TCOM 513 Optical Communications Networks

1. TCOM 513Optical Communications Networks Spring, 2006 Thomas B. Fowler, Sc.D. Senior Principal Engineer Mitretek Systems

2. Topics for TCOM 513 • Week 1:Wave Division Multiplexing • Week 2: Opto-electronic networks • Week 3: Fiber optic system design • Week 4: MPLS and Quality of Service • Week 5: Heavy tails, Optical control planes • Week 6: The business of optical networking: economics and finance • Week 7: Future directions in optical networking

3. Future directions in optical networking • Solitons • New types of optical fiber • Photonic multilayer routers • New uses for MEMS • Ethernet Passive Optical Networks • Wavelength converters

4. Solitons • Soliton = “solitary solution” • Name suggests particle-like behavior • Basic idea • Ordinary pulse smears out as it propagates, due to dispersion • If dispersion could be compensated, pulse might diminish but still retain its shape • Discovered in 1834 by John Scott-Russel, in canals of Edinburgh • Water solitons • Studied in detail in 1960s

5. Solitons—how they work • Chromatic dispersion in ordinary fiber causes different wavelengths to travel at different speeds • Higher RI means slower speed of propagation • RI = speed in medium / speed in vacuum Travel faster Travel slower Source: Corning

6. Frequency content of pulse Original pulse

7. Solitons—how they work (continued) • Chromatic Dispersion • Shorter wavelengths move to beginning of pulse • Longer wavelengths move to end of pulse • Presence of light in fiber, at sufficiently high intensity, causes transient change in RI • Decreases RI slightly • Light travels faster in region of lower RI

8. Solitons—how they work (continued) • If light pulse is strong enough, this will affect its own phase and frequency • Called “self-phase modulation” (SPM) • Caused by nonlinear Kerr effect • Results in a “chirp” • Longer wavelengths move to beginning of pulse • Shorter wavelengths move to end • Exactly opposite to chromatic dispersion • Idea is to exactly balance the two effects • Pulse must be short: 30-50 ps ~ 20-30 GHz • Resulting pulse, a soliton will not smear but will still suffer attenuation

9. Solitons—how they work (continued) • Stable equilibrium: two dispersive forces balanced • If pulse disperses slightly forces will compress it • If pulse compresses slightly forces will smear it • Pulse launched onto fiber underthese conditions will quickly assumestable shape • Shape is that of hyperbolic secant Source: eFunda

10. Solitons—practical problems • Though solitons do not disperse, they are attenuated • Must be amplified periodically • Problem is that their high signal level (much higher than ordinary pulses) leads to SRS and SBS effects in amplifiers

11. Solitons—time line • 1973: First suggested by Akira Hasegawa of AT&T Bell Labs • First demonstrated in optical fiber by Linn Molenauer of Bell Labs in 1988 • 1991: Bell Labs research team transmitted solitons error-free at 2.5 gigabits over more than 14,000 kilometers, using optical amplifiers • 1998: Thierry Georges and his team at France Telecom R&D Center, combining optical solitons of different wavelengths demonstrated a data transmission of 1 Tb per second • 2001, the practical use of solitons became a reality when Algety Telecom deployed submarine telecom equipment in Europe carrying real traffic

12. Solitons—current efforts • In theory, multiple solitons can exist on a single mode fiber, giving rise to a new type of WDM • Being commercialized by Algety, spinoff from France Telecom • Doing DWDM with solitons of different colors ~ different distributions of light • Record: 1 Tbit/second over 1000 km • Bell Labs claims 80 Gbit/second over 10,000 km • NTT (Japan) • 10 Gb/s over 106 km • 40 Gb/s over 70,000 km • 640 Gb/s over 100 km • Use for information bearing purposes will likely be done with time division multiplexing • For comparison, Bell Labs claims that maximum fiber capacity is 100 Tb/s

