* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Chapter 2
Gaseous detection device wikipedia , lookup
Optical coherence tomography wikipedia , lookup
Dispersion staining wikipedia , lookup
Spectral density wikipedia , lookup
Photon scanning microscopy wikipedia , lookup
Birefringence wikipedia , lookup
3D optical data storage wikipedia , lookup
Magnetic circular dichroism wikipedia , lookup
X-ray fluorescence wikipedia , lookup
Astronomical spectroscopy wikipedia , lookup
Anti-reflective coating wikipedia , lookup
Ultrafast laser spectroscopy wikipedia , lookup
Fiber-optic communication wikipedia , lookup
Passive optical network wikipedia , lookup
Phase-contrast X-ray imaging wikipedia , lookup
Silicon photonics wikipedia , lookup
Ultraviolet–visible spectroscopy wikipedia , lookup
Diffraction grating wikipedia , lookup
Optical rogue waves wikipedia , lookup
Nonlinear optics wikipedia , lookup
Fiber Bragg grating wikipedia , lookup
Opto-isolator wikipedia , lookup
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] 1 α 1-α α:coupling ratio 3dB couple α= 1/2 α = 0.95 (for monitoring) 2 For multiplexing 1310nm 1310nm 1550nm 1550nm For EDFA 1550nm 1550nm 980nm or 1480nm 980nm or 1480nm Def: excess loss: the loss of the device above the fundamental loss introduced by the coupling ratio α Example: A 3dB coupler may have 0.2dB excess loss 3 a1→ 3.1.1 Principle of Operation → b1 → b2 a2→ E: electrical field S-parameters For lossless couplers E01( f ) j cos( k ) i sin( k e E ( f ) i sin( k ) cos( k 02 ) Ei1( f ) ) Ei 2 ( f ) (3.1) b1 s11 s12 a1 b s s 2 21 22 a2 : the coupling length k : coupling coefficient depending on width, shape of waveguides, n1 distance... 4 The power transfer function Tij ( f ) E0 j 2 Eii 2 i : input, j : output T11( f ) cos2 ( kf ) 2 T ( f ) 12 sin ( kf ) (3.2) For a 3dB coupler 1 T11( f ) T12 ( f ) 2 1 sin ( kf ) cos ( kf ) 2 2 k (2n 1) 2 4 n0 5 3.1.2 Conservation of Energy (S-parameter) E01 s11 s12 Ei1 E02 s21 s22 Ei 2 (3.3) The scattering matrix is s11 s12 S s21 s22 sij : complex Denote E 0 ( E01 E02 )T E i ( Ei1 Ei 2 )T E0 S Ei The sum of input power is proportional to T E 0 E 0 Ei1 2 Ei 2 complex conjugate 2 6 Similarly the sum of output power is proportional to 0 T 0 2 E E E01 E02 2 If it is lossless T E 0 E 0 ( S E i )T ( S E i ) T T T E i ( S S )E i Ei Ei This relation holds for arbitraryE i ST S I (3.4) I : identity matrix Eq(3.4) can be extended to any number of ports 7 For a 2 x 2 symmetrical coupler s21 s12 a s22 s11 b ST S I a 2 b 2 1 ab ba 0 a cos( x ) b sin( x ) (3.5) (3.6) (3.7) let a cos( x )eia , b sin( x )eib ab ba 0 cos( x ) sin( x ) ei (a b ) ei (b a ) 0 cos( a b ) 0 2k 1 a b 2 lossless combination is impossible 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 1. Low insertion loss 2. Polarization-independent loss 3. Low temperature coefficient 4. Reasonable broad passbands 5. Sharp passband skirts passband skirt 6. Low cost a. integrated-optic (may be polarization dependent) b. all-fiber devices 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 i d : diffraction angle The slits are small compared to λ, phase changes across a slit is negligible 18 AB CD a sin i a sin d a sin i sin d For construction interference at λ occurs at the image plane if a sin i sin d m m : the order of the grating 19 20 The energy at a single λ 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 θi= θd The wavelengths are not separated. blazed reflection grating maximize the light energy at α 21 3.3.2 Diffraction Pattern Relax the constrain a <<λ, the phase change