LEDs are of interest for fibre optics because of five inherent characteristics.. How it works? Spectrum of an LED Modulation of LED LED Vs. Laser diode disadvantages of LED

1. PRESENTED BY:
ANNI PRIYANKA
Topic: Modulation of LED
Satellite and Optical Communication

2. Requireme
nt of
Optical
Source
Output
power
must be
high, using
lowest
possible
current
and less
heat
High
output
directionali
ty, narrow
spectral
width.
Bandwidth
should be
wide
enough
Low
distortion

3. LED
It is a semi-conductor and has polarity
Light emitters converts the electrical signal into a corresponding light
signal that can be injected into the fibre.

4. LEDs are of interest for fibre optics
because of five inherent characteristics
 They are small
 They possess high radiance
 The emitting area is small
 They have a very long life
 They can be modulated

5. How it works?
When current flows across a
diode
Negative electrons move one
way and positive holes move
the other way

6. How it works?
Here, the holes exist at a
lower energy level than the
free electrons
So, when a free electrons falls, it
losses energy

7. How it works?
This energy is emitted in a
form of a photon, which
causes light
The color of the light is determined by the
fall of the electron and hence energy level of
the photon

8. Inside a LED

9. Spectrum of an LED
LED
Density
-15 to -25 dBm
60-100nm
λ
Different frequency = different wavelength = different colors

10. Modulation of LED
In order to code information as pulses, it is
necessary to modulate the output of the LED
 This is done by switching the current on and off

11. Modulation of LED
• The frequency response of an LED depends on:
1- Doping level in the active region
2- Injected carrier lifetime in the recombination region, .
3- Parasitic capacitance of the LED
• If the drive current of an LED is modulated at a frequency of 𝜔 the
output optical power of the device will vary as:
• Electrical current is directly proportional to the optical power, thus we
can define electrical bandwidth and optical bandwidth, separately.
2
0
)(1
)(
i
P
P





12. Modulation of LED













)0(
)(
log10
)0(
)(
log10BWOptical
I
I
P
P 

13. LED Vs. Laser diode

14. The disadvantages
lower output optical power is coupled into a fiber
 lower modulation bandwidth
 harmonic distortion

15. Thank You..