1. CSC/ECE 778: Optical Networks Rudra Dutta, Fall 2007 Fiber-Optical Communication and Switching

2. Copyright Rudra Dutta, NCSU, Fall, 20072 Outline We want/need to understand effect on networking – What components are possible, limitations Quick overview of representative technology – Optical Connection and Power Budget – Fundamentals of Fiber Optic Transmission – Transmission Impairments and Solutions – Lasers and Photodetectors – Other Optical Components (Couplers, Filters, Multiplexers, Switches, OADMs, Amplifiers)

3. Copyright Rudra Dutta, NCSU, Fall, 20073 Layering and Optical Services Generalized protocol layering can create complicated multi-layer networks In this context, “optical layer” is another layer close to physical layer, but possibly implementing network semantics of its own Network Data Link Physical Optical SONET ATM IP User Apps Network Data Link Physical Network Data Link Physical

4. Copyright Rudra Dutta, NCSU, Fall, 20074 Why Fiber? Huge bandwidth: 30-50 THz Low losses (intrinsic): 0.2 db/Km Low bit error rates (BER): 10 -11 Low power requirements: 100 photons/bit Immunity to electromagnetic interference (EMI) Low cross-talk Repeater-less amplification (EDFAs) Low cost, maintenance

5. Copyright Rudra Dutta, NCSU, Fall, 20075 Optical Endpoint

6. Copyright Rudra Dutta, NCSU, Fall, 20076 Optical Power Budget Finite power available at source (laser) Minimum detectable receiver power Must account for all losses between source and receiver Optical networks are power-budget limited, not bandwidth limited

7. Copyright Rudra Dutta, NCSU, Fall, 20077 Optical Power Budget (cont'd)

8. Copyright Rudra Dutta, NCSU, Fall, 20078 Wavelengths of Importance

9. Copyright Rudra Dutta, NCSU, Fall, 20079 Optical Fiber Optical waveguide Cylindrical core surrounded by cladding (+ protective covering) – made of same transparent material (glass, plastic) – difference is value of refractive index n = c / v Single-mode vs. multimode fiber – single-mode: core diameter 8-12µm, link length > 2Km – multimode: core diameter 50µm, link length < 2Km Step-index vs. graded-index fiber – step-index: refractive index constant across core diameter – graded-index: refractive index varies along core diameter

10. Copyright Rudra Dutta, NCSU, Fall, 200710 Refractive Index Profiles

11. Copyright Rudra Dutta, NCSU, Fall, 200711 Geometric Optics: Snell's Law n 1 sin  i = n 2 sin  t

12. Copyright Rudra Dutta, NCSU, Fall, 200712 Geometric Optics: Total Reflection Critical angle:  c = sin -1 (n2 ÷ n 1 )

13. Copyright Rudra Dutta, NCSU, Fall, 200713 Maximum Cone of Acceptance

14. Copyright Rudra Dutta, NCSU, Fall, 200714 Transmitter-to-Fiber Coupling

15. Copyright Rudra Dutta, NCSU, Fall, 200715 Modes: The Wave Picture

16. Copyright Rudra Dutta, NCSU, Fall, 200716 Allowed Ray Angles Only allowed ray angles result in guided modes AB = d sin  m = m /2 leads to half wavelength in the core – m : integer, : optical wavelength in the core Mode: one possible path that a guided ray can take

17. Copyright Rudra Dutta, NCSU, Fall, 200717 Transmission Impairments Factors affecting transmission distance and bandwidth: – attenuation – dispersion – non-linear effects Must minimize their effects for high performance – improvement and redesign of fiber itself – compensating for these factors Attenuation problem solved  dispersion effects significant Dispersion effects reduced  non-linear effects dominant

18. Copyright Rudra Dutta, NCSU, Fall, 200718 Attenuation Decrease in optical power along the length of the fiber Varies with wavelength Attenuation coefficient: a dB = - 10/L log 10 (P R ÷P T ) (dB/Km) – L : length of fiber – P T : power launched into the fiber – P R : power received at end of fiber

