Patent Description:
Sound, which propagates as a pressure wave, can be induced in virtually any material, including biologic tissue, whenever time-varying electromagnetic energy is absorbed. When the stimulating radiation that induces these thermally generated acoustic waves is optical, the term "photoacoustic" applies to this effect.

Photoacoustic application is a known technique to inspect an objection. Specifically, a photo-induced acoustic wave is used to detect the optical absorption coefficient at different locations of the object or the propagation property of the object. Consequently, photoacoustic application can be used to monitor various properties (e.g. sound speed dispersion) of the object. Moreover, by using the scattering of the generated acoustic wave, the internal structure of the object can be reconstructed using various photoacoustic (PA) imaging techniques.

Photoacoustic spectroscopy is another important field for material analysis. It uses the absorption spectrum of light within a material, i.e. within an object, to determine the concentration or distribution of a certain material. This can be done by directly measuring the generated acoustic waves based on the photoacoustic effect. Photoacoustic spectroscopy is advantageous compared to normal spectroscopy in situations in which light signals can be strongly scattered within the object since acoustic signals are not easily scattered due to their long wavelength such that the acoustic signal can be more easily measured compared to the transmission light signal in normal spectroscopy.

<CIT>, <CIT> and <CIT> disclose systems to perform photoacoustic spectroscopy. These systems use a light source with a diffraction grating to direct an optical light beam towards the object. A detector is used to detect a sound wave generated within the object.

A downside of the known systems is the efficiency of the PA effect. Specifically, the PA effect is not efficient enough to generate sufficiently strong acoustic signals. This is mainly due to a low energy transfer efficiency from photo-thermal effect to an acoustic signal. Moreover, as the acoustic wave propagates to all directions, the measured vibration at the detection location site will be even weaker.

From a theoretical point of view, investigations have been done to increase the efficiency of the PA effect. <NPL> disclose that the acoustic wave amplitude may be increased when the motion of the optical source is synchronized to that of the acoustic wave. However, no practical application of this theory is proposed.

The document <CIT> discloses time-delayed photoacoustic excitation at different line-shaped locations on an object, with appropriately triggered laser diodes, for generating predetermined acoustic modes.

The article <NPL> discloses the generation of directive ultrasound waves in an inspection object by constructive interference in predetermined directions, using delayed photoacoustic excitation at different points.

It is an object of the present invention to provide a system for photoacoustic inspection, in particular photoacoustic inspection, of an object having an improved signal-to-noise ratio (SNR).

This object is achieved according to the invention with a system according to claim <NUM> or according to claim <NUM>.

By using a direction apparatus to sequentially direct the emission beam to different locations on the object, multiple acoustic waves are generated within the object. By appropriately choosing the locations and times, the multiple acoustic waves are such that they at least semi-constructively interfere to generate a propagating acoustic wave having an amplitude that is greater than any one of the individual acoustic waves. Consequently, the resulting propagating acoustic wave has an enhanced signal strength such that it is easier to detect by the vibration sensing system than any of the individual acoustic waves thereby improving the SNR when compared to a system where only a single acoustic wave is generated.

The use of a broadband source and at least one spectrum splitter allows to generate multiple (i.e. the first and the second) propagating acoustic wave within the object for different wavelengths. This improves the speed at which a PA spectrum may be generated for the object when compared to an emission source emitting a single wavelength.

The direction apparatus comprises a plurality of spectrum splitters, each of the spectrum splitters corresponding to one of the plurality of locations.

Providing a spectrum splitter at each location avoids having to provide one or more moving spectrum splitters to cover all locations. It will be readily appreciated that such moving spectrum splitters would make the system more complicated both during set-up and during operation as the movement of the spectrum splitters would have to be coordinated with the locations and times at which the emission beam needs to impact the object.

In the system of claim <NUM>, the broadband source comprises a plurality of broadband sources, each broadband source corresponding to a spectrum splitter, and the direction apparatus comprises a switching array configured to sequentially activate at least one of the plurality of broadband sources.

Such a system has the advantage that no moving broadband source is required, which, as described above, leads to a less complicated system.

In the system of claim <NUM>, the direction apparatus comprises routing means configured to sequentially direct the emission beam from the broadband source to one of the plurality of spectrum splitters.

Moreover, both alternative systems provide flexibility in designing the system. While a switching array reduces the total number of moving parts, the routing means allow to only use a single broadband source.

In an embodiment of the present invention, the vibration sensing system comprises a vibration sensor, such as an accelerometer, preferably a contactless vibration sensor, such as an on-chip interferometer with at least one membrane, more preferably a laser Doppler vibrometer.

