WO2014026929A1

WO2014026929A1 - A nosepiece for an optical probe and an optical probe comprising said nosepiece

Info

Publication number: WO2014026929A1
Application number: PCT/EP2013/066749
Authority: WO
Inventors: Berdinus Christianus Maria VROLIJK; Gerrit Jan Pels; Hendrikus Gerardus Maria De Wit
Original assignee: Element Six NV
Current assignee: Element Six NV
Priority date: 2012-08-13
Filing date: 2013-08-09
Publication date: 2014-02-20
Anticipated expiration: 2015-02-13
Also published as: GB201214364D0; GB2506514B; GB2506514A; GB201314318D0

Description

A NOSEPIECE FOR AN OPTICAL PROBE AND AN OPTICAL PROBE COMPRISING SAID NOSEPIECE

Field of Invention

The present invention relates to a nosepiece for an optical probe and an optical probe comprising said nosepiece. The nosepiece and optical probe are particularly configured for use with optical windows fabricated of diamond material. Certain embodiments relate to an optical probe for chemical analysis, particularly in harsh chemical and/or thermal environments.

Background of Invention

The use of diamond material as an optical component such as a flat window, prism, or lens in an optical probe is known. For example, JP 9028715, JP 5095962, US 4,170,997 and US 6,104,853 disclose the use of diamond as a window material disposed in the end of a medical laser probe. Diamond material is useful as a window material as it has low optical absorption. Diamond material has the additional advantage over other possible window materials in that it is mechanically strong, inert, and biocompatible. For example, the inertness of diamond material makes it an excellent choice for use in reactive chemical environments where other optical window materials would not be suitable.

One problem with using diamond as a window material is that the diamond window has a tendency to de-bond from the optical tool to which it is attached, for example due to chemical and/or thermal conditions. Another related problem when faced with designing an optical tool for use in reactive chemical environments is how to improve diamond window bonding whilst also ensuring that the optical tool is chemically inert to the reactive chemical environments in which it is to be used.

An optical probe configured to address these problems is described in the present inventors' earlier publications GB2483768 and WO2012/034926. In those documents it is described that the problem of diamond window de-bonding in a tubular optical probe has been traced to a thermal mismatch between the diamond window and the tubular body to which it is bonded. It has been found that the small end tip of the optical tool can become very hot during operation. Temperatures of approximately 200°C have been observed. This heating effect is exacerbated by the small size of the end tip which requires relatively little energy to rapidly increase in temperature. The increase in temperature causes the diamond window and the tubular body to expand at different rates causing stress at the join between the diamond window and the tubular body. This is because diamond has a much lower coefficient of thermal expansion compared with standard materials used to form the tubular body such as stainless steel. If the stress becomes too large, then the join fails and the window de-bonds from the tubular body. Repeated heating and cooling in use can exacerbate this problem. Furthermore, autoclaving the apparatus in order to sterilize the apparatus between uses can also exacerbate this problem.

A similar problem can also occur during manufacture of the component if any thermal processing steps are utilized. For example, if the diamond window is welded or brazed to the tubular body, strain generated during cooling of the component due to a mismatch in thermal expansion coefficient can cause the diamond window to de- bond.

One way to alleviate some of the aforementioned problems is to provide an efficient cooling system to prevent the end tip from heating too rapidly or becoming too hot. However, incorporating an efficient cooling system can increase the size and complexity of the device which is not desirable.

An alternative possibility is to provide a flexible join between the diamond window and the tubular body. However, flexible joins such as polymer adhesives are susceptible to melting and burning during use and/or are not suitable for autoclaving and/or reactive chemical environments.

