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1 Semiconductor Optical Sources. 2 Source Characteristics Important Parameters –Electrical-optical conversion efficiency –Optical power –Wavelength –Wavelength.

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Presentation on theme: "1 Semiconductor Optical Sources. 2 Source Characteristics Important Parameters –Electrical-optical conversion efficiency –Optical power –Wavelength –Wavelength."— Presentation transcript:

1 1 Semiconductor Optical Sources

2 2 Source Characteristics Important Parameters –Electrical-optical conversion efficiency –Optical power –Wavelength –Wavelength distribution (called linewidth) –Cost Semiconductor lasers –Compact –Good electrical-optical conversion efficiency –Low voltages –Los cost

3 3 Semiconductor Optoelectronics Two energy bands –Conduction band (CB) –Valence band (VB) Fundamental processes –Absorbed photon creates an electron-hole pair –Recombination of an electron and hole can emit a photon Types of photon emission –Spontaneous emission Random recombination of an electron-hole pair Dominant emission for light emitting diodes (LED) –Stimulated emission A photon excites another electron and hole to recombine Emitted photon has similar wavelength, direction, and phase Dominant emission for laser diodes

4 4 Basic Light Emission Processes Pumping (creating more electron-hole pairs) –Electrically create electron-hole pairs –Optically create electron-hole pairs Emission (recombination of electron-hole pairs) –Spontaneous emission –Simulated emission

5 5 Semiconductor Material Semiconductor crystal is required Type IV elements on Periodic Table –Silicon –Germanium Combination of III-V materials –GaAs –InP –AlAs –GaP –InAs … –Periodic Table of ElementsPeriodic Table of Elements

6 6 Direct and Indirect Materials Relationship between energy and momentum for electrons and holes –Depends on the material Electrons in the CB combine with holes in the VB Photons have no momentum –Photon emission requires no momentum change –CB minimum needs to be directly over the VB maximum –Direct bandgap transition required Only specific materials have a direct bandgap

7 7 Light Emission The emission wavelength depends on the energy band gap Semiconductor compounds have different –Energy band gaps –Atomic spacing (called lattice constants) Combine semiconductor compounds –Adjust the bandgap –Lattice constants (atomic spacing) must be matched –Compound must be matched to a substrate Usually GaAs or InP

8 8 Direct and Indirect Materials Only specific materials have a direct bandgap Material determines the bandgap MaterialElement GroupBandgap Energy E g (eV) Bandgap wavelength g (  m) Type GeIV I SiIV I AlPIII-V I AlAsIII-V I AlSbIII-V I GaPIII-V I GaAsIII-V D GaSbIII-V D InPIII-V D InAsIII-V D AnSbIII-V D

9 9

10 10 Common Semiconductor Compounds GaAs and AlAs have the same lattice constants –These compounds are used to grow a ternary compound that is lattice matched to a GaAs substrate (Al 1-x Ga x As) –0.87 < < 0.63 (  m) Quaternary compound Ga x In 1-x As y P 1-y is lattice matched to InP if y=2.2x –1.0 < < 1.65 (  m) Optical telecommunication laser compounds –In 0.72 Ga 0.28 As 0.62 P 0.38 ( =1300nm) –In 0.58 Ga 0.42 As 0.9 P 0.1 ( =1550nm)

11 11 Optical Sources Two main types of optical sources –Light emitting diode (LED) Large wavelength content Incoherent Limited directionality –Laser diode (LD) Small wavelength content Highly coherent Directional

12 12 Light Emitting Diodes (LED) Spontaneous emission dominates –Random photon emission Implications of random emission –Broad spectrum (  ~30nm) –Broad far field emission pattern Dome used to extract more of the light –Critical angle is between semiconductor and plastic –Angle between plastic and air is near normal –Normal reflection is reduced –Dome makes LED more directional

13 13 Laser Diode Stimulated emission dominates –Narrower spectrum –More directional Requires high optical power density in the gain region –High photon flux attained by creating an optical cavity –Optical Feedback: Part of the optical power is reflected back into the cavity –End mirrors Lasing requires net positive gain –Gain > Loss –Cavity gain Depends on external pumping Applying current to a semiconductor pn junction –Cavity loss Material absorption Scatter End face reflectivity

14 14 Lasing Gain > Loss Gain –Gain increases with supplied current –Threshold condition: when gain exceeds loss Loss –Light that leaves the cavity Amount of optical feedback –Scattering loss –Confinement loss Amount of power actually guided in the gain region

15 15 Optical Feedback Easiest method: cleaved end faces –End faces must be parallel –Uses Fresnel reflection –For GaAs (n=3.6) R=0.32 Lasing condition requires the net cavity gain to be one – g: distributed medium gain –  : distributed loss – R 1 and R 2 are the end facet reflectivities

