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September 2004

SEMICONDUCTOR LASERSSEMICONDUCTOR LASERS

AND LIGHT EMITTING DIODESAND LIGHT EMITTING DIODES

CHAPTER 4CHAPTER 4

FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIA

ASMS05 FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIA

http://en.wikipedia.org/wiki/Diode_laser http://en.wikipedia.org/wiki/LED


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September 2004 Prof. John Watson

4.1 The Semiconductor Laser4.1 The Semiconductor Laser

In many ways the ultimate optoelectronic source.

Provides high optical power

in a small package

at a low cost low electrical power

The laser diode

has become the standard source for opticalcommunications.

We first deviate slightly from our discussion of coherent light sources

to bring in the light emitting diode (LED). Both the LED and the Diode laser are based on forward biased

pn-junctions

both rely on phenomenon of injection luminescence for

their operation




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4.2 Injection Luminescence4.2 Injection Luminescence

Luminescence - phenomenon of light emission from solidmaterials

on excitation by some form of applied energy

The Light Emitting Diode (LED)

Emits light when a current is injectedinjected across a forward

biased pn-junction

Injection luminescence

In a normal pn-junction diode

spontaneous radiation (luminescence) represents lost

energy Normal diode is designed to minimise this loss.

In a LED or LD

We want to exaggerate this effect.





4/50September 2004 Prof. John WatsonFACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIAFACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIAASMS05 FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIA

A thin depletion region or layer is formed at

the junction through carrier recombination

which effectively leaves it free of mobilecharge carriers (both electrons and holes).

This establishes a potential barrier between the p and n

type regions which restricts the inter diffusion of majority

carriers from their respective regions

Equilibrium situation


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On applying a forward voltageacross the ends of a pn-junction

the p-type is made positive withrespect to the n-type

The equilibrium situation isdisturbed.

The energy barrier is reduced

Because the barrier energy hasdecreased, the diffusion current

must now exceed the drift current

gives rise to net flow of currentfrom p-side to n-side.

This current is known as the

injection currentinjection current

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Holes in p-side and electrons in n-side can cross over to oppositesides of junction

Holes which cross to n-side recombine with electrons in n-type

Electrons which cross to p-side recombine with holes in p-type

Excess energy produced by recombination emitted asphotons

spontaneous emission of radiation.

Effectively electrons drop from CB into VB

The longest wavelength which can be emitted,corresponds to an electron dropping from bottom of CB to

top of VB.

Shorter wavelengths emitted when the electron dropsfrom higher energy states in CB.

c = hc/Eg ;

where Eg is the energy of the bandgap.


Eg = E2 - E1 = hf , where h = 6.626 x 10-34 J s is Planck's constant



4.3 Light Emitting Diodes (LED's)4.3 Light Emitting Diodes (LED's)

All pn junctions emit light on passage of forward biased current

Si & Ge are not efficient producers of light

Electrons & holes must first lose momentum before they canrecombine

indirect semiconductors

Compound semiconductors are bettere.g. GaAs, GaP, GaAlAs

Electrons & holes can directly recombine

direct semiconductors

In GaAs with a bandgap energy of 1.43 eV

Longest emitted wavelength is 860 nm

In practice, doping of the materials creates energy states insidethe bandgap

Gives shorter wavelengths than those predicted above

Peak emission wavelengths depend on material


http://en.wikipedia.org/wiki/Doping_(semiconductors)



4.3 The LED4.3 The LED

More complex compounds such as GaAsxP1-x allowselection of the bandgap width and,

hence, emission wavelength by varying the As:P ratio.

For pure GaP (x = 0) the band gap is 2.26 eV,whereas, for pure GaAs (x = 1) the bandgap is about

1.44 eV,

providing a range of wavelengths between 550 and 860 nm.

Typical emission wavelengths for LEDs GaAs 1.44 eV - 880 nm

GaP 2.26 eV - 550 nm or 700 nm

GaAsP - 580 nm or 660 nm

Si - 1100 nm

Ge - 1810 nm


Sb-Stibium



4.3 The LED4.3 The LED

LEDs can often prove a useful low cost alternative to the laserdiode.

Radiation emitted from diode results mainly from spontaneoustransitions.

low intensity radiation is emitted.

Construction of LEDs is slightly different from the laser diode

a shallow junction is fabricated to allow as much radiant emission

as possible to escape.

