Regulation – 2022 (CBCS Scheme) BEC515D– SATELLITE AND OPTICAL COMMUNICATION
MODULE V
OPTICAL SOURCES AND DETECTORS
Optical Sources
Optical transmitter coverts electrical input signal into corresponding optical signal.
The optical signal is then launched into the fiber. Optical source is the major component in an optical
transmitter .Popularly used optical transmitters are Light Emitting Diode (LED) and semiconductor
Laser Diodes (LD).
Characteristics of Light Source of Communication
To be useful in an optical link, a light source needs the following characteristics
It must be possible to operate the device continuously at a variety of temperatures for
many years.
It must be possible to modulate the light output over a wide range of modulating
frequencies.
To couple large amount of power into an optical fiber, the emitting area should be small.
To reduce material dispersion in an optical fiber link, the output spectrum should be
narrow.
The power requirement for its operation must be low.
The light source must be compatible with the modern devices.
1
Prepared by: Ms.Mohanthi K, Dr.Martin Joel Rathnam Sri Sairam College of Engineering, Anekal Page
, Regulation – 2022 (CBCS Scheme) BEC515D– SATELLITE AND OPTICAL COMMUNICATION
The optical output power must be directly modulated by varying the input current to
the device.
Better linearity of prevent harmonics and intermodulation distortion.
High coupling efficiency.
High optical output power.
High reliability.
Low weight and low cost.
Two types of light sources used in fiber optics are light emitting diodes (LEDs) and
laser diodes (LDs).
Light Emitting Diodes (LEDs)
p-n Junction
Conventional p-n junction is called as homojunction as same semiconductor material
is used on both sides junction. The electron-hole recombination occurs in relatively layer = 10
μm. As the carriers are not confined to the immediate vicinity of junction, hence high current
densities can not be realized.
The carrier confinement problem can be resolved by sandwiching a thin layer ( 0.1 μm)
between p-type and n-type layers. The middle layer may or may not be doped. The carrier
confinement occurs due to bandgap discontinuity of the junction. Such a junction is called
heterojunction and the device is called double heterostructure.
Prepared by: Ms.Mohanthi K, Dr.Martin Joel Sri Sairam College of Engineering, Anekal Page 2
, Regulation – 2022 (CBCS Scheme) BEC515D– SATELLITE AND OPTICAL COMMUNICATION
LED Structures
Heterojunctions:
Heterojunction is an interface between two adjoining single crystal semiconductors
with different bandgap.
Heterojunctions are of two types, Isotype (n-n or p-p) or Antisotype (p-n).
Double Heterojunctions (DH):
In order to achieve efficient confinement of emitted radiation double heterojunctions are used in
LED structure. A heterojunciton is a junction formed by dissimilar semiconductors. Double
heterojunction (DH) is formed by two different semiconductors on each side of active region.
Fig. 3.1.1 shows double heterojunction (DH) light emitter.
Prepared by: Ms.Mohanthi K, Dr.Martin Joel Sri Sairam College of Engineering, Anekal Page 3
, Regulation – 2022 (CBCS Scheme) BEC515D– SATELLITE AND OPTICAL COMMUNICATION
The crosshatched regions represent the energy levels of free charge. Recombination
occurs only in active In GaAsP 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.
A double heterojunction (DH) structure will confine both hole and electrons to a narrow
active layer. Under forward bias, there will be a large number of carriers injected into active
region where they are efficiently confined. Carrier recombination occurs in small active region
so leading to an efficient device. Another advantage DH structure is that the active region has a
higher refractive index than the materials on either side, hence light emission occurs in an optical
waveguide, which serves to narrow the output beam.
LED configurations
At present there are two main types of LED used in optical fiber links
Surface emitting LED
Edge emitting LED.
Both devices used a DH structure to constrain the carriers and the light to an active layer.
Surface Emitting LEDs
In surface emitting LEDs the plane of active light emitting region is oriented perpendicularly to
the axis of the fiber. A DH diode is grown on an N-type substrate at the top of the diode as shown
in Fig. 3.1.2. A circular well is etched through the substrate of the device. A fiber is then
connected to accept the emitted light.
