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Ch 6: Optical Sources

Ch 6: Optical Sources. Variety of sources. LS considerations:. Wavelength. Dl. Output power. Modulation. Coupling. Cost/performance. LED. Operation. Spontaneous emission. LED. Recent advances:. l =hc/ e PH. Porous silicon. Semiconductor polymers. LED: Light Emission.

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Ch 6: Optical Sources

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  1. Ch 6: Optical Sources • Variety of sources • LS considerations: • Wavelength • Dl • Output power • Modulation • Coupling • Cost/performance • LED • Operation • Spontaneous emission

  2. LED • Recent advances: • l=hc/ePH • Porous silicon • Semiconductor polymers

  3. LED: Light Emission

  4. LED: Coupling

  5. LED: Characteristics • Low cost • Low power: 100 uW (recently: 75 mW) • Wide spectrum: 50-100 nm • Incoherent light: neither directional nor coherent • Digital modulation: up to 300 Mbps • Analog modulation: simply

  6. LD • LASER: Light Amplification by the Stimulated Emission of Radiation • For stimulated emission to happen: • Material capable of st. emission (have a metastable high energy state • Laser active media • Laser action: Absorption, spont. Emission, stimulated emission • Population inversion: pumping

  7. LD • Lasing requires: • Active medium • Supplying energy • Confinement

  8. Fabry-Perot Laser • LED with couple of mirrors • Fabry-Perot resonator • Operation • L=lx/2n

  9. FPL Modes Spectral Characteristics

  10. FPL Gain threshold

  11. DFB Laser • Disadvantages of FPL: • Spectral width of 5-8 nm • Mode hopping • WDM • DFB: Operation

  12. DFB Laser • DFB: Phase shifted grating • DFB: Spectral characteristics

  13. DFB Characteristics • Very narrow linewidths • Low chirp • Low relative intensity noise (RIN) • Sensitive to reflections • Temperature sensitive • Output power fluctuations • High cost • Speed?

  14. DFB • Integrated Electro-Absorption Modulator • Q-Switching • Stability?

  15. Distributed Bragg Reflector (DBR) • More stable • Less efficient • Quantum Well Lasers (QW) • When light is confined to an area ~ l, it behaves like a particle • Fundamental differences of SQW over non QW • Reduction in lasing threshold • Low output power • No lateral modes • Narrower linewidth

  16. MQW/Tunable DBR • MQW vs SQW • Strained layer QW • High power • Construction • Low threshold • Advantages • Breadth of materials • Broader linewidth • Tunability • Tunable DBR Lasers • 3 section tunable DBR • Complex electronics • 10 nm range

  17. Tunable DBR • Sampled grating TDBR • 100 nm range • External Cavity TDBR • Wide range • Slow • Expensive

  18. Frequency stabilized DBR • External fiber cavity DBR • Low cost • Accurate wavelength control • Narrow linewidth • Temepreture control is shifted out • Difficult positioning • Coherence collapse operation

  19. Vertical Cavity Surface Emitting Laser • Low power • Scarce of wavelengths • MM and SM lasers • Easy coupling • Low threshold current • Simple electronics • High modulation BWs • Very stable

  20. In Fiber Lasers • Characteristics • High output power • Low noise • Tunability • Very narrow linewidths • Good soliton generation • External modulation is required • Preselected wavelength • Mode hopping • Upconversion (double pumping)

  21. Comparison

  22. Optical Amplifiers: EDFA Materials: Praseodymium Neodymium L Band: CoDopants Longer fiber 2nd generation: Gain control Gain equalization Wavelengths: 1480 980

  23. EDFA: Gain

  24. EDFA

  25. Raman Amplifiers • Multistage

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