CA2241470C - Diamond detection using coherent anti-stokes raman spectroscopy - Google Patents
Diamond detection using coherent anti-stokes raman spectroscopy Download PDFInfo
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- CA2241470C CA2241470C CA002241470A CA2241470A CA2241470C CA 2241470 C CA2241470 C CA 2241470C CA 002241470 A CA002241470 A CA 002241470A CA 2241470 A CA2241470 A CA 2241470A CA 2241470 C CA2241470 C CA 2241470C
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Abstract
The invention concerns a method and apparatus for detecting diamonds. In the method, particles (32) undergoing analysis are irradiated by a beam (30) of laser light formed by focusing multiple laser beams (12, 14). At least two of these beams have frequencies differing from one another by a value characteristic of diamond so that, in the focused beam, at least some components of the laser beams are coherently phase-matched. The scattered signal emitted by each particle is then collected and a determination is made as to whether such signal is a CARS signal characteristic of diamond.
Description
BACKGROUND TO THE INVENTION
THIS invention relates to diamond detection using coherent anti-Stokes Raman spectroscopy (CARS).
It has already been proposed to detect and sort diamonds on the basis of Raman response. In the known technology, particles which are to be sorted are irradiated with a laser beam. In the case of a diamond, interaction of the laser beam with vibrational modes of the diamond crystal results in absorption of energy from the laser photons and produces scattered photons of slightly longer wavelength than the incident laser beam. The corresponding frequency shift corresponds to the vibrational energy which, for diamonds, amounts to a wave number of 1332 cni'.
THIS invention relates to diamond detection using coherent anti-Stokes Raman spectroscopy (CARS).
It has already been proposed to detect and sort diamonds on the basis of Raman response. In the known technology, particles which are to be sorted are irradiated with a laser beam. In the case of a diamond, interaction of the laser beam with vibrational modes of the diamond crystal results in absorption of energy from the laser photons and produces scattered photons of slightly longer wavelength than the incident laser beam. The corresponding frequency shift corresponds to the vibrational energy which, for diamonds, amounts to a wave number of 1332 cni'.
The so-called Raman shift is independent of the laser frequency, but the intensity of the spontaneous Raman scattered light is frequency dependent and is generally weak. Although shorter exciting wavelengths produce somewhat stronger signals such wavelengths typically also excite fluorescence in diamonds which can swamp the characteristic Raman signal and make it extremely difficult to detect reliably.
CARS is a third order variant of the Raman technique as outlined above. In the known CARS technique, two laser beams are simultaneously directed at the particle, with the frequencies of the two beams differing from one another by an amount characteristic of the material which is to be to be detected, i.e. 1332 cm' in the case of diamond. Coherence is achieved by ensuring that the two beams are at a specified angle to one another so that phase matching is ensured. The beams interact with the diamond or other mineral crystal lattice and produce a third, resultant beam at a specified angle. The resultant signal, which has an intensity substantially greater than the spontaneous Raman signal, has a frequency higher than that of the input laser frequencies. In the case of diamond, there is a 1332 cm' shift to higher frequency, relative to one of the exciting frequencies. The characteristic, higher frequency signal is outside the fluorescence band and hence can be detected without a fluorescence background being present.
The problem with the known CARS technique, as outlined above, for the purposes of detecting diamonds, is the fact that the particles which are presented for analysis have rough surfaces which refract the laser beams so that angular separation between the beams cannot be ensured.
This problem is addressed by the present invention.
CARS is a third order variant of the Raman technique as outlined above. In the known CARS technique, two laser beams are simultaneously directed at the particle, with the frequencies of the two beams differing from one another by an amount characteristic of the material which is to be to be detected, i.e. 1332 cm' in the case of diamond. Coherence is achieved by ensuring that the two beams are at a specified angle to one another so that phase matching is ensured. The beams interact with the diamond or other mineral crystal lattice and produce a third, resultant beam at a specified angle. The resultant signal, which has an intensity substantially greater than the spontaneous Raman signal, has a frequency higher than that of the input laser frequencies. In the case of diamond, there is a 1332 cm' shift to higher frequency, relative to one of the exciting frequencies. The characteristic, higher frequency signal is outside the fluorescence band and hence can be detected without a fluorescence background being present.
The problem with the known CARS technique, as outlined above, for the purposes of detecting diamonds, is the fact that the particles which are presented for analysis have rough surfaces which refract the laser beams so that angular separation between the beams cannot be ensured.
