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ENE 623 Optical Networks

ENE 623 Optical Networks. Lecture 5. Tunable Filters. Tunable Filters. Δ f defines as the frequency difference between the lowest- and the highest-frequency channels and f as the spacing between channels.

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ENE 623 Optical Networks

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  1. ENE 623 Optical Networks Lecture 5

  2. Tunable Filters

  3. Tunable Filters • Δf defines as the frequency difference between the lowest- and the highest-frequency channels and f as the spacing between channels. • The maximum number of equally spaced channels can be calculated as Nmax = Δf / f . • The access time is the speed which a tunable filter can be reset from one frequency to another. This should be in the unit of microsecond. • The filter’s transfer function T(f) is not generally equal to unity due to its internal losses.

  4. Tunable Filters • Good filters should be independent to light polarizations. Tunable-filters have an advantage over the coherent-detection due to that. • With the help of lithography, the low cost filter can be fabricated, but the fiber loss attachment exists. However, with other methods, high-cost processes are involved and that is the great barrier to develop such a technology.

  5. Tunable Filters • Wavelength selective filters might be categorized into: • 2-port filter • 1  N WDM filter

  6. Tunable Filters • 4-port add-drop filter

  7. Crosstalk

  8. Filters for WDM • Requirements: • Center wavelength near 1.55 μm or 1.3 μm. Some local network might have a center wavelength of 0.8 μm. • Frequency Spacing: about 100 GHz. • Number of channels: having been increased to more than 256. • Tuning speed: less than 1 μs.

  9. Some of tunable filters • Fabry-perot filters • Mach-Zehnder chain • Grating • Acousto-optic tunable filter (AOTF) • Electro-optic tunable filter (EOTF)

  10. Fiber Fabry-Perot Filter

  11. Fiber Fabry-Perot Filter • Consider a single mirror

  12. Fiber Fabry-Perot Filter • After a round trip

  13. Fiber Fabry-Perot Filter • After two round trips • After N round trips

  14. Fiber Fabry-Perot Filter • At steady state (N  ∞)

  15. Fiber Fabry-Perot Filter • At steady state (N  ∞)

  16. Fiber Fabry-Perot Filter • Plot T vs 

  17. Fiber Fabry-Perot Filter

  18. Fiber Fabry-Perot Filter • We can find the bandwidth of the peak by looking at the denominator expression for T.

  19. Fiber Fabry-Perot Filter • Another important parameter to characterize a FP filter is the finesse, F. • This can determine the maximum number of channels in WDM system.

  20. Example • If we have 10 channels with 100 GHz spacing for each channel. What shoud the length of FPI filter be?

  21. Example • Consider a Fabry Perot filter with an air cavity of length L and R = 0.99 for each mirror, with a free spectral range of 3.2 THz. • What is L in μm? • For the value of L determined in (a), what is the wavelength λ0 nearest 1.53 m for which the transmittance is a maximum.

  22. Mach-Zehnder Chain

  23. Mach-Zehnder Chain

  24. Mach-Zehnder Chain

  25. Mach-Zehnder Chain

  26. Example • Consider a 7-stage MZ chain with a FSR of 3.2 THz produced in single mode fiber with n = 1.46 for the fundamental mode and a transmittance maximum for λ1 = 1.53 m. What is the shortest and longest path difference for any interferometer in the chain?

  27. Gratings • Spatial period d diffracted waves interface

  28. Example • Find the allowed mode and angles for each mode for I =30,  = 1.53 μm, and d = 1.61 μm.

  29. Gratings

  30. Gratings • Calculating wavelength dependence of focused spot position.

  31. Example • From previous example, if channel spacing is 100 GHz. What should be a value of h?

  32. Spectral resolution • Spectral resolution is an ability to separate light into wavelength components.

  33. Gratings

  34. Gratings

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