Preparatory School to the Winter College on Fibre Optics, Fibre Lasers and Sensors

Transcription

1 SMR Preparatory School to the Winter College on Fibre Optics, Fibre Lasers and Sensors 5-9 February 2007 Fiber-Optic Communications Hugo L. Fragnito Optics and Photonics Research Center UNICAMP-IFGW Brazil

2 Fiber-Optic Communications Hugo L. Fragnito Optics and Photonics Research Center UNICAMP-IFGW Tel. (xx55-19) Fiber-Optic Systems Basics on Optical Communication Systems Systems with Optical amplifiers DWDWM systems System s components System performance 2

3 Topics Basic system concepts Some system components System performance WDM Systems 3 H. Fragnito UNICAMP IFGW System concepts Elements of transmission systems Multiplexing Modulation Formats Digital Modulation Keying TDM standards Optically amplified systems WDM systems 4

4 Elements of a Communication System Transmitter Receiver Input Tx Transmission Line Rx Output Laser + modulator Optical fiber Photodiode + Filter + clock recovery H. Fragnito UNICAMP IFGW Multiplexing Multiplexer Demultiplexer Channel 1 Channel 1 Tx Rx Channel N MUX Voice, video, or data channels DEMUX Channel N Multiplexing can be in time domain (TDM), frequency domain (FDM), Polarization (PDM), Wavelength (WDM),... TDM = Time Division Multiplexing, FDM = Frequency Division Multiplexing, 6 H. Fragnito UNICAMP IFGW

5 Digital Modulation Formats Binary Word Non-Return to Zero NRZ B/2 Return to Zero RZ B T = 1/B (bit slot) Better for clock recovery time 7 H. Fragnito UNICAMP IFGW Spectrum of NRZ pulses 1.0 NRZ pulses, 400 ps (2.5 Gb/s) Linear scale Log scale 10-4 Spectral power density Frequency, GHz Frequency, GHz No Fourier component at f = B (2.5 GHz) 8 H. Fragnito UNICAMP IFGW

6 Digital Modulation Keying Optical field: E(t) = A(t) cos[(t)t + (t)] Electrical binary data Optical modulated signal ASK - Amplitude Shifting Keying: A(t) = 0 or 1 PSK - Phase Shifting Keying: (t) = or FSK - Frequency Shifting Keying: (t) = or 9 H. Fragnito UNICAMP IFGW TDM Standards SONET - Synchronous Optical Network SDH - Synchronous Digital Hierarchy SONET SDH B(Mb/s) Channels OC OC-3 STM ,016 OC-12 STM ,064 OC-48 STM-16 2, ,256 OC-192 STM-64 9, ,024 OC = Optical Carrier STM = Synchronous Transport Mode 10 H. Fragnito UNICAMP IFGW

7 Absorption Scattering Bending Attenuation 11 Decibel units Input power P in System Output power P out System Transmission: T = P out /P in T db = 10 log(p out /P in ) -10 db means P out = P in /10-3 db means P out = P in /2-40 db means P out = 10-4 P in dbm: Power in db relative to 1 mw P dbm = 10 log (P/mW) -10 dbm means P = 0.1 mw 3 dbm means P = 2 mw 40 dbm means P = 10 W T db = P out -P in (P in and P out in dbm) 12 H. Fragnito UNICAMP IFGW

8 Attenuation L/ 10 Optical Power at a distance L: PL ( ) P( 0) 10 P ( L) P ( 0) L Loss coefficient: (db/km) dbm dbm Loss, (db/km) st window 820 nm Rayleigh scattering = C/ 4 2nd 1.3 µm OH overtones 3rd 1.55 µm IR absorption 0.2 UV absorption tail Wavelength, (µm) 13 H. Fragnito UNICAMP IFGW Attenuation of Multi-Mode Mode (MM) and Single Mode (SM) fibers Multimode (MM) fibers attenuate more than single mode (SM) fibers Light in higher order modes travels longer optical paths Lower order mode Higher order mode 4 Loss, db/km Single mode fibers Multimode fibers Wavelength, µm 14 H. Fragnito UNICAMP IFGW

