Abstract:
A Raman spectroscopy system has a filter arrangement comprising two filters ( 16, 26 A) in series, to reject light of the illuminating wavenumber from the scattered light of interest. The filters are tilted and have different characteristics for light of first and second different polarisation states. To counter this, the filters are arranged so that their respective effects on the respective polarisation states at least partially cancel each other out. This may for example be done by arranging their tilt axes ( 32, 34 ) orthogonally to each other.

Description:
FIELD OF THE INVENTION 
     This invention relates to spectroscopy systems, for example, Raman spectroscopy systems. 
     BACKGROUND ART 
     A known Raman spectroscopy system is described in U.S. Pat. No. 5,442,438, which is incorporated herein by reference.  FIG. 1  of the accompanying drawings shows a commercially-available embodiment of such a system. 
     In  FIG. 1 , light  13  from a laser source  10  is reflected by mirrors  12 , 14  and by a notch or edge filter  16  which acts as a dichroic beamsplitter. This directs it along an optical path  15  into a microscope  18 , where it is deflected by a mirror  20  through an objective lens  22  and focused on a sample  24 . Raman scattering takes place at the sample, producing Raman-shifted light at different wavenumbers from the incident laser line. The Raman-shifted light is collected by the objective lens  22  and passed back along the optical path  15  via the mirror  20  to the filter  16 . 
     Whereas the filter  16  reflects light of the laser wavelength, it transmits the Raman-shifted wavenumbers. While doing so, it rejects the much more intense laser line. Further rejection of the laser line takes place in a second, identical filter  26 . The Raman-shifted light then passes through a Raman analyser  28 , which as described in U.S. Pat. No. 5,442,438 may comprise a diffraction grating, or filters which accept specific Raman lines of interest. The resulting light is then passed to a detector  30 . This may for example comprise a charge-coupled device (CCD), across which a Raman spectrum may be dispersed by a diffraction grating. Or a filter may pass a two-dimensional image of the sample to the CCD, in light of a selected Raman wavenumber. 
     The notch or edge filters  16 , 26  may be holographic filters, as described in U.S. Pat. No. 5,442,438. Or they may be thin film multi-layer dielectric filters, such as for example the hard oxide filters supplied by Semrock Inc, Rochester, N.Y., USA under the trademark RazorEdge. Such filters are described in U.S. Pat. No. 7,068,430, incorporated herein by reference. 
     The filter  16  is necessarily placed at an angle to the optical path, in order to inject the light from the light source  10  towards the sample  24 . However, in order to provide a sharp cut-off between the rejection of the laser line and the acceptance of Raman-scattered light at wavenumbers close to the laser line, U.S. Pat. No. 5,442,438 describes that this angle should be a low angle of incidence, such as 10°. The second filter  26  is similarly placed at the same low angle of incidence, to provide matching performance. In practice, angles of between 7.5° and 13° are used, but other angles are also possible. 
     Even at such low angles of incidence, however, polarisation effects reduce the sharpness of the cut-off. The larger the angle of incidence, the greater the problem. Specifically, the cut-off for p-polarised light is different from that for s-polarised light, by an amount which depends on the angle of incidence. It follows that the transmission characteristic of the filter shows a step or shelf in the cut-off edge at around 50% transmission for randomly polarised light. This results in polarisation artefacts in the resulting spectra measured by the device. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention provides a spectroscopy system comprising:
         a light source for illuminating a sample and causing scattering at wavenumbers shifted from that of the light source;   an optical path for collecting the scattered light and passing it to a detector;   a pair of filters in series in the optical path, arranged to reject light of the illumination wavenumber and accept light of the shifted wavenumbers, the filters having different characteristics for light of first and second different polarisation states;   characterised in that the filters are arranged so that their respective effects on the respective polarisation states at least partially cancel each other out.       

     A second aspect of the invention provides a filter arrangement, comprising:
         a pair of filters in series in an optical path, arranged to reject light of a first wavenumber and accept light of a second wavenumber, the filters having different characteristics for light of first and second polarisation states;   characterised in that the filters are arranged so that their respective effects on the respective polarisation states at least partially cancel each other out.       