Several different methods of encapsulation of the junction are

employed to maximise the amount of light that can be emitted. The type of encapsulation used influences the spatial profile of the

output beam.




4.3.1 LED Structure4.3.1 LED Structure

Light emission

p-region

n-region

Metal contactsSURFACE EMITTING LED

Light emission

p-region

n-region

Metal contactsEDGE EMITTING LED




Surface emitting LEDSurface emitting LED

Light emission(some light will escape

into substrate)

Oxide

p-GaAs0.6P0.4

n-GaAs0.6P0.4

n-GaAs substrate

Metal contacts

50 m

5 m

200 m



12/50September 2004 Prof. John WatsonFACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIAFACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIAASMS05 FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIA

Te-Tellurium



4.3.2 Typical LED Parameters4.3.2 Typical LED Parameters

The GaAs0.6P0.4 LED shown earlier Emits in the red part of spectrum (650 nm)

Powers can be a few hundred W to tens of mW

Newer high irradiance LEDS can produce Watts ofcw power

Radiation is broadband around 50 nm full width

Drive currents between few mA to a few hundred mAneeded

Beam divergences often determined by the

encapsulation Geometries devised to reduce losses at

interfaces and to minimise total internalreflection



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4.4 Semiconductor Lasers (Laser Diodes)4.4 Semiconductor Lasers (Laser Diodes)

Fundamentally different from other lasers (gas lasers) Formed from heavily doped pn-junctions

Based on modified LED structure

To achieve laser action, need to ensure high concentration of e-hpairs available for recombination

Achieved by high doping concentrations across junction

Long spontaneous li fetime materials enhance stimulated emission

Laser diodes constructed so that light emerges from ends ratherthan through the wide gap

narrow active layer contains holes across the whole length

ends are cleaved, polished and made flat & parallel

light, which is spontaneously generated, is reflected back & forthcausing stimulated emission

High current densities are needed to produce stimulated emission& population inversion



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4.4 Laser Diode4.4 Laser Diode

Light emission

Oxide

p-region

n-region

Metal contacts

Active region

Heavily doped n-region(light emission)

END VIEW SIDE VIEW

LASER DIODE

Sides are roughened Ends are cleaved and polished



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4.4.1 Practical Laser Diodes4.4.1 Practical Laser Diodes

The first operational laser diode consisted of a single crystal ofGaAs

To create a population inversion and enhance possibility ofrecombination

high levels of doping are necessary to ensure that, in the depletion

region,

filled states in the CB are directly above empty states in VB

This applies only across a very narrow region of the depletionarea,

about 1 nm wide, known as the active layer.

The narrow active layer contains holes across the whole length

of the crystal.

Its ends are cleaved, polished and made flat & parallel and thesides are roughened to trap light inside crystal.

This forms the optical cavity


http://en.wikipedia.org/wiki/Optical_cavity


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Light, which is spontaneously generated,

is reflected back & forth causing stimulated

emission.

The onset of lasing is characterized by a

specific injection current known as the

threshold currentthreshold current.

Below this threshold, Ith, light emission will bespontaneous and incoherent.

For a significant gain, a high current density of

the order of several hundred A mm-2 is

necessary. These early lasers had lifetimes of only a few

hundred hours and required cooling by liquid

nitrogen for efficient operation.

These are known as homojunctionhomojunction lasers

I

P

Ith

Laser action

LED action


4.4.1 Practical Laser Diodes4.4.1 Practical Laser Diodes


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4.4.3 Double4.4.3 Double HeterojunctionHeterojunction Laser DiodesLaser Diodes

Further improvements with double heterojunction(DH) diode.

Reducing the active region even further and

sandwiching between a double layer Confines gain to an even narrower region

threshold currents down to hundreds of milliamps

efficient operation in both pulsed and continuous modes.

n-GaAs

P-GaAlAs

p-GaAs

Radiation field

Gain

N-GaAlAs Loss

Loss

Refractive index

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4.4.3 Double4.4.3 Double HeterojunctionHeterojunction -- ExampleExample

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4.4.4 Stripe (Index4.4.4 Stripe (Index--guided) Lasersguided) Lasers

Further confinement of the gain improves this further

Gain region confined to a narrow vertical stripe as well as inthe active area

Index-guiding or Gain-guiding

Collectively known as stripe lasers

IndexIndex--guidedguided structures vary refractive in vertical as well ashorizontal plane

