GB2423816A

GB2423816A - High Temperature ATR Probe

Info

Publication number: GB2423816A
Application number: GB0504148A
Authority: GB
Inventors: Steven Etienne; Ian Weaver
Original assignee: AstraZeneca UK Ltd; Spectraprobe Ltd
Current assignee: AstraZeneca UK Ltd
Priority date: 2005-03-01
Filing date: 2005-03-01
Publication date: 2006-09-06
Also published as: AR052678A1; WO2006092252A3; GB0504148D0; WO2006092252A2

Abstract

The invention provides an attenuated total reflection ATR probe 10, comprising an elongate body 15 with a head 100 at one end, which probe is adapted for use with hot sample fluids and has particular benefits when utilising infra-red radiation in the mid-infra-red region ("MIR") of the spectrum; generally accepted as extending over the wavelength range from 3-10žm. The ATR probe 10 comprises hollow waveguides 60, 70 disposed in a region of the body exposed to relatively high temperatures, and solid fibres 20, 30 disposed in a region of the body exposed to relatively low temperatures. The waveguides and the fibres are arranged to sequentially convey infra-red radiation to and/or from the probe head. The hollow waveguide preferably comprises a light pipe. Also provided is a diamond ATR head (200, Fig. 3) constructed and configured to be utilised at high temperatures, and which enhances the chemical resistance of the ATR probe as a whole to potentially corrosive materials comprised in the sample fluids.

Description

High Temperature ATR Probe This invention relates to attenuated total

reflection (so-called "ATR") probes.

Such probes typically comprise an elongated, tubular body having a specially configured head at one end. Such probes also typically comprise means, remote from the head, for generating infra-red ("IR") radiation, and for passing it along the elongated body to the head, which is placed in communication with a fluid which it is desired to investigate (hereinafter called "sample fluids"); the relative refractive indices of the head material and the sample fluid being contrived such that most of the IR energy is reflected back into the probe, where it is directed onto a sensor device capable of recording the amount of IR energy incident thereon.

Depending upon the composition of the sample fluid, some of the IR energy escapes from the probe head through absorption of energy in the evanescent wave. Hence, the amount of lR radiation received by the sensor, when compared with the known amount of IR radiation originally generated and passed along the probe, provides an indication as to whether or not the sample fluid contains certain constituents, and can also be used to evaluate the relative amounts of such constituents in the sample fluid. The wavelength of the IR radiation used can be varied in a known manner to produce a plot of radiation loss versus wavelength which can be used in known manner to identify or otherwise characterise the sample fluid and/or constituents thereof.

The invention has especial, though not exclusive, application to the transmission of radiation in the mid infra-red wavelength range down a conduit to and from an ATR head in a manner which facilitates usage of the ATR probe in contact with hot sample fluids.

An additional aspect of the invention relates to the incorporation of a diamond AIR head constructed and configured to be utilised at high temperatures, and which enhances the chemical resistance of the ATR probe as a whole to potentially corrosive materials comprised in the sample fluids.

The mid-infra-red region (hereinafter "MIR") of the IR spectrum; generally accepted as extending over the wavelength range from 3-lOj.tm, is of increasing interest to industry for gas or liquid detection or monitoring and/or process control, due to the relative simplicity of the spectra compared to those encountered in the near-infra- red ("NIR"). However, difficulties arise as regards operation in the MIR since, unlike the NIR, where established robust fibre optics are commonplace and can be used at elevated temperatures, currently only two types of fibre are known to be effectively usable within the MIR.

Neither of these fibre types, however, is particularly robust and they can only be operated at moderate temperatures. One such fibre type (chalcogenide) can only be operated at temperatures up to around IOOC, whilst the second type (silver halide) suffers from cold flow, a phenomenon which causes the putty-like silver halide material to flow gradually with time, thereby potentially disrupting the integrity of MIR transmission. Cold flow is exacerbated as the temperature is increased.

Prior attempts to address this vulnerability to high temperature operation have focussed on the provision of cooling systems, typically involving the use of plumbed-in pipe-work to sleeve the tubular body of the AlP probe, but this is costly and adds bulk to the structure, restricting its accessibility to certain environments.

Moreover, although hollow waveguides do not, in general, suffer from the above difficulties, and can be employed in the MIR, their use in ATR probes has hitherto been inhibited, since their use renders it difficult to make the probe as a whole substantially explosion proof. In this connection, it will be appreciated that whilst solid fibres running in and out of a tubular probe can be encapsulated, thereby eliminating flame paths, so rendering the probe substantially explosion proof, this is significantly more difficult to achieve with hollow waveguides.

