EP1604248A2

EP1604248A2 - Method and device for opto-acoustical imagery

Info

Publication number: EP1604248A2
Application number: EP04720896A
Authority: EP
Inventors: Michel Jean Gross
Original assignee: Centre National de la Recherche Scientifique CNRS
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 2003-03-19
Filing date: 2004-03-16
Publication date: 2005-12-14
Also published as: WO2004085978A3; US20070151343A1; FR2852700A1; US7623285B2; FR2852700B1; JP4679507B2; JP2006520893A; WO2004085978A2

Abstract

The inventive method for opto-acoustical imagery of an image object (OBJ) consists in a) generating an incident optical wave (INC) and a reference optical wave (REF) coherent therewith, b) oscillating the image object (OBJ) area at an acoustic frequency, c) sending the incident wave (INC) to said image object (OBJ), thereby generating a diffused signal wave (DIF), d) sending at least one part of said diffused signal wave (DIF) to a detection device (DET), e) sending the reference optical wave (REF) to the detection device (DET) avoiding the image object (OBJ), thereby generating an interferogram I (r, t), f) extracting digital information from said interferogram I (r, t), and in g) obtaining co-ordinates (U, V, W) from one measurement point of the image object (OBJ) associated to said digital information.

Description

ACOUSTO-OPTICAL IMAGING PROCESS AND INSTALLATION

The present invention relates to a method and an installation for acousto-optical imaging. In acousto-optical imaging, the beat of a local oscillator is detected with an acoustic component of a signal wave scattered by an object to be imaged, shifted in frequency by vibration to an acoustic frequency. From a point of said object to image from which we seek to obtain information of an optical nature.

In mono-pixel acousto-optical imaging, the measurement of the beat between the component of the signal wave diffused without frequency offset, which serves as a local oscillator, and the acoustic component of the signal wave, which carries the information, presents a significant noise because the measurement of said beat is carried out at a single point of the detection plane whereas these two components each vary randomly in this plane. It is necessary to perform a summation over time of the square of the amplitude of said beat to obtain information having a better signal / noise ratio.

To overcome this drawback, a multi-pixel detection device can be used, by summing on the pixels of the detection device, rather than over time, as described in "Ultrasonic tagging of photon paths in scattering media: parallel speckle modulation processing ”, Optics Letters, Vol. 24, No. 3, February 1, 1999, page 181. In this context, it is necessary to modulate the laser power at a frequency close to the acoustic frequency, so that the beat between the acoustic component of the wave signal, carrying information, and side modulation strip

(the local oscillator) is of sufficiently low frequency to be detected by a multi detection device pixels, which generally has a low acquisition frequency. Nevertheless, a major problem remains, in that the weight of the local oscillator is generally too low. The heterodyne gain is then too low to be able to perform heterodyne detection with optimal noise.

The present invention aims in particular to overcome these drawbacks.

To this end, according to the invention, provision is made for an acousto-optical imaging method of an object to be imaged comprising the steps consisting in:

(a) generating an incident optical wave, of frequency f _τ , and a reference optical wave, of frequency f _R , this reference wave being coherent with the incident wave, and having with it a phase difference φι (t) known,

(b) vibrating in a first object direction and at an acoustic frequency f _A , an area of the object to be imaged using a vibration generating device,

(c) applying said incident wave to the object to be imaged, and thereby generating a scattered signal wave,

(d) applying at least part of the signal wave diffused on a detection device, (e) applying the reference wave on the detection device without passing it through the object to be imaged, which generates at point r of the detection device an interferogram I (r, t) varying over time t,

(f) extracting digital information from the interferogram I (r, t), and

(g) obtain the coordinates of a measurement point of the object to be imaged, to which the digital information is relative. This prevents the reference wave, which serves as a local oscillator, from passing through the object to be imaged. This makes it possible to have a sufficient level of local oscillator, and to extract, with a better signal / noise ratio, information relating to the measurement point for example for imaging purposes, in particular medical imaging. In addition, this imaging method makes it possible to obtain an exploitable signal even with low acoustic or optical powers, for example compatible with the security standards for imaged tissues associated with medical imaging.

In preferred embodiments of the invention, it is optionally possible to have recourse to one and / or the other of the following arrangements: - during step (f), an acoustic component of the part of the scattered signal wave applied to the detection device, this acoustic component being at a frequency corresponding to the sum of the frequency f of the incident wave and of a harmonic of the acoustic frequency f _A (fi ± Hf _A , H non-zero integer); during step (a), said reference wave is generated at a frequency f _R equal to or substantially equal to the sum of the frequency f _τ of the incident wave and said harmonic of the acoustic frequency f _A (f _R ≈ f _x ± Hf _A , H non-zero integer); during step (b), a focused acoustic wave is generated at a focal point located in the object to be imaged and during step (g), the coordinates of the measurement point are obtained, as being the coordinates of said focal point; repeating steps (a) to (g) for different focal points of the acoustic wave in the object to be imaged, these different focal points being aligned in the first direction of object; during a first iteration, steps (a) to (f) are carried out for a first frequency f _A of the acoustic wave and a first frequency f _R of the reference wave, during at least one second iteration, steps (a) to (f) are repeated for a second frequency f ' _A of the acoustic wave and a second frequency f' _R of the reference wave, these second frequencies being coded respectively with the first frequencies , the method further comprising a step during which:

(f ') at least one digital item of information is obtained by decoding said digital information obtained during steps (f) of each iteration as a function of the frequencies used, and, during step (g), the coordinates d are obtained '' at least one measurement point of the object to be imaged to which the digital information obtained during step (f ') is relative, by decoding said digital information obtained during steps (f) of each iteration as a function frequencies used; the following sequence of operations is carried out: the frequency of the acoustic wave is scanned, which is focused on an interval of coordinate points ([U-Dx, U + Dx], V, W) extended around the coordinate point (U, V, W) along the first direction of the object, a scan of the frequency f _R of the reference wave is carried out jointly so as to maintain f _R substantially equal to or equal to f _τ ± Hf _A , H being a non-zero integer, we record for each pixel r and for each frequency f _A an interferogram I (f _A , V, W, r) associated with the set of points ([U-Dx, ϋ + Dx], V, W) of the extended interval, we perform, for each pixel r, a 1D Fourier transformation frequency → time according to the frequency f _A of the interferogram I (f _A , V, W, r), and at least one interferogram I (r) associated with at least one coordinate measuring point (U ', V, W) is obtained by replacing the time obtained after the Fourier transform by the dimension U 'in the first direction of the object using the speed of propagation of the acoustic wave in the object to be imaged (U ¹ possibly being worth U); at least steps (a) to (g) are repeated after having imposed - a displacement of the vibration generating device relative to the object to be imaged in a direction not parallel to the first object direction of

.the object to be imaged;

- during step (f), the complex amplitude E _s (r) of the acoustic component is estimated from the interferogram I (r, t); the detection device used is a mono-pixel detector and, during step (f), the digital information is obtained as being the intensity of the field of complex amplitude E _s (r) scattered by the object ; the detection device used is a multi-pixel detector, and during step (f), the digital information is extracted as being the sum over at least part of the pixels r of the intensity detection device the complex amplitude field E _s (r) scattered by the object; during step (d), a spatial filtering device is used, so as to limit, in at least one direction, the angular extent of the part of the scattered signal wave which is seen by each pixel of the detection device (an average angular direction can thus be defined for the part of the scattered signal wave which is seen by each pixel of the detection device); a spatial filtering device is used comprising a diaphragm, of dimensions X in a first diaphragm direction and Y in a third diaphragm direction, and a focal lens L, of object focus located directly downstream of the object to be imaged for limit the angular extent of the part of the scattered signal wave which is seen by each pixel of the detection device, and the reference wave applied to the detection device is generally a plane wave (the direction of application of the acoustic wave, incident wave, and the directions of the diaphragm are not necessarily related); a spatial filtering device is used comprising a diaphragm of dimensions X along the first diaphragm direction and Y along the third direction of diaphragm, disposed between the object to be imaged and the detection device at a distance L from it, to limit the angular extent of the part of the scattered signal wave which is seen by each pixel of the detection device, and the reference wave applied to the detection device is a spherical wave coming from a source point located in the plane of the diaphragm (the direction of application of the acoustic wave, of the incident wave, and the directions of the diaphragm are not necessarily linked); the reference wave and the scattered signal wave interfere with the detection device by forming a non-zero angle θ _γ , θ _γ being measured in the plane of incidence of these two waves on the detection device; the detection device used is a multi-pixel detector, and the part of the acoustic component, of complex amplitude E _s (r), which varies rapidly in space in the plane of the detection device (the detection device, is isolated) detection is in a plane quasi-orthogonal to the direction of the reference wave, and the part that one seeks to isolate corresponds to the components of the interferogram I (r) which vary rapidly in space and slowly in time ); the detection device comprises pixels arranged in a matrix comprising lines in a first detector direction and columns in a third detector direction, and step (f) comprises the following steps: (fl) we do for at least a line or column a one-dimensional Fourier transform along this line or column of the detection device towards the space of wave vectors, of complex amplitude

