WO2009092391A1

WO2009092391A1 - Ultrasound detection using a gas-liquid interface

Info

Publication number: WO2009092391A1
Application number: PCT/EP2008/000455
Authority: WO
Inventors: Helmut Rathgen
Original assignee: Universiteit Twente
Priority date: 2008-01-22
Filing date: 2008-01-22
Publication date: 2009-07-30

Abstract

The present invention relates to ultrasound characterization. According to the present invention an ultrasound detector is provided that comprises a light source (2), a light detector (3), and a substrate (4) which is in contact with a liquid (5), said liquid having a first gas-liquid interface (6), wherein said light source (2) is arranged to emit light on said gas-liquid interface (6) and said light detector is arranged to detect light from said interface (6), further comprising means to substantially confine said light from said interface towards said detector. By using a gas-liquid interface much higher sensitivities can be obtained compared to known detectors. The present invention further provides a device for ultrasound characterization that comprises such a detector.

Description

Ultrasound detection using a gas-liquid interface

The present invention relates to an apparatus and detector for ultrasound characterization. Ultrasound characterization is a technique known in the art and applications include medical imaging (medical sonography) , materials characterization and industrial process analysis.

A typical ultrasound apparatus comprises an ultrasound source that uses a piezoelectric material to generate an ultrasound wave. Applying a voltage to the piezoelectric material results in a deformation, which, given the right frequencies, results in an ultrasound wave. This wave is then focused and directed to the object under test, where it is reflected. The reflected wave is detected by a detector and the resulting signal can be further processed, for instance to form an image on a screen. Most ultrasound detectors also comprise a piezoelectric material to perform the inverse conversion between mechanical deformation and electrical signal. To achieve a good resolution, the sensitivity of the detector must be high. Sensitivity of a piezoelectric detector is related to the ratio between the voltage generated by the impinging wave and the pressure exerted by that wave. It depends largely on the material properties of the piezoelectric element and the dimensions of the detector.

A drawback of known piezoelectric ultrasound detectors, or known ultrasound detectors in general, is the relatively low sensitivity. Consequently, ultrasound waves must be sent out at relatively high powers to still achieve a good signal to noise ratio. For medical imaging, the use of high power ultrasound waves is not desirable as the impact of these waves is not yet fully known, especially in the case of visualizing fetal tissue. It is an object of the present invention to provide a detector and apparatus for ultrasound characterization in which the abovementioned problems do not occur or at least in a lesser degree. The ultrasound detector according to the present invention comprises a light source, a light detector, and a substrate. The substrate is in contact with a liquid, which has a gas-liquid interface. The light source is arranged to emit light on this interface. Light from the gas-liquid interface is detected by the light detector. The ultrasound detector further comprises means to confine the light from the gas-liquid interface towards the light detector. Without confinement, much light would be lost, resulting in a low sensitivity. The gas-liquid interface is highly sensitive to ultrasound waves. Surface tension in the liquid causes the interface to act as an elastic interface. An applied pressure causes a deflection of the interface, and an ultrasound wave causes an oscillatory deflection. The deflection of the surface results in a change of the optical properties of the light. These changes are detected by the light detector and are used for further processing such as imaging. Possible optical properties that can be used for detection are intensity and phase of the light but the invention is not limited hereto.

To improve sensitivity, it is advantageous if the ultrasound detector comprises means to confine the light emitted from the light source towards the interface. For instance, an optical lens could be used to focus the light onto the gas-liquid interface. It should be noted that these confinement means can be part of or are identical to the confinement means to confine the light from the interface towards the detector. Although the change in optical properties of the light can be determined using light transmitted through the gas-liquid surface, the use of reflected light is more convenient. For instance, a change in optical path length, i.e. the phase of the light, is related. to the deflection of the reflecting surface. Such a change can be detected by the detector and used for subsequent processing and characterization .

In a first embodiment of the present invention, a lens, for instance an optical lens, is used to focus light onto the gas-liquid interface. The so generated light field has the well known intensity minima and maxima in the neighborhood of the focus. The interface is preferably positioned between the focus and an intensity minimum, for instance, the first minimum. The amount of light that is reflected by the gas-liquid interface depends on the position of the interface between the focus and said minimum. Consequently, an ultrasound wave impinging on the interface can be detected by measuring the intensity changes of the reflected light.