A contactless vibration sensor is preferred as this avoids having to place the vibration sensor in direct contact with the object. A laser Doppler vibrometer (LDV) is advantageous as it has a much broader bandwidth when compared to an accelerometer. Moreover, the LDV is also very sensitive which aids in detecting the propagating acoustic wave.

In a preferred embodiment of the present invention, the vibration sensing system comprises a multi-beam laser Doppler vibrometer configured to detect each of the different propagating waves, the multi-beam laser Doppler vibrometer preferably being realized with a photonic integrated circuit.

Using a multi-beam LDV is an easy way to measure the different propagating waves in case of a broadband source while only requiring a single detection device. Moreover, such an LDV may be realized with a photonic integrated circuit (PIC) as disclosed in <NPL>.

In an embodiment of the present invention, the plurality of first/second locations are substantially aligned along a signal direction and are separated by a distance based on the sound speed within the object, the first/second detection location being located further along said signal direction when viewed from a last one of said plurality of first/second locations.

In this embodiment, the system set-up is simplified by using a straight line along which the propagating acoustic wave is generated. Moreover, using the sound speed within the object as a basis for the locations improves the constructive interference effect of the multiple individual acoustic waves.

In an embodiment of the present invention, the system further comprises a mechanical cavity and/or membrane placed close to each detection location, the mechanical cavity preferably having a mechanical resonance frequency that is similar to the acoustic frequency of the respective propagating acoustic wave within the object to be detected at the detection location.

This embodiment further enhances the signal amplitude by using a mechanical cavity and/or membrane. By having a mechanical resonance frequency that is similar to the acoustic frequency of the propagating acoustic wave, the measured vibration can be further enhanced.

The invention will be further explained by means of the following description and the appended figures.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes. The terms so used are interchangeable under appropriate circumstances and the embodiments of the invention described herein can operate in other orientations than described or illustrated herein.

The present invention generally relates to photoacoustic inspection of an object and may be used for various purposes. Examples include measuring food compounds (e.g. to detect trace quantities of pollution or other unwanted substances), oil compound evaluation, non-contact glucose sensing in biological tissue, gas compound evaluation, etc. It will thus be readily appreciated that the term "object" should be interpreted broadly and may include solid, liquid, gaseous, plasma and other states of matter.

Photoacoustic inspection is based on the photoacoustic (PA) effect, which is the formation of acoustic waves following light absorption in a material sample, i.e. the object.

The main idea of the present invention is to introduce a sonic boom in the object by using a photo-acoustic sound source that moves along the object. In other words, a plurality of individual acoustic waves are induced in the object due to the PA effect. These individual acoustic waves interfere with one another and generate a propagating acoustic wave (i.e. a sonic boom). This wave may then be detected by a vibration sensing system at a detection location on the object.

As used herein, the term "semi-constructive interference" means that the acoustic amplitude of the propagating acoustic wave is higher than the acoustic amplitude from any single acoustic source alone. The amplitude comparison is made preferably at the detection location. Mathematically this may be represented as |A_res| > IA_kl for k = <NUM>,. , N where A_res is the amplitude of the propagating acoustic wave at the detection location, A_k is the amplitude of individual acoustic wave k at the detection location and N is the total number of individual acoustic waves.

<FIG> shows a system <NUM> for photoacoustic inspection of an object <NUM>, which does not form part of the present invention.

The system <NUM> comprises a scanning source <NUM> that emits an optical beam <NUM>. A direction apparatus <NUM> is integrated within the scanning source <NUM> and allows the optical beam <NUM> to be rotated as indicated by arrow <NUM>, i.e. the direction apparatus <NUM> comprises routing means (not shown) that rotate the optical beam <NUM>. This rotation enables to move the optical beam <NUM> in a continuous fashion across a surface area <NUM> (i.e. a straight line) of the object <NUM>. Due to the PA effect, the optical beam <NUM> causes the generation of multiple individual acoustic waves within the object that interfere to generate a propagating acoustic wave as indicated by arrow <NUM>. The system <NUM> further includes a vibration sensing system <NUM> that detects the propagating acoustic wave within the object at at least one detection location <NUM>.

The interference of the individual acoustic waves is optimized in case the movement speed of the optical beam <NUM> across the surface <NUM> is similar to that of the sound speed within the object. Although it will be readily appreciated that the system <NUM> also achieves an improved SNR at the detection location <NUM> when the interference of the individual acoustic waves is sub-optimal, i.e. when there is only semi-constructive interference.