across the slit is not negligible, consider a slit of length from y w 2 to w 2 , w w 2 y w ( 2 The relative phase shift of the diffracted light from y at an angle θ compared to that from y=0 is given by ( y ) 2 y sin 22 The amplitude A(θ) at θ A A 0 w A 0 w w 2 w 2 w 2 w 2 (Ref: Optics, page401) exp i ( y ) dy exp i 2 (sin ) y dy A 0 sin( w sin ) (3.10) w sin Fourier Transform of rectangular slit. For any diffracting aperture f(y) y A A 0 f ( y )exp 2 i(sin ) dy (3.11) where A intensity distribution 2 23 If f ( y )dy 1 normalized For a rectangular slit 1 y w f ( y) = w otherwise 0 For a pair of narrow slits (infinite long ) with spacing d 2 y d 2 y dy d d y y exp 2 i (sin ) 2 2 f ( y ) 0.5 y d A A 0 2 A 0 d d exp 2 i (sin ) exp 2 i (s in ) 2 2 2 = A 0 cos( sin d ) 24 3.3.3 Bragg Gratings (BGs) BGs are widely used in WDM BGs: any periodic perturbation in the propagating medium. (periodic variation of n) (Fiber BGs are written by UV) BGs can also be formed by acoustic waves. 25 Principle of Operation Consider two waves with β0 and β1 propagating in opposite directions. If the Bragg phase-matching condition is satisfied 0 1 2 when Λ= the period of the grating Consider β1 wave propagating from left to right, Then the energy from this wave is coupled onto a scattered wave traveling from right to left at the same wavelength provided. 2 0 ( 0 ) 20 let 0 2 neff / 0 0 : wavelength of the incident wave neff : effective refractive index 0 2neff 0 Bragg wavelength 26 These reflections add in phase, when the path length in λ0 each period is equal to half the incident wavelength λ0 neff 0 2 Bragg condition 27 28 Δλ: detuning from λ0 Δ is inversely proportional to the length of the grating Apodized grating: the refractive index change is made small toward the edges of the grating => 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) A. B. 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 C. Made from photosensitive fiber (Ge-doped) UV intensity ↑ n↑] change of n ~ 10-4 D. Two kind of FGs a. short period (Bragg Grating Λ~ 0.5μm) b. long period (Λ~ 100+μm – 1000+μm) 30 Fiber Bragg Gratings (FBG) A. B. C. D. E. extremely low loss ~ 0.1dB high wavelength accuracy (±0.05nm) high crosstalk suppression (Fig 3.8) (40dB) flat tops typical temperature coefficient ~1.25x102nm/℃ For passive temperature-compensated ~ 0.07x10-2nm/℃ 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 λ coupling occurs depending on Λ β= propagation constant of the core mode cP1: propagation constant of the path order cladding mode The phase matching condition cP 2 Because cP is very small long a few hundred m 34 p n Let eff and neff be the refractive indices of the core and the path-order cladding modes 2 neff 2 P neff nneff 2 P (neff nneff ) P c core cladding P Given neff , nneff , obtain It is a wavelength dependent loss element. 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. r1 r3 t1 t2 Ei E i e i (1 A R ) Ei ei 3 R(1 A R ) Ei ei 5 R 2 (1 A R ) l Assume r1=r2 t1=t2 The reflectance R=r1r2 A: absorption loss of mirror T=t1t2=transmission : One way delay n e n : reflective index 37 E0 Ei (1 A R )e i 1 Rei 2 R 2ei 4 ... =Ei (1 A R )e i 1 1 Rei 2 Ei (1 A R )ei 1 Rei 2 TFP ( f ) E0 Ei 2 2 (1 A R )e i 1 Re i 2 2 1 A R 1 R cos 2 iR sin 2 (1 A R )2 (1 R cos 2 )2 R 2 sin2 = (1 A R )2 1+ R 2 -2 R cos 2 cos 2 (1 A R )2 1+ R -2 R 4 R sin2 (1 A R )2 (1- R )2 +4 R sin2 ( 1 2 sin 2 2 A 1 1 R ) 2 2 R 2 n 1 sin 1 R 2 2 n : one way delay n c , TFP ( f ) A 1 1 R 2 2 2 R 1 sin(2 f ) 1 R For maximam TFP ( f ) sin 2 f 0 (3.12) 38 f k / 2 A=0, R=0.75, 