Preferably, the gas pertaining to said gas-liquid interface is trapped in a cavity that is at least partly bound by said gas-liquid interface and said substrate. In this situation, the gas pressure prevents the gas-liquid interface from collapsing.

It is advantageous, if the gas-liquid interface comprises a meniscus. The meaning of the word meniscus should in the context of the present invention be construed as the gas-liquid interface pertaining to a liquid that is restrained by a solid, e.g. a substrate. Its interpretation should therefore not be limited to a gas-liquid interface of a liquid within a container such as a bottle, but should also incorporate other forms or means to restrain the liquid. The gas-liquid interface, and a meniscus in particular, may act as a spring having a spring constant that depends inter alia on the surface tension. In addition, the actual shape of the gas-liquid interface also depends on the interplay between the liquid and the substrate. Such an interface demonstrates one or more resonance frequencies. High sensitivities can be obtained if the frequency of the ultrasound wave approaches such a resonance frequency. By choosing a suitable substrate material and liquid as well as choosing a suitable size for the gas-liquid interface, the design of the ultrasound detector can be optimized for a particular frequency or range of frequencies.

To enable formation of the gas-liquid interface, the substrate comprises a structural discontinuity. The gas- liquid interface is obtained by the co-acting of the liquid with said structural discontinuity. Structural discontinuity in this aspect refers to any discontinuity that can be used in conjunction with a liquid to form a gas-liquid interface. The interface that is used for detection results from the interplay between the substrate, and more specifically the discontinuity, and the liquid. However, it is also possible to use a gas-liquid interface not directly related to a substrate or discontinuity. For instance, a droplet of liquid or a gas bubble embedded in the liquid can be used in which the gas-liquid interface used for detection is not related to any discontinuity of the substrate. However, there should be means, for instance in the form of a substrate, to restrict the movement of the bubble or droplet. Nevertheless, the person skilled in the art should appreciate that the gas- liquid interface in case of a bubble or droplet is substantially different from the interface that results from the interplay between liquid and discontinuity. In case of a bubble or droplet, the gas-liquid interface used for characterization is not necessarily the interface that is in contact with the substrate.

The aforementioned structural discontinuity of the substrate could refer to discontinuities at the surface or inside of the substrate. It could be part of a profiled surface of the substrate. For example, a profile could be realized on the surface by means of etching the substrate, with or without lithographic patterning, or by milling, e.g., with a focused ion beam device. The discontinuity could also comprise a hole in the substrate. A hole should be distinguished from a cavity, which constitutes a further possibility for a discontinuity, because the latter in general is not accessible from the outside of the substrate. To improve the stability of the gas-liquid interface, e.g. a meniscus, it is advantageous if the opening pertaining to the discontinuity, e.g. hole or cavity, in which the gas- liquid interface is arranged, widens in a direction away from said liquid. For instance, a meniscus could be present in a hollow cylinder which widens towards the bottom. In this configuration gas is trapped by the meniscus and the cylinder. Alternatively, a hole could be comprised of a top hole and a much wider hole connected thereto resulting in a step discontinuity. It should be appreciated that many more variations are possible within the context of the present invention.

As mentioned above, the light detector is used to detect changes in optical properties of light from the gas- liquid interface, which preferably comprises light reflected from the interface. It is convenient if the light that is emitted onto the gas-liquid surface by the light source is substantially coherent. Further or alternative advantages can be obtained if the light source comprises a monochromatic light source. The term monochromatic should be attributed its usual practical meaning, implicating that a monochromatic source may include several distinctive wavelengths. Preferably, the light source comprises a laser.

Examples of liquids that can be used are water, mercury. Additionally or alternatively, the liquid may comprise a partially wetting liquid on said substrate. Other liquids or composition of liquids are not excluded. Additionally, additives can be used to change the surface tension. Similarly, a suitable choice for the gas is ambient air, Nitrogen and/or an inert gas such as Argon.

