GB2275348A - Optical fibre probe - Google Patents

Abstract

The probe has a number of multimode fibres, there being at least one delivery fibre 11 adjacent to at least one return fibre 12, the fibres being parallel and in close conjunction at least in the vicinity of their tips, there being optical isolation means eg. a layer of Al 16 between the delivery and the return fibres. There will usually be a plurality, for example three, return fibres surrounding a single delivery fibre. <IMAGE>

Description

OPTICAL SENSORS The present invention relates to optical sensors of the type where a beam of light is projected onto a substance to physically affect that substance such that the substance emits an analysable signal.

One class of sensors of this type uses a fibre optic probe in which a beam of light, for example from a laser, passes through at least one optical delivery fibre before projection onto the substance and the signal is collected by at least one optical collecting fibre adjacent to the delivery fibre.

The Applicant was involved with the analysis of hydrogen isotopes using a probe of the above type and the technique of Laser Raman Spectroscopy. In this technique light is projected at a substance and occasionally light photons are inelastically scattered by molecules within substance as a result of energy being transferred to, or from, a quantised vibrational and/or rotational state of a molecule. The result is a decrease, or an increase, in the energy (or an increase, or decrease, in the wavelength) of a scattered photon, relative to the energy of an incident photon. Such scattered photons can be detected, and are known as Stokes, or anti-Stokes, radiation respectively.

For the purposes of this Specification the light causing excitement of molecules will be referred to as incident light, the light generated as a result of inelastic collisions with molecules will be referred to as signal, and the portion of the signal captured by the collecting fibre or fibres will be referred to as the return light. (It is well known in the art that there is also some scattered light accompanying the return light, but the wavelengths of the return light and scattered light are different).

It will be realised that the ratio of return to incident light is extremely small; for instance in the region of 1x10-15:1. In consequence not only is a powerful incident light required but also it is important that the amount of return light is maximised. The amount of signal that is collected as return light depends, amongst other factors, on the geometry of the optical fibres at the sensing end.

This is a maximum when the tips of the optical fibre carrying the return light are positioned as closely as possible to the tips of the optical fibre carrying the incident light in the vicinity of a sample under test. A convenient way of meeting this requirement is by having the optical fibres parallel to one another and in touching relationship, at least adjacent to the tips thereof.

The Applicant carried out tests using an argon ion laser delivering 1 watt of light of 488 nanometre wavelength through a multimode optical delivery fibre 100 microns in core diameter to a sample gas containing a mixture of hydrogen isotopes. The delivery fibre was surrounded by a number of multimode optical return fibres, each 600 microns in core diameter, the latter being parallel to and in contact with the delivery fibre, the tips of the fibres lying in a common plane. The return fibres were connected to a sensor tuned to detect a set of wavelengths appropriate to the isotopes in the sample gas.

The tests were unsatisfactory, as the amount of background noise, at wavelengths similar to those of the Raman signal, in the return fibres tended to mask the return signal.

The present invention overcomes the problem of background noise.

According to the present invention an optical probe includes a number of multimode optical fibres, there being at least one delivery fibre and at least one return fibre, the fibres being parallel and in close conjunction at least in the vicinity of tips thereof, there being optical isolation means between the delivery and the return fibres.

The optical isolation means might be formed, for example, from a soft metal foil. The metal might be, for example, aluminium.

In one form of the invention one or more delivery fibres might be surrounded by a plurality of return fibres, in which case the isolation means might be in the form of a sheath round the or each delivery fibre.

The tips of the fibres will usually lie in a common plane.

A composite probe might consist of a pack of optical probes according to the invention.

It will be realised that in a probe according to the invention the distance between the tips of the delivery and of the return fibres cannot be minimised as was considered to be significantly advantageous in conventional probes used in this type of work, as the optical isolation means must of necessity have some thickness. However, the thickness of material required to effect the optical isolation is very thin, for example of the order of one micrometre, and this small enforced separation has been found not to significantly reduce the size of the return signal.

