COMPONENTS OF
OPTICAL
COMMUNICATION
SYSTEMS-TRANSMITTER
Optical Transmitter – Continuous Wave Laser
A laser is a device that emits or amplifies coherent radiations in
the infrared, visible or ultraviolet region of the spectrum
A laser source exploits the principle of stimulated emission,
where excited electrons in a higher energy state are stimulated
by an impinging photon
The external pumping stimulates the electrons of the atoms to
jump to higher energy levels, a condition which is generally
known as population inversion
Optical Transmitter – Continuous Wave Laser
Optical Transmitter – Continuous Wave Laser
The electrons in the excited state can also emit photons
spontaneously, i.e without any stimulation
This spontaneous emission of photons is a source of noise in
lasers
For a laser source to emit light continuously, the rate of
stimulated emission should always be higher than the rate of
spontaneous emission:
where h is Plank’s constant, f is the frequency of the light
emitted, kB is the Boltzmann constant and T is the absolute
temperature
Continuous Wave Semiconductor Laser
In case of semiconductor lasers, intrinsic semiconductor
materials constitute the gain medium
Since intrinsic semiconductor materials have a low
concentration of free electrons and holes, they are doped with
impurities to form n-type and p-type materials, which have a
high concentration of electrons and holes, respectively
When the n-type and p-type materials are brought in contact
with each other, a p-n junction is formed
Every time an electron crosses the junction and diffuses into a
hole, it releases energy in the form of light
Continuous Wave Semiconductor Laser
The wavelength of the emitted light is dependent upon the
electric field strength across the junction, which in turn can be
controlled by the level of doping used in the n-type and p-type
materials
Continuous Wave Semiconductor Laser
In case of semiconductor lasers, light reflection required for
optical feedback is achieved through the cleaved ends of the
semiconductor block
It is the change of refractive index between the laser end and
the outside material, that typically reflects 30% of light back to
the gain medium
Semiconductor Laser Noise
Like all other semiconductor components, lasers also generate
noise, which is commonly referred to as Relative Intensity
Noise (RIN)
The RIN terminology collectively represents the noise due to
variations in the intensity, phase and frequency of the laser light
There are two main reasons for these variations, namely
spontaneous emission and electron-hole combination
Semiconductor Laser Noise
Again, spontaneous emission is a phenomenon, where the
excited electron in a higher energy state jumps to a lower
energy state without stimulation, hence releasing a photon of
random phase and frequency
The electron-hole combination, which is also termed as Shot
Noise occurs due to the filling of holes with the free electrons in
the semiconductor material
Distributed Feedback Semiconductor Laser
Most common type of semiconductor lasers used in optical
communications is constituted by the Distributed Feedback
(DFB) Laser
As the terminology implies, the feedback in case of DFB lasers
is achieved not only by the end facets, but it is distributed along
the entire length of the active region
The reflection of light is achieved through appropriate grating,
which is composed of portions of alternate low and high
refractive indices repeated periodically along the length of the
semiconductor
Distributed Feedback Semiconductor Laser
Due to periodicity of the high/low-index grating, the reflected
light of a certain wavelength may add constructively due to
interfering phase-coherently and hence the laser emitted
predominantly consists of light of this particular wavelength
Direct Modulation of Semiconductor Laser
The simplest and cheapest method of EO conversion is that of
directly modulating a CW laser
As mentioned earlier, semiconductor lasers are biased
appropriately in order for them to emit light
If the electronic signal to be transmitted over the fiber is applied
to an appropriately biased laser, the output optical power of the
laser varies almost linearly with the signal applied to it
However, there is a limited linear region in the laser’s response
to an RF voltage applied to it
Direct Modulation of Semiconductor Laser
Direct Modulation of Semiconductor Laser
In order to accommodate the entire peak-to-peak variations in
the input electronic signal, the laser has to be biased to the
operating point of Ibias using a DC source
The RF signal is then superimposed on the DC source
The output optical power is related to the input current by the
following relation:
Direct Modulation of Semiconductor Laser
Here ηL denotes the laser’s quantum efficiency,
h denotes Plank’s constant,
f is the frequency of the emitted photon and
q is an electron’s charge
The laser’s quantum efficiency is defined as the average
number of photons generated per electron
Pulsed Semiconductor Laser
In baseband optical communication systems relying on TDM for
transmitting multiple channels typically pulsed semiconductor
lasers are used
The optical pulses are generated by gain switching applied to
the DFB laser diode (LD) (shown in next slide)
Gain switching uses a periodic high-frequency sinusoidal signal
to generate optical pulses of the same repetition rate as the
applied sinusoidal signal
Pulsed Semiconductor Laser
Pulsed Semiconductor Laser
The DFB LD is biased to an operating point that is just below its
threshold level, where only spontaneous emission is released
by the laser
When for example a sinusoidal RF signal is superimposed on
the DC bias, the positive voltage cycle of the sinusoidal signal
adds to the DC bias and increases the current beyond the
threshold level
Hence the DFB LD’s stimulated emission is triggered, which we
might refer to as lasing
Pulsed Semiconductor Laser
During lasing period, a large number of photons are generated
which results in a fall of the carrier density below the threshold
level
Therefore, the DFB LD stimulated emission is curtailed before
the positive cycle of the RF signal is completed
Then again, the positive cycle of the RF signal is followed by a
negative cycle, during which the injection current is reduced to
a low level
Hence, the carrier density is increased during the period, when
the DFB LD is not lasing
Pulsed Semiconductor Laser
On the next positive cycle, another laser pulse is emitted
This process continues until the electronic signals applied to
the laser are switched off
Therefore, the process of gain-switching can be used for the
efficient generation of short duration optical pulses at the
repetition rate of the applied sinusoidal RF signal
External Modulation
Although the direct modulation of semiconductor lasers is a
low-complexity and cost-effective method of generating
intensity modulated optical signals, it is limited to low-frequency
electronic signals
Directly modulating a laser at high frequencies that are close to
the laser’s relaxation resonance frequency, increases the
generation of laser RIN, which then gives rise to variations in
the phase of the optical signal, an effect termed as phase noise
In case of digital optical communications transmitting optical
pulses over long fibers, the laser’s phase noise tends to
become the factor limiting the maximum distance before
regeneration is required.