In relation further to the construction of the ATR head itself, only materials in a limited range, including zinc selenium, AMTIR and silicon, are usable as ATR probe tips at MIR wavelengths. These standard materials are optically efficient because, in addition to transmitting in the MIR, they can easily be fashioned into the required shape or form, such as a corner cube reflector. These materials show varying degrees of chemical resistance. If a particularly aggressive and reactive chemical product is to be probed, however, a diamond tip must be used. This is not only extremely expensive but is optically inferior to probes using tips fabricated from the aforementioned standard materials. Diamond, by necessity of cost and manufacture, tends to be used either in the form of small crystals or in thin sheets. Coupling IR radiation to and from a small crystal or a thin sheet is not optically efficient, and optical throughputs are significantly reduced compared with those achievable for materials that can be made into large corner cube designs. It is also difficult to configure a narrow probe to have a high number of internally reflecting interactions with the sample fluid under investigation.

The present invention aims to address at least one of the foregoing problems or difficulties associated with the fabrication of AIR probes, particularly for use at MI R wavelengths.

In accordance with one aspect of the present invention, an AIR probe formed with an elongate, generally tubular body with a head at one end comprises at least one high temperature tolerant hollow waveguide means disposed in a region of the body exposed, in use of the probe, to relatively high temperatures and at least one solid fibre means disposed in a region of the body exposed, in use of the probe, to relatively low temperatures; the waveguide means and the fibre means being disposed to sequentially convey infra-red ("IR") radiation to and/or from said head.

The, or each, hollow waveguide may conveniently comprise a light pipe, and an internal surface of at least one such light pipe may be coated to reduce radiation losses associated with reflections thereat.

In a preferred embodiment of the invention, a waveguide means and a fibre means are optically coupled together in end-to-end relationship.

In a further preferred embodiment, go and return paths for IR radiation along said tubular probe comprise respectively: (a) a first fibre means optically coupled to a first hollow waveguide means; and (b) a second hollow waveguide means optically coupled to a second fibre means.

It is preferred that the optical coupling comprises at least one lens means, and it is further preferred that the waveguide means is optically coupled to a tip device incorporated in said head of the probe.

The tip device is typically, in operation of the probe, disposed in direct contact with a sample fluid.

In a further preferred embodiment, encapsulant means is provided around the said fibre means and configured and located so as to seal said tubular body.

If desired, a diamond tip device may be provided; said device being resistive to chemical attack by the sample fluid and/or configured to enhance optical coupling and to enable a variable number of interacting internal reflections to be achieved.

A further aspect of the invention provides an AIR probe comprising a tubular body provided at one end with a head intended to be placed in communication with a sample fluid and comprising a substantially planar member formed substantially of diamond; the body containing respective channels for conveying IR radiation towards and away from the head and means for coupling said radiation from and to a coupling zone of said head; wherein the substantially planar member extends away from said coupling zone towards a tip-like extremity of the member; the width of said member being relatively broad in the region of said coupling zone and relatively narrow at said extremity, characterised in that the member is inclined to the axis of the tubular body, causing in use radiation coupled into the member to repeatedly bounce between opposite surfaces thereof to enhance interaction of the radiation with the sample fluid, and that the coupling zone of the member is angled to accommodate the inclination.

In order that the invention may be clearly understood and readily carried into effect, an embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 shows, in longitudinal cross sectional view, components of an ATR probe in accordance with one example of the invention; Figure 2 shows graphs explanatory of relationships between certain parameters of the probe shown in Figure 1; and Figures 3, 4, 5 and 6 show respective views of a preferred configuration usable at the head of an ATR probe.

A standard ATR probe typically constitutes a tubular body with solid fibre optics disposed to run, in a generally longitudinal direction, along a substantial part of the length of the body. The fibre optic material is, as mentioned above, typically either chalcogenide or silver halide. IR radiation, from a suitable source, is focussed into a first fibre and then collimated at the fibre exit plane by an appropriately placed first lens. This radiation is directed toward, and transits, an ATR head which is typically formed with a tip-like extremity, and is then focussed back into a second (return) fibre, disposed parallel to the first fibre, by a second lens placed at an equivalent, but offset, position to the first lens. It is to be noted that the lenses are positioned so as to abut the ATR head. Part at least of the region of the tubular body around the solid fibres is encapsulated to render the probe substantially explosion proof.

It will be appreciated that, throughout this specification, references made to an optical fibre are intended to encompass not only single fibres, but also multi-cored structures and indeed any and all forms of fibrous carrier usable for conveying infra-red radiation in an AIR context.

In accordance with an example of the present invention, and as shown in Figure 1, high temperature operation of an AIR probe 10 is facilitated by incorporating into a tubular body 15 of the probe, made from Hastelloy (Registered Trade Mark) or stainless steel or any other self-supporting and otherwise suitable material, solid fibres 20, 30 and collimating lenses 40, 50 substantially as described above. In this example of the invention, however, the fibres 20 and 30 do not approach the head (100) of the probe; instead, collimated radiation exiting from lens 40 is directed into a hollow waveguide, such as a first light pipe 60, which optically couples the lens 40 to the AIR head 100.

