WO2005064293A1

WO2005064293A1 - Optical hydrophone for a stretched shock wave field provided with several receivers

Info

Publication number: WO2005064293A1
Application number: PCT/EP2004/053142
Authority: WO
Inventors: Bernd Granz; Christian Meinert; Ralf Nanke
Original assignee: Siemens Aktiengesellschaft
Priority date: 2003-12-18
Filing date: 2004-11-26
Publication date: 2005-07-14
Also published as: DE10359663A1

Abstract

The invention relates to an optical hydrophone which is used to measure acoustic sound distribution in a fluid medium (10), in particular for measuring an ultrasonic shock wave field. Said optical hydrophone comprises a light source (2) which is used to generate light (LS) and to illuminate a boundary surface (8) which is disposed between an optical transparent body (4) and the fluid medium (10). According to the invention, a light receiving arrangement (14), which comprises a plurality of light receivers (14i) and which is used to measure the intensity distribution of the light (LR) reflected onto said boundary surface (8), is provided, thus enabling a hydrophone having a long service life and high spatial selectivity to be produced.

Description

OPTICAL HYDROPHONE FOR AN EXTENDED SHOCK WAVE FIELD WITH SEVERAL RECEIVERS

Optical hydrophone for measuring the sound pressure distribution in a fluid medium 5 The invention relates to an optical hydrophone for measuring the sound pressure distribution in a fluid medium, in particular for measuring an ultrasonic shock wave field. 0 With acoustic shock waves, such as those used in litotripsy, high pressures of up to about 10 ⁸ Pa occur with rise times in the range of a few ns. The measurement of such high pressures requires sensors with high mechanical stability. In addition, these sensors should be largely miniaturized in order to be able to measure the sound pressure distribution in a shock wave field with the highest possible local resolution. 0 From EP 0 354 229 B1 or DE 38 02 024 AI and from J. Staudenraus,. Eisenmenger, "Fiber-optic probe hydrophone for ultrasonic and shock-wave measurements in water", Ultrasonics 1993, Vol.31, No.4, page 267-273, measuring arrangements are known in each case for measuring the spatial and temporal distribution the pressure of ultrasonic shock waves in a liquid, the light reflected at the free end of an optical fiber is used. This known fiber-optic measuring arrangement takes advantage of the fact that the high pressure amplitude produces a change in density and thus a change in the refractive index of the liquid in the immediate vicinity of the free end, which modulates the proportion of the light reflected back into the optical waveguide at the interface. The optical waveguides used for the measurement have a diameter that does not significantly exceed 5 mm. The free end of the optical waveguide, which determines the reflectivity of the liquid / optical waveguide interface, becomes vertical by means of a spherical or plane end surface standing to the optical fiber axis. The small size of this end surface creates a high spatial resolution, low directional sensitivity and high bandwidth required for the measurement of focused shock waves.

From DE 39 32 711 AI a fiber optic shock wave sensor is known in which the free end of the optical waveguide is designed as a body of revolution, the envelope of which can be described by a third degree polynomial. This measure is also intended to improve both the sensitivity and the spatial resolution when using optical fibers with a larger diameter.

From Koch, Gh., "Coated fiber-optic hydrophone for ultrasonic easurement", Ultrasonics 34, 1996, pages 687-689, a fiber-optic hydrophone is known which not only changes the refractive index of the surrounding fluid but also changes the properties of an the fiber tip uses interferometers formed by dielectric layers in order to increase the sensitivity of the measuring arrangement.

A disadvantage of the known fiber-optic hydrophones, however, is that they are very sensitive to breakage and can be destroyed after only 10 to 100 shock waves at about 50 MPa. In addition, a high manufacturing outlay is required in order to reproducibly produce the free ends of the optical waveguides with the shape required in each case.

Especially when used in medical therapy

Lithotriptors must know the lateral intensity distribution of the shock wave in the area of the focus with a local resolution that is less than 1 mm. For this it is necessary to measure the time course of the intensity of the shock wave pulse at a large number of measuring points. The total intensity or the total energy of the shock wave pulse in the focus is then determined from the measured intensities. Corresponding to the number of measuring points, generally about 20, a number of successive measuring steps must be carried out, with the hydrophone being exposed to a shock wave in each measuring step. With such a measurement, there is therefore the risk that the hydrophone will either be destroyed during the measurement or at least so damaged that its sensitivity changes to an extent that is outside the permitted tolerance range, which is 10% in the application example.