9 Macrobending loss Shedding of power by the effect of bending Frustrated TIR Power Loss In multimode fibers bending causes mode coupling. Higher order modes are scattered out of fiber. In single mode fibers, bending losses are appreciable for curvature radii < 1 cm. 15 H. Fragnito UNICAMP IFGW Microbending loss Scattering loss caused by rugosity of fiber Power Loss Small axial distortions along the fiber axis Causes mode mixing and/or loss of optical power Can be induced by fiber jacketing, cabling, or environment 16 H. Fragnito UNICAMP IFGW

10 Intermodal dispersion Multimode fibers Intramodal dispersion Multimode and singlemode Dispersion Dispersion parameter Measurement of Group Velocity Dispersion (GVD): Input pulses (different wavelength) Single mode fiber, length L Received pulses + + time D 1 L time Dispersion parameter [ps/nm/km] Standard SM fibers at 1550 nm: D = 17 (ps/nm)/km

11 Group Velocity Dispersion (GVD) Modes propagate as exp(-iz). is the propagation constant Dispersion relation - Taylor series expansion ! ( ) ( ) ( ) ( ) : gives group delay 2 : GVD (Group Velocity dispersion) 2 vgroup v g 1/ d n 0 0 D c d c 3 : Third order dispersion coefficient 4 : Fourth order dispersion coefficient Group velocity, v g ()/c fs pulse spectrum Wavelength, (µm) Synthetic fused silica \\hugo\cursos\f641\dispers.org 18-apr (µm) Refractive index, n() Modal Dispersion Input pulse MM fiber - step index Output pulse time Transmitted Byte time Received Byte time time Dispersion limits the transmission capacity of fiber Capacity of MM-step-index fibers 20 Mb/s km

12 Dispersion in step index MM fibers Relation between the propagation constant and frequency is called the Dispersion Relation: Normalized Propagation Constant ( / n b n n 2 k ) Normalized frequency 2 2 V k0a n1 n2 [k / c / ] 0 2 Normalized propagation constant, b At a given frequency, optical field is a superposition of modes, each one with a different propagation constant Single mode region Cutoff: V = Normalized frequency, V Dispersion in graded index MM fibers Input pulse Graded Index MM fiber 1 2 Output pulse time time Mode 1 travels a longer physical path than mode 2, but through regions of lower index; The optical path is approximately the same for both modes. physical path = d Capacity of MM- graded index fibers 2 Gb/skm z optical path = nd z

13 Dispersion in single mode fibers Chromatic dispersion: group velocity depends on frequency Material dispersion: refractive indices depend on frequency Waveguide dispersion: boundary conditions depend on frequency Propagation constant, b Normalized propagation constant for single mode step index fiber Frequency, V Material dispersion Electronic transitions ( ) n( )/ c Refractive index Rotational bands (gases & liquids) n() Vibrational bands FIR IR VIS UV 1 0 Frequency,

14 GVD in silica Optical fibers for communications are made of silica Group velocity dispersion, D (ps/nm/km) Normal GVD region Synthetic fused silica Anomalous GVD region Wavelength, (µm) Transparent materials exhibit a particular 0 where D( 0 ) = 0. In pure silica, 0 = 1.27 µm (Zero Dispersion Wavelength). Chromatic dispersion (another name for GVD) Group velocity (v g ) depends on Dispersion parameter: D = D M + D W D 2 d 1 2c d 2 2 d vg d D M (Material Dispersion): n 1 and n 2 depend on D W (Waveguide Dispersion): v g depends on waveguide geometry D (ps/nm/km) D M D (total) D W Zero dispersion wavelength