19. Copyright Rudra Dutta, NCSU, Fall, 200719 Power Losses Material absorption: due to – resonances of silica molecules – impurities -- most serious is peak at 1390 nm due to OH ions Rayleigh scattering: medium is not absolutely uniform – refractive index fluctuates  light is scattered – scattering proportional to -4  dominant at < 800 nm Waveguide imperfections: relatively small component – nonideal fiber geometries – due to bending, manufacturing imperfections

20. Copyright Rudra Dutta, NCSU, Fall, 200720 Low Loss Region of An Optical Fiber

21. Copyright Rudra Dutta, NCSU, Fall, 200721 Erbium-Doped Fiber Amplifiers

22. Copyright Rudra Dutta, NCSU, Fall, 200722 EDFA Principle of Operation E i : energy level N i : population of erbium ions at energy level E i – normally (no pump/signal): N 1 > N 2 > N 3 – pump/signal present: population inversion N 2 > N 1

23. Copyright Rudra Dutta, NCSU, Fall, 200723 EDFA Properties Emission: – stimulated  amplification – spontaneous  noise  amplified spontaneous emission  limit on number of EDFAs along the fiber Energy levels are narrow bands  each transition associated w/ a band of wavelengths  amplify wide band around 1550nm Replace expensive and complicated electronic units Signal remains in optical form  transparency “Distributed” amplifiers

24. Copyright Rudra Dutta, NCSU, Fall, 200724 Semiconductor Optical Amplifiers (SOAs) Similar to semiconductor laser Consist of active medium (p-n junction) Energy levels of electrons confined to 2 bands  EDFA E 1, E 2 Mobile carriers (holes, electrons) play the role of erbium ions Has several disadvantages compared to EDFAs Useful when combined with other components into optoelectronic integrated circuits (OEICs) – preamplifier in optical receiver – power amplifier in optical transmitter

25. Copyright Rudra Dutta, NCSU, Fall, 200725 Dispersion A narrow pulse spreads out as it propagates along the fiber Intersymbol interference: – pulse overlaps neighboring pulses – sharply increases the BER Dispersion imposes a limit on the bit rate that can be supported Intermodal vs. chromatic dispersion

26. Copyright Rudra Dutta, NCSU, Fall, 200726 Intermodal Dispersion Most serious form of dispersion Occurs in multimode fibers Different modes of a wavelength travel at different speeds Multimode fibers limited to low bitrate-distance products Solutions: – use single-mode fibers for large bitrate-distance products (8 µm < 2a < 10 µm  only one mode is guided) – use graded-index fibers

27. Copyright Rudra Dutta, NCSU, Fall, 200727 Graded Index Fibers

28. Copyright Rudra Dutta, NCSU, Fall, 200728 Propagation in Graded Index Fibers Rays are bent as they approach the cladding Rays further from core travel faster (due to lower n) Intermodal dispersion reduced by several orders of magnitude

29. Copyright Rudra Dutta, NCSU, Fall, 200729 Chromatic Dispersion Two sources of chromatic dispersion: – material dispersion, D M – waveguide dispersion, D W Chromatic dispersion: D = D M + D W

30. Copyright Rudra Dutta, NCSU, Fall, 200730 Material Dispersion The physical effect that allows raindrops to form rainbow Refractive index of a material changes with wavelength  different wavelengths travel at different speeds along the fiber Different delays cause spreading of output pulse, depending on: – wavelength span of source – length of fiber

31. Copyright Rudra Dutta, NCSU, Fall, 200731 Waveguide Dispersion D W is a function of fiber geometry Dispersion-shifted fibers: – D W causes zero-dispersion point to shift to 1550 nm range – min dispersion range coincides with min loss range Dispersion-flattened fibers: dispersion profile close to zero for a wide spectral range