Beam is confined both vertically and horizontally

Substrate n-GaAlAs

n-GaAlAs n-GaAlAs

n-GaAlAs

Active region n-GaAlAsp-GaAlAs

Metal contacts

Oxide

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4.4.5 Stripe (Gain4.4.5 Stripe (Gain--guided) Lasersguided) Lasers

GainGain--guidedguided lasers achieve a similar performance by restricting the current flow through the diode

Highly resistive regions channel the current through a

narrow strip emission of light confined to narrow active region

n-GaAs

Highly

resistive

Highly

resistive

n-GaAlAsActive region n-GaAs

p-GaAlAs

Metal contactsCurrent through device



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HeterojunctionHeterojunctionLaserLaser

StructuresStructures



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The Laser DiodeThe Laser Diode

Laser diodes are produced across a wide range of wavelengths

such as 633, 770, 809 nm, 1.1 or 1.3 m depending on the

material and structure.

For example, AlGaInP, GaAlAs, InGaAsP

Powers range from a few mWs to several Ws cw.

Most semiconductor lasers are edge emitters

Typical active layers are 500 m long by 1 m thick and tensof m wide

Newer structures like Multiple Quantum Well (MQW) andDistributed Feedback (DFB)

Pulsed powers up to 100s of Watts peak.

The lower injection currents, tens of milliamps, prolong

operational life.



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DFB Laser DiodeDFB Laser Diode


An optical grating is incorporated

into the heterostructure waveguide

to provide periodic variations in

refractive index along the direction

of wave propagation so that

feedback of optical energy. The

corrugated grating may be applied

over the whole active length of the

device where it gives what is known

as distributed feedback andeliminates the need for end mirrors.

The characteristics of this type of

laser is that it has a smaller spectral

width and its output wavelength ismore stable and linear. It is also less

temperature independent.


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Diode Laser StructuresDiode Laser Structures

Semiconductor Lasers

Edge emitters(single-element & arrays)

Surface emitters(mostly arrays)

Homojunction DH SH Planar cavity Vertical cavity

Stripe Broad area

Gain-guided

Index-guided

Variety of structures



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Other StructuresOther Structures Quantum WellQuantum Well

Quantum well lasers Utilise fact that an energy well can be formed at bottom of

CB and top of VB

High densities of electrons & holes can collect in respectivebands

Population inversion obtained more easily

Lower threshold currents, smaller active areas

Low temperature sensitivity Replacing DH lasers

Also seen as multiple quantum well (MQW) lasers

Adjacent quantum wells couple together and increasethickness of active layer

Reduces loss in surrounding regions



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Other StructuresOther Structures Vertical CavityVertical Cavity

Vertical cavity lasers

Vertical Cavity Surface Emitting Lasers (VCSEL)

Resonant cavity is in plane of active layer

Photons have a very short path length (< 1 m) in active

region

Need high reflectance mirrors to overcome losses

Active layer could be SQW or MQW Low thresholds, symmetrical beam profiles, high

temperature stability

Divergence angle of 7 - 10



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Sony LaserSony Laser

DiodeDiode



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September 2004 Prof. John WatsonFACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIAASMS05 FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIA


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Sony LaserSony Laser

DiodeDiode



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4 6 Efficiency4 6 Efficiency


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4.6 Efficiency4.6 Efficiency


1. Total efficiency (external quantum efficiency), T is defined as:

There are a number of ways in which the operational efficiency of thesemiconductor laser may be defined.

2. The external power efficiency of the device ep

in converting electrical

input to optical output is given by:

where P=IV is the d.c. electrical input power and Pe = power emitted

%100=

P

Peep

%100

=

V

EgT

T = total number of output photons

total number of injected electron


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The optical power emitted, Pe into a medium of low refractive

index, n from the face of a planar LED fabricated from a material of

refractive index nx , is given approximately by:

where Pint is the power generated internally and F is the

transmission factor of the semiconductor-external interface.

4.6 Efficiency4.6 Efficiency

Hence it is possible to estimate the percentage of optical power emitted.

2

2

int

4 xe

n

FnPP =


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4.7 MODULATION BANDWIDTH4.7 MODULATION BANDWIDTH

The modulation bandwidth in optical communications may be defined ineither electrical or optical terms.

