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Episode V: Optical Systems and Performances

Episode V: Optical Systems and Performances. Topics to be covered. 5.1 Mutliplexing Schemes 5.2 Point-to-Point Optical System 5.3 Point-to-Multipoint Optical System 5.4 Multipoint-to-Multipoint Optical System 5.5 Existing Transport Network 5.6 Wide and Metropolitan Area Network

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Episode V: Optical Systems and Performances

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  1. Episode V: Optical Systems and Performances

  2. Topics to be covered 5.1 Mutliplexing Schemes 5.2 Point-to-Point Optical System 5.3 Point-to-Multipoint Optical System 5.4 Multipoint-to-Multipoint Optical System 5.5 Existing Transport Network 5.6 Wide and Metropolitan Area Network 5.7 Local Area Network 5.8 Subscriber Loop 5.9 Broadband Access

  3. 5.1 Multiplexing Schemes

  4. A2 A2 C2 B2 B1 C2 C1 A1 A1 C1 B1 B2 A time B l C Multiplexing Schemes - Time Division Multiplexing Multiplexing: transmits information for several connections simultaneously on the same optical fiber Time Division Multiplexing (TDM) • single wavelength channel (one laser) • if each channel data rate is R bits/sec, for N channels, the aggregated data rate is (R  N) bits/sec

  5. lA lA lB lC A lB wavelength B lC C wavelength multiplexer Multiplexing Schemes - WDM Wavelength Division Multiplexing (WDM) wavelength spacing: 0.8 nm (100-GHz) • Multiple wavelength channels. • wavelength multiplexer/demultiplexer are needed to combine/separate wavelengths • if data rate per wavelength is R bits/sec, for N wavelengths, the total data rate is (R  N) bits/sec • high capacity transmission • Latest 1,024 WDM channels from Lucent. Total throughput >1Tb/s

  6. fA fB fC fA freq freq A fB freq B fC l freq C Multiplexing Schemes - Subcarrier Multiplexing Subcarrier Multiplexing (SCM) • multiple frequency carriers (subcarriers) are combined together • only require one wavelength (one laser) (optical carrier) • suitable for video distribution on fiber • analog modulation - requires very linear laser

  7. Subcarrier Multiplexing - 1 digital I b analog • Exact output requires • linear output

  8. f1 ~ ~ ~ S f2 fk Subcarrier Multiplexing - 2 f1 f2 f3 • squeeze many ch. Into 1 optical ch. • Within modulation bandwidth of Tx

  9. ~ Subcarrier Multiplexing - 3 Think of sinusoidal waves f1 - f2 - f3 - + At the receiver baseband 1xN f1-fk • powerful concept • combine with WDM yields multiple ch. optional

  10. Constraints of Subcarrier Modulation 1. All modulations must be within the modulation bandwidth of the laser. 2. Nonlinearity in laser gives rise to Intermodulation distortion (IMD): 1.Composite Second Order (CSO) --> 2fi + fj for all i, j. There are k(k-1) ways 2.Composite Third Order Beating (CTB) --> fi + fj + fk. There are k(k-1)(k-2)/2 ways 3.Clipping Modulation index m per ch. Total number of ch. : k Total max mod mi=mj, i.e. mk statistically, modulation = m k phase amp freq

  11. Composite Nonlinear Distortions

  12. Carrier to Noise Ratio For AM-VSB (vestigial sideband) TV modulation signal (cable TV) constant But how can one extract the BER? Or evaluate the SNR? SNR = Noise power Easier to calculate Carrier to Noise Ratio Optical Carrier Power CNR = Noise power

  13. CNR Calculation Example where m : modulation index per channel I dc : d.c. photo current BW : receiver bandwidth Ft: electronic preamp noise figure R eq: receiver equivalent resistance RIN: laser relative intensity noise The last term (laser intensity contribution) in the denominator is introduced since the noise becomes non-negligible when I dc is large. Note that the above CNR is per channel.

  14. Multiplexing Schemes - CDM and PDM • Code-division multiplexing (CDM) • a specific code-word (key) for each channel • the data are first coded using the key before transmission. • after detection, the data from each channel can be recovered by • correlating the signal with the key. • Polarization-division • use different polarization states (TE and TM) • need polarization control to maintain the state of polarization • difficult to implement

  15. TDM/WDM lA lB lC lA lB lC SCM/WDM f1 f2 f3 f1 f2 f3 f1 f2 f3 wavelength wavelength TDM stream A A TDM stream B B TDM stream C C wavelength multiplexer wavelength multiplexer Multiplexing Schemes - Hybrid Hybrid Types (TDM/WDM, SCM/WDM) higher capacity lA lA lB lB lC lC • Channel number = wavelength x TDM/subcarrier ch. • But complexity in multiple access

  16. 5.2 System Design Issues

  17. Types of Optical System • Nomenclature can be confusing, classified by • Topology: Ring • Bus • Mesh • Functions: Point-to-point (e.g. long-haul transmission) • Broadcast-and-select network • Broadcast-and-distribution (e.g. Cable TV) • Routing network • Network size - local area network (LAN) link distance: • metropolitan-area network (MAN) diameter ~50 km • wide-area network (WAN) backbone network • Characteristics: Optical transport network • All-optical wavelength routing network

