MANAKULA VINAYAGAR INSTITUTE OF TECHNOLOGY
DEAPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
Unit IV
Optical Sources, Detectors and
Amplifiers
Unit IV Syllabus
• LED- structures -Light source materials
efficiency and LED power, Modulation of LED.
• Laser Diodes-Modes
equations
-External
frequencies.
-Quantum
and Threshold condition -Rate
Quantum
efficiency
-Resonant
• Optical detectors – PIN diode and APD –operation and
characteristics
• Erbium Doped Fiber Amplifiers - principle, operation and
applications.
• Link Power budget and Rise time budget calculations
Light Emitting Diode
• An LED is a semiconductor P-N junction under forward
bias.
• LED is the best suitable optical source which
guarantees the bit rate of100-200Mbps.
• Instead of generating heat at the PN junction, light is
generated and passes through a lens.
• LEDs can operate at visible spectrum or infrared
spectrum.
• In forward bias condition, electrons cross the PN
junction from the n-type material and recombine with
holes in the p-type material.
• The recombining electrons release energy in the form of
light.
Specifications
• LEDs are typically used at the 850-nm and 1310-nm
transmission wavelengths
• LEDs are typically used in lower-data-rate, shorterdistance multimode systems because of their inherent
bandwidth limitations and lower output power.
• The output spectrum of a typical LED is about 40 nm,
which limits its performance because of severe
chromatic dispersion.
• LEDs operate in a more linear fashion than do laser
diodes. This makes them more suitable for analog
modulation.
• Two basic structures for LEDs are used in fiber optic
systems:
– surface emitting and
– edge emitting
Light Source Materials
Energy Band of LED
•In LED, the dominant photon generation is spontaneous emission in which
the electron drops to the lower energy level in an entirely random way. The
output spectrum of an LED is relatively wide.
•Spontaneous emission: An atom in an excited level can make a downward
transition spontaneously (i.e., on its own) by emitting a photon
corresponding to the energy difference between the two levels.
Configuration: Edge and Surface
LED Structures
Heterojunction:
• A heterojunction is a junction formed by dissimilar
semiconductors.
• It is an interface between two adjoining single crystal
semiconductors with different band gap.
• Heterojunction are of two types, Isotype (n-n or p-p) or
Antistype (p-n).
LED Structures
• Double Heterojunction (DH): In order to achieve efficient
confinement of emitted radiation, double heterojunction are
used in LED structure.
• Double heterojunction (DH) is formed by two different
semiconductors on each side of active region.
• The crosshatched regions represent the energy levels of free
charge.
• Recombination occurs only in active InGaAsP layer.
• The two materials have different band gap energies and
different refractive indices. The changes in band gap energies
create potential barrier for both holes and electrons.
• The free charges can recombine only in narrow, well defined
active layer side.
Quantum efficiency and LED
power
Modulation of LED
Figure 4.16 frequency response of optical source
LASAR - Light Amplification by
Stimulated Emission of Radiation
• The most frequently used light source in
optical communication systems is
laser.Why we use a laser as the light
source?monochromatic: suitable for
elimination of white noisecoherent:
suitable for coherent detection(c) high
power: improves signal to noise ratio(d)
small divergence: improves efficiency of
transmission(e) small source size: suitable
for use with optical fibres
• LaserAn acronym for light amplification
by stimulated emission of
radiation.Stimulated emissionAn atom in
an excited level can make a downward
transition in the presence of external
stimulation by emitting a photon
corresponding to the energy difference
between the two levels. The emitted
photon is in phase with the incident
photon.
Configuration