Abstract:
The spectrum of light, inelastically scattered by a sample ( 16 ) is measured. The light is guided through a capillary ( 12 ) from and to the sample, at least in one of these directions, through the channel no inelastic scattering of light occurs which can form an interfering background when measuring on the sample. By guiding the light through the channel, inelastic scattering of this light is prevented and it becomes possible to guide scattered light back through the channel to spectral analysis equipment ( 14 ) without problems with inelastic scattering during the guidance of the light. Preferably, the light is guided through the channel of the capillary in both directions.

Description:
This application is the U.S. National Phase of International Application Number PCT/NL02/00386 filed on 12 Jun. 2002, which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The invention relates to a spectrometer for measuring inelastically scattered light. Raman spectrometers are an example of spectrometers for measuring inelastically scattered light. When a sample is irradiated with monochromatic light, the light scattered by the sample will contain wavelength components different from those present in the incident light. So-called Raman scattering of light on molecules present in the sample creates this effect. With a Raman spectrometer, a sample is illuminated with a strong, essentially monochromatic light source. The light which is scattered by the sample is collected and the spectral distribution is analysed for spectral components with a wavelength different from that of the light source. 
   U.S. Pat. No. 5,862,272 describes the use of optic fibres for guiding Raman spectra of samples to and from the sample, while the sample is at a distance from the light source which is used for excitation and/or at a distance from the equipment for detection of the Raman signal. The use of optic fibres enables measurements at locations which are otherwise difficult to reach or cannot be reached at all, as is the case, for instance, in in vivo measurements on tissue, for instance in blood vessel examination or more generally in endoscopic applications, or as is the case in industrial applications such as in-line process control. 
   U.S. Pat. No. 5,862,273 describes the use of a filter at the extremity of the fibre (excitation-fibre) which guides the monochromatic light from the light source to the sample, adjacent the sample to be examined. This filter (excitation filter) only transmits light of the wavelength of the light source and no Rams scattered light from the fibre, which is generated therein by inelastic scattering of light from the monochromatic light source. This enables sensitive measurement, without interference by inelastically scattered light from the fibre which returns to the analysis unit after scattering or reflection on the sample. 
   Further, the use of an excitation filter in or an the excitation fibre necessitates the use of a different fibre or fibres (collection fibres) for collecting light scattered by the sample and its guidance to the analysis unit, since otherwise the Raman scattered light of interest would be stopped by the excitation filter so that it cannot reach the analysis unit. Special measures are necessary to obtain a good overlap between the volume or surface of the sample irradiated by light from the excitation fibre and the volume or surface of the sample from which scattered light is intercepted by the collection-fibre(s). Also, the use of separate fibres for supply of light to and discharge of light from the sample renders Raman probes complex. The signal collection efficiency is suboptimal and far-reaching miniaturisation is impeded. 
   The light scattered by the sample largely consists of elastically scattered light, having the same wavelength as the incident light from the monochromatic light source. This elastically scattered light of considerable intensity can tin give rise to the generation of Raman signal in the collection fibres, which leads to an interfering signal background. For that reason, for sensitive measurements, a filter (emission filter) is utilized before, on or in the collection fibre(s) on the side where the sample is located, which serves to stop the light scattered elastically by the sample and to transmit only the scattered light which is shifted in wavelength with respect to the wavelength of the monochromatic light source. 
   Using excitation and emission filters limits the use of the spectrometer to a light source of a particular wavelength and/or measurement of the Raman signal in a particular wavelength range. The necessity of the use of optic filters before, in or on the fibres close to the sample renders fibre optic probes for Raman spectroscopy complex. 
   Also from U.S. Pat. No. 6,208,887, a Raman spectroscope is known. This patent discusses the possibility of using a multimode laser. The sample is illuminated via an optical fibre, without the use of optic filters, so that the spectrometer is only suitable for low sensitivities. For receiving scattered light the possibility of any light guide (waveguide) is mentioned, without, however, mentioning the advantages thereof over optic fibres. 
