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Semiconductor Optical Sources

Semiconductor Optical Sources. 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.

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Semiconductor Optical Sources

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  1. Semiconductor Optical Sources

  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. 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. 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. 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 Elements

  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. 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. Direct and Indirect Materials • Only specific materials have a direct bandgap • Material determines the bandgap

  9. 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 (Al1-xGaxAs) • 0.87 < l < 0.63 (mm) • Quaternary compound GaxIn1-xAsyP1-y is lattice matched to InP if y=2.2x • 1.0 < l < 1.65 (mm) • Optical telecommunication laser compounds • In0.72Ga0.28As0.62P0.38 (l=1300nm) • In0.58Ga0.42As0.9P0.1 (l=1550nm)

  10. 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

  11. Light Emitting Diodes (LED) • Spontaneous emission dominates • Random photon emission • Implications of random emission • Broad spectrum (Dl~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

  12. 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

  13. 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

  14. 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 • a: distributed loss • R1 and R2 are the end facet reflectivities

  15. 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

  16. 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

  17. Longitudinal Modes

  18. 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 Dl depends on the length of the cavity

  19. 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

  20. Cleaved Cavity Laser Example • A laser has a length of L=500mm and has a gain of • Solving this for wavelength gives (1550-5.65) nm < l < (1550+5.65) nm • The supported modes are calculated based on the constructed interference condition • The minimum and maximum orders are • mmin=2249 • mmax=2267 • The number of modes is 18 • With a wavelength separation of Dl=0.69nm

  21. 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 (C3) laser • Distributed feedback laser (DFB) laser

  22. Cleaved Coupled Cavity (C3) Laser • Longitudinal modes are required to satisfy the phase condition for both cavities

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

  24. Laser Wavelength Linewidth

  25. 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

  26. Example Laser Specifications • Let look at an example specification sheet • Phasebridge “Wideband Integrated Laser Transmitter Module” • Laser + External Modulator • Specifications • Wavelength: 1548 nm < l < 1562 nm • Average power: 5 < Pt < 9 mW • Threshold current Ith=40mA • TEC cooler • Line width: 10 MHz • We need to convert from Df to Dl • Dl=0.008 nm

  27. Semiconductor Optical Detectors

  28. 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 i2 • Electrical power is proportional to optical power squared • Called square law device • Important characteristics • Modulation bandwidth (response speed) • Optical conversion efficiency • Noise • Area

  29. 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

  30. I-V Characteristic of Reversed Biased pin • Photocurrent increases with incident optical power • Dark current, Id: current with no incident optical power

  31. 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 (a) requires the photon energy to be smaller than the material band gap

  32. 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

  33. Detector Responsivity • Each absorbed photon generates an electron hole pair Iph = (Number of absorbed photons) * (charge of electron) • Rate of incident photons depends on • Incident optical power Pinc • Energy of the photon Ephoton= hf • Generated current • Detector responsivity • Current generated per unit optical power l in units of mm

  34. Responsivity • Depends on quantum efficiency h, and photon energy

  35. Avalanche Photodiode (APD)

  36. Minimum Detectable Power • Important detector Specifications • Responsivity • Noise Equivalent noise power in or noise equivalent power NEP • Often grouped into minimum detectable power Pmin at a specific data rate • Pmin scales with data rate • Common InGaAs pin photodetector • Pmin=-22 dBm @B=2.5 Gbps, BER=10-10 • Common InGaAs APD • Pmin=-32 dBm @B=2.5 Gbps, BER=10-10 • Limited to around B=2.5 Gbps

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