DISTANCE MEASURING METHOD AND DEVICE AS WELL AS LASER LITHOTRIPSY DEVICE

Abstract

A measuring method for optically determining a distance (z) of a surface located in a medium from an end of an optical waveguide is described and has the following steps: emitting electromagnetic measuring radiation of a first wavelength (λ1) and of a second wavelength (λ2) from the end of the waveguide towards the surface, wherein the medium more strongly absorbs the electromagnetic measuring radiation of the second wavelength (λ2) than the electromagnetic measuring radiation of the first wavelength (λ1); measuring a first reflection signal (I.sub.1) of the electromagnetic measuring radiation of the first wavelength (λ1) reflected from the surface, and measuring a second reflection signal (I.sub.2) of the electromagnetic measuring radiation of the second wavelength (λ2) reflected from the surface, and determining the distance (z) from a ratio (I.sub.2:I.sub.1) of the second and the first reflection signal. Furthermore, a measuring device and a laser lithotripsy device are described.

Claims

1. A measuring method for optically determining a distance (z) of a surface of a body stone located in a medium from an end of an optical waveguide, having the steps: emitting electromagnetic measuring radiation of a first wavelength (λ1) and of a second wavelength (λ2) from the end of the waveguide towards the surface, wherein the medium more strongly absorbs the electromagnetic measuring radiation of the second wavelength (λ2) than the electromagnetic measuring radiation of the first wavelength (λ1); measuring a first reflection signal (I.sub.1) of the electromagnetic measuring radiation of the first wavelength (λ1) reflected from the surface, and measuring a second reflection signal (I.sub.2) of the electromagnetic measuring radiation of the second wavelength (λ2) reflected from the surface, and determining the distance (z) from a ratio (I.sub.2:I.sub.1) of the second and the first reflection signal.

2. The measuring method according to claim 1, wherein the determination of the distance (z) comprises determining whether a predetermined maximum distance (z.sub.limit) is undercut and/or a predetermined minimum distance is exceeded.

3. The measuring method according to claim 2, wherein a trigger signal is generated when the measurement results in the predetermined maximum distance (z.sub.limit) being undercut and/or the predetermined minimum distance being exceeded.

4. The measuring method according to claim 2, wherein the second wavelength (λ2) is selected such that the second reflection signal (I.sub.2) at a distance from the end of the waveguide below the predetermined maximum distance (z.sub.limit) decreases to a fraction of less than 20%, preferably of less than 10%, further preferably of less than 5%, of the maximally measurable intensity I.sub.02.

5. The measuring method according to claim 2, wherein, as a function of the predetermined maximum distance (z.sub.limit), a first threshold value (S.sub.λ2) is predetermined for the second reflection signal (I.sub.2) and wherein the ratio (I.sub.2:I.sub.1) of the second reflection signal and the first reflection signal is multiplied by 1 when the second reflection signal (I.sub.2) exceeds the threshold value (S.sub.λ2) for the second reflection signal (I.sub.2) and otherwise is set to zero and wherein, as a function of the predetermined maximum distance (z.sub.limit), a second threshold value (S.sub.quot) is predetermined for the ratio (I.sub.2:I.sub.1) of the second reflection signal and the first reflection signal.

6. The measuring method according to claim 3, wherein the trigger signal is generated when the ratio (I.sub.2:I.sub.1) of the second reflection signal and the first reflection signal exceeds the second threshold value (S.sub.quot).

7. The measuring method according to claim 1, wherein the second wavelength (λ2) is selected such that the absorption coefficient (a) of the medium in the case of the second wavelength (λ2) differs from the absorption coefficient in the case of the first wavelength (λ1) by a factor of at least 100, preferably of at least 1000, further preferably of at least 10000.

8. The measuring method according to claim 1, wherein the first wavelength (λ1) is in the visible spectral range.

9. The measuring method according to claim 1, wherein the second wavelength (λ2) is in the near-infrared spectral range.

10. The measuring method according to claim 1, wherein the measuring radiation of the first wavelength (λ1) is coupled into the waveguide at a first opening angle, which differs from an opening angle at which the measuring radiation of the second wavelength (λ2) is coupled into the waveguide.

