EP0193457B1 - Multilens acoustic device with variable magnification and focus - Google Patents

Abstract

1. Device with acoustic lenses, of the type wherein two acoustic lenses (L1, L2) are arranged coaxially in a mounting (M) which also contains an acoustic coupling medium (MC1) between the internal faces (L12, L21) of the lenses, whereas the external face (L11) of the first lens is intended to be acoustically coupled to a transducer (T), and the external face (L22) of the second lens is intended to be acoustically coupled (MC2) to a specimen (EC) which one wishes to examine, characterised in that the first lens (L1), which is constituted by a concave dioptre (L11)** hollowed out on the end of a bar (L1) whose other end is provided with the acoustic transducer (T), focuses the radiation of this transducer, in that the second lens (L2) is composed of an internal concave dioptre (L21) whose first focal point (F2) is adjacent to the second focal point of the concave dioptre (L12) of the first lens, and of an external concave dioptre (L22) appropriate for being acoustically coupled to the object, and in that the mounting (M) is constructed (M21, M22; M31, M32; M41, M42) in such a manner as to enable a relative axial displacement of the two lenses (L1, L2) which is continuously adjustable on a predetermined range, thereby defining an acoustic sensor with variable focal length and magnification.

Description

The invention relates to acoustic microscopy.

Acoustic microscopy offers certain advantages over traditional optical microscopy. First of all, it allows you to explore not only the surface of a body, but also the areas close to the surface, which are not visible when the body is opaque. In addition, for transparent bodies; it allows acoustic contrasts to appear, where the optics give little information.

Thus, at an acoustic frequency of the order of a gigahertz, we obtain an acoustic spatial resolution comparable to that of optics, and which opens up new horizons, especially in biology.

At slightly lower frequencies, of the order of a hundred megahertz, acoustic microscopy is capable of providing a means of non-destructive testing of materials in their volume. But this path is still little explored at present.

It should be recalled first of all that, if one seeks in reasoning to reason as in optics, there are nevertheless differences between these two disciplines.

In traditional optics, the word "lens" is associated with a meniscus shape. It is different in acoustics, where the "lens" function is achieved by a diopter, that is to say by the interface between a solid and a coupling medium. However, we will keep the word “lens”, as is customary, to define the solid body which carries one or more diopters of this type.

Furthermore, it is usual in optical reasoning to work on refractive indices greater than unity. We will therefore call here "diopter index" the relative index of the coupling medium with respect to the solid, and this index is defined by the ratio of the speed of the acoustic waves in the solid to that in the coupling medium.

It has been indicated previously that the use of acoustic microscopy for the purpose of non-destructive testing of materials in their volume is not widespread at present. There is in fact a prejudice, according to which the crossing of the diopter constituted by the acoustic coupling medium and the sample to be examined creates aberrations on the acoustic beam which prevent it from reaching a satisfactory resolution.

The Applicant has observed that in fact the very inclined "rays", which would create this aberration, are strongly attenuated when crossing the interface between coupling medium and sample. It is then possible to reduce said aberrations, provided that the aberrations due to the lenses themselves are not predominant. This factor, and other considerations developed below, lead to prefer lenses with a high index, in particular greater than 4.

It has been proposed in document EP-A-150 843 (a European patent application as defined in Article 54, paragraph 3 of the EPC) to make use of a focused electro-acoustic probe, operating around 100 megahertz.

This previous patent application highlighted another difficulty: when a sample is observed using a focused acoustic beam (to have good resolution), the examination time quickly becomes prohibitive if the volume of the sample is large.

The Applicant has found that, to carry out an image or a detection throughout the volume of a sample, it is necessary to use several lenses whose depths of field make it possible to cover the entire thickness of the sample. For example, more than ten lenses would be required to test a 10 millimeter thick ceramic sample with uniform resolution.

Another difficulty then arises: the depth of field and the resolution obtained in the sample material depend on the aperture angle presented by the focused beam in the coupling medium. However, this opening angle is in turn related to the size of the transducer used, and consequently to the electrical impedance thereof. However, it is desirable that this electrical impedance has a nominal value of 50 ohms, which is the internal impedance of the excitation generator. It is therefore necessary to have an additional degree of freedom in order to be able to make the desired depth of field and resolution compatible with this electrical impedance of 50 ohms.

The present invention provides a solution to these problems, a solution which essentially consists of an acoustic device with several lenses, forming a probe capable of variable magnification and focal length.

By the French patent published under No. 2,241,228, a “complex acoustic lens” is known comprising a frame which coaxially houses at least two acoustic lenses, and which contains an acoustic coupling medium in the interval separating these two lenses.