     Preferably, both filters are tilted about respective axes with respect to the optical path, the axes being generally orthogonal to each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of a prior art spectroscopy system; 
         FIG. 2  shows a novel arrangement of a pair of filters in such a system; 
         FIGS. 3 and 4  are graphs of the transmission characteristics of the respective filters against wavenumber; 
         FIG. 5  is a graph of the combined transmission characteristic of the two filters; and 
         FIGS. 6 and 7  show further novel arrangements of filters for use in a system such as that of  FIG. 1 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present embodiments of the invention are in most respects the same as described above with respect to  FIG. 1 .  FIG. 2  illustrates one way in which  FIG. 1  is modified, in a first embodiment of the invention. 
     The first notch or edge filter  16  is of the same type as in  FIG. 1 . It is arranged in the same way, tilted about an axis  32  with respect to the optical path  15 , suitably at a low angle of incidence such as 10°. As described above, other angles may be used, such as between 7.5° and 13°, or even 450°. As shown in  FIG. 2 , the axis  32  is generally horizontal. 
     However, the second filter  26  is arranged differently, as shown at  26 A in  FIG. 2 . Instead of being tilted about an axis parallel to the axis  32 , it is tilted about an axis  34  which is generally orthogonal to the axis  32 . Thus, the axis  34  is generally vertical. The second filter  26 A is however preferably tilted to the same angle of incidence as the first, e.g. 10°. This ensures that its performance matches that of the filter  16 , except as discussed below. 
     It would of course be possible to arrange the filters the other way around, so that the first filter was tilted about a vertical axis while the second filter was tilted around a horizontal axis. Any other substantially orthogonal arrangement could be used instead. Where the incident laser light  13  is to be injected into the optical path  15  by the filter  16 , then of course it would need to be delivered to the filter  16  at an appropriate angle. 
       FIG. 3  shows the transmission characteristic (transmission versus wavenumber) of the first filter  16 . It also shows the laser line L. In the following discussion, the s- and p-polarisation directions are all defined relative to the angle of incidence of the light as shown for the first filter  16  in  FIG. 3 . 
     As can be seen in  FIG. 3 , the transmission characteristic P for p-polarised light shows a sharp edge close to the laser line L. The transmission characteristic S of the s-polarised light has a similar sharp edge, but it is further away from the laser line L. As a result, the transmission characteristic M for light of mixed polarisation exhibits a flat shelf or step at around 50% transmission, because in this region the filter transmits p-polarised light but not s-polarised light. 
     In the prior art arrangement of  FIG. 1 , both the filter  16  and the filter  26  will exhibit characteristics similar to  FIG. 3 . Therefore, as discussed above, polarisation artefacts are introduced into the resulting spectra taken close to the laser line L. However, since the filter  26 A in  FIG. 2  is tilted about an axis which is orthogonal to the filter  16 , the characteristics relative to s-polarised and p-polarised light are reversed. This is shown in  FIG. 4 , where the sharp cut-off edge S for the s-polarised light is closer to the laser line than the cut-off edge P for the p-polarised light. 
     Therefore, when the scattered light passes through the filters  16  and  26 A in series as it travels along the optical path  15 , the combined transmission characteristic is as shown in  FIG. 5 . The overall transmission characteristics S and P for the s-polarised and p-polarised light are much closer together. The resulting characteristic M for light of mixed polarisation therefore does not exhibit the shelf or step shown in  FIGS. 3 and 4 . The polarisation artefacts in the resulting spectra are reduced or eliminated. 
     In an advantageous arrangement, the axis of tilt ( 32  or  34 ) for the first filter  12  is chosen such that the rejection curve which is further from the laser line L matches the input laser polarisation. E.g. a filter with the  FIG. 3  characteristic would be used when the laser is s-polarised. Or if the laser is p-polarised, then the axis of the first filter would be arranged so as to have the characteristic of  FIG. 4 . By adjusting the angle of incidence, the filter  12  can then be set so that its sharp rejection edge is closer to the laser line. The combined filters then give good laser rejection performance, whilst accepting Raman scattered light closer to the laser line than if the first filter was arranged with the other axis of tilt relative to the input laser polarisation. 
     The invention is not restricted to the use of filters tilted about orthogonal axes as shown in  FIG. 2 . Other arrangements may be envisaged to ensure that the effects of the two filters on the different polarisation states partially or wholly cancel each other out. 
     For example, such an arrangement is shown in  FIG. 6 . Here, there are two filters  16 ,  26 , of any of the types mentioned above, and they are tilted to provide an angle of incidence as also discussed above. The tilt axes of the filters are parallel to each other, as in  FIG. 1 . However, a half-wave plate  36  is placed between the two filters. This rotates the polarisation of the scattered light in the path  15  by 90°, as it passes from one filter to the other. The effect is similar to the use of orthogonal tilt axes in  FIG. 2 . 
     Alternatively, as shown in  FIG. 7 , a composite optical component  38  may be used. This comprises a half-wave plate  40 , sandwiched between two filters  42 . The filters  42  may be any of the types discussed above, and may simply be cemented on opposing faces of the half-wave plate  40 . Advantageously, however, the filters  42  are of the thin film multi-layer dielectric type, e.g. of hard oxide as supplied by Semrock Inc. They may then be formed as coatings directly on the opposing faces of the half-wave plate  40 . 
     If it is desired to return the light to its original polarisation state, a second half-wave plate could be included in the path  15 , before or after the filter/half-wave plate arrangements shown in  FIGS. 6 and 7 . 
     The embodiments described may incorporate the various alternatives discussed above in relation to  FIG. 1 . In a further alternative, instead of the microscope  18 , a remote sample may be analysed using a fibre optic probe of known type. With one type of such a fibre optic probe, the incident laser light may not be injected via the filter  16 , but may instead be taken directly to the probe via a first optical fibre. The Raman scattered light is then returned to the filter  16  via a second optical fibre. 
     Furthermore, the system is not restricted to Raman spectroscopy. It may be used for other kinds of spectroscopic analysis, such as fluorescence, narrow-line photoluminescence and cathodoluminescence.