13. Soliton—transmission diagram • Diagram from NTT illustrates advantages of Solitons

14. New generation of optical fiber • Dispersion-managed fiber • Objective: reduce dispersion over long distances • Low-water-peak fiber • Objective: remove attenuation due to water • Non-Zero Dispersion Shifted Fiber • Reduces PMD and allows longer lengths

15. Dispersion-managed fiber • Match fibers with opposite chromatic dispersion slopes • Done during manufacturing process • Two fibers spliced at appropriate points • Requires almost perfect matching of slopes • Primary application is undersea cable • Also being pushed for terrestrial applications • Opens up S-band for Raman amplification • Made by several manufacturers • Corning: Vascade R1000 • Furukawa (acquired Lucent OFS in 2001): Ultrawave

16. Dispersion-managed fiber (continued) Source: Lindstrom/Photonics Spectra, 4/02

17. Low water peak fiber • Reduce absorption peak at 1400 nm due to residual OH- present in fiber • Opens up E-band • Essentially opens up entire spectrum from 1260-1625 nm • Much greater potential for DWDM • But present equipment not ready • Better overall specs • Low attenuation: 0.22 dB/km at 1550 nm • Low PMD: 0.08 ps/nm/km (vs. 0.09 for standard fiber) • Extends reach of fiber by 50%

18. Low water peak fiber (continued) • Multiple vendors • Lucent: Allwave • Alcatel • Furukawa

19. Low water peak fiber (continued) Source: Lindstrom/Photonics Spectra, 4/02

20. Non-Zero Dispersion Shifted Fiber (NZ-DSF) • Ideal fiber for long-haul DWDM systems • Moves zero dispersion point away from 1550 nm, where it is with Dispersion-shifted fiber • Low dispersion, reduced 4-wave mixing • Increases maximum distance between compensation modules from 40-80 km to 200 km • Designed for 10 and 40 Gbps systems • Can be used in large rings or long-haul applications • Manufacturers • Alcatel: TeraLight • Corning: Leaf, MetroCor

21. NZ-DSF (continued) • Latest types have negative dispersion curves • Dispersion goes down with increasing wavelength • Primarily for metro area networks • Allows use of inexpensive, positively-chirped lasers • Combination can actually compress pulses, eliminating need for compensation • Cuts cost by eliminating expensive OEO equipment (repeaters)

22. NZ-DSF (continued) Source: Lindstrom/Photonics Spectra, 4/02

23. GMPLS Photonic multilayer routers • Also known as Hikari routers • Combine layer 1, 2, and 3 functions • Do IP routing and switching, wavelength routing • Optical label switched path network topology optimized • To minimize network cost • In response to fluctuations in IP traffic • Photonic platform with additions • 3R functions • Reclocking • Reshaping • Reamplification • Wavelength conversion • Layer 3 functions (MPLS)

24. Router specifications Source: NTT

25. Router structure Source: NTT

26. Cost reduction with Hikari router Source: NTT

27. New uses for MEMS • Variable attenuator • Spectral equalizer • OLS monitors • Dispersion compensators • Data modulators • Protection switches • Add/drop mux

28. MEMS chips Source: Lucent

29. Where MEMS devices will be used in lightwave systems Source: Lucent

30. New MEMS devices 1x2 optical switch Mechanically actuated reconfigurable slitmask (MARS) device 2 axis tilting micromirror MARS variable attenuator Near field scanning optical microscope (NSOM) Source: Lucent Tilting mirror for use in variable attenuator Mirror array for ADM

31. 1024 port optical switch fabric with capacity of 2 Pb/s Source: Lucent

32. Ethernet Passive Optical Network (EPON) • “Last mile” renamed “first mile” • Intended to solve local access problem once and for all • Neither DSL nor cable modems can meet all demands

33. Fiber to the home deployment scenarios Source: Alloptic

34. Advantages of passive optical networks • Long distance from central office to customer premises ~ 20 km • Minimize fiber deployment in local exchange office, local loop • High bandwidth • Can do video broadcast in downstream direction • No need to install active multiplexers at splitting locations • More info can be found at Passive Optical Networks Forum website, http://www.ponforum.org/