An alternative possibility for solving the aforementioned problem it to manufacture the tubular body from a material which has a coefficient of thermal expansion closer to that of diamond than, for example, standard stainless steel. In this regard, stainless steel has a coefficient of linear thermal expansion a of approximately 17 x 10^~6 K^"1 at 20°C (this value varies depending on the particular formulation of stainless steel). Diamond has a coefficient of linear thermal expansion α o f 1.1 x lO^ K^"1 at 20°C. As such, manufacturing the tubular body from a material such as titanium (which has a coefficient of linear thermal expansion α o f 8.6 x 10^"6 K^"1 at 20°C) would be expected to alleviate the problem of diamond window delamination. In practice however, it was found that this did not solve the problem.

The inventors traced the problem with the titanium tubular body arrangement to the relatively poor thermal conductivity of titanium. Titanium has a thermal conductivity of about 21 Wm^K^"1 at 20°C. Heat build-up at the tip of the laser tool around the diamond window is exacerbated if the heat cannot be conducted away from the diamond window. As such, even though the thermal expansion coefficient of titanium and diamond are better matched, a large and rapid increase in temperature during use due to the poor thermal conductivity of titanium offsets this benefit.

One possible way to solve the aforementioned problem is to use a material for the tubular body which has a very high thermal conductivity such as silver (which has a thermal conductivity of 429 Wm 'K^"1 at 20°C) or copper (which has a thermal conductivity of 393 Wm 'K^"1 at 20°C). However, it was found that even though local heat build-up around the diamond window can be reduced using such materials, the thermal expansion coefficients of these materials are too large (19.5 x 10^~6 K^"1 at 20°C for silver and 16.6 x 10^~6 K^"1 at 20°C for copper). As such, a lower and slower increase in temperature is still sufficient to generate enough stress for the diamond window to delaminate from the tubular body.

In light of the above, the inventors found that in order to solve the problem of diamond window delamination, the tubular body to which it is bonded must be made of a material having a relatively low thermal expansion coefficient and a relatively high thermal conductivity. The inventors found that tubular body can be made of a material having a coefficient of linear thermal expansion a of 14 x 10^~6 K^"1 or less at 20°C and a thermal conductivity of 60 Wm^K^"1 or more at 20°C. Various materials fall within these ranges, molybdenum being an example. However, while the use of such a tubular body can solve the problem of diamond window delamination, materials such as molybdenum are relatively reactive and are not suitable for use in reactive chemical environments such as highly acidic environments.

One possible way of getting around this further problem is to coat the tubular body with a non-reactive coating such as a gold coating. However, it has been found that such coatings can be readily scratched thus exposing the underlying material which then can react with the external chemical environment degrading the optical probe and contaminating the chemical environment in which the probe is placed. This problem can be remedied by making the tubular body of a chemically inert material such as hastelloy C-276. However, such chemically inert materials do not generally meet the dual requirements of a low thermal expansion coefficient and a relatively high thermal conductivity. As such, the diamond window will tend to delaminate for the reasons previously described.

In order to solve the aforementioned problem, the inventors have realized that to provide a chemically inert optical probe with a diamond window requires both a chemically inert tubular body and a nosepiece or mounting ring which is made of a material having a coefficient of linear thermal expansion a of 14.0 x 10^"6 K^"1 or less at 20°C and a thermal conductivity of 60 Wm ' ^"1 or more at 20°C. The mosepiece/mounting ring is mounted within the chemically inert tubular body and the diamond window is bonded to the nosepiece/mounting ring. An optical fibre is mounted within the chemically inert optical probe to address a rear surface of the diamond window. In such an arrangement, the chemically inert tubular body protects the mounting ring from the external chemical environment and the mounting ring provides a reliable bonding to the diamond window to prevent delamination. If any exposed areas of the mounting ring remain around the diamond window they can be coated with an inert material such as gold to prevent adverse reactions.

While the aforementioned configuration as described in GB2483768 and WO2012/034926 has been found to work well at temperatures below 400°C, at temperatures above 400°C it has been found that the optical fibre within the probe tends to melt. It is an aim of certain embodiments of the present invention to solve the aforementioned problem. In particular, certain embodiments of the present invention seek to provide an optical probe which is stable, reliable, has improved lifetime, and can be made small in size. Particular embodiments are aimed at providing a chemically inert optical probe with a diamond window, particularly for insertion into harsh hot chemical environments to perform spectroscopic analysis such as in a chemical reactor. Application areas include the pharmaceuticals industry where manufacturers are required to provide process data including a chemical analysis of their processes.

