB.30 dB gain means1000 photons outfor 1 photon in
Power levelPower levelPower exchange980 nmsignal1550 nmdata signalIsolatorWavelengthmultiplexerInput980 nmsignal1550 nmdata signalNarrowbandoptical filterOutputPumpEnergy Transitions in Er3 - Doped Silica Fibre
Pumping ConfigurationsPumping Configurationsforward pumpingbackward pumpingbidirectional pumpinggives higher gainpopulation inversionrelatively uniform alongamplifier lengthForward-pumping(same direction as signal)gives less noiseBackward-pumping(opposite direction to signal)Bidirectional-pumping(both directions)59Gain as a function of length of erbium-doped fibreIf the fibre is too long, there will be more absorption than gain, but if thefibre is too short we will not have as much gain as we could. Optimumlength depends on the pump power.60Two-stage EDFASome new EDFA designs concatenate two or even three amplifier stages. Anamplifier “stage” is considered to consist of any unbroken section of erbium dopedfibre. Multistage amplifiers are built for a number of reasons:1. To increase the power output whilst retaining low noise2. To flatten the total amplifier gain response3. To reduce amplified stimulated emission noise
61Random spontaneousemission (SE)Amplifiedspontaneousemission (ASE)Amplification along fibreErbium randomly emits photons between 1520 and 1570 nmNOISE & GAIN COMPRESSION INERBIUM-DOPED FIBRE AMPLIFIERS Spontaneous emission (SE) is not polarized or coherent Like any photon, SE stimulates emission of other photons With no input signal, eventually all optical energy is consumed intoamplified spontaneous emissionOptical Amplifier ChainsAmplifier Chains and Signal LevelLink between a transmitter andOptical amplifiers allow one to extend linkFibredistancereceiverExample: system uses fibre with 0.25 dB/kmattenuation, 80 km fibre sections,Fibre Linkamplifiers with 19 dB gain a noise figure of 5 dBAmplifier can compensate for attenuation10Amplifiers also introduce noise, as each amplifier reduces the Optical SNR by a smallamount (noise figure)OpticalReceiverTransmitter1Optical Amplifiers2Signal level (dBm)Cannot compensate for dispersion (and crosstalk in DWDM on (km )Fibre SectionEach amplifier restores the signal level to a value almost equivalent to the levelat the start of the section - in principle reach is extended to 700 km
EDFA Behaviour at Gain SaturationAmplifier Chains and Optical SNRThere are two maindifferences between thebehaviour of electronicamplifiers and of EDFAs ingain saturation:FibreLink noise figure of 5 dB,Same system: Transmitter SNR is 50 dB,amplifier60Optical SNR (dB)501) As input power is increasedon the EDFA the total gain ofthe amplifier increases slowly.4030201000100200300400500600700800Location (km)Optical SNR drops with distance, so that if we take 30 dB as a reasonable limit, themax distance between T/X and R/X is only 300 kmSaturation in EDFAsTotal output power:Amplified signal Noise (Amplified Spontaneous Emission ASE)Total P outMax-3 dBGainAn electronic amplifier operates relatively linearly until its gain saturates. This means that anelectronic amplifier operated near saturation introduces significant distortion into the signal (it justclips the peaks off).2) An erbium amplifier at saturation simply applies less gain to all of its input regardless of theinstantaneous signal level. Thus it does not distort the signal. There is little or no crosstalk betweenWDM channels even in saturation.Gain Compression Total output power:Amplified signal ASE– EDFA is in saturation if almost allErbium ions are consumed foramplification– Total output power remains almostconstant– Lowest noise figureTotal P outMax-3 dBGain- 30- 20- 10P in (dBm)EDFA is in saturation if almost all Erbium ions are consumed for amplificationTotal output power remains almost constant, regardless of input power changes Preferred operating point– Power levels in link stabilizeautomatically-30-20P in (dBm)-10
69EDFA Output Spectra 10 dBmAmplified signal spectrum(input signal saturates theoptical amplifier)ASE spectrum when noinput signal is presentGAIN PROFILE OF ERBIUM-DOPEDFIBRE AMPLIFIERS-40 dBm1575 nm1525 nmEDFA Gain SpectrumGain Characteristics of EDFAErbium can provide about 40-50 nm of bandwidth, from 1520 to 1570 nmGain spectrum depends on the glass used, eg. silica or zblan glassGain spectrum is not flat, significant gain variations (basically because ofdifferent population levels in different bands).Gain (amplifier) - is the ratio in decibels ofinput power to output power.Gain at 1560 nm is some 3 dB higher thangain at 1540 nm (this is twice as much).In most applications (if there is only asingle channel or if there are only a fewamplifiers in the circuit) this is not toomuch of a limitation.30EDFA gain spectrumGain(dB)201001520153015401550Wavelength (nm)1560WDM systems use manywavelengths within the amplifiedband. If we have a very long WDMlink with many amplifiers thedifference in response in variouschannels adds up.