15 Dispersion Shifted Fibers Index profile 20 n 2 n 1 Standard Fiber (SMF) n 2 n 1 Dispersion Shifted Fiber (DSF) D (ps/nm/km) S = 0.09 ps/nm 2 /km S = ps/nm 2 /km Standard Fiber (SMF) Dispersion Shifted Fiber (DSF) Wavelength, (µm) Zero dispersion wavelength ( 0 ): Standard fiber: 0 = 1310 nm Dispersion Shifted Fiber: 0 = 1550 nm Dispersion slope: S dd d Fibers for Telecom Review Multimode fibers Used in local area networks (LANs) Capacity limited by intermodal dispersion: 20 Mb/sxkm (step index) 2 Gb/sxkm (graded index) Single Mode Fibers Used for long distance 25 THz bandwidth in 1.55 µm window Capacity limited by chromatic dispersion Dispersion (D) can be positive, negative or zero D = 1.3 µm in standard silica fibers Waveguide dispersion can be adjusted by index profile Dispersion shifted fiber (DSF): D = 1.55 µm DSF: combines D = 0 and minimum loss so, is DSF the ideal fiber?? Optical nonlinearities are very large in DSF - See next lectures

16 Some system components Fiber Couplers Fiber Connectors Fiber Cables Fiber pigtailed devices Lasers Detectors Modulators Integrated optics: Mux Amplifiers 29 Optical Couplers Planar waveguide device Input Mode coupling through evanescent field Fiber device fibers Fuse and stretch 30 H. Fragnito UNICAMP IFGW

17 Fiber connectors Various Types FC/PC, SC, LC, SMA, ST Polishing quality SPC (Super), UPC (Ultra) Fiber Alumina ferrule 8 PC = Physical Contact Insertion loss 0.2 db Back reflection 20 db APC = Angled Physical Contact Insertion loss 0.2 db Back reflection 40 db 31 H. Fragnito UNICAMP IFGW Optical cables for the lab Simplex 2mm Duplex Patch cords Duplex connectors DSC ESCON 32 H. Fragnito UNICAMP IFGW

18 Cables 33 H. Fragnito UNICAMP IFGW Collimator Fiber pigtail Grin lens Collimated light Miniature bulk optics (polarizer, isolator, filter, anything) 34 H. Fragnito UNICAMP IFGW

19 MEMS Micro-Electro-Mechanical Systems Optical Switching Optical Cross Connect (OXC) 35 E. Kruglick, WDM, March 2001 H. Fragnito UNICAMP IFGW MEMS (2) Polarization Rotators Chuan Pu, Tellium, February 2001 Variable reflectivity Gain slope compensation of amplifiers 36 H. Fragnito UNICAMP IFGW K.W. Goossen, Lucent, October 2000

20 Lasers for transmission Semiconductor Diode Laser Fabry-Perot Laser DFB 37 Laser diode p-type current Low gap semiconductor 0.1 mm n-type 0.3 mm cooler 0.2 mm Fabry-Perot (FP) laser cavity 38 H. Fragnito UNICAMP IFGW

21 L Cavity modes Cavity modes = c/2nl Fabry-Perot lasers ~ 30 modes (2-10 nm linewidth) Mode jumping Mode partition noise DFB and DBR lasers: ~ 50 MHz linewidth DFB: Distributed Feedback laser DBR: Distributed Bragg Reflector 39 H. Fragnito UNICAMP IFGW Spontaneous emission spectrum Wavelength (nm) Emission spectrum of LEDs (Light Emission Diode) Powert (arbitrary units) InGaAsP T = 280 K ASE or noise spectrum of Lasers (Amplified Spontaneous Emission) 40 H. Fragnito UNICAMP IFGW Energy (ev)

22 Modulation bandwidth and chirp Direct Modulation BW limited by carrier diffusion RC of wiring, contacts, packaging Relaxation oscillations Instantaneous frequency varies (chirp) Dependence of refractive index on injection current (changes effective length of optical cavity) Positive chirp 41 H. Fragnito UNICAMP IFGW Photodiodes 42

23 Photodiodes p-n junction - p-type + (reverse bias) n-type p-i-n p i (intrinsic) n APD avalanche photodiode p + i p n + absorption gain (10X) 43 H. Fragnito UNICAMP IFGW Materials for Photodiodes First communications window (830 nm) Second window (1300 nm) Third window (1550 nm) G.P. Agrawal, Fiber-Optic Communication Systems, Wiley, New York (1997) 44 H. Fragnito UNICAMP IFGW