11. The measuring method according to claim 1, wherein the measuring radiation of the first wavelength (λ1) or the measuring radiation of the second wavelength (λ2) is coupled into the waveguide obliquely to the waveguide axis.

12. A measuring device to optically determine a distance (z) of a surface of a body stone located in a medium from an end of an optical waveguide, having: a measuring radiation source to generate electromagnetic measuring radiation of a first wavelength (λ1) and of a second wavelength (λ2), wherein the medium more strongly absorbs the electromagnetic measuring radiation of the second wavelength (λ2) than the electromagnetic measuring radiation of the first wavelength (λ1), the optical waveguide to emit the electromagnetic measuring radiation from the end of the waveguide towards the surface; a detection device to measure a first reflection signal (I.sub.1) of the electromagnetic measuring radiation of the first wavelength (λ1) reflected from the surface, and for measuring a second reflection signal (I.sub.2) of the electromagnetic measuring radiation of the second wavelength (λ2) reflected from the surface, and an evaluation unit to determine the distance (z) from a ratio (I.sub.2:I.sub.1) of the second and the first reflection signal.

13. The measuring device according to claim 12, wherein the measuring radiation source has a first laser (Lk′) for generating the electromagnetic measuring radiation of the first wavelength (λ1) and a second laser (L.sub.λ2) for generating the electromagnetic measuring radiation of the second wavelength (λ2).

14. The measuring device according to claim 12, wherein the optical waveguide has a numerical aperture of greater than 0.1, preferably greater than 0.2.

15. A laser lithotripsy device for breaking up body stones, with a treatment laser for emitting treatment laser light and a measuring device according to claim 12.

16. The laser lithotripsy device according to claim 15, wherein the evaluation unit of the measuring device generates a trigger signal for enabling the treatment laser when a distance (z) of the end of the waveguide is measured from the surface of a body stone to be broken up that is shorter than a predetermined maximum distance (z.sub.limit).

17. The laser lithotripsy device according to claim 16, wherein the trigger signal is generated when the measured distance (z) of the end of the waveguide from the surface is greater than a minimum distance that is shorter than the maximum distance.

18. The laser lithotripsy device according to claim 15, wherein a control device for the treatment laser adjusts the pulse energy of the treatment laser light as a function of the measured distance (z).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Exemplary embodiments of the invention are represented in the drawing and are described in more detail with reference to them hereafter, in which:

[0060] FIG. 1 is a block diagram of a measuring device for optically determining a distance of a surface located in a medium from an end of an optical waveguide;

[0061] FIG. 2 is a flow diagram of a measuring method for optically determining a distance of a surface located in a medium from an end of an optical waveguide;

[0062] FIG. 3a) is a diagram showing reflection signals of surfaces of different target objects as a function of the distance of the respective surface from the end of an optical waveguide for two different wavelengths in each case;

[0063] FIG. 3b) is a diagram showing the respective ratio of the reflection signals for the two different wavelengths from FIG. 3a);

[0064] FIG. 4a)-e) are further diagrams showing the distance dependency of reflection signals on surfaces of different target objects for two different wavelengths as well as the ratio of the reflection signals for the two different wavelengths;

[0065] FIG. 5 is a further diagram showing the ratios of reflection signals of surfaces of different target objects for two different wavelengths in a practical implementation of the measuring method;

[0066] FIG. 6 is a schematic diagram of different couplings of measuring radiation in an optical waveguide;

[0067] FIG. 7 is a further schematic diagram of the coupling of measuring radiation into an optical waveguide;

[0068] FIG. 8a)-e) are different radiation profiles of measuring radiation exiting an optical waveguide, which have been generated by different couplings of the measuring radiation into the waveguide;

[0069] FIG. 9 is a diagram of reflection signals for the different radiation profiles in FIG. 8a) to e) as a function of the distance of the end of the waveguide from a surface of a target object; and

[0070] FIG. 10 is a schematic design of a laser lithotripsy device with the measuring device in FIG. 1.

DETAILED DESCRIPTION

[0071] FIG. 1 shows an exemplary embodiment of a measuring device 10 for optically determining a distance z of a surface 12 of a target object 14, which is located in a medium 16, from an end 18 of an optical waveguide 20. The medium 16 is in particular a fluid medium, for example water or an aqueous solution. The medium 16 can for example be a bodily fluid, in particular urine.