32. Copyright Rudra Dutta, NCSU, Fall, 200732 Dispersion Profile of Single-Mode Fiber

33. Copyright Rudra Dutta, NCSU, Fall, 200733 Non-Linear Effects Stimulating Raman Scattering (SRS): – light interacts with fiber medium  inelastic collisions – not important in single-channel systems (thresh. about 500mW) – involves transfer of power: hi freq. wave  lo freq. wave – introduces cross-talk in multiwavelength systems Stimulating Brillouin Scattering (SBS): – no cross-talk, low threshold power (few mW for 20-Km fiber) Four-Wave Mixing – three signals present at neighboring freq: f 1, f 2, f 3 – new signal produced, e.g., f 4 = f 1 + f 2 - f 3

34. Copyright Rudra Dutta, NCSU, Fall, 200734 Solitons Distortion, non-linearities: distort, broaden a propagating pulse Right combination of distortion, non-linearity: – compensate each other – produce a narrow, stable pulse (soliton) – solitons travel over long distances without any distortion – solitons in opposite directions pass thru transparently Ideal situation for long-distance communication EDFAs needed to maintain solitons over long distances

35. Copyright Rudra Dutta, NCSU, Fall, 200735 Lasers Light amplification by stimulated emission of radiation Schawlow and Townes, 1958 First solid-state laser by Maiman, 1960 Today, lasers exist in myriad forms

36. Copyright Rudra Dutta, NCSU, Fall, 200736 Semiconductor Energy State Diagrams

37. Copyright Rudra Dutta, NCSU, Fall, 200737 Fabry-Perot Cavity Part of light leaves cavity through right facet, part is reflected Resonant wavelengths: L = m /2

38. Copyright Rudra Dutta, NCSU, Fall, 200738 Single-Wavelength Operation FP laser cavity supports many modes/wavelengths of operation Monochromatic light needed for high bitrate- distance products Geometry is modified to achieve single- wavelength operation Distributed Bragg Reflector (DBR) lasers Distributed Feedback (DFB) lasers Expensive, widely used in long-distance communication

39. Copyright Rudra Dutta, NCSU, Fall, 200739 Tunability Laser tunability important in WDM network applications: – slow tunability (ms range): set up lightpaths in wavelength routing networks – fast tunability (µs or ns range): multiple access (T- WDMA) applications

40. Copyright Rudra Dutta, NCSU, Fall, 200740 Tunability (cont'd) Mechanically tuned: change FP cavity length – (tuning range: 10-20 nm, tuning time: 100-500 ms) Injection current tuned: change refr. index in DFB/DBR lasers – (tuning range: 4 nm, tuning time: 10s of ns) Multiwavelength laser arrays – built in single chip – one or more lasers can be activated simultaneously – light from each laser fed to star coupler

41. Copyright Rudra Dutta, NCSU, Fall, 200741 Optical Receivers

42. Copyright Rudra Dutta, NCSU, Fall, 200742 Photodetectors

43. Copyright Rudra Dutta, NCSU, Fall, 200743 Filters Various technologies: – Fabry-Perot filters – Multilayer interference (MI) filters – Mach-Zehnder interferometers – Arrayed waveguide grating – Acousto-optic tunable filter Tunability important Can be used as MUX/DEMUX, wavelength routers

44. Copyright Rudra Dutta, NCSU, Fall, 200744 MI Filters Bandpass filter Passes thru particular wavelength, reflects all other Cascade multiple filters to create a MUX/DEMUX

45. Copyright Rudra Dutta, NCSU, Fall, 200745 MI Filters as MUX/DEMUX

46. Copyright Rudra Dutta, NCSU, Fall, 200746 MUX/DEMUX: Logical View

47. Copyright Rudra Dutta, NCSU, Fall, 200747 Directional Couplers Coupling possible when waveguides placed close together Coupling ratio controlled by voltage

48. Copyright Rudra Dutta, NCSU, Fall, 200748 Couplers: Logical View P 1’ = a 11 P 1 + a 12 P 2, P 2’ = a 21 P 1 + a 22 P 2 For ideal symmetric couplers: a 11 = a 22 = a, a 12 = a 21 = 1-a

