光纤通信课程设计课件Class8-03.pptx

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1、Class 8 Class 8Brillouin ScatteringStimulated Brillouin Scattering Specifics in fiber Equations Scattering GainExerciseWhats happening:Brillouin scattering,named after Lon Brillouin,occurs when light in a medium(such as air,water or a crystal)interacts with time dependent optical density variations

2、and changes its energy(frequency)and path.The density variations due to acoustic modes,such as phonons,magnetic modes,such as magnons,or temperature gradients.Electrostriction:when the medium is compressed,its index of refraction changes,and a fraction of the traveling light wave,interacting with th

3、e periodic refraction index variations,is deflected as in a threedimensional diffraction grating.Since the sound wave,too,is travelling,light is also subjected to a Doppler shift,so its frequency changes.Brillouin Scattering PhenomenonElastic scatteringInelastic scatteringThe phonons present inside

4、a solid move in thermal equilibrium with very small amplitudes creating fluctuations in the dielectric constant,which is viewed as a movingdiffractiongratingby an incident light wave.Therefore Brillouin scattering can be explained by the two concepts of Braggs reflection and Doppler shift:Fluctuatio

5、n in Fluctuation in density densityMoving MovingObservations in Fiber For a standard SMF SRS threshold:100 W,SBS threshold:1 W The primary method of achieving this involves increasing the mode area of the fibers(largemodearea fibers).To accomplish this and still maintain singlemode operation,however

6、,the NA of the core must be decreased.Reducing the NA of the fiber creates a smaller difference between the refractive index of the core and cladding.Fibers with core diameters of 30 m and NAs as low as 0.06 are available.Another method of maintaining singlemode operation is to use a bendloss mechan

7、ism to filter out higherorder modeslossBrillouin Scattering v.s.Rayleigh and Raman Rayleigh scattering occurs from nonpropagating density fluctuations(random inhomogeneities)scattering from entropy fluctuations.Raman scattering arises from the interaction of light with the vibrational/rotational mod

8、es of the constituent molecules in the scattering medium(glass)scattering of light from optical phonons.Brillouin scattering arises from the interaction of light with propagating density waves or acoustic phonons(inhomogeneities).Each of these scattering processes are always present in optical fiber

9、 as no fiber is free from microscopic defects or thermal fluctuations which drive the three processes.Elastic scatteringInelastic scatteringBrillouin Scattering for sensing,data storage,Stimulated Brillouin Scattering in Silicate GlassEnergy and momentum Energy and momentum conservation conservation

10、Phonon spectrum Phonon spectrumEvolution Equations Evolution EquationsDensity or pressure Density or pressure variation:variation:Electrostriction ElectrostrictionStimulated Brillouin Scattering in Silicate Glass Dispersion relation of phonon angular dependencemultipleorder Gain effectPeak gainThres

11、hold Power for SBS Define:Ps(0)=P0=1 or 0.01 Measurement deviated from theory:difficult to model depends on material,fiber length,temperature,strain,Measurement:threshold power Big difference observed between below and above threshold!Signal Signal depleted depletedMeasurementGain Spectrum:Downshift

12、 11GHz from spontaneous to stimulated Brillouin scattering gain narrowingGain v.s.input power Counterpropagation pumping Difficult to modelSBS Effects in optical fiber Nonlinear interaction between the pump and Stokes wave through an acoustic waveOnly observe in backscattering in fiber(shift in forw

13、ard scattering is zero)Reduces signal strength by directing most portion of the transmitted light back toward the transmitter Introduces distortion and noise into both backscattered and transmitted light main impairment in longdistance optical comm.Systems long distance cant be achieved by power inc

14、rease!A=11GHz;p=1.55 mAn ExampleAfter SBS After SBSLaunched LaunchedAn exampleBER,Noise and Effective AreaEffective Area Increase Effective Area IncreaseWith SBS With SBSThreshold and Effective Area,Length Threshold power:at which the scattered power grows as large as the input power To avoid channe

15、l power below thresholdBrillouin and Brillouin and Pump Pump Linewidth LinewidthEffective Effective Length LengthEffective Effective Area AreaNoise,Noise,Noise SBS:a parametric coupling among a backscattered Stokes wave,pump wave,and electrostrictioninduced traveling acoustic wave significantly incr

16、eases intensity noise in both the backscattered and transmitted light.Experimental and theoretical study confirm:The intensity noise in the transmitted light is phase noise of the laser source,converted by SBS The phase to intensity conversion is attributed to modification of the pump optical spectr

17、um by the frequencydependent imaginary part of the Brillouin loss spectrum.A nonlinear phase shift at the optical carrier is extended to conversion of phase modulation to intensity modulation(PMIM)Ref:Intensity noise induced by stimulated Brillouin scattering in optical fiber transmission systems.Zh

18、ang,Jinye.Supervisor:Phillips,Mary R.Northwestern UniversityMeasures to reduce SBS noise Compensation of PMIM conversion by chromatic dispersion is predicted by the model and successfully observed in an experiment with a dispersion compensating fiber By transmitting the SBSdegraded optical signal th

19、rough a Brillouin amplifier,significant reduction of intensity noise(improvement of more than 6 dB of the signaltonoise ratio(SNR)of the transmitted signal)is achievedThe dependence of the SNR output on the gain and the frequency detuning of the amplifier indicates that the nonlinear group delay at the carrier is the most reliable predictor of output intensity noise Inclass Exercise Design a setup to observe Brillouin back scattering