光通信技术 课件Lecture5

A Review check list
Fiber
• Fiber mode • What is it? • Why it is discrete? • Condition for single propagation mode
• Attenuation • Function of wavelength • Calculation • Causes
I Ne / t
P(mW) [int Ep (eV )]I (mA)
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Radiation wavelength - energy gap
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Some popular III-V compounds
•Diffusion of majority carriers across the depletion region
•Carrier injection
•Injected minority carriers recombine with majority
•Radiative/non-radiative recombination
Lambertian Source radiance distribution
Generalized Coupled Power
Source Fiber Coupling - II
P0=100W ->Pin=7.56 W
•Spectral width
•SLED 170 nm •ELED 65 nm (LD 1nm)
Refractive index changes with temperature
•Wavelength
•Increase with temperature 0.38nm/ºC
1/ 1/ rad 1/ nonrad int / r
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Direct-coupling efficiency
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•When temperature is raised, some electrons are excited cross the gap
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Thermal population
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Direct or indirect bandgap: if min lines up with max
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How to choose k’s: electron confined in a box
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Majority carrier is holes Minority carrier is electrons
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P-N Junction
•N region –+ve charge
•p region –-ve charge
•Electrical neutral •Diffusion potential
=forward-biasing
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Effect of forward bias
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Radiative recombination
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Photon emission unlikely:
10-8 – 10-10 s
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Popular materials
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Light-emitting Diodes
Holes
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Both free electrons and holes conduct currents
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Electrons: particle and wave
➢ Particle nature
➢ Drift current=diffusion current
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E-k diagram: Si & GaAs
Indirect
direct
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Carrier Recombination
Light source
• LED • LD
Coupling optics Electronics
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Classification of materials
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Power v.s. driving current
➢ Power v.s.
photon numbers -> excited (injected) electrons N
P (Nint Ep ) / t
int internal quantum efficiency: electron->photon
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Radiation Patterns
➢ Surface-emitting LED(SLED)
A Lambertian source P=P0cos
➢ Edge-emitting LED (ELED)
A Lambertian source in a plane
Fiber connecterization •Connectors •Connector types / structure •Splices
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Light source and transmitters
➢ A transmitter consists of three parts:
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Two types of packaging
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Coupling, spectral width and efficiencies
•Coupling of SLED into a step-index fiber Pin=P0(NA)2 62.5/125 MM fiber: NA=0.275
• Dispersion • Intramodal dispersion • Intermodal dispersion • Polarization mode dispersion
• Higher order dispersion
Fiber Cables •Construction elements •Optical characteristics
Discrete k values
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More than one state have the same energy -degeneracy or density of state
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Blue LED: a challenge
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Blue LED: a challenge
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White LED: a magic
➢ Reduces electrostatic barrier at the junction, thus diffusion occur again
➢ Splitting Fermi level
Apply a E-field to reduce built-in field: + on p-type, - on n-type
What is a possible measure to make it
happen?
That’s what we
want
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Effect of forward-biasing
➢ Produces a current and interrupts the equilibrium of carrier population
•Increase with driving current 0.69 nm/mA
•Rise time •0.4-10 ns
tr 2.2[ (1.7 104T K C) / I p
•Determined by recombination time •Modulation bandwidth