   It is one object of the invention to provide a spectrometer for inelastic scattering with a simplified construction of the probe. 
   It is another object of the invention to provide such a spectrometer wherein further-reaching miniaturisation of the probe is possible. 
   It is a further object of the invention, among still further objects, to provide such a spectrometer which is designed such that sensitive measurements are possible, while no filters are required or fewer filters than in the prior art. 
   SUMMARY OF THE INVENTION 
   The spectrometer according to the invention is described in claim  1 . This spectrometer utilizes a probe with a capillary with a channel through which at least one of the light transports of the light from the light source and the light scattered by the sample is guided. The other light transport can take place through the same channel or through a wall of the capillary if it is made of transparent material. Within the context of this invention, a capillary is understood to mean a flexible or non-flexible tube or pipe of any material (materials), with one or more channels in the longitudinal direction of the tube or pipe, which may be evacuated or filled with a gas (gasses) or a liquid (liquids). 
   Due to this construction, the probe is very compact. One capillary can suffice for guiding light to and from the sample. The location where the light leaves the capillary to illuminate the sample and the location where the light returns scattered from the sample into the capillary are in each other&#39;s immediate vicinity. No complex constructions are required for creating an overlap between the area of the sample which is irradiated and the area from which light returns to the analysis unit. Moreover, for the light transport through the channel in sensitive measurements, no filter is required because in the channel, due to the absence of glass, no interfering inelastic scatterings occur. 
   Preferably, at least the light from the light source is guided to the sample through the channel in the capillary. In the prior art, inelastic scattering of this light in the light guide is the chief source of disturbances. By guiding at least this light through the channel, a narrow band pass filter becomes superfluous. Preferably, however, both light transports, from and to the sample, are guided through the channel. Hence, automatically, there is a complete overlap between the area of the sample which is irradiated and the area from which light returns to the analysis unit. Moreover, thus, for sensitive measurements, no filters are required at the distal end of the light guide, so that the construction remains very simple. Moreover, when filters can be dispensed with, use of the probe is not limited to a particular wavelength. Ultimate miniaturisation of Raman probes is possible when only one light guide needs to be utilized for supply and discharge of light to and from the sample. 
   For manufacturing the hollow light guide according to this principle, different techniques can be used. A smooth metal coating can be applied on the inside of a capillary, optionally with a dielectric coating thereon to enhance the resection. Also, an accumulation of dielectric layers can be used, which, together, form a dielectric mirror with a high reflection efficiency. Further, a hollow metal capillary can be utilized, whose inside wall is smooth to such an extent that specular reflection is ensured. 
   In one embodiment, a capillary is filled with a liquid of a higher refractive index than the material of which the capillary is made, so that for light in the liquid propagating in a certain solid angle through the capillary, total internal reflection occurs at the liquid-capillary transition and the arrangement functions as an optical waveguide. The liquid in the present invention is preferably confined in the capillary, at least at the distal end (adjacent the sample), so that it cannot be easily replaced without opening the capillary. Preferably, the liquid is selected such that it exhibits no significant Raman scattering in the spectral region relevant to the analysis of the sample. This in contrast with optical glass fibres which in large parts of the spectrum cause a continuous high, often broadband signal background. Further, a coating of low refractive index can be applied to the inside of the capillary, thus enabling total internal reflection. 
   It should be noted here that measuring Raman scattering on liquids in a capillary is known per se. This involves scattering on the liquid in the capillary and not on a sample located beyond the extremity of the capillary. In this known measurement on the liquid, scattering of light from the capillary on a sample outside the capillary is undesired. 
   The use of hollow light guides based on this invention as a basis for Raman probes renders the construction of these probes simple and increases the possibilities for simple adaptation of probes to application-specific requirements, with regard to the light source to be used and properties of the sample to be examined as well as with regard to spatial resolution and location of the measured volume, by using specific optical elements at the distal extremity of the Raman probe. 