To obtain better mechanical stability, it is advantageous if the substrate is hydrophobic. Hydrophobicity can be an intrinsic property of the material that is used or it may result from a treatment, e.g. a surface treatment, of the substrate. Additionally or alternatively, the substrate may comprise a single layer, e.g. a hydrophobic coating layer such as Teflon, Alkylsilane, or Alkylthiol. This layer can be applied using known techniques such as plasma deposition, sputtering or evaporation. Preferably the gas-liquid interface has a contact angle on said substrate larger than 90 degrees. It should be noted here that the term hydrophobic refers to the property of the surface as to repel the liquid. By no means should this term be construed as to limit to the property as to repel water. Rather, it should be understood as the property to repel the particular liquid co-acting with the substrate to form a gas-liquid interface, i.e., it should be understood as oleophobic, in case the liquid is mainly non-polar, such as an oil, or hydrophobic in case the liquid is mainly polar, such as water.

Preferably the substrate comprises a material out of the group of Silicon, Silicates, Gold and polymers. Especially Silicon or another semiconductor or semiconductor compound allows for integrating the substrate with other functionality or components of the detector. For instance, the light source, the gas-liquid interface and the light detector may all be integrated into or onto a single substrate. However, the different components could also be combined on a single carrier using known integration techniques from the semiconductor or thin-film industry. Semiconductor lasers, such as vertical cavity surface emitting lasers (VCSELs) could be used in combination with a light sensitive diode (such as a PIN diode) . Integration could be further enhanced using techniques as micro machining or photonic crystals for light confinement and steering.

The detector may comprise a second interface, wherein the light source is further arranged to emit light onto this second interface. In this case, the light detector is arranged to detect the changes in the interference pattern between light reflected from the second interface and light reflected from the gas-liquid interface. If the second interface is static, e.g. a solid-gas interface that does not substantially alter under the influence of the impinging ultrasound wave, light reflected from this interface constitutes a reference wave. It should be noted that such a reference wave could also be constructed from the light emitted on the gas-liquid surface without said second interface, for instance by means of known beam splitting techniques. The use of interference provides a suitable way to determine the phase of the light from the first surface, albeit relatively.

As mentioned above, confinement means are used to confine and or guide the light from the gas-liquid interface to the detector. As stated before, such means could also be employed to confine and or guide the light from the light source to the gas-liquid interface. The confinement means preferably comprise an optical waveguide such as an optical fiber. Optical fibers are known in the art and generally comprise two ends.

It is convenient if the aforementioned substrate is connected to a first end of said fiber and if the light source and light detector are optically coupled, or connected, to a second end of this fiber. In this way, the fiber guides the light from the light source towards the substrate, where it is reflected, and back to the light detector.

The gas-liquid interface may face the optical fiber or it may reside at the face of the substrate that is directed away from the optical fiber. In case of the latter, the substrate itself may comprise means to guide the light towards the interface, e.g. a hole, or the substrate may comprise translucent material. It should be noted that the use of translucent substrates is not limited to this specific situation only.

Preferably, the ultrasound detector comprises means, for instance an optical directional coupler, which are arranged in between said optical fiber and said light detector and light source, to direct light emitted from said light source into said optical fiber, and to direct light that has been reflected from said gas-liquid interface from said optical fiber to said light detector. As a result, the same optical fiber can be used to transmit the emitted and reflected light.

Additionally or alternatively, the substrate may be an integral part of the optical fiber. In the optical fiber mentioned above, a lens may be included to focus the light onto the interface or to direct or guide the light reflected, or in general emitted, from the interface . The aforementioned discontinuity could comprise a hole in the core in an end face of the optical fiber. Such a hole, could co-act with the liquid such that a meniscus is formed that at least partly, but preferably entirely, covers the hole. For convenient construction it is advantageous if the liquid is encapsulated and if the optical fiber is such that gas can be trapped by the liquid in the hole. In this preferred embodiment, the interface between the core of the optical fiber and the gas that is trapped by the liquid constitutes the aforementioned second interface. Detection is then based on the interference of the light reflected from this second surface and light reflected from the gas-liquid interface .

An example process to construct the fiber could include a focused ion beam milling step to produce a hole in the end face of the optical fiber, a surface treatment to increase the hydrophobicity of the surface, an immersion of the fiber in a suitable liquid, and to subsequently encapsulate the liquid using for instance a cover or a seal. It should be obvious that the ultrasound properties of such encapsulation should be borne in mind.