This problem is addressed by the present invention.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a method of detecting diamonds, the method comprising the steps of irradiating particles in a beam of laser light formed by focusing two or more laser beams, at least two of which have frequencies differing from one another by a value characteristic of diamond, whereby at least some components of the laser beams focused to form the irradiating beam of laser light are coherently phase-matched, collecting the scattered signal emitted by each particle and determining whether such signal is a CARS signal characteristic of diamond.
According to another aspect of the invention there is provided an apparatus for detecting diamonds comprising means for producing beams of laser light at least two of which have frequencies differing from one another by a value characteristic of diamond, means for focusing the laser beams to form an irradiating beam of laser light by means of which particles undergoing analysis are irradiated, at least some components of the laser beams being coherently phase-matched by the focusing means, means for collecting the scattered signal emitted by each particle and means for determining whether such signal is a CARS signal characteristic of diamond.
Other features of the method and apparatus are set forth in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail, by way of example only, with reference to the accompanying diagrammatic drawings in which Figures 1 to 3 each illustrate a different embodiment of the invention.
DESCRIPTION OF EMBODIMENTS
In each of Figures 1 to 3, the same symbols are used to identify corresponding components. In the embodiment of Figure 1, laser sources indicated generally by the numeral 10 produce two laser beams 12 and 14 which are polarised by respective polarisers 16 and 18. The beam 12 has a wavelength of 532 nm and the beam 14 a wavelength of 573 nm. The wavelength difference corresponds to the characteristic frequency difference of 1332 cm' for diamond.
The beam 12 is reflected by a mirror 20 and a dichroic mixing plate 22, while the beam 14 passes through the dichroic mixing plate 22. The beams 12 and 14 then combine to form a collinear beam 24 which is reflected by a mirror 26 and focused by a lens 28 to produce a cone of laser light 30 with which a particle 32 undergoing analysis is irradiated.
In the light cone 30 the two laser frequencies, i.e. the frequencies of the beams 12 and 14, are in effect at a range of angles to one another, such range being defined by the limiting cone angle.
Within this range of angles, at least some intersecting components of the beams will satisfy the phase matching criterion required for successful implementation of the CARS technique, irrespective of the fact that the particle 32 may have rough and uneven surfaces.
The scattered CARS signal leaving the particle 32 has a conical shape, as indicated by the numeral 34. The signal passes through a collecting lens 36 which restores a collimated beam. The signal is passed through a filter 38 which removes all wavelengths other than a characteristic wavelength of 497 nm for diamond. Any signal passed by the filter is reflected by a mirror 40 to a spectrometer 42 tuned to the characteristic wavelength. A suitable electronic processor, not shown, assesses whether the signal received by the spectrometer is indicative that the particle 32 is a diamond.
Figure 2 illustrates a modified embodiment of the invention. In this Figure, components corresponding to those of Figure 1 are designated by the same reference numerals. In this case, the dichroic mixing plate is replaced by a mirror 23 which reflects the laser beam 12 and which is bypassed by the laser beam 14. Thus there is no production of a collinear, combined laser beam as in Figure 1. The respective beams 12 and 14, at an angle to one another, are independently reflected by the mirror 26 to the focusing lens 28 which produces a cone 30 of laser light with which the particle 32 undergoing analysis is irradiated.
The frequencies of the beams 12 and 14 are as in the first embodiment, so there is a similar effect in the cone 30, i.e. the respective frequencies are at a range of angles, allowing the CARS condition of phase matching to be achieved by at least certain components of the beams.
The scattered CARS signal is, as before, condensed to a collimated beam by the collecting lens 36 and passed through the filter 38 which removes _ 7 _ wavelengths other than the 497 nm wavelength characteristic of diamond.
The resulting signal is reflected by the mirror 40 to the spectrometer 42 which, in this case, incorporates a third polariser 44. Once again, if the characteristic CARS signal is detected by the spectrometer, the particle 32 may be identified as a diamond.
In the embodiment of Figure 3, the laser beams 12 and 14, at the same frequencies as before, are arranged parallel to one another and are reflected by a prism 46 to the focusing lens 28 which produces the mixed cone 30 of laser light in which the particle 32 is irradiated. As in the first two embodiments, the range of angles present in the cone 30 between the different frequencies enables the CARS condition of beam coherence to be achieved by at least some components of the respective frequencies.
The scattered CARS signal which is produced is condensed by the collector lens 36 to a collimated beam and is filtered by the filter 38 allowing passage of the characteristic 497 nm signal. The signal is directed to the spectrometer 42 by the mirror 40 as before, and an assessment made as to whether such signal is indicative of a diamond particle 32.