[0072] The optical waveguide 20 can for example be an optical fiber, in particular a multi-mode fiber. The optical waveguide 20 can also be a fiber bundle.

[0073] The measuring device 10 has a measuring radiation source 22 for generating electromagnetic measuring radiation of a first wavelength and of a second wavelength. In the exemplary embodiment shown, the measuring radiation source has a first laser L.sub.λ1 for generating electromagnetic measuring radiation 24 of a wavelength λ1 and a second laser L.sub.λ2 for generating electromagnetic measuring radiation 26 of a wavelength λ2, wherein the wavelengths λ1 and λ2 are different in such manner that the measuring radiation of the wavelength λ2 is more strongly absorbed by the medium 16 than the measuring radiation of the wavelength 1. In other embodiments, the measuring radiation 24 and the measuring radiation 26 can also be generated together by a single light source, for example a broadband light source, which is arranged after one or a plurality of spectral filters which has or have a transmission range for the wavelengths λ1 and λ2.

[0074] The measuring radiation 24 and the measuring radiation 26 are, simultaneously or slightly offset in time, coupled into an entry end 28 of the optical waveguide 20 and exit the end 18, which is the exit end for the measuring light, of the waveguide.

[0075] The measuring device 10 can have a plurality of optical elements, as shown. The measuring radiation 26, which is generated by the laser L.sub.λ2, can for example be collimated by a lens L.sub.1, then passes through a beam splitter PS.sub.1, which can be a polarization beam splitter, and a dichroic mirror S.sub.1 and is bundled by a further lens L.sub.2 to an end 28 of the optical waveguide 20 opposite the end 18 and coupled into said optical waveguide. The measuring radiation 24, which is generated by the laser L.sub.λ1, is collimated by a lens L.sub.3, passes through a beam splitter PS.sub.2, which can be a polarization beam splitter, is deflected by the dichroic mirror S.sub.1 to the lens L.sub.2 and is also bundled by said lens to the end 28 of the optical waveguide 20 and coupled into said optical waveguide.

[0076] The measuring device 10 also has a detection device 30 for measuring a first reflection signal of the electromagnetic measuring radiation 24r of the first wavelength λ1 reflected from the surface 12 and for measuring a second reflection signal of the electromagnetic measuring radiation 26r of the second wavelength λ2 reflected from the surface 12. The reflected measuring radiation 24r of the first wavelength λ1 enters the end 18 of the optical waveguide 20 and exits the end 28, is collimated by the lens L.sub.2, deflected by the dichroic mirror S.sub.1 to the beam splitter PS.sub.2 and deflected thereby to a further lens L.sub.4, which bundles the reflected measuring radiation 24r to a first detector 32 in order to measure a first reflection signal I.sub.λ1. The reflected measuring radiation 26r of the second wavelength λ2 is also captured by the end 18 of the optical waveguide 20, exits the end 28, passes through the lens L.sub.2 and the dichroic mirror S.sub.1, is deflected by the beam splitter PS.sub.1 to a further lens L.sub.5 and is bundled thereby to a detector 34 in order to measure a second reflection signal Ike. In other embodiments, the detection device 30 can also have only one single detector which receives both reflected measuring radiations 24r and 26r and is sensitive to both wavelengths λ1 and λ2.

[0077] The measuring device 10 also has an evaluation unit 36, which evaluates the reflection signals I.sub.λ1 and I.sub.λ2 in order to determine the distance z from a ratio of the two reflection signals I.sub.λ1 and I.sub.λ2, as will be described in more detail later. The evaluation unit 36 can also be integrated into the detection device 30 and can be implemented as a microprocessor or as software.

[0078] FIG. 2 shows a flow diagram of a measuring method 40 for optically determining a distance z of a surface 12 of a target object 14 located in a medium 16 from an end 18 of an optical waveguide 20. The measuring method 40 can be carried out using the measuring device 10 in FIG. 1, but without it being limited thereto.