The present invention uses an apparatus having a certain relationship with this anterior complex lens, it being observed that it uses it in an entirely different context.

First, the prior patent recommends the use of polystyrene lenses, and a filling medium chosen from freons, silicone oils, and fluorinated hydrocarbons. These choices are based on the search for a high acoustic propagation speed, for a frequency of a few megahertz. Outside the lenses at least, the prior patent uses the most conventional coupling medium, namely water, but lenses made of the aforementioned materials do not have a high relative index with respect to water.

Besides this, the device described in the previous French patent n ° 2 241 228 cannot be used at frequencies of the order of 50 MHz or more. In addition, it aims to treat simultaneously all points of an image, which is very unfavorable in terms of signal / noise ratio. Finally, it cannot work at a wide aperture, which compromises its resolution.

The present invention makes use of an acoustic device with several lenses having the general structure defined above, but in which the acoustic frequency can be made higher, and above all the diopters have a high relative index, the coupling medium being such that the water. In addition, the device is suitable for high-resolution scanning imaging applications, and in particular it is suitable for acoustic microscopy.

According to a first aspect of the present invention, a first lens, consisting of a concave diopter, hollowed out at the end of a rod of a solid material such as corundum, the other end of which is provided with a transducer acoustic material made of a piezoelectric material, focuses the radiation from the transducer. This radiation then consists of a plane acoustic wave propagating in the material of the lens. The second lens is composed of an internal concave diopter (that is to say facing the other lens) whose object focal point is close to the image focal point of the concave diopter of the first lens, and of a concave diopter external suitable to be coupled to the sample. Finally, the frame is arranged to allow relative axial displacement of the two lenses, continuously-adjustable over a predetermined range. An acoustic probe with variable focal length and magnification is thus obtained.

The lenses are made of a material, preferably monocrystalline, which has a high relative index. The currently preferred material is corundum.

Other types of materials may also be suitable, such as silica, and sintered ceramics such as alumina or silicon carbide.

Very advantageously, said predetermined adjustment range comprises the confocal position of the two internal diopters.

The concave diopters can be spherical or cylindrical dioptres.

According to another aspect of the invention, the coupling medium provided between the two lenses is preferably mercury or liquid gallium.

It is then advantageous to provide, between the two parts in relative movement of the frame, a controlled leak towards a reservoir of the coupling medium.

It is also advantageous, at least when the coupling medium is mercury, for the internal dioptres, and preferably the internal faces of the frame, to be coated with a layer of gold with a bonding undercoat (Chrome or Titanium for example).

For its part, the external diopter of the second lens is advantageously provided with a quarter-wave layer, preferably made of glass, to form an adaptation with the external coupling medium (with respect to the sample) if this medium coupling is water.

According to another aspect of the invention, the transducer, which is in particular a lithium niobate transducer, has an active surface such that it has a radiation resistance of approximately 50 ohms, and the characteristics of the lenses are chosen for obtain the desired beam opening while respecting this radiation resistance. Although the present invention essentially considers the use of two lenses, it goes without saying that a higher number of lenses can be provided, at least for certain applications.

In a first family of applications, the combination of the two lenses (or more) provides an enlargement of the acoustic beam.

In another family of applications, the combination of two (or more) lenses provides a reduction in the acoustic beam.

According to another aspect of the invention, the transducer, which is excited in pulses, is chosen of a type capable of delivering an acoustic wave of frequency between a few tens of megahertz and a gigahertz.

More particularly still, the device is designed to deliver an acoustic beam into the external coupling medium, the opening of which can be made optimal with respect to the total angle of reflection on the sample.

The device of the invention applies in particular to the examination, in volume, of ceramic or metallic samples.

• - la figure 1 illustre schématiquement une sonde acoustique classique,
• - la figure 2 illustre schématiquement le couplage de la sonde de la figure 1 à un échantillon,
• - la figure 3 est le schéma de principe d'un dispositif selon l'invention, dans une première position relative des deux lentilles qu'il contient,
• - la figure 4 est le schéma du même dispositif dans une deuxième position relative des deux lentilles,
• - la figure 5 est un schéma du montage de la seconde lentille sur un tube,
• - les figures 6 et 7 sont deux variantes de réalisation d'un dispositif à deux lentilles selon l'invention,
• - la figure 8 est le schéma détaillé, en coupe, d'un mode de réalisation particulier de l'invention, tandis que les figures 8A et 8B sont des vues en demi-coupe de parties de la figure 8, et
• - les figures 9 à 11 sont des diagrammes servant à expliquer le fonctionnement du dispositif de l'invention.