  18. System Design Issues • Multi-wavelength System • (Additional Requirements) • Channel Crosstalk • Cross Phase Modulation • Gain/Power Equalization • Frequency Chirping Single Wavelength System • Power budget • Rise-time budget • Transmission Distance • Data rate / bandwidth • Bit-error-rate / SNR • Sources of Degradation • Self Phase Modulation

  19. System Limitations - 1 • Dispersion  pulse broadening  inter-symbol interference  pulse energy within the bit slot reduces • Fixes: dispersion-compensating fibers (DCF), • dispersion-shifted fibers (DSF), • pre-chirping, • soliton transmission (dispersion + nonlinear effect) • Attenuation  system power budget • optical amplifiers, coherent detection • Polarization  polarization dependent gain/loss, • polarization mode dispersion (PMD), • polarization sensitive  power penalty • polarization diversity, use polarization-maintaining fibers

  20. System Limitations - 2 • Nonlinear effects  • four-wave-mixing (FWM), • stimulated Raman scattering (SRS), • stimulated Brilluoin scattering (SBS), • self-phase modulation (SPM), • cross-phase modulation (XPM)  system degradation • Fix: power control, phase modulation; frequency assignment • Noises  reflection noise, phase noise, back-scattering, modal noise, mode partition noise, thermal noise, shot noise, amplifier beat noise, RIN, etc.  power penalty

  21. Example Problem The following parameters are established for a long-haul single-mode optical fiber system operating at a wavelength of 1.3 mm. Mean power launched from laser transmitter -3 dBm Cabled fiber loss 0.4 dB/km Splice loss 0.1 dB/km Connector loss at the transmitter and receiver 1 dB each Mean power required at the APD receiver : at 35 Mbit/s (BER 10-9) -55 dBm at 400 Mbit/s (BER 10-9) -44 dBm Required safety margin 7 dB

  22. Vin Vo 90% Linear System 10% Vout Vin time Tr Rise-time budget • rise-time (Tr) = 10-90% time Example : RC low-pass filter circuit The Tr is found to be Tr=(ln 9) RC ~ 2.2 RC The transfer function H(f)=(1+jwRC)-1 à Df = (2pRC)-1 (the frequency that |H(f)|2=0.5 ) àTr = 2.2/(2p Df )=0.35/Df • For RZ : Df = Br (Br = bit-rate) • For NRZ : Df = Br/2 à

  23. The overall system rise-time Tsys The overall system rise-time T2sys = T2tr + T 2f +T2rec Ttr : transmitter rise-time Tf : fiber rise-time Trec : receiver rise-time T2f = T2modal + T2GVD Tmodal : modal dispersion induced rise-time TGVD : group velocity dispersion

  24. Example of rise-time budget calculation Ex : 1.3 mm SM fiber communication system, 50 km, 1 Gb/s Assume Ttr =0.25 ns, Trec =0.35 ns, Dl=3 nm, D=2 ps/(nm × km) àTGVD = 2 (ps/nm km) × 50 (km) × 3 (nm) = 0.3 ns = Tf àTsys =0.524 ns 0.35/B £ Tsys £ 0.7/Bè only NRZ is suitable

  25. Power budget - a revision • Power budget - to ensure the received power is enough for reliable performance. • Need a system margin, MS (6-9 dB) to tolerate some system or component degradation. af : fiber attenuation (dB/km) acon : connector loss (dB) a splice : fiber splice loss (dB) ; • a splice may be expressed in (dB/km), then the equation a splice àa splice L

  26. Bit-rate vs. Distance • At low bit-rate, system is loss-limited; • At high bit-rate, pulse broadening ->L -> dispersion-limited. • Use af=2.5, 0.4, 0.25 dB/km and =300, 500, 500 for 0.85, 1.3, 1.55mm =1 mW for all cases.

  27. Digital Link • characterized by the bit-error-rate (BER) at 10-9 for optical link • bandwidth efficiency is not as good as analog transmission • common modulation scheme : • baseband ASK/OOK (SNR~16 dB) - 2 bits/s/Hz of electrical BW (Nyquist rate) • SCM, FSK, PSK, QPSK (SNR~16 dB ) - <1 to 2 bits/s/Hz of electrical BW • coherent FSK, PSK, DPSK (~12 dB SNR) • signal can be regenerated at 100% fidelity • Rise-time budget • BER floor • Eye diagram

  28. Channel/Line Coding - a revision • eliminate DC baseline drifting due to a long string of zeros or ones • maintain a minimum number of transitions per time interval for clock recovery • introduce redundancy to have errors detection/correction capability • interference • common used line codes : • non-return-to-zero (NRZ) • return-to-zero (RZ) • Manchester/biphase • block codes mBnB

  29. Example of Line codes 0 1 0 1 0 1 1 0 0 1 0 NRZ data clock RZ data Manchaster transition

  30. Receiver sensitivity - a revision • The minimum average received power (or receiver sensitivity) is the average received power required to achieve certain BER. • a function of Q.

