Lecture7 - Laser Diodes; Light Emitting Diodes EE4035...

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1 Laser Diodes; Light Emitting Diodes EE4035 Optical Communications Semester A 2010-11 Lecture 7
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2 Intended Learning Outcomes (ILOs) Describe the transmission characteristics of laser diodes Explain the operation principles of various types of laser diodes. Calculate the basic transmission characteristics of laser diodes. Explain the transient effects and noise effects in laser diodes. Explain the principles of narrow-line lasers. Explain the principles of wavelength tuning. Describe the transmission characteristics of light emitting diodes.
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3 Laser Diodes n L d I p Light Double Heterosructure (DH Laser) Buried Double Heterostructure (BH Laser) p n Light w d L I p n I L Light Surface Emitter p n -y Light I x z Homostructure
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4 Laser Diodes P-I Curve
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5 Three basic conditions: • Active medium • Population inversion Stimulated emission in the optical • Optical feedback spectrum Threshold condition for laser oscillation: α : Loss coefficient in cm 1 : Gain coefficient in cm 1 g Fractional loss = ) 2 exp( 2 1 L r r Fractional gain = ) 2 exp( L g Laser Operation Steady-state condition for laser oscillation is achieved when the gain in the active medium exactly balances the total losses. Threshold gain per unit length: 2 1 1 ln 2 1 r r L g th + = Transmission loss through the mirrors 1 ) 2 exp( ) 2 exp( 2 1 × L r r L g 1 ] ) ( 2 exp[ 2 1 L g r r or L r 1 r 2 Active Medium g ,
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6 Condition for setting up standing waves in the laser cavity n q L 2 λ = nL qc f 2 = q : Integer : Emission wavelength or n : Refractive index c : Speed of light nL c f 2 = δ i.e., nL 2 2 δλ = There may also be transverse modes. Each value of q defines an axial or longitudinal mode . Laser Modes Finite linewidth due to the change in refractive index produced by a change of carrier density in laser diode. The refractive index of a semiconductor is inversely proportional to the carrier density. Frequency spacing or wavelength spacing between two modes:
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7 Laser Modes Intensity Frequency Gain Curve Intensity Frequency c 2 nL Finite linewidth due to the change in refractive index produced by a change of carrier density in laser diode Review : Fabry-Perot filter
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8 Homostructure Light Emitter Depletion region Potential barrier E f Energy Fermi level A A A AA A DD D D D D D D Depletion region p-type n-type Free carriers: holes Free carriers: electrons A A A D D D D D D D D A A A
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9 Homostructure Light Emitter Potential barrier E f Fermi level hf hf Population inversion Emitted wavelength: h : Planck's constant E f : Bandgap energy λ = hc / E f Energy vs position in the case of forward biasing Forward biasing Injection current (pump)
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10 Major considerations: Direct bandgap semiconductors (efficiency) Bandgap energy (wavelength) Lattice matching (fabrication) Examples: GaAs/Al x Ga 1-x As on GaAs for λ ~ 0.8 0.9 μ m In y Ga 1-y As/In x Ga 1-x P on GaAs for ~ 0.85 1.1 μ m Ga 1-y Al y As 1-x Sb x /GaSb on GaSb for ~ 1.0 1.7 μ m In 1-x Ga x As y P 1-y- /InP on InP for ~ 0.92 – 1.7 μ m Semiconductor Material for Laser Diodes In
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This note was uploaded on 04/17/2011 for the course EE 4035 taught by Professor Prof.chiang during the Spring '10 term at City University of Hong Kong.

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Lecture7 - Laser Diodes; Light Emitting Diodes EE4035...

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