Sumitesh Majumder

What are the Light Sources ?The most common light sources in this systems are

Light Emitting Diode (LED)Light Emitting Diode (LED) and Laser Diode (LD)Laser Diode (LD) Convert the electrical signal into a corresponding light electrical signal into a corresponding light

signalsignal Small size Solid structure Low power requirements

Light-Emitting DiodesThe combinationcombination between a free electron and a

hole, returning the atom to its neutral atom, releases energyreleases energy. Light is a form of this energyLight is a form of this energy.

Absorption

Absorption

Spontaneous

Emission

Spontaneous

Emission

Stimulated EmissionStimulated Emission

A light emitting diode (LED) is essentially a PN junction opto-semiconductor that emits a monochromatic (single color) light when operated in a forward biased direction.

LEDs converts electrical energy into light energy. They are frequently used as "pilot" lights in electronic appliances to indicate whether the circuit is closed or not.

h­ ­Eg

Eg (b)

V

(a)

p n+

Eg

eVo

EF

p n+

Electron­in­CBHole­in­VB

Ec

Ev

Ec

Ev

EF

eVo

Electron­energy

Distance­into­device

(a)­The­energy­band­diagram­of­a­ p-n+ ­(heavily­n -type­doped)­junction­without­any­bias.Built-in­potential­ Vo ­prevents­electrons­from­diffusing­from­ n+ ­to­p ­side.­(b)­The­appliedbias­reduces­Vo ­and­thereby­allows­electrons­to­diffuse,­be­injected,­into­the­ p -side.

Recombination­around­the­junction­and­within­the­diffusion­length­of­the­electrons­in­thep -side­leads­to­photon­emission.

How Does A LED Work? (1/2)• When sufficient voltage is applied to the chip across the leads of the LED, electrons can move easily in only one direction across the junction between the p and n regions.

• In the p region there are many more positive than negative charges.

• When a voltage is applied and the current starts to flow, electrons in the n region have sufficient energy to move across the junction into the p region.

How Does A LED Work? (2/2)

• Each time an electron recombines with a positive charge, electric potential energy is converted into electromagnetic energy.

• For each recombination of a negative and a positive charge, a quantum of electromagnetic energy is emitted in the form of a photon of light with a frequency characteristic of the semi-conductor material (usually a combination of the chemical elements gallium, arsenic and phosphorus)..

Properties of LED Emits incoherent light through spontaneous emission.

Used for Multimode systems w/ 100-200 Mb/s rates.

Broad spectral width and wide output pattern.

850nm region: GaAs and AlGaAs

1300–1550nm region: InGaAsP and InP

Two commonly used types: ELEDs and SLEDs

Double hetero junction structure

Two different bandgap energies and refractive indice

The change in different bandgap energies create potential energy for holes and electrons

The electrons can meet and recombine in the well-defined active layer.

[ double hetero junction ]

[ double hetero junction ]

2­eV

2­eVeVo

Holes­in­VB

Electrons­in­CB1.4­eV

No­bias

Withforwardbias

Ec

EvEc

Ev

EFEF

(a)

(b)

(c)

(d)

pn+ p

Ec

GaAs AlGaAsAlGaAs

ppn+

~­0.2­m

AlGaAsAlGaAs

(a)­A­doubleheterostructure­diode­hastwo­junctions­which­arebetween­two­differentbandgap­semiconductors(GaAs­and­AlGaAs)

(b)­A­simplified­energyband­diagram­withexaggerated­features.­EF

must­be­uniform.

(c)­Forward­biasedsimplified­energy­banddiagram.

(d)­Forward­biased­LED.Schematic­illustration­ofphotons­escapingreabsorption­in­theAlGaAs­layer­and­beingemitted­from­the­device.

©­1999­S.O.­Kasap,­Optoelectronics (Prentice­Hall)

GaAs

SLEDs – Surface Emitting LEDsPrimary active region is a small circular area

located below the surface of the semiconductor substrate, 20-50µm diameter and up to 2.5µm thick.

Emission is isotropic and in lambertian pattern.

A well is etched in the substrate to allow the direct coupling of emitted light to the optical fiber

Emission area of substrate is perpendicular to axis of optical fiber

Coupling efficiency optimized by binding fiber to the substrate surface by epoxy resin with matching refractive index

[surface-emitting LED ][surface-emitting LED ]

ELEDs – Edge Emitting LEDsPrimary active region is a narrow strip that lies

beneath the semiconductor substrateSemiconductor is cut and polished so emission

strip region runs between front and back.Rear face of semiconductor is polished so it is

highly reflective while front face is coated with anti-reflective, light will reflect from rear and emit through front face

Active Regions are usually 100-150µm long and the strips are 50-70µm wide which are designed to match typical core fibers of 50-100µm.

Emit light at narrower angle which allows for better coupling and efficiency than SLEDs

Edge Emitting LED

(a)­Surface­emitting­LED (b)­Edge­emitting­LED

Doubleheterostructure

Light

Light

2. LED Operating characteristics

Light­output

Insulator­­(oxide)p

n+ Epitaxial­layer

A­schematic­illustration­of­typical­planar­surface­emitting­LED­devices.­(a)­p-layergrown­epitaxially­on­an­n+­substrate.­(b)­First­n+­is­epitaxially­grown­and­then­p­regionis­formed­by­dopant­diffusion­into­the­epitaxial­layer.

Light­output

pEpitaxial­layers

(a) (b)

n+

Substrate Substrate

n+

n+

Metal­electrode

©­1999­S.O.­Kasap,­Optoelectronics (Prentice­Hall)

Light­output

p

Electrodes

LightPlastic­dome

Electrodes

Domedsemiconductor

pn­Junction

(a) (b) (c)

n+n+

(a)­Some­light­suffers­total­internal­reflection­and­cannot­escape.­(b)­Internal­reflectionscan­be­reduced­and­hence­more­light­can­be­collected­by­shaping­the­semiconductor­into­adome­so­that­the­angles­of­incidence­at­the­semiconductor-air­surface­are­smaller­than­thecritical­angle.­(b)­An­economic­method­of­allowing­more­light­to­escape­from­the­LED­is

to­encapsulate­it­in­a­transparent­plastic­dome.

Substrate

V

2

1

(c)

0 20 40I­(mA)0

(a)

600 650 700

0

0.5

1.0

Relativeintensity

24­nm

655nm

(b)

0 20 40I (mA)0

Relative­light­intensity

(a)­A­typical­output­spectrum­(relative­intensity­vs­wavelength)­from­a­red­GaAsP­LED.(b)­Typical­output­light­power­vs.­forward­current.­(c)­Typical­I-V­characteristics­of­ared­LED.­The­turn-on­voltage­is­around­1.5V.

©­1999­S.O.­Kasap,­ Optoelectronics (Prentice­Hall)

Multimode­fiberLens

(a)

ELED

Active­layer

Light­from­an­edge­emitting­LED­is­coupled­into­a­fiber­typically­by­using­a­lens­or­aGRIN­rod­lens.

GRIN-rod­lens

(b)

Single­mode­fiberELED

©­1999­S.O.­Kasap,­Optoelectronics (Prentice­Hall)

• Sensor Applications

• Mobile Applications

• Signal Applications

• LED Signals

• Illuminations

• Indicators