0.9 and 0.99 TFP (f) is periodic function with period FSR Where FSR: free spectral range = The spectral range between two successive passband = 1/2τ 39 ( f ' f ) k , k 1 2 FSR f ' f 1 2 Define finesse F FSR FWHM R 1 R proof : (3.12) Assume A 0 2 R sin 2 f ' 1, for 3 dB point 1 R 1 R sin 2 f ' 2 R 1 R 1 sin 2 f ' 2 f ' , f ' is the smallest value satisfied the condition 1 R f ' , FWHM 2 f ' 4 R FSR 1 F 2 FWHM R 1 R 1 R 2 R (3.13) 40 Tunability 1. change cavity length 2. change refractive index n k k : positive integer Recall f0 2 The wave with frequency f 0 will be selected. n c 1. mechanical tuning 2. 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: a. wavelength drift caused by aging or temperature variation b. not exact 50:50 c. not flat top passbands Change temperature (or refractive index) of one arm=> tuning 45 Recall Principle of Operation E01( f ) i cos( k ) i sin( k e i sin( k ) cos( k E02 ( f ) let Ei 2 ( f ) 0 for DEMUX ) Ei1( f ) ) Ei 2 ( f ) (3.1) E01( f ) e i cos( k ) E02 ( f ) e i i sin( k ) phase lag due to i let L length difference in lower arm L : another phase lag 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 2 L 2 L At the lower output the phase difference L L 2 2 through low arm 由第一個3dB coupler 產生delayπ/2 through upper arm 由第二個coupler 到第二個output 產生delayπ/2 所以互相cancel 47 If L k k is odd k (2n 1) (2n 1) 2( n 2) in phase The signals at the upper arm add in phase at upper arm At the lower output, the phase difference is (2n 1) L (2n 1) out of phase ( no signal ) If L (2n ) At the upper arm output, the phase difference is (2n) (2n 1) out of phase no signal At the lower arm output L (2n) signals add in phase The transfer function of MZI is 2 L T11 ( f ) sin 2 (3.14) T ( f ) 12 cos2 L 2 1 hint : cos( k ) sin( k ) and 2 The input and output relation of the middle section is ' E i'1 ( f ) 1 0 E 01 ( f ) i L e i L ' E' ( f ) 0 e E 02 ( f ) i2 let E i 2 ( f ) 0 and Multiply three matrices (3.14) 48 consider K MZI interconnected The path length difference for the kth MZI is assumed to be L 49 2k 1 50 MZI can be used as a 1x2 demultiplexer or multiplexer λ1 λ 2 λ1 MZI λ2 λ1 λ2 chosen to be coincide with the peaks or troughs of the transfer function 2 neff let L 2neff sin2 L If 1 mi i 2 neff mi i , L mi i 2neff sin2 2 2Lneff mi mi : integer 2 , and mi is odd, say mi=1 output 1 has signal, output 2 has no signal, 2Ln If and mi is even, output 1 has no signal. m mi eff 2 i 51 3.3.8 Array wavelength Grating (AWG) Usage: a. nx1 multiplexer 目前除了用Rowland circle之外尚可用 b. 1xn demultiplexer multimode interference (MMI) 做coupler 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 SiO2-Ta2O5 Because the temperature coefficient = 0.01nm/℃ is large 52 Temperature control may be needed. 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 i n k m n m ΔL=length difference between two adjacent waveguides. d i ink = difference in distance between input i and array waveguide k =difference in distance between array waveguide k and output dk out 54 j j The relative phase 2 in out ijk n d n k L n d 1 ik 2 1 kj (3.15) k= 1. 