Other characteristics and advantages of the invention will appear on examining the detailed description below, and the appended drawings, in which:

• FIG. 1 schematically illustrates a conventional acoustic probe,
• FIG. 2 schematically illustrates the coupling of the probe of FIG. 1 to a sample,
• FIG. 3 is the block diagram of a device according to the invention, in a first relative position of the two lenses which it contains,
• FIG. 4 is the diagram of the same device in a second relative position of the two lenses,
• FIG. 5 is a diagram of the mounting of the second lens on a tube,
• FIGS. 6 and 7 are two alternative embodiments of a device with two lenses according to the invention,
• FIG. 8 is the detailed diagram, in section, of a particular embodiment of the invention, while FIGS. 8A and 8B are views in half-section of parts of FIG. 8, and
• - Figures 9 to 11 are diagrams used to explain the operation of the device of the invention.

Reference has already been made to French Patent No. 73 30,024, published under No. 2,241,228. While specifically targeting the examination of living organisms, and the associated ultrasound imaging, this prior patent appears to relate more generally many other applications of imaging and concentration of acoustic waves. It should also be noted that this prior patent- allows the formation of images relating to macroscopic objects.

The first proposals for acoustic microscopy were made in United States patents 4,012,950 and 4,028,933.

These patents make use of an ultrasound source corresponding to the diagram in FIG. 1. An ultrasonic transducer T is bonded to the flat rear face L1 of a cylindrical bar L1 the front face of which is machined to form a diopter L12, spherical or cylindrical, and centered in C1.

où R est le rayon de courbure du dioptre L12, tandis que n est l'indice relatif du dioptre que porte la lentille L1, cette formule, et celles qui suivent, correspondent à une approximation du même type que l'approximation d'optique géométrique paraxiale.This results in a focusing of the plane acoustic wave emitted by the transducer inside the bar L1, in a focus F1. The image focal length, counted from the top of the diopter, is given by the relation:

where R is the radius of curvature of the diopter L12, while n is the relative index of the diopter carried by the lens L1, this formula, and those which follow, correspond to an approximation of the same type as the approximation of geometric optics paraxial.

In the cited United States patents, a thin section of the sample to be examined is placed transversely at the focal point F1, in an acoustic coupling medium with the lens L1. A receiving device, symmetrical with the preceding one with respect to the point F1, is placed on the other side, in order to detect the acoustic waves transmitted through the sample.

And the excitation of the transducer T is done in principle in continuous waves.

These Patents also describe variants in which one operates in reflection.

This Patent Application can also apply to the examination of thin sections of samples, but it preferably relates to the non-destructive testing of thicker samples, more precisely the high-resolution non-destructive testing of materials in their volume. This application is particularly interesting for metals and ceramics. The rest of the description will focus mainly on the examination of ceramic materials, which better highlight the advantages of the invention, because the low toughness of ceramic materials gives, under identical stress conditions, critical defect sizes. about a hundred times weaker than in metals (typically about 0.1 mm).

In the following description, it is also assumed that the lenses are made of corundum, unless otherwise stated. The coupling medium between the lenses is mercury, while the coupling medium with the sample is mercury or water, it being observed that these two materials have similar acoustic propagation speeds.

It is also recalled that the coupling medium, such as mercury or water, has a high index, namely about 7.3, compared to corundum.

où k est une constante voisine de 1, et liée à la forme de la pupille et à son éclairement,

• λ est la longueur d'onde dans le milieu de focalisation.
• f est la distance focale image du dioptre déjà défini;
• et a est le diamètre de la pupille avec a = 2.R.sin 0_m où O_m est l'ouverture angulaire du dioptre.

Returning now to FIG. 1, it has been observed that the device illustrated gives a focal spot practically free from spherical aberrations, and whose diameter at -6dB is given by the following relation:

where k is a constant close to 1, and related to the shape of the pupil and its illumination,

• λ is the wavelength in the focusing medium.
• f is the image focal length of the diopter already defined;
• and a is the diameter of the pupil with a = 2.R.sin 0 _m where O _m is the angular opening of the diopter.

It is known that in light optics, the depth of field is defined as the distance over which the focal spot remains the same width, that is to say truly the area where the light beam remains parallel.

The Applicant has observed that in acoustics, the depth of field must be defined differently, because, by controlling the structure of a material, one is essentially interested in whether or not one will know how to detect the echo. Even if the echo is due to a structural element on which it is not focused, the important thing is to have been able to detect this element correctly. It therefore seems preferable to refer to a depth of acoustic efficiency, defined according to the reduction in the amplitude of the field created by the lens.

où k' est une constante de forme voisine de 1.From there, the Applicant was able to observe that the depth of efficiency 2p is given, for a reduction in the amplitude of the acoustic field by 6 dB by the formula:

where k 'is a constant of form close to 1.

