Download Acousto-optics Devices for High

WDS'12 Proceedings of Contributed Papers, Part III, 65–68, 2012.
ISBN 978-80-7378-226-9 © MATFYZPRESS
Acousto-optics Devices for High-power Laser Beam
D. Yu. Velikovskiy, V. E. Pozhar
Scientific and Technological Centre of Unique Instrumentation (STC UI RAS), Moscow, Russia.
M. M. Mazur
Federal State Unitary Enterprise (FSUE) “National Research Institute for Physicotechnical and Radio
Engineering Measurements “VNIIFTRI”, Solnechnogorsk, Russia.
Abstract. We present a study of the actuality of acousto-optics devices capable to
withstand high-power laser emission. Some applications and specifics were considered, and
the first assessment of parameters of the devices on KGd(WO4)2 single crystal was made.
New acousto-optics modulator on that crystalline material for non-polarized high-power
light radiation was developed, tested and shown almost 100% of diffraction efficiency.
Introduction
As maximum power of lasers increases that appears a need for power-resistant instruments to
control it. Acousto-optics (AO) devices are widely used for modulation and deflection of laser beam.
However, variety of efficient AO materials is not rich and their "laser" properties are not of the firstpriority for their initial selection. In particular, their radiation resistance is limited by the intrinsic
properties of the crystals. For example, paratellurite crystal (TeO2), which is the most effective and
widely used acousto-optics material, could withstand the radiation exposure up to 2 kW per cell in a
continuous mode, however getting warm. For applications requiring greater radiation exposure it is
still used fused quartz (SiO2), which has very low AO efficiency. As on high acoustic power should be
driven and piezo-transducer in needs of water cooling. Therefore, the problem of searching for
acousto-optically effective materials capable to withstand intensive laser radiation is highly important.
To solve this problem we have investigated the elastic and photo-elastic properties of crystals
used in the laser technologies. The results show that some materials with chemical formula
KRe(WO4)2 [1], where Re is a rear-earth metal, exhibit rather good acousto-optics properties and could
be used for development of the AO devices for laser techniques. We prefer the KGd(WO4)2 [2] single
crystal (short: KGW) because this material is widely used in lasers techniques and widespread. Also
KGW is a crystal with monoclinic system, which provides biaxial optical properties. AO interaction in
biaxial media is still new and needed side of optoelectronics [3], due to opportunity of making other
types of AO devices based on optical biaxiality [4, 5].
AO devices for laser technics have some specifics. Filters need to switch wavelength in
parametric lasers and they are in needs of a very acousto-optically effective medium with prejudice of
the withstand of high power optical emission. Q-swithers with deflectors and modulators should work
with high power laser radiation first of all. Other specifics are shown in Table 1.
Table 1. Specifics of AO devices for laser technics
DEVICES
Tunable optical
filters
AO Q-switch
(in-resonator)
Deflectors
Modulators
Frequency shifters
SPECIFICS
Collinear diffraction
Non-colinear (anisotropic)
diffraction
Running ultrasonic wave
Standing ultrasonic wave
(isotropic diffraction)
Anisotropic diffraction
Isotropic diffraction
Anisotropic diffraction
(out-resonator devices)
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FUNCTIONS
Tunable wavelength in
parametric lasers
Q-switching
Mode locking
Beam deflector
Q-switching
Shifting of optical
heterodyne frequency
VELIKOVSKIY ET AL.: ACOUSTO-OPTICAL DEVICES FOR HIGH POWER LASER BEAM
Observations
To develop an AO device it needs to determine the geometry of the phase-matching and evaluate
the performance. So, optical properties, acoustic field and diffraction efficiency should be known.
According to the monoclinic system, medium has expressed anisotropy, which provides some
difficulties for investigation of it properties Refractive indices of KGW single crystal are well known.
We have measured sound velocities for different acoustical modes and elastic tensor was determined.
Our results show that KGW has the same level of isotropic diffraction efficiency with LiNbO3 single
crystal and anisotropic diffraction efficiency is still under investigation.
Case 1
It is provided all the characteristics, which are necessary for developing acousto-optics devices
based on isotropic diffraction. New AO modulator made of KGd(WO4) single crystal (Fig. 1a) was
developed and tested.
It is double-polarization (Fig. 1b) and could modulate non-polarized light, with maximum driving
HF power on the piezo-transducer P = 4 W at λ = 1.064 μm with the beam diameter d = 2 mm (Fig. 2).
Also the modulator is capable to withstand intensive laser radiation.
Case 2
New types of AO devices on KGd(WO4)2 crystal based on specific look of axial section of
refractive surface (optical biaxiality) are possible. It could be design a “fast” modulator [4] or wideangle modulator with great angular aperture [6]. And we suppose that both ideas could be combined.
