1. 1 Chapter 5 Transmission System Engineering Design the physical layer Allocate power margin for each impairment Make trade-off

2. 2 5.1 System Model Only digital systems are considered Using NRZ codes BER is the measurement factor

3. 3 5.2 Power penalty Power penalty (1)The increase in Signal power required (dB) to maintain the same BER in the presence of impairment. (2)The reduction in SNR due to a specific impairment (used in this course) Recall (p.260) For PIN receiver with Gaussian noise where the decision threshold is optimal

4. 4 When impairments appear, let denote the received powers and noise standard deviations, at the same SNR, we have

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6. 6

7. 7 5.3 Transmitter Design parameters 1.Output optical power (laser output, eye safety) 2.Rise/fall time 3.Extinction ratio (ER) 4.Modulation type 5.Side-mode suppression ratio 6.Relative intensity noise (RIN) 7.Wavelength stability and accuracy

8. 8 In general the output power of a laser is about 1mw ~10mw Output power of optical amplifiers~50mw ER is defined as

9. 9 Practical case

10. 10 5.4 Receiver Key system parameters: sensitivity and overload Dynamic range: P max -P sen Sensitivity is usually measured at BER = 10 -12, and using a pseudo-random 2 23 -1 bit sequence.

11. 11 5.5 Optical Amplifiers C-band and L-band EDFAs, Raman Amplifiers are available. EDFAs have BW=35nm at 1550nm and they can amplify multiple wavelength in a WDM system. Impairments of EDFA 1.Inducing noise 2.Nonlinear gain (depending on power) 3.Nonflat gain profile

12. 12 5.5.1 Gain Saturation in EDFAs

13. 13

14. 14 5.5.2 Gain Equalization in EDFAs The gain flatness becomes an important issue in WDM systems with cascaded amplifiers 1.Preequalization (preemphasis) 2.Equalization at each stage The saturation power (~10mw to 100mw) is proportional to pump and other parameters) Operating an EDFA in saturation has no fundamental problem Practically it is operated in saturation

15. 15

16. 16 5.5.3 Amplifier Cascades Let the loss between two stages = where α: attenuation coefficient : amplifier spacing In general G ≧ gain > loss Recall

17. 17 At the first a few stages, the input power (signal + noise) to a stage increases as the number of stages increases, consequently, the amplifiers begin to saturate and gain drops. (Fig 5.3)

18. 18 It reached steady-state condition where the amplifier output power,, and gain remain the same from stage to stage The total input power +ASE = the total output power

19. 19 Consider the case There are amplifiers (Fig 5.5) Using the equation (4.5), we have the total noise power at the output as

20. 20

21. 21 5.5.4 Amplifier Spacing Penalty In a cascaded Amplifiers WDM system, If is small we may use a small gain amplifier. In this section, we will study the relation between penalty and spacing. The ASE noise power at the output of a cascade of amplifiers is

22. 22 Ideally when G=1 the minimum noise power is achieved. (perfectly distributed gain) (N=∞ NInG=αL) The power penalty for using lumped amplifier is given

23. 23 For α = 0.25dB/km We reduce the spacing from 80km→40km However we have double the number of amplifiers Recall (4.11) page. 257 Noise figure F n =2n sp If an amplifier with F n =3.3dB is used It can be viewed as having an effective NF = 3.3dB - 13.3dB = -10dB

24. 24 5.5.5 Power Transients and Automatic Gain Control If some of channels fail, input power and the amplifier gain In Fig 5.7 λ 8 will be amplified unusually => receiver overloaded => We need an AGC.

25. 25 (a)

26. 26 (b) (c) monitoring wavelength

27. 27 5.5.6 Lasing Loops In ring networks, if the amplifier gain is larger than the loss, the ring may lase. Lasing may occur even for a single wavelength Solutions ： a.Gain is less than the loss being compensated for => degrade SNR a. No loop

28. 28 5.6 Crosstalk Filters, Mux/Demuxs, switches, optical amplifiers and fibers can induce crosstalk. Two kinds of crosstalk ： (a) interchannel crosstalk, (b) intrachannel crosstalk (coherent crosstalk) Crosstalk results in a power penalty. 5.6.1 Intrachannel Crosstalk Causes ： (a) reflection (b) leakage The penalty is high when the polarization is matched or out of phase.

29. 29 In the worst case (polarization matched, out of phase) Let be the average received signal power and be the average crosstalk power from other signal channel. The electrical field at the receiver is

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31. 31

32. 32 5.6.2 Interchannel Crosstalk Source ： leakage of filter, switches, Mux/Demux

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34. 34 5.6.3 Crosstalk in Networks Crosstalk may accumulate.

35. 35 5.6.4 Bidirectional Systems The near end crosstalk is more severe than the far end crosstalk.