Comparatively, in optics, the depth of field would have been given by the formula 2.f. ² / a ^z .

It should also be emphasized that this evaluation of the depth of effectiveness of the lens corresponds to a zone of propagation medium which can give acoustic echoes for a fixed lens geometry. Outside this zone, the conditions for phasing the echoes, at the level of the lens, are no longer sufficiently respected, the detection of faults in the medium analyzed will be difficult (echoes are only observed in certain cases) , or even impossible.

The depth of efficiency thus obtained is of course limited. To carry out an image or a detection throughout the volume of a sample, it therefore appears necessary to use several lenses whose respective depths of efficiency make it possible to cover the entire thickness of the sample. We can thus establish that it would take more than ten lenses to control a ceramic sample over a thickness of ten millimeters while maintaining the same resolution regardless of the depth of investigation.

Indeed, the total reflection angle on the ceramic is approximately 7 °, under the conditions set. To have a focal spot diameter measuring approximately a wavelength at mid-height, it is necessary to take a lens aperture of approximately 5 °. This opening gives, in ceramics, a depth of effectiveness at mid-height which is about nine to ten wavelengths.

Reference is now made to FIG. 2, where the opening of the beam at the entry into the material constituting the sample EC is noted. d is the equivalent depth that the wave would travel from the interface between the coupling medium MC1 and the sample EC, in the absence of the sample, while Om is the opening angle which then remains the same as before. d 'is the distance that the wave will actually travel taking into account the presence of the sample EC, while Oi is the opening angle modified due to the refraction. Finally n _m is the relative index of water with respect to the material of the sample EC.

We can write as a first approximation:

It follows that at the initial opening angle O _m = 5 ° corresponds the modified opening angle Oi = 32 ° in the material, which brings 2p = 9.7 λ _m , where k ,, is the acoustic wavelength in ceramic at the frequency of 100 megahertz, or about 0.1 millimeter. This gives a depth of field of about 1 mm for each lens, hence the need to have ten or so lenses to cover the entire thickness of a 10 mm sample.

où d' est la distance de focalisation dans le matériau. Dans le milieu de couplage, qui est par exemple de l'eau, on observe que la résolution est de l'ordre de 6 à 7 longueurs d'onde (longueurs d'onde acoustiques dans l'eau).At the focus, the resolution is then given at 6dB by

where d is the focusing distance in the material. In the coupling medium, which is for example water, it is observed that the resolution is of the order of 6 to 7 wavelengths (acoustic wavelengths in water).

In the material of the sample, we observe, for a beam whose opening is 5 ° inside the coupling medium, a resolution in the material of the sample which is of the order of propagation wavelength in the material.

It goes without saying that this need to use a large number of lenses poses a major problem for those skilled in the art.

The document EP-A-150 843 proposed a first solution consisting in machining several diopters at the exit of a bar such as that of FIG. 1. Although this solution allows certain progress, it is not without also posing problems, in particular of realization and implementation, when the dioptres must have neighboring radii of curvature.

The present invention offers another solution, the block diagram of which is illustrated in FIG. 3.

A high frequency pulse generator G10 is connected by a directional coupler CD to the two electrodes of an ultrasonic transducer T. The echoes received by this transducer are transmitted by the coupler CD to an echo reception and analysis circuit noted R10 .

For the rest, this device has in common with that of FIG. 1 the fact that a lens L1 is formed of a cylindrical bar whose front face L11, flat, receives the ultrasonic transducer T. Its rear face L12 is machined like a spherical or cylindrical diopter centered in C1. The image focal point associated with this diopter L12 is at F in FIG. 3.

But instead of being applied directly to the sample, the acoustic radiation from this first lens L1 is applied to a second lens L2, which is here a biconcave lens, formed of two spherical or cylindrical diopters, L21 on the inside, and L22 on the outside, facing the EC sample.

The internal diopter L21, centered at C2, is positioned here so that its object focal point is the image focal point F of the internal diopter L12. The acoustic lens L2 is then crossed by a parallel acoustic beam, in other words a plane wave, which passes through the material constituting the lens L2, namely corundum.

The combination of lenses L1 and L2 makes it possible to obtain focusing at variable distance.

It will first be observed that the internal diopter L12 of the first lens L1, associated with the internal diopter L21 of the second lens L2, constitutes a beam-expanding system in the application presented, which is the acoustic inspection of a sample of ceramic material. It can also be used as a beam reducer in other applications such as measuring the thickness of coatings (paints).

