DIAMOND DETECTION USING COHERENT ANTI-STOKES RAMAN SPECTROSCOPY

DETECTION DES DIAMANTS A L'AIDE DE LA DIFFUSION RAMAN ANTI-STOKES COHERENTE

English 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.

Claims

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THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:-
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.
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.
3.
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.
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.

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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.
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.
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.
9.
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.
11.
A method according to claim 10 wherein the filtered signal is polarised.

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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.
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.
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.
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.
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

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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.
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.
19.
An apparatus according to claim 18 comprising a polariser for polarising the
filtered signal.

Description

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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'.

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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.

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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

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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

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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

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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.

Representative Drawing