External Modulation
In order to avoid the impairments imposed by direct modulation
of the laser using high-bandwidth signals, typically external
modulators are used
We will study two types of external modulators that are most
commonly used:
1. Single Drive Mach-Zehnder Modulator (MZM)
2. Electro-absorption Modulator (EAM)
Mach-Zehnder Modulator
The most popular commercially used type of modulator
Mach-Zehnder Modulator
In order to achieve amplitude modulation, the optical input of
the MZM is split in two paths, path 1 and path 2 with the aid of
the optical waveguides
Initially, the optical signals in each path have the same phase
∅1 and ∅2 , therefore, ∅1 − ∅2 = 0
Each optical waveguide is surrounded by electrodes which are
connected to the DC bias voltage and the modulating signal
voltage
These voltages are used to vary the phase of the optical signal
in in path 1 and path 2 with the aid of the so-called linear
electro-optic effect
Mach-Zehnder Modulator
This electro-optic phenomenon typically occurs in nonlinear
optical mediums such as optical crystals, where the application
of an external electric field changes the refractive index of the
crystal
When light is passed through such a crystal, its phase is
modulated due to variation in the refractive index caused by the
amplitude variations of the applied modulating voltage
In the absence of the modulating voltage, the DC bias voltages
keep the refractive index of the crystals at values such that no
phase variation is imposed on the signals in path1 and path2
Mach-Zehnder Modulator
With the application of the modulating voltage, the phase of the
optical signals in path 1 and path 2 changes in proportion to the
amplitude of the applied modulating voltage
The phase shifts induced by the drive signals in the two arms
are of the opposite sense, but equal in magnitude
When the phase-modulated optical signals in path1 and path2
are superimposed, they result in an optical signal whose
amplitude depends upon the phase difference
So difference in the phase ∅1 and ∅2 results in amplitude
variations of the optical signal at the output of the MZM
Mach-Zehnder Modulator
The optical power at the output of the MZM in terms of the input
optical power may be written as:
𝑃𝑜 𝑡 = 𝑃𝑖 𝑡 ∙ 𝑐𝑜𝑠 2 ∆∅ 𝑡
Where 𝑃𝑖 (𝑡), 𝑃𝑜 (𝑡) and ∅(𝑡) are the input optical power, the
output optical power and the total phase difference between the
optical signals in the two arms of the MZM
The phase difference in the two arms can be written as:
Electroabsorption Modulator
The second major type of optical intensity modulators is
constituted by the Electro-absorption Modulator (EAM)
In EAM, the absorption coefficient of a material is modulated in
response to the external voltage
The light-absorption of the material directly modulates the
intensity of the optical signal passing through it
EAMs are primarily manufactured using semiconductor
materials, such as for example Indium Gallium Arsenide
(InGaAs) and Indium Aluminium Arsenide (InAlAs)
Electroabsorption Modulator
The mathematical relationship of the output optical field of an
EAM versus the input optical field can be written as:
Where 𝐸𝑜 (𝑡) and 𝐸𝑖 (𝑡) represent the input and output optical
fields respectively,
𝑇𝐹𝐸𝐴𝑀 represents the power transfer function of the modulator,
while 𝜌 is the chirp factor
Electroabsorption Modulator
The power transfer function of the EAM can be written as:
Where m is the modulation index of the modulator
D(t) is the instantaneous value of the data signal applied to it
EAMs are eminently suitable for integration with the
semiconductor lasers and PDs used in the optical link since all
of them are manufactured from Indium Gallium Arsenide
(InGaAs) and Indium Aluminium Arsenide (InAlAs)