• - obtenir l'ouverture de faisceau désirée sur le dioptre de sortie L22 de la seconde lentille. En l'espèce, cette ouverture est 0₀₁ = 5° environ, ce qui, pour un rayon de courbure R3 donné du dioptre L22, impose:
  
• - conserver une résistance de rayonnement du transducteur T qui soit proche de 50 ohms (impédance interne du générateur d'émission G10 de la figure 3), afin d'en faciliter l'adaptation d'impédance électrique.

By this means consisting in enlarging or reducing the size of the acoustic beam on the lens L2, it becomes possible to make the following two requirements compatible:

• - obtain the desired beam opening on the output diopter L22 of the second lens. In this case, this opening is approximately 0 ₀₁ = 5 °, which, for a given radius of curvature R3 of the diopter L22, requires:
  
• - keep a radiation resistance of the transducer T which is close to 50 ohms (internal impedance of the emission generator G10 of FIG. 3), in order to facilitate the adaptation of electrical impedance.

We will now focus on the variation in focus that the device of the invention allows.

où e est l'épaisseur du milieu de couplage, que l'on peut négliger en pratique. Par ailleurs, n désigne ici l'indice relatif de la lentille L2, n_m désigne l'indice du milieu de couplage relativement au matériau de l'échantillon examiné.When the two lenses are in the relative afocal position, as shown in FIG. 3, the acoustic beam is substantially parallel to the axis of the system inside the lens L2, and the focusing depth is given by:

where e is the thickness of the coupling medium, which can be neglected in practice. Furthermore, n denotes here the relative index of the lens L2, n _m denotes the index of the coupling medium relative to the material of the sample examined.

With the approximation of neglecting the thickness e of the coupling medium, we obtain:

In the case where the lens L2 is made of corundum, and where the sample is ceramic, we have _m ≅ n - 7, which gives a focusing distance roughly equal to one sixth of the radius of curvature R ₃ . Consequently, for an average observation distance of 5 millimeters, there comes a radius of curvature of the output diopter L22 of value R _3. = 30 millimeters.

For the reasons indicated above, the beam opening 0 _m should be equal to approximately 5 °, in order to obtain in the material a resolution close to λ _m = 0.1 millimeter at 100 megahertz in the ceramic.

The diameter of the output diopter L22 is then given by the relation:

A lithium niobate transducer which would have the shape of Figure 1, and which should illuminate such a large pupil, would have a radiation resistance of about 2 ohms. By obtaining an expansion factor a = 5, the invention makes it possible to produce said pupil, while retaining the electrical impedance adaptation required for the transducer, at the usual value of 50 ohms.

où D et a sont les largeurs de faisceau dans les lentilles L₁ et L₂ (figure 3).He comes now:

where D and a are the beam widths in the lenses L ₁ and L ₂ (Figure 3).

The ratio between the radii of curvature of the diopters L21 and L12 is therefore equal to 5.

However, the optimization of the geometry of the device also requires that the distance between the lenses L1 and L2 be minimized, in order to reduce as far as possible the travel time of the acoustic waves inside the device. Indeed, in acoustic microscopy, as in any other imaging application, it is necessary to keep a sufficient number of points on each line of the image.

The creation of images using the device according to the invention can be done in the manner described in the French Patent Application n ° 84 01 597 already cited. Its descriptive content is in this respect integrated into the present patent application. In short, line scanning is obtained by vibrating the object or sample to be examined, using a tuning fork type assembly. The image is scanned using a controlled micrometric movement (motorized table) of the sample.

où e₁ et e₂ sont respectivement les épaisseurs des lentilles L1 et L2, v_c est la vitesse de propagation des ultrasons dans le corindon, les deux lentilles étant supposées réalisées en corindon dans cet exemple, Δ est l'écart que présentent les deux lentilles par rapport à la position confocale, et v_H est la vitesse de propagation des ondes acoustiques dans le milieu de couplage entre les lentilles L1 et L2.Thus, it was possible to operate a device according to the invention with a line scanning frequency of 80 hertz. The number of points on a line was fixed at 512. This implies a repetition frequency of the ultrasound pulses at least equal to 40 kHz, which corresponds to a maximum period of approximately 25 microseconds. The transit time throughout the probe of the invention must therefore be much less than this value. This transit time Tp is written:

where e ₁ and e ₂ are the thicknesses of the lenses L1 and L2 respectively, v _c is the speed of propagation of the ultrasound in the corundum, the two lenses being assumed to be made of corundum in this example, Δ is the difference between the two lenses with respect to the confocal position, and v _H is the propagation speed of the acoustic waves in the coupling medium between the lenses L1 and L2.