A second hollow light pipe 70, disposed parallel to the first light pipe 60, is used to collect radiation returning from the ATR head and the second lens 50 re-focuses the returning radiation into the second fibre 30. It will thus be appreciated that, so far as radiation travelling along the probe 10 towards the head 100 is concerned, the fibre 20 and the waveguide 60 are disposed to sequentially convey MIR radiation to the head. For radiation returning from the tip 100, on the other hand, the waveguide 70 and the fibre 30 are disposed to sequentially convey MIR radiation away from the head 100, along the probe and towards a suitable sensor (not shown). The fibres 20, 30 and the light pipes 60, 70 are all disposed so as to run generally longitudinally of the tubular body 15 of the probe 10.

An interface adapter 95 locates the waveguides 60, 70 relative to the head 100 and the end of the tubular body 15.

It will be seen from Figure 1 that each fibre 20, 30 and its respective hollow waveguide 60, 70 are optically coupled together, via the lenses 40, 50 respectively, in end-to-end relationship.

Each of the hollow waveguides 60, 70 is, moreover, optically coupled to the head 100 of the probe.

When the head 100 of the probe, which may be configured as a corner cube or a cone, for example, or which may take any other convenient form, is inserted into a hot liquid or exposed to a hot gas, the hollow light pipes 60, 70 are subjected to the high temperature of the sample fluid, but the solid fibres 20, 30 are disposed in a cooler region. Part at least of the region of tubular body 15 surrounding the solid fibres 20, 30 is encapsulated, as indicated at 90, to render the probe 10 substantially explosion proof.

The collimating lens 40, which directs outgoing radiation into light pipe 60 does not yield a perfectly collimated beam of IR due, mainly, to the finite size of the optical components in the probe.

The radiation exiting from lens 40 therefore exhibits a range of angles, centred on a collimated beam, causing some radiation to "bounce" down the inside of the light pipe 60. To minimise losses associated with such "bouncing", the inside of the light pipe 60 is preferably coated with gold, which is highly reflective of PR radiation, and the total transmission of the light pipe can then be estimated as equal to the average reflectance value R) to the power of the average number of reflections (N), T=R. It is clear that the number of reflections down the light pipe should be minimised in order to maximise the transmission of the light pipe.

The average number of reflections can be written as: N = (LID)tanO, where L is the length of pipe, D is the internal diameter and 0 is the average glancing angle. Thus, as the diameter of the light pipe is increased, the transmission of the light pipe will increase as a consequence of a reduced number of reflections.

Generally, the overall system will be characterised by a figure of merit "F/#", with radiation directed from the exit plane of the fibre onto a detector (not shown) at a particular F/# being matched to the FI# of radiation being directed into the fibre 20; radiation overfilling the refocusing collimating' lens, giving a lower FI# than the system F/#, will be lost. If the optical arrangement is symmetrical, with two identical collimating lenses and two light pipes of equal diameter, then the optimum diameter of the light pipes is that which maintains the system F/#. To achieve this, the internal diameters of the light pipe 60 should match the diameter of the radiation beam exiting the first collimating lens 40. If this diameter is 3mm, Figure 2 details the transmission of light pipes of varying length as a function of diameter for radiation of divergence half angIe 12.5 degrees, indicating that, for these particular parameters, optimum transmission occurs at a pipe diameter of 3mm. Here radiation is coupled from a first fibre 20 via a collimating lens 40 to the light pipe 60 - ATR head 100 - light pipe complex and then back to a second fibre 30 using a second (focussing) lens 50, which effectively provides the reverse optical function to the collimating lens 40, to maintain the F/#.

Instead of using collimating/focussing optics it is possible, if the free space radiation divergence of radiation in the system is relatively small, to simply couple optical fibres directly to the respective light pipes. The divergence half angle within the ATR probe is decreased by the ratio of free space to material refractive index. If the free space divergence half-angle is 12 degrees and the fibre material is ZnSe, the equivalent divergence angle within the ATR probe is 5 degrees, which is acceptable. Using this option, a large number of optical fibres can be used to both couple radiation to the output light pipe and collect radiation from the return light pipe.

An additional mafter to be considered in relation to ATR probes used for some purposes is the resistance of the heads to certain chemicals to which they may be exposed.