2. …m input output through k If we design that dikin diin k iin out d kjout d out k j j 2 in out Then ijk n1diin n2 k L n1d out n k n k j 1 i 1 j 2 n1 in 2 k out di d j n1 iin n2 L n1 out j = + (3.16) 55 Rowland circle construction grating circle Rowland 56 If j appears at input i, and n1 iin n2 L n1 out p j , j 2 k j p is integer p j 2 kp j will add in phase at output j ( prob 3.16) j will be present at output j If n1 iin n2 L n1 out ( p 1) j ' j j ' will be also present at output j let n in 1 i n2 L n1 c FSR f ' f n2 L out j pc ( p 1)c p f f' 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 nTM nTZ 1 (3.17) TE mode is converted to TM mode. For LiNbO3, |nTE-nTM|=0.07=Δn. at 1.55μm λ=ΛΔn (3.18) At 170MHz Λ=22μm, acoustic wavelength 59 The transfer function is sin2 1 (2 )2 2 T ( ) 1 (2 )2 where Δλ=λ-λ0 λ0 satisfies (3.17) Δ=λ02/lΔn l : the length of acoustic-optic interaction FWHM bandwidth=0.8Δ -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 A. Erbium-doped fiber amplifiers (EDFA) B. Raman amplifiers (RA) C. Semiconductor optical amplifiers (SOA) 67 3.4.1 Stimulated Emission (EDFA or SOA) 68 Two energy levels E2>E1 hfc= E2-E1, h: Planck's constant= 6.63x10-34JS (absorption) E1→E2 excitation (by photons or population inversion) E2→E1 emission photons a. stimulated emission b. spontaneous emission If emission > absorption => amplification N1: Population (number of atoms) at E1 N2: population at E2 rate of E1 E2 r rate of E2 E1 If N2 > N1, population inversion occurs. 69 3.4.2 Spontaneous Emission E2 hf=E2-E1 E1 noncoherent amplified spontaneous emission (ASE) (noise) If ASE is very large => Saturate the amplifier 70 3.4.3 EDAF Erbium fiber = Er3+ doped silica fiber Pumping wavelength = 980nm or 1480nm Advantages (1) Availability of high power pump lasers (2) All fiber device, polarization independent, ease to couple, reliable (3) Simple (4) 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. hfc E2 E1 32 E3 E2 1 sec 21 E2 E1 10m sec atoms stay at E2 longer, 980nm pump is usable 1520nm f c 1570nm BW 50nm page 39, c-band from 1530~1565nm 73 When 980nm pump is used τ32≈ 1μsec << τ21 We have population inverse between E2 and E1 We can amplify 1530-1570nm signals When 1480nmpump is used the absorption from the bottom of E1 to the top of E2 1480nm pump is less efficient Less population inversion Higher noise figure 980nm for low noise EDFA High power 1480nm pump is available => 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) For example 1550~1600nm signal 1460~1480nm pump RA can provide gain about 100nm band (13THz) above the pumping wave λp<λs (Signal Wavelength) A. RA is a distributed device and can provide gain in different bands B. No special fibers are needed C. Required high pump power~1w D. Pump power fluctuations induce noise (propagating in same direction), propagating in opposite directions will have lower noise E. Crosstalk (modulated signals will deplete the pump power => fluctuation => noise) so, pumping opposite direction will lower the noise. (average out) F. Another noise is due to Rayleigh scattering of the pumping84 signal 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: a. Reasonably high power 0~10dBm, low threshold current, high slop efficiency b. Narrow spectral width c. Wavelength stability (low aging effect) d. 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) 107 Reference: John Gowar “Optical Communication Systems” PP262~323 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 Longitudinal Modes Multiple-longitudinal mode (MLM) lasers have large spectral widths~10nm (Fabry-Perot lasers) =>cause chromatic dispersion Single—longitudinal 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 a. Injecting current (change n) tuning range ~10~15 nm at 1550nm b. Temperature tuning 0.1nm/℃ c. Mechanical tuning (wide range but bulky) Desirable properties a. Short tuning time b. Wide tuning range (100nm) c. Stable over its lifetime d. Easily controllable and manufacturable 128 Two-and Three-Section DBR Lasers Problems a. Aging