光纤通信导论（英文）课件_179页.ppt

按一下以編輯母片標題樣式,按一下以編輯母片,第二層,第三層,第四層,第五層,*,*,*,Chapter 3,Components,Couplers,Isolators and Circulators,Multiplexers and Filters,Optical Amplifiers,Transmitters,Detectors switches,Wavelength converters.,1,3.1 Couplers,wavelength independent,wavelength selective for 1.31/1.55 multiplexing,：,coupling ratio,3dB couple=1/2,=0.95(for monitoring),1,1-,2,Def:excess loss:the loss of the device above the,fundamental loss introduced by the coupling ratio,Example:A 3dB coupler may have 0.2dB excess loss,For multiplexing,1310nm,1550nm,1310nm,1550nm,For EDFA,1550nm,1550nm,980nm,or 1480nm,980nm,or 1480nm,3,3.1.1 Principle of Operation,E:electrical field,S-parameters,For lossless couplers,a,1,a,2,b,1,b,2,4,The power transfer function,5,3.1.2 Conservation of Energy(S-parameter),6,Similarly the sum of output power is proportional to,If it is lossless,This relation holds for arbitrary,Eq(3.4)can be extended to any number of ports,7,For a 2 x 2 symmetrical coupler,8,3.2 Isolators and Circulators(nonreciprocal devices),Isolators are for transmitter,circulators are for add and drop or others.,The insertion loss should be small 1dB,A circulator is similar to an isolator except it has multiple ports.,9,3.2.1 Principle of Operation of an Isolator,SOP=state of Polarization,10,A spatial walk-off polarized splits the signal into two orthogonally polarized components.,11,3.3 Multiplexer and Filters,Multiplexers and filters are for WDM,add/drop.WXC,12,Dynamic,WXCs,use optical switches and,mux/demux,.,13,The desired characteristics of filters,Low insertion loss,Polarization-independent loss,Low temperature coefficient,Reasonable broad,passbands,Sharp,passband,skirts,Low cost,a.integrated-optic(may be polarization dependent),b.all-fiber devices,passband,skirt,14,15,3.3.1 Gratings,Any device whose operation involves interference among multiple optical signals originating from the same source but with different relative phase shifts.An exception is a device where the multiple optical signals are generated by repeated traversals of a single cavity (etalons).,F-P,16,17,Principle of Operation,The pitch of the grating(distance between adjacent slits)=a,Assuming plane wave is incident at angle,:diffraction angle,The slits are small compared to,phase changes across a slit is negligible,18,For construction interference at occurs at the image plane if,19,20,The energy at a single is distributed over all the discrete angles that satisfy(3.9).,For WDM only light of a certain order m will be collected,the remaining energy is lost.,m=0 has most energy,i,=,d,The wavelengths are not separated.,blazed reflection grating maximize the light energy at,21,3.3.2 Diffraction Pattern,Relax the constrain a increasing the main lobe width,The index distribution over the length of BG is analogous to the grating aperture in sect3.3.2.,The side lobes arise due to the abrupt start and end of the grating,which result in a,sinc,(.)behavior for the side lobes.,Apodization,is similar to pulse shaping to reduce the side lobes of signal spectrum.,29,3.3.4 Fiber Gratings(,FGs,),Useful for filter,add/drop compensating dispersion,Advantages:,a.low loss(0.1dB),b.ease of coupling,c.polarization insensitivity,d.low temperature coefficient,e.simple packaging,f.extremely low cost,Made from photosensitive fiber(,Ge,-doped),UV intensity n,change of n 10,-4,Two kind of,FGs,a.short period(Bragg Grating 0.5m),b.long period(100,+,m 1000,+,m),30,Fiber Bragg Gratings(FBG),extremely low loss 0.1dB,high wavelength accuracy(0.05nm),high crosstalk suppression(Fig 3.8)(40dB),flat tops,typical temperature coefficient 1.25x10,-2,nm/,For passive temperature-compensated 0.07x10,-2,nm/,31,32,Long-Period Fiber Grating(a few,intermeters,),Useful for EDFA