35. List of current vendors

36. How PON works • All active components between central office exchange and customer premises are eliminated • Passive optical components are put into the network to guide traffic based on splitting the power of optical wavelengths to endpoints along the way • Replacement of active with passive components provides a cost-savings to service provider by eliminating need to power and service active components in the transmission loop • Passive splitters or couplers are merely devices working to pass or restrict light • Have no power or processing requirements and have virtually unlimited Mean Time Between Failures

37. How PON works (continued) • Main fiber run on a PON network can operate at 155 Mps, 622 Mbps, 1.25 Gbps or 2.5 Gbps using APON/ BPON, EPON or the emerging GPON standards • Bandwidth allocated to each customer from this aggregate bandwidth can be static or dynamically assigned in order to support voice, data and video applications. • With PON, a single fiber from the carrier’s exchange can service 16, 32 or more buildings through the use of both passive devices to split the optical signal • PON protocols to control the sending and transmission of signals across the shared access facility 

38. Passive optical network topologies OLT=optical line terminator ONU=optical network user Source: Alloptic

39. Another flavor of PON Source: PON Forum

40. Downstream traffic split using MAC address Source: Alloptic

41. Issues • Signal attenuation different for each user (ONU) • Security • Need to handle different types of traffic • Standards • Being done in IEEE P802.3AH group

42. Wavelength converters • Types • Laser converters • Coherent or nonlinear converters • Four-wave mixing converters • Difference frequency mixing converters • Cross-phase modulation converters • Optically controlled amplifiers • Cross-gain modulated converter • Cross-phase modulated (XPM) converter • Delayed interference converter

43. Laser converters • Strong input signal at one wavelength directed into another laser • Output laser is continuous wave single-frequency laser • Input signal causes gain saturation • Light energy drained from oscillation wavelength • When input is “on”, output laser generates much lower power at its normal wavelength • Filter blocks input wavelength • Disadvantages • Inverts original signal • Speed limited to 10 Gbit/sec • Narrow input power range (0 – 10 dBm)

44. Coherent or nonlinear converters • Based on coherent nonlinear processes • Two or more wavelengths interact to generate other wavelengths • Types • Four wave mixing converters: combine pump signal at frequency npump with input signal at ninput to generate output at 2npump-ninput = nout • Difference frequency mixing converters: generate output signal as difference frequency between pump light and laser light • Cross-phase modulation converters: use input signal to modulate phase of signal at second wavelength passing through long fiber • Then convert phase modulation into intensity modulation

45. Cross-phase modulation Source: WDM Solutions, 4/02

46. Optically controlled amplifiers • Based on optically controlled gates • Relatively weak input signal modulates output of second wavelength from semiconductor optical amplifier (SOA) • Input at second wavelength is from continuous wave source • Effect is to transfer signal from input to modulation of second wavelength • Typically the two wavelengths transmitted in opposite directions • 3 modulation methods • Cross-gain: input signal saturates gain of SOA • Modulates second wavelength by reducing its intensity (inverting it) when input is strong • Fast: up to 100 Gbits/sec • Has wavelength chirp

47. Optically controlled amplifiers (continued) • Cross-phase modulation (XPM) • Low power input signal modulates phase rather than intensity of continuous wave at second wavelength • Occurs because input signal depletes carrier intensity in SOA, changing RI and thus shifting phase • Interferometer stage converts phase modulation to intensity modulation • Works up to 10 Gbits/sec • Can do 2R regeneration • May be able to do reclocking in future

48. Optically controlled amplifiers (continued) • Delayed interference • Input signal phase-modulates amplification of continuous wave beam • Amplifier output divided into two beams • One beam time delayed by loop • Two beams combined, regenerating input signal at new wavelength • Works up to 100 Gbits/sec

49. Delayed interference converter Source: WDM Solutions, 4/02