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

LED Recent advances: Recent advances: Porous silicon Porous silicon Semiconductor polymers Semiconductor polymers =hc/  PH =hc/  PH

LED: Light Emission

LED: Coupling

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

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

LD Lasing requires: Lasing requires: Active medium Active medium Supplying energy Supplying energy Confinement Confinement

Fabry-Perot Laser LED with couple of mirrors LED with couple of mirrors Fabry-Perot resonator Fabry-Perot resonator Operation Operation L= x/2n L= x/2n

FPL Modes Spectral Characteristics

FPL Gain threshold

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

DFB Laser DFB: Phase shifted grating DFB: Phase shifted grating DFB: Spectral characteristics DFB: Spectral characteristics

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

DFB Integrated Electro- Absorption Modulator Integrated Electro- Absorption Modulator Q-Switching Q-Switching Stability? Stability?

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

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

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

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

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

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

Comparison

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

EDFA: Gain

EDFA

Raman Amplifiers Multistage Multistage