24 Photodiode Characteristics parameter symbol unit Si Ge InGaAs wavelength nm Responsivity R A/W Quantum eff. % Dark current I d na Rise time t r ps Bandwidth f GHz Bias (p-i-n) V b V Bias (APD) V b V Gain (APD) M H. Fragnito UNICAMP IFGW WDM Multiplexers Free Space Grating MUX Fibers Lens Grating AWG: Arrayed Waveguide Grating 46 H. Fragnito UNICAMP IFGW

25 AWG AWG: Arrayed Waveguide Grating 47 H. Fragnito UNICAMP IFGW Modulators 48

26 Electro-Optic Switch P in d Control Voltage V Nonlinear crystal P out = P in (1+cos)/2 Refractive index change: n 1 rn 3 ( V / d) 2 r = EO - coefficient V / V Switching voltage: 49 H. Fragnito UNICAMP IFGW V d rn 3 Waveguide Electro-Optic Switch Waveguide (Ti in-diffused) V Mach-Zehnder Switching voltage: 5-8 V Integrated optics Fiber pigtail 0 LiNbO 3 50 H. Fragnito UNICAMP IFGW

27 Electroabsorption modulator Franz-Keldysh effect 50 GHz, 2V Energy gap of semiconductors depends on applied electric field Effect is enhanced in Multiple Quantum Wells Integrated with laser in the same chip G.P. Agrawal, Fiber-Optic Communication Systems, Wiley, New York (1997) 51 H. Fragnito UNICAMP IFGW Direct versus External Modulation Direct modulation I(t) laser External modulation V(t) laser modulator Broad spectral source Fabry-Perot Laser: = 2 nm D = 34 ps/km t LD DFB Laser: ( = 20 MHz) = 0.16 pm D = ps/km But frequency chirp at 2.5 Gb/s modulation is ~ 20 GHz: = 0.17 nm D = 2.7 ps/km Transform limited pulses Examples: D = 17 ps/nm/km; = 1550 nm Bandwidth limited pulses: B = 2.5 Gb/s D = 0.34 ps/km B = 10 Gb/s D = 1.4 ps/km B = 40 Gb/s D = 5.5 ps/km 52 H. Fragnito UNICAMP IFGW

28 Optical Amplifiers 53 Er +3 energy levels Pump: 980 nm or 1.48 µm pump power > 5 mw F 9/2 (nm) 659 Emission: µm long living upper state (10 ms) gain 30 db Energy (ev) I 9/2 4 I 11/2 = 10 µs emission 4 I 13/ absorption 0.5 = 10 ms Absorption (db/m) (µm) 54 H. Fragnito UNICAMP IFGW I 15/2

29 Gain Saturation and Noise of EDFA Signal to Noise Ratio (SNR) degradation Due to Amplified Spontaneous Emission (ASE) Noise Figure: NF = SNR in /SNR out > 3 db Gain (db) Pump Power 10 mw 30 mw 20 mw Power (dbm) P out + P ASE = GP in + P ASE P ASE Output Signal Power (dbm) Wavelength (nm) 55 H. Fragnito UNICAMP IFGW SOA Semiconductor Optical Amplifier Laser diode with antireflection coating Fast response (recombination time) ~ ns Very sensitive to light polarization (PDG: polarization dependent gain) Wide bandwidth Operation in 1300 nm or 1500 nm windows 56 H. Fragnito UNICAMP IFGW

30 PDG compensation in SOAs 57 H. Fragnito UNICAMP G.P. Agrawal, Fiber-Optic Communication Systems, Wiley, New York (1997) IFGW Thulium Doped Fiber Amplifiers nm normal nm gain shifted Special host glass M. Islam and M. Nietubyct.WDM, March 2001 A. Aozasa et. al., OFC 2001, PD1, March H. Fragnito UNICAMP IFGW