E _s (r), and we thus obtain for this row or column a TFi field E _s (k),

(f2) several summation zones are defined in the space of wave vectors,

(f3) the intensities of the field TF _X E _s (k) are summed in at least one zone at each point k of this zone, and

(f4) the digital information is extracted as being a linear combination of the sums thus obtained in each zone (this linear combination possibly comprising only one term); the detection device comprises pixels arranged in a matrix comprising lines in a first detector direction and columns in a third detector direction, and step (f) comprises the following steps: (fl) a two-dimensional Fourier transform of the complex amplitude E _s (r) is made, from the plane of the detection device to the space of the wave vectors, and a TF ₂ E _s field is thus obtained (k), 5 (f2) several summation zones are defined in the space of wave vectors,

(f3) the intensities of the field TF ₂ E _s (k) are summed in at least one zone at each point k of this zone, and 10 (f4) the digital information is extracted as being a linear combination of the sums thus obtained in each zone (this linear combination possibly comprising only one term); the angle θ _γ is approximately equal to 3Y / 2L, during

15 of step (f2), a first summation zone, called the central zone, a second summation zone, called the left zone, and a third summation zone called the right zone are defined, and, during the step (f4), we extract the digital information as being a combination

20 linear of the value of the sum of the left zone and the sum of the right zone (this linear combination possibly comprising only one term); during step (a), a laser source of wavelength λ, emits '25 an emission wave, of frequency f _L , means of amplitude modulation of the emission wave, generate a carrier wave of incident frequency fi, and at least one side band of amplitude modulation, which corresponds to a wave of frequency f _R , 30. a semi-reflecting mirror, transmits part of the side band wave and part of the carrier wave forming the incident wave, and reflects part of the carrier wave and part of the side band wave forming the wave reference ; during step (a), a laser source of wavelength λ, emits an emission wave, of frequency f _L , a first acousto-optical modulator transmits part of the emission wave to form the incident wave on the object to be imaged, and moreover generates a first frequency-offset wave, the frequency of which is offset by a value δfi, possibly negative, with respect to the emission wave, and. a second acousto-optical modulator intercepts the first wave shifted in frequency and generates a second wave shifted in frequency, the frequency of which is shifted by a value δf ₂ , possibly negative, with respect to the shifted wave, the second wave shifted in frequency forming the reference wave, the frequency of which is thus shifted in frequency with respect to the incident wave by a value δf = δfι + δf ₂ , thus determining a known phase difference <pi (t) between these two waves ; (most often δfi and δf ₂ are of opposite sign) - during step (a), two independent laser sources, locked in phase by an electronic servo, generate the incident and reference waves, presenting a phase difference cpι (t) known to each other. - during step (a), a laser source of wavelength λ, emits an emission wave, of frequency f _L , a semi-reflecting mirror transmits part of the emission wave to form l wave incident on the object to be imaged, and transmits a second part of the emission wave, a first acousto-optical modulator intercepts the second part of the emission wave and generates a first wave shifted in frequency, frequency offset by a value δfi, possibly negative, with respect to the emission wave, and a second acousto-optical modulator intercepts the first wave offset in frequency and generates a second wave offset in frequency, the frequency of which is offset by a value δf ₂ , possibly negative, with respect to the offset wave, the second wave offset in frequency forming the reference wave, the frequency of which is thus offset in frequency with respect to the incident wave by a value δf = δfι + δf, thus determining a known phase difference φi (t) between these two waves; (most often δfi and δf ₂ are of opposite sign)

- the object to be imaged is biological tissue; the vibration generating device is used to obtain acoustic information of the area of the object to be imaged, and the digital information extracted in step (f) is used in conjunction with said acoustic information.

According to another aspect, the invention relates to an installation for acousto-optical imaging of an object to be imaged (OBJ) comprising:

means for generating an incident optical wave, of frequency f _lr and a reference optical wave of frequency f _R , this reference wave being consistent with the incident wave and having with it a known phase difference φι (t), a vibration generating means for vibrating in a first direction and an object at an acoustic frequency f _a a zone of the object to be imaged, - means for applying said incident wave to the object to be imaged, thereby generating a wave broadcast signal, a detection device, means for applying at least part of this signal wave diffused on the detection device, means for applying the reference wave to the detection device without passing it through the object to be imaged, which generates at the point r of the detection device an interferogram I (r, t) varying over time t, and means for extracting from the interferogram digital information and the coordinates of a measurement point of the object to be imaged, at which l digital information is relative.

In preferred embodiments of the invention, it is possible optionally to have recourse to one and / or the other of the following arrangements:

the installation also comprises the following elements: means for viewing said digital information relating to said measurement point of the object to be imaged, and means for moving the object to be imaged; the installation also includes a spatial filtering device, located downstream of the object to be imaged.

Other aspects, aims and advantages of the invention will appear on reading the description of several of its embodiments given by way of nonlimiting examples.

The invention will also be better understood using the drawings, in which: - Figure 1 describes a first example of implementation of the method according to the present invention, Figure 2 is a detailed diagram of an example of the device generation of two coherent waves according to the present invention, FIGS. 3 and 4 are graphs representing the phase difference between two coherent waves as a function of time, generally and in a particular case, FIG. 5 describes the first example of implementation of the method according to the present invention with the generation device of FIG. 2, FIG. 6 represents a map of the signal obtained, - FIG. 7 represents a second example of implementation of the method according to the present invention, FIG. 8 is a detailed diagram of another example of the device for generating two coherent waves according to the present invention, - Figure 9 shows a detail of a third embodiment of the invention, and Figure 10 shows the rear face of the diaphragm used in the third embodiment .

FIG. 1 shows a device for generating GEN waves, which generates: an incident optical wave INC, of wavelength λ, of frequency fj, applied to an object to be imaged OBJ, and an optical reference wave REF of frequency f _R.

The incident INC and REF reference waves are consistent with each other and have a known phase difference φι (t). These optical waves can be emitted in the visible range, or possibly in the infrared or the ultraviolet.

The generation device GEN is adjusted so that the reference wave REF is offset in frequency with respect to the incident wave by a value equal to δf. There are many possibilities for producing such a generation device, and it is possible, for example, to use a generation device as shown in FIG. 2. This generation device comprises: - a LAS laser source emitting an optical wave of EMI emission at the frequency f _L , a first acousto-optical modulator MAOl transmitting part of the emission wave EMI to form the incident wave INC at the frequency f _lr and also generating a wave DEC shifted by δfi in frequency, and a second acousto-optical modulator MA02 intercepting the frequency-shifted DEC wave, of frequency f _L + δfi, in order to again shift this wave by a frequency δf ₂ to thereby generate a reference wave REF at the frequency f _R = f + δf _x + δf ₂ , δfi and / or δf ₂ being possibly negative (δf _x and δf ₂ are most often of opposite sign).

