Optical SourcesRecombination of an electron and hole can emit a photon
Types of photon emission
Dominant emission for light emitting diodes (LED)
Stimulated emission
Emitted photon has similar wavelength, direction, and phase
Dominant emission for laser diodes
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Electrically create electron-hole pairs
Optically create electron-hole pairs
Spontaneous emission
Simulated emission
Silicon
Germanium
Depends on the material
Electrons in the CB combine with holes in the VB
Photons have no momentum
CB minimum needs to be directly over the VB maximum
Direct bandgap transition required
608.pdf
Semiconductor compounds have different
Combine semiconductor compounds
Adjust the bandgap
Usually GaAs or InP
Material determines the bandgap
GaAs and AlAs have the same lattice constants
These compounds are used to grow a ternary compound that is lattice matched to a GaAs substrate (Al1-xGaxAs)
0.87 < l < 0.63 (mm)
Quaternary compound GaxIn1-xAsyP1-y is lattice matched to InP if y=2.2x
1.0 < l < 1.65 (mm)
Optical telecommunication laser compounds
Light emitting diode (LED)
Dome used to extract more of the light
Critical angle is between semiconductor and plastic
Angle between plastic and air is near normal
Normal reflection is reduced
3423.pdf
Requires high optical power density in the gain region
High photon flux attained by creating an optical cavity
Optical Feedback: Part of the optical power is reflected back into the cavity
End mirrors
Gain > Loss
Cavity gain
Cavity loss
Material absorption
Threshold condition: when gain exceeds loss
Loss
Amount of optical feedback
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Uses Fresnel reflection
Lasing condition requires the net cavity gain to be one
g: distributed medium gain
616.pdf
Cleaved Cavity Laser
The cavity can be produced by cleaving the end faces of the semiconductor heterojunction
This laser is called a Fabry-Perot laser diode (FP-LD)
Semiconductor-air interface produces a reflection coefficient at normal incidence of
For GaAs this reflection coefficient is
Threshold condition is where the gain equals the internal and external loss
Longer length laser has a lower gain threshold
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Resulting in modes given by
Where m is an integer and n is the refractive index of the cavity
627.pdf
The optical cavity excites various longitudinal modes
Modes with gain above the cavity loss have the potential to lase
Gain distribution depends on the spontaneous emission band
Wavelength width of the individual longitudinal modes depends on the reflectivity of the end faces
Wavelength separation of the modes Dl depends on the length of the cavity
629.pdf
ga in
p ro
fil e
1 .5 2 1 .5 3 1 .5 4 1 .5 5 1 .5 6 1 .5 7 1 .5 8
wavelength, λ (µm)
The wavelength separation of the modes is
A longer cavity
Decrease the threshold gain
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Cleaved Cavity Laser Example
A laser has a length of L=500mm and has a gain of
Solving this for wavelength gives
(1550-5.65) nm < l < (1550+5.65) nm
The supported modes are calculated based on the constructed interference condition
The minimum and maximum orders are
mmin=2249
mmax=2267
With a wavelength separation of Dl=0.69nm
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Multimode laser have a large wavelength content
A large wavelength content decrease the performance of the optical link
Methods used to produce single longitudinal mode lasers
Cleaved-coupled-cavity (C3) laser
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Cleaved Coupled Cavity (C3) Laser
Longitudinal modes are required to satisfy the phase condition for both cavities
638.pdf
λ
λ
λ
For l=1550 nm, L=220 nm
Distributed feedback (DFB) laser
Distributed Bragg reflector (DBR) laser
Grating replaces mirror at end face
640.pdf
p
DFB Laser
DBR Laser
DFB Laser
DFB: More expensive, smaller linewidth
Optical characteristics
Optical wavelength
Optical linewidth
Optical power
Electrical characteristics
Phasebridge “Wideband Integrated Laser Transmitter Module”
Laser + External Modulator
Threshold current Ith=40mA
Dl=0.008 nm
Source: convert electric current to optical power
Detector: convert optical power to electrical current
Use pin structures similar to lasers
Electrical power is proportional to i2
Electrical power is proportional to optical power squared
Called square law device
pin Photodiode
p-n junction has a space charge region at the interface of the two material types
This region is depleted of most carriers
A photon generates an electron-hole pair in this region that moves rapidly at the drift velocity by the electric field
Intrinsic layer is introduced
3424.pdf
Dark current, Id: current with no incident optical power
3409.pdf
Depends on the energy band gap (similar to lasers)
Absorption (a) requires the photon energy to be smaller than the material band gap
3412.pdf
Ge
Si
Absorption requires
Be absorbed
Photon absorbed before it gets to the depletion region
Photon gets absorbed in the depletion region
Fraction of incident photons that are absorbed
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Rate of incident photons depends on
Incident optical power Pinc
Generated current
Detector responsivity
l in units of mm
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3420.pdf
0.2 0.4 0.6
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Noise Equivalent noise power in or noise equivalent power NEP
Often grouped into minimum detectable power Pmin at a specific data rate
Pmin scales with data rate
Common InGaAs pin photodetector
Common InGaAs APD
Limited to around B=2.5 Gbps
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1
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