Preferably, the encapsulation is obtained with a container, which is mainly transparent to ultrasound and which is attached to the optical fiber. The optical fiber could also be a hollow core fiber. Also in this case, a meniscus could be formed. In a hollow core fiber, a significant amount of light propagates in the cladding of the fiber. This light is reflected at the top of the fiber. Because this interface is not significantly affected by the ultrasound wave, light reflected from it can be used to serve as a reference light wave for interference based detection. Additional advantages can be achieved if the fiber is a single-mode fiber and if the wavelength of the emitted light lies in the range from 400 nm to 10000 nm.

The aforementioned second interface may also be dynamic. It could for instance comprise a second meniscus. By having two or more dynamic interfaces, the sensitivity can be increased. It is beneficial if the operation of these menisci is anti-parallel, meaning that deflection-related phase difference of one meniscus should not be cancelled by the phase difference of another meniscus.

Preferably, an optical interferometer should be used to determine the interference of light from said gas-liquid interface and said reference light wave, which can originate from said second interface. These devices are known in the art and therefore do not require a detailed description.

In general, not relating to any specific embodiment, to obtain a compact arrangement of light source, light detector and substrate it is advantageous if said gas-liquid interface has a first side facing the liquid and a second side facing the discontinuity, wherein said light source and said light detector are arranged on said second side. In this arrangement, an ultrasound wave impinging from the first side of the gas-liquid interface is not obstructed by the light source and or light detector. As mentioned before, the gas-liquid interface acts as a spring which may have a resonance frequency. Apart from the surface tension of the liquid, in conjunction with the substrate, this frequency depends on the dimensions of the discontinuity, e.g. the diameter of a hole in the substrate. It is therefore convenient if the detector comprises means to control the size of said discontinuity. In this way, the sensitivity of the detector could be adapted to the frequency of the ultrasound wave or it could be optimized for other techniques, such as a Doppler measurement. Possible means include but are not limited to piezoelectric actuators. Furthermore, these means could comprise an gas pressure regulation system which is arranged to control the size of the discontinuity by regulating a static pressure exerted on the gas-liquid interface.

The present invention further provides a device for ultrasound characterization comprising an ultrasound detector as described above. Such a device could employ an ultrasound source emitting ultrasound at a frequency in the range of 50 kHz - 50 MHz. Such a device could for instance be used as a medical sonograph.

To improve the bandwidth of the device, a multiplicity of ultrasound detectors as previously described could be used. These detectors could have different sizes of discontinuities resulting in different resonance frequencies.

In the following, the present invention will be described in more detail under reference to the accompanying drawings in which: Figure 1 illustrates a cross section of a general embodiment according to the present invention;

Figure 2 illustrates a cross section of a further embodiment according to the present invention;

Figure 3 shows a cross section of a preferred embodiment of the present invention comprising an optical fiber to confine the light;

Figure 4 depicts a schematic overview of an optical setup used in conjunction with the embodiment of figure 3.

A general embodiment of an ultrasound detector 1 according to the present invention is depicted in figure 1. In this embodiment, the ultrasound detector 1 comprises a light source 2, a light detector 3, and a substrate 4 which is in contact with a liquid 5. A gas-liquid interface 6 is formed between the liquid and a gas environment, the latter being for example ambient air. The light source 2 is arranged to shine light 7 (dotted line) onto gas-liquid interface 6. Light from this interface 7' is collected and subsequently guided towards the optical detector 3 by an optical lens 8. The optical lens 8 substantially confines the light from interface 6. An advantage of confining the light is that the power density of the light signal that falls onto the optical detector is increased compared to the situation in which no confinement is done, thereby increasing the sensitivity of the detector.

In figure 1, an ultrasound wave 9 traveling within the liquid 5 in direction 10 falls onto the gas-liquid interface 6. Because the gas-liquid interface 6 is very sensitive to mechanical vibrations, it will start to vibrate, e.g. oscillate, corresponding to the mechanical excitation caused by the ultrasound wave 9. It should be noted at this point that the term ultrasound wave refers to a mechanical vibration that propagates in a medium, in this case the liquid. By no means should this term be construed as to limit the present invention to waves as such, i.e. for instance excluding ultrasound pulses. In addition, the actual propagation direction of the wave or the medium in which it travels could differ from the situation as illustrated in figure 1 without deviation from the scope of the present invention. However, it is advantageous if the wave impedance pertaining to this medium is such that reflection of the wave at various interfaces in the path from the source of the ultrasound wave, being direct or indirect, e.g. an organ in the human body, is kept at a minimum.

