Procedia Engineering

Procedia Engineering 5 (2010) 299–302
Procedia Engineering 00 (2009) 000±000
Procedia
Engineering
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Proc. Eurosensors XXIV, September 5-8, 2010, Linz, Austria
Determination of nanometer vibration amplitudes by using a
homodyne photorefractive crystal interferometer
Saeid Zamiria*, Bernhard Reitingera,b, Thomas Berera,b, Siegfried Bauerc,
Peter Burgholzera,b
a-Christian Doppler Laboratory of Photoacoustic Imaging and Laser Ultrasonic, TECHCENTER Linz-Winterhafen Hafenstraße 47-51, 4020
Linz, Austria
b-Research Center for Non Destructive Testing GmbH (RECENDT), TECHCENTER Linz-Winterhafen Hafenstraße 47-51, 4020
Linz, Austria
c- Department of Soft Matter Physics, Johannes Kepler Universität, Altenbergerstraße 69, 4040 Linz, Austria
Abstract
We report a low cost and high accuracy interferometric technique for detecting nanometer vibrations by using a
photorefractive crystal interferometer based on two±wave mixing within a Bi12SiO20 (BSO) crystal. The results of small
displacement detection on the sample (1-2nm) and comparison between the sensitivity of Michelson and photorefractive adaptive
interferometers are presented. Furthermore, the effect of an external electric field applied to the photorefractive crystal on the
Signal to Noise Ratio (SNR) and on the vibrometer sensitivity is investigated.
c 2009
2010 Published
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PublishedbybyElsevier
Elsevier
Keywords:PhotoRefracrive Crystal(PRC); Piezoceramic transducer (PZO); Interferometer; Reference beam; Signal beam;
1. Introduction
Non-destructive testing techniques are able to detect position and size of cracks and defects in material. For
industrial applications this can be achieved by utilizing optical interferometers and measuring small ultrasonic
displacements [1,2]. For this purpose, photorefractive crystal (PRC) interferometers based on two±wave mixing
show good sensitivity, especially on the rough industrial surfaces [3]. By using this type of interferometers, one is
able to measure very small displacements in order of several nanometers or even below [4,5]. Also by detecting the
ultrasonic displacements of a sample one is able to measure the thicknesses of various kinds of sheet materials [3].
* Corresponding author. Tel.: +43(0)732 9015-5630; Fax: +43 (0) 732 9015-5618.
E-mail address: [email protected].
c 2010 Published by Elsevier Ltd.
1877-7058 doi:10.1016/j.proeng.2010.09.107
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2. Experimental results
In our experiments, ultrasound waves with different frequencies and amplitudes are generated with a
piezoceramic transducer (PZO, 2mm×5mm×5mm). The transducer has a resonance frequency of 500 kHz and a
maximum stroke of 3µm/150V. As sample, a small circular mirror of diameter 4mm connected to PZO is used. In
figure (1), an image of the PZO structure can be seen.
Piezoceramic transducer
(PZO, 2mm×5mm×5mm)
Connected mirror of 4mm diameter
Fig 1: Image of PZO and the connected mirror
By applying sinusoidal voltages (0.05-5V) on the PZO, displacements in the order of nanometers with
frequencies between 1Hz and 70kHz are generated. To measure the vibrations, we used a Michelson and a
photorefractive crystal interferometer.
For the latter, a 532nm detection laser beam is divided into reference and signal beam [3]. The signal beam is
focused on the sample surface. By surface vibrations, the reflected beam gets modulated and interferes with the
reference beam in a BSO (Bismuth Silicon Oxide: Bi12SiO20) photorefractive crystal with a [110] crystallographic
axis. The crystal has a size of 5mm×5mm×5mm. An external electric field is applied in the [001] direction by
evaporated gold contacts. It is notable that reversing the applied electric filed direction will change the sign of the
photorefractive gain. Silver paste was used to contact wires from a high voltage power supply to the gold electrodes
on both surfaces. We used this BSO crystal because of its high photorefractive gain, its fast response time and the
reasonable cost.
Due to the photorefractive grating generated in the crystal, the reference beam gets diffracted in the direction of
the signal beam. By interfering both beams on a photodiode, one can measure the phase shifts via amplitude
modulations of the laser intensity and thus the ultrasonic displacements on the sample surface. In Fig. 2, a simple
schematic of the interferometer is shown. We optimized this interferometer by changing the signal and reference
beam intensities ratio (R=10), their beam diameter on the crystal (Asignal-in= 0.03cm2, Areference-in=0.2cm2) and their
incident angle (2ˁ=10°). It is worth noting that all the physical parameters (beam spot size on the sample, laser
beam power on the detector, detector type and object beam power) for the photorefractive interferometer and the
Michelson interferometer are chosen similarly.
We found a flat frequency response for Michelson interferometer. In the case of the adaptive interferometer, the
response frequency is approximately constant for high frequencies (1kHz-40kHz) while for frequencies lower than
1kHz (fcut) the sensitivity is decreased. By using the adaptive interferometer and applying a high voltage of 1.5-2kV
across the BSO crystal, we found a minimum detectable displacement (sensitivity) of 1-2nm. In Fig.3 and Fig. 4,
one can see the PRC frequency response and dependence of the SNR on the vibration amplitudes at 20kHz for both
types of interferometers respectively.
The photorefractive crystal interferometer shows in comparison to the classical homodyne (Michelson)
interferometer with the same physical parameters chosen, 4-5 times less sensitivity for mirror like surfaces.
However, for high rough industrial specimens the sensitivity of the photorefractive crystal interferometer is much
higher than that of the Michelson interferometer.
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Polarizing beam splitter
Quarter wave plate
Half wave plate
Sample
Detection laser
PZO
Mirror
Lenses
Signal beam
Reference beam
Puls generator
Photorefractive BSO crystal
Lens
Oscilloscope
Preamplifier
Detector
Fig 2: Schematic of the photorefractive interferometer based on the two-wave mixing in a BSO crystal
Fig 3: Frequency response of the BSO photorefractive crystal at a laser intensity of 0.1W/cm2
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Fig 4: Comparison of the sensitivity of a photorefractive (left) and Michelson interferometer (right) on a mirror surface at 20kHz
3-Conclusion
By using a simple photorefractive crystal interferometer small displacements and vibrations in order of a few
nanometers on the surface of specimens were detected. The sensitivity of the interferometer can be improved by
applying moderate external electric fields of 4kV/cm along the crystal. These results show the potential of such
interferometers for non-destructive testing in industrial applications.
Acknowledgements
This work has been supported by the Christian Doppler Research Association, by the Federal Ministry of
Economy, Family and Youth, by the industrial partner INPRO Innovationsgesellschaft für fortgeschrittene
Produktionssysteme in der Fahrzeugindustrie mbH, by the European Regional Development Fund (EFRE) in the
framework of the EU-program Regio 13, and the federal state Upper Austria.
References
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