1. OPTICAL SOURCE : Light Emitting Diodes (LEDs)

2. INTRODUCTION

3. Fiber Optic Communication (1)
FOC is a method of transmitting information from one
place to another by sending pulses of light through an
optical fiber.
The physics behind the use of light as the transmitter is the
amount of information that can be transmitted is directly
proportional to the frequency range over which the carrier
operates.

4. a)
Information
source
Transmitter
(modulator)
Transmission
medium
Receiver
(demodulator)
Destination
b)
Aa
Information
source
Electrical
transmit
Optical
source
Optical fiber
cable
Optical
detector
Electrical
receive
Figure 1 (a)The general communication system.
(b) The optical fiber communication system.
Destination

5. Fiber Optic Communication (2)
As the carrier frequency increases, the available
transmission bandwidth will increases as well.
When the bandwidth becomes larger, it will provides a
larger information capacity.

6. Objective
To explore design and operation of optical
communication components : light emitting diode (LEDs).

7. Problem Statement
The improvement of designation and operation of LEDs
should be practiced in the optical communication world.
This kind of alternative can bring us to the most effective
optical source that will save more energy and time, but at
the same time will gain more benefit.

8. THEORY

9. Requirement of Optical Source
Output
wavelength
must
coincide with
the loss
minima of
the fiber
Low
distortion
Requirement
of Optical
Source
Bandwidth
should be
wide enough
Output
power must
be high,
using lowest
possible
current and
less heat
High output
directionality
, narrow
spectral
width.

10. Properties of LEDs (1)
Light emitters converts the electrical signal into a
corresponding light signal that can be injected into the fiber.
The disadvantages of LEDs compared to the laser:
generally lower optical power coupled into a fiber
usually lower modulation bandwith
harmonic distortion

11. Properties of LEDs (2)
But, LEDs have a number of distinct advantages which
have given it a prominent place in optical fiber
communications.
Simpler fabrication
• No mirror facets
• No striped geometry
Low cost
• Simpler construction
Reliability
Less temperature
dependence
Simpler drive circuitry
Linearity
• Less-sensitive to gradual degradation
• Immune to self-pulsation and modal noise problems
• Light output against current less affected by temperature
• LEDs are not threshold device
• Temperature compensation circuits unnecessary.
• Linear output against current characteristic

12. WORKING PRINCIPLE OF
LEDs

13. Power & Efficiency of LEDs (1)
In LEDs, the light emitting region consists of a p-n junction
constructed of a direct band gap III-V semiconductor, which
when forward biased, experiences injected minority carrier
recombination, resulting in the generation of photons.

14. Power & Efficiency of LEDs(2)
Other than radiative recombination, non-radiative
recombination may be occur sometimes and generate
phonons.
LEDs will produce at best internal quantum efficiency of
50% for simple homojunction devices.
Whereas for heterojunction devices, the internal quantum
efficiency range from 60 to 80%.

15. Power & Efficiency of LEDs (3)
Internal quantum efficiency, ηint : the ratio of the radiative
recombination rate to the total recombination rate.
ηint = (Pint / hf ) / (I/e)

16. Power & Efficiency of LEDs(4)
External quantum efficiency, ηext : ratio of the number of
photons emitted from the device to the photons internally
generated.

17. Power & Efficiency of LEDs(5)
There are several loss mechanisms that affect the external quantum
efficiency:
Absorption within LEDs
Fresnel losses
Critical angle loss

18. Power & Efficiency of LEDs(6)
The optical power emitted Pe into a medium :
The coupling efficiency :

19. The Operating Principle of LEDs (1)
There are homostructure, single heterostructure and
double heterostructure.

20. The Operating Principle of LEDs (2)
When a forward bias is applied, electrons from the n-type layer are
injected through the p–n junction into the p-type GaAs layer where they
become minority carriers.
These minority carriers diffuse away from the junction, recombining
with majority carriers (holes) as they do so.
light is emitted from the device without re-absorption because the
bandgap energy in the AlGaAs layer is large in comparison with that in
GaAs.

21. The Structure of LEDs

22. The Surface Emitter LEDs (1)

23. The Surface Emitter LEDs (2)
This kind of structure provides a low thermal impedance in
the active region.
The internal absorption in this device is very low due to the
larger bandgap-confining layers, and the reflection
coefficient at the back crystal face is high giving good
forward radiance.
The surface emitter LEDs can transmit the data rate less
than 20 MHz.
It contains the short optical link with large NA.

24. The Edge Emitter LEDs (1)

25. The Edge Emitter LEDs (2)
This type of LEDs make use of the transparent guiding
layers with a very thin active layer (50 to 100 μm) in order
that the light produced in the active layer spreads into the
transparent guiding layers.
Majority of the propagating light are emitted at one end
face with the light reflected back from the other end face.
Its coupling efficiency is higher than the surface emitter
LEDs due to the smaller NA fiber.
The edge emitter LEDs radiate less power to the air
compared to the surface emitter LEDs because of the
reabsorption and interfacial recombination.
The edge emitter LEDs can transfer higher data rate, as
much as 100 MHz than the surface emitter LEDs.

26. CHARACTERISTICS OF LEDs

27. LEDs Characteristics (1)
The spectral profile of LEDs is broader than the laser.

28. LEDs Characteristics (2)
The graph of light output into the air against the drive
current for LEDs.

29. LEDs Characteristics (3)
The effect of temperature to the LEDs performance.

30. SUMMARY

31. Summary (1)
The double heterostructure of LEDs give the best
performance due to the high radiative recombination occur.
The edge emitter LEDs give rise to the high coupling
coefficient with smaller NA, thus produce high radiance into
the fiber.