74Gain Flattening ConceptRAMAN AMPLIFICATIONRaman AmplifiersRaman Fibre Amplifiers (RFAs) rely on an intrinsic nonlinearity in silica fibreRaman Effect Amplifiers Stimulated Raman Scattering (SRS) causes a new signal (a Stokes wave) to begenerated in the same direction as the pump wave down-shifted in frequency by 13.2THz (due to molecular vibrations) provided that the pump signal is of sufficientstrength.Variable wavelength amplification:Depends on pump wavelengthFor example pumping at 1500 nm produces gain at about 1560-1570 nmRFAs can be used as a standalone amplifier or as a distributedamplifier in conjunction with an EDFA In addition SRS causes the amplification of a signal if it is lower in frequency than thepump. Optimal amplification occurs when the difference in wavelengths is around13.2 THz. The signal to be amplified must be lower in frequency (longer in wavelength) thanthe pump. It is easy to build a Raman amplifier, but there is a big problem:we cannot build very high power (around half a watt or more) pump lasers at anywavelength we desire! Laser wavelengths are very specific and high power lasersare quite hard to build.
Distributed Raman Amplification (I)Distributed Raman Amplification (II)Raman pumping takes place backwards over the fibreGain is a maximum close to the receiver and decreases in the transmitterdirectionWith only an EDFA at the transmit end the optical power level decreases over thefibre lengthWith an EDFA and Raman the minimum optical power level occurs toward themiddle, not the end, of the fibre.Long Fibre SpanEDFAOpticalReceiverOptical PowerTransmitterEDFA RamanEDFA onlyRamanPump LaserDistanceBroadband Amplification using Raman AmplifiersRaman amplification can provides very broadband amplificationAdvantages and Disadvantages of Raman AmplificationAdvantagesVariable wavelength amplification possibleMultiple high-power "pump" lasers are used to produce veryhigh gain over a range of wavelengths.Compatible with installed SM fibreCan be used to "extend" EDFAs93 nm bandwidth has been demonstrated with just two pumpssources400 nm bandwidth possible?Can result in a lower average power over a span, good for lower crosstalkVery broadband operation may be possibleDisadvantagesHigh pump power requirements, high pump power lasers have only recently arrivedSophisticated gain control neededNoise is also an issue
Optical Amplifiers All-opticalcomponents (i.e. optical input/output). Fibre-based amplifiers also contain lasers, but this is to create a population inversion in the gain medium. Have replaced electronics-based regenerators, in which optical signals had to be photodetected, amplified electronic
RF/IF Differential Amplifiers 5 Low Noise Amplifiers 7 Low Phase Noise Amplifiers 10 Gain Blocks 11 Driver Amplifiers 13 Wideband Distributed Amplifiers 13 Power Amplifiers 15 GaN Power Amplifiers 18 Variable Gain Amplifiers 19 Analog Controlled VGAs 19 Digitally Controlled VGAs 20 Baseband Programmable VGA Filters 21 Attenuators 22
Semiconductor optical amplifiers (SOAs) Fiber Raman and Brillouin amplifiers Rare earth doped fiber amplifiers (erbium – EDFA 1500 nm, praseodymium – PDFA 1300 nm) The most practical optical amplifiers to date include the SOA and EDFA types. New pumping methods and materials are also improving the performance of Raman amplifiers. 3
optical networks have been made possible by the optical amplifier. Optical amplifiers can be divided into two classes: optical fibre amplifiers (OFA) and semiconductor optical amplifiers (SOAs). The former has tended to dominate conventional system applications such as in-line amplification used to compensate for fibre losses.
Optical Amplifiers vs Regenerators (1 of 2) Transparent: Regenerators specific to bit rate and modulation format used; O-Amps are insensitive Easily upgraded: A system with optical amplifiers can be more easily upgraded to higher bit rate without replacing the amplifiers Optical amplifiers
Semiconductor Optical Amplifiers (SOAs) have mainly found application in optical telecommunication networks for optical signal regeneration, wavelength switching or wavelength conversion. The objective of this paper is to report the use of semiconductor optical amplifiers for optical sensing taking into account their optical bistable properties .
Mar 14, 2005 · Background - Optical Amplifiers zAmplification in optical transmission systems needed to maintain SNR and BER, despite low-loss in fibers. zEarly optical regeneration for optic transmission relied on optical to electron transformation. zAll-optical amplifiers provide optical g
RF/IF Differential Amplifiers 9 Low Noise Amplifiers 9 Gain Blocks and Driver Amplifiers 10 Wideband Distributed Amplifiers 11 Linear and Power Amplifiers 12 GaN Power Amplifiers 13 Active Bias Controllers 13. Variable Gain Amplifiers 14. Analog Controlled VGAs 14 Digitally Controlled VGAs 14 Baseband Programmable VGA Filters 14. Attenuators 15
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