Advanced Modulation Formats for High-Capacity Optical Transmission

1. www.optcom.polito.it Advanced Modulation Formats for High-Capacity Optical Transmission Pierluigi Poggiolini Politecnico di Torino Winter School 2005

2. special thanks to: Roberto Gaudino, Andrea Carena, Vittorio Curri, Gabriella Bosco

3. Preface: a grim situation... • The optical transmission system market situation has been dismal since 2001 • Both component and system sales have fallen to 25% of their peak 2000 values! • Capex by carriers has not substantially recovered yet • R&D has been brutally cut worldwide • Lit capacity still exceeds required capacity • System and device manufacturers are still in trouble everywhere and many have not survived

4. Global Components & Networks Market (Actual and Forecast) $40 $35.6 $35 Optical Components Optical Networks $30 $28.2 $25 USD Billions $20 $ in billions $15 $11.2 $10.5 $10.2 $10 $9.0 $8.3 $8.0 $5 $4.4 $2.5 $2.2 $1.9 $1.6 $1.5 $0 2000 2001 2002 2003 2004 2005 2006 www.nren.nasa.gov/workshop7/pps/03.birkner.keynote.ppt

5. ...but there’s hope • Internet traffic has been consistently growing at a fast pace even in 2001-2004 • Forecasts claim it will keep growing strong • Capacity in deployed long-haul links of core networks is expected to saturate between 2005 and 2006, perhaps quicker than expected

6. Internet Edge Traffic 5.00 4.24 4.00 0.46 2.98 IP Traffic Non-IP Traffic 3.00 Edge Traffic, Tbps 0.41 1.91 2.00 3.78 0.34 1.19 2.57 1.00 0.30 0.73 1.57 0.46 0.31 0.22 0.26 0.11 0.89 0.23 0.05 0.47 0.20 0.17 0.23 0.00 1999 2000 2001 2002 2003 2004 2005 2006 www.nren.nasa.gov/workshop7/pps/03.birkner.keynote.ppt

7. Hitting the capacity lid U.S. Long-haul capacity (Terabps) required capacity Range of Estimate 93 Lit capacity does not intersect with required capacity until 2005 installed litcapacity 60 81 74 42 50 26 76 72 65 15 54 41 9 35 6 21 13 7 5 End of 2000 2001 2002E 2000 2001 2002E 2003E 2004E 2005E 2006E Year www.nren.nasa.gov/workshop7/pps/03.birkner.keynote.ppt

8. Capacity utilization in the US, early 2003 41.8% of total capacity already lit

9. Is recovery round the corner? • If Internet growth data proves correct, already in early 2005 we should see signs of capex recovery • Someone claims they are in fact visible • Biq question: • will carrier be willing to deploy more cables? • will they be willing to light up more fibers? • or will they prefer to upgrade existing WDM systems to a higher capacity?

10. A foreseeable scenario • It seems to be more likely that carriers willrather upgrade than dig or light up more fibers • Upgrade means that the installed EDFA bandwidth will be the constraint (typically 20-30 nm) • DWDM systems capable of delivering the highest capacity within the installed amplifier bandwidth will be of interest • Those who will be able to sell the most bandwidth-efficient systems at the lower cost, will win

11. it is difficult to use this band because RAMAN amplification is needed 4 THz S 0.2 dB/km Fiber attenuation vs l attenuation [dB/km] C L wavelength [nm]

12. Another scenario • Assume carriers are willing to light up more fibers • Even so, they will want to make sure they need to light up the minimum amount • Again, bandwidth efficiency is the key • This presentation concentrates on new modulation formats with a specific slant: increased bandwidth efficiency • … but not only…

13. Resilience to Impairments • Key to the new wave of system installation is cost reduction • Cost reduction means: • using the minimum amount of optics • come up with systems that are robust and tolerant to transmission impairments such as: • fiber dispersion • non-linearity • crosstalk and cascaded filtering