These acousto-optical modulators MAOl and MA02 are for example made up of an acousto-optical cell of Tellurium dioxide (Te0 ₂ ), oriented at a given angle with the wave applied to it, namely the emission wave EMI and the shifted wave DEC respectively, and vibrating under the action of a high frequency generator, of frequency δfi and δf ₂ respectively, and transmit at the same time a non-diffracted beam and a diffracted beam shifted in frequency. The two acousto-optical modulators

MAOl and MA02 performing the frequency shift of the reference wave REF with respect to the laser wave for example have frequencies close to 80 MHz, so that these two waves can be offset between them by a frequency shift δf = δfi + δf ₂ , which can be set between a few hertz and a few tens of megahertz approximately.

Alternatively, it is also possible to use another generation device (not shown) where one extracts the incident wave INC from the laser source LAS using a transparent mirror (or any other device) placed upstream of the first acousto-optical modulator MAOl, which then generates the offset wave DEC from the part of the incident wave transmitted by the transparent mirror i (or any other similar device placed between the laser source LAS and the first acousto-optical modulator).

It is also possible optionally to use two independent laser sources, locked in phase by an electronic servo, and generating incident waves INC and reference REF having a phase difference φ ± (t) known between them.

In these embodiments of a generation device, the LAS laser source (s) may for example comprise a laser diode stabilized by an external cavity with network, emitting an EMI laser wave of wavelength λ = 850 nm and delivering a maximum power of 20 mW for a current of 65 mA. It is optionally possible to use a power of said laser of approximately 15 mW which, taking into account the optical losses, gives in this example a power of approximately 7 mW of the incident wave INC on the object to be imaged OBJ.

The two waves thus obtained, the incident wave INC at the frequency f _τ and the reference wave REF at the frequency f _R are thus two coherent waves offset in frequency by a value δf = δfi + δf ₂ . It is thus possible to control the phase difference cp (t) between these two waves. FIG. 5 represents the first embodiment of the invention with the generation device GEN of FIG. 2.

The object to be imaged OBJ on which the incident wave INC is applied is a scattering object for waves optical, for example a sample of biological tissue. In the example considered, this sample can for example have a thickness of approximately 20 mm in the direction of propagation of the incident wave INC. This sample can in particular be compressed between a front plate and a downstream plate, perpendicular to the direction of propagation of the incident wave INC, these two plates being part of a sample holder (not shown). The upstream plate is for example entirely transparent and produced in particular from PMMA (Plexiglas®), while the downstream plate can for example be opaque and produced in particular from black bakelite. This rear plate is pierced, for example, with a circular hole with a diameter of approximately X = 20 mm.

An acoustic coupling is carried out between the object to be imaged OBJ and a TRANS vibration generator device located outside the object. To this end, the object to be imaged OBJ and its sample holder can be installed in the center of a tank 1, for example 180 mm in diameter and 150 mm in height. This tank can be fitted with flat glass windows 50 mm in diameter, distant from

240 mm, and can optionally be filled with liquid used to accomplish the acoustic coupling between the object to be imaged OBJ and the TRANS vibration generating device.

If the object to be imaged OBJ cannot be immersed, or for any other reason, other means, known to those skilled in the art, can be used to guarantee the acoustic coupling. We can for example mount the vibration generator device directly on the object to be imaged

OBJ.

The vibration generating device TRANS can be a PZT transducer having an acoustic vibration frequency f _A = 2.2 MHz, and a variable focal length fixed for example at 75 mm. It is excited for example by a sinusoidal signal produced at a maximum power of 22 dBm on 50 Ω, or 6.3 Vdc, harmless for biological tissues. It is possible to use a different maximum power, for example of the order of 34 dBm or other.

This vibration generating device is oriented along a first direction of the object x ₀ , and emits an acoustic wave of frequency f _A along this first direction of the object x ₀ . The vibration generator device TRANS then vibrates at the frequency f _A a zone (Dx, Dy, Dz) of the object to be imaged, centered on a point of coordinates (ϋ, V, W) from which one seeks to obtain information. . The extent (Dy, Dz) of the vibrating object zone corresponds approximately to the dimension of the focal zone of the acoustic wave, in the transverse directions y and z, that is to say in the plane normal to the propagation direction x ₀ of the acoustic wave emitted by the transducer. In this plane, the focal area is centered in (V, W). U corresponds to the distance between the acoustic transducer and the focal point thereof along the direction of propagation of the acoustic wave. Around this focal point, a certain area, of extent Dx variable depending on the type of transducer used and the nature of the object to be imaged, inter alia, vibrates at the acoustic frequency f _A. Thus, the position and orientation of the transducer and the position of its focal point determine a measurement point of the object to be imaged OBJ, with coordinates (U, V, W). The incident wave INC is applied to the object to be imaged OBJ, in a second direction of object z ₀ , possibly identical to the first direction of object x ₀ , to form a scattered signal wave DIF which is scattered by object in all directions. AT inside the object to be imaged OBJ, part of the wave passes through the area (Dx, Dy, Dz) of the object to be imaged OBJ vibrating at the acoustic frequency f _A. The movement of the points of the object liable to scatter generates a modulation at the acoustic frequency f _A of the phase of the scattered wave. The vibration also produces a modulation of the optical index of the medium (also at the frequency f _A ). These two effects result in the generation of an acoustic lateral band offset in frequency by f _A vis-à-vis the incident wave of frequency f _τ which crosses the medium. The diffused signal wave DIF therefore has an acoustic component offset in frequency with frequency f _0A = fi ± Hf _A (where H = 1, 2, ... is the harmonic rank, in general H = 1). The principle of acousto-optical imaging then consists in detecting on a DET detection device this acoustic component of the signal wave scattered by the object, by making this acoustic component of the signal wave scattered interfere with a local oscillator of neighboring frequency. This is carried out according to the invention, using the reference wave REF not passing through the object to be imaged OBJ, as a local oscillator for the detection device.

The detection device has at least one detection cell in a plane x _D , y _D almost normal to the direction of observation z _D (in the illustrative examples given in the figures, x _D , y _D , z _D correspond to x ₀ , γo, z ₀ ).

A multi-pixel DET detection device can be used, for example a digital CCD camera (12 bits) made up of 1280 x 1024 square pixels of size d _x = d _γ = 6.7 μm. The CCD camera is chosen to have a quantum yield sufficient for a wave at 850 nm, for example 5% or more. The camera can be of the “full frame” type (not interleaved) or frame transfer, with a detection frequency for example of f _c = 12.5 Hz.

A device is used to cause the diffused signal wave DIF coming from the object to be imaged OBJ and the reference wave REF to interfere on the detection device DET. To perform an effective detection on pixels of finite dimension, these two waves must be almost collinear (forming an angle of 5 ° at most). It is possible, for example, to use a semi-reflecting plate or a beam-splitting prism to guide the reference wave REF to the detection device DET.

We will see that it can be useful to angularly shift the direction of the reference wave REF from the mean direction of the signal wave DIF, by an angle θy. For the description which follows and in FIGS. 1, 5, 7 and 9, the angular shift is carried out in the direction y, the direction ′ of the acoustic wave is x, the direction of observation is the same as that of the incident wave INC (direction z) and the matrix of the CCD detector is oriented along x and y, but these orientations are only defined by way of example.

The scattered signal wave DIF and the reference wave REF are made to interfere with the detection device, and an interferogram I (U, V, W, r, t) is recorded using this device, taken at time t, at point r of the detection device, and which corresponds to the measurement point of coordinates (U, V, W) of the object to be imaged, vibrating at the acoustic frequency f _A.

A CALC processing device can be coupled to this installation, capable of extracting from the recorded temporal interferogram digital information relating to the measurement point, with coordinates (U, V, W), this information being able to be subsequently displayed in an image of the object. This processing involves the calculation of the complex amplitude E _s (ϋ, V, W, r) of the acoustic component of the scattered field, shifted in frequency by the acoustic vibration, on the detector.

In a first nonlimiting embodiment of the determination of the complex amplitude E _s (ϋ, V, W, r), the acoustic wave, of fixed frequency f _A , is focused at the measurement point, with coordinates (U , V, W), and the frequency f _R of the reference wave REF is chosen so as to perform a demodulation with N phases, in particular with 4 phases (N = 4), as explained below.

