GB2454192A

GB2454192A - Fibre optic discrimination sensor

Info

Publication number: GB2454192A
Application number: GB0721264A
Authority: GB
Inventors: Jeffrey Marshall; Ross Elliott
Original assignee: Evanesco Ltd
Current assignee: Evanesco Ltd
Priority date: 2007-10-30
Filing date: 2007-10-30
Publication date: 2009-05-06
Also published as: GB0721264D0

Abstract

An evanescent wave fibre optic sensor for discriminating between compositions of a fluid (liquid or gas) under test comprises an optical fibre (Sen) configured to expose to said fluid an evanescent wave generated by light propagating within said optical fibre, an optical transmitter (Tx) to launch said light into said fibre, and an optical receiver (Rx2) to receive light which has propagated from said optical transmitter through said optical fibre, wherein said optical transmitter and said optical receiver are at opposite ends of an optical path through said optical fibre, and wherein said light makes only a single pass through said optical fibre, in a single direction along said optical path. The optical fibre may comprise a plurality of bends each providing a separate evanescent wave sensor region. The fluid may, for example, comprise fuel, lubricant or air. A method of draining surplus water from a fuel tank using such a sensor is also disclosed.

Description

1 2454192 Fibre Optic Discrimination Sensor

FIELD OF THE INVENTION

This invention relates to fibre optic-based sensing systems and methods for discriminating between different compositions of a fluid (liquid or gas), in particular using evanescent waves.

BACKGROUND TO THE INVENTION

We have previously described various techniques for evanescent wave-based sensing, for example in: WO 2006/054117, WO 2006/008557, WO 2005/088270, WO 2005/088274, WO 2005/088278, WO 2005/088277, and WO 04/068123 (all hereby incorporated by reference in their entirety). Further background prior art can be found in: "Optically based biological and chemical detection for defence", Proceedings of SPIE Vol. 6739, Conference 6739B, "September 2007, "Fibre optic system for the detection of uranyl ions in the solution phase", N.W. Hayes, C.J. Tremlett, A.M. Shaw, P.J. Melfi, J.L. Sessler; "Performance optimised optical fibre sensor for humidity sensing", Optical Engineering 44(3) 034401 (March 2005) -Sunil K. Khijwania, Kirthi L. Srinivansan, Jagdish P. Singh; "What Everyone needs to know about evanescent fields", Tom Hunt 01-01-03, www.phvsics.harvard.edu/-tomhunt/nubs/evanescent.pdf "A novel fibre-optic gas sensing configuration using extremely curved optical fibre and an attempt for optical humidity detection", Soichi Otsuki, Kimihiro Adachi, Takahisa Taguchi -Sensors and Actuators B 53 (1998) 9 1-96; "Optical fiber humidity sensor based on a tapered fiber coated with agarose gel", Candido Bariain, Ignacio R.Matias, Francisco J. Arregui, Manuel Lopez-Amo - Sensors and Actuators B 69 (2000) 127- 131; "Effects of geometry on transmission ands sensing of tapered fibre sensors", Angela Leung, Kishan Rijal, P. Mohana Shankar, Raj Mutharasan -Biosensors and Bioelectromes 21(2006) 2202-2209; "Maximum achievable sensitivity of the fibre optic evanescent field absorption sensor based on a U-shaped probe", S.K. KHijwania, B.D. Gupta -Optical communications 175 (2000) 135-137; "Fibre optic evanescent field absorption senor with high sensitivity and linear dynamic range", B.D. Gupta -Optical communications 152 (1998) 259-262; "Plastic optical fiber sensors and devices", Rebecca J Barlett, Rekha Philip-Chandy, Piers Eldridge, David F. Merchant, Roger Morgan, Patricia J. Scully; "Transactions of the institute of measurements and control" 22,5 (2000) pp. 431-457; "Multiple bending loss of inner core light in a special optical fibre for force sensing", Ramesh B. Malla, Eakchat Deerungroy. -16th ASCE Engineering Mechanics Conference. July 16-18 2003; US 2004/0047535; US 5,131,062; US 4,770,047, and US 5,712,934.

However there remains a need for an inexpensive sensing system, with low manufacturing costs.

SUMMARY OF THE INVENTION

According to the invention there is therefore provided an evanescent wave fibre optic discrimination sensor for discriminating between compositions of a fluid under test, the sensor comprising an optical fibre configured to expose to said fluid an evanescent wave generated by light propagating within said optical fibre, an optical transmitter to launch said light into said fibre, and an optical receiver to receive light which has propagated from said optical transmitter through said optical fibre, wherein said optical transmitter and said optical receiver are at opposite ends of an optical path through said optical fibre, and wherein said light makes only a single pass through said optical fibre, in a single direction along said optical path.