  31. Dispersion-Limited system- A Revision (1). MM-step index fiber At 0.85mm for n1=1.46 and D=0.01, dispersion limited is ~ 1Mb/s -km. (2). MM-graded index At 0.85mm, loss-limited is up to ~ 100 Mb/s-km. This corresponds to 1st generation optical communication system (3). Large sl (negligible initial pulse width) where D: fiber dispersion ; sl: source linewidth (r.m.s.) Ex. Using typical value D=1~2 ps/nm km, sl~2nm, àBL £ 125 Gb/s -km.

  32. Dispersion-Limited system - a revision (4). Small spectral width (sws0 <<1 case) But loss-limited up to 5 Gb/s for NDSF. • Use DSF loss limited can be extended to 20 Gb/s with repeater spacing 80 km for |D|~1.6ps/(km nm). 30 km, 8 Gb/s, 1.3 mm 120 km, 2.5 Gb/s, 1.55 mm

  33. Power Penalty from Dispersion • Complex calculation depends on exact pulse shape at receiver • Use Gaussian pulse shape, single mode fiber and multimode laser or LED • Design: <0.2

  34. BER=10-9 Mode Partition Noise Penalty • Multimode laser • DFB laser

  35. Extinction Ratio and Chirp Penalty

  36. Reflection Noise Penalty • RIN enhances at freq corresponding to multiples of external cavity mode spacing. • Feed-back induced chaos • Can be improved by isolator • r < 30 dB effective intensity noise N = number of external cavity modes r = feedback reflection into the laser Side mode fluctuations enhance the transmitter intensity noise BER as a function of feedback. VCSEL (vertical cavity surface emitting laser) cavity length ~ 1 mm, 500 Mb/s

  37. Channel Crosstalk in WDM Systems • Crosstalk occurs: • Optical filter - FP, MZI, AOTF, etc • Optical Mux/Demux - AWG, spectrograph • Optical Crossconnects • Semiconductor Laser Amplifier • Add-drop Multiplexer/Demultiplexer

  38. Channel Crosstalk on WDM System

  39. Fiber nonlinearity induced impairments - a revision • system performance degrades under high optical power in fiber. • inelastic scattering process • stimulated Raman scattering (SRS) • stimulated Brillouin scattering (SBS) • intensity dependent refractive index • self-phase modulation (SPM) • cross-phase modulation (XPM) • four-wave mixing (FWM)

  40. 5.2 Point-to-Point Optical Link System

  41. Encoder Decoder Mod Electronics Demod Opt. Tx Opt. Rx Optics 5.2.2 Point-to-Point Optical Link Single wavelength or WDM • Issues of interested: • Power budget • Link span between optical amplifier • Bit-rate distance limit • Digital and analog transmission • Gain sharpening in cascading OA • Optical link: • Short distance: inter-building, inter-hub, • computer- optical interconnect • Medium distance: backup site • Long distance: transoceanic cable

  42. Point-to-Point Optical Link: Short Distance • 0.87 mm - single l per multimode waveguide • 4 ch. X 200 Mb/s each • Flip chip packaging / C-4 bonding • Computer interconnect application • IBM Research-’90

  43. Point-to-Point Optical Link: Short Distance • IBM Optical Interconnect ‘95 • 32 ch. X 1 Gb/s Manchester coding • Electronic and optical packaging • MSM detectors, GaAs MESFET • VSEL - 0.87 mm • Monolithically integrated • Low cost packaging - standard IC • packaging

  44. Point-to-Point Optical Link: Medium Distance • Probably the first WDM product circa 1995 • 20 channel bi-directional • (10 ch each way) • up to 622 Mb/s per channel • protocol independent - FDDI, OS3, etc. • managed by simple Network • Management Protocol • link distance up to 75 km • applications - disaster recovery, remote • storage. IBM 9729

  45. 132 Ch 1 Ch TDM Transmission Capacity Progress

  46. Point-to-point Optical System: Long Distance Undersea Fiber Systems AT&T • repeater/amplifier spacing • fault tolerance, system monitoring • component reliability • cost • span distance • data rate

  47. SYSTEM TIME BANDWIDTH/ NUMBER OF COMMENTS BIT-RATE BASIC CHANNELS TAT-1/2 1955/59 0.2 MHz 48 HAW-1 1957 COPPER COAX TAT-3/4 1963/65 ANALOG HAW-2 1964 1.1 MHz 140 VACUUM TUBES H-G-J 1964 TAT-5 1970 HAW-3 1974 6 MHz 840 Ge TRANSISTORS H-G-O 1975 TAT-6/7 1976/83 30 MHz 4,200 Si TRANSISTORS TAT-8 1988 OPTICAL FIBER HAW-4 1989 280 Mb/s 8,000 DIGITAL TPC-3 1989 l = 1.3 mm TAT-9 1991 16,000 TPC-4 1992 560 Mb/s 24,000 l = 1.55 mm TAT-10/11 1992/93 TAT-12 1995 5 Gb/s 122,880 OPTICAL AMPLIFIERS TPC-5 1995 l = 1.55 mm TAT: Trans-Atlantic Telecommunications TPC: Trans-Pacific Cable Undersea Fiber Systems -1

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