The measurement point, with coordinates (U, V, W), of the object to be imaged OBJ vibrating at the acoustic frequency ^" f _A , the diffused signal wave DIF contains an acoustic component of frequency f _r + f _A. L The CALC processing unit processes the time-varying interferogram I (U, V, W, r, t) by a four-phase demodulation as follows: The frequency shift décalagef is first of all chosen between the incident wave INC and the reference wave REF such that δf = f _a + f _c / 4 = f _A +3.125 Hz. The detection could however also be carried out for any number N at least equal to 2 of phases, and the frequency of the reference wave adapted by taking δf = f _A + f _c / N.

N interferograms are then measured, each for a time T _int , each interferogram corresponding to a distinct phase difference cpi known between the incident wave INC and the reference wave REF. The reference wave being at the frequency f _R = fι + f _A + f _c / N, and the acoustic component of the signal wave being at the frequency i + fAr the phase difference between the reference wave REF and the acoustic component of the signal wave is, as shown in FIG. 3 in general and in FIG. 4 for N equal 4, linear by interval as a function of time and goes from zero to 2τr over a time interval equal to N / f _c . We thus detect N interferograms Iι ¹ (ϋ, V, W, r, t), ..., I ^ U, V, W, r, t), ..., I _N ^ U, V, W, r , t) corresponding to N distinct values cpi of the phase difference, N being at least equal to 2, and in the case of FIG. 4 being equal to 4.

This operation can be performed a number n at least equal to 1 of times, and for example for n = 3, in order to obtain, for each phase difference (known pi, n interferograms Iι ^k (ϋ, V, W, r , t) (i = l ... N, k = l ... n) For 12 interferograms, i.e. 3 cycles of 4 phases

(N = 4, n = 3), the total duration of the measurement is for example of the order of a second.

The 4-phase demodulation calculation can be carried out separately for each of the pixels r of the detection device. The CALC processing device in fact performs the following operations for each pixel r of the detection device: integration of the intensity measured by the detector during the duration of a frame of the CCD camera (1 / 12.5 s): Ii ^k (U, V, W, r) ≈ (1 / T _int ) r I (U, V, W, r, t) dt taken between instants t = ti _k and t = ti _k + Ti _nt . for each given phase φ, an averaging of the n interferograms Ii ^k (U, V, W, r) (with k = l ... n) detected for this phase to obtain N average interferograms Iι (ϋ, V, W, r ), a N-phase demodulation of the N interferograms measured in order to obtain information on the complex amplitude E _s (ϋ, V, W, r) of the acoustic component of the DIF signal wave scattered by the object. For example, for n equal to 1, and in the particular case of Figure 4 where N is equal to 4 and where φ _x = τr / 2, φ ₂ = TT, φ ₃ = 3TT / 2, φ = 2ττ, l complex amplitude E _S (U, V, W, r) of the acoustic component is proportional to (I ₄ - I ₂ ) + j (Ii - I ₃ ) (where j is the complex number such that j ² = -l ), and the intensity associated with the object to be imaged OBJ too, because we consider that the reference wave has a constant complex amplitude in space and in time.

To determine the complex amplitude E _s (ϋ, V, W, r) of the acoustic component of the diffused signal wave DIF, a number of other techniques are commonly available, known to those skilled in the art.

A second method for determining the complex amplitude E _S (U, V, W, r) of the acoustic component consists, for example, in using the method known as "frequency chirping". In this case, instead of focusing at dimension U in the direction x an acoustic wave of fixed frequency f _A , the following sequence of operations can be carried out: - the frequency f _A of the acoustic wave is scanned , which is focused on an interval in [U-Dx, U + Dx] extended around U; a scan of the frequency f _R of the reference wave is carried out in conjunction with the first scan so as to maintain the condition f _R = fχ ± Hf _A (where H is the harmonic rank, in general 1) (it will be noted that the reference wave here has the same frequency as the acoustic component that one seeks to detect); for each pixel of dimension r and for each frequency f _A, the interferogram I (f _A , V, W, r) associated with the set of points ([U-Dx, U + Dx], V, W) is recorded the extended interval; we carry out, for each pixel of dimension r, a Fourier transformation 1D frequency -> time, according to the frequency f _A , of the interferogram I (f _A , V, W, r), and we obtain the interferograms I (U , V, W, r) complexes associated with the different values of the dimension U along the direction x by replacing the time obtained after the Fourier transform by the dimension U along x using the speed of propagation of the acoustic wave in the object to be imaged.

The reference wave being approximately a plane wave of constant amplitude, the complex interferogram I (U, V, W, r) thus decoded is directly proportional to the complex amplitude E _s (U, V, W, r) of the acoustic component of the DIF scattered signal wave that one seeks to determine.

There are many variants of the "frequency chirping" technique which consists in replacing the coding in the direction x, by a coding in frequency according to f and f _R. These techniques, or others, may as well be used to determine E _Ξ (U, V, W, r) in the context of this invention. In this first embodiment, it is also possible to have a spatial filtering device COL

(Figures 1 and 5), which limits the angular extent of the part of the signal wave seen by each pixel of the detection device. We will see that this device is useful for controlling the size of the speckle grains associated with the DIF scattered signal wave, in the plane of the detection device.

To carry out the detection, it may be advantageous to ensure that the size of the speckle grains is adapted to the size of the pixels of the camera.

This first condition corresponds to the so-called “anti-aliasing” condition.

In addition, the judicious choice of the geometry of this spatial filtering device will make it possible to isolate, according to the invention, the useful signal from the various noise components.

This filtering device is for example constituted by a rectangular diaphragm 2, positioned for example directly downstream of the object to be imaged OBJ, perpendicular to the direction of observation (and therefore almost parallel to the detection device), for example between the object to be imaged OBJ and the downstream plate of the sample holder, and elongated in a direction. This diaphragm 2 can for example be made up of two thin sheets of aluminum sheet 0.5 mm thick separated by about Y = 4 mm. The observable zone upstream is thus of quasi-rectangular shape of approximately X mm and Y mm follow two directions perpendicular to the direction of observation. We can for example take z as the direction of observation, and x and y as axes for the diaphragm.

In the embodiment of the spatial filtering device COL shown in FIG. 5, this device COL further comprises a lens 3 placed between the tank 1 and the detection device DET. The focal point of lens 3 is located in the plane of the diaphragm

(possibly taking into account the optical index of the liquid used for the acoustic coupling of the TRANS vibration generating device, if the signal wave is scattered

DIF passes through this liquid). In the embodiment presented, a focal lens L = 250 mm is used, but other focal lengths may be suitable.

This spatial filtering device reduces the angular extent of the GIS part of the DIF scattered signal wave which reaches the DET detection device, which can be useful for adapting the size of the speckle grains to the dimension of the pixels of the camera.

In fact, in the absence of a spatial filtering device, the signal wave DIF scattered by the object to be imaged OBJ can occupy a wide solid angle, of the order of Tr steradants, and can be broken down into a superposition of plane waves very different K _s wave vector elements. Each Kg wave vector has, in the plane of the detector (x _D , y _D ) _ι two coordinates K _x and K _y . To obtain exploitable information, it is necessary that the density of the fringes corresponding to the spatial modulation of the interference signal of the signal wave diffused with the reference wave does not exceed the resolution of the matrix detector made up of elementary detectors

(condition known as "anti-aliasing"). In particular, for a plane REF reference wave, of wave vector Ko, of coordinates in the plane of the detector K _x0 , Kyo, this condition of density of the fringes is expressed by the condition of “anti-aliasing” defined by :

SKO = SINC (d _x . ( _X - K _x0 )). SINC (d _γ . (K _y - K _y0 )) ~ 1 - Ea where the SINC function of a dummy variable xx is defined as being equal to 1 for xx = 0 and to sin xx / xx otherwise, where Ea is a extinction factor quantifying the loss of contrast of the fringes linked to the spatial integration of the detector, and where d _x and d _γ respectively represent the characteristic dimensions of the elementary detectors of the detection device in the directions x _D and y _D. Thus, the measurement must be limited to an elementary angular field of the signal wave SIG, corresponding to a cone of angle (+ α _x ; ± α _y ) around the direction of the wave vector Ko of the wave of REF reference, the dimensions α _x and α _y of this elementary angular field having to be appreciably less than or equal to λ / 2d _x and λ / 2d _γ respectively to respect said condition of “anti-aliasing”, where λ is the length d 'reference wave REF.