The optical fibre may be configured in a number of different ways to expose the evanescent wave at a physical interface of the fibre, for example by bending the fibre, by stretching the fibre to provide a tapered region, and/or by removing part of the cladding of the fibre to expose the core. It will be appreciated that the light propagating within the fibre need not be visible light.

It has been found that for simple discrimination tasks a single path down the fibre is sufficient and no cavity or mirror need be employed. Some preferred embodiments employ plastic fibre. The electronics can also be straightforward, for example having two modes one in which a reference value is stored and a second in which changes from this reference value are detected, preferably at greater than a threshold level, in order to detect differences in composition of the fluid under test. Optionally multiple stored sample readings and/or an average may be employed. These may be stored in non-volatile memory such as Flash and a simple microprocessor such as a PlC (Registered Trade Mark) may be employed to process the data from the sensor. Despite the very high sensitivity of the sensor a simple processing method where discrimination between compositions of a fluid under test is performed by making successive measurements in time and/or space at least of which acts as a reference, has proven to be effective in practice. In some preferred embodiments an optically closed loop is employed at the source (transmitter) end for stability.

In some particularly preferred embodiments an evanescent wave sensing region is formed by bending the optical fibre, which again has the advantage of low manufacturing costs. In embodiments multiple bends are employed to provide multiple separate sensing regions, for example to provide a different sensor on each curve. If the refractive index of the sensed fluid is greater than that of the cladding the cladding may be partially or wholly removed and/or the fibre may be tapered at a bend in order to expose the evanescent wave more fi.illy to the sensed fluid. In embodiments each separate bend is provided with a separate functionalisation ("smart surface") so that each bend is thereby configured for specific sensing of a parameter of the fluid. The functionalising material may comprise a chromophore and is preferably bound to the evanescent wave interface by means of a molecular tether or link. The skilled person will understand that there is a range of techniques which may be employed -for example where the interface comprises silica the tether may be attached by means of an Si-O-Si bond.

In a partially or wholly functionalised version of the sensor the sensed target(s) may be biological or non-biological, living or non-living including, for example, elements, ions, small and large molecules, groups of molecules, bacteria and viruses; targets may comprise a single substance, species or entity or a group of substances, species or entities. In embodiments a protein or a monoclonal (or polyclonal) antibody may be used as the functionalising material, using a natural chromophore or modified to include a chromophore. As the skilled person will understand an antibody may be deposited by many known techniques; an antibody-based sensor may be used, for example, for sensing oestrogen. More generally the functionalising material may comprise a host for a guest species or ligand; in embodiments the flinctionalising material may comprise a crown ether. For radiological sensing ioamethyrin may be employed, which binds to uranyl. Other functionalising materials which may be employed include (c)DNA, RNA, fluorescent molecules; for a pH sensor an amino derivative may be bonded to the surface, for example 3-aminopropyltrimethoxysilane, with an indicator chromophore, comprising, for example, a derivative of medola blue (which changes colour on protonation, shifting the absorbance spectrum).

The invention also provides use of a sensor as described above for discriminating between different compositions of a fluid under test, for example to discriminate between compositions of the fluid with differing water content. Such compositions may comprise fuel in water or water in fuel -for example a dipstick-type sensor configuration may be dipped into a fuel tank, calibrated by establishing a reference value from the optical receiver, and then used to detect, say, a boundary between fuel and water within the tank.

The invention still further provides a method of draining surplus water from a fuel tank containing a combination of water and a substantially colourless fuel less dense than water, said fuel and water being substantially immiscible, the method comprising: draining fluid from the bottom of said fuel tank; monitoring said drained fluid using the apparatus described herein to detect when said drained fluid changes from water to fuel; and ceasing said draining on detecting said change.

In embodiments the apparatus is configured to provide a simple binary indication of whether the drained fluid comprises fuel or water. Embodiments of the technique are particularly suitable for removing surplus water from an aircraft fuel tank containing an aviation fuel such as kerosene.

