Compare of direct modulation ,external modulation and
principles of coherent transmission systems
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By: 
Sura Sa’ad
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CONTENTS
1- Fiber optic communication
2- Advantage of Fiber optic communication
3- Optical Distributing and Generating
3-1 Direct Modulation
3-1-1 Advantages
3-1-2 Disadvantages
4- Modulation in communication systems
5- Optical modulator
6- Classification of optical modulators
7- External Modulators
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1_FIBER OPTIC COMMUNICATION:
Communication in which pulses light travels with its original speed i.e. 3*10^8
m/sec from one place to another with the help of optical fiber, and delay in speed
occurs due to reflection ,refraction ,scattering ,dispersion ,macro bending ,macro
bending and intermodal-dispersion.[1]
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2_ADVANTAGE OF FIBER OPTIC
COMMUNICATION:
The biggest advantage is that communication is very quick since data travels with
the speed of light [1]
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3.OPTICAL DISTRIBUTING AND GENERATING:
Several techniques for distributing and generating microwave signals via optical
fiber exist.The techniques may be classified into two main categories namely
Intensity– Direct Detection (IM-DD) and Remote Heterodyne
Detection (RHD) techniques.
The electrical signal at the Head end of the optical link may be one of three kinds
namely, baseband, modulated IF, or the modulated RF signal itself. Whatever the
case,
the aim is to produce appropriate RF signals at the remote station, which meet
the specifications of the wireless application..
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3-1 DIRECT MODULATION:
The simplest method for optically distributing RF signals is simply to directly
modulate the intensity of the light source with the RF signal itself and then to
use direct detection at the photo detector to recover the RF signal. This method
falls under the IM-DD technique [3].
There are two ways of modulating the light source. The laser diode can itself
be modulated directly by using the appropriate RF signal to drive the laser bias
current.
The second option is to operate the laser in continuous wave (CW) mode and
then use an external modulator such as the Mach-Zehnder Modulator
(MZM), to modulate the intensity of the light. In both cases, the modulating
signal is the actual RF signal to be distributed. The RF signal must be
appropriately pre-modulated with data
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3-1-1 ADVANTAGES
The advantage of this method is that it is simple. Secondly, if low dispersion fiber
is used together with a (linearized) external modulator, the system becomes
linear. Consequently, the optical link acts only as an amplifier or attenuator and is
therefore transparent to the modulation format of the RF signal
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3-1-2 DISADVANTAGES:
The disadvantage of this method depends on the fact that only low RF frequency
signals can be generated (distributed). This is so because to generate higher
frequency signals such as mm waves, the modulating signal must also be at the
same high frequency.
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3-1-2 DISADVANTAGES:
A further disadvantage has to do with the fact that analogue applications are
more sensitive to system non-linearity. As a result, linearity requirements of system
components are more stringent in IMDD based RoF systems. For instance, drive
amplifiers must compensate for incoherent static and dynamic non
frequencies are involved. This leads to more complex systems.[4]
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4_MODULATION IN COMMUNICATION SYSTEMS:
Modulation is defined as a process by which any characteristics of a wave is
varied as a function of the instantaneous value of another wave.The first wave
which is a high frequency sine wave is known as carrier wave. Second wave is
known as modulating wave and the resultant wave is known as the modulated
wave. [1]
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5_OPTICAL MODULATOR :
An optical modulator is a device which is used to modulate a beam of light.The
beam may be carried over free space, or propagated through an optical
waveguide. Depending on the parameter of a light beam which is manipulated,
modulators may be categorized into amplitude modulators, phase modulators,
polarization modulators etc. Often the easiest way to obtain modulation of
intensity of a light beam, is to modulate the current driving the light source, e.g.
a laser diode.This sort of modulation is called direct modulation, as opposed to
the external modulation performed by a light modulator. For this reason light
modulators are, e.g. in fiber optic communications, called external light
modulators.
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5_OPTICAL MODULATOR :
With laser diodes where narrow linewidth is required, direct modulation is avoided
due to a high bandwidth "chirping" effect when applying and removing the
current to the laser[2]
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6_CLASSIFICATION OF OPTICAL MODULATORS:
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According to the properties of the material that are used to modulate the light
beam, modulators are divided into two groups: absorptive
modulators and refractive modulators. In absorption coefficient of the material is
changed, in refractive modulators refractive index of the material is changed.
The absorption coefficient of the material in the modulator can be manipulated
by the Franz-Keldysh effect, the Quantum-confined Stark effect , excitonic
absorption, or changes of free carrier concentration. Usually, if several such effects
appear together, the modulator is called an electro-absorptive modulator.
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7_EXTERNAL MODULATORS:
When data rates were in the low gigabit range and transmission distances were
less than 100 km or so, most fiber optic transmitters used directly modulated
lasers. However, as data rates and span lengths grew, waveguide chirp, caused by
turning a laser on and off, limited data rates. Dispersion problems resulted when
the wavelength chirp widened the effective spectral width of the laser. A laser
source with no wavelength chirp and a narrow linewidth provide one solution to
the problem.
This solution took the form of external modulation which allows the laser to be
turned on continuously; the modulation is accomplished outside of the laser
cavity. [4]
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Thank you for listening
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