Summary of Invention

According to a first aspect of the present invention there is provided an optical probe comprising:

an outer tube defining an internal channel and an opening, wherein the outer tube is made of a chemically inert material;

a nosepiece mounted within the outer tube at the opening, the nosepiece defining an aperture for an optical window and being formed of a material having a coefficient of linear thermal expansion a of 14.0 x 10^"6 K^"1 or less at 20°C and a thermal conductivity of 60 Wm 'K^"1 or more at 20°C;

an optical window disposed across the aperture of the nose piece and bonded to the nosepiece around the aperture, the optical window being formed of a diamond material,

an optical fibre extending through the internal channel of the outer tube and the aperture of the nosepiece to a rear surface of the optical window providing an optical light path to the rear surface of the optical window, and

a coolant system comprising:

an inlet channel extending through the internal channel of the outer tube to the nosepiece;

an outlet channel extending through the internal channel of the outer tube from the nosepiece; and

a linking channel configured to link the inlet and outlet channels at the nosepiece. As described in the background section, an optical probe configured to alleviate problems of window de lamination due to thermal mismatch has been previously described in GB2483768 and WO2012/034926. These documents describe the use of a nosepiece mounted in an inert tube, the nosepiece being formed of a material having a coefficient of linear thermal expansion a of 14.0 x 10^"6 K^"1 or less at 20°C and a thermal conductivity of 60 Wm 'K^"1 or more at 20°C. As further described in the background section, while such a configuration has been found to work well at temperatures below 400°C, at temperatures above 400°C it has been found that the optical fibre within the probe tends to melt. Accordingly, the present invention solves this problem by combining a low thermal expansion coefficient, high thermal conductivity nosepiece with a cooling system. It has been found that this combination of features leads to a more thermally stable optical probe configuration which is resistant to both diamond window delamination and optical fibre melting.

As described in GB2483768 and WO2012/034926, incorporating an efficient cooling system can increase the size and complexity of a device which is not desirable. GB2483768 and WO2012/034926 teach away from the use of a cooling system and instead suggest selecting suitable materials to alleviate thermal mismatches thereby allowing the optical probe to be made small and simple in design. Given the new problem of optical fibre melting at very high temperatures it would appear that some cooling system is required to alleviate this new problem.

One further technical challenge is then how to configure such a cooling system to prevent melting of the optical fibre while retaining a small and simple design for the optical probe. Preferred embodiments of the present invention have solved this further technical challenge by modifying the optical probe as previously disclosed in GB2483768 and WO2012/034926 to include a cooling system which fits neatly into the optical probe and allows cooling without unduly increasing the size and complexity of the device. This has been achieved according to certain embodiments by modifying the high thermal conductance / low thermal expansion coefficient nosepiece to include a groove on an exterior surface thereof which is linked to an interior of the nosepiece via two through holes. The through holes and the groove thus form a cooling channel extending from the interior of the nosepiece, through a first through hole, along a linking channel defined by the groove and the interior surface of the tube into which the nosepiece is mounted, and through a second through hole back to the interior of the tubular body. The through holes can readily be addressed by inlet and outlet channels extending into an interior of the nosepiece. As such, it is possible to provide a cooling system which is sufficient to prevent melting of the optical fibre without increasing the size of the optical probe and without unduly increasing its complexity.