   An example of such provisions is a cone at the extremity of the light guide and tapering towards the sample, which makes it possible to capture light coming from the sample at a very large solid angle, whereupon in the cone, the solid angle is reduced, so that the light can be more readily guided further in the capillary. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the spectrometer according to the invention will be further described with reference to the following figures. 
       FIG. 1A  shows a spectrometer suitable for using Raman probes based on a hollow capillary as light guide; 
       FIG. 1B  shows a coupling unit for light source, Raman probe and analysis unit; 
       FIG. 2  shows the principle of the use for Raman spectroscopy of a probe based on a capillary; 
       FIGS. 3A–3C  show different embodiments of a light guide suitable for Raman spectroscopy; 
       FIGS. 4–8  show different embodiments of Raman probes based on light conducting capillaries; 
       FIGS. 9–13  show different embodiments of Raman probes based on thick-walled capillaries; and 
       FIG. 14  shows an alternative embodiment of a coupling unit for light source, Raman probe and analysis unit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A  shows a sample  16  and a spectrometer for measuring scattering on the sample  16 . The spectrometer includes a light source  10 , a probe with a hollow capillary  12 , an analysis unit  14  and a coupling unit  18  which provides the coupling between the light source  10  and the light guide  12  on the one hand and between wave guide  12  and analysis unit  14  on the other hand. 
   In operation, with the light source  10 , essentially monochromatic light is generated, which is guided via the coupling unit into the hollow light guide  12 . This light is guided through the hollow light guide  12  to the sample  16 . Light that has been scattered by the sample is guided through the probe, preferably through the hollow light guide, to the analysis unit. In the analysis unit  14 , the intensity distribution of the light scattered by the sample  16  is measured as a function of the wavelength. Preferably, measurements are carried out on scattered light whose frequency is shifted by scattering with a shift expressed in inverted centimeters between −4000 and +4000. 
   Optionally, the polarisation of the scattered light is measured. Optionally, only the intensity and/or polarisation of the scattered light in one or several selected subregions of the spectrum is/are measured. Optionally, the coupling unit  18  can be connected via optical light guides (not shown) to light source  10  and analysis unit  14 , respectively. 
     FIG. 1B  shows a possible embodiment of the coupling unit  18 . Light from the light source  10  (represented in full line), preferably a narrow band laser which emits light of a wavelength lo, is coupled-in in an optic fibre  11 , preferably a fibre with a small numerical aperture, for instance a monomode fibre. Light issuing at the distal end of this fibre is collimated with the aid of a lens  13  and filtered by a filter  15  so that only the narrow band laser light l 0  is coupled-in with the aid of lens  17  into the hollow light guide  12  and reaches the sample. The hollow light guide  12  is attached to coupling unit  14  by means of a mechanic coupling  20 , for instance by means of an SMA-coupling or an FC-coupling, so that simply and at will, different light guides can be attached to coupling unit  14 , or be exchanged. Light which is scattered by the sample (represented in broken lines) is guided back through the hollow light guide  12  to the coupling unit  18 . After collimation by lens  17 , this scattered light issuing from the hollow light guide  12  reaches filter  15 . Light l 0  not shifted in wavelength is largely transmitted through the filter, preferably by more than 99%. Raman scattered light with wavelengths l 0,2 . . . N  is largely reflected by filter  15 , preferably by more than 99%. Filter  22  provides for further suppression of the component lo, whereupon the Raman scattered light is focussed by lens  19  onto a bundle of optic fibres  21 . This bundle is for instance circular on the side where the Raman scattered light is focussed, but different shapes are possible. Preferably, the bundle is designed such that as much light as possible is coupled into the core of the fibres of which the bundle consists. The fibres can be circular, square or otherwise multiangular. At the distal side, on which side the light is coupled into the analysis unit  14 , the fibres of fibre bundle  21  are arranged in a straight row, perpendicular to the dispersion direction of a dispersive spectrometer. 