The use of the diaphragm downstream of the object to be imaged OBJ, makes it possible inter alia to eliminate the components of the signal wave not respecting this condition of "anti-aliasing".

A judicious choice of the geometry of the COL spatial filtering device, and of the detection device allows furthermore to isolate the useful signal from the different terms which appear in the signal resulting inter alia from the analysis of the interferograms I (U, V, W, r, t). The discussion is carried out in the case of 4-phase demodulation but a similar discussion could be made in the case of detection by “frequency chirping”, or other similar technique.

The interferogram I (U, V, W, r, t) corresponds to the total intensity I _τ seen by the detection device, ie the square of the module of the complex amplitude E (

I _T = | E | ² = EE * where E * is the conjugate complex of E). To simplify the discussion we will consider only one acoustic component of frequency f _A0 = fi + H. f _A (with

H = 1). The complex amplitude E results from the sum of the amplitude E _R of the reference wave REF, the amplitude Ei of the part of the signal wave scattered SIG at the frequency of the incident wave f _lΛ and the amplitude E _s of the acoustic component of the wave signal of frequency f _A0 . So we have E = Eτ + E _R + E _s . The total intensity I _τ , which is used to extract the information sought, corresponds to the sum of 6 terms (I = EE * = (EÏ + ER + ES). (Eτ + E _R + E _s ) *): the term EÏ.EI * corresponds to the interference between the part of the signal wave diffused at the frequency f _τ and itself, that is to say the interference between the ordinary speckle and the ordinary speckle, the term E _S .E _S * corresponds to the interference between the acoustic component of the signal wave scattered at the acousto-optical frequency f _A0 = fι + f and itself, that is to say the interference between the acousto-optic speckle and the acousto-optic speckle, the term E _R .E _R * corresponds to the interference between the reference wave and itself, the term E _R .Eι * corresponds to the interference between the reference wave and the ordinary speckle, the term Eτ _. .E _s * corresponds to the interference between the ordinary speckle and the acousto-optic speckle, and the term E _R .E _S * corresponds to the interference between the reference wave and the acousto-optic speckle, which constitutes the term carrying relevant information.

The spatial filtering device COL makes it possible to reduce the angular extent of the wave coming from the object which can behave at the level of the detection device like a quasi-plane wave. This is in particular the case of the parts of the scattered signal wave having a complex amplitude Ei and E _s . Furthermore, the reference wave REF is, in the present embodiment, a plane wave.

Due to the spatial filtering device the 3 diagonal terms E _R .E _R *, Ei.Ej. * and E _S .E _Ξ * (as well as the term

Ei. E _s *) vary slowly in space along the x and y directions of the detection device. Furthermore, the term E _R .E _r * varies rapidly over time (at a frequency close to f) and is average at zero due to the low acquisition frequency of the detector. It is therefore easy to isolate by a suitable digital processing the relevant term E _R .E _S *, (which makes it possible to determine E _s ) - If we choose an angular offset θ _γ sufficient between the reference wave and the wave signal broadcast, this term is the only one to vary slowly in time, and rapidly in space along the y direction. This direction y corresponds to the direction of the width of the diaphragm, and to the direction y _D of the plane of the detector.

One way of extracting the relevant information consists in carrying out a Fourier transformation of the complex amplitude E _S (U, V, W, r) calculated above, along the directions x and y of the plane of the detector (or possibly the only direction y). A signal TF E _S (U, V, W, k) is then obtained, k being the space coordinate of the wave vectors. A map of the signal TF E _s (U, V, W, k) obtained after the Fourier transform is shown in FIG. 6, which is an angular representation in the space of the wave vectors. The various terms contributing to the interferogram which are represented therein, are discussed below.

N-phase demodulation (or “frequency chirping”) should in theory make it possible to completely eliminate the interference term between the reference wave and the reference wave (E _R .E _R *), if the experience was perfectly stable over time. This is never perfectly the case, and there therefore remains a fairly large parasitic component. However, this term varies slowly along the x and y directions of the detector plane, which leads, in the space of wave vectors, due to the two-dimensional Fourier transform, to a narrow peak centered on the origin of the coordinates (zone 4 of figure 6).

The demodulation with N phases (or “frequency chirping”) should make it possible to eliminate the term of interference between the ordinary speckle and the speckle - ordinary (Eι.Eι *)> if the experiment was perfectly stable over time, and if the speckle remained static without decorrelating. This is never perfectly the case, and there therefore remains a fairly large parasitic component (zone 2 of FIG. 6). This term is even the dominant noise term for certain objects to be imaged in which the speckle does not remain static (for example for certain biological tissues). This interference term between the ordinary speckle and the ordinary speckle is, like the interference term between the reference wave and the reference wave, centered on the origin of the vector space. wave. Due to the use of the previously defined spatial filtering device, the amplitude of the ordinary speckle field has a finite angular extent, which corresponds to the interval [-Y / 2L; Y / 2L]. As the interference term between the ordinary speckle and the ordinary speckle corresponds to the intensity of the field, that is to say to the product of the complex amplitude with the conjugate complex amplitude, it is necessary, to evaluate l In the space of wave vectors, convolve in the space of wave vectors the speckle field with itself. This interference term therefore has an angular extent twice as wide as the field itself (interval [-Y ₂ = -Y / L; Y ₂ = Y / L]). Similarly, in height in the space of wave vectors, this term corresponds to the interval [-X ₂ = -X / L; X ₂ = + X / L], if this satisfies the aliasing condition in the optimum case (X / L = λ / 2d _x ). This noise thus presents an envelope of delimited pyramidal shape which is centered on the origin of the coordinates of the space of the wave vectors. The interference between the acousto-optic speckle and the acousto-optic speckle (E _S .E _S *) is a diagonal second order term. Apart from its lower intensity, this term does not differ from the interference term between the ordinary speckle and the ordinary speckle described above. The reference wave REF being at the frequency f _R and the ordinary speckle being at the frequency f _τ , the interference term between the reference wave and ^' the ordinary speckle (E _R .Eι *) is at a frequency approximately f _R - f _τ approximately equal to f _A or approximately 2.2 MHz. This interference term is thus averaged at zero during the duration of acquisition of each image due to the low acquisition frequency of the detection device and can therefore be neglected. Similarly, the interference term between the ordinary speckle and the acousto-optic speckle (E _T _.E _S *), in addition to being a second order term, also has a frequency approximately equal to the frequency of l acoustic wave f _A and can therefore average zero at the time of acquisition of each image. It can therefore be overlooked.

The relevant term to extract from the interference between the signal wave SIG and the reference wave REF is therefore the term of interference between the reference wave and the acousto-optic speckle (E _R .E _S *). This term corresponds to zone 3 and to the interval [Y3-; Y3 ₊ ] of Figure 6. As this term corresponds to the interference of a speckle from diaphragm 2 and a plane wave, the interval [Y ₃ _; Y ₃₊ ] has the same angular width as the viewing angle of the diaphragm is Y ₃ - - Y ₃₊ = Y / L. This term changes over time at the detection frequency f _R -f _A o (= fc / 4 in the case of 4-phase demodulation, and = 0 in the case of "frequency chirping"). It can therefore be measured by the electronic detection device. Furthermore, as the reference wave is angularly offset, the center of the area 3 is angularly offset by θ _γ (where the angle θ _γ is, as defined above, the angle between the reference wave REF and l signal wave SIG). The choice of the angle θ _γ defines the position of the center of the zone 3, which is in any case of extent Y / L, when a diaphragm of width Y is used. It is therefore necessary to ensure that the zones 2 and 3 do not overlap in order to obtain only useful information on the pixels of zone 3.