Each of the apparatuses described above can form part of a sorting apparatus used to sort diamond particles from associated gangue particles. The particles may be analysed on-line with means being provided to separate those particles identified as diamonds from the other particles. Also, in each of the embodiments described above, the focusing achieved by the lens 28 is such as to limit the intensity of the radiation to which the particles are subjected, thereby to reduce the possibility of radiation-induced damage to diamond particles.
According to one aspect of the present invention there is provided a method of detecting diamonds, the method comprising the steps of irradiating particles in a beam of laser light formed by focusing two or more laser beams, at least two of which have frequencies differing from one another by a value characteristic of diamond, whereby at least some components of the laser beams focused to form the irradiating beam of laser light are coherently phase-matched, collecting the scattered signal emitted by each particle and determining whether such signal is a CARS signal characteristic of diamond.
According to another aspect of the invention there is provided an apparatus for detecting diamonds comprising means for producing beams of laser light at least two of which have frequencies differing from one another by a value characteristic of diamond, means for focusing the laser beams to form an irradiating beam of laser light by means of which particles undergoing analysis are irradiated, at least some components of the laser beams being coherently phase-matched by the focusing means, means for collecting the scattered signal emitted by each particle and means for determining whether such signal is a CARS signal characteristic of diamond.
Other features of the method and apparatus are set forth in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail, by way of example only, with reference to the accompanying diagrammatic drawings in which Figures 1 to 3 each illustrate a different embodiment of the invention.
DESCRIPTION OF EMBODIMENTS
In each of Figures 1 to 3, the same symbols are used to identify corresponding components. In the embodiment of Figure 1, laser sources indicated generally by the numeral 10 produce two laser beams 12 and 14 which are polarised by respective polarisers 16 and 18. The beam 12 has a wavelength of 532 nm and the beam 14 a wavelength of 573 nm. The wavelength difference corresponds to the characteristic frequency difference of 1332 cm' for diamond.
The beam 12 is reflected by a mirror 20 and a dichroic mixing plate 22, while the beam 14 passes through the dichroic mixing plate 22. The beams 12 and 14 then combine to form a collinear beam 24 which is reflected by a mirror 26 and focused by a lens 28 to produce a cone of laser light 30 with which a particle 32 undergoing analysis is irradiated.
In the light cone 30 the two laser frequencies, i.e. the frequencies of the beams 12 and 14, are in effect at a range of angles to one another, such range being defined by the limiting cone angle.
Within this range of angles, at least some intersecting components of the beams will satisfy the phase matching criterion required for successful implementation of the CARS technique, irrespective of the fact that the particle 32 may have rough and uneven surfaces.
The scattered CARS signal leaving the particle 32 has a conical shape, as indicated by the numeral 34. The signal passes through a collecting lens 36 which restores a collimated beam. The signal is passed through a filter 38 which removes all wavelengths other than a characteristic wavelength of 497 nm for diamond. Any signal passed by the filter is reflected by a mirror 40 to a spectrometer 42 tuned to the characteristic wavelength. A suitable electronic processor, not shown, assesses whether the signal received by the spectrometer is indicative that the particle 32 is a diamond.
Figure 2 illustrates a modified embodiment of the invention. In this Figure, components corresponding to those of Figure 1 are designated by the same reference numerals. In this case, the dichroic mixing plate is replaced by a mirror 23 which reflects the laser beam 12 and which is bypassed by the laser beam 14. Thus there is no production of a collinear, combined laser beam as in Figure 1. The respective beams 12 and 14, at an angle to one another, are independently reflected by the mirror 26 to the focusing lens 28 which produces a cone 30 of laser light with which the particle 32 undergoing analysis is irradiated.
The frequencies of the beams 12 and 14 are as in the first embodiment, so there is a similar effect in the cone 30, i.e. the respective frequencies are at a range of angles, allowing the CARS condition of phase matching to be achieved by at least certain components of the beams.
The scattered CARS signal is, as before, condensed to a collimated beam by the collecting lens 36 and passed through the filter 38 which removes _ 7 _ wavelengths other than the 497 nm wavelength characteristic of diamond.
The resulting signal is reflected by the mirror 40 to the spectrometer 42 which, in this case, incorporates a third polariser 44. Once again, if the characteristic CARS signal is detected by the spectrometer, the particle 32 may be identified as a diamond.
In the embodiment of Figure 3, the laser beams 12 and 14, at the same frequencies as before, are arranged parallel to one another and are reflected by a prism 46 to the focusing lens 28 which produces the mixed cone 30 of laser light in which the particle 32 is irradiated. As in the first two embodiments, the range of angles present in the cone 30 between the different frequencies enables the CARS condition of beam coherence to be achieved by at least some components of the respective frequencies.
