Optical Fiber Communications

1. Optical Fiber Communications

2. Outline • History • Types of fiber • Light propagation • Losses in optical fiber • Optical fiber classification • Sources • Detectors • Optical fiber system link budget

3. Introduction • EM waves are guided through media composed of transparent material • Without using electrical current flow • Uses glass or plastic cable to contain the light wave and guided them • Infinite bandwidth – carry much more information

4. History • Photophone • Alexander Graham Bell • Mirrors and detectors transmit sound wave via beam light • Awkward, unreliable, no practical application • Smoke signals and mirrors • Uncoated fiber cables • 1930, J.L. Baird and C.W. Hansell • scanning and transmitting TV image

5. History • 1951 – light transmission via bundles of fibers – leads to fiberscope – medical field • 1958 – light amplification – stimulated emission • 1960 – laser invention • 1967 – fiber cable with clad • 1970 – low loss optical cable. < 2 dB/km • 1980 – optical cable refined – affordable optical communication system • 1990 – 0.16dB/km loss

6. History • 1988 – long haul transmission system • 1988 – SONET • 1990 – optical voice and data network are common

7. Advantages • Wider bandwidth • Better than metallic cables • Up to several thousand GHz • Speed up to several Gbps • Immunity to crosstalk • glass fiber/plastic are non-conductor to electrical current • immune to adjacent cables • Immunity to static interference • immune to static noise – EMI, lightning etc.

8. Advantages • Environmental Immunity • more resistant to environment, weather variations • wider temperature range operation • less affected by corrosive liquids and gases • Safety and convenience • safer and easier to install and maintain • no current and voltage associated • no worry about explosion and fire caused • lighter and compact, flexible, lesser space required

9. Advantages • Lower transmission loss • lesser loss compared to metallic cables • 0.19 dB/km loss @ 1550 nm • amplifiers can be spaced more farther apart • Security • virtually impossible to tap into a fiber cable • Durability and reliability • last longer, higher tolerance to changes in environment and immune to corrosion • Economics • Approximately the same cost as metallic cables • less loss between repeaters. Lower installation and overall system’s cost

10. Disadvantages • Interfacing cost • Optical cable – transmission medium • Needs to be connected to standards electronics facilities – often to be expensive • Strength • lower tensile strength • can be improved with kevlar and protective jacket • glass – fragile – less required for portability • Remote electrical power • need to be include electrical line within fiber cable for interfacing and signal regeneration

11. Disadvantages • Loss due to bending • bending causes irregularities in cable dimension – the light escapes from fiber core – loss of signal power • prone to manufacturing defect • Specialized tools, equipment and training • tools to splice, repair cable • test equipment for measurements • skilled technicians

12. Optical Spectrum

13. Optical Communication systems

14. Types of fiber • Optical fiber construction

15. Types of fiber • Optical fiber construction • special lacquer, silicone, or acrylate coating – outside of cladding – to seal and preserve the fiber’s strength, protects from moisture • Buffer jacket – additional cable strength against shocks • Strength members – increase a tensile strength • Outer polyurethane jacket

16. Types of fiber • fiber cables – either glass, plastic or both • Plastic core and cladding (PCP) • Glass core – plastic cladding (PCS) • Glass core – glass cladding (SCS) • Plastic core – more flexible - easier to install • but higher attenuation than glass fiber – not as good as glass • Glass core – lesser attenuation – best propagation characteristics • but least rugged • Selection of fiber depends on its application – trade off between economics and logistics of particular application

17. Physics of light • Physics of light • Einstein and Planck – light behaves like EM wave and particles – photon – posses energy proportional to its frequency

18. light propagation • the lowest energy state – grounds state • energy level above ground state – excited state • if energy level decays to a lower level – loss of energy is emitted as a photons of light • The process of decaying from one level to another – spontaneous decay or spontaneous emission • Atoms can absorbs light energy and change its level to higher level – absorption

19. light propagation • Optical power • flow of light energy past a given point in a specified time

20. light propagation • Optical power • generally stated in decibel to define power level (dBm) • Question • 10 mW in dBm?