The invention is based on the object of specifying an optical hydrophone for measuring the sound pressure distribution in a fluid medium, which is easy to manufacture in terms of production technology, has a long service life and whose spatial resolution is comparable to the spatial resolution as described by the prior art fiber optic hydrophones known in the art can be achieved.

The stated object is achieved according to the invention with an optical hydrophone with the features of claim 1. Such an optical hydrophone comprises at least one light source for generating light and for illuminating an interface between an optically transparent body and the fluid medium, the optically transparent body has a refractive index, the dependence of which on the sound pressure is negligible, and a plurality of light receivers for measuring the intensity of the light reflected at the interface in the light receiver as a measure of the sound pressure.

This measure allows the shock wave field to be measured at several locations simultaneously, so that the sound pressure distribution can be determined in a larger area at a large number of measuring points with high local resolution with a single measurement. This increases the service life of the optical hydrophone, since the number of hydrophones required to measure the shock wave field phon mechanically stressing shock wave pulses is reduced according to the area. The search for the location of the focus is also simplified.

The invention is also based on the consideration that in order to achieve a high local resolution, only the size of the illuminated area or the size of the partial area of the illuminated area detected by the respective light receiver is important. In order to enable a high local resolution, the transparent body does not necessarily have to be designed as a light guide in which the light is guided by reflection on the walls. Rather, it is sufficient to generate one or more light beams by appropriate beam shaping and guidance, which spread freely in the transparent body and which have a beam cross section adapted to the respective need in the area of the interface. In an advantageous embodiment, the transparent body therefore has dimensions that are very much larger than the illuminated area, so that it can be an order of magnitude smaller than the interface formed between the body and the fluid medium. As a result, the transparent body can be made solid, so that it is more resistant to shock waves, such as may occur in the focus of a lithrotripter. In addition, the interface can be machined without any problems, so that a high level of reproducibility can be achieved with little manufacturing effort.

In an advantageous embodiment of the invention, the light strikes the interface at an angle of incidence which is significantly smaller than the critical angle of total reflection and in particular is smaller than half the critical angle of total reflection. Due to the incidence with an angle of incidence which differs significantly from the critical angle of total reflection, the sensitivity to an arrangement with light incident close to the critical angle of total reflection decreases, but it is advantageous that the measuring arrangement is less sensitive against slight changes in the incidence, since the reflectivity for angles of incidence that are significantly smaller than the critical angle of the total reflection is almost independent of the angle of incidence. In addition, at such an angle of incidence and especially in the area of vertical incidence (angle of incidence 0 °) the reflectivity changes almost linearly with the refractive index of the fluid medium and thus also with the sound pressure, so that the reflected intensity is also approximately linear to the sound pressure.

In an advantageous embodiment of the invention, the area is illuminated over its entire area, i. H. a single, contiguous, at least approximately homogeneously illuminated light spot is formed at the interface. In this embodiment, only a single light source is required and the structure is correspondingly simplified.

In an alternative embodiment, the area is illuminated simultaneously on a plurality of spatially separated partial areas, so that a large number of light spots arranged next to one another are formed at the interface. A light receiver is assigned to each partial area and receives at least part of the light reflected by this partial area. If the reflected light rays belonging to each partial area are guided in optical waveguides, a high spatial resolution is possible with the aid of one or two-dimensional light receiver arrays, without this requiring a special geometric adaptation of the position and size of the light receiver array to the distribution and size of the partial areas ,

An advantageous embodiment provides for the body to be arranged such that it can be moved relative to the path of the light propagating in it to the interface, so that the illuminated surface area can be positioned at different locations on the interface depending on the position of the body. This can in the event of any damage the interface in the illuminated area of this area to be moved to another location. In the case of a cuboid body, this is done by moving parallel to the interface. The body can also have the shape of a polygon with flat flat sides lying opposite one another. In this case, the position of the illuminated surface area at an interface of the body can be varied by rotating the body about an axis of symmetry parallel to these flat sides.

The optically transparent body preferably has a refractive index that is as close as possible to the refractive index of the fluid medium. If the fluid medium is water (n = 1.33), the refractive index n _{κ of} the body is preferably approximately n = 1.45 (glass) or less. Then the static reflectivity, ie the reflectivity in the absence of an ultrasound field, is minimal and the signal-to-noise ratio is maximal.

Further advantageous embodiments are given in the further subclaims.