Yet another technical problem has also been identified when providing a cooling system in close proximity to the optical fibre in the optical probe according to embodiments of the present invention. For infrared spectroscopic applications silver halides are considered the material of choice for the optical fibre. However, silver halides are extremely reactive and degrade when placed in contact with most metals. This has been found to be problematic when it is desired to place coolant tubes in close proximity to the optical fibre if the coolant tubes are made of standard metal materials such stainless steel or copper. To solve this problem, the present inventors have found that it is advantageous to form the inlet and outlet channels of the coolant system using a chemically inert material and/or provide a shielding tube formed of a chemically inert material around the optical fibre.

Brief Description of the Drawings

For a better understanding of the present invention and to show how the same may be carried into effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 illustrates a perspective view of a nosepiece for an optical probe according to an embodiment of the present invention;

Figure 2 illustrates a side cross-sectional view of a nose piece for an optical probe according to an embodiment of the present invention; Figure 3 illustrates a side cross-sectional view of an optical probe according to an embodiment of the present invention;

Figure 4 illustrates a side view of an optical probe according to an embodiment of the present invention; and

Figure 5 illustrates an end view of an optical probe according to an embodiment of the present invention.

Detailed Description of Certain Embodiments

Figure 1 illustrates a perspective view of a nosepiece for an optical probe according to an embodiment of the present invention. The nosepiece 2 comprises a tubular body 4 and an end plate 6. The end plate 6 includes an aperture 7 for an optical window. The nosepiece 2 is preferably formed of a unitary body of material although could in principle be made of separate component parts. The material of the nosepiece is selected to have: (i) a coefficient of linear thermal expansion a of 14.0 x 10^"6 K^"1 or less, 12.0 x 10^"6 K^_1or less, 10.0 x 10^"6 K^"1 or less, 8.0 x 10^"6 K^"1 or less, 6.0 x 10^"6 K^"1 or less, or 4.0 x 10^"6 K^"1 or less at 20°C; and (ii) a thermal conductivity of 60 Wm ' ^"1 or more, 80 Wm^K^"1 or more, 100 Wm^K^"1 or more, 120 Wm^K^"1 or more, and 140 Wm 'K^"1 or more at 20°C. The nosepiece is preferably formed of at least 50% of said material, at least 70% of said material, at least 80% of said material, at least 90% of said material, or at least 95% of said material. The material may be a metal, an alloy, a ceramic, or a composite material. For example, the material may comprise one or more of molybdenum, chromium, tungsten, nickel, rhodium, ruthenium, diamond, silicon carbide (SiC), tungsten carbide (WC), aluminium nitride (A1N), titanium zirconium molybdenium (TZM), tungsten nickel iron (WNiFe), and tungsten nickel copper (WNiCu). Another possibility is to manufacture the mounting ring from a diamond material such as polycrystalline CVD diamond.

Molybdenum has been found to be particularly preferred due to its thermal properties (a low thermal expansion coefficient of 5 x 10^"6 K^"1 and a relatively high thermal conductivity of 144 Wm^"' ) and because it is relatively easy to work the material to form the nosepiece.

In the above respects, the nosepiece is similar to that described in GB2483768 and WO2012/034926. The nosepiece differs in that a groove 8 has been provided on an exterior surface of the tubular body 4. This groove 8 is linked to the interior of the nosepiece via two through holes which are better seen in the cross-sectional view of Figure 2. As can be seen in Figure 2, the first and second through holes 10, 12 and the groove 8 form a cooling channel from an interior of the nosepiece, through the first through hole 10, along the groove 8 in the exterior surface of the tubular body, and through the second through hole 12 to the interior of the nosepiece. The groove 8 forms a spiral path in the exterior surface of the tubular body 4 from the first through hole 10 to the second through hole 12. However, other shapes of groove are also envisaged.

As illustrated in Figure 2, the nosepiece may further comprise an internal tubular section 14 which extends from a rear surface of the end plate 6 around the aperture 7 to form a central channel 16 for receiving an optical fibre and outer channels 18 for receiving inlet and outlet coolant tubes. The outer channels 18 include the first and second through holes 10, 12 which link the outer channels 18 to the groove 8 on the exterior surface of the tubular body 4. This internal structure allows optical fibres and coolant tubes to be correctly positioned, aligned, and mounted within the nosepiece for optically addressing the aperture and aligning coolant tubes with the through holes for coolant to be directed around the external groove.