   In this embodiment, the coupling unit  14  is suitable for coupling the hollow light guide  12  to a dispersive analysis unit, in which the width dimension of the row of fibres is partly determinative of the spectral resolution of the device. This is not the case when the analysis unit  14  is a Fourier-Transform Raman spectrometer or when, with the aid of optic fibres, the intensity of the scattered light is measured ill only one or several wavelength intervals. In these two cases, fibre bundle  21  can be optionally omitted. 
   In a different embodiment of the coupling unit, filter  15  is replaced with a mirror which reflects the light issuing from the hollow light guide in the direction of filter  22 . The light from light source  10  reaches the hollow light guide through a hole in the mirror of a diameter which is relatively small with regard to the total mirror surface. 
   In a different embodiment (not shown) laser light is coupled into the hollow light guide via a small mirror. The surface of the mirror is small with respect to the diameter of the light beam issuing from the hollow light guide and collimated by a lens. In still another embodiment (not shown) laser light is coupled into the hollow light guide via a small mirror placed between a lens and the hollow light guide. 
     FIG. 2  shows the principle of the use of a capillary  12  as hollow light guide for Raman spectroscopy light of wavelength l 0  from light source  10  (not shown) is coupled in via coupling unit  18  (not shown) in capillary  12  and is reflected on the inner wall of the capillary and reaches the sample  16 . The sample scatters the incident light through elastic and inelastic scattering. A portion of this scattered light, consisting of a strong component of, a wavelength l 0  and, in addition, components l 1,2 . . . ,N  shifted in wavelengths, is intercepted by capillary  12  and guided back to the coupling unit  14  (not shown) to which capillary  12  is connected by means of a mechanical coupling  20  (not shown). 
     FIGS. 3A–C  shows different possible embodiments of the hollow light guide. 
     FIG. 3A  shows a capillary  32  made of a material having an intrinsically high reflection coefficient for the relevant part of the electromagnetic spectrum, for instance of metal. The inside of the capillary is sufficiently flat, so that specular reflection from the walls dominates. Gold has a high reflection in the near-infrared (&gt;800 nm) of 98–99%. For applications in the ultraviolet, aluminum is preferred. When the length of the hollow light guide remains limited, such a capillary suffices for the application described here. In a longer capillary, the reflection losses could become a drawback when signal is intercepted from a sample which inherently emits a weak signal. For a light ray  31 , entering through the centre of the capillary  32  at an angle φ to the axis of the capillary, having a radius R and a length L, and made of a material having reflection coefficient ρ, the weakening as a result of reflection losses can be assessed with the aid of formula 1.
   I   out   /I   in ˜≅ρ L tan φ/2R    (1) 
   For a gold capillary of a diameter of 1 mm and a length of 50 cm and light incidence at an angle φ of maximally 6°, this reflection loss is then approximately 64%. 
     FIG. 3B  shows a capillary made of fused silica, glass or a plastic  33 , whose inner wall is provided with a reflecting metal coating  34 . For the metal, in these embodiments, aluminum (particularly suitable for use in the ultraviolet), silver (for use with visible light and in the near-infrared) or gold (near-infrared) can be utilized. The inside has been smoothed such that specular reflection occurs. In a further embodiment a dielectric layer can be applied on the reflecting layer to increase the reflection coefficient. Light guides according to this principle are known per se which exhibit a transmission of approximately. 90%/m. Such light guides are suitable in particular for applications where the light guide does not need to be bent or hardly so. Sharp bends lead to strong decrease of the transmission. 
   Further, on the inner wall of the capillary, an accumulation of dielectric layers can be applied, selected such that in the spectral region of interest an optimum reflection is accomplished at every angle of incidence. Such a hollow light guide, whose walls function as so-called omni-directional reflector is, described in Fink, Y. et al J. Lightwave Technology 17:2039–2041 (1999). 
