sumitesh majumder. what are the light sources ? light emitting diode (led)laser diode (ld) the most...
TRANSCRIPT
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