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Semiconductor Optical Sources
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Source Characteristics
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Important Parameters
– Electrical-optical conversion efficiency
– Optical power
– Wavelength
– Wavelength distribution (called linewidth)
– Cost
Semiconductor lasers
– Compact
– Good electrical-optical conversion efficiency
– Low voltages
– Los cost
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Semiconductor Optoelectronics
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Two energy bands
– Conduction band (CB)
– Valence band (VB)
Fundamental processes
– Absorbed photon creates an electron-hole pair
– Recombination of an electron and hole can emit a photon
Types of photon emission
– Spontaneous emission
• Random recombination of an electron-hole pair
• Dominant emission for light emitting diodes (LED)
– Stimulated emission
• A photon excites another electron and hole to recombine
• Emitted photon has similar wavelength, direction, and phase
• Dominant emission for laser diodes
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Basic Light Emission Processes
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Pumping (creating more electron-hole pairs)
– Electrically create electron-hole pairs
– Optically create electron-hole pairs
Emission (recombination of electron-hole pairs)
– Spontaneous emission
– Simulated emission
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Semiconductor Material
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Semiconductor crystal is required
Type IV elements on Periodic Table
– Silicon
– Germanium
Combination of III-V materials
– GaAs
– InP
– AlAs
– GaP
– InAs
…
– Periodic Table of Elements
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Direct and Indirect Materials
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Relationship between energy and momentum for electrons and holes
– Depends on the material
Electrons in the CB combine with holes in the VB
Photons have no momentum
– Photon emission requires no momentum change
– CB minimum needs to be directly over the VB maximum
– Direct bandgap transition required
Only specific materials have a direct bandgap
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Light Emission
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The emission wavelength depends on
the energy band gap
E g  E2  E1

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•
hc
1.24
 m

Eg Eg eV 
Semiconductor compounds have
different
– Energy band gaps
– Atomic spacing (called lattice
constants)
Combine semiconductor compounds
– Adjust the bandgap
– Lattice constants (atomic spacing)
must be matched
– Compound must be matched to a
substrate
• Usually GaAs or InP
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Direct and Indirect Materials
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Only specific materials have a direct bandgap
Material determines the bandgap
Material
Element Group
Bandgap Energy
Eg (eV)
Bandgap wavelength
g (m)
Type
Ge
IV
0.66
1.88
I
Si
IV
1.11
1.15
I
AlP
III-V
2.45
0.52
I
AlAs
III-V
2.16
0.57
I
AlSb
III-V
1.58
0.75
I
GaP
III-V
2.26
0.55
I
GaAs
III-V
1.42
0.87
D
GaSb
III-V
0.73
1.70
D
InP
III-V
1.35
0.92
D
InAs
III-V
0.36
3.5
D
AnSb
III-V
0.17
7.3
D
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Common Semiconductor Compounds
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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 <  < 0.63 (m)
Quaternary compound GaxIn1-xAsyP1-y is lattice matched to InP if y=2.2x
– 1.0 <  < 1.65 (m)
Optical telecommunication laser compounds
– In0.72Ga0.28As0.62P0.38 (=1300nm)
– In0.58Ga0.42As0.9P0.1 (=1550nm)
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Optical Sources
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Two main types of optical sources
– Light emitting diode (LED)
• Large wavelength content
• Incoherent
• Limited directionality
– Laser diode (LD)
• Small wavelength content
• Highly coherent
• Directional
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Light Emitting Diodes (LED)
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Spontaneous emission dominates
– Random photon emission
Implications of random emission
– Broad spectrum (D~30nm)
– Broad far field emission pattern
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
– Dome makes LED more directional
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Laser Diode
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Stimulated emission dominates
– Narrower spectrum
– More directional
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
Lasing requires net positive gain
– Gain > Loss
– Cavity gain
• Depends on external pumping
• Applying current to a semiconductor pn junction
– Cavity loss
• Material absorption
• Scatter
• End face reflectivity
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Lasing
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Gain > Loss
Gain
– Gain increases with supplied current
– Threshold condition: when gain exceeds loss
Loss
– Light that leaves the cavity
• Amount of optical feedback
– Scattering loss
– Confinement loss
• Amount of power actually guided in the gain region
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Optical Feedback
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Easiest method: cleaved end faces
– End faces must be parallel
– Uses Fresnel reflection
 n 1 
R

n

1


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2
– For GaAs (n=3.6) R=0.32
Lasing condition requires the net cavity gain to be one
R1 R2 expg  a  L  1
– g: distributed medium gain
– a: distributed loss
– R1 and R2 are the end facet reflectivities
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Cleaved Cavity Laser
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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
 n2  n1 