This formula introduces an additional relationship between R ₁ and R ₂ . These values are then fixed, because, as will be seen below, the thickness of the output lens is imposed by the variation of the focusing distance which is desired as a function of the spacing Δ. For its part, the thickness of the lens L1 is chosen so that the multiple echoes give a periodicity equal to or greater than that of the multiple echoes in the lens L2, from which it results e ₁ greater than or equal to e ₂ .

Corundum has various advantages including that of offering excessively low acoustic attenuation. There is therefore no constraint on the maximum dimensions to be given to the lenses L1 and L2. And we can even operate these at frequencies much higher than 100 megahertz. On the other hand, such constraints on the dimensions may occur if the output lens L2 is made of silica or sintered alumina.

Corundum still has the distinction of being an anisotropic material, unlike silica or sintered alumina, and it belongs to the trigonal crystallographic system. We choose to propagate the acoustic wave along the Z axis of the trigonal system, which is a pure mode axis. For the L1 emitting lens the acoustic wave is practically a plane wave.

It has also been observed that, along its Z axis, the corundum has a slightly focusing section for energy, that is to say that the diffraction is less important than expected, and that the performances are a little better than that which is given by the relationships set out above.

The situation is a little different in the lens L2 output. In particular, if it is also made of corundum, when the distance Δ is varied between the focal points of the lenses L1 and L2, the acoustic beam takes significant inclinations in the output lens L2 relative to the axis Z. Those skilled in the art will understand that the angle between the wave vector and the direction of the energy then becomes also important. As a result, the overall image focal length F of the device, expressed in the coupling medium, differs slightly from that which can be predicted from the following relationship:

In this relation, A is as before the distance between the focal point of the diopter L12 of the lens L1, and that of the diopter L21 of the lens L2; d is the thickness of the exit lens, R ₂ is the radius of curvature of the diopter L21, R ₃ is the radius of curvature of the diopter L22, and n is the refractive index (assumed to be the same) associated with the diopters L12 , L21 and L22.

FIG. 4 illustrates the same lenses L1 and L2 in a different relative position, where the focal point F1 of the diopter L12 is spaced from the focal point F2 of the diopter L21. This results in the above-mentioned curvature of the propagation of the waves in the material of the lens L2, and the fact that the focusing in the sample EC takes place closer to the interface between this sample EC and the coupling medium MC2 which connects it to the L2 lens. The thick dashed line M provisionally schematizes the frame which will connect the lenses L1 and L2 to allow their relative displacement, as will be seen below.

FIG. 9 illustrates the relationship between the focusing distance f, counted from the top of the diopter L22 of the lens L2 and the spacing A between the focal points F1 and F2, this spacing being positive when F2 is to the right of F1. FIG. 10 does the same for a lens L2 made of silica, it being observed that in both cases the radii of curvature of L21 and L22 have been fixed at R ₂ = 10 mm and R ₃ = 30 mm. It appears that, in both cases, there is a fairly large area in which the variation of the focal distance f as a function of A is substantially linear. If we examine more particularly FIG. 9, it appears that a lens L2 with a thickness of 5 mm makes it possible to obtain the desired focal length variation. In particular, for a variation of A which goes from +3 to -6 mm, the equivalent focal distance of the lens in water varies from 0 (focusing on the output diopter L22) to 7 cm. This corresponds to a variation in the focusing distance in a ceramic sample which goes practically from the surface of the sample up to 10 mm in depth, which solves the problem posed.

For a satisfactory use of the device, it is necessary to minimize the losses inside the probe, while preserving the index of the equivalent lens, in order to also preserve a focal spot whose dimensions are close to the length of wave in the material, to minimize aberrations. It has been observed that this condition can be fulfilled by using mercury or even gallium as the coupling medium MC1. Mercury is particularly advantageous in that it constitutes a liquid whose propagation speed is practically equal to that of water, while its acoustic impedance is high. This results in a considerable reduction in transmission losses in each interface between the coupling medium and the lens.

For example, if corundum lenses coupled with water are used, the losses at each diopter crossing are approximately 9 dB. With mercury, they reduce to less than 1 dB. (If we switch to silica lenses, these losses would be 4.5 dB and 0.2 dB respectively). The better transmission which results from this choice of the coupling medium is also accompanied by a significant reduction in the amplitude of the multiple echoes in the probe.

Finally, mercury also has the advantage of attenuating very little the acoustic waves, which allows large variations in the parameter. This does not result in a significant modification of the losses in the probe. In particular, for the above-mentioned variation from +3 to -6 mm, the losses in the probe would only vary by around 4.5 dB.

