TCOM 503 Fiber Optic Networks

1. TCOM 503Fiber Optic Networks Spring, 2007 Thomas B. Fowler, Sc.D. Senior Principal Engineer Mitretek Systems

2. Topics for TCOM 503 • Week 1: Overview of fiber optic communications • Week 2: Brief discussion of physics behind fiber optics • Week 3: Light sources for fiber optic networks, cable types and propagation of light in fiber • Week 4: Fiber optic components fabrication and use • Week 5: Modulation of light, its use to transmit information • Week 6: Noise and detection • Week 7: Optical fiber fabrication and testing of components

3. Week 2: Brief discussion of physics behind fiber optics • Brief history of the physics of light • Nature of light • Basic principles of optics • Reflection and refraction • Interference and diffraction • Types of optical fiber • Devices used in fiber optics

4. Brief history of the physics of light • Atomists in ancient Greece (5th c. BC): formulated an emission theory • Pictured light as torrent of minute, high-speed particles • Aristotle (4th c. BC) • Added fifth element to traditional four (fire, air, earth, water): the aether • All void had to be filled with something, hence the aether • Proposed that human vision arises from movement of the aether produced by the body we perceive

5. Brief history of the physics of light (continued) • Robert Hooke (1660s) proposed that only a wave could account for observed properties of light • Pattern of colors in (thin films) soap bubbles • Fact that two beams of light can cross without scattering • Light is self-sustaining vibration of some medium without transport of matter • Newton favored corpuscular theory, primarily because of optics • Beams of light don’t diverge, as he thought wave theory required • Didn’t realize small size of light waves • Theory required speed of light to be greater in water than in air

6. Brief history of the physics of light (continued) • In 19th century, wave theory reborn with work of Thomas Young (1773-1829) • Discovered interference • Augustin Fresnel (1788-1827) did extensive experiments on interference and diffraction • Put wave theory on mathematical basis • Showed that rectilinear propagation of light due to short wavelength • 1850 – Foucault measured speed of light in water and showed it was less than in air • 1860 — Maxwell develops electromagnetic theory, shows that light is form of electromagnetic radiation • 1887 — Hertz confirms Maxwell’s theory, but also discovers photoelectric effect • Ability of light to dislodge electrons • Found to be independent of intensity, dependent on frequency

7. Brief history of the physics of light (continued) • 1905 — Einstein explains photoelectric effect by reverting to particle theory of light • Light particles called photons • Energy given by famous formula E = hf • h = Planck’s constant • Also in 1905, Einstein proposes Special Theory of Relativity • Speed of light, c, is universal constant • 1920s — Development of Quantum Mechanics elucidates nature of light and matter

8. Speed of light • Galileo attempted to measure, but his equipment was too crude, leading to assumption of infinite speed • 1675 — Römer measured using eclipses of Jupiter’s moons • Obtained result of 125,000 miles/sec (2.02 x 108 m/sec) • Used incorrect value for diameter of earth’s orbit • 1849 — Fizeau measured speed using lenses and mirrors • Obtained value of 3.133 x 108 m/sec • 1850 — Foucault measures speed with improved method • Obtained value of 2.98 x 108 m/sec

9. Speed of light (continued) • 1926 — Michaelson used similar method • Obtained value of 2.99786 x 108 m/sec • Current value: 299,792,458 m/sec source observer

10. Nature of light • Sometimes a particle, sometimes a wave • Electromagnetic spectrum • Particles: ray optics, lenses, reflection, refraction • Waves: interference, diffraction

11. Visible spectrum—Newton’s experiment Demo: http://micro.magnet.fsu.edu/primer/java/scienceopticsu/newton/

12. Types of waves • Longitudinal (sound waves) Source: C. R. Nave, Hyperphysics, Georgia State Univ.

13. Types of waves (continued) • Transverse (light, waves on rope) Source: C. R. Nave, Hyperphysics, Georgia State Univ.

14. Types of waves (continued) Wave motion demo http://www.matter.org.uk/schools/Content/seismology/longitudinaltransverse.html

15. Light is type of electromagnetic radiation • Electric, magnetic fields orthogonal to each other and direction of propagation • Eye (and most other things) affected primarily by electric field • Magnetic field much weaker, by factor of c E = cB

16. Electromagnetic Spectrum

17. Electromagnetic spectrum in vicinity of visible light used for fiber optics • Seven regions, called windows, lie at infrared wavelengths of relatively low attenuation in glass • First at 850 nm • First developed • Used now only for short distance multimode fiber • Second (O band) at 1310 nm (1260-1310 nm) • Second developed • Lower attenuation than 850 window • Third (C band) at 1550 nm (1530-1565 nm) • Third developed • Superior to other two • Fourth (L band) at 1625 (1565-1625) nm • Currently under development