31 Optical Amplifiers EDFA ( nm) Erbium TDFA ( nm) Thulium Gain (db) C-band EDFAs L-band SOA (60 nm bandwidth) Semiconductor Optical Amplifier Raman Amplifiers (40 nm bandwidth; choice of spectral region) Limited by Nature to ~ 100 nm Physics: Quantum energy levels Chemistry: Special materials Raman Gain, db nm Wavelength (nm) Raman Gain Signal Wavelength, nm 59 H. Fragnito UNICAMP IFGW Components REVIEW Lasers and photodiodes Linewidth depends on laser type: Fabry-Perot: 2-8 nm (1 THz) DFB: 50 MHz ( nm) Modulation bandwidth to 20 GHz but chirp may be a problem Avalanche photodiodes provide gain p-i-n photodiodes respond to 50 GHz integrated, waveguide devices (ex. AWG MUX) Compact, highly stable devices High speed modulators: electro-optic (Lithium Niobate) electroabsorption (Semiconductor) Optical Amplifiers: EDFA: 35 nm bandwidth, NF = 4-6 db, = 10 ms L-band EDFA: nm SOAs: Compact and inexpensive, but polarization dependent and = ns Raman: nm bandwidth, we can choose the gain region Other Rare earths: Thulium (S-band), Praseodymium (1300 nm band), 60 H. Fragnito UNICAMP IFGW

32 System performance Power Budget Detection BER and Q factor Eye diagram 61 System design Power budget (db units) P RX = P TX L N splices splice System margin Dispersion Average splicing insertion loss Keep pulse duration < 1.25 T ( T = 1/B = bit slot ) DLB < 0.25 What about noise? 2 5 db (aging) Attenuation and dispersion can be compensated with optical amplifiers and dispersion compensators 62 H. Fragnito UNICAMP IFGW

33 Optical Field Noise Optical communications region Microwaves region ( = µm) h (vacuum field noise) Noise power density (W/Hz) T = 300 K T = 1000 K T = 1 K Thermal Noise (Bose-Einstein statistics) 2h e h / kt wavelength, (µm) At optical telecom frequencies: quantum fluctuations of vacuum field >> thermal noise (even at 1273 ºC) In practice, thermal noise of electronic part of receiver dominates 63 H. Fragnito UNICAMP IFGW Total current Shot noise Thermal noise I ( e / h) P i 1 Responsivity (A/W) P = optical power (W) 2 2 S i S 1 2 T Photodetection noise S 2eI f 4kT FN f R L i T i Noise figure of electrical amplifier d Dark current Thermal noise Shot noise Signal photocurrent In practice T S R L G = 1 f Low pass filter (cut-off = 70% B) 64 H. Fragnito UNICAMP IFGW

34 BER BER = p(1) P(0/1) + p(0) P(1/0) Bit Error Rate Signal current I 1 I D I 0 P(1/0) P(0/1) 1 0 Q parameter Q I 1 I D 1 Q I 1 I Q ~ Signal/Noise time Probability 11 I 00 I I D = decision threshold I D optimum H. Fragnito UNICAMP IFGW BER for Gaussian random processes IF fluctuations around I 1 and I 0 follow Gaussian distributions 11 I 0I0 AND (optimum) decision threshold I D 1 0 THEN exp( Q / 2) BER Q BER < 10-9 for Q > 6 Q I I Q parameter BER Q 2 SNR/2 (Electrical signal-to-noise ratio) H. Fragnito Q UNICAMP IFGW

35 Eye diagram 10 Gb/s NRZ PRBS Before transmission After transmission (DSF 80 km) 67 H. Fragnito UNICAMP IFGW 25 ps/div Using the eye diagram I 1 I 0 Q = (I 1 I 0 )/( ) 7/(1+0.5) = 5 ( BER 10-7 ) 68 H. Fragnito UNICAMP IFGW