It is therefore important to use as the coupling medium MC1 between the lens L1 and the lens L2 a liquid medium, of low propagation speed, of high acoustic impedance, and having a low attenuation at the acoustic waves. It has been observed that mercury and possibly gallium meet these conditions.

Example of particular realization

The diopters L21 and L22 are spherical or cylindrical, and of center located on the axis of the probe Z. While the centering of the diopter L22 is not excessively critical, it appeared that any inaccuracy on the position of the center of curvature of the diopter L21 causes very significant losses. Indeed, working at 100 megahertz, the lens concentrates in mercury the acoustic energy on a focal spot of less than 50 micrometers, which must be collected at a distance n R _{2 /} (n-1) using the L2 lens (with typically R ₂ = 5 to 10 mm). Take for example R, = 1 mm.

As shown in FIG. 5, the lens L2 having a diopter L21 of radius R ₂ = 10 mm and a diopter L22 of radius R ₃ = 30 mm is placed in a frame part M22 which is a cylindrical tube of stainless steel by example, in such a way that the axes of the dioptres L22 and especially L21 are merged with the axis Z of the frame.

Using a quarter-wave glass layer deposited by evaporation, the lens L22 is mechanically adapted to the coupling medium with the sample, which is water. This quarter wave layer performs an anti-reflection function. It is not necessary with mercury.

The diopter L21 is covered with a layer of a few hundred nanometers of a chromium-gold alloy, which facilitates the wetting of the mercury on its surface by the formation of an amalgam. The same is done for the inner face of the frame M22, as well as of the frame M21 (FIG. 6) which cooperates with it, and the inner diopter L12 of the lens L1.

The lens L1, for its part, can be of the type of conventional lenses used in acoustic microscopy. A corundum bar is provided at one of its ends with a piezoelectric transducer, for example made of lithium niobate, provided with suitable armatures, and capable of generating a longitudinal plane wave inside the bar. At the other end, an L12 diopter is machined which is polished and rigorously centered on the axis of the corundum bar. The diopter 12 is also well centered on its mount M21, which moves around the mount M22 in the embodiment of FIG. 6.

It is of course possible to conceive of a reverse embodiment, as illustrated in FIG. 7, in which the frame M32 associated with the lens L2 is external and serves to guide the frame M31 associated with the lens L1.

In practice, the diopter L12 is first machined, so that it is centered on the axis of the corundum cylinder L1. The electrode is evaporated on the L11 face of the corundum bar. The transducer is placed on this electrode. A counter electrode is then evaporated on the transducer T, taking care that this counter electrode is well centered on the axis of the corundum cylinder. The corundum bar is then placed in the frame M 21 or M31 so that the axis of the latter coincides with that of the cooperating frame M22 or M32 as the case may be.

An alternative embodiment consists in machining the lens L1 once mounted in its frame M31, and in doing the same for the evaporation of the counter-electrode.

Experiments have shown that it is undesirable to use a lens L1 with a large aperture, since its small radius of curvature then makes it difficult and in certain cases uncertain to establish acoustic contact with the other lens L2. In addition, given the enlargement of the acoustic beam, it is not in fact necessary to work at a large aperture: with the aforementioned numerical values, an opening of 20 ° on the diopter L21 (or the lens L1) gives a useful opening 0 _m from the output beam (in the coupling medium) which varies from 3.5 ° to 9 ° for a focusing distance ranging from 12 to 70 mm.

FIGS. 8, 8A and 8B illustrate a detailed embodiment of the device of the invention, in which provision is made for a controlled leak of the coupling medium, the latter being typically mercury. The assembly is similar to that of FIG. 7. The corundum bar L1 is mounted on the frame M41, of which an external thread 410 cooperates with an internal thread 420 of an enlarged part 425 of the frame M42 of the biconcave lens L2. A leak exists between the M41 and M42 mounts, through one or more flats 415 of the M41 mount. This slot ends in the reservoir 400 formed between 425 and M41. This avoids almost all of the problems encountered with mercury, given the variable volume that will exist between the lenses L1 and L2. In particular, the presence of air bubbles, or the introduction of vibrations would be very damaging, especially for imaging applications.

The bore 417 receives an adapter body of the transducer T, held by a transverse screw 418 (FIG. 8B). The larger terminal bore 419 is used to house the adapter electronics for the transducer.

• - Rayons de courbure: R, = 1 mm, R₂ = 10 mm, R₃ = 30 mm.
• - Ouvertures (dans le milieu de couplage, qui est du mercure) 0₁ = 20°, 0₂ = 20°, O_s = 7°.
• - Lentilles L, et L₂ en corindon, l'épaisseur de L₂ étant 4,5 mm.