For example, we can choose θ _γ = 3Y / 2L, which gives Y ₃₊ = 2Y / L, and Y ₃ - = Y / L = Y ₂ . Thus, the outer edge of zone 2 and the inner edge of zone 3 overlap without the two zones overlapping. There is then also no blank part between zones 2 and 3 of figure 6. One thus separated in the space of the vectors wave the interference signal of the reference wave with the acousto-optic speckle of the terms of the intensity of the reference and the intensity of the speckle, which are located in the center (zones 4 and 2 of the figure 6) the space of wave vectors.

The extent of zone 3 being directly proportional to Y, we will be tempted to increase the width of the diaphragm so much. that we respect the condition of "aliasing" Y ₃₊ <λ / 2d _γ . There is however a compromise to be made between on the one hand maximizing the surface of zone 3 by increasing the width Y of the diaphragm, and on the other hand maximizing the integral of the product between surface and efficiency in this zone. This efficiency corresponds to the loss of contrast of the interferograms due to the integration of the interferograms on pixels of finite size. For contiguous pixels, there is an efficiency factor following a sine law similar to that introduced previously for "aliasing". In this case, the efficiency is canceled when the angle of "aliasing" λ / 2d _γ is reached. For example, Y ₃₊ could be chosen to be significantly lower than the "aliasing" limit, for example Y ₃₊ = (2/3) .λ / 2d _γ approximately, to leave approximately one sixth of the surface of the detection device inactive on each side, so that the effectiveness is sufficient. Y ₃₊ being equal to 2Y / L, this makes it possible to adapt, according to the laser used and the detector used, the characteristics of the spatial filtering device.

FIG. 6 thus represents the cartography of the zones obtained in the space of the wave vectors by the present invention after the two-dimensional Fourier transform. This map can be broken down into a central column or zone 2, of extent [-Y / L, Y / L], a left column or zone 1, and a right column or zone 3. Inside the zone 2, zone 4 represents the interference term between the reference wave and itself (E _R .E _R *). In the first order, if there is no acoustic signal, the noise observed on the image outside of zones 2 and 4 corresponds to the “shot-noise” noise associated with the reference wave. In the presence of an acoustic signal, zone 3 represents the region of the space of wave vectors where the useful signal according to the invention is detected. According to the sign of the angle of incidence θ _γ , zone 3 could of course be located on the left of FIG. 6. • The relevant information, which makes it possible to calculate the complex amplitude E _s of the acoustic component, corresponds to the interference between the reference wave and the acousto-optic speckle (E _R .E _S *). Numerical information relating to the measurement point (U, V, W) of the object to be imaged is for example obtained by summing the intensities calculated on the pixels of the area 3 (. | TF E _s | ² ). The ^'sum of the intensities of pixels in zone 1 of the symmetric region 3 with respect to the vertical axis of Figure 6 can be used as control. It is also possible to subtract from the sum of the intensities of the pixels of zone 3 the sum of the intensities of the pixels of zone 1, which makes it possible to reduce any systematic measurement errors. The sum of the intensities measured in zone 2 is also carrying information, because it characterizes the noise level associated with the decorrelation of the speckle.

It is also possible to use conventional image processing means, to average, after 4-phase demodulation, Fourier transform and calculation of the intensity, the intensities of the pixels for example 8 to 8, 16 to 16, or 32 to 32, in order to display a map controlling the signal obtained for the measurement point, with coordinates (U, V, W) (to check, for example, the positioning of the different zones). We can also be content to carry out for each line of the detection device, a one-dimensional Fourier transform (according to y for the examples considered), in which case the signal Fi E _s (U, V, W, k) is not exactly that shown in FIG. 6, but the zone 4 extends over the entire extent in X of this figure. The rest of the processing remains valid.

This gives the digital information sought, relating to the measurement point, of coordinates (U, V, W) of the object to be imaged OBJ, in the case of 4-phase demodulation. For “frequency chirping”, or the like, the digital information thus obtained is relative to the acoustic frequency f _A used, and the coordinates and the digital information relating to different points of the interval are obtained [U-Dx; U + Dx] by the means described above.

The choice of the angle θ _γ makes it possible to properly separate the signals obtained, but the adequate positioning of the device making it possible to use a given angle θ _γ , may require a control step. In this control step, an image is obtained, for example by suppressing the acoustic wave and by adjusting the frequency f _R of the reference wave, so as to detect the component of the field scattered at the frequency f _τ . We choose for example f _{R ≈} fx + f _c / 4 and we carry out a 4-phase demodulation. The lens is positioned precisely so that a clear image of the area seen through the diaphragm 4 is obtained by Fourier transform of the signal detected in the plane of the detection device. As seen above, in a Fourier transform calculation, the edge of the calculation matrix corresponds to the “aliasing” condition. The control step ensures, for example, that the outer edge of the area containing the useful signal is not too near the edge of the calculation matrix, and / or that the noise-containing area and the area containing the useful signal are in contact at the internal edge of this area, but do not overlap. FIG. 7 represents a second example of implementation of the method according to the invention in which the spatial filtering device is not used. Indeed, for low laser intensities, and if the speckle decorrelation term is not too large, the “shot noise” becomes the dominant noise. This is the case when, with the first example of implementation (FIG. 5), the values of the sums, measured in zones 1 and 2, are close. This configuration is also useful for other applications where the acoustic wave is of sufficient power to achieve a sufficient acousto-optical conversion efficiency. The signal associated with the acoustic component can then be greater than the speckle decorrelation noise.

We can bypass the spatial filtering device because, even if the interference term between the ordinary speckle and the ordinary speckle (E _I .E _I *) can no longer be spatially separated from the relevant term

(E _R .E _S *), it remains much lower than the “shot-noise” of the reference wave. It is also possible in this case to choose a zero angle θ _γ , for example using a semi-reflecting plate. Only the interference term between the reference wave and itself (E _R .E _R *), which is very centered on the few pixels of zone 4 of FIG. 6, constitutes a noise term to be filtered. This filtering is carried out simply by digitally eliminating the components of the space of the neighboring wave vectors of

(0,0) by excluding zone 4 from the pixel summation of the signal intensities. This noise term may even be negligible, and we can then perform the sum of the intensities on the pixels no longer in the space of the wave vectors, but in the real space (and it is no longer necessary in this case to carry out a Fourier transformation). FIG. 8 represents another generation device used to implement the method according to the invention in the particular case of the second example of FIG. 7. The EMI emission wave, of wavelength λ, emitted by the LAS laser, is intercepted by an AM amplitude modulation device. This device generates a carrier wave POR, of frequency f _τ , and two lateral bands modulated in amplitude LATMOD and LATMOD ', of frequency f _R. These three waves are applied to a semi-transparent slide, which transmits part of each of these waves, applied to the object to be imaged OBJ. The transmitted part of the carrier wave constitutes the incident wave INC. In addition, the semi-transparent plate reflects a part of each of these waves towards the detection device, the reflected part of LATMOD constituting the reference wave REF.

This device is only suitable for the second embodiment, where no spatial filtering device is used, because in the case of a lateral intensity modulation band, spatial filtering does not allow the different terms d 'interference.

FIG. 9 describes a detail of a third embodiment of the present invention and of the spatial filtering device. Many characteristics have already been described during the presentation of the first embodiment, and will therefore not be described again. In this third embodiment, the reference wave REF is no longer a plane wave, but a spherical wave of frequency f _R. Such a spherical reference wave can for example be obtained from the reference wave plane generated by a GEN generation device previously described, by focusing the plane reference wave, using a lens 5, on a small mirror 4 situated for example in the plane of the diaphragm 2. The reference wave REF can also arrive at the mirror 4 at an angle θ _γ so that it is reflected and arrives at the detection device DET by forming an angle θ _γ with the signal wave SIG. In this embodiment, a converging lens 3 is not necessarily used to collimate the signal wave in the direction of the DET detector, in fact, the divergent nature of the reference wave fulfills an analogous role.

In addition, the diaphragm 2 can be removed from the sample holder of the object to be imaged OBJ, and placed between the tank and the DET detection device, or possibly fix it on the downstream face of the tank 1. This arrangement can be advantageous if the acoustic coupling between the transducer TRANS, of acoustic frequency f _A , and the object to be imaged OBJ is produced by a tank filled with water 1, and this latter is located on the light path traveled by the REF reference wave before reaching the mirror 4. (This arrangement is also valid for the other embodiments where a diaphragm is used.)

