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

12 EEEg

meVEE

hc

gg

24.1

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

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

9

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 < < 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

• 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)

• Spontaneous emission dominates

– Random photon emission

• Implications of random emission

– Broad spectrum (~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• 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

• 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

• 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– : distributed loss– R1 and R2 are the end facet reflectivities

1exp21 LgRR

2

1

1

n

nR

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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

2

12

12

nn

nnR

32.016.3

16.32

R

21ln1

rrL

g

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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

mL z 22

m

nL2

18

Longitudinal Modes

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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 depends on the length of the cavity

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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

nL

m

2

1

1121 mm

nLmm

nLm

nL

2

2 2

2

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Cleaved Cavity Laser Example

• A laser has a length of L=500m and has a gain of

– Solving this for wavelength gives

(1550-5.65) nm < < (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

• The number of modes is 18

• With a wavelength separation of =0.69nm

2

10

1550exp15001090

g

m

nL2

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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

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Cleaved Coupled Cavity (C3) Laser

• Longitudinal modes are required to satisfy the phase condition for both cavities

21

22

m

nD

m

nL

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Periodic Reflector Lasers

• Periodic structure (grating) couples between forward and backward propagating waves

– For =1550 nm, =220 nm

n2

• Distributed feedback (DFB) laser

• Grating distributed over entire active region

• 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

• 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• 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 f to

• =0.008 nm

f

c f

f

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Semiconductor Optical Detectors

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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

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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

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I-V Characteristic of Reversed Biased pin

• Photocurrent increases with incident optical power

• Dark current, Id: current with no incident optical power

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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 ( requires the photon energy to be smaller than the material band gap

gEhc

meVEE

hc

gg

24.1

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

le

R 1

de 1

dl eeR 11

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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

in units of m

fh

qPI incph

WAfh

q

24.1

35

Responsivity

• Depends on quantum efficiency , and photon energy

WAhf

q

24.1

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Avalanche Photodiode (APD)

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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 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