As further illustrated in Figure 2, the internal tubular section 14 preferably has an internal taper 20 for receiving an optical fibre or fibres. The internal taper 20 aids in aligning and mounting an optical fibre within the nosepiece.

As also shown in Figures 1 and 2, the aperture 7 in the end plate 6 is recessed, the end plate 6 forming a recessed border region 22 around the aperture 7 for receiving a bonding material to bond an optical window made of diamond material over the aperture 7. The recessed border region 22 aids in retaining the bonding material in the correct location while bonding the optical window and can also aid in maintaining correct alignment of the optical window. For example, the optical window can be bonded to the nosepiece by a metal braze join such as a gold and/or tantalum braze which is disposed within the recessed border region. Such braze joining has been found to be particularly useful for adhesion of an optical window formed of diamond material. The window may be flat or may be some other shape such as a prism or curved lens.

Figure 3 illustrates a side cross-sectional view of an optical probe according to an embodiment of the present invention. The optical probe comprises an outer tube 30 defining an internal channel and an opening in which the nosepiece 2 is mounted. A window 32 disposed across the aperture of the nose piece 2 and bonded 33 to the nosepiece 2 around the aperture. An optical fibre 34 extends through the internal channel of the outer tube 30 and the tubular body of the nosepiece 2 to a rear surface of the window 32 providing an optical light path to the rear surface of the window. The optical probe further comprises a coolant system including an inlet channel 36 extending through the internal channel of the outer tube 30 to an interior of the nosepiece 2 and an outlet channel 38 extending through the internal channel of the outer tube 30 from an interior of the nosepiece 2.

As previously described in relation to Figure 2, Figure 3 also illustrates the nosepiece structure comprising an internal tubular section 14 which extends from a rear surface of the end plate 6 to form a central channel 16 for receiving the optical fibre 34 and outer channels 18 for receiving the inlet and outlet coolant tubes 36, 38. The outer channels 18 include the first and second through holes 10, 12 which link the outer channels 18 to the groove 8 on the exterior surface of the nosepiece. The internal tubular section 14 also has an internal taper for receiving the optical fibre.

The outer tube 30 is formed of a chemically inert material. The nosepiece may be mounted within a chemically inert outer tube in a variety of ways. However, bolting or clamping arrangements can be complex and unreliable, especially under thermal cycling. It has been found that a press-fit (pressure fitted) mounting ring provides a simple and reliable configuration using the previously described materials. A metallic braze such as a gold nickel braze may also be used to bond the nosepiece into the outer tube and prove a seal between the nosepiece and the outer tube at an end portion 40. In the illustrated embodiment the outer tube 30 is provided in two pieces include an end piece 42 in which the nosepiece is mounted and a second tubular section 44. These two pieces of the outer tube can be laser welded together at an outer tube join 45. The end piece 42 can be specifically configured to receive the nosepiece. For example, in the illustrated embodiment the end piece 42 includes a step portion 46 located at a distance from the end of the outer tube which is equivalent to the length of the nosepiece. This step portion 46 aids in mounting and locating the nosepiece within the outer tube.

By providing a nosepiece made of a material which is selected to achieve reliable bonding to a diamond window, and using the above described mounting configuration, the outer tube can then be selected from a range of chemically inert materials according to the optical probes intended use. By chemically inert, we mean that the outer tube is more chemically inert than the nosepiece. For example, the outer tube may be made of a material which is less reactive to acidic environments than the nosepiece. Example materials include inert and/or corrosion resistant alloys, ceramics, or composite materials. Nickel alloys have been found to be useful and particularly the commercially available Hastelloy™ superalloys such as Hastelloy C- 276, a corrosion resistant Nickel-Molybdenum-Chromium alloy with addition of Tungsten. However, it will be understood that the chemically inert material may be selected according to the end use of the optical probe.

