lect19-20

lect19-20 - LASER DIODES Injection laser diodes (ILD):...

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Injection laser diodes (ILD): Stimulated emission + Optical feedback LED: Spontaneous emission LASER DIODES
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LASER DIODE: GAIN COEFFICIENT Peak gain coefficient: Attenuation (without injection) Min injection: transparency
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Peak gain coefficient: LASER DIODE: GAIN COEFFICIENT Transparent current density: Active layer thickness
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LASER DIODE: FEEDBACK d R SC-air interface: Reflection loss coefficient: Total loss coefficient: Confinement factor Other losses
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Lasing: LASER DIODE: LASER THRESHOLD Threshold gain: Active layer thickness Threshold current density:
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Active layer of ILD’s and LED’s Narrow bandgap (small Eg) Large refractive index Cladding layers Wide bandgap (large Eg) Small refractive index Substrate region SC starting material Mechanical support Electrical contact and heat conduction Contact layer Electrical contact SC STRUCTURES TWO JUNCTIONS
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Amplifying medium + optical feedback + pump Pump: optical pumping electron beam pumping current injection Gain is high modest amount of feedback is needed Feedback – cavity: SC (n=3)/Air (n=1) high enough power reflection coefficient (0.3) Optical cavity – two cleaved sufraces SC INJECTION LASER DIODES
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SC INJECTION LASER DIODES Electrons: injected from n to p Electrons and holes recombine
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Semiconductor injection laser diodes Are ILDs three- or four- level lasers? Take a PpN diode laser as an example. E FN E 3 E cp E 2 E vp E a E FP E 1 Answer: ILDs are four-level lasers E 3 E 2 E a E 1 SC INJECTION LASER DIODES Filled valence band P Cond band N Cond band p Val band p
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hf E cp -E vp ~ E g of narrow bandgap SC Forward biased diodes PI characteristic Threshold current, I th I < I th Broad spectral line width
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lect19-20 - LASER DIODES Injection laser diodes (ILD):...

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