As a result of success of the invention, it is now believed that the previous background noise problem was caused by light being transmitted directly from the delivery fibre to the return fibres, despite the fact that with optical fibres of the type used and with the wavelengths of light used such transmission should be impossible.

One embodiment of the invention will now be described, by way of example only. with reference to the accompanying diagrammatic drawings, of which: Figure 1 is an elevation of an optical probe according to the invention connected to a sample box, and Figure 2 is an end view, in section along line II-II of Figure 1.

An optical probe 10 according to the invention has an optical delivery fibre 11, typically 100 microns in core diameter, surrounded by three optical return fibres 12, each typically 600 microns in core diameter. At one end of the probe 10, leading from a plane in which lie tips 13, 14 of the fibres 11, 12, there is a length 15 where the return fibres 12 would contact the delivery fibre 11 were it not for an optically isolating layer 16 (Figure 2) of aluminium foil round the delivery fibre 11. Over the remainder of the length the delivery and return fibres 11, 12 are separated.

In use the probe 10 is positioned with the tips 13, 14 of the fibres 11, 12 in a sample box 17. An end 18 of the delivery fibre 11 remote from the tip 13 is connected to a laser 19, and the return fibres 12 are separated from the delivery fibre 11 to allow ends 20 thereof, remote from tips 14, to be connected to a sensor 21.

The sensor 21 is tuned according to frequencies which might be generated as a result of operation of the sensor, and sample gas is introduced to the sample box 17. The laser is activated, and if there is a gas in the sample gas generating a frequency for which the sensor is tuned then that gas will be detected.

A probe substantially as described above was used to deliver 0.5 Watt of light at a wavelength of 488 nanometres from an argon ion laser 19 to a gas containing hydrogen, deuterium and nitrogen.

Nitrogen generates a signal at a wavelength of 551 nanometres when excited by 488nm light. The sensor 21 was tuned for this wavelength and the presence of the nitrogen was confirmed.

It will be realised that many variations of the above described embodiment are possible within the scope of the invention. For example there are many alternatives to aluminium foil for the optically isolating layer 16. The layer 16 should, of course, be as thin as possible and preferrably should be of a pliable material to resist rupture. Alternatively the metal could be applied to the delivery fibres by a standard coating technique such as electron beam evaporation, sputtering, or even dip coating the optical fibre into molten metal.

A typical probe according to the invention might be two to five metres in length when used for the purpose described above. Clearly, however, there are many alternative uses for probes of this type - for example in the detection and environmental monitoring of gases generated in land fill sites, in the non- invasive testing of gases and liquids for the water, food and semi-conductor industries, in the in- situ analysis of exhaust gases in the automotive industry and in environmental monitoring of industrial waste gases. Probes for some of these uses might be, for example, up to fifty metres in length.

Claims (9)

1. What is claimed is: 1. An optical probe including a number of multimode optical fibres, there being at least one delivery fibre adjacent to at least one return fibre, the fibres being parallel and in close conjunction at least in the vicinity of tips thereof 1 there being optical isolation means between the delivery and the return fibres.
2. 2. An optical probe as claimed in Claim 1 wherein the delivery fibre or fibres are surrounded by return fibres.
3. 3. An optical probe as claimed in Claim 2 wherein the or each delivery fibre has optical isolation means in the form of a sheath.
4. 4. An optical probe as claimed in any one of Claims 1 to 3 wherein the optical isolation means comprises a soft metal foil.
5. 5. An optical probe as claimed in Claim 4 wherein the metal is aluminium.
6. 6. An optical probe as claimed in any on of Claims 1 to 5 wherein the tips of the fibres lie in a common plane.
7. 7. An optical probe substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.
8. 8. An optical probe substantially as herein described.
9. 9. A composite probe consisting of a pack of optical probes according to any one of Claims 1 to 8.