The improvement and evaluation of a laser interferometer for the absolute measurement of ultrasonic displacements in the frequency range up to 15 MHZ

European Union

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Industrial research and development

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• Sciences et techniques

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• Publié par: EUROPEAN-UNION
• Nombre de lectures: 24
• Langue: English
• Poids de l'ouvrage: 1 Mo

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Commission of the European Communities
BCR information
APPLIED METROLOGY - REFERENCE MATERIALS
THE IMPROVEMENT AND EVALUATION
OF A LASER INTERFEROMETER
FOR THE ABSOLUTE MEASUREMENT
OF ULTRASONIC DISPLACEMENTS
IN THE FREQUENCY RANGE UP TO 15 MHz
Report
EUR 10910 EN
Blow-up from microfiche original Commission of the European Communities
BCR
APPLIED METROLOGY - REFERENCE MATERIALS
THE IMPROVEMENT AND EVALUATION
OF A LASER INTERFEROMETER
FOR THE ABSOLUTE MEASUREMENT
OF ULTRASONIC DISPLACEMENTS
IN THE FREQUENCY RANGE UP TO 15 MHz
D.R. BACON
National Physical Laboratory
UK - Teddington, Middlesex, TW11 OLW
Contract No 784/1/000/056/82/4-BCR-UK(30)
Acoustics Special Report S15
(Second Edition) July 1985
Directorate-General Science, Research and Development
1986 EUR 10910 EN Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Telecommunications, Information Industries and Innovation
Bâtiment Jean Monnet
LUXEMBOURG
LEGAL NOTICE
Neither the Commission of the European Communities nor any person acting on behalf
of then is responsible for the use which might be made of the following
information
ECSC—EEC—EAEC Brussels - Luxembourg, 1987 III
CONTENTS
ABSTRACT 1
1 INTRODUCTION 2
1.1 Background
1.2 Principles of Operation 3
1.3 Development of the Interferometer 5
2 DESCRIPTION OF THE SÏSTEM 8
2.1 Acoustic Tan* 10
2.2 Pellicles2
2.3 Block Diagrams3
2.4 Method of Operation8
3 INVESTIGATION OF SYSTEM PERFORMANCE 2
3.1 Sampling Voltmeter
3.2 Photodiode Performance4
3.3 Performance of the Pockels Cell6
3.4e of the Phase-locked Loop7
3.5 Frequency Response of the Interferometer
3.6 Linearity of the Interferometer 3
3.7y of the Digitiser
3.8 Noise Level 3
4 EXPERIMENTAL INVESTIGATION OF SYSTEMATIC EFFECTS
4.1 Position of the Laser Beam7
4.2n of the Hydrophone in the Ultrasonic Field 38
4.3 Orientation of thee andc Beam 40
4.4 Pellicle Transmission Coefficient 42
4.5 Temperature Coefficient of Hydrophone Sensitivity
5 THEORETICAL INVESTIGATION OF SYSTEMATIC EFFECTS9
3.1 Acousto-optic Interaction
5.2 Spatial Averaging 51
5.3 Nonlinear Propagation
5.4 Pellicle Transmission Coefficient 53
5.5 Performance of the Pockels Cell4
5.6 Effect of Noise5
6 RESULTS6
6.1 Calculation of Results
6.2 Systematic Uncertainties7
6.3 Randoms8
6.4 Comparison with other Calibration Methods 59
7 CONCLUSIONS AND RECOMMENDATIONS 61
8 ACKNOWLEDGEMENT3
9 REFERENCES 61»
APPENDIX 1 PERFORMANCE SPECIFICATION6
A1.1 Performance and Hardware Requirement 6IV
A1.2 Achieved Performance 68
APPENDIX 2 THEORETICAL CALCULATIONS 70
A2.1 Acousto-optical Interaction - General Linear Fields 7
A2.2 Plane Waves1
A2.3 Field of a Plane Piston2
A2.4 Amplitude Variation3
A2.5 Spatial Averaging4
A2.6 Nonlinear Propagation5 - 1 -
ABSTRACT
This report describes the application of a laser interferometer to the
absolute calibration of hydrophones in the frequency range 0.5 to
15 MHz. The interferometer, previously developed and improved at
AERE Harwell, has now been assessed in terms of its performance
characteristics. The optimum experimental arrangement and method of
calibration has been defined and corrections for various effects which
influence the measurement have been studied experimentally and
theoretically. The reproducibility of the method is approximately 1$ and
the estimated systematic uncertainty ranges from 2.1$ at 0.5 MHz to 6.3$
at 15 MHz. A hydrophone has been calibrated using the interferometer and
using two other methods, and the results are in agreement to within the
estimated uncertainties. - 2 -
1 INTRODUCTION
The absolute determination of the temporal and spatial characteristics
of the field is a crucially important ultrasonic measurement. In medical
ultrasound it is necessary for reasons of clinical safety, for
biomedical research and for equipment specification. In the area of
η η-destructive testing the requirements are currently less stringent,
but there is a growing need for the characterisation and standardisation
of equipment. The devices most commonly used for the above measurements
are miniature hydrophones but as these are not absolute devices they
must be calibrated. There are several alternative methods of doing this,