The vibration of the interface causes a change in the optical properties of the light from the interface 6. In figure 1, the light 7' from the interface 6 corresponds to the light that is reflected by that interface. Other embodiments that fall within the scope of the present invention are possible in which the light is transmitted instead of reflected, or a combination of both. The optical detector 3 is arranged to detect the change or changes in optical properties of the light 7 ' from the interface, thereby detecting the ultrasound wave 9. An example of an optical property that could be used to detect the ultrasound wave 9 is the intensity of the light 7 ' that is reflected from the interface 6. The vibration of the interface 6 results in a corresponding change of the measured intensity of the light 7¹ at the detector 3.

It should be noted at this point, that the term lens refers to a magnifying system. By no means should this term be construed as to limit to a lens as such, i.e., for instance, excluding compound lenses, such as arrangements of several lenses or optical elements acting as a lens such as diffractive elements or a device known in the art as a Fresnel lens. The gas-liquid interface 6 in figure 1 could be a meniscus, which is formed as a result of the surface tension properties of the -liquid 5 itself as well as the interplay with the gas environment and the substrate 4. In this situation, the substrate 4 confines and/or contains part of the liquid 5 such that a meniscus is formed. However, other gas-liquid interfaces are possible, e.g. a droplet on a substrate, or a bubble, that are equally applicable in, and therefore falling within the ambit of, the present invention. Figure 2 presents a further or alternative development of the general embodiment from figure 1. A light source (not illustrated) now shines light 7 towards a beam splitter 11, which, in this direction, will mainly transmit the light 7 towards the optical lens 8, which in this embodiment focuses the light 7 onto either of the gas-liquid interfaces 6, 6', which in this embodiment comprise menisci (focusing on meniscus 6 is shown in the figure) . Light 7 ' reflected from the menisci 6, 6¹ travels through the same splitter 11, albeit in an opposite direction. The splitter 11 will reflect this incoming light 7' towards the optical detector 3.

The menisci 6, 6' in this embodiment are obtained by- containing a liquid 5, 5' within an area defined by an outer casing of which walls 12 are depicted. Inside this casing, a substrate 4 having an opening 13 is provided. Due to the surface tension of the liquid 5, 5' in combination with the material and shape of the substrate 4, menisci 6 and 6' are formed. The lens is positioned such that the rest position of either of the menisci is between the focus of the lens and an intensity minimum, e.g. the first minimum, of the illuminating light field. A deflection of the interface results in a change in the reflected intensity which is measured with the detector, thereby detecting the ultrasound wave. The use of the narrow spaced intensity minima in the vicinity of the focus of the illuminating light field provides a sensitive detector.

As illustrated, to prevent the ultrasound wave 9 to reach the optical lens 8 and/or detector 3, absorbing material 14, which is known in the art, could be applied to part of the casing walls 12.

This embodiment, as well as the embodiment from figure 1, demonstrate that an ultrasound wave can be detected using a single gas-liquid interface. Combinations of multiple interfaces, as for instance will be shown next in figure 3, are also possible. According to the present invention as defined in claim 1, the substrate is in contact with the liquid. Contact between the substrate and the gas-liquid interface is not necessary per se. For example, the present invention is equally applicable to a gas bubble trapped in a liquid or a droplet of liquid.

In the aforementioned examples, the intensity of light was used to detect the presence of ultrasound waves, although these embodiments are not restricted to this use only. It is also possible to use the phase of a light signal or light wave to determine the presence of an ultrasound wave. This could be based on an absolute measurement of the phase in which the difference in optical path length due to the movement of the gas-liquid interface is measured, or it could be based on a relative measurement between two different light waves of which at least one is affected by the movement of the gas-liquid interface. A possible implementation could be to use a modulated light source that emits light pulses at a certain frequency and at a certain time interval. The detected pulse pattern will differ from the emitted pattern. For example, a pulse emitted while the gas-liquid interface is bent upwards towards the optical detector, will take less time to reach the detector than a pulse emitted while the interface is bend downwards. Hence, differences will occur both in the shape of the measured pulse and the time intervals between pulses.

