ACOUSTO-OPTIC IMAGING IN DIFFERENT FIELDS OF ACOUSTICS W. Mayer To cite this version: W. Mayer. ACOUSTO-OPTIC IMAGING IN DIFFERENT FIELDS OF ACOUSTICS. Journal de Physique Colloques, 1990, 51 (C2), pp.C2-641-C2-649. 10.1051/jphyscol:19902150. jpa-00230452 HAL Id: jpa-00230452 https://hal.archives-ouvertes.fr/jpa-00230452 Submitted on 1 Jan 1990 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. COLLOQUE DE PHYSIQUE Colloque C2, supplement au n02, Tome 51, Fevrier 1990 ACOUSTO-OPTIC IMAGING IN DIFFERENT FIELDS OF ACOUSTICS W.G. MAYER Ultrasonics Research Laboratory, Physics Department GeOrgetaJn University, Washington DC 20057, U.S.A. Resume - Une introduction tr&s rapide aux principes de la diffrac- tion de la lumiere par les ondes ultrasoniques est suivie par une discussion de la technique de mise en image acousto-optique (schlieren). Cette methode est souvent utile A l'obtention de resultats qualitatifs ayant trait h divers ph&nom&nes d'acoustique ultrasonique ou sous-marine,, A lY@talonnage de transducteurs et h d'autres domaines d'acoustique. On donne des exemples provenant de diffkrents domaines d'acoustique ainsi que quelques etudes de modeles reduits, illustrant les conditions sous lesquelles cette technique peut-Stre utilishe afin d'obtenir'des renseignements sur les champs acoustiques sans avoir a introduire de sonde, d'hydro- phone ou d'autre appareil. Abstract - A very short introduction of the principles of light diffraction by ultrasonic waves is followed by a discussion of acousto-optic imaging (schlieren) techniques. This method is often useful to obtain qualitative results of various acoustic phenomena in ultrasonics, underwater sound, material characterization, trans- ducer performance and other areas of acoustics. Examples from different fields of acoustics and some scale model studies will be given, illustrating under what conditions this method can be used to obtain information about sound fields, without having to intro- duce a probe, hydrophone or other devices. 1 - INTRODUCTION Most people who work in various fields of acoustics are very often interested in the shape, direction, intensity, absorption and other parameters of the sound field with which they work. This is particularly true in the domain of ultrasonics, underwater acoustics, medical acoustics, physical acoustics, material characterization, non-linear acoustics, mode analysis, interfacial waves - to name only the most obvious. Reflectivity as well as transmission phenomena, tone burst and pulsed sonic signals, all have to be analyzed if a study of these special sound fields and their propagation are to be made. These characteristics of sound fields can be measured in most cases, usually by means of scanning the field with probes, hydrophones or other recording devices and gathering many numerical results from which the sound field can be mapped. But there are many instances where the introduction of a probe into the sonic field disturbs the field (by possibly scattering or reflec- ting parts of the intercepted sound energy) and this invasion may contribute to errors which may already exist because we usually do not know what changes have been introduced when a mechanical vibration is "translated" into an electrical signal from the transducer to an electronic display device like an osc i l loscope. So there are different potential errors in making a measurement. Therefore, one might wish to use a method which is non-invasive, does not disturb the sound field, and does not depend on a conversion from mechanical vibration energy to an electronic signal which can then be processed. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19902150 C2-642 COLLOQUE DE PHYSIQUE Fortunately, such a method exists. In its usual application it is limited to frequencies in the inaudible range, starting at about 100 kHz and ending, in principle, in the gigahertz region. This method is based on phonon-photon interactions, and is referred to as acoqsto-optics. If it is used the sound field never knows that it is being measured, so the method is non-invasive. The method is not new, it was first suggested as a possibility in 1922 by Brillouin /l/. However, it was not successfully tried until the 1930s. 2 - PRINCIPLES OF ACOUSTO-OPTIC INTERACTION A sound wave is a pressure wave. As it travels through a transparent medium, it changes the index of refraction of the medium in a periodic fashion, determined by the sound wavelength S*. If a beam of monochromatic light with wavelength S travels through the sound.wave, the originally plane wave fronts of the light beam will emerge with a very slight corrugation. The sound wave has acted as a phase grating for the light beam; constructive end destructive interference will create a light diffraction pattern where the angle between the orders, 8, is given by sine = S/&*. Since diffraction patterns usually have angular spacing5 of a few degrees or less, it is evident that the sound frequency must be rather high (short S*) if visible light is to be used and the interaction is to take place in a transparent liquid where the velocity is around 1500 m/s or in a transparent solid where the longitudinal velocity may be 10 km/s or higher. The principle of the interaction is shown in Fig. 1 together with the basic setup for the experiment where light is collimated by a lens into a beam of a few cm diameter to cover many ultrasonic wavelengths and a second lens focu- ses focus the resulting diffraction orders onto a screen for visual inspec- tion as indicated in Fig. lb. Similar basic setups were first used by Lucas and Biquard /2/ and by Debye and Sears /3/ to measure sound velocities in various liquids. The complete theory for low-intensity and low-MHz waves transversed by visible light was first worked out correctly by Sir Raman and N. Nath /4/, resulting in an expression which also gives an indication of the sound pressure which determines the light intensity in the various diffr-action pattern orders. Accordingly, the light intensity I, in the nth order (for progressive waves) is given by where 3, is the nth order Bessel function and the parameter v is directly proportional to the sound pressure. It is obvious that only those parts of the light beam which travel through the sound field will be affected by the sound field. This means that any light which is present in the diffraction orders outside the zero order has gone through a portion in the liquid which is a part of the sound field. This is indicated schematically in Fig. 2a which sketches a cross-section of the light beam going through an ultrasonic beam which is incident on a solid plate, is partially reflected and partially transmitted. The incident and transmitted beam have the same k-vector direction which is different from the direction of the reflected beam k-vector. So two diffraction patterns are produced, as shown in Fig 2b, where the alignment of the pattern corresponds to the sound travel direction. If we now block the light in the zero order but let the light of the other orders continue and use another lens behind the diffraction plane (Fig.3) we create a light image of the entire sound field which was illuminated by the expanded light beam. The amount of illumination in such an image (or, as it is sometimes called, the schlieren image) is related .to the local sound pressure via the parameter v in the Raman-Nath equation shown above. This then is the basic idea behind the visualization of an entire ultrasonic field. We should, however, be aware of the fact that one does not necessa- rily obtain an accurate picture of the exact pressure levels which exist in the field at any point (after all, the light integrates over a depth of the field and the diffraction order light intensity is not p'roportional to the pressure, as is seen from the Bessel function formulation above). However, a visual image of the entire illuminated ultrasonic field can be obtained where the brightest portions generally correspond to the highest intensities in the field and the dark portions clearly are the locations where no sound energy is present. One thus produces an integrated quantita- tive intensity map of the ultrasonic field in the planes perpendicular to the light propagation direction. We will now discuss some application in different fields of acoustics where a knowledge of the sound field configuration is of interest and value. From above it is evident that a visual image (schlieren image) cannot be produced unless a sufficiently strong light diffraction pattern has been formed. Much can be learned about the sound field from the diffraction pattern all by itself, without investigating the schlieren image. 3 - INFORMATION CONTAINED IN DIFFRACTION PATTERN The first evaluations of sound fields, in this case the velocity of sound in various liquids, were made in 1932. An example of these early efforts is shown in Fig. 4, taken from Lucas and Biquard /S/. No lasers were available then, and the light sources were mercury or sodium arc lamps, focused on a slit and then the light was collimated. These were some of the first measure- ments of sound velocity in liquids.