[0079] In a step 42, electromagnetic measuring radiation of a first wavelength λ1 and of a second wavelength λ2 is emitted from the end 18 of the waveguide 20 towards the surface 12, wherein the medium 16 more strongly absorbs the electromagnetic measuring radiation of the second wavelength λ2 than the electromagnetic measuring radiation of the first wavelength λ1. In a step 44, a first reflection signal I.sub.λ1 of the electromagnetic measuring radiation of the first wavelength λ1 reflected from the surface 12 and a second reflection signal I.sub.λ2 of the electromagnetic measuring radiation of the second wavelength λ2 reflected from the surface 12 are measured. In a step 46, the distance of the surface 12 from the end 18 of the optical waveguide 20 is determined from a ratio of the second reflection signal I.sub.λ2 to the first reflection signal I.sub.λ1.

[0080] The wavelength λ1 is preferably selected such that the absorption of the medium 16 in the case of this wavelength λ1 is only weak. The intensity of the reflection signal I.sub.λ1 in the case of the wavelength λ1 can be described as follows:

I.sub.λ1=I.sub.01.Math.r.sub.1.Math.d.sub.λ1(z)  (1)

[0081] In equation (1), I.sub.01 is the reflection signal measured upon contact between the end 18 of the optical waveguide 20 and a highly-reflective surface (for example of a mirror) (that is to say, the maximum measurable intensity or the maximum measurable reflection signal), r.sub.1 is the reflectance of the surface 12 and d.sub.λ1 (z) a function, which describes the decrease of the reflection signal with increasing distance z due to the illumination and detection geometry and depends on the numerical aperture (as described for example in the article by Komives C, Schultz J. S.: “Fiber-Optic Fluorometer Signal Enhancement and Application to Biosensor Design”, Talanta 1992, 39(4): 429-441, or in the article by V. Svyryd et al.: “An analysis of a displacement sensor based on optical fibers”, Revista Meixana de Fisica S 52(2): 61-63 (2006)).

[0082] Since the reflectance r.sub.1 of different surfaces, as can be the case with surfaces of body stones, can vary significantly, a conclusion cannot be made from the reflection signal I.sub.λ1 about the distance z between the end 18 and the surface 12, even if d.sub.λ1 (z) is known.

[0083] The wavelength λ2 is preferably selected such that the measuring radiation of the wavelength λ2 is more strongly absorbed by the medium 16 than the measuring radiation of the wavelength λ1. In particular, the wavelength λ2 is selected such that the reflection signal I.sub.λ2 decreases within a desired distance z.sub.limit to less than a fraction p of the maximally measurable intensity. λ2 is preferably selected such that the second reflection signal I.sub.λ2 at a distance from the end of the waveguide below a predetermined maximum distance decreases to a fraction of less than 20%, preferably of less than 10%, further preferably of less than 5%, of the maximally measurable intensity I.sub.02.

[0084] For the wavelength λ2, the equation (1) is expanded by an exponential factor following the Beer-Lambert law:

I.sub.λ2=I.sub.02.Math.r.sub.2.Math.d.sub.λ2(z).Math.exp(−2αz)  (2)

[0085] α is the absorption coefficient of the medium 16 in the case of the wavelength λ2. The factor 2 in the exponent emerges from twice passing through the distance z between the end 18 of the waveguide 20 and the surface 12.

[0086] In the case of the same illumination and detection geometry for both wavelengths λ1 and λ2, it is d.sub.λ1(z)˜d.sub.λ2(z). If the ratio of the two reflection signals I.sub.λ2:I.sub.λ1 is formed, this ratio thus depends on the distance z according to an exponential course:

I.sub.λ2:I.sub.λ1=(I.sub.02:I.sub.01).Math.(r.sub.2:r.sub.1).Math.exp(−2αz)  (3)

[0087] If I.sub.01 and I.sub.02 are the same or roughly the same for the two wavelengths λ1 and λ2, and the reflectances r.sub.1 and r.sub.2 are the same or approximately the same for the two wavelengths λ1 and λ2, the equation (3) is reduced to the exponential factor. When the absorption of the medium 16 in the case of the wavelength λ2 is at least approximately known or determinable, the distance z can therefore be precisely determined by solving the equation (3) for z. The above-mentioned assumptions are, however, not applicable in all cases. In practical applications, it is also not required to precisely know the value of the distance z, but rather it may be sufficient to detect whether the distance z is within a distance range below a predetermined maximum distance and/or above a predetermined minimum distance. The measuring method 40 can therefore be carried out such that it is determined whether a predetermined maximum distance is undercut and/or a predetermined minimum distance is exceeded in order to then, if this is the case, generate a trigger signal, for example to trigger a process or an action, such as for example enabling a device. This will be described below.