     FIG. 3C  shows a capillary  35  non-absorbing in the relevant spectral region and transparent, filled will liquid  36  non-absorbing in the relevant spectral region, having no Raman signal in the spectral regions of interest to the user. The light guide with liquid is based on the principle of total internal reflection with which very high transmission-efficiency can be attained, which is moreover little influenced by bends in the light guide. This means that the refractive index of the liquid needs to be higher than that of the capillary material. The numerical aperture of this light guide is determined by formula 2:
   N.A. =√( n   1   2   −n   2   2 )  (2) 
   A possible option for liquid  36  is tetrachloroethylene. Use of this agent having a refractive index of ˜15.05 and only Raman scattering at ˜1571 cm−1 and below 525 cm−1 leaves a large part of the Raman spectrum free for measurements on a sample. When a fused silica (refractive index ˜1.46) capillary is filled with this liquid, a light guide is formed with a numerical aperture of −0.36. Another possible choice is CC14. It will be clear that, depending on the application (i.e. depending, inter alia, on which part or which parts of the spectrum have to be clear of background signal contributions of the Raman probe) a particular liquid can be chosen, and that there is a large freedom in that respect. 
   It will be clear that many other variations on the above-mentioned examples are possible, such as, for instance, a capillary filled with a liquid of a lower refractive index than the material of which the capillary is made. The principle of light guidance through total internal reflection can then be realized by employing a coating of the inside wall of the capillary with a material having a refractive index lower than the refractive index of the liquid. 
   In principle, it is also possible to guide light through the capillary by using total internal reflection on the transition of capillary material and air at the outer side of the capillary. However, this is a less advantageous solution. On the one hand, also a portion of the Raman signal generated in the capillary material will be guided into the light guide. On the other hand, total internal reflection will (can) be lost at locations where the outside of the capillary is in contact with other materials. 
   To obviate this last-mentioned problem, a coating of a material with a lower refractive index than the refractive index of the liquid can be applied on the outside of the capillary. The disadvantage remains that a portion of the Raman signal generated in the capillary material is transmitted into the liquid core. Onto the coating on the outside of the capillary a strongly absorbing coating has to be applied because, otherwise, total internal reflection on the transition of coating and air occurs. 
   In the embodiments of  FIGS. 2 and 3 , the Raman capillary is simply * cut off at the exit. In the case of a light guide with a liquid core as described, a transparent sealing of the capillary will be used at both ends. In this case, the choice of diameter of the capillary determines the magnitude of the measured surface and/or of the measured volume in the sample of which scattered light is captured. 
   Other embodiments of the extremity are possible. The shape of the extremity of the Raman capillary, i.e. the side of the hollow light guide where the sample is located, can be varied. This makes it possible, for instance, to use the Raman capillary for illumination of a sample and/or signal collection from a sample at an angle or angles deviating from the direction of the longitudinal axis of the capillary and/or are not centered about the longitudinal axis of the Raman capillary. It can also be provided that control is obtained over the location of the measured volume from which preferentially light is intercepted and/or the spatial resolution of the Raman capillary. It can also be provided that light scattered by the sample can be intercepted within a large solid angle, whereupon this solid angle is transformed to a smaller solid angle which can be (efficiently) guided through the capillary. A number of examples are discussed in  FIGS. 4 to 8 . 
     FIG. 4  shows an embodiment of the light guide with an element  41  at the distal end (the part of the probe which is in contact with the sample) which brings about a solid angle adaptation. The element  41  is for instance manufactured from a material which gives off no or only a relatively weak Raman signal in the relevant part of the Raman spectrum. An example of such a material is UV-grade CaF 2 , as can be supplied by, for instance, the firm Crystran Ltd, 27 Factory Road, Poole, Dorset BH16 5SL, United Kingdom. 