Prepared by: Ms.Mohanthi K, Dr.Martin Joel Sri Sairam College of Engineering, Anekal Page 4
MODULE V
OPTICAL SOURCES AND DETECTORS
Optical Sources
Optical transmitter coverts electrical input signal into corresponding optical signal.
The optical signal is then launched into the fiber. Optical source is the major component in an optical
transmitter .Popularly used optical transmitters are Light Emitting Diode (LED) and semiconductor
Laser Diodes (LD).
Characteristics of Light Source of Communication
To be useful in an optical link, a light source needs the following characteristics
It must be possible to operate the device continuously at a variety of temperatures for
many years.
It must be possible to modulate the light output over a wide range of modulating
frequencies.
To couple large amount of power into an optical fiber, the emitting area should be small.
To reduce material dispersion in an optical fiber link, the output spectrum should be
narrow.
The power requirement for its operation must be low.
The light source must be compatible with the modern devices.
1
Prepared by: Ms.Mohanthi K, Dr.Martin Joel Rathnam Sri Sairam College of Engineering, Anekal Page
, Regulation – 2022 (CBCS Scheme) BEC515D– SATELLITE AND OPTICAL COMMUNICATION
The optical output power must be directly modulated by varying the input current to
the device.
Better linearity of prevent harmonics and intermodulation distortion.
High coupling efficiency.
High optical output power.
High reliability.
Low weight and low cost.
Two types of light sources used in fiber optics are light emitting diodes (LEDs) and
laser diodes (LDs).
Light Emitting Diodes (LEDs)
p-n Junction
Conventional p-n junction is called as homojunction as same semiconductor material
is used on both sides junction. The electron-hole recombination occurs in relatively layer = 10
μm. As the carriers are not confined to the immediate vicinity of junction, hence high current
densities can not be realized.
The carrier confinement problem can be resolved by sandwiching a thin layer ( 0.1 μm)
between p-type and n-type layers. The middle layer may or may not be doped. The carrier
confinement occurs due to bandgap discontinuity of the junction. Such a junction is called
heterojunction and the device is called double heterostructure.
Prepared by: Ms.Mohanthi K, Dr.Martin Joel Sri Sairam College of Engineering, Anekal Page 2
, Regulation – 2022 (CBCS Scheme) BEC515D– SATELLITE AND OPTICAL COMMUNICATION
LED Structures
Heterojunctions:
Heterojunction is an interface between two adjoining single crystal semiconductors
with different bandgap.
Heterojunctions are of two types, Isotype (n-n or p-p) or Antisotype (p-n).
Double Heterojunctions (DH):
In order to achieve efficient confinement of emitted radiation double heterojunctions are used in
LED structure. A heterojunciton is a junction formed by dissimilar semiconductors. Double
heterojunction (DH) is formed by two different semiconductors on each side of active region.
Fig. 3.1.1 shows double heterojunction (DH) light emitter.
Prepared by: Ms.Mohanthi K, Dr.Martin Joel Sri Sairam College of Engineering, Anekal Page 3
, Regulation – 2022 (CBCS Scheme) BEC515D– SATELLITE AND OPTICAL COMMUNICATION
The crosshatched regions represent the energy levels of free charge. Recombination
occurs only in active In GaAsP 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.
A double heterojunction (DH) structure will confine both hole and electrons to a narrow
active layer. Under forward bias, there will be a large number of carriers injected into active
region where they are efficiently confined. Carrier recombination occurs in small active region
so leading to an efficient device. Another advantage DH structure is that the active region has a
higher refractive index than the materials on either side, hence light emission occurs in an optical
waveguide, which serves to narrow the output beam.
LED configurations
At present there are two main types of LED used in optical fiber links
Surface emitting LED
Edge emitting LED.
Both devices used a DH structure to constrain the carriers and the light to an active layer.
Surface Emitting LEDs
In surface emitting LEDs the plane of active light emitting region is oriented perpendicularly to
the axis of the fiber. A DH diode is grown on an N-type substrate at the top of the diode as shown
in Fig. 3.1.2. A circular well is etched through the substrate of the device. A fiber is then
connected to accept the emitted light.
Prepared by: Ms.Mohanthi K, Dr.Martin Joel Sri Sairam College of Engineering, Anekal Page 4