14. This talk • Preface • The system context • What is “bandwidth efficiency”? • Conventional IM/DD • Alternative formats: Duobinary and DPSK • Future prospects: multilevel systems • Conclusion

15. This talk • Preface • The system context • What is “bandwidth efficiency”? • Conventional IM/DD • Alternative formats: Duobinary and DPSK • Future prospects: multilevel systems • Conclusion

16. The system context • The majority of links, even in core networks are medium-long haul: 500-1500 km (EU) • Thanks to FECs (Forward Error Correcting codes) the launched power has been dramatically reduced • In this scenario, non-linear fiber propagation effects are often rather mild

17. The system context - II • So, we primarily concentrate on linear effects, and specifically on: • inter-channel crosstalk • TX/RX filter distortion • fiber chromatic dispersion • However, we will also touch on non-linearity tolerance to get a feel of the potential of the new formats for ultra-long-haul in core networks

18. Bit rate per channel • In 2000, 40 Gbit/s was on the brink of being commercially deployed • It is not clear whether in 2004 there is any manufacturer that sells 40 Gbit/s systems • Nonetheless, it is still believed that 40 Gbit/s will follow suit when the recovery takes place • So we assume 40 Gbit/s • However, most results are easily scalable to 10 Gbit/s since we assume negligible non-linear effects

19. The OSNR • A key parameter affecting the error probability of an optical system is the OSNR • The Optical Signal to Noise Ratio at the receiver is defined in this talk as: • noise is typically EDFA or RAMAN ASE noise • noise is typically white and the noise bandwidth must always be specified

20. noise must be associated to a bandwidth dB [mW/THz] f dB [mW/THz] f Signal and noise dB [mW/THz] f

21. OSNR operating level • We assume an OSNR at the receiver due to optical ASE noise of 11 dB over 0.5 nm, typically yielding, in an actual (good) system:  uncorrected BERs on the order of 10-5  10-6 • We assume FECs are used, with 7% overhead FEC corrected BERs better than10-15  42.65 Gbit/s per channel

22. What is a FEC? • The bit stream is transformed into another bit stream, with more bits in it • The extra bits (called “redundancy”) make it possible to correct for reception errors • The Reed-Solomon (RS) (255,239) code is now commonly used and standardized in ITU-T G.975 and G.709 • This code provides 3.3 dB coding gain at 10-6 corrected bit error rate and 5.8 dB coding gain at 10-13 corrected bit error rate with a redundancy of 7%. • More are being standardized

23. FEC Performance Bit Error Rate uncoded 3.3 dB RS(255,239) 5.8 dB Optical Signal to Noise Ratio OSNR

24. This talk • Preface • The system context • What is “bandwidth efficiency”? • Conventional IM/DD • Alternative formats: Duobinary and DPSK • Future prospects: multilevel systems • Conclusion

25. Bandwidth efficiency • How many channels can we put in any given amount of optical bandwidth? • It depends on the systemBANDWIDTH EFFICIENCY:

26. Frequency spacing Df Power Spectrum of a WDM signal frequency

27. Examples • RB =10 Gbit/s, Df =100 GHz rB=0.1 • RB =10 Gbit/s, Df =50 GHz rB=0.2 • RB =40 Gbit/s, Df =100 GHz rB=0.4

28. Where do we stand with rB? • 40 Gbit/s is not commercial • 10 Gbit/s is currently offered at a minimum channel spacing of 50 GHz  rB=0.2 • Ciena is claiming to be shipping 10 Gbit/s at 25 GHz spacing  rB=0.4 (but that was announced back in 2001 and it is not clear if today you can really buy the system)

29. Current record • Many laboratory experiments have been performing better, one of them up torB=2 !!! • They have used advanced modulation formats and a number of additional tricks • We will try to work our way up to that result • It is going to be a long trip so buckle up, sit back and enjoy the ride!