The processing of the interferograms measured is then identical to the processing explained above, where L now represents the distance between the diaphragm 2 and the DET detection device.

FIG. 10 represents the diaphragm 2 provided with the mirror 4, viewed from the DET detection device. The diaphragm 2 has a slot, for example central, of width Y and height X. To obtain an image as shown in FIG. 6, a mirror 4, for example circular, is placed at mid-height of the slot. The mirror 4 is moreover laterally offset with respect to the slot of the diaphragm 2. This lateral offset is linked to the offset of zones 2 and 3 of FIG. 6 that one seeks to achieve, once the calculations have been made by the processing device CALC. By choosing for example an angular lateral deflection equal to θ _γ between the source point of the reflected reference wave and the edge closest to the mirror of the diaphragm slot 2, seen from the detection device, zones 2 are obtained. and 3 of Figure 6 juxtaposed, but not overlapping. In this case, we will of course be careful to choose a mirror 4 of dimensions such that the mirror does not encroach on the slot of the diaphragm 2. Thus, it is easy to separate the zones 2 and

3 of figure 6.

This embodiment also makes it easy to find an optimal position of the DET detector making it possible to obtain good quality information for the point studied (U, V, W) of the object to be imaged OBJ. Indeed, by increasing L, that is to say by moving the detection device DET away from the diaphragm 2, the angle between the signal wave SIG and the reference wave REF decreases, so that the condition of "Aliasing" is better respected.

On the other hand, the signal intensity decreases with this distance L. The compromise to be made between the efficiency of the detection and the size of the area 3 of FIG. 6, already mentioned, is simply achieved by the positioning of the detection device DET with respect to the diaphragm 2.

In this embodiment, the optimal positioning of the detector is achieved independently of the relative positioning of the zones 2 and 3 of FIG. 6, which is itself achieved by the adequate placement of the focal point of the reference wave (center of the mirror 4 on the diaphragm 2).

By implementing the method described above, according to any one of its embodiments represented in FIGS. 1 to 10, digital information has been obtained relating to the measurement point, of coordinates (U, V, W) of the object (in the case of demodulation with N phases) or relating to each point of the interval ([U-Dx, U + Dx], V, W) of the points of the object (in the case of "frequency chirping" or other analogous method), that is to say a point information or 1D.

To obtain an image of the 2D or 3D object, it is necessary to repeat all the operations described above after having moved the position of the focal point of the acoustic wave in the other directions (x or y or z for demodulation to N phases) (y or z for “frequency chirping”). The displacement of the position of the focal point can be obtained, either by moving the transducer (in the x, y or z direction considered) while maintaining the acoustic coupling between the transducer and the object to be imaged, or by using several transducers of different focal points, for example. It may be advantageous to move the spatial filtering device COL (if one is used), the DET detector, or even the device used to make the reference wave interfere with the detector, and all or part of the GEN generation device jointly to the displacement of the position of the focal point ie of the coordinate measuring point (U, V, W). Alternatively, it may possibly be preferable to move only the object to be imaged in the opposite direction so as not to have to perform a movement similar to the rest of the installation.

As an acoustic transducer is used, any device according to the invention can also be coupled to a conventional acoustic imaging device. The coupling of these two devices thus makes it possible to obtain the object to imaging OBJ purely acoustic information supplied by the acoustic transducer by a conventional ultrasound imaging technique, and optical information supplied by the device according to the invention.

Claims

1. A method of acousto-optical imaging of an object to be imaged (OBJ) comprising the steps consisting in: (a) generating an incident optical wave

(INC), of frequency f _τ , and a reference optical wave (REF), of frequency _ f _R , this reference wave being coherent with the incident wave (INC), and having with it a phase difference φi ( t) known, (b) vibrate in a first object direction (x ₀ ) and at an acoustic frequency f _A , an area (Dx, Dy, Dz) of the object to be imaged (OBJ) using a vibration generating device (TRANS),

(c) applying said incident wave (INC) to the object to be imaged (OBJ), and thus generating a scattered signal wave (DIF),

(d) applying at least part of the signal wave scattered on a detection device (DET),

(e) applying the reference wave (REF) to the detection device (DET) without passing it through the object to be imaged (OBJ), which generates an interferogram at point r of the detection device (DET) I (r, t) varying over time t,

(f) extracting digital information from the interferogram I (r, t), and

(g) obtain the coordinates (U, V, W) of a measurement point of the object to be imaged (OBJ), to which the digital information is relative.

2. An acousto-optical imaging method according to claim 1, in which during step (f), an acoustic component of the part of the scattered signal wave applied to the detection device (DET) is detected, this acoustic component being at a frequency corresponding to the sum of the frequency f _x of the wave incident (INC) and a harmonic of the acoustic frequency f _A.

3. The acousto-optical imaging method according to claim 2, in which, during step (a), said reference wave (REF) is generated at a frequency f _R equal or substantially equal to the sum of the frequency fi of the incident wave (INC) and of said harmonic of the acoustic frequency f _A.

4. The acousto-optical imaging method according to claim 3, in which during step (b), an acoustic wave is generated focused at a focal point located in the object to be imaged (OBJ), and in which , during step (g), we obtain the coordinates (U, V, W) of the point. as the coordinates of said focal point.

.

5. The acousto-optical imaging method according to claim 4, in which steps (a) to (g) are repeated for different focal points of the acoustic wave in the object to be imaged (OBJ), these different focal points being aligned in the first direction of object (x ₀ ).

6. An acousto-optical imaging method according to any one of claims 1 to 3, in which, during a first iteration, steps (a) to (f) are carried out for a first frequency f _A of l acoustic wave and a first frequency f _R of the reference wave (REF), during at least a second iteration, steps (a) to (f) are repeated for a second frequency f ' _A of the acoustic wave and a second frequency f ' _R of the reference wave (REF), these second frequencies being coded respectively with the first frequencies, the method further comprising a step during which:

(f ') at least one digital information is obtained by decoding said digital information obtained during steps (f) of each iteration as a function of the frequencies used, and in which, during step (g), the coordinates (U, V, W) of at least one point ^' of ^measuring the object to be imaged (OBJ) to which the digital information obtained during step (f ') is related, by decoding said digital information obtained during steps (f) of each iteration as a function of frequency used.

7. Method according to claim 6, in which the following series of operations is carried out:

- the frequency of the acoustic wave is scanned, which is focused on a range of coordinate points ([U-Dx, U + Dx], V, W) extended around the coordinate point (U, V, W) along the first direction of the object (x ₀ ), a scanning of the frequency f _R of the reference wave (REF) is carried out jointly so as to maintain f _R substantially equal to or equal to fi ± Hf _A , H being a non-zero integer,

- for each pixel r and for each frequency f _A, an interferogram I (f _A , V, W, r) associated with the set of points ([U-Dx, U + Dx], V, W) of l is recorded 'extended interval; - for each pixel r, a 1D frequency - time Fourier transformation is carried out according to the frequency f _A of the interferogram I (f _A , V, W, r), and at least one associated interferogram I (r) is obtained at least at a coordinate measuring point (U ', V, W) by replacing the time obtained after the Fourier transform by the dimension U' following the first direction of object (x ₀ ) using speed propagation of the acoustic wave in the object to be imaged.

8. An acousto-optical imaging method according to any one of the preceding claims, in which at least steps (a) to (g) are repeated after having imposed a displacement, of the vibration generating device relative to the object to be to image (OBJ) in a direction not parallel to the first object direction (x ₀ ) of the object to be imaged (OBJ).

9. An acousto-optical imaging method according to claim 1, in which during step (f), the complex amplitude E _s (r) of the acoustic component is estimated from the interferogram. I (r, t).

10. The acousto-optical imaging method according to claim 9, in which the detection device (DET) used is a mono-pixel detector, and in which, during step (f), the information is obtained numerical as being the intensity of the complex amplitude field E _s (r) scattered by the object.