In this respect, one particularly preferred head arrangement utilises a thin sheet-like member 200 of diamond, shaped and configured as shown in Figure 3. The member 200 tapers to a tip 210, having a 90 degree apex angle, and is formed with an optical insertion/extraction surface 220 (facing into the tubular probe body), angled such that the diamond member 200 as a whole is inclined at 45 degrees to the longitudinal axis of the tubular probe body, and thus to the insertion and extraction radiation beam lines, Jo whilst the insertion/extraction optical surface 220 is disposed normal to these beam line directions. This is shown in Figures 4 and 5. This configuration promotes "bouncing" of radiation inserted into the optical face 220 from a lens collimating from a fibre or from a light pipe (e.g. the light pipe 60 of the probe shown in Figure 1) or any other suitable optical conduit, whereby the radiation propagates through the member 200 by undergoing repeated total internal reflections at 45 degrees at the top and bottom surfaces 230, 240 as shown in figure 4. Radiation is reflected across the tip 210 at its 90 degree apex; travels back to the optical surface 220 and is collected by an element of an optical extraction system (such as the light guide 70 of the probe shown in figure 1). The repeated "bouncing" of the radiation between the top and bottom surfaces of member 200 enhances interaction between the radiation and the sample fluid. By varying the length and width of the diamond member 200, it will be appreciated that different numbers of interacting reflections can be realised.

The diamond member 200 can conveniently be mounted to a probe 10 in the manner indicated in Figure 6, such that its front surface 230 is framed by a suitably angled and knife-edged window aperture 260 supported by the tubular probe body 15 and thereby exposed to the sample fluid. The inner lip of the knife- edged window aperture 260 is provided with a gold frame, against which the front surface 230 is urged by a pressure fitment applied from behind the rear surface 240 of the member 200; and applied to the rear surface 240 by way of a second gold frame squeezed between the periphery of the rear surface 240 and the rear pressure fitment such that the main portion of the rear surface 240 is free standing.

By this means, the diamond member 200 is sealed into the window 260 with low risk of damage, and the majority of its rear surface 240 is left clear of contact with any of the fitting components, thus avoiding any significant compromise on the internal reflection performance of that surface of the diamond 200.

The entrance/exit surface 220 of the diamond member 200 is, in this example, similarly mounted, using a gold frame, to the ATR probe's tubular body. In an alternative constructional technique, the diamond member 200 may be brazed into the body 15 of the probe 10.

The present invention provides a high temperature, substantially explosion proof ATR probe and enables operation of an efficient and easily configurable chemically resistant ATR probe, where these novel concepts can be configured separately or together as a high temperature, substantially explosion proof and chemically resistant ATR probe.

Claims

Claims: 1. An ATR probe formed with an elongate, generally tubular body

with a head at one end comprises at least one high temperature tolerant hollow waveguide means disposed in a region of the body exposed, in use of the probe, to relatively high temperatures and at least one solid fibre means disposed in a region of the body exposed, in use of the probe, to relatively low temperatures; the waveguide means and the fibre means being disposed to sequentially convey infra-red ("IR") radiation to and/or from said head.
2. A probe according to claim 1, wherein the or each hollow waveguide comprises a light pipe.
3. A probe according to claim 2, wherein an internal surface of at least one such light pipe is coated to reduce radiation losses associated with reflections thereat.
4. A probe according to any of claims I to 3, wherein go and return paths for IR radiation along said tubular probe comprise respectively: (a) a first fibre means optically coupled to a first hollow waveguide means; and (b) a second hollow waveguide means optically coupled to a second fibre means.
5. A probe according to any preceding claim, wherein a waveguide means and a fibre means are optically coupled together in end-to-end relationship.
6. A probe according to claim 5, wherein the optical coupling comprises at least one lens means.
7. A probe according to any preceding claim, wherein encapsulant means is provided around the said fibre means and configured and located so as to seal said tubular body.
8. A probe according to any preceding claim, wherein a hollow waveguide means is optically coupled to a tapering head device incorporated at a head of the probe.
9. A probe according to claim 8, wherein the tapering head device is, in operation of the probe, disposed in direct contact with a sample fluid.
10. A probe according to claim 8 or claim 9, wherein said tapering head device comprises a sheet of diamond material.
11. A probe according to claim 10, wherein said sheet is configured to enhance optical coupling and to enable a variable number of internal reflections of the IR radiation to be achieved.
12. A probe according to any preceding claim constructed and configured to utilise mid-infra-red ("MIR") radiation.
13. An ATR probe comprising a tubular body provided at one end with a head intended to be placed in communication with a sample fluid and comprising a substantially planar member formed substantially of diamond; the body containing respective channels for conveying lR radiation towards and away from the head and means for coupling said radiation from and to a coupling zone of said head; wherein the substantially planar member extends away from said coupling zone towards a tip-like extremity of the member; the width of said member being relatively broad in the region of said coupling zone and relatively narrow at said extremity, characterised in that the member is inclined to the axis of the tubular body, causing in use radiation coupled into the member to repeatedly bounce between opposite surfaces thereof to enhance interaction of the radiation with the sample fluid, and that the coupling zone of the member is angled to accommodate the inclination.
14. An ATR probe substantially as herein described with reference to and/or as shown in the accompanying drawings.