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ENE 623 Optical Networks

ENE 623 Optical Networks. Lecture 6. Polarization splitter based Filters. Acoustooptic Tunable Filters. Electrooptic Tunable Filters. Tunable Add-Drop Filters. Phase matched AOTF : changing f for tuning EOTF: changing V for tuning. Laser Diodes. Laser Diodes.

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ENE 623 Optical Networks

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  1. ENE 623 Optical Networks Lecture 6

  2. Polarization splitter based Filters

  3. Acoustooptic Tunable Filters

  4. Electrooptic Tunable Filters

  5. Tunable Add-Drop Filters • Phase matched • AOTF : changing f for tuning • EOTF: changing V for tuning.

  6. Laser Diodes

  7. Laser Diodes • Direct modulation: easy to implement but causing spectral broadening which can reduce bandwidth for long distance transmission. • External modulation: Overcoming excess spectral broadening, at cost of increased transmitter cost of complexity.

  8. Laser Diodes • Two key features of laser operation • Gain: stimulated emission of light. • Oscillation: resonant cavity.

  9. Fabry-Perot model of laser

  10. Fabry-Perot model of laser • After one round trip • After N round trips

  11. Fabry-Perot model of laser • N  : steady state

  12. Fabry-Perot model of laser

  13. Fabry-Perot model of laser

  14. Fabry-Perot model of laser • Relate Δto spectral characteristic

  15. Fabry-Perot model of laser • Recall N = group refractive index

  16. Fabry-Perot model of laser

  17. Fabry-Perot model of laser

  18. Fabry-Perot model of laser • How does total output power in a mode depend on ?

  19. Fabry-Perot model of laser • Output power in mode varies as 1/.

  20. Fabry-Perot model of laser

  21. Example 1 • What is the longitudinal mode spacing in Angstroms and Hz, for an InGaAsP Fabry-Perot laser emitting at a wavelength of 1.53 μm, with N = 4 and L = 300 μm?

  22. Example 2 • From previous example, what is the total spectral width of the laser emission, in Angstroms and Hz, if the laser emission contains seven longitudinal modes?

  23. Laser Rate Equations • N = number of carriers (e-h pairs) in active region. • S = number of photons in cavity in lasing mode. • J = current for pumping diode. • e = electronic charge = 1.6 x 10-19 C. • sp = spontaneous lifetime of carriers. • N0 = number of carriers for transparency •  = fraction of spontaneous emission coupled into lasing mode. • ph = photon lifetime in cavity. • g = gain coefficient.

  24. Laser Rate Equations • Steady state: • For small current (S  0)

  25. Laser Rate Equations • Lasing threshold:

  26. Laser Rate Equations • Above threshold:

  27. Laser Rate Equations

  28. Example 3 • Parameters for a semiconductor laser are: • What is the photon lifetime? • What is the number of carriers at lasing threshold?

  29. Laser Rate Equations • How long does a photon stay in cavity?

  30. Laser Rate Equations

  31. Example 4 • What is the power gain coefficient in cm-1 in a semiconductor FP laser operating above threshold with a cavity length of 250 μm and facet reflectances of R1=R2 = 1%. In both cases assume that the gain is a constant within the cavity.

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