gain(equalization),They may be cascaded to obtain the desired profile.,33,Principle of Operation,The propagating mode in core couples onto the modes in the cladding=induce loss,For a given,coupling occurs depending on,=propagation constant of the core mode,:propagation constant of the path order cladding mode,The phase matching condition,34,Let and be the refractive indices of the core and the path-order cladding modes,core,cladding,35,3.3.5,Fabry,-Perot Filters,This filter is called,Fabry,-Perot interferometer or etalon.,Principle of Operation,The wavelengths for which the cavity length is an integral multiple of half the wavelength in the cavity are called resonant wavelengths.,36,A round trip through the cavity is an integral multiple of the wavelength.,The light waves add in phase.,Assume r,1,=r,2,t,1,=t,2,The reflectance R=r,1,r,2,A:absorption loss of mirror,T=t,1,t,2,=transmission,37,38,A=0,R=0.75,0.9 and 0.99,T,FP,(f)is periodic function with period FSR,Where FSR:free spectral range,=The spectral range between two successive,passband,=1/2,39,is the smallest value,satisfied the condition,40,Tunability,1.change cavity length,2.change refractive index n,Recall,The wave with frequency will be selected.,mechanical tuning,piezoelectric tuning,=thermal instability,hysteresis,41,3.3.6 Multilayer Dielectric Thin-Film Filters,A thin-film resonant,multicavity,filter(TFMF)consist of two or more,cavitied,.,Advantages:flat top,sharp skirt,low loss,insensitive to the polarization,42,43,44,3.3.7 Mach-,Zehnder,Interferometers(MZI),Usage:,filter,MUX/DEMUX,modulator,switch,Problems:,wavelength drift caused by aging or temperature variation,not exact 50:50,not flat top,passbands,Change temperature(or refractive index)of one arm=tuning,45,Principle of Operation,46,At the upper output.,The signal all through the upper arm as reference.,The signal through the lower arm and the upper output has phase lag,At the lower output the phase difference,through low arm,由第一個,3dB coupler,產生,delay/2,through upper arm,由第二個,coupler,到第二個,output,產生,delay/2,所以互相,cancel,47,48,consider K MZI interconnected,The path length difference for the,kth,MZI is assumed to be,49,50,MZI can be used as a 1x2,demultiplexer,or multiplexer,1,2,chosen to be coincide with the peaks or troughs of the transfer function,If ,and m,i,is odd,say m,i,=1 output 1 has signal,output 2 has no signal,If and m,i,is even,output 1 has no signal.,1,2,1,2,MZI,51,3.3.8 Array wavelength Grating(AWG),Usage:a.nx1 multiplexer,b.1xn,demultiplexer,c.,crossconnect,(wavelengths and FSR,must be chosen),Advantages:low loss,flat,passband,ease to realized on a integrated-optic substrate(silicon),the waveguides are silica.,Ge,-doped silica,or SiO,2,-Ta,2,O,5,Because the temperature coefficient=0.01nm/is large,Temperature control may be needed.,目前除了用,Rowland circle,之外尚可用,mu,ltimode interference(MMI),做,coupler,52,53,Principle of Operation,Let number of inputs and outputs be n,and the numbers of inputs and outputs of the couplers be,nxm,and,mxn,L=length difference between two adjacent waveguides.,=difference in distance between input i and array waveguide k,=difference in distance between array waveguide k and output j,n,i,k,m,m,n,54,The relative phase,input,output,through k,k=1.2.m,55,Rowland circle construction,grating circle,Rowland,56,57,3.3.9 Acoustic-Optic tunable Filter(AOTF),polarization-dependent,polarization-independent.,58,Principle of Operation,As Fig 3.27 AOTF is constructed from a,birefringent,material and only supporting the lowest-order TE and TM modes.,If an acoustic wave is launched,the n varies to form gratings.,The Bragg condition is satisfied,TE mode is converted to TM mode.,For LiNbO3,|,n,TE,-n,TM,|=0.07=,n,.at 1.55m,=,n,(3.18),At 170MHz =22m,acoustic