This document discusses different types of optical modulation techniques, including direct modulation and external modulation. Direct modulation involves superimposing the modulating signal directly onto the driving current of the light source, such as a laser diode. External modulation separates the light generation and modulation processes, where a continuous wave light source is modulated externally after generation. Some key advantages of external modulation are wider modulation bandwidth and better performance, though it is more complex and expensive. The document also covers modulation parameters, direct and analog modulation, limitations of direct modulation, and external modulation techniques like Mach-Zehnder and electro-absorption modulators.

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Modulation Formats : Direct
and External Modulation
S u b m i t t e d B y : P r a j j w a l R o h e l a
M . E ( E . C . E ) – R e g u l a r – 1 s t S e m e s t e r
D a t e & Ti m e : 0 4 . 0 2 . 2 0 2 1 ( 11 . 3 0 A M - 1 . 0 0 P M )

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Table of Content:
 I n t r o d u c t i o n t o M o d u l a t i o n
 N e e d o f M o d u l a t i o n
 Ty p e s o f O p t i c a l M o d u l a t i o n
 M o d u l a t i o n Ty p e s
 I m p o r t a n t p a r a m e t e r s u s e d t o
c h a r a c t e r i z e a n d c o m p a r e
d i f f e r e n t m o d u l a t o r s
 D i r e c t M o d u l a t i o n
 D i r e c t A n a l o g M o d u l a t i o n
 L E D A n a l o g M o d u l a t i o n
 M o d u l a t i o n o f L E D
 M o d u l a t i o n o f L A S E R D i o d e
 L A S E R N o i s e
 L i m i t a t i o n s o f D i r e c t
M o d u l a t i o n
 E x t e r n a l O p t i c a l M o d u l a t i o n
 M a c h - Z e h n d e r m o d u l a t o r
 P a r a m e t e r s t o c h a r a c t e r i z e
p e r f o r m a n c e o f o p t i c a l
m o d u l a t i o n
 C h a r a c t e r i s t i c s o f M a c h –
Z e h n d e r M o d u l a t o r
 E l e c t r o - A b s o r p t i o n ( E A )
M o d u l a t o r
 C h a r a c t e r i s t i c s o f E A
M o d u l a t o r
 A d v a n t a g e s o f E A M o d u l a t o r
 I n t e g r a t i o n o f E A M o d u l a t o r
w i t h L D
 S u m m e r y o f O p t i c a l
M o d u l a t i o n Te c h n i q u e s
 R e f e r e n c e s 2

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Introduction to Modulation
 Mo du lation is th e pro cess o f en cod ing inf or mation fr o m a
messag e so u r ce in a w ay th at is su itab le f o r tr an smissio n .
 I n th e mo du lation p rocess, a p arameter of th e carr ier w ave
( su ch as amp litu d e, fr eq u en cy o r p h ase) is v ar ied in
acco r d an ce w ith th e mo d u latin g sig n al .
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Needs of Modulation
A communication link is established by transmission of information
reliably.
Optical modulation is embedding the information on the optical carrier
for this purpose.
The information can be digital (1,0) or analog (a continuous waveform).
The bit error rate (BER) is the performance measure in digital systems.
The signal to noise ratio (SNR) is the performance measure in analog
systems.
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Types of Optical Modulation
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Direct Modulation and External Modulation
Direct modulation is done by superimposing the modulating (message) signal on the driving
current.
External modulation is done after the light is generated; the laser is driven by a dc current
and the modulation is done after that separately.
Both these schemes can be done with either digital or analog modulating signals.
In external modulation, the laser emits continuous wave (CW) light and the modulation is done
in the fiber.

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Important parameters used to characterize
and compare different modulators
Modulation efficiency: Defined differently depending on whether we
modulate intensity, phase or frequency.
For intensity it is defined as (Imax – Imin)/Imax.
Modulation depth: For intensity modulation it is defined in decibel by 10
log (Imax/Imin).
Modulation bandwidth: Defined as the high frequency at which the
efficiency has fallen by 3dB.
Power consumption: Simply the power consumption per unit bandwidth
needed for (intensity) modulation.
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Direct Modulation
It is 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.
The message signal (ac) is superimposed on the bias current (dc) which
modulates the laser.
Robust and simple, hence widely used.
Issues: laser resonance frequency, chirp, turn on delay, clipping and laser
nonlinearity
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Direct Analog Modulation
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LED LASER