In a similar way the relative humidity (RH) of a fluid may be sensed or the water content in liquid foodstuffs such as alcohol. Embodiments of the sensor can, for example, be employed to determine the specific gravity of beer. In other embodiments the sensor maybe used to determine a composition of an alcoholic drink, for example whisky, or of other liquid foodstuffs, for example oil such as olive oil. In this latter case a reference level for calibration may be established using one sample, say from a first vat, and this may then be compared with a sensed signal level from a second vat to, for example, determine one or both of oil quality and dilution (the oil may, for example, have been diluted with a cheaper oil such as sunflower seed oil).

In still other embodiments the sensor may be used to determine a degree of lubricant degradation, which may include a component due to particulate scattering (which increases with the increasing degradation). A sensor of the type described above has particular advantages for sensing the condition of lubricant since this is generally black (optically very dense) and thus impossible to sense using conventional optical techniques. By contrast evanescent wave sensing penetrates only a few 100 nm into the oil and hence can sense changes even in fluids with an optical density of greater than 1.0 such as engine oil and crude oil, vistar and the like. In embodiments a sensor as described above may be used as a real-time sensor for monitoring the quality of engine oil in a working engine; data from such a sensor may, for example, be provided to an engine management system and/or used to provide an oil-change warning indication.

As described above, embodiments of the sensor may also be employed for pH sensing and for sensing the growth of living organisms such as bacteria. For example sensing a change in pH may be employed to sense a change in bacteria growth rate and hence a sensor array may be employed to monitor an array of biological growth media and provide an indication of living organism growth or growth rate within the media, for example to provide a go/no-go indication of growth for each sensed growth medium.

In embodiments the sensor can be used to discriminate between compositions of a gaseous fluid by employing a medium to solvate a component of the fluid at the evanescent wave interface. Thus, for example, embodiments of the sensor may be employed to determine the presence of gas in air and/or to discriminate between different gas compositions, in particular where these have a different refractive index (real and/or imaginary component).

The invention still further provides an evanescent wave fibre optic discrimination sensor for discriminating between compositions of a fluid under test, the sensor comprising an optical fibre configured to expose to said fluid an evanescent wave generated by light propagating within said optical fibre, an optical transmitter to launch said light into said fibre, and an optical receiver to receive light which has propagated from said optical transmitter through said optical fibre, wherein said optical transmitter and said optical receiver are at opposite ends of an optical path through said optical fibre, and wherein said optical fibre has a configuration comprising a plurality of bends each providing a separate evanescent wave sensor region.

BRIEF DESCRIPTION OF THE DRAWiNGS

These and other aspects of the invention will now be fI.irther described, by way of example only, with reference to the accompanying figures in which: Figure 1 shows a simplified illustration of a fibre optic sensor system according to an embodiment of the invention; Figure 2 shows an embodiments of an electronic control system for the sensor of Figure I; Figure 3 shows a preferred embodiment of fibre optic discriminator control electronics; Figure 4 shows an example of a curved optical fibre sensor; Figure 5 shows an example of a curved optical fibre sensor with a fibre cladding structure removed to expose the fibre; Figure 6 shows an optical fibre sensor with a chemical or biological agent attached to the surface to functionalise the sensor; Figure 7 shows an optical fibre sensor in the form of a circle or helix with one or more complete turns optionally wrapped around a mandrel; Figure 8 shows an example of an optical fibre sensor with multiple curved sensing regions which may be employed with surface functionalising material to provide a different sensor on each curve; Figure 9 shows an example of an optical fibre sensor with a tapered evanescent wave sensing region; and Figure 10 shows a photograph of a constructed embodiment of a fibre optic discrimination sensor according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Figure 1, the optical discrimination system is based on the use of a fibre optical sensor, and optical transmitter and receiver, and level discriminating electronics.

As an optical signal from the transmitter (Tx), migrates (pmpagates) towards an optical receiver (Rx), it will form an optical evanescent field around the fibre structure. The intensity of the field is governed by the bend radius of the fibre, the angle of the optical ray at the boundary interface, the optical source wavelength, and the difference in the material refractive index between each side of the boundary layer.

The change in RI at this interface causes the optical loss to vary. This optical variation is detected at the optical receiver, converted to an electrical signal, which can be compared to a reference signal. The difference allows the measurement system to discriminate between the test samples, providing the user with a go / no go indication.

The combination of sensor types described, within this application, and electronics, enables an adaptable, small form factor, cost effective system to be realised.

Figure 2 details a schematic for a simple discriminator system. Used in conjunction with the sensors detailed within this application, enables the discrimination between two distinct test samples.

System operation: - * The system is activated using switch (SW 1). This connects the battery to system via a voltage regulator (vreg).