[0088] To verify the equation (3), exemplary measurements have been carried out.

[0089] FIG. 3a) shows a diagram of curves of exemplary measurements, which have been carried out with a measuring device similar to the measuring device 10 in FIG. 1 using electromagnetic measuring radiation of a first wavelength λ1=520 nm and of a second wavelength λ2=1310 nm on a mirror and two human stones as objects 14 in water as medium 16. The absorption coefficient of water at a temperature of 25° C. is 3.035×10.sup.−5 mm.sup.−1 at 520 nm and 0.7708 mm.sup.−1 at 1310 nm. The curve 50 shows the distance dependency of the reflection signal I.sub.λ1 for the mirror, the curve 52 the distance course of the reflection signal I.sub.λ1 for the first stone and the curve 54 the distance course of the reflection signal I.sub.λ1 for the second stone. The curve 56 shows the distance course of the reflection signal Ike for the mirror, the curve 58 the distance course of the reflection signal I.sub.λ2 for the first stone and the curve 60 the distance course of the reflection signal Ike for the second stone. The reflection signals I.sub.λ1 are standardized in FIG. 3a) to I.sub.01 and the reflection signals I.sub.λ2 to I.sub.02. FIG. 3b) shows the distance course of the ratio I.sub.λ2:I.sub.λ1 for the mirror (curve 62), the first stone (curve 64) and the second stone (curve 66). I.sub.λ1 and I.sub.λ2 are in turn standardized to I.sub.01 or I.sub.02. The curves 56, 58 and 60 in FIG. 3a) decrease, in comparison to the curves 50, 52, 54, notably more quickly with increasing distance between the end 18 of the waveguide 20 and the surface 12 of the respective target object. The exponential decrease of the ratio I.sub.λ2:I.sub.λ1 described according to equation (3) emerges from the curves 62, 64 and 66 in FIG. 3b). All measuring curves have been standardized to the respective maximum. The extension of the abscissa in FIG. 3b) compared to the abscissa in FIG. 3a) should be noted here.

[0090] Generally, the second wavelength λ2 can be selected such that the absorption coefficient of the medium 16 in the case of the wavelength λ2 differs from the absorption coefficient in the case of the wavelength λ1 by a factor of at least 100, preferably of at least 1000, further preferably of at least 10000. In the exemplary measurement, the absorption coefficient of water for λ2 differs from the absorption coefficient for λ1 at a temperature of 25° C. even by a factor of over 25,000.

[0091] If, in the equation (3), the reflectances r.sub.1, r.sub.2 and/or the maximum signal intensity I.sub.01, I.sub.02 are not substantially the same and are also not known, it may be appropriate to select the wavelength λ2 such that reflection signals can only be measured from when a predetermined or desired maximum distance z.sub.limit is undercut. This can be ensured by a correspondingly high absorption coefficient. For example, in the case of a wavelength λ2=1310 nm, the reflection signal Ike of a mirror in water, depending on the numerical aperture of the waveguide and angle of incidence on the mirror, can decrease within 1 mm to roughly 4% of the maximum value, as emerges from FIG. 4a) from a measuring curve 68 for the reflection signal I.sub.λ2. FIG. 4a) shows by way of example the measured distance course of the reflection signal I.sub.λ1 (curve 70) for λ1=520 nm and the distance course of the ratio I.sub.λ2:I.sub.λ1 (curve 72) for a mirror. Since the intensity of the measuring radiation for the wavelength λ1 at the end 18 of the waveguide 20 was lower by a factor of 10 than in the case of the other measurements, the reflection signals have been multiplied accordingly by 10 for the wavelength λ1. FIG. 4b) to e) show further exemplary measurements on different body stone samples in water/buffer solution. The measurement values have only been substrate-corrected, not standardized. In FIG. 4b) to e), the measurement scales have been omitted at the ordinate for reasons of clarity. The measurements have, however, resulted in the absolute values of the reflection signals of the different body stones differing by a factor of >5 both for the wavelength λ1=520 nm and for the wavelength λ2=1310 nm and the ratios I.sub.λ2:I.sub.λ2 substantially following the course of the curves for the wavelength λ2, but being closer to one another in terms of value. It can follow from this that the reflectance r.sub.1 for λ1 is sufficiently similar to the reflectance r.sub.2 for λ2 for the different body stones to be able to “standardize” the reflection signals for λ2 with the reflection signals for λ1.