49. Copyright Rudra Dutta, NCSU, Fall, 200749 Couplers Star Coupler: – a = 1/2, 2x2 star coupler (3-dB coupler) – Cascade 2x2 couplers to build NxN star coupler Power Splitter: – P 2 = 0, a = 1/2 Switches: – a = 0,1; 2x2 switch – cascade 2x2 switches to build NxN switch Real devices are lossy: – a 11 + a 12 < 1, a 21 + a 22 < 1

50. Copyright Rudra Dutta, NCSU, Fall, 200750 Internal Structure of Star Coupler

51. Copyright Rudra Dutta, NCSU, Fall, 200751 Gratings

52. Copyright Rudra Dutta, NCSU, Fall, 200752 Gratings: Principle of Operation Multiple narrow slits spaced equally apart on the grating plane Light incident on one side of grating transmitted through slits Diffraction: light through each slit spreads out in all directions Different s interfere constructively at different points of imaging plane  separate WDM signal into constituent wavelengths

53. Copyright Rudra Dutta, NCSU, Fall, 200753 Bragg Gratings Bragg grating: any periodic pertrubation in propagating medium Perturbation is usually periodic variation of refractive index Bragg gratings used in many photonic devices: – DBR lasers: Bragg gratings written in waveguides – Fiber Bragg gratings (FBG): written in fiber – Acousto-optic tunable filters: Bragg grating formed by propagation of an acoustic wave in the medium

54. Copyright Rudra Dutta, NCSU, Fall, 200754 FBG as Add-Drop Multiplexers

55. Copyright Rudra Dutta, NCSU, Fall, 200755 OADM: Logical View

56. Copyright Rudra Dutta, NCSU, Fall, 200756 Optical Switches Mechanical switches – directional couplers, ratio modified by bending (ms range) – MEMS mirrors moved in and out of path (100s of ns range) Bubble-Based switches – bubbles in optical fluid reflect beam (10s of ms range) Electro-Optic switches – couplers, ratio modified by changing refr. index (ns range) Thermo-Optic switches – refractive index function of temperature (ms range) Semiconductor Optical Amplifier (SOA) switches – SOA, change in voltage to use as on-off switch (ns range)

57. Copyright Rudra Dutta, NCSU, Fall, 200757 MEMS Optical Switching MEMS = micro-electro-mechanical system Movable mirrors to reflect light 2D MEMS: a 2-state pop-up MEMS mirror – state ``0'': popped up position light reflected – state ``1'': flat (folded) position light passes through

58. Copyright Rudra Dutta, NCSU, Fall, 200758 2D MEMS Switches

59. Copyright Rudra Dutta, NCSU, Fall, 200759 Analog Beam-Steering Mirror Mirror can be freely rotated on two axes to reflect a light beam

60. Copyright Rudra Dutta, NCSU, Fall, 200760 3D MEMS Switch

61. Copyright Rudra Dutta, NCSU, Fall, 200761 Static Optical Switches

62. Copyright Rudra Dutta, NCSU, Fall, 200762 Reconfigurable Optical Switches

63. Copyright Rudra Dutta, NCSU, Fall, 200763 Wavelength Converters

64. Copyright Rudra Dutta, NCSU, Fall, 200764 Spectrum Partitioning c = f,  f - c  / 2 100 Ghz is about.8 nm at 1,550 nm range 10-Ghz spacing: – very dense by current standards – can accommodate 1 Gbps digital bit rates – can accomodate 1 Ghz analog bandwidths – OK for receivers, but too close for wavelength routing 100 Ghz spacing OK for optical switches – WDM limit today Waveband routing alleviates throughput loss – But better switching technology nullifies advantage – However, continue to be useful because needs “coarser” filters

65. Copyright Rudra Dutta, NCSU, Fall, 200765 Spectrum Partitioning (cont'd)

66. Copyright Rudra Dutta, NCSU, Fall, 200766 Waveband vs. Wavelength