1 Sources and detectors of light 1)Revision: semiconductors 2)Light emitting diodes (LED) 3)Lasers 4)Photodiodes for integrated optics and optical communications

2 1) Revision: Energy band diagrams for metal… Metals characteristically have partially filled energy bands.

3 … and semiconductor

4 Generation of electron-hole pair

5 Semiconductor statistics DOSFD area = area = p

6 n-type semiconductor

7 p-type semiconductor

8 Intrinsic, n-type, and p-type semiconductor

9 Degenerate semiconductor heavily doped n-type semiconductor donors form a band that overlaps the CB heavily doped p-type semiconductor

10 E-k diagram band diagramE-k diagram

11 Direct and indirect bandgap

12 pn junction

13 pn junction open circuitforward bias V 0

14 2) Light emitting diodes (LED)

15 Surface emitting LED

16 How to get the light out?

17 External and internal quantum efficiency Task: Estimate the external quantum efficiency of GaAs LED.

18 Double heterostructure LED two junctions between materials with different bangaps

19 LED materials direct bangap

20 LED materials

21 LED materials

22 LED characteristics

23 LED characteristics

24 surface emitting LED (SLED) Edge-emitting LED (ELED) Double- heterosturuture light - LEDs are preferred for short haul applications - more economical - wider output spectrum, i.e. not suitable for wide bandwidth systems LEDs for optical fiber communications

25 Coupling the radiation from a SLED into an fiber

26 DH ELED

27 Coupling the radiation from a ELED into an fiber

28 3) Lasers - population inversion - optical feedback (optical cavity)

29 Stimulated emission absorptionspontaneous emission Stimulated emission The emitted photon has the same energy, polarization, direction and phase as the incoming photon.

30 Population inversion 1. (optical) pumping 2. rapid decay to the long-lived (metastable) state 3. population inversion4. Stimulated emission

31 Stimulated emission: Optical amplifiers E.g.: Erbium doped fiber amplifier (EDFA)

32 Erbium doped fiber amplifier (EDFA)

33 Semiconductor optical amplifier (SOA) Fabry-Perot Traveling wave

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35 population inversion Laser diode (LD)

36 DOS for electrons a holes in SCL under forward bias

37 cleaved surface = mirror Optical feedback

38 Laser oscillation conditions

39 Laser oscillation conditions FP cavity must be the same in the steady state phase condition amplitude condition = = threshold condition

40 stimulated emission spontaneous emission Optical power diode current

41 Principle of a double heterostructure LD

42 Example: DH stripe contact LD

43 Example: A buried DH LD

44 LD characteristics

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48 LDs for optical fiber communications

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50 DBR Laser

51 Distributed feedback (DFB) laser

52 Distributed feedback (DFB) laser

53 Distributed feedback (DFB) laser

54 Cleaved-coupled-cavity (C 3 ) laser

55 Tunable lasers

56 Quantum well devices

57 Single quantum well (SQW)

58 Multiple quantum well (MQW)

59 Vertical cavity surface emitting laser (VCSEL)

60 Vertical cavity surface emitting laser (VCSEL)

61 Vertical cavity surface emitting laser (VCSEL)

62 4) Photodiodes

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67 Absorption in direct and indirect semiconductor

68 Responsivity

69 p-i-n photodiode

70 Avalanche photodiode electrode

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