The scattered CARS signal which is produced is condensed by the collector lens 36 to a collimated beam and is filtered by the filter 38 allowing passage of the characteristic 497 nm signal. The signal is directed to the spectrometer 42 by the mirror 40 as before, and an assessment made as to whether such signal is indicative of a diamond particle 32.
Each of the apparatuses described above can form part of a sorting apparatus used to sort diamond particles from associated gangue particles. The particles may be analysed on-line with means being provided to separate those particles identified as diamonds from the other particles. Also, in each of the embodiments described above, the focusing achieved by the lens 28 is such as to limit the intensity of the radiation to which the particles are subjected, thereby to reduce the possibility of radiation-induced damage to diamond particles.
Claims (19)
1.
A method of detecting diamonds, the method comprising the steps of irradiating particles in a beam of laser light formed by focusing two or more laser beams, at least two of which have frequencies differing from one another by a value characteristic of diamond, whereby at least some components of the laser beams focused to form the irradiating beam of laser light are coherently phase-matched, collecting the scattered signal emitted by each particle and determining whether such signal is a CARS signal characteristic of diamond.
A method of detecting diamonds, the method comprising the steps of irradiating particles in a beam of laser light formed by focusing two or more laser beams, at least two of which have frequencies differing from one another by a value characteristic of diamond, whereby at least some components of the laser beams focused to form the irradiating beam of laser light are coherently phase-matched, collecting the scattered signal emitted by each particle and determining whether such signal is a CARS signal characteristic of diamond.
2.
A method according to claim 1 wherein the particles are irradiated by a cone of laser light formed by focusing the two or more laser beams.
A method according to claim 1 wherein the particles are irradiated by a cone of laser light formed by focusing the two or more laser beams.
3.
A method according to claim 2 wherein laser beams are combined collinearly and are focused to form the cone of laser light.
A method according to claim 2 wherein laser beams are combined collinearly and are focused to form the cone of laser light.
4.
A method according to claim 3 wherein one laser beam is reflected by a dichroic mixing plate and another laser beam is passed through the dichroic mixing plate to be combined collinearly with the reflected beam.
A method according to claim 3 wherein one laser beam is reflected by a dichroic mixing plate and another laser beam is passed through the dichroic mixing plate to be combined collinearly with the reflected beam.
5.
A method according to claim 2 wherein laser beams are inclined angularly with respect to one another and are focused to form the cone of laser light.
A method according to claim 2 wherein laser beams are inclined angularly with respect to one another and are focused to form the cone of laser light.
6.
A method according to claim 5 wherein one laser beam is reflected by a mirror and another laser beam is caused to bypass the mirror at an acute angle to the reflected beam.
A method according to claim 5 wherein one laser beam is reflected by a mirror and another laser beam is caused to bypass the mirror at an acute angle to the reflected beam.
7.
A method according to claim 2 wherein laser beams are arranged to be parallel to and spaced apart from one another and are focused to form the cone of laser light.
A method according to claim 2 wherein laser beams are arranged to be parallel to and spaced apart from one another and are focused to form the cone of laser light.
8.
A method according to claim 7 wherein one laser beam is reflected by a dichroic mixing plate and another laser beam is passed through the dichroic mixing plate so as to be parallel to and spaced apart from the reflected beam.
A method according to claim 7 wherein one laser beam is reflected by a dichroic mixing plate and another laser beam is passed through the dichroic mixing plate so as to be parallel to and spaced apart from the reflected beam.
9.
A method according to claim 1 wherein the laser beams are polarised.
A method according to claim 1 wherein the laser beams are polarised.
10.
A method according to claim 9 wherein the scattered signal emitted by each particle signal is filtered to remove wavelengths which are not characteristic of diamond and the filtered signal is analysed to determine whether it is a CARS signal characteristic of diamond.
A method according to claim 9 wherein the scattered signal emitted by each particle signal is filtered to remove wavelengths which are not characteristic of diamond and the filtered signal is analysed to determine whether it is a CARS signal characteristic of diamond.
11.
A method according to claim 10 wherein the filtered signal is polarised.
A method according to claim 10 wherein the filtered signal is polarised.
12.
An apparatus for detecting diamonds comprising means for producing beams of laser light at least two of which have frequencies differing from one another by a value characteristic of diamond, means for focusing the laser beams to form an irradiating beam of laser light by means of which particles undergoing analysis are irradiated, at least some components of the laser beams being coherently phase-matched by the focusing means, means for collecting the scattered signal emitted by each particle and means for determining whether such signal is a CARS signal characteristic of diamond.