* The optical source (Ix) is biased via a resistor (Ri). This source can be any component that emits either a visible coherent or non-coherent light of either polarized or no polarized light.

* The optical signal from Tx passes through the optical sensor (Sen) and is detected by an optical receiver (Rx).

* Rx is biased directly from the stabilized voltage.

* Comparator (Cp 1) measures the voltage across R2, and compares this to the voltage set on the variable resistor (Rvl). Rvl is varied at setup with the sensor in the reference sample so as to trigger the indicator L2.

* Cp2 in effect is an inverter, providing the means to power Li. The input is obtained from Cpl output, with the trigger threshold set using Rv2.

* Li and L2 provide the user with system feed back.

Figure 3 adds further enhancements to the simple form of the discriminator detailed in figure 2. This system adds a higher degree of complexity enabling the user to expand the discriminator from two states, to n states. It also offers a means of storing recorded data, and wirelessly communicating to a PC. Again this circuit in conjunction with the sensors described provides a unique cost effective use of this sensing technology.

System operation: - * The system is activated using switch (SW 1). This connects the battery to the system via a voltage regulator (vreg).

The optical source (Tx) is in a closed loop control system, which enables the Tx drive current to vary so as to ensure a constant optical power to be maintained on receiver Rxl. The initial Rxl optical power is adjusted using the variable resistor Iset. This provides for a greater degree of sensors stability, dynamic range and smaller detectable differences between sample types. This source can be any component that emits either a visible coherent or non coherent signal of either polarized or no polarized light.

* The optical signal from Tx passes through the optical sensor (Sen) and is detected by an optical receiver (Rx2).

* Rx2 is Ov biased to improve its dynamic range and optical sensitivity. The electrical signal is feed into a variable gain trans-impedance (Tza) amplifier.

This further improves sensitivity and dynamic range of the system.

* The output from the Tza is feed into and analogue to digital converter. The output of which is read by a Programmable Intelligent Computer (PlC).

* The PlC is able to store the ADC values of n samples, and compare the current values to those in this stored table.

* The function selection feature optionally provides the user with the means to select more than one mode of operation or reference/calibration value to employ, that is a nonnal measurement mode or the means to capture reference sample readings 1 to n. The read button is pressed to enable the PlC to capture the required values at the correct time period.

The PlC provides the user with an output in the form of simple indicators, or an LCD screen. The optical sensors used in this application are manufactured from a formed optical fibre.

Figure 4 details the simplest form of sensor used in this application. The curved sensing region is formed by the application of heat while the fiber is formed around a mandrel of the radius.

Figure 5 details the same optical senor detailed in figure 4 but has had the cladding removed either chemically of by the application of heat. Removing the cladding increases the sensors refractive index dynamic range.

Figure 6 shows a sensor which has had its surface chemistry altered. This enables the sensor to detect, depending on the properties of the chemistry, a very wide range of chemical interactions, or biological microbes. The surface can be attached to either the cladding or core surface.

Figure 7 details a sensor formed around a former of the required diameter. The number of complete turns is varied depending on the sensitivity required.

Figure 8 shows a sensor with three sensing regions these can be expanded to n, (n is limited by the overall dynamic range of the measurement electronics). Each sensing region may be modified, with different bend radii, or chemical or biological surface modifications to form a complex (multiplexed) detection system. In embodiments alternate bends are in opposite directions and/or bends are linked by (short) straight sections.

Figure 9 shows a sensor with a tapered sensing region. The taper is formed by locally heating the fiber, and stretching it to reduce the overall local fiber diameter. This modifies the fiber structure which varies the optical critical angle required for total internal reflection, enabling a greater amount of light to leak into the test samples. The further improves the potential dynamic range of the sensing system.

The sensor types described in figures 4 -9 may all be formed into curved sensors, with the cladding structure removed, and / or with a chemically or biologically modified surface.

The sensors detailed above are manufactured from glass, or plastic optical fibres of both stepped and graded index, single or multi mode construction, covering a core diameter of 3um -I 500um. The operating wavelength range covers I OOnm to 2Oum. The sensor is formed into a curve around a former covering the range of 0.3mm to 50mm ii diameters. Depending on the sensor application the optical fibre cladding may be removed to expose the core.

Figure 10 shows an embodiment of an actually constructed device.

We have described a small form factor, low cost, hand-held fibre optic based sensor instrument. Utilises curved or tapered, plastic or glass optical fibres, with or without a smart surfaces. The combination enables a discrimination sensing system for applications requiring simple go/no-go user signals.