but all of the current methods are difficult and time-consuming to
inplement, and consequently there are very few centres in Europe where
the> are available. Moreover, no formal intercomparison of measurement
metnods has been undertaken in Europe (or indeed anywhere in the world),
•■-hat calibrations have not been verified by international
o .~_f>boration.
Th report describes the development and implementation of an optical
inlerferometric method for hydrophone calibration which demonstrates
higher reproducibility, accuracy and speed of use than existing
techniques. As such, it is a significant contribution to the facilities
available within Europe, and could be used in an international
intercomparison of measurement techniques. In addition to its use in
calibration, the laser interferometer can be employed to „measure
ultrasonic fields directly; here it has the advantage of high spatial
resolution (less than 0.1 mm).
1.1 Background
The work described below has been partially funded by the Community
Bureau of Reference (BCR) of the Commission of the European
Communities and involves a laser interferometer which was developed
under a previous BCR contract. Under the present contract, the
interferometer was applied to the calibration of hydrophones and its
performance was improved to meet the required specifications. The
improvement and manufacture of the interferometer were undertaken at
AERE Harwell in consultation with the National Physical Laboratory (NPL)
and this work is described in a separate report [1]. NPL was responsible
for defining and verifying the performance specifications, for applying - 3 -
the device to the calibration of hydrophones, and for studying the
sources of systematic and random uncertainty in the calibration
technique. These areas of work are described below, along with a
description of the system and a comparison of calibration results with
those of other techniques.
1.2 Principles of Operation
An ultrasonic wave can be characterised in terms of various field
parameters such as acoustic pressure, velocity, displacement or
temperature and these parameters are related through the characteristic
equations of the medium. Under conditions of low-amplitude plane
progressive wave propagation, the acoustic pressure amplitude, p, is
related to the displacement amplitude, a, by the equation
1.1 ρ = ρ ema
where ρ is the density of the medium, c the sound speed and ω the
angular frequency of the wave. Ρ and c are properties of the medium
that are known accurately and ω is easily measured. If the displacement
at a particular point in an acoustic field can be measured reliably,
then the acoustic pressure, p, can be derived and a hydrophone
calibrated by placing it at the same point in the field and measuring
its electrical output. Interferometric techniques are widely used for
the accurate measurement of displacement, so a calibration method based
on interferometry should give reliable results.
The ultrasonic wave is detected by placing a plastic pellicle in the
beam from a transducer [2]; the pellicle is sufficiently thin that it
does not perturb the acoustic field significantly, so the displacement
of its surface follows that of the wave. One surface of the pellicle is
covered with an optically reflective coating and the motion of its
surface is detected by the interferometer. The pellicle is then replaced
by a hydrophone, which is positioned at the same field point; the output
voltage from the hydrophone is measured and the sensitivity calculated
in terms of voltage per unit pressure. There are several effects to be
accounted for when implementing this technique and these have been
studied in some detail both experimentally and theoretically. This work
is described more fully in sections 4 and 5, but effects include the
interaction of the optical beam directly with the ultrasound, the - 4 -
(small) perturbation of the field due to the pellicle and the spatial
averaging of the field over the active element of the hydrophone. The
effect of these phenomena on the results has been minimised by choosing
the experimental conditions to reduce their significance or by
determining an appropriate correction by some independent method. At
present, the accuracy of the calibration is limited more by uncertainty
e be ut the performance of the interferometer than by difficulties in
implementing the calibration method.
Two practical difficulties that have to be overcome in the
interferometer design arise from the small displacements which have to
te measured and from the presence of environmental vibration. For a
given acoustic pressure, the displacement is inversely proportional to
frequency (equation 1.1) and at 15 Μ ζ the amplitu