36. 36 5.6.5 Crosstalk reduction A.For switches 1. better switch device 2. spatial dilation 3. wavelength dilation

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38. 38 B.For Mux/Demux add a filter between the demux and the mux

39. 39 5.6.6 Cascaded Filters Required 1. wavelength stability 2. wavelength accuracy

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41. 41 5.7 Dispersion 1.Intermode dispersion (multimode fibers) 2.Polarization mode dispersion (imperfect core) 3.Chromatic dispersion (different wavelengths) 5.7.1 Chromatic Dispersion Limits: NRZ Modulation Let the pulse spreading due to chromatic dispersion be a fraction of the bit period. is specified by ITU(G.957) and Telcordia(GR- 253) for 1dB and 2dB penalty

42. 42

43. 43 Narrow Source Spectral Width For SLM DFB lasers, the unmodulated lasers Δλ ≦ 50MHz Ideally a directly modulated laser, Δλ≈ bit rate e.g 2.5GHz for 2.5Gb/s ook (B=1/2 B e ) When chirping occurs. Δλ≈ 10GHz Reducing reflection, Isolator or reducing extinction ratio can reduces Δλ For external modulated lasers Δλ≈ 2.5 × bit rate

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45. 45

46. 46 5.7.3 Dispersion Compensation Methods to reducing the impact of dispersion 1.External modulation (reduce chirping) 2.Small dispersion fiber 3.Dispersion compensation fiber

47. 47 If 80km fiber with 17 ps/nm-km dispersion is used, we have 1360 ps/nm dispersion. Then 13.5km DCF fiber with 100 ps/nm-km can compensate the dispersion to zero as shown in Fig 5.20. However DCF fiber has high loss about 0.5dB/km 0.5dB/km × 13.6km=7dB Figure of merit (MOF) for DCF fiber is If the DCF fiber has -100 ps/nm-km dispersion and loss = 0.5dB/km then FOM = = 200 ps/nm-dB Larger FOM is desirable.

48. 48 Chirped Fiber Bragg Gratings In a regular fiber, chromatic dispersion introduces larger delays for the lower frequency components in a pulse, we can design a chirped grating fiber with larger delays for the higher frequency components to compress the pulse.

49. 49 For WDM systems, we need to use a different grating for each wavelength as shown in Fig 5.22.

50. 50 5.7.4 Polarization-Mode Dispersion (PMD) Because of the ellipticity of the fiber core, different polarizations travel with different group velocities. Polarization changes with time. So PMD varies with time. The time-averaged differential time delay is given by

51. 51 Δτ is a random variable (Appendix H) For the outrage probability (PMD ≧ 1dB) is less than 4×10 -5 (accumulative outage of about 20 minutes/yer)

52. 52 Limitations for various dispersions (PMD limitations is not significant)

53. 53 5.8 Fiber nonlinearities A.Scattering a. stimulated Brillouin scattering (SBS) b. stimulated Raman scattering (SRS) B.Refractive index change a. four-wave mixing (FWM) b. self-phase modulation (SPM) (page 83) c. cross-phase modulation (CPM) (page 89) SBS, SRS, FWM transfer energy from one channel to the others SPM and CPM affect the phase of signals and cause spectral broadening => dispersion

54. 54 5.8.1 Effective Length in Amplified Systems on page 79 For a link without amplifies In a link of length L km with amplifies and spaced l km apart The total effective length is

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57. 57

58. 58 5.8.3 Stimulated Raman Scattering (SRS)

59. 59 To calculated the effect of SRS in WDM systems. We approximate the Raman gain shape as a triangle, where the Raman gain coefficient is given by the system bandwidth

60. 60 Channel 0 is affected the most. Consider the worst case, all channels are transmitting “1” bit with the same power and no interaction between the other channels. The fraction of the power coupled from channel 0 to channel i is given by

61. 61 The fraction of the power coupled out from channel 0 is The power penalty is

62. 62 The total system bandwidth The total input power If chromatic dispersion appears, Raman scattering decreases, we have

63. 63 To alleviate the effects of SRS, we can (1) have small Δλ s (2) lower P (reduce the amplifier spacing)

64. 64 5.9 Wavelength Stability In general, Mux/Demux made of Silica/Silicon have coefficients of 0.01nm/ ℃ A thermistor and a thermo-electric cooler can be used to control DFB laser temperature. In addition aging may change around ±0.1nm. If high wavelength stability is needed, OPLL can be employed.

65. 65 5.12 Overall Design Considerations 1.Fiber type a. Single channel high speed systems use dispersion shifted fibers which is hard to use for WDM for upgrading the link capacity in the future due to four-wave mixing. b. WDM systems use standard single-mode fibers or NZ/DSF. 2.Transmit Power and Amplifier Spacing Old systems have spacing about 80 km, New systems don’t have this restriction.

66. 66 3.Chromatic Dispersion Compensation 4.Modulation Most systems use NRZ Ultra-long-haul systems use chirped RZ modulation 5.Nonlinearities Reducing power or having larger effective area can reduce the effect of nonlinearities.

67. 67 6.Interchannel spacing and number of wavelength a. 100 GHz is common b. For loop application coarse WDM is used c. We have to consider the bandwidth of amplifiers d. Because the output power of amplifiers is limited to 20~25dBm, when the number wavelength increases, the input power per channel decreases, such that the total system span is reduced. 7.All-optical Networks

68. 68 9.Transparent Bit rate, protocols, modulation formats 8. Wavelength Planning Typical spacing ≈ 0.8nm=100 GHz at 1.55μm