The chemically inert outer tube may be selected to have a variety of possible shapes according to its intended use. For many applications a circular or oval cross-sectional shape will be suitable. As previously described, the present invention is particularly useful for small components. Accordingly, the largest external diameter of the outer tube may be 30 mm or less, 20 mm or less, 15 mm or less, or 10 mm or less. The term "largest external diameter" is simply the diameter when the outer tube has a circular cross section. When the outer tube has a non-circular cross-section, such as an oval cross-section, then the largest external diameter refers to the largest distance across the cross-section in a direction perpendicular to a longitudinal axis of the outer tube. The outer tube may have a wall thickness of 0.5 mm to 4 mm, 0.5 mm to 3.0 mm, 0.5 mm to 2.0 mm, 0.7 mm to 1.2 mm, or 0.8 mm to 1.0 mm. Furthermore, the internal diameter of the outer tube may be 0.2 mm to 20 mm, 0.3 mm to 15 mm, 0.4 mm to 10 mm, or 0.5 mm to 5 mm. Diamond window de-bonding has been found to be problematic for such a small chemically inert tubular body unless provided with an internal nosepiece according to the present invention.

In addition to the provision of a chemically inert outer tube 30, it has also been found to be advantageous to fabricate components disposed within the outer tube using chemically inert materials. As indicated in the summary of invention section, silver halide optical fibres are considered the material of choice for infrared applications but such materials have been found to be extremely reactive and degrade when placed in contact with most metals. This has been found to be problematic when it is desired to place coolant tubes in close proximity to the optical fibre if the coolant tubes are made of standard metal materials such stainless steel or copper. To solve this problem, the present inventors have found that it is advantageous to form the inlet and outlet channels of the coolant system using a chemically inert material and/or provide a shielding tube formed of a chemically inert material around the optical fibre. As such, optical probes according to embodiments of the present invention are configured to have both internal and external thermal and chemical stability.

The diamond window may be formed of single crystal or poly crystalline diamond material. Furthermore, the diamond may be CVD diamond material, HPHT diamond material or natural diamond. The diamond material should preferably be of an optical grade and may have an absorption coefficient β equal to or less than 0.1 cm^"1, 0.05 cm^"1, 0.01 cm^"1, 0.005 cm^"1, or 0.001 cm^"1 at an operating wavelength of the laser tool. An exemplary operating wavelength is in the mid- infrared in the range 2 to 10 μιη. For example, the optical probe may be configured to perform infrared spectroscopy, more particularly attenuated total reflectance (ATR) spectroscopy. In such an application, the optical component may be configured to couple to two optical fibres. One fibre is arranged to transmit an infrared beam into the sample and the other fibre is arranged to receive light reflected from the sample. This light may then be analysed to perform infrared spectroscopy. However, it should be noted that due to diamond material's low absorption across the infrared and visible region of the spectrum, other operating wavelengths may be utilized and the present invention is not limited to this infrared application.

The diamond window is advantageously provided with a metallization coating on an internal surface in an area around the aperture. The metallization coating can be provided on the diamond window between the diamond window and an inert metal braze join. The metallization coating may comprise a layer of a carbide- forming metal. The metallization coating may further comprise an inert metal barrier layer between the layer of carbide forming metal and the braze join. By inert metal barrier we mean a material which is less reactive with the braze material than the carbide forming metal. Such a metallization coating aids bonding between the diamond window and the nosepiece. A preferred metallization coating comprises a layer of a carbide forming material such as titanium, an inert barrier layer such as platinum, and a metal layer such as gold for soldering or brazing to the nosepiece. The titanium provides a good bond with the diamond forming titanium carbide at an interface with the diamond. The gold provides a good bond to the nosepiece. The platinum functions as an inert shield between the gold and titanium. Furthermore, as previously stated, a braze join can be provided between the metallization coating and the nosepiece for bonding the diamond window to the nosepiece. A gold-tantalum braze has been found to be useful as it is chemically inert and non-corrosive. The braze material can extend around the periphery of the diamond window to cover any exposed portions of the mounting ring around the diamond window and prevent adverse reaction of the mounting ring material with the surrounding chemical environment in use.