b. Temperature changes c. Current recalibration d. Mode hopping 129 Vertical grating-assisted coupler filter (VGF) Lasers The coupling condition (3.17) λ=ΛB(n1-n2) ΛB: The period of the Bragg grating n1 and n2 are refractive indices of two waveguides. If n1 changes to n1+Δn1 ' B ( n1 n1 n2 ) B n1 B n1 n1 n1 n2 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 T11( f ) cos2 ( k ) 2 T12 ( f ) sin ( k ) (3.2) k : coupling coefficient depending on width of the waveguide, refractive indices, distance of two waveguides 135 T11( f ) sin2 ( L 2) 2 T12 ( f ) cos ( L 2) (3.14) 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. hf c fc eEg (3.19) fc 1.24 ( m.ev ) eEg Eg 138 139 Pabs (1 e L )Pin (3.20) Pin : incident power : absorption coefficient L : Thickness of the semiconductor The efficiency Pabs L 1 e Pin Example 10 4 cm (3.21) , L 10 m, 0.99 140 The responsivity R IP A W Pin IP e R = Pin hf c e A W hf c e hc A 1.24 W The quantum efficiency I ph e ( electrons / sec) / Pin / E ph ( photons / sec) = ( I ph e ) Pin / E ph I ph hc hc R Pin e e 141 142 PIN Photodiodes a. 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. b. 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. Gm: multiplicative gain M: multiplication factor (Gm: M-1) Large Gm will induce large noise. If Gm→∞, avalanche breakdown occurs. 147 148 149 150 151 152 153 154 155 156 157 3.6.2 Front-End Amplifiers a. High-impedance amplifier b. Transimpedance amplifier 158 159 160 3.7 Switches Important parameters a. Number of ports b. Switching time c. The insertion loss d. The crosstalk e. Polarization-dependent loss f. Latching (maintaining its switch state) g. Monitoring capability h. Reliability 161 3.7.1 Large Optical Switched The main considerations a. Number of switch elements required b. Loss uniformity c. Number of crossovers d. Blocking characteristics blocking and nonblocking (strict sense, wide sense, rearrargeable) e. 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 a) b) c) d) e) f) 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. A device converters data from one incoming wavelength to another outgoing wavelength. b. 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 c. Types i. fixed-input, fixed-output ii. Variable-input, fixed-output iii. Fixed-input, variable-output iv. Variable-input, variable-output 171 d. Other important characteristics i. convertion range ii. Transparent to data rate or modulation format iii. Loss (efficiency) iv. Noise, crosstalk e. Mechanism to achieve wavelength convertion i. optoelectronic (commercial available) ii. Optical gating iii. Interferomatric iv. Wave mixing 172 3.8.1 Optoelectronic Approach (O/E, E/O) 173 3.8.2 Optical Grating Using the principle of cross-gain modulation in a SOA. (For high input signal power, the carrier will be depleted => less gain for the probe wavelength) 174 Disadvantages i. small extinction ratio ii. High input signal power to deplete the carriers (simultaneously changes n) iii. Requiring to filter this high-powered signal iv. Changing refractive index inducing pulse distortion 175 3.8.3 Interferometric Techniques 1 1 176 Principle of Operation (cross phase modulation CPM) When λs presents, the carrier densities (or n) change to induce different phase changes of λp. At the port A, the intensity of λp will be modulated. i. digital signal only ii. Higher extinction ratio iii. Providing reamplification and reshaping iv. Low input power 177 Stage1 samples the data Stage2 reshapes and retimes the data (inverse) Stage3 reamplifies 178 3.8.4 Wave Mixing 179