Thus we have described a small form factor hand-held fibre optic sensing device, which combines fibre optic sensors and cost effective electronics into a measurement system.

The electronics system discriminates between levels of optical loss as sensed by the optical sensor and displays this to the user. The sensor is useful in measurement applications for liquid, gas, chemical and biological areas. The electronics discrimination is provided by a comparator circuit or a PlC. The sensor may use glass or plastic optical fibres and fibre core sizes in the range of 3um to 1 500um and optical source wavelengths in the range of! Onm to 2Oum. Curved or tapered optical sensors, with fibre cladding removed and/or with cladding in place, may be employed. Curved sensors may have diameters or bend radius/radii in the range of 0.3mm to 50mm (i.e. "macrobends"). Sensing regions with chemical or biological smart surfaces may be employed.

Embodiments provide a means of optical discriminating the difference between various test samples. These samples may be in the form of, for example, a liquid, gas, chemical composition or biological cell.

The detection uses a combination of fibre optic sensors, combined with a cost effective electronics measurement system. This combination enables the manufacture of a small form factor cost effective optically based discrimination system. The sensor may be used in extreme application areas including: liquid/gas fraud detection, chemical type detection, radiological sensing, fluid contamination, fuel type and quality, water quality monitoring, and wine and beer fermentation as well as those previously mentioned.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

CLAIMS: I. An evanescent wave fibre optic discrimination sensor for discriminating between compositions of a fluid under test, the sensor comprising an optical fibre configured to expose to said fluid an evanescent wave generated by light propagating within said optical fibre, an optical transmitter to launch said light into said fibre, and an optical receiver to receive light which has propagated from said optical transmitter through said optical fibre, wherein said optical transmitter and said optical receiver are at opposite ends of an optical path through said optical fibre, and wherein said light makes only a single pass through said optical fibre, in a single direction along said optical path.
2. An evanescent wave fibre optic discrimination sensor as claimed in claim I further comprising control electronics coupled to said optical transmitter and to said optical receiver to control said launch of said light and to process said received light to discriminate between different compositions of said fluid.
3. An evanescent wave fibre optic discrimination sensor as claimed in claim 2 wherein said control of said launch comprises control with an optically closed loop.
4. An evanescent wave fibre optic discrimination sensor as claimed in claim 1, 2 or 3 wherein said optical fibre has a configuration comprising a plurality of bends each providing a separate evanescent wave sensor region.
5. An evanescent wave fibre optic discrimination sensor as claimed in claim 4 wherein said separate sensor regions are differently functionalised to provide a multiple sensor system.
6. Use of a sensor as claimed in any one of claims ito 5 to discriminate between compositions of said fluid with differing water content.
7. Use as claimed in claim 6 wherein said fluid comprises fuel.
8. Use of a sensor as claimed in any one of claims ito 5 to detect lubricant degradation in real-time.
9. Use as claimed in claim 8 to provide an oil-change warning indication.
10. Use of a sensor as claimed in any one of claims 1 to 5 to discriminate between compositions of a gaseous fluid.
11. Use as claimed in claim 10 wherein said gaseous fluid comprises air.
12. A method of draining surplus water from a fuel tank containing a combination of water and a substantially colourless fuel less dense than water, said fuel and water being substantially immiscible, the method comprising: draining fluid from the bottom of said fuel tank; monitoring said drained fluid using the apparatus of any one of claims I to 5 to detect when said drained fluid changes from water to fuel; and ceasing said draining on detecting said change.
13. Use of a sensor as claimed in any one of claims 1 to 5 to discriminate between chemical or biological compositions, and wherein a physical interface of said optical fibre at which said evanescent wave is present is provided with a functionalising material such that different said chemical or biological compositions differently affect said functionalising material to modulate an absorption of said evanescent wave.
14. Use as claimed in claim 13 to provide a go/no-go indication of biological growth in a growth medium.
15. An evanescent wave fibre optic discrimination sensor for discriminating between compositions of a fluid under test, the sensor comprising an optical fibre configured to expose to said fluid an evanescent wave generated by light propagating within said optical fibre, an optical transmitter to launch said light into said fibre, and an optical receiver to receive light which has propagated from said optical transmitter through said optical fibre, wherein said optical transmitter and said optical receiver are at opposite ends of an optical path through said optical fibre, and wherein said optical fibre has a configuration comprising a plurality of bends each providing a separate evanescent wave sensor region.
16. An evanescent wave fibre optic discrimination sensor substantially as herein described and as shown in one or more of Figures 2 to 9.