2

fB =

where

B = electrical BW

f = optical BW

Assuming a Gaussianfrequency response:

Other StructuresOther Structures LEDLED


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4 S CO C O S S S4 7 SEMICONDUCTOR LASER VERSUS LED


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4.7. SEMICONDUCTOR LASER VERSUS LED4.7. SEMICONDUCTOR LASER VERSUS LED

The power supplied by both devices

is similar (about 10 20 mW).

However, the maximum coupling

efficiency of a fiber is much smaller

for an LED than for an LD; for an LED

it is (5 10)%, but for an LD it can beup to 90%.

This difference in coupling efficiency

has to do with the difference in

radiation geometry of the twodevices.

Temperature Effects on LD Output Power

4 7 SEMICONDUCTOR LASER VERSUS LED4 7 SEMICONDUCTOR LASER VERSUS LED


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As an LED emits spontaneous radiation, the speed of modulation

is limited by the spontaneous recombination time of the carriers.

LEDs have a large capacitance and modulation BW are not very

large (a few hundred MHz).

For a LD above the threshold the electrons remain in the CB for a

very short time, due to the stimulated recombination; therefore,

very fast modulation is possible (up to 10 GHz).

LDs have narrower spectra than LEDs, and the single-mode lasers,

in particular have a very narrow spectrum.

This explains why the pulse broadening at transmission throughan optical fiber is very small.

Therefore, with an LD as a light source, wideband transmission

systems can be designed.

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Changes of power output for an LD with temperature can be

prevented by stabilizing the heat sink temperature with a Peltier

element and a control circuit.

LD generally requires more complicated electronic circuits than

for an LED.

LEDs can withstand power overloading for short duration betterthan LDs.

At current prices, LEDs are less expensive than LDs.

LEDs are generally very reliable and lifetime of 105 hours or 11

years are comman for good LEDs if operated within the limits

(power, voltage, current and temperature).


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Table 1 omparison of LEDs and Lasers

Characteristic LEDs Lasers


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Characteristic LEDs Lasers

Output Power

Linearly proportional

to drive current

Proportional to current

above the threshold

CurrentDrive Current: 50 to

100 mA PeakThreshold Current: 5

to 40 mA

Coupled Power Moderate High

Speed Slower Faster

Output Pattern Higher Lower

Bandwidth Moderate High

Wavelengths

Available0.66 to 1.65 m 0.78 to 1.65 m

Spectral WidthWider (40-190 nm

FWHM)Narrower (0.00001 nm

to 10 nm FWHM)

Fiber Type Multimode Only SM, MM

Ease of Use Easier Harder

Lifetime Longer Long

Cost Low ($5-$300) High ($100-$10,000)

LED/LD Performance Characteristics

S l k h i i LED/l d i h i f l i i


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Several key characteristics LED/lasers determine their usefulness in a given

application.

Peak Wavelength: This is the wavelength at which the source emits the most power.

It should be matched to the wavelengths that are transmitted with the least

attenuation through optical fiber. The most common peak wavelengths are 1310,

1550, and 1625 nm.

Spectral Width: Ideally, all the light emitted from a laser would be at the peak

wavelength, but in practice the light is emitted in a range of wavelengths centered

at the peak wavelength. This range is called the spectral width of the source.

Emission Pattern: The pattern of emitted light affects the amount of light that can becoupled into the optical fiber. The size of the emitting region should be similar to

the diameter of the fiber core.

Power: The best results are usually achieved by coupling as much of a source's

power into the fiber as possible. The key requirement is that the output power of thesource be strong enough to provide sufficient power to the detector at the receiving

end, considering fiber attenuation, coupling losses and other system constraints. In

general, lasers are more powerful than LEDs.

LED/LD Performance Characteristics


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Speed: A source should turn on and off fast enough to meet the bandwidth limits of

the system. The speed is given according to a source's rise or fall time, the time

required to go from 10% to 90% of peak power. Lasers have faster rise and fall times

than LEDs.

Linearity is another important characteristic to light sources for some applications.

Linearity represents the degree to which the optical output is directly proportionalto the electrical current input. Most light sources give little or no attention to

linearity, making them usable only for digital applications. Analog applications

require close attention to linearity. Nonlinearity in lasers causes harmonic distortion

in the analog signal that is transmitted over an analog fiber optic link.


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Analog LED Drive Circuits

Digital LED Drive Circuits


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Analog Laser Drive Circuits

Digital Laser Drive Circuits

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