The present invention is particularly useful when applied to a small diamond window which is more susceptible to heating. Accordingly, the diamond window may have a thickness in the range 0.1 mm to 0.5 mm, 0.1 mm to 0.3 mm, 0.1 mm to 0.2 mm, or 0.1 mm to 0.2 mm. Furthermore, the diamond window may have a longest dimension in the range 1.0 mm to 10.0 mm, 1.0 mm to 8.0 mm, 1.5 mm to 5.0 mm, or 2.5 mm to 3.5 mm. Further still, the diamond window may have a width in the range 1.0 mm to 5.0 mm, 1.0 mm to 3 mm, or 1.5 mm to 2.5 mm. The overlap between the diamond window and the nosepiece around the aperture may be 0.2 mm to 1.0 mm, 0.3 mm to 0.8 mm, 0.35 mm to 0.6 mm, or more preferably 0.4 mm to 0.5 mm. The relative size of the diamond window and the aperture can be selected to achieve a reliable bond between the diamond window and the nosepiece around the aperture and to provide an optimal thermal contact between the diamond window and the nosepiece.

One further problem which the present inventors have identified is that the diamond window can be damaged if it is knocked by an external member, such as a stirring apparatus within a chemical reactor in which the probe is placed. As such, it has been found to be advantageous to position the diamond window in a recess of the outer tube such that at least a portion of the outer tube extends beyond the diamond window to protect it from damage. The diamond window is thus disposed in a recess across the aperture, the diamond window having an area less than an area of the recess and greater than an area of the aperture. This arrangement ensures that the diamond window extends across the entire aperture while being wholly located within the recess such that that diamond window is protected.

The aforementioned recess may be formed by a plurality of projections with a plurality of openings disposed therebetween around the diamond window. The openings allow fluid to flow more readily into and out of the recess so that fluid adjacent to the diamond window is representative of the composition of a sample which is being analysed. Without such openings, it has been found that fluid can become trapped in the recess such that it doesn't mix and react with other components in the sample resulting in misleading readings for applications such as attenuated total reflection spectroscopy. Such an arrangement is illustrated in Figure 4 which illustrates a side view of an optical probe according to an embodiment of the present invention. The diamond window 32 is disposed in a recess formed by a plurality of projections 50 of the outer tube 30. Figure 5 illustrates an end view of such an optical probe including the diamond window 32 mounted on the nosepiece 2 and disposed in a recess formed by the plurality of projections 50 of the outer tube. In Figure 5 cross- section A- A corresponds to the cross-sectional view shown in Figure 3. The diamond window is preferably bonded to the nosepiece around the aperture to form a seal all around the aperture. This will prevent fluid or other debris entering the tubular body and fouling the optical fibre.

In addition to the problem of the diamond window de-bonding, the present inventors have identified another problem associated with undue heating of the diamond window in an optical tool. As stated in US 4,170,997, diamond has a high refractive index. As such, US 4,170,997 teaches that it is advantageous to apply an antireflective coating to the diamond window to maximize light transmission. This may be applied on one or both sides of the diamond window. Indeed, it is well known to use such an antireflective coating when using diamond as a window material for optical applications. However, when used in small devices such as optical probes, the present inventors have found that heating during operation leads to de lamination of the antireflective coating. Delamination of the antireflective coating can lead to the formation of hot-spots in the window material and possible fracture of the window. Accordingly, for small devices it can be advantageous to use a diamond window without an antireflective coating.