To further explain the invention, reference is made to the exemplary embodiments of the drawing. Show it:

1 shows an optical hydrophone according to the invention in a schematic diagram, FIGS. 2a, b each show an advantageous shape of the illuminated area, FIG. 3 shows an alternative embodiment of an optical hydrophone according to the invention also in a schematic diagram, FIGS. 4a-c each advantageous Arrangements of the subareas on the interface.

According to FIG. 1, the optical hydrophone comprises a light source 2 for generating light LS, in the exemplary embodiment a laser diode which is embodied in a transparent body 4. Example, an approximately cubic block consisting of glass (in the exemplary embodiment quartz glass with a refractive index n _κ = 1.45 at a wavelength of 800 nm) is coupled. Both the thickness and the lateral dimensions of the body 4 are in the range from a few mm to 100 mm. The transmitted light LS propagates freely within the body 4, ie without reflection on the walls of the body 4, and illuminates an area 6 of a plane wall or boundary surface 8 opposite the entry surface 7 to a fluid medium 10 at an angle of incidence θ deviating from 0 ° This angle of incidence θ is significantly smaller than the critical angle θ _{g of} the total reflection. An angle of incidence θ, in which the dependence of the reflectivity on the angle of incidence θ is only weak, is significantly smaller than the critical angle θ _g of total reflection in the sense of the invention. In practice, this is the case for angles of incidence θ which are in particular smaller than θ _g / 2, preferably smaller than θ _g / 3. In the present case, ie at n _κ = 1.45 and n _M = 1.34 (refractive index of the fluid medium 10 located outside the body 4, in the present case water, for light of the wavelength 800 nm) and a resulting angle angle θ _{g of} the total reflection of 67 °, these are angles of incidence θ <33 ° or θ <22 °. In a practical embodiment, an angle of incidence of approximately 10 ° has proven to be particularly suitable.

An ultrasound wave 12 incident on the interface 8 generates a temporal and spatial modulation of the refractive index n _{M of} the fluid medium 10 at the interface 8 (the modulation of the refractive index n _{κ of} the body 4 generated by the ultrasound wave is negligible) and thus a temporal and spatial modulation of the intensity of the light LR reflected at the interface 8. The temporal course of the intensity of the reflected light LR and its intensity distribution in the beam cross section is recorded in a light receiver arrangement 14, in the exemplary embodiment a one- or two-dimensional array of light receivers 14j _. , measured and is a direct measure of the time course and the spatial distribution of the sound pressure in the illuminated area 6. In the exemplary embodiment, photodiodes are provided as light receivers 14 _± . Alternatively, a CCD array can also be used. A section of the reflected light beam LR and thus a section of the illuminated area 6 is assigned to each light receiver 14 _± , so that the lateral resolution due to the geometric relationships of the light receiver arrangement 14, ie the arrangement and size of the entrance apertures of the light receivers 14j _. is determined.

In the exemplary embodiment, the transmitted light LS and the reflected light LR also propagate freely outside the body 4. The angle of incidence and exit angle θ, which is inclined to the vertical, leads to a decoupling of the two light paths after a distance dependent on the angle of incidence and the beam diameter, without the need for intensity-reducing beam splitters, as would also be necessary with a possible vertical incidence. For reasons of clarity, a refraction of the transmitted and of the reflected light LS, LR that occurs at the entry surface 7 is also not shown in the figure.

In order to collimate the transmitted light LS onto the interface 8, imaging optics 20 are provided which correspond to those of the

Light source 2 emits divergent light beam into an approximately parallel beam that converges the area 6, i. H. illuminated over its entire area. The transverse extent of the area 6 is typically 1 to 20 mm.

The imaging optics 20 can also be arranged inside the body 4, so that a compact and insensitive construction of the hydrophone is possible. Under certain circumstances, it may also be expedient to arrange imaging optics, for example a collimator, between the body 4 and the light receiver arrangement 14. Through appropriate imaging optics between the body 4 and the light receiver arrangement 14, it is also possible to adapt the desired spatial resolution in the illuminated area to the geometric conditions of the light receiver arrangement 14, ie the pitch of the light receiver 14 _± .

The body 4 is displaceable relative to the imaging optics 20 transversely to its optical axis (transversely to the light path or transversely to the normal of the region 6 or the interface 8), as is illustrated by the arrow 26. If the surface of the body 4 in the illuminated or used surface area 6 is damaged by the ultrasound pulse or by cavitation bubbles, the body 4 can be shifted by a few mm with the light source 2, imaging optics 20 and light receiver 14 arranged in a fixed manner until the illuminated surface area 6 comes to rest on an undamaged point of the body 4.