11. The acousto-optical imaging method according to claim 9, in which the detection device used is a multi-pixel detector, and in which during step (f), the digital information is extracted as being the sum over at least part of the pixels r of the device for detecting the intensity of the field of complex amplitude E _s (r) scattered by the object.

12. The acousto-optical imaging method according to claim 9, in which, during step (d), a spatial filtering device (COL) is used, so as to limit, in at least one direction, the angular extent of the part of the scattered signal wave (DIF) which is seen by each pixel of the detection device.

13. The acousto-optical imaging method according to claim 12, in which a spatial filtering device (COL) is used comprising a diaphragm, dimensions X in a first diaphragm direction and Y in a third diaphragm direction, and a focal lens L, of object focus located directly downstream of the object to be imaged (OBJ) to limit the angular extent of the part of the scattered signal wave (DIF) which is seen by each pixel of the detection device, and in which the reference wave (REF) applied to the detection device (DET) is generally a plane wave.

14. A method of acousto-optical imaging ^'according to claim 12, wherein a spatial filtering device is used (COL) comprising a diaphragm dimensions X along the first direction of the diaphragm and Y along the third direction of the diaphragm, disposed between the object to be imaged (OBJ) and the detection device (DET) at a distance L from it, to limit the angular extent of the part of the scattered signal wave which is seen by each pixel of the detection device , and in which the reference wave (REF) applied to the detection device (DET) is a spherical wave coming from a source point located in the plane of the diaphragm.

15. An acousto-optical imaging method according to any one of claims 12 to 14, in which the reference wave (REF) and the scattered signal wave (DIF) interfere with the detection device (DET) by forming a non-zero angle θ _γ , θ _γ being measured in the plane of incidence of these two waves on the detection device.

16. An acousto-optical imaging method according to any one of claims 12 to 15, in which the detection device used is a multi-pixel detector, and in which the part of the acoustic component, of complex amplitude, is isolated. E _s (r), which varies quickly in space in the plane of the detection device.

17. An acousto-optical imaging method according to any one of claims 12 to 16, in which the detection device (DET) comprises pixels arranged in a matrix comprising lines in a first detector direction (xD) and columns in a third detector direction (yD), and in which step (f) comprises the following steps: (fl) _. at least one line (1) or column (c) is made a one-dimensional Fourier transform along this line (1) or column (c) of the detection device (DET.) towards the vector space wave, of the complex amplitude of the field E _s (r), and one thus obtains for this row or column a field TFi E _s (k),

(f2) several summation zones are defined in the space of wave vectors,

(f3) the intensities of the field TFi E _s (k) are summed in at least one zone at each point k of this zone, and

(f4) the digital information is extracted as being a linear combination of the sums thus obtained in each zone.

18. An acousto-optical imaging method according to any one of claims 12 to 16, in which the detection device (DET) comprises pixels arranged in a matrix comprising lines in a first detector direction (xD) and columns according to a third detector direction (yD), and in which step (f) comprises the following steps:

(fl) a two-dimensional Fourier transform of the complex amplitude E _s (r) is made, from the plane of the detection device (DET) to the space of the wave vectors, and a TF field is thus obtained ₂ E _s (k), (f2) several summation zones are defined in the space of wave vectors,

(f3) the intensities of the field TF ₂ E _s (k) are summed in at least one zone at each point k of this zone, and

19. An acousto-optical imaging method according to any one of claims 15 to 18, in which the angle θ _γ is approximately equal to 3Y / 2L, in which, during step (f2), we define a first summation zone, called the central zone, a second summation zone, called the left zone, and a third summation zone called the right zone, and in which, during step (f4), the numerical information as a linear combination of the value of the sum of the left area and the sum of the right area.

20. An optical imaging method according to any one of claims 1 to 11, in which during step (a), a laser source of wavelength λ, emits an emission wave (EMI), of frequency f, amplitude modulation means (MA) of the emission wave (EMI), generate a carrier wave (POR) of incident frequency f _{I r} and at least one side band of amplitude modulation ( LATMOD), which corresponds to a wave of frequency f _R , a semi-reflecting mirror, transmits part of the lateral band wave (LATMOD) and part of the carrier wave (POR) forming the incident wave (INC), and reflects part of the carrier wave (POR) and part of the sideband wave (LATMOD) forming the reference wave (REF).

21. An acousto-optical imaging method according to any one of claims 1 to 19, in which during step (a),

- a laser source of wavelength λ, emits an emission wave (EMI), of frequency f _L a first acousto-optical modulator (MAOl) transmits part of the emission wave (EMI) to form l incident wave (INC) on the object to be imaged (OBJ), and also generates a first frequency-shifted wave (DEC), whose frequency is shifted by a value δfi, possibly negative, relative to the wave emission, and

- a second acousto-optical modulator (MA02) intercepts the first frequency-shifted wave (DEC) and generates a second frequency-shifted wave, the frequency of which is offset by a value δf ₂ , possibly negative, relative to the wave offset (DEC), the second wave offset in frequency forming the reference wave (REF), the frequency of which is thus shifted in frequency with respect to the incident wave (INC) by a value δf = δfι + δf ₂ , thus determining a known phase difference cpi (t) between these two waves.

22. An acousto-optical imaging method according to any one of claims 1 to 19, in which, during step (a), two independent laser sources, locked in phase by an electronic servo, generate the incident waves (INC) and reference (REF), having a phase difference φi (t) known between them.

23. An acousto-optical imaging method according to any one of claims 1 to 19, in which during step (a), a laser source of wavelength λ, emits an emission wave (EMI ), of frequency f _L , a semi-reflecting mirror transmits part of the emission wave (EMI) to form the incident wave (INC) on the object to be imaged (OBJ), and transmits a second part of the emission wave (EMI) ), - a first acousto-optical modulator

(MAOl), intercepts the second part of the emission wave and generates a first frequency-offset wave (DEC), of frequency offset by a value δfi, possibly negative, with respect to the emission wave, and - a second acousto-optical modulator

(MA02) intercepts the first frequency offset wave (DEC) and generates a second frequency offset wave, the frequency of which is offset by a value δf ₂ , possibly negative, with respect to the offset wave (DEC), the second wave shifted in frequency forming the reference wave (REF), the frequency of which is thus shifted in frequency with respect to the incident wave (INC) by a value δf = δfι + δf ₂ , thus determining a phase difference φ ± (t) known between these two waves.

24. An acousto-optical imaging method according to any one of the preceding claims in which the object to be imaged (OBJ) is a biological tissue.

25. Acousto-optical imaging method according to any one of the preceding claims, in which the vibration generator device (TRANS) is used to obtain acoustic information of the area (Dx, Dy, Dz) of the object to be imaged. (OBJ), and in which the digital information extracted in step (f) is used in conjunction with said acoustic information.

26. Installation for acousto-optical imaging of an object to be imaged (OBJ) comprising: means for generating an incident optical wave (INC), of frequency fi, and a reference optical wave (REF) of frequency f _R , this reference wave being coherent with the incident wave (INC) and having with it a known phase difference φ ± (t), a vibration generating device for vibrating in a first object direction (x ₀ ) and at an acoustic frequency f _To a zone (Dx, Dy, Dz) of the object to be imaged (OBJ), means for applying said incident wave (INC) to the object to be imaged (OBJ), thus generating a scattered signal wave (DIF), - a detection device (DET), means for applying at least a part (SIG) of this scattered signal wave (DIF) to the detection device (DET), means for applying the reference wave (REF) to the detection device (DET) without passing it through the object to be imaged (OBJ), which generates at point r of the detection device (DET) ^' an interferogram I (r, t) varying over time t , and means (CALC) for extracting digital information and the coordinates (U, V, W) of a point from the interferogram measurement of the object to be imaged, to which the digital information is relative.

27. Installation of acousto-optical imaging according to claim 26 further comprising the following elements: means for viewing said digital information relating to said measurement point of the object to be imaged, and means for moving the object to be imaged ( OBJ).

28. Installation of acousto-optical imagery according to claim 26 or 27, further comprising a spatial filtering device located downstream of the object to be imaged.