In the illustrated embodiment, the direction apparatus <NUM> rotates the optical beam <NUM> along a straight line <NUM> on the surface of the object <NUM>. This causes the propagating wave <NUM> to also propagate along this same direction, also called the signal direction. It is thus advantageous to place the detection location further along this signal direction to detect the peak amplitude of the propagating wave <NUM>.

The system <NUM> comprises a switching source array <NUM> that emits a plurality of optical beams <NUM><NUM>,. , <NUM>N where N is a natural number greater than one that denotes the total number of optical beams <NUM> that may be output from the switching source array <NUM>. A direction apparatus <NUM> is integrated within the switching source array <NUM> and controls which optical beams <NUM><NUM>,. , <NUM>N are output at a certain moment in time. Each output from the switching source array <NUM> has a corresponding impact location <NUM><NUM>,. , <NUM>N on the object <NUM>. The direction apparatus <NUM> is able to control the positions and times at which the optical beams <NUM><NUM>,. , <NUM>N impact the target location <NUM> on the object <NUM>. Due to the PA effect, the optical beams <NUM><NUM>,. , <NUM>N cause the generation of multiple individual acoustic waves within the object that interfere to generate a propagating acoustic wave as indicated by arrow <NUM>. The system <NUM> further includes a vibration sensing system <NUM> that detects the propagating acoustic wave within the object at at least one detection location <NUM>.

The use of a switching source array <NUM> allows to vary the configuration of the impact locations <NUM><NUM>,. An advantageous configuration is where the impact locations are aligned with one another with a distance based on the sound speed within the object <NUM> and the switching source array <NUM> outputs subsequent optical beams <NUM><NUM>,. , <NUM>N with a timing to cause the impact location of the next beam to move along the signal direction with a speed similar to that of the sound speed within the object <NUM>. Although it will be readily appreciated that the system <NUM> also achieves an improved SNR at the detection location <NUM> when the interference of the individual acoustic waves is sub-optimal, i.e. when there is only semi-constructive interference, which may be obtained by a variety of configurations conceivable.

The emission source in the systems <NUM>, <NUM> is typically a laser that emits a single-wavelength beam. Preferably a tuneable laser is used which allows multiple absorption lines of the object to be tested.

<FIG> and <FIG> show an embodiment of a system <NUM> for photoacoustic inspection of an object <NUM> according to the present invention. The system <NUM> comprises a plurality of broadband optical sources <NUM><NUM>,. , <NUM>N, where N is a natural number greater than one that denotes the total number of broadband sources. Each broadband source <NUM><NUM>,. , <NUM>N emits an optical beam <NUM><NUM>,. , <NUM>N towards a plurality of spectrum splitters <NUM><NUM>,. , <NUM>N that form part of a direction apparatus <NUM>. Each spectrum splitter <NUM>i splits the broadband emission 315i into multiple components <NUM>i1,. , <NUM>iM where M is a natural number greater than one that denotes the total number of components (M is equal to three in the illustrated embodiment). The direction apparatus <NUM> further includes optical focussing means <NUM><NUM>,. , <NUM>M for each component that focus the component onto a single target location <NUM>i1,. , <NUM>iM on the object <NUM>. In the illustrated embodiment, the broadband sources <NUM><NUM>,. , <NUM>N are integrated into an optical switching array <NUM> similar to the embodiment of <FIG>. The direction apparatus <NUM> further includes means to control this switching array <NUM> in order to control the activation of the various sources <NUM><NUM>,. The direction apparatus <NUM> is able to control the positions and times at which the optical beam components <NUM><NUM>,. , <NUM>NM impact the target locations <NUM><NUM>,. , <NUM>NM on the object <NUM>. Due to the PA effect, the optical beam components <NUM><NUM>,. , <NUM>NM cause the generation of multiple individual acoustic waves within the object that interfere to generate M propagating acoustic wave as indicated by dotted lines <NUM><NUM>,. <NUM>M in <FIG>. The system <NUM> further includes a vibration sensing system <NUM> that detects the propagating acoustic waves within the object at at least M detection locations <NUM><NUM>,.

