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AOTF Set Up Manual
Acousto Optic Tunable Filter
1. What is an AOTF?
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Brief Description
Figure 1. Schematic representation of a non-collinear AOTF.
An AOTF acts as an electronically tunable spectral bandpass filter. It is a
solid state electro-optical device with no moving parts. It consists of a crystal in
which acoustic (vibrational) waves, at radio frequencies (RF) are used to
separate a single wavelength of light from a broadband or multi-color source. The
wavelength of light selected is a function of the frequency of the RF applied to
the crystal. Thus, by varying the frequency of the RF, the wavelength of the
separated or filtered light can be varied. This wavelength is independent of
device geometry.
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Technical Details
The most common types of AOTF which operate in the NIR region use a
crystal of Tellurium Dioxide (TeO2) in a so-called non-collinear configuration - the
acoustic and optical waves propagate at quite different angles through the
crystal. Figure 1 is a schematic representation of a TeO2 AOTF. A transducer is
bonded to one side of the TeO2 crystal. This transducer emits vibrations (acoustic
waves) when RF is applied to it.
The frequency of the vibrations equals the frequency of the applied RF. As
these acoustic waves pass through the TeO2, they cause the crystal lattice to be
alternately compressed and relaxed. The resultant refractive index variations act
like a transmission diffraction grating or Bragg diffracter. Unlike a classical
diffraction grating, however, the AOTF only diffracts one specific wavelength of
light, so that it acts more like a filter than a diffraction grating. This is a result of
the fact that the diffraction takes place over an extended volume, not just at a
surface or plane, and that the diffraction pattern is moving in real time. The
Brimrose Corporation of America
19 Loveton Circle, Hunt Valley Loveton Center,
Sparks, MD 21152-9201 USA.
Phone: +1 410 472-7070 . Fax: +1 410 472-7960
E-Mail: [email protected] . Web: http://www.brimrose.com
DOC: SUMan-AOTFIntro-20081212
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AOTF Set Up Manual
wavelength of light that is diffracted is determined by the "phase matching"
condition as described:
Where ∆n is the birefringence of the the TeO2 crystal, Vα and fα are the velocity
and frequency of the acoustic wave, and α is a complex parameter depending
on the design of the AOTF. The wavelength of the light that is selected by this
diffraction can therefore be varied simply by changing the frequency of the
applied RF. As indicated in the figure, the diffracted light intensity is directed into
two first order beams, termed the (+) and (-) beams. These beams are
orthogonally polarized, which is utilized in certain applications. To use the AOTF
as a tunable filter, a beam stop is used to block the undiffracted, broadband light
and the (+) and/or (-) monochromatic light is directed to the experiment. The
angle between the beams is a function of device design, but is typically a few
degrees. The bandwidth of the selected light depends on the device and the
wavelength of operation, and can be as narrow as 1nm FWHM.
Transmission efficiencies are high (up to 98%), with the intensity divided between
the (+) and (-) beams. Another useful and unique feature of the AOTF is its ability
to precisely and rapidly adjust the intensity of the diffracted (filtered) light by
varying the RF power.
2. AOTF Set-Up
AOTF Set-Up with a White Light Source
Polarized
White Light
Source
Collimating Optics
Detector
AOTF
Figure 2. The input light into the AOTF must be collimated with an angle equal or
smaller than the acceptance angle. The polarizers can be eliminated when the
input white light acceptance angle is smaller than the acceptance angle.
Brimrose Corporation of America
19 Loveton Circle, Hunt Valley Loveton Center,
Sparks, MD 21152-9201 USA.
Phone: +1 410 472-7070 . Fax: +1 410 472-7960
E-Mail: [email protected] . Web: http://www.brimrose.com
DOC: SUMan-AOTFIntro-20081212
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AOTF Set Up Manual
A typical wavelength vs. frequency curve for the fabricated AOTF is shown in
Figure 1.
3. Test Data
3.1 Spectral Resolution.
The AOTF device spectral resolution (Full Width Half Maximum) was measured
using a linearly polarized laser light. The optical set-up for this measurement is
shown in Figure 3, and test results are in Figure 5.
Spectral Resolution and Diffraction Efficiency AOTF Set-Up
Detector
Laser
AOTF
Figure 3.
3.2 Optical Surface Finish.
The Surface flatness is λ / 10 with a scratch and dig specification of 20 -10.
3.3 Diffraction Efficiency.
The diffraction efficiency was measure using the same set-up in Figure 3.
with a linearly polarized laser and is shown in Figure 6. The diffraction
efficiency is defined as follow:
I - Is
DE (%) =
* 100%
Io
Where I is the deflected light intensity. Is is the scatter light intensity and the
Io is the input light intensity.
The white light response can be found in Figure 7.
Brimrose Corporation of America
19 Loveton Circle, Hunt Valley Loveton Center,
Sparks, MD 21152-9201 USA.
Phone: +1 410 472-7070 . Fax: +1 410 472-7960
E-Mail: [email protected] . Web: http://www.brimrose.com
DOC: SUMan-AOTFIntro-20081212
AOTF Set Up Manual
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3.4 Electrical Text Data.
The electrical text data of the AOTF device describe the conversion
efficiency from the RF source into the transducer. The following test data was
measures and shown in Figure 8.
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SMITH CHART
POLAR CHART
VSWR CHART
RETURN LOSS CHART
Brimrose Corporation of America
19 Loveton Circle, Hunt Valley Loveton Center,
Sparks, MD 21152-9201 USA.
Phone: +1 410 472-7070 . Fax: +1 410 472-7960
E-Mail: [email protected] . Web: http://www.brimrose.com
DOC: SUMan-AOTFIntro-20081212