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Analog LED Modulation
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Modulation of LED
The frequency response of an LED depends on:
1- Doping level in the active region
2- Injected carrier lifetime in the recombination region, .
3- Parasitic capacitance of the LED
Electrical current is directly proportional to the optical power, thus we can define electrical bandwidth
and optical bandwidth, separately.
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Modulation of Laser Diodes
Internal Modulation: Simple but suffers from non-linear effects.
External Modulation: for rates greater than 2 Gb/s, more complex, higher
performance.
Most fundamental limit for the modulation rate is set by the photon life time in the
laser cavity:
Another fundamental limit on modulation frequency is the relaxation oscillation
frequency given by:
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Laser Noise
 Modal (speckle) Noise: Fluctuations in the distribution of energy among
various modes.
 Mode partition Noise: Intensity fluctuations in the longitudinal modes of a
laser diode, main source of noise in single mode fiber systems.
 Reflection Noise: Light output gets reflected back from the fiber into the laser,
couples with lasing modes, changing their phase, and generating noise peaks.
Isolators & index matching fluids can eliminate these reflections.
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Limitations of Direct Modulation
Turn on delay and resonance frequency are the two major factors that
limit the speed of digital laser modulation.
Saturation and clipping introduces nonlinear distortion with analog
modulation (especially in multi carrier systems).
Nonlinear distortions introduce second and third order
intermodulation products.
Chirp: Laser output wavelength drifts with modulating current.
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External Optical Modulation
Modulation and light generation are separated.
Offers much wider bandwidth up to 60 GHz.
More expensive and complex.
Used in high end systems.
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Mach- Zehnder Modulator
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Parameters to characterize performance of
optical modulation
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Modulation Depth
Bandwidth
Insertion Loss
Degree of Isolation
Power Handling
Induced Chirp

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Mach- Zehnder Modulator
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Applying voltages to electrodes to change the refractive indices of Light paths 1&2.
The optical paths of 1&2 vary with the applied voltage.
In phase: strong output light; out of phase: weak output.
Output Light is then modulated by voltage signal.

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Continue….
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Characteristics of Mach-Zehnder modulator
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Material Used : LiNbO3
Modulation Depth : better than 20 dB
Bandwidth : Could be 60 GHz
Insertion Loss > = 4 dB
Power Handling : 200 mW
Induced Chirp : Negligible

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Electro-absorption (EA) Modulator
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EA modulator is a semiconductor device with the same structure as the
laser diode.
In laser diodes, we inject large enough current to achieve stimulated
emission. While in EA modulator, we apply electric field (reverse bias) to
change the absorption spectrum. No carriers are injected into the active
region. However, carriers are generated due to absorption of light.

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Continue….
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When the P-N structure in LED is reverse
biased, it becomes light absorption .
At zero-bias, absorption is weak. Under
strong reverse-biased, absorption is strong.
Light intensity is then modulated by the
voltage signal.

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Characteristics of EA Modulator
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Material : Semiconductor QWs
Modulation depth : better than 10 dB
Bandwidth : Could be 40 GHz
Insertion Loss : Almost Zero
Power Handling : 1 mW
Induced Chirp : Negligible
Operation Voltage : 2 V

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Advantages of EA Modulators
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Zero biasing voltage
Low driving voltage
Low/negative chirp
High bandwidth
Integrated with DFB

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Integration of EA Modulator with LD
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Summery of Optical Modulation Techniques
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Direct Modulation External Modulation
In this modulation type, output power of
device depends directly on input drive current.
This means light is emitted from the device
when "1 (binary one)" is being transmitted
and no light is emitted when "0 (binary zero) "
is being transmitted.
This optical modulation is simple. It is
cheaper as no complex circuitry is involved
during modulation process.
This method is slower compare to indirect or
external modulation type. It can be used
below 3 GHz.
In this method, external device is incorporated
to modulate the intensity/phase of the light
source. Here light source is kept ON and
external modulator is used which functions as
switch/shutter. This switch is controlled by
information to be transmitted.
It is much faster in processing. It can be used
with high power laser devices. It can be
employed in high speed applications e.g. long
haul telecom or cable TV head ends.
It is more expensive. High frequency RF
modulation circuit is required for operation
which usually will be complex.

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References
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https://www.rfwireless-world.com/Terminology/Direct-modulation-vs-
External-modulation.html
http://www.ece.ucy.ac.cy/courses/ece645/docs/08_modulators_2015.pdf
https://iopscience.iop.org/article/10.1088/1742-6596/1038/1/012035/pdf
https://en.wikipedia.org/wiki/Optical_modulator

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Thank You

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Direct Modulation and External Modulation
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Queries