The use of a switching source array <NUM> allows to vary the configuration of the impact locations <NUM><NUM>,. An advantageous configuration is where the impact locations of the different optical beam components <NUM><NUM>,. , <NUM>NM are aligned with one another with a distance based on the sound speed within the object <NUM> and the switching source array <NUM> outputs subsequent optical beams <NUM><NUM>,. , <NUM>N with a timing to cause the impact location of the next beam to move along the signal direction with a speed similar to that of the sound speed within the object <NUM>. Although it will be readily appreciated that the system <NUM> also achieves an improved SNR at the detection locations <NUM><NUM>,. <NUM>M when the interference of the individual acoustic waves is sub-optimal, i.e. when there is only semi-constructive interference, which may be obtained by a variety of configurations conceivable.

The vibration sensing system <NUM>, <NUM>, <NUM> of the systems described above typically comprises a vibration sensor to detect the propagating acoustic wave(s) <NUM>, such as an accelerometer. A contactless vibration sensor is preferred as this avoids having to place the vibration sensor in direct contact with the object <NUM>. A laser Doppler vibrometer (LDV) is advantageous as it has a much broader bandwidth when compared to an accelerometer. Moreover, the LDV is also very sensitive which aids in detecting the propagating acoustic wave. In the system of <FIG> and <FIG>, a multi-beam LDV (e.g. realized by a photonic integrated circuit PIC as disclosed in <NPL>) is advantageous as this allows simultaneous measurement of the different propagating acoustic waves <NUM><NUM>,. , <NUM>M using a single device. It will be readily appreciated that each propagating acoustic wave <NUM> may also be detected by multiple vibration sensors simultaneously, for example multiple LDVs that each detect a different component of displacement of the same propagating acoustic wave.

The systems <NUM>, <NUM>, <NUM> may be further enhanced by providing a mechanical cavity and/or membrane placed close to the detection location(s) <NUM> as this enhances the signal amplitude of the propagating acoustic wave(s) <NUM>. Preferably, the mechanical cavity has a mechanical resonance frequency that is similar to the acoustic frequency of the propagating acoustic wave within the object as this further enhances the vibration amplitude. In practice, this may be achieved by having a mechanical cavity resonance frequency within the 6dB band of the frequency of the propagating acoustic wave within the object <NUM>.

It will be readily appreciated that the system <NUM>, <NUM>, <NUM> described above may be used for photoacoustic inspection, such as photoacoustic application or photoacoustic spectroscopy.

<FIG> illustrates a non-claimed method <NUM> for photoacoustic inspection of an object <NUM>, preferably by using the system <NUM>, <NUM>, <NUM> described above.

The method starts by generating <NUM> the optical beam <NUM>, <NUM>, <NUM>. At an initial time t<NUM>, the optical beam <NUM>, <NUM>, <NUM> is directed <NUM> towards an initial location <NUM> on the object <NUM> to generate an initial acoustic wave within the object <NUM>. In the systems of <FIG> and <FIG>, the initial position may be the first position <NUM><NUM>, i.e. the position farthest away from the detection location <NUM>. Subsequently, one or more further optical beams <NUM>, <NUM>, <NUM> are directed <NUM> towards further location on the object <NUM>. In the system of <FIG>, the initial and further locations form a continuous line, while in the systems of <FIG> and <FIG> these positions are distinct from one another and are separated by a certain distance. The times and positions (i.e. the movement speed of the impact locations along the object <NUM>) is such that the initial and the further acoustic wave(s) at least semi-constructively interfere to generate a propagating acoustic wave <NUM> within the object <NUM>. Finally, the generated acoustic wave <NUM> is detected <NUM> at the detection location <NUM>. Steps <NUM>, <NUM> and <NUM> may be jointly performed by a switching array <NUM>, <NUM> as described above by reference to <FIG> and <FIG>.

By sequentially directing the emission beam to different locations on the object, multiple acoustic waves are generated within the object. By appropriately choosing the locations and times (i.e. based on the sound speed within the object), the multiple acoustic waves are such that they at least semi-constructively interfere to generate a propagating acoustic wave having an amplitude that is greater than any one of the individual acoustic waves. Consequently, the resulting propagating acoustic wave has an enhanced signal strength such that it is easier to detect than any of the individual acoustic waves thereby improving the SNR when compared to a method where only a single acoustic wave is generated.

<FIG> will be used to illustrate simulation results when using the system <NUM> shown in <FIG> with a varying number of individual acoustic waves N.

