1. Light Emitting Diode
Sumitesh Majumder

2. What are the Light Sources ?
The most common light sources in this systems are Light Emitting Diode (LED) and Laser Diode (LD)
Convert the electrical signal into a corresponding light signal
Small size
Solid structure
Low power requirements

3. Light-Emitting Diodes
The combination between a free electron and a hole, returning the atom to its neutral atom, releases energy. Light is a form of this energy.

4. Light-Material Interaction
Spontaneous Emission
Stimulated Emission
Absorption

5. 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.

6. l c t r i C B H (a) The energy band diagram of a p-n (heavilyu E g ( b ) V a p n + e o F l c t r i C B H v y D s d
(a) The energy band diagram of a
p-n
(heavily
-type doped) junction without any bias.
Built-in potential
prevents electrons from diffusing from to
side. (b) The applied
bias reduces
and thereby allows electrons to diffuse, be injected, into the
-side.
Recombination around the junction and within the diffusion length of the electrons in the
-side leads to photon emission.

7. 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.

8. 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)..

9. Properties of LED Emits incoherent light through spontaneous emission.
Used for Multimode systems w/ 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

10. 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 ]

12. SLEDs – Surface Emitting LEDs
Primary 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

13. Surface Emitting LED

14. [surface-emitting LED ]

15. ELEDs – Edge Emitting LEDs
Primary active region is a narrow strip that lies beneath the semiconductor substrate
Semiconductor 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 µm long and the strips are 50-70µm wide which are designed to match typical core fibers of µm.
Emit light at narrower angle which allows for better coupling and efficiency than SLEDs

16. Edge Emitting LED

17. L i g (a) Surface emitting LED (b) Edge emitting LED D o u b l e h t r

18. 2. LED Operating characteristics

19. 2. LED Operating characteristics

20. Planar LED

21. Dome LED L i g h t o u p E l e c r d s P a m D n J ( ) b +
(a) Some light suffers total internal reflection and cannot escape. (b) Internal reflections
can be reduced and hence more light can be collected by shaping the semiconductor into a
dome so that the angles of incidence at the semiconductor-air surface are smaller than the
critical angle. (b) An economic method of allowing more light to escape from the LED is
to encapsulate it in a transparent plastic dome. S

23. g h p G R I N . M u l t i m o d e f b r L n s ( a ) E D - Sc v y g h p G R I N . - S
© 1999 S.O. Kasap,
Optoelectronics
(Prentice Hall)

24. Characteristics of commercial LEDs

25. 1. Light-Emitting Diodes

26. Applications Sensor Applications Mobile Applications
Signal Applications
LED Signals
Illuminations
Indicators