A preferred method of detection comprises the determination of the relative phase of the light signal from the gas-liquid interface compared to a reference light wave. The interference pattern generated by combining these waves can be used to detect the presence of an ultrasound wave. It should be remembered that a change in optical characteristics is detected, and that in most cases the absolute value is of minor importance.

An example of a detector based on an interference measurement is illustrated in figure 3. This detector comprises a optical fiber 15 having a core 16 and cladding 17. A hole 18 replaces part of the core at one end of the fiber 15. A second larger hole 18' substitutes part of the end face of the optical fiber. It constitutes the walls of a casing that contains the liquid 5. The casing is completed by a cover 19, which is mainly transparent to ultrasound. By choosing the material properties of both the liquid 5 and the fiber 15, in connection with the geometry and the surface properties of the hole, a meniscus 6 is formed that covers the hole 18. Consequently, gas is trapped between the meniscus 6 and the bottom wall 20 of the hole 18. The other side of the fiber 15 is optically connected to a laser such that the light, which is schematically illustrated by bar 1,1', is coupled into the optical fiber 15 where it is guided by the core and cladding towards the bottom wall 20 of the hole 18. At this interface, part of the light 7 will be reflected as a first reflected light wave (reference wave) . The transmitted part will travel towards the meniscus 6 where it will be reflected as a second reflected light wave. Part of this second reflected light wave will be transmitted through the bottom wall towards the opposite end of the fiber 15. The reflected light waves in the core will interfere with each other. This combined wave can be coupled out of the fiber 15 towards the optical detector 3 (not shown) . Changes in intensity due to the interference are then for example used to determine the presence of an ultrasound wave 9.

In this embodiment, the liquid casing is fabricated from the optical fiber itself, however, it should be noted, that this is not the only possibility. The casing can also be provided by a previously independent casing that becomes connected to the fiber 15. Although encased, the detector could also be used without the casing as used and shown in figure 3. For instance, to determine the presence of ultrasound waves in a given liquid, e.g. the sea, the fiber could be immersed in the liquid, thereby forming the meniscus, and the ultrasound wave could subsequently be detected.

Though, the hole 18 as is illustrated in figure 3 has the same diameter as the core 16 of the fiber 15, it should be noted here, that its diameter may in general be larger or smaller than the diameter of the core. The diameter of the hole is chosen according to the desired frequency characteristics and sensitivity of the ultrasound detector. It should also be noted here, that the rest position and stability of the meniscus are related to the geometry and the surface properties of the hole. For instance, in the case of water and a cylindrical hole in an untreated glass fiber, the meniscus may collapse within a time that is related to the solubility of the gas in the water. In contrast, the meniscus can be made stable if a hydrophobic coating is provided to the surface.

A further development is the use of an opening that widens towards the bottom of the hole. This increases the maximum possible deflection of the meniscus, and thereby enlarges the dynamic range of the detector. A further development involves the use of a pressure line, indicated by the dotted line 21, to change the gas pressure in the hole 18 in the fiber 15. This would allow to press liquid out of the cavity, and thereby refresh a collapsed meniscus. Moreover, changing the gas pressure changes the resonance behavior of the meniscus 6. This opens up the possibility to tune the resonance behavior of the meniscus 6 to the frequency of the incoming ultrasound wave 9. Sensitivity can thereby be increased considerably.

A complementary way of tuning the resonance frequency involves a control of the hole diameter. This could be realized by piezoelectric means. The optical fiber 15 in figure 3 could also be a hollow core fiber. In that case, the incident light is not only confined to the core 16 but also extends partly in the cladding 17. A wave is reflected at the interface of the cladding 17 with the liquid 5. At the same time, any reflection from a hole bottom is absent. Consequently, the interferes takes place between with the wave reflected from the meniscus, and the wave reflected from the adjacent cladding-liquid interface . It should be noted that in the embodiments presented in figure 3, the meniscus 6 could be replaced by other gas- liquid interfaces such as but not limited to an interface corresponding to a gas bubble or droplet of liquid. A gas bubble or droplet of liquid could be brought into intimate contact with the fiber 15. Light from the core 16 could be reflected by the gas-liquid interface closest to the core 16 as well as the interface with is on the opposite side. The interference of these two waves can again be used to determine the presence of the ultrasound wave 9. In this particular case, the hole 18 in the optical fiber 15 could serve the purpose to attach the bubble or droplet. However, it should be noted that it can also be omitted.