wavelength,59,The transfer function is,where,=-,0,0,satisfies(3.17),=,0,2,/,l,n,l,:the length of acoustic-optic interaction,FWHM bandwidth=0.8,-10dB down is not enough,=cross talk,60,61,Disadvantages:high loss,large crosstalk,bulky,wide,passband,100GHz,dynamic,crossconnect,response time millisecond,62,3.3.10 High Channel Count Multiplexer Architectures,A.Serial(only for small number of ports),不同,channel,有不同,insertion loss,63,B.Single stage(AWG),最好的選擇,64,C.Multistage banding,65,D.Multistage Interleaving,66,3.4 Optical Amplifiers,Advantages:transparent to bit rate,pulse format,large bandwidth,high gain,Disadvantages:noise accumulates,Erbium-doped fiber amplifiers(EDFA),Raman amplifiers(RA),Semiconductor optical amplifiers(SOA),67,3.4.1 Stimulated Emission(EDFA or SOA),68,Two energy levels E,2,E,1,hf,c,=E,2,-E,1,h:Plancks constant=6.63x10,-34,JS,(absorption),E,1,E,2,excitation(by photons or population inversion),E,2,E,1,emission photons,a.stimulated emission,b.spontaneous emission,If emission absorption=amplification,N,1,:Population(number of atoms)at E,1,N,2,:population at E,2,If N,2,N,1,population inversion occurs.,69,3.4.2 Spontaneous Emission,If ASE is very large,=Saturate the amplifier,70,3.4.3 EDAF,Erbium fiber=E,r,3+,doped silica fiber,Pumping wavelength=980nm or 1480nm,Advantages,Availability of high power pump lasers,All fiber device,polarization independent,ease to couple,reliable,Simple,Less crosstalk,71,Principle of Operation,72,Stark splitting:an isolated ion of erbium is split into multiple energy levels.,Each stark splitting level is spread into a band.,Thermalization,:the erbium ions are distributed in the various levels within the band.,Capable of amplifying several wavelengths simultaneously.,page 39,c-band from 15301565nm,73,When 980nm pump is used,32,1,sec High output power and pump can be located remotely,74,75,76,77,Gain Flatness,78,79,80,81,Multistage Designs,The first stage:high gain,low noise,The second stage:high output power,Two-stage amplifier is more reliable(pump failure),The inserted loss element can be gain compensation,add/drop or dispersion compensation,L-band EDFA needs high pumping power and produces high ASE,82,83,3.4.4 Raman Amplifiers(RA),RA can provide gain about 100nm band(13THz)above the pumping wave,p,fluctuation=noise)so,pumping opposite direction will lower the noise.(average out),Another noise is due to Rayleigh scattering of the pumping signal,For example,15501600nm,signal,14601480nm pump,84,85,86,3.4.5 Semiconductor Optical Amplifiers(,SOAs,),Amplifier,Switches,wavelength converters,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,3.5 Transmitters,A transmitter includes a driving circuit and a light source.,The light source can be laser or LED.For WDM systems,a laser needs to have the following important characteristics:,Reasonably high power 010dBm,low threshold current,high slop efficiency,Narrow spectral width,Wavelength stability(low aging effect),Small chirping(direct modulation),103,104,105,Lasers,Semiconductor lasers,fiber lasers,gas lasers,solid state lasers(Ruby lasers),free electron laser,106,Principle of Operation(semiconductor laser),Reference:John,Gowar,“Optical Communication Systems”PP262323,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,Longitudinal Modes,Multiple-longitudinal mode(MLM)lasers have large spectral widths10nm(,Fabry,-Perot lasers)=cause chromatic dispersion,Singlelongitudinal mode(SLM)lasers have very narrow spectral widths,Side-mode suppression ratio is an Important parameter for SLM lasers.