Measurements were made to determine variations in focal length. For a sample of known thickness e, the amplitude of the echo reflected on its rear face (which would be located on the right in FIGS. 3 and 4) has been noted as a function of the deviation Δ from the afocal position. of the two lenses, while keeping the distance between the probe and the sample constant. The position of the maximum amplitude of the echo on the rear part of the sample makes it possible to deduce the equivalent focal distance F of the lens. The results illustrated in FIG. 11 for corundum show the possibility of focusing an acoustic beam in ceramic samples of thickness between 10 and 1 mm, with the type of lens described. This figure 11 corresponds to the following conditions:

• - Bending radii: R, = 1 mm, R ₂ = 10 mm, R ₃ = 30 mm.
• - Openings (in the coupling medium, which is mercury) 0 ₁ = 20 °, 0 ₂ = 20 °, O _s = 7 °.
• - L lenses, and L ₂ in corundum, the thickness of L ₂ being 4.5 mm.

The amplitudes A (in millivolts) are illustrated as a function of for sample thicknesses e = 10.5 mm, 5.2 mm, 3.1 mm and 0.7 mm.

Claims (15)

1. Device with acoustic lenses, of the type wherein two acoustic lenses (L1, L2) are arranged coaxially in a mounting (M) which also contains an acoustic coupling medium (MC1) between the internal faces (L12, L21 ) of the lenses, whereas the external face (L11) of the first lens is intended to be acoustically coupled to a transducer (T), and the external face (L22) of the second lens is intended to be acoustically coupled (MC2) to a specimen (EC) which one wishes to examine, characterised in that the first lens (L1), which is constituted by a concave dioptre (L11)** hollowed out on the end of a bar (L1) whose other end is provided with the acoustic transducer (T), focuses the radiation of this transducer, in that the second lens (L2) is composed of an internal concave dioptre (L21) whose first focal point (F2) is adjacent to the second focal point of the concave dioptre (L12) of the first lens, and of an external concave dioptre (L22) appropriate for being acoustically coupled to the object, and in that the mounting (M) is constructed (M21, M22; M31, M32; M41, M42) in such a manner as to enable a relative axial displacement of the two lenses (L1, L2) which is continuously adjustable on a predetermined range, thereby defining an acoustic sensor with variable focal length and magnification.

2. Device according to claim 1, characterised in that the lenses (L1, L2) are manufactured from a material which is preferably monocrystalline, wherewith the coupling medium defines a strong relative index.

3. Device according to claim 2, characterised in that at least one of the lenses (L1, L2) is made of corundum.

4. Device according to any one of the claims 2 and 3, characterised in that at least one of the lenses (L1, L2) is made of silica.

5. Device according to any one of the claims 2 to 4, characterised in that at least one of the lenses (L1, L2) is made of sintered ceramics.

6. Device according to any one of the claims 1 to 5, characterised in that the transducer (T) is excited into pulses in order to emit an acoustic wave whose frequency is comprised between several tens of megahertz and approximately one gigahertz.

7. Device according to any one of the claims 1 to 6, characterised in that the aforesaid predetermined adjustment range comprises the confocal position of the two internal dioptres (L12, L21 ).

8. Device according to any one of the claims 1 to 7, characterised in that the concave dioptres (L12, L21, L22) are spherical or cylindrical dioptres.

9. Device according to any one of the claims 1 to 8, characterised in that the transducer (T), which is in particular a niobate of lithium transducer, has an active surface such that it possesses a radiation resistance of approximately 50 ohms, and in that the characteristics of the lenses (L1, L2) are selected so as to obtain the desired beam opening whilst respecting this radiation resistance.

10. Device according to any one of the claims 1 to 9, characterised in that the coupling medium (MC1) provided between the two lenses is mercury, or liquid gallium.

11. Device according to claim 10, characterised in that there is provided between the two parts (M41, M42), in relative movement of the mounting, a controlled leakage towards a reservoir (M40) of the coupling medium.

12. Device according to anyone of the claims 10 and 11, wherein the coupling medium is mercury, characterised in that the internal dioptres (L12, L21 ), as well as preferably the internal faces of the mounting (M), are coated with a layer of gold, with an undercoat for bonding purposes.

13. Device according to any one of the claims 1 to 12, characterised in that the combination of the two lenses (L1, L2) procures a magnification of the acoustic beam.

14. Device according to any one of the claims 1 to 12, characterised in that the combination of the two lenses (L1, L2) procures a reduction of the acoustic beam.

15. Application of the device according to any one of the preceding claims, for the examination, by volume, of ceramic or metallic specimens.

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