It should be appreciated that the use of a fiber as means to confine the light should serve to demonstrate the invention. Other solutions are possible to guide and confine the light from the light source towards the gas-liquid interface. For instance, semiconductor material could be processed to accommodate an optical wave guide, e.g. photonic crystals or waveguide in a deposited layer stack. The particular advantage of semiconductor materials is the possibility to integrate different components such as the light detector and light source in a single die, wafer or integrated circuit, allowing for further size reduction. Figure 4 illustrates a possible implementation to couple the light source 2 and light detector 3 to the optical fiber 15, although it could find similar use for other optical waveguides. Light source 2 is connected to an optical coupler 22 having four ports of which port 3 is terminated such that any light incident on that port will be absorbed by adsorbent material 23. Light incident on port 1 will mainly be transmitted to port 2 and port 3, whereas light incident on port 2 will be transmitted to port 1 and port 4. By inserting an optical isolator 24 in between port 1 and the light source 2, only light incident on port 1 will be transmitted, light exiting port 1 will be absorbed by the absorbent material 23 that is connected to the isolator 24.

The optical detector 3 is connected to port 4 whereas the optical fiber 15 is connected to port 2. Light exiting port 2 will enter the fiber 15 where it is subjected to reflection and interference. The light signal exiting the fiber 15 is coupled into port 2 where it is distributed over ports 1 and 4. As mentioned above, the light exiting port 1 is absorbed and the light exiting port 4 is coupled into the optical detector 3.

Although the present invention has been disclosed by means of several embodiments thereof, the skilled person should appreciate that various modification, adjustments and substitutions may be possible without departing from the scope of the invention as defined by the following claims.

Claims

1. An ultrasound detector comprising a light source, a light detector, and a substrate which is in contact with a liquid, said liquid having a first gas-liquid interface, wherein said light source is arranged to emit light on said gas-liquid interface and said light detector is arranged to detect light from said interface, further comprising means to substantially confine said light from said interface towards said detector.

2. The ultrasound detector according to claim 1, wherein said light from said gas-liquid interface comprises light reflected from said gas-liquid interface.

3. The ultrasound detector according to claim 2, further comprising means to confine said emitted light from said light source towards said gas-liquid interface.

4. The ultrasound detector according to any of the claims 1-3, wherein said confinement means comprise an optical lens.

5. The ultrasound detector according to claim 4, wherein said gas-liquid interface is positioned between a focus and a minimum of a light field of said lens.

6. The ultrasound detector according to any of the claims 1-5, wherein gas pertaining to said gas-liquid interface is trapped in a cavity which is at least partly bound by said gas-liquid interface and said substrate.

7. The ultrasound detector according to any of the claims 1-6, wherein said gas-liquid interface comprises a meniscus .

8. The ultrasound detector according to any of the claims 1-7, wherein said substrate comprises a structural discontinuity, wherein said liquid co-acts with said discontinuity to form said gas-liquid interface.

9. The ultrasound detector according to claim 8, wherein said discontinuity is part of a profiled surface of said substrate.

10. The ultrasound detector according to claim 8 or 9, wherein said discontinuity comprises a hole.

11. The ultrasound detector according to any of the claims 8-10, wherein said discontinuity comprises a cavity.

12. The ultrasound detector according to any of the claims 10-11, wherein said gas-liquid interface is arranged in an opening pertaining to said discontinuity, wherein said opening widens in a direction away from said liquid.

13. The ultrasound detector according to any of the claims 1-12, wherein said light source is arranged to emit light which is substantially coherent.

14. The ultrasound detector according to any of the claims 1-13, wherein said light source comprises a monochromatic light source.