30. Next generation commercial systems • First, we will try to beat the current state of the art for commercial systems • To do so it is necessary to do better than 0.2 • A challenging goal could be: done either: RB =10 Gbit/s, Df =12.5 GHz RB =40 Gbit/s, Df =50 GHz, rB=0.8 RB =42.7 Gbit/s, Df =50 GHz, rB=0.8

31. Assessing rB • How do people assess the system rB ? • First, they set a tolerable level of system penalty (say, 1 dB on the OSNR) • Then, they findthe maximum rB compatible with such penalty • This is done either experimentally or through simulations

32. The simulations • In the following we will look at a series of simulation results that we obtained at Politecnico • These results have been confirmed in many cases by several experiments performed by various groups • In the simulations we assumed: • a “center” channel • two adjacent interfering channels • ASE noise-limited performance • no non-linearity • The penalty was evaluated on the center channel

33. An example of 3-channel IM/DD NRZ spectrum Df=75 GHz

34. Pierluigi Poggiolini: perché non 50 GHz???? An example of 3-channel IM/DD NRZ spectrum Df=75 GHz with ASEnoise

35. Evaluating the penalty • We estimated the center channel BER using appropriate techniques • We computed the OSNRpenalty with respect to the best possible BER performance • What is the “best possible performance”?

36. Best possible performance • The best possible performancefor IM/DD is obtained with: • optimum (matched filter) receiver • single channel transmission • all TX and RX components ideal • no propagation effects (over 0.5 nm)

37. What is “penalty”? • PENALTY is defined as:how much the OSNR must be increased from 11 dB to get back to BER=2 10-8

38. Example • Optimum system: • OSNR=11 dB  BER=2 10-8 • Actual multi-channel system with impairments • OSNR=11 dB  BER=5 10-5 • OSNR=14 dB  BER=2 10-8 The resulting penalty is 3dB

39. This talk • Preface • The system context • What is “bandwidth efficiency”? • Conventional IM/DD • Alternative formats: Duobinary and DPSK • Future prospects: multilevel systems • Conclusion

40. IM/DD • Despite all the hype regarding alternative modulation formats, IM/DD is still used in 100% of installed systems • To date, there is no single installed commercial system using anything else (any evidence of the contrary?) • The situation is bound to change, starting with submarine systems • Tyco will probably deploy DPSK in its next generation of trans-oceanic links • However, IM/DD is and will remain the leader for a long, long time and so let’s see what it can do for us

41. optical power at the TX output optical spectrum at the TX output 20 10 1 0 1 0 0 1 1 0 0 1 0 1 0 optical power density [dB] optical power -10 -20 -30 frequency time The IM/DD spectrum IM/DD - NRZ FORMAT

42. 20 10 0 -10 -20 -30 The IM/DD spectrum optical spectrum at the TX output optical power spectral density [dB (mW/THz)] frequency [THz]

43. channel selection filter Reducing the side lobes • Bandwidth efficiency is directly related to how compact the spectrum can be made • With high sidelobes, when channels are pulled together they interfere and penalty occurs:

44. Reducing the side lobes • Bandwidth efficiency is directly related to how compact the spectrum can be made • With high sidelobes, when channels are pulled together they interfere and penalty occurs:

45. Are there ways to reduce the side lobes? • it is the sharp edges in the time pulses that generate high side-lobes • one possibility is to smooth out the pulses • are there easy ways to smooth out the pulses??

46. FILTER sharp electrical pulses at driver  sharp optical pulses Practical ways of smoothing out pulses • a simple and practical way to smooth out pulses is to insert an electrical filter before the modulator electrical filter after the driver  smooths out pulses

47. Electrical TX filter • Typically, 3-6 pole Bessel filters are used • One can then tune the filter bandwidth to decrease the spectrum side lobes • Eye distorsion must be kept under control

48. 5-Pole Bessel TX filter -3dB bandwidth equal to 1.0*RB pulse power at TX spectrum at TX frequency time

49. 5-Pole Bessel TX filter -3dB bandwidth equal to 0.9*RB pulse power at TX spectrum at TX frequency time

50. 5-Pole Bessel TX filter -3dB bandwidth equal to 0.8*RB pulse power at TX spectrum at TX frequency time