AU2001276600B2

AU2001276600B2 - Diamond detection using coherent anti-stokes raman spectroscopy

Info

Publication number: AU2001276600B2
Application number: AU2001276600A
Authority: AU
Inventors: Hendrik Gerhard Coenraad Human; Nicholaas Johannes Adolf Lubbe; George William Over; Narendra Balagury Viranna
Original assignee: De Beers Consolidated Mines Ltd
Current assignee: De Beers Consolidated Mines Ltd
Priority date: 2000-08-11
Filing date: 2001-08-08
Publication date: 2005-05-19
Anticipated expiration: 2021-08-08
Also published as: AU7660001A; WO2002014837A3; WO2002014837A2; CA2418858A1

Description

WO 02/14837 PCT/IB01/01415 -1- "DIAMOND DETECTION USING COHERENT ANTI-STOKES RAMAN

SPECTROSCOPY"

BACKGROUND TO THE INVENTION THIS invention relates to a diamond detection using coherent anti-Stokes Raman spectroscopy (CARS).

ZA 98/5496, to which-reference should be made for the theory and details, describes a method of detecting diamonds in which particles suspected of containing diamond are irradiated in a beam of laser light formed by focusing multiple laser beams at least two of which have frequencies differing from one another by a value characteristic of diamond. The result is that at least some components of the laser beams which are focused to produce the irradiating beam are coherently phase-matched. The scattered signal emitted by a particle undergoing analysis is collected and analysed to determine whether a CARS signal characteristic of diamond is present.

As explained in the specification of the aforementioned reference, this technique was a development of the basic CARS detection technique in which particles are irradiated by laser beams at a specified angle to one another to ensure phase matching, but in which the problem was encountered that roughness of the particles presented for analysis led to difficulties in controlling beam direction in the particles.

The present invention seeks to provide an enhancement of the method described in ZA 98/5496.

CONFIRMATION COPY WO 02/14837 PCT/IB01/01415 -2- SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a method of detecting diamonds in which each particle which is to be analysed is irradiated by a focused particle irradiating beam of laser light formed by combining initial laser beams having frequencies differing from one another by a value characteristic for diamond, such that components of the initial laser beams are coherently phase-matched, and the scattered signal from the particle is collected and analysed for the presence of a CARS signal characteristic of diamond, wherein the particle irradiating beam and/or at least one of the initial laser beams is focused by a lens system including a cylindrical lens.

Various alternative arrangements are possible. For instance, the initial laser beams could first be combined with one another to form the irradiating beam which is then passed through a cylindrical lens, or an initial laser beam could be passed through a cylindrical lens before being combined with the other initial laser beam. Preferably however the lens system is arranged to focus the irradiating beam to spaced apart focal points, thereby forming a "soft focus" in a focal volume of relatively less intense laser light.

According to another aspect of the invention there is provided an apparatus for detecting diamonds, the apparatus comprising: particle irradiating means for irradiating each particle which is to be analysed by a focused particle irradiating beam of laser light formed by combining initial laser beams having frequencies differing from one another by a value characteristic for diamond, such that components of the initial laser beams are coherently phasematched; and detection means for collecting and analysing the scattered signal from the particle for the presence of a CARS signal characteristic of diamond, WO 02/14837 PCT/IB01/01415 the particle irradiating means comprising a lens system, including a cylindrical lens, for focusing the particle irradiating beam and/or at least one of the initial laser beams.

BRIEF DESCRIPTION OF THE DRAWING 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 and 2 illustrate different embodiments of the invention and Figures 3 shows a perspective view of a cylindrical lens as used in these embodiments.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS In Figure 1 a frequency doubled Nd:YAG laser 10 produces an initial laser beam 12 at a wavelength of 532nm. The beam 12 is expanded by a concave lens 14 and collected by a convex lens 16 which directs the resultant beam onto a beam splitter 18. A portion of the beam 12 is then directed by further mirrors 20 and 22 through a collimator consisting of a lens 24 and pinhole 26, and through an attenuator 28. The attenuator 28 is an optional component which, if present, is used to control the beam intensity and diameter. The beam is then reflected by a mirror 30 through a concave lens 31. The resultant, expanded beam is converged by a convex lens 32 onto a dichroic beam combiner 34. The beam 35 produced by the components 20, 22, 24, 26, 28 (if present) and 30 is referred to below as the stimulating beam.

The portion of the beam 12 which passes through the beam splitter 18 is absorbed by dye in a dye laser 38 which produces a second laser beam 36, referred to below as the probe beam, at a frequency of 572nm. The beam 36 is expanded by a concave lens 40 and the expanded beam is converged by a convex lens 42 onto the dichroic beam combiner 34.

WO 02/14837 PCT/IB01/01415 -4- The beam combiner combines the two initial laser beams to produce a combined particle irradiating beam 44 which is focused as a cone of laser light into a particle analysis zone in which each particle 46 undergoing analysis is presented. The lens system which achieves focusing includes a cylindrical lens 48 in accordance with this invention.

The scattered signal which is emitted by the particle is then collected and analysed for the presence of a CARS signal characteristic of diamond, as described in ZA 98/5496.

Compared to the technique described in ZA 98/5496, it has been observed that enhanced detection of a CARS signal characteristic of diamond is possible as a result of the use of the cylindrical lens 48 to focus the laser beam 44 by which the particle is irradiated.