R  
 n2  n1 
2
For GaAs this reflection coefficient is
 3.6  1 
R
  0.32
 3.6  1 
2
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Threshold condition is where the gain equals the internal and external loss
g a 
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1
ln r1 r2 
L
Longer length laser has a lower gain threshold
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Phase Condition
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The waves must add in phase as given by
2 L  z  2 m
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Resulting in modes given by

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2Ln
m
Where m is an integer and n is the refractive index of the cavity
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Longitudinal Modes
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Longitudinal Modes
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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 D depends on the length of the cavity
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Mode Separation
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Wavelength of the various modes

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m
2Ln
The wavelength separation of the modes is
1 
1
D  m  m 1  2 L n  

m
m

1


2Ln
2
D  2 
m
2Ln
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A longer cavity
– Increases the number of modes
– Decrease the threshold gain
There is a trade-off with the length of the laser cavity
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Cleaved Cavity Laser Example
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A laser has a length of L=500m and has a gain of
    1550 2 
g  1090  1500 exp  
 
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 
 
– Solving this for wavelength gives
(1550-5.65) nm <  < (1550+5.65) nm
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The supported modes are calculated based on the constructed interference
condition
2Ln

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m
The minimum and maximum orders are
– mmin=2249
– mmax=2267
The number of modes is 18
With a wavelength separation of D=0.69nm
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Single Longitudinal Mode Lasers
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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
– Distributed feedback laser (DFB) laser
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Cleaved Coupled Cavity (C3) Laser
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Longitudinal modes are required to satisfy the phase condition for both
cavities

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2Ln 2Dn

m1
m2
Periodic Reflector Lasers
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Periodic structure (grating) couples between forward and backward
propagating waves
L

2n
– For =1550 nm, L=220 nm
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Distributed feedback (DFB) laser
• Grating distributed over entire
active region
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Distributed Bragg reflector (DBR)
laser
• Grating replaces mirror at end
face
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Laser Wavelength Linewidth
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Summary of Source Characteristics
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Laser type
– FP laser: Less expensive, larger linewidth
– DFB: More expensive, smaller linewidth
Optical characteristics
– Optical wavelength
– Optical linewidth
– Optical power
Electrical characteristics
– Electrical power consumption
– Required voltage
– Required current
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Example Laser Specifications
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Let look at an example specification sheet
Phasebridge “Wideband Integrated Laser
Transmitter Module”
– Laser + External Modulator
Specifications
– Wavelength: 1548 nm <  < 1562 nm
– Average power: 5 < Pt < 9 mW
– Threshold current Ith=40mA
– TEC cooler
– Line width: 10 MHz
• We need to convert from Df to D

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c
f
D 
D=0.008 nm
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
f
Df
Semiconductor Optical Detectors
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Semiconductor Optical Detectors
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Inverse device with semiconductor lasers
– 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
Important characteristics
– Modulation bandwidth (response speed)
– Optical conversion efficiency
– Noise
– Area
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pin Photodiode
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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 electronhole pair in this region that moves
rapidly at the drift velocity by the
electric field
Intrinsic layer is introduced
– Increase the space charge
region
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I-V Characteristic of Reversed Biased pin
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Photocurrent increases with incident optical power
Dark current, Id: current with no incident optical power
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Light Absorption
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Dominant interaction
– Photon absorbed
– Electron is excited to CB
– Hole left in the VB
Depends on the energy band gap
(similar to lasers)
Absorption (a requires the photon
energy to be smaller than the
material band gap
hc


 Eg
hc
1.24
 m

Eg Eg eV 
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Quantum Efficiency
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Probability that photon generates an electron-hole pair
Absorption requires
– Photon gets into the depletion region
– Be absorbed
Reflection off of the surface
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Photon absorbed before it gets to the depletion region
  1  R
  e a l
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Photon gets absorbed in the depletion region
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Fraction of incident photons that are absorbed
  1  ea d 
  1  R  ea l 1  ea d 
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Detector Responsivity
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Each absorbed photon generates an electron hole pair
Iph = (Number of absorbed photons) * (charge of electron)
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Rate of incident photons depends on
– Incident optical power Pinc
– Energy of the photon Ephoton= hf
Generated current
q
I ph   Pinc
hf
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Detector responsivity
– Current generated per unit optical power
q

 

AW
hf
1.24
 in units of m
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Responsivity
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Depends on quantum efficiency , and photon energy
q

 

AW
hf
1.24
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Avalanche Photodiode (APD)
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Minimum Detectable Power
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Important detector Specifications
– Responsivity
– 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
– Pmin=-22 dBm @B=2.5 Gbps, BER=10-10
Common InGaAs APD
– Pmin=-32 dBm @B=2.5 Gbps, BER=10-10
– Limited to around B=2.5 Gbps
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