16 16 Cleaved Cavity Laser The cavity can be produced by cleaving the end faces of the semiconductor heterojunction This laser is called a Fabry-Perot laser diode (FP-LD) Semiconductor-air interface produces a reflection coefficient at normal incidence of For GaAs this reflection coefficient is Threshold condition is where the gain equals the internal and external loss Longer length laser has a lower gain threshold

17 17 Phase Condition The waves must add in phase as given by Resulting in modes given by Where m is an integer and n is the refractive index of the cavity

18 18 Longitudinal Modes

19 19 Longitudinal Modes The optical cavity excites various longitudinal modes Modes with gain above the cavity loss have the potential to lase Gain distribution depends on the spontaneous emission band Wavelength width of the individual longitudinal modes depends on the reflectivity of the end faces Wavelength separation of the modes  depends on the length of the cavity

20 20 Mode Separation Wavelength of the various modes The wavelength separation of the modes is A longer cavity –Increases the number of modes –Decrease the threshold gain There is a trade-off with the length of the laser cavity

21 21 Cleaved Cavity Laser Example A laser has a length of L=500  m and has a gain of –Solving this for wavelength gives ( ) nm < < ( ) nm The supported modes are calculated based on the constructed interference condition The minimum and maximum orders are –m min =2249 –m max =2267 The number of modes is 18 With a wavelength separation of  =0.69nm

22 22 Single Longitudinal Mode Lasers Multimode laser have a large wavelength content A large wavelength content decrease the performance of the optical link Methods used to produce single longitudinal mode lasers –Cleaved-coupled-cavity (C 3 ) laser –Distributed feedback laser (DFB) laser

23 23 Cleaved Coupled Cavity (C 3 ) Laser Longitudinal modes are required to satisfy the phase condition for both cavities

24 24 Periodic Reflector Lasers Periodic structure (grating) couples between forward and backward propagating waves –For =1550 nm,  =220 nm Distributed feedback (DFB) laser Grating distributed over entire active region Distributed Bragg reflector (DBR) laser Grating replaces mirror at end face

25 25 Laser Wavelength Linewidth

26 26 Summary of Source Characteristics Laser type –FP laser: Less expensive, larger linewidth –DFB: More expensive, smaller linewidth Optical characteristics –Optical wavelength –Optical linewidth –Optical power Electrical characteristics –Electrical power consumption –Required voltage –Required current

27 27 Example Laser Specifications Let look at an example specification sheet Phasebridge “Wideband Integrated Laser Transmitter Module” –Laser + External Modulator Specifications –Wavelength: 1548 nm < < 1562 nm –Average power: 5 < P t < 9 mW –Threshold current I th =40mA –TEC cooler –Line width: 10 MHz We need to convert from  f to   =0.008 nm

28 28 Semiconductor Optical Detectors

29 29 Semiconductor Optical Detectors Inverse device with semiconductor lasers –Source: convert electric current to optical power –Detector: convert optical power to electrical current Use pin structures similar to lasers Electrical power is proportional to i 2 –Electrical power is proportional to optical power squared –Called square law device Important characteristics –Modulation bandwidth (response speed) –Optical conversion efficiency –Noise –Area

30 30 pin Photodiode p-n junction has a space charge region at the interface of the two material types This region is depleted of most carriers A photon generates an electron- hole pair in this region that moves rapidly at the drift velocity by the electric field Intrinsic layer is introduced –Increase the space charge region

31 31 I-V Characteristic of Reversed Biased pin Photocurrent increases with incident optical power Dark current, I d : current with no incident optical power

32 32 Light Absorption Dominant interaction –Photon absorbed –Electron is excited to CB –Hole left in the VB Depends on the energy band gap (similar to lasers) Absorption (  requires the photon energy to be smaller than the material band gap

33 33 Quantum Efficiency Probability that photon generates an electron-hole pair Absorption requires –Photon gets into the depletion region –Be absorbed Reflection off of the surface Photon absorbed before it gets to the depletion region Photon gets absorbed in the depletion region Fraction of incident photons that are absorbed

34 34 Detector Responsivity Each absorbed photon generates an electron hole pair I ph = (Number of absorbed photons) * (charge of electron) Rate of incident photons depends on –Incident optical power P inc –Energy of the photon E photon = hf Generated current Detector responsivity –Current generated per unit optical power  in units of  m

35 35 Responsivity Depends on quantum efficiency , and photon energy

36 36 Avalanche Photodiode (APD)

37 37 Minimum Detectable Power Important detector Specifications –Responsivity –Noise Equivalent noise power i n or noise equivalent power NEP –Often grouped into minimum detectable power P min at a specific data rate P min scales with data rate Common InGaAs pin photodetector –P min =-22 Gbps, BER= Common InGaAs APD –P min =-32 Gbps, BER= –Limited to around B=2.5 Gbps


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