Semiconductor Optical Sources

woundcallousSemiconductor

Nov 1, 2013 (3 years and 7 months ago)

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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

E
g

(eV)

Bandgap wavelength


g

(



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 (Al
1
-
x
Ga
x
As)


0.87 <


< 0.63 (

m)


Quaternary compound Ga
x
In
1
-
x
As
y
P
1
-
y

is lattice matched to InP if y=2.2x


1.0 <


< 1.65 (

m)


Optical telecommunication laser compounds


In
0.72
Ga
0.28
As
0.62
P
0.38

(

=1300nm)


In
0.58
Ga
0.42
As
0.9
P
0.1

(

=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 (
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

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



R
1

and R
2

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
D

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=500

m 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


m
min
=2249


m
max
=2267


The number of modes is 18


With a wavelength separation of
D
=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 (C
3
) laser


Distributed feedback laser (DFB) laser

23

Cleaved Coupled Cavity (C
3
) 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

=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 <


< 1562 nm


Average power: 5 < P
t

< 9 mW


Threshold current I
th
=40mA


TEC cooler


Line width: 10 MHz


We need to convert from
D
f to
D






D
=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 i
2


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, I
d
: 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

I
ph

= (Number of absorbed photons) * (charge of electron)



Rate of incident photons depends on


Incident optical power
P
inc


Energy of the photon
E
photon
= hf


Generated current




Detector responsivity


Current generated per unit optical power










in units of

m

35

Responsivity


Depends on quantum efficiency

, and photon energy


36

Avalanche Photodiode (APD)


37

Minimum Detectable Power


Important detector Specifications


Responsivity


Noise Equivalent noise power i
n

or noise
equivalent power NEP


Often grouped into minimum detectable
power P
min

at a specific data rate


P
min

scales with data rate


Common InGaAs pin photodetector


P
min
=
-
22 dBm @B=2.5 Gbps, BER=10
-
10


Common InGaAs APD


P
min
=
-
32 dBm @B=2.5 Gbps, BER=10
-
10


Limited to around B=2.5 Gbps