   The element  41  is for instance cone-shaped. The sides of the element make an angle φ with the longitudinal a of the Raman capillary  42 . When light is intercepted by element  41  at an angle γ, larger than φ, with the longitudinal axis of the capillary, then, after reflection from the wall of element  41 , the light will travel further at an angle γ′ with the longitudinal axis of the capillary which is smaller than the original angle of incidence γ. When angle γ is large, more reflections with the sides of element  41  can occur before the light proceeds in the Raman capillary  42 . Many variations on the shape of element  41  with a view to adjusting the solid angle at which intercepted signal is guided further, are possible. For Raman capillaries in which use is made of a reflecting coating on the inside wall of capillary, this has the great advantage that this limits the number of reflections of the intercepted Raman light against the walls of the hollow light guide en route to the detector, and hence also reflection losses. For Raman capillaries in which use is made of total internal reflection, this has the advantage that signal from the sample can be intercepted at a larger solid angle than could be guided further by the Raman capillary. 
   Also at the detector side of the hollow light guide such a cone or an optimized non-imaging parabolic light collector can be attached. There, it serves as a numerical aperture adapter, for the analysis unit. 
     FIG. 5  shows a light guide with a tapering hollow element  51  with a reflecting wall at the distal end which effects a similar sort of solid angle adjustment to element  41  in  FIG. 4 . 
     FIG. 6  shows a light guide with a reflecting element  61  at the distal end having for an object to make the light guide view sideways. This reflecting element can for instance be a microprism with a reflecting layer  62  applied thereon. Various other embodiments are possible. 
     FIG. 7  shows a capillary  70  whose extremity is depicted through a lens  71  in a volume of the sample  72  or on a surface of the sample  72 . In particular the light which is scattered in the same volume of the sample or on the same part of the surface of the sample is, again, coupled-in with great efficiency in capillary  70 . In this way, in a simple manner, a confocal measurement arrangement is created with which accurate control is obtained over which part of the sample is examined. The choice of the lens and capillary deter together determine the spatial resolution of a probe in this embodiment. Also, this renders it possible to place an extra window  73  at the extremity of the Raman probe, which falls outside the measured volume of the probe, and therefore does not cause an interfering Raman signal background. For the same reason, with such a probe, it becomes possible to measure a sample through a transparent container wall  74  without measuring an interfering Raman signal from the wall material. The housing  75  of this embodiment can be adapted to the specific application. 
     FIG. 8  shows an embodiment as in  FIG. 7  in which the lens is replaced by a gradient index lens  80 . Lens and core diameter of capillary  81  together are determinative of the spatial resolution. 
     FIG. 9  shows the use of a thick-walled capillary  90  as a guide for light to and from a sample to be examined. On the outside, the capillary is coated with a material  91  of lower refractive index than the material of the capillary, while the layer thickness of this coating is sufficiently great for the effects on the interface between capillary material and coating to be determined by the bulk properties of the coating material. The capillary is filled with a liquid  92  of a higher refractive index than the material of which the capillary is made. Light from light source  10  (not shown) is introduced into the capillary within a solid angle which is smaller than the maximum solid angle at which the liquid core can guide light (given by formula 2). Further, the liquid  92  is selected such that in the spectral region relevant to the measurement, no significantly interfering overlap occurs between the scattering spectrum of the sample and the scattering spectrum of the liquid. Through this suitable choice of liquid and wall material, a light guide is created through which light from light source  10  (not shown) can be guided to the sample to be examined. Light scattered by the sample can be guided back through the wall of the capillary to the analysis unit  14  (not shown), via a Coupling unit  18  (not shown). At the top and bottom, the capillary is closed off with the aid of thin transparent windows  93 ,  94 . If the Raman spectrum of the capillary material overlaps the spectrum of the sample to be examined in a significantly interfering manner, a filter  95  can be provided on the capillary on the side of the sample, which filter screens light elastically scattered by the sample. Accordingly, this prevents an interfering Raman signal being generated in the wall of the capillary. Eligible for serving as a substrate for the filter is a thin, smoothly polished disk sawn off from the capillary having the same dimensions. Such a filter can then be positioned and fastened onto the capillary in a simple manner; The refractive indices of liquid and capillary material determine the solid angle at which light from light source  10  (not shown) can be coupled-in into the light guide and at which the sample can be illuminated, according to formula 2 for the numerical aperture of a light guide. 