(30dB),125,Distributed-Feedback Lasers(DFB Lasers)Distributed Bragg reflector(DBR)Lasers,The temperature coefficient 0.1nm/at 1550nm.,126,External Cavity Lasers,Grating External Cavity Lasers,127,3.5.3 Tunable lasers,Tunable lasers are useful to reduce the inventory,(spare parts),to reconfigure the network,to be used for optical packet switched networks and for laboratory testing.,Tuning mechanisms,Injecting current(change n)tuning range 1015 nm at 1550nm,Temperature tuning 0.1nm/,Mechanical tuning(wide range but bulky),Desirable properties,Short tuning time,Wide tuning range(100nm),Stable over its lifetime,Easily controllable and,manufacturable,128,Two-and Three-Section DBR Lasers,Problems,Aging,Temperature changes,Current recalibration,Mode hopping,129,Vertical grating-assisted coupler filter(VGF)Lasers,The coupling condition(3.17),=,B,(n,1,-n,2,),B,:The period of the Bragg grating,n,1,and n,2,are refractive indices of two,waveguides.,If n,1,changes to,n,1,+n,1,130,Sample Grating and Super-Structure Grating DBR lasers,131,Grating Coupled sampled Reflection lasers,132,3.5.4 Direct and External Modulation,Direct modulation,Advantage:Simple,Disadvantage:induce chirping,Biasing above the threshold will reduce chirping but decrease the extinction ratio.,133,External Modulation,a.Lithium,niobate,modulator,b.electro-absorption modulator,134,:coupling coefficient depending on width of the waveguide,refractive indices,distance of two waveguides,135,MZI can achieve high extinction ratio 15 20dB with almost on chirping.Polarization control is needed.,136,3.6 Detectors,137,3.6.1,Photodetectors,Photons incident on a semiconductor are absorbed by electrons in the valence band.These are excited into the conduction band and leave holes in the valence band.When a reversed bias voltage is applied,these electron hole pairs produce photo current.,138,139,140,141,142,PIN Photodiodes,A very lightly doped intrinsic semiconductor between the p-type and n-type Layers can improve the efficiency.The depletion region extends across the intrinsic layer.,If the p-type or n-type layer is transparent the efficiency can be further improved.,143,144,145,146,Avalanche Photodiodes(APD),When the generated election in a very high electric field,it can generate more secondary electron-hole pairs.This process is called avalanche multiplication.,G,m,:multiplicative gain,M:multiplication factor(G,m,:M-1),Large G,m,will induce large noise.,If G,m,avalanche breakdown occurs.,147,148,149,150,151,152,153,154,155,156,157,3.6.2 Front-End Amplifiers,High-impedance amplifier,Transimpedance,amplifier,158,159,160,3.7 Switches,Important parameters,Number of ports,Switching time,The insertion loss,The crosstalk,Polarization-dependent loss,Latching(maintaining its switch state),Monitoring capability,Reliability,161,3.7.1 Large Optical Switched,The main considerations,Number of switch elements required,Loss uniformity,Number of crossovers,Blocking characteristics,blocking and,nonblocking,(strict sense,wide sense,rearrargeable,),Synchronous or asynchronous,162,Crossbar,163,Spanke,164,3.7.2 Optical Switch Technologies,165,MEMS Switches,166,167,Bubble-Based Waveguide Switch,168,Liquid Crystal Switches,169,A.Thermal-Optic Switches(MZI),B.Semiconductor Optical Amplifier Switches,C.Large Electronic Switched,Single stage,Multistage,Line rate,Total capacity(line rate x number of ports),Circuit switching V.S.packet switching,Cross bar V.S.shared memory,170,3.8 Wavelength Converters,A device converters data from one incoming wavelength to another outgoing wavelength.,Used in WDM networks,i.input wavelength is not suitable for the networks,ii.Improving the wavelength utilization in WDM networks,iii.Converting to suitable outgoing wavelengths,Types,i.fixed-input,fixed-output,ii.Variable-input,fixed-output,iii.Fixed-input,variable-output,iv.Variable-input,variable-output,171,Other important characteristics,i.,convertion,range,ii.Transparent to data rate or