[0092] If the distance measurement is carried out for the purpose of detecting whether the end 18 of the waveguide is located at a distance to the surface 12 of the target object, which is shorter than or at best the same as a predetermined maximum distance z.sub.limit, it is advantageous for the practical implementation of the measuring method according to the invention when a threshold value S.sub.λ2 is predetermined for the reflection signal I.sub.λ2 as a function of the predetermined maximum distance z.sub.limit. Furthermore, it is advantageous to also predetermine a threshold value S.sub.quot for the ratio I.sub.λ2:I.sub.λ1 as a function of z.sub.limit. In FIG. 5, the distance course of the ratio I.sub.λ2:I.sub.λ1 is shown for I.sub.λ2>S.sub.λ2 from the curves in FIG. 4d) to e). The threshold value S.sub.λ2 used for FIG. 5 has been set at 0.0025 V and the curves for the ratio I.sub.λ2:I.sub.λ1 taken from FIG. 4b) to e) have been multiplied by a step function which is 1 for I.sub.λ2>S.sub.λ2 and otherwise is 0. The threshold value S.sub.quot can for example be set to 0.02. When the measuring method is used in this manner for laser lithotripsy, this means, according to FIG. 5, that body stones are treated with lithotripsy only at a distance of <0.5 mm between the end 18 of the waveguide 20 and the surface 12 of the respective body stones.

[0093] As emerges from the above description, it is advantageous when the measured reflection signal I.sub.λ2 strongly decreases as a function of the distance. A change in the distance behavior of the reflection signal can be achieved, instead of using long-wave measuring radiation, in which the medium 16 has a high absorption, also by using different ways of coupling the measuring radiation into the waveguide 20, in particular by utilizing the numerical aperture of the waveguide 20 in different manners and/or by coupling obliquely in relation to the longitudinal axis of the waveguide, with which donut modes can be generated.

[0094] If the measuring radiation is coupled into the waveguide 20 at an opening angle (aperture) that is smaller than the angle of acceptance of the waveguide, the opening angle (aperture) of the exiting light beam is generally also smaller and the reflection signal decreases more slowly with increasing distance. If, conversely, the measuring radiation is coupled into the waveguide 20 at a greater opening angle, for example the same as the angle of acceptance of the waveguide 20, the opening angle of the exiting light beam is generally also greater and the reflection signal decreases more quickly with increasing distance. In the case of coupling in the measuring radiation 26 obliquely to the waveguide axis, a donut-shaped beam profile is created, whose reflection signal decreases even more quickly. This possibility of adjusting the coupling may be helpful if the measuring light source 22 in FIG. 1 cannot provide a measuring radiation with a wavelength that is absorbed by the medium 16 to the desired extent. In principle, the wavelengths λ1 and λ2 in the case of this embodiment of the measuring method can be the same or virtually the same, wherein the respective measuring radiation generated by the light sources L.sub.λ1 and L.sub.λ2 is coupled into the waveguide 20 simply in a different manner (opening angle and/or orientation).

[0095] FIG. 6 shows the coupling of measuring radiation 24 at a large opening angle and the coupling of the measuring radiation 26 at a smaller opening angle and FIG. 7 shows the coupling of measuring radiation 24 into the waveguide 28 obliquely to the waveguide axis 27 of the waveguide 20, while the measuring radiation 26 (not shown) is for example coupled collinearly to the waveguide axis 27.