(a)
(b)
Figure 1. Acousto-optics modulator on KGd(WO4)2 single crystal (a) and vector diagram of doublepolarized modulator (b).
Figure 2. Diffraction efficiency vs. ultrasound power measured for KGd(WO4)2 modulator.
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VELIKOVSKIY ET AL.: ACOUSTO-OPTICAL DEVICES FOR HIGH POWER LASER BEAM
(a)
(b)
Figure 3. Section of refractive surface (a) and vector diagram of “fast” wide-angle modulator with
large angular aperture (b).
Table 2. Characteristics of “fast” wide-angle modulator with large angular aperture.
KGW
λ = 1.064 µm
λ = 1.55 µm
λ = 5.0 µm
λ = 0.633 µm
1.0 GHz
675 MHz
200 MHz
1.8 GHz
HF frequency
○
○
○
68.2
68.0
68.3
68.9○
Θ2 angle
○
○
○
32.7
33.8
29.3
29.0○
Θ1 angle
BIVO4
λ = 0.633 µm
10 GHz
79.7○
46.0○
It should be noticed that AO device with that vector diagram (Fig. 3b) could work with large
angular aperture beam. For example it’s capable to modulate Gaussian-profiled laser beam with high
diffraction efficiency. Available data are enough to predict some characteristics of such AO deflector
on KGW single crystal. Also we observed BiVO4 single crystal for this configuration. Results are
shown in Table 2.
Unfortunately, that geometry of AO interaction (Fig. 3b) produces high driving frequency. It’s
essential technical problem to make a piezo-transducer to excite ultrasound more than 1 GHz. So we
propose to use this deflector in IR region. It could be seen (Table 2) that driving frequencies for
wavelengths λ = 1.55 µm and λ = 5.0 µm are “normal” for AO devices.
Discussion
We make a comparison of device’s characteristics based on KGW and BiVO4 single crystals. The
second of them is known as a very effective biaxial AO material with expressed temperature
dependence, so probable conclusion – it couldn’t withstand high power laser emission. Could be
noticed that “fast” wide-angle modulator with large angular aperture on BiVO4 single crystals in needs
of 10 GHz driving frequency on λ = 0.633 µm wavelengths, and modulator on KGW needs 1.8 GHz
driving frequency on the same wavelengths. We prefer modulator on KGW, of course it has lower
diffraction efficiency and that means higher driving ultrasound power. But it has lower driving
frequency and could withstand high-power laser emission. According to this it could be in-resonator
device for mode-locking or Q-switching with high (almost 100%) diffraction efficiency. Also it’s
capable to modulate Gaussian-profiled laser beam and that could be useful to minimize AO interaction
time notable make “fast” deflector.
Conclusion
New AO modulator on KGd(WO4)2 single crystal for non-polarized high-power light radiation
was developed and tested. Other opportunities of making deflector on this material based on optical
biaxiality were analyzed. Some characteristics were estimated and two sentences could be interesting
and promising for further work – “fast” wide-angle modulator, which can manage high-power
Gaussian-profiled laser beams and in-resonator device for mode-locking or Q-switching with high
(almost 100%) diffraction efficiency.
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VELIKOVSKIY ET AL.: ACOUSTO-OPTICAL DEVICES FOR HIGH POWER LASER BEAM
Acknowledgments. The present work was supported by the Russian scientific and educational action
program “Kadri” under Contract No П803 and by the Russian’s president State encouragement of leading
scientific groups under Contract НШ-4705.2012.9.
References
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Properties of KY(WO4)2 Single Crystals. Inorganic Materials, 2012, 48, 1, p. 74–80.
[2] M. M. Mazur, D. Yu. Velikovskiy, F. A. Kuznecov, L. I. Mazur, A. A. Pavlyuk, V. E. Pozhar,
V. I. Pustovoit. Elastic and Photoelastic Properties of KGd(WO4)2 Single Crystal. Acoustical Journal,
2012, 58, 6, p. 701–709.
[3] A. Yu. Tchernyatin. Analysis and application of Bragg acousto-optic diffraction in biaxial media // Proc. of
SPIE. 2005, 5953, p. 1–8.
[4] V. I. Balakshiy. Acoustoopticheskie modulatory s anisotropnoy difrakciey sveta (Acousto-optics modulators
with anisotropic diffraction). Izvestiya of the Academy of Sciences of the USSR, physics, 1981, 45, 3,
p. 636–639.
[5] H.-C. Lee and H.E. Meissner Walk-off correction in biaxial crystals, Proc. of SPIE. 2007, 6740, p. 1–12.
[6] H. Lee. Acoustooptic Light Modulation with Large Bandwidth and Angular Aperture. IEEE Transactions on
Ultrasonics, Ferroelectrics and Frequency Control, 1987, Vol. UFFC-34, 4, p. 485–486.
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