Optical fiber Transmission Media

1. Optical fiber Transmission Media MIRZA IRFAN BAIG Asst Prof Department of Information Technology Balaji Institute of Technology & Sciences Laknepally, Narsampet zissiar2@yahoo.com +91-9985287172

2. ADVANTAGES • Wider bandwidth and greater information capacity • Immunity to crosstalk • Immunity to static interference • Environmental immunity • Safety and convenience • Lower transmission loss • Security • Durability and Reliability • Economics

3. DISADVANTAGES • Interfacing Costs • Strength • Remote electrical power • Losses through Bending • Specialized tools

4. Optical fiber communication link

5. Optical Fiber - Transmission Characteristics • Act as wave guide for 1014 to 1015 Hz • Portions of infrared and visible spectrum • Light Emitting Diode (LED) • Cheaper • Wider operating temp range • Last longer • Injection Laser Diode (ILD) • More efficient • Greater data rate • Wavelength Division Multiplexing

6. Optical Fiber cross-section Protection cladding Plastic (250 or 900 µm) Cladding pure silica (125 µm) Core Doped silica (9 – 62.5 µm) Index step typical values: n = 0.001 - 0.01 Index Profile

7. Electromagnetic Spectrum  = 1550 nm = 1.55 m f = 193 1012 Hz = 193 THz  = wavelength (meters) c = velocity of light (300.000.000 meters per second) f = frequency (hertz)

8. Bandwidth of an optical fiber What is the bandwidth of and optical fiber?  = 1 – 1.6 m

9. Bandwidth of optical fiber communications What is the bandwidth of an optical fiber?  = 1 – 1.6 m f = 3 x 1014 – 1.9 x 1014 Hz BW = 1.1 x 1014 Hz = 110 THz

11. Communications: Photonics versus Electronics Capacity of transmission lines: Coaxial Cable: 10 Mb/s×km, Limited by attenuation Optical fiber: > 10 Tb/s×km, Limited by DISPERSION (?) Implications in high speed signal processing: Assuming that an electronic chip could process at 10 Tb/s, it could not transmit information to another chip 1mm apart (!) Implications in $$ (telecom services): Metallic line (10 Mb/s): 0.2 M$/month Optical Fiber (10 Gb/s): 200 M$/month (!!) (at 1¢/min per voice channel used 8 h/day) Substitute cables and electronics by fibers and photonics

12. Optical Fiber & Chip

13. Refraction & Dispersion

14. Refractive index n = refractive index (unitless) c = velocity of light in free space (300.000.000 meters per second) v = velocity of light in a given material

15. Snell’s law n1 = refractive index material 1 n2 = refractive index material 2 1= angle of incidence 2= angle of refraction

16. Snell’s law From dense to less dense What happens if 1 increases ?????

17. Snell’s law: Critical angle Where does the power go?

18. Snell’s law: Critical angle

19. Acceptance angle

20. Acceptance angle Acceptance angle (θin(max) ) is the maximum angle in which external light rays may strike the air/fiber interface and still propagate to the fiber with a response which is smaller than 10dB from the peak value. Mathematically , it is related by indexes n1 and n2

21. Acceptance cone

22. Numerical Aperture • Numerical aperture (NA) • describe the light-gathering/light collecting ability • For a step-index fiber • NA = sin θin = ( n12 - n22 )1/2 • For a graded index • NA = sin θc • See numerical example 11-3

23. Optical Fiber Configurations:Modes of propagation Multimode Number of modes

24. Core index profiles Single-mode step index Multimode step index Multimode graded index

25. Cladding Air cladding • Glass cladding • stronger • smaller acceptance angle

26. Multimode Step-index Optical Fiber multiple rays follow different paths

27. Multimode Graded-Index Optical Fiber multiple rays follow different paths What is the modal dispersion of a graded index fiber?

