1. 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. 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. After one round trip After N round trips

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

12. Fabry-Perot model of laser

14. Relate Δ  to spectral characteristic

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

16. Fabry-Perot model of laser

18. 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 μ m, with N = 4 and L = 300 μ m?

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. N 0 = 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 μ m and facet reflectances of R 1 =R 2 = 1%. In both cases assume that the gain is a constant within the cavity.