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We now talk about optical sources for the design of optical transmitters. The optical source
should have the capability of encoding the information (‘0’ and ‘1’) as multiple amplitudes or
phase levels or a combination of both. There are several critical pa
in the datasheets of the sources that

The emission wavelength of the source, since the attenuation of the channel
case) is dependent on the wavelength. It is also a relevant
wavelength source or a multiple/tunable wavelength source, which could be critical in
cases where multiple carrier frequencies are required.
Modulation: The source should have the capability of amplitude and phase modulated
Additionally, the bandwidth,
direct modulation, since that decides the maximum data rate provided by the source.
Power: The power emitted by the source is an important parameter. It is also relevant to
look at the wall-plug efficiency

We now talk about optical sources for the design of optical transmitters. The optical source
should have the capability of encoding the information (‘0’ and ‘1’) as multiple amplitudes or
phase levels or a combination of both. There are several critical parameters, which can be found
sources that need to be taken into account for choosing a relevant source.

of the source, since the attenuation of the channel

is dependent on the wavelength. It is also relevant to decide whether it is a single
wavelength source or a multiple/tunable wavelength source, which could be critical in
cases where multiple carrier frequencies are required.

The source should have the capability of amplitude and phase-modulated
bandwidth or the speed of modulation is also important
since that decides the maximum data rate provided by the source.
the power emitted by the source is an important parameter. It is also relevant to the efficiency of the source, which is the ratio of

We now talk about optical sources for the design of optical transmitters. The optical source
should have the capability of encoding the information (‘0’ and ‘1’) as multiple amplitude or
rameters, which can be found
need to be taken into account for choosing a relevant source.

of the source, since the attenuation of the channel (fiber in our
decide whether it is a single
wavelength source or a multiple/tunable wavelength source, which could be critical in
The source should have the capability of amplitude and phase modulation.
or the speed of modulation is also important, in case of
since that decides the maximum data rate provided by the source.
the power emitted by the source is an important parameter. It is also relevant to
the ratio of electrical power


consumed to the optical power generated. It is an important parameter, since in an optical
communication system it is desirable to have low power consumption.
Noise: The noise characteristics of the source are important parameters. The source may
have two kinds of noise – amplitude noise and phase noise. The phase noise is
characterized by the linewidth of the source. It is also important to look at whether
The source is a broadband emitter or a narrowband emitter.
Capability to couple into a fiber: The divergence of the beam emitted from the source
and the ease of coupling the beam into an optical fiber is also a relevant property of the
source.
Semiconductor optical sources are typically considered for optical communication. The possible
sources are LEDs and laser diodes, and optical communication system became a huge
commercial success only because of the availability of compact, reliable, cost-effective
semiconductor sources. One may find several sources which may have all the above-listed
features, but for commercial deployment, the most important parameters are compactness,
reliability and cost-effectiveness along with the requisite speed of modulation. Availability of
reliable, compact, energy-efficient, long lifetime, easily deployable semiconductor sources is one
of the key reasons for the commercial success of optical communication.
Next, we move to the basic working principles of semiconductor sources – light-matter
interaction in semiconductors. The energy levels in a semiconductor are represented by the E-k
diagram, where the x-axis and the y-axis represent the momentum and energy respectively. The
Energy levels in a semiconductor have a distinct valence band and a conduction band. The
The conduction band is expected to have free electrons. When a photon is incident on the
semiconductor, it may excite an electron from the valence band to the conduction band, leaving
behind a hole in the valence band, thus generating an electron-hole pair. This process is called
absorption. The energy of this photon must be equal toEଶ − Eଵ, where E2 is the energy
occupied by the electron in the conduction band and E1 is energy occupied by the hole in the
valence band. Alternatively, an electron from the conduction band may recombine with a hole in
the valence band. This electron-hole recombination is accompanied by the release of the
corresponding energy Eଶ − Eଵ, as a photon. This process is called emission.
The difference of the lowest energy of the conduction band and the highest energy of the valence
a band is known as the band-gap (Eg). Materials for which the momentum corresponding to the
lowest energy in the conduction band and the highest energy in the valence band is the same are
known as direct band-gap materials. The materials for which this condition is not satisfied are
known as indirect band-gap materials. Emission process essentially requires a direct band-gap
structure.