15. The ultrasound detector according to any of the claims 1-14, wherein said light source comprises a laser.

16. The ultrasound detector according to any of the claims 1-15, wherein said liquid comprises mainly water.

17. The ultrasound detector according to any of the claims 1-16, wherein said liquid comprises mainly mercury.

18. The ultrasound detector according to any of the claims 1-17, wherein said liquid is a partially wetting liquid on said substrate.

19. The device according to any of the claims 1-18, wherein said gas comprises mainly ambient air.

20. The ultrasound detector according to any of the claims 1-19, wherein said substrate is hydrophobic.

21. The ultrasound detector according to claim 20, wherein said hydrophobicity of said substrate is provided by a surface coating.

22. The ultrasound detector according to claim 21, wherein said surface coating is a material out of the group consisting of Teflon, Alkylsilane, Alkylthiol.

23. The ultrasound detector according to any of the claims 1-22, wherein said gas-liquid interface has a contact angle on said substrate larger than 90 degrees.

24. The ultrasound detector according to any of the claims 1-23, wherein said substrate comprises a material out of the group consisting of Silicon, Silicates, Gold and polymers .

25. The ultrasound detector according to any of the claims 1-24, comprising a second interface, wherein said light source is further arranged to emit light onto said second interface and wherein said light detector is arranged to detect the interference between light from said second surface and light from said gas-liquid interface.

26. The ultrasound detector according to any of the claims 1-25, wherein said confinement means comprise an optical waveguide.

27. The ultrasound detector according to claim 26, wherein said optical waveguide comprises an optical fiber.

28. The ultrasound detector according to claim 27, wherein said substrate is connected to a first end of an optical fiber and wherein said light source and said light detector are optically coupled to a second end of said optical fiber.

29. The ultrasound detector according to claim 28, further comprising means, arranged in between said optical fiber and said light detector and light source, to direct light emitted from said light source into said optical fiber, and to direct light that has been reflected from said gas- liquid interface from said optical fiber to said light detector.

30. The ultrasound detector according to claim 29, wherein said means comprise an optical directional coupler.

31. The ultrasound detector according to any of the claims 28-30, wherein said substrate is an integral part of said optical fiber.

32. The ultrasound detector according to claim 31, wherein said discontinuity comprises a hole in the core in an end face of said optical fiber.

33. The ultrasound detector according to claim 32, wherein said second surface as defined in claim 25 is provided by a bottom of said hole.

34. The ultrasound detector according to any of the claims 27-33, wherein said optical fiber is a hollow core fiber.

35. The ultrasound detector according to claim 34, wherein said second surface as defined in claim 25 is provided by an end face of said optical fiber.

36. The ultrasound detector according to any of the claims 27-35, wherein said liquid is enclosed by a container, said container being mainly transparent to ultrasound and said container being attached to said optical fiber.

37. The ultrasound detector according to any of the claims 27-36, wherein said optical fiber is a single-mode fiber .

38. The ultrasound detector according to any of the claims 14-37, wherein the wavelength of said light source lies in the range between 400 nm and 10000 nm.

39. The ultrasound detector according to any of the claims 25-38, wherein said second surface is provided by a second meniscus.

40. The ultrasound detector according to any of the claims 1-39, comprising an optical interferometer.

41. The ultrasound detector according to any of the claims 1-40, said gas-liquid interface having a first side facing the liquid and a second side facing the discontinuity, wherein said light source and said light detector are arranged on said second side.

42. The ultrasound detector according to any of the claims 1-41, comprising means to control the size of said discontinuity.

43. The ultrasound detector according to claim 42, wherein said means comprise a piezoelectric actuator.

44. The ultrasound detector according to claim 42 or 43, wherein said means are arranged to control said size by regulating a static gas pressure exerted on said gas-liquid interface .

45. A device for ultrasound characterization comprising an ultrasound detector as defined in any of the claims 1-44.

46. The device of claim 45, wherein the frequency of said ultrasound wave is 50 kHz- 50 MHz.

47. The device according to claims 45 or 46, wherein said device is a medical sonograph.

48. The device according to any of the claims 45-47, comprising a multiplicity of ultrasound detectors as claimed in 1-44, of which the corresponding gas-liquid interfaces are different in size.