36 System Performance REVIEW Noise in optical field gives shot noise, but in photodetected, electrical signals, thermal noise dominates Eye diagram is a powerful tool for system diagnostics BER is determined by Q factor (Q 2 ~ SNR/2) BER < 10-9 for Q > 6 What are the physical limits of fiber optic transmission systems? Attenuation? Dispersion? Nonlinearities? 69 H. Fragnito UNICAMP IFGW WDM Systems Traditional Systems 3R Optically Amplified systems WDM 70

37 Traditional Optical Communication System Loss compensation: Repeaters at every km Optical fiber (20-50 km) Transmitter Tx Repeater 3R Receiver Rx Optoelectronic repeater Photodiode Timing & Shaping circuits Laser 3R = Retiming, Reshaping & Regeneration System capacity limited by speed of electronic circuits 71 H. Fragnito UNICAMP IFGW Optically Amplified Systems EDFA = Erbium Doped Fiber Amplifier Laser Booster Fiber ( km) Pre-Amp Photodiode EDFA EDFA EDFA EDFA Signal (1.55 µm) Erbium Doped Fiber (10-50 m) Amplified Signal WDM WDM isolator Bits continue in photonic format Pump Lasers (1.48 µm or 980 nm) 72 H. Fragnito UNICAMP IFGW

38 Characteristics of EDFA based Systems Bandwidth > 4 THz depends on glass host (100 nm in Telurite glass) Low noise NF ~ 4 7 db Gain (db) nm Optical power > 10 mw larger distance between EDFAs Wavelength (nm) Transparent to bit rate, modulation format, and bit coding Ideal for WDM (Wavelength Division Multiplexing) 73 H. Fragnito UNICAMP IFGW DWDM Systems DWDM = Dense Wavelength Division Multiplexing Transmitters (Bit rate = B) Optical Amplifiers Receivers MUX DeMUX N N Power density = 100 GHz, 50 GHz, (up to 40 Gb/s) DWDM 25 GHz, or 12.5 GHz (up to 10 Gb/s) UDWDM (Ultra Dense) ITU grid n = + n = THz ( nm) (n = 0, ±1, ±2,...) Frequency, (THz) 74 H. Fragnito UNICAMP IFGW

39 Bandwidth of WDM systems with EDFAs 4 THz Optical Bandwidth EDFA = Erbium Doped Fiber Amplifier C band ( nm) WDM channels x 40 Gb/s (50 GHz channel spacing) Total capacity: 3.2 Tb/s Gain (db) nm Important issues: Bandwidth Gain flatness Output power Wavelength (nm) Capacity of DWDM systems determined by bandwidth of optical amplifiers 75 H. Fragnito UNICAMP IFGW Chirped Fiber Bragg Gratings L 12 Dispersion compensation Optical Delay = nl 12 /c 1 2 Gain flattening (varying index modulation) 76 M. Guy and F. Trépanier, WDM, March H. Fragnito 2001 UNICAMP IFGW

40 Systems Concepts REVIEW Optical Communication system use several Multiplexing techniques, all include TDM (Time Division Multiplexing) TDM standards (SONET, SDH): ex. OC-192 = STM-64 = 10 Gb/s Modulation is ISK Intensity Shifted Keying and NRZ format DWDM systems use Gb/s lasers at different lambdas Aggregated bit rate = NxB Channel spacing GHz; ITU grid Total optical bandwidth limited by bandwidth of optical amplifiers Total power NxP (nonlinear optical effects are important) 77 H. Fragnito UNICAMP IFGW Where to get more information G.P. Agrawal, Fiber-Optic Communication Systems, 2 nd ed., J. Wiley & Sons, New York (1998). Lightwave Magazine, PennWell, free subscription: B.E.A. Saleh and M.C. Teich, Fundamentals of Photonics, Wiley, New York, K. Iizuka, Elements of Photonics, Vol. II: For fiber and integrated optics, Wiley, New York, G. Keiser, Optical Fiber Communications, 2nd ed., Mc Graw Hill, New York, J.A. Buck, Fundamentals of optical Fibers, Wiley, New York, Journal of Lightwave Technology 24(12), special issue, Dec H. Fragnito UNICAMP IFGW