18. Electromagnetic spectrum in vicinity of visible light (continued) • Others • E band (1360-1460 nm) • S band (1460-1530 nm) • U band (1625-1675 nm)

19. Electromagnetic spectrum in vicinity of visible light (continued) Source: Cisco

20. Electromagnetic spectrum in vicinity of visible light (continued) • All the bands Source: Networkmagazine.com

21. Electric and magnetic fields of light wave Source: Dutton, Figure 3

22. Propagation of light – 3d view Source: Dutton, Figure 4

23. Propagation of electromagnetic waves • Demonstration http://www.phy.ntnu.edu.tw/java/emWave/emWave.html

24. Polarization Source: Hecht, Physics

25. Polarization Demo http://micro.magnet.fsu.edu/primer/java/polarizedlight/filters/index.html

26. Circular polarization • Electric, magnetic fields can rotate as wave propagates • Referred to as “circular polarization” Source: Dutton, Figure 5

27. Basic principles of optics • Light propagation • Law of refraction

28. Refraction Demo http://micro.magnet.fsu.edu/primer/java/refraction/index.html

29. Reflection Source: Zona Land, http://id.mind.net/~zona/index.html

30. Reflection Demo http://micro.magnet.fsu.edu/primer/java/specular/index.html

31. Basic relationships • Frequency — n or f (Hz or cycles/second) • Angular frequency — w = 2pf • Wavelength — l (m, cm, nm) • Wave number — k (dimensionless), proportional to number of waves per unit length • Period — T (seconds, msec, microsecond, nanosecond) • Amplitude — A • Velocity — v (m/sec) v = f l k = 2p/l • Propagation of a wave y(x,t) = A sin (kx-kvt) = A sin (kx – wt)

32. Refraction (continued) • Snell’s law:

33. qi qi Incident medium, ni refraction medium, nr Incident ray Refracted ray refraction medium, nr Incident medium, ni Refracted ray Incident ray qr qr (b) Snell’s Law for light entering a less dense medium Figure 2. (a) Snell’s Law for light entering a denser medium Law of refraction

34. Refraction of light rays n2 < n1 n1 Source: Tipler, Physics

35. Refraction and total internal reflection Source: Tipler, Physics

36. qr=90o refraction medium, nr < ni refraction medium, nr < ni Refracted ray Reflected ray Incident medium, ni qi Incident ray Incident medium, ni Incident ray qcr (b) Critical angle for refraction (b) Total internal reflection, qi > qcr Total internal reflection

37. Total internal reflection • Demo http://www.phy.ntnu.edu.tw/java/propagation/propagation.html

38. Optical fiber construction n2 < n1 Source: Nortel

39. Optical fiber construction (continued) Source: Corning

40. Light propagation in a glass fiber Source: Hecht, Physics

41. Total internal reflection in fiber optic cables • Note that, in the case of optical fiber (and most other cases), cladding is not a conductor • Therefore electric and magnetic fields of light wave penetrate some distance into it • Sharp cutoff assumes ray optics, not actual wave optics and quantum mechanics

42. Dispersion • Newton's experiments illustrated the dispersion of sunlight into a spectrum (and recombination into white light). • Sunlight consists of a mixture of light with different wavelengths. • A dispersive medium is one in which different wavelengths of light have slightly different indices of refraction • Crown glass is a dispersive medium since the index of refraction for violet light in crown glass is higher than for red light • This is responsible for chromatic aberration • Manufacturers of optical glass customarily specify the refractive index of a material for yellow sodium light, the D line

43. Dependence of index of refraction on l • Index of refraction not constant • Since index of refraction is determined by speed of light in the medium, follows that speed of light in medium is function of l • Shorter wavelengths travel slower because index of refraction is greater • Will lead to dispersion of information bearing light waves over distance • Called “material dispersion”

44. Dependence of index of refraction on l Source: Hecht, Physics

45. Dispersion (continued) • Waveguide dispersion • Light travels in both core and inner cladding at slightly different speeds (faster in cladding) • Material and waveguide dispersion opposite effects • Can be balanced to allow for zero dispersion at a particular wavelength between 1310nm and 1650 nm • Total effect called “chromatic dispersion” Source: Corning

46. Effect of chromatic dispersion Source: Nortel

47. Interference and Diffraction • Extremely important for fiber optics • Both effects limit performance of optical fiber

48. Interference • Interference from two point sources • Originates because waves from two sources are in phase or out of phase, depending on position (distance from the two sources • Gives rise to series of alternating light and dark bands on target at fixed distance from the sources • Basic relationships • Maxima at angle q given by d sin q = ml, m = 0, 1, 2… • Minima at angle q given by d sin q = (m+1/2)l, m = 0, 1, 2…

49. Interference – Young’s experiment Nowadays this would be replaced by a laser Source: Dutton, Figure 6

50. Intensity of interference pattern Source: Dutton, Figure 7