Fiber Optic Communication (1)
FOC is a method of transmitting information from one
place to another by sending pulses of light through an
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
Figure 1 (a)The general communication system.
(b) The optical fiber communication system.
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.
To explore design and operation of optical
communication components : light emitting diode (LEDs).
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.
Requirement of Optical Source
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
Properties of LEDs (2)
But, LEDs have a number of distinct advantages which
have given it a prominent place in optical fiber
• No mirror facets
• No striped geometry
• Simpler construction
Simpler drive circuitry
• 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
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.
Power & Efficiency of LEDs(2)
Other than radiative recombination, non-radiative
recombination may be occur sometimes and generate
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%.
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)
Power & Efficiency of LEDs(4)
External quantum efficiency, ηext : ratio of the number of
photons emitted from the device to the photons internally
Power & Efficiency of LEDs(5)
There are several loss mechanisms that affect the external quantum
Absorption within LEDs
Critical angle loss
Power & Efficiency of LEDs(6)
The optical power emitted Pe into a medium :
The coupling efficiency :
The Operating Principle of LEDs (1)
There are homostructure, single heterostructure and
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
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
The surface emitter LEDs can transmit the data rate less
than 20 MHz.
It contains the short optical link with large NA.
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.
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