In order to have an emission process, it is necessary to have an electron
electron in the conduction band and the hole in the valence band
can be generated by injecting additional
this carrier injection is bypassing
generates excess electrons in the conduction band and excess holes in the valence band.
electron-hole pairs recombine, thus generating a photon with the energy

be a situation where an electron-
a photon with energy Eଶ − Eଵ can cause the electron

photon of the same energy. This process is kn
three processes can occur due to
Absorption: An incident photon get
generating an electron-hole
where the electron-hole pair generated by the incident photon is extracted in an electrical
circuit, giving rise to a
photons.
Spontaneous emission:
recombines spontaneously (on its own) to emit a photon of


In order to have an emission process, it is necessary to have an electron-hole pair, i
electron in the conduction band and the hole in the valence band. Such extra electron
can be generated by injecting additional carriers (electrons and holes). A practical way of doing
passing a current through a forward-biased p-n junction diode, which
excess electrons in the conduction band and excess holes in the valence band.
hole pairs recombine, thus generating a photon with the energy Eଶ − E

-hole pair is generated through carrier injection and an incident
can cause the electron-hole pair to recombine, thus emitting another

the photon of the same energy. This process is known as stimulated emission. Thus,
light-matter interaction in a semiconductor material.

An incident photon gets absorbed by an electron in the valence band, thus
hole pair. Photodetectors work on the principle of absorption,
hole pair generated by the incident photon is extracted in an electrical
circuit, giving rise to a photocurrent that is proportional to the number of incidents
An electron-hole pair (generated through carrier injection)

spontaneously (on its own) to emit a photon of energy correspond

hole pair, i.e., an extra
. Such extra electron-hole pairs
the practical way of doing
n junction diode, which
excess electrons in the conduction band and excess holes in the valence band. The
Eଵ. There can also
hole pair is generated through carrier injection and an incident
hole pair to recombine, thus emitting another
Thus, the following

matter interaction in a semiconductor material.
absorbed by an electron in the valence band, thus
work on the principle of absorption,
hole pair generated by the incident photon is extracted in an electrical
that is proportional to the number of incidents
hole pair (generated through carrier injection)
corresponding to the

difference between the energy values corresponding to the transitioning states. This process is dependent on the lifetime of the electrons in the excited state (conduction
band).
LEDs work on the principle of spontaneous emission, where a current is passed through
The diode and light are emitted as photons. If the current is modulated in an on-off fashion,
the emitted light is also modulated in a similar fashion, thus realizing the on-off-keyed
the signal generated through direct modulation of an LED.
Stimulated emission: The recombination of an electron-hole pair (generated through
carrier injection) is stimulated by an incoming photon, and as a result, another photon of
the same energy is generated. Laser diodes work on the principle of stimulated emission.
In all these processes, the energy, as well as the momentum, has to be conserved. In order to have
an emission process, it is essential to have excess electrons in the conduction band and holes in
the valence band. This condition is known as population inversion. This population inversion is
generated by passing the forward-biased drive current in the p-n junction diode, which generates
the excess electron-hole pairs and results in emission.
Another difference between spontaneous emission and stimulated emission is that in case of
spontaneous emission, the electron-hole recombination happens randomly, and there is no phase
relationship between two different recombination processes. Thus the photons emitted as a result of
This recombination has random phases. On the other hand, in case of stimulated emission, the
recombination processes are stimulated by another photon, and the photon emitted due to that is
in phase with the incident photon. Thus, the light emitted due to stimulated emission is coherent,
whereas that generated due to spontaneous emission, as in the case of LED, is incoherent.
We now talk about the mathematical formulation of the process. Let n represent the carrier
density, which is the number of carriers per unit volume. The rate at which the carriers recombine is
similar to radioactive decay and can be represented mathematically as,

dn
dt = −
n
τ

Negative sign indicates that the recombination process reduces the carrier density, and τ is the
time-constant of the process which is dependent on several factors. Integrating this equation, we
can find n at any time as,

n(t) = n଴e
ି

ഓ.