21. light propagation • Velocity of Propagation • in vacuum – 3 x 108 m/s • but slower in a more dense material than free space • when it passes through different medium or from one medium to another denser material – the ray changes its direction due to the change of speed

22. light propagation • from less dense to more denser material – the ray refracted closer to the normal • from more denser material to less denser material – the ray refracted away from the normal

23. light propagation • Refraction • Occurs when the light travels between two different material density and changes it speed based on the light frequency • Refractive Index • the ratio of the velocity of propagation of a light ray in a given material

24. light propagation

25. light propagation • Snell’s Law • how a light ray reacts when it meets the interface of two transmissive materials that have different indexes of refraction

26. light propagation • Snell’s Law • angle of incidence • angle at which the propagating ray strike the interface with respect to the normal • angle of refraction • the angle formed between the propagating ray and the normal after the ray entered the 2nd medium

27. light propagation • Snell’s Law

28. light propagation • Question • medium 1 – glass = 1.5 • medium 2 – ethyl alcohol = 1.36 • angle of incident – 30o • determine the angle of refraction

29. light propagation • Critical Angle • the angle of incident ray in which the refracted ray is 90o and refracted along the interface

30. light propagation • Critical Angle • the minimum angle of incident at which the refracted angle is 90o or greater • the light must travel from higher refractive index to a lesser refractive index material

31. light propagation • Critical Angle

32. light propagation • Acceptance Angle • the maximum angle in which external light rays may strike the air/glass interface and still propagate down the fiber

33. light propagation • Acceptance Angle

34. light propagation • Numerical Aperture - NA • to measure the magnitude of the acceptance angle • describe the light gathering or light-collecting ability of an optical fiber • the larger the magnitude of NA, the greater the amount of external light the fiber will accept

35. light propagation • Numerical Aperture - NA

36. Optical Fiber Configurations • Mode of propagation • single mode • only one path for light rays down the fiber • multimode • many higher order path rays down the fiber

37. Optical Fiber Configurations • Index Profile • graphical presentation of the magnitude of the refractive index across the fiber • refractive index – horizontal axis • radial distance from core – vertical axis

38. Optical Fiber Configurations • Index Profile • step index – single mode • step index – multimode • graded index - multimode

39. optical fiber classification • Single Mode Step Index • dominant – widely used in telecommunications and data networking industries • the core is significantly smaller in diameter than multimode cables

40. optical fiber classification • Multimode Step Index • similar to single mode – step index fiber • but the core diameter is much larger • light enters the fiber follows many paths as it propagate down the fiber • results in different time arrival for each of the path

41. optical fiber classification • Multimode Mode Graded Index • non uniform refractive index – decreases toward the outer edge • the light is guided back gradually to the center of the fiber

42. optical fiber classification • Comparison • Single mode step index • (+) minimum dispersion – same path propagation – same time of arrival • (+) wider bandwidth and higher information txn. rate • (-) small core – hard to couple light into the fiber • (-) small line width of laser required • (-) expensive – difficult to manufacture

43. optical fiber classification • Comparison • Multimode step index • (+) relatively inexpensive, simple to manufacture • (+) easier to couple light into the fiber • (-) different path of rays – different time arrival • (-) less bandwidth and transfer rate • Multimode graded index • intermediate characteristic between step index single and multimode

44. losses in optical fiber • Attenuation • power loss – reduction in the power of light wave as it travels down the cable • effect on system’s performance by reducing: • system’s bandwidth • information tx rate • efficiency • overall system’s capacity

45. losses in optical fiber • Attenuation

46. losses in optical fiber • Attenuation • depends on signal’s wavelength • generally expressed as decibel loss per km • dB/km

47. losses in optical fiber • Attenuation optical power in decibel units is P(dBm)= Pin(dBm)-A(dB) P= measured power level (dBm) Pin =transmit power (dBm) A= cable power loss, attenuation (dB)

48. losses in optical fiber • Question • Single-mode optical cable • input power 0.1 mW light source • 0.25 dB/km cable loss • determine • optical power 100 km from the transmitter side

49. losses in optical fiber • Absorption Loss • absorption due to impurities – absorb lights and convert it into heat • contributors: • Ultraviolet – ionized valence electron in the silica material. • infrared – photons of light absorbed by glass’s atom – converted into random mechanical vibrations - heating • ion resonance – caused by OH- in in the material. OH- trapped in the glass during manufacturing process

50. losses in optical fiber • Absorption Loss