The document discusses optical modulators, specifically acousto-optic and electro-optic modulators. It describes how acousto-optic modulators use the acousto-optic effect to diffract and modulate a laser beam using a sound wave, while electro-optic modulators use the electro-optic effect to change an optical property like phase with an electric field. It provides details on the operating principles, materials used, and applications of these two types of optical modulators.

1. Optical Modulator
Course coordinator: Arpan Deyasi
3/3/2021 1
Arpan Deyasi, India

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Optical Modulator
A device which can modulate amplitude, phase or
polarization of optical signal in a controlled manner
so that desired intensity or color can be achieved
electrical signal
optical
modulator
LASER beam
modulated
optical signal

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Optical factors affected
 Refractive index
 Transmission/reflection factor
 Degree of deflection
 Coherency of light

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Why it is required?
Direct modulation of LASER
pulse spreading frequency chirp information loss
solution: optical modulation

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Classification
Optical Modulator
refractive modulator absorptive modulator
electro-optic
acousto-optic
phase amplitude polarization

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Acousto-optic Modulator

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Acousto-optic effect
Change in material permittivity due to mechanical strain
It is a more general effect of
photo-elastic effect
This effects diffracts light which
may be utilized to modulate a
beam

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AOM
Device which can be used for controlling the power
of a laser beam with an electrical drive signal
based on the acousto-optic effect, i.e. the modification
of the refractive index of some crystal or glass material
by oscillating mechanical strain of a sound wave
(photo-elastic effect)

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Source: Kasap
Schematic Diagram

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What AOM basically does?
RF signal is converted into sound waves via crystal
Sound absorber
Sound transducer
RF signal
aperture

11. Photoelastic Effect
2
1
e
p S
n
 
∆ =
 
 
Strain
Refractive index
Change
Photo-elastic
coefficient
The strain changes the density of the crystal and distorts the bonds
(and hence the electron orbits), which lead to a change in the
refractive index n

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Process Strain is generated in a crystal by applied
electric field (piezoelectric effect)
Modulating voltage generates Surface Acoustic
Wave (SAW) via piezoelectric effect
SAW propagates by compression and
rarefaction of the crystal surface
causes periodic variation of density of the medium
causes periodic variation of refractive index
in synchronization wit acoustic wave amplitude

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Acousto-Optic Modulation Regime
Raman-Nath regime ---- Diffraction occurs from a line grating
Occurs at relatively low frequency
[f< 10 MHz]
Smaller acousto-optic interaction
length
Occurs for arbitrary angle of
incidence
Multiple diffracted beam at
output
Source: Kasap

14. Raman-Nath Regime
Wavelength of light
Acoustic wavelength
Acoustic frequency
Acoustic velocity
2
L
λ
Λ
<<
beam length
where
a
v
f
Λ =

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Acousto-Optic Modulation Regime
Source: Kasap
Bragg regime ---- Diffraction occurs from through beam
Occurs at relatively higher
frequency [f< 10 MHz]
Larger acousto-optic
interaction length
Only one diffracted beam at
output
Scattering effect is negligible

16. Bragg Regime
Wavelength of light
Acoustic wavelength
2
L
λ
Λ
>>
beam length

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Features of acousto-optic modulation process
Diffracted beam is increased or decreased by the frequency of
the sound wave (depending on the propagation direction of the
acoustic wave relative to the beam) and propagates in a slightly
different direction
The frequency and direction of the scattered beam depend on
the frequency of the sound wave, whereas the acoustic power is
the control for the diffracted optical power.
For most applications, the slight change of optical frequency is
irrelevant.
Diffraction process may or may not be polarization-dependent,
depending on the device designs

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Acousto-optic modulator

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Properties of acousto-optic modulator
The material should have a high transparency at the relevant
wavelengths, and parasitic reflections should be minimized
High diffraction efficiency is lower for acousto-optic materials
with high elasto-optic coefficients
Depending on the device design, the diffraction efficiency can
be polarization dependent
Input aperture size limited the usable beam radius

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Applications of acousto-optic modulator
Q switching of solid-state lasers
Cavity dumping of solid-state lasers, generating either
nanosecond or ultrashort pulses
Active mode locking is often performed with an AOM
Pulse picker for reducing pulse repetition rate of a pulse train
In laser printers and other devices, an AOM can be used for
modulating the power of a laser beam
AOMs can be used as external modulators in certain laser
communications systems

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Electro-optic Modulator

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Electro-optic effect
Change in optical properties of a material in response to
electric field
If refractive index changes linearly with electric field,
called Pockel’s effect
If refractive index changes non-linearly with electric field,
called Kerr’s effect

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Electro-optic effect
2
0 1 2 ...
n n E E
α α
= + + +
1
n E
α
∆ = Pockel’s effect
2
2
n E
α
∆ = Kerr’s effect

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EOM
Device which can be used for modulating beam of
light by electro-optic effect
Types of Electro-optic modulator
 Amplitude modulator
 Phase modulator
 Frequency modulator
 Polarization modulator
Principle of operation is based on linear electro-optic effect

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Pockel’s cell

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Materials used
Potassium Di-deuterium Phosphate (KDP)
Potassium Titanyl Phosphate (KTP)
β-Barium Borate (BBO)
EOM: half-wave voltage
voltage required for inducing a phase change of π

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Phase Electro-optic modulator
High-performance, low-drive voltage modulator
Increased bandwidth allows for chirp control in high-speed
data transmission
simplest type of electro-optic modulator
polarization of the input beam often has to be aligned with
one of the optical axes of the crystal, so that the polarization
state is not changed

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Electro-optic crystal
Polarizer
Polarizer
Phase Electro-optic modulator
Phase
modulated
output
beam
oscillator

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Polarization Electro-optic modulator
Depending on the type and orientation of the nonlinear
crystal, and on the direction of the applied electric field,
phase delay may depend on polarization direction
Pockel’s cell can be used for modulating the
polarization state

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V(t)
0° quarter
waveplate
45° polarizer
Polarization Electro-optic modulator

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Amplitude Electro-optic modulator
based on a Pockels cell for modifying the polarization state
and a polarizer for subsequently converting this into a
change in transmitted optical amplitude and power

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Vertical electric field
Electro-optic crystal
Polarizer Polarizer
45° -45°
Amplitude Electro-optic modulator

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Properties of electro-optic modulator
Device must have a sufficiently large open aperture,
particularly in cases with high peak powers
Depending on the device design, the polarization of the
incoming beam may or may not be maintained in the
output
Phase modulator may generate unwanted amplitude
modulation, and vice versa. This depends strongly on the
design

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Applications of electro-optic modulator
Modulating the power of a laser beam, e.g. for laser
printing, high-speed digital data recording, or high-speed
optical communications
Q switching of solid-state lasers (where the EOM serves to
block the laser resonator before the pulse is to be emitted)
Active mode locking (where the EOM modulates the
resonator losses or the optical phase with the round-trip
frequency or a multiple thereof)
Switching pulses in pulse pickers, regenerative amplifiers
and cavity-dumped lasers