The following assumptions and notations will be used. We assume that the input optical power at an impact location is Pin and the power generated to the acoustic energy is Pa = kPin, where k is the energy transfer ratio. We assume k to be the same for all cases. The generated sound wave power <MAT>, where A is the area of the surface, p is the sound pressure, ρ is the mass density and c is the speed of sound. If we split the input optical power into N impact locations (also termed spots), the sound power for each spot is Pae = Pa/N and the corresponding sound pressure will be <MAT>. The volume of each spot is assumed to be the same. The radius of the spot is denoted as rs. This radius is determined by the size of the light spot, the absorption depth and the scattering size of the light. The area at the boundary of the spot <MAT>. The pressure at rs will be <MAT>. When the sound pressure propagates to a region with a radius r, the sound pressure will be p(r) = p · <MAT>. Therefore the sound pressure decrease inversely proportional to the radius. The wavelength of the sound wave is <MAT>, where f is the sound frequency. The vector of the sound pressure at the radius rs and at time t can be expressed as: <MAT> The sound propagating on the surface is a transverse wave with a vibration direction in the normal direction of the surface. If we have several sound sources, the sound pressure can be summed to obtain the final pressure. <MAT> We also assume the sound pressure wave is not changed by another sound source when it propagate through the various impact locations. This is valid when the photo-acoustic source is not strong. N spots are used to enhance the sound pressure of the propagating acoustic wave with the spots being in a line with a uniform spacing d. To create the optimal interference, the generation of the optical signals <NUM> have a controlled time delay to ensure that it is compatible to the traveling time of the sound pressure between the two adjacent spots. For example, if d = λ/α, then the time delay Δt of the two generation beams is <MAT>. The sound pressure of all spots thus have constructive interference at one direction. The following values are used in the simulation: rs = <NUM> mm, λ = <NUM> mm, <MAT>, f = <NUM> MHz such that c = λf = <NUM>km/s.

<FIG> shows the maximal sound pressure level (in dB = <NUM>*log(P)) for different locations on the object when there is only a single spot, i.e. N = <NUM>. <FIG> shows the same plot for two spots (i.e. N = <NUM>) with a separation distance d = λ/<NUM>. <FIG> shows that the sound pressure level at positive y direction is much stronger, while that it is much weaker at the negative y direction. <FIG> show results for <NUM> spots, <NUM> spots, and <NUM> spots respectively. It is clear that the sound pressure power is focused into one direction by increasing the number of spots.

<FIG> also shows a line at y = <NUM>. <FIG> shows the sound pressure along this line for different values of N. <FIG> shows that by spreading the sound source into more spots, there is a constructive enhancement of the sound pressure in one direction, i.e. the sound is focused more locally in the space domain. It can be seen in <FIG> that at <NUM> away from the centre in the x direction, the sound pressure dropped by around <NUM> dB for N=<NUM>, while for N = <NUM>, the sound pressure level dropped by <NUM> dB. Therefore, if we put several lines (i.e. several parallel switching arrays <NUM>) with a spacing of <NUM>, the cross-talks of different lines will be -30dB. <FIG> shows a similar view as <FIG> but at y = <NUM>. A comparison illustrates that the size of the propagating acoustic wave shrinks when the distance of the vibration measurement location to the pressure source is reduced.

Claim 1:
System (<NUM>) for photoacoustic inspection of an object (<NUM>), the system comprising:
- a broadband emission source (<NUM>) configured to generate an emission beam (<NUM><NUM>, ..., <NUM>N);
- a direction apparatus (<NUM>) connected to the broadband emission source, the direction apparatus comprising N spectrum splitters (<NUM><NUM>, ..., <NUM>N) each configured to split the emission beam into at least a first and a second component (<NUM><NUM>, ..., <NUM>M) having a different wavelength, N being a natural number exceeding one, wherein the direction apparatus is configured to sequentially direct the first component to N first locations (<NUM><NUM>, ..., <NUM>N1) on the object at N times to generate N first acoustic waves within the object, wherein the N first locations and N times are such that the N first acoustic waves at least semi-constructively interfere to generate a first propagating acoustic wave (<NUM><NUM>) within the object, wherein the direction apparatus is configured to sequentially direct the second component to N second locations (<NUM>M1, ..., <NUM>MM) on the object at said N times to generate N second acoustic waves within the object, wherein the N second locations and N times are such that the N second acoustic waves at least semi-constructively interfere to generate a second propagating acoustic wave (<NUM>M) within the object, the first N locations and the second N locations being different from one another, each of the N spectrum splitters corresponding to respective ones of the N first and second locations, wherein the direction apparatus comprises routing means configured to sequentially direct the emission beam from the broadband emission source to one of the N spectrum splitters; and
- a vibration sensing system (<NUM>) configured to detect the first propagating acoustic wave at a first detection location (<NUM><NUM>) on the object and to detect the second propagating acoustic wave at a second detection location (<NUM>M) on the object.