   In the same manner, the solid angle at which light scattered by the sample can be intercepted and guided further to the coupling unit  18  (not shown) is determined by the refractive indices of the capillary material and the coating on the outside of the capillary. 
   In an alternative embodiment, the capillary is coated on the outside with a highly reflective material, preferably a gold or silver coating or an omni-directionally reflecting dielectric coating. Optionally, depending on the application, a coating can be dispensed with, so that total internal reflection can occur at the transition of capillary and air. 
   In a different embodiment, conversely, the excitation light is guided  through the wall of the capillary to the sample, with optional filtering at the extremity of the capillary, and light scattered by the sample is guided via a channel of the capillary to the analysis unit. 
     FIG. 10  shows a further embodiment, in which the capillary is not filled with a liquid but in which the inside wall of the capillary has been made strongly reflective, for instance through application of a silver layer and a dielectric layer  100 . 
     FIG. 11  shows a further embodiment in which the inside of the capillary is coated with a material  110  of lower refractive index than both the liquid in the core of the capillary and the material of the capillary. In this manner, the liquid core of the Raman capillary through which light from the light source  10  (not shown) is guided to the sample and the wall of the capillary through which light Raman scattered by the sample is discharged to the analysis unit  18  (not shown) are optically separated from each other. 
     FIG. 12  shows a further embodiment, in which the capillary according to  FIG. 9 ,  10  or  11  is provided with an element  121  on the side where the sample to be examined is located. Use of this element provides for optimal overlap between irradiated sample-volume and sample-volume from which light is intercepted. Further, the element serves for conversion of the large solid angle at which light scattered by the sample is intercepted, to a smaller solid angle, which light can be guided back through the capillary in the direction of the analysis unit  18  (not shown). The element has, for instance, the shape of a cone, but other shapes are also possible. Preferably, the element is manufactured from a material which does not emit a significantly interfering Raman signal in the relevant spectral range. For several applications, for instance UV-grade calcium fluoride is a good option, because this material has a strong Raman band only at 312 cm−1. The element can be coated with a strongly reflecting coating, for instance gold, or with a material of lower refractive index so that total internal reflection occurs. A suitable choice is, for instance, Teflon AF (produced by DuPont) having a refractive index of 1.29–1.31. 
     FIG. 13  shows a further embodiment in which the capillary according to  FIG. 10  or  11  is provided with an element  131  as in  FIG. 12 , but for which a  10  same capillary as of which the light guide is made serves as base material, which is subsequently brought into the desired shape by grinding and polishing or in a different mater. This offers advantages in view of a simpler manner of production, which can be utilized if the Raman signal background of the capillary material which is generated in the element does not have a significantly interfering effect. Additionally, this embodiment offers the possibility of displacement of the measured volume to a location deeper into the sample, since signal does not necessarily need to be intercepted directly from the surface of the sample (for instance when the inside of the element is coated with a reflecting metal layer). 
     FIG. 14  shows an alternative embodiment of coupling unit  18 . The extremity of the light guide  12  is coupled to an element  140  which reduces the solid angle at which light  146  issues from the hollow light guide, which enables this light being coupled-in and guided further with the aid of standard optic glass fibres  141  with, for instance, a numerical aperture of 0.22. A filter  142  serves for blocking light of the same wavelength as the light from light source  10  (not shown). Through a relatively saw hole in the filter, light  147  is coupled in from light source  10  (not shown) in capillary  143 . This capillary can have one of the embodiments as described hereinabove or derived therefrom. Elements  140  can for instance be manufactured by taking as a basis so-called fibre optic ferrules, as sold, for instance, by Vitrocom (Mountain Lakes, N.J., USA). The inside wall is made strongly reflective, for instance by applying a gold or silver coating  144 . 
   In an alternative embodiment, element  140  is filled with a liquid  145  of a higher refractive index than the wall material of element  140 , so that total internal reflection occurs at the transition between liquid and wall material.