An apparatus for detecting diamonds comprising means for producing beams of laser light at least two of which have frequencies differing from one another by a value characteristic of diamond, means for focusing the laser beams to form an irradiating beam of laser light by means of which particles undergoing analysis are irradiated, at least some components of the laser beams being coherently phase-matched by the focusing means, means for collecting the scattered signal emitted by each particle and means for determining whether such signal is a CARS signal characteristic of diamond.
13.
An apparatus according to claim 12 wherein the focusing means focuses the laser beams to form a cone of laser light to irradiate the particles.
An apparatus according to claim 12 wherein the focusing means focuses the laser beams to form a cone of laser light to irradiate the particles.
14.
An apparatus according to claim 13 comprising a dichroic mixing plate arranged to reflect one laser beam and to pass another laser beam collinearly with the reflected laser beam and a lens for focusing the collinear laser beams.
An apparatus according to claim 13 comprising a dichroic mixing plate arranged to reflect one laser beam and to pass another laser beam collinearly with the reflected laser beam and a lens for focusing the collinear laser beams.
15.
An apparatus according to claim 13 laser comprising means for producing laser beams inclined at an acute angle to one another and a lens for focusing the inclined beams.
An apparatus according to claim 13 laser comprising means for producing laser beams inclined at an acute angle to one another and a lens for focusing the inclined beams.
16.
An apparatus according to claim 13 comprising a dichroic mixing plate arranged to reflect one laser beam and to pass another laser beam parallel to but spaced apart from the reflected beam and a lens for focusing the parallel beams.
An apparatus according to claim 13 comprising a dichroic mixing plate arranged to reflect one laser beam and to pass another laser beam parallel to but spaced apart from the reflected beam and a lens for focusing the parallel beams.
17.
An apparatus according to claim 12 comprising polarisers arranged to polarise the laser beams.
An apparatus according to claim 12 comprising polarisers arranged to polarise the laser beams.
18.
An apparatus according to claim 17 comprising a filter for filtering the scattered signal emitted by each particle signal to remove wavelengths which are not characteristic of diamond and means for analysing the filtered signal to determine whether it is a CARS signal characteristic of diamond.
An apparatus according to claim 17 comprising a filter for filtering the scattered signal emitted by each particle signal to remove wavelengths which are not characteristic of diamond and means for analysing the filtered signal to determine whether it is a CARS signal characteristic of diamond.
19.
An apparatus according to claim 18 comprising a polariser for polarising the filtered signal.
An apparatus according to claim 18 comprising a polariser for polarising the filtered signal.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| ZA975676 | 1997-06-26 | ||
| ZA97/5676 | 1997-06-26 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2241470A1 CA2241470A1 (en) | 1998-12-26 |
| CA2241470C true CA2241470C (en) | 2005-06-21 |
Family
ID=25586458
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002241470A Expired - Fee Related CA2241470C (en) | 1997-06-26 | 1998-06-23 | Diamond detection using coherent anti-stokes raman spectroscopy |
Country Status (3)
| Country | Link |
|---|---|
| AU (1) | AU732189B2 (en) |
| CA (1) | CA2241470C (en) |
| RU (1) | RU2180108C2 (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU7660001A (en) * | 2000-08-11 | 2002-02-25 | De Beers Cons Mines Ltd | Diamond detection using coherent anti-stokes raman spectroscopy |
| AU2003212060A1 (en) * | 2002-07-10 | 2004-01-29 | Opdetech Pty Ltd | Sorting Assembly |
| CN109863388B (en) * | 2016-10-24 | 2020-12-25 | 陶朗分选有限责任公司 | Method and system for detecting diamond mark |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4936823A (en) * | 1988-05-04 | 1990-06-26 | Triangle Research And Development Corp. | Transendoscopic implant capsule |
| GB2219394B (en) * | 1988-05-06 | 1992-09-16 | Gersan Ets | Sensing a narrow frequency band of radiation and examining objects or zones |
| ZA955745B (en) * | 1994-08-19 | 1996-02-20 | De Beers Ind Diamond | Classification of particles according to their Raman response |
-
1998
- 1998-06-23 CA CA002241470A patent/CA2241470C/en not_active Expired - Fee Related
- 1998-06-25 RU RU98112329/28A patent/RU2180108C2/en not_active IP Right Cessation
- 1998-06-26 AU AU73205/98A patent/AU732189B2/en not_active Ceased
Also Published As
| Publication number | Publication date |
|---|---|
| RU2180108C2 (en) | 2002-02-27 |
| AU7320598A (en) | 1999-01-07 |
| AU732189B2 (en) | 2001-04-12 |
| CA2241470A1 (en) | 1998-12-26 |
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| EEER | Examination request | ||
| MKLA | Lapsed |