The lens system described above is set up to produce two spaced apart focal points, referred to herein as a vertical and a horizontal focal point. The vertical focal point is defined by light rays converged in a vertical plane inter alia by the cylindrical lens 48 and is indicated in Figure 3(a) by the numeral 54. The horizontal focal point is attributable to the convergence of the light rays in a horizontal plane by the lenses 32 and 42 and is indicated in Figure 3(b) by the numeral The use of the cylindrical lens 48 produces a beam comprising light rays at a range of angles. This, together with the fact that the two laser beams are individually refracted at slightly different angles on passing through a diamond surface, ensures that amongst all the angular deviations at least some light rays are still in a phase matching relationship to one another.

Compared to the technique described in ZA 98/5496, it is also believed that the technique described above will have the further advantage that there is less likelihood of damage to diamond particles presented for analysis. This is because the presence of two focal points creates, in effect, a threedimensional focal volume in which the particle is positioned for analysis, WO 02/14837 PCT/IB01/01415 compared to a single, well-defined, intense focal point in the technique of ZA 98/5496. In the latter technique, the particle is presented for analysis at a position close to the single, well defined focal point for effective emission of the required CARS signal, thereby creating the potential for possible graphitisation damage to the diamond. In the present technique the required signal can be emitted from the particle at any suitable position within the less intense focal volume provided by the spaced focal points, thereby reducing the chances of such damage to diamond. In this regard it is noted that within the focal volume defined by the spaced focal points of the cylindrical lens, it is not necessary for the phase matching to be exact or for the relevant light rays to be perfectly coincidental. In other words it is not necessary for the particle 46 to be perfectly positioned between the respective focal points.

The focal length of the cylindrical lens determines the extent of the cone of laser light coming to the focal points. The optimal angle between the combined beams in air for adequate phase matching to take place is estimated to be 3,50 in situations where the combined beam is presented at a near normal angle of incidence to the particle surface, but it is recognised that this angle may increase significantly as the angle of incidence on the particles moves away from a near normal value, for example towards the edges of most particles. As in the technique described in ZA 98/5496, it will be understood that the required angular relationships can exist between outer rays in the cone or between inner and outer rays.

In the illustrated embodiment, the combined irradiating beam 44 is focused towards the particle 46 by the lenses 32 and 42 and by the cylindrical lens 48. It is envisaged that advantageous results can also be obtained in alternative embodiments in which a cylindrical lens is placed in the path of the stimulating beam 35 between the mirror 30 and the lens 31, as indicated in broken outline in Figure 1 by the numeral 60, and/or in the path of the probe beam 36 between the dye laser 38 and the lens 40, as indicated in broken outline in Figure 1 by the numeral 62.

WO 02/14837 PCT/IB01/01415 -6- Figure 2 illustrates another possibility in which the stimulating and probe beams are combined with one another before passing through the cylindrical lens. As shown, the cylindrical lens 64 is placed between a dichroic beam combiner 65 and a beam expander 66. It will be understood that the cylindrical lens 64 once again produces a vertical focal point while the beam expander 66 and subsequent beam focusing lens 68 produce the -horizontal focal point.

In both embodiments described above, a dye laser is used to produce the probe beam at the appopriate frequency. The invention is not however limited to the use of dye lasers. For instance, it is envisaged that optical parametric oscillators could be used in other embodiments to produce the probe beam from the stimulating beam. Alternativey separate solid state diode lasers could be used to produce the stimulating and probe beams at the required frequencies.

Claims

1. A method of detecting diamonds in which each particle which is to be analysed is irradiated by a focused particle irradiating beam of laser light formed by combining initial laser beams having frequencies differing from one another by a value characteristic for diamond, such that components of the initial laser beams are coherently phase-matched, and the scattered signal from the particle is collected and analysed for the presence of a CARS signal characteristic of diamond, wherein the particle irradiating beam and/or at least one of the initial laser beams is focused by a lens system including a cylindrical lens.

2. A method according to claim -1 wherein the initial laser beams are first combined with one another to form the irradiating beam which is then passed through a cylindrical lens.

3. A method according to claim 1 or claim 2 wherein an initial laser beam is passed through a cylindrical lens before being combined with the other initial laser beam.

4. A method according to any one of the preceding claims wherein the lens system is arranged to focus the irradiating beam to spaced apart focal points.

WO 02/14837 PCT/IB01/01415 -8- An apparatus for detecting diamonds, the apparatus comprising: particle irradiating means for irradiating each particle which is to be analysed by a focused particle irradiating beam of laser light formed by combining initial laser beams having frequencies differing from one another by a value characteristic for diamond, such that components of the initial laser beams are coherently phase- matched; and detection means for collecting and analysing the scattered signal from the particle for the presence of a CARS signal characteristic of diamond, the particle irradiating means comprising a lens system, including a cylindrical lens, for focusing the particle irradiating beam and/or at least one of the initial laser beams.

6. An apparatus according to claim 5 wherein the cylindrical lens is located in the path of the particle irradiating beam after combination of the initial beams.

7. An apparatus according to claim 5 wherein a cylindrical lens is located in the path of an initial beam before the initial beams are combined.

8. An apparatus according to any one of claims 5 to 7 wherein the lens system is arranged to focus the irradiating beam to spaced apart focal points. WO 02/14837 PCT/IB01/01415 A method of detecting diamonds substantially as herein described with reference to the accompanying drawings. An apparatus for detecting diamonds substantially as herein described with reference to the accompanying drawings.