Download 1 Electro-optic modulators vs. acousto

Electro-optic modulators vs. acousto-optic modulators
Electro-optic modulators (EOM) and acousto-optic modulators (AOM) are optical devices that can be used
for controlling the intensity of a laser beam. Being driven by an electrical signal and working without
moving parts makes them faster and more precise than mechanical control devices. However, their
principle of operation is based on completely different physical effects, which again explains their different
optical properties.
EOM
EOMs use the linear electro-optic or Pockels effect, which describes the variation of the refractive index of
an optical medium under the influence of an external electrical field. In this case certain crystals become
birefringent in the direction of the optical axis, which is isotropic when no voltage is applied. Thus, the
state of linear polarization of light can be manipulated: When voltage is applied to the crystal the Pockels
effect introduces a phase shift between two polarization components of the propagating light, whereas the
polarization remains unchanged when no voltage is applied.
This way the Pockels effect makes the crystal an electrically switchable and adjustable wave plate. The
magnitude of this phase shift scales linearly with the applied voltage. Positioned between two crossed
Polarizers, the Pockels cell forms an electro-optical modulator which allows for continuous intensity
adjustment of a laser beam.
Switching speed and modulation frequency are limited by the high voltage driver electronics and can be
faster than 5ns and several hundred kilo Hertz.
The aperture size is only restricted by the electro-optical crystals available and has no influence on the
switching speed of the modulator.
High voltage driver
Polarizer
Polarizer
Pockels
cell
Pockels
cell
No voltage applied
λ/2 - voltage applied
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AOM
Acousto-optic modulators (AOM) introduce a periodic modulation of the refractive index in a transparent
medium, of which light is scattered similar to the Bragg diffraction. The periodic index modulation is
generated by sound waves which form a periodic density grating when propagating through the medium.
The sound waves are created by a Piezo electric transducer which in turn is driven by a radio frequency
signal. An acoustic absorber on the other end of the crystal prevents the acoustic wave from travelling
back to the transducer.
Due to the Bragg diffraction the laser beam changes its direction slightly. Therefore, one has to distinguish
between the “transmission” in the original beam direction and the “efficiency” which gives the fraction of
the original beam diffracted into the first order beam. The intensity of the sound wave determines the
efficiency of the AOM and is therefore used to modulate the light intensity.
The switching speed of an AOM is limited by the time the sound wave needs to cross the beam diameter.
So in order to achieve fast modulation, the beam diameter has to be small which on the other hand
generates a conflict between the light intensity and the laser induced damage threshold of the modulator.
Unlike to ordinary Bragg diffraction, the light is scattered from a moving refractive index grating, which
generates a slight frequency shift of the diffracted light, equal to the frequency of the sound wave.
Piezo electric
transducer
Radio
frequency
generator
Crystal or
Glass
Diffracted first
order beam
Blocked beam
Absorber
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Comparison of EOM and AOM in terms of application / properties
Application / properties
EOM
AOM
Q-switching
Yes, possible
Yes, possible
Intensity modulation
Yes, possible
Yes, possible
Phase modulation
Yes, possible
Not possible
Transmission
98%
transmission: 98%
efficiency: 90%
Extinction ratio
3000:1
2000:1
Rise / fall time of applied
modulation
driver limited (approx. 5ns)
beam diameter limited
-> conflict with laser damage
induced threshold
spectral filtering
up to 500kHz – 1MHz (Pockels
Cells)
up to 20 MHz (Modulators)
Not possible
diffraction
Not possible
Yes, possible
spectral range
250nm - 5µm
250nm-1600nm
9.3µm, 11µm
Modulation Bandwidth
3
Up to 50 – 100 MHz
Yes, possible