Here, n଴ is the initial carrier density (at t = 0) after the carrier injection. As the electron-hole
Recombination process starts, the carrier density decays with a time constant τ.
Emission of photons is not possible in indirect band-gap materials, because it is essential to
conserve the momentum to facilitate the electron-hole recombination, which is not an easy task.

Silicon, for example, is an indirect band
common silicon p-n junction diode does not emit light when employed in electrical circuits such
as rectifiers.

It is important to understand that all electron
a photon. Thus it is essential to find out the fraction of the recombinations that lead to photon
emission, which is the efficiency
photon emission is called Radiative
represented as τ௥

, and the corresponding recombination rate is given as

radiative recombinations are those which result in
Phonons are the discrete units (
visible spectrum. There can be several mechanisms of non-result in light emission. For example, there are certain
semiconductor materials, which could lie between
Electron/hole transitions from the conduction/valence band to these defect states result in
emission, which is non-radiative recombination. Alternatively, there can be a condition where
the energy released by an electron
excited to a further higher energy state. In other words, the electron gains kinetic energy from the

Silicon, for example, is an indirect band-gap material. It is because of this reason that the
n junction diode does not emit light when employed in electrical circuits such

It is important to understand that all electron-hole recombinations do not result in the emission of
Thus it is essential to find out the fraction of the recombinations that lead to a photon
of the system. The electron-hole recombination which results in
Radiative recombinations. The radiative recombination lifetime is

, and the corresponding recombination rate is given as R௥
are those which result in the emission of phonons instead of photons.
Phonons are the discrete units (quanta) of thermal vibrations, and hence do not exist in the
There can be several mechanisms of non-radiative recombination which do not
result in light emission. For example, there are certain defect states (energy levels) in t
which could lie between the valence and conduction bands
Electron/hole transitions from the conduction/valence band to these defect states result in
radiative recombination. Alternatively, there can be a condition where
the energy released by electron-hole recombination is absorbed by another electron that gets
excited to a further higher energy state. In other words, the electron gains kinetic energy from the

cause of this reason that the
n junction diode does not emit light when employed in electrical circuits such

hole recombinations do not result in the emission of
Thus it is essential to find out the fraction of the recombinations that lead to photon
hole recombinations which result in
recombination lifetime is
=

ఛೝ

. The non-
instead of photons.

, and hence do not exist in the
radiative recombination which does not
(energy levels) in the
the valence and conduction bands.
Electron/hole transitions from the conduction/valence band to these defect states result in phonon
radiative recombination. Alternatively, there can be a condition where
the combination is absorbed by another electron that gets
excited to a further higher energy state. In other words, the electron gains kinetic energy from the

process. This phenomenon is known as
radiative recombinations.

The non-radiative recombination lifetime is represented as
defined as R௡௥ =

ఛ೙ೝ
. We know that the non

the system. Hence, we define the
radiative recombination rate and the total recombination rate.

η௜௡௧ =
The non-radiative recombination is
temperature, larger is the probability of phonon emission
lifetime (), which is a combination of radiative and non
defined as follows.

process. This phenomenon is known as Auger recombination. Such processes contribute to non

radiative recombination lifetime is represented as τ௡௥ and the corresponding rate is
We know that the non-radiative recombination reduces the efficiency of
the system. Hence, we define the internal quantum efficiency of the system as the ratio of the
radiative recombination rate and the total recombination rate.

R௥
R௥ + R௡௥
=
n
τ௥ ൗ
n
τ௥ ൗ + n
τ௡௥ ൗ
=
τ௡௥
τ௡௥ + τ௥

radiative recombination is a temperature-dependent quantity because a larger
probability of phonon emission. Another useful parameter, which is a combination of radiative and non-radiative recombination

1
τ
=
1
τ௥
+
1
τ௡௥


Such processes contribute to non-
and the corresponding rate is

radiative recombination reduces the efficiency of
of the system as the ratio of the

, because larger the
. Another useful parameter is the carrier
radiative recombination lifetimes,