28. Optical Fiber Comparison Single-mode step-index fiber + Minimum dispersion (no modal dispersion) - Small aperture - Expensive ???? Multimode step-index fiber + Relatively inexpensive + Large aperture - Multimode dispersion Multimode graded-index fiber - Expensive + Large aperture + Small multimode dispersion

29. Decibel units System Pin Pout System Transmission: T = Pout/Pin - 10 dB means Pout = Pin/10 - 3 dB means Pout = Pin/2 - 40 dB means Pout = 10-4 Pin TdB = 10 log(Pout/Pin) dBm: Power in dB relative to 1 mW - 10 dBm means P = 0.1 mW 3 dBm means P = 2 mW 40 dBm means P = 10 W PdBm = 10 log(P/1 mW) TdB = Pout - Pin (Pin and Pout in dBm)

30. Losses in Optical Fiber Cables

31. Losses in Optical Fiber Cables 1st 0.82 m 2nd 1.3 m 3rd 1.55 m BW = 13 THz

32. Rayleigh scattering loss

33. Multimode step-index fiber modal dispersion Bandwidth length product BLP = 20 Mb/s x km

34. Single-mode step-index fiber Only one angle (one mode) !!!!! No modal dispersion (but chromatic dispersion) 13 THz bandwidth in 1.55 µm window (1.5 – 1.6)

35. Multimode graded-index fiber More angles (many modes) !!!!! Small modal dispersion 2 Gb/s x km

36. Pulse spreading UPRZ signal UPNRZ signal

37. Losses and distortion in Optical Fiber Cables Absorption loss - Ultraviolet absorption - Infrared absorption - ION resonance absorption (OH-) Material, or Rayleigh, Scattering Losses - Submicroscopic irregularities Chromatic, or Wavelength, Dispersion - Not loss Radiation Losses - Bending loss - Microbending Modal Dispersion - Only in multimode fibers

38. Pulse spreading Original bits Difficult to distinguish between bits

39. Coupling Losses • Loss splice (or fuse) < 0.02 dB • Loss connector ± 0.5 dB

40. Review • Multimode fibers • Used in local area networks (LANs) • Capacity limited by intermodal dispersion: • 20 Mb/s x km (step index) • 2 Gb/s x km (graded index) • Single Mode Fibers • Used for long distance • 13 THz bandwidth in 1.5 – 1.6 µm window • Capacity limited by chromatic dispersion • Dispersion (D) can be positive, negative or zero • D = 0 @ 1.3 µm in standard silica fibers • Waveguide dispersion can be adjusted by index profile • Dispersion shifted fiber (DSF): D = 0 @ 1.55 µm • DSF: combines D = 0 and minimum loss

41. Optical fiber Transmission Media • Light Sources • Light Emitting Diodes • Injection Laser Diodes • Detectors • PIN photodiode • Avalanche photodiode • Photodiode Characteristics • Power budget

42. Light sources • 1: Light -emitting diodes (LED) • Burrrus etched-well surface-emitting LED > 100Mbs • Edge-emitting LED • 2. Injection Laser Diode(ILD) • below threshold current : similar to LED • above threshold current : ILD oscillates; lasing occurs

43. ILD’s advantages & disadvantages • Advantages of ILDS • easy coupling light into fiber due to directive radiation pattern • radiated power greater than LED • higher data bit rate than LED • monochromatic light is generated • Disadvantages • more expensive than LED • shorter lifetime and more temperature dependent

44. Light detectors • PIN Diodes • the most common device used in optic fiber communication • photoelectric effect • Avalanche Photodiodes • more sensitive than PIN diodes • long transit time, additional internally generated noise • Characteristics of Light Detectors • Responsivity (conversion efficiency), Dark current, Transit time, Spectral response.

45. Laser Types • 1. Gas lasers : • mixture of helium and neon in a glass tube • continuous light-wave output is monochromatic • 2. Liquid lasers : • organic dyes enclosed in a glass tube. • 3. Solid lasers : • solid, cylindrical crystal for active medium • continuous wave output • 4. Semiconductor lasers : ILD, light is easily modulated

46. Laser Type Characteristics • Employing active material to convert energy to light • A pumping source providing power • Optics directing the beam through the active material • Optics directing the beam into narrow powerful cone of divergence • Feedback mechanism providing continuous operation • Output coupler transmitting power out of laser