[0096] FIG. 8a) to e) show different beam profiles of measuring radiation at a distance of 50 mm from the end 18 of the waveguide 20. FIG. 8a) shows a beam profile 80a with a small aperture, FIG. 8b) a beam profile 80b with a medium aperture, and FIG. 8c) a beam profile 80c with a maximum aperture. FIG. 8d) shows a beam profile 80b in the form of a donut profile with a small aperture and FIG. 8e) a beam profile 80e in the form of a donut profile with a large aperture. The uninterrupted circle lines illustrate the maximally usable aperture of the waveguide 20.

[0097] FIG. 9 shows the distance behavior of a reflection signal I.sub.λ for different ways of coupling measuring radiation into the waveguide 20. The wavelength observed here is 520 nm, the medium is water. The two curves without symbols in FIG. 9 show the calculated distance behavior of the reflection signal I.sub.λ for a waveguide in the form of a fiber with a core diameter of 365 μm and a numerical aperture 0.11 (curve 82) and a numerical aperture of 0.22 (curve 84). It can be shown from this that the selection of a waveguide with a higher numerical aperture leads to a quicker decrease of the reflection signal I), with increasing distance. The curves with symbols in FIG. 9 show measurements on a mirror in water, in which the numerical aperture of the waveguide 20 has been utilized differently or the measuring radiation has been coupled into the waveguide 20 in a skewed manner. The curve 86 shows the distance behavior of the reflection signal I), for coupling the measuring radiation into the waveguide 20, which generates the beam profile according to FIG. 8a). The curve 88 shows the distance behavior of the reflection signal I), for coupling, which generates the beam profile 80b according to FIG. 8b). The curve 90 shows the distance behavior of the reflection signal I), for coupling the measuring radiation, which generates the beam profile 80c according to FIG. 8c). The curve 92 shows the distance behavior of the reflection signal I.sub.λ for coupling, which generates the beam profile 80d according to FIG. 8d). The curve 94 shows the distance behavior of the reflection signal I), for coupling, which generates the beam profile 80e according to FIG. 8e). It follows that the reflection signal as a function of the distance decreases increasingly more quickly with increasing utilization (or filling) of the aperture of the waveguide, and that oblique coupling, which generates a donut beam profile, causes an even quicker decrease of the reflection signal as a function of the distance.

[0098] FIG. 10 shows a block diagram of a laser lithotripsy device 100, which can have the measuring device 10 according to FIG. 1 as a component. In addition to the measuring device 10, the laser lithotripsy device 100 has a treatment laser 102, which generates the treatment laser light for breaking up body stones. The treatment laser 102 can be a Holmium laser. The treatment laser light can be coupled into the waveguide 20, which is also used for the measuring radiation, via a waveguide 104, a further lens L.sub.6 and a further dichroic mirror S.sub.2 and via the lens L.sub.2 such that the treatment laser light can exit the end 18 of the waveguide 20. The treatment laser 102 can be enabled by a trigger signal of the evaluation unit 36 (FIG. 1, omitted in FIG. 10 for clarity), when the measuring device 10 detects a distance z of the end 18 of the waveguide 20 from the surface 12 of the target object 14 that is shorter than the maximum distance z.sub.limit. The maximum distance z.sub.limit can be shorter than 1 mm, for example shorter than 0.5 mm. After being enabled, the treatment laser can be activated to emit treatment light. In the case of the lithotripsy device 100, it is therefore ensured that the treatment laser light is only emitted when a body stone is located at a distance in front of the end 18 of the waveguide 20 that is shorter than the maximum distance z.sub.limit. It may also be advantageous to generate the trigger signal only when the measured distance of the end of the waveguide 20 from the surface 12 of the target object 14 is greater than a minimum distance that is shorter than the maximum distance z.sub.limit. Additionally, the laser lithotripsy device 100 can have a controller for the treatment laser 102 designed to adjust the pulse energy of the treatment laser light as a function of the measured distance z. For example, the control device can increase the pulse energy if the end 18 of the waveguide 20 is located at a greater distance from the surface of a body stone to be treated, or reduces the pulse energy in the case of a shorter distance. The controller can be integrated into the treatment laser or into the measuring device 10, for example into the evaluation device 36, as hardware or software.