1. Semiconductor Optical Sources

2. Source Characteristics
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

3. Semiconductor Optoelectronics
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

4. Basic Light Emission Processes
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

5. Semiconductor Material
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

6. Direct and Indirect Materials
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

7. Light Emission The emission wavelength depends on the energy band gap
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

8. Direct and Indirect Materials
Only specific materials have a direct bandgap
Material determines the bandgap
Material
Element Group
Bandgap Energy
Eg (eV)
Bandgap wavelength
lg (mm)
Type Ge IV
0.66
1.88 I Si
1.11
1.15
AlP
III-V
2.45
0.52
AlAs
2.16
0.57
AlSb
1.58
0.75
GaP
2.26
0.55
GaAs
1.42
0.87 D
GaSb
0.73
1.70
InP
1.35
0.92
InAs
0.36
3.5
AnSb
0.17
7.3

10. Common Semiconductor Compounds
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
In0.72Ga0.28As0.62P0.38 (l=1300nm)
In0.58Ga0.42As0.9P0.1 (l=1550nm)

11. Optical Sources 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

12. Light Emitting Diodes (LED)
Spontaneous emission dominates
Random photon emission
Implications of random emission
Broad spectrum (Dl~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

13. Laser Diode 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

14. Lasing 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

15. Optical Feedback Easiest method: cleaved end faces
End faces must be parallel
Uses Fresnel reflection
For GaAs (n=3.6) R=0.32
Lasing condition requires the net cavity gain to be one
g: distributed medium gain
a: distributed loss
R1 and R2 are the end facet reflectivities

16. 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

17. Phase Condition The waves must add in phase as given by
Resulting in modes given by
Where m is an integer and n is the refractive index of the cavity

18. Longitudinal Modes

19. Longitudinal Modes
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

20. Mode Separation Wavelength of the various modes
The wavelength separation of the modes is
A longer cavity
Increases the number of modes
Decrease the threshold gain
There is a trade-off with the length of the laser cavity

21. Cleaved Cavity Laser Example
A laser has a length of L=500mm and has a gain of
Solving this for wavelength gives
( ) nm < l < ( ) nm
The supported modes are calculated based on the constructed interference condition
The minimum and maximum orders are
mmin=2249
mmax=2267
The number of modes is 18
With a wavelength separation of Dl=0.69nm

22. Single Longitudinal Mode Lasers
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

23. Cleaved Coupled Cavity (C3) Laser
Longitudinal modes are required to satisfy the phase condition for both cavities

24. Periodic Reflector Lasers
Periodic structure (grating) couples between forward and backward propagating waves
For l=1550 nm, L=220 nm
Distributed feedback (DFB) laser
Grating distributed over entire active region
Distributed Bragg reflector (DBR) laser
Grating replaces mirror at end face

25. Laser Wavelength Linewidth

26. Summary of Source Characteristics
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

27. Example Laser Specifications
Let look at an example specification sheet
Phasebridge “Wideband Integrated Laser Transmitter Module”
Laser + External Modulator
Specifications
Wavelength: 1548 nm < l < 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 Dl
Dl=0.008 nm

28. Semiconductor Optical Detectors

29. Semiconductor Optical Detectors
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

30. 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
Increase the space charge region

31. I-V Characteristic of Reversed Biased pin
Photocurrent increases with incident optical power
Dark current, Id: current with no incident optical power

32. Light Absorption 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

33. Quantum Efficiency
Probability that photon generates an electron-hole pair
Absorption requires
Photon gets into the depletion region
Be absorbed
Reflection off of the surface
Photon absorbed before it gets to the depletion region
Photon gets absorbed in the depletion region
Fraction of incident photons that are absorbed

34. Detector Responsivity
Each absorbed photon generates an electron hole pair
Iph = (Number of absorbed photons) * (charge of electron)
Rate of incident photons depends on
Incident optical power Pinc
Energy of the photon Ephoton= hf
Generated current
Detector responsivity
Current generated per unit optical power
l in units of mm

35. Responsivity
Depends on quantum efficiency h, and photon energy

36. Avalanche Photodiode (APD)

37. Minimum Detectable Power
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 Gbps, BER=10-10
Common InGaAs APD
Pmin=-32 Gbps, BER=10-10
Limited to around B=2.5 Gbps