Optical probes according to embodiments of the present invention are configured to have internal and external thermal and chemical stability. Thermal stability is provided by the use of a low thermal expansion coefficient, high thermal conductivity nosepiece in combination with a coolant system to alleviate problems of diamond window delamination and optical fibre melting. Chemical stability is provided by the use of a chemically inert outer tube and a chemically inert cooling system and/or shielding to alleviate problems of external corrosion in harsh chemical environments and internal corrosion due to adverse reaction of the optical fibre material with other internal components.

While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appendant claims.

Claims

1. An optical probe comprising:

a coolant system comprising:

a linking channel configured to link the inlet and outlet channels at the nosepiece.

2. An optical probe according to claim 1, wherein the nose piece comprises a tubular body and an end plate, the end plate defining the aperture for the optical window and the tubular body comprising a groove on an exterior surface of the tubular body forming the linking channel of the coolant system, wherein a first through hole in the tubular body links the inlet channel of the coolant system in an interior of the tubular body to the groove on the exterior surface of the tubular body, and wherein a second through hole in the tubular body links the groove on the exterior surface of the tubular body to the outlet channel in the interior of the tubular body.

3. An optical probe according to claim 1 or 2, wherein the nosepiece further comprises an internal tubular section forming a central channel for receiving the optical fibre and an outer channel for receiving inlet and outlet coolant tubes.

4. An optical probe according to claim 3, wherein the internal tubular section has an internal taper for receiving the optical fibre.

5. An optical probe according to any preceding claim, wherein the aperture in the nosepiece is recessed forming a recessed border region around the aperture for receiving a bonding material to bond the optical window over the aperture.

6. An optical probe according to any preceding claim, wherein the coefficient of linear thermal expansion a is one of: 12.0 x 10^"6 K^_1or less; 10.0 x 10^"6 K^"1 or less; 8.0 x 10^"6 K^"1 or less; 6.0 x 10^"6 K^"1 or less; and 4.0 x 10^"6 K^"1 or less.

7. An optical probe according to any preceding claim, wherein the thermal conductivity is one of: 60 Wm 'K^"1 or more; 80 Wm 'K^"1 or more; 100 Wm-'K^"1 or more; 120 Wm^K^"1 or more; and 140 Wm 'K^"1 or more.

8. An optical probe according to any preceding claim, wherein the tubular body and end plate are formed of at least 50% of said material, at least 70% of said material, at least 80% of said material, at least 90% of said material, or at least 95% of said material.

9. An optical probe according to any preceding claim, wherein the material of the tubular body and end plate is a metal, an alloy, a ceramic, or a composite material.

10. An optical probe according to any preceding claim, wherein the material of the tubular body and end plate comprises one or more of molybdenum, chromium, tungsten, nickel, rhodium, ruthenium, diamond, silicon carbide (SiC), tungsten carbide (WC), aluminium nitride (A1N), titanium zirconium molybdenium (TZM), tungsten nickel iron (WNiFe), and tungsten nickel copper (WNiCu).

11. An optical probe according to any preceding claim, wherein the optical window is bonded to the nosepiece by a metal braze join.

12. An optical probe according to claim 11, wherein the metal braze join comprises gold and/or tantalum.

13. An optical probe according to any preceding claim, wherein the optical window is in the form of a prism.

14. An optical probe according to any preceding claim, wherein the outer tube comprises an inert and/or corrosion resistant alloy, ceramic, or composite material.

15. An optical probe according to any preceding claim, wherein the optical fibre is formed of a silver halide.

16. An optical probe according to any preceding claim, further comprising a second optical fibre arranged to receive light reflected from a sample.

17. An optical probe according to any preceding claim, wherein the inlet and outlet channels of the coolant system are formed of a chemically inert material.

18. An optical probe according to any preceding claim, further comprising a shielding tube provided around the optical fibre, the shielding tube formed of a chemically inert material.

19. An optical probe according to any preceding claim, wherein the optical probe is configured to function as a reflectance spectrometer.

20. An optical probe according to any preceding claim, wherein the optical probe is configured to perform attenuated total reflectance spectroscopy.