According to Figure 2a, the interface is on an approximately circular disc-shaped, due to the different from zero

Incidence angle T illuminated in practice slightly elliptical} area 6a. In this case, a two-dimensional arrangement of light receivers 14 ^ adapted to this surface shape is provided as the receiver arrangement. The entrance aperture of the light receiver 14ι defines the size of the partial area 6j assigned to it _. of area 6a.

Appropriate beam-shaping imaging optics (cylindrical lenses) can also be used to produce approximately rectangular beam shapes which illuminate the interface 8 in a rectangular region 6b, as illustrated in FIG. 2b.

In the exemplary embodiment according to FIG. 3, a plurality of light sources 2 to 2 _{n are} provided. Each of these light sources 2ι to 2 _n is assigned a light receiver 4_ to 14 _n _ι, 14 _n . In the example, the light sources 2χ to 2 _n each generated light beams LSi with the aid of a light guide arrangement 16 to the body 4, which is made up of a number of light sources 2 ₁ to 2 _n corresponding to the number of light guides 16ι to 16 _n . Likewise, the reflected light beams LR _α to LR _{n are} directed to the light receivers 14 with the aid of a light guide arrangement 18 made up of light guides 18 1 to 18 _n _! transmitted to 14 _n . In the figure it is also indicated by dashed lines that each optical waveguide 16ι or 18χ has an imaging optics 30 _A or 32j _. can be assigned, which maps the exit aperture of the light guide 16ι on the interface 8 or the image of the exit aperture on the interface 8 in the entry aperture of the light guide I8 _3rd

Instead of the light guide arrangement shown in the figure, which in practice is constructed by a correspondingly divided optical fiber bundle, there is also free beam propagation of the transmitted light LSj _. and the reelected light LRj _. also possible outside the body 4. Likewise, instead of a large number of light sources, a single one can also be used

Light source can be provided and the light generated by this can be coupled into a plurality of light guides.

With such a separation of the transmitted light beams LSi, it is possible to illuminate the illuminated area 6 on a large number of spatially separated small partial areas 6 ₁ to 6 _n . The arrangement of these partial areas 6 ₁ to 6 _n can then be adapted to the respective needs. FIG. 4a illustrates a linear arrangement, FIG. 4b a cross-shaped arrangement and FIG. 4c a rectangular arrangement of the partial areas 6 ₁ to 6 _n . By guiding the reflected light LRi with light guides 18i, the geometric arrangement of the light receivers 14i in the light receiver array 14 no longer has to match the arrangement of the partial areas at the interface 8, so that it is fundamentally possible to use a rectangular pattern even with a linear light receiver array - Order of the measurement spots, as illustrated for example in FIG. 4c.

Claims

claims

1. Optical hydrophone for measuring the sound pressure distribution in a fluid medium (10) using the modulation of the refractive index of the fluid medium (10) dependent on sound pressure, in particular for measuring an ultrasonic shock wave field, with at least one light source (2; 2i) for generating light (LS) and for illuminating an interface (8) between an optically transparent body (4) and the fluid medium (10), the optically transparent body (4) having a refractive index, the dependence of which

Sound pressure is negligible, and with a plurality of light receivers (14i) for measuring the intensity of the light (LRj _. ) Reflected at the interface (8) in the light receiver (14 _A ) as a measure of the sound pressure.

2. Optical hydrophone according to claim 1, wherein the at least one light source (2; 2i) illuminates an area (6) of the interface (8) formed between the body (4) and the fluid medium (10) that is smaller than this ,

3. Optical hydrophone according to claim 1 or 2, wherein the light (LS) illuminates the interface (8) with an angle of incidence (θ) which is smaller than half the critical angle (θ _g ) of the total reflection.

4. Optical hydrophone according to one of claims 1 to 3, wherein the area (6) is illuminated over its entire surface.

5. Optical hydrophone according to one of claims 1 to 3, bezL the area (6) is simultaneously illuminated on a plurality of spatially separate sub-areas (6ι), and each sub-area is assigned a light receiver (14ι).

6. Optical hydrophone according to one of the preceding claims, in which for guiding the light (LSi, LRj _. ) A Lichtlei— Teranordnung with spatially separate optical fibers (16i, 18i) is provided.

7. Optical hydrophone according to one of the preceding claims, in which the body (4) is arranged such that it can be moved relative to the path of the light (LS) propagating in it to the interface (8).

8. Optical hydrophone according to one of the preceding claims, in which the optically transparent body (4) has a refractive index which is as close as possible to the refractive index of the fluid medium (10).