OPTICAL MODULATION
Types of Optical Modulation Direct
modulation is modulation is done by superimposing the modulating (message) signal on the driving current
External
modulation is modulation is done after the light is generated; the laser is driven by a dc source and it is modulated using an external modulator
Both
these schemes may be implemented with either digital or digital or analog analog modulating modulating signals
Direct Modulation of Semiconductor Laser The
simplest and cheapest method of EO conversion is that of directly modulating a CW laser
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
The
message signal (ac) is superimposed on the bias current (dc) which modulates the laser
Robust
and simple, hence widely used
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 I bias using a DC source
The
The
RF signal is then superimposed on the DC source
output optical power is related to the input current I by the following relation:
Direct Modulation of Semiconductor Laser Here ηL
h
denotes Plank’s constant,
f is
q
denotes the laser’s quantum efficiency,
the frequency of the emitted photon and
is an electron’s charge
The
laser’s quantum efficiency is defined as the average number of photons generated per electron
Direct Analog Modulation
Direct Digital Modulation Optical Power P(t)
(P) I th I 1
I 2 I Current ( I) ( t )
t
t
Issues With Direct 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 (< 1 Gbps)
Changes
in modulating signal changes the carrier density which in turn changes the refractive index
The
changes in refractive index causes variations in the center frequency of the laser – chirp
Delay
between the time electrical power is applied to the time laser starts to produce coherent light – Turn on delay
The Chirped Pulse Modulation - Chirp Issues With Direct
This pulse increases its frequency linearly in time (from red to blue)
External Optical Modulation
Modulation Offers
and light generation are separated
much wider bandwidth
Expensive
up
to 60 GHz
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. Mach-Zehnder Modulator (MZM) 2. Electro-absorption Modulator (EAM)
Mach-Zehnder modulator
Single
drive MZM
Mach-Zehnder modulator v Pi
Applying
mode
Po
V will cause a phase shift for the propagating
Mach-Zehnder modulator Applied
voltage causes variation in propagation constant.
u ( x, y, z ) u ( x, y )e i z • where k = constant
2 neff
neff neff V 0 kV
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 Pout A
2 out
e
i1
e
i 2
2
Ain2 4
e
i1
e
i 2
2
2
cos 1 cos 2 sin 1 sin 2
cos 2 1 cos 2 2 2 cos 1 cos 2
sin 2 1 sin 2 2 2 sin 1 sin 2
P out
P in
2
1 cos 1 2 2
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 Aluminum Arsenide (InAlAs)
Electroabsorption Modulator
D.G. Moodie, A.D. Ellis, C.W. Ford, Generation of 6.3 ps optical pulses at a 10 GHz repetition rate using a packaged electroabsorption modulator and dispersion decreasing fibre. Electron. Lett. 30(20), 1700 (1994)
Electroabsorption Modulator The
mathematical relationship of the output optical field of an EAM versus the input optical field can be written as:
Where
() and () represent the input and output optical fields respectively,
represents
the power transfer function of the modulator, while is the chirp factor
Electroabsorption Modulator The
power transfer function of the EAM can be written as:
Where
D(t) is
EAMs
m is the modulation index of the modulator
the instantaneous value of the data signal applied to it
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)
Electroabsorption Modulator EAM
can operate with much lower voltages (a few volts instead of ten volts or more)
They
can be operated at very high speed; a modulation bandwidth of tens of gigahertz can be achieved, which makes these devices useful for optical fiber communication
A
convenient feature is that an EAM can be